Carbon-containing Impurities Research Articles

Behavior of carbon-containing impurities during plasma synthesis of trichlorosilane

The behavior of CCl4 and CHCl3 admixtures during the plasma synthesis of trichlorosilane via the hydrogenation of SiCl4 in a capacitively coupled radiofrequency (40.68 MHz) discharge was studied. It was shown that the main portion of the impurities undergo chemical transformations yielding silicon carbide and carbon. The degree of conversion determined from the impurity concentrations in the reactant SiCl4 and its hydrogenation products is strongly dependent on the processing parameters and reaches 99.9% for CCl4 and 96% for CHCl3.

Removal of carbon-containing impurities from hydrogen using an electrical discharge

A process is described for removing carbon-containing impurities from hydrogen with the use of an electrical discharge. Hydrogen purified by this process contains no more than 0.1 ppm carbon.

Gas-chromatographic determination of carbon-containing impurities in high-purity sulfur

A gas-chromatographic procedure was developed for the determination of carbon-containing impurities in sulfur with a detection limit of 6 x 10-6%. Carbon-containing substances and elemental carbon can be effectively separated from sulfur samples as CO2 and COS.

XPS investigation of the interaction between ECR-excited hydrogen and the native oxide of GaAs (100)

The composition and the chemical element states of the native oxide of GaAs(100) during ECR-hydrogen cleaning have been investigated. It was found that carbon-containing impurities and arsenic oxide are removed from the surface at a substrate temperature equal to 150°C, while gallium oxide is removed at 350°C. The reduction of arsenic oxide at low substrate temperatures is accompanied by the formation of gallium oxide which diminishes the etching rate.

An XPS study of adsorption of methyl and amyl xanthates on copper

Adsorption of methyl and amyl xanthate ions from aqueous solution onto clean copper substrates has been followed by electron-spectroscopic measurements (XPS and AES techniques). Measurements of methyl xanthate adsorption layers confirm the mechanism of xanthate adsorption previously presented on the basis of ethyl xanthate layer observations. Replacement of preadsorbed hydroxyl groups with xanthate ions with simultaneous removal of the carbon-containing impurities is found to be the predominant adsorption mechanism. The velocity of the adsorption is in the case of amyl xanthate much slower than for methyl xanthate, and the resulting amyl xanthate layer is more heterogeneous. Some oxidation of the substrate surface during the first stage of the contact with xanthate solution is observed for amyl xanthate. No signs of dixanthogen can be observed in the adsorbed layers. This indicates adsorption of individual xanthate ions, without further oxidation to dixanthogen, during the formation of the first monolayer, and formation of copper xanthate molecules in the subsequently deposited part of the adsorption layer.

Ultrapurification of Nitrogen Monitored by Metastable Transfer Emission Spectroscopy

Metastable transfer emission spectroscopy (MTES) has been applied for on-line monitoring of nitrogen purity. The relative efficiencies of various purification techniques for removing oxygen- and carbon-containing impurities have been compared. Total oxygen impurities present initially at several hundred ppm are reduced to <50 ppb with a reactive resin. Several other conventional laboratory purification methods reduce, but do not eliminate, oxygen contamination. Hydrocarbon impurities as methane are reduced only to the 3 ppm level by a liquid-nitrogen cold trap. Further reduction is obtained by the use of the microwave discharge as a chemical reactor prior to liquid-nitrogen trapping. An on-line system for the direct purification and reliable monitoring of nitrogen purity in a flowing gas stream is reported.

Adhesion of pyrolytic chromium coatings to steels

The cohesional rupture of the coatings is a result of growth of normal tensile cracks perpendicular to the basis metal and shear cracks propagating along the boundaries of layers formed owing to an uneven distribution of carbon-containing impurities over the coating thickness. The adhesional rupture of the coatings is brought about by steady growth of shear cracks propagating over the coating/basis interfaces and which are initiated by normal tensile cracks. The mechanism of fracture of the coating/basis composite can change depending on the ratio between the fracture toughness of the coating and the slip toughness of the layer boundaries. Macroscopically, this manifests itself in the fracture or removal of the coating from the basis material or in the fracture of the basis.

Assessment of the relative contribution of atomic mixing and selective sputtering to beam induced broadening in SIMS depth profiling

A thin native silicon oxide layer, covered with hydrogen- and carbon-containing impurities, was sandwiched between a crystalline silicon substrate and an evaporated 35 nm layer of amorphous silicon. SIMS depth profiles of H −, C −, O − and Si − were measured under 4 to 12 keV Cs + impact. The impurity profiles exhibit exponential slopes at the leading and the trailing edges. The characteristic widths describing the exponential tails increase with increasing probe energy. At the leading edges the characteristic widths for H, C and O are identical whereas at the trailing edges the decay lengths are strongly element-specific. The results are interpreted in terms of the relative importance of atomic mixing and selective sputtering. The oscillatory behaviour of the Si − and H − yields, observed in the vicinity of the interface, is also discussed.

An X-Ray Photoelectron Study of Doped and Supported Nickel Oxide

The surfaces of lithium-and chromium-doped nickel oxide and of nickel oxide supported on alumina have been examined by X-ray photoelectron spectroscopy. The concentration of the nickel(III) species increased for the lithium-doped oxide and decreased for the chromium-doped oxide relative to the undoped oxide. The effects of doping were manifested most clearly however by the amount of oxygen-containing species adsorbed on the oxide surface rather than by variations in the nickel(III) peak intensity. Lithium-doped oxides were also shown to reduce more readily than undoped or chromium-doped oxides in the presence of carbon-containing impurities. The rate of reduction is influenced by the activation energy for electron transport which is related to the availability of the nickel(III) species. Supported oxides showed significant surface enhancement of nickel(II) as well as an absence of the nickel(III) species. The nickel(II) species in the supported oxide was not easily reduced to nickel(0).

Oxidation of carburized hastelloy X

In a high temperature gas-cooled reactor (HTGR), small quantities of impurities such as H/sub 2/, H/sub 2/O, CO, CO/sub 2/, and CH/sub 4/ are present in the helium coolant. Prolonged exposure to these impurities at high temperature can result in undesirable mechanical properties due to corrosion. Extensive research has been performed on many candidate alloys to study their properties in HTGR helium, and in addition to oxidation, it has been found that carburization is also a possible problem in HTGR. Carbon in the graphite core, carried out by carbon-containing impurities such as CO and CH/sub 4/, can deposit on and diffuse into metallic components and cause carburization. The purpose of this research was to study the effects of carburization on an accelerated test.

Kaolin refractories for the lining of the iron notch of a 5000-m3 blast furnace

The formation of the black core in large kaolin bricks during the firing process is caused mainly by the reducing medium which develops in the central part of these high-density refractories, graphitizes the carbon, and converts some of the high-valence iron and titanium oxides to low-valence oxides. The principal sources of the reducing medium inside the brick are the spent sulfite liquor and the sulfur- and carbon-containing impurities of the starting material. The likelihood of a black core being formed increases with the density of the green product and the rate at which the kiln temperature is raised. The properties of the light-colored outer and black inner zones are dissimilar so that cracks are formed during heating and cooling. In a joint project with the Zaporozhe Refractories Plant the Ukrainian Scientific-Research Institute for Refractories developed a method of producing high-density kaolin brick without black core. The method was used to produce about 200 tons of bricks measuring 345×150×150 mm and 230×150 ×150 mm for the lining of the iron notches of the 5000-m3 No. 9 blast furnace of the Krivoi Rog Metallurgical Plant.

Thermal and low energy electron bombardment induced oxygen loss of V 2O 5 single crystals: Transition into V 6O 13

Thermal treatment in UHV of clean V 2O 5 single crystals results in homogeneous oxygen loss, involving a rate-limiting surface reaction. Depending upon the pretreatment, aircleaved samples transform topotactically into V 6O 13, or into what we call a phase Q of probable composition V 4O 9 or V 6O 13.5. Low energy electron bombardment of clean UHV-cleaved V 2O 5(010) surfaces produces the transition V 2O 5 → V 6O 13 at room temperature. This effect is attributed to electron beam stimulated reactions. The influence on the transition of carbon-containing impurities is discussed. The nucleation of V 6O 13 on V 2O 5 is explained by a model based on a surface reaction, the rate of which is enhanced by the interaction with contaminating molecules and low energy electron bombardment. The presence of shear planes at the boundary between V 2O 5 and the V 6O 13 nuclei locally enhances the oxygen loss rate and allows the V 6O 13 nuclei to grow into the bulk. The enhanced mobility of the oxygen at these boundaries is thought to influence favorably the oxidation-regeneration rate of the V 2O 5-catalyst.