Dielectric Grain-Size Effect in High-Permittivity Ceramics

Abstract

On the basis of existing experiments, the grain-size effect on the permittivity and dielectric function in the case of ferroelectric and relaxor ferroelectric dense single-phase ceramics is reviewed and critically discussed. Relevant data exist for BaTiO3 (BTO), SrTiO3 (STO), Pb(Zr,Ti)O3 (PZT), Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). In the case of displacive ferroelectrics, the grain-size effect (decrease of the permittivity with decreasing grain size) can be attributed to a narrow low-permittivity grain-boundary layer (dead layer), whose effect can be modeled as a core-shell composite using an effective-medium approach. The strongest size effect was observed for STO, for which a model of two dead layers with a microscopic meaning was suggested. In the case of relaxor ferroelectrics it appears that rather thick layers near grain boundaries are influenced in such a way that the slow relaxation dispersion is there missing, which could be interpreted as pinning of the polar nano-regions by the grain boundaries, which prevents their flipping. In the ferroelectric phases, the grain-size effect is dominated by the domain structure (domain-wall contribution), which is processing and grain-size dependent and is present even in nano-grain ceramics.