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Formation of Mullite by Rapid Expansion of High Pressure Suspensions of Alumina and Silica in Supercritical CO2

Daniel To1, Sameer Dalvi2, Rajesh Davé2, and Sankaran Sundaresan3. (1) New Jersey Center for Engineered Particulates, New Jersey Institute of Technology, 138 Warren Street, YCEES 208, Newark, NJ 07102, (2) Chemical Engineering, New Jersey Institute of Technology, 138 Warren Street, YCEES 222, Newark, NJ 07102, (3) Chemical Engineering, Princeton University, Princeton, NJ 08544

Nanocomposite materials are gaining popularity because of their unique properties. High interfacial area and the interactions at these interfaces between the different constituents, result in properties that are not present in the individual bulk components. Mullite is one of such nanocomposite material and finds wide applications because of its high creep resistance and refractory properties. It is a naturally occurring aluminum silicate and is often produced synthetically because of its rarity. One common method of producing mullite is to mix aluminum oxide and silicon oxide particles in liquid solvents and then firing the mixture at temperatures as low as 1250 oC. However, removing the suspending liquid can be expensive in terms of both time and resources. Therefore, the objective of this work was to use dry mixing methods to improve the quality of mixing of alumina and silica for the purpose of reducing the temperature for mullite formation and to increase % conversion of a mixture to mullite.

Various environmentally friendly dry mixing methods have been used to prepare nanoparticle mixtures of alumina (dp = 13 nm) and silica (dp = 16 nm) at the stoichiometric ratio of 3:2 for the purpose of forming mullite at sintering temperatures above 1250 oC. One such method is Magnetically Assisted Impact Mixing, which uses sub-millimeter magnets propelled by oscillating magnetic field to mix the nanopowders. The quality of mixing, which was determined by Energy Dispersive X-Ray Spectroscopy (EDS) was correlated to the amount of mullite formed as measured by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC).