Diluent Gas Research Articles

OH Reaction Kinetics of Gas-Phase α- and γ-Hexachlorocyclohexane and Hexachlorobenzene

Rate constants for the gas-phase reactions of the hydroxyl radical (OH) with α- and γ-hexachlorocyclohexane (α- and γ-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346−386 K. The experiments were carried out in He diluent gas at approximately 1 atm, in a 160-mL quartz chamber. OH was produced by the photolysis of ozone in the presence of H2O, and reactants in the chamber were monitored by online mass spectrometry. The rate constants measured at elevated tem peratures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298 K were estimated for each compound. These rate constants at 298 K (in units of 10-13 cm3 s-1) were α-HCH, 1.4; γ-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions (τOH) were also estimated (in units of days): α-HCH, 120; γ-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicate that these compounds can be transported great distances through the atmosphere before removal processes associated wit...

Influence of Plasma Chemistry on the Properties of Amorphous (Si,Ge) Alloy Devices

ABSTRACTWe report on the growth and properties of a-(Si,Ge):H films and p-i-n solar cell devices prepared using a remote, low pressure ECR plasma deposition technique. The films and devices were prepared using either He or H2 as the diluent gas. The plasma conditions were controlled so as to induce significant ion bombardment during growth. We find that there is a dramatic influence of plasma chemistry on the growth and properties of a-(Si,Ge):H films and devices. In particular, with hydrogen as the diluent gas, changing the pressure in the reactor dramatically changes both the Germanium incorporation in the film, and the electronic properties. Lower pressures lead to less Ge being incorporated, and higher mobility-lifetime product for holes for a given Tauc gap, as well as better p-i-n devices. In contrast, changing the pressure when He is the diluent gas does not produce such large changes. We speculate that the changes in device and film properties are due to the influence of ion bombardment on growth chemistry, and that both efficient energy and momentum transfer to the growing surface are necessary to achieve the best devices. The differences between He and hydrogen may simply be due to the fact that He plasma is much more energetic than a comparable hydrogen plasma, and there is more efficient momentum transfer when He is used as compared to when hydrogen is used. We have also produced very good single junction a-(Si,Ge) devices using the ECR technique.

Numerical simulation of diluent effects on flame balls

The effects of various diluent gases on steady, source-free spherical premixed flames (“flame balls”) were modeled using a time-dependent numerical code with detailed chemical, transport, and radiation submodels. The diluent gas affects the Lewis number ( Le ) and radiative properties of the flames. Numerical solutions for the steady properties and stability limits were obtained for lean H 2 -air, H 2 −O 2 , H 2 −O 2 −CO 2 , and H 2 −O 2 −SF 6 mixtures. When results were nondimensionalized by the properties at the dynamic stability limit, all results collapsed onto one of two curves, depending only on whether the dominant source of radiative loss is from the product H 2 O, whose concentration decays to zero in the far-field, or diluent gas, whose concentration is constant in the far-field. No significant Le effect was found. Numerical predictions were compared with recent Space Shuttle experiments. For CO 2 and SF 6 diluents, experimental results lie between computational predictions obtained with diluent radiation included and with diluent radiation artificially suppressed, indicating that radiation models including reabsorption effects are needed in these cases. Significant influences of the chemical mechanism were found, even for mechanisms that properly predict the burning velocities of H 2 -air mixtures away from extinction limits. Sensitivity analysis showed that the discrepancies are due mainly to differences in the H+O 2 +H 2 O→HO 2 +H 2 O rate parameters for these mechanisms.

Surface Science Studies of NF3 Plasma and Ion Beam Interactions with Silicon

ABSTRACTThe interaction of nitrogen trifluoride (NF3) with silicon (Si) surfaces has been investigated by x-ray photoelectron spectroscopy (XPS). Si (100) surfaces were subjected to plasmas created by NF3 gas mixtures and ion bombardment from NF3-generated ion beams as a means of approximating the ion component of plasma processing under controlled conditions. NF 3 plasma processing has lead to the appearance of an interesting high binding energy component in F 1s XPS spectra. Investigation into the mechanism responsible for the high binding energy component suggests that the unusual component is directly influenced by diluent gas ions in the gas mixtures. It is postulated that the high binding energy component is the result of neutral fluorine incorporated in the Si lattice as a result of an ion damage mechanism.

Partial oxidation of methane to syngas using NiO-supported catalysts

Claims for the partial oxidation of methane to CO and H2 have been clouded by the possibility of the indirect reaction sequence occurring at hot spots in the catalyst bed. In this report, we claim that the direct reaction occurs on NiO/silica at 500°C with methane conversions as high as 80% and selectivities for H2 of 80% and CO of 70%. Arguments are presented to show that the amount of diluent gas employed prevents formation of local hot spots with temperatures high enough for the indirect reaction to occur. Consequently, this study constitutes one of the most convincing reports for the conversion of CH4 to syngas by the partial oxidation of methane. The catalyst deactivates in 10 h. Surface studies show coke formation and aggregation of NiO contribute to the loss of activity.

Study of Factor and Interaction Effects During Programmed Rate Chemical Vapor Deposition of Aluminum

ABSTRACTResults from a statistically designed experiment studying the effects of initial substrate temperature (A), precursor pulse time (B), diluent gas flow rate (C), and their interactions on film properties of programmed rate chemical vapor deposited aluminum are presented. Deposition rate, reflectivity, absolute and normalized roughness, and grain orientation were the response variables investigated. A two level, three factor (23) full factorial experimental design was used. Five center cell replicates were used to estimate cell standard deviations. The experiments were conducted using tri-isobutyl aluminum (TIBA) as the precursor in a lowpressure chemical vapor deposition (LPCVD) cold wall reactor. Effects were considered significant at the 95% confidence level. Initial substrate temperature (A) affected deposition rate and reflectivity. Diluent gas flow rate (C) affected deposition rate, normalized roughness and film texture. The precursor pulse time/diluent gas flow rate interaction (BC) affected film reflectivity and absolute surface roughness, while the initial substrate temperature/diluent gas flow rate interaction (AC) affected film texture.

Stable carbon dioxide reforming of methane over modified Ni/Al2O3 catalysts

CO2 reforming of methane was studied over modified Ni/Al2O3 catalysts. The metal modifiers were Co, Cu, Zr, Mn, Mo, Ti, Ag and Sn. Relative to unmodified Ni/Al2O3, catalysts modified with Co, Cu and Zr showed slightly improved activity, while other promoters reduced the activity of CO2 reforming. Mn-promoted catalyst showed a remarkable reduction in coke deposition, while entailing only a small reduction in catalytic activity compared to unmodified catalyst. The catalysts prepared at high calcination temperatures showed higher activity than those prepared at low calcination temperature. The Mn-promoted catalyst showed very low coke deposition even in the absence of diluent gas and the activity changed only slightly during 100 h operation.

Enhanced boron doping in amorphous and microcrystalline silicon by Ar dilution

Thin films of boron-doped silicon - hydrogen alloy materials have been deposited by plasma enhanced chemical vapour deposition using argon as the diluent gas. The effect of hydrogen and helium dilution on the film properties is also reported for comparison. Films deposited by dilution with argon showed a higher conductivity than those deposited with hydrogen or helium dilution at any gas phase doping and dilution level. Bonded boron content in the films was studied by measuring SiB with the help of secondary ion mass spectroscopy. The increase of conductivity even with a lowering of the SiB/Si ratio in the deposited films at high argon dilution indicates an enhancement of doping efficiency. Moreover, boron-doped microcrystalline samples were prepared by argon dilution at low rf power density () by decreasing the silane flow. The structural and electrical properties of the deposited films revealed a high degree of microcrystallinity. A higher deposition rate with less degradation of the transparent conducting oxide (TCO) coated substrates compared to hydrogen or helium dilution indicates compatibility of the argon dilution technique with solar cell fabrication processes.

Addition and Cyclization Reactions in the Thermal Conversion of Hydrocarbons with an Enyne Structure, 4. Formation and Rearrangements of Bicyclic C10H8 Aromatics from 1‐Phenyl‐1‐buten‐3‐yne

AbstractThe thermal conversion of 1‐phenyl‐1‐buten‐3‐yne (1) into the cycloisomerization products naphthalene (2), azulene (3), and 1‐methylene‐1H‐indene (4) has been studied at temperatures between 550 and 1000°C, a reaction time of approximately 0.3 s at 13 Torr (FVP) and at low partial pressures in a quartz flow system. The results obtained by FVP as well as in the presence of chemically modified diluent gases suggest a predominance of unimolecular cycloisomerizations to 2 and 3 which are, however, always accompanied by radical cyclization reactions to 2 and 4 even under FVP conditions if temperatures above 550°C are applied.

Pyrolytic carbon coating of Zircaloy-4 tubes at relatively low temperatures

A new technique has been carried out to coat the inner surface of Zircaloy-4 tubes with pyrolytic carbon at relatively low temperatures. The coating gas was a mixture of commercial butane gas as a source for carbon and argon gas as a carrier and diluent gas. Some factors affecting the coating process have been investigated. These factors were: (i) coating temperature (250–450°C), (ii) coating time (1.8–5.4 ks) and (iii) flow rate of commercial butane gas (0.83 × 10 −6 and 1.5 × 10 −6 m 3/s) at a nearly constant flow rate of argon gas (1 × 10 −6 m 3/s). A photo-cell-reflection system has been used to determine the low coating weight gain. The results of this investigation indicated that the coating weight gain increased by increasing the coating temperature, time and flow rate of commercial butane gas in the studied ranges. The coating efficiency of pyrolytic carbon coatings increased by increasing the coating temperature and decreasing the flow rate of commercial butane gas.

Thermal decomposition of immobilized compounds for the generation of gaseous standard mixtures containing ammonia and amines

The thermal decomposition of phthalamic acid derivatives chemically bonded to the surface of silica gel was examined and utilized for the generation of single-component gaseous standard mixtures of ammonia, methylamine, diethylamine and triethylamine. The conditions of the thermal decomposition (temperature, time, diluent gas flow rate) were optimized to ensure complete liberation and rapid elution of the compounds from the bed of modified silica gel. The total amounts of these four compounds that can be released from unit mass of the modified silica gel are in the range of several mg.

Gas-Phase Ion Chemistry of Oxalyl Chloride: An Electron Bombardment Matrix Isolation FTIR Spectroscopic Study

The gas-phase ion chemistry of oxalyl chloride has been studied by bombardment of dilute mixtures of oxalyl chloride in rare gases with fast electrons followed by isolation of the decomposition products in the diluent rare gases at 12 K. The major products observed in the matrix by FTIR absorption spectroscopy are phosgene, carbonyl chloride radical, and carbon monoxide. Some CCl was observed when the diluent gas was argon. No ions were identified in the matrix, and it is proposed that the initially isolated ions are neutralized by secondary electrons impinging on the positively biased matrix. A change in the relative intensities of products was observed when the diluent rare gas was changed from argon to krypton and when the mixture was doped with varying concentrations of CO or N2. These doping experiments give direct evidence for at least two mechanisms for formation of the carbonyl chloride cation, one of which involves secondary decomposition of phosgene radical cation. The products obtained here are compared with those obtained by EI mass spectrometry. It is suggested that the different product intensities observed in EI and CE mass spectra are due to the fact that product decomposition is efficiently quenched in the source following CE ionization.

The Effect of Sulphur Dioxide Exposure under Controlled Environmental Conditions on the Challenge Performance of Copper and Chromate Impregnated Active Carbon against Hydrogen Cyanide

An ASC/T (Cu2+, Cr6+, Ag and triethylenediamine impregnated) Whetlerite activated carbon sample was exposed to a flow rate of 1 l/min, 0.746 mg/l SO2 in 80% RH air at 22°C for up to 510 min. Samples were subsequently challenged with 2 mg/l HCN in an identical diluent gas stream. Increasing SO2 exposure resulted in accelerated HCN and (CN)2 bed penetration. The basic shapes of the breakthrough profiles were however essentially unchanged. This observation is in accordance with numerical analysis of these results using Hinshelwood's adsorption model, which suggested that the adsorption rate constant was not significantly affected by SO2 but rather the pollutant exposure resulted in the number of active centres on the carbon surface being reduced and the effective bed depth of the sample being shortened. This loss in active centres was thought most likely to result from the reduction of Cr6+ to Cr3+.

Optimization of light-induced metastable changes in hydrogenated amorphous silicon-germanium alloys

Abstract Light-induced degradation of hydrogenated amorphous silicon-germanium (a-Si x Ge1-x: H) alloys, prepared in an ultrahigh-vacuum (about 10−10 Torr) plasma-enhanced chemical vapour deposition system having load-lock facility, has been studied. Metastable changes under prolonged light soaking can be controlled by varying the different deposition parameters such as the rf power density and the flow rates of diluent and sources gases. Appropriate choice of deposition parameters can reduce the bonded hydrogen content, microvoid fraction and heterogeneity of a-Six Ge1-x: H alloys and hence increase the stability of the materials. Light-induced degradation of electronic properties of a-SixGe1-x: H alloy films can be correlated with the p-i-n a-SixGe1-x, solar cells to some extent. However, discrepancies between the degradation of material and cell have also been observed. After light soaking for 100 h, a-SixGe1-x: H films, prepared at Hf= 30 sccm and Pw = 30 mW cm−2 degrades slowly while the internal qua...

Energy distribution of H atom and C2 radical during diamond growth in H2/ArCH4O2 plasma

The source gas for diamond deposition is hydrocarbon, normally diluted by an amount of hydrogen. Recently, diamond has been successfully deposited in rare-gas based plasma, i.e. ArCH 4 O 2 , using a conventional microwave CVD device. In this paper, the emission spectra have been investigated during diamond deposition in H 2 /ArCH 4 O 2 plasmas. In particular, Balmer series of H atoms and Swan bands of C 2 radicals were examined carefully in order to estimate the energy distributions of these radicals. Diamond particles could be deposited under any conditions investigated here, which were confirmed by Raman spectra. Both the rotational temperature and the vibration temperature of the C 2 radical could be estimated by the intensity distributions of the Swan bands. These values for H 2 -10%CH 4 -4%O 2 plasma at 9.0 kPa were 3100 and 4800 K, respectively. These temperatures were almost independent both of diluent gas species and of oxygen concentration in the source gas. However, the distribution equilibrium was not established in the case of Balmer series excitation levels of H atoms. This could be due to the fact that the energy gap between levels was fairly large in this case.

Scattering dynamics in HF+He, Ne, and Ar: State-to-state cross sections, Dopplerimetry, and alignment measurement via direct infrared laser absorption in crossed supersonic jets

Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm−1, respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering states are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J>0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration. Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably good, but significant discrepancies are noted for the less accurately determined Ne+HF surface. Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of |P|<2%. High resolution IR laser Dopplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1←0 state-to-state differential cross section.

The Kinetics of the Catalyzed CVD of Monocrystalline α‐Silicon Nitride Filaments

AbstractCatalyzed chemical vapor deposition of α‐silicon nitride filaments from silicon subhydrides and ammonia was studied using calculation of nitrogen solubility in selected catalysts (silicon‐containing iron alloys), both in the solid and liquid‐state, by filament deposition experiments with catalysts of various composition and size, and by variation of the flow rates and partial pressures of precursor gases, the silicon subhydrides and ammonia. The results show that neither nitrogen solubility in, nor nitrogen diffusion through the catalyst plays a determining role in the deposition kinetics, as has been previously suggested. Instead, the deposition rate is determined by the partial dissociation of ammonia, which can be accelerated by adding diluent gases with a sufficiently high collision cross section. The catalyst material has an additional, strongly accelerating effect. The present state of research suggests that nitrogen is the preferred diluent gas and chromium an optimum alloying element.

Methane oxidative coupling in an undiluted reaction mixture in a reactor-adsorber system with gas recirculation

Results of the oxidative coupling of methane obtained in a reactor-adsorber system with gas recirculation, with a continuous supply on the methane-oxygen reaction mixture without a diluent gas ( p(CH 4 + O 2) = 1 atm, CH 4:O 2 = 20:1), are described. At total methane conversion equal to 94%, the C 2+ hydrocarbon selectivity of the process was 70%, while ethylene contents was 92% in the obtained hydrocarbons.

Effect of argon dilution on the structure of microcrystalline silicon deposited from silane

The structure of the hydrogenated amorphous and microcrystalline silicon films deposited by radio-frequency (rf) glow discharge decomposition of silane diluted in argon has been studied by transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy techniques. It has been observed that argon acts not only as a passive diluent gas but also plays an important role in the growth of the amorphous or microcrystalline network. Calculation of the variation of equilibrium concentration of Ar*(3P0,2) and Ar+ with argon dilution shows that at high argon dilution (≳90%) equilibrium concentration of Ar* rises sharply with increase in argon dilution. Variation of structural properties with argon dilution suggests involvement of these excited states of argon in the growth process. A model has been proposed based on the energy exchange between the Ar* states and the growth zone of the materials to explain the structural changes observed in the presence of argon in the plasma.

Fullerenic nanostructures in flames

High resolution transmission electron microscopy (HRTEM) was used to characterize nanostructures in soots produced in flames of benzene, acetylene, or ethylene premixed with oxygen and an inert diluent gas. The nanostructures ranged from ∼2 nm to ∼30 nm in size with a hollow core measuring about ∼ 1 nm to ∼10 nm in diameter and containing 5 to 20 shells. The shapes of the nanostructures included spherical, spheroidal, tubular, and trigonous.