Piezoelectric and Ferroelectric materials

Zaahir Salam

Some Basic Terms

• Dielectric Material- The Cumulative effect of microscopic displacements (charges,ions,electrons) results in Net Polarization due to setting up of induced dipole moments or due to rotation of permanent electric dipoles which are already present in the material.

• Dielectrics are the materials having electric dipole moment permantly.

• Polarization- Neutral Atom In DC field +ve Nucleus is pushed in direction of Electric field. -ve Charged Electrons Pushed opposite to electric field. Hence -ve and +ve Centres don’t coincide and undergo net displacement r .

Dipole moment p= (Ze)r. p=αE α - is known as polarizability of the atom(or molecule) The induced charge on the surface of dielectric is polarization.

P= net dipole moment/ volume

• Polarization is due to shifting of

– Electron charge cloud (electronic polarization).

– Shifting of +ve and –ve ions (ionic polarization).

– Due to orientation of dipoles(orientation polarization).

• When there is shifting of ions or charge in there orientation of dipoles there will be a slight change in the dimension of the material- electrostriction effect(occurs in all dielectrics).

Cetro-Symmetric Piezoelectrics (Non-Centro Symmetric)

Non-pyroelectrics Pyroelectrics

Non-Ferroelectrics Ferroelectrics

Dielectrics

Don’t Posses Inversion centre

A Hierarchical Overlook

The History of Piezo

• The name Piezo originates from the Greek word piezein, which means to squeeze or press.

• The piezoelectric effect was first proven in 1880 by the brothers Pierre and Jacques Curie.

What is Piezoelectric Material?

Piezoelectric Material is one that possesses the property of converting mechanical energy into electrical energy and vice versa.

Piezoelectric materials can be divided in 2 main groups: crystals and cermaics.

Direct Piezoelectric Effect

• Piezoelectric Material will generate electric potential when subjected to some kind of mechanical stress.

• The direct Effect : Strain Sensor, microphones, gas lighters, ultrasonic detectors

Compression Effect: Decrease in volume and it has a voltage with the same polarity as the material

Tension Effect: Increase in volume and it has a voltage with opposite polarity as the material

Inverse Piezoelectric Effect

• If the piezoelectric material is exposed to an electric field (voltage) it consequently lengthens or shortens proportional to the voltage. E.g Crystal Oscillators, crystal Speakers, record player Pic ups, actuators etc.

If the applied voltage has the same polarity then the material expands.

If the applied voltage has the opposite polarity then the material contracts.

The necessary condition for the piezoelectric effect is the absence of a center of symmetry in the crystal structure. Of the 32 crystals classes 21 lack a center of symmetry, and with the exceptions of one class, all of these are piezoelectric.

If lead zirconate titanate (PZT), a piezoceramic, is placed between two electrodes and a pressure causing a reduction of only 1/20th of one millimeter is applied, a 100,000-volt potential is produced.

The basic equations of piezoelectricity are:

P = D x stress and E = strain/D

Where,

P = Polarization,

E = electric field generated and

D = piezoelectric coefficient in metres per volt.

Naturally occurring crystals: Berlinite (AlPO4), cane sugar, Quartz, Rochelle salt, Topaz, Tourmaline Group Minerals, and dry bone (apatite crystals) Man-made crystals: Gallium orthophosphate (GaPO4), Langasite (La3Ga5SiO14) Man-made ceramics: Barium titanate (BaTiO3), Lead titanate (PbTiO3), Lead zirconate titanate (Pb[ZrxTi1-x]O3 0<x<1) - More commonly known as PZT, Potassium niobate (KNbO3), Lithium niobate (LiNbO3), Lithium tantalate (LiTaO3), Sodium tungstate (NaxWO3), Ba2NaNb5O5, Pb2KNb5O15 Polymers: Polyvinylidene fluoride (PVDF)

Quartz(crystalline form of SiO2)

• Most abundant and widely used.

• Non- ferroelectric.

• Alternating field applied to the crystal-it vibrates with a characteristic frequency which depends on the crystal geometry.

• Used as a dielectric-excellent frequency standard.

• Hexagonal structure.

• E.g crystal oscillators are used as frequency standards in watches, electronic clocks, computer clocks.

Polyvinylidene fluoride

• In 1961 polyvinylidene fluoride, a piezoelectric plastic was invented. It is one of the most widely used piezopolymer from which substantial electricity can be generated. It is cheap and physically quite strong.

• In 2001 researchers found that PVDF becomes supersensitive to pressure when impregnated with very small quantity of nanotubes, thus PVDF with its inherent superior mechanical properties when upgraded with nano-technology produces a new generation of piezopolymer, which are durable and can generate large quantity of electricity economically.

• Although a number of polymers possess piezoelectric properties, none match the magnitude of the effects in polyvinylidene fluoride (PVDF), which is the most widely studied and commercially used piezoelectric polymer. PVDF has been commercially available since 1965.

• Substantial piezoelectricity can be permanently induced by heating stretched films of PVDF to about 1000C followed by cooling to ambient temperature with a strong DC electric field (about 300kVcm-1) applied. This treatment is called “Polling”.

• Such polarization, attributed to redistribution of electronic or ionic charges within the solids or injected from electrodes, characteristically vanishes on exceeding some polarization temperature, Tp. The effect in PVDF is totally different in that the induced polarization is thermally reversible and polarizations current are, produced on either heating or cooling.

• When a sheet of PVDF is compressed or stretched, an electric charge is generated and collected on the surfaces. The PVDF sheet is metallized on both sides which acts as electrodes

PHYSICAL PROPERTIES OF PVDF Specific gravity: 1.75 -1.80;

melting point: 154-1840 C;

water absorption: 0.04-0.06%;

tensile strength at break: 36-56 Mpa;

elongation at break: 25-500%,

hardness shores D: 70-82;

low temperature embrittlement; -62 to 640 C.

Electrical Properties of PVDF (with out nanotubes impregnation)

Volume resistivity: 2x1014 ohm-cm;

Dielectric constant at 60 Hzs: 8.40 pm/V

Piezoelectric stress constant: 0.23V/ (m. pa)

Sonic and Ultrasonic Applications

• Sonar with Ultrasonic time-domain reflectometers.

• Materials testing to detect flaws inside cast metals and stone objects.

• Measure elasticity or viscosity in gases and liquids

• Used in Compact sensitive microphones and guitar pickups.

• Loudspeakers.

Pressure Applications

• Transient pressure measurement to study explosives, internal combustion engines (knock sensors), and any other vibrations, accelerations, or impacts.

• Piezoelectric microbalances are used as very sensitive chemical and biological sensors.

• Transducers are used in electronic drum pads to detect the impact of the drummer's sticks.

• Energy Harvesting from impact on the ground

• Atomic force and scanning tunneling microscopes.

• Electric igniters and cigarette lighters

Consumer Electronics Applications

• Quartz crystals resonators as frequency stabilizers for oscillators in all computers.

• Phonograph pick-ups

• Accelerometers: In a piezoelectric accelerometer a mass is attached to a spring that is attached to a piezoelectric crystal. When subjected to vibration the mass compresses and stretches the piezo electric crystal. (iPhone)

Motor Applications

• Piezoelectric elements can be used in laser mirror alignment, where their ability to move a large mass (the mirror mount) over microscopic distances is exploited. By electronically vibrating the mirror it gives the light reflected off it a Doppler shift to fine tune the laser's frequency.

• The piezo motor is viewed as a high-precision replacement for the stepper motor.

• Traveling-wave motors used for auto-focus in cameras.

Ferroelectrics

All Ferroelectric materials exhibit Piezoelectric effect because –lack of symmetry.

Special Class of Piezoelectric Material- exhibit certain other characteristics also.

Exhibit spontaneous polarization i.e., polarization in the absence of an electric field.

Ferroelectrics are the electric analog of the ferromagnets, which may display permanent magnetic behaviour.

Valasek discovered the first ferroelectric material, namely Rochelle salt.

In ferroelectrics, the polarization can be changed and even reversed by an external electric field.

Ferroelectrics Continued

Properties Spontaneous polarization in the absence applied

electrical field. Extremely high dielectric constant (~500-15,000). Strong non-linear dielectric response to an applied

electrical field. High strain response to applied electrical field

piezoelectricity Strong variation in polarization with temperature

pyroelectricity

Polarisation vs. E-field

• If we apply a small electric field, such that it is not

able to switch domain alignments, then the material will behave as a normal dielectric:

PE

• As E is increased, we start to flip domains and rapidly increase P.

• When all domains are switched, we reach saturation.

What happens if the E-field is now removed?

Spontaneous Polarization

• The value at zero field is termed the remnant polarisation.

• The value of P extrapolated back from the saturation limit is the spontaneous polarisation.

• Reversal of the field will eventually remove all polarisation – The field required is the coercive field.

• Further increasing the reverse field will completely

reverse the polarisation, and so a hysteresis loop is formed…

Perovskite Crystal Structure

General Formula: ABX3

Phase Transition

Charges Coincide Charges doesn’t Coincide

> 120°C < 120°C

Pyroelectric Materials

A special class of material which is subset of piezoelectric material.

Are polarized spontaneously but they do not respond to an electric field like ferroelectronics –require very high electric field for orienting the dipoles.

The field required is so high that the material reaches electric breakdown before it can get polarized. But When temperature is changed the polarization of crystal changes. e.g LiNbO3

Pyroelectricity

0 100 200 300 400 5000.3

0.4

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PbTiO3

TC=490oCS

ponta

neous P

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C/m

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Temperature [oC]

The spontaneous polarization is strongly dependent on the temperature. It dissapears completely at the phase transformation temperature TC. The variation in the polarization with respect to the temperature is called the pyroelectric effect.

TE

T

P

T

Dp S

E

Applications of Ferroelectrics

Non-Volatile RAMs (memory)

Dynamic Capacitors.

Tunable Microwave Devices

Pyroelectric Detectors/Sensors

Optical Waveguides

Non-Volatile RAMs (memory)

The two possible orientations in these materials make the materials attractive to researchers developing computer memory because one orientation could correspond to a 1 and the other to a 0. (Computer memory stores information in 1’s and 0’s.)

Non volatile RAM continued…

• These materials could help address the very expensive upkeep of cloud computing. Facebook, Google, Web-based email and other services are stored in the cloud and rely on volatile memory.

• With this type of memory, if the power is turned off, the information is retained. If the cloud and electronic devices operated on non-volatile memory, $6 billion in electricity costs would be saved in the U.S. annually.

Non-Volatile RAMs (memory)

Smart cards use ferroelectric memories. They can hold relatively large amounts of information and do not wear out from use, as magnetic strips do, because they use contactless radio frequency input/output. These cards are the size and shape of credit cards but contain ferroelectric memory that can carry substantial information, such as its bearer's medical history for use by doctors, pharmacists and even paramedics in an emergency. Current smart cards carry about 250 kilobytes of memory.

Tunable Microwave Devices / Optical Waveguides

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Die

lect

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Electric Field, E3 [kV/cm]

(E=0)

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Etunability

Innovative mobile communication applications is driven by the combination of different functionalities as cell phones, GPS, Bluetooth, and WLAN at varying carrier frequencies and band widths in a single device. The use of integrated microwave

components with characteristics tunable by an applied voltage is a suitable strategy to meet the required challenges.

• PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range.

Pyroelectric Detectors/Sensors

• The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR.

• When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected.

All Ferroelectric materials are Piezoelectric, But all Piezoelectric materials are not

Ferroelectric!

Ferroelectrics are spontaneously polarised, but are also piezoelectric, in that their polarisation changes under the influence of a stress. This is because while all ferroelectrics are piezoelectric, not all piezoelectrics are ferroelectric.

Any Questions?

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