CH4 H2 Mixtures Research Articles

Study of the Preparation of Bulk Powder Tungsten Carbides by Temperature Programmed Reaction with CH4+ H2Mixtures

The synthesis of bulk tungsten carbides by carburization of W metal or of WO3with mixtures of CH4in hydrogen at various pressures has been studied in temperature programmed experiments. The resulting solids have been characterized by elemental analysis, X-ray diffraction, XPS analysis, and specific surface area measurements. The carburization occurs in two distinct steps: W2C is formed in the first step taking place at about 650°C at atmospheric pressure with a 20% CH4–H2mixture, while the formation of WC occurs only at higher temperatures. During carburization some free carbon is deposited, the importance of which is very much dependent on CH4partial pressure and on the temperature of carburization. It has also been shown that direct carburization of WO3by CH4–H2does not take place, but that the carburization occurs via the reduction of WO3to W metal. The rate of reduction of WO3and that of carburization of W metal are very much dependent on, respectively, hydrogen partial pressure and CH4partial pressure. The extent of reduction of WO3into W metal required for carburization which takes place also depends on CH4partial pressure, indicating a competition between carburization of W metal at the surface and diffusion of W metal into the bulk of the solid.

Optical and electrical characterization of boron-doped diamond films

Diamond films synthesized from CH4H2 mixtures using microwave-assisted plasma deposition were prepared on Si substrates under different growth conditions resulting in 〈100〉 and 〈110〉/〈111〉 mixed textured diamond films. p-Type doping with boron was achieved by adding trimethylborate to the process gas at concentrations ranging from 0.05 to 500 ppm. The boron concentration in the film, as determined by secondary ion mass spectroscopy (SIMS), depended strongly on the texture of the films. For gas phase concentrations of less than 1 ppm, the boron incorporation rate in 〈100〉 textured films was found to be two orders of magnitude lower than for 〈110〉/〈111〉 textured films. Higher boron concentrations led to a deterioration of the 〈100〉 texture, whereas the structure and morphology of the 〈110〉/〈111〉 textured films remained almost unaffected even by the highest boron concentrations. The boron concentration of 〈110〉/〈111〉 textured films varied linearly with gas phase concentrations below 50 ppm. The boron incorporation saturates for gas phase concentrations exceeding 50 ppm, resulting in a maximum boron concentration of 2.1 × 1020cm−3 in the diamond film. Temperature-dependent Hall measurements were performed to obtain information on the carrier concentration and activation energy. The data for the highest doped samples reveal electrical properties which can be attributed to films with doping levels near the metal-insulator Mott transition. Raman spectra of the more heavily doped 〈110〉/〈111〉 textured samples show an increasing asymmetry of the 1332cm−1 zone centre optical phonon line with increasing boron concentration, which is assigned to a Fano-type interference. The shape of the phonon line can be fitted by a superposition of lorentzian and Fano lineshapes which can be explained by a growth sectordependent boron incorporation. The Fano asymmetry, increasing in strength with increasing dopant concentration, was also found in the Raman spectra of homoepitaxial boron-doped 13C diamond films which were grown on (111) natural diamond substrates. For the 〈110〉/〈111〉 textured films, strong IR absorption around 3000 cm−1 was observed due to acceptor ionization, the strength of which correlates with the boron concentration.

Atomic force microscope imaging for process characterization in diamond film deposition

Atomic force microscope (AFM) images are presented for diamond films deposited using two processes. Films were deposited on polished Si from a CH4–H2 mixture by microwave plasma chemical vapor deposition (CVD) for CH4 concentrations in the range 0.4%–2.0%. At the lowest CH4 concentration, the film surface was rough (Rq = 80 nm) and contained many grains wider than 1 μ and taller than 0.4 μ. At the highest CH4 concentration, the growth of large grains was suppressed, producing much smoother films (Rq = 25 nm), which may be suitable as mask substrates for x-ray lithography. Filament-assisted CVD was also used; deposition was halted before a continuous film formed. The tallest grains (100–200-nm high) were clustered in intersecting linear arrays corresponding to marks made by diamond-polishing (seeding). Features a few nm high found in flat regions may be diamond nuclei on bare Si formed much later.

Dry-etching techniques and chemistries for III–V semiconductors

Abstract Dry etching of III–V materials using both chlorine-based (CCl2F2, SiCl4, BCl3 and Cl2) and CH4-H2 discharges is briefly reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4-H2, but the latter gives smoother surface morphologies for indium-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional r.f. (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both indium- and gallium-based III–V semiconductors using CCl2F2-O2 and CH4-H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage in the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Effect of oxide scale on carbon deposition on Fe-Ni alloys in carburizing gas

The behavior of carbon deposition on preoxidized Fe-Ni alloys containing 0 to 57.0 mass pct Ni in 10 pct CH4-H2 mixture at 1203 K was studied by metallography and thermogravimetry. Nickel retarded carburization and carbon deposition by lowering the solubility limit of graphite in austenite and by reducing catalytic activity for the pyrolytic reaction of CH4. On oxidation in air, the addition of nickel to iron depressed the development of FeO and, thereby, caused a significant decrease in the thickness of the scale. The exposure of the alloys to 10 pct CH4-H2 mixture after the oxidation in air led to a sudden mass loss in the early stage and then a rapid mass gain. This mass change is primarily ascribed to mass loss by reduction of iron oxides and to mass gain by carbon deposition. The rapid mass gain by carbon deposition is probably due to the formation of active iron by reduction of iron oxides and to the increase in the reaction area by spallation of the scale; the active iron formed may promote filamentous carbon deposition through Fe3C formation and decomposition. Carbon deposition on the alloys containing 27.2 mass pct Ni or more was considerably retarded because of the formation of a thin oxide scale which consists of α-Fe2O3 and spinel (NixFe3−xO4) and the reduction of catalysis by enrichment of nickel in the subscale. However, the amounts of carbon deposition increased compared with those on the as-polished alloys, owing to the presence of reducible iron oxides.

Surface reaction kinetics as a probe for studying carbon-induced surface structural changes on nickel and Ni-Cr alloys

During carburization of polycrystalline Ni and Ni-Cr alloys (cCr≤13.1 wt.% Cr) in CH4-H2 gas mixtures at 1150 to 1400 K, the formation of a surface graphitic carbon monolayer with 2 C Atoms per 1 metal atom (accompanied by surface restructuring on faces other than (111) and probably (110) and (311)) occurs if the carbon activity ac exceeds a critical value (0.75 for Ni) and if the exposure time ta is long enough. The fraction of the materials surface which is covered with the carbon monolayer after a definite carburization treatment has been determined quantitatively as a function of ta, ac, temperature T, chromium concentration cCr and additions of H2S to the carburizing CH4-H2 mixture. This was done by decarburizing the materials in a H2 or H2-H2S atmosphere immediately after carburization and using the kinetic features of the decarburization process (which is controlled by surface reaction steps) as a probe. The cyclic carburization/decarburization experiments were carried out in a flow apparatus by using thin foils with 5 μm thickness and applying the resistance-relaxation method.

Effect of aluminum oxide on carbon deposition of Fe-Al alloys in carburizing gas.

The behavior of carbon deposition on as polished or oxidized Fe–Al alloys containing 05.7 mass% Al in 10% CH2–H2 mixture at 1 203 K was studied by metallogrphy and thermogravimetry to elucidate the effect of alumina on carbon deposition on the alloys in carburizing gas. Aluminium works to accelerate the deposition of filamentous carbon by promoting Fe3C decomposition. However, carbon deposition on as polished alloys containing 2.0 mass% Al or more is markedly retarded because of the formation of Al2O3 on the surface despite of the carburizing atmosphere. Under this condition, Al2O3 may have formed by the reaction of aluminum in the alloy with small amounts of oxidizing gases such as O2 and H2O present in the atmosphere. On oxidation in air, Fe2O3, Fe3O4, and FeO form on iron and small additions of aluminum (≤2.7 mass% Al) retard the formation of Fe3O4 and FeO and a further addition of aluminum up to 5.7 mass% causes the formation of Al2O3 besides FE2O3. The exposure of the alloys containing 2.0 mass% Al or less to 1–%CH4–H2 mixture after the oxidation in air causes a rapid mass gain. It may be due not only to the promoting action of aluminum for filamentous carbon deposition but also to the formation of active iron by reducing iron oxides and to the increase in the reaction area by spalling the scale. Carbon deposition on the alloys containing 2.7 mass% Al or more, however, is greatly depressed because Al2O3 forms or remains even during carburization.

Pressure-induced absorption coefficients for radiative transfer calculations in Titan's atmosphere

For the purpose of calculating the far-infrared thermal radiation from Titan's atmosphere, the pressure-induced absorption in pure N2, CH4 and in N2Ar, N2CH4, and N2H2 mixtures has been modeled using recent laboratory measurements. The results are tabulated as a function of wavenumber and temperature, and gaseous absorption spectra are calculated for the composition and temperatures known to be typical of the troposphere of Titan.

ＣＨ４‐Ｈ２系ガスの高温高カーボンポテンシャル雰囲気における鉄表面上の炭素析出挙動

The deposition behavior of carbon on iron from CH4-H2 mixtures in the form of pyrolitic carbon and soot as well as filmy carbon and filamentous carbon at 1203-1373K has been studied. Deposition of filmy carbon and filamentous carbon on iron was observed under most reaction conditions, while pyrolitic carbon and soot were found to form in nearly pure CH4 at temperatures above 1273K and to be significantly influenced by the residence time of the gas. In the region of deposition of filmy carbon and filamentous carbon, the amount of carbon deposit decreased when a certain CH4 concentration was exceeded, and as temperature increased the CH4 concentration shifted downward. This may be caused by the fact that under such conditions the nucleation of filamentous carbon is retarded because Fe3C formed on iron dissolved into the substrate rather than decomposing. Also, even in the region of formation of pyrolitic carbon and soot, filmy carbon was found to deposit on iron at an early stage and to retard the subsequent formation of pyrolitic carbon and soot since it prevents contact between the iron substrate and CH4 gas.

An analytical equation derived from a perturbation theory to calculate Henry's constants for square well mixtures

An analytic form for Henry's constant is derived and applied to several systems. The derivation is based on the use of Leonard-Henderson-Barker perturbation theory for a square well pair potential assuming the Ponce-Renon analytical solution of the square well fluid. Computed values of Henry's constants for CH4-Ar, CH4-N2, CH4-He, CH4-H2, C2H6-N2 and C2H6-CH4 mixtures are compared with experiment. The agreement is quite satisfactory, with mean relative deviations between 2.5 and 8 per cent. Heats of solutions are also computed and compared with experiment.

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH4-H2 mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M7C3 and M23C6 are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H2O-H2 mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H2O-H2 atmosphere where Cr2O3 is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.

Deuterium isotope enrichment in methane-hydrogen mixtures subjected to a low-temperature glow discharge

Significant enrichment of deuterium has been demonstrated in flowing CH4-H2 mixtures subjected to a low-temperature glow discharge. In the best case, over 500% enrichment per stage was measured. A pronounced translational temperature effect was observed with the optimum enrichment occurring at Twall=−105 °C.

Carburization of tungsten powder by carburizing gases. (2nd Report)

In the 1st. report, the effect of carburization temperature, carburization time, the specific surface area of W powder, CH4/H2 ratio in volume, and the thickness of W powder on the carburization of W powder by H2-CH4 mixture has been discussed. In this paper, the effect of heating of imperfectly carburized W powder at lower temperatures for 30 min in the temperature range of 1200°C-1400°C in a stream of H2 and H2-CH4 mixture have been studied and the change of lattice constants of residual W with progress fo carburization has been studied. The mechanism of carburization of W powder by H2-CH4 mixture has been also discussed. The results obtained are summarized as follows: (1) The lattice constant of the residual W in gaseously carburized powder is smaller than that of normal W and approaches to normal value with progress of carburization. (2) By heating at higher temperatures the diffracted X-ray of WC in carburized powder becomes sharper. (3) When WC powder which is carburized at lower temperature, is used in popuration of WC-Co alloy, abnormaly grown grains appear and the number of these grains increases with the broadening of the diffracted X-ray of the used WC.

Carburization of Tungsten Powder by Carburizing Gases (1st Report)

A study was made to investigate carburization of tungsten powder by H2-CH4. The results obtained are summarized as follows:(1) According to X-ray examination, fine tungsten powder is carburized to WC at 700°C by H2-CH4.(2) The thermal decomposition rate of CH4 on the surface of tungsten powder is larger on the initial stage of carburization process, and then decreases with carburization time at the temperature range between 800-1000°C. (3) The CH4 concentration of H2-CH4 mixtures on the range of CH4/H2=1/1-1/5 in volume have no remarkable effects on the total carbon contents of the products. (4) The thickness of the tangsten powder layer has remarkable effect on the total carbon contents of the products. (5) According to X-ray examination, the crystal lattice of WC formed between 700-1000°C are abnormal.