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

The formation of AI2O3 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 SiOx 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.