Spinel-Mullite Composites with Optical Properties

The aim of this paper was to study the synthesis and characterization of spinel-containing mullite based materials, using sol-gel techniques. Several gels were prepared, with nominal compositions 3(Al_2−2xM_x Ti_xO₃)·2SiO₂ and 3(Al_2−xM_xO₃)·2SiO₂, with M=Ni⁺² or Co⁺² and 0.0≤x≤0.2, by hydrolysis and condensation of mixtures of aluminum, silicon and titanium alkoxides and nickel chloride. Dried gels were homogeneous and displayed a glass transition at around 750°C, which indicated that the system could be described as an amorphous silicoaluminate network. Crystallization pathway of gels were followed using differential thermal analysis and X-ray diffraction patterns of samples thermal treated at temperatures in the range between 800 and 1400°C. A two-phase aluminate spinel-mullite arrangement was detected at temperatures around 1200°C. The microstructure of the final product was interesting, because the minor secondary phase was homogeneously dispersed in the mullite matrix. Chemical and thermal resistance of diphasic materials were tested and the results indicate that these materials can be used as high temperature ceramic pigments.