Microstructure and nonlinear electrical properties in Na2CO3-ZnO varistor ceramics

Herein, (1-x)ZnO + xNa2CO3 (x = 0, 0.5, 1.0, and 1.5 mol%) varistor ceramics were prepared via conventional solid-state sintering at a low sintering temperature of 900 degrees C. The microstructure and electrical properties of these varistor ceramics were systematically investigated as a function of Na2CO3 concentration. Results revealed that the prepared varistor ceramics exhibit the hexagonal structure without a secondary phase. The increase of Na2CO3 concentration promotes the ZnO grain growth. Owing to an increasement in the barrier height at the grain boundaries, the nonlinear electrical properties of the ceramics considerably improve with increasing Na2CO3 concentration. The optimal composition of 98.5 mol%ZnO + 1.5 mol%Na2CO3 corresponds to a nonlinear coefficient of 3.55, leakage current density of 450 mu A/cm2, breakdown strength of 241.2 V/mm, and barrier height of 0.57 eV. Moreover, the dielectric properties of the varistor ceramics are considerably affected by Na2CO3 concentration.