Showing papers on "Carbide published in 2000"

Method for depositing nanolaminate thin films on sensitive surfaces

Abstract: The present method provides tools for growing conformal metal nitride, metal carbide and metal thin films, and nanolaminate structures incorporating these films, from aggressive chemicals. The amount of corrosive chemical compounds, such as hydrogen halides, is reduced during the deposition of transition metal, transition metal carbide and transition metal nitride thin films on various surfaces, such as metals and oxides. Getter compounds protect surfaces sensitive to hydrogen halides and ammonium halides, such as aluminum, copper, silicon oxide and the layers being deposited, against corrosion. Nanolaminate structures (20) incorporating metal nitrides, such as titanium nitride (30) and tungsten nitride (40), and metal carbides, and methods for forming the same, are also disclosed.

Molybdenum carbide catalysts for water-gas shift

Abstract: Molybdenum carbide (Mo2C) was demonstrated to be highly active for the water–gas shift of a synthetic steam reformer exhaust stream. This catalyst was more active than a commercial Cu–Zn–Al shift catalyst under the conditions employed (220–295°C and atmospheric pressure). In addition, Mo2C did not catalyze the methanation reaction. There was no apparent deactivation or modification of the structure during 48 h on‐stream. The results suggest that high surface area carbides are promising candidates for development as commercial water–gas shift catalysts.

Deposition of transition metal carbides

Abstract: The present invention relates generally to a method of depositing transition metal carbide thin films. In particular, the invention concerns a method of depositing transition metal carbide thin films by atomic layer deposition (ALD), in which a transition metal source compound and a carbon source compound are alternately provided to the substrate. A variety of metal and carbon source gases are disclosed. The methods are applicable to forming metal carbide thin films in semiconductor fabrication, and particularly to forming thin, conductive diffusion barriers within integrated circuits.

A process for converting a metal carbide to carbon on the surface of the metal carbide by etching in halogens

Abstract: A process for the synthesis of carbon coatings on the surface of metal carbides, preferably SiC, by etching in a halogen-containing gaseous etchant, and optionally hydrogen gas, leading to the formation of a carbon layer on the metal carbide. The reaction is performed in gas mixtures containing about 0 % (trace) amounts to 100 % halogen-containing gaseous etchant, e.g., Cl2, and about 0 % to 99.9 % H2 (hydrogen gas) at temperatures from about 100 °C to about 4,000 °C, preferably about 800 °C to about 1,200 °C, over any time range, maintaining a pressure of preferably about one atmosphere, to about 100 atmospheres.

Synthesis, microstructure and properties characterization of disintegrated melt deposited Mg/SiC composites

Abstract: In the present study, elemental and silicon carbide reinforced magnesium materials were synthesized using an innovative disintegrated melt deposition method followed by hot extrusion. Microstructural characterization studies revealed the presence of minimal porosity and completely recrystallized matrix in all the unreinforced and reinforced samples. In the case of reinforced magnesium samples, a fairly uniform distribution of SiC particulates and good SiC-Mg interfacial integrity was realized. The results of microhardness measurements revealed an increase in the brittleness of the SiC-Mg interfacial region with an increase in the amount of SiC particulates. Results of physical and mechanical properties characterization revealed that the increasing presence of SiC particulates led to an increase in hardness and elastic modulus, does not affect 0.2% yield strength and reduces the ultimate tensile strength, ductility, work for fracture and coefficient of thermal expansion. An attempt is made to correlate the results of physical and mechanical properties testing with that of the microstructural characterization.

Study of the Temperature-Programmed Reaction Synthesis of Early Transition Metal Carbide and Nitride Catalyst Materials from Oxide Precursors

Abstract: The synthesis of high surface area carbide and nitride materials from binary and ternary oxides of vanadium, niobium, tantalum, molybdenum, and tungsten, suitable for use as catalysts for a wide ra...

Dry reforming of methane to synthesis gas over supported molybdenum carbide catalysts

Abstract: The dry reforming of methane at elevated pressure over supported molybdenum carbide catalysts, prepared from oxide precursors using ethane TPR, has been studied. The relative stability of the catalysts is Mo2C/Al2O3>Mo2C/ZrO2>Mo2C/SiO2>Mo2C/TiO2, and calcination of the oxide precursor for short periods was found to be beneficial to the catalyst stability. Although the support appears to play no beneficial role in the methane dry reforming reaction, the alumina-supported material was stable for long periods of time; this may be important for the production of pelletised industrial catalysts. The evidence suggests that the differences in the stabilities may be due to interaction at the precursor stage between MoO3 and the support, while catalyst deactivation is due to oxidation of the carbide to MoO2, which is inactive for methane dry reforming.

Titanium Carbide Nanocrystals in Circumstellar Environments

Abstract: Meteorites contain micrometer-sized graphite grains with embedded titanium carbide grains Although isotopic analysis identifies asymptotic giant branch stars as the birth sites of these grains, there is no direct observational identification of these grains in astronomical sources We report that infrared wavelength spectra of gas-phase titanium carbide nanocrystals derived in the laboratory show a prominent feature at a wavelength of 201 micrometers, which compares well to a similar feature in observed spectra of postasymptotic giant branch stars It is concluded that titanium carbide forms during a short (approximately 100 years) phase of catastrophic mass loss (>0001 solar masses per year) in dying, low-mass stars

Preparation of Molybdenum Carbides Using Butane and Their Catalytic Performance

Abstract: The synthesis of high surface area molybdenum carbides from molybdenum oxide and butane has been studied via temperature-programmed reaction (TPRe), X-ray diffraction (XRD), scanning electron microscopy (SEM), 13C solid-state NMR, infrared (IR), and Raman spectroscopy (LR) The molybdenum oxygen/carbon system passes through four phase transitions before transforming into the pure Mo2C carbide Carbon exists in two forms within high surface area molybdenum carbide The initially produced molybdenum carbide has a face-centered-cubic (fcc) structure but is gradually converted into the hexagonal-close-packed (hcp) structure with increasing carburization temperature, and eventually at high temperature coke is deposited During the early stages, MoO3 is reduced by H2, but at higher temperatures, butane also takes part in the reduction and, besides being consumed in the formation of carbide, is catalytically converted into methane, ethane, propane, and benzene The high surface area of the molybdenum carbide mat

The formation of carbon filaments upon decomposition of hydrocarbons catalysed by iron subgroup metals and their alloys

Abstract: The structure of filamentous carbon formed upon catalytic decomposition of hydrocarbons on iron subgroup metals and their alloys is considered. The regularities of the deposition of carbon on these metals are generalised. The carbide cycle mechanism of carbon formation is considered in detail. The growth models of some morphological modifications of filamentous carbon are discussed. The bibliography includes 151 references.

Synthesis and characterization of Hf2PbC, Zr2PbC and M2SnC (M=Ti, Hf, Nb or Zr)

Abstract: Predominantly single phase (92–94 vol.%), fully dense samples of Hf2SnC, Zr2SnC, Nb2SnC, Ti2SnC, Hf2PbC and Zr2PbC were fabricated by reactively HIPing the stoichiometric mixture of the corresponding elemental powders in the 1200–1325°C temperature range for 4–48 h. The latter two, fabricated here for the first time, required a further anneal of 48–96 h to increase the volume fraction of ternary phases. Hf2PbC and Zr2PbC are unstable in ambient atmospheres at room temperature. As a family these compounds are good electrical conductors; the lowest and highest values of the electrical conductivities were, respectively, 2.2×106 (Ω.m)−1 for Hf2SnC and 13.4×106 (Ω.m)−1 for Hf2PbC. The Vickers hardness values range from 3 to 4 GPa. All compounds are readily machinable. The Young's moduli of Zr2SnC, Nb2SnC and Hf2SnC are, respectively, 178, 216 and 237 GPa. The thermal coefficients of expansion, TCE's, of the ternaries scale with those of the corresponding binaries, and are relatively low for such readily machinable solids. The lowest TCE belonged to Nb2SnC [(7.8±0.2)×10−6 K−1], and the highest to Ti2SnC [(10±0.2)×10−6 K−1]. The TCE's of Hf and Zr containing ternaries cluster around (8.2±0.2)×10−6 K−1. All the synthesized ternary carbides were found to dissociate into the transition metal carbide and the A-group element in the 1250–1390 °C temperature range.

Carbon‐Induced Corrosion of Nickel Anode

Abstract: In high‐temperature solid oxide fuel cells where natural gas is used as a fuel, high‐carbon‐activity environments can be encountered in the anode compartment. Under these conditions, nickel could corrode by a process known as metal dusting. In the present study, metal dusting corrosion of pure nickel is simulated in high‐carbon‐activity environments at temperatures between 350 and 1050°C. The focus of this research is to understand reaction mechanisms by characterizing interfacial processes at the nanometer level. Nickel corrodes by a combination of carbon diffusion and precipitation in the bulk metal and atom migration through surface carbon deposits. The nature of the carbon deposit is important in the overall corrosion process. At lower temperatures closer to about 350°C, nickel forms a carbide. , which is rather stable and does not decompose. © 2000 The Electrochemical Society. All rights reserved.

Oxidation and dissolution of tungsten carbide powder in water

Abstract: The oxidation and dissolution of tungsten carbide powder dispersed in water was investigated using X-ray photoelectron spectroscopy (XPS) and leaching studies. We found that the WO 3 surface layer on the oxidised tungsten carbide powder dissolves readily at pH > 3 with the tungsten concentration increasing linearly with time. Adding cobalt powder to the tungsten carbide suspension resulted in a significant reduction of the dissolution rate at pH 3 solution chemistry and the Co 2+ adsorption at oxide/water interfaces.

Grain-boundary structure and precipitation in sensitized austenitic stainless steel

Abstract: Grain-boundary carbide precipitation and intergranular corrosion in sensitized austenite stainless steel were examined by transmission electron microscopy to clarify the effect of grain-boundary structure on precipitation and corrosion The propensity to intergranular precipitation depends strongly on the grain-boundary structure Carbide precipitates tend to be detected at grain boundaries with higher Σ values or larger deviation angles (Δθ) from low-Σ coincidence site lattice misorientations The more ordered boundary requires a longer time for intergranular carbide precipitation and corrosion than less ordered or random boundaries

Synthesis of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel thermal spraying

Abstract: A nanostructured WC-12 pct Co coating was synthesized using mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying. The variation of powder characteristics with milling time and the performance of the coatings were investigated using scanning electron microscope (SEM), X-ray, transmission electron microscope (TEM), thermogravimetric analyzer (TGA), and microhardness measurements. There is no evidence that indicates the presence of an amorphous phase in the sintered WC-12 pct Co powder, and the binder phase in this powder is still crystalline Co. Mechanical milling of up to 20 hours did not lead to the formation of an amorphous phase in the sintered WC-12 pct Co powder. During the initial stages of the milling, the brittle carbide particles were first fractured into fragments and then embedded into the binder phase. This process gradually formed polycrystal nanocomposite powders of the Co binder phase and W carbide particles. The conventional cold welding and fracturing processes primarily occurred among the Co binder powders and polycrystal composite powders. The nanostructured WC-12 pct Co coatings, synthesized in the present study, consist of an amorphous matrix and carbides with an average particle diameter of 35 nm. The coating possesses an average microhardness of 1135 HV and higher resistance to indentation fracture than that of its conventional counterpart.

Electronic structure and bonding properties in layered ternary carbide Ti3SiC2

Abstract: Ab initio calculations based on the density-functional pseudopotential approach have been used to study the electronic structure and chemical bonding in layered machinable Ti3SiC2 ceramic. The calculations reveal that all three types of bonding-metallic, covalent and ionic-contribute to the bonding in Ti3SiC2 The high electric conductivity is attributed to the metallic bonding parallel to the basal plane and the high modulus and high melting point are attributed to the strong Ti-C-Ti-C-Ti covalent bond chains in the structure.

Low temperature deposition of nanocrystalline silicon carbide thin films

Abstract: Silicon carbide thin films have been deposited by reactive magnetron sputtering in a pure hydrogen plasma at substrate temperatures, Ts, ranging between 100 and 600 °C. The infrared (IR) absorption spectra and the transmission electron microscopy observations reveal an onset of crystallization at Ts as low as 300 °C. The crystalline fraction increases with Ts and reaches a value of about 60% for Ts=600 °C. Both refractive index n and room temperature dark conductivity σd(RT) show quite consistent behaviors with the structural evolution of the layers. Thus n increases from 1.9 to 2.4 and σd(RT) improves by six orders of magnitude when Ts is raised from 100 to 600 °C.

Composite vapor deposited coatings and process therefor

Abstract: A vapour deposited coating on the surface of a metal, such as a stainless steel substrate, obtained in a plasma vapour depositing device having magnetic vapour stream filtering means. The vapour deposited coating comprises pairs of a metal layer and a ceramic metal compound (“ceramic”) layer. The metal layer deposited is one of titanium, chromium, vanadium, aluminum, molybdenum, niobium, tungsten, zirconium, hafnium or alloys ofthese metals. The ceramic layer is a nitride, carbonitride, carbide, oxycarbide or oxynitride of one or an alloy of the above metals. The substrate surface is optionally ion nitride prior to the vapour deposition of the metal—ceramic layers. The substrate bearing pairs of metal—ceramic layers may be subsequently heat treated. The obtained coating on a metal substrate exhibits high wear resistance and hardness and low surface roughness.

Sputtered silicon carbide thin films as protective coating for MEMS applications

Abstract: There is a need for chemically resistant coatings that protect the exposed surface of microfluidics components. Pinhole free films with low stress and a good uniformity on flat and inclined surfaces are required. In this study, amorphous silicon carbide (SiC) thin films have been deposited by RF magnetron sputtering on hat surfaces and into micromachined cavities of Si (100). The variation of RF power, deposition pressure and substrate bias voltage have been studied. Depending on the deposition conditions, the film stress can be adjusted from - 1400 MPa to + 100 MPa. Modifications of the deposition rate and the morphology between normal and inclined (54.7 degrees) planes have been observed. Optimal chemical stability was found with slightly compressive (-100 MPa) SiC thin films. No degradation of the protective layer has been observed after 3 h in KOH at 80 degrees C. (C) 2000 Elsevier Science S.A. All rights reserved.

Structure and phases in nickel-base self-fluxing alloy coating containing high chromium and boron

Abstract: The structure of a nickel-base, self-fluxing alloy coating, containing chromium and boron thermal sprayed and fused, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). A lumpy M6C carbide, a rodlike M3B2 boride of tetragonal structure, a rodlike M7C3 carbide of hexagonal structure, and a Ni-Ni3B eutectic phase formed in the coating after fusing. Metals of M6C, M3B2, and M7C3 phases are composed of chromium, molybdenum, and nickel; chromium and molybdenum; and mainly chromium, respectively. The nickel phase in the coating has the L12 type superlattice structure.

A novel approach for the preparation of B4C-based cermets

Abstract: Additions of TiO2 affect significantly the sintering behavior of B4C. The two powders react at approximately 1500°C according to the reaction: B4C+TiO2→B4C1−x+TiB2+CO↑ (or CO2↑). Above 2000°C, the resulting two-phase, B4C1−x+TiB2, mixture sinters at a higher rate than the single-phase boron carbide. The rate of sintering increases dramatically for mixtures that contained initially 40 wt% TiO2, yielding after sintering at 2190°C for 1 h a 95% dense, fine-grained composite material, consisting of sub-stoichiometric B4C1−x and TiB2. The boron-rich carbide displays very high affinity to carbon and reverts rapidly to stoichiometric composition when treated at high temperature in a carburizing atmosphere. The two-phased ceramic can be sintered to various levels of density and infiltrated with a molten metal. After infiltration with molten aluminum, the hardness of the resulting cermets ranges from 800 to 2500 HV and the flexural strength varies from 750 to 350 MPa depending on their ceramic-to-metal ratio.

Straight β-SiC nanorods synthesized by using C–Si–SiO2

Abstract: Straight beta-silicon carbide nanorods have been grown on silicon wafers using hot filament chemical vapor deposition with iron particles as catalyst. A plate made of a C–Si–SiO2 powder mixture was used as carbon and silicon sources. Hydrogen, which was the only gas fed into the deposition system, acts both as a reactant and as a mass transporting medium. The diameter of the β-SiC nanorod ranged from 20 to 70 nm, while its length was approximately 1 μm. A growth mechanism of beta-silicon carbide nanorods was proposed. The field emission properties of the beta-silicon carbide nanorods grown on the silicon substrate are also reported.

Adsorption and Reaction of Methanol Molecule on Nickel Cluster Ions, Nin+ (n = 3−11)

Abstract: The reactions of methanol on a size-selected nickel cluster ion, Nin+ (n = 3−11), were investigated at collision energies less than 10 eV in a beam-gas geometry Dominant reactions were methanol chemisorption, demethanation, and carbide formation The absolute cross sections of these different reactions were measured and found to change dramatically with the cluster size; the demethanation proceeds preferentially on Ni4+, the carbide formation on Ni7,8+, and the chemisorption on Ni6+ A kinematic model explains the size-dependent characteristics that the chemisorption proceeds efficiently if the barrier height between the physisorbed and the chemisorbed states is low, and the demethanation and the carbide formation proceed otherwise The rate of the carbide formation depends sensitively on an Ni−Ni distance of Nin+ so that it proceeds only on Ni7,8+

Chemistry of SO2, H2S, and CH3SH on Carbide-Modified Mo(110) and Mo2C Powders: Photoemission and XANES Studies

Abstract: The chemistry of SO2, H2S, and CH3SH on carbide-modified Mo(110) and powders of bulk Mo2C was studied using synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) These studies reveal that molybdenum carbide is very reactive toward sulfur-containing molecules, being able to break S−O, S−H, and S−C bonds at temperatures below 300 K Upon adsorption of sulfur dioxide on molybdenum carbide at 150 or 300 K, there is dissociation of the adsorbate into S and O, and also formation of SO3 or SO4 by reaction with O adatoms or disproportionation of SO2 The adsorbed SO3 and SO4 species decompose upon heating to 500 K leaving a heavily sulfided and oxidized carbide In the case of H2S adsorption, the cleavage of the first H−S bond occurs at 80−100 K and the resulting HS intermediate transforms into S at temperatures below 250 K Finally, for CH3SH on MoCx, the breaking of the C−S bond takes place at temperatures between 250 and 400 K From 100 to 350 K, CH3S and a second

Improved Ni based composite Ohmic contact to n-SiC for high temperature and high power device applications

Abstract: Ni/WSi/Ti/Pt Ohmic contacts to n-SiC were investigated as a function of annealing temperatures up to 1000 °C. Annealing at temperatures between 950 and 1000 °C yielded excellent Ohmic behavior. At these temperatures the contact–SiC interface was smooth, defect free, and characterized by a narrow Ni2Si reaction region. The annealed contacts possessed atomically smooth surface morphologies and exhibited minimal contact expansion. The residual carbon, resultant from SiC decompositon and reaction with Ni to form Ni2Si, was constrained by reaction with the WSi and Ti layers forming carbide phases of W and Ti spatially distant from the metal semiconductor interface. Our results demonstrate that the Ni/WSi/Ti/Pt composite Ohmic contact maintains the desirable electrical properties associated with Ni contacts and possesses excellent interfacial, compositional, and surface properties which are required for reliable high power and high temperature device operation.