Impurity control in sapphire whiskers

Abstract

The effect of selective chemical removal of impurity elements on the structure and high temperature stability of sapphire whiskers has been investigated. For Compagnie Thomson Houston (CTH) sapphire whiskers (with an “as-grown” impurity concentration of 6% silicon and approximately 2% (sodium + potassium + calcium)), treatment in a 20% H₂SO₄-20% HF solution (Solution A) effectively removed the grown-in second phase particles and formed a surface reaction product. The reaction product could be removed from the whiskers with distilled water and the “cleaned” whisker annealed at 1100° C for 17 h (in high purity argon) without attendant whisker disintegration. A similar effect was noted for Thermokinetic Fibers Inc. (TFI) whiskers (approximately 0.2% silicon) after treatment in Solution A. In both cases the improvement in high temperature stability is correlated with a reduction in silicon concentration to < 0.15%.

An alternative treatment in H₃PO₄ (Solution B) resulted in preferential removal of (sodium + potassium + calcium) and aluminium from CTH whiskers. The grown-in second particles were retained in the whiskers and coarsened into a variety of configurations during an 1100° C/17 h anneal, which may be attributed to the geometry produced by differential whisker thinning.