Fabrication of magnesium aluminate (MgAl2O4) spinel foams

This paper reports on a novel-processing route for fabricating magnesium aluminate (MgAl₂O₄) spinel (MAS) foams from aqueous suspensions containing 30–35 vol.% solids loading. A stoichiometric MAS powder formed from alumina (71.8%) and magnesia (28.2%) at 1400 °C was surface passivated against hydrolysis in an ethanol solution of H₃PO₄ and Al(H₂PO₄)₃ at 80 °C for 24 h. Stable aqueous suspensions with 30–35 vol.% solids loading were prepared using the surface passivated MAS powder with the help of tetra-methylammonium hydroxide (TMAH) and an ammonium salt of polyacrylic acid (Duramax D-3005) employed as dispersing agents. An aqueous solution of N-cetyl-N,N,N-trimethylammonium bromide (CTMAB) was utilized to create foam in aqueous MAS suspensions by mechanical frothing. Liquid foam was then consolidated in non-porous moulds by introducing a polymerization initiator and a catalyst under ambient conditions. Dried (at >90 °C for 24 h) MAS foams were then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS bodies out of an un-passivated stoichiometric MAS powder, and, dense as well as foams out of alumina were also prepared in this study. The sintered properties of MAS and alumina ceramics were characterized by various means and thus obtained results are presented and discussed in this paper. The sintered MAS foams exhibited a porosity of about 74–76% and a compressive strength of about 4–7.2 MPa inline to values reported for other ceramic foams in the literature.