Carbon Carbide Research Articles

Reverse Water Gas Shift Reaction over Molybdenum Carbide

The reverse water gas shift reaction on molybdenum carbides was studied at 573 K and atmospheric pressure. Molybdenum carbides with various C/Mo atomic ratios were synthesized on γ-Al 2 O 3 using a vertical tube hot-wall chemical vapor deposition (CVD) reactor in a stream of MoCl 5 , benzene, and hydrogen at 1000 K and total pressure of 0.13 kPa. The activities of the molybdenum carbide catalysts prepared by the CVD method were compared with those by the temperature-programmed reaction (TPR). The ratio of the C Is peak of carbidic carbon to the Mo 3d peak of the alumina-supported molybdenum carbide, prepared by changing the benzene/MoCl 5 ratio, was determined by XPS spectroscopy. The activities of the CVD and TPR catalysts with the C/Mo ratio of molybdenum carbides for CO 2 hydrogenation were discussed. The turnover frequencies of the CVD catalysts were 0.347-0.498 s -1 and higher than those of the TPR catalysts.

Silicon-incorporated diamond-like coatings for Si 3N 4 mechanical seals

Amorphous silicon carbide (a-SiC) and silicon-incorporated diamond-like carbon films (DLC-Si) were evaluated as protective and friction reduction coatings onto Si 3N 4 rings. Unlubricated tribological tests were performed with a pin-on-disk apparatus against stainless steel pins with loads ranging from 3 to 55 N and sliding velocities from 0.2 to 1.0 m/s under ambient air and 50–60% relative humidity. At the lowest loads, a-SiC coatings present a considerable improvement with respect to the behavior of uncoated disks since the friction coefficient is reduced to about 0.2 and the system is able to run stably for thousands of meters. At higher loads, however, a-SiC coatings fail. DLC-Si-coated rings, on the other hand, presented for loads up to 10 N a steady-state friction coefficient below 0.1 and very low wear rates. The lowest steady-state mean friction coefficient value of only 0.055 was obtained with a sliding velocity of 0.5 m/s. For higher loads in the range of 20 N, the friction coefficient drops to values around 0.1 but no steady state is reached. For the highest loads of over 50 N, a catastrophic behavior is observed. Typically, wear rates below 5×10 −6 and 2×10 −7 mm 3/N m were obtained for the ceramic rings and pins, respectively, with a load of 10 N and a sliding velocity of 0.5 m/s. Analysis of the steel pin contact surface by scanning electron microscopy (SEM)–energy dispersive X-ray spectrometry (EDS) and Auger spectroscopy revealed the formation of an adherent tribo-layer mainly composed by Si, C and O. The unique structure of DLC-Si films is thought to be responsible for the formation of the tribo-layer.

Evaluation of Young modulus of CVD coatings by different techniques

The silicon carbide (SiC) and pyrolytic carbon (PyC) thin films, deposited by Chemical Vapor Deposition in a fluidized bed, are used in the manufacture of nuclear fuels of high-temperature gas-cooled reactors. The integrity of these coatings ensures the absence of gas-coolant contamination. Therefore, it is important to be able to simulate the thermomechanical behaviour of these layers that involves the exact knowledge of the mechanical characteristics (fracture stress, Young modulus, Poisson's ratio, etc.). The elastic modulus of the SiC and PyC thin films has been evaluated by various techniques, at ambient and hot temperature (until 1600 °C). The studied mechanical tests are the nanoindentation, the acoustic microscopy and the impulse excitation technique. From these tests, the most adapted techniques have been discussed and defined, according to the nature of the layer, the temperature and the method of examination. This work is original because few comparative data exist in the literature for these kinds of layers.

Formation of endothermic carbides on iron and nickel

The formation of endothermic carbides on Fe and Ni is studied using X-ray photoelectron spectroscopy (XPS) by deposition of carbon films from the vapor phase and subsequent annealing steps. The reaction between carbon and metal substrates is measured by shifts in the C 1s photoelectron peaks. By comparison with two elementary carbon photoelectron energies determined from carbon films on unreactive Au substrates, a carbide peak in the C 1s spectra on reactive Fe and Ni substrates is identified. The carbides formed after deposition of carbon films at room temperature are located at the interface between carbon film and metal substrate. We report on the behavior of elementary carbon films with respect to film thickness and thermal treatment leading to carbide formation and carbon diffusion into the bulk.

Metal–support interaction effects on the growth of filamentous carbon over Co/SiO 2 catalysts

The addition of BaO, La 2O 3 and ZrO 2 to the SiO 2 support of a 12 wt.% Co/SiO 2 catalyst modifies the reduction behavior of Co species and leads to changes in metal dispersion. These changes are due to a modified metal–support interaction (MSI) between cobalt species and the Me x O y /SiO 2 (Me x O y : BaO, ZrO 2 and La 2O 3) support. Catalyst characterization by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) have been used to determine the relative strength of the MSI and the results suggest an increasing MSI in the order Co/SiO 2≈Co/BaO/SiO 2<Co/La 2O 3/SiO 2<Co/ZrO 2/SiO 2. The rate of catalyst deactivation during methane decomposition (CH 4⇔C+2H 2) is shown to increase with increasing MSI. Analysis of the used catalysts also shows that an increasing rate of deactivation correlates with an increasing amount of graphitic carbon versus carbidic carbon on the used catalyst. It is suggested that an increase in graphitic carbon is a consequence of a strong MSI that limits carbon removal from the metal surface by filament formation. Consequently, graphitic, encapsulating carbon is formed from the carbon deposited during methane decomposition, leading to deactivation of the catalyst.

Formation and Erosion of Carbon-Containing Mixed Materials on Metals

In this paper we discuss the formation and erosion of multi-component materials (‘mixed materials’) on metals with respect to carbon deposition from the vapor phase. The carbide formation and carbon accumulation at the surface are governed both by chemical reactions and by carbon diffusion into the bulk metal. Carbide formation at room temperature is restricted to several monolayers at the interface between carbon and supporting metal. Additional energy entry into the system either by ion bombardment or by annealing enhances the carbide formation. The carbon amount present at the surface decreases after a material-dependent temperature threshold for diffusion in the case of metallic carbides of transition metals. Taking into account oxygen as a reactive plasma impurity, e.g. by starting with an oxidized surface before carbon deposition, new erosion channels open up in these ternary systems due to chemical interactions, rendering interaction simulations by kinematic Monte Carlo models insufficient. Chemical erosion mechanisms additional to mere kinematic processes are also responsible for carbon erosion by deuterium ion impact, shown for film thicknesses smaller than the ion range. They lead to enhanced carbon film erosion rates compared to erosion by sputtering.

Titanium boron nitride films grown by ion beam assisted deposition: chemical and optical characterization

Titanium boron nitride films with a functionally graded underlayer of Ti/TiN were grown at low temperatures (<200 °C) on a silicon substrate using ion beam assisted deposition. These coatings had a total thickness of 1.5±0.2 μm. They were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The primary phases in the film were identified using XRD and were found to be in agreement with the equilibrium phase diagram. The surface morphology and nanocrystalline nature of the coating were deduced using AFM. The chemical and phase composition was determined from XPS measurements. The refractive indices were deduced from the investigation of the ellipsometric data and were subsequently correlated to the elemental and phase composition. The hardness and elastic modulus were measured and were found to depend on phase composition. The films were implanted with carbon ions. XPS measurements revealed that new phases formed at the surface as a result of carbon implantation, namely, titanium carbide and elemental carbon. Carbon implantation significantly reduced the friction coefficient from 0.5–0.7 to 0.15–0.25. The hardness of the films decreased by 2–5%.

Multiscale modeling of CVD film growth—a review of recent works

Multiscale modeling is a promising route towards the prediction of the microstructure and properties of materials prepared by chemical vapor deposition. The recent advances made in this field of investigation are briefly reviewed in this article. Some basic principles of multiscale modeling are first reminded and numerical simulation strategies are illustrated through a few selected examples, including deposition over patterned substrates and growth of carbon-based materials such as diamond and silicon carbide films and carbon nanotubes. Self-consistent approaches linking macroscopic reactor models and microscopic or atomic scale growth models are particularly emphasized. Actual limitations and future trends in multiscale modeling of film deposition or etching are finally discussed.

Different modification effects of carbidic and graphitic carbon on Ni surfaces

The dehydrogenation of cyclohexene was used as a probe reaction to examine the effect of carbide and graphite modifications on Ni surfaces. The carbon-modified surfaces were first characterized with Auger electron spectroscopy (AES) and near edge X-ray absorption fine structure (NEXAFS), where the transformation from carbidic to graphitic carbon was detected after heating the carbon overlayer to 900 K. The reaction pathways of cyclohexene on the modified Ni surfaces were then compared with those on the unmodified Ni surface using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Thermal desorption studies showed that on the clean Ni surface, 39% of the adsorbed cyclohexene underwent dehydrogenation to benzene, while the remaining 61% completely decomposed to surface carbon and gas-phase hydrogen. The formation of carbide significantly modified the surface chemistry of Ni. For example, upon carbide modification, the selectivity toward benzene increased to 79%. After converting the carbide to graphite, the overall surface activity was reduced by a factor of 2.2, but the selectivity toward benzene remained at 79%. Vibrational studies further revealed that cyclohexene was converted to benzene by 200 K on the modified and unmodified Ni surfaces, and that the degree of interaction was different between benzene and the three surfaces.

The thermal chemistry of saturated layers of acetylene and ethylene on Ni(100) studied by in situ synchrotron x-ray photoelectron spectroscopy

Temperature-programmed x-ray photoelectron spectroscopy was used to study the thermal chemistry of acetylene (C2H2) and ethylene (C2H4) on Ni(100) in the temperature range 90–530 K. The use of a third generation synchrotron light source facilitated the measurement of high-resolution C 1s photoelectron spectra within a few seconds, approaching the ideal of real-time analysis. In a quantitative and quasi-continuous manner, the thermal dehydrogenation pathways are followed. For the acetylene decomposition, acetylide (CCH) and methylidyne (CH) are confirmed as intermediates. For the dehydrogenation of ethylene, a vinyl species (C2H3) is observed. Using the fingerprint capabilities of x-ray photoelectron spectroscopy, acetylene can be identified as the subsequent dehydrogenation product. Upon further heating, acetylide and methylidyne are successively formed on the surface, as in the decomposition experiment starting with acetylene adsorbed at 100 K. For both systems carbidic carbon is formed as the final dehydrogenation product, although with different transition temperatures. Species identification is based on observed vibrational fine structure data and correlation of core-level binding energies with previous literature.

Porous silicon photoluminescence modification by surface treatments and impregnation of carbon based nanoclusters

It has been shown that the photoluminescence (PL) intensity of a porous silicon (PS) sample, covered by a thin SiC film is six times larger compared with the initial PS subjected to rapid thermal annealing (RTA), and a strong up-shift (∼100 nm) is observed relative to the non-treated PS + SiC sample. Clustering processes with participation of SiC compounds on quantum wires cause these spectral changes. Silicon carbide and diamond-like carbon (DLC) films deposited onto PS have been used as modification and functional layers. The long-wave and short-wave region of light emission was observed after their deposition on the PS surface. The SiC film deposition leads to a decreasing of the PL intensity up to three times in the long-wave range (700 nm) compared to the initial PS, and to the appearance of high energy emission (450 nm) attributed to the emission of SiC compounds. The DLC film deposition leads to a modification of the PS properties. The result of RTA is a substantial decreasing of the PL intensity, and a considerable transformation of the spectral bands.

Development of metal–carbon high-temperature fixed-point blackbodies for precision photometry and radiometry

High-temperature fixed-point blackbodies based on Re–C and TiC–C metal–carbon eutectic alloys are being investigated for use as radiance and irradiance sources for precise measurements in radiometry, photometry and radiation thermometry above the conventionally assigned values of temperatures on the ITS-90 scale. Graphite crucibles having inner diameters varying between 4 mm and 10 mm were used to prepare the metal–carbon and metal carbide–carbon eutectics; the cells were designed and manufactured at VNIIOFI, Russia using high-purity materials.The melting and solidification temperatures of the cells were measured. Their reproducibility was investigated. The radiance reproducibility of the Re–C and TiC–C fixed points was found to be from 0.01% to 0.03% at 650 nm wavelength depending on the cell. Preliminary investigations of ZrC–C fixed-point reproducibility have been carried out. The radiance of all measured cells agreed at the solidification point within 0.02%.

Deuterium bombardment of carbon and carbon layers on titanium

Interaction of deuterium ions with highly oriented pyrolytic graphite and thin carbon layers on titanium are studied by X-ray photoelectron spectroscopy (XPS) measurements. The effects induced by the deuterium beam are compared to bombardment with chemically inert argon ions. XPS allows the identification of several carbon states: graphitic carbon, disordered graphitic carbon, ion-induced radiation defects in carbon, and carbidic carbon in the carbon films on titanium. Erosion of the carbon film by deuterium proceeds through two paths: first a chemical erosion with a cross-section of 4.7×10 −17 cm 2 removes elementary carbon. At the same time, carbidic carbon is eroded with a linear yield of 0.003 within the fluence range between 8×10 16 and 1.4×10 17 cm −2.

Structural changes of AgO chains on Ag(1 1 0) by photo- and CO-induced oxygen elimination

We have investigated the structural changes in the added-rows of Ag–O chains at a Ag(1 1 0)(2×1)-O surface due to photo- and CO-induced elimination of O by using scanning tunneling microscopy. The photo-induced elimination occurs only on the surface containing carbidic carbon, resulting in the structural change of the added-rows from (2×1) to (4×1) according to the reduction of O coverage ( θ O). The structural change due to the CO-induced elimination depends on the carbon coverage: the (2×1) structure is retained in spite of the decrease of θ O for the C-contained surface, while the structure changes sequentially from (2×1) to (4×1) and (6×1) for the carbon-free surface. Furthermore, the CO-induced elimination rate in the low θ O on the carbon-free surface is much faster than that on the C-contained surface. These results indicate that the small amount of C atoms play an important role not only in the structural changes associated with the oxygen elimination reactions but also in the kinetics of the oxidation reaction of CO.

Role of sulphur in carburization, carbide formation and metal dusting of iron

AbstractThe kinetics and mechanisms of reactions on iron in CH4–H2 and CO–H2–H2O mixtures have been studied, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. Rate equations and mechanisms of the carburization in CH4–H2 and CO–H2–H2O have been elucidated. Both reactions are retarded by adsorbed sulphur, the rates becoming inversely proportional to the sulphur activity aS ∼ p(H2S)/p(H2) with increasing aS. At carbon activities aC &gt; 1, cementite growth can be started in both gas mixtures on iron but the decomposition of this unstable carbide gives rise to a corrosion process called ‘metal dusting’, i.e. a disintegration of iron and steels to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulphur, which hinders the nucleation of graphite. The stabilizing effect of sulphur on cementite and higher carbides such as Hägg carbide allows fundamental studies to be carried out on their thermodynamics, non‐stoichiometry and diffusional growth mechanisms, and the presence of sulphur will allow iron carbide production in a process of great present interest—the direct reduction of iron ores in carburizing gas mixtures. Kinetic studies in the system Fe–C–S are supplemented by AES and low‐energy electron diffraction investigations on the adsorption and segregation of sulphur on iron and cementite. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Carbonaceous Deposition on Mo/HMCM-22 Catalysts for Methane Aromatization: A TP Technique Investigation

TPO (temperature-programmed oxidation), TPSR (temperature-programmed surface reaction), and TPH (temperature-programmed hydrogenation) techniques are used to investigate carbonaceous deposition on Mo/HMCM-22 catalysts for methane aromatization. Different carbonaceous species and their amounts are distinguished. There are at least three kinds of cokes: carbidic carbon in molybdenum carbide, molybdenum-associated coke, and aromatic-type cokes on acid sites. It is suggested that the aromatic-type cokes on Brønsted acid sites are responsible for catalyst deactivation. Catalytic activity can be restored after temperature-programmed hydrogenation.

Formation mechanism of C/SiC/C multi-layers during self-propagating high-temperature synthesis

In order to study the formation mechanism of a silicon carbide layer during the self-propagating high-temperature synthesis (SHS) reaction between graphite and silicon layers, multi-layers of carbon–silicon carbide–carbon on UO 2 pellets were prepared and analyzed. The carbon and silicon were deposited by thermal decomposition of propane in a chemical vapor deposition (CVD) unit and a microwave-pulsed electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) unit using silane and propane gases, respectively. The activation energy for the formation of silicon carbide determined by differential scanning calorimetry (DSC) depended on the initial carbon precursors. The final product layer was a fine, crystalline beta-silicon carbide. The numerically estimated value of the combustion limit was 0.06 at an initial temperature of 1200 °C, which supported the finding that pre-heating above the temperature was required for the formation of the silicon carbide. From thermal analysis and microstructure observations, the formation mechanism of the silicon carbide layer included carbon diffusion at the interface between liquid silicon and silicon carbide.

Simultaneous growth of silicon carbide nanorods and carbon nanotubes by chemical vapor deposition

Silicon carbide nanorods and carbon nanotubes are grown simultaneously on Ni-coated Si(001) substrates by chemical vapor deposition. The nanorods have a non-uniform diameter (average ≈100 nm) significantly larger than that of the nanotubes (<30 nm). Tips of the nanorods are decorated with catalyst particles. Both these features indicate that the SiC nanorods are not obtained through the chemical conversion of nanotubes. We demonstrate that the relative amounts of nanorods and nanotubes can be controlled by adjusting the Ni film thickness. Variants of our approach are potentially useful for creating hybrid architectures of high-aspect ratio nanostructures in one growth step.

Stepwise Conversion of Methane over Supported Metal-Boron Amorphous Alloy Catalysts

The stepwise conversion of CH4 to higher hydrocarbons over HMCM-22 zeolite supported metal-boron amorphous alloy catalysts has been investigated, including: (1) the influence of metals (Co, Ni, Pt, Rh and Ru) of the catalysts on the reaction; (2) the promotional effect of V on the catalytic behavior of the catalysts; (3) the influence of hydrogenation pressure and CH4 decomposition temperature; and (4) the nature of carbon species. It is found that the yield of C+ 2 hydrocarbons is strongly dependent on the metals. Good yields of C+ 2 hydrocarbon are reached only on the supported NiB and CoB catalysts. The probability of C--C chain growth is increased by V promotion without seriously affecting the activities of CH4 decomposition and hydrogenation. The ease of carbon removal via hydrogenation is strongly affected by the CH4 decomposition temperature. Increasing hydrogenation temperature has a negative effect on the yield of C+ 2 hydrocarbons. XPS measurements show that a carbide(-like) carbon species is active and selective for hydrogenation to produce higher hydrocarbons. Its activity/selectivity is greatly reduced at high CH4 decomposition temperatures, mainly due to transition of the reactive carbidic to unreactive graphitic form. Graphite/filamental carbons were found to be formed at high CH4 decomposition temperature.

Effect of carbon on the tensile properties of Nb–Mo–W alloys at 1773 K

Ternary Nb–Mo–W alloys of various composition and carbon and oxygen contents were prepared to investigate the deformation and fracture behavior by tensile and compression tests at 1773 K. The fracture mode changes from transgranular to intergranular with increasing molybdenum and tungsten contents as substitutional elements of the VIA group, and with increasing oxygen content. A certain amount of carbon addition to Nb–5Mo–15W alloy causes niobium carbide (Nb 2C) precipitation along grain boundaries and improves the ductility accompanied by a change in the fracture mode. It is suggested that carbide precipitation and carbon segregation at grain boundaries are responsible for the ductility improvement in carbon-doped Nb–5Mo–15W alloys.