Decrease In Hydrogen Content Research Articles

Transpolyacetylene chains in hydrogenated amorphous carbon films free of nanocrystalline diamond

The microstructure of distributed electron cyclotron resonance plasma-deposited hydrogenated amorphous carbon films (a-C:H) was investigated using electron diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Experimental evidence of the existence of transpolyacetylene (TPA) chains in a-C:H films free of nanocrystalline diamond is presented. The values of the mean bond angles and lengths and first neighbor numbers are consistent with the TPA data. The Raman spectra were fitted using the G and D bands and the bands centered at 1140, 1233, and 1475 cm−1 assigned to TPA chains modes. The relative intensity of the latter decreases while hydrogen content decreases. A significant sp2-CH olefinic mode contribution to the infrared stretching band is observed for the low-density films (∼1.2 g/cm3). TPA chains growth is enhanced when ion current density and energy decrease.

Analysis of Pore and Pore-Related Properties in Plasma-Enhanced Chemical Vapor Deposition Low Dielectric Constant Films

The effects of bias power on the pore size, pore distribution, relative dielectric constant density, composition, and leakage current of plasma enhanced chemical vapor deposited (PE-CVD) low-k film were studied. The pore diameter of the PE-CVD film changes from 1.23 to 0.73 nm with the application of bias power from 0 to 100 W. Pores with an average diameter larger than 0.63 nm were found to decrease the k value of the PE-CVD low-k film. The pore volume ratio of the PE-CVD low-k film with different compositions was calculated from the measured pore size, k value, and density. The relationships between the k value and the density, and the calculated pore volume ratio, were estimated with a simple model. The k value and the density were found to increase with increasing silicon and oxygen contents and decreasing hydrogen content in the film at the same pore volume ratio. The pore volume ratio of the PE-CVD low-k film with the lowest k value, 2.66, was estimated as 31% from the relationship between the k value and the calculated pore volume ratio. © 2002 The Electrochemical Society. All rights reserved.

Effect of Nitrate and Sulfate on Dechlorination by a Mixed Hydrogen-Fed Culture

A novel hollow-fiber membrane remediation technology developed in our laboratory for hydrogen delivery to the subsurface was shown to support the dechlorination of perchloroethene (PCE) to cis-dichloroethene. In previous research, the presence of nitrate or sulfate has been observed to inhibit biological reductive dechlorination. In this study hollow-fiber membranes were used to supply hydrogen to a mixed culture to investigate whether adequate hydrogen could be added to support dechlorination in the presence of alternative electron acceptors. By continuously supplying hydrogen through the membrane, the hydrogen concentrations within the reactor were maintained well above the hydrogen thresholds reported to sustain reductive dechlorination. It was hypothesized that by preventing nitrate and sulfate reducers from decreasing hydrogen concentrations to below the dehalorespirer threshold, the inhibition of PCE dechlorination by nitrate and sulfate might be avoided and dechlorination could be stimulated more effectively. Enough membrane-fed hydrogen was supplied to completely degrade the alternative electron acceptors present and initiate dechlorination. Nevertheless, nitrate and sulfate inhibited dechlorinating activity even when hydrogen was not limiting. This suggests that competition for hydrogen was not responsible for the observed inhibition. Subsequent microcosm experiments demonstrated that the denitrification intermediate nitrous oxide was inhibitory at 13 µM.

Mechanism for low temperature activation of Mg-doped GaN with Ni catalysts

The activation mechanism of Mg-doped GaN with Ni catalysts has been investigated by thermal desorption spectroscopy. It has been revealed that Ni deposited on Mg-doped GaN enhances the hydrogen recombination reaction at temperatures below 200 °C with the activation energy of 1.3 eV. The hydrogen desorbed at this temperature can be attributed to a part of the passivating hydrogen in GaN with a weak binding energy. The enhancement of the hydrogen recombination reaction on the GaN surface is essential to decreasing hydrogen concentration efficiently at low temperatures.

Sound velocity and Young’s modulus in plasma deposited amorphous hydrogenated carbon

The first carbon-film sound velocities obtained with the piezoelectric method are given. The a-C:H films were deposited from methane using rf plasma chemical vapor deposition at different substrate biases and thus contain varying hydrogen concentrations. Measurements of density allowed the Young’s modulii of the films to be calculated. Both the sound velocity and the Young’s modulus reached a maximum as the substrate bias changed from −47 to −175 V. The film with the maximal properties occurred at a bias of −76 V and had a sound velocity of 16.4 km/s, a Young’s modulus of 589 GPa, an optical gap of 2.16 eV and a density of 2.19 g/cm3. Although the modulus is 52% that of the directionally averaged value of diamond, it produced a sound velocity 91% that of diamond due to the lower film density. The film densities were in the range of 1.81–2.43 g/cm3 with the densest films occurring at the highest bias magnitudes. Optical gap measurements were taken with a photospectrometer and yielded a gap that decreased from 3.86 to 1.40 eV with increasing bias magnitude and thus decreasing hydrogen content. These values indicated sp3 concentrations that increased with the gap from 43% to 94% and these were consistent with Raman spectroscopy results.

Ab initiostudy of the initial growth mechanism of silicon nitride onSi(100)−(2×1)usingNH3

Density functional theory (DFT) is used to examine the reaction mechanisms of nitridation of the $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface by ${\mathrm{NH}}_{3}.$ The surface is modeled using the cluster approximation. A detailed reaction mechanism that includes ammonia adsorption and decomposition, insertion of nitrogen into Si-Si bonds, hydrogen diffusion on the surface and ${\mathrm{H}}_{2}$ desorption is investigated. We find that nitrogen-containing species prefers to be on the surface, bonded to the two surface Si atoms. The energy barriers are also calculated. We find that the activation barrier of the nitridation rate-limiting step is 0.43 eV higher than the activation barrier for ${\mathrm{NH}}_{3}$ desorption. This confirms the experimental observation that a large fraction of ${\mathrm{NH}}_{3}$ that dissociates upon adsorption recombines and desorbs. We also find the hydrogen desorption barrier from the nitrided silicon surface to be 3.34 eV. As the surface hydrogen concentration decreases, we find that the activation barriers of the nitridation reactions become smaller. Our calculations show that the dissociation barrier decreases from 2.65 to 1.95 eV when two surface hydrogen atoms are removed from the reactive silicon dimer. This result agrees with the experimental findings that further nitride growth does not occur until hydrogen desorption takes place. Therefore, we conclude that the rate-limiting step for the nitridation reaction is the hydrogen desorption. Further dissociation reactions leading to stable species such as bridge-bonded $\mathrm{NH}(b)$ and $\mathrm{N}(b)$ species are also studied. Vibrational frequencies of the surface species are calculated and found to agree with experiments. This confirms our prediction of the nature of the surface species and the reaction mechanisms leading to the initial growth of silicon nitride films on $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1).$

Study of porous development in pyrolysis chars obtained from a low-volatile coal

Oxidation produces important transformations in the chemical composition and textural properties of bituminous coals. A higher-rank coal — semianthracite, 14.7% volatile matter — was oxidised in an oven with a forced air circulation, at 473 and 543 K, for periods of 1–7 days. The pyrolysis of fresh and oxidised coal samples was performed under nitrogen up to 1123 K at a heating rate of 60 K min −1 without a soaking time. As oxidation progressed, increases in volatile matter and oxygen content were observed for all the coals. Preoxidation led to a densification of carbonaceous matter with an increase in the true and apparent densities, the increase being most pronounced for the series oxidised at 543 K. The pyrolysis of raw material and oxidised coals caused a decrease in oxygen and hydrogen contents and an increase of up to 93% in carbon content. The true density of the chars was higher than that of the oxidised coals. Total pore volume and porosimetry values show that oxidation has a strong influence on pore-size distribution in the chars, a longer oxidation favouring a pronounced microporous character. Both BET-N 2 and DR-CO 2 surface areas of the chars increase correspondingly, showing the positive effect of oxidation on the evolution of the solid texture.

Tribological and mechanical properties of diamond-like carbon prepared by high-density plasma

Abstract Diamond-like carbon films (DLC) have been deposited by plasma-enhanced chemical vapor deposition using a commercial r.f. high-density plasma (HDP). The hardness and elastic modulus have been determined from force–displacement curves using a nanoindentation instrument. The friction coefficients and wear rates have been measured using a pin-on-flat tribometer in ambient air and UHV at maximum Hertzian contact pressure of 1.09 GPa. It was found that the hardness, Young's modulus, and wear resistance of the films increased with decreasing hydrogen content of the films. A viscoplastic behavior has been observed for films with hydrogen content higher than 30% indicating that these films have a softer ‘polymer-like’ structure. The unbound hydrogen present in the HDP films contributes to the enhancement of the viscoplastic behavior of the film and to the decrease of the friction in UHV. The correlation between viscoplastic properties, film structure and tribological behavior of the HDP films is presented and compared to those obtained with samples prepared by d.c. PECVD.

Thermoelectric power of hydrogenated palladium and some of its dilute alloys, between 80 and 300 K

Thermoelectric power and electrical resistance of Pd and PdMe (Me=Ti, Nb, Zr, Ce, Be and Ge) saturated with hydrogen at high pressure have been simultaneously measured between 80 and 300 K. Several experimental runs have been carried out on samples of successively decreasing hydrogen content. The results have been analysed in terms of the Nordheim–Gorter rule. The phonon and disorder diffusion contribution to the thermoelectric powers could be estimated.

K-0704 a-Si:C:H薄膜の微小押込み試験による機械的特性評価(S07-1 薄膜の創製と評価)(S07 表面改質とその機能特性評価)

This paper describes an evaluation of mechanical properties for a-Si:C:H thin films by means of nano-indentation test. This research focuses on revealing the relationship between mechanical properties of films, depositing conditions and film structure. The a-Si:C:H films were prepared by the plasma-enhanced CVD method of the hot cathode PIG discharge type. The microstructure of the films was examined by XPS and Raman spectroscopy. Young's modulus of the films increased with an increase of bias voltage and with decreasing hydrogen content in the film. Better mechanical properties of the a-Si:C:H thin films was achieved by the higher bias voltage of the deposition condition.

Aqueous speciation of sulfuric acid–cupric sulfate solutions

This work presents the development and use of an ion-association multicomponent equilibrium model and the application of an ion-interaction model developed by Pitzer, to simulate the distribution and concentration of chemical species (speciation) in sulfuric acid–cupric sulfate solutions in the 0– 200 g/ l H 2SO 4 and 0– 50 g/ l Cu(II) concentration range and 15–70°C temperature range. The model consists of a set of equations that represent the equilibrium relationships for the ionic reactions and the mass balances for the components present in the system. The effect of ionic strength was taken into account by correcting the equilibrium constants using a relationship proposed by Davies. Several species can be formed at various pH and temperature values, the principal ones being: HSO 4 −, H +, SO 4 2−, Cu 2+, and CuSO 4(aq). Simulations show that concentrations are highly dependent on pH. Sulfuric acid speciates mainly as bisulfate ion (HSO 4 −) and hydrogen ion (H +) at pH values lower than 1. Calculations including Davies’ relationship show very good agreement with experimental values for the ionic conductivity of the solution. The conductivity of the aqueous CuSO 4–H 2SO 4 solution decreases as cupric sulfate is added, due to the resulting decrease in hydrogen ion concentration. The model presented in this work can be applied to predict and to analyze the solution composition in electrolytic copper processes such as leaching, electrowinning and electrorefining, as well as the purification of industrial solutions used in hydrometallurgical processing.

Precipitation and dissolution peaks of hydride in Zr–2.5Nb during quasistatic thermal cycles

Two internal friction peaks and corresponding discontinuity (‘knee’) points of elastic modulus have been observed in hydride-forming metals upon heating and cooling, respectively. In the present work, measurements of Young’s modulus as functions of temperature and hold time during a quasistatic thermal cycle were made in Zr–2.5Nb samples containing hydrogen using a composite oscillator technique. The increment of modulus during an isothermal hold is proportional to the decrease in hydrogen concentration in solid solution of the Zr alloy. As a result, elastic modulus measurements provide a means to determine the amount of transformed hydride during the transition. It is confirmed experimentally that the two peaks reflect the variation of the hydride transformation rate during heating or cooling. It is demonstrated by the present work that the maximum-slope point of each peak, on the high temperature side, is coincident with the knee point on the curve of modulus versus temperature and this point provides the most reliable and physically sensible indicator of the end of hydride dissolution during heating, or the beginning of hydride precipitation during cooling.

Vibrational lifetime of bond-center hydrogen in crystalline silicon

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.

Deposition of Amorphous Carbon Films using ECR Plasma by Varying the Substrate Temperature

Amorphous hydrogenated carbon thin films have been deposited with benzene plasma in an electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition system. The characteristic of Benzene discharge plasma has been monitored by Mass spectrometry. It shows that the majority of the plasma species in the downstream ECR plasma with benzene as gas source are acetylene, ethylene and higher mass species. In the experiments, the effects of the substrate temperature on the deposition rates have been emphatically studied. The structures of the films were analyzed by FTIR and Raman spectrum. The results show that when the substrate temperature rises, the deposition rate drops down, the hydrogen content decreases, with the higher SP3 content being presented in the film.

Compositional and structural properties of deuterated plasma enhanced chemical vapour deposited silicon-carbon alloys

Abstract Hydrogenated and deuterated amorphous silicon carbon films were prepared by plasma enhanced chemical vapour deposition (PECVD) starting from silane and deuterated methane gas mixtures. The gas percentages was varied in order to produce films with different carbon and silicon content. The elemental composition was determined by Rutherford backscattering and elastic recoil detection analysis and the bonding structure by infrared spectroscopy. The hydrogen plus deuterium atomic fraction, in the grown films, is about 0.36, almost independent of the film composition. However, the concentration of hydrogen or deuterium depends on the carbon content. In silicon-rich samples both hydrogen and deuterium atoms are contained in the films; with increasing carbon content, the hydrogen concentration decreases and the deuterium concentration increases. At the highest carbon concentration (0.28) the resulting films are fully deuterated with the deuterium atoms attached both to silicon and carbon. From infrared absorption analysis, information on plasma chemistry and surface or bulk reactions during film growth was obtained. For the first time the experimental determination of the origin of the bonded hydrogen in amorphous SiC: H films, grown by methane-silane PECVD, is reported. Comparison of elemental composition and infrared spectra of films grown from hydrogenated and deuterated methane shows that hydrogen exchange occurs between carbon and silicon atoms. Moreover, as the films approach stoichiometry the hydrogen incorporated into the sample originates mainly from methane gas.

Formation of 6H–SiC due to subsequent annealing of sputtering a-SiC:H films

In this paper, the phase formation of 6H–SiC is studied through a 60 min isochronal annealing of a-SiC:H prepared at the substrate temperature of about 100°C by reactive rf sputtering. Infrared transmission, Raman scattering and X-ray diffraction analyses are employed to characterize the films before and after annealing. It shows that a-SiC:H deposited at higher rf power changes into 6H–SiC phase, along with the disappearance of silicon or carbon clusters existed in as-prepared films, at annealing temperature of 800°C, while 1000°C is required for the phase change of those deposited at lower rf power. It is suggested that the bombardment of energetic argon ions during deposition is responsible for the enhancement of 6H–SiC formation, decrease of hydrogen content, and homogeneity of the films.

Temperature and growth-rate effects on the hydrogen incorporation in a-Si:H

Hydrogen incorporation in amorphous hydrogenated silicon produced by the expanding thermal plasma is investigated as a function of substrate temperature at three different deposition rates of 0.3, 3 and 11 nm/s. The increase of the refractive index with increasing substrate temperature is attributed to decreasing hydrogen concentration. The latter result is explained by a model which assumes a thermally activated hydrogen cross-linking step immediately after the chemisorption of a silyl radical. The activation energy for this process is about 150 meV. For growth rates larger than 1 nm/s the hydrogen content is significantly growth-rate dependent.

Structural and hydrogen sorption studies of NdNi 5− xAl x and GdNi 5− xAl x

Crystallographic and hydrogen sorption studies of the NdNi 5− x Al x and GdNi 5− x Al x systems have been carried out. It is found that up to x<2 the prototype hexagonal crystal structure of the CaCu 5-type is preserved. However, at x≥2, a larger unit cell of the HoNi 2.6Ga 2.4-type is observed. In both structures, a site preference occupancy between Al and Ni atoms occurs. The structural transition with x is explained by considering steric effects: Al being bigger than Ni. All alloys were subjected to hydrogen sorption. The decrease of maximum hydrogen content and plateau pressure with increasing aluminium content (from 4.1 H/f.u. and 0.87 MPa for GdNi 4.5Al 0.5 to 2.6 H/f.u. and 0.01 MPa for GdNi 3.26Al 1.74) is explained from both the thermodynamic and crystallographic point of view. These results are consistent with: (i) previous observations on the LaNi 5− x Al x system and (ii) the `Westlake criterium' except when the HoNi 2.6Ga 2.4-type structure appears. The huge decrease of the hydrogen content observed when the superstructure appears is explained by the change of possible hydrogen tetrahedral sites from RE 2Ni 2 to RE 2Al 2 (RE=Nd, Gd).

MICROSTRUCTURES OF THE MICRO-CRYSTALLINE SILICON THIN FILMS PREPARED BY HOT WIRE CHEMICAL DEPOSITION WITH HYDROGEN DILUTION

Microcrystalline silicon thin films were prepared by hot wire chemical vapor deposition with hydrogen dilution. Structures of the films were examined by Raman scattering, Fourier transform infrared (IR) and small angle X-ray scattering (SAXS)etc. It is shown that with increasing flow ratio RH=H2/(H2+SiH4) the volume fraction of crystalline increases while the hydrogen content decreases. The result from SAXS indicates that with increasing dilution ratio, the density of the film is increased, which implies the volume fraction of micro-voids is reduced. Combining with the data from IR and SAXS, we conclude that SiH2 vibration mode in hydrogenated microcrystalline silicon thin film is located at the grain boundaries rather than in the internal surface of the micro-voids.

Transport properties of intrinsic hydrogenated amorphous silicon produced by the hot-wire technique investigated by the photomixing technique

The transport properties of hydrogenated amorphous silicon (a-Si:H) with a hydrogen content ranging from 12% to less than 1%, which were produced by the hot-wire technique, varying the deposition substrate temperature, 290 °C&amp;lt;TS&amp;lt;400 °C, were systematically studied by the photoconductive frequency mixing technique. With an increase of the deposition substrate temperature, and consequent decrease of hydrogen content, the photoconductivity, σpc, and the drift mobility, μd, are found to decrease, while the width of the conduction band tail, ε, increases. Continuous degradations of photoconductivity, drift mobility, and photomixing lifetime, τ, were found during light soaking experiments. In addition, it was found that the drift mobility increases and the photomixing lifetime decreases with an increase of the applied electric field, while the photoconductivity is essentially independent of the electric field within the range of 1000–10 000 V cm−1. Furthermore, the electric field dependence of the drift mobility in the annealed state is always larger than in the light-soaked state. The results for the electric field dependence are explained using the model of long-range potential fluctuations, whose range can be determined by employing an analysis previously developed.