engleski

The influence of nickel on the crystallization path of mullite gel

The impurities in sol-gel obtained mullite gels can strongly influence its crystallization paht and microstructure, as well as physical properties of obtained composites. During the last decade, a variety of mullite matrix composites with transition metal oxides have been intensively examined. It has been shown that the cations of smaller ionic size and higher valence can be incorporated relatively easily in the mullite structure, Substituting for Al3+ in the octahedral sites. On the other side, the entrance of cations of larger size and lower valence is still doubtful. The same ions have been shown to react at temperatures below mullite crystallization temperature with gamma-Al2O3, forming the corresponding metal aluminates, MeAl2O4, with spinel structure. The aim of this work was to examine the possibility of entrance of Ni2+ into mullite structure and to study the influence of nickel on the crystallization path of mullite gel. The sample with nominal composition in mol.%: 0.572 Al2O3, 0.393 SiO2 and 0.035 NiO has been prepared by the sol-gel synthesis. Nitrates (Al(NO3)3*9H2O and Ni(NO3)2*6H2O, were dissolved in water, and nitrate solution was mixed with tetraetoxysilane (TEOS) dissolved in ethanol. The crystallization path has been studied by the differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis. The structure characterization has been performed throug the Rietveld structure refinement. The microstructure and morphology has been investigated by the high-resolving transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrometry (EDXS). It has been determined that the splitting of spinel DSC peak occurs in the course of thermal treatment. The XRD scans show no distinctive patterns of Ni-spinel and Al-Si spinel at temperature <1000C. The spinel phase enriched with Ni can be identified by XRD with certainty at 1270C. However, HRTEM and EDX analysis established the existence of spinel grains differing in composition even at 900C. The non-isothermal kinetic analysis yielded activation energies of 625+-20kJmol-1 and 1360+-52kJmol-1 for the first and second spinel phase, respectively. The activation energies obtained are in accordance with the literature data for nickel spinel and Al-Si spinel. Therefore, the first exothermal DSC peak is attributed to the nickel-spinel and second to the Al-Si spinel crystallization process. The activation energy of the mullite crystallization process, obtained in the same manner, is 1198+-36kJmol-1. Ni-spinel remain stable up to 1600C. According to the Rietveld structure refinement, Ni does not enter the mullite structure.

mullite; sol-gel; nickel; DSC; XRD

nije evidentirano