Microstructure, crystal structure and electrical properties of Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics obtained at different sintering conditions

Abstract

Details of the formation of Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics under different sintering conditions have been studied by optical microscopy, scanning electron microscopy (SEM), electron probe and energy dispersive spectroscopy (EDX) microanalyses, X-ray diffraction (XRD) and electrical resistivity measurements. Microstructure studies of samples sintered at 1170 °C for 1 h indicated the presence of a secondary phase besides the main spinel phase with modified composition. XRD measurements showed that the spinel phase exhibits a tetragonally distorted spinel structure (space group I4 1/ amd, a=5.9410(5) Å, c=8.4196(15) Å). The secondary phase (solid solution based on NiO) crystallizes with the NaCl-type structure (space group Fm 3m , a=4.1872(3) Å). The content of the secondary phase in ceramics is 10.61 mass%. For NiMn 2O 4 ceramics, prepared under the same sintering conditions, the decomposition with Ni 1− x Mn x O solid solution (NaCl-type structure) and spinel phase formation have been observed. The tetragonal modification of the spinel phase for NiMn 2O 4 ceramics is more preferable (space group I4 1/ amd, a=5.9764(5) Å, c=8.4201(8) Å). The distribution of atoms in the structure has been proposed for both ceramics. According to XRD results the Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramic samples, sintered at 920 °C for 8 h (program 1), at 920 °C for 8 h and at 750 °C for 24 h (program 2), at 920 °C for 8 h, at 1200 °C for 1 h and at 920 °C for 24 h (program 3) and at 920 °C for 8 h, at 1200 °C for 1 h, at 920 °C for 24 h and at 750 °C for 48 h (program 4), contain a single phase with the cubic spinel structure (space group Fd 3m ). Small residuals of the secondary phase for the ceramics, prepared via programs 3 and 4, have been observed by SEM investigations. The structure transformations of the spinel phase for Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics sintered at 1170 °C are attributed to a Jahn–Teller-type distortion due to a compositional change as a result of the secondary phase separation.