A MODEL FOR SWITCHING AND HYSTERESIS IN FERROELECTRIC CERAMICS

In ferroelectric ceramics every grain has an average polarization aris ing from the spontaneous polarization of all domains. Since the domain configuration is known in fine grained as well as in coarse grained p erovskite ceramics the average polarization can be easily determined. An electric field can invert the average polarization by 180 degrees i f it exceeds a critical field strength E(2crit). At fields above E(2cr it) a new 90 degrees domain wall being perpendicular to the regular 90 degrees domain walls nucleates at the boundary of the grain. It trave rses the grain driven by the external field and disappears at the oppo site grain boundary where its energy is dissipated. This transient dom ain wall is highly mobile, because, on its passage through the grain i t does not change the gross shape of the grain. The calculated values of the critical electric field and its temperature dependence are in g ood agreement with observations. In addition, the new domain wall is t he origin of dielectric nonlinearities in ferroelectric ceramics for l arge signal ac-fields or for de-biased fields. The very high permittiv ity which is observed in fine grained ceramics can also be ascribed to the presence of these easily movable walls. Internal depolarization f ields in this case cause the generation of the new walls. The electric hysteresis curve P(E) is described by a progress parameter delta such that P(delta) and E(delta) are separate hysteresis curves. A model fo r the determination of P(delta) and E(delta) is presented. The final h ysteresis curve results from the elimination of delta.