Piezoelectric Materials

By

Sayantan Das

Dept. of Metallurgy & Material Engg.

Jadavpur University

History of Piezoelectric Materials

• Pierre and Jacques Curie discover in 1880 that some crystals got electrically polarized when compressed, the charge induced being proportional to pressure and disappearing when the charge is withdrawn.

• This was based on Curie’s correspondence between pyro electricity and crystal symmetry and marked the important discovery of the existence of hemihedral axis within these crystals.

• The direct piezoelectric effect did not come into picture until the First World War where quartz crystals were used in SONAR . This was replaced in the Second World War by Barium Titanate, and this has become one of the most important piezoelectric materials in the world

Piezoelectricity: what is it?

• Direct piezoelectric effect :- Electric polarization produced by mechanical Strain in crystals belonging to a certain class and changing sign with it.

• Converse piezoelectric effect :-Phenomenon by which crystals become strained when electrically polarized , amount being proportional to strength of external field and changing sign along with the field.

Simple mathematical treatment of piezoelectricity

Piezoelectricity is the combined effect of:-• The linear electrical behavior of the material:D=ε.E where D is the electric charge density displacement (electric displacement), ε is permittivity (free-body dielectric constant), E is electric field strength.

• Hooke's Law for linear elastic materials: S= s.T where S is strain, s is compliance under short-circuit conditions, T is stress.

Crystallographic aspects of piezoelectric materials

Piezoelectric materials are those whose crystal lattice do not have a center of symmetry (i.e. non-centro symmetric ). For them, the center of gravity of the positive and negative charges do not intersect and any application of stress results in unsymmetrical movement of atoms and leads to the creation of an electric dipole. The axis joining the centers of gravity of the positive and negative charges are the hemihedral axes.

Differences between ferro, pyro and piezoelectric materials

Piezoelectric materials are a broader class under which come pyroelectrics and Ferro electrics. Only those pyroelectric materials which are spontaneously polarized and polarization is switchable with direction of electric field are known as ferro electric

materials, while those whose polarization direction is not switchable are called pyroelectrics.

Types of piezoelectric materials

• 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:- Poly vinylidene fluoride (PVDF).

Quartz: The Most Abundant Piezoelectric material

•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.•Hexagonal structure(β- quartz)•E.g. crystal oscillators are used as frequency standards in watches, electronic clocks, computer clocks

Gallium orthophosphate

• Discovered in 1980 mainly to meet the requirement of a high temperature sensors

• Non- ferroelectric• Isotypic with quartz but piezoelectric effect is doubled .• Triagonal crystal structure( similar to α-quartz)• Thermal stability up to 970oC • E.g. piezoelectric pressure sensors at high temperatures and

for high temperature microbalance

Barium Titanate –First Manmade ferroelectric crystal

• Discovered during World War II in 1941 and 1944 in the United States, Russia, and Japan.

• Ferroelectric• Has a low curie temperature of about 170o C• Tetragonal crystal structure• E.g. microphones and other transducers, thermistors

and self-regulating electric heating systems, thermal cameras.

Polyvinylidene fluoride-The piezopolymer

• In 1961 polyvinylidene fluoride, a piezoelectric plastic was invented.• Substantial piezoelectricity can be permanently induced by heating

stretched films of PVDF to about 100oC followed by cooling to ambient temperature with a strong DC electric field (about 300kVcm-1) applied. This treatment is called “Polling”.

• melting point: 154-184oC , tensile strength at break: 36-56 MPa, elongation at break: 25-500%

• 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

• E.g. tactile sensor arrays, inexpensive strain gauges and lightweight audio transducers, binder material used in the production of composite electrodes for lithium ion batteries

Application of Piezoelectric Materials

Sonic and Ultrasonic Applications

•Sonar with Ultrasonic time-domain reflectometers.•Materials testing to detect flaws inside cast metals and stone objects(Ultrasonic probe)•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.

Pyro electric application: A special mention

Pyroelectric Detectors/Sensors• PIR sensors allow you to sense motion, almost always used to detect

whether a human has moved in or out of the sensors range.

Concluding Remarks

Piezoelectric Materials are an important class of dielectrics that find use in a wide range of instruments and gadgets because of their unique property to convert pressure to electricity and vice-versa. Its one of the most important discoveries that were made in the world of electronics in particular and materials in general