Increasing Background Pressure Research Articles

Electrospray plume divergence: Background pressure influence

The influence of background pressure on electrospray plume evolution is observed by simulating the emission and propagation of an electrospray particle population into an electric field at a range of relevant background pressures. Differences in plume evolution from atmospheric pressure to one hundredth of atmospheric pressure are evident from plume characteristics such as (1) the overall domain of the resulting plumes and (2) the terminal angle at a downstream terminus of one standard deviation and three standard deviations of particle number density. Plume divergence and terminal angle are shown to correlate strongly with background pressure for pressures above which plume-background collision rates are significant, consistent with experimental observations of increased plume divergence with increased background pressure. The results suggest a simple expression for the pressure below which a system achieves minimum plume divergence: Pth=kT/7.7299σfld for a system of temperature T, background fluid molecules with cross-section σfl, and plume species of diameter d.

The Seebeck Coefficient of Sputter Deposited Metallic Thin Films: The Role of Process Conditions

Because of their reduced dimensions and mass, thin film thermocouples are a promising candidate for embedded sensors in composite materials, especially for application in lightweight and smart structures. The sensitivity of the thin film thermocouple depends however on the process conditions during deposition. In this work, the influence of the discharge current and residual gas impurities on the Seebeck coefficient is experimentally investigated for sputter deposited copper and constantan thin films. The influence of the layer thickness on the film Seebeck coefficient is also discussed. Our observations indicate that both a decreasing discharge current or an increasing background pressure results in a growing deviation of the film Seebeck coefficient compared to its bulk value. Variations in discharge current or background pressure are linked as they both induce a variation in the ratio between the impurity flux to metal flux towards the growing film. This latter parameter is considered a quantitative measure for the background residual gas incorporation in the film and is known to act as a grain refiner. The observed results emphasize the importance of the domain size on the Seebeck coefficient of metallic thin films.

Background Pressure Effects on Ion Velocity Distributions in an SPT-100 Hall Thruster

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (), with vacuum facility background pressures ranging from to . Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal . Time-resolved measurements, implemented using a sample-hold scheme with resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at , where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at . These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.

Kinetic simulations of laser-induced plume expansion into a background gas under conditions of spatial confinement

Expansion of vapor plume induced by short-pulse laser irradiation of a bottom of a cylindrical cavity or planar trench in a copper target into helium, argon, and xenon background gases at pressure varying from zero to 1 bar is studied numerically. The simulations are performed based on a hybrid computational model that includes an unsteady heat conduction equation, Hertz-Knudsen model of evaporation, and a kinetic model of two-component gas flow implemented in the form of the Direct Simulation Monte Carlo method. It is found that increasing background pressure results in the formation of a complex structure of shock waves, which drastically changes the flow compared to the case of expansion into a vacuum. The moving shocks strongly affect the deposition of the ablated material inside the cavity. In particular, they induce the formation of a near-surface layer with a reduced fraction of vapor, which protects the cavity wall from vapor redeposition. The overall effect of the increasing background gas pressure on the flow structure and redeposition of vapor inside the cavity can be described as a trade-off between the confinement effect, i.e. a reduction in the propagation speed of the primary shock and overall rate of the plume expansion, and the focusing effect of moving shock waves that induce the radial flow towards the cavity axis, increase vapor density around the axis of symmetry, and reduce vapor density at the cavity wall. The efficiency of vapor removal out of the cavity can increase or decrease with increasing molar mass of the background gas depending on whether the flow is dominated by the focusing or confinement effect. These findings suggest that tuning the background gas parameters and geometrical parameters of spatial confinement can be used to effectively control the laser-induced plume expansion process in applications of laser ablation.

Nonstoichiometric transfer during laser ablation of metal alloys

Large angular variations in film composition have been found for ablation of a metallic AuCu alloy (Au/Cu ratio $\ensuremath{\sim}1)$ in vacuum and background gases of Ne and Xe. The AuCu films grown in vacuum at a laser fluence of $5\phantom{\rule{0.16em}{0ex}}\mathrm{Jc}{\mathrm{m}}^{\ensuremath{-}2}$ exhibit a large loss in the Cu content, with the Au/Cu ratio $\ensuremath{\sim}2.4$ at angles close to normal incidence. At this fluence, a distortion of the plume front is observed followed by the appearance of a secondary emission at the substrate, suggesting that resputtering of the film by energetic ions and reflection of ions/atoms at the substrate can lead to a nonstoichiometric transfer in pulsed laser deposition. Further, we have found that depending on the mass of the background gas employed during growth (Ne or Xe), the ratio of elements in the film can vary significantly over a wide range of angles of deposition. In the presence of the light gas Ne, the degree of nonstoichiometric transfer is gradually reduced with increasing background pressure, resulting in a nearly stoichiometric AuCu films at a Ne pressure of 2 mbar. The behavior in the heavy gas Xe is more complex, and both theoretical and experimental data indicate that the loss of Cu in the deposits is caused by the preferential scattering, as well as by backscattering of the light Cu atoms in the plume upon collisions with the background gas.

Particulate photocatalyst sheets for Z-scheme water splitting: advantages over powder suspension and photoelectrochemical systems and future challenges.

Water splitting using semiconductor photocatalysts has been attracting growing interest as a means of solar energy based conversion of water to hydrogen, a clean and renewable fuel. Z-scheme photocatalytic water splitting based on the two-step excitation of an oxygen evolution photocatalyst (OEP) and a hydrogen evolution photocatalyst (HEP) is a promising approach toward the utilisation of visible light. In particular, a photocatalyst sheet system consisting of HEP and OEP particles embedded in a conductive layer has been recently proposed as a new means of obtaining efficient and scalable redox mediator-free Z-scheme solar water splitting. In this paper, we discuss the advantages and disadvantages of the photocatalyst sheet approach compared to conventional photocatalyst powder suspension and photoelectrochemical systems through an examination of the water splitting activity of Z-scheme systems based on SrTiO3:La,Rh as the HEP and BiVO4:Mo as the OEP. This photocatalyst sheet was found to split pure water much more efficiently than the powder suspension and photoelectrochemical systems, because the underlying metal layer efficiently transfers electrons from the OEP to the HEP. The photocatalyst sheet also outperformed a photoelectrochemical parallel cell during pure water splitting. The effects of H+/OH- concentration overpotentials and of the IR drop are reduced in the case of the photocatalyst sheet compared to photoelectrochemical systems, because the HEP and OEP are situated in close proximity to one another. Therefore, the photocatalyst sheet design is well-suited to efficient large-scale applications. Nevertheless, it is also noted that the photocatalytic activity of these sheets drops markedly with increasing background pressure because of reverse reactions involving molecular oxygen under illumination as well as delays in gas bubble desorption. It is shown that appropriate surface modifications allow the photocatalyst sheet to maintain its water splitting activity at elevated pressure. Accordingly, we conclude that the photocatalyst sheet system is a viable option for the realisation of efficient solar fuel production.

Effect of background pressure on Co/C multilayers.

Co/C multilayers with a period thickness of 3.54 nm and 30 bilayers were deposited by direct current magnetron sputtering with different background pressures. The effects of residual background gases were investigated. The films were characterized by using grazing incidence hard x-ray reflectivity, soft x-ray reflectivity, and x-ray photoelectron spectroscopy. The results indicate that the x-ray reflectivity of Co/C multilayers decreases with increasing background pressure as well as the increasing interlayer roughness. The inclusion of more residual background air increases the interdiffusion of Co and C layers.

Study on Interfacial Bonding Strength of SiO<sub>2</sub>/PET Composite Packaging Film

SiO2/PET composite packaging film was prepared by depositing SiO2on PET film substrates through magnetron sputtering technology. The state of SiO2particles and its dispersion properties in PET substrates were characterized by scanning electron microscope (SEM) and bonding strength tester. The effects of the sputtering process parameters and intermediate layer on the bonding strength of the coating-substrate system were investigated. Results show that the bonding strength of coating-substrate system firstly increases and then decreases with the increase of sputtering process parameters. It is up to a maximum at the argon flow of 10 mL/min, radio-frequency power of 250 W and sputtering time of 70 min; and it is 0.14 MPa, 0.22 MPa and 0.19 MPa, respectively. The bonding strength of coating-substrate system increases with the increasing background pressure. Meanwhile, we find that the bonding strength of coating-substrate system is improved by depositing A12O3intermediate layer between SiO2film and PET substrates.

Velocity map imaging using an in-vacuum pixel detector

The use of a new type in-vacuum pixel detector in velocity map imaging (VMI) is introduced. The Medipix2 and Timepix semiconductor pixel detectors (256 x 256 square pixels, 55 x 55 microm2) are well suited for charged particle detection. They offer high resolution, low noise, and high quantum efficiency. The Medipix2 chip allows double energy discrimination by offering a low and a high energy threshold. The Timepix detector allows to record the incidence time of a particle with a temporal resolution of 10 ns and a dynamic range of 160 micros. Results of the first time application of the Medipix2 detector to VMI are presented, investigating the quantum efficiency as well as the possibility to operate at increased background pressure in the vacuum chamber.

Surface morphology and evolution of amorphous carbon thin films

The surface morphology of disordered carbon films grown by nanosecond pulsed laser ablation of graphite is reviewed. It is shown that the presence of a background gas can have a profound effect on the plume of material ejected during ablation. At low pressures smooth films are produced but at higher pressures rough films with an evolution from a nodular morphology to a large area cluster-assembled morphology occurs. The surface morphology changes with increasing background pressure as a result of collisions, which reduce the kinetic energy of the ejected material and allow for cluster formation within the plume. It is shown that the energy of some of the carbon ablated species in vacuum can exceed 100 eV. The nature of the species present in the plume is discussed in terms of electron–ion recombination and impact ionisation/excitation. The cluster-assembled films are shown to be useful as a scaffold for supporting metal nanoparticles to produce substrates for surface enhanced Raman spectroscopy.

Decoherence in a Talbot–Lau interferometer: the influence of molecular scattering

We study the interference of C70 fullerenes in a Talbot-Lau interferometer with a large separation between the diffraction gratings. This permits the observation of recurrences of the interference contrast both as a function of the de Broglie wavelength and in dependence of the interaction with background gases. We observe an exponential decrease of the fringe visibility with increasing background pressure and find good quantitative agreement with the predictions of decoherence theory. From this we extrapolate the limits of matter wave interferometry and conclude that the influence of collisional decoherence may be well under control in future experiments with proteins and even larger objects.

Non-neutral plasma expansion induced by electron-neutral collisions in a Malmberg–Penning trap

Single species non-neutral plasmas have very robust confinement properties because the conservation of canonical angular momentum in a system with azimuthal symmetry provides a powerful constraint on the allowed radial positions of the particles. If no external torques act on the plasma, the plasma cannot expand radially to the wall. However, collisions with a background neutral gas will exert a torque on the rotating plasma thus allowing the mean-square radius to increase. In the electron diffusion gauge experiment, a pure electron plasma is confined in a Malmberg–Penning trap and the radial density profile is measured as a function of time. The base pressure is 5×10−10 Torr and purified helium is injected to pressures ⩾5×10−9 Torr. The magnetic field is varied between 100 and 600 G. The experimentally measured radial density profile shape is found to match closely the theoretically predicted (expanding) equilibrium profile, where a single free parameter proportional to the electron temperature T is varied to best fit the experimental data. The best-fit value of the temperature T is found to stay approximately constant even as the plasma expands and the electrostatic energy decreases. The measured plasma expansion rate is found to scale with magnetic field strength as 1/B3/2 instead of the expected 1/B2 scaling. This modification in scaling may be caused by field asymmetries, which are believed to be an important factor in plasma expansion for the pressure ranges examined here. Nevertheless, the expansion rates are observed to increase with increasing background pressure, and the absolute scaling with pressure is consistent with theoretical predictions.

Residual Gas Effect on Photofield Emission from Tungsten (111) Face

Using the photofield emission method, surface states and bulk excitations from clean and residual gas adsorbed (111) face of tungsten were measured. Residual gas shaded the surface states and diminished photoemission from the bulk excitations. Gas of the artificially increased background pressure 10 -8 Pa adsorbed during the measurement deforms the photocurrent—voltage characteristic; the optical excitations observed for clean surfaces are exhibited, diminished or lost. For the adsorbed (111) face the state related to the metal—gas surface has been observed. PACS numbers: 79.70.+q, 79.60.Dp, 73.20.Hb

Macroparticle interaction with a substrate in cathodic vacuum arc deposition

The presence of macroparticles (MP) in the vacuum arc cathodic jet is the critical problem of cathodic vacuum arc deposition technology. The MP distribution on the substrate depends on MP generation, MP transport to the substrate, and the MP—substrate interaction. In the present work different aspects of the interaction of the MP with a substrate are studied theoretically. The MP temperature evolution during transport to the substrate was studied and it was found that the MP is liquid in the case of low melting point cathode materials. It was found that the MP temperature does not depend on the initial MP temperature in the near cathode region. Possible MP acceleration in the substrate direction resulting from ion friction was calculated by solving the equation of motion for the MP taking into account the plasma density and the velocity distribution obtained in previous analyses of the 2-D plasma expansion. The voltage drop in the substrate sheath as a function of the background pressure was calculated assuming charge exchange between ions and gas atoms. It was found that the voltage drop in this sheath increases with increasing background pressure p g by factor of 2 for p g ∼ 1 Pa. The MP motion in this sheath was calculated taking into account MP charging and reflection from the substrate. The probability of reflection decreases with MP size, velocity, angle of the MP velocity vector with respect to the substrate, and increases with the background pressure.

Wechselwirkung kondensierter Molekularstrahlen mit festen Oberflächen

The reflection of a beam of nitrogen clusters from a polished stainless steel surface is investigated. The scattered flux shows a strong maximum at an angle of reflection almost 90°, independent of angle of incidence. The mean velocity of the reflected beam is about equal to the tangential velocity component of the incident beam. Measurements with increased background pressure demonstrate that the reflected beam still consists essentially of clusters.

Observations of Instabilities in Electron-Cyclotron Plasmas

Two types of instabilities have been studied in an electron-cyclotron plasma confined by magnetic mirrors and heated by continuous wave microwave power at 10.6 Gc. One type of instability results in the loss of energetic electrons (Ee > 100 keV) across magnetic field lines, accompanied by oscillations in the 3–30 Mc range. Suppression of this instability is accomplished by increased background pressure. The second type is characterized by loss of fast electrons along field lines through the magnetic mirrors and accompanied by radio-frequency oscillations at 5.3 Gc (=½ωce). The loss is also characterized by a threshold in magnetic field corresponding to locating the heating zones in the mirror throats. The first type of instability appears to be flute-like and driven by ▿B drifts. The second type may be initiated by the mirror instability in the plasma.