Structure and electrical properties of single-phase cobalt manganese oxide spinels Mn3−xCoxO4 sintered classically and by spark plasma sintering (SPS)

Abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

Structural variations and cation distributions in Mn3−xCoxO4 (0≤x≤3) dense ceramics using neutron diffraction data

“…In order to prepare powders of cobalt manganese oxide spinels suitable for sintering techniques, cobalt manganese oxalic precursors were obtained by the co-precipitation method of an aqueous solution of 0.2 mol L À1 ammonium oxalate and a mixture of manganese and cobalt nitrates in the desired proportions (4 mol L À1 ) [22]. After 30 min of ageing, the solution was filtered, washed several times with water and dried in air at 90 C. A thermal decomposition in air for 4 h at 800 C of the oxalate precursors was chosen for the formation of oxygen stoichiometric oxide powders (cooling rate of 150 C/h).…”

“…For x ¼ 0, a manganese sesquioxyde was synthesized while a mixture of (Mn,Co) 2 O 3 and a tetragonal spinel appeared for x ¼ 0.58. For 0.98 x < 1.78 (compositions determined by ICP-AES measurements [22]), a mixture of tetragonal and cubic spinels were obtained, while for 1.78 x 3 a single cubic spinel was formed at room temperature.…”

“…The oxide powders were mixed with an aqueous solution of polyvinylic alcohol (17 wt%) used as a binder and pressed at [22]. For 0 x < 1.78, all phases were pure tetragonal spinels.…”

“…A series of papyex/alumina/papyex layers were put on both sides of the powder for lowering the amount of (Mn, Co)O reduced phase that appeared on the surface of the pellet during a cycle in carbon graphite. In order to remove any oxide impurities, each side of the highly densified ceramics (densification > 95%) was then polished until a single cubic spinel phase (of the same composition than the starting powder) could be identified via XRD and EDS/SEM measurements [22].…”

“…An important remark is that refinements performed on Mn 1 w1150 C in the conventional route) and shorter heating time in the SPS process allow preserving the small grain size from the powders to the ceramics. The number of grain boundaries is larger and the ceramic densification higher after SPS treatment [22]. However, it is known that these differences observed in the NTC ceramic microstructures do not have a significant contribution in the conduction mechanisms [25].…”