engleski

CoAl2O4-mullite composites prepared by sol-gel process

Mullite, 3Al2O3*2SiO2, is attractive ceramic material for advanced applications. Variety of mullite matrix composites have been recently studied. The aim of the work was to examine in situ formation of spinell-mullite composites by sol-gel synthesis of diphasic gels doped with 1, 2 and 3 at.%, Co2+ (introduced on account of aluminium). The gels were prepared by mixing Al and Co nitrate water solutions with tetraetoxysilane, dissolved in ethanol. The structural evolution with temperature has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Rietveld structure refinement. The microstructure and morphology has been investigated by scanning electrion microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). DSC analyses provide two exothermic effects, the first attributed to the spinel crystallization around 1000C, and second caused by the mullite crystallization at higher temperatures. Shift of peaks towards lower temperatures with the increase of Co2+ content has been observed. At lower taemperaturtes XRD patterns show no distinction between Co and Al-Si spinels, while sperate diffraction lines could be spotted at 1350C. SEM micrographs of samoples thermally treateed at these temperatures revealed the presence of particles with different morphology: fewer coarser particles embedded in nano-sized mullite matrix. Using EXD analises it was determined that cobalt is concentrated in coarser particles while matrix is composed of alumosilicate phase. At 1650C samples are composed of mullite, Co-spinel and traces of alpha-alumina. The differences in the mullite lattice parameters between samples could be the consequence of cobalt entering the mullite structure, on the other hand, they could be explained by variations in Al2O3 to SiO2 ratio. Rietveld analyses of the sample with 3% Co2+ indicated that the majority of Co2+ is in spinel phase. The kinetic analysis showed certain decrease in activation energy for mullite crystallization in comparison with non-doped sample.

cobalt spinel; mullite; sol-gel

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