Influence of zirconia addition on the microstructure of K0.5Na0.5NbO3 ceramics

We prepared (K0.5Na0.5)NbO3 (KNN) ceramics with the addition of 1 mass% ZrO2 with the aim to hinder the exaggerated grain growth encountered in KNN. Both KNN and KNN-ZrO2 ceramics sintered at 1115 °C and 1125 °C, respectively, had relative density exceeding 95%. KNN had a bimodal microstructure, with a population of fine grains of a few 100 nm and the large grains of about 20 μm, while the microstructure of KNN-ZrO2 was fine and uniform, with the median grain size of 0.3 μm and the largest grains of about 1.3 μm. We attribute the refinement of the microstructure to the matrix grain-growth inhibition by ZrO2 addition. The influence of ZrO2 is twofold: sub-100 nm sized ZrO2 particles, located at the KNN grain junctions hinder the matrix grain growth. Additionally, the enrichment of the boundary regions of the matrix grains with Zr relative to the grain interiors, confirmed by TEM/EDXS analysis, is also a probable reason for the decreased mobility of the grain boundaries. The dielectric permittivity and losses, measured at 10 kHZ, and piezo d33 constant of KNN-ZrO2 are 905, 0.04 and 100 pC/N, respectively.