Thermal processes of volatile RuO2 in nanocrystalline Al2O3 matrixes involving γ→α phase transformation

Abstract

Thermal processes of the volatile compound RuO2 in Al2O3 matrixes involving phase transformation have been investigated. In this model system, the secondary phase, rutile RuO2, can be formed and preserved in Al2O3 matrixes throughout a wide temperature range (400-1000 °C) due to the nanocrystalline Al2O3 that turns on or off matrix reactions with the surrounding RuO2. At 300-400 °C, nanocrystallites (2 nm) of γ-Al2O3 are formed in the presence of ruthenium. Single-phase RuO2 is formed from its precursor compound at 400 °C, while the intermediate species of Ru0 and RuO2·xH2O are detected at lower temperatures. The imbedded ruthenium shows a catalytic effect on the early formation of nanocrystalline γ-Al2O3 matrix, and the resultant high-surface-area matrix in turn allows the ruthenium to be fully oxidized to RuO2 in air. The γ-Al2O3 matrix is converted to α-phase at ca. 1000 °C. This transformation is promoted by the nanosized RuO2. A lowering of 60 °C in the transformation temperature has been observed with the presence of only 0.5 wt % of ruthenium. During the phase transformation, an abrupt matrix-crystallite growth takes place (from 2-3 to 40 nm), which leads to a denser α-Al2O3 matrix, protecting RuO2 from thermal evaporation at high temperature. The utilization of other important thermal processes observed (such as evaporation-condensation) in fabrication of nanostructured materials is also addressed.