Oxidation behavior of rhenium at high temperatures

Abstract

Oxidation of polycrystalline Re has been studied at temperatures from 1,500 to 1,900 C. During oxidation volatile Re-oxides were emitted in the form of smoke and resulted in dramatic surface recessions of the samples. XRD analysis indicated that ReO[sub 3] was the primary oxide present in the condensed vapor deposits. Preferential oxidation of Re, manifested by the formation of crystallographic facets, was noted on the oxidized surfaces. Etchpits and islands bounded by high-symmetry planes showing a 6-fold symmetry were formed thereon, suggesting that the kinetics of oxidation are slower on close-packed planes. It is demonstrated that surface recession rate, dR/dt, which is equivalent to weight change per unit area and time (dW/A[center dot]dt), can be used to characterize oxidation behavior. The overall surface recessions of both the PM-Re and CVD-Re generally increased with oxidation duration and temperature. The CVD-Re exhibits lower recession rates than the PM-Re in the temperature range examined, which is attributable to the stronger basal-plane texture and larger grain size of CVD-Re. Oxidation of PM-Re was observed to be anisotropic. At 1500[degree]C, oxidation rates on the direction I (rolling plane) were higher. At higher temperatures (1,700 and 1,900 C), on the other hand, an opposite result wasmore » obtained. The differential oxidation rate of the PM-Re is suggested to originate from the synergistic effects of temperature-dependent oxidation behavior and basal-plane texture that have evolved during sample processing. This hypothesis is consistent with the fact that similar activation energies were obtained for the oxidation of CVD-Re and PM-Re (I).« less