Effect of mechanochemical treatment on the crystallization behaviour of diphasic mullite gel
Introduction
Mullite is an important compound in material science and technology because of its excellent properties (high temperature strength, low thermal expansion, creep resistance and good chemical stability)1, 2. Mullite can be synthesized from monophasic and diphasic gel obtained either by sol–gel or coprecipitation methods3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15leading to the different precursor types described by Schneider et al.[16]The main difference between monophasic and diphasic gels is their homogeneity; monophasic gels are homogeneous at the atomic level and mullite formation occurs by exothermic reaction around 980°C3, 4, 12, 14, 15. In diphasic gels, homogeneity is in the nanometer range (1 to 100 nm) and mullite formation occurs above 1200°C3, 5, 6, 8, 9, 10, 12, 14, 15. Diphasic gels consisting of discrete alumina and silica particles do not show an exothermic effect around 980°C, the silica and alumina components reacting independently to form mullite at temperatures >1300°C for complete crystallization3, 10, 15.
In our previous experiments[17]we have shown that mechanochemical treatment of mixtures of gibbsite and amorphous silica promoted the formation of Al–O–Si bonds, enhancing the homogeneity of the system, and leading to crystallization of spinel-phase at about 980°C. A further consequence of mechanochemical processing was the lowering of the mullitization temperature. The presence of surface excess hydroxyl groups in the starting material leads to a decrease in the mullitization temperature at about 150–200°C after mechanochemical processing[18].
Since diphasic gel obtained via a liquid medium contains two discrete types of hydroxide particles with homogeneity on a nanometric scale, it is possible that mechanochemical treatment of such a system might produce a precursor with an increased mullitization rate and a reduced mullitization temperature by comparison with an untreated diphasic gel. The mechanochemical process should also be facilitated by starting with gibbsite and silica gel17, 18, which has better initial homogeneity and smaller particle size than hydroxide mixtures.
The aim of this investigation was to experimentally verify the predicted effect of mechanochemical treatment of diphasic gel.
Section snippets
Preparation
The diphasic gel was prepared by dissolving aluminum nitrate nonahydrate Al(NO3)39H2O (Wako Pure Chemical Industries Ltd) in deionized water and adding fumed silica (Aerosil 200, Aerosil Co. Ltd) during ultrasonic treatment. The dispersion was stirred vigorously and dilute NH4OH was added to precipitate of the aluminium and silicon hydroxides. The pH of the solution was adjusted to about 8.2 and stirred continuously during the time of the precipitation (30 min). The precipitated gel was
Results and discussion
Fig. 1 shows the DTA–TG traces of the unground and ground powders. Unground diphasic gel shows endothermic peaks at 83 and 468°C due to the dehydration of adsorbed water and dehydroxylation of aluminum hydroxide, respectively. There is very little weight change above 600°C. The DTA and TG curve of ground diphasic gel is distinctly different; the DTA endothermic peak is shifted to 168°C and broadened, due to mechanochemical dehydration and increased adsorption of surface water.
Conclusions
Mechanochemical treatment of diphasic gel is found to promote Al–O–Si polymerization between the silica and alumina gel components. Mullite formation in ground diphasic gel begins at a lower temperature (1150°C) than in the unground diphasic gel. The crystallization behaviour of mechanochemically-treated diphasic gel is similar to that of type III mullite precursors. Initially, the mullite is alumina-rich in composition, but gradually approaches the stoichiometric composition at higher
Acknowledgements
J.T. would like to thank UNESCO/MONBUSHO for the award of a research fellowship.
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