Direct observation of the formation of alumina phase by metallic Al solid-SiO₂ solid reaction

Abstract

The formation of Al₂O₃ phases by the solid-solid reaction of a metallic Al layer evaporated on a SiO₂ amorphous grain has been induced by heating above 600°C in vacuum (1×10^-6 Pa). The distortion process of the amorphous SiO₂ grains by the formation of Al₂O₃ have been directly imaged by in-situ TEM observation. A partly deposited Al layer covered the SiO₂ grains after heating at 750°C, and γ-Al₂O₃ grains of about 25 nm diameters were formed on the SiO₂ surface. Upon the growth of Al₂O₃, the SiO₂ grain decomposed into a mixture of metallic Si and SiO₂ and disappeared as a result of sublimation due to the formation of SiO_x at high temperatures. The present result on dust surface dynamics will become an important field with respect to the metamorphism of grains from the astromineralogical viewpoint.