Fundamentals of Piezo Technology

The word "piezo" is derived from the Greek word for pressure. In 1880 Jacques and Pierre Curie discovered that pressure generates electrical charges in a number of crystals such as quartz and tourmaline; they called this phenomenon the "piezoelectric effect". Later they noticed that electrical fields can deform piezoelectric materials. This effect is called the "inverse piezoelectric effect".

The industrial breakthrough came with >> Piezoelectric Ceramics, when scientists discovered that barium titanate adopts piezoelectric characteristics on a useful scale when an electric field is applied.

The piezoelectric effect is nowadays used in many everyday products such as lighters, loudspeakers and signal transducers. Piezo actuator technology has also gained acceptance in automotive technology, because piezo-controlled injection valves in combustion engines reduce the transition times and significantly improve the smoothness and exhaust gas quality.

Pressure generates charges on the surface of piezoelectric materials. This direct piezoelectric effect, also called generator or sensor effect, converts mechanical energy into electrical energy.

Vice versa, the inverse piezoelectric effect causes a change in length in this type of materials when an electrical voltage is applied. This actuator effect converts electrical energy into mechanical energy.

The piezoelectric effect occurs both in monocrystalline materials and in polycrystalline ferroelectric ceramics. In single crystals, an asymmetry in the structure of the unit cells of the crystal lattice, i.e. a polar axis that forms below the Curie temperature T_C , is a sufficient prerequisite for the effect to occur.

Piezoelectric ceramics additionally have a spontaneous polarization, i.e. the positive and negative charge concentration of the unit cells are separate from each other. At the same time, the axis of the unit cell extends in the direction of the spontaneous polarization and a spontaneous strain occurs.

These relationships apply only to small electrical and mechanical amplitudes, so-called small signal values. In this range, the relationships between mechanical, elastic deformation S or stress T and electrical field E or electrical flux density D are linear, and the values for the coefficients are constant.

These small-signal coefficients can be found in the material data table:

The directions are designated by the axes 1, 2, and 3 (corresponding to the axes X, Y, and Z of the Cartesian coordinate system). The rotational axes, known as U, V, W in the coordinate system, are designated with 4, 5, and 6.

The direction of polarization (axis 3) is established during the polarization process by means of a strong electrical field applied between the two electrodes. This is where the largest displacement of the piezoceramic is reached.

Since the piezoelectric material is anisotropic, the corresponding physical quantities are described by tensors. The piezoelectric coefficients are therefore indexed accordingly.