Piezoelectricity is a characteristic exhibited by certain solids which allows for a linear, reversible relationship between mechanical stress and electrical charge. When a piece of piezoelectric material undergoes physical deformation, an electric charge is generated; conversely, when a charge is passed through the material, a physical change can be observed.
In normal crystals, the unit cell (repeating “building block” of the crystal) is symmetric at its center. In piezoelectric crystals, they are asymmetrical. Under no mechanical stress, the net charge of the crystal would be zero. However, due to their asymmetrical basal structure, an application of mechanical stress forces misalignment of the charges, thus creating a charge on opposite ends or faces of the material. Conversely, the introduction of an electric current realigns the unit cells of the crystal, causing deformation.
These properties allow for very practical applications of piezoelectric materials, providing the ability to precisely control voltage and mechanical operations down to the micrometer. Examples include sound detection and production, extremely high-resolution microscopes, and simple cigarette lighters ignition.
Preparation and Properties
Creation and Limitations
Piezoceramic materials only exhibit their unique properties under conditions where they are polarized. This is usually accomplished by heating the crystal to a certain temperature and creating a strong electric field in the direction of desired polarization. Even when the electric field is removed, the crystal will retain most of its polarity, demonstrating the properties of piezoelectricity mentioned above.
Naturally, these materials have their limitations, one of which being temperature. Once heated above a certain point, known as the Curie temperature (this is the temperature referred to above), polarization is lost. Another limitation is voltage. If a strong enough electric field in the opposite direction of a material’s polarization is applied, then the material will fail to revert to its original polarized state.
Along with physical wear, the above limitations, if surpassed, can cause problems in piezoelectric materials and machines, as piezoelectric properties are based upon the material’s polarization.
Running an alternating current (AC) through a piece of piezoelectric ceramic allows its size to oscillate in a cyclic manner. At a certain point, impedance is minimized and mechanical energy is maximized; this is the resonant frequency.
This is most useful in producing and receiving sound; its reversibility also allows for a great amount of versatility in its use. It can also be used as a transducer to create ultrasonic vibrations in various industrial and commercial applications including atomization, welding, or drilling. Knowledge of materials’ resonance frequencies is extremely crucial in creating an efficient machine.