Microstructure, crystal structure and electrical properties of Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics obtained at different sintering conditions

Details of the formation of Cu0.1Ni0.8Co0.2Mn1.9O4 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 degreesC 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 14(1)/amd, a=5.9410(5) Angstrom, c=8.4196(15) Angstrom). The secondary phase (solid solution based on NiO) crystallizes with the NaCl-type structure (space group Fm3m, a=4.1872(3) A). The content of the secondary phase in ceramics is 10.61 mass%. For NiMn2O4 ceramics, prepared under the same sintering conditions, the decomposition with Ni1-xMnxO solid solution (NaCl-type structure) and spine] phase formation have been observed. The tetragonal modification of the spinel phase for NiMn2O4 ceramics is more preferable (space group I4(1) /amd, a=5.9764(5) Angstrom, c=8.4201(8) Angstrom). The distribution of atoms in the structure has been proposed for both ceramics. According to XRD results the Cu0.1Ni0.8Co0.2Mn1.9O4 ceramic samples, sintered at 920 degreesC for 8 h (program 1), at 920 degreesC for 8 h and at 750 degreesC for 24 h (program 2), at 920 C for 8 h, at 1200 C for 1 h and at 920 degreesC for 24 h (program 3) and at 920 degreesC for 8 h, at 1200 degreesC for 1 h, at 920 degreesC for 24 h and at 750 degreesC for 48 h (program 4), contain a single phase with the cubic spinel structure (space group Fd (3) over barm). 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 Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics sintered at 1170 degreesC are attributed to a Jahn-Teller-type distortion due to a compositional change as a result of the secondary phase separation. (C) 2002 Elsevier Science B.V. All rights reserved.