Showing papers on "Argon published in 1987"

Amorphous water ice and its ability to trap gases

Abstract: The trapping of argon by amorphous water ice at 19--80 K and its release from the ice were studied experimentally, by flowing gas into ice, or by codepositing a gas-water vapor mixture. Upon warming the gas-laden ice, the trapped argon is released from it in seven temperature ranges: (a) 23 K, (b) 35 K, (c) 44 K, (d) \ensuremath{\sim}80 K, (e) 136.8 K, (f) 160.0 K, and (g) \ensuremath{\sim}180 K. The amount of internally trapped argon, in (b)--(g), can be as high as 3.3 times the amount of the ice itself. By using argon to probe the ice's structure and dynamics, it was found that the highly porous amorphous ice anneals at all temperatures, at a rate which is strongly temperature dependent, and transforms into the cubic form at 136.8\ifmmode\pm\else\textpm\fi{}1.6 K and then into the hexagonal form at 160.0\ifmmode\pm\else\textpm\fi{}1.0 K. When gas is made to flow into the amorphous ice, it fills open holes and cracks in it with gaseous or frozen argon, depending upon the temperature. The slow creeping of the ice closes some gas-filled holes and squeezes out some of the gas, while locking the rest and letting it escape only during the transformations. The last range, (g), is attributed to the simultaneous evaporation of gas and water from the argon clathrate-hydrate. Gas flow at a pressure exceeding 2.6 dyn ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, results in a very fluffy ice, with a density of only 0.17 g ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The release of gas from this kind of ice, or from ice codeposited with gas, is accompanied by massive ejection of 0.1--1-\ensuremath{\mu}m ice grains and by argon jets, which propel them. Many of the experimental findings could be important for interpreting observations on comets, icy satellites, icy ring particles, and interstellar grains.

Recombination of electron-ion pairs in liquid argon and liquid xenon.

Abstract: We have measured the electric field dependence of electron-ion recombination in liquid argon and xenon. The observed relationship is incompatible with Onsager's geminate theory [Phys. Rev. 54, 554 (1938)] of recombination so we have developed a new, single-parameter model to describe the data. The model is based on realistic assumptions about liquid argon and xenon and yields a simple, analytic result. This work is part of a program to build a high-resolution liquid-xenon detector.

Pulsed beam Fourier transform microwave measurements on OCS and rare gas complexes of OCS with Ne, Ar, and Kr

Abstract: A pulsed molecular beam Fourier transform microwave spectrometer, which has been recently constructed at NBS, was employed for measurements on several monomer and van der Waals species of OCS. The absorption–emission cell consists of a Fabry–Perot resonant cavity inside a high vacuum chamber. A pulsed nozzle is used to generate a supersonic molecular beam of a seeded inert gas. New spectra of Ne–OCS, Ar–OC34S, and for four Kr isotopes of Kr–OCS are reported, as well as structure analyses of each species. The nuclear electric quadrupole hyperfine structure of 83Kr–OCS has been resolved and yields eQqaa=1.601(7) MHz and eQqbb=−1.857(3) MHz.

Quenching rate coefficients for N2(a′ 1Σ−u)

Abstract: We have studied the kinetics of the lowest excited singlet state of molecular nitrogen, N2(a′ 1Σ−u,v=0), in a discharge flow reactor. The metastables were generated in a hollow cathode dc discharge through molecular nitrogen highly diluted in argon, and detected by VUV flourescence of the forbidden N2(a′ 1Σ−u–X 1Σ+g) band system. Observations of the equilibrium between the N2(a 1Πg) and N2(a′ 1Σ−u) states in our reactor indicate that the metastable has a radiative lifetime ≳23+11−6ms, assuming a radiative lifetime of 80+40−20μs for N2(a 1Πg). Rate coefficients for quenching by NO, CH4, CO, and N2O are approximately gas kinetic, those by H2, O2, and CO2 roughly tenth gas kinetic, and that by molecular nitrogen (1.9±0.5)×10−13 cm3 molecule−1 s−1. The interaction between N2(a′) and CO produced CO(A 1Π–X 1Σ) emission in about 21+10−8% of the quenching events.

X-Ray photoemission study of plasma modified polyethylene surfaces

Abstract: X-Ray photoelectron spectroscopy (XPS) was used to determine plasma induced chemical species on the surface of polyethylene (PE). Argon plasmas were found to have no detectable chemical effect on the PE surface, whereas oxygen and nitrogen plasmas created new chemical species which altered the chemical reactivity of the PE surface. Oxygen plasmas were found to react more rapidly with the PE surface than nitrogen plasmas. The degree of incorporation of new chemical species in the near surface region is approximately 20 at. % at the saturation level for both oxygen and nitrogen plasmas. Core level spectra for oxygen and nitrogen plasma treated PE suggest the formation of primarily C-O-C species in the former and C-N species in the latter. Angle-resolved XPS measurements indicate that the depth of incorporation of new chemical species is confined to the top 25 A.

A two‐temperature model of the inductively coupled rf plasma

Abstract: A two‐temperature model is proposed for the computation of the two‐dimensional flow and temperature fields in a rf inductively coupled plasma torch. The model is applicable to monatomic gases. The results obtained for an argon plasma indicate that, while at atmospheric conditions, deviations from local thermodynamic equilibrium (LTE) are relatively small, the situation is different under reduced pressure conditions, where substantial deviations from LTE have been noted, particularly in the energy addition region.

Measurement of the universal gas constant R using a spherical acoustic resonator

Abstract: A spherical acoustic resonator has been used to redetermine the universal gas constant R with an uncertainty of 1.8 ppm (standard deviation). To accomplish this, three subtasks were completed. (1) The volume of a spherical shell was determined by weighing the mercury required to exactly fill it at the temperature of the triple point of water, 273.16 K. (2) With the resonator filled with commercially supplied argon, the resonance frequencies of the radial modes were measured as a function of pressure. Using our theoretical model for the cavity, the frequency measurements were combined with the mean resonator radius determined in subtask (1) to obtain the speed of sound in commercially supplied argon. (3) Finally, the speed of sound in the commerically supplied argon was compared to the speed of sound in a “standard” sample whose chemical and isotopic composition was accurately established.

The diffusion of hydrogen in silicon and mechanisms for “unintentional” hydrogenation during ion beam processing

Abstract: Experiments are described in which hydrogen is injected into silicon by various techniques and detected by the neutralization of boron acceptor sites. Wet chemical etching is shown to inject protons several microns in a few seconds; this experiment is used to set a lower limit on the diffusivity of hydrogen of ⋍2⊠10−11 cm2/s at 300 K, a number in reasonable agreement with prior estimates deduced by Van Wieririgen and Warmholtz from high-temperature permeation measurements. A number of experiments are reported to elucidate the mechanism for “unintentional” hydrogenation occurring during argon ion bombardment. The data suggest that this effect is caused by bombardment-induced injection of hydrogen from surface H2O/hydrocarbon contaminants.

Degradation of tin‐doped indium‐oxide film in hydrogen and argon plasma

Abstract: The degradation of tin‐doped indium‐oxide (ITO) films in glow‐discharge plasmas of hydrogen and argon have been investigated. Parameters which have been varied for the study include the temperature of ITO under ion bombardment, the rf power density, the time of exposure to plasma, and the gas flow rate. After bombardment, scanning electron micrograph observation, measurement of sheet resistance, transmittance and reflectance, and Auger analysis have been carried out to decide the extent of degradation. Magnetron‐sputtered ITO films have been found to be more resistant to ion bombardment damage compared to electron‐beam evaporated films. The degradation of ITO under the plasma of the reducing species such as hydrogen has been found to take place at lower temperature and power density compared to argon plasma.

Electron impact excitation of rare gases: differential cross sections and angular correlation parameters for neon, argon, krypton and xenon

Abstract: Distorted-wave Born approximation results for the differential cross sections and various angular correlation parameters for electron impact excitation of the rare gases Ne, Ar, Kr and Xe are presented and compared with recent experimental data and other theoretical calculations. The sensitivity of the results to different static and optical potentials in the calculation of the distorted waves is analysed and the importance of relativistic effects both in the description of the target states (intermediate coupling) as well as in the wavefunction for the continuum electron is investigated. The overall agreement with the available experimental data is very satisfactory over a wide range of incident-electron energies.

The use of Langmuir probes and optical emission spectroscopy to measure electron energy distribution functions in RF-generated argon plasmas

Abstract: A single Langmuir probe technique has been used to measure the electron energy distribution function (EEDF) in an RF (13.56 MHz)-generated argon plasma in a reactive ion etcher. This was achieved by applying a driving RF signal to the probe to compensate for the effects of RF fluctuations in the plasma potential and then using the second differential of the probe characteristics to obtain the EEDF. It is observed that the EEDF is not well represented by a Maxwellian, Optical emission spectra (4600-4900 AA) were also recorded. In argon plasmas, generated at a constant power level, the average electron energy deduced from the Langmuir probe increases as the pressure is reduced below 20 mTorr. At 5 mTorr the average energy is 8.5 eV whereas above 20 mTorr the average energy lies in the range 3.5-5.3 eV. This is found to correlate with the variation in the intensity of an emission line (4s'(1/2)10 from 5p(1/2) at 4702.32 AA) in an excited argon atom, which can be accounted for by the presence of an increasing fraction of higher-energy electrons at lower gas pressures.

An electrostatic probe technique for RF plasma

Abstract: The use of electrostatic probes in radio frequency (RF) generated plasmas is discussed. A technique is demonstrated in which the RF potential difference between probe and plasma is removed by driving the probe with synchronous RF of suitable phase and amplitude. An application of this technique is the measurement of electrical structure within a 13.56 MHz argon discharge.

Microwave discharges produced by surface waves in argon gas

Abstract: Reports on an experiment in argon discharges produced by surface waves at 2.45 GHz. A self-consistent description of these discharges at low pressure (p<10 Torr) is presented. Characteristics such as the electron density, the maintaining electric field (E), the collision frequency for momentum transfer v and the mean power required for maintaining the electron-ion pair theta are determined as functions of the product of the pressure and the plasma diameter. In particular scaling laws are verified for v and theta . It is concluded that experimental surface-wave discharges behave similarly to positive-column discharges.

Spectroscopic studies of plasma during cw laser materials interaction

Abstract: The role of plasma during cw laser materials interaction was studied using emission spectroscopy. The experiments were carried out in two stages: first, with a pure argon plasma; and second, with an aluminum target in an argon atmosphere. The electron‐temperature distribution and electron density were determined. The experimental results were used to estimate laser attenuation and refraction as the beam propagated through the plasma column. Thus, transmitted energy and energy distribution on the target surface were determined. This was expected to be a more realistic input for transport phenomena models based on absorbed energy.

Method of fabricating stabilized threshold switching material

Abstract: A method of stabilizing the switching characteristics of a thin film chalcogenide glass material by subjecting said material to a hydrogenated atmosphere, preferably activated hydrogen and argon. It is also preferred to provide a post hydrogenation anneal step.

Ceramic coatings produced by means of a gas tunnel‐type plasma jet

Abstract: A gas tunnel‐type plasma jet has a higher temperature than any other plasma jets; a high‐power stabilized plasma beam can be easily obtained. Moreover, it is possible to inject various materials from the center electrode toward the center of this plasma beam. These properties of the gas tunnel‐type plasma jet apparatus are of great advantage to the application to the thermal spraying of high melting point materials such as ceramics. The fundamental characteristics of this new type of plasma spraying apparatus were determined through experimentation, and a variety of ceramic coatings produced using this apparatus were studied. Since this system is very effective in heating and melting sprayed powder, high‐quality ceramic coatings can be easily produced even at a small power using argon as the working gas. At 30 kW or more, the Vickers hardness of the various ceramic coatings is 30% greater than that produced with a conventional‐type plasma spraying apparatus. For example, a Vickers hardness value of 1200 has been achieved for Al2O3 coatings.

Equilibria of nitrogen, oxygen, argon, and air in molecular sieve 5A

Abstract: Pure-component equilibrium data for nitrogen, oxygen, and argon, and multicomponent equilibrium data for air in 20 × 40 mesh 5A molecular sieve were measured at 297.15, 233.15, and 203.15 K and up to pressures of 4 bar. The multicomponent data were analyzed by a statistical thermodynamic model, ideal adsorbed solution theory, an extended Langmuir-Sips equation, and an extended Langmuir equation. The multicomponent system exhibited nonideal behavior at 233.15 and 203.15 K. Isotherm parameters determined in this work may be useful in the design and evaluation of molecular sieve air separation systems.

In situ silicon-wafer temperature measurements during RF argon-ion plasma etching via fluoroptic thermometry

Abstract: In situ silicon-wafer temperature measurements during RF argon-ion plasma etching are reported for four different wafer cooling environments: (i) lying on the electrode; (ii) clamped to the electrode; with additional helium gas between wafer and electrode in both (i) and (ii), in order to enhance heat transfer. Fluoroptic thermometry is applied to measure the local surface temperature of the wafer during plasma processing. The measured heating and cooling curves can be fitted using a model which considers conductive heat losses only. Deviations are observed when high RF power is applied and surface temperatures higher than 120 degrees C are reached. The authors report on effective cooling via helium gas of the rear side of the wafer which is lying on an electrode without any clamping.

The thermal conductivity of argon, carbon dioxide and nitrous oxide

Abstract: The paper presents new, absolute measurements of the thermal conductivity of three gases: argon (Ar), carbon dioxide (CO 2 ) and nitrous oxide (N 2 O). The measurements have been carried out with a transient hot-wire instrument in the temperature range 308 to 430 K and at pressures up to 11 MPa. For most of the range of thermodynamic states covered by the measurements it is estimated that the accuracy of the thermal conductivity data is one of ±0.3%. However, for carbon dioxide and nitrous oxide near their critical conditions the accuracy is degraded and the uncertainty may be as much as ±2%. The new experimental data for argon confirm the accuracy claimed for the thermal conductivity in the limit of zero-density. The thermal conductivity of the polyatomic gases in the limit of zero-density is used in conjunction with information on other transport cross-sections for the same systems, to extract a consistent set of cross-sections sensitive to the anisotropy of the intermolecular pair potential for use in the testing of proposed potential surfaces. Cross-sections for both of the available formulations of the thermal conductivity of a polyatomic gas due to Wang Chang and Uhlenbeck and Thijsse et al. are derived. For both gases it is shown that the Mason-Monchick approximation breaks down in either formulation. However, the effect of the failure on the formulation of Thijsse et al. is smaller and it is possible to represent the data with the aid of a single cross-section which has a very simple temperature dependence. The analysis also demonstrates that all available high-temperature thermal conductivity data for carbon dioxide are in substantial error. In the moderately dense gas the concept of a temperature-independent excess thermal conductivity is confirmed to a high degree of precision for carbon dioxide and argon when due allowance is made for the critical enhancement. A generalized correlation of the temperature dependence of the first density coefficient of thermal conductivity is broadly confirmed.

A gas ion source for radiocarbon dating

Abstract: An ion source for operation with carbon dioxide gas has been designed and built. We report on the operation of this source on an ion source test bench at Oxford. The ion source is capable of giving C − currents of up to 24 μA at an efficiency of approximately 10% by sputtering the gas adsorbed on to a titanium surface. Different titanium targets are used for each sample and with sample changing times of 5 min the cross contamination between samples is measured to be less than 0.1%. Background currents have been reduced to about 70–100 nA by cutting down the hydrocarbons present in the vacuum system. These background currents are found to be constant to within 10% when argon or air is fed into the ion source instead of carbon dioxide. The background currents can therefore be subtracted when the ion source is used for radiocarbon measurements. A sample changing mechanism for twelve targets is used which operates in vacuum and, together with the rest of the ion source, is under computer control for unattended operation.

Experimental Study of the Effect of Gas Pressure on Arc Cathode Erosion and Redeposition in He, Ar, and SF6 from Vacuum to Atmospheric Pressure

Abstract: Experimental data on the erosion rates of a copper cathode in He, Ar, and SF6 from 10-6 to 760 torr are presented. The work performed by the cathodic-erosion plasma expanding against the gas is found experimentally to be constant, the volume of the expanding plasma cloud being linked to the gas pressure by the relation R3p = constant. These data agree with a redeposition model based on condensation of the metallic vapor produced by the arc on the cathode surface. The redeposited mass on the cathode is found to be proportional to the cube root of the gas mass density.

Argon recovery from hydrogen depleted ammonia plant purge gas utilizing a combination of cryogenic and non-cryogenic separating means

Abstract: An improved process is disclosed for argon recovery from an ammonia synthesis plant purge gas stream comprising hydrogen, nitrogen, argon, methane, and ammonia. This purge gas is conventionally subjected to ammonia absorption and a first membrane separation of hydrogen for recycle to the ammonia plant. The hydrogen depleted non-permeate gas stream from the first membrane separator, comprising the aforesaid four components, and any residual moisture from the ammonia absorption, is subjected, according to a first embodiment of the present invention, to the following steps: (i) Separation of methane and residual moisture and most of the nitrogen in the gas stream in a pressure swing adsorption system using molecular sieve or activated carbon material. (ii) Separation of most of the hydrogen in a second membrane separator. The separated hydrogen may be used as purge gas for regeneration of the pressure swing adsorption systems of step (i). (iii) Separation of the nitrogen and residual hydrogen by cryogenic distillation to obtain essentially pure liquid argon product. In a second embodiment, step (ii) above is eliminated and separation of hydrogen is accomplished by cryogenic separation in combination with step (iii). In a third embodiment, separation of most of the hydrogen is accomplished by metal hydride adsorption in place of membrane separation. The present process is equally applicable with minor modifications for the recovery of argon from ammonia synthesis plant purge gas streams, wherein PSA or cryogenic units are employed instead of the first membrane separator, in order to recover hydrogen for recycle to the ammonia synthesis plant.

Nitrogen, oxygen, and argon incorporation during reactive sputter deposition of titanium nitride

Abstract: For the reactive sputter deposition of titanium nitride, stress and resistivity of the films has been measured as a function of the processing variables target power, substrate bias, pressure, and N2/Ar ratio. These studies were limited to the conditions that produce titanium nitride of stoichiometry near 1. Through Rutherford backscattering spectroscopy, the changes in stress and the conductivity of the films as a function of the processing variables were interpreted in terms of nitrogen, argon, and oxygen concentration in the films. The increase in resistivity of the films correlates with increased oxygen incorporation and the increase in compressive stress of the films correlates with increased argon incorporation. The amount of oxygen in the films appears to produce a unique value of resistivity but the argon concentration that produces a given compressive stress is a function of the processing parameters that control argon incorporation.

Doubly charged argon clusters and their critical size

Abstract: The electron impact ionization of neutral Ar clusters results in the formation of stable doubly charged Ar2+n only if their size exceeds a critical value. The observed minimum cluster size for argon is Ar2+91 . This result is compared with theoretical predictions.