Sol-emulsion-gel synthesis of hollow mullite microspheres

Hollow mullite microspheres were obtained from emulsified diphasic sols by an ion extraction method. The surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined microspheres were investigated by using thermogravimetry analysis (TGA), differential thermal analysis (DTA), Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), optical and scanning electron microscopy (SEM) and particle size analysis. TGA indicated the removal of most of the volatiles, i.e. 30.77 wt% up to about 500degreesC. Crystallization of the Si-Al spinel at 900degrees-970degreesC in gel microspheres was confirmed by DTA and XRD. XRD results also showed the formation of orthorhombic mullite at 1200degreesC. FTIR indicated the sequence of transformations taking place during heat-treatment of gel microspheres at different temperatures. The optical and scanning electron microscopy confirmed the spherical morphology of the gel and calcined particles. Formation of hollow microspheres with a single cavity was identified by SEM. The particle size distribution of the mullite microspheres calcined at 1300degreesC/1h exhibited a size range of 6-100 mum with an average particle size (d(50)) of 22.5 mum. (C) 2002 Kluwer Academic Publishers.