High-pressure Argon Research Articles

Spectroscopic and Kinetic Studies of the Reaction of Bromopropanesulfonate with Methyl-coenzyme M Reductase

Methyl-coenzyme M reductase (MCR) catalyzes the final step of methanogenesis in which coenzyme B and methyl-coenzyme M are converted to methane and the heterodisulfide, CoMS-SCoB. MCR also appears to initiate anaerobic methane oxidation (reverse methanogenesis). At the active site of MCR is coenzyme F430, a nickel tetrapyrrole. This paper describes the reaction of the active MCR(red1) state with the potent inhibitor, 3-bromopropanesulfonate (BPS; I50 = 50 nM) by UV-visible and EPR spectroscopy and by steady-state and rapid kinetics. BPS was shown to be an alternative substrate of MCR in an ionic reaction that is coenzyme B-independent and leads to debromination of BPS and formation of a distinct state ("MCR(PS)") with an EPR signal that was assigned to a Ni(III)-propylsulfonate species (Hinderberger, D., Piskorski, R. P., Goenrich, M., Thauer, R. K., Schweiger, A., Harmer, J., and Jaun, B. (2006) Angew. Chem. Int. Ed. Engl. 45, 3602-3607). A similar EPR signal was generated by reacting MCR(red1) with several halogenated sulfonate and carboxylate substrates. In rapid chemical quench experiments, the propylsulfonate ligand was identified by NMR spectroscopy and high performance liquid chromatography as propanesulfonic acid after protonolysis of the MCR(PS) complex. Propanesulfonate formation was also observed in steady-state reactions in the presence of Ti(III) citrate. Reaction of the alkylnickel intermediate with thiols regenerates the active MCR(red1) state and eliminates the propylsulfonate group, presumably as the thioether. MCR(PS) is catalytically competent in both the generation of propanesulfonate and reformation of MCR(red1). These results provide evidence for the intermediacy of an alkylnickel species in the final step in anaerobic methane oxidation and in the initial step of methanogenesis.

Modelling of non-equilibrium DC argon and xenon arcs

Numerical modelling of a wall-stabilised low-current arc at high-pressure argon and xenon is performed taking into account a deviation of the electron temperature from the heavy particles temperature. The model is based on the energy conservation equations for electrons and heavy particles. The electron density is derived from the law of mass action. The degree of thermal non-equilibrium is determined by the ratio of electron and heavy particles temperatures. Results are presented for cylindrical channel of 2 mm in diameter, the pressure of 330 kPa and with a dc of 15, 20 and 30 A. The electron and heavy particle temperatures and density are calculated. The deviation from LTE in a wall layer occurs due to the different electron and heavy particles thermal conductivity and temperature gradients. The non-equilibrium wall layer is larger for lower arc current. In case of xenon arc the electron density is higher than in argon arc and equilibrium is reached close to the axis for lower current.

Modelling of microwave sustained capillary plasma columns at atmospheric pressure

In this work we present a model of argon microwave sustained discharge at high pressure (1 atm), which includes two self-consistently linked parts - electrodynamic and kinetic ones. The model is based on a steady-state Boltzmann equation in an effective field approximation coupled with a collisional-radiative model for high-pressure argon discharge numerically solved together with Maxwell's equation for an azimuthally symmetric TM surface wave and wave energy balance equation. It is applied for the purpose of theoretical description of the discharge in a stationary state. The phase diagram, the electron energy distribution function as well as the dependences of the electron and heavy particles densities and the mean input power per electron on the electron number density and wave number are presented.

Selective water vapor cryopumping through argon

A selective cryopumping process for water vapor control takes place in vacuum systems for web coating or plasma operations, such as sputter deposition, etching, etc. Excessive water vapor content will affect the quality of the processes and final products. These vacuum systems typically operate at pressures corresponding to transitional or viscous flow regimes, and water vapor cryopumping is highly limited by diffusion of water vapor molecules through a noncondensable process gas (argon, air). An analytical model was created to describe water vapor condensing process through a noncondensable gas diffusion barrier. The model accounts for the collisions of different molecules by means of Boltzmann kinetic equations for two-component rarefied gas. It was assumed that water vapor content is about three orders of magnitude lower than that of the noncondensable gas (argon). Cryopumping process was analyzed for two simplified cases when water vapor source and cryosurface are parallel plates and coaxial cylinders. The calculations were conducted for different water vapor outgassing rates and argon pressures ranging from 0.5×10−3to20.0×10−3torr. At certain parameters a strongly nonlinear distribution of water vapor pressure and density versus distance between source and cryosurface was obtained. At high argon pressures an increase of water vapor pressure was observed near an outgassing surface. The results were used for calculation of water vapor cryopumping rates.

3D modelling of heating of thermionic cathodes by high-pressure arc plasmas

Numerical investigation of steady-state interaction of a high-pressure argon plasma with a cylindrical tungsten cathode is reported. A whole ‘zoo’ of very diverse modes of current transfer is revealed. Detailed results are given for the first five (three-dimensional) 3D spot modes, four of them branching off from the diffuse mode and one from the first axially symmetric spot mode. Divergences in the general pattern of solutions, which have been present in preceding works, are resolved. Hypotheses on stability of steady-state solutions, available in the literature, are analysed. It is found that these hypotheses provide an explanation of the fact that the transition between diffuse and spot modes is difficult to reproduce in the experiment but they do not explain the indication that it is the low-voltage branch of the first 3D spot mode that seems to occur in the experiment. Thus, the question of stability of steady-state solutions remains open: an accurate stability analysis, as well as additional experimental information is required.

Cryosurgery for Prostate Cancer

This article describes a new minimally invasive treatment for men with recurrent cancer post radiotherapy and localised prostate cancer utilising argon and helium gases. Cryosurgery utilises high pressure argon and helium gas linked by a computer-controlled delivery system to cryoneedles which are introduced through the perineum into the prostate gland. Transrectal ultrasound allows accurate placement of the needles. Temperature needles are used to protect the sphincter muscle and rectum.

The molecular structure and a Renner-Teller analysis of the ground and first excited electronic states of the jet-cooled CS2+ molecular ion

The A 2Pi(u) - X 2Pi(g) electronic band system of the jet-cooled CS2 + ion has been studied by laser-induced fluorescence and wavelength-resolved emission techniques. The ions were produced in a pulsed electric discharge jet using a precursor mixture of carbon disulfide vapor in high-pressure argon. Rotational analysis of the high-resolution spectrum of the 2Pi32 component of the 0(0) 0 band gave linear-molecule molecular structures of r0" = 1.5554(10) A and r0' = 1.6172(12) A. Renner-Teller analyses of the vibronic structure in the spectra showed that the ground-state spin-orbit splitting (A = -447.0 cm(-1)) is much larger than that of the excited state (A = -177.5 cm(-1)), but that the Renner-Teller parameters are of similar magnitude and that a strong nu1 - 2nu2 Fermi resonance occurs in both states. Previous analyses of the vibronic structure in the ground and excited states of the ion from pulsed field-ionization-photoelectron data are shown to be substantially correct.

Visible continuum radiation in high-pressure argon and argon–mercury plasmas

The sources for continuum emission are investigated in the visible wavelength range for a high-pressure argon plasma as well as for a high-pressure argon–mercury plasma used for light application. The mechanisms responsible for the continuum emission depend differently on the pressure, the temperature and the ionization degree. By changing these conditions it is shown that one can modify the dominant continuum radiation mechanism.Operational improvement of the high-pressure argon–mercury test lamp are considered with respect to the continuum spectrum in the visible wavelength range.

Study of deposition and post-oxidation of d.c. magnetron sputtered W/Fe bilayers

W/Fe bilayer metallic coatings are deposited by d.c. magnetron sputtering on an Fe substrate. Correlation between the deposition parameters and the film characteristics are investigated. The coatings exhibit small grain sizes and dense structure for high target power and low argon pressure, leading to well adhesive films. Bilayer post oxidation process is preformed in order to form Fe3O4 at the surface (W/Fe3O4). The ability of the Fe layer to be oxidised is also studied depending on the bilayer deposition parameters.

Magnetic Properties of Pr–Fe–Al Alloys Produced by the Metallic Mold Casting Method

The structures and magnetic properties of Pr-Fe-Al alloys produced by the metallic mold casting method were investigated. The Pr-Fe-Al alloy consisted of the amorphous phase together with the non-ferromagnetic Pr phase. Magnetic measurements confirmed that the origin of coercivity in the alloys was the amorphous phase. It was found that the Curie temperature and coercivity of the alloys were strongly dependent on the composition of the amorphous phase. In this study, cylindrical bulk specimens of Pr-Fe-Al alloys have been produced by the metallic mold casting technique. The magnetic and microstructural studies of the resultant bulk materials were carried out to investigate the magnetic properties of the Pr-Fe-Al alloys produced by the metallic mold casting method. The relationship between the coercivity and Curie temperature of the resultant Pr-Fe-Al alloys are discussed. 2. Experimental Procedures Pr1� xFex (x ¼ 0:1{0:4) and Pr1� xyFexAly (x ¼ 0:1{0:4, y ¼ 0{0:2) alloy ingots were prepared by arc melting in an argon atmosphere. Small amounts of the ingots were placed in a quartz crucible with an orifice of 0.6 mm at the bottom. The ingots were induction melted in an argon atmosphere and then ejected through the orifice with high-pressure argon into a water-cooled copper mold. The resultant specimens had a cylindrical shape of 1-3 mm in diameter and 50 mm in length. In spite of the high oxidation tendency of rare-earth- containing alloys, the cylindrical specimens had a smooth surface. The composition of the specimens was determined by chemical analysis. The phases of the specimens were identified by X-ray diffraction (XRD) using Cu Kradiation. The thermal properties of the specimens were measured in an argon atmosphere at a heating rate of 0.16 K/s using a differential scanning calorimeter (DSC). The magnetization of the specimens was examined in a helium atmosphere at an applied field of 40 kA/m by a vibrating sample magnetom- eter (VSM). The VSM was calibrated using a pure nickel sphere. The hysteresis loops of the specimens (3 mm in diameter and 10 mm in length) were measured by a recording fluxmeter with a maximum applied field of 1.6 MA/m. 3. Results and Discussion

Physics of high-pressure helium and argon radio-frequency plasmas

The physics of helium and argon rf discharges have been investigated in the pressure range from 50 to 760Torr. The plasma source consists of metal electrodes that are perforated to allow the gas to flow through them. Current and voltage plots were obtained at different purity levels and it was found that trace impurities do not affect the shape of the curves. The electron temperature was calculated using an energy balance on the unbound electrons. It increased with decreasing pressure from 1.1 to 2.4eV for helium and from 1.1 to 2.0 for argon. The plasma density calculated at a constant current density of 138mA∕cm2 ranged from 1.7×1011 to 9.3×1011cm−3 for helium and from 2.5×1011 to 2.4×1012cm−3 for argon, increasing with the pressure. At atmospheric pressure, the electron density of the argon plasma is 2.5 times that of the helium plasma.

Continuum radiation spectroscopy in a high-pressure argon–mercury lamp

A primary study of the continuum emission from the arc of a high-intensity discharge (HID) lamp with an argon–mercury chemistry at pressures higher than 1 × 10 5 Pa is reported. Spectrally and spatially resolved calibrated continuum spectra from a vertically mounted test lamp (an MHN150 vessel filled at room temperature with 12 mg mercury and 400 mbar argon) are reported for the visible and infrared wavelength range 0.3– 5.5 μ m . These spectra were analyzed to quantify the infrared (IR) losses and to determine the temperature profile inside the lamp.

In situ high pressure FT-IR spectroscopy on alkene hydroformylation catalysed by RhH(CO)(PPh 3) 3 and Co 2(CO) 8

Catalytic hydroformylation of olefins has been carried out in a HP FT-IR cell using RhH(CO)(PPh 3) 3 catalytic precursor. A different behaviour was noticed between a terminal (hex-1-ene) and an internal alkene (cyclohexene) and different rate-determining steps of the catalytic cycle have been hypothesised. The hydroformylation of hex-1-ene has also been tested in the presence of Co 2(CO) 8 as catalyst. In this case, only the catalytic precursor is evidenced by HP FT-IR. Finally, the influence of an additional gas (helium, nitrogen or argon) in the reaction medium was evaluated: a high pressure of argon or nitrogen affects the initial rate of the reaction as shown by a decrease of the rate of the aldehyde formation.

Free Growth of 4H-SiC by Sublimation Method

4H-SiC crystals of cylindrical shape 25 mm and 45 mm in diameter have been grown by the Modified Lely method in a graphite crucible with a guide. The crystals had not mechanical contacts with the walls of the crucible and no formation of polycrystalline SiC during the growth to decrease the stresses. This was confirmed by the facets observed at the edges of the crystals. Numerical simulations of the growth process have been performed. Introduction Silicon carbide is a wide band gap semiconductor with unique properties that make it suitable for application in high frequency, high temperature and high power devices. These applications prompted increased interest in growth of high quality, large diameter single crystals with defect reduction. The objective of this paper is the study of 4H-SiC growth by the Modified Lely method without polycrystalline rim and mechanical contact with graphite walls of the crucible in order to decrease the stresses. Besides, the aim was to obtain crystals with a cylindrical shape and flat top surface to minimize the number of low grain boundaries. Numerical modeling has been used to optimize the design of the crucible. Experimental details 4H-SiC crystals , 25 and 45 mm in diameter, have been grown by the Modified Lely method over the 2000-2050°C temperature range (measured by the top pyrometer : 1 in Fig. 2) in argon [1] (Fig.1). An experimental set-up with RF heating and graphite crucible was used. The crucible (Fig.2) was wrapped in a graphite felt for thermal insulation, and the whole assembly was placed inside a water cooled quartz reactor. Under these experimental conditions we obtained 4H single crystals with a thickness of 5-10 mm. The growth rate was 200-300 μm/h. An increase of the growth rate resulted in 6H-SiC growth. The SiC source powder was loaded both between a dense graphite crucible and a thin walled inner graphite cylinder and inside of the inner cylinder. With this configuration it is possible to reduce the influence of the leakage of Si and the reaction of Si from the main central source with the walls in order to maintain an excess of Si over the seed. To eliminate the influence of the periphery we cut out the seed after gluing to the graphite holder and avoided 6H polycrystalline SiC growth at the periphery. 4H-SiC wafers of 25 and 45 mm in diameter were used as seeds. The ingots were grown on the C-face of seeds which are 8° off-axis. The distance between the powder source and the seed was 15-20 mm. The growth process consisted of : (a) annealing in vacuum at a temperature lower than 1000°C, (b) heating of the crucible up to growth temperature at a high argon pressure, (c) decrease of the pressure (3-5 torr) and growth at low argon pressure. Fig. 2 : Schem right-half par Numerical d Intense effor profiles in th especially rad without a gr minimize the in growth ca Knudsen equ total mass flu Here γi is th equilibrium p ) 25 mm (thickness 5mm Fig.1. Photographs of “as-grown” 4H-SiC atic representation of the Fig. 3 : Compar t of the reactor with guide in the growth ca etails t has been focused on the crucible design to optim e growth area [1-7]. Electromagnetodynamics must iative heat transfer within the growth cavity [1]. The aphite guide (3 on Fig. 2) was optimized by heat radial temperature gradient (Fig. 3). In order to consi vity, we have used a model based on the heterogeneo ations (Eq. 1) to relate the partial pressures of the spec x i R : ( ) RT 2 M P P R i eq i i i i S i π α − γ = (i=Si,SiC2,Si2C) e sticking coefficient and αi is the evaporation c ressures eq i P obey to mass action law. 2605 K without guide radial gradient : axial gradient : 2.Graphite crucible 4. Graphite foam 5. SiC seed 6. SiC powder 7. Induction coils 3. Graphite guide 1.Top pyrometer measure 45 mm (thickness 10mm)

Continuum radiation in a high pressure argon–mercury lamp

In typical high pressure high intensity discharge lamps about 60% of the deposited power is released in non-useful channels, non-radiative heat transfer from the hot arc core, ultraviolet radiation and infrared radiation, of which the last is the dominant one.To reduce the amount of infrared radiation emitted in high pressure argon–mercury lamps the sources of continuum radiation are identified, quantified and compared with results from emission spectroscopy. Objective data, especially for mercury, are lacking, which makes experimental verification necessary.Three possible continuum radiation sources were examined: electron–atom free–free bremsstrahlung, electron–ion free–free bremsstrahlung and electron–atom free–bound recombination radiation. The radiance caused by each of these processes can be calculated from test lamp fillings and the temperature profile in the lamp using a computer program. From comparing experimental results with computer calculations, the electron–mercury atom bremsstrahlung process has been determined as dominating the infrared radiation.

Influence of the ambient gas in laser structuring of the titanium surface

Surface structuring of titanium was performed by multipulse high repetition rate Nd:YAG ( λ=1.064 μm, τ=∼300 ns, ν=30 kHz) laser irradiations in vacuum as well as reactive or inert ambient gases at an intensity value below the single-pulse melting threshold. After irradiations, the surface morphologies have been investigated by scanning electron microscopy (SEM). The surface structuring is strongly influenced by the ambient gas. In high-pressure nitrogen, rippled, and with the increase of the laser pulse number, columnar microrelief formation takes place. In vacuum or in high-pressure argon, the surface morphology is characterised by smooth polyhedral structures developing in the surface plane. In air, the initial surface morphology consisting of a network of microcracks evolves with further irradiation towards a porous microrelief. The physical phenomena involved in the specific surface structure formation are discussed.

Surface processing of CdTe crystals by an excimer laser and its application in fabricating nuclear radiation detectors

Excimer laser based surface processing of CdTe crystals is investigated in two different ways, namely laser ablation and laser enhanced impurity doping, for their applications in fabrication of nuclear radiation detectors. Micro-pattering on CdTe surface is performed by irradiating the laser through a shadow mask placed on the CdTe in a high vacuum condition. On the other hand, single pulse laser irradiation on the thin In-layer evaporated CdTe surface at a high-pressure argon gas environment produced an In-doped n-type thin surface layer. Finally, a strip type CdTe nuclear imaging detector was constructed by utilizing these two distinct features of excimer laser. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Persistence rewarded: successful observation of the B 2Sigmau+-X 2Pig electronic transition of jet-cooled BS2.

The B-X electronic transition of jet-cooled BS2 has been observed using laser-induced fluorescence techniques. The boron disulfide radical was produced in a pulsed electric discharge jet using a mixture of BCl3 and CS2 in high-pressure argon as the precursor. The spectrum consists of a strong 0(0)(0) band with the 2Sigma-2Pi(3/2) component at 24,393.2 cm(-1) and short progressions in the symmetric stretching (nu1' = 506.7 cm(-1)) and bending (nu2' = 303.2 cm(-1)) modes. A rotational analysis of both spin-orbit components of the 0(0)(0) band gave an upper state B value of 0.0932779(19) cm(-1) and a ground-state spin-orbit coupling constant of A = -405.163(4) cm(-1). The ground-state bond length of 1.66492 angstroms increases to 1.6812(1) angstroms on sigmau --> pig electronic excitation. The B-X data have been used to further refine the Renner-Teller analysis, which is in good agreement with our previous work [J. Chem. Phys. 119, 2047 (2003)].

Deposition and structure of W–Cu multilayer coatings by magnetron sputtering

W–Cu–W multilayer metallic coatings are designed and deposited by dc magnetron sputtering on an Fe substrate. Correlations between the deposition parameters, such as target power and Ar gas pressure, and the film characteristics are investigated. Especially, deposition parameters for a dense W–Cu multilayer coating are discussed. The coatings exhibit small grain sizes and a dense surface structure for high target power and low argon pressure, leading to dense and well adhesive films.

Microstructure evolution during solid-state foaming of titanium

Solid-state foaming of commercially pure titanium was achieved by high-temperature expansion of high-pressure argon bubbles trapped in titanium by a powder-metallurgy technique. The foaming step was performed at constant temperature, where creep of the titanium matrix controls pore expansion, or during thermal cycling around the α⧸β allotropic temperature, which induces transformation superplasticity of the matrix. Superplastic foaming led to significantly faster pore growth and higher terminal porosity than isothermal creep foaming. During thermal cycling, the porosity remains nearly fully closed to the surface of the specimen up to the point where the maximum porosity (44%) is obtained, despite the presence of some internal pore coalescence. With continued thermal cycling, pores coalesce further by fracture of thin interpore walls and pores finally open to the surface, but without a significant increase in the amount of total porosity. The remnants of these walls result in a jagged pore morphology. Under isothermal conditions, pores remain small, equiaxed and unconnected with no pore surface roughness. However, after long annealing times, they exhibit faceting due to surface diffusion.