Effect of MgO addition on mullite synthesis "in situ"

Reference	Presenter	Authors (Institution)	Abstract
14-045	Thays Allana Olcoski	Olcoski, T.A.(universidade estadual de ponta grossa); Chinelatto, A.S.(Universidade Estadual de Ponta Grossa); Chinelatto, A.L.(Universidade Estadual de Ponta Grossa); Salem, R.E.(Universidade de São Paulo);	Mullite is a crystalline phase formed in the silica-alumina binary system. It can be used in traditional and advanced applications. However, it is necessary to use high temperatures and long times, due to the low interdiffusion rate of Si4+ and Al3+ ions within the lattice and due to the difficulty of sintering. Thus, one way to optimize the production process and reduce the cost is to reduce the mullitization temperature. This can be achieved using submicrometer grain size precursors or using a sintering aid, which promotes the formation of a liquid phase during sintering. The aim of this study was to verify the effect of MgO addition in the obtention and "in situ" sintering of mullite. For this, colloidal silica and aluminum hydroxide were used as raw materials for production of mullite, with MgO as sintering aid. In the stoichiometric composition of mullite, there were added different amounts of MgO (0.8; 1.6; 2.4; 3.1; 4 wt %). The compositions were milled in ball mill and, after drying, were pressed and sintered at different temperatures (1350, 1450, 1550°C). The compositions were characterized by dilatometry, X-ray diffraction, scanning electron microscopy (SEM) and apparent density measurements. The results of X-ray diffraction showed that the mullite phase was obtained at all the sintering temperatures studied, and that, the spinel phase also appeared with increase of the MgO amount. The increasing on the amount of MgO also promoted an increase in sample densification, and by the dilatometry up to 1500°C, it could be seen that the starting temperature of linear shrinkage of the compositions decreases considerably with MgO addition. From the SEM images, it was observed that in addition to a higher densification, the presence of MgO caused grain growth. Thus, it can be concluded that the presence of MgO was effective to aid in the formation and densification of the mullite at lower temperatures, reducing the porosity and altering the formed microstructure.
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