First let’s overview the main key performance indicators of piezoelectric materials.
The coupling factor, k, shows the ratio of converted energy to the total input energy. A higher coupling factor is desired; however, datasheets usually only contain theoretical maximum values for a specific device configuration, direction and resonant mode.
More direct information can be obtained from the charge constant and the voltage constant. The charge constant, d, shows either the ratio of the acquired strain, e, to the applied electrical field, E, or the ratio of the resultant displacement field, D, to the applied stress, σ. Note this also emphasizes the reciprocity of the piezoelectric effect; in both cases the units are C/N (or m/V which is equivalent).
The voltage constant, g, shows either the ratio of the resultant strain to an applied displacement field or the ratio of the resultant electric field to the applied mechanical stress. The unit in this case is V∙m/N (or m2/C). Furthermore, both d and g are direction dependent as all piezoelectric materials are anisotropic; directions are usually given as subscripts.
Note the duality of d and g when comparing the figures.
The quality factor of a material gives the ratio of the stored energy over the lost energy over one cycle of oscillation. A higher Q factor simply means the material can store energy for a longer period of time.
Finally, the dielectric constant in this case can be viewed as a measure of the charges generated on the surfaces of the material when an electric field is applied. Well below resonance the material acts as a capacitor, and the product of the dielectric constant, the voltage constant and the vacuum permittivity gives the charge constant.
Large charge constants are ideal for actuation purposes (such as in an atomic force microscope), as the same amount of applied voltage generates larger strain in these materials. Ceramics offer high coupling coefficients and are well-suited for these purposes. But their low Q-factor makes them not the best choice for resonator devices, where a longer energy storage would be ideal. Example materials in this group are barium titanate (BaTiO3) or lead zirconate titanate (PZT).
On the other hand, natural crystalline structures, such as quartz (silicon dioxide, SiO2) exhibit large Q factor, and therefore well suited for resonators or frequency stabilizers in watches. The charge constant of quartz is two orders of magnitude lower than ceramics, quartz is not commonly used for actuation or sensing purposes.
Polymers (such as PVDF) offer charge constants between ceramics and crystalline materials, with low quality factor, and applicable for low-cost sensing purposes.
In summary, for high Q factor resonators quartz-like materials, for transducers PZT-like materials are recommended.
Next the different device functions will be overviewed depending on the orientation of the applied stress or voltage with respect to the poling axis.