Low-pressure Environment Research Articles

Modification of oxide layer in plasma-sprayed thermal barrier coatings

Formation of Ni(Cr,Al) 2O 4 and NiO in plasma-sprayed thermal barrier coatings (TBCs) is believed to be detrimental to the durability of TBC systems. Therefore, a heat treatment to the as-sprayed TBC in a low-pressure oxygen environment is developed to produce a uniform thin alumina (Al 2O 3) layer at the ceramic/bond coat interface, which suppresses the growth of Ni(Cr,Al) 2O 4 and NiO during later thermal exposure in air.

Surface modification of polymeric substrates by plasma-based ion implantation

Plasma-based ion implantation (PBII) as a tool for polymer modification is studied. Polymeric films have good performances for flexible use, such as food packaging or electronic devices. Compared with inorganic rigid materials, polymers generally have large permeability for gases and moisture, which causes packaged contents and devices to degrade. In order to add a barrier function, surface of polymeric films are modified by PBII. One of the advantageous features of this method over deposition is that the modified surface does not have peeling problem. Besides, micro-cracks due to mechanical stress in the modified layer can be decreased. From the standpoint of mass production, conventional ion implantation that needs low-pressure environment of less than 10−3Pa is not suitable for continuous large-area processing, while PBII works at rather higher pressure of several Pa. In terms of issues mentioned above, PBII is one of the most expected techniques for modification on flexible substrates. However, the mechanism how the barrier function appears by ion implantation is not well explained so far. In this study, various kinds of polymeric films, including polyethyleneterephthalate (PET), are modified by PBII and their barrier characteristics that depend on the ion dose are evaluated. In order to investigate correlations of the barrier function with implanted ions, modified surface is analyzed with X-ray photoelectron spectroscopy (XPS). It is assumed that the diffusion and sorption coefficients are changed by ion implantation, resulting in higher barrier function.

System for In Situ Characterization of Nanoparticles Synthesized in a Thermal Plasma Process

We have designed a particle diagnostic system that is able to measure particle size and charge distributions from low stagnation pressure (≥746 Pa) and high temperature (2000–4000 K) environments in near real time. This system utilizes a sampling probe interfaced to an ejector to draw aerosol from the low pressure chamber. Particle size and charge distributions are measured with a scanning mobility particle sizer. A hypersonic impactor is mounted in parallel with the scanning mobility particle sizer to collect particles for off-line microscopic analysis. This diagnostic system has been used to measure size and charge distributions of nanoparticles (Si, Ti, Si–Ti–N, etc.) synthesized with our thermal plasma reactor. We found that the mean particle size increases with operating pressure and reactant flow rates. We also found that most particles from our reactor are neutral for particles smaller than 20 nm, and that the numbers of positively and negatively charged particles are approximately equal.

Generation of focused electron beam and X-rays by the doped LiNbO3 crystals

Generation of focused electrons beam with energies up to 100keV from undoped LiNbO3 (LN) crystals have been observed [J.D. Brownridge, Nature 358 (1992) 287; J.D. Brownridge, S.M. Shafroth, Appl. Phys. Lett. 79 (2001) 3364] during heating–cooling cycles in a low-pressure environment. This paper reports about similar results that were observed in doped crystals of LN. Generation of electrons by crystals with thicknesses of 1 and 6mm, was visualized by ZnS screen [ZSS] during heating–cooling cycles in a vacuum chamber (P=1–10mTorr). Generation of X-rays from both thin and thick crystals was evident from registered images from dental X-ray film [DXF]. The possibility of X-ray imaging was demonstrated, using different metal masks. Imaging of X-rays reveals that both focusing and wide-angle scattering modes of operation exist in the electron beams generation pattern during heating–cooling cycles.

Inclusion of Aerodynamic Non-Equilibrium Effects in Supersonic Plasma Jet Enthalpy Probe Measurements

Low pressure plasma spraying (LPPS) is a thermal spraying technique that has found a niche for low oxidation products. It uses a low pressure environment (i.e., chamber pressure between 2 and 90 kPa) and yields supersonic plasma jets. The enthalpy probe technique is a common measurement method in plasmas. However LPPS jets are difficult to diagnose as their supersonic nature forces the apparition of a shock wave in front of any measuring device inserted in the jet. Incomplete or erroneous assumptions are usually invoked to overcome the difficulties associated with this shock wave and carry out the LPPS jet diagnosis from enthalpy probe measurements. In this work, a new device is designed to gain access to an additional physical quantity, which is needed to assess the aerodynamic non-equilibrium state of the jet. It is combined with enthalpy probe measurements, and the resulting set of experimental data is used with a numerical procedure based on gas dynamics theory, yielding free-stream supersonic plasma jet values from the measurements behind the induced shock wave. The results agree well with the phenomenology of supersonic jets in aerodynamic nonequilibrium. However this new method is restricted by the local thermodynamic equilibrium assumption, which is directly linked with the pressure and temperature conditions of the plasma jet.

Nanocrystalline Cr 2O 3–TiO 2 thin films by pulsed laser deposition

In the present work, pulsed laser deposition (PLD) technique was applied to Cr 2O 3–TiO 2 mixed oxide (5–20 at.% TiO 2) gas sensing materials with the aim to find the feasibility and optimal conditions for the growth of nanostructured thin films. The films were deposited by KrF laser ( λ = 248 nm) on Si(1 0 0) substrates in low-pressure oxygen environment, and characterised by X-ray diffraction, X-ray reflection, reflection high-energy electron diffraction and atomic force microscopy. The substrate temperature (450–700 °C), oxygen pressure (10 −3 to 5 × 10 −2 mbar), and laser energy density (1.5–4.5 J/cm 2) were varied for optimising the growth conditions. Structural and morphological analysis showed that the films started to crystallize at oxygen pressures above 10 −2 mbar. At the highest oxygen pressure the first traces of crystallization appeared at raising the temperature over 400 °C (20 at.% of Ti) or 450 °C (at 5 at.% Ti). Smallest particle sizes (10–20 nm) were obtained at 500–550 °C, whereas the best degree of crystallinity of nanostructured films was observed at higher temperature but at lower laser energy densities. When the films were grown or annealed at temperatures over 600 °C the crystallinity further improved but the particle size grew over 100 nm.

Diagnostic considerations for optical laser-extinction measurements of soot in high-pressure transient combustion environments

Laser-extinction diagnostics can provide spatially and temporally resolved measurements of attenuation from combustion-generated soot within the path of the beam. When laser-extinction techniques are utilized in high-pressure combustion environments, however, a number of complications may be encountered that are not present in low-pressure environments. Several of these experimental difficulties were investigated in diesel engine environments, and solutions that facilitated acquisition of reliable laser-extinction data were demonstrated. Beam steering due to refractive index gradients within the combusting gases was observed, and a full-angle beam divergence of over 100 mrad was measured. A spatial-filtering scheme was employed to reduce the collection of forward-scattered light and background combustion luminosity while ensuring full collection of the steered beam. To further reject combustion luminosity, a narrow-bandpass laser-line filter was employed, after diffusing the transmitted light sufficiently to avoid the effects of significant spatial non-uniformities of the filter. As the windows were subjected to thermal and mechanical stresses, dynamic etaloning effects due to the photoelastic properties of synthetic fused silica were observed. Dynamic changes in the polarization of the exit beam were also observed, as stress-induced birefringence in the windows caused dynamic phase retardation of the transmitted beam. Although these photoelastic effects could not be eliminated, they were mitigated by introducing curvature to the wavefronts in the laser-extinction beam and using polarization-insensitive elements in the detection optics. Soot deposits on window surfaces were removed ablatively using a coaxial, high-energy, pulsed Nd:YAG laser beam.

Axial Flow Cyclone for Segregation and Collection of Ultrafine Particles: Theoretical and Experimental Study

In this study, an axial flow cyclone was designed, fabricated, and evaluated at different conditions of air flow rates (Q0) and low-pressure environments (P), especially for the segregation and collection of ultrafine particles. An evaporation/condensation type of aerosol generation system consisting of tube furnace and mixing chamber was employed to produce test aerosols. The test aerosol was then classified by a differential mobility analyzer (DMA) and number concentration was measured by a condensation nuclei counter (CNC) and an electrometer upstream and downstream of the cyclone, respectively. The s-shaped curve of the collection efficiency in submicron particle size range was obtained to be similar to the traditional cyclone found in the literatures when the particles were largerthan 40 nm at Q0 = 1.07, 0.455 L(STP)/min, and P = 4.8-500 Torr. The curve was found to be fitted very well by a semiempirical equation described in this paper. For particles smaller than 40 nm, however, the collection efficiency was unusually increased as the particle diameter was decreased due to the fact that the diffusion deposition becomes the dominant collection mechanism in the low-pressure conditions. A model composed of centrifugal force and diffusion deposition is presented and used to fit the experimental data. The cyclone was demonstrated to separate and collect ultrafine particles effectively in the tested vacuum conditions.

Ｈｏｌｌｏｗ Ｃａｔｈｏｄｅ Ａｒｃにおけるアーク柱の特性

Hollow Cathode Arc (HCA) was developed as a plasma source under the condition of low pressure in the 1960s, and various researches were performed to make clear the mechanism of the HCA. Since the HCA is a plasma source under low pressure environment, it attracts attention as a welding heat source in space. Moreover, the HCA is expected to be useful for the industrial application on the earth, because the melting process by HCA is very active and the penetration is huge in comparison with that by the conventional GTA.In the previous paper, the discharge and melting process by the HCA has been discussed, and it has been shown that a concentrated columnar arc and a huge penetration are obtained in the case of low gas flowrate.In the present paper, the current distribution on the anode surface and the floating potential of the HCA column have been made clear and the plasma structure of the HCA has been discussed to understand the heat input to the anode by the HCA. It is concluded that the huge penetration by the HCA in the case of low gas flowrate is closely related to concentrated current distribution on the anode surface and the strong flow of electrons in the HCA.

Investigation of windage splits in an enclosed test fixture having a high-speed composite rotor in low air pressure environments

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted a series of composite rotor spin tests to measure the windage losses and temperature distributions of a test setup at high rotor speeds and low air pressures. The intent of the windage tests is to validate the windage loss predictions and investigate how the air-gap windage is distributed between the rotor and stator. The findings of the spin tests will then be used to perform windage-related thermal design and analysis of a high-speed electrical machine. The radial air-gap flows under the test conditions, a low rotor cavity air pressure of 1 torr and high rotor surface velocities of 333 and 614 m/s, were in a laminar flow regime. Transient rotor and stator finite-element thermal analyses, using the measured windage losses and predicted laminar-flow windage splits, have been carried out to analyze the rotor and stator temperature distributions. This paper shows the detailed thermal analysis and compares the predictions with the measurements. The predicted and measured transient rotor and stator temperatures are in good agreement.

Splits of windage losses in integrated transient rotor and stator thermal analysis of a high-speed alternator during multiple discharges

For a high-speed electrical alternator, the rotor outer banding and stator inner liner are typically made of high strength graphite epoxy composites due to their high strength and stiffness. Machine structural integrity at high rotating speeds degrades significantly as the composite resins lose their strength at high temperatures. The magnitude of the frictional windage losses generated in the air gaps and the splits of the windage losses between the rotor and stator become crucial to the machine design since these windage losses greatly influence the rotor outer and stator inner surface temperatures. Splits of windage losses generated by an enclosed high-speed composite rotor in low air pressure environments were investigated by The University of Texas at Austin Center for Electromechanics and described in a companion paper. The windage splits are dictated by the air temperature gradients at the rotor outer and stator inner surfaces. Unique heating, cooling, and component material properties of a typical high-speed alternator during repetitive-discharge events make its transient air-gap windage splits very much different from those of the test setup. This paper describes transient windage splits in integrated rotor and stator thermal analyses of a high-speed alternator designed for multiple discharges. The transient windage splits in the air-gap air flow were obtained through multiple iterations on windage losses, air-gap air temperatures, and rotor and stator surface temperatures.

Influence of Percutaneous Coronary Intervention on Coronary Microvascular Resistance Index

Coronary microvascular resistance during maximal hyperemia is generally assumed to be unaffected by percutaneous coronary interventions (PCIs). We assessed a velocity-based index of hyperemic microvascular resistance (h-MR(v)) by using prototypes of a novel, dual-sensor (Doppler velocity and pressure)-equipped guidewire before and after PCI to test this hypothesis. Aortic pressure, flow velocity (h-v), and pressure (h-P(d)) distal to 24 coronary lesions were measured simultaneously during maximal hyperemia induced by intracoronary adenosine. Measurements were obtained in the reference vessel before PCI and in the target vessel before and after PCI, stenting, and ultrasound-guided, upsized stenting. h-P(d) increased from 57.9+/-17.0 to 85.5+/-15.6 mm Hg, and h-MR(v) (ie, h-P(d)/h-v) decreased from 2.74+/-1.40 to 1.58+/-0.61 mm Hg x cm(-1) . s after stenting (both P<0.001). The reduction in h-MR(v) accounted for 34% of the decrease in total coronary resistance achieved by PCI. h-MR(v) of the target vessel after PCI was lower than that of the corresponding reference vessel despite a higher h-P(d) in the reference vessel (P<0.01). Post-PCI baseline MR(v) was correlated with baseline P(d) before PCI (P<0.01). PCI-induced restoration of P(d) resulted in a reduction of h-MR(v) in accordance with the pressure dependence of h-MR(v). The decrease in h-MR(v) to a level below that of the corresponding reference vessel in the immediate post-PCI period and a lowered baseline MR(v) suggest microvascular remodeling induced by long-term exposure to a low-pressure environment.

Oxidation behavior of a single phase γ-TiAl alloy in low-pressure oxygen and hydrogen

The microchemistry and morphology of the oxide layer produced on a single phase γ-TiAl alloy (Ti–52.1Al–2Ta at.%) following anneals at 823 and 1173 K in low-pressure oxygen and hydrogen environments were studied. The oxide structure on the electropolished surface consisted of two layers, an outer one of Al2O3 and an inner one of TiO2. Annealing in low partial pressures of oxygen retained the same oxide structure but increased the total thickness. After the low-pressure oxygen anneals at 823 K, the oxide surface was smooth, whereas after anneals at 1073 K and above the surface consisted of small, round particles, whose size and density increased with increasing annealing temperature and oxygen partial pressure. In contrast, the oxide structure produced by annealing in a hydrogen environment after the low-pressure oxygen treatment consisted of an outer layer of TiO2 and a sub-oxide of Al2O3. The morphology of this oxide consisted of elongated, rod-like particles, whose size increased with annealing temperature. Absorption of H from gaseous hydrogen atmospheres is discussed in relation to the surface and sub-surface oxide structures.

Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al) 2O 3·Ni(Cr,Al) 2O 4·NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al 2O 3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al) 2O 4 and NiO particles embedded in the chromia.

Is Spin Conserved in Heavy Metal Systems? Experimental and Theoretical Studies of the Reaction of Re+with Methane

A guided ion beam tandem mass spectrometer is used to study the reactions of atomic 187Re+ with CH4 and CD4 and collision-induced dissociation (CID) of ReCH4+ with Xe. These studies examine the activation of methane by Re+ in a low-pressure environment free of ligand supports or other reactive species. In the bimolecular reaction, ReCH2+ is efficiently produced in a slightly endothermic process and is the only ionic product observed at low energies, whereas at higher energies, ReH+ dominates the product spectrum. Other products observed include ReC+, ReCH+, and ReCH3+. Modeling of these endothermic reactions yields 0 K bond dissociation energies in eV of D0(Re+−C) = 5.12 ± 0.04, D0(Re+−CH) = 5.84 ± 0.06, D0(Re+−CH2) = 4.14 ± 0.06, D0(Re+−CH3) = 2.22 ± 0.13. Analysis of the behavior of the cross sections suggests that formation of ReH+, ReCH2+, and ReCH3+ occurs via an H−Re+−CH3 intermediate. CID of ReCH4+ reveals a bond energy of 0.53 ± 0.15 eV for Re+−CH4. The experimental bond energies compare favorably with theoretical calculations at the B3LYP/HW+/6-311++G(3df,3p) level with the exception of the singly bonded species (ReH+, ReCH3+), where the Becke-half-and-half-LYP functional performs much better. Theoretical calculations also elucidate the reaction pathways for each product and provide their electronic structures. Overall we find that the dehydrogenation reaction, which occurs with an efficiency of 86 ± 10%, must involve three facile spin changes (2s + 1 = 7 → 5 → 3 → 5) indicating that little hint of spin conservation remains in this heavy-metal system.

Characterization of supersonic low pressure plasma jets with electrostatic probes

Measurement of key plasma jet properties, such as the Mach number, electron density and temperature, in a low pressure environment, were performed using double Langmuir probes and Mach probes. In particular, under-expanded jets are studied in detail by performing complete mappings of plasma jet properties at chamber pressures of 10 and 2 mbar. These results show that the measured physical properties are consistent with the jet flow phenomenology, such as the presence of periodic expansion and compression zones, the effect of the pressure and the location of the shocks. It is shown, in particular, that for a highly under-expanded jet at 2 mbar, the Mach number reaches 2.8 in the first expansion zone followed by a strong drop to subsonic flow revealing the presence of a Mach reflection. The flow is accelerated further and a periodic structure of compression/expansion cells is observed until the local static pressure is in equilibrium with the surrounding pressure. These results contribute to the understanding of the supersonic plasma jet behaviour at low pressure and can be used to quantify the deviation from local thermodynamic equilibrium. The extensive mapping of the measured physical properties of the jet will also serve as input for modelling.

Effect of process parameters on sidewall roughness in polymeric optical waveguides

Most polymeric optical devices are mainly fabricated by photolithography and reactive ion etching (RIE). This paper describes the fabrication of fluorinated polyether (FPE) waveguides by using a metallic hard mask and proximity exposure followed by RIE. In particular, we focused on the effects of various fabrication parameters on the sidewall roughness. According to our study, a high-power, low-pressure oxygen RF plasma environment will provide vertical and smooth sidewalls desired in a cladded waveguide. The sidewall roughness increases with pressure in pure oxygen plasma, while adding nitrogen gas to oxygen during RIE, creates a more vertical profile and smoother sidewalls along with low vertical and lateral etch rates. A simple technique is presented to quantify sidewall roughness using an atomic force microscope (AFM).

Simulation of emission molecular spectra by a semi-automatic programme package: the case of C 2 and CN diatomic molecules emitting during laser ablation of a graphite target in nitrogen environment

Some emission spectra of diatomic molecules were calculated by a semi-automatic programme package in order to infer the rotational and vibrational temperatures in Boltzmann distribution by comparing them with the corresponding experimental ones. The calculation procedure was applied in the case of CN radical and C 2 molecule whose optical emission spectra were recorded during pulsed excimer laser ablation of a graphite target in low-pressure nitrogen environment. Computed similar or dissimilar values of rotational and vibrational temperatures let to verify the existence or not of local thermodynamic equilibrium and to hypothesise the temporal range necessary to establish it in such experiments.

Characteristics of supersonic nitrogen/hydrogen-mixture and ammonia plasma jets and nitrided material properties

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low pressure environment. Heat fluxes into the plate from the plasma were also evaluated. Ammonia and mixtures of nitrogen and hydrogen were used as a working gas. The H-atom electronic excitation temperature and the N 2 molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. As approaching the titanium plate, the thermodynamical non-equilibrium plasma came to be a temperature-equilibrium one. Both the electron number density near the plate and the heat flux increased with H 2 mole fraction for mixtures gases. In cases with mixtures of N 2 and H 2, a radical of NH with a radially wide distribution was considered to contribute to the better nitriding as a chemically active and non-heating process.

Plasma Characteristics of Supersonic Nitrogen/Hydrogen-Mixture and Ammonia Plasma Jets and Nitrided Material Properties

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low pressure environment. Heat fluxes into the plate from the plasma were also evaluated. Ammonia and mixtures of nitrogen and hydrogen were used as the working gas. The H-atom electronic excitation temperature and the N2 molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. As approaching the titanium plate, the thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one. Both the electron number density near the plate and the heat flux increased with H2 mole fraction for mixtures gases. In cases with mixtures of N2 and H2, a radical of NH with a radially wide distribution is considered to contribute to the better nitriding as a chemically active and non heating process.