Torr Pressure Research Articles

High-rate growth of single crystal diamond in AC glow discharge plasma

The detailed experimental behavior of an AC abnormal glow discharge plasma assisted chemical vapor deposition reactor for single crystal diamond (SCD) synthesis operating within the wide-range (20–250 Torr) pressure regime was presented. Within the discharge operating window absorbed power densities of 250–9000 W/cm3 were achieved and high quality SCD synthesis with growth rate up to 80 μm/h was demonstrated. At the power densities over 2500 W/cm3 the diamond substrates were located directly on water-cooled substrate holder without any intermediate molybdenum holder. The influence of several input experimental variables including pressure, N2 and Ar concentration, CH4 percentage, and discharge current on SCD deposition was explored. The quality of synthesized crystals approved competitive purity level of glow discharge CVD process.

Measurements of Metastable Molecules in Nonequilibrium Supersonic Flows

Nonintrusive laser diagnostics are used for the measurements of metastable nitrogen molecules in the lowest excited electronic state, , in a nonequilibrium flow, blowdown supersonic wind tunnel. The tunnel is operated with nitrogen at plenum pressures of Torr, with the flow expanding through a two-dimensional contoured nozzle to the Mach number of . The steady-state run time of the tunnel is approximately 10 s. The flow is excited by a repetitive nanosecond pulse discharge operated in the plenum or in the nozzle throat. Time-resolved absolute populations in the plenum are measured by single-pass, continuous wave tunable diode laser absorption spectroscopy (TDLAS). In the supersonic test section, absolute populations are measured by cavity ring down spectroscopy (CRDS), using a tunable pulsed laser system operated at 10 Hz. During each run, 50 single-shot ring down traces are acquired, demonstrating good shot-to-shot reproducibility. The results demonstrate that the cavity ring down time is not affected by the supersonic flow. populations and the flow temperature are inferred from the single-shot CRDS data. At the conditions when the flow is excited by the discharge in the nozzle throat, population in the supersonic test section is measured by both CRDS and TDLAS diagnostics. The two diagnostic techniques are complementary and can be used for characterization of nonequilibrium reacting flows over a wide range of pressures, including short-run-time high-enthalpy flow facilities.

Obtaining the solid solution Sb2S3-xSex by selenization of Sb2S3 film and identifying the thermal processing parameters to achieve recrystallization while maintaining phase-purity

In this work, thin films of the ternary material Sb2S3-xSex with promising optical and electrical properties were developed for its application as absorber material in photovoltaic devices. It was found that 400 °C is the lowest recrystallization temperature at which phase-pure polycrystalline films of Sb2S3-xSex can be obtained. Films were recrystallized in a mixture of S/Se vapor by varying the proportion of Se and the film properties were studied as a function of Se content. The conductivity and photosensitivity were increased with the increase in the amount of Se in films. The activation energies associated with the defects or traps in the films decrease with the amount of Se. The morphology of the film changes significantly depending on the annealing pressure. Films recrystallized in an ambient of S/Se vapor generated from 10 mg each of S and Se at 400 °C and 460 Torr pressure are found to have the optimum band gap, highest photosensitivity, and phase-purity. Assuming an appropriate window layer, the modeling result shows that the obtained film Sb2S0.6Se2.4 has the suitable material properties that can result in solar cells with efficiencies in the range of 27%.

CO and O2 Adsorption and CO Oxidation on Pt Nanoparticles by Indirect Nanoplasmonic Sensing.

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

Confirm Suitability of the 3D Positive Ion Detector to Use in the Field of Radiation Protection, and Gamma Spectrometry Applications

It is well known that the environmental radiation surrounding us consists of various types of radiation that could be dangerous to human health, causes air pollution and also has importance in industry and medicine. There are different types of gamma spectrometry and radiation protection technologies are available. Even though these technologies have been used for several decades; there is a need to improve the sensitivity and selectivity for these detectors. The Multilayer PCB technology based Nanodosimeter was designed to capture positive ions produced by the interaction of ionizing radiation with low pressure gas medium. In order to confirm the signal, the present signal captured under methane medium at 1 to 10 Torr pressure. The indigenously fabricated 3D positive ion detector of thickness 3.483 mm with optimized cathode was using the same Nanodosimeter experimental setup using the Co-60 source. To conform the performance of the 3D positive ion detector as gamma spectrometry and Radiation protection.

Radiation Effects on Thin Flexible Superconducting Cables

We have studied the effects of gamma radiation on flexible superconducting microstrip transmission line structures, to explore their suitability for use in harsh environments where they may be subjected to radiation. In this work, we used two conductor variants: one with only Nb and another with a conductor stack of Al/Nb/Al. Spin-on polyimide HD-4110 was used as the dielectric substrate and both the variants were encapsulated with a layer of HD-4110 to increase the robustness of the structure by protecting the superconducting trace from potential mechanical damage. The resonators were irradiated at room temperature in a vacuum sealed chamber at a pressure of 1E-6 Torr using cobalt-60 as a source of gamma radiation. Samples were exposed to doses up to 60.8 Mrad (608 kGy). Quality factors of the resonators were extracted in the frequency range from 2 to 20 GHz, at various cryogenic temperatures between 1.2 K and 4.2 K, using a closed-cycle cryostat. We observed a small increase in microwave loss for higher temperature ranges, which points to a small change in the superconducting properties of the conductors. The results of this work show that low mass, flexible superconducting cables may be suitable alternatives to bulky coaxial cables for signal transmission at cryogenic temperatures in extreme (radiation) environments.

A 220–300 GHz Twin-FET Detector for Rotational Spectroscopy of Gas Mixtures

In this paper, a twin-FET (field effect transistor) THz detector monolithically integrated with a dual-feed antenna is presented. The antenna-detector configuration can be extended for 'N' FET elements to relax the input impedance requirement for the detector readout circuitry while maintaining good responsivity. A dual-feed octagonal slot antenna is used to couple THz radiation and provide differential excitation to the twin-FET detector. The multi-feed antenna-FET detector interface can be designed for an arbitrary phase excitation of one or more FET elements. Fabricated in Global Foundries 22nm FD-SOI process, the proposed detector covers a measured frequency range from 220 GHz to 300 GHz. Such bandwidth is sufficiently wide to capture reliable spectral information for distinguishing a number of gases in a mixture. The absorption spectra of pure methanol, chloromethane and acetonitrile are measured using a rotational spectroscopy setup incorporating the twin-FET detector. Detection of a small concentration of acetonitrile, at 4 Torr pressure, is demonstrated in a binary mixture with predominantly chloromethane. Detection of ethanol and methanol in their binary mixture is also demonstrated at 4 Torr pressure. This is a significant step towards the development of all-electronic THz spectroscopy for the detection of volatile organic compounds (VOCs) in the presence of contaminants with a strong THz absorption signature at >1 Torr pressure.

Line parameters for hot methane [formula omitted] band broadened by H[formula omitted] from 296 to 1100 K

Methane (CH4) spectra in the ν3 band near 3.3 m were measured for 0, 50, 150, 240, 320, and 400 Torr pressure of added hydrogen. The spectra were recorded using a high resolution Fourier transform spectrometer. The CH4 spectra were measured at 5 different temperatures from room temperature up to ∼ 1100 K. A multi-spectrum non-linear least squares fit method was used to determine the line parameters at each temperature. Voigt lineshape functions were used to determine the broadening and shifting of methane lines in the P and R branches. The temperature dependence of exponent parameters for the line width and the linear frequency shift coefficients were determined from a fit for temperatures ranging from 296 to 1098 K. The temperature dependence of the retrieved line broadening parameters was observed to follow the Double Power Law (DPL) proposed by Gamache and Vispoel. The temperature dependence of the pressure shift coefficients follows a linear trend for the first three temperatures. The pressure broadening parameters decrease with increasing temperature, and the pressure shift parameters increase with increasing temperature, especially for the first three temperatures. Finally, the dependence of pressure broadening (γ0) and shift (δ0) parameters on the rotational quantum number (J) was studied. The pressure broadening coefficients decrease about 20–30% for temperatures up to 894 K and about 40% for 1098 K, with increasing J quantum number. A complete set of fitted parameters including line position (σ), line intensity (S), pressure broadening (γ0), shifting (δ0), and their corresponding fitting errors are provided in supplementary tables.

Systematic growth of carbon nanotubes on aluminum substrate for enhanced field emission performance

For more than two decades, carbon nanotubes (CNTs) have shown great potential for a wide range of applications. Several methods are known to synthesize CNTs, though only a few of them are able to produce good quality and economically available CNTs. Chemical vapor deposition (CVD) is one of those methods that produce economically feasible and good quality CNTs onto specific substrates, even with nanopatterning. However, growing CNTs by CVD at temperatures below 700 °C remained a long-time challenge, as this meant keeping a host of low-melting materials out of bounds for direct CNT growth on them. In this work, CNTs have been synthesized directly onto a low-melting, conducting substrate, aluminum, by thermal CVD, at a temperature as low as 550 °C and up to as high as 650 °C (just below the melting point of aluminum). The diameters of the grown CNTs were observed to be influenced by process parameters, e.g., temperature and pressure. The effect of synthesis parameters on CNT diameters was verified by scanning electron microscopy and transmission electron microscopy. The quality of the CNTs was checked by Raman spectroscopy, selected area electron diffraction pattern of transmission electron microscopy, and XPS. It was observed that an increase in temperature and pressure had a significant effect on the diameters of the CNTs. Randomly entangled CNTs were measured to have an average diameter of 28 nm at 550 °C and one atmospheric (760 Torr) pressure, whereas it was observed to be 78 nm at a temperature of 650 °C and pressure of 0.01 Torr. The field emission response, i.e., the turn-on field (2.5 V/μm) and the maximum emission current density (2.17 mA/cm2) of the CNTs synthesized at the temperature of 550 °C and pressure of 1 atm (760 Torr) was found to be excellent.

Analytical characterization of laser induced plasmas towards uranium isotopic analysis in gaseous uranium hexafluoride

To perform direct enrichment assay on gaseous uranium hexafluoride (UF6) with laser induced breakdown spectroscopy (LIBS), the dominant spectral-line features, evolution of the signal and background of the U II 424.437 nm line, and its Stark width and shift, were studied as a function of UF6 gas pressure and pulse energy of a nanosecond Nd:YAG laser. Vapor pressure of UF6 was found to be the most important parameter for LIBS analysis of gaseous UF6. Spectral congestion with numerous U lines of high excitation potential was observed and signal-to-background ratio (SBR) was low for measurements with 80 Torr UF6. Only when both UF6 vapor pressure and laser pulse energy were low, for example, less than 20 Torr pressure and 30 mJ pulse energy, the resultant LIBS spectra from gaseous UF6 resembled those obtained from solid U samples. The experimental data also suggest that U and F atoms recombine back to UF6 after the laser pulse. The U emission was found to decay fast with a persistent background signal, degrading SBR with delay time. Systematic positive biases were found for UF6 enrichment assays performed with the 235U–238U line pair at 424.412–424.437 nm, which was confirmed to be caused by self-absorption. Even with optimization of experimental parameters and incorporation of a self-absorption term into the spectral-fitting algorithm, to reduce and compensate for self-absorption, self-absorption is still a main factor limiting accurate UF6 enrichment assay. The use of another spectral window which contains no resonance lines is a prospective solution for the self-absorption issue.

Superatomic Signature and Reactivity of Silver Clusters with Oxygen: Double Magic Ag 17 – with Geometric and Electronic Shell Closure

Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision. Herein, we report a systematic study o...

Optical, electrical, and structural properties of Ta-doped SnO2 films against substrate temperature using direct current magnetron sputtering

Ta-doped SnO2 (TTO) films were deposited at 3 × 10−3 Torr pressure from the 6 wt. % Ta2O5-doped SnO2 target at temperatures of 30–500 °C in Ar sputtering gas. The Ta dopants in the SnO2 host lattice were detected via X-ray photoemission, EDX mapping, and photoluminescence spectra. The Ta5+–Sn4+ substitution formed the OOx at VOxsites in the host lattice and the substitution occurred at deposition temperatures greater than 200 °C. The substitution leads to a decrease in the amount of VOx in the SnO2 lattice corresponding to the preferred rutile SnO2 (110) lattice reflection in the X-ray diffraction patterns. The ultraviolet-visible transmittance in visible light was approximately 80%. The lowest resistivity achieved was 2.0 × 10−3 Ω.cm, with a carrier concentration of 1.28 × 1020 cm−3 and carrier mobility of 24.5 cm2V−1s−1. The integration of the TTO-400 film with (n- and p-) Si exhibited an excellent photoelectric effect.

Primary photodissociation mechanisms of pyruvic acid on S1: observation of methylhydroxycarbene and its chemical reaction in the gas phase.

Pyruvic acid, a representative alpha-keto carboxylic acid, is one of the few organic molecules destroyed in the troposphere by solar radiation rather than by reactions with free radicals. To date, only its stable final products were identified, often with contribution from secondary chemistry, making it difficult to elucidate photodissociation mechanisms following excitation to the lowest singlet excited-state (S1) and the role of the internal hydrogen bond in the most-stable Tc conformer. Using multiplexed photoionization mass spectrometry we report the first direct experimental evidence, via the observation of singlet methylhydroxycarbene (MHC) following 351 nm excitation, supporting the decarboxylation mechanism previously proposed. Decarboxylation to MHC + CO2 represents 97-100% of product branching at 351 nm. We observe vinyl alcohol and acetaldehyde, which we attribute to isomerization of MHC. We also observe a 3 ± 2% yield of the Norrish Type I photoproducts CH3CO + DOCO, but only from d1-pyruvic acid. At 4 Torr pressure, we measure a photodissociation quantum yield of 1.0+0-0.4, consistent with IUPAC recommendations. However, our measured product branching fractions disagree with IUPAC. In light of previous calculations, these results support a mechanism in which hydrogen transfer on the S1 excited state occurs at least partially by tunneling, in competition with intersystem crossing to the T1 state. We present the first evidence of a bimolecular reaction of MHC in the gas phase, where MHC reacts with pyruvic acid to produce a C4H8O2 product. This observation implies that some MHC produced from pyruvic acid in Earth's troposphere will be stabilized and participate in chemical reactions with O2 and H2O, and should be considered in atmospheric modeling.

Kinetic fall-off behavior for the Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction.

Rate coefficients, k, for the gas-phase Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction were measured over the 15-500 torr (He and N2 bath gas) pressure range at temperatures between 283 and 323 K. Kinetic measurements were performed using pulsed laser photolysis (PLP) to produce Cl atoms and atomic resonance fluorescence (RF) to monitor the Cl atom temporal profile. Complementary relative rate (RR) measurements were performed at 296 K and 620 torr pressure (syn. air) and found to be in good agreement with the absolute measurements. A Troe-type fall-off fit of the temperature and pressure dependence yielded the following rate coefficient parameters: ko(T) = (9.4 ± 0.5) × 10-29 (T/298)-6.3 cm6 molecule-2 s-1, k∞(T) = (3.4 ± 0.5) × 10-11 (T/298)-1.4 cm3 molecule-1 s-1. The formation of a Cl·C4H2O3 adduct intermediate was deduced from the Cl atom temporal profiles and an equilibrium constant, KP(T), for the Cl + C4H2O3 ↔ Cl·C4H2O3 reaction was determined. A third-law analysis yielded ΔH = -15.7 ± 0.4 kcal mol-1 with ΔS = -25.1 cal K-1 mol-1, where ΔS was derived from theoretical calculations at the B3LYP/6-311G(2d,p,d) level. In addition, the rate coefficient for the Cl·C4H2O3 + O2 reaction at 296 K was measured to be (2.83 ± 0.16) × 10-12 cm3 molecule-1 s-1, where the quoted uncertainty is the 2σ fit precision. Stable end-product molar yields of (83 ± 7), (188 ± 10), and (65 ± 10)% were measured for CO, CO2, and HC(O)Cl, respectively, in an air bath gas. An atmospheric degradation mechanism for C4H2O3 is proposed based on the observed product yields and theoretical calculations of ring-opening pathways and activation barrier energies at the CBS-QB3 level of theory.

Near-Surface Imaging of the Multicomponent Gas Phase above a Silver Catalyst during Partial Oxidation of Methanol

Fundamental chemistry in heterogeneous catalysis is increasingly explored using operando techniques in order to address the pressure gap between ultrahigh vacuum studies and practical operating pressures. Because most operando experiments focus on the surface and surface-bound species, there is a knowledge gap of the near-surface gas phase and the fundamental information the properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species and temperature distributions in operando catalysis experiments using complementary near-surface optical and mass spectrometry techniques. The partial oxidation of methanol over a silver catalyst demonstrates the value of these diagnostic techniques at 600 Torr (800 mbar) pressure and temperatures from 150 to 410 °C. Planar laser-induced fluorescence provides two-dimensional images of the formaldehyde product distribution that show the development of the boundary layer above the catalyst under different flow conditions. Raman scattering imaging provides measurements of a wide range of major species, such as methanol, oxygen, nitrogen, formaldehyde, and water vapor. Near-surface molecular beam mass spectrometry enables simultaneous detection of all species using a gas sampling probe. Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether, and methyl formate, provides insights into catalytic mechanisms of the partial oxidation of methanol. The combination of these techniques provides a detailed picture of the coupling between the gas phase and surface in heterogeneous catalysis and enables parametric studies under different operating conditions, which will enhance our ability to constrain microkinetic models of heterogeneous catalysis. (Less)

Numerical investigation of the effect of insulator permittivity on the initial stage of dielectric barrier discharge

We present a numerical simulation of the simplified ferroelectric plasma source operating in a gas discharge mode. The plasma source contains two electrodes separated by a ferroelectric. High voltage pulse applied to the cathode leads to formation of a dielectric barrier discharge in gas (argon at 1 Torr pressure). The main point of the study is to clarify how the high permittivity of the ferroelectric affects the initial stage of the discharge. Two cases are considered with different values of barrier relative permittivity. The results show that the process of compensation of bound charge affects the electron avalanche formation near the dielectric surface. The electron concentration in the avalanche drops down for materials with higher permittivity.

High-Density Plasma Diagnosis Using Quasi-Optical Millimeter-Wave Systems

This report proposes a quasi-optical millimeter-wave system based on a noninvasive measurement technique to diagnose high-density plasmas. The millimeter wave in the F-band (90–140 GHz) can interact with high-density plasma (1013–1014 cm−3) in the quasi-optical system, such that the plasma characteristics (e.g., plasma density and effective collision frequency) obtained using the Drude model can be measured with a high signal-to-noise ratio. High-density inductive radio frequency (RF) discharge argon plasma is generated in a helix of inductively coupled plasma using RF power of up to 1000 W at 13.56 MHz with a pressure of a few Torr. The millimeter-wave signals can be analyzed to estimate the plasma-density distribution by using the proposed axisymmetric 2-D multilayered plasma modeling. The experimental results agreed well with the model calculations. Qualitatively, the plasma density tends to increase with increasing RF power and gas pressure. The effective collision frequency decreases only with increasing gas pressure. The proposed measurement technique will also be applicable to measuring localized regions (with areas of a few square millimeters) of higher density plasma (1014–1017 cm−3) by using terahertz waves for various applications.

The Influence of Temperature on the Tribological Properties of the Ti<sub>6</sub>Al<sub>4</sub>V Alloy Treated by Plasma Oxidation

The purpose of the plasma oxidation process is to increase the hardness, corrosion resistance and to improve the biocompatibility properties of Ti6Al4V alloys by thickening the natural oxide in the material, which is produced by this treatment. The aim of this work is to verify the effect of temperature on the thickness, hardness and wear resistance of the Ti6Al4V alloy treated with plasma oxidation. The treatment was performed using a Pulsed DC vacuum reactor, with a gas ratio of 60% Ar and 40% O2and 1.65 torr pressure for 1 hour of treatment, at temperatures of 480°C, 520°C, 670°C and 705°C. In regards to the multilayer formation of anatase and rutile, it was observed that the layer thickness increased as the treatment temperature increased. The increase of surface hardness provided by the treatment caused a considerable increase in the wear resistance of the studied material. The greatest layer thickness and surface hardness were obtained for the material treated at 705°C, but the lowest wear volume was obtained for the material treated at 520°C.

Parameters of a Subthreshold Microwave Discharge in Air and Carbon Dioxide as a Function of Microwave Field at Different Gas Pressures

Propagation velocity of a subthreshold microwave discharge in air and carbon dioxide is measured at various gas pressures and intensities of microwave radiation. At air pressures of 200, 390, and 738 Torr and carbon dioxide pressures of 390 and 750 Torr, the propagation velocity of the head part of the self-non-self-sustained discharge closely follows a quadratic power law as a function of microwave-beam intensity in the range from 4 to 16 kW/cm2, while decreasing directly proportional to the initial gas density. In the process, the discharge propagation velocities in carbon dioxide are twice lower that those in air at equal intensities of the microwave radiation. The temperature in the head part of the discharge in air reaches 3.5–5.5 kK, while that in carbon dioxide reaches 9–15 kK.

Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H2−20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.