Synthesis of spinel powders using hydrothermal techniques involves reacting
the MgO- and Al2O3-bearing precursors in water/steam at appropriate pressures
and temperatures. After reaction, the resulting spinel-forming precursors can
be pyrolysized at appropriate temperatures to obtain reactive spinel powders.
Fujiyoshi et al. (11) recently synthesized MA spinel precursors by hydrothermally
heating gibbsite and magnesium acetate [Mg(CH3COO)2] for 48 hr at
15.2 MPa and 2008C. They found that spinel could be prepared by heating the
precursors at temperatures as low as 5008C. Similar to the sol-gel process, hydrothermal
synthesis markedly decreases the spinel synthesis temperature while
the resulting spinel powders are homogeneous and reactive. The drawbacks
with this technique are that it sometimes uses similar precursors to the sol-gel
process and requires a high-pressure autoclave. Furthermore, long reaction
times are generally needed to complete the reaction (often several days).