Effect of Nb2O5 on ZnBiMnO varistor ceramic prepared by solid-state sintering at 850°C

Abstract

Nb 2 O 5 is a commonly used donor dopant for ZnO-based varistor ceramics, but its effect, especially on the low-temperature sintered ZnO varistor ceramics, is not fully understood. To provide a possible answer to this problem, ZnO–Bi 2 O 3 –MnCO 3 (ZnBiMnO) based varistor ceramics with 0.05%–0.3% (in mole ratio) Nb 2 O 5 were fabricated by solid-state sintering at 850 °C for 3 h. Their microstructure and nonlinear electrical properties were studied by XRD, SEM and the standard current-voltage (I–V) tests to reveal the effect of Nb 2 O 5 . With the increase of Nb 2 O 5 from 0.05 mol% to 0.3 mol%, more solid Bi 5 Nb 3 O 15 inter-granular particles form within the ceramic during sintering, thereby decreasing the Bi-rich liquid phase. As a result, the average size of ZnO grain decreases from 4.35 μm to 1.67 μm. This microstructural change leads to the increase of the breakdown voltage in the range of 821 V/mm to 1851 V/mm. The ZnBiMnO varistor ceramic with 0.1 mol% Nb 2 O 5 shows the best nonlinear properties. The optimum nonlinear coefficient is 35.81, the breakdown voltage is 907.51 V/mm, and the leakage current is 7.72 μA/cm 2 . The result of this study provides a promising candidate material for manufacturing the multilayered low-voltage varistor that may use Ag, Ni or even Cu as the inner electrodes.