Shock-induced disproportionation of mullite (3Al_2O_3·2SiO_2)

摘要

Shock-recovery experiments have been performed on high-purity mullite polycrystals in the pressure range up to 72 GPa. The recovered samples have been examined by an x-ray diffraction method, nuclear magnetic resonance spectroscopy, and transmission electron microscopy. The samples shocked above 65 GPa contain an amorphous SiO_2 phase and a γ-Al_2O_3 phase, indicating that a rapid disproportionation reaction of mullite is induced by shock compression. The recovered amorphous SiO_2 has a mean Si-O-Si bond angle roughly 7° narrower than that of the fused SiO_2 glass, indicating the formation of the densified amorphous SiO_2 phase. The γ-Al_2O_3 phase is crystallized as very fine particles with grain sizes less than 10 nm in the matrix of the desified amorphous SiO_2 phase. The crystallization of γ-Al_2O_3 is expected to occur during a pressure-release process owing to the crystal size effect concerning the phase stability of Al_2O_3.