CH4 H2 Gas Mixture Research Articles

The Raman signature of H2 in the uv spectra of Uranus and Neptune: Constraints on the haze optical properties and on the para-H2 fraction

The geometric albedos of Uranus and Neptune, inferred from archived Hubble Space Telescope observations and from the ground-based measurements of Karkoschka, 1994, are modeled in the wavelength range 2200–4200 Å. The radiative transfer model, which includes Rayleigh–Raman scattering and Mie scattering by haze particles, aims at reproducing the fine structure of the geometric albedos at a resolution of 2–10 Å. The steep variation of the total optical depth allows to investigate the influences of both the stratospheric and tropospheric haze layers and that of the deep tropospheric cloud, although their relative importance is difficult to estimate accurately. Using the haze models of Baines et al., 1995, the optical properties of the Mie scatterers are inferred. The haze material on Uranus is characterized by a slowly decreasing imaginary index of refraction: ni varies from about 0.10 to 0.01–0.02 between 2200 and 4200 Å. Below 3000 Å, the absorptivity of Neptuness haze material is comparable to that on Uranus or slightly lower (ni ∼ 0.03–0.10). Above 3000 Å, it exhibits a steeper decrease (from 0.30 to 0.003). The main source of uncertainty at longer wavelengths is the reflectivity of the underlying (H2S ?) cloud. At shorter wavelengths, molecular scattering strongly dominates Mie scattering and the determination of the absorptivities is estimated to be accurate within a factor of 2. For Neptune, there is an additional uncertainty due to the inability of the initial haze model to provide a fit to the observed albedo. The Baines et al. model was modified by multiplying the number-densities of the hydrocarbons haze layers by a factor of 2.5–4.8, making it more consistent with the results of Pryor et al., 1992. For Uranus, these results suggest a darkening of the southern hemisphere since the Voyager epoch, in agreement with recent HST imaging. As a whole, the Neptunian haze seems to be more transparent than that of Uranus, possibly owing to the more turbulent dynamical state of the troposphere. Longwards of 3000 Å, the inferred absorptivities are consistent with laboratory measurements on tholins produced from CH4–H2 gas mixtures (Khare et al., 1987). The para-H2 mole fraction on both planets is constrained from the strength of a prominent H2 Raman feature at 2853 Å. On Uranus, at latitudes between 45 and 75°S and in the 50–500 mbar pressure range, the best agreement is obtained with an equilibrium para-H2 distribution. On Neptune, there is an indication of a slight departure from equilibrium in the same pressure range at mid-southern latitudes. Although this new method is significantly less accurate, its results are consistent with those of previous investigations based on the analysis of H2 quadrupole lines (Baines et al., 1995) and of the Voyager IRIS spectra (Conrath et al., 1998).

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

Abstract We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH4-H2 gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H2 flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH4. The highest growth rate, 5 μm h−1, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm−1 K−1. Diamond disks with very low optical absorption (loss tangent tgδ

Influence of the composition of a H2-CH4 gas mixture on the catalytic synthesis of carbon nanotubes-Fe/Fe3C-Al2O3 nanocomposite powders

C NTs -Fe/Fe 3 C-Al 2 O 3 nanocomposite powders have been prepared by selective reduction of an α-Al 1.9 Fe 0.1 O 3 solid solution in H 2 -CH 4 gas mixtures (0, 1.5, 3, 4.5, 6, 9, 12, 14, 16, 18, 24, 30 and 45 mol% CH 4 ). The powders have been studied using macroscopic and microscopic techniques. The C NTs are arranged in very long bundles homogeneously dispersed in the composite powder. Most C NTs have less than four walls and are free of pyrolytic or amorphous carbon deposits. The inner diameter is in the range 1-6 nm, which could indicate that the catalyst particles active for C NTs formation are in this size range. The reduction of the Fe 3+ ions to metallic Fe is highly favoured by the presence of CH 4 in the reduction atmosphere. There are more Fe 3 C than α-Fe particles located at the surface of the matrix grains for CH 4 contents higher than 4.5 mol%, however, the exact nature of the catalytically active particles remains an open question. Compositions in the range 9-18 mol% CH 4 give the best results.

Nature of band-A cathodoluminescence inCVD diamond films

Several sets of diamond films have been grown by Microwave Plasma-Enhanced Chemical Vapor Deposition (MWPECVD) using CH4-H2 and CO2-CH4 gas mixtures. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations and crystal qualities. In particular, the CH4 concentration in the gas mixture was varied in a wide range to obtain different preferential orientations. Substrate growth temperatures as high as 950 °C were used to intentionally obtain highly defective films. The resulting films were characterized by X-Ray Diffraction, Raman spectroscopy and Scanning Electron Microscopy. Cathodoluminescence has been investigated in the energy range 1.55-6.20 eV (200-800 nm). A strong dependence of the 2.85 eV (435 nm) band-A emission on the CH4 concentration is found. Moreover, this dependence does not substantially change with the growth conditions (substrate temperature, nucleation pretreatment, gas mixture etc.). The position of the diamond Raman peak is also correlated with the band-A emission. These results are discussed in terms of crystal defects and diamond film texturing induced by the growth process.

Diagnostics of plasma emission spectra during electron assisted chemical vapor deposition of diamond films

Diamond films were deposited by hot cathode direct current discharge plasma chemical vapor deposition from a CH4–H2 gas mixture. Optical emission spectroscopy was employed to investigate in situ the plasma emission characterization during diamond synthesis. The dependence of plasma emission spectra on the input CH4/H2 ratio and the substrate temperature was investigated. A significant variation in the emission intensity of the CH radical was measured with a change in the CH4 concentration. C2 was detected only at high CH4 concentration. In addition, the relative emission intensity of the C2 species was sensitive to a high substrate temperature. The correlation between the spectra of some species and the quality of diamond films was studied. These results suggest that CH and CH+ are all important precursor species in the diamond deposition reaction, while C2 is associated with the presence of a nondiamond phase in the films.

Synthesis of carbon nanotube–Fe-Al2O3 nanocomposite powders by selective reduction of different Al1.8Fe0.2O3 solid solutions

Al1.8Fe0.2O3 solid solutions have been prepared as amorphous, η (cubic) and α (corundum) phases. The oxides have been reduced in a H2–CH4 gas mixture at 900 or 1000 °C, giving rise to composite powders containing alumina, α- and γ-Fe, Fe3C and different forms of carbon including nanotubes, thick tubes and spheroidal particles. The powders have been investigated using a combination of chemical analysis, X-ray diffraction, Mossbauer spectroscopy, scanning and transmission electron microscopy, thermogravimetric analysis and specific surface area measurements. Using the stable form (corundum) of Al1.8Fe0.2O3 as starting material favours the formation of carbon nanotubes compared to the other forms of carbon. This could partly result from the fact that the metal nanoparticles formed upon reduction of the α solid solution, which act as a catalyst for CH4 decomposition and possibly nanotube nucleation, are smaller than when using amorphous or η solid solutions. Moreover, the crystallization of these latter compounds during the reduction in some way provokes the entrapment of carbon within the oxide grains. The nanotubes, most of which are less than 10 nm in diameter, are arranged in bundles several tens of micrometers long.

Iron Carbide Synthesis by Reaction of Iron Ore with H2-CH4 Gas Mixtures Containing Traces of Sulfur.

Particles of hematite ores in a porcelain boat were reacted at 873∼1 273 K by H2-CH4 mixtures having low sulfur activities as based on Fe/FeS equilibrium where metallic iron remained stable. In the tests without sulfur, much free carbon appeared beside metallic iron and iron carbide. The free carbon increased with temperatures. In the tests with sulfur, nearly one hour reaction could convert their particles mostly to iron carbide such as Fe 3 C, especially favorable in a s =0.1 ∼ 1.0 at 1173∼1223 K. The conversion yield was sensitive to inlet flow volume ratio H 2 /CH 4 and ore type, while the authors' previous test in H 2 -CO gas mixtures had already indicated their insensitivity.

Optical emission spectroscopy during the bias-enhanced nucleation of diamond microcrystals by microwave plasma chemical vapor deposition process

Microwave plasma chemical vapor deposition (MWPCVD) process has been used to grow diamond thin films on silicon substrates from CH4–H2 gas mixture. Bias-enhanced nucleation (BEN) pretreatment has been used to increase the density of diamond nuclei. Various species in the CH4–H2 plasma have been identified using optical emission spectroscopy (OES), and their effect on the film microstructure has been studied. During the pretreatment process the emission intensities of CH, CH+, C2, H, and H2* species have been found to increase significantly for a negative dc bias voltage |VB| > 60 V. The higher concentration of excited species and the associated effects play a significant role in the growth process. A very thin layer of a-C containing predominant sp3 bonded carbon species in the initial stages of the growth is found to be present in these films. The microstructure of the films has been found to be very sensitive to the biasing conditions.

Characterization of Diamond Films Grown on Amorphous SiO 2

Continuous diamond film growth on SiO2 was achieved by an effective pretreatment. Carbon implantation and diamond powder abrasion were used to enhance diamond nucleation on SiO2. The possibility of using two gas mixtures (CH4–CO2 and CH4–H2) in order to achieve the growth of continuous diamond films on SiO2 was examined. The diamond film growth rate obtained using the CH4–CO2 gas mixture was 0.6 µm/h which is roughly three times higher than that of the CH4–H2 gas mixture. CH4–H2 gas mixtures had a lower etching reaction rate than that of CH4–CO2 gas mixtures. The higher etching rate of the CH4–CO2 gas mixture was considered to be due to a higher concentration of active carbon radicals or carbon-containing species in the plasma. Scanning electron microscopy (SEM) and optical emission spectroscopy (OES) indicated that active carbon radicals and carbon-containing species are the major cause of pits arising at the surface. Using CH4–H2 gas mixtures at a low microwave power gave the optimum reaction conditions for depositing diamond films on SiO2 layers.

Bias-enhanced diamond nucleation on silicon—a TEM study

The bias-enhanced nucleation and early growth of diamond on (100) silicon by microwave plasma-enhanced chemical vapour deposition (MPCVD) from a CH4H2 gas mixture were investigated by transmission electron microscopy (TEM). Hillocks containing β-SiC, diamond and amorphous components are the most prominent features at the substrate surface after bias-enhanced nucleation. The hillocks are formed not only by the deposition of carbon, but also by the re-deposition of Si etched from the substrate surface. In between the hillocks, β-SiC clusters, with a distinct texture parallel to the heteroepitaxial orientation, are formed at the interface between the silicon substrate and a thin amorphous layer. During subsequent diamond growth, only the hillocks act as nucleation sites.

Comparison of diamond-like carbon films deposited from 40 kHz and 13.56 MHz r.f. plasmas

R.f. plasma discharges are a standard method for depositing diamond-like carbon (DLC) films. This work looks at the effect of the r.f. driving frequency by studying films deposited at both 40kHz and 13.56 MHz. Films were grown on a 10 cm diameter driven electrode from CH4H2 and C2H2H2 gas mixtures onto 75 × 25 mm2 soda glass substrates at process pressures in the range 6–80 Pa (0.05–0.6 Torr). Film growth was monitored by in situ ellipsometry (at 675 nm) which allows measurement of the refractive index, extinction coefficient and thickness. The refractive index has been shown previously to be a reliable measure of film quality as determined by the hardness, scratch resistance and wear resistance. Films are assessed as a function of deposition pressure and r.f. frequency. The film quality is seen to improve as the pressure decreases and as the r.f. power increases for both driving frequencies. It is necessary to deposit 13.56 MHz films at low pressure, typically less than 12 Pa (0.1 Torr). Similar quality films may be deposited at 40 kHz at pressures up to 80 Pa (0.6 Torr).

Band-A emission in synthetic diamond films: A systematic investigation

Several sets of diamond films were grown by microwave plasma enhanced chemical vapor deposition (CVD) using a CH4–H2 gas mixture. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations, and crystal qualities. In particular, the substrate temperature Ts and the CH4 concentration in the gas mixture were varied in the ranges 850–950 °C and 0.4%–2.0%, respectively. The resulting films were characterized by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Room-temperature cathodoluminescence has been investigated in the energy range 1.55–6.20 eV (200–800 nm). A clear correlation of the 2.85 eV (435 nm) emission band, the so-called band A, with both the substrate temperature and CH4 content in the gas mixture during deposition, was observed. An explanation is given in terms of crystal defects and diamond film texturing induced by the growth process.

Investigation of the process factor space on bias-enhanced nucleation of diamond on silicon

The influence of the process parameters on bias-enhanced nucleation of diamond on silicon was studied. When low pressures (< 15 Torr) and/or high bias voltages (more than 350 V d.c.) were used, no significant diamond nucleation was observed; in some cases diamond was found to be removed under these conditions. Low bias voltages (below −150 V d.c.) had very little effect on diamond nucleation, and higher process pressures (> 25 Torr) resulted in poor diamond film uniformities. A well defined process zone was determined in which a short bias duration (< 20 min) may be utilized to obtain enhanced diamond nucleation densities and improved diamond film uniformities. The process factors responsible for these optimum responses were a pressure of ∼20 Torr and a bias voltage of ∼−320 V d.c. using a 5%CH4-H2 gas mixture.

Diamond Synthesis by Microwave-Plasma Chemical Vapor Deposition Using CH3Cl and CH2Cl2 as Carbon Source

Polycrystalline diamond films have been synthesized from gas mixture systems of CH2Cl2-H2 and CH3Cl-H2 by the microwave-plasma chemical vapor deposition (CVD) technique. The gas reaction speed was promoted by using CH3Cl and CH2Cl2, although the deposition rate decreased, due probably to surface etching by the chlorine in the case of high-concentration CH2Cl2. A broad band centered at about 1500 cm-1 in the Raman spectra decreased considerably in the films compared with those synthesized from the CH4-H2 gas mixture. Very little residual chlorine was detected in the deposited films. The use of chlorine-permuted methane as a carbon source was found to be advantageous for high-purity diamond deposition at low temperature and high concentration.

Dimensional changes in a sinter hardening alloy steel: R.W. Kiefer et al (Pitney Bowes Inc, Stamford, Connecticut, USA)

Different powder mixtures have been used to analyse the effect of presintering heat treatments on the composition and microstructure of TiMoCN–Ni based cermets. When methane is present in the atmosphere C contents after presintering can be associated to the C activity calculated from the C + 2H2 = CH4 equilibrium. H2 based atmospheres produce high C losses whereas N2–CH4–H2 gas mixtures induce strong carburization of the compacts. When no methane is present in the atmosphere, the C activity could be related to the C content of the compacts by the Boudouard equilibrium. Changes in C and N contents are also observed to depend critically on the phases present in the powder mixtures. Those containing free C, TiC, Mo2C or TiMoC phases show higher C and N changes after presintering than those based on quasi-monophasic TiMoCN powders. These data confirm that presintering treatments can be designed to modify the C/N ratio of TiMoCN–Ni cermets in order to avoid graphite precipitation and to have a closer control on the composition of carbonitride and metallic binder phases after sintering.

Thermodynamic and experimental study of diamond deposition from a CH4H2 gas mixture

Abstract The effects of deposition variables on the nucleation and growth of diamond particles were investigated using the tungsten-filament chemical vapor deposition method. Deposition was performed under the conditions of 0.3%–1.5% methane in hydrogen, 2000–2200°C filament temperature, 840–1040°C substrate temperature, 50–300 mbar reactor pressure, and 50–150 standard cm 3 min −1 flow rate. The nucleation density and growth rate increased with increasing methane concentration, filament temperature, and flow rate. By increasing the substrate temperature, the nucleation density and growth rate increased first and showed maxima around 950°C. The free energy change, calculated under the assumption that diamond was deposited from the supercooled gas mixture with no solid to the equilibrium gas and solid mixture at the substrate temperature, agreed well with the experimental results.

TEM observations of diamond films produced by hot filament thermal CVD

Diamond films were provided by a hot filament thermal chemical vapour deposition method with an H2-CH4 gas mixture under various reaction conditions: CH4/H2 ratios of 0.5% and 1.0%, Si and Cu substrates, a substrate temperature of 750 °tC, a pressure of 7 torr and a reaction time of 12 h. TEM observation showed that the films produced have many defects such as twins, stacking faults and large distortion of lattices. These defects, which increase with increasing CH4 concentration, seem to be introduced during the crystal growth process. Fivefold symmetry twinned crystals were often observed in the diamond films.

Carbon-induced surface structural and compositional changes on nickel and Ni-Cr alloys as studied by an analysis of surface reaction kinetics

Carburization of polycrystalline foils (5 μm thickness) of Ni and low-chromium Ni-Cr alloys (cCr ⩽ 13.1wt%Cr) in CH4-H2 gas mixtures at 1150 to 1400 K and high carbon activities aC above a critical value aC,cr (> 0.75 for Ni) leads to the formation of a surface graphite monolayer with an atomic ratio of 2 carbon atoms per 1 metal atom. This process is accompanied by surface restructuring if the crystallite orientations are different from (111) (and probably (110) and (311)) and proceeds in dependence on the time ta of sample exposure to the carburizing atmosphere. The fractional area of the surface covered with the carbon monolayer has been determined by decarburizing the samples in H2 immediately after the carburization treatment and analyzing the surface-reaction controlled decarburization kinetics as a function of ta, aC, temperature, alloy composition and H2S additions. Moreover, the rate equation for decarburization of the materials in H2 was extended to apply also to decarburization via a surface which is partially covered with the carbon monolayer. The cyclic carburization/decarburization experiments were carried out in a flow-apparatus by applying the resistance-relaxation method.

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.

A finite element study of the mechanical response of a multilayer system (TiNTiTin…/Steel) to indentation by rigid tips or wedges

We report here the improvement of the adhesion of CVD diamond films on WC-Co (6%) tools and the enhancement in the cutting tool life-time by the introduction of CF4 in the CH4H2 gas mixture. By the use of this halogen precursor it was possible to reduce the substrate temperature during the deposition without degradation in the diamond film quality. The low-temperature deposition improves the smoothness of the coating and minimizes the thermal stress induced by the CVD process.