Homogeneous precipitation and thermal phase transformation of mullite ceramic precursor

A fine pure mullite powder was prepared by homogeneous precipitation, mixing a fumed silica suspension in water and aqueous solutions of ammonium bisulphite and aluminium sulphate. This method produced a nanocomposite like structure, consisting of a fumed silica core coated with a thin layer of basic aluminium sulphate. The thermal transformation sequence of this mullite precursor was studied in this work in detail by means of scanning electron microscopy, X-ray powder diffraction technique, BET, thermal analysis (DTA and TG), and infrared spectroscopy, dilatometry and particle size distribution analysis. This nanocomposite lost water accompanied by a shrinkage at temperatures lower than 400°C. The sulphate ion elimination occurred at a temperature range of 500–850°C. Although most of the sulphate ion was eliminated, the powder was still amorphous at 800°C. The γ-Al₂O₃ was formed at 850°C increasing the particle size by the pyroexpansion process (eliminating sulphur trioxide gas). A spinel phase and traces of crystalline mullite were formed at 950°C. A fine mullite whisker was observed in the samples calcined at 1000–1100°C. A total transformation to mullite required temperatures higher than 1250°C. Grain growth occurred at these temperatures at the same time.