Abstract
Shock-recovery experiments have been performed on high-purity mullite polycrystals in the pressure range up to 72GPa. The recovered samples have been examined by an x-ray diffraction method, nuclear magnetic resonance spectroscopy, and transmission electron microscopy. The samples shocked above 65GPa contain an amorphous SiO2 phase and a γ-Al2O3 phase, indicating that a rapid disproportionation reaction of mullite is induced by shock compression. The recovered amorphous SiO2 has a mean Si–O–Si bond angle roughly 7° narrower than that of the fused SiO2 glass, indicating the formation of the densified amorphous SiO2 phase. The γ-Al2O3 phase is crystallized as very fine particles with grain sizes less than 10nm in the matrix of the desified amorphous SiO2 phase. The crystallization of γ-Al2O3 is expected to occur during a pressure-release process owing to the crystal size effect concerning the phase stability of Al2O3.
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