We adapt the level-set method to simulate epitaxial growth of thin films on a surface that consists of different reconstruction domains. Both the island boundaries and the boundaries of the reconstruction domains are described by different level-set functions. A formalism of coupled level-set functions that describe entirely different physical properties is introduced, where the velocity of each level-set function is determined by the value of the other level-set functions.

The synthesis of bulk tungsten carbides by carburization of W metal or of WO3with mixtures of CH4in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4–H2mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3by CH4–H2does not take place, but that the carburization occurs via the reduction of WO3to W metal. The rate of reduction of WO3and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4partial pressure. The extent of reduction of WO3into W metal required for carburization which takes place also depends on CH4partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid.

Starting from bulk carbides, the removal by hydrogen of free carbon, oxygen from passivation and from carbidic carbon was studied in temperature-programmed experiments. Treatment of WC1+x Oy with flowing hydrogen shows that the production of methane is maximum at about 700°C, above which it decreases. However, at the final pretreatment temperature, a residual partial pressure of methane is still observed, but this residual methane pressure decreases with temperature and disappears. After a few hours' treatment in hydrogen the final solid corresponds approximately to the stoichiometry WC without extra carbon. Such a treatment was monitored by X-ray diffraction in a diffractometer allowing temperature-programmed experiments from room temperature up to 900°C. It was shown that no metallic tungsten appears below 700°C. The amount of metal formed during treatment at 750°C is very small, and above 750°C it increases with temperature. However, even after a two hour treatment at 800°C, followed by treatment at 900°C, large proportions of carbide are maintained. No W2C formation is detected. W2C, after passivation, is very sensitive to hydrogen treatment, since a thermogravimetric experiment showed that, after a five hour treatment in hydrogen at 600°C, W2C was completely decarburized into W. W2C seems relatively stable at 400°C. A treatment of Mo2C containing large amounts of free carbon in hydrogen at 700°C leads to MoC0.47, that is, to clean Mo2C. On the other hand, Cr3C2 behaves differently in hydrogen since some decarburization occurs before complete elimination of excess free carbon. By temperature-programmed X-ray diffraction in a O2(2%)-N2 mixture from room temperature to 500°C, it is shown that oxidation of WC to WO3 starts at 400°C and is very fast at 500°C. No oxycarbides or intermediary oxides such as WO2 were detected.

In this paper, results of a research work conducted in four laboratories in the frame of the ECC Research Science Program are presented (Contract ST 2J 0467C(TT)).The preparation of bulk tungsten carbide by reaction of WO3 with a CH4/H-2 mixture is studied by TPR. After passivation, the material is characterized by ESCA, X-Ray diffraction, electron microscopy and electron diffraction, physical adsorption (BET using N2 and Kr) and chemical adsorption of H-2 and CO, TPR under H-2, O2, and inert gaz. These last studies allowed to evidence the importance of the protection of the material by surface free carbon, in opposition with the high sensitivity of the material to oxygen when ''liberated'' of its carbon protection. The test reactions used are reforming reactions of hexanes and alkylcyclopentanes. Labelled hexanes with C-13 are used for the determination of the reaction mechanisms and comparison with the Pt series metals. Surface free carbon resulting namely from the preparation procedure inhibits all catalytic activity. In situ activation under H-2 at 1073K produces clean hexagonal WC, characterized by a catalytic activity essentially for cracking. This last catalyst is highly sensitive to traces of O2, that induces important modifications in selectivity. The oxycarbidic type catalyst exhibits selectivities similar to traditional bifunctional catalysts like Pt on zeolites.