Gelcasting of Magnesium Aluminate Spinel Powder

A stoichiometric MgAl2O4 spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41-45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of alpha-Al2O3 and calcined caustic MgO at 1400 degrees C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H3PO4 and Al(H2PO4)(3). The as-treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly-acrylic acid (Duramax D-3005) as dispersing agents. The as-obtained stable suspensions were gelcast, dried, and sintered at 1650 degrees C for 1-3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze-dried granules and sintered along with gelcast samples. Near-net-shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650 degrees C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties.