Effects of grain size on the dielectric properties of Pb(Mg1/3Nb2/3)O3-30 mol % PbTiO3 ceramics

Abstract

The effects of the average grain size on the phase transformation and consequent changes in the dielectric properties of Pb(Mg1/3Nb2/3)O3-30 mol % PbTiO3 were investigated. It was observed that the size refinement down to ∼90 nm impeded the phase transformation of the cubic phase formed in the course of sintering either into rhombohedral or into tetragonal one, and a series of phase transformation from cubic to rhombohedral and then to tetragonal phase took place as the average grain size increased from ∼90 nm to ∼5.7 μm. This dependence of the crystal structure on the grain size was qualitatively explained in terms of the capillarity effect and the free volume at the grain boundaries exerting a negative hydrostatic pressure on the grains. With the change in crystal structure the typical ferroelectric behavior in dielectric properties evolved into that of a relaxor and finally into a paraelectric one. The overall changes in the dielectric behavior were interpreted on the basis of the size-driven phase transformation. An anomalous dielectric behavior, i.e., a relatively high dielectric constant over a wide range of temperature (30–330 °C), was observed at the average grain size of ∼330 nm possibly due to a state of a single domain in a single grain, while below ∼90 nm all the phases in the system practically transformed into the paraelectric cubic phase.