CoAl2O4–mullite composites prepared by sol–gel processes

The formation of CoAl2O4–mullite composites from diphasic sol–gel precursors with 3:2 mullite composition doped with 1, 2 and 3 at.% Co2+ was studied by differential scanning calorimetry (DSC), X-ray diffraction and Rietveld structure refinement. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different composition and activation energies. The former phase with smaller activation energy (822 kJ mol−1) is attributed to cobalt-containing spinel structure and the latter with larger activation energy (about 1200 kJ mol−1) to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl2O4, while Al–Si spinel forms mullite above 1100 °C. Mullite lattice parameters, Rietveld refinement data and the CoAl2O4/Co2+ ratio in annealed samples points out that the majority of cobalt is incorporated in CoAl2O4 and only about 0.6 at.% enters mullite structure or the glassy phase, or both.