engleski

NiAl2O4-mullite composites prepared by sol-gel processes

Mullite composites have been extensively investigated during the last decade. Several aspects involving synthesis, characterisation and properties have been examined. Sol-gel process is a convenient method for syntheses of mullite precursors, as well as mullite composites. Depending on the starting materials and methods applied, the synthesized precursors have different properties, which in turn affect the resulting properties of the ceramics. The aim of the work was to examine in situ formation of spinel-mullite composites by sol-gel synthesis of diphasic gels doped with 1, 2 and 3 at.%, Ni2+ (introduced on account of Al3+). The gels were prepared by dissolving nitrates (Al(NO3)3×9H2O and Ni(NO3)2×6H2O, in water, and by mixing nitrate solutions with tetraetoxysilane (TEOS) dissolved in ethanol. The structural evolution with temperature has been studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis and Rietveld structure refinement. The microstructure and morphology of calcined powders has been investigated by scanning and high-resolving transmission electron microscopy (SEM and HRTEM) and energy dispersive X-ray spectrometry (EDXS). DSC analyses provide two exothermic effects of spinel crystallization followed by the mullite crystallization peak at higher temperatures. Although XRD scans show no distinctive patterns of Ni and Al-Si spinels at temperature <1000°C, by means of SEM and EDX analysis the existence of two spinels has been confirmed. Due to relative changes in DSC peak intensities with the increase in Ni content, the first peak was attributed to Ni-spinel. The kinetic analysis results, conducted in non-isothermal conditions are in concordance with this hypothesis. The Ni-spinel remains its stability up to 1650°C, although nickel also enters the mullite structure. SEM micrographs revealed coarser grains of Ni-spinel embedded in nano-sized mullite matrix.

Ni-spinel; Mullite; DSC; XRD

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