Synthesis of Mullite by Means of Transferred and Nontransferred Arc Plasma Melting

Arc plasma melting technique is a simple method for the synthesis of high melting point materials. In this article, mullite was synthesized by transferred arc plasma (TAP) and nontransferred arc plasma (non-TAP) melting processes, and the results were compared. The mixes of alumina and silica powders (3:2 mole ratios) were ball milled for four hours and then melted in an arc plasma torch, used in transferred and nontransferred mode, at 5 kW input power and two minutes of processing time. Argon gas was used as a plasma-forming gas. The crystalline phases and the microstructural features of the melted samples were determined by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. A complete crystallization of mullite, with dense, thick whiskers-shaped crystals, was achieved in TAP processing of the alumina/silica system. On the other hand, the non-TAP process produced porous mullite along with a small amount of residual alumina phase. Differential thermal analysis (DTA) curves of the synthesized mullite samples allowed a deeper understanding of the mechanisms of mullite formation during the two different processes.