Effects of raw particle size and annealing on microstructure, electrical and mechanical behaviors of ZnO-based varistors

In this paper, the effect of the raw particle size and the annealing process on the compositions, microstructure, mechanical, thermal, and electrical properties of ZnO varistors was investigated using X-ray diffraction, infrared, X-ray photoelectron spectroscopy, scanning electron microscopy, electric field–current density, capacitance–voltage, mechanical, and thermal expansion measurements. Furthermore, the ZnO grain growth mechanism during the annealing process was discussed in detail. Results show that when the raw particle size equaled 90 nm and sintering was performed at 1100 °C for 2 h, the obtained ZnO-based varistors possessed excellent mechanical, thermal, and electrical properties; the E1mA, α, JL, and K values were 1015.3 V mm–1, 37.98, 2.45 μA cm–2, and 1.37, respectively, due to the highest bulk density and the finest uniform particle size distribution. When annealing occurred at 300 °C for three times, values of σf, KIC, E1mA, α, JL, and K reached 175.35 MPa, 1.96 MPa m1/2, 1180.2 V mm–1, 43.58, 2.15 μA cm–2, and 1.2, indicating improvements of approximately 14.7%, 4%, 16.2%, 14.7%, − 12.2%, and 11.11% compared with those of the non-annealed ZnO-based varistors, respectively, being due to the improved uniformity of grain size distribution and the re-distribution of donor Al3+ and Y3+ in the ZnO crystal and grain boundary. The annealing process lead to the transformation of Sb3+ to Sb5+ in the ZnO crystal, which could generate two additional coordination sites. The newly generated sites could combine with O2- in other crystals to increase the grain growth, resulting in clear grain growth on the surface, improved grain size distribution, and regular grain shape in the interior of ZnO-based varistors.