Effects of radius and electronegativity of donors on the microstructure and mechanical, thermal, and electrical properties of ZnO varistors

Abstract

In this paper, influence of radius and electronegativities of seven kinds of donors on the composition, microstructure, mechanical, thermal and electrical properties of ZnO varistors were investigated via XRD, SEM, XPS, mechanical, thermal and electrical properties tests. Results showed that when sintered at 1100 °C for 2 h, the obtained varistors exhibited a dense microstructure, where radius of donors played an important role on the grain size and size distribution of varistors; Mechanical properties and coefficient of thermal expansion of varistors were mainly affected by the radius of donors, with specimen 6# possessing the highest values of 123.46 MPa (σ_f), 80.47 GPa (E_f), and 6.58 × 10⁻⁶ °C⁻¹ (λ_c), respectively; Values of E_1mA and α initially increased and then decreased with the increase of donors’ radius, whereas those of J_L and K exhibited opposite trends. E_1mA and α had maximum values of 569.94 V mm^–1 and 26.70, whereas J_L and K possessed the lowest values of 2.31 μA⋅cm⁻² and 1.35, respectively, when Co²⁺ worked as the donor (specimen 6#). The DC aging process of ZnO varistors obeyed the ion migration mechanism, the radius of donors can affect the number of it entering the ZnO lattice, which can limit the migration of zinc gaps via the formation of substitution and filling defects in the ZnO lattice. The electronegativity of donors can reduce the ion migration speed via the stronger electrostatic force, and specimen 6# possessed the lowest DC aging properties (K_T = 1.68).