Synthesis and sintering behaviour of spinel-type CoxNiMn2−xO4 (0.2 ≤ x ≤ 1.2) prepared by the ethylene glycol-metal nitrate polymerized complex process

CoxNiMn2−xO4 (0.2 ≤ x ≤ 1.2) spinel-type powders were prepared by auto-combustion of ethylene glycol-metal nitrate polymerized gel precursors. A pure spinel-type phase, with no intermediate compounds, was attained from the burning of the polymerized gel precursor and subsequent calcining at 600-700 °C. The formation and the structural evolutions of the spinel-type phase have been studied by simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Powder characteristics such as particle size and specific surface areas of the calcined powders were dependent of the chemical compositions. In the same way, the crystalline structure of the synthesized spinel-type phases was strongly dependent of the Co content, and a tentative cationic distribution is proposed. Sintering of the CoxNiMn2−xO4 oxides have been studied by both the constant rate of heating (CRH) and the conventional ramp-and-holding methods. Density increased with the increasing of the Co content, and theoretically dense bodies (≥99.9% of the theoretical density) with submicronic average grain sizes were obtained at a temperature as low as 1050 °C for 6 h in the case of the Co1.2NiMn0.8O4 composition. Above that temperature, and coinciding with the apparition of an amorphous cobalt oxide liquid-phase, a slow bloating phenomena as consequence of the evolution of oxygen gas with the corresponding decreasing in density and the beginning of an exaggerated grain growth in the sintered samples was present. Microstructural evolution of the samples as a function of the sintering temperature has also been studied.