Ambient Gas Pressure Research Articles

Diverse effects of gases on surface flashover in a wide pressure range from atmospheric to near vacuum pressure

Investigations into surface flashover characteristics and underlying mechanisms in the range from atmospheric to near vacuum pressure are significant for developing high-power equipment based on the application of gases used for flashover prevention. In this work, polyetherimide (PEI) films with controllable outgassing properties were prepared. Then, the gas-related factors, namely, gas pressure, the type of gases, and outgassing were clarified by investigating the DC surface flashover of films in a wide pressure range (10−4–105 Pa) in different gas atmospheres. In all conditions, the variation of flashover voltage was found to be divided into three regions with pressure increasing, while its deeper reason is that the number density of molecules of the gas layer is affected by outgassing and ambient gas pressure, which affects the collision of electrons. In region I, the surface flashover voltage is closely related to outgassing and shows the highest value. In region II, the surface flashover is jointly affected by outgassing and ambient gas, along with a rapid decrease of flashover voltage with increasing pressure. In region III, the surface flashover is dominated by ambient gas, and the surface flashover voltage shows a U-shaped curve with increasing pressure. Transition points of the three regions shift significantly with the change of outgassing properties. In region I, the desorption of adsorbed gas has little effect on the surface flashover voltage, while the escape of dissolved gas dominates. Finally, a model of the diverse effects of gases on surface flashover in a wide pressure range was established.

Dynamics of angularly colliding laser-produced plasmas in air and argon ambient

The effects of ambient gas pressure on the dynamics of angularly colliding laser-produced plasmas are investigated in air and Ar ambient. Fast formation of intense stagnation region is observed even in the low-pressure regime, and the tendency of interpenetration is also greater because of the involvement of the fast forward expansion velocity of seed plasmas. An intense and spherical stagnation region is observed at an intermediate pressure regime of Ar ambient. At higher pressure regime, shock front interaction in air ambient and at 10−1 mbar of Ar ambient is due to the regular reflection of shock waves. At 1 mbar of Ar ambient, a channel is formed at the collision front, and its width increases with time which is due to the Mach reflection of shock waves. Increasing inter-plume separation or the increase of angle between the targets significantly influences the shock wave reflections. Mach reflection of shock waves is observed even at 10−1 mbar of Ar ambient in higher angular separation. A peculiar plume splitting is also observed at 750 ns in 10−1 mbar of Ar ambient. Optical emission spectroscopic analysis was also done to understand the spectral emission from interaction region and its variation with ambient gas pressure.

Adjustment of resistivity for phosphorus-doped n-type multicrystalline silicon

In this work we present a novel approach to reduce inhomogeneity of resistivity for n-type phosphorus-doped directionally solidified silicon ingots by using complex arrangements on influencing dopant transport during growth process, which involves adjustments of melt flow and gas transport. In order to obtain more uniform resistivity profile over ingot's height, different process conditions were tested on G1-sized (22 × 22 × 12 cm 3 ) n-type directionally solidified ingots, including altering of ambient gas pressure, enhanced melt mixing with travelling magnetic fields and variations of argon flow above the melt. The quality of ingots was characterised in terms of electrical resistivity and minority carrier lifetime, as well as the presence of impurities and inclusions. The influence of process parameters on material quality will be discussed. It will be shown that a complex approach to melt stirring and gas transport is an effective way to control resistivity distribution along the height of phosphorus-doped multicrystalline ingots, which makes it possible to provide n-type ingots with resistivity profiles comparable to p-type boron-doped ones. • The mechanisms of phosphorus transport during directional solidification of silicon is investigated. • Process parameters influencing phosphorus transport in G1-size system are defined. • An optimized growth recipe, which ensures more uniform resistivity distribution along the ingot length, is developed. • A multicrystalline ingot showing 45% narrower resistivity variation and high material quality is grown.

Evaluation of self-absorption effect of He I resonance line by measurement of forbidden emission in helium arc jet plasma

We have proposed a new method to estimate the optical escape factor (OEF) in high-density helium (He) plasma. Plasma with an electron temperature of ca. 3 eV and density of 5 × 1013 cm−3 was generated by a cascaded arc discharge and rapidly cooled by the introduction of additional He gas, which resulted in a transition from ionizing to recombining plasma. With an increase in the gas pressure, the plasma became optically thick, and the He I forbidden line (spin-exchange intercombination line, 1 1S-2 3P: 59.1 nm) with resonance lines were simultaneously observed using a vacuum ultraviolet spectrometer. Comparison of the intensity ratio of the He I 58.4 nm resonance line to the forbidden emission with those by determined from the collisional-radiative model considering the self-absorption enabled the successful estimation of the OEF. The OEF was decreased with the ambient He gas pressure and was 6.6 × 10−4 and 3.5 × 10−5 for He gas pressures of 1.59 and 20.22 Pa, respectively.

Ultrasensitive gas pressure sensor based on two parallel Fabry-Perot interferometers and enhanced Vernier effect

An ultra-high sensitivity optical fiber gas pressure sensor based on enhanced Vernier effect is proposed. The sensor consists of FPI1 and FPI2 in parallel. FPI1 is composed of single mode fiber, quartz capillary and polydimethylsiloxane film. FPI2 consists of single mode fiber, large inner-diameter capillary, and small inner-diameter capillary. Because FPI1 and FPI2 have different sensing mechanisms for gas pressure, the interference fringes of FPI1 and FPI2 drift in the opposite direction with the change of ambient gas pressure. When FPI1 and FPI2 with a similar free spectral range are connected in parallel, an enhanced Vernier effect is generated. Compared with the traditional Vernier effect gas pressure sensor, the enhanced Vernier effect gas pressure sensor realizes ultra-high sensitivity gas pressure measurement without increasing the difficulty of sensor manufacturing. The sensitivity of the enhanced Vernier effect gas pressure sensor is as high as 283.32 nm/MPa, which is about 1.7 times that of the traditional Vernier effect barometric sensor (170.03 nm/MPa). In addition, the sensor has the advantages of good repeatability and stability, low cost, small temperature cross-sensitivity and easy manufacture. The sensor structure designed provides a new design idea and scheme for the ultra-high sensitivity optical fiber gas pressure sensor.

Hypervelocity penetration behavior and fireball radiation characteristics of tungsten-zirconium reactive projectile

Reactive damage elements has the dual properties of kinetic energy penetration and strong combustion/explosion and thus enhanced damage ability potentially. At present, research on the damage characteristics of reactive projectile is mostly limited to the velocity of projectile impacting target below 2km/s, and there are few reports on hypervelocity impact. The damage ability of high-density and high-strength tungsten-zirconium (W/Zr) alloy projectiles is enhanced in hypervelocity impact, because the degree of material being fragmented is high, the particle size is small, and the flying speed of particles is fast, which are all conducive to the combustion of reactive components to form continuous high-temperature fireballs. In order to investigate the hypervelocity penetration behavior of W/Zr reactive projectile and the radiation characteristics of fireball, the failure characteristics of W/Zr reactive projectile impacting titanium alloy plate with velocities of 1.842-4.039km/s were obtained by hypervelocity impact test. The effects of projectile impact velocity and ambient gas pressure on the combustion characteristics of impact fireball are analyzed. The evolution characteristics of equivalent radiation temperature and equivalent radiation area are obtained by fitting experimental data. The effects of environmental gas on radiation characteristics under different impact conditions are identified and analyzed.

Effect of gas pressure on the quality-factor of single-crystal diamond micro cantilevers

Single-crystal diamond (SCD) is a prominent semiconductor material due to its superior mechanical and physical properties. From the viewpoint of practical application, a low vacuum or atmospheric environment are preferred. Here we systematically investigated the effect of ambient gas pressure on the quality-factor (Q-factor) of SCD micro cantilevers, and analyzed the disparate energy dissipation mechanisms. The Q-factor around 7500 was found to be almost stable for ambient pressure <10 Pa, then decreased sharply by about three orders of magnitude with the pressure increasing up to 105 Pa gradually. The ambient pressure ranging from 10−3 to 105 Pa can be defined as intrinsic, molecular, and viscous regimes by Knudsen number. We found that the Q-factors of the SCD cantilevers can be well described by the analytical models corresponding to the three regimes.

Surface Charge Characteristics of DC-GIL Insulator Under Multiphysics Coupled Field: Effects of Ambient Temperature, Load Current, and Gas Pressure

In this article, the surface charge characteristics of the direct current gas-insulated transmission lines (DC-GIL) insulator under thermal–electric coupled fields are investigated. A horizontally installed GIL with a basin-type insulator is employed. The 3-D charge distribution under different ambient temperatures, load currents, and gas pressures is obtained based on the finite-element method. It is found that the charge density increases with increasing ambient temperature since the bulk conduction is promoted due to the increasing insulator temperature. At lower ambient temperature, the surface charge distribution shows a more obvious difference in different radial directions along the insulator due to the increasing difference in electric conductivity in different directions. With increasing load current, the charge density increases, and the difference in charge density in different radial directions becomes more obvious. The charge density increases with increasing gas pressure although the temperature decreases, which is attributed to the domination of decreasing ion mobility over decreasing bulk conduction. It can be concluded that attention should be paid to these conditions when dealing with the insulation property of DC-GIL considering the effect of thermal gradient on the charge accumulation. Besides, simulation based on a 3-D model is necessary, especially for horizontal GILs.

Understanding the influence of unsteady and convection terms in heat transfer model on the predicted heat flux in engines operated under different combustion modes

The aim of current research is to develop an enhanced heat transfer model to accurately predict the heat flux through the cylinder walls during the working process of internal combustion engines operated under different loads and combustion modes. Two enhanced models, i.e., the unsteady model and the steady convective contribution (SCC) model were respectively constructed based on the thermal wall function. The effects of the transient ambient gas temperature and pressure variations, as well as the chemical reaction source term, were included in the unsteady model. Then the contributions of the unsteady terms, such as the temporal derivative of gas temperature, the pressure work, and the chemical heat source in the energy equation were analyzed in detail. The results indicate that the mechanism of the unsteady heat transfer process is different in the engine operated under motored conditions and different combustion modes, including conventional diesel combustion (CDC), homogeneous charge compression ignition (HCCI), and reactivity controlled compression ignition (RCCI). Afterward, the influence of the gas velocity and the pressure difference perpendicular to the walls were included in the SCC model to consider their contributions to the heat fluxes near the chamber walls. It is found that the heat fluxes predicted by the SCC model are in better agreement with the experimental data, especially in the CDC mode under different swirl ratios and loads. The peak of the heat flux predicted by the SCC model shows superiority to that of the previous steady heat transfer models and the unsteady heat transfer model.

Fundamental physics effects of background gas species and pressure on vapor plume structure and spatter entrainment in laser melting

First-principles modeling, based on the direct simulation Monte Carlo (DSMC) method, is applied to quantitatively explain the physics of stainless-steel (SS316L) vapor plumes and jets and powder particle entrainment under conditions relevant to laser welding and laser powder bed fusion additive manufacturing. The DSMC is a particle-based gas kinetic simulation approach, with few built-in assumptions, that resolves the Knudsen and diffusion layers in vapor microjets. The simulations are performed for different background gas species (argon, helium, and neon) at background gas pressures from 0.1 to 5 bar and molten pool temperatures from 3250 K to 4000 K. The simulations capture the evolution of the vapor jet structure from unsteady to steady-state vapor flow, the transition from subsonic flow at low surface temperatures to supersonic flow at surface temperatures close to boiling, as well as the effects of the background gas species and pressure. The simulations show that the vapor jet structure, as well as the magnitude and direction of the entrained gas close to the laser spot, greatly affects the process environment, creating particle spatter and denudation around the laser track. High pressure and lighter background gas can decrease the amount of spatter. At high pressure, the evaporation is reduced, which forms a low-velocity subsonic plume. For lighter gas species, the gas-entrainment velocity increases while the jet radius decreases which forces entrained particles to merge with the melt pool. The powder particle simulations reveal that the width of the denudation zone strongly depends on particle diameter and increases at low pressure and lighter background gas. This study emphasizes that the ambient gas species and the pressure in the build chamber should be treated as process parameters that are as important as controlling laser power and scan speed when trying to prevent defects. • Kinetic modeling reveals strong effect of molecular diffusion in AM vapor microjets. • The laser power and surface temperature stronlgy affect the transition from subsonic to supersonic vapor jet. • Ambient gas chamber species and pressure strongly affect the dynamics (gas, powder) near the laser spot. d. • Strong gas flow (helium) does not necessarily create more spatter as these merge with the melt pool and disappear. • The width of the vapor plume is more strongly dependent on the ambient gas pressure than on the laser beam size.

Transient surface charge of SF 6 ‐filled direct current gas‐insulated transmission lines insulator under thermal‐electric coupled fields: effects of ambient temperature, load current and gas pressure

The transient surface charge characteristics of a basin-type insulator in horizontally installed SF6-filled direct current gas-insulated transmission lines (DC-GIL) are investigated based on a three-dimension (3D) simulation model under thermal-electric coupled stress. The influence of ambient temperature, load current and gas pressure on the surface charge and electric field characteristics is investigated. Simulation results indicate that the surface charge accumulation process is accelerated due to the promotion of bulk conduction under high ambient temperature, high load current and low gas pressure. The transient field variation is accelerated as the consequence. The highest field strength on the convex surface increases and moves towards the enclosure with increasing time. With increasing load current, the field strength increases due to the influence of charge accumulation and temperature-dependent conductivity of the insulator. It is revealed that the transient surface charge under coupled fields should be focused when dealing with the insulation characteristics of DC-GIL insulators during long-term operation. The mentioned operating conditions should be paid attention. This method introduced in this study can help to evaluate the 3D transient charge and field characteristics under various conditions.

Parallel-structured Fabry-Perot interferometers gas pressure sensor with ultraviolet glue sensitization based on dual Vernier effect

A parallel-structured Fabry-Perot interferometers (FPIs) gas pressure sensor with ultraviolet glue sensitization based on dual Vernier effect has been proposed and experimentally investigated, which was fabricated based on the FPI coated with ultraviolet glue film and the FPI fully filled with ultraviolet glue. Owing to the unique features of the parallel-structured FPIs, the reflective spectra of them shift to opposite directions as ambient gas pressure varies. Two FPIs serve as sensing reference for each other in gas pressure sensing, generating the dual Vernier effect to enhance Vernier effect. The proposed gas pressure sensor exhibits a high sensitivity of 131.88 nm/MPa and provides new strategies and ideas for fiber-optic sensing filed.

A testbed for investigation of laser powder bed fusion at elevated atmospheric pressure

Metal additive manufacturing (AM) by laser powder bed fusion ( L -PBF) builds upon fundamentals established in the field of laser welding which include the influence of gas and plume dynamics on weld depth and quality. L -PBF demands a thorough investigation of the complex thermophysical phenomena that occur where the laser interacts with the metal powder bed. In particular, melt pool turbulence and evaporation are influenced by the ambient gas chemistry and pressure, yet typical L-PBF equipment lacks flexibility to alter the ambient pressure within the process chamber. This paper presents the design and validation of high pressure laser melting (HPLM) testbed. This system accommodates bare metal plate samples as well as manually-coated single powder layers, and operates at up to 300 psig. The open architecture of this testbed allows for full control of all relevant laser parameters in addition to ambient gas pressure and gas flow over the build area. Representative melt tracks and rasters on bare plate and powder are examined in order to validate system performance, and preliminary analysis concludes that pressure has a significant impact on melt pool aspect ratio. The HPLM system thus enables careful study of the effects of pressure on the outcomes of the L-PBF process, and can be applied in the future to materials that are challenging to process by L-PBF under ambient pressure, such as those with high vapor pressures.

SIMULATION STUDY OF BREAKUP AND ATOMIZATION CHARACTERISTICS OF GASOLINE-ETHANOL (GASHOLS) FUEL BLENDS

To meet the increasingly stringent constraints on mobile and point emissions in the wake of the dwindling fortunes of fossil fuels, evaluation of hybrid fuels for application in combustion engines has become an imperative, especially in the third world countries. This paper presents, modeling of atomization characteristics of gasoline-ethanol fuel blends. Fuel breakup models and correlations between flow patterns and droplet characteristics were adopted and implemented in OpenFOAM Computational Fluid Dynamics (CFD) modeling suite for direct gasoline injector using a simple cylindrical mesh structure at constant volume. The Rosin Rammler distribution model was used to generate the number of spray particles injected into the cylinder. The spray modeling and atomization employed the use of blob sheet model and Kelvin-Helmholtz-Rayleigh-Taylor (KH-RT) model while the numerical technique for simulating atomization process by CFD included the use of governing equations such as Eulerian for gas phase, Lagrangian for disperse phase and turbulence modeling. The effects of fuel blends, injection pressure, ambient gas pressure and spray cone angle were evaluated on the axial spray tip penetration, spray width, and overall Sauter Mean Diameter (SMD). The SMD was seen to be affected by varying the degree of injection cone angle. The spray tip penetration lengths were larger for higher injection cone angles while higher penetration lengths were obtained at higher injection pressures. One salient conclusion drawn from the modeling is that as the number of particle increased, the density of clusters became smaller.

Shock front interaction and dynamics of laterally colliding laser-produced plasmas

The effects of ambient gas pressure on the propagation dynamics of laterally colliding laser-produced aluminum plasma is investigated under various ambient conditions. In lower pressure range the interaction zone formed in between the seed plasmas exist for a short span of time. A strong stagnation region is observed at 10−2 mbar pressure in Ar ambient. Interaction zones are formed due to the shock waves generated at the seed plasmas at pressures above 0.1 mbar. A peculiar type of plume splitting is also observed at 0.15 mbar in air ambient at 150 ns. In Ar ambient, a channel like structure is observed in between the plasma plumes whose width increased with time. Shock interaction in air ambient is explained by regular reflection of shock waves where an illuminated interaction zone was formed where as in heavier Ar ambient, Mach reflection of shock waves occurs.

A newly designed compact CEY-XAFS cell in the soft X-ray region and its application to surface XAFS measurements under ambient-pressure conditions without photoinduced side effects.

We report a newly designed compact cell to measure XAFS spectra with the conversion electron yield (CEY) method in the soft X-ray region under ambient-pressure gas conditions. Secondary electrons generated from the gas and sample by collision of X-ray-absorption-induced Auger electrons are collected by a positively biased collector electrode to obtain XAFS spectra. It was confirmed that this cell is applicable to soft X-ray surface XAFS measurements for different types of materials such as insulating organic materials and metal oxides under 1 bar gas conditions. During the measurements, photoinduced side effects were observed; i.e. photoinduced degradation of organic materials and photoinduced reduction/oxidation of metal oxides. We found that these photoinduced side effects can be sufficiently suppressed by controlling the measuring conditions. The presented measuring approach will enable surface XAFS spectra to be obtained in the soft X-ray region for various types of functional materials under ambient-pressure working conditions.

Thermal Fluctuations of the Optical Properties of Nanomechanical Photonic Metamaterials

AbstractThe combination of optical and mechanical resonances offers strong hybrid nonlinearities, bistability, and the ability to efficiently control the optical response of nanomechanical photonic metamaterials with electric and magnetic field. While optical resonances can be characterized in routine transmission and reflection experiments, mapping the high‐frequency mechanical resonances of complex metamaterial structures is challenging. Here, it is reported that high‐frequency time‐domain fluctuations in the optical transmission and reflection spectra of nanomechanical photonic metamaterials are directly linked to thermal motion of their components and can give information on the fundamental frequencies and damping of the mechanical modes. This is demonstrated by analyzing time‐resolved fluctuations in the transmission and reflection of dielectric and plasmonic nanomembrane metamaterials at room temperature and low ambient gas pressure. These measurements reveal complex mechanical responses, understanding of which is essential for optimization of such functional photonic materials. At room temperature the magnitude of the observed metamaterial transmission and reflection fluctuations is of order 0.1% but may exceed 1% at optical resonances.

RHEED analysis of the oxidized M′2M″xXyene sheets by ablated plasma thrust method in pulsed laser deposition chamber

M′2M″xXyene (M′ and M″ are the early transitional metals and X is carbide with x = 1 for y = 2 and x = 2 for y = 3) are the ordered double transitional metal layered carbides derived from their parent MAX phases M′2M″xAlXyene by a wet chemical etching method. Their oxides are predicted to have topological properties for which they should be annealed at around 800 °C in an oxygen background. This paper includes the new ablated plasma thrust method for the ionization and adsorption of oxygen on the M′2M″xXyene substrate in the pulsed laser deposition chamber. We have found that the background pressure has a negative effect and the substrate temperature has a positive effect on plume expansion. The density profile of the background gas is highly affected by deposition temperature. Similarly, it is found that the density of plasma generated by longer wavelengths is not affected significantly due to the inverse bremsstrahlung process. A shorter wavelength produces the bremsstrahlung process as photoionization takes place. At a certain time (200 ns), the pressure of background gas and plasma pressure are equal (snow-plogh effect) so that all the wavelengths then produce electrons (highest for shorter wavelengths), thereby increasing its density. The energy necessary for the oxidation of the substrate is provided by the energy of the ablated species. The adsorption is assured by the reflective high electron energy diffraction technique, and it is found that the ambient gas pressures p = 0.1 mbar and 0.2 mbar are appropriate for the adsorption process. The obtained M′2M″xXyene oxides can be used for their topological test.

Morphology and photoluminescence properties of silicon nanoparticles deposited in helium-nitrogen mixtures maintained at low residual pressures

We elaborated a technique of pulsed laser ablation in gas mixtures (He-N2), maintained under residual pressures of 0.5–5 Torr to deposit silicon (Si)-based nanostructured films on a substrate. We show that the deposited films can exhibit strong photoluminescence (PL) emission with the position of peaks depending on the pressure of ambient gas and the ratio of gases in the mixture. Nanostructured films prepared in pure He gas exhibited a strong band in the infrared range (around 760 nm) and a weak band in the green range (550 nm), which were attributed to quantum-confined excitonic states in small Si nanocrystals and radiative transitions via the localized electronic states in silicon suboxide coating, respectively. In contrast, nanostructured films prepared in He-N2 mixtures exhibited more intense “green-yellow” PL band centered at 580 nm, which was attributed to a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals. We also present a detailed analysis of morphology of nanostructures Si-based films prepared by laser ablation. Finally, we show that the nanocrystals can be removed from the substrate and milled by ultrasound to form aqueous solutions of colloidal Si nanopartiles. The fabricated Si-based nanocrystals present a promising object for theranostics, combining imaging functionality based on PL emission and a series of therapy functionalities (photo and radiofrequency hyperthermia, photodynamic therapy).

Experimental and theoretical study of the characteristics of LSPR peaks for metal NPs produced by controlling Ar ambient gas pressure to enhance the efficiency of solar cells

In this study, silver (Ag) nanoparticle thin films were deposited on microscope slide glass and Si wafer substrates using the pulsed-laser deposition (PLD) technique in Ar ambient gas pressures of 1 × 10−3 and 7.5 × 10−1 mbar. AFM analysis has shown that the number of Ag nanoparticles reaching the substrate decreased with increasing Ar gas pressure. As a result of Ar ambient gas being allowed into the vacuum chamber, it was observed that the size and height of Ag nanoparticles decreased and the interparticle distances decreased. According to the absorption spectra taken by a UV–vis spectrometer, the wavelength where the localised surface plasmon resonance (LSPR) peak appeared was shifted towards the longer wavelength region in the solar spectrum as Ar background gas pressure was decreased. This experiment shows that LSPR wavelength can be tuned by adjusting the size of metal nanoparticles, which can be controlled by changing Ar gas pressure. The obtained extinction cross section spectra for Ag nanoparticle thin film was theoretically analysed and determined by using the metal nanoparticle–boundary element method (MNPBEM) toolbox simulation program. In this study, experimental spectrum and simulation data for metal nanoparticles were acquired, compared, and determined to be in agreement.