Synthesis, phase composition and characterization of Co-diopside ceramic pigments
Rositsa Titorenkova, Vladislav Kostov‐Kytin, Ts. I. Dimitrov
Abstract
Ceramic pigments in the system CaO – CoO – MgO – 2SiO2 were synthesized through solid-state high temperature sintering at 1000, 1100 and 1200 °C. The starting compositions were determined from the stoichiometric mineral diopside, following the expression CaO.xCoO.(1-x)MgO·2SiO2, where x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 0.7 and 0.9. The effect of sintering temperature and initial cobalt concentration on the phase composition and colour were studied by powder X-ray diffraction analysis, scanning electron microscopy (SEM), infrared (FT-IR), ultraviolet–visible - (UV–Vis) and electron paramagnetic resonance (EPR) spectroscopy. Poly-phase ceramics were obtained depending on synthesis parameters, which in addition to Co-diopside, may contain wollastonite, Co-åckermanite – Ca2(Co, Mg)Si2O7, Co-olivine – CoMgSiO4, Co-monticellite – Ca(Mg, Co) SiO4, and cristobalite. Quantitative ratios of detected mineral phases and the degree of cobalt isomorphic substitution were determined by applying Rietveld refinement approach. Trends and consistencies were outlined revealing that concentration of cobalt in the initial synthesis batch and its isomorphous incorporation in the crystal structure of diopside affect the structural perfection of the cation polyhedra. This fact obviously plays a key role in controlling the amount of the main constituent phase in the run-product. Ceramics of highest diopside content and lowest measured degree of structural distortion were only formed at 1200 °C and initial cobalt concentration in the range x = 0.2–0.3. These samples are pink in colour and have the highest value of red coordinate in the CIELab system due to Co2+ ions in octahedral coordination in diopside structure.