Preparation of Codeposited Al₂O₃-TiO₂ Powders by Vapor Phase Reaction Using Combustion Flame

Abstract

Spherical ultrafine (average 30-50nm) codeposited Al₂O₃-TiO₂ powders were prepared by the oxidation of vapor mixture of AlCl₃ and TiCl₄ in a combustion flame. γ-Al₂O₃ and anatase and rutile TiO₂ were detected in the as-prepared powders, but β-Al₂TiO₅, which was one of the major phases in the Al₂O₃-TiO₂ powders prepared from AlBr₃ and TiCl₄ by r.f. plasma or in a tubular reactor, was scarcely observed. The simultaneous deposition of Al₂O₃ and TiO₂, leading to the formation of β-Al₂TiO₅, became difficult due to the adoption of less reactive AlCl₃. It appeared that the anatase TiO₂ particles were not accompanied by Al₂O₃, while the rutile TiO₂ crystallites dispersed in the Al₂O₃ contained noticeable amounts of Al₂O₃ metastably. The solution of Al₂O₃ into TiO₂ appeared to promote the phase change of TiO₂ from anatase to rutile. The calcination experiment was performed to investigate the stability of phases in the codeposited powders. The TiO₂ appeared to accelerate the formation of α-Al₂O₃ due to the solution of TiO₂ into γ-(or δ-)Al₂O₃. An unknown phase was detected in the powders calcined at 1200°C, whose X-ray diffraction pattern was similar to that of Na₂Ti₂Ti₆O₁₆. The new phase was considered to be a bronze-type compound containing H⁺ and Al³⁺, possibly expressed by H_2-X(Al_YTi_1-Y)₂Ti₆O_16-Z.