Previous materials of this kind have had to offer a compromise between piezoelectric activity
and maximum operating temperature, with rapid thermal degradation taking place at temperatures above 200[degrees]C.
Since the discovery of its strong piezoelectric activity
in 1969 by Kawai , poly(vinylidene fluoride) (PVDF) and its copolymers, in particular the copolymers of vinylidene difluoride and trifluoroethylene P(VDF-TrFE) have dominated the science and technology of piezoelectric polymers.
On the other hand, the soft materials, such as PZT-5A (DOD type II) and PZT-5H (DOD type VI), offer higher sensitivity and piezoelectric activity.
Note that the piezoelectric activity becomes nonlinear at high stress levels.
The application of a mechanical deformation or an electrical field changes the dipole density, which confers to the material a piezoelectric activity
The piezoelectric activity
is described in the adiabatic case by: