Grain size effect on structure and electrical properties of lead-free Na0.4K0.1Bi0.5TiO3 ceramics

Sodium potassium bismuth titanate ceramics (Na0.4K0.1Bi0.5)TiO3 with grain sizes of 0.58, 0.93, 1.19 and 1.83 mu m were prepared by varying sintering time. Grain size dependent structural and electrical properties of NKBT ceramics at morphotropic phase boundary composition have been investigated for the first time. Large internal compressive stress preclude the formation of tetragonal phase in sub-micron sized samples. Phase coexistence in 1.83 mu m sample has been well modelled with rhombohedral (R3m) and tetragonal (P4mm) using Rietveld refinement. Relative permittivity increases with grain size and all the samples exhibits relaxor behavior. Strong coupling between grain boundary and domain walls hinder domain switching and polarization reversal. Hence reduction in the remnant polarization was observed in sub-micron sized samples. Excellent piezoelectric constant of 215 pC/N for sample with grain size of 1.83 mu m is due to large amount of non-180 degrees domain variants than that of samples with lowest grain size.