Influence of grain boundary insulation on nonlinear and dielectric discrepancy in Ni²⁺/Ni³⁺ doped CaCu_3-xNi_xTi₄O₁₂ ceramics

Two series of CaCu_3-xNi_xTi₄O₁₂ (x = 0.0, 0.5, 1.0, 2.5) ceramics doped with Ni²⁺ and Ni³⁺ were prepared via the solid-state reaction method. Despite the similar micro-morphologies of the samples, the nonlinear electrical characteristics were enhanced in the Ni³⁺-doped samples but deteriorated in their Ni²⁺-doped counterparts. With Ni content increasing, the breakdown field and nonlinear coefficient of Ni³⁺-doped samples enhanced from 0.54 to 6.14 kV cm⁻¹ and 2.68 to 4.92 respectively while dropped from 0.98 to 0.21 kV cm⁻¹ and 3.48 to 1.03 for Ni²⁺-doped samples. Impedance spectroscopy analysis revealed significant discrepancies in terms of grain boundaries between the two series of samples, which resulted in their different dielectric properties. The oxygen loss owing to the in situ formation of NiTiO₃ was considered to be the crucial factor impeding grain boundary insulation, which was key to the potential barrier formation. In contrast with that of the Ni²⁺-doped samples, the oxygen loss of the Ni³⁺-doped samples was compensated by the reduction of Ni³⁺ to Ni²⁺. This helped maintain a high potential barrier height and resulted in different nonlinear characteristics of the two series.