thermal conductivity: a perspective from nanotechnology diego a gomez-gualdron seminar ii...
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THERMAL CONDUCTIVITY: A perspective from Nanotechnology
Diego A Gomez-GualdronSeminar II
Nanotechnology CHEN 689-601Texas A&M University
April 13th 2010
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PART I
INTRODUCTION & CONTEXTUALIZATION
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Definition
The thermal conductivity relates to the ability of a material to transfer heat
Fourier’s law
3
RelevanceUnsuitable values of thermal conductivity might render a new material useless for an application.
POWER DISSIPATION
INSULATIONTHERMO
ELECTRICITY
HEAT EXCHANGE
FLUIDS
4
Overview: Power Dissipation Decrease in size of electronic devices requires ingenuous ways
to dissipate heat and protect the device components structure and performance
THINGS TO LOOK FOR:
Good thermal contact between components and heat sink
Materials with high thermal conductivity and low coefficient of thermal expansion
www.epotek.com
5
Overview: Insulation The basic principle is the protection of a system from the harsh
(hot or cold) conditions in a neighboring region, while fulfilling additional requirements
A MATTER OF COMPROMISE
Space suits require insulating materials, while being light enough to be handled by the astronaut
Skylights require insulating characteristics, while allowing light to pass through
www.wikipedia.com www.mygreenhomeblog.com
6
Overview: Thermoelectricity In many technologies a vast quantity of heat is eliminated as
waste. Nonetheless, the efficiency of the process would be much higher if some of the heat were transformed into electricity
www.iav.com
THE FIGURE OF MERIT
Materials with a high Seebeck coefficient (S=∆V/∆T) are needed
Also a low thermal and a high electrical conductivity would be ideal
7
Overview: Heat-Exchange Fluids Conventional heat-transfer fluids have inherently poor thermal
conductivity compared to solids. Several industries would benefit from increasing their thermal conductivity to reduce heat exchanger sizes and pumping needs
TO HAVE IN MIND
High thermal conductivity
Low friction coefficient
Clogging of microchannels is undesired
Lubricating behavior is a plus
www.engadget.com
8
Preliminary Approaches:
INSULATION
bricks asbestos fiber glass
Evolution of new materials from ceramics to modern composites
www.wikipedia.com www.scrapetv.com www.coolandquiet.com
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Preliminary Approaches
POWER DISSIPATION
BJT transistorvacuum tube CMOS technology
Changes in the electronics technology rather than in cooling methods
www.noveltyradiocom www.digitalcounterproducer.comwww.solarbotics.com
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Preliminary Approaches
THERMOELECTRICS
Thermoelectric Module Radioactive heating
Not much interest until the 90’s, because of conflicting characteristics of materials (figure of merit)
www.thermoelectrics.caltech.edu
www.thermoelectrics.caltech.edu
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www.thermoelectrics.caltech.edu
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Preliminary Approaches
HEAT EXCHANGE Playing with the design equation Q=UA (Ti-To) and making heat
integration
www.cerematec.com
Microchannel heat exchangerHelically baffled heat exchanger
www.alltecho.co.uk
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Contextualization
The intelligent design of the nanostructure of a material can provide all the desired properties, including the thermal conductivity
REQUIREMENTS
Understanding the heat transfer phenomena at the molecular level
Modification of the structure of the material accordingly
Nanotechnology-based revolution!!!
Computational and experimental resources to determine k at the nanolevel
www.salaswildthoughts.blogspot.com
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Current Research: Nanotechnology
Aerogels/Insulation
www.boingboing.net
Deionized water prior to(left) and after (right)dispersion of Al2O3
nanoparticles
Oil prior to (left) andafter (right) evaporationof Cu nanoparticles
Nanofluids/Heat Exchange
www.kostic.niu.edu
Reduce k Increase k
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Current Research: Nanotechnology
Thin Film/Thermoelectrics
Nature Materials (2008) Vol 7, 105
MEMS/Power Dissipation
Nature Nanotechnology(2008) Vol 3, 275
Reduce k
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Emphasis: Polymer Industry
www.wikipedia.com
www.epotek.com
www.batchglow.co.uk
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Motivation: Polymer Industry
One of the most pervasive materials in modern society
• Ease of processing and versatility
• Attractive for the development of new materials
• Integral part of high-tech applications
Bayern chemical Plant, Baytown, Texas
Nature Materials (2008) Vol 7, 261
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Research Status: Polymer Industry
Structural Reinforcement
Increase of Electrical Conductivity
Increase of Thermal Conductivity
www.silmore.cn
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PART II
THEORETICAL BACKGROUND
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Mechanism: Electron Heat Transport
Characteristic of metallic compoundsFree Electrons
Metal Atoms
HOT REGION
Strong vibration
High Kinetic Energy Electrons
Interaction between energetic
electron and atom
Increased vibration
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Mechanism: Electron Heat Transport
Very effective heat transport mechanism
Characterized by electron mean free path
Not so sensitive to lattice defects
Typically 20-400 W/m.K
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Mechanism: Phonon Heat Transport
Characteristic of most compounds
A Diamond lattice
HOT REGION
Strong vibration
Vibrational excitation being transmitted
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Mechanism: Phonon Heat Transport
Heat is transferred through lattice vibrations
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Mechanism: Phonon Heat Transport
Phonons are quantized analogous to the vibrations of a guitar string
www.wikipedia.com
L
k=1/3(CV v l)
Heat capacity
Phonon velocity (sound speed)
Mean free path length
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Mechanism: Phonon Heat Transport
Imperfections in the structure enhance phonon scattering and decrease k
Scattering point
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Mechanism: Phonon Heat Transport
Not as efficient as electron heat transport
Characterized by phonon free path and velocity
Very sensitive to defects (e.g. amorphous structure of polymers)
Typical values range from 0.01-50 W/m.K
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Molecular SimulationThe Green-Kubo expression for thermal conductivity
is widely used
k= V ∫dt <JQ(t)JQ(0)>kBT2
www.zeolites.nqs.northwetern.edu
• Force Field defining potential energy
• Instantaneous velocities related to kinetic energy
• Sometimes and external field
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Thermal Conductivity Design
Serial Resistances
www.boingboing.net
Analogy with electric circuits with R ~ 1/kAerogel structure
www.aip.org
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Thermal Conductivity Design
Analogy with electric circuits with R ~ 1/k
Parallel Resistances
www.ntu.edu.vn
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Thermal Conductivity Design
Altering the value of the resistances…
www.chemistryland.com
Incr
ease
res
ista
nce
Adding defects
Nature Materials (2008) Vol 7, 105
decr
ease
res
ista
nce
Improving crystallinity
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PART III
CASE STUDY:A Polymer more conductive than metal
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Alternative work: Polymer Composites
Embedding thermally conductive nanostructures in a polymeric matrix
Nature (2007), Vol 447, p. 1066 www.physorg.news
35
TEM image of a composite
Alternative work:Carbon Nanotube Conductivity
Phys. Rev. Let. (2000) Vol 84, p. 4663
Molecular simulations reveal a thermal conductivity of ~ 104 W/m.K
Nanotube (10,10)
Green-Kubo relation
36
Alternative Work: Nanotube-Polymer Composites
Ideal structure model
An effort to conduct through the nanotube network instead of the polymer matrix
TEM side view
Adv. Mat. (2005) Vol 17, p. 1562
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Alternative Work:Nanotube-Polymer Composites
Even the most promising results only enhance 6.5 W/m.K
Adv. Mat. (2005) Vol 17, p. 1562
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Preliminary Work: Conduction in Molecular Chains
Experimental work shows ultrafast thermal transport in self-assembled molecules
Self-assembly
Set-up schematics
Science (2007) Vol 317, p. 787
• Sample is heated with a pulsed laser
• Sum Frequency Generation (SFG) spectroscopy is performed
Summary
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Preliminary Work: Conduction in Molecular Chains
Heat is transferred in a time frame of picoseconds
Molecular excitations Heat transfer
Science (2007) Vol 317, p. 787
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Preliminary Work: Conduction in Molecular Chains
Science (2007) Vol 317, p. 787
Molecular Dynamics results provide further inside
Thermal Disorder after 10 ps
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Preliminary work:Thermal Conductivity of Polymer ChainsPolyethylene chains were shown to have k in
the order of 103 W/m.KThermal conductivity for different
domain sizes
Phys. Rev. Let. (2008) Vol 101, p. 235502
Polyethylene chain
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Motivation
Modification of thermal properties in polymers composites not as good
Molecular simulations and experiments suggest high thermal conduction in hydrocarbon chains
Thermal conductivity enhancement done on microfibers
43
Featured Paper:Synthesis Procedure
Fiber Drawing Schematics a) Polyethylene gel preparation
b) Gel sample heating
c) Tungsten tip contact wit gel
d) Tungsten tip withdrawing
e) Microscope inspection
f) Secondary heating activatedNature nanotechnology (2010), Vol. 5, p. 251
44
Featured Paper:Nanostructure Changes
Molecular chains are expected to align, thus approaching the ideal case of a thermal transport on a single chain
nanostructure in gel sample nanostructure in nanofiber
Nature Nanotechnology (2010), Vol. 5, p. 251
45
Featured Paper:Nanostructure Changes
The structure achieves crystallinity as confirmed by diffraction measurements
TEM image of the fiber Diffraction pattern of the fiber
Nature Nanotechnology (2010), Vol. 5, p. 251 Orthorhombic Structure
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Featured Paper:Thermal Conductivity Measurements
Measurement Setupa) Cantilever holds the fiber
b) Fiber cut at 300µm from the tip
c) Loose end joined to thermocouple
d) Thermocouple heated up
e) Cantilever is stimulated
f) Laser picks up the signal
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Featured Paper:Thermal Conductivity Results
A thermal conductivity around 110 W/m.K was achieved. This is higher than for most pure metals!!!
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General Challenges
Improve uncertainty in measurements
Understand mechanism in nanostructures
Trade-off in design of material properties
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Particular Challenges
Structure uniformity along the nanofiber
Adapt process for future scaling up
Vanish thermal resistance among fibers
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Follow-up Research
Dependence of fiber structure from process parameters:
1) Heating rate and strategy 2) Nature of gel preparation 3) Drawing rate 4) Composition
Is it possible to make ‘Doped’ nanofibers?
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Follow-up ResearchExploration of fillers that reduce thermal contact
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Design of processes exploiting 1-D heat transport
Electrical Component HEAT SINK
Q
Nanofiber
Nanofiber
Thermal Contact
Questions?
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G4Rebuttal: Thermal Conductivity
Diego A. Gómez-Gualdrón
Reviewer G1: “The presenter gave an overview and contextualization of the topic. However, this part of the talk lasted too long and was a little disorganized and there was not direct relation with the papers he talk”
A:/ 1) The thermal conductivity is an important parameter in the design of an overwhelming number of applications and worth of a careful review. The reviewer is assessing part I as it were and introduction to part III. The three sections of the presentations are meant to be independent, and were timed accordingly.
2) I invite the reviewer to check the slides again and he will clearly see the following structure for part I: a) Definition of thermal conductivity b) Relevance and fields of application c) Overview: Power Dissipation → Insulators → thermoelectricity → Heat Exchange d) Preliminary Approach: Power Dissipation → Insulators → thermoelectricity → Heat Exchange e) Nanotechnology Approach: : Power Dissipation → Insulators → thermoelectricity → Heat Exchange Then, It is stated the interest and motivation of manipulating TC in polymers in particular, and the featured paper is announced
A:/ 3) There was not direct relation, because the three sections are independent. Part I reviews the role of thermal conductivity in several fields, and the role that nanotechnology has started playing in them. Part II visits the theoretical background needed to be able to understand and manipulate thermal transport at the nanoscale. Parts III explores the latest progress in manipulating the thermal conductivity of a material (a polymer in this case) using nanotechnology.
Reviewer G1: “The presenter gave an overview and contextualization of the topic. However, this part of the talk lasted too long and was a little disorganized and there was not direct relation with the papers he talk”
Reviewer G1: “The overall presentation was good, however I think he had the opportunity of exploiting a little more the topic since there are some recent applications of thermal transport to create logical circuits using the rectification capability of designed graphene sheets. This phenomenon opens the possibility to a large variety of applications”
A:/ There has been more than enough examples of nanotechnology applications in electronics during the class. I can understand that due to the academic background of the reviewer he prefers focusing on circuits and whatnot. However, I think that for a class of chemical engineers, an application involving polymers is much more attractive. Besides, the featured application is a beautiful example how nanotechnology can alter commonplace conceptions such as polymers being poor thermal conductors.
Reviewer G2: “It would have been good mentioning the reason for the difference on the nature of main thermal carriers when comparing metals and polymers”
A:/ During the oral presentation, from slides 22 through 24, this was explained. In slide 22 the graph shows the existence of free electrons in metallic compounds and described a mechanism based on them. In slide 24, the mechanism in all other compounds (this includes polymers) is explained. Diamond was used as a example of a material with no free electrons, hence featuring a phonon-controlled thermal transport
Reviewer G2: “The typical or approximate values of electron and phonon mean free path for metal and polymers were not mentioned”
A:/ I agree. Here are the values : mean free path of electrons varies between 5-50 Å; mean free path of phonons varies from 500 to 700 nm
Reviewer G2: “The Green-Kubo expression for thermal transport was mentioned but not well depicted, neither its relation with Fourier’s law”
A:/ The impact this would have had on the overall presentation is not worth the additional time needed to go into the mathematical details of the equation. The term autocorrelation function was briefly explained, as well what the terms of the equation were, and what you needed to run the simulation. The gist of that slide is that there exists an equation to calculate the thermal conductivity using molecular simulations
G1Review:Thermal Conductivity
Edson Bellido
The presenter gave an overview and contextualization of the topic. However, this part of the talk lasted too long and was a little disorganized and there was not direct relation with the papers he talk. He talk in the second part about the difference between electron and phonon heat transport and theoretical background that help to understand the topic.
He showed some attempts to improve the thermal conductivity of polymer using carbon nanotubes. He also showed some Molecular Dynamics simulations and how Polyethylene chains were shown to have k in the order of 103 W/mK. In the actual paper he described the synthesis of the nanofibers. He explained how they were able to measure the thermal conductivity on the nanofibers that was in the range of 110W/mK .
The overall presentation was good, however I think he had the opportunity of exploiting a little more the topic since there are some recent applications of thermal transport to create logical circuits using the rectification capability of designed graphene sheets. This phenomenon opens the possibility to a large variety of applications.
http://images.iop.org/objects/ntw/news/7/3/21/070321-right.jpg
G2Review:Thermal Conductivity
Alfredo Bobadilla
Thermal conductivity lecture review
It would have been good mentioning the reason for the difference on the nature of main thermal carriers when comparing metals and polymers.The typical or approximate values of electron and phonon mean free path for metal and polymers were not mentioned.
The Green-Kubo expression for thermal transport was mentioned but not well depicted, neither its relation with Fourier’s law.
It’s noticeable the effort of the presenter on trying to explain the concepts as far as possible using graphic illustrations.
It was well emphasized the challenges when trying to integrate the polymernanofiber in ‘networks’ for potential applications, because it’s desired not loosing the outstanding 1-D thermal conductivity of a single nanofiber.
Alfredo D. Bobadilla
REVIEW:THERMAL
CONDUCTIVITYG4 PRESENTATION
Mary Coan, G3
Chemical Engineering
Review Defines Thermal Conductivity and it’s
applicationsNew Nanostructure Materials
○ PolymersStructural ReinforcementIncrease Electrical ConductivityIncrease of Thermal Conductivity
- Polyethylene Nanofibres
DefinedElectron Heat TransportPhonon Heat Transport
Review Thermal Conductivity design
Can be viewed as an electrical series of resistors or Parallel Resistances○ Increase defects or Decrease defects to increase or
decrease resistance
Polymer compositesEmbedded thermally conductive nanostructure into
polymer matrix Nanotube-Polymer Composites
Uses a Nanotube matrix instead of a Polymer matrix
Review Conduction through Molecular Chains
Polyethylene Chains, k = 103 W/(m*K)○ Addition of nanofibers might help
Polyethylene NanofibersSynthesis
○ Nanostructure Changes as Nanofiber is pulledThermal Conductivity Measured
○ K = 110 W/(m*K)Higher than most pure metals
Challenges○ Understand Mechanisms, Scale Up, Uniformity issues
Future work was discussed
G5Review: Thermal Conductivity
Norma L. Rangel
Thermal Conductivity, by Diego Gomez-Gualdron
• Diego did an excellent job in his presentation, he has very good skills that he implements well in his oral presentations. Very fluent, well prepared, organized and able to deliver concepts and ideas to the audience.
• The information presented was highly oriented for undergraduates and Chemical Engineers, I understand his motivation to do that but I believe he underestimated the audience capability to digest more state of the art and deep information.
G6Review: Thermal Conductivity
Jung Hwan Woo
• The preparation was very well organized.• The oral presentation was also very good. It
flowed very well and was sequenced nicely to let the audiences to understand the presentation.
• There were very interesting ideas such as aerogels.
• The introduction was quite well organized as the topic was a very broad and hard to gather and present ideas.