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This paper reports on the synthesis of magnesium aluminate spinel (MAS) powders and consolidation of MAS ceramics by hydrolysis-induced aqueous gelcasting (GCHAS). The MAS powders surface passivated against hydrolysis were dispersed in an aqueous-organic premix solution using suitable dispersants. The consolidation of green bodies occurred under ambient conditions by adding a polymerization initiator, a catalyst, and AlN (0-4.06 wt%) as a cosetting agent. Green samples with exceptionally high green strength (B21 MPa) were obtained in the presence of 4.06 wt% AlN. Sintered (16501C, 1 h) MAS ceramics consolidated by GCHAS exhibited mechanical properties that are comparable to those consolidated by dry pressing.

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A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.