16.15 Piezoelectricity

Piezoelectric materials are ones that undergo lattice deformation (change shape) when subjected to an applied electrical field, and conversely develop an electrical charge when strained by an applied stress. Also in piezoelectric materials the polarisation is proportional to the strain and changes sign with it. The name “piezo” is derived from the Greek meaning “to press”. Piezoelectricity is the effect that converts the mechanical energy you impart into the piezoelectric device in your gas lighter into the electrical spark to light the gas. A great step forward for piezoelectricity was the discovery of ferroelectricity in single crystal barium titanate by Von Hippel in 1945.

For several years the phenomenon of piezoelectricity was only considered possible in such single crystal ferroelectrics, as it was thought that the randomly oriented piezoelectric crystallite domains in a sintered ceramic would cancel each other out. This proved not to be the case for ferroelectric ceramic crystallites as, in 1952, it was found they could be permanently aligned by poling in an electric field. Once poled the piezoelectric ceramic acts rather like a single crystal, typically with the equivalent of 80 to 90 % alignment.  Polycrystalline ceramics such as barium titanate and lead zirconate titanate were found to have very high electromechanical coefficients once they were poled. To make piezoelectric ceramics such as PZT (lead zirconate titanate) highly effective, they need to be as dense as possible with the minimum of contaminants. Shapes are formed by die pressing, extrusion and slip casting.

As these piezoelectric materials can transform dynamic mechanical energy to electrical signals and vice versa, so they can make very effective transducer devices. They can also be doped to tailor them to particular applications. For example, just a few percent of niobium, lanthanum or iron oxides can be added to produce a ceramic having specific piezoelectric characteristics and dielectric constants. In particular, PZT and PLZT (lead lanthanum zirconium titanate) are easily poled, are relatively easy to sinter and readily form mixtures with other materials.

Piezoelectric ceramics are used in four main application areas, electromechanical actuators and sensors; the generation of voltages; frequency control; and the generation and detection of ultrasonic electromechanical energy. They are the active components in a wide range of applications, including: accelerometers; ignition systems (gas lighter); voltage transformers (used in laptops/notebooks); automobile devices (knock sensors, tyre pressure indicators, tread wear indicators, wheel balancers, keyless door entry, fuel atomisation and active car suspension systems to cancel vibration); medical applications (imaging, therapy, insulin pumps and vaporisers); microphones, loudspeakers and alarms; vibration reduction in fighter aircraft; reduction of earthquake damage by friction dampers in buildings; ink printers; touch controls; ultrasonic cleaning and milling; ultrasonic welders; quartz watch timers; depth finders; fish locators and very sensitive submarine sonar echo rangers (using complex arrays and multiple ceramic stacks, some towed arrays weighing several tons). They are also used in cameras for auto focussing and if intense enough as a military/police noise deterrent.

Piezoelectric igniter - source Wikipedia

Internals of a Sandia accelerometer