What Are Piezoelectric Materials?

If you've ever used a cigarette lighter, experienced a medical ultrasound in a doctor's office or turned on a gas burner, you've used piezoelectricity.

​Piezoelectric materials are materials that have the ability to generate internal electrical charge from applied mechanical stress.​ The term ​piezo​ is Greek for "push."

Several naturally occurring substances in nature demonstrate the piezoelectric effect. These include:

•Bone
•Crystals
•Certain ceramics
•DNA
•Enamel
•Silk
•Dentin, and many more.

Materials that exhibit the piezoelectric effect also demonstrate the inverse piezoelectric effect (also called the reverse or converse piezoelectric effect). The ​inverse piezoelectric effect​ is the internal generation of mechanical strain in response to an applied electrical field.

Crystals were the first material used in early experimentation with piezoelectricity. The Curie brothers, Pierre and Jacques, first proved the direct piezoelectric effect in 1880. The brothers expanded upon their working knowledge of crystalline structures and pyroelectric materials (materials that generate an electric charge in response to a temperature change).

They measured the surface charges of the following specific crystals:

•Cane sugar
•Tourmaline
•Quartz
•Topaz
•Rochelle salt (sodium potassium tartrate tetrahydrate)

Quartz and Rochelle salt demonstrated the highest piezoelectric effects.

However, the Curie brothers didn't predict the inverse piezoelectric effect. The inverse piezoelectric effect was deduced mathematically by Gabriel Lippmann in 1881. The Curies then confirmed the effect and provided quantitative proof of the reversibility of electric, elastic and mechanical deformations in piezoelectric crystals.

By 1910, the 20 natural crystal classes in which piezoelectricity occurs were completely defined and published in Woldemar Voigt's ​Lehrbuch Der Kristallphysik​. But it remained an obscure and highly technical niche area of physics without any visible technological or commercial applications.

​World War I:​ The first technological application of a piezoelectric material was the ultrasonic submarine detector created during World War I. The detector plate was made from a transducer (a device that transforms from one type of energy into another) and a type of detector called a hydrophone. The transducer was made of thin quartz crystals glued between two steel plates.

The resounding success of the ultrasonic submarine detector during the war stimulated intense technological development of piezoelectric devices. After World War I, piezoelectric ceramics were used in the cartridges of phonographs.

​World War II:​ Applications of piezoelectric materials advanced significantly during World War II due to independent research by Japan, the USSR and the United States.

In particular, advancements in the understanding of the relationship between crystal structure and electromechanical activity along with other developments in research shifted the approach toward piezoelectric technology entirely. For the first time, engineers were able to manipulate piezoelectric materials for a specific device application, rather than observing properties of the materials and then searching for suitable applications of the observed properties.

This development created many war-related applications of piezoelectric materials such as super-sensitive microphones, powerful sonar devices, sonobuoys (small buoys with hydrophone listening and radio-transmitting capabilities for monitoring movement of ocean vessels) and piezo ignition systems for single cylinder ignitions.

As mentioned above, piezoelectricity is the property of a substance to generate electricity if a stress such as squeezing, bending or twisting is applied to it.

When placed under stress, the piezoelectric crystal produces a polarization, ​P​, proportional to the stress that produced it.

The ​main equation of piezoelectricity is​

\(P=d\times\text{stress}\)

where ​d​ is the piezoelectric coefficient, a factor unique to each type of piezoelectric material. The piezoelectric coefficient for quartz is 3 × 10^-12. The piezoelectric coefficient for lead zirconate titanate (PZT) is 3 × 10^-10.

Small displacements of ions in the crystal lattice create the polarization observed in piezoelectricity. This only occurs in crystals that do not have a center of symmetry.

The following is a non-comprehensive list of piezoelectric crystals with some brief descriptions of their use. We'll discuss some specific applications of the most frequently used piezoelectric materials later.

​Naturally occurring crystals:​

•Quartz. A stable crystal used in watch crystals and frequency reference crystals for radio transmitters.
•Sucrose (table sugar)
•Rochelle salt. Produces a large voltage with compression; used in early crystal microphones.
•Topaz
•Tourmaline
•Berlinite (AlPO₄). A rare phosphate mineral structurally identical to quartz.

​Man-made crystals:​

•Gallium orthophosphate (GaPO₄), a quartz analog.
•Langasite (La₃Ga₅SiO₁₄), a quartz analog.

​Piezoelectric ceramics:​

•Barium titanate (BaTiO₃). The first piezoelectric ceramic discovered.
•Lead titanate (PbTiO₃)
•Lead zirconate titanate (PZT). Currently the most commonly used piezoelectric ceramic.
•Potassium niobate (KNbO₃)
•Lithium niobate (LiNbO₃)
•Lithium tantalate (LiTaO₃)
•Sodium tungstate (Na₂WO₄)

​Lead-free piezoceramics:​

The following materials were developed in response to concerns about harmful environmental exposure to lead.

•Sodium potassium niobate (NaKNb). This material has properties similar to PZT.
•Bismuth ferrite (BiFeO₃)
•Sodium niobate (NaNbO₃)

​Biological piezoelectric materials:​

•Tendon
•Wood
•Silk
•Enamel
•Dentin
•Collagen

​Piezoelectric polymers:​ Piezopolymers are lightweight and small in size, thus growing in popularity for technological application.

Polyvinylidene fluoride (PVDF) demonstrates piezoelectricity that is several times larger than quartz. It is often used in the medical field such as in medical suturing and medical textiles.

Piezoelectric materials are used in multiple industries, including:

•Manufacturing
•Medical devices
•Telecommunications
•Automotive
•Information technology (IT)

​High-voltage power sources:​

•Electric cigarette lighters. When you depress the button on a lighter, the button causes a small spring-loaded hammer to hit a piezoelectric crystal, producing a high-voltage current that flows across a gap to heat and ignite the gas.
•Gas grills or stoves and gas burners. These work similarly to the lighter, but on a larger scale.
•Piezoelectric transformer. This is used as an AC voltage multiplier in cold cathode fluorescent lamps.

Ultrasound transducers are used in routine medical imaging. A ​transducer​ is a piezoelectric device that acts as both a sensor and an actuator. ​Ultrasound transducers​ contain a piezoelectric element that converts an electrical signal into mechanical vibration (transmit mode or actuator component) and mechanical vibration into electric signal (receive mode or sensor component).

The piezoelectric element is usually cut to 1/2 of the desired wavelength of the ultrasound transducer.

Other types of Piezoelectric sensors include:

•Piezoelectric microphones.
•Piezoelectric pickups for acoustic-electric guitars.
•Sonar waves. The sound waves are both generated and sensed by the piezoelectric element.
•Electronic drum pads. The elements detect the impact of the drummers' sticks on the pads.
•Medical acceleromyography. This is used when a person is under anesthesia and has been administered muscle relaxants. The piezoelectric element in the acceleromyograph detects force produced in a muscle after nerve stimulation.

One of the great utilities of piezoelectric actuators is that high electric field voltages correspond to tiny, micrometer changes in the width of the piezoelectric crystal. These micro-distances make piezoelectric crystals useful as actuators when tiny, accurate positioning of objects is needed, such as in the following devices:

•Loudspeakers
•Piezoelectric motors
•Laser electronics
•Inkjet printers (crystals drive the ejection of ink from the print head to the paper)
•Diesel engines
•X-ray shutters

Smart materials are a broad class of materials whose properties can be altered in a controlled method by an external stimulus such as pH, temperature, chemicals, an applied magnetic or electric field, or stress. ​Smart materials are also called intelligent functional materials.​

Piezoelectric materials fit this definition because an applied voltage produces a stress in a piezoelectric material, and conversely, the application of an external stress also produces electricity in the material.

Additional smart materials include shape memory alloys, halochromic materials, magnetocaloric materials, temperature-responsive polymers, photovoltaic materials and many, many more.