Diwakar Garg

Kinetics of chemical vapor deposition of tungsten carbide

Abstract Hard tungsten carbide based coatings were deposited by chemical vapor deposition (CVD) at low temperatures onto metal substrates by hydrogen reduction of tungsten hexafluoride (WF 6 ) in the presence of dimethyl ether (DME). These coatings contain a mixture of tungsten and tungsten carbide in the form of W 3 C, W 2 C, or a combination of W 3 C and W 2 C depending on reaction conditions. They exhibit layered morphology, which is primarily responsible for their excellent erosion and wear-resistance properties. The deposition of these coatings with layered morphology is believed to involve two simultaneous reactions, leading to the deposition of tungsten and tungsten carbide phases with crystallite size varying between 50 and 150 A. Since the deposition of these coatings in a large-scale commercial reactor requires knowledte about the rate and order of simultaneous reactions, a pilot-scale CVD reactor was designed and operated to investigate reaction kinetics. This paper describes an experimental approach to determine the apparent reaction order and kinetic rate constant for depositing the coating with optimal properties. It also discusses the significance of these kinetic parameters on scaling up reactor design.

A first principles study of water dissociation on small copper clusters

Water dissociation on copper is one of the rate-limiting steps in the water-gas-shift (WGS) reaction. Copper atoms dispersed evenly from freshly made catalyst segregate to form clusters under the WGS operating conditions. Using density functional theory, we have examined water adsorption and dissociation on the smallest stable 3-dimensional copper cluster, Cu(7). Water molecules are adsorbed on the cluster sequentially until full saturation at which no direct water-copper contact is sterically possible. The adsorption is driven mainly by the overlap between the p-orbital of O atom occupied by the lone pair and the 3d-orbitals of copper, from which a fractional charge is promoted to the 4s-orbital to accommodate the charge transfer from water. Water dissociation on the Cu(7) cluster was investigated at both low and high water coverage. It was found that water dissociation into OH and H is exothermic but is inherently a high temperature process at low coverage. At high coverage, the reaction becomes more exothermic with fast kinetics. In both cases, water can catalyze the reaction. It was found that direct dissociation of the OH species is endothermic with a significantly higher barrier at both low and high coverage. However, the OH species can readily react with another adjacent hydroxyl group to form an O adatom and water molecule. Our studies indicate that the basic chemical properties of water dissociative chemisorption may not change significantly with the size of small copper clusters. Similarities between water dissociation on copper clusters and on copper crystalline surfaces are discussed.

Effect of catalyst distribution in coal liquefaction

Abstract The effect of the mode of catalyst addition in the liquefaction of eastern Kentucky Elkhorn #2 coal was studied in a continuously stirred tank reactor. Particulate addition of iron in the form of pyrite significantly catalyzed the liquefaction reaction; coal conversion increased, and oil yield was greater by more than a factor of two, both at 825 and 850°F (714 and 728 K, respectively). Although pyrite concentration had a negligible effect on product distribution, the mode of catalyst addition was significant. Impregnation and molecular dispersion with 1 wt. % iron based on coal gave either similar or improved product distribution compared to the addition of 3.5 wt. % iron in the form of particulate pyrite. Significantly lower hydrocarbon gas make and hydrogen consumption were noted with impregnation and molecular dispersion than with particulate addition. SRC sulfur content was marginally higher with impregnation, but was lower with molecular dispersion. Solvent hydrogen content increased with particulate addition, but decreased with impregnation and molecular dispersion.

Relative activity of transition metal catalysts in coal liquefaction

Abstract The catalytic activity of transition metals in coal liquefaction was studied and compared. Impregnation of coal with transition metals significantly increased oil production and asphaltene and preasphaltene conversion in coal liquefaction. Overall, coal conversion increased marginally and hydrocarbon gas production decreased slightly with metals. Iron impregnation was more active than cobalt, nickel, and molybdenum in preasphaltene conversion, whereas the other metals were more active than iron in asphaltene conversion. Hydrogen consumption decreased with all metals. The quality of generated solvent decreased with iron, but increased with other metals. Significant benefits were observed by using iron and molybdenum together; simultaneous impregnation of coal with iron and molybdenum significantly increased coal, asphaltene, and preasphaltene conversion, as well as oil production compared to individual metals. In addition, a mixture of iron and molybdenum decreased hydrocarbon gas production and increased hydrogen consumption and the quality of generated solvent over iron alone.

Catalyst performance in hydroprocessing solvent-refined coal

Abstract The hydroprocessing of solvent-refined coal (SRC) was studied in a fixed-catalyst basket reactor using modified Shell 324 NiMo catalyst supported on alumina. At a reaction temperature of 775°F (413°C), a hydrogen pressure of 2000 psig, and a space velocity of 0.9 g feed/(g catalyst h), the SRC conversion was 57%. As expected, SRC conversion, hydrocarbon gas production, total desulfurization, and hydrogen consumption increased with increasing reaction temperature. However, oil production, which was also expected to increase, changed only slightly with increasing temperature. The effect of time on stream on SRC conversion and catalyst activity was determined at 775°F (413°C), while other process conditions were maintained constant. Catalyst activity changed only slightly during the first 294 h (catalyst age = 246 g SRC/g catalyst) of SRC hydroprocessing. However, catalyst activity decreased drastically thereafter. Analysis of the spent catalyst revealed that catalyst surface area, pore volume, and pore diameter decreased significantly, while chemical analysis showed an unusually high level of sodium deposition (4.2%), which was probably due to the high sodium level in the feed. A mass balance on the metals in the feed revealed that only small amounts of iron and titanium from the feed SRC were deposited on the catalyst, whereas most of the sodium was retained by the catalyst. The decrease in catalyst activity could have been due to a reduction in intraparticle diffusion due to coke and metals deposition, to catalyst poisoning by sodium, or both. Finally, it was concluded that the SRC hydroprocessing experiments should be carried out long enough to cover the entire range of catalyst activity to obtain a meaningful catalyst replacement rate.

Boron-Doped Plasma Enhanced Chemical Vapor Deposition of ZnO Thin Films

Highly conducting boron-doped ZnO thin films have been grown at low temperature (200°C) by plasma enhanced chemical vapor deposition using diethyl zinc [Zn(C 2 H 5 ) 2 ], carbon dioxide (CO 2 ), triethylboron, and argon gas mixtures. The minimum resistivity is 85% for the visible spectrum). The free-electron concentration, determined by Hall effect measurement, was as high as 1 × 10 21 /cm 3 with a mobility of 13.5 cm 2 /V s. For this deposition approach, the low-reactivity oxidant CO 2 allows a uniform film growth over a large area, and the low-toxicity triethylboron allows a simple and convenient boron doping.

Plasma assisted chemical vapor deposition on ‘‘three‐dimensional’’ substrates

A novel plasma assisted chemical vapor deposition reactor forms ceramic coatings on ‘‘three‐dimensional’’ surfaces of complicated parts such as cutting tools and bearing surfaces. The reactor chamber features a unique rf‐driven electrode configuration to establish uniform, large volume, low temperature plasmas from which the coatings are deposited. Highly adherent silicon nitride coatings formed from silane/ammonia/argon plasmas have hardness values up to 30 GPa. Variations of methane/silane feed gas ratios produce microcrystalline silicon carbide films with C/Si atomic ratios ranging from 0.4 to 3; the coatings consist of free Si and stoichiometric SiC; SiC, free Si and diamond‐like carbon (DLC); or SiC and DLC, depending on the methane/silane fraction. Hardness varies monotonically between about 10 and 60 GPa as the carbon fraction increases. SiC coatings significantly reduced wear rates in abrasive wear tests; Si–SiC coatings effectively protected molybdenum samples during a 180 h oxidation test in 650...

Reactor design for chemical vapor deposition of tungsten carbide coatings

Abstract A chemical vapor deposition (CVD) technique has been developed to deposit hard tungsten carbide coatings at low temperatures onto a variety of metal substrates by hydrogen reduction of tungsten hexafluoride (WF6) in the presence of dimethyl ether (DME). These coatings exhibit excellent erosion and wear-resistance properties when deposited onto parts with uniform composition and thickness. They were deposited onto three-dimensional parts with uniform chemical composition by carefully controlling the deposition temperature and reagent partial pressures, but attempts to deposit them with uniform thickness and properties in a conventional tubular CVD reactor were not successful. The non-uniform deposition of these coatings had been thought to be caused by poor distribution of gas flow and temperature in the reactor. Therefore, a program was undertaken to develop a pilot-scale CVD reactor to deposit these coatings with uniform thickness and properties. The program involved devising a method to control fluid flow and temperature profiles in the reactor by iterative interaction between computerized flow and temperature simulation and experimentation. This led to the development of specially designed reactor internals to deposit coatings with uniform thickness and properties. This paper discusses efforts involved in developing a reactor design to deposit tungsten carbide coatings with uniform thickness and properties.

Simulation of the Growth of Copper Films for Micro and Nano-Electronics

Copper is a desirable material to replace aluminum-based alloys in the metallization of very large-scale integrated circuits, due to its high conductivity and reduced electromigration. However, practical ways to grow high quality copper layers on top of the common materials used as barrier layers on silicon is problematic, because of several issues, like poor adhesion and reduced coverage of high aspect-ratio surface features. We will describe efforts in developing procedures and chemical compounds for the growth of high quality films of copper on barrier layers. Our work is based on ab-initio calculations of the energetics and dynamics of the growth processes involved, including the interaction of the chemicals with the surfaces. The calculations presented use density functional theory, and in particular the SIESTA code.