Piezoelectric Actions

The piezoelectric effect for a given item depends on the type of piezoelectric material and the mechanical and electrical axes of operation can be precisely orientated within the shape of the ceramic. These axes are set during "poling"; the process that induces piezoelectric properties in the ceramic. The orientation of the dc poling field determines the orientation of the mechanical and electrical axes.

The poling field can be applied so the ceramic exhibits piezoelectric responses in various directions or combination of directions.

The poling process permanently changes the dimensions of a ceramic element. The dimension between the poling electrodes increases and the dimensions parallel to the electrodes decrease. The effect is shown in the fig 1, albeit greatly exaggerated.

Axes

Piezoelectric materials are anisotropic - their electrical, mechanical, and electromechanical properties differ for electrical or mechanical excitation along different directions. Thus for systematic tabulation of properties, we require a standardized means for identifying directions. Where crystals are concerned, we use the orthogonal axes originally assigned by crystallographers. However, we identify the axes by numerals:
1 corresponds to x axis,
2 corresponds to y axis,
3 corresponds to z axis.

Piezoelectric ceramics are isotropic and are not piezoelectric before poling. Once they are polarized, however, they become anisotropic. The direction of the poling field is identified as the 3 direction. In the plane perpendicular to the 3 axis, the ceramics are non directional. Accordingly, the 1 and 2 axes may be arbitrarily located but, of course, they must be perpendicular to each other.