Aging of PLZT and the space charge arising in hot poling

Abstract

Measurements of P versus T by integration of the pyro-electric charge and the decreasing of P_r at constant temperature are obtained for PLZT 8/65/35 ceramic samples. From the P versus T diagrams the arising of an space charge is shown to be unlike the pyroelectric one and very dependet of the poling temperature. That charge is revealed in the hot poled sample by measuring the thermally stimulated currents which present a time evolution as i = kt^-n that resembles the dielectric discharge law. That sort of charge may easily give a wrong P_r measure by the integration method. On the other hand, the presence of that charge is responsible for the P_r recovery after the sample has been thermoly depoled in part, by opposition to a poled sample at room temperature, since it does not appear any more.

From the P_r time decreasing measurements and after having got hysteresis loop, evidence of the space charge role on the aging rate is shown. The P_r evolution follows the law -ΔP_r/P_r = A + Blnt where the B slope decreases with the poling temperature when the P_r sign is the one of the former polarization. Otherwise, B increases if the polarization sign is opposite.

The experimental results point out that from the very first measurements in the aging process it is possible to know the P_r time evolution. Further more it would be possible to explain the aging behaviour by assuming that the space charge arising in hot poling would fix on P_r direction as well as impurities and deffects in these materials do.

Note: This paper has been submitted to The Journal Material Science.