Relation Between Electrical Properties and Microstructure of Barium Titanate

Abstract

A simple theoretical appraisal shows how the square hysteresis loop of a ferroelectric crystal must be altered when randomly oriented crystals are assembled into a polycrystalline ceramic. The effect of particle size is discussed for grain boundary depolarization and demonstrated in barium titanate ceramics with average grain diameters differing by a factor of 100. The expected decrease in saturation polarization and susceptibility with decreasing grain size is observed in these samples. Contrary to the predictions of this model, the initial susceptibility can show a drastic increase with decreasing grain size. It returns to normal values only after aging. The explanation of this anomaly requires the application of domain theory, suggesting that it is associated with imperfections on an atomic scale.

Polycrystalline ferroelectric materials have not received the theoretical attention given to single crystals. Since they are finding increasing application, some insight into the relation between the much-investigated single crystals and the polycrystalline material is of interest. A few guiding principles are provided in the following account. Particular attention is given to the depolarizing effect due to imperfect flux closure across the grain boundaries, which is negligible for coarse-grained samples but can become quite appreciable when the grains are very small.