Flowing Gas System Research Articles

Evaluation of tantalum-based materials for hydrogen separation at elevated temperatures and pressures

The hydrogen permeability of bulk tantalum and tantalum coated with thin films of palladium was measured at temperatures from 623 to 1173 K and hydrogen partial pressures from 0.1 to 2.6 MPa in a flowing gas system. Palladium coatings were deposited by both electroless plating (1–2 μm thick Pd layer) and cold plasma-discharge sputtering with two different thicknesses, 0.04 or 1.2 μm. All samples studied showed declining permeability values with surface fouling over time. The highest absolute values of permeability were observed at the lowest temperatures of study (623–773 K). However, permeability data taken at 1173 K tended to be more consistent with less scatter. The overall trend exhibited declining permeability values as a function of temperature. As confirmed by surface analysis, the palladium coating was lost from the coated tantalum samples. In one test, a 1.2 μm sputter-coated palladium film was observed peeling off the tantalum surface after 49 h at 773 K. The tantalum surface of all tested samples exhibited oxidation, distortion and cracking. Tantalum hydride formation was observed at 623 K on a palladium sputter-coated sample. Surface fouling limited the hydrogen permeability of all samples studied.

Using a Wavelength-Modulated Quantum Cascade Laser to Measure NO Concentrations in the Parts-per-Billion Range for Vehicle Emissions Certification

Measurements of NO concentrations at sub-ppm levels in vehicle exhaust are needed for emissions certification of future ultra-low emission vehicles. We demonstrate a wavelength-modulation, laser-based, NO detection system suitable for this purpose. A quantum cascade distributed feedback laser (QC-DFB) operating continuous wave (cw) at ∼100 K is frequency modulated at f = 10 kHz and locked to the center of a transition at ∼1921 cm−1 in the fundamental band of NO. The demodulated signal at 2 f of the beam passing through the sample cell directly measures the NO concentration. The cell is a multipass Herriott-type with a 100-m path length. Doppler broadening, pressure broadening, and unresolved Λ doubling combine to yield a pressure for optimum sensitivity of 100 torr and a modulation amplitude of ∼600 MHz. A flowing gas system is used to avoid problems with adsorption and desorption of NO from the cell walls. The reduced pressure eliminates interference from other gas species. Detection of NO concentrations in the few parts-per-billion (ppb) range is demonstrated in diluted exhaust-gas bag samples collected in the vehicle certification process.

Mass spectrometric analysis of a N 2/H 2 microwave discharge plasma

Using Li +-ion attachment mass spectrometry, we studied the products of microwave discharges through N 2/H 2 in a flowing gas system at a pressure of 840 Pa (6.3 Torr) and a total flow rate of 6 mL/min. The discharges produced various neutral and ionic species in amounts that depended upon the experimental arrangement and the discharge parameters. Ionic products were N +, N 2 +, N 2H +, N 2H 2 +, NH 3 +, and (NH 3) n H + ( n=1–3). In addition to NH 3 as a major neutral product, we observed a significant peak at m/ z 65 and tentatively assigned it to the Li +-ion adduct species N 4H 2Li +. This interesting product species has not been reported before.

Methane decomposition properties of ZrNi alloy in flowing gas system

With the intention of developing the decomposition process of tritiated compounds, ZrNi alloy was taken up as one of proper materials and its methane decomposition properties were experimentally investigated in a laboratory-scale flowing gas system. Methane was selected as a typical tritiated compound because of being known to be hardly decomposed and little data. The performance tests of methane decomposition using ZrNi alloy under various process conditions of gas flow rates (20, 35, 70 and 100 cc min −1), granular sizes (70–200, 200–400 and <400 mesh) and reaction temperatures (823, 848 and 873 K), have been carried out. As a result, the methane and hydrogen concentrations after undergoing decomposition process showed qualitatively similar four-stage-time change profiles for all of the independent experimental conditions. However, the definite dependence of decomposition properties on the decomposition conditions was recognized by observing a duration time of a briskest decomposition rate of more than 70%, volumes of decomposed methane and generated hydrogen.

Sulfuric Acid‐Induced Corrosion of Aluminum Surfaces

The sulfuric acid‐induced corrosion of smooth (2 nm average roughness) aluminum surfaces has been studied in real times using an in situ Fourier transform infrared reflection absorption spectrometer and a quartz crystal microbalance. Submicron thick, 35 to 55 weight percent (5 to 12 molal), sulfuric acid films were formed on room temperature metal surfaces by the reaction of gas‐phase and vapor in a flowing gas system at a total pressure of ∼200 Torr. The deposition of the acid films and subsequent changes in their chemical composition resulting from corrosion of the aluminum substrate could be monitored using characteristic infrared absorption features. The corrosion process always significantly perturbed the spectral signature of the films from that which was observed on inert gold surfaces. Using changes in spectral features that are linked to the production of Al3+ as indicators of corrosion, we conclude the rate of corrosion of the metal is strongly enhanced by both higher relative humidities and increased rates of sulfuric acid deposition.

Properties of reactively sputtered tin oxide films as CO gas sensors

SnO 2 thin films for CO sensor applications have been obtained by r.f. reactive sputtering from a target of the same compound in an ArO 2 atmosphere. The films are polycrystalline with mean grain sizes in the range 60–100 nm. An energy gap of about 4.14 eV is determined from the analysis of the spectral dependence of the absorption coefficient. The electrical measurements are carried out in a flowing gas system where dry air at ambient pressure is used as the carrier and reference gas. The CO sensitivity is found to be dependent on the sputtering conditions. The samples with relatively high initial resistance ( > 2 kΩ ) are sensitive to CO injection. The film sensitivity is found to reach the highest values in the temperature range 500–600 K.

Dissociation of nitrogen in flowing DC glow plasmas

Results of a mass spectrometric investigation of dissociation of nitrogen in DC glow plasmas in a flowing gas system in the pressure range 1-3 Torr, electron density range (1-8)*1010 cm-3 electron temperature range 2.0-2.7 eV and linear flow rate range 1.1-5.8 m s-1 are presented. The extent of dissociation is of the order of approximately 1%. A simple collisional-radiative model is presented which incorporates several molecular electronic excited states, nitrogen-containing ions, and N atoms. The N atom production is presented as a function of electron density; the predictions of the model are in agreement with the experimental results within an order of magnitude.

An X-ray photo-electron spectroscopic investigation of the air-formed film on copper

The composition and the structure of the film formed on copper by exposure to ambient air for 24 h were investigated by X-ray photo-electron spectroscopy (XPS). This characterization of the native oxide on copper provides a basis for subsequent exposures in mixed flowing gas systems. Using angle-resolved XPS, it was observed that only the univalent, diamagnetic Cu + species was present in the film. The film was modelled as a two-layer structure with an inner layer of Cu 2O and an outer layer comprising adventitious hydrocarbon and bound hydroxyl and water, with determined thicknesses of approximately 1.6 and 0.8 nm, respectively.

Dissociation of nitrogen in dc glow plasmas

Dissociation of nitrogen in dc glow plasmas in flowing-gas systems has been studied quantitatively in the pressure range 1–3 Torr, n e range 3–8 × 10 10 cm −3, and kT e range 2–3 eV. The extent of dissociation, which is of the order of 1% and varies with experimental conditions, has been for the first time expressed fully as a function of electron density and electron temperature. A simple collisional-radiative model predicts well the trends of dissociation as a function of plasma parameters, and predicts absolute values of per cent dissociation, in the worst case, within an order of magnitude.

Atomic fluorescence in a pulsed hollow cathode discharge with a copper vapor pumped dye laser

Application of a pulsed glow discharge as an atomizer for atomic fluorescence spectroscopy with a copper vapor laser pumped dye laser excitation source was investigated. The discharge system was synchronously pulsed with the laser to provide a low noise background and the low pressure, inert gas atmosphere contributed to an almost ideal environment for fluorescence measurement. The high repetition rate of the copper vapor laser (6000 Hz) improved the detection efficiency over lower repetition rate laser systems. Disposable graphite cup electrodes were used as demountable hollow cathodes for measurement of aqueous samples. The samples were injected into the cathode cups and dried using a programmable electrothermal device for reproducibility. The discharge chamber was a commercially available vacuum chamber adapted for both flowing and static gas environments. Operational parameters, such as chamber pressure, discharge voltage, sample volume, and cathode cup diameter were evaluated for maximum fluorescence signal. The maximum signals were observed at discharge voltages of 480 V for solid samples and above 1000 V for aqueous samples. The chamber pressure (2–14 Torr) was not critical. Sample sizes of over 40 μl gave no signal increase. Maximum fluorescence signals were obtained 10–15 s (60 000–90 000 shots) following the initiation of laser-discharge pulsing. Linear calibration curves were obtained for lead and iridium with limits of detection of 15 fg and 2 pg, respectively. Although the signal was higher in a static gas environment, the signal to noise ratio was lower than in a flowing gas system.

Excimer laser deposition of thin amorphous carbon films

A new carbon film deposition technique, based upon excimer laser vaporization of graphite in a flowing gas system has been developed. The low temperature vapor (LTV) technique alleviates high temperatures occurring in most other deposition methods. In this technique the UV laser ablation occurs in an inert flowing gas atmosphere. Atoms and molecules evaporated from graphite are cooled by gas entrainment before condensing on a substrate. The resulting films of amorphous carbon or hydrogenated amorphous carbon are free from strain. Measurement of the optical band gap of these films shows that E g can be controlled by the hydrogen content of the carrier gas.

Transfer of electronic excitation in sodium vapour

Excitation of Na 2 molecules to the B 1Π u state gives (in a vapour) also molecules in the A 1Σ u + state. To study this transf of excitation we use a flowing-gas system (free jet) in which we excite the molecules locally and analyse the resulting fluorescence both spectrally and spatially. Using models for the flow of gas and for the spreading of trapped atomic excitation we find that the dominant mechanisms consists of two inelastic collisions; the first one transferring the excitation to an atom Na 2 (B) + Na(3s) → Na 2(X) + Na(3p) and the second one transferring it to another dimer Na(3p) +Na 2(X) → Na(3s) + Na 2(A). The effective cross sections found for these processes are respectively σ Bp = 43 Å 2 and σ pA = 400 Å 2. A smaller contribution involving the (2) 1Σ g + state is also identified.

The rapid oxidation of atmospheric CO to CO 2 by soils

Gas exchange rates over soils were measured in a closed, flowing-gas system. 14CO was rapidly oxidized to 14CO 2 with only a minor loss in atmospheric radioactivity. Incorporation of 14C into the soil was slight and was via 14CO 2 rather than 14CO. CO oxidation was a microbial process and no oxidation occurred when soils had been autoclaved. The rate of CO depletion was concentration dependent and followed Michaelis-Menten kinetics. The rate constants K m and V max ranged from 18 to 51 μ 1 −1 CO and from 0.58 to 4.35 mg C kg −1 dry soil h −1 respectively. The maximum rate of reaction for Hubbard Brook soil was about an order of magnitude greater than any soil previously reported. The oxidation reaction was accompanied initially by a reduction in net soil respiration. This was then followed by a period of high respiration which continued until CO levels were reduced to about 5μll −1. Thereafter respiration fell below the pretreatment rate and only returned to that rate 45 min after CO had been depleted from the atmosphere. The data suggest that at high CO concentrations (40–100 μll −1CO) autotrophic carboxydobacteria comprise the main component of the CO-oxidizing population and, as the concentration declines towards ambient levels they are replaced by heterotrophic microorganisms possessing a cometabolic process.

Investigation of the reactions of SiCl4 and O2 at elevated temperatures by infra-red spectroscopy

The modified chemical vapour deposition (MCVD) process for making fibre optic preforms uses direct oxidation of SiCl4, together with dopant chlorides to produce core glass. This reaction has been studied by direct infra-red absorption spectrophotometry of the flowing gas system. The spectrum of the SiCl4 reactant is recognizable, its decrease with increasing temperature was measured, and the absorption spectra of SiO2 and of silicon oxychloride reaction products were recognized in the flowing gas stream. The oxychlorides began to appear in the spectra at 900° C and increased with temperature to 1110° C. Their concentration decreased drastically between 1110° C and 1160° C above which only the spectrum of SiO2 was observed. The identities of the oxychloride molecular species produced are uncertain, but small amounts of Si2OCl6 and the cyclic Si4O4Cl8 were present together with several unidentified higher molecular weight compounds. The particulate matter formed in the MCVD process shows infra-red spectra varying with conditions of formation. At temperatures below 1160° C the particulate matter shows absorption bands attributable to silicon oxychlorides. At higher temperatures, the particulate matter is free of oxychloride bands and shows only the proper SiO2 spectrum. If moisture is present, these spectra are modified.

Compact flowing gas system for CO2 lasers

A miniature air-actuated vacuum pump has been utilized to effect gas flow at partial atmospheric pressures for waveguide CO(2) lasers. Implications are that flowing gas laser systems can be acceptable for many applications requiring miniaturization of components.

Chemiluminescence of CaH and AIH in the reaction of the metal atoms and formaldehyde

Chemiluminescence from the reaction of calcium and aluminum with various hydrogen containing compounds in a flowing gas system and in a heat pipe oven are described. Red chemiluminescence of CaH was observed in the reaction of calcium, and weak chemiluminescence of AlH was seen in the reaction of aluminum with formaldehyde (H 2CO). It is proposed that a reaction between metal atoms and formaldehyde may be used as a source of diatomic metallic hydrides.

The a3Σ+ → X1Σ+ and b3Π → X1Σ+ band systems of SiO and the a3Σ+ → X1Σ+ band system of GeO observed in chemiluminescence

Chemiluminescence from SiO and GeO has been produced in a flowing gas system utilizing the reactions Si(3P) + N2O → SiO(b3Π,a3Σ+) + N2 ΔH = −150 kcal/mole Ge(3P) + N2O → GeO(a3Σ+) + N2 ΔH = −130 kcal/mole. An analysis of the chemiluminescence spectrum has shown that the emission originates in the b3Π and a3Σ+ states and terminates in the ground electronic state of SiO. The measured value of the spin–orbit coupling constant for the b3Π state was 75 cm−1. No vibrational constants for either the a3Σ+ or b3Π states could be determined. T0 for the b3Π state was found to be 33 853 cm−1 while that for the a3Σ+ state was found to be 33 409 cm−1. An analysis of the chemiluminescence of GeO was found to originate from the a3Σ+ state and terminate in the electronic ground state. T0 for the a1Σ+ state of GeO was found to be 27 552 cm−1. Good agreement between the experimentally observed vibronic band intensities and those calculated using the Rydberg–Klein method has been obtained.

Measurement of gas temperature gradients using Raman scattering spectroscopy

The application of Raman scattering (inelastic light scattering) spectroscopy to the measurement of gas temperature and its gradient in hot flowing gas systems is described, using experiments on a chemical vapor deposition system as an example. Apparatus is described and data are presented showing temperature gradients in the range 280 C to 950 C with RMS deviations of 14°C on a spatial scale of 100μm. Advantages of the system are its ability to make measurements in a small well defined volume and to do this totally without perturbation of the system. The principle disadvantage is a relatively low rate of data acquisition.

A Simple Very High Temperature Cell for Raman Spectroscopy

For accurate measurements of the temperature dependence of Raman cross sections up to high temperatures, flowing gas systems generate excessive temperature gradients and are subjected to refractive scintillation. At the same time, pyrolysis of even small traces of contaminants in such systems leads to a gradual buildup of their decomposition products in the cell. If, on the other hand, a closed cell is employed, either the cell must be sealed (which is inconvenient if many samples are to be used in a non-throwaway cell of good optical quality), or a high temperature valve with excellent sealing qualities must be used.

Carbon monoxide fixation by plants

Leaves of 35 species of temperate and tropical plants absorbed CO in light from air containing 6 ppm CO at an average rate of 0.19 μl/h g fresh weight. CO absorption was measured by the uptake of 14CO from a closed flowing gas system. CO uptake by bean leaves varied considerably with age. Uptake by nine species having widely different rates of absorption was exactly proportional to CO concentration in the range 0 to 100 ppm CO. Absorbed CO was metabolized either by oxidation to CO2 and fixation as such or by reduction and incorporation into serine. Corn, a C4 plant, emphasized the former pathway and bean, a C3 plant, emphasized the latter pathway. CO had various effects on the photosynthesis of leaves of different species; ranging from being inhibitory at concentrations as low as 65 ppm to exerting no influence, or even permitting an increase in net CO2 fixation at 99% CO because of the absence of O2.Plants do not contribute significantly to the global CO balance because their uptake rate is low at the CO concentration normally encountered in interurban areas. However, their contribution may become very important in or near urban and polluted areas, where elevated CO concentrations are frequently found.