Study of electrical and mechanical contribution to switching in ferroelectric/ferroelastic polycrystals

Polarization switching in a polycrystalline ferroelectric/ferroelastic ceramic is simulated with a finite element model. It is assumed that a crystallite switches if the reduction in potential energy of the polycrystal exceeds a critical energy barrier per unit volume of switching material. Each crystallite, represented by a cubic element in a finite element mesh, is a single domain that switches completely without a simulated domain wall motion. The possible dipole directions of each crystallite are assigned randomly subject to crystallographic constraints. The model accounts for electric field induced (i.e. ferroelectric) switching and stress induced (i.e, ferroelastic) switching without piezoelectric interaction. Different weights for the mechanical and electrical contribution to switching are selected phenomenologically to simulate electric displacement vs electric field and strain vs electric field of a ceramic lead lanthanum zirconate titanate (PLZT). Although the critical energy barriers for 90 degrees and 180 degrees switching are assumed to be the same, 90 degrees switching is favored when the electrical contribution to switching (i.e. electrical energy) is dominant, but 180 degrees switching is favored when the mechanical contribution to switching (i.e. elastic strain energy) is dominant. With increasing mechanical contribution and decreasing electrical contribution, the simulated electric displacement deviates from the Rayleigh law under a low applied electric field, and the shape of a switching region (or a process zone) changes from a prolonged ellipsoid to a sphere. (C) 2000 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc.