The Pennsylvania State University

The Graduate School

Department of Physics

A SOLID-STATE HEAT PUMP USING ELECTROCALORIC

CERAMIC ELEMENTS

A Dissertation in

Physics

by

Matthew G. Hilt

c© 2009 Matthew G. Hilt

Submitted in Partial Fulfillmentof the Requirements

for the Degree of

Doctor of Philosophy

May 2009

The dissertation of Matthew G. Hilt was reviewed and approved* by the following:

J. D. MaynardProfessor of PhysicsDissertation AdviserChair of Committee

G. D. MahanProfessor of Physics

Peter SchifferProfessor of Physics

Victor SparrowProfessor of Acoustics

Jayanth BanavarProfessor of PhysicsHead of the Department of Physics

*Signatures are on file in the Graduate School.

iii

Abstract

The thermoacoustic cycle is a robust thermodynamic cycle that can be generalized

to describe and develop an all-solid-state heat pump using generic caloric elements.

Ferroelectric barium strontium titanate (BST) and relaxor lead magnesium niobate -

lead titanate (PMN-PT) are two candidate materials for the caloric elements using the

electrocaloric effect. I developed a procedure to repeatably produce high quality BST and

PMN-PT ceramics so that the electrocaloric and dielectric properties could be accurately

measured. The measured electrocaloric properties serve as the baseline numbers for

calculating the performance of a proposed all-solid-state cooler based on thermoacoustic

principles.

iv

Table of Contents

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii

Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.1 Early history . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.2 Modern Refrigerators . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Thermoacoustics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.1 Thermoacoustics, quantified . . . . . . . . . . . . . . . . . . . 9

1.3 Solid-state refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4 Perovskite ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4.1 Ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.4.2 Relaxors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.4.3 Ferroelectric thermodynamics . . . . . . . . . . . . . . . . . . 15

Chapter 2. General comments about ceramics and their processing . . . . . . . . 20

2.1 Preparing ceramic-grade powder . . . . . . . . . . . . . . . . . . . . 21

2.1.1 Mixing the components . . . . . . . . . . . . . . . . . . . . . 24

2.1.2 Calcining the powder . . . . . . . . . . . . . . . . . . . . . . . 27

v

2.1.3 Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.2 Preparing ceramic samples . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.1 Binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.2 Pressing ceramic pellets . . . . . . . . . . . . . . . . . . . . . 33

2.2.3 Sintering the pellets . . . . . . . . . . . . . . . . . . . . . . . 35

2.3 Preparing samples for measurement . . . . . . . . . . . . . . . . . . . 38

2.3.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.4 Measurements on ceramic wafers . . . . . . . . . . . . . . . . . . . . 40

2.4.1 Physical property measurement methods . . . . . . . . . . . . 41

2.4.2 Electrical property measurement methods . . . . . . . . . . . 42

Chapter 3. Electrocaloric effect in barium strontium titanate . . . . . . . . . . . 47

3.1 Ferroelectric-Paraelectric transition in BST . . . . . . . . . . . . . . 48

3.2 Sample preparation protocol . . . . . . . . . . . . . . . . . . . . . . . 50

3.3 Electrical properties measurement . . . . . . . . . . . . . . . . . . . 56

3.4 Comparison to other results . . . . . . . . . . . . . . . . . . . . . . . 61

Chapter 4. PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2 Relaxor transition in PMN-PT . . . . . . . . . . . . . . . . . . . . . 67

4.2.1 Transition temperature for PMN-PT . . . . . . . . . . . . . . 67

4.3 Single crystal PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.4 Ceramic PMN-PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.4.1 XRD of PMN-PT powder . . . . . . . . . . . . . . . . . . . . 75

vi

4.4.2 Electrical property measurements on ceramic PMN-PT . . . 76

4.4.3 Results for (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 . . . . . 79

4.5 Comparison to other measurements . . . . . . . . . . . . . . . . . . . 82

Chapter 5. An electrocaloric solid state heat pump . . . . . . . . . . . . . . . . . 90

5.1 A theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.1.1 A quick review of thermoacoustics . . . . . . . . . . . . . . . 93

5.1.2 Generalizing the thermoacoustic cycle . . . . . . . . . . . . . 94

5.2 Description of device . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

5.3 Predicted performance using ferroelectric ceramic elements . . . . . . 102

Chapter 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Appendix A. Protocol for preparing ceramics . . . . . . . . . . . . . . . . . . . . 108

A.1 BST Protocol, July 2007 . . . . . . . . . . . . . . . . . . . . . . . . . 108

A.2 PMN-PT Protocol, July 2008 . . . . . . . . . . . . . . . . . . . . . . 109

A.2.1 Stage I: MgNb2O6 preparation . . . . . . . . . . . . . . . . . 109

A.2.2 Stage II: 0.92PMN- 0.08PT preparation . . . . . . . . . . . . 110

A.2.3 Stage III: Pressing 1/2” ceramic disks . . . . . . . . . . . . . 111

Appendix B. Perkin Elmer 4400 Operator’s Reference . . . . . . . . . . . . . . . 113

B.1 Start up procedure: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

B.1.1 Start up from shutdown: . . . . . . . . . . . . . . . . . . . . . 113

B.1.2 Start up from overnight: . . . . . . . . . . . . . . . . . . . . . 113

B.2 Automatically controlled operating procedures: . . . . . . . . . . . . 114

vii

B.3 Sputtering Procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . 115

B.3.1 DC Sputter Deposition: . . . . . . . . . . . . . . . . . . . . . 115

B.3.2 RF Sputter Deposition: . . . . . . . . . . . . . . . . . . . . . 115

B.3.3 RF Sputter etching: . . . . . . . . . . . . . . . . . . . . . . . 116

B.4 Shutdown procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . 117

B.4.1 Overnight: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

B.4.2 Longterm shutdown: . . . . . . . . . . . . . . . . . . . . . . . 117

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

viii

List of Tables

2.1 Compositional analysis of PMN-PT crystal . . . . . . . . . . . . . . . . 22

2.2 Furnace instructions for cleaning crucibles . . . . . . . . . . . . . . . . . 23

2.3 Powder to make 50 gm Ba0.67Sr0.33TiO3 . . . . . . . . . . . . . . . . . 25

2.4 Powder to make 50 gm MgNb2O6 . . . . . . . . . . . . . . . . . . . . . 25

2.5 Powder to make 50 gm (PbMg1/3Nb2/3O3)0.92 − (PbTiO3)0.08 . . . . 25

2.6 Furnace instructions for calcining BST . . . . . . . . . . . . . . . . . . . 29

2.7 Furnace instructions for calcining magnesium niobate . . . . . . . . . . . 29

2.8 Furnace instructions for calcining PMN-PT . . . . . . . . . . . . . . . . 29

2.9 Furnace instructions for binder burnout . . . . . . . . . . . . . . . . . . 33

2.10 Furnace instructions for sintering BST . . . . . . . . . . . . . . . . . . . 36

2.11 Furnace instructions for sintering PMN-PT . . . . . . . . . . . . . . . . 37

2.12 Furnace instructions for annealing PMN-PT ceramics . . . . . . . . . . 39

3.1 Expected and observed XRD angular peaks in BST . . . . . . . . . . . . 51

3.2 Corresponding grain sizes of different sintering conditions . . . . . . . . 56

4.1 Effect of crystal orientation on the electrocaloric effect in PMN-PT . . . 70

4.2 Compositional analysis of PMN-PT crystal . . . . . . . . . . . . . . . . 74

5.1 Cooling power at a given temperature span (lift) of a thermoacoustic-like

device operating at 30 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . 99

5.2 SSHP performance using 1.0 K caloric elements . . . . . . . . . . . . . . 102

ix

5.3 SSHP performance using BST and PMN-PT ceramic elements . . . . . 103

x

List of Figures

1.1 The vapor compression cycle[1] . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 The thermoacoustic cycle along a solid-fluid interface . . . . . . . . . . . 8

1.3 Perovskite crystal structure[27] . . . . . . . . . . . . . . . . . . . . . . . 12

1.4 Ferroelectric and relaxor polarization . . . . . . . . . . . . . . . . . . . . 14

2.1 Die used to press ceramic pellets . . . . . . . . . . . . . . . . . . . . . . 34

2.2 Setup to measure the electrocaloric properties of BST . . . . . . . . . . 43

2.3 Schematic of dielectric constant measurement . . . . . . . . . . . . . . . 44

3.1 A compilation of measured Curie temperatures in BST systems . . . . . 49

3.2 XRD analysis of BST powder . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3 Particle size analysis of BST powders . . . . . . . . . . . . . . . . . . . . 54

3.4 SEM micrograph showing carbon contamination in BST ceramics . . . . 55

3.5 SEM micrographs of fracture cross-sections of BST ceramics . . . . . . . 57

3.6 Dielectric constant of BST ceramics with different electrodes . . . . . . 59

3.7 The dielectric constant of Ba0.67Sr0.33TiO3 . . . . . . . . . . . . . . . . 62

3.8 Electrocaloric effect in BST at 1 MV/m . . . . . . . . . . . . . . . . . . 63

3.9 Electrocaloric effect in BST as a function of temperature and applied field 64

4.1 Phase diagram for PMN-PT mixtures[62] . . . . . . . . . . . . . . . . . 68

4.2 Mask for electroding rectangular plates . . . . . . . . . . . . . . . . . . . 71

4.3 High temperature electrocaloric measurement setup . . . . . . . . . . . . 73

xi

4.4 Electrocaloric effect in single crystal (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2 74

4.5 XRD of calcined 0.90PMN-0.10PT powder . . . . . . . . . . . . . . . . . 77

4.6 XRD of a sintered 0.90PMN-0.10PT ceramic . . . . . . . . . . . . . . . 78

4.7 Effect of different electrodes on dielectric constant in ceramic 0.85PMN-

0.15PT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4.8 Effect of different electrodes on electric effect in ceramic 0.85PMN-0.15PT 81

4.9 Dielectric constant of ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 . 83

4.10 Electrocaloric effect in ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15 84

4.11 Strip chart recording of the electrocaloric effect in PMN-PT . . . . . . . 86

4.12 Strip chart recording of joule heating in a ceramic sample . . . . . . . . 87

5.1 A heat switching heat pump . . . . . . . . . . . . . . . . . . . . . . . . . 91

5.2 The thermoacoustic cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.3 A generalized Stirling-like heat pump cycle . . . . . . . . . . . . . . . . 95

5.4 Proposed design of all solid state heat pump . . . . . . . . . . . . . . . . 101

xii

Acknowledgments

I would like to thank my advisor and my committee for their guidance and their

assistance throughout my graduate research. I would like to thank Omega Piezo Tech-

nologies, Inc. for technical support and the use of their facilities while preparing mate-

rials. I would like to thank the technicians at the Materials Research Lab for countless

hours of discussion and help refining my sample preparation and characterization tech-

niques. Finally, I would like to thank my patient wife for supporting me through the

completion of this research.

1

Chapter 1

Introduction

Perhaps the most influential technological developments in human history are

agriculture, medicine, and the atomic theory of matter. Supplementing all of these great

advances, refrigeration has emerged as one of the greatest recent advancements. Refriger-

ated storage allows for better food preservation and transportation over longer distances,

mitigating the effects of seasonal variance in farming conditions. Local droughts and re-

gional crop failures no longer lead to mass starvation and famine. The preservation

of medicines and vaccines have led to radical advances in the treatment and even the

elimination of many diseases. Study of matter at its coldest temperatures has given us

insight in to fundamental interactions of the structure of matter. Despite this progress,

problems arising from our implementation of refrigeration are threatening each of these

advances.

For over one hundred years vapor compression techniques have been the gold

standard for the design of refrigeration and air conditioning systems. The most efficient

fluids to drive these systems were Freon gases or chlorofluorocarbons (CFCs). As CFCs

entered the atmosphere they catalyzed a reaction with the atmospheric ozone, depleting

the natural barrier against ultraviolet (UV) radiation. The rise in UV radiation exposure

has led to increased rates of skin cancers, impacting public health[9]. Eventually the

world adopted the Montreal Protocol effectively banning CFC use.

2

Though the replacement for CFCs, hydrochlorofluorocarbons (HCFCs), do not

deplete the ozone, they feed a growing problem: atmospheric global warming. While

carbon dioxide is the most publicized contributor to global warming, HCFCs have global

warming potential 3000 times that of an equivalent volume of carbon dioxide[14]. New

advancements in refrigeration are needed to extend the benefit of artificial cooling, but

also limit, eliminate, or hopefully reverse the disastrous environmental consequences.

Liquids and solids are attractive candidate media for new refrigeration techniques, both

because their higher mass density allows a higher energy density, and because they avoid

the release of harmful gases. The main drawbacks to current alternatives are twofold:

energy efficiency and scalability. This thesis explores an all-solid-state alternative to

existing refrigeration technology with several target goals:

1. Identify an appropriate solid-state caloric effect which could be used in a scalable

refrigeration device.

2. Find a specific material which maximizes the magnitude of the solid-state caloric

effect.

3. Determine a design for a device incorporating the material so that its caloric prop-

erty results in a temperature lift and cooling power suitable for a real refrigerator.

To acheive target 1, the remainder of this chapter reviews various refrigeration

schemes and shows that a property of solids referred to as the “electrocaloric effect” could

be used for refrigeration. For target 2, a search of the literature indicated that there

were some electrocaloric materials which could be used to make a real, practical refriger-

ator. Specifically, materials literature implied or reported extremely large values of the

3

electrocaloric effect in both barium strontium titanate, BaxSr1−xTiO3, (BST) and lead

magnesium niobate-lead titanate, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x, (PMN-PT). At

this point it was believed that one could simply locate a company capable of making

these materials and order a quantity suitable for research. With considerable suprise,

it was discovered that no such company is currently in existance worldwide; a company

proposed to use its “best effort” to make the desired material but fell considerably short.

The results are documented in Chapter 2.

A search to collect the necessary information for fabricating these materials “in

house” was undertaken; this proved to be far more difficult than it should have been.

Many papers omitted what turned out to be critical steps or reported procedures that

were at odds with every other protocol in the field. At least one thesis presumably con-

taining detailed steps was not made available even after both a direct, informal request

and a formal request through the library of the granting institution. Despite the diffi-

culties, excellent materials, which were in many aspects superior to those found in the

materials literature, were made in house. Contrary to what seems to be the apparent

practice in the field of materials research, the full details, as well as general concepts of

the materials processing are provided in Chapter 2 and in Appendix A.

It was with further dismay that the values of the electrocaloric effect measured in

the high quality materials of this thesis were somewhat smaller than values claimed in

some of the materials literature. This seems to be another feature of materials literature:

papers which report unbelievable results should, in fact, not be believed. The reputation

of the research lab preparing the material must be known before any result reported in the

4

materials literature can be accepted. Another problem with some results in the literature

may have been poor practices used to measure and report the material properties.

In this thesis, the methods used to characterize the materials followed the high

standards of condensed matter and low temperature physics, and are reported in full

detail in latter part of Chapter 2. Furthermore, the raw data are presented unmodified,

and any calculations based on the data are fully explained. Data from measurements on

BST samples are presented in Chapter 3 and data from PMN-PT samples are presented

in Chapter 4.

Concerning target 3, a device based on the “thermoacoustic effect” [discussed

below] was designed. However, setbacks encountered from having to reproduce and

verify the materials literature precluded the construction of an actual device. A first

principles theory of operation was developed to predict the performance of the device

and an analysis of using BST and PMN-PT as caloric elements in such a device is

presented in Chapter 5.

To properly introduce a conceptually new heat pump, one must have a background

on refrigeration techniques and the relevant thermodynamics of the electrocaloric effect,

and this is presented next.

1.1 Refrigeration

Any physical mechanism which can displace heat further than it will naturally

diffuse can be adapted to form a heat pump or refrigerator. Many thermodynamic effects

exist which can repeatedly generate changes in heat. A most fundamental difference

5

between all forms of heat engine or pump is the manner in which heat is used to do work

or the manner in which work is done to move heat.

1.1.1 Early history

Early examples of heat engines based off natural work are waterfalls and wind-

mills. Evaporation and condensation do work against gravity, creating a reservoir of

water at a higher altitude. As the water flows downhill to a reservoir at a lower grav-

itational potential, the current is used to drive mechanical wheels. Windmills similarly

rely on rotors driven by a fluid flow, though the moving gas comes from nonuniformity

in atmospheric temperature and pressure.

The earliest forms of refrigeration were also based on natural mechanisms. An-

cient Egyptians used evaporative cooling systems[12]. Advances in shipping and land

transportation lead to a boom in the ice trade during the eighteenth and nineteenth

centuries[12]. While ice has been stored under insulation since ancient times, by the

eighteenth century it could be transported from colder regions. Ice was sold in large

blocks which were placed in chests, creating a reservoir at 0 C. A great boom in refrig-

eration came at the end of the nineteenth century with the advent of mechanical vapor

compression refrigerators.

1.1.2 Modern Refrigerators

The refrigerator as it is currently known started with the vapor compression cycle

in the 1850s with ammonia-based refrigerants. In the 1920s, CFC gasses were discovered

with very large thermal expansion coefficients near room temperature. This expansion

6

Fig. 1.1. The vapor compression cycle[1]

7

was exploited by compressing the gas in one location and then allowing the gas to expand

through a long network of tubes. A schematic of a vapor compression refrigerator is

shown in figure 1.1[1]. A mechanical compressor condensed the CFC gas, dumping heat

into the environment. The condensed gas was then forced through a long network of

coils wherein the gas expanded drawing heat into the gas from the coils. The gas was

then returned to the compressor. As the compressor is maintained at room temperature,

the free expansion of the gas in the coils cools the coils below room temperature.

Though the technology is over 150 years old, vapor compression refrigeration is

still the workhorse in most refrigeration applications. CFC gas has been banned by the

Montreal Protocol due to the catalyzing effect CFCs have in breaking down atmospheric

ozone[9]. A different class of gases, HCFCs, had less ideal thermodynamic and transport

properties; however, they were good substitutes for CFC gases in vapor compression

refrigerators. Unfortunately, because of the relatively low concentration of HCFC gases

in the atmosphere, the infrared absorption spectrum of HCFC gases make them 1000 to

3000 times worse than carbon dioxide as green house gases. As HCFCs are emitted, they

settle in the upper atmosphere. HCFCs are man-made; they have never been produced

naturally. The photoabsorbtion spectrum of HCFCs is different than carbon dioxide,

with the result that light that had previously passed through the atmosphere is now

being absorbed. As the atmosphere is normally 1% carbon dioxide, additional releases

of carbon dioxide are only slowly increasing the amount of light absorbed because it is in

the same frequency spectrum that has always been absorbed. More nations are focussing

on legislation to limit emissions to combat global warming, and more research will be

needed in alternate refrigeration cycles based on completely environmentally inert fluids

8

or solid state techniques. One technology using environmentally-safe fluids as a working

medium is thermoacoustics.

1.2 Thermoacoustics

A simple picture of a sound wave is a small pressure oscillation within a fluid

about some equilibrium coordinate. Considering the ideal gas law, PV = nRT , if

volume were held constant that pressure oscillation would correspond to a temperature

oscillation. This is not a useful means of transporting heat, however, because these

oscillations happen on a time scale faster than the heat is able to be displaced. Normal

sound propagation is therefore adiabatic.

Fig. 1.2. The thermoacoustic cycle along a solid-fluid interface

9

If the parcel of gas is placed next to a solid plate, parallel to the displacement

of the gas, the gas is able to transfer heat to and from the solid. The gas can expand

above the plate, drawing a small amount of heat from the solid. The gas then moves and

compresses further down the plate, dumping a small amount of heat into the solid. The

gas then returns to its initial position, repeating the cycle. This is the thermoacoustic

effect and the cycle is shown in figure 1.2. The displaced heat would naturally travel back

down the solid to restore thermal equilibrium. However, the gas displaces more heat per

unit time than the solid is able to transport, breaking a symmetry. On either side of

the gas parcel is another gas parcel, and each individual temperature oscillation acts as

a bucket brigade converting the small local temperature oscillations into a temperature

gradient along the length of the solid. Quantitatively, the presence of the fluid-solid

interface introduces a phase shift between the pressure oscialltion, pa, and the volume

oscillation, ua.

1.2.1 Thermoacoustics, quantified

Using the approximation that a fluid is oscillating parallel to a solid plate in a very

open channel, the cooling power for a thermoacoustic cycle is found by the relation[50]:

Q =12ΥδκTmβppaua(1−RT ) (1.1)

where Υ is the perimeter of the channel, δκ =√

2κ/ρω is the thermal penetration depth,

Tm is the average temperature, βp is the thermal expansion coefficient of the fluid, pa is

the amplitude of the pressure oscillation, ua is the amplitude of the velocity oscillation

10

and RT is the ratio of established temperature gradient to a characteristic or critical

temperature gradient.

The acoustic wave equations have been more properly solved including thermal

and viscous loss effects and geometries beyond open parallel plates[50, 51]. As devices

are being scaled to kilowatt levels of refrigeration power, the thermoacoustic equations

are being tested to incorporate non-linear effects such as mass streaming[46]. In addition

to switching to inert gases using thermoacoustic refrigerators, research into alternatives

to vapor compression refrigeration has focussed on solid-state refrigeration.

1.3 Solid-state refrigeration

Most existing solid-state coolers are based on one of two physical effects: thermo-

electrics and adiabatic demagnetization of magnetocaloric materials. The basis of the

thermoelectric effect is the work of Seebeck, Peltier and Thomson during the mid-1800s.

When a junction of dissimilar metals experience a temperature gradient, a small cur-

rent is generated. This effect is reversible, where the flow of a current through a metal

junction warms or cools the junction. Advances in the 1950s led to the discovery of

semiconductor-based thermoelectric materials which allowed the creation of coolers ca-

pable of operating at temperature spans of over 30 K. Ongoing thermoelectric research is

seeking to tailor the physical properties of semiconducting thermoelectric materials with

the ultimate goal of achieving larger temperature changes with smaller applied currents

in an effort to overcome the Joule heating limits of thermoelectric materials[30].

Magnetocalorics is the interaction of magnetism and heat. On the application

of an external magnetic field, the magnetic domains in a ferromagnetic material orient

11

along the direction of the applied field, raising the entropy and therefore the temperature

in an isolated system. As the magnetic field is removed, the magnetic dipoles are able

to relax and absorb some heat from the materials crystal lattice, lowering the material’s

temperature. The largest observed effects occur in gadolinium-based composites with 11

to 17 K magnetocaloric temperature changes occurring upon the application and removal

of up to 5 T magnetic fields[21, 54, 61, 69].

Cryogenic adiabatic demagnetization refrigerators have been built to continuously

maintain a sensor at 4 K, cooled from 10 K[11]. Current magnetocaloric refrigerators

need either large superconducting magnets or heavy permanent magnet arrays to gen-

erate the large magnetic fields needed to run the devices[21]. Even these devices can

only cool over a 20 to 30 K span. A promising alternative to magnetocaloric ferromag-

netic materials are electrocaloric ferroelectric materials. Most of the interesting new

electrocalorics come from a class of materials known as perovskites.

1.4 Perovskite ferroelectrics

Perovskite refers to both the mineral perovskite CaTiO3 and crystalline material

with cubic structure and the generic unit cell ABO3. Oxygen atoms form an octahedral

cage around the B-site ions while the A-site ions form a simple cubic lattice, as shown

in figure 1.3. The atoms in the B-site “cages” can often be pulled slightly above and

below the equilibrium position which give rise to polarizability and ferroelectric and

paraelectric properties.

12

Fig. 1.3. Perovskite crystal structure[27]

13

1.4.1 Ferroelectrics

Ferroelectricity is analogous to ferromagnetism in magnetic materials. Weak per-

turbative fields lead to the formation of permanantly polarized electric dipole domains

at low temperatures. As the temperature reaches the Curie temperature, the ferroelec-

tric ordering is quickly lost and the electric domains enter a paraelectric phase. In the

paraelectric phases, the electric dipoles are highly mobile and align in response to an

applied electric field, but will not maintain a net polarization when the field is removed.

1.4.2 Relaxors

What separates relaxor materials from similar ferroelectric materials is the nature

of the phase transition. As the temperature is lowered below the Curie temperature,

ferroelectrics see a sudden rise in permanent polarization that begins to saturate about

10 K below the Curie temperature. This rise manifests itself as a very sharp peak

in the dielectric constant, often occurring in the span of 20 K and almost no thermal

hysteresis. Relaxors, on the other hand, see a gradual rise in polarization at the Curie

temperature followed by a transition to a ferroelectric-like rise in polarization below the

Curie temperature. This manifests itself as a very broad peak in the dielectric constant

with a strong hysteresis and a dependence on the frequency at which the dielectric

constant is measured. These two behaviors are shown in figure 1.4. The data have been

scaled to normalize dielectric peak heights, but the temperature data is not scaled. The

ferroelectric dielectric constant data have been plotted as the dashed curve towards the

left hand side of the graph. The relaxor dielectric constant data are plotted as the solid

curve towards the right hand side of the graph. The ferroelectric has a Curie temperature

14

Fig. 1.4. Ferroelectric and relaxor polarization

15

of 21 C. The relaxor has a freezing temperature which occurs around 100 C and a Curie

temperature which occurs at 72 C. For both the ferroeletric and the relaxor, the lower

curve represents data taken upon heating the sample and the upper curve represents data

taken upon cooling the sample. Arrows have been imposed over the data to indicate the

curves corresponding to heating and cooling the sample. Both samples were measured

at a frequency of 1000 Hz.

Relaxors are analogous to a spin glass in magnetic systems. The Curie tempera-

ture refers to the temperature at which the onset of permanent polarization begins; the

temperature at which the transition to a ferroelectric-like rise in polarization occurs is

called the “freezing” temperature, a term borrowed from spin-glass literature.

Another characteristic of relaxors is a frequency dependence of the dielectric con-

stant. In the region of the phase transition, a larger dielectric constant is measured at

lower frequencies. Above the transition region, the dielectric constant is independent

of frequency. The divergent results typically begin at the Curie temperature and the

dielectric constant is largest at the freezing temperature.

1.4.3 Ferroelectric thermodynamics

In any material, there exists an electric susceptibility describing the capacitance

of a parallel-plate capacitor made with that material between the plates. The dielectric

constant, ε is found from the relation:

ε =

(∂D

∂Eq

)

T

(1.2)

16

where D is the electric displacement, Eq is the electric field[17].

Ferroelectrics exhibit very large dielectric properties near the phase transition. In

ferroelectric materials, there is a permanent polarization, Ps, in addition to the electric

displacement. In the following discussions on ferroelectrics, polarization P refers to the

total contribution, defined as:

P = Ps + κEq = Ps + (1 + ε)εoEq (1.3)

The temperature derivative of polarization is called the pyroelectric coefficient, pq, given

as:

pq =(

∂P

∂T

)

Eq

. (1.4)

Given that the polarization is the charge per unit area, consider polarization changing

as a function of time in the material. Expanding the derivative:

dP

dt=

1A

dq

dt=

dP

dT

dT

dt(1.5)

where A is the area. We call Ipyro = dq/dt the pyroelectric current. We can rearrange

this equation,

Ipyro = A(

∂P

∂T

)

Eq

dT

dt= Apq

dT

dt(1.6)

to show the dependence of the pyroelectric current on the pyroelectric coefficient and a

time-dependent temperature. The converse effect of pyroelectricity is the electrocaloric

effect.

17

The electrocaloric effect is derived by considering a temperature change resulting

from the application of an electric field at constant entropy. An electrocaloric coefficient,

Υq can be defined as follows:

Υq(Eq) =

(∂T

∂Eq

)

S

=∂(T, S)

∂(Eq, S))=

∂(Eq, T )

∂(Eq, S)∂(T, S)∂(Eq, T )

= T

1

T(

∂S∂T

)Eq

(∂S

∂Eq

)

T

=T

CEq

(∂S

∂Eq

)

T

(1.7)

where CEqis the heat capacity at constant electric field and S is the entropy. Through

a Maxwell relation we find that

(∂S

∂Eq

)

T

=(

∂P

∂T

)

Eq

= pq. (1.8)

To find an electrocaloric temperature change, ∆TEqresulting from the application of an

electric field, Eq, integrate the electrocaloric coefficient in equation 1.7,

∆TEC =∫ Eq

oΥqdE′

q=

∫ Eq

0

T

CEq

pq(E′q)dE′

q' T

CEq

∫ Eq

0pq(E

′q)dE′

q. (1.9)

It has been suggested[20] that below the ferroelectric transition temperature, the

pyroelectric coefficient is independent of electric field. Using this assumption, equa-

tion 1.9 simplifies to

∆TEC =T

CEq

pqEq (1.10)

and the observed electrocaloric effect should be linear with respect to the applied field.

18

Well above the critical region the material should be paraelectric and there should

be no remnant polarization. The only contribution from the polarization would then be

P = εεoEq (1.11)

where ε is the dielectric constant defined in equation 1.2 and εo is the permittivity of

free space. Integrating equation 1.9 using this assumption yields

∆T ' εo2

T

CEq

∂ε

∂TE2

q. (1.12)

Equation 1.12 implies that the electrocaloric effect should increase as the square of the

applied field above the transition temperature.

The polarization would be more properly defined by including the internal con-

tribution of the polarization using the electric displacement Dq defined as

Dq = εoEq + εεoEq. (1.13)

The dielectric susceptibility, χ, is defined as

χ = 1 + ε. (1.14)

Using χ to define the electrocaloric effect in the perovskite ceramics is not necessary,

however, because ε tends to be very large. At the Curie temperature in BST, ε is 20,000

or larger. Even well outside the critical region ε is typically over 1,000.

19

Just a fundamental description of the thermodynamics of ferroelectric materials

is not enough to produce electrocaloric materials. An extensive study in the process

of preparing high-quality ceramic materials was undertaken to produce electrocaloric

ceramics capable testing the electrocaloric effect as a refrigeration mechanism. A pro-

tocol to prepare two ferroelectric ceramics, barium strontium titanate (BST) and lead

magnesium niobate-lead titanate (PMN-PT), was developed. Samples of both BST and

PMN-PT were measured to determine the maximum possible electrocaloric effects. Fi-

nally, a complete theoretical framework and a design for an electrocaloric refrigerator

was developed to evaluate the practicality of using the electrocaloric effect in a solid-state

refrigerator.

20

Chapter 2

General comments about ceramics and their processing

To prepare research-grade ceramics, one must pay careful attention to seemingly

insignificant details. Often exotic elements with complex reaction mechanisms are com-

bined in a precise order to form a new material. Impurities on the order of 1 part in

1000 can radically alter ceramic properties. Even the materials forming electrodes for

measurements of electrical properties can be the difference between observing a physical

effect and observing practically nothing. Through an exhaustive process, a protocol to

reproduce high-quality ferroelectric ceramics was established. Protocols for both barium

strontium titanate (BST) and lead magnesium niobate-lead titanate (PMN-PT) were

developed through this thesis; step-by-step details of both protocols are presented as

Appendix A.

BST, BaxSr1−xTiO3, is made in a single calcination process using barium car-

bonate, BaCO3, and strontium carbonate, SrCO3, and titanium dioxide, TiO2. The

decomposition of the carbonates in to metal oxide and carbon dioxide regulates the re-

action of the barium oxide, strontium oxide and titanium dioxide so that the powder

ends up in the proper crystalline phase.

PMN-PT, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x, must be made in a double cal-

cination process to avoid unwanted crystalline phases. First, magnesium oxide, MgO,

and niobium (V) oxide, Nb2O5 are reacted to form magnesium niobate, MgNb2O5. The

21

magnesium niobate (MN) is then mixed with lead oxide, PbO, and titanium dioxide

and reacted to form PMN-PT. The preparation of high-quality ceramics begins with

preparing high-quality reacted powder.

2.1 Preparing ceramic-grade powder

Many considerations must be taken into account to make high-quality, ceramic-

grade powder. Proper stoichiometry is essential to ensure that there is no batch-to-batch

variance in sample properties. Many steps involve transfer of ingredients and exposure

to solvents, each of which runs a risk of contaminating the powder. Even uncontrolled

environmental factors, such as loss of air conditioning during powder processing can

produce unexpected results.

Ferroelectric properties, such as the phase transition temperature, depend highly

on mixing components in the proper ratio. Table 4.2 shows the full compositional analysis

of a PMN-PT single crystal that had a phase transition at an unexpected temperature.

The ratio of ions suggest that this is (PbMg1/3Nb2/3O3)0.82 − (PbTiO3)0.18, however,

using this concentration the sample has a magnesium deficiency of 22%. Theoretically,

the transition temperature should occur at 88 C; the transition in the sample occurred

near 141 C. Note, the calculation to find the composition assumes the number of oxygen

molecules in the unit cell and then finds the amounts of other ions present relative to

the assumed number of oxygen molecules.

Small levels of contaminants can cause other undesirable effects. Some of our

early ferroelectric samples exhibited the behavior of a semi-conductor. The resistivity of

BST should be greater than 1013 Ω−m. When measured under the small bias field of

22

Ion Number per unit cell Expected Percent differencePb 0.98 1.00 2.1Mg 0.21 0.27 21.8Nb 0.57 0.55 3.6Ti 0.20 0.18 8.3O 3 3 0

Table 2.1. Compositional analysis of PMN-PT crystal

a digital multimeter, the sample resistivity was 109 Ω−m or greater. Under an electric

field of 0.2 MV/m the resistivity dropped to 104 Ω−m. Further analysis showed these

samples were contaminated with < 0.5% mol of calcium. The calcium contamination was

eventually traced to crucibles that had been washed in State College water, a richer source

of calcium than milk. To avoid contamination, all tools, crucibles, storage containers and

mixing vessels that contact powder should be cleaned with the following procedure: wash

the item in reagent grade acetone (with a purity greater than 99.5%) then dry the item

with helium gas. Any component that will go inside a furnace, such as a crucible, should

be cleaned as follows: heat the item to 1200 C in a furnace and let it set for 6 hours at

that temperature. This procedure will help burn off any contaminants that may have

become bound to the surface of the object. Any object used in the making of lead-free

powders must only be used to make lead-free powders. If a furnace has ever been used in

the production of lead-based powders, it must be assumed to be contaminated with lead

and can be used for only the production of lead-based powders. Also, any tool, crucible,

storage container or other item used in the making of a lead-based powder must be used

only in the making of other lead-based powders.

23

All powders and ceramics were prepared in a Sentro-Tech model ST-1600 furnace

with a programmable interface. To clean crucibles the program in table 2.2 should be

used. The C0X instructions are temperature set points in C and the T0X instructions

are times in minutes. The programming logic is: temperature, time to next temperature,

repeat. A time instruction of -121 is the furnace’s code to shutdown.

Instruction SettingC01 0T01 200C02 1400T02 200C03 1400T03 200C04 0T04 -121

Table 2.2. Furnace instructions for cleaning crucibles

In addition to a clean working environment, high-quality ingredients are needed

to produce high-quality powder. All metal-oxide powders used to create the ceramic

powders should be reagent grade or better. All solvents throughout the process also

need to be reagent grade or better. No water should ever be allowed to contact powder

of any kind.

Even the water vapor in a humidity-controlled air conditioned environment can

degrade BST ceramics. If a BST ceramic wafer is left exposed to air for six months, the

skin of the pellet will change from a light brown color to a dark brown color, and then

24

eventually turn purple. All powders should be stored in sealed containers. Ceramics

without electrodes should be stored in a desiccator. With close attention to cleanliness,

purity and environmental control, the process to make ceramic samples begins with the

mixing of constituent metal oxide powders.

2.1.1 Mixing the components

Given all of the attention that must be paid to consistently produce powder with

similar properties, the mixing of the constituent metal oxide powders is perhaps the

riskiest step in the procedure. To minimize the risk of contaminating the powder, all

constituent metal oxide precursors should be massed onto a fresh waxed massing paper

using a balance sensitive to a tenth of a milligram. The powders required to make BST

are listed in table 2.3. The total amount adds up to more than 50 grams because the

recipe yields 50 grams of reacted powder and 10.1 grams of carbon dioxide is given off

as exhaust. Since PMN-PT is made in two stages, the mixture amounts for making

magnesium niobate are presented separately in table 2.4 for the mixture amounts for

0.92PMN-0.08PT in table 2.5. The MN numbers are slightly larger than 50 grams

because the calculation is performed for mixing in a ratio of 1 mol of MgO to 2 mol of

Nb2O5. It was previously found that in addition to a two-stage reaction the addition of

2% mol of MgO to the stoichiometric ratio almost prevents unwanted pyrochlore phases

of PMN-PT from forming[48, 49].

A 1 l Nalgene bottle should be prepared with a layer of duct tape wrapped twice

around both the top and bottom of the wide part of the bottle. This will aid in giving

the bottle better traction as the powder mixes. After the powders are massed they are

25

Component Mass (gm)BaCO3 30.364SrCO3 11.358TiO2 18.436

Table 2.3. Powder to make 50 gm Ba0.67Sr0.33TiO3

Component Mass (gm)MgO 6.715

Nb2O5 43.421

Table 2.4. Powder to make 50 gm MgNb2O6

Component Mass (gm)PbO 34.501

MgNb2O6 14.549TiO2 0.988

Table 2.5. Powder to make 50 gm (PbMg1/3Nb2/3O3)0.92 − (PbTiO3)0.08

26

placed in the prepared bottle. Once a bottle is used for mixing a particular powder, that

bottle should be used only for mixing that same powder in all future mixings. For BST,

150 ml of reagent- grade acetone should be added to the powder to form a slurry and

approximately 450 ml of yttria-stabilized zirconia spheres should be added to the slurry.

For MN and PMN-PT, 75 ml of reagent-grade ethanol should be added to the powder to

form a slurry and then approximately 220 ml of yttria-stabilized zirconia spheres should

be added to the slurry. In both cases, the level of the spheres should be just below the

level of the slurry. More solvent or spheres can be added if needed to get the proper

level.

Once the proper slurry level is achieved, the bottle should be sealed and placed

on a roller mill to mix the slurry. The mill should be set so that the bottle rotates

at about 22 rpm. For the 1 l Nalgene bottles, this is a roller speed of approximately

44 rpm. The roller mill has a spring-loaded nylon bolt to maintain a downward force

on the bottle to improve traction. Tension should be adjusted so that the bottle rolls

smoothly with minimal slipping. At the proper speed, the grinding media should be

pulled about one-quarter of the way up the side of the bottle as it rotates. The powder

should be mixed for at least 12 hours.

Once the mixing has completed, the slurry must be recovered from the bottle and

the media and the powder must be dried. The slurry should be poured from the Nalgene

bottle through a stainless steel strainer into a large Pyrex mixing bowl. The strainer

will separate the zirconia media from the slurry. The Nalgene bottle should be rinsed

twice with approximately 25 ml of solvent and the bottle should be drained through

the strainer. Additionally, about 50 ml of solvent should be drizzled over the zirconia

27

media in the strainer to wash any remaining powder off the media. It is important to not

use too much solvent because this will prolong the drying time, increasing the powder’s

exposure to possible contamination. The media in the strainer should be carefully shaken

over the Pyrex bowl to promote the drip drying of the media. The strainer should then

be set aside to allow the media to finish drying.

To dry the powder, the mixing bowl of slurry should be placed under a 60 W

incandescent lamp. As the powder dries, it must be stirred every 15 minutes to prevent

the constituent powders from separating as the slurry dries and settles. Once the slurry

has dried to a paste-like consistency, stirring frequency can be reduced to once per hour

to help complete the drying. Large chunks should be broken up with a stainless steel

spatula. Once the powder has finished drying, the dried chunks should be broken up in

an agate mortar. The BST and MN powder should be placed in a clean yttria-stabilized

zirconia crucible. PMN-PT should be placed in an alumina crucible, that has previously

been seasoned with lead-based powder. The powder is now ready for calcination.

2.1.2 Calcining the powder

Calcination is the process of heating mixed metal oxide powders to near their

melting points to catalyze a solid state reaction which results in the formation of small

crystals of BST, MN, or PMN-PT. The temperatures and duration of calcination is

chosen as the ideal conditions under which perovskite structured crystals will form[23,

48, 49]. All calcination was carried out in a Sentro-Tech Model ST-1600 furnace. The

BST calcination program is found in table 2.6. The calcination program for MN is found

in table 2.7. The calcination program for PMN-PT is found in table 2.8. Calcining for

28

longer times and at higher temperatures can result in the growth of larger crystals which

will take significantly longer to mill.

After the powder has been calcined, x-ray diffraction (XRD) measurements should

be performed on the powder to verify that the reaction has completed and that the pow-

der is not contaminated. According to the Powder Diffraction File (PDF-4) maintained

by the International Center for Diffraction Data (ICDD), BST powder should have an

XRD pattern with six strong peaks at angles of 2θ equal to 22.399o, 31.866o, 39.312o,

45.733o, 51.486o, and 56.802o. Extraneous peaks in BST data are often the result of

unreacted carbonate powders. Good BST powder should also have a white color.

According to PDF-4, PMN-PT powder should have one strong central peak at an

angle of 2θ equal to 32.3o and two smaller peaks at angles of 22.1o and 38.8o. Peaks at

angles from 30.2o to 30.5o often accompany powder with a red-yellow salt and pepper

color. These peaks are the result of unreacted lead oxide. Good PMN-PT powder

should have a faint yellow color. Non-ferroelectric pyrochlore phased material, such as

Pb6MgNb6O23 can also form during the PMN-PT reaction. This pyrochlore will show

up in the XRD spectrum with peaks at 29.2o and 33.9o. Small amounts of pyrochlore in

the starting powder often disappear during the sintering process. However, if pyrochlore

is present in the powder, XRD should be performed on sintered ceramics to verify that

this reaction has completed. The XRD results for BST are presented in Chapter 3 and

the results for PMN-PT are presented in Chapter 4. If the calcined powders pass both

visual and XRD inspection, they are ready to be ground to a fine size.

29

Instruction SettingC01 0T01 240C02 1200T02 360C03 1200T03 240C04 0T04 -121

Table 2.6. Furnace instructions for calcining BST

Instruction SettingC01 0T01 200C02 1000T02 360C03 1000T03 200C04 0T04 -121

Table 2.7. Furnace instructions for calcining magnesium niobate

Instruction SettingC01 0T01 160C02 800T02 600C03 800T03 160C04 0T04 -121

Table 2.8. Furnace instructions for calcining PMN-PT

30

2.1.3 Grinding

As the calcined powder comes out of the furnace, it is typically comprised of

polycrystalline granules several microns in size. In order to form a dense ceramic, the

reacted powder will need to be packed as tightly together as possible. The ideal packing

density is between 55 and 60% of the single crystal density. To accomplish this packing,

the powder must be ground to a fine size of 100 nm to 1 µm. A distribution of particle

sizes can help improve the packing packing density, as will be discussed in section 3.2.

The same 1 l Nalgene bottle in which the powder was mixed should be used for

grinding. The bottle should be free of unreacted powder and dry. Pour the calcined

powder into the bottle and add approximately 150 ml of zirconia spheres to the bottle.

Seal the Nalgene bottle and place it in an empty can on the vibratory mill. Secure the

bottle in place and grind for 4 hours. After 4 hours, stop the mill and add solvent to the

powder: 150 ml of acetone for BST or 75 ml of ethanol for either MN or PMN-PT. The

level of the zirconia media should be just below the level of the solvent; add more media

to the bottle if needed. Replace the bottle in the mill and grind for an additional 6 hours.

After grinding, retrieve the powder in the same manner as outlined in section 2.1.1. It is

not essential to frequently stir the ground, calcined powder as it should be homogenous

and settling is not an issue. Once the powder has formed a thick paste, however, it

should be stirred to complete the drying.

After the powder is dry, it should be in large, solid chunks. These chunks should

be broken up in an agate mortar and the powder should be filtered through a #50 sieve.

It may be necessary to grind the powder several times in the mortar to allow it to pass

31

through the sieve. Using a recipe to make 50 grams of powder, at least 45 grams of

reacted powder should be recovered after this step. After sieving, the ground powder

should be stored in a clean 125 ml Nalgene bottle. The lid should be secured tightly.

If the powder will not be used immediately or if the atmosphere is not environmentally

controlled, the powder should be stored in a desiccator or under vacuum.

2.2 Preparing ceramic samples

The first step in forming a ceramic from the calcined powder is to shape the

powder by pressing it into what is known as a “green pellet”. Here, “green” refers to

any pressed powder pellet that has not been fired in a furnace. To form a dense ceramic,

it is often necessary to create a green pellet with a density of 55 to 60% of the single

crystal density so that the powder is close enough to condense during firing. To aid

in the formation of high density green pellets, organic binders are often added to the

powders.

2.2.1 Binder

The addition of organic binder to ceramic powders as an aid to forming green parts

is a common industrial practice for the manufacture of most ceramics. The binder helps

lubricate both the powder and the die as the pellet is being shaped, minimizing stress

gradients in the green pellets and allowing tighter packing. Binders typically improve

cohesion of the unsintered pellets making them easier to handle.

The most common binders are poly vinyl alcohol (PVA) and poly acrylate (PA).

PVA is water soluble and often requires a dispersant to properly bond with ceramic

32

powder. Dispersants often contain elements such as sodium which can interfere with

electrical properties. PA is acetone soluble and requires no dispersants to bond to PMN-

PT. As a result, PA was used as the binder for making PMN-PT ceramics. Because

of the reactivity of barium, the process of adding binder was omitted for making BST

ceramics.

Binder typically constitutes as little as 1% of the total weight of pressed pellets

to as much as 40% of the weight of pressed pellets for some special ceramic applications.

For making PMN-PT, 4% weight of liquid binder is added to the powder, which becomes

1.5 to 1.7% of the total weight of the binder plus powder once the binder has dried.

To add the binder, first weigh 4% of the weight of the powder of binder and add

it to a small, clean Pyrex dish. Mix in just enough acetone so that the binder completely

dissolves. The more acetone that is added beyond this amount will increase the time

it takes the binder to dry. Stir the powder into the dilute binder using a stainless steel

spatula. Place the dish under an incandescent heat lamp and stir continuously until the

mixture becomes too tough to stir. It is important to keep stirring because the binder

can separate from the powder forming a skin on the mixture. If a skin forms, the binder

may not properly attach to the PMN-PT powder, eventually resulting in poor sintered

ceramics. If this has happened, the binder must be removed from the powder and the

process needs to be restarted.

To remove binder from powder, place the bindered powder in an open crucible.

Place the crucible in the furnace and run the program found in table 2.9. The 3 hours

at 325 C breaks down the compounds in the binder and the 10 hours at 500 C removes

the carbon ash from the powder.

33

Instruction SettingC01 0T01 300C02 325T02 180C03 325T03 90C04 500T04 600C05 500T05 20C06 0T06 -121

Table 2.9. Furnace instructions for binder burnout

If the addition of binder was successful, the Pyrex dish should contain several

large hard clumps of binder and powder. These clumps need to be broken up in an agate

mortar and the bindered powder should be passed through a #50 sieve to filter out any

large clumps of binder that may have formed. The bindered powder should pass readily

through the sieve compared to the non-bindered powder.

2.2.2 Pressing ceramic pellets

To form dense ceramics, the powder must first be shaped as compactly as possible

into a pellet. For the property evaluations carried out in this thesis, the powder was

pressed into 1/2” cylinders using a high strength steel die with high strength steel anvils

on a uniaxial press. A schematic of the die is presented in figure 2.1. The anvil is placed

inside the die. The powder is poured into the die and then the press forces the hammer

down on the powder, compacting the powder between the hammer and the anvil.

34

Anvil

Hammer

Die

Force gauge

Fig. 2.1. Die used to press ceramic pellets

35

To make BST ceramics, 1.5 grams of powder were pressed. The pressure was

slowly increased to 200 MPa, which corresponded to a force of 5700 lbs on the press.

The pressure was maintained for 30 seconds and then released. The compression cycle

was then repeated two more times.

To make PMN-PT ceramics, 0.7 grams of powder were pressed. The pressure was

slowly increased until the press was just starting to measure a force on the die. Then,

in one fluid motion, the pressure was raised to 150 MPa, a force of 4500 lbs, and then

immediately released. The compression was not repeated.

Once compressed the pellets were extracted from the die. First, the die was

inverted on the press and a stainless steel spacer was placed between the die and the

press. The anvil was pushed three-quarters of the way out of the die so that it could be

removed by hand. The inverted die was then placed back on the press with the spacer

and the hammer was pushed through the die until the sample was clear of the die. Then

a fine paint brush was used to push the green pellet onto a razor blade to be transported

to the sintering vessel.

2.2.3 Sintering the pellets

Sintering is the process of heating green pellets to just below the melting point

of the material so that the pressed powder condenses into a dense ceramic. All sintering

was performed in the same furnace as the calcination.

BST pellets were placed on a sheet of platinum foil in a yttria-stabilized zirconia

calcination tray. The tray was then covered with another tray and placed in the furnace.

Oxygen was fed into the furnace to prevent oxygen depletion in the ceramics at high

36

temperatures. To set the oxygen flow rate, a tube from a regulated oxygen cylinder was

fed into a water bath. A needle valve on the regulator was adjusted until the oxygen

flowed out of the regulator at a rate sufficient to produce one to three 3 mm bubbles per

second in the water bath. The oxygen line was then connected to the oxygen inlet on

the furnace. The furnace was programmed with the program found in table 2.10. The

BST ceramics had to be cooled very slowly from high temperatures to avoid cracking

and other structural damage.

Instruction SettingC01 0T01 80C02 400T02 60C03 400T03 367C04 1500T04 360C05 1500T05 750C06 0T06 -121

Table 2.10. Furnace instructions for sintering BST

PMN-PT ceramics were first placed on platinum foil on a zirconia brick. The

brick was placed in the furnace and the binder burnout program from table 2.9 was

run. The pellets were then weighed to determine that they had lost approximately

1.5% of their weight and transferred to a piece of platinum foil on the lid of a small

37

alumina crucible that had been previously seasoned with lead oxide-based powder. The

pellets were covered with another piece of platinum foil, the crucible was placed over

the lid, covering the platiunum wrapped pellets. The crucible was placed in the furnace.

Oxygen was fed into the furnace in the same manner as described above. The pellets

were sintered according to the program in table 2.11. Lead oxide can evaporate out of

PMN-PT solutions at temperatures above 1000 C, so the ceramics had to be cooled as

quickly as possible to avoid changes in elemental composition. To combat this lead loss,

industrial ceramics are often sintered while buried in some sort of lead donor powder.

This was attempted with the PMN-PT samples, but this often lead to bizarre structural

deformations of the ceramics.

Instruction SettingC01 0T01 128C02 1280T02 120C03 1280T03 20C04 0T04 -121

Table 2.11. Furnace instructions for sintering PMN-PT

High quality sintered ceramics should have a lustrous shine. BST should be a

brown color and PMN-PT should be light tan to faint yellow color. Both BST and

38

PMN-PT should be translucent below a thickness of 1 mm. Samples meeting these

visual properties are ready to be prepared for electrical property measurements.

2.3 Preparing samples for measurement

PMN-PT is a more stable ceramic than BST and is therefore more forgiving in

the preparation of thin slices. PMN-PT ceramics thicker than 1.0 mm can be cut on

a wire saw. The ceramics should be mounted to an aluminum right angle jig with hot

wax. A small piece of graphite should be placed just below the ceramic to help support

the slices as they are cut and to keep the blade from falling into the aluminum jig. The

ceramic should be sliced into 400 µm or thicker slices and then each piece should be

polished to thickness. The wire saw cuts very slowly compared to the wafering saw and

the wire blade is more subject to travel. Sometimes the cut surface from a wire saw cut

sample is wedge shaped or bowled, which requires careful polishing.

Ceramics thinner than 1.0 mm should be hand polished down to a thickness of

250 µm. The ceramics should be mounted to a flat stainless steel polishing base by

heating a small amount of wax on the base and pressing the sample firmly into the wax.

Weight should be placed on the ceramic as it cools to ensure that the ceramic lies flat

against the steel base. After the wax has hardened, the sample can be polished using a

South Bay polishing jig, starting with 120 grit emery paper, followed by 400 grit emery

paper and finally with 9 µm grit paper. In all cases, distilled water should be used to

lubricate the polishing surface. Further polishing beyond 9 µm grit paper is usually

unnecessary because the friction between the polishing paper and the ceramic tends to

pull grains out of the ceramic rather than break down the ceramic grain, itself.

39

After polishing, the samples need to be annealed to help repair any scratches,

gouges, or microfractures that may have occurred during polishing. The samples should

be placed on platinum foil on a zirconia brick. The brick is placed in the furnace and

oxygen should be flowing into the furnace at a rate of one to three 3 mm diameter bubbles

of gas per second. The furnace program is shown in table 2.12. At a temperature of

900 C the material should not be in danger of decomposing. After the samples have

cooled, they are ready for the application of electrodes.

Instruction SettingC01 0T01 180C02 900T02 600C03 900T03 180C04 0T04 -121

Table 2.12. Furnace instructions for annealing PMN-PT ceramics

2.3.1 Electrodes

Electrodes turn out to have significant influence on the measurement of ferro-

electric properties. The dielectric constant and electrocaloric effect depend on a high

mobility of surface charges both at ceramic grain surfaces and along ferroelectric domain

boundaries. Industrially manufactured piezoelectric ceramics typically use a silver paint

40

electrode that is screen-printed, dried, then fired onto the ceramic. The organic backing

of the paint can leave behind a thin insulating film, spoiling electrical property measure-

ments. Because of the high reactivity of barium, it is best to minimize the contact of

BST with any possible surface contaminant. The ideal electrode solution is to directly

sputter metal film electrodes directly onto each surface of the polished ceramic disk.

The electrodes in the research were applied using a Perkin-Elmer model 4400

commercial sputter deposition system. Detailed operating instructions of the sputtering

system are found in appendix B. To form the electrode, a layer of chrome was deposited

for 10 minutes at a power of 500 W using an RF magnetron. Then, a layer of aluminum

was deposited for 10 minutes at a power of 1000 W using a DC magnetron. The samples

were then flipped over and a similar electrode was applied to the other side. The resulting

electrodes had a thickness of one to two microns and a surface resistance of 1 to 3 Ω.

Because the sputtered metal can redeposit as it accumulates on the sample, a thin bridge

of metal usually forms around the edge of the sample. After sputtering, this bridge of

metal needs to be polished from the sample with some fine grit emery paper to isolate

the two electrodes. With the application of the electrodes, the ceramic samples are ready

for measurement of electrical properties.

2.4 Measurements on ceramic wafers

Both physical and electrical properties were measured to evaluate the ferroelectric

ceramics. Density and structure of the materials were probed first to determine that

the ceramics compared to those found in the literature. After the physical parameters

were met, the dielectric constant and electrocaloric effect of the samples were measured.

41

Physical property measurement methods will be discussed in section 2.4.1. Electrical

property measurement methods will be discussed in section 2.4.2. The results of these

measurements will be presented and discussed in chapters 3 and 4.

2.4.1 Physical property measurement methods

The first test typically performed on the materials was x-ray diffraction (XRD)

As this was first performed just after calcination, this was discussed previously in sec-

tion 2.1.2. Occasionally, XRD was used as a spot check on ceramic wafers to ensure

that the phase and composition of the powder had not changed during processing. No

significant differences were ever found between powder or ceramic XRD measurements.

SEM microscopy was also used to characterize the structure of the grains within

the ceramic. SEM micrographs along fractures of ceramic samples were made to deter-

mine the average grain size and porosity of the samples. After a reliable method for

reproducing large grained, dense ceramics was established, most of the SEM microscopy

was deemed unnecessary.

The density of the ceramic samples is the fastest test outside of a visual inspection

that can provide clues to the quality of the sample. Good ceramics for ferroelectric

measurements should have a density 98 to 99 % of the single crystal density. The

single crystal density of BST is 5700 kg/m3 and the single crystal density of PMN-PT

is 8000 kg/m3. Lower densities suggest that the ceramic has either large open pores

between the ceramic grains or large empty regions inside the sample. Both high-porosity

and voids in the sample increase the risk of dielectric breakdown and spoil the electrical

properties.

42

2.4.2 Electrical property measurement methods

Electrical properties were directly measured using the setup shown in figure 2.2.

Copper leads were attached to the metal films on the ceramic wafers using a conducting

silver paint; one side was connected to ground and the other to a high voltage power

supply. A thermocouple was attached to the grounded electrode. Figure 2.2 shows

two PVDF sensors on the ground electrode of the sample. These were initially used

to check the thermocouple data. As the sample was heated, the pyroelectric PVDF

sensors produced a current corresponding to the heating rate. The heating rate from

the thermocouple was always in excellent agreement with the PVDF sensors, so for

simplicity, the PVDF sensors were deemed unnecessary for most ceramic samples. The

thermocouple was connected in series with a second thermocouple which was attached

to a large copper block with an alcohol thermometer attached to it. This block served

as an isothermal reference and all temperature measurements were made relative to the

isothermal block. Both dielectric and electrocaloric measurements were carried out under

high vacuum to minimize the risk of arcing when large electric fields were applied to the

sample and to maximize the thermal isolation between the sample and the environment,

giving the longest possible time constant to observe thermal effects before the sample

returned to thermal equilibrium.

The dielectric constant was measured by connecting the sample in series with a

56.6 kΩ resistor and applying a 100 mV AC electric field at a frequency of 1 kHz. A

schematic of the measurement is shown in figure 2.3. The voltage across the capacitor

was measured using a computer controlled digital volt meter. Using Vo as the applied

43

Fig. 2.2. Setup to measure the electrocaloric properties of BST

44

voltage, VC as the voltage across the capacitor, R as the series resistance, and ω as the

angular frequency of the applied AC field, the capacitance of the sample is given as

C =1

ωR

√(VoVC

)2− 1. (2.1)

Since the sample is a parallel-plate capacitor, the dielectric constant can be found from

ε =Ct

εoA(2.2)

where t is the thickness of the sample, A is the cross-sectional area of the sample and εo

is the permittivity of free space, 8.85 F/m.

V

Fig. 2.3. Schematic of dielectric constant measurement

45

The electrocaloric effect was measured by applying a large DC electric field across

the sample and recording the resulting temperature change observed on the thermocou-

ple. The high resistivity of the ceramics mitigates any possible Joule heating. Also,

Joule heating shows up like an exponential curve (as with an RC time constant) in the

temperature data; there is an initial rapid change in temperature that gradually levels off

as the system comes into thermal equilibrium. Under the conditions in this experiment,

the time constant for this equilibrium was on the order of one minute. The observed elec-

trocaloric temperature change established itself on the order of two seconds, the response

time of the thermocouples. As the electric field was kept on the sample, the tempera-

ture of the sample slowly returned to thermal equilibrium on a several minute thermal

time constant scale. If the electric field was removed after the system had returned to

thermal equilibrium, the temperature of the sample would instantly drop below room

temperature and recover to thermal equilibrium on a several minute time scale.

The electrocaloric effect can also be measured as the heat required to restore a

system to equilibrium in a calorimeter[26]. In the calorimeter, the sample and a reference

material are mounted to heaters with a compensation system to ensure that they remain

at the same temperature. To study phase transitions, the materials are heated at a slow

consistent rate; if the sample has a change in heat capacity, the reference heater must

change its output to match the effects in the sample.

To measure the electrocaloric effect, the sample and the reference are heated at

a slow, uniform rate. An electric field is applied to the sample, inducing a temperature

change in the sample. The reference heater must then do work to compensate for the

change in the sample. The power of the heater can be recorded to find the heat flow

46

in and out of the sample. It must be noted that this process is not inducing a phase

transition and this is not latent heat. The proper term for measuring the electrocaloric

effect through calorimetry is measuring the heat of polarization.

Both direct temperature change data and calorimetry data can be compared by

either converting the temperature change in an isolated environment to a heat of polar-

ization or by converting the heat of polarization to a temperature change in an isolated

environment. Assuming the temperature change occurs in isolation, the heat of polar-

ization is given from

∆hEC =cp

ρ∆TEC . (2.3)

Direct observation of the electrocaloric effect is not practical in thin film systems.

Polarization of thin films can be mapped as a function of applied external electric field

and temperature. This surface can then be fit numerically to calculate derivatives and

these can be integrated to determine electrocaloric temperature change. Unfortunately,

this calculation is only useful as an intellectual exercise and cannot be translated into

real-world devices. Any exploitation of the electrocaloric effect needs to consider the

thermodynamics of the entire system. If there is a 2 µm thin film on both sides of a

100 µm thick rigid substrate there is only a 4% electrocalorically active region. Ther-

modynamically, there is 25 times more material that will be heated but not positively

contribute to the electrocaloric effect. Any electrocaloric element based off a thin film

will have a useable temperature change of the theoretical thin film effect divided by the

ratio of electrocalorically inactive material to electrocalorically active material.

47

Chapter 3

Electrocaloric effect in barium strontium titanate

Recent trends have prompted global research for more environmentally friendly

sources of energy and refrigeration. While ozone-depleting chlorofluorocarbon refriger-

ants have been phased out, their replacements, hydrochlorofluorocarbons tend to have

global warming potentials 3000 times that of an equivalent amount of carbon dioxide[14].

Alternatives to vapor compression refrigeration suffer from serious technical challenges.

Thermoelectric technology provides an all-solid-state solution, but thermoelectric de-

vices have typical efficiencies around 7% of the Carnot limit. Although some recent

advances[30] are pushing the technology to near 10% of the Carnot limit, typical house-

hold refrigerators operate with efficiencies near 25% of the Carnot limit. As most elec-

trical power in the United States is provided by carbon emitting sources, the reduced

efficiency contributes to the potential for global warming. Magnetocaloric materials are

capable of very large temperature changes, greater than 17 K; however, they require mag-

nets capable of generating large fields (1-5 T) which are not conducive to scalability[21].

The electrocaloric effect is a possible replacement for thermoelectrics in the devel-

opment of all-solid-state heat pumps and refrigerators. Modern ceramic and single crystal

material can withstand large electric fields, and high voltages can be generated from in-

expensive sources. Most of the electrocaloric materials are perovskite ferroelectrics, and

the general properties of these materials will be discussed next.

48

3.1 Ferroelectric-Paraelectric transition in BST

Ferroelectric materials exhibit large electrocaloric effects near the ferroelectric-to-

paraelectric phase transition. Barium strontium titanate, (BST), has a phase transition

temperature which can be tuned from ∼ −233 oC, the transition temperature of SrTiO3,

to 118 C, the transition temperature of BaTiO3. The chemical symbol is usually given

as BaxSr1−xTiO3, where x is the molar concentration of barium.

The simple approximation of the transition temperature in C as a function of

doping can be found with

Tc = 371x− 253.0[3] (3.1)

This assumes a linear shift in transition from pure SrTiO3 to pure BaTiO3. Careful

study of the transition temperature shows multiple regions of interest in the transition

temperature[24, 58]. Below a Ba concentration of 20.0%, the Curie temperature increases

as x1/2 with the temperature in C given by[58]

TC = 274(x− 0.0002)1/2 − 273.15. (3.2)

Above a concentration of 20.0% a better approximation of the Curie temperature in C

is given by[24]

Tc = 339x− 221. (3.3)

Room temperature phase transitions occur with a doping of 0.65 < x < 0.70. A broad

survey of transition temperatures found throughout the literature is presented in figure

3.1[2, 3, 4, 7, 6, 8, 16, 18, 19, 22, 23, 24, 25, 26, 28, 29, 33, 37, 41, 44, 45, 47, 52,

49

Fig. 3.1. A compilation of measured Curie temperatures in BST systems

50

53, 55, 56, 57, 58, 59, 63, 65, 66, 68]. The dashed line plots the linear shift in phase

transition temperature that is predicted by equation 3.1. The solid curve plots the phase

transition temperatures predicted by equations 3.2 and 3.3. It should be noted that both

the theoretical curves usually tend to under-predict observed Curie temperatures. The

Curie temperature can be very sensitive to both small variations from the assumed

stoichiometric ratio of barium and strontium, and no paper surveyed made any effort to

verify the exact composition of the sample studied.

Small levels of impurity can have strong effects on the transition temperature

and physical properties near the transition. Small levels of nickel and calcium can lead

to strong changes in resistivity near the phase transition, creating a semiconductor-

like material. At large electric fields, if the resistivity drops below 1012 Ω−m, Joule

heating can begin to overwhelm the electrocaloric response. The electrocaloric materials,

therefore, need to be good insulators. To achieve consistent, high-quality electrocaloric-

grade BST ceramics, a precise preparation protocol is crucial. A considerable portion of

the research of this thesis was directed toward developing such a protocol.

3.2 Sample preparation protocol

BST powder was prepared from carbonate precursors; the procedure is docu-

mented in Chapter 2 and a step by step summary is presented in Appendix A. Briefly:

powders were mixed for 4 hours in an acetone solution using a roller mill and dried. The

dried powder was calcined at 1200 C for 6 hours. To verify the crystal phase of the BST

powder, X-ray diffraction (XRD) was performed on the calcined powder. The results of

the XRD analysis are shown in figure 3.2. Table 3.1 lists the angular locations of the

51

expected XRD peaks for BST found in the PDF-4 in addition to the measured locations.

The measured angles are all slightly below the expected angles for BST, however, it is

likely that the sample in the reference database has a slightly different stoichiometric

composition. The data from figure 3.2 are in good agreement with the expected XRD

signature from the PDF-4. Also, there are no additional peaks beyond the six anticipated

peaks. Large quantities of unreacted carbonate powder would show up as two additional

peaks. As the calcination appeared to be complete the BST powder was ground to a

suitable size for making ceramic pellets.

2θtheory [o] 2θobserved [o]22.399 22.35231.866 31.85039.312 39.28745.733 45.62351.486 51.41456.802 56.760

Table 3.1. Expected and observed XRD angular peaks in BST

The optimal procedure to grind the BST powder was found to be a 4 hour dry

grinding using yttria-stabilized zirconia media, followed by a 6 hour grinding in an ace-

tone slurry. A particle size analysis was performed using a laser diffraction particle size

analyzer. Figure 3.3 shows the distribution of grain sizes. The time listed is the total

time for which the powder was ground. The 2 hour and 4 hour ground powder was only

ground dry. The 6 hour, 8 hour and 10 hour all included both the 4 hour dry grinding

52

Fig. 3.2. XRD analysis of BST powder

53

in addition to acetone grinding to complete the balance of the time. As grinding time

is increased, the mean particle size decreases and the distribution of particle sizes tends

to increase. It is only at 10 hours of total grinding time that a significant fraction of

powder particles appear below 250 µm. It is important to maintain a broad distribution

of particle sizes to achieve optimal packing of powder when the powder is pressed into a

pellet.

Ceramic pellets were prepared by pressing 0.5 grams of BST powder in a 1/2”

diameter die to a pressure of 200 MPa. As barium is highly reactive, even in solid solu-

tion, no binder was used to hold the pressed powder together. While binder is typically

used in industrial ceramic manufacture, many binders are either water or solvent-based.

Barium is highly reactive with water; even limited exposure to water vapor in a humid-

ity controlled environment for more than three months can turn the surface of ceramic

BST from brown to purple. In addition to the problems presented by water, the organic

compounds that comprise the binder can inhibit the sintering of high-quality ceramics.

The presence of carbon inhibits grain growth and leaves large deposits throughout

the sample. Figure 3.4 shows the surface of a BST ceramic pellet made from powder

that has been contaminated with an organic compound. The open porosity and small,

poorly connected grains increase the occurence of dielectric breakdown.

It is suggested that the pyroelectric coefficient in BST increases as the ceramic

grain size increases, with optimal values occurring between 10 and 20 µm[23]. Figure 3.5

shows SEM micrographs of a cross-sectional fracture of BST ceramics. Table 3.2 shows

the estimation of the average grain diameters of these ceramics. The sintering time

54

Fig. 3.3. Particle size analysis of BST powders

55

Fig. 3.4. SEM micrograph showing carbon contamination in BST ceramics

56

and temperature is sufficient to reach the predicted sweet spot for optimal pyroelectric

activity.

Sintering conditions Average grain size ( µm)1460 C, 8 hours 12.71500 C, 9 hours 16.2

Table 3.2. Corresponding grain sizes of different sintering conditions

As grain size increases, the ceramics become increasingly brittle. Above a grain

size of 15 µm the ceramics could not be sliced thinner than 330 µm. Polishing of the

ceramics typically had a negative impact on the larger grained samples. As the grain size

increased, the strength holding the grain together seemed to be stronger than the strength

connecting the grains. This leads to grains pulling out of the ceramic causing a rough,

cloud-like texture on the surface. The rough surface can create an uneven thickness,

which leads to local spots of larger than average electric field. These hotspots increase

the likelihood of catastrophic dielectric breakdown. Once ceramic pellets exhibiting

good physical properties were crafted repeatedly, the pellets were prepared for electrical

properties measurements.

3.3 Electrical properties measurement

The sintered pellets were sliced to a thickness of 300-400 µm and approximately

1-2 µm thick chrome and aluminum electrodes were sputtered onto the wafers. Detailed

57

Fig. 3.5. SEM micrographs of fracture cross-sections of BST ceramics

58

instructions for operating the sputtering machine can be found in Appendix B. While it is

more traditional in the ceramic industry to apply electrodes by firing a conductive silver

paste onto the ceramic, the BST samples showed degraded properties after contact with

the silvering compound. This degradation is most likely due to either the reactivity of

barium, or a thin layer of the organic carrier of the paint remaining between the ceramic

and the electrode. This paint layer could degrade charge mobility from the ceramic to the

electrode, spoiling the measurement of dielectric and electrocaloric properties. Figure 3.6

shows a comparison of dielectric constants measured on samples prepared from the same

powder: the top trace is for a ceramic with sputtered electrodes, the bottom trace is

for a ceramic with silver paint electrodes. While the peak temperature is visible, the

sharp peak is muted, and the maximum value of dielectric constant in the paint electrode

samples are almost an order of magnitude smaller than the sputtered electrode samples.

The electrocaloric effect was directly measured using the setup shown in figure 2.2.

Copper leads were attached to the metal films on the BST wafers using a conducting silver

paint; one side was connected to ground and the other to a high voltage power supply.

A thermocouple was attached to the grounded electrode. Figure 2.2 shows two PVDF

sensors on the ground electrode of the sample. These were initially used to check the

thermocouple data. As the sample was heated, the pyroelectric PVDF sensors produced

a current corresponding to the heating rate. The heating rate from the thermocouple

was always in excellent agreement with the PVDF sensors, so for simplicity, the PVDF

sensors were deemed unnecessary for most ceramic samples.

The dielectric constant of Ba0.67Sr0.33TiO3 is shown in figure 3.7. The lower

curve was recorded as the sample was heated and the upper curve was recorded as

59

Fig. 3.6. Dielectric constant of BST ceramics with different electrodes

60

the sample cooled. Arrows have been imposed over the data to show the direction

in which the data were recorded. The difference in the two curves can be explained

by the process of measuring the dielectric constant. The polarization does not truly

spontaneously appear, especially because of the presence of the small AC electric field

present to measure the dielectric constant. Just as ”permanent” ferromagnets can be

degaussed with an oscillating magnetic field, the dielectric measurement fights the onset

of the ”permanent” polarization. Over time, small fluctuations will begin ordering the

electric dipole. As this remnant polarization sets in, the measured dielectric constant

slowly decays to the lower curve.

The sharp peak in the dielectric constant of just under 20000 occurs at a temper-

ature of 25 C; this is the Curie temperature. Below 18 C and above 35 C the dielectric

constant levels off. These upper and lower temperatures set the bounds for observing a

large electrocaloric effect.

Figure 3.8 shows the electrocaloric response of this sample under the application

and removal of a 1 MV/m electric field. The effect peaks at 24.5 C with an electrocaloric

temperature change of 0.38 K. The electrocaloric response is larger than 0.25 K from

20 C to 33 C. Outside of this range, the effect drops quickly.

The electrocaloric effect in the phase transition temperature region was mapped as

a function of both temperature and applied field and is presented in figure 3.9. As the field

is increased the maximum electrocaloric effect is observed at larger temperatures. This

is due to the nature of the polarization in this region. Below the transition temperature,

there is some net polarization, even at zero applied field. There is a limit to the ability

of the applied field to polarize the material; at low fields, the effect is linear with applied

61

field but the electrocaloric effect begins to saturate. Above the transition temperature,

the BST has become a paraelectric and there is a growing contribution from the square

of the electric field, as mentioned in equation 1.12. At much higher temperatures, the

electrocaloric response should scale exactly as the square of the applied electric field.

The largest absolute electrocaloric response observed in the BST sample was 0.45 K,

which occurred at 24 C and an applied field of 1.33 MV/m.

3.4 Comparison to other results

While this is not the first experiment to measure the electrocaloric effect in

BST[26], the measurements are superior to those found in the literature. Lin measured a

bulk BST ceramic using differential scanning calorimetry. Lin reports his electrocaloric

measurement as a latent heat; however, the terminology is not proper. The effect is

more properly called a heat of polarization[10], which can be converted to an equivalent

electrocaloric temperature change for easy comparison, as follows: assuming that the

electrocaloric effect occurs in isolation, the electrocaloric heat of polarization is given as,

∆hEC =cp

ρ∆TEC . (3.4)

The density of BST is 5700 kg/m3 and the specific heat is 2.7 MJ/m3 −K[5], the ob-

served heat of polarization is approximately 0.189 kJ/kg for a temperature change of

0.40 K. The only latent heat observations made on the electrocaloric phase transition in

BST observe a heat of polarization of 0.12 kJ/kg at the transition temperature under

62

Fig. 3.7. The dielectric constant of Ba0.67Sr0.33TiO3

63

Fig. 3.8. Electrocaloric effect in BST at 1 MV/m

64

Fig. 3.9. Electrocaloric effect in BST as a function of temperature and applied field

65

an applied field of 1.0 MV/m[26]. Lin’s measurement corresponds to an electrocaloric

temperature change of 0.25 K in an isolated sample.

Extrapolating the peak electrocaloric effect to 2.5 MV/m, close to the limit of

the onset of dielectric breakdown in bulk ceramics, we would anticipate to see an elec-

trocaloric temperature change of 0.65 K and a heat of polarization of 0.3 kJ/kg.

66

Chapter 4

PMN-PT

During the last 10 years, one of the most interesting perovskite compounds to

emerge is lead magnesium niobate-lead titanate, (PbMg1/3Nb2/3O3)x−(PbTiO3)1−x

(PMN-PT). Both ceramic and single crystal PMN-PT exhibit an interesting structure

which have led to the observation of extremely large electrocaloric and piezoelectric

properties. At lower PT concentrations, from x = 5 − 15%, PMN-PT has a near-

room-temperature pseudo-cubic to cubic phase transition, around which a very large

electrocaloric effect occurs. At higher PT compositions, a morphotropic region exists

between rhombohedral and cubic phases; in this region large piezoelectric effects are

observed, making PMN-PT an ideal material for sensors and actuators. This uniquely

structured material is at the forefront to revolutionize several technical fields[31].

4.1 Crystal structure

Though not obvious from its canonical chemical formula, PMN-PT has the same

perovskite unit cell structure as BST, ABO3, shown in figure 1.3. To highlight this

structure an alternate chemical formula, Pb(Mgx/3Nb2x/3Ti1−x)O3, could be used.

The A sites in the unit cell are clearly occupied by Pb atoms. The oxygen atoms form

octahedral cages about the B site atoms. The B sites are filled in proportion to the ratios

set within the parentheses; for example, for (PbMg1/3Nb2/3O3)0.9 − (PbTiO3)0.1, or

67

(Pb(Mg0.3Nb0.6Ti0.1)O3) in the alternate scheme, there would be 30% Mg atoms, 60%

Nb atoms and 10% Ti atoms occupying the B site. Below the Curie temperature, it

becomes energetically favorable for the B site atoms to settle either slightly above or

below the centers of the oxygen octahedra, leading to a remnant polarization at low

temperatures.

4.2 Relaxor transition in PMN-PT

Just as ferroelectrics exhibit large electrocaloric effects near the ferroelectric-to-

paraelectric phase transition, relaxors exhibit large electrocaloric effects in the region

of the freezing temperature. As the ferroelectric-to-paraelectric phase transition is very

sharp, the region of large electrocaloric effect in a ferroelectric such as BST is limited to a

range of about 10 K. Because the relaxor transition is very broad, a large electrocaloric

effect can be observed over a broader range, though the peak effect may be smaller

because the changes in the polarization are smoother.

4.2.1 Transition temperature for PMN-PT

As PMN is a relaxor and PT is a ferroelectric, the structural transition becomes

rather complicated. Room temperature phase transitions occur for two different con-

centrations of PT. At PT concentrations of 27-33%, PMN-PT exhibits a morphotropic

transition from the low-temperature rhombohedral phase to a low temperature tetrago-

nal phase. This transition allows very large piezoelectric coupling coefficients[15, 62, 64].

Figure 4.1 is based on the phase diagram presented by Zekria, et. al. At a composition

of 6-12% PT there is a transition from the rhombohedral phase to the high-temperature

68

Fig. 4.1. Phase diagram for PMN-PT mixtures[62]

69

cubic phase. It should be noted that diffuse neutron scattering experiments fail to show

the distinct change in the physical structure along this transition[61] that is seen in XRD

measurements[62, 64]. Gehring has speculated that this transition is really a skin effect,

the part sensitive to XRD, and that the bulk of the material, the part sensitive to neutron

scattering, does not experience any change[13]. This could explain some observations of

larger electrocaloric effects in thinner samples[31, 40].

4.3 Single crystal PMN-PT

Recent data suggests a strong dependence of crystal orientation on the observation

of large electrocaloric effects[39]. Table 4.1 shows the electrocaloric effect observed by

Sebald, et al., in three orientations of single crystal 0.75PMN-0.25PT as well as ceramic

PMN-PT. The orientations investigated were [111], [110], and[100]. While Sebald fails to

draw the connection between crystal orientation and electric field, the result is straight

forward. The energy in and electric dipole in an electric field is E = ~P · ~Eq, where ~P is

the dipole moment vector and ~Eq is the applied electric field vector. For a single crystal,

the dipole moment vector is the direction of net polarization, which seems to occur in

the [111] orientation. The column in table 4.1 labeled cos θ[111] is the cosine of the angle

between the listed crystal orientation and the [111] orientation. Sebald’s data show good

agreement with the simple estimate of maximum polarization occurring along the [111]

orientation.

Note that for the ceramic sample mentioned in table 4.1 the fraction of the po-

larization which remains when the bias field is removed is presented in lieu of the cosine

70

Orientation cos θ[111] ∆TEC,[111] cos θ[111] Observed ∆TEC (J/g)[39]

[111] 1.000 0.40 0.40[110] 0.817 0.33 0.32[100] 0.578 0.23 0.18

Ceramic 0.866 0.35 0.36

Table 4.1. Effect of crystal orientation on the electrocaloric effect in PMN-PT

of the rotation angle. Because the largest electrocaloric response seems to occur in

[111]− oriented, single crystal PMN-PT was initially investigated.

Single crystal PMN-PT plates were provided by Omega Piezo Technologies, Inc.

of State College, PA. The plates measured 18 mm by 22 mm by 0.2 mm thick. The plates

were very fragile and the edges could not be safely polished if metal from the electrode

redeposited around the side of the sample. A special deposition mask was prepared for

use in the sputtering system and is shown in figure 4.2. The edges of the mask overlap the

sample edges by 0.5 mm to maintain a large gap to prevent arcing through air between

the electrodes when large fields were applied to the plates. The mask had holes over one

corner of the plate to ease connection to the electrical leads in the proposed solid-state

heat pump.

When the first sample was mounted in the electrocaloric apparatus, no peak in

either dielectric constant or electrocaloric effect was observed below 80 C. A special

high temperature setup was constructed in an oven and is shown in figure 4.3. The

sample is still suspended by the electrical connections with a thermocouple attached

to the grounded electrode; however, the sample was not measured under high vacuum.

71

Fig. 4.2. Mask for electroding rectangular plates

72

The presence of air increased both the uncertainty of the absolute temperature and

diminished the size of the measured electrocaloric effect. These considerations are re-

flected by the large error bars in figure 4.4. The lower set of data, presented as boxes,

show the electrocaloric effect measured at 1 MV/m. The upper set of data, presented

as “X’s”, show the electrocaloric effect measured at 1.5 MV/m. The peak effect ob-

served was 0.56 K at 1 MV/m and 0.74 K at 1.5 MV/m, both occurring at 141 C. A

transition temperature of 141 C would suggest that the composition of the material is

(PbMg1/3Nb2/3O3)0.73 − (PbTiO3)0.27. The manufacturer of the sample quoted the

sample as having between 12 and 15% PT.

An electron backscatter measurement was made on the ceramic, and an analy-

sis of the electron absorption measured the relative concentration of constituent ions.

The results are presented in table 4.2. The magnesium, niobium and titanium ratios

suggest that the composition is closest to (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2 though

this material would be significantly magnesium deficient. This deficiency could explain

why the Curie temperature was observed to occur at 141 C when it should occur at

88 C in (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2. To help control compositional variance

and produce PMN-PT with a Curie temperature near room temperature, a protocol to

produce ceramic PMN-PT samples was developed.

4.4 Ceramic PMN-PT

PMN-PT powder was prepared in a two reaction process[48, 49]; the procedure

is documented in Chapter 2 and a step-by-step summary is presented in Appendix A.

Briefly: magnesium niobate was prepared first by mixing magnesium oxide and niobium

73

Fig. 4.3. High temperature electrocaloric measurement setup

74

Fig. 4.4. Electrocaloric effect in single crystal (PbMg1/3Nb2/3O3)0.8 − (PbTiO3)0.2

Ion Number per unit cell Expected Percent differencePb 0.98 1.00 2.1Mg 0.21 0.27 21.8Nb 0.57 0.55 3.6Ti 0.20 0.18 8.3O 3 3 0

Table 4.2. Compositional analysis of PMN-PT crystal

75

(V) oxide for 12 hours in an ethanol solution using a roller mill and drying the powder.

The dried powder was calcined at 1200 C for 6 hours and ground for a total of 10 hours in

a vibratory mill. The magnesium niobate was then mixed with lead oxide and titanium

dioxide for 12 hours in an ethanol solution in a roller mill and dried. This powder was

then calcined at 800 C for 10 hours and ground for a total of 10 hours in a vibratory

mill. The ground powder was run through a #50 mesh sieve and binder was added to the

powder to aid the pressing of ceramic pellets. For the ceramics measured in this research,

a PT concentration of x = 0.15 was chosen. PMN-PT powders with concentrations of

x = 0.10 were also prepared; however, they were found to contain extremely high levels

of lead, and the ceramic samples were not studied in detail. Properly reacted PMN-PT

powder should have a uniform faint yellow color. Excess amounts of lead typically show

up as a reddish color throughout the powder. To ensure there is minimal excess lead

and that the PMN-PT powder has the proper crystalline structure, XRD must first be

performed on any calcined powder.

4.4.1 XRD of PMN-PT powder

To verify the crystal phase of the PMN-PT powder, XRD was performed on the

calcined powder. The results of the XRD analysis are shown in figure 4.5. The vertical

lines superimposed over the intensity data are the expected locations and relative intensi-

ties for the XRD peak in perovskite PMN-PT found in the PDF-4. Perovskite PMN-PT

has one very strong peak at an angle of 2θ equal to 32.3o and two small peaks at 2θ equal

to 22.1o and 38.8o. The small peaks at 29.2o and 33.9o are evidence of a small amount

of non-ferroelectric pyrochlore material in the powder, most likely Pb6MgNb6O23. The

76

small peak at 30.5o could be unreacted lead oxide powder. Figure 4.6 shows XRD data

from a sintered ceramic. The parasitic peaks have mostly disappeared, except for tiny

peak around 30.2o. The reaction to form PMN-PT does not always complete during cal-

cination but often will complete during the sintering process. With the proper chemistry

and structure ensured, the PMN-PT cermaics can be prepared for electrical property

measurements.

4.4.2 Electrical property measurements on ceramic PMN-PT

To form thin samples for measurement, sintered ceramics thicker than 1.0 mm can

be cut on a wire saw using the procedure from section 2.3. Sometimes the cut surface

from a wire saw cut sample is wedge shaped or bowled, which requires careful polishing

by hand. Sintered ceramics thinner than 1.0 mm should be hand polished down to a

thickness of 250 µm using the procedure documented in section 2.3..

Initially, electrodes were applied by silk screening a conducting silver paste onto

each face of the ceramic. These electrodes were then dried and fired onto the ceramic.

This process is standard in the piezoelectrics industry and avoided when working with

BST only because of barium’s known high reactivity. Later, electrodes were applied to

the PMN-PT ceramics using the same sputter deposition process that was developed for

BST. Figure 4.7 shows dielectric constant measurements in a PMN-PT sample with fired

silver electrodes and sputtered chrome/aluminum electrodes. The arrows represent the

direction in which the data were taken. Both curves show the heating and cooling asym-

metry typical of relaxors and both have the freezing and Curie temperatures occurring

at roughly the same temperature. However, the dielectric constant is three times larger

77

Fig. 4.5. XRD of calcined 0.90PMN-0.10PT powder

78

Fig. 4.6. XRD of a sintered 0.90PMN-0.10PT ceramic

79

when sputtered electrodes were used compared to fired silver electrodes. The effects were

not limited to the dielectric constant.

Figure 4.8 shows the electrocaloric effect measured in two identical samples, one

with sputtered electrodes, (the data are presented as the “X’s”), the other with fired

electrodes, (the data are presented as the crosses). With the exception of one data point

taken at high temperatures, the sputtered electrode sample consistently showed a 20%

larger electrocaloric effect. These electrical and thermodynamic effects arise from the

mobility of electric dipoles in the ceramics. In addition to dipole boundaries, there are

also grain boundaries within the ceramic. It is possible that large surface charges could

move over these grain boundaries to attempt to cancel changes in polarization. If a very

thin layer of non-ferroelectric insulating material, such as remnants of the organic carrier

of the silver paint, is left behind between the electrode and the ferroelectric material, it

is conceivable that surface charges in this insulating layer could screen the macroscopic

fields applied to the sample, mitigating the dielectric and electrocaloric measurements.

4.4.3 Results for (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15

Both the dielectric constant and electrocaloric effect were measured in a 0.85PMN-

0.15PT ceramic. The dielectric constant data are shown in figure 4.9. The lower curve

was taken heating the sample and the upper curve was taken cooling the sample. Ar-

rows have been imposed over the data to show the direction the data were taken. All

measurements were made at a frequency of 1 kHz; at lower frequencies larger dielectric

constant values would be expected below the Curie temperature. The curves begin to

diverge at a temperature of 90 C. The expected Curie temperature for 0.85PMN-0.15PT

80

Fig. 4.7. Effect of different electrodes on dielectric constant in ceramic 0.85PMN-0.15PT

81

Fig. 4.8. Effect of different electrodes on electric effect in ceramic 0.85PMN-0.15PT

82

is 70 C. The peak of just under 25000 occurs at a temperature of 72 C, the freezing

temperature for this sample. As this is where polarization changes most rapidly, this is

the temperature around which the largest electrocaloric effect is expected.

The electrocaloric data are shown in figure 4.10. The effect peaks at 60 C with a

temperature change of 0.375 K. The electrocaloric effect is larger than 0.35 K from 55 C

to 81 C, and is larger than 0.30 K from 50 C to above 95 C. While the absolute maximum

electrocaloric effect is about the same in PMN-PT as it is in BST, large electrocaloric

effects occur in a 45 degree temperature span rather than a 10 degree temperature

span. At lower PT concentrations, similar dielectric and electrocaloric effects would be

expected to occur at lower temperatures.

4.5 Comparison to other measurements

The largest reported electrocaloric is effect in bulk 0.85PMN-0.15PT is 1.8 K

under an applied electric field of 1.6 MV/m at 18 C[40]. This peak temperature con-

tradicts most reports of peak properties in 0.85PMN-0.15PT, which should occur above

63 C[15][62][64]. In Shaobo’s description of the electrocaloric effect, the large temper-

ature gradient establishes itself over a period of many seconds to several minutes [40].

However, the electrocaloric effect should be an almost instantaneous change in the tem-

perature of the sample. Figure 4.11 shows a typcial electrocaloric measurement taken on

a PMN-PT ceramic. The horizontal axis shows the time and the vertical axis shows the

temperature reading of the thermocouple. A DC electric field of 1 MV/m is applied for

25 seconds and removed. A DC electric field of -1 MV/m is then applied for 25 seconds

and removed. The circuit supplying the electrical field has a time constant of 20 ms to

83

Fig. 4.9. Dielectric constant of ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15

84

Fig. 4.10. Electrocaloric effect in ceramic (PbMg1/3Nb2/3O3)0.85 − (PbTiO3)0.15

85

prevent damage to the sample. The temperature response of the thermocouple occurs

with a time constant of less than a second. The temperature change provided by Shaobo

could, however, be described if the sample was joule heating because of the application

of the electrical field.

Ferroelectrics and relaxors are supposed to have very high resistance, though con-

taminents can change the structure of the material. Contaminated BST powder created

semiconductor-like ceramics, as was mentioned in section 3.1. While the low voltage

resistance measurement of a typical multimeter measured high resistance, a measure-

ment of the resistance of the contaminated sample under a large electric field showed

resistances less than 10 MΩ. When attempting to measure the electrocaloric effect in

this sample by applying a 200 V electric field, the result was a 5 mW of joule heating.

Figure 4.12 shows the thermocouple response due to joule heating of a ceramic sample.

The temperature rises quickly, but with a time constant on the order of several seconds.

Even though this particular sample was dissipating only 5 mW of power, this would have

led to a steady-state temperature gradient of 10 K if the electric field would have been

left on for several minutes. Other groups using measurement techniques similar to both

Shaobo and the techniques described in this research report electrocaloric temperature

changes ranging from 0.4 to 1.2 K under an applied electric field of 1.5 MV/m[67]; how-

ever, these data show very erratic behavior, large amounts of scatter and peak effects

outside the expected temperature range for the stated concentration.

One group has measured the electrocaloric effect in both single crystal and ce-

ramic 0.75PMN-0.25PT using a calorimetry technique[38, 39], which measures the heat

generated by the electrocaloric effect. In calorimetry-based electrocaloric measurements,

86

Fig. 4.11. Strip chart recording of the electrocaloric effect in PMN-PT

87

Fig. 4.12. Strip chart recording of joule heating in a ceramic sample

88

samples are slowly heated in a temperature regulated environment. A temperature-

regulated heater is connected directly to the sample. If the temperature of the sample

starts to drop, the power to the heater is increased; if the temperature of the sample

starts to rise, the power is decreased. The power input to the heater is recorded. When

an electrocaloric measurement is made, the power of the heater is measured and then

integrated to find the total heat released by the sample when the field is applied and

the total heat absorbed by the sample when the electric field is removed. In ceramic

PMN-PT a peak electrocaloric effect of 0.13 J/g was measured under an applied field of

1.0 MV/m at a temperature of 130 C[38]. This corresponds to an electrocaloric temper-

ature change of 0.35 K, slightly below the average peak electrocaloric effect of 0.38 K

reported in this dissertation. In [111]− oriented single crystal 0.75PMN-0.25PT the

largest reported electrocaloric effect is 0.23 J/g under an applied field of 1.0 MV/m at

a temperature of 110 C[39]. This corresponds to an electrocaloric temperature change

of 0.61 K, compared to the peak electrocaloric effect of 0.56 K reported in this disserta-

tion. While none of the samples in this research have the same compostition as Sebald’s

samples, the directly observed electrocaloric temperature is consistent with calorimetry

measurements in similar materials.

Other groups have estimated the electrocaloric effect of PMN-PT in thin-film

samples[31]. While the thin-films are too small to directly probe, they are grown on a

substrate with an electrode and the polarization is mapped as a function of temperature

and applied electric field. The presented data is the electric field integral of the temper-

ature derivative of the polarization. While the astounding result of a 12 K electrocaloric

89

effect is presented, this must be taken with a mine of salt. This estimation of the tem-

perature change in the thin film neglects the substrate, which account for nearly all of

the thermodynamics. The proper way to treat the thin film is as a latent heat source;

however, the heat capacity of the substrate plus the thin film must be considered to

derive the useful contribution. For a 1 µm thin film on a 100 µm substrate, there would

be a two order of magnitude reduction in the “useful” electrocaloric contribution.

Polarization measurements in PMN-PT suggest that it should be capable of pro-

ducing an electrocaloric effect greater than 1 K over a temperature span of 30 to 50 K.

While 1 K has yet to be reliably observed, single crystal PMN-PT is capable of pro-

ducing electrocaloric effects greater than 0.5 K over a 30 K temperature span. Better

ceramic processing techniques may allow ceramics to withstand electric fields greater

than 2.0 MV/m, increasing the electrocaloric effect. Dependence of the electrocaloric

effect on concentration of PT should also be studied. Even though the maximum elec-

trocaloric effect in a PMN-PT plate may be limited to 0.5 to 0.75 K, this may be more

than enough to serve as the working medium for a refrigerator capable of cooling an

object by 30 K.

90

Chapter 5

An electrocaloric solid state heat pump

While vapor compression refrigeration has tended towards safer, more environ-

mentally friendly working fluids, better environmental benefits would come from the de-

velopment of high-efficiency all-solid-state refrigerators. Currently thermoelectric heat

pumps are used as chip coolers for computer processors, and they have been adapted as

plug-in refrigerated chests. A key drawback to more widespread use of thermoelectric

technology are low efficiencies, typically 7 to 11% of the Carnot limit. Thermoelec-

tric research has focused on designing better materials with larger temperature changes

to raise efficiencies. While there is no theoretical upper limit to the performance of

thermoelectrics, few breakthroughs have occurred since the discovery of semi-conductor

thermoelectrics. The slow pace of discovery for thermoelectrics has helped spur research

into other solid-state cooling technologies.

The focus in cooler design has been to develop new materials capable of producing

the largest possible temperature changes. Larger temperature changes allow simpler

designs. If a working material can produce a 30 K temperature change, it can cool an

object almost 30 K through direct contact and relatively slow cycles. Figure 5.1 shows

a schematic of a typical solid state cooler. A heat switch connects a caloric element

to the object to be cooled as the element is engaged. As the two objects come into

thermal equilibrium, the switch connects the caloric element to the hot heat exchanger

91

Fig. 5.1. A heat switching heat pump

92

to recharge the caloric element. The process is then repeated. If the caloric elements

can produce temperature changes smaller than the desired temperature span, the caloric

elements could be run in series to create a step-up cooling system[36]. This simplicity

has focussed development towards a search for caloric elements with larger and larger

temperature changes.

Recent research into magnetocaloric and electrocaloric materials[21, 34, 35, 42,

43, 31, 39] has led to the discovery of many new compounds capable of generating large

temperature changes. Gadolinium-based materials exhibit a magnetocaloric temperature

change of 11 to 17 K under the application of 5 T magnetic fields[34]. Demonstration

coolers based off these materials can produce cooling spans of 30 to 50 K[21]. The limit

to magnetic refrigeration, however, is the ability to scale devices. Large magnetic fields

are not easy to create. Most 5 T magnets require superconducting wires with a separate

cryogenic cooling system, making it difficult to build a small, portable system. It is

also difficult to raise and lower a magnetic field quickly. Electrocaloric materials avoid

most of the problems with magnetocalorics; large electric fields are relatively cheap and

easy to produce and electrical power supplies are highly scalable. The drawback to the

electrocaloric effect has been the lack of discovery of materials capable of producing large

temperature changes. In this research, the largest measured electrocaloric effects were

0.75 K at electric fields of 1.5 MV/m in single crystal PMN-PT. The largest reports in

the literature of a directly measured electrocaloric effect is 1.8 K at 2.0 MV/m in lead

scandium tantalate at temperatures around 300 K[42]. The limitation may not be in the

thermal properties of the materials, but in the design of devices to exploit the cooling

power of these materials.

93

If the thermoacoustic effect discussed in section 1.2 could be generalized to de-

scribe a device using a generic thermally-active element in a process analogous to the

Stirling cycle, such a device could achieve greater cooling power and larger temperature

spans than current solid-state cooler designs. In this chapter, an all-solid-state heat

pump analogous to the thermoacoustic effect in fluids will be described. Though the de-

sign may be adapted to exploit one of many thermal effects, this heat pump is designed

to use electrocaloric elements as a working medium.

5.1 A theoretical model

So long as some physical symmetry is broken[60], a small, cyclical temperature

change can be used to establish temperature gradients much larger than the local tem-

perature change. Thermoacoustics provides a model for a novel thermodynamic cycle to

more effectively use solid-state thermodynamic effects.

5.1.1 A quick review of thermoacoustics

Fig. 5.2. The thermoacoustic cycle

94

Literally, thermoacoustics is the interaction of heat and sound. Specifically, ther-

moacousticians design closed-cycle heat engines using the inertia, compliance, and dissi-

pation of acoustic media to replace the mechanical pistons and linkages of conventional

heat engines and refrigerators. While normal sound is adiabatic, standing waves along a

solid interface can interact with the interface to pump heat. Figure 5.2 shows a diagram

of the thermoacoustic cycle.

In the first panel, the gas moves to the right and is compressed. In the next panel,

the compressed gas transfers a small amount of heat to the solid, compressing further. In

the third panel, the gas moves to the left and expands. In the final panel, the gas absorbs

a small amount of heat from the solid and expands. A small amount of heat has been

moved from left to right, and the cycle repeats. Just before and just after this parcel

of gas is another small parcel of gas, which allows a bucket-brigade-like effect to occur,

building a large temperature gradient from a small local temperature oscillation in the

gas. Schematically, it is easy to envision a generic temperature oscillation interacting

across an interface. In the next section I will generalize the heat transport equations in

thermoacoustics to present a theory for a general cycle analogous to the thermoacoustic

cycle.

5.1.2 Generalizing the thermoacoustic cycle

While figure 5.2 shows a schematic of the actual thermoacoustic effect, figure 5.3

shows an abstraction of the effect. The fluid half of the interface has been replaced

by a solid capable of producing a temperature oscillation. Every other solid plate is

free to move parallel to the interface. Provided that the move plate travels farther

95

Fig. 5.3. A generalized Stirling-like heat pump cycle

96

laterally than the heat generated by the temperature oscillation, the resulting effect

should be completely analogous to the thermoacoustic effect. Two length scales would

need to be observed for this analogy to hold. First, the plates would have to be within

one thermal penetration depth of each other. This would allow heat to be transferred

between the plates within the time span of the oscillation. Second, as previously stated,

the relative displacement of the two plates would have to be much greater than the

thermal penetration depth, breaking a physical symmetry that would cancel out any net

heat transport.

For BST and PMN-PT, the thermal pentration depth for a 30 Hz oscillation is

about 100 µm. A 200 µm thick plate would be nearly totally thermally active. 2 cm

square, single crystal PMN-PT plates 200 µm thick have already been produced. Ceramic

PMN-PT and BST samples have been polished thinner than 250 µm, though the largest

areas made have been 1 cm cylinders. To break the heating symmetry, the plates would

need to be displaced a distance greater than 1 mm relative to each other. This could

easily be accomplished using a loudspeaker cone as an actuator. Thermoacoustics also

provides a framework to estimate the maximum possible cooling power of an analogous

relative motion device.

The cooling power for a fluid-based thermoacoustic cycle is found by the relation[50]:

Q =12ΥδκTmβppaua(1−RT ) (5.1)

where Υ is the perimeter, δκ =√

2κ/ρ2πf is the thermal penetration depth, Tm is

the average temperature, βp is the thermal expansion coefficient of the fluid, pa is the

97

amplitude of the pressure oscillation, ua is the amplitude of the velocity oscillation and

RT is the ratio of established temperature gradient to a critical temperature gradient

given by

RT =ρmcpua∆T

2πTmβppafl(5.2)

where ρm is the mean density of the fluid, cp is the specific heat at constant pressure,

∆T is the temperature span, f is the frequency of the oscillation and l is the length of

the interface. The critical temperature gradient

∆Tcrit =2πfTmβppa

ρmcpua(5.3)

is the point in thermoacoustics where temperature changes due to pressure oscillations

exactly cancel the temperature changes due to displacement oscillations[50].

The expressions for Q and RT can be elucidated with a few substitutions. First,

the pressure amplitude pa can be converted to a temperature oscialltion Ta with the

relation

Ta =Tmβp

ρmcppa (5.4)

The velocity amplitude ua can be expressed in terms of the displacement ampli-

tude ψa = ua/(2πf). Finally, the thermal working volume of the device is V = lΥδκ.

This simplifies the temperature gradient ratio to

RT =(∆T/l)(Ta/ψa)

(5.5)

98

The numerator in the temperature gradient ratio is the mean temperature gra-

dient established across the device while the denominator is an effective temperature

gradient given by the temperature oscillation amplitude Ta and the displacement ampli-

tude ψa.

The cooling power density Q/V can now be written as

Q

V=

(πfρmcpT 2

a

) 1∆T

RT (1−RT ) (5.6)

This density has a maximum value when the ratio RT = 1/2.

The maximum cooling density can then be defined as

Q

V=

π

4

(ρmcpTa

)f

Ta∆T

(5.7)

The factor ρmcpTa is the amount of heat per unit volume generated in one cycle

of the device. The frequency converts the heat per cycle to the heat per second. Ta/∆T

is the fraction of the total temperature lift ∆T provided by the temperature oscillation

Ta.

Table 5.1 shows the theoretical maximum cooling power density for a heat pump

described by equation 5.7. The maximum cooling power density and total span achieved

by the device are inherently trade-offs; the larger a temperature span sustained, the

less cooling power available for the device. As this is a first principles theory, loss

effects have been neglected. As a result, cooling power density is not reported when

it drops below 1.0 W/cm3 −K. Since the cooling power increases as the square of the

99

(Q/V )Max (W/cm3)Lift (K) 0.50 0.66 1.00 1.50 2.00 Ta

0.5 35.3 61.6 141 318 5651.0 23.5 30.8 94.3 212 3771.5 11.8 20.5 47.1 106 1892.0 8.8 15.4 35.3 79.5 1412.5 7.1 12.3 28.3 63.6 1133.0 5.9 10.3 23.6 53.0 94.34.0 4.4 7.7 17.7 39.8 70.75.0 3.5 6.2 14.1 31.8 56.67.5 2.4 4.1 9.4 21.2 37.7

10.0 1.8 3.1 7.1 15.9 28.312.5 1.4 2.5 5.7 12.7 22.615.0 1.2 2.1 4.7 10.6 18.920.0 1.5 3.5 8.0 14.125.0 1.2 2.8 6.4 11.330.0 1.0 2.4 5.3 9.440.0 1.8 4.0 7.150.0 1.4 3.2 5.7

Table 5.1. Cooling power at a given temperature span (lift) of a thermoacoustic-likedevice operating at 30 Hz

100

driving temperature oscillation Ta, even small improvements in the caloric elements could

significantly reduce the size of a potential device.

It is important to note that the theoretical performance summarized in table 5.1

is for any physical mechanism capable of producing temperature oscillation Ta at a fre-

quency of 30 Hz. While in the traditional thermoacoustic picture only the fluid is capable

of generating a temperature oscillation, one could double the effective temperature os-

cillation using two thermally active solids properly phased with each other. Hence, a

temperature oscillation of 0.5 K in one element translates to a working temperature

oscillation of 1.0 K.

5.2 Description of device

Figure 5.4 shows a plan for a demonstration solid-state heat pump. At the heart

of the heat pump is a stack of rectangular caloric plates. Every other plate is rotated 90o

so that the ends of the plates stick out slightly from each side. Beryllium copper flexures

are attached to the protruding ends of the rotated plates. A conductive paint is used to

electrically connect an electrode on the plate stack to the copper flexure. Four electrical

connections are needed to apply opposite electric fields on every other plate but have

no potential difference in the small gap between electrodes. Copper blocks are attached

to the stationary plates for structural support and also serve as heat exchangers. The

moving plates can be driven by a piezoelectric actuator, a loudspeaker with a linkage,

or some other type of piston. The caloric elements could be either BST or PMN-PT;

estimations of the performance of each of these materials in this proposed device are

presented in the next section.

101

Fig. 5.4. Proposed design of all solid state heat pump

102

5.3 Predicted performance using ferroelectric ceramic elements

Max Ta 2.0 KSpan at Q/V = 1.0W/cm3 150 K

Q/V at 30 K Span 9.4 W/cm3

Table 5.2. SSHP performance using 1.0 K caloric elements

Table 5.2 shows the performance characteristics of a hypothetical relative motion

heat pump using caloric elements capable of producing a temperature oscillation of 1.0 K.

If each layer could generate this temperature change and the device could be properly

phased, the total net temperature oscillation would be 2.0 K. Given this temperature os-

cillation, equation 5.7 predicts that the device could produce a temperature span greater

than 200 K at almost no cooling load. To sustain a cooling over a 30 K temperature

span, the device could theoretically achieve a cooling density of 9.4 W/cm3. Cooling the

volume the size of a human body would require about 150 W of cooling power. If the

caloric elements could achieve the 1.0 K oscillation, the proposed device would require

16 cm3 of active material to achieve this cooling over the 30 K span; this would require

a device with a length of 2.5 cm on each side.

While measurements made on BST and PMN-PT ceramics only observed max-

imum electrocaloric effects between 0.4 and 0.5 K, these ceramics may still be useful

103

BST PMN-PTMax ∆T 0.45 K 0.38 K

∆T > 0.25K 10 K 50 K∆T > 0.33K 5 K 35 K

Span at Q/V = 1.0W/cm3 15 K 30 KQ/V at 10 K Span 1.8 W/cm3 3.1 W/cm3

Q/V at 30 K Span 1.0 W/cm3

Table 5.3. SSHP performance using BST and PMN-PT ceramic elements

in designing and building small-scale coolers. Table 5.3 shows performance characteris-

tics of a relative motion solid-state heat pump using both BST and PMN-PT elements.

While the maximum electrocaloric effect observed in BST is slightly larger than that

measured in PMN-PT, the electrocaloric effect in BST drops off rapidly as the tempera-

ture differs from the peak effect temperature. PMN-PT has an electrocaloric effect larger

than 0.25 K over a 50 K temperature span compared to a 10 K temperature span in

BST. PMN-PT has an electrocaloric effect greater than 0.33 K over a 35 K temperature

span compared to only a 5 K temperature span in BST. This limits a BST-based heat

pump to a maximum operating span of 15 K whereas PMN-PT could operate over a

30 K span. Q/V > 1.0W/cm3 was chosen as the threshold of operation because the

theory of operation currently does not account for losses, such as thermal conduction,

which must be overcome in a practical device. Over a 10 K operating span, a PMN-PT

solid-state heat pump should be able to achieve a cooling density of 3.1 W/cm3. If the

device was constructed from 10 layers with dimensions of 22 mm by 18 mm by 0.2 mm,

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a demonstration cooler should be able to produce 12 W of cooling power about the same

power as other thermoacoustic demonstration coolers[50].

105

Chapter 6

Conclusion

Many challenges in the realm of physics, materials science and engineering exist

before a practical electrocaloric refrigerator is ready for the market. First and foremost,

we need to construct a working prototype to verify AC-to-DC temperature conversion.

With a working prototype, research in better electrocaloric materials will need to be

conducted to further understand ferroelectric properties.

To bridge the electrocaloric measurements in bulk materials and estimated elec-

trocaloric effect in thin films, a detailed study of polarization in bulk materials should

be conducted to confirm the integral relation. The mapping would need to include mea-

surements of the dielectric constant, ε(Eq, T ), as a function of temperature and external

electric field. In addition, the pyroelectric effect will need to be measured with no exter-

nal electric field to determine any temperature effects of the constant of integration. The

electrocaloric temperature change, ∆TEq(Eq, T ), could then be mapped as a function of

temperature and external electric field to confirm the relation. Thin films should also

be tested to try to directly observe any electrocaloric effect. Thin films can reportedly

withstand electric fields in excess of 20 MV/m[31, 32]. The mechanism which supports

larger fields should be investigated further, beacuse if thinner single crystal or ceramic

plates could be made which could safely accept electric fields larger than 2 MV/m, larger

electrocaloric effects may be measured in bulk BST or PMN-PT. While it is dubious to

106

calculate electrocaloric temperature changes from thin-film polarization measurements,

it may be possible to observe some temperature change.

If a thin film 2 µm thick can truly create a 12 K electrocaloric temperature

change[31], some direct temperature change should be observable on a 150 µm thick

substrate. Provided that the specific heats of both the thin film and the substrate

are comparable, a 12 K temperature change in an isolated film would translate to a

0.16 K temperature change in a thin film-substrate system. The direct electrocaloric

temperature measurement setup has a resolution capable of seeing temperature changes

as small as 0.4 K, so direct observations of the effect from a thin film are possible.

While thin films may lead to larger temperature changes in BST or PMN-PT, there are

other reported ferroelectrics capable of producing 1 K electrocaloric effects. If reports of

electrocaloric effects larger than 1 K in perovskite ferroelectrics such as lead scandium

titanate are true[42], these materials could open the door for better exploitation of

thermoacoustic-based solid state heat pumps.

In addition to a better understanding of the materials themselves, processing

techniques need to be developed to produce thinner samples with larger areas. Previous

research[23], confirmed by data in chapter 3, shows that better electrocaloric effects are

achieved by larger grained ceramics. However, larger grained ceramics tend to be more

brittle. A detailed study of mechanical properties such as Young’s modulus must be

performed to better understand the limits to aspect ratio of caloric plates for building

larger devices.

Achieving a sustained temperature gradient through a moving temperature oscil-

lation is still just a first step. Even with advances in elements themselves, an effective

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heat exchanger would need to be designed to convert the temperature gradient into use-

ful cooling power in a practical device. Each element added in the process of converting

work into cooling power introduces new loss mechanisms and engineering challenges.

While a caloric element capable of 30 K temperature changes remains the holy grail of

solid-state cooling, making better use of easier to control caloric effects can open up a

new field of development for all-solid-state devices capable of producing a greater cooling

density compared to current technology.

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Appendix A

Protocol for preparing ceramics

A.1 BST Protocol, July 2007

Preliminary steps, as necessary: a) Burn out oven, crucibles, and mixing/grindingmedia. b) Use high purity metal oxide and carbonate powders, 99.99% or better. c) Useonly reagent grade acetone (99.5% purity or better) to mix the powder or to clean anyvessel contacting the BST.

1. Weigh powder. For 50 g Ba0.67Sr0.33TiO2: 25.346 g BaCO3; 9.339 g SrCO3;15.313 g TiO2.

2. Form a slurry with 50 g powder and 150 ml acetone in a 1 l Nalgene bottle.

3. Add 500 ml Zr media; media should be just below level of slurry.

4. Mix on roller mill at 22 rpm for 4 hours.

5. Pour through SS strainer (to separate media) into a 1 l Pyrex bowl; rinse media instrainer with acetone once into bowl. Wash media, discarding acetone, for futureuse.

6. Dry slurry at 80 C; stirring occasionally to promote drying. Place under vacuumafter initial drying.

7. Grind to fine powder in agate mortar and pestle.

8. Sieve through 250 µm mesh into Zr trays.

9. Calcine in oven. Ramp up at 5 C/min to 1200 C for 6 hours; ramp down at5 C/min.

10. Put dry calcined powder in 1 l Nalgene bottle with 150 ml Zr media. Dry-grind inbalanced vibratory mill for 4 hours.

11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if calcining eliminated non-BST materials.

12. Add 150 ml acetone to the grinding bottle, then add more Zr media to reach justbelow slurry level.

13. Wet-grind in balanced vibratory mill for 6 hours.

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14. Dry slurry as in step 6.

15. Grind to fine powder in mortar and pestle.

16. Clean die with lint-free tissue.

17. Weigh 0.5 g powder on small square of waxed massing paper and sieve through#50 mesh screen into 1/2 inch die.

18. Uniaxially press to 5700 lbs (29,000 psi = 200 MPa for 1/2 inch die) and hold for30 seconds.

19. Cold isostatic press (CIP) to 200 MPa (if available).

20. Place green pellets on Zr tray and cover with second tray; place in oven.

21. With O2 diverted to water in beaker, adjust needle valve for flow rate of 1 bubble/s;reconnect O2 to oven.

22. Heat oven at 5 C/min to 400 C and hold for 1 hour; heat at 3 C/min to 1500 Cand hold for 6 hours; cool at 2 C/min.

23. Cut 250 µm thick wafers of sintered pellets using commercial ceramic wafering saw.

24. Sputter Cr/Al electrodes on each wafer face using Perkin-Elmer 4400 sputteringmachine.

A.2 PMN-PT Protocol, July 2008

Preliminary steps, as necessary: a) Burn out oven, crucibles, and mixing/grindingmedia. b) Use high purity components, 99.9% or better.

To avoid the pyrochlore phase, the powder is made in two stages.

A.2.1 Stage I: MgNb2O6 preparation

1. Weigh powder. For 50 g MgNb2O6: 6.697 g MgO; 43.302 g Nb2O5. (2% mol MgOis added to inhibit the pyrochlore phase.)

2. Form a slurry with 50 g powder and 150 ml ethanol in a 1000 ml Nalgene bottle.

3. Add 500 ml Zr media; media should be just below level of slurry.

4. Mix on roller mill at 22 rpm for 12 hours.

5. Pour through SS strainer (to separate media) into 1000 ml Pyrex bowl; rinse mediain strainer with ethanol once into bowl. Wash media, discarding ethanol, for futureuse.

6. Dry slurry at 80 C. Stir occasionally to promote drying.

7. Grind to fine powder in agate mortar and pestle.

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8. Sieve through a #50 mesh into a zirconia crucible.

9. Calcine in oven. Ramp up at 5 C/min to 1000 C for 6 hours; ramp down at5 C/min.

10. Put dry calcined powder in 1000 ml Nalgene bottle with 150 ml Zr media. Dry-grind in balanced vibratory mill for 4 hours.

11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if powder has the proper columbite phase and is free of unreacted powder.

12. Add 150 ml ethanol to grinding bottle, then add more Zr media to reach just belowslurry level.

13. Wet grind for 6 hours in a balanced vibratory mill.

14. Recover the powder following steps 5 through 8.

15. Dry the powder in the oven at 200 C for 12 hours.

16. Transfer dry powder to a 250 ml Nalgene bottle.

17. Store powder under vacuum to prevent absorption of moisture.

A.2.2 Stage II: 0.92PMN- 0.08PT preparation

1. Weigh powder. For 50 g PMN-PT: 34.501 g PbO; 14.549 g MgNb2O6; 0.988 gTiO2. (5% mol of PbO is added to compensate for loss during ceramic processing.)

2. Form a slurry with 75 g powder and 150 ml ethanol in a 1000 ml Nalgene bottle.

3. Add 450 ml Zr media; media should be just below level of slurry.

4. Mix on roller mill at 22 rpm for 12 hours.

5. Pour through SS strainer (to separate media) into 1000 ml Pyrex bowl; rinse mediain strainer with ethanol once into bowl. Wash media, discarding ethanol, for futureuse.

6. Dry slurry at 80 C. Stir occasionally to promote drying.

7. Grind to fine powder in agate mortar and pestle.

8. Sieve through a #50 mesh into Al2O3 crucible and cover.

9. Calcine in oven. Ramp up at 5 C/min to 800 C for 4 hours; ramp down at 5 C/min.

10. Put dry calcined powder in 1000 ml Nalgene bottle with 100 ml Zr media. Dry-grind in balanced vibratory mill for 4 hours.

11. Perform x-ray diffraction (XRD) analysis on sample of dry-ground powder. Deter-mine if powder has pyrochlore phase.

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12. Add 150 ml ethanol to grinding bottle, then add more Zr media to reach just belowslurry level.

13. Wet grind in balanced vibratory mill for 6 hours.

14. Recover the powder following steps 5-7.

15. Sieve all the powder through a #50 mesh screen and store in a 250 ml Nalgenebottle.

16. Add 35 g of PMN-PT powder into a 200 ml Pyrex dish.

17. Measure out 4% of powder weight of binder resin into another 200 ml Pyrex dish.For 35 g of powder, this corresponds to 1.4 g of binder resin.

18. Dilute binder resin with acetone until the resin completely dissolves into a clearliquid.

19. Pour the diluted binder resin into the dish containing the powder and stir the liquidinto the powder using a stainless steel spatula.

20. Continue stirring until the acetone has completely evaporated. Transfer the powderto a mortar.

21. Grind the powder with a mortar and pestle, sieve through a #50 mesh screen andstore powder until it is ready to be pressed.

A.2.3 Stage III: Pressing 1/2” ceramic disks

1. Measure 0.7 g of binderized powder.

2. Wipe off die with lint-free tissue.

3. Increase the pressure in the press until the force gauge is just beginning to get areading.

4. In one motion, press to a force of 4500 lbs and hold for 10 seconds.

5. Remove the pellets from the die and measure their mass.

6. Place the pressed pellets on Pt foil on a Zr plate. Place the plate in the oven.

7. Burn out the binder by heating the oven at 1 C/min to 325 C and hold for 3 hrs.Heat oven at 2 C/min to 500 C and hold for 10 hrs to remove the ash. Turn offthe heating elements and let the oven cool.

8. Measure the mass of the pellets just after coming out of the furnace. The pelletsshould have lost about 1.5 to 1.7% of the initial mass. Measure the thickness ofthe pellets to compute the green density. The green density should be between4.4 g/cc and 4.8 g/cc.

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9. Place the pellets on Pt foil in the lid of an Al2O3 crucible. Cover with a secondsheet of Pt foil. Cover with the crucible and place in the oven.

10. Sinter the pellets heating the oven at 10 C/min to 1280 C. Hold the oven at 1280 Cfor 2 hours. Turn off the heating elements and let the oven cool.

11. Polish the sample on one face to create a flat, smooth surface.

12. Polish the sample’s other face until the sample has a total thickness of 300 µm orless.

13. Measure the mass of the pellets just after coming out of the oven. Measure thethickness and diameter to compute the density. The ceramics should have a densitywithin 2% of 8.0 g/cc.

14. Anneal the polished samples to remove any excess lead that may have built up inthe ceramic.

15. Sputter Cr/Al electrodes on each wafer face using Perkin-Elmer 4400 sputteringmachine.

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Appendix B

Perkin Elmer 4400 Operator’s Reference

B.1 Start up procedure:

B.1.1 Start up from shutdown:

1. Open the N2 purge gas regulator.

2. Turn on the cryopump water using the middle valve located on the wall between the twowindows about 10 ft high.

3. Turn on the compressed air line, located behind the sputtering unit about 12 feet high.

4. Make sure that all keys are set to ‘Manual’ and the all switches are set to off or closed.

5. Turn on the main breaker on the back of the sputtering unit under the load lock.

6. Locate the Ultek Auto Pumpdown Control (APC).

7. Turn on the mechanical pump.

8. Turn on the Hi-Vac pump.

9. Open the throttle valve.

10. Open the Plexiglas door on the lower right side of the unit. Locate the cryo-pump regen-eration control.

11. Turn the ‘cycle time minutes’ control knob to the right, slightly off the ‘0’ mark and pushthe red start button. At the end of the cycle, all adsorbed gases in the cryo-pump shouldbe vented and the temperature should reach 10 K.

B.1.2 Start up from overnight:

1. Turn on the network water by opening the valve below the cryopump water valve. Checkthe flow meters on the cryopump panel below the Ultek Lock Control (ULC) to make surethat water is flowing.

2. Make sure that the vent fan is on and the purge gas cylinders are open.

3. Check the key on the APC. If the key is set to ‘Manual’ or ‘Manual (interlock)’, set thekey to ‘Auto’. Pump down the chamber by holding the ‘Start’ button on the APC andpressing the ‘Pump’ button. When the trip light comes on the hi-vac valve should open.

4. When the convectron gauge reads 0-1 mTorr, turn on the ion gauge. If the chamber iskept under vacuum the ion gauge should read below 5.0 × 10−7 Torr after 2-3 hours. Ifthe unit has not been used in a while, the ion gauge may take a while to reach this value.

5. Vent the load lock. When the ‘ATM’ light turns on, close the vent line and set the ULCkey to ‘Auto’.

6. Open the gas regulators on the Ar and N2 cylinders.

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7. Turn on the target water using the valve located below the cryopump water valve.

8. Turn on the RF generator and the DC power supply located to the right of the sputteringunit. If the DC sputtering controller is beeping, press the blinking red light to silence thealarm. The light will continue to flash until the gas solenoid valve is opened.

B.2 Automatically controlled operating procedures:

1. With the ‘ATM’ light on, depress both ‘Up’ switches on the load lock lid. As the lid opens,the elevator will automatically raise the pallet for loading substrates.

2. Load substrates on the pallet. If working with a non-standard substrate, the total heightmust be kept below 3/4” or the pallet will not clear the gate valve.

3. Depress both ‘Down’ switches on the load lock lid. Wait for the elevator to fully lower andseat before completely closing the lid. When the lid closes, the automatic loading sequenceshould begin.

4. To interrupt the pumpdown and loading sequence hold down ‘Start’ and press ‘Standby’on the ULC. To resume loading hold ‘Start’ and press ‘Load’.

5. Monitor the progress of the pallet as it loads into the sputtering chamber. Look throughthe viewport to make sure that the pallet is evenly balanced on the table and that nothingwill catch on the chamber shutter as the table rotates.

6. If you need to use the DC sputtering target, wait for the chamber pressure to drop below7.0× 10−7Torr as read by the ion gauge.

7. To add gas to the sputtering chamber, hold down ‘Start’ on the APC, press ‘Gas’ and thenquickly press ‘Pump’. Pressure can build up behind the gas solenoid valve causing thechamber pressure to rise too quickly when the valve is opened. If the chamber pressurerises to much and the hi-vac valve closes, pump the system down by holding ‘Start’ andpressing ‘Pump’ on the APC. Wait for the trip light to illuminate and the hi-vac valve toreopen.

8. Repeat ‘Gas’, ‘Pump’, ‘Gas’ bursts until the convectron holds at 0 mTorr with the gaslight indicated. Open the flow controller cutoff valve of the desired sputtering gas, locatedon the right side of the sputtering unit. Make sure the flow controller of the desired gasis turned on and set to flow. Run the gas pressure up to the desired level for sputtering.The convectron should read 8-10 mTorr. For Ar, the flow controller should be set pointshould be 3.7.

9. With the gas running, make sure the motor speed control is set at the yellow mark. Look inthe viewport and confirm that the table is rotating. The pallet is now ready for sputtering.Detailed instructions can be found in the ‘Sputter Procedures’ section of this guide.

10. When you have finished sputtering, turn the flow controller set point back to 0.0. Whenthe convectron reads 0 mTorr close the flow controller cutoff valve. Shut the gas solenoidby holding ‘Start’ and pressing ‘Pump’ on the APC panel. Check the chamber viewport tomake sure that the pallet table has stopped rotating. If the table is still rotating 5 secondsafter turning off the gas, tap on the front of the APC to free the relay. If you cannot freethe relay, turn the rotation control all the way counterclockwise to stop the motor. Neverattempt to load or unload the pallet if the table is rotating as this can damage the system.Contact the sputtering unit supervisor immediately.

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11. With the pallet table stationary, unload the pallet by holding ‘Start’ and pressing ‘Unload’on the ULC panel. The pallet will automatically unload and the load lock will vent toatmospheric pressure.

12. The purge gas line will run until the load lock is opened. To conserve N2, open the loadlock as soon as the ‘ATM’ light is illuminated.

B.3 Sputtering Procedures:

B.3.1 DC Sputter Deposition:

1. Set the selector switches on the RF control panel to ‘Sputter Deposit’ and ‘Target 1’.

2. The DC power supply is controlled by an interface panel in the bottom right corner of thesputtering unit. After ‘Target 1’ has been selected an alarm on this panel should sound.Press the flashing red button to silence the alarm. If the button is still flashing, check thatgas is flowing and that water is flowing through the switches in the lower left panel.

3. Make sure that all of the yellow ‘Remote’ lights are out and that the regulation mode isset to ‘Power’ (left green regulation button lit).

4. Set the cathode power by holding down the ‘Level’ button and turning the knob. Forcoarse adjustments, the ‘Vernier’ button should not be lit. For finer adjustments, pressthe ‘Vernier’ button.

5. Set the power ramp up time by holding the ‘Ramp’ button and turning the knob. Thesystem can easily handle a ramp of several kV per minute.

6. Set the sputtering time by holding the ‘Ramp’ button, pressing the ‘Set Pt’ button, andturning the knob. Note: All time settings are displayed in 1

100 ’s of a minute.

7. If the target has not recently been used, clean the target as follows:

8. set the shutter position switch to ‘Close’ and wait for the light above the shutter positionswitch to go out. The open section of the shutter should be closest to the sputteringchamber viewport.

9. Begin sputtering by pressing the green ‘On’ button on the DC control panel. You canmonitor power, voltage, and current on the top monitor of the panel and power, voltage,current, energy (kW-hrs), and time elapsed on the bottom monitor.

10. When the process stops, press the blinking red ‘Off’ button to silence the alarm.

11. To sputter onto a substrate, set the shutter position switch to ‘Open’. Wait until the lightabove the switch goes out. The open section of the shutter should be toward the left sideof the chamber. The target is now ready for sputtering. If needed, adjust the depositionpower and time by following the above procedure.

B.3.2 RF Sputter Deposition:

1. Set the selector switches to ‘Sputter Deposit’ and either ‘Target 2’ or ‘Target 3’

2. Make sure the RF power knob is turned all the way down and the ‘Auto’ toggle switch onthe digital clock timer panel is down (off).

3. If the selected target has not been recently used, clean the target as follows:

4. Set the shutter switch to ‘Close’ and wait for the light above the shutter switch to go out.The open section of the shutter should be by the viewport of the sputtering chamber.

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5. Tune the system by turning on the ‘RF power on/off’ button and slowly turning up the‘RF power adjust’ knob to the desired forward power. Carefully monitor the ‘RF reflectedpower’ meter and adjust the ‘Load’ and ‘Tune’ switches to minimize reflected power, ad-justing the ‘RF power adjust’ as needed to maintain forward power. Keep the reflectedpower below 10 Watts.

6. When the system is tuned, press the ‘RF power on/off’ button to turn off the power. Flipthe ‘Auto’ toggle switch on. Set the sputtering time on the timer by holding down theblue button and punching in the desired time in tenths of seconds. Start sputtering bypressing the RF power button. The plasma impedance will vary slowly over time; monitorthe reflected and forward power and adjust tuning settings as necessary. The system willautomatically turn off when the timer has finished counting down. After the power hasbeen shut off, press the RF power button once to cycle the button.

7. Set the shutter switch to ‘Open’ and wait for the shutter indicator light to go out. Thetarget is now ready for RF sputtering. If necessary, forward power and deposition timecan be changed using the above procedures.

B.3.3 RF Sputter etching:

(Note: As the auto tune system is currently offline, Etch mode is not recommended atthis time)

1. Set the selector switches to ‘Sputter Etch’ and ‘Etch’

2. Make sure the RF power knob is turned all the way down and the ‘Auto’ toggle switch onthe digital clock timer panel is down (off).

3. During sputter etching mode, there is no way to prevent plasma from contacting thesubstrate. Keep in mind that the plasma starts bombarding the substrate as soon as it islit. It is best to use the auto tune system if there is concern of thermally damaging thesubstrate due to prolonged exposure to hot gas ions. To manually tune the system, seethe procedure for RF sputtering.

4. When the system is tuned, turn off the RF power. Flip the ‘Auto’ toggle switch on. Setthe etching time on the timer in tenths of seconds. Start etching by pressing the RF powerbutton. The system will automatically turn off when the timer finishes counting down.When the etch has finished, press the RF power button once to cycle the button.

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B.4 Shutdown procedures:

B.4.1 Overnight:

1. Turn off the RF generator and DC power supply.

2. Close all sputtering gas feed and make sure that the purge gas cylinder remains open.

3. Check that the pallet is unloaded.

4. Set the ULC key to ‘Manual’.

5. Pump down the load lock until the pressure falls below the trip point.

6. Turn off the ion gauge.

7. Check that the hi-vac and mechanical pumps switches are on and that only the throttlevalve is switched open.

8. Close only the target water line. Water must remain flowing through the cryo-pumpcompressor whenever the compressor is on to prevent damage.

9. If the cryo-pump temperature is above 50 K, the cryo-pump needs to be regenerated.Set the APC key to ‘Manual’ and follow the procedure outlined in the “start up fromshutdown” section to regenerate the cryo-pump.

B.4.2 Longterm shutdown:

1. Check that the RF generator and DC power supply are turned off.

2. Check that the pallet is unloaded.

3. Pump down the load lock until the pressure falls below the trip point.

4. Check that the ion gauge is turned off.

5. Set all keys to ‘Manual’.

6. Close all valves.

7. Turn off the hi-vac pump.

8. Turn off the mechanical pump.

9. Close all gas cylinders and shut off the regulators.

10. Turn off the vent fan and compressed air line.

11. Make sure that the target water line is closed.

12. When the cryo-pump compressor has cooled (after about 30 minutes) close the cryo-pumpwater line.

13. Turn off the main breaker on the power distribution box located in the back of the sput-tering system under the load lock.

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Vita

Matthew G. Hilt

EDUCATION:

May 2009 Ph. D. Physics The Pennsylvania State University

May 2002 B. S. Physics Worcester Polytechnic Institute

PRESENTATIONS:

Matthew G. Hilt, K. A. Pestka II, Jin H. So, and J.D. Maynard (2007, March). Elastics

constants and sound velocities in single crystal transition metal scandates. Paper presented at

the annual meeting of the American Physics Society, Denver, CO.

Matthew G. Hilt, K. A. Pestka II, G. D. Mahan, J. D. Maynard, D. Pickrell, B. Na,

and J. Tamburini (2006, May). Unconventional thermoacoustic heat engines (A). Journal of the

Acoustical Society of America, 119, 5, p. 3414.

J. D. Maynard, Matthew G. Hilt, and Logan Marcus (2005, September). Thermophysical

properties, as functions of pressure and temperature, for over 300 fluids, in vapor or liquid phase

(A). Journal of the Acoustical Society of America, 118, 3, p. 1927.

TEACHING EXPERIENCE:

2008 Instructor, Fluids and Thermal Physics (PHYS 213), The Pennsylvania State Uni-

versity

2008 Instructor, Wave Motion and Quantum Physics (PHYS 214), The Pennsylvania

State University

2005 Teaching Assistant, Mechanics (PHYS 211), The Pennsylvania State University

2004 Lab Instructor, Senior Labs (PHYS 458/402), The Pennsylvania State University