Showing papers on "Argon published in 2001"

Characterization of Lanthanum-Doped Barium Titanate Ceramics Using Impedance Spectroscopy

Abstract: The electrical properties of two single-phase, lanthanum-doped BaTiO3 compositions, x= 0.03 and x= 0.20, in Ba1–xLaxTi1–x/4O3 were investigated by impedance spectroscopy after heat treatment in oxygen, argon, and air at 1350°C. Samples heated in oxygen were electrically insulating, whereas those heated in argon lost oxygen and were semiconducting at room temperature, irrespective of x. Samples heated in air showed intermediate electrical properties and also were electrically inhomogeneous; the two compositions showed different electrical behaviors, and a model for each, based on oxygen nonstoichiometry within the ceramics, is proposed. Oxygen deficiency in samples sintered in air was avoided by heating at 1200°C, instead of 1350°C. Alternatively, oxygen lost from ceramics heated in air at 1350°C was regained by postannealing in oxygen at 1350°C.

Dramatic extension of the high-order harmonic cutoff by using a long-wavelength driving field

Abstract: We present an experimental demonstration of extending the high-order harmonic cutoff photon energy more than a factor of 2 when the driving-field wavelength is changed from 0.8 to 1.51 μm with an optical parametric amplifier. With argon gas, the cutoff has been extended from 64 to 160 eV. We predict that coherent keV x rays can be generated by exciting helium gas with the long-wavelength driving pulses. Experiments on xenon gas with several pump wavelengths also showed the dramatic cutoff extension, as well as full tunability of the generated XUV wavelengths.

Molecular simulation of adsorption: Gibbs dividing surface and comparison with experiment

Abstract: At low temperature near the normal boiling point, computer simulations of adsorption of gases can be compared directly with experiment. However, for adsorptive gas separations in which the gas is adsorbed near or above its critical temperature, absolute simulation variables must be converted to excess variables for comparison with experiment. The conversion of absolute to excess variables requires the helium pore volume of the adsorbent. Lennard-Jones potential parameters for helium gas molecules interacting with the oxygen atoms of silicalite are ϵ/k = 28.0 K and σ = 2.952 A. The helium pore volume of silicalite is 0.175 cm3/g. Lennard-Jones potential parameters derived for Ar-O interactions in silicalite are ϵ/k = 93.0 K and σ = 3.335 A. Density profiles for adsorption of argon in silicalite at 300 K show that the local density is highest in the middle of the channels where the gas-solid potentials overlap.

Gas Breakdown in an Atmospheric Pressure Radio-Frequency Capacitive Plasma Source

Abstract: Gas breakdown is studied in an atmospheric pressure rf capacitive plasma source developed for materials applications. At a rf frequency of 13.56 MHz, breakdown voltage is largely a function of the product of the pressure and the discharge gap spacing, approximating the Paschen curve. However, breakdown voltage varies substantially with rf frequency due to a change in the electron loss mechanism. A large increase in breakdown voltage is observed when argon, oxygen, or nitrogen is added to helium despite their lower ionization potential. Discussion is given for optimal breakdown conditions at atmospheric pressure.

Application of atmospheric pressure dielectric barrier discharges in deposition, cleaning and activation.

Abstract: Dielectric barrier discharges (DBDs) at atmospheric pressure are obtained using mixtures of He and Ar as carrier gasses and various reactive additives such as hydrocarbons, hydrogen and nitrogen. These DBDs are used in three applications: deposition of polymer films; cleaning of Ag and Cu substrates; and activation of polyurethane and steel surfaces. In the case of the film deposition, several process conditions are investigated and the resulting films are analysed by scanning electron microscope, Fourier transform infrared spectroscopy and NMR. In another series of experiments Ag and Cu surfaces, covered with sulfide and oxide layers, are treated by means of a DBD in helium or argon with hydrogen added. The surfaces are analysed with X-ray photoelectron spectroscopy. Finally, a He–N2 plasma is used as an activator of polyurethane.

On the effect of the electron energy distribution on the plasma parameters of an argon discharge : a global (volume-averaged) model study

Abstract: On the effect of the electron energy distribution on the plasma parameters of argon discharge : A global (volume averaged) model study

Ag8 Fluorescence in Argon

Abstract: The fluorescence of Ag8 in an argon matrix and in argon droplets is reported. This is the first unambiguous assignment of the fluorescence of a metal cluster larger than the tetramer, indicating that the excited state lifetime is longer than previously thought. It is discussed as a possible result of a matrix cage effect. The excitation spectrum is compared with two-photon-ionization measurements of Ag8 in helium droplets and to known absorption data. The agreement is excellent. We propose that the excited states relax rapidly through vibrational coupling to a long-lived state, from which the fluorescence occurs.

Depth profiling of multi-layer samples using femtosecond laser ablation

Abstract: An all solid-state femtosecond laser (λ0 ∼775 nm, pulse duration ∼170 fs, maximum pulse energy ∼0.5 mJ) with a Gaussian beam profile was used for depth profiling of Cu–Ag and TiN–TiAlN multi-layers on silicon and iron substrates. Laser-induced breakdown spectroscopy (LIBS) in argon was used for characterisation of the Cu–Ag samples, while laser ablation in a vacuum with time-of-flight mass spectrometry (TOF-MS) was applied for the characterisation of the TiN–TiAlN samples. The thickness of the individual Cu and Ag layers was 600 nm. Each individual TiN and TiAlN layer was 280 nm thick. The LIBS experiment was performed in the pressure range 10–1000 mbar. Variation of the pulse fluence from 0.8 to 1.5 J cm−2 caused a change of the ablation rate from 15 to 30 nm per pulse. The first layers of Cu and Ag could be satisfactorily resolved by LIBS. In femtosecond laser ablation TOF-MS a lower fluence (about 0.3 J cm−2) than in LIBS could be applied. The TiN–TiAlN multi-structures were well resolved. The Gaussian-type beam of the femtosecond laser limited the contrast of the detected depth profiles in both schemes. The complementary sensing techniques enable study of technical and physical limitations in the use of femtosecond laser ablation.

Bubble formation during horizontal gas injection into downward flowing liquid

Abstract: Bubble formation during gas injection into turbulent downward-flowing water is studied using high-speed videos and mathematical models. The bubble size is determined during the initial stages of injection and is very important to turbulent multiphase flow in molten-metal processes. The effects of liquid velocity, gas-injection flow rate, injection hole diameter, and gas composition on the initial bubble-formation behavior have been investigated. Specifically, the bubble-shape evolution, contact angles, size, size range, and formation mode are measured. The bubble size is found to increase with increasing gas-injection flow rate and decreasing liquid velocity and is relatively independent of the gas injection hole size and gas composition. Bubble formation occurs in one of four different modes, depending on the liquid velocity and gas flow rate. Uniform-sized spherical bubbles form and detach from the gas injection hole in mode I for a low liquid speed and small gas flow rate. Modes III and IV occur for high-velocity liquid flows, where the injected gas elongates down along the wall and breaks up into uneven-sized bubbles. An analytical two-stage model is developed to predict the average bubble size, based on realistic force balances, and shows good agreement with measurements. Preliminary results of numerical simulations of bubble formation using a volume-of-fluid (VOF) model qualitatively match experimental observations, but more work is needed to reach a quantitative match. The analytical model is then used to estimate the size of the argon bubbles expected in liquid steel in tundish nozzles for conditions typical of continuous casting with a slide gate. The average argon bubble sizes generated in liquid steel are predicted to be larger than air bubbles in water for the same flow conditions. However, the differences lessen with increasing liquid velocity.

Si-rich/SiO2 nanostructured multilayers by reactive magnetron sputtering

Abstract: Silicon-rich (SR)/SiO2 multilayered systems were produced by reactive magnetron sputtering, through an approach based on the ability of hydrogen, when alternatively mixed to the argon of the plasma, to reduce the oxygen originated from the SiO2 target. Optimum values of both hydrogen partial pressure (45 mTorr) and deposition temperature (500 °C) have led to the highest incorporation of Si in the SR layer which crystallizes after annealing. The SR/SiO2 superlattices grown with such conditions showed that the size of the Si nanocrystals is limited by the thickness of the SR layer. Considering the difference observed between the photoluminescence peak position and the predicted band gap for Si nanocrystals, the results suggest that we deal with a quantum-size confinement assisted by a Si–O vibration located at the interface.

Evaluation of porosity in porous copper fabricated by unidirectional solidification under pressurized hydrogen

Abstract: Lotus-type porous copper with elongated pores has been fabricated by a unidirectional solidification method in a mixture of hydrogen and argon gases with high pressure. The porosity and the pore morphology in the Lotus-type porous copper are significantly affected by the partial pressures of hydrogen and argon. The porosity has been evaluated on the basis of the idea that the gas pores are formed by the insoluble hydrogen during solidification of the liquid copper dissolving hydrogen. The evaluation gives the porosity as a function of the partial pressures of hydrogen and argon. The evaluated values are in good agreement with the experimental results.

Detection of particles of less than 5 nm in diameter formed in an argon-silane capacitively coupled radio-frequency discharge

Abstract: A method for the detection of dust particles occurring in silane–argon gas mixture plasmas is presented. It is based on the spectral analysis of the radio-frequency current. The amplitudes of the fundamental (13.56 MHz) and second harmonics (40.68 MHz) are very sensitive to the presence of the earlier nanoparticles when their size is in the range of 2–3 nm even if their influence on the capacitive character of the impedance is negligible. This method is nonperturbative, with a temporal resolution in the microsecond range, very easy to implement, and can thus be used for industrial reactors.

Atmospheric pressure glow discharge in neon

Abstract: The dielectric barrier discharge burning at atmospheric pressure usually has a filamentary non-homogeneous form. However, it was found that uniform dielectric barrier discharge can be generated in helium, nitrogen and in the mixture of argon with acetone under specific conditions. Such uniform discharge is called atmospheric pressure glow discharge (APGD). We studied dielectric barrier discharge burning in neon at atmospheric pressure and we found that the APGD can also be generated in neon. We measured the electrical characteristics of APGD in neon for different voltages and frequencies of power supply and different gas flows. We found that higher gas flow stabilizes the APGD and we determined the area of parameters in which the APGD burns. The images of discharges were recorded by a video camera and emission optical spectra were measured by a spectrometer. The properties of the discharges in neon were compared with the properties of discharges burning in argon and in the mixture of neon with argon.

High-pressure melting curves of argon, krypton, and xenon: deviation from corresponding states theory.

Abstract: The melting curves of argon, krypton, and xenon were measured in a laser heated diamond-anvil cell to pressures of nearly 80 GPa reaching melting temperatures around 3300 K. For the three gases, we observed a considerable decrease in the melting slopes $(dT/dP)$ from the predictions based on corresponding states scaling starting near 40, 30, and 20 GPa, respectively. The melting anomaly can be understood in terms of a model in which hcp stacking faults act as solutes in a binary system.

Diameter control of single-walled carbon nanotubes using argon–helium mixture gases

Abstract: A method is reported for controlling the diameter of single-walled carbon nanotubes (SWCNTs) during the electric-arc-discharge process. Using argon as inert atmosphere provides smaller diameters as compared with those when pure helium is used. Varying the gas mixture from argon to helium changes the diameter distribution to higher values. A linear fit of the average diameter shows a 0.2 A diam decrease per 10% increase in the argon–helium ratio.

a-C:H thin films deposited by radio-frequency plasma: influence of gas composition on structure, optical properties and stress levels

Abstract: a-C:H thin films are deposited by plasma-enhanced chemical vapor deposition (PE-CVD) at 13.56 MHz at room temperature. Three different precursor gas mixtures are used (CH 4 , CH 4 /He, CH 4 /Ar). Structure, optical properties and stress levels are evaluated by elastic recoil detection analysis (ERDA), IR absorption, UV/vis spectrometry, Raman spectroscopy. We observe a loss of hydrogen content (bonded and not bonded) from 38 to 24 at.%, as well as an increasing of sp 2 content (from 14 to 29%) with the increase of self-bias voltage for all mixtures. Argon and helium addition to methane induce a greater graphitization of a-C:H thin films. These modifications induce a decrease of the optical gap which is set between 1.4 and 1.1 eV and an increase of the Urbach gap from 0.6 to 0.8 eV. The internal stresses are controlled by subplantation model and decrease from 4 to 1 GPa with the increase of the bias voltage. The use of argon and helium as carrier gas induce lower stress in the films.

Silicon Particle Formation by Pyrolysis of Silane in a Hot Wall Gasphase Reactor

Abstract: The formation of silicon powder by pyrolysis of silane diluted in argon at different concentrations has been studied. A hot wall gas-phase reactor was used for the thermal decomposition of SiH 4 at 1000°C and atmospheric pressure. The composition, morphology, size, and shape of the particles produced has been studied utilizing electron microscopy, X-ray diffraction, infrared spectroscopy, and BET gas adsorption. Depending on the experimental conditions, agglomerates of polycrystalline, sintered particles have been obtained, which are composed of nanocrystallites of about 25 nm in size.

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: On-line studies with a laser ion source

Abstract: The application of a gas cell filled with noble gas (helium or argon) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in on-line conditions. Radioactive ions produced in nuclear reactions and stable energetic ions have been resonantly re-ionized by laser light via a two-step resonant process after thermalization and neutralization in high-pressure noble gas. The influence of the ion–electron density created by the projectile beam on the recombination of exotic ions has been investigated in different experimental conditions, including DC and RF electrical fields in the gas cell. Results for the laser ion source efficiency and selectivity for heavy-ion induced fusion reaction products are given. For the Rh isotopes the efficiency reaches up to 12%.

Rotational temperature measurements in air and nitrogen plasmas using the first negative system of N2

Abstract: Recent Fourier-transform spectrometry measurements of a wide array of rovibrational lines of the first negative system of N 2 + were used to develop a quantitative line-by-line radiation code that can be used for accurate rotational temperature determinations in nitrogen and air plasmas. The model is applied to the interpretation of spectral measurements obtained in a nonequilibrium nitrogen/argon plasma produced by a 50 kW radio-frequency inductively coupled plasma torch. Two rotational temperature determination techniques are presented that consist either in performing a global fit of the spectral region 3700–3920 A or in comparing the intensity of a single group of isolated lines at 3759.5 A to the intensity of the (0–0) band head. Both techniques yield a rotational temperature of 4850 K with an accuracy of better than 2%.

Laser induced fluorescence of argon ions in a plasma presheath

Abstract: The characteristics of presheaths near an electrically floating plate in weakly collisional argon multidipole plasmas are investigated with a combination of data from laser induced fluorescence using a diode laser, Mach probes, emissive probes, and Langmuir probes. It is shown that ion–neutral collisions result in an increase in ion temperature from approximately room temperature in the bulk plasma to 0.13 eV, 0.5 cm from the plate, the location of the closest measurement. In addition, at that point, the presheath plasma potential drop is greater than Te/2, and the drift velocity is equal to 0.5 cs, where cs is the ion sound velocity.

Sputter-deposited nanocrystalline Cr and CrN coatings on steels

Abstract: Chromium and chromium-nitride coatings on steels have been deposited with a magnetron sputter-deposition system. The deposition power was 200 W pulsed DC with a frequency of 185 kHz and 96% deposition duty. The substrate temperature was maintained at 200°C. The sputtering gas was argon mixed with 0, 3, 5 or 7% nitrogen. Results show that the average deposition rate was 1.2 μm/h and was not affected by the nitrogen content in the sputtering gas. Using X-ray diffraction analysis, it was observed that the deposited Cr under pure argon condition, was nanocrystalline bcc chromium with a particle size in the range of 7–8 nm. With an increasing nitrogen content in the sputtering gas the amount of CrN increased. The measured microhardness of the chromium-coated steel increases with the increasing nitrogen content. With less than a 2-μm Cr coating, the steel hardness increases from 129 to 255 HV when the nitrogen content in the gas is 7%. The microhardness of Cr/CrN-coated steel prepared with sputter deposition is superior to that prepared with electroplating. The hardness of the coating layer calculated from these data is 1270 HV. Scratch tests were used to characterize coating adhesion. The critical load was determined to be the applied load under which an acoustic noise was found and cracks in the scratch track were first observed. The critical loads of deposited films with 0, 3, 5 and 7% N 2 in the gas, were 1.57, 5.68, 8.33 and 20.29 N, respectively.

Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe

Abstract: A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.