KINETIC MEASUREMENTS OF OXYGEN DISSOLUTION INTO NIOBIUM SUBSTRATES - INSITU X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES

The stabilities of Nb2O5 films occurring on niobium or Nb-1%Zr substrates were examined during ultrahigh vacuum annealing. In situ X-ray photoelectron spectroscopy (XPS) was used to determine the concentration of species present at the surface. The metal-oxide systems investigated showed similar behavior. They did not form stable layers and surface reduction from Nb2O5 to NbO due to dissolution of oxygen into the bulk metal was observed at temperatures above 598 K. The kinetics of this reaction were determined using anodically deposited Nb2O5 overlayers (33 nm thick). Kinetic data were obtained by continuously monitoring the O 1s peak during annealing in the temperature range 623-723 K at base pressures of around 10-8 Pa. The reaction was first order and proceeded with a measured activation energy of 174±6 kJ mol-1 and a frequency factor of 8.9×1010.0±0.3s-1. Similar kinetic data using oxide overlayers 11 and 66 nm thick demonstrated that the dissolution process was independent of initial oxide thickness and controlled by reaction at the oxide-metal interface. The shift of Nb 3d 3 2- 5 2 spectra to lower binding energies during vacuum annealing suggested the following reduction sequence. {A figure is presented} Continuous monitoring and deconvolution of the Nb 3d 3 2- 5 2 spectra allowed independent rate measurements of each step. The reaction for the Nb5+ to Nb4+ transition was first order and proceeded with a measured activation energy of 134 ±16 kJ mol-1 and a frequency factor of 8.0×107.0±0.4 s-1. The Nb4+ to Nb2+ transition was also first order and the activation energy and the frequency factor were calculated using two different methods. A single-point method, using the maximum concentration of Nb4+, yielded an activation energy of 180±18 kJ mol-1 and a frequency factor of 5.4×1010.0±0.5 s-1. A multi-point method, using all measured Nb4+ concentrations in a computer simulation, yielded an activation energy of 177±16 kJ mol-1 and a frequency factor of 1.0×1011.0±0.4 s-1. These two methods compare favorably with one another and based on the rate constants, the Nb4+ to Nb2+ transition was determined to be the rate-limiting step in the overall reduction. The presence or absence of 1% zirconium did not affect the kinetics of oxygen dissolution during our studies. © 1990.