Preparation and properties of barium mono- and hexaaluminate

Abstract

Several barium compounds are of interest as raw materials for refractories and ceramics. Barium oxide is refractory (m.p. ca 1920 8C), as are the binary oxides of barium and aluminum. Barium monoaluminate (m.p. ca 1800 8C), which hydrates [1], can be the basis of a cement or grouting material, and the hexaaluminate BaO . 6Al2O3 (m.p. ca 1900 8C) is a possible engineering ceramic [2±6]. Pressed and sintered barium hexaaluminate can produce mechanically strong components (MOR ca 600 MPa), and barium hexaaluminate ceramic ®bres may be prepared by sol-gel processing [7]. Barium hexaaluminate, like calcium and strontium hexaaluminate, has the space group D6h ± hexagonal crystal system with two molecules per unit cell [8]. A gel precursor of barium monoaluminate or barium hexaaluminate was prepared [2] by mixing a solution of a barium salt with a solution of an aluminum salt to give the oxide stoichiometry BaO . Al2O3 or BaO . 6Al2O3 and forming a gel; this was ®red to obtain barium monoaluminate or hexaaluminate. The preferred aluminum salt is an aluminum chlorohydrate Al2(OH)6-m Clm, with m in the range of 1±5. Desirably, m is 1 or 2; when m ˆ 1 the salt is available as an aqueous solution (`Chlorhydrol' solution, nominally 22±23 wt % Al as the Al2O3 equivalentÐWilfrid Smith Ltd., Edgware, UK). The barium salt may be barium chloride (hydrated or anhydrous); but barium bromide dihydrate, which is more soluble in water than barium chloride dihydrate [9], is usually preferable for preparing barium monoaluminate gel precursors. Acetates that can form alkaline solutions, ammonium acetate for instance, are known [10] to gel aluminum chlorohydrate solutions, and aluminum chlorohydrate solutions containing barium. Table I gives the preparation and properties of barium monoand hexaaluminate precursor gels. Gel times were determined at ambient temperature, unless otherwise stated. The precursor gels were air-dried, and then sintered after milling, to give barium monoaluminate or barium hexaaluminate. Barium acetate is a basic acetate, suitable for forming a gel from `Chlorhydrol' solution. It gives better control of gel formation and was investigated [2] as a gel-inducing agent. For precursor gels of oxide stoichiometry BaO . Al2O3, the gel-precursor solution was prepared by using barium bromide dihydrate. Table II shows the effect of the barium bromide: barium acetate ratio on the properties of the resulting gels. Gel times were measured at ambient temperature. The clear, rigid gels were airdried and then milled, to give barium monoaluminate when sintered; for use as a cement or grouting composition, the gel should be air-dried and then milled to a ®ne particle size. For precursor gels of oxide stoichiometry BaO . 6Al2O3, either barium chloride or barium bromide may be used with barium acetate to prepare the gel precursor solution. Table III shows the effect of the barium chloride: barium acetate ratio on the gel time, determined at ambient temperature. The