Shock-induced disproportionation of mullite (3Al2O3•2SiO2)

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 SiO2 phase and a gamma-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 gamma-Al2O3 phase is crystallized as very fine particles with grain sizes less than 10 nm in the matrix of the desified amorphous SiO2 phase. The crystallization of gamma-Al2O3 is expected to occur during a pressure-release process owing to the crystal size effect concerning the phase stability of Al2O3. (C) 2009 American Institute of Physics. [DOI:10.1063/1.3177320]