Simultaneous Carbon Deposition Research Articles

MOCVD Growth of Tungsten Ditelluride Thin Films

The growth of tungsten ditelluride (WTe2) thin films on c-plane sapphire substrates was demonstrated by metalorganic chemical vapor deposition (MOCVD) using tungsten hexacarbonyl (W(CO)6) and diethyltelluride (DETe) as precursors for W and Te, respectively, in a H2 carrier gas. The effects of substrate temperature, reactor pressure, precursor and carrier gas flow rates, and chalcogen-to-metal ratio on the WTe2 growth rate and film properties were determined. Substrate temperatures less than 500 °C were required to deposit WTe2 which was found to be beneficial to suppress simultaneous carbon deposition from the DETe precursor. The films exhibited a fine-grained morphology independent of growth temperature. The layered crystalline 1 T’ phase of WTe2 was confirmed using high resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (TEM) which revealed that the films were predominantly c-axis oriented with in-plane twist. X-ray photoelectron spectroscopy (XPS) demonstrated that the WTe2 films were stable in air for a few days but eventually exhibited oxidation.

Experimental studies of catalyst deactivation due to carbon and sulphur during CO2 reforming of CH4 over Ni washcoated monolith in the presence of H2S

AbstractThis study presents the CO2 reforming of CH4 over Ni coated monolith catalyst at 800°C and 101.325 kPa. The high CH4 to CO2 ratio employed in this study is similar to the CH4:CO2 ratio of >1 found in biogas. Cordierite monolith samples (0.258 channels per m2) washcoated with alumina are used for the experimental purpose. The study considers the combined deactivation effect due to sulphur poisoning and fouling due to carbon deposition. Four different cases with respect to the introduction and removal of H2S are considered. The rate of deactivation due to simultaneous carbon deposition and sulphur poisoning is much faster than the individual poisoning processes. The catalyst shows almost stable operation for 6 h without the presence of in the feed stream. From the conversion studies, it appears that the pre‐treatment of catalyst samples with H2S leads to negligible sulphur coverage. The sulphur poisoning effect is also found to be reversible.

Mechanistic Insights for Dry Reforming of Methane on Cu/Ni Bimetallic Catalysts: DFT-Assisted Microkinetic Analysis for Coke Resistance

Density functional theory (DFT) calculations have been utilized to evaluate the complete reaction mechanism of methane dry reforming (DRM) over Ni2Cu (111) bimetallic catalyst. The detailed catalytic cycle on Ni2Cu (111) catalyst demonstrated superior coke resistance compared to pure Ni (111) and Ni2Fe (111) reported in the literature. Doping Cu in the Ni–Ni network enhanced the competitive CH oxidation by both atomic O and OH species with the latter having only 0.02 eV higher than the 1.06 eV energy barrier required for CH oxidation by atomic O. Among the C/CH oxidation pathways, C* + O* → CO (g) was the most favorable with an energy barrier of 0.72 eV. This was almost half of the energy barrier required for the rate-limiting step of CH decomposition (1.40 eV) and indicated enhanced coke deposition removal. Finally, we investigated the effect of temperature (800~1000 K) on the carbon deposition and elimination mechanism over Ni2Cu (111) catalyst. Under those realistic DRM conditions, the calculations showed a periodic cycle of simultaneous carbon deposition and elimination resulting in improved catalyst stability.

Simultaneous carbon and tungsten thin film deposition using two thermionic vacuum arcs

Carbon and tungsten films were deposited sequentially and simultaneously from two separate plasmas produced by the thermionic vacuum arc (TVA) method. Total film thickness and atomic concentration of C and W were measured for different substrate positions and arc parameters and a comparison between sequential and simultaneous deposition of carbon and tungsten is presented. For the simultaneous case, while keeping the same discharge parameters as in the sequential case, the carbon composition percentage is enhanced despite the presence of the tungsten plasma. When only the carbon deposition rate is increased 10 times, the tungsten atomic concentration drops to a lower level, while the content of carbon becomes dominant for the substrates positioned near the carbon anode.

Novel Nano- and Micro-Processing by Photoactivation of Methylene Iodide Precursor

In this letter we present methylene iodide as a versatile precursor for nano- and microprocessing. It allows the deposition of Carbon micro/nano-fibers and nanotubes via chemical vapor deposition (CVD) using different metal catalysts (Pd, Ni, Fe, Co and Mn) on Si (100) substrates. Here, a versatile and low-cost IR lamp technique is employed to induce the deposition process. With this method carbon features could be obtained already at temperatures much lower than with common techniques. When utilizing 193nm excimer laser pulses, the precursor is activated to induce etching of Si and simultaneous deposition of carbon, at high local separation. The existence of thermally defined chemical boundaries suggests the possibility of high confinement and the formation etching/deposition structures of sub-micron sizes.

Boron erosion and carbon deposition due to simultaneous bombardment with deuterium and carbon ions in plasmas

Erosion of boron out of a thin film exposed to deuterium edge plasmas and the simultaneous carbon deposition have been investigated in the tokamak TEXTOR-94 and simulated by means of a dynamic Monte Carlo code. The calculated results are compared with some observations (colorimetry, spectroscopy and AES) during and after the exposures. The implantation of carbon impurities strongly changes the effective boron sputtering yield of the film, which results into a lowering of the film erosion and a formation of thick carbon deposits. A strong decrease in the observed BII line emission around a surface location far from the plasma edge can be explained by a carbon deposition on the film. The calculated carbon depth profiles in the film, depending on the distance of the exposed surface from the plasma edge, are in reasonable agreement with measurements by AES after the exposures. Although simultaneous surface erosion and carbon deposition can be simulated, the calculated erosion rate is larger, by a factor of 2, than the observations by colorimetry at the early stage of the exposure.

Investigation of Erosion and Deposition on Wall Components of TEXTOR-94

The paper describes in the first part the formation of carbon flakes up to 10–20 µm thickness (average growth rate 2 nm/s) on the graphite tiles of the toroidal belt limiter. This occurred as a consequence of a slight change of the geometry and turned parts of the surface area from net erosion into net deposition zones. The possible influence of the morphology on this behaviour is discussed in the second part by means of an erosion experiment where the gradual disappearance of a boron substrate could be discriminated from the simultaneous carbon deposition on the surface. The two counter-acting processes co-exist within 10–30 µm distance and lead to an extremely non-uniform carbon deposition even in net erosion zones. The carbon agglomeration coincides with surface imperfections, e.g. grooves, but agglomeration by temperature enhanced mobility is not excluded. The changeover from net deposition to net erosion averaged over larger distances can still be observed and is due to the hydrogen and carbon fluxes in the SOL. This is confirmed by Monte-Carlo code calculations.

Phenomenological model of ion irradiation effects on the possibilities of diamond nucleation enhancement

The possibilities of diamond nucleation enhancement during a PVD process involving simultaneous carbon deposition and ion irradiation are investigated via a phenomenological model including processes of carbon adsorption, sputtering and formation of activated sites of nucleation. The kinetics of the development of the surface topography is studied on the basis of the stochasticnature processes of sputtering and deposition. It is shown that in the case of high deposition and low sputtering rates the maximum of the distribution of activated atoms is located on the sides of the surface microstructures. During irradiation by high-energy heavy ions, where sputtering prevails, the deposition of activated carbon atoms is found to be maximum on the top of microstructures. The theoretical conclusions are in qualitative agreement with CVD experimental ones, showing that diamond nucleation preferentially occurs on the edges of surface topography.

Two-dimensional measurements of thin film erosion and deposition by colorimetry of interference colors and its application in fusion devices

The paper describes time resolved observations of the erosion of a thin (≈ 105 nm) and transparent a-B:D film on a roof-like graphite test probe (60 mm × 63 mm) in the scrape-off layer plasma of the tokamak Textor-94. Steady state of the erosion pattern was almost achieved after 22 discharges (121.3 s total exposure time). After each of the pulses the interference color pattern in illuminating light of 3200 K color temperature was observed by means of conventional video technique and the color coordinates R, G, B measured. Since they can be related uniquely to the film thickness contour maps and profiles could be evaluated and erosion rates determined. Net deposition on parts of the surface is also observed. Most of the results can be described by erosion due to the hydrogen flux and the simultaneous deposition of carbon and other impurities from the background plasma. The deposition of impurities onto the a-B:D bulk seems to reduce the erosion rates which prolongs its lifetime. This interpretation is backed by AES and NRA analysis of the boron content and by EPMA.

Nucleation of diamond in the channels of electrical breakdown in SiC films

Abstract Experimental studies were performed on the nucleation of diamond crystals on the surface of dielectric SiC layers enriched by carbon during simultaneous carbon deposition and 20 keV H + irradiation at 800 °C. The stochiometric SiC layer with thickness 10 nm was formed employing processes of dynamic and cascade mixing of deposited carbon with silicon substrate under 160 keV Xe + irradiation at 800 °C. After deposition of a 20 nm carbon layer at room temperature onto irradiated Si-SiC-amorphous-C, well-shaped diamond crystals of size 7–10 μm were found, randomly distributed on the surface. A diamond nucleation mechanism in the channels of electrical breakdown in SiC is proposed. This is followed by coalescence of the nuclei formed on the surface into large crystals, activated by surface discharge currents.

Electron beam deposition of gold nanostructures in a reactive environment

Electron beam deposition (EBD) is a maskless technique suitable for the fabrication of nanometer scale structures. Metals can be deposited from an organometallic gas, but simultaneous carbon deposition typically yields grossly impure (∼25% metal) deposits. We have found that the metal content of the deposited solid is dramatically improved by performing the whole EBD process in a reactive gaseous environment containing a source of oxygen (O2 or H2O) in addition to the organometallic gas. With simple procedures we prepared Au deposits showing significantly diminished C content (up to 50% metal) as the partial pressure of O2 (or H2O) is increased in the gas.

Wear reduction by ion implantation-assisted deposition

The investigation of hard carbon coatings resulting from a simultaneous nitrogen ion implantation process in an oil diffusion pumped vacuum chamber is described. The thin surface layer system produced is a good examples for studying the depth profiling of light elements up to charge number 20 by elastic recoil detection analysis (ERDA) in a Bragg ionization chamber. The characterization of the surfaces at different conditions by ERDA depth profiling and the measurement of the correlated wear and friction behaviour allow optimization of the technological conditions for simultaneous carbon deposition and nitrogen implantation.

An activation-deactivation model for catalytic deposition of carbon

A sequence of reaction steps that corresponded to the fundamental phenomena of adsorption/desorption of a hydrocarbon and hydrogen, hydrogenation/dehydrogenation surface reactions, carbon diffusion and precipitation, catalytic site activation, and deactivation involved in catalytic deposition of carbon from hydrocarbons is proposed. Rate expressions derived from the reaction sequence yielded a kinetic model for simultaneous carbon deposition, catalytic site activation, and catalyst deactivation, which exhibited trends qualitatively similar to experimental results for Ni-catalyzed catbon deposition from propylene/hydrogen mixtures. It was found that increasing the H 2 concentration decreased the rates of deposition and poisoning but did not affect the rate of site activation. Thus, higher H 2 concentrations lead to lower initial deposition rates, but the total level of deposition increases because of reduced deactivation rates. The hydrocarbon concentration affected the rates of activation, poisoning, and deposition similarly so that the ultimate level of deposition was independent of the hydrocarbon concentration. However, as the hydrocarbon concentration increased, the time over which deposition occurred decreased due to an increased rate of deactivation. Optimized fits of the model to experimental data at various temperatures showed that the model agreed well with data at H 2 concentrations from 0.0 to 45%, propylene concentrations up to 3%, and temperatures up to 550°C.

Carbon-boron-nitrogen compounds obtained between 800 and 1000°C by low temperature chemical vapor deposition

The conditions of simultaneous deposition of carbon and boron nitride from a gaseous mixture of ammonia, boron trichloride, and acetylene, are investigated between 800 and 1000°C under reduced pressure. Using a hot-wall horizontal reactor, three successive steps and the corresponding zones are identified. From the characterization of the deposits it is shown that C-B-N mixtures of various composition can be obtained. Thermodynamic calculations are used to improve the understanding of the behavior of this complex chemical system. A schematic understanding of the reaction process is proposed.

Application of thermodynamics to the examination of the diamond CVD process II. A model of diamond deposition process from hydrocarbon-hydrogen mixtures

A model of the diamond deposition process from gas phases activated by thermal methods, consisting of the four following stages, is proposed: (1) activation of a hydrocarbon-hydrogen mixture; (2) transport of the activated gas phase to the substrate surface; (3) simultaneous deposition of carbon in sp 3 and sp 2 hybridization at the growth surface; (4) etching of co-deposited sp 2 hybridized carbon. Carbon deposition with maintenance of sp 3 hybridization can proceed only on the condition that the gas phase is in a non-equilibrium state, generally called the activated state, in which a non-equilibrium partial pressure of atomic hydrogen is present. A definition of the activated state is proposed. Depending on the degree of activation carbon is deposited in stage (3) by potentially many different reactions. When the gas phase is activated to a “high”, “very high” or “extremely high” degree, the most important reactions, according to which carbon is deposited, are the reactions C 2H 2(g) + 2 H(g) = C(diamond & graphite) + CH 4(g) , C 2H(g) + 3 H(g) = C(diamond & graphite) + CH 4(g) , or 2 CH(g) + 2 H(g) = C(diamond & graphite) + CH 4(g) , respectively. The most important reaction, according to which graphite is etched or gasified in stage (4), is reaction C( graphite) + 4 H( g) = CH 4( g). Atomic hydrogen in a super-equilibrium concentration is necessary not only to etch graphite in stage (4) but also to precipitate carbon in stage (3). Conclusions following from the model are in good agreement with the results of experimental investigations reported in the current literature.

Coating Microspheres with Zirconium Carbide-Carbon Alloy by Iodide Process

Simultaneous deposition of carbon and zirconium from vapor produced by the reaction between methyl iodide vapor and zirconium sponge was studied with the application of a spouted bed constituted by a funnel carrying a charge of alumina microspheres, which were blown upward and held in dynamic suspension by a jet of the vapor and gases spouting from the funnel. The purpose of the experiment was to determine the conditions favorable for obtaining a coat of zirconium carbide-carbon alloy on the microspheres. Deposition of the vapor on the microspheres, leading to the formation of the carbon alloy coating, was found to take place at temperatures exceeding 1,100°C. The C/Zr ratio of the deposited coat was found to increase with deposition temperature. The hydrogen concentration in the spouting gas affected both the deposition yield and the chemical composition of the deposit. Repeated use of the sponge was found to impair its performance due to deactivation by premature deposition of carbon.