| abstract |
A precursor, in gel form, of an oxide having the formula (I): BaO.n(Al2xCr2yO3), where 1 « n « 6.6, (x + y) = 1, and 0 « y « 0.5, said oxide being derivable from the precursor gel by the application of heat, is prepared by mixing a solution of a barium salt with a solution of an aluminium salt or a solution of an aluminium salt and a solution of a chromium salt, and polymerising the mixture to produce said precursor gel. A mixture suitable for firing to an oxide of the formula (II): BaO.m(Al2xCr2yO3), where 4.6 « m « 6.6; (x + y) = 1; and 0 « y « 0.5, comprises at least one of: (a) barium oxide; (b) a clean thermal precursor of barium oxide; and (c) barium mono-aluminate, BaO.Al2O3; with at least one of: (A) alumina, Al2O3; (B) hydrated forms of alumina, such as boehmite, Al2O3.H2O; and (C) a clean thermal precursor of aluminium oxide; and, where y^_ is not zero, with at least one of (D) chromium (III) oxide, Cr2O3; (E) hydrous chromium (III) oxide; and (F) a clean thermal precursor of chromium (III) oxide, the relative amounts of the components of the mixture being adjusted to give the desired values of m^_ and y^_ when the mixture is heated. |