Copper manganite Cu2+1+x(Mn3+2-2xMn4+x)O4 as black ceramic pigment: microwave and coprecipitation synthesis

Three representative stoichiometries of copper manganite, Cu 2+ 1+x (Mn 3+ 2-2x Mn 4+ x )O 4 (x = 0, 0.2, and 0.5) were synthesised by solid-state method through both conventional electric and microwave-assisted firing and evaluated as black ceramic pigments. All fired powders crystallised predominantly in the primitive cubic Cu 1 . 5 Mn 1 . 5 O 4 spinel, with minor secondary phases of bixbyite, tenorite, and crednerite (CuMnO 2 ), the latter being more prominent in microwave-fired samples. Overall, microwave firing produced blacker powders, attributed to the formation of an effective composite spinel–tenorite structure with the presence of crednerite. When powders were 5 wt% glazed in a double firing glaze (1000 °C), blackness increased compared with the powders. In particular, the x = 0 electrically fired and x = 0.2 microwave-fired glazed samples showed a deep-black colour, with visible reflectance values (R Vis ) of 1.59% and 1.65%, respectively. When powders were 20 wt% glazed in a third firing glaze for porcelain (860 °C), electrically fired samples exhibited pinhole defects caused by partial pigment decomposition in the molten glaze, defects that were absent in microwave-fired samples probably associated to a non-thermal effect of microwaves. The x = 0.2 composition, fired and glazed using microwaves, achieved the highest blackness in the study (R Vis = 1.54%). Additionally, the x = 0 composition electrically fired, with optimal coloration in double firing glaze, was prepared by coprecipitation. This route produced a spinel phase with a finer grain size and greater photocatalytic activity toward Orange II compared with the samples synthesized by the solid-state method anda deep-black glazed surfaces (R Vis = 2.80%).