Abstract. 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(A12-2xMx TixO3) 9 2SIO2 and 3(Al2_xMxO3) 9 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~ 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~ A two-phase aluminate spinel-muUite arrangement was detected at temperatures around 1200~ 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.
NiA1204 spinel-mullite composites were prepared by simultaneous replacement of Al by Ti and/or Ni in 3:2 stoichiometric mullite. Specimens having nominal compositions 3(A12-2xNi,Ti,03) -2Si02 (x = 0, 0.025, 0.05, 0.2) and 3(A12-3,0J * 2Si02 (M = Ni" or Ti& and x = 0.05) were synthesized by sol-gel techniques, which provide homogeneous gels in the Si02-A1203 system. Gel structures investigated by infrared (IR) spectroscopy revealed the formation of AI-OSi bonds in dried gels. The reaction sequence of gel-derived glasses, previously obtained by preheating gels at 750°C for 3 h, was evaluated by X-ray powder diffraction (XRD) and ultraviolet-visible (W-vis) spectroscopy. AU samples crystallized at around 1OOO"C from an amorphous state, but unexpectedly the first crystalline phase was AESi spinel in all aluminum-substituted specimens; i.e., a change in the sequence of reaction with respect to the 3:2 stoichiometric m d t e was produced. NiA1204 spinel was almost simultaneously detected. Two processes of mullite crystallization were observed. The temperature of formation of mullite was the lowest for the higher substituted sample. The microstructure of the final NiA1204 spinel-mullite composites found in all Ni-containing samples after annealing for 96 h was examined by scanning electron microscopy with energy dispersive X-ray analysis (SEMDX), which revealed the presence of small particles of NiA1204 spinel dispersed in a mullite matrix. For annealed compositions with the larger Al replacement, i.e., when x = 0.2, a small amount of very small AI,TiO, particles was also detected by XRD and transmission electron microscopy (TEM).
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