APPLICATION OF MICROWAVE-INDUCED PLASMA EMISSION SPECTROSCOPY (MIPES) TO THE STUDY OF GAS/SOLID REACTIONS (KINETICS, FLUORINATION, TANTALUM/PLATINUM).

摘要

This work describes the development of an emission spectroscopic technique which facilitates (i) instantaneous gas/metal reaction rate measurements over a large temperature range in a single experimental run, and (ii) surface mass balances which are necessary for mechanistic understanding of high temperature gas/solid reactions. In this technique, a low pressure microwave induced plasma (MIP) excites characteristic emission from the atoms formed by the dissociation of the gaseous product species of the gas/metal reaction in a low pressure flow reactor.;The results of the metal evaporation and F-atom gasification experiments have revealed that the metal atom (Pt, Ta) emission line intensity is proportional to the metal flux from the surface over at least two decades (log-log plot). This linear response confirms that the concentration of product species in the plasma is sufficiently low as to not affect its nature. Moreover, our sublimation kinetics experiments are evidently sensitive to metal atom fluxes at the gas/metal interface as low as about 10('13) atom/cm('2) sec.;Flash evolution experiments have revealed that fluoride films form on both platinum and tantalum with thicknesses varying from 10 to 2000 monolayers. Fluorine adsorption on the film, dissolution into the film, diffusion in the film, reaction at the film/metal interface are reaction steps common to the kinetic models discussed in the present work. A diffusion coefficient of 10('-9) cm('2)/sec has been estimated for fluorine atom diffusion through the tantalum fluoride scale.;The MIPES flash evolution experiments are the first that directly show that thick fluoride films form on platinum and tantalum during reaction with fluorine. The new technique has been used to rapidly obtain instantaneous relative rate measurements in transient as well as steady-state experiments. It can also be applied to the study of chemical reactions with condensed stable product phases, so long as the product phase has a higher vapor pressure than that of the substrate. (Abstract shortened with permission of author.).;The reaction systems of choice involve fluorine, for which adequate reaction rates have been reported. The low dissociation energy of the molecule (157kJ/mol) makes the study of the heterogeneous reactions of atomic fluorine important because nearly complete dissociation can be achieved and the resulting atoms usually undergo heterogeneous reactions more rapidly than do molecules.