Volatile Metal Oxides Research Articles

Spectroscopic studies on transition metal fluorides

The use of matrix isolation techniques for the complete structural characterisation of volatile transition metal fluorides and oxide fluorides will be illustrated with particular reference to Ruf6, RuOF4, OsF7 and RhF6.

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO( β 1) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ⩽ n ⩽ 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)−c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than 7 6 monolayers. At higher sulfur coverages the dimer, S 2, was observed to desorb at 1700 K in addition to atomic sulfur desorption.

High-Temperature Oxidation of NbC and TaC at Low Oxygen Pressures

The oxidation of hot-pressed NbC and TaC was studied between 2000 and 2500 K at 10/sup -5/ to 10/sup -3/ mbar oxygen pressure by gravimetric methods. By CO-formation a metallic surface layer is formed which simultaneously reacts with oxygen forming volatile metal oxides. A steady-state layer thickness is reached if the rates of layer formation and metal evaporation are equal. Far away from the steady-state conditions the weight loss and the metal layer thickness deviate essentially from the values predicted by a reaction model. This suggests that for Va metal carbides the model should be modified by considering the early stage of the oxidation.

5180. Volatile metal oxide incorporation in layers of GaAs and Ga 1- xAl xAs grown by molecular beam epitaxy : P D Kirchner et al,J Vac Sci Technol,19 (3), 1981, 604–606

5180. Volatile metal oxide incorporation in layers of GaAs and Ga 1- xAl xAs grown by molecular beam epitaxy : P D Kirchner et al,J Vac Sci Technol,19 (3), 1981, 604–606

Volatile metal oxide incorporation in layers of GaAs and Ga1−xAlxAs grown by molecular beam epitaxy

A model is presented which relates the observed effects of substrate temperature and growth flux magnitudes upon layer quality to the presence of volatile oxides and the thermodynamics of the formation of nonvolatile oxides on the growth surface. A means for reducing oxide contamination is presented and the consequent benefits explored. Ga2O is shown to be a major contaminant of the gallium growth flux, on the order of 0.1%. Certain conditions, specifically low substrate temperature or high arsenic fluxes, can favor the formation of Ga2O3, a nonvolatile contaminant, on the growth surface and hence in the layer. The presence of aluminum, magnesium, or other highly reactive materials makes oxide incorporation increasingly probable. Drastically lowering the Ga2O pressure by several orders of magnitude by adding 0.1% aluminum to the gallium effusion cell should allow growth at lower substrate temperatures and higher rates. Magnesium doping of GaAs at unity efficiency is demonstrated as proof of the relevance of the model. Photoluminescence data from the doping experiment are presented.

Contribution of Activation Products to Fusion Accident Risk: Part I. A Preliminary Investigation

Release of neutron-activation products in severe hypothetical fusion-reactor accidents may constitute a larger health hazard than that of the tritium released at the same time. Significant escape of activation products could result from lithium fires hot enough to melt and partly vaporize activated first-wall materials, or from other accident sequences that bring air into contact with activated structure hot enough to cause the formation of volatile metal oxides. Analysis of three combinations of structural materials and severe accident scenarios has been undertaken for an early conceptual tokamak reactor, using a simple consequence model based on that of the Nuclear Regulatory Commission's Reactor Safety Study (the Rasmussen report) to determine conceivable radiation doses near the plant boundary. No attempt was made to estimate probabilities for such severe events. In the cases of stainless-steel and molybdenum structures subject to massive lithium fires, the boundary doses far exceed those that would be produced by release of the entire plant inventory of tritium and are comparable to the doses similarly calculated for ''worst case'' light water reactor accidents. The case of niobium fusion reactor structure is more favorable. These results, based on an early fusion-reactor design not optimized with respect to safety characteristics, maymore » well portray a worst case picture of fusion accident consequences. They suggest, however, that the large potential safety advantages of fusion compared to fission are not necessarily inherent for all designs and choices of materials, and they motivate attention to the several available strategies for greatly reducing the potential for activation-product release from fusion reactors.« less