Phase evolution in Al₂O₃ fibre prepared from an oxychloride precursor

Abstract

The formation of polycrystalline α-alumina during the heat treatment of aluminium oxychloride fibres occurs via transitions through various intermediate crystalline alumina phases. The different phases show a strict pseudomorphism, i.e. the external shape of the crystal is retained and there is an orientational relationship between the crystal axes, before and after the phase transition. Consequently, the phase sequences and microstructure are interdependent and controlled by the heat-treatment conditions, which determine the final properties of the fibres. The effects of water content in the fibre formulation, and sintering temperature and heating rate on the phase development from amorphous oxychloride to crystalline α-alumina fibres, were investigated. The transition path for the phase evolution during heat treatment to 1200°C (when the formation of α-alumina is completed) was established and correlated with fibre microstructure. In fibres with higher water content, the transition temperature for γ- and δ-alumina phase formation was higher. Only for a very high heating rate was θ-alumina observed as an intermediate phase during phase evolution. The axial porosity appeared to be eliminated more rapidly in the fibres at temperatures below 1200°C. Consequently, residual porosity, comprising closed pores, in the final stage of sintering, was mainly observed radially oriented toward, and on, the fibre surface. A high diametral shrinkage was measured and plotted as a function of the sintering temperatures used for fibre heat-treatment.