Showing papers on "Argon published in 1985"

Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low‐pressure argon positive column

Abstract: The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low‐pressure argon positive column is investigated theoretically and experimentally. A simple yet self‐consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady‐state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05

Distribution of the heat and current fluxes in gas tungsten arcs

Abstract: The distribution of heat flux on a water-cooled copper anode as a function of welding process parameters has been determined experimentally following an experimental technique developed previously. The results indicate that arc length is the primary variable governing heat distribution and that the distribution is closely approximated by a gaussian function. The half width of the heat flux is defined by a distribution parameter, σ, which was determined from the experimental data and is expressed as a function of arc length, current, and electrode tip angle. The distribution parameter, σ, increases from 1.5 mm to 3.6 mm as the arc length increases from 2 mm to 9 mm for a 100 A arc. The experimental data also show that arc energy transfer efficiency is greater than 80 pct on the water-cooled anode which is much higher than has been measured in the presence of a molten metal pool. For this reason, it is believed that the distribution of the heat flux and not the magnitude is the most useful information obtained in this study. The effect of helium additions to the argon on the heat distribution is also reported.

Absolute cross sections for multi-electron processes in low energy Arq+−Ar collisions: Comparison with theory

Abstract: Absolute cross sections have been measured for a variety of multi-electron processes in low-energy collisions of multiply charged argon recoil ions with neutral argon. The cross sections are compared with theoretical estimates based on an extension of the classical barrier model. Comparison is also made with the statistical theory of Muller et al.

Spatial dependence of particle light scattering in an rf silane discharge

Abstract: A capacitively coupled, rf glow discharge of silane in argon was studied to determine the spatial dependence of particle light scattering. Light scattering from a pulsed laser at 500 nm shows sharp spatial peaks of scattered intensity near the electrodes. These scattering peaks are correlated with the plasma sheath edges because they exhibit spatial changes with discharge pressure characterized by a constant product of pressure and distance from the electrode. Light scattering at various silane mole fractions and gas flow rates is compatible with concepts of particle growth in sharp spatial zones near the plasma sheaths.

Large-area lanthanum hexaboride electron emitter

Abstract: A large‐area cathode assembly which is capable of continuous, high‐current electron emission is described. The cathode utilizes an indirectly heated lanthanum hexaboride (LaB6) disk as the thermionic electron emitter. The LaB6 cathode emits over 600 A of electrons at an average of 20 A/cm2 continuously with no observable lifetime limits to date after about 400 h of operation in a plasma discharge. Proper clasping of the LaB6 disk is required to avoid impurity production from chemical reactions with the holder and to provide adequate support if the disk fractures during rapid thermal cycling. Modification of the LaB6 surface composition due to preferential sputtering of boron by hydrogen and argon ions in the plasma discharge has been observed. The surface appearance is consistent with the formation of LaB4 as a result of boron depletion. The electron emission capability of the cathode is not significantly altered by the surface change. This surface modification by preferential sputtering is not observed i...

Superheating of small solid-argon bubbles in aluminum.

Abstract: Small solid-Ar bubbles, formed by the implantation of 50-keV Ar+ ions into Al, are observed to melt at 730 K, compared with an equilibrium bulk melting temperature of 250 K. A model based on the suppression of Ar thermal vibrations at the Al interface is used to account for the superheating of 480 K. Measurements on the temperature dependence of diffracted electron intensities from these bubbles yield Debye-Waller factors consistent with an effective Debye temperature of about 140 K, compared with a bulk value of 110 K.

The conversion of HCN to NO and N2 in H2−O2−HCN−Ar flames at low pressure

Abstract: We have investigated the kinetic mechanism for converting HCN to NO and N 2 in low pressure (30 Torr) H 2 −O 2 flames doped with HCN and diluted in argon. Major stable species concentrations were measured by mass spectrometric techniques and CN concentrations and temperature were measured by laser induced fluorescence. The three flames investigated had equivalence ratios of Φ=0.5, 1.0, and 1.5. The experimental results are compared to complete flame structure calculations based on a reaction mechanism developed almost solely from direct measurements of rate coefficients reported in the literature. The agreement is good. From the analysis we deduce that the nitrogen atom in HCN predominantly follows the following route to NO and N 2 , HCN→NCO→NH→N→NO→N 2 . The most important and rate controlling step for HCN removal is the reaction with oxygen atoms, HCN + O ⇄ products. Our results are not sensitive to the products formed: NCO+H or NH+CO. Most important and rate controlling for NO production and conversion of NO to N 2 is the extended Zel'dovich mechanism.

Quantum Monte Carlo study of the thermodynamic properties of argon clusters: The homogeneous nucleation of argon in argon vapor and ‘‘magic number’’ distributions in argon vapor

Abstract: The thermodynamic properties of clusters of argon atoms are studied by a combination of classical and quantum mechanical Monte Carlo methods. The argon atoms are represented by Lennard‐Jones interactions and internal energies, free energies, and entropies are calculated as a function of temperature and cluster size. For the argon system quantum effects and anharmonicity corrections are found to be simultaneously important for a temperature range from 15 to 20 K. By examining local minima in the free energy of formation of argon clusters as a function of cluster size, magic numbers in the Boltzmann mass distribution are observed at n=7, 13, and 19 under some conditions of temperature and pressure. In some cases magic numbers are predicted in the quantum and not in the classical calculation. The entropy changes associated with cluster growth are found to be insensitive to cluster size. Quantum corrections are calculated to nucleation rates and found to be very important at low temperatures.

Electronic quenching, rotational relaxation, and radiative lifetime of NH(A 3Π, v’=0, N’)

Abstract: Using ArF (193 nm) laser photolysis of NH3, excited triplet NH(A 3Π, v’=0, N’) radicals were generated with a hot rotational distribution (N’max=8–13). From the decays of the excited radicals, the average lifetime for this rotational distribution was determined to be 465±14 (3σ) ns. The values of the rate constants for the electronic quenching by NH3, NO, CH4, H2, CO, Xe, O2, CO2, Ar, N2, and He were found to decrease in the given order. Rotational relaxation was found to be so efficient to compete with the electronic quenching. The electronic quenching of rotationally relaxed NH(A 3Π, v’=0, Trot=300 K) by NH3 and H2 was observed to be 1.6 and 1.5 times faster than that of the unrelaxed system. The trend in the rate data can be described by Parmenter’s correlation for most molecules studied, hence indicating attractive forces being effective during the quenching process.

Two-dimensional analysis of free burning arcs in argon

Abstract: The authors present a theoretical model of a free burning argon arc. Equations describing the conservation of mass, momentum and energy are solved together with Ohm's law and Maxwell's equation for the magnetic field in a cylindrical coordinate system. The material properties used in these solutions are derived theoretically, using the assumption of local thermodynamic equilibrium (LTE). Calculations are made for an axisymmetric, laminar arc burning between a rod electrode and a plane electrode. Comparisons between predicted and measured temperatures and velocities show good agreement for 100 A, 200 A and 400 A arcs in argon for a pre-specified current density of 108 A m-2 at the rod electrode and a plasma pressure of 1 atm. They describe the consequences of variations in the current density at the rod electrode and in the net radiation losses. They also discuss the importance of energy transfer due to the electron convection current and analyse in detail the various components which contribute to the momentum and energy conservation.

Surface treatment of plastics materials

Abstract: Discontinuities, e.g. scratches, in a plastics surface are removed by exposing the surface to a pulsed radio frequency plasma comprising a mixture of argon and hydrogen. After melting has been effected the surface is exposed to an argon plasma to effect cross-linking to form a smooth surface layer.

Direct observation of the fine structure transitions in the Ne+ and Ar+ ions with diode lasers

Abstract: The 2P1/2–2P3/2 fine structure transitions in the ground (np5) state of the neon (n=2) and argon (n=3) atomic ions have been observed in absorption in the positive column of a liquid–nitrogen cooled discharge cell, by employing tunable infrared diode lasers as sources. The transition frequencies were measured to be 780.4240±0.0011 and 1431.5831±0.0007 cm−1 for Ne+ and Ar+, respectively. A few transitions have also been observed for the neutral Ne and Ar atoms.

A study of ionization electrons drifting large distances in liquid and solid argon

Abstract: A two liter gridded ionization chamber was built and operated repeatedly to study electron transport in liquid and solid argon over distances up to 10 cm in drift fields of 0.01–2 kV/cm. A detailed analysis of the ionization pulse shape was performed, yielding information on both the degree of purity of the liquid and the electron drift velocity. Argon with an impurity concentration less than 0.2 ppb oxygen equivalent was obtained with a simple purification system. The measured attenuation length extrapolates to 7.5 m for liquid argon and to 1 m for solid argon at a drift field of 1 kV/cm.

Low temperature reactive sputter deposition of vanadium oxide

Abstract: In the present study, vanadium oxide films were grown on water‐cooled sapphire, silicon, and glass substrates by reactive sputter deposition using a vanadium target and rf‐excited argon discharges containing 0%–25% oxygen. Optical emission spectroscopy was used for in situ discharge diagnostics. Films were characterized by x‐ray diffraction to determine crystallography and by double‐beam spectrophotometry to determine optical absorption behavior in the 200–1200 nm spectral region. The results show that film crystallography evolves from single (110) orientation body‐centered cubic vanadium to single (001) orientation orthorhombic vanadium pentoxide to a material in which there is no long range atomic order detectable by x‐ray diffraction as the sputtering gas oxygen content is increased. Changes in optical absorption behavior which accompany the decrease in atomic order are presented and discussed.

An investigation of non-equilibrium effects in thermal argon plasmas

Abstract: The parameters and transport properties of a wall stabilized argon arc (40-200 A) at atmospheric pressure with diameters of 5 and 8 mm are studied by spectroscopy and interferometry. The plasma is assumed to be partial local thermal equilibrium and this assumption is verified with the aid of a collisional-radiative model. The departures from Saha-equilibrium of the argon neutral ground state are found to be associated with particle diffusion and the escape of recombination radiation. The measurement of the total excitation rate, from the ground level, including direct ionization, of neutral argon is in reasonable agreement with the literature value

Process for producing high concentration oxygen by a pressure-swing-adsorption method

Abstract: In production of high concentration oxygen by a pressure-swing-adsorption method using air as a raw material, air is introduced into a first step adsorption apparatus packed with a zeolite molecular sieve and is subjected to a pressure-swing-adsorption operation to obtain a non-adsorbed gas comprising oxygen as the major component, argon and nitrogen; the non-adsorbed gas is introduced into a second step adsorption apparatus packed with a carbon molecular sieve and is subjected to a pressure-swing-adsorption operation to separate argon and nitrogen from oxygen as a non-adsorbed gas; and the desorbed oxygen is obtained as high concentration oxygen.