Ion Momentum Transfer Research Articles

The transient thermomechanical behavior of solid oxide fuel cell during the operation condition variations

Solid oxide fuel cell (SOFC) is a promising energy conversion technology with high efficiency, low emission and fuel flexibility. In practical and dynamic operation, the thermal stress at the interface of different layers can cause cracks or even thermal mechanical failure. This study investigates the transient thermomechanical behavior of SOFC under changing operating conditions. A three-dimensional dynamical model is developed by considering the electrochemical reaction, electron and ion transport, gas diffusion, heat and momentum transfer, and thermal-mechanical interaction. When the operating conditions vary, the time it takes for the thermal stress to reach a steady state is consistent with the time required for heat transfer to stabilize. The electrolyte layer is subjected to the maximum gradient of thermal stress change after the voltage is switched. With an increase in the inlet gas temperature, the change in the cell temperature can be divided into two stages: a rapid rise and a slow rise. Appropriate prolongation of the temperature rise time is beneficial to the thermal stability of SOFC. The increase in inlet gas temperature changes the distribution trend of cell temperature and thermal stress, while the 50 % reduction in cathode and anode inlet velocity changes its magnitude.

Mathematical modeling of a submerged jet of electric wind

Technological processes of electro-gas dynamics of dispersed systems are based on charging and transporting dispersed raw material particles in a strong electric field of a corona discharge. It is accompanied by turbulent gas motion caused by momentum transfer of ions to gas molecules. This accompanying motion is called electric wind. It must be considered when calculating the trajectories of particles and devices design. In recent years, equipment is being developed, the operation of which is based on the direct use of electric wind. In most cases these studies are based on experimental research and empirical calculation formulas, therefore, to make the design and construction of this equipment science oriented, it is necessary to develop mathematical models and methods for calculating this phenomenon. The object of the research is a unipolar electric corona discharge of direct current between a negative corona wire and a flat electrode in the form of a mesh. The calculation of a turbulent jet of an electric wind is considered both within the framework of the boundary layer theory and in a full-scale formulation using the k- и k- turbulence models in the Comsol program. Two new solutions for the velocity field of a submerged flat jet of electric wind are found and compared. They are an analytical solution based on the boundary layer theory and a numerical solution in a full-scale formulation based on the Reynolds equation integration. The novelty of the solutions is that they are applied for a two-dimensional problem and consider the turbulent motion of gas. The phenomenon of electric wind is widely applied in modern technologies that allow electric and gas cleaning, disinfection, and water purification from organic impurities, as well as treatment and disinfection of surfaces and air which is especially important recently. In the case of jet spread of electric wind in a closed channel, the boundary layer approximation conditions are satisfied, and a self-similar solution can be used. In the case of an open jet, the calculation should be carried out in a full-scale formulation of the problem based on the numerical solution of the Reynolds equation.

Dynamics of the gas flow turbulent front in atmospheric pressure plasma jets

In this paper, dynamic characterizations of the turbulent flow field in atmospheric pressure plasma jets (APPJs) are investigated by focusing on the effect of different APPJ parameters, such as gas flow rate, applied voltage, pulse repetition frequency, and time duration of the pulse. We utilize Schlieren photography and photomultiplier tubes (PMT) as a signal triggering of an intensified charge coupled device (ICCD) and also a high speed camera to examine the formation of the turbulent front and its dynamics. The results reveal that the turbulent front will appear earlier and closer to the tube nozzle by increasing the gas flow rate or the applied voltage amplitude. However, the pulse time duration and repetition frequency cannot change the dynamics and formation of the turbulent front. Further investigation shows that every pulse can excite one turbulent front which is created in a specific position in a laminar region and propagates downstream. It seems that the dominating mechanisms responsible for the formation of turbulent fronts in plasma jets might not be ion momentum transfer.

Schlieren imaging investigation of the hydrodynamics of atmospheric helium plasma jets

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a “Z-type” Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed.

Nonlinear gravito-electrostatic waves in self-gravitating complex plasma in presence of ion-drag effects

We present theoretical model analysis to study fully nonlinear behavior of gravito-electrostatic fluctuations in unmagnetized self-gravitating collisional dust cloud in presence of the ion-drag forces methodologically on the Jeans scales of space and time. The ion-drag effect as a result of streaming plasma ions arises here due to the ion orbital motion (scattering effect by dust) and the ion momentum transfer (capturing effect by dust) processes in opposite phase with the electrostatic force field. All the realistic astrophysical processes, such as electron impact-ionization of the neutral atoms, volume recombination, attachment of the electrons and ions to the dust grains and the collective plasma particle collisions are jointly considered. The Sagdeev pseudo-potential formulation is methodologically carried out in modified form to derive a new pair of gravito-electrostatically coupled energy integral equations. A numerical analysis is made to see the fluctuation features in judicious plasma parameter window. It is shown that the fluctuation dynamics evolves as self-gravitational rarefactive solitary structures and electrostatic compressive shock-like spectral patterns. The new features brought about by the considered ion-drag effects are discussed in the light of the existing theoretical, experimental and satellite-based predictions. The relevance of our results to understand the dynamics of self-gravitational collapse leading to galactic structure formation in interstellar space is briefly summarized.

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H + and ionospheric O + points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow. The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H + mass-flux and increasing ionospheric O + mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O + ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O + outflow does not escape, but remains near Venus as a fast (km/s) O + zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O + mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10 −14 kg/(m 2 s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O +, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O + plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.

Ion acceleration in bias-enhanced nucleation of diamond at relatively high pressures

In order to explain the bias-enhanced nucleation (BEN) of diamond at pressures of approximately 6 kPa, the flux and energy of hydrogen ions to a negatively biased substrate in microwave plasma chemical vapor deposition (MWP-CVD) were investigated. Ion flux increased rapidly upon increasing the negative bias voltage to greater than 150 V when the pressure was higher than 3 kPa. This phenomenon was attributed to the additional ionization in the vicinity of the substrate due to biasing. Optical emission spectroscopy at 6 kPa clarified that the emission intensity of the Swan band of C 2 radicals was reduced, and intramolecular temperatures were enhanced in this bias region. A numerical simulation on the basis of the measured ion flux and density revealed that the most frequent energy was less than 2 eV under the conditions of 200 V and 6 kPa, and that the average number of collisions in the plasma sheath reached 60. We propose, on the basis of these results, that the substrate biasing in BEN at pressures above 3 kPa causes the additional ionization and activation of neutral species in the sheath, while the direct momentum transfer of ions to the substrate is strongly suppressed in BEN in this pressure range.

Effect of ion–neutral collision on the deduction of Mach number in collisional plasmas

A new unmagnetized collisional Mach probe theory is developed in order to resolve the collisional effect on Mach probe analysis by including ionization, charge and momentum transfer of ions in the perturbation region of a Mach probe. A fluid model is established by assuming Boltzmann electrons and taking the moments of the one-dimensional Boltzmann transport equation, which contains the two-dimensional transport information as a source, after adding a collisional term. A new relation between the flow velocity and the ratio of the ion sheath current densities is obtained, and is compared with those by a collisionless kinetic theory and a particle-in-cell simulation in the applicable range of these theories. A new relation between flow velocity and the ratio of the ion sheath current densities shows that ion–neutral collisions have a very strong effect to produce much smaller Mach numbers from the same ratio than those by collisionless models.

Angular momentum transfer and polarization degree of ions with one-valence electron by electron impact

We carry out the R-matrix calculations for electron-impact excitations of ions with one valence electron. The integral cross sections and polarization degree are obtained for the excitation process from the ground state to the first 2P° state of Li2+, B2+ and Al2+ as functions of electron incident energy. The differential cross sections and angular momentum transfer are also shown at non-resonant low-energy points. As for the angular momentum transfer (L⊥) at small scattering angles, they are negative for B2+ and Al2+, while it is positive for Li2+. Thus L⊥ of doubly charged ions with one-valence electron is not simple.

Angular momentum transfer and polarization degree of ions with two-valence electrons by electron impact

We study for electron-impact excitation of ions with two valence electrons (Be2+, C2+ and Si2+) from the ground state to the first 1P° state using the R-matrix method. The integral cross sections and polarization degree for this transition of each ion are obtained. The differential cross sections and angular momentum transfer are also shown at a few energies in the non-resonant region. The present angular momentum transfer for Be2+ and C2+ at small scattering angles has positive values, while it is negative for Si2+.

Cumulative processes on a dust particle in plasma

Processes of asymmetric ionization and cumulation of electric field and electron and ion flows can develop near the surface of charged dust particles in plasma. In the region of cumulation asymmetry on heating, the particle surface and ion momentum transfer arises; as a result, the dust particle moves in the plasma with high velocity.

Discharge characteristics of plasma sheet actuators

The electrical characteristics of a plasma sheet device used for subsonic airflow control are studied in this paper. Experiments are undertaken with a two-wire asymmetrical (different diameters, opposite polarity) electrode configuration connected to dc high voltage sources in the presence of a dielectric plate and under different gases (dry air, nitrogen and oxygen). For large distances electrode-plates it has been found that the discharge current consists of a purely dc component. The proximity of the plate reduces notably this dc current component until a limit situation for which the electrodes practically lay on the plate and a current pulsed regime is superimposed on the dc (small) component, thus establishing a plasma sheet regime. This regime could be reached only when the small wire was positive. This work establishes that the pulsed regime may be associated with a succession of positive streamers (cathode directed) which formation is promoted by different parameters of the gas and surface characteristics (thresholds of photoionization and photoemission, charge deposition, …). The dc component seems to be produced by a small number of electrons originated in the ionization region of the negative corona that are amplified in the ionization region of the positive corona. The charged particles produced during the streamer propagation could contribute appreciably to the ion momentum transfer to the gas. This transfer should be due very likely to the drift of the charged species present in the streamer channel during the streamer collapsing phase. The source of momentum transfer associated with the dc current would always persist with a magnitude that depends on the intensity of this current.

Wavefront control of SiO_2-based ultraviolet narrow-bandpass filters prepared by plasma ion-assisted deposition

Metal oxide layers produced by plasma ion-assisted deposition are extensively used for complex optical coatings due to the availability of materials, the high packing density of films, and the smooth surfaces. Stringent optical surface figure specifications necessary for both laser optics and precision optics require film stress to be well-controlled and surface deformation to be corrected or compensated. SiO(2)- based single-cavity UV narrow-bandpass filters were prepared by plasma ion-assisted deposition. The correlation between film stress, refractive index, deposition parameters, and postdeposition annealing was established. The film stress was calculated based on interferometric surface deformation. The refractive index and film thickness were determined by means of variable angle spectroscopic ellipsometry. The center wavelength of the filters was obtained through spectral transmission measurement. The results suggest that the wavefront distortion of the multilayer coatings is dominated by the compressive stress of the SiO(2) layers and can be controlled and corrected by the amount of plasma ion momentum transfer, substrate temperature, postdeposition annealing, and stress compensation via backside SiO(2) coating. Based on the understanding of the mechanical and optical properties, the wavefront correction technique enables us to satisfy stringent surface figure specifications.

First direct evidence of meso-scale variability on ion-neutral dynamics using co-located tristatic FPIs and EISCAT radar in Northern Scandinavia

Abstract. This paper presents the first direct empirical evidence that mesoscale variations in ion velocities must be taken into consideration when calculating Joule heating and relating it to changes in ion temperatures and momentum transfer to the neutral gas. The data come from the first tristatic Fabry-Perot Interferometer (FPI) measurements of the neutral atmosphere co-located with tristatic measurements of the ionosphere made by the European Incoherent Scatter (EISCAT) radar which were carried out during the nights of 27-28 February 2003 and 28 February until 1 March 2003. Tristatic measurements mean that there are no assumptions of uniform wind fields and ion drifts, nor zero vertical winds. The independent, tristatic, thermospheric measurements presented here should provide unambiguous vector wind information, and hence reduce the need to supplement observations with information obtained from models of the neutral atmosphere, or with estimates of neutral parameters derived from ionospheric measurements. These new data can also test the assumptions used in models and in ion-neutral interactions. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia, which is a region well covered by a network of complementary instruments. These provide a larger scale context within which to interpret our observations of mesoscale variations on the scales of tens of kilometres spatially and minutes temporally. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Calculations using the tristatic volume measurements show that the magnitude of the neutral wind dynamo contribution was on average 29% of Joule heating during the first night of observation. At times it either enhanced or reduced the effective electric field by up to several tens of percent. The tristatic experiment also presents the first validation of absolute temperature measurements from a common volume observed by independently calibrated FPIs. Comparison of EISCAT ion temperatures at an altitude of 240km with FPI neutral temperatures show that Ti was around 200K below Tn for nearly 3h on the first night during a period of strong geomagnetic activity. This is inconsistent with energy transfer. Comparison with FPI temperatures from surrounding regions indicate that it could not be accounted for by height variations. Indeed, these first results seem to indicate that the 630-nm emission did not stray too far from 240km. There were also apparent drops in Te at the same time as the anomalous Ti values which are energetically implausible. Incorrect assumptions of composition or non-Maxwellian spectra are likely to be the problem.

Wave Drag Due to Dust Acoustic Waves in Collisional Dusty Plasmas

The ion drag force on dust grains in a dusty plasma contributes significantly to the formation of equilibrium dust layers and void regions in radio-frequency discharges. In addition to the drag due to the direct collection of ions and to momentum transfer of ions in the electric field near grains, dust grains can also be subject to a drag force due to the effect of coherent waves, such as dust acoustic waves that can grow because of the drift of plasma ions relative to charged grains induced by an externally imposed electric field. Here, we examine wave drag due to unstable dust acoustic waves in a collisional dusty plasma using numerical simulations. We study the drag as a function of grain size as well as neutral-ion and neutral-dust collisions in both one-dimensional periodic systems, in which it is easier to study the instability properties per se, and in aperiodic configurations in order to assess effects associated with dust drag and void formation. For the parameter range considered, we find that in the absence of background collisions, the instability tends to saturate by trapping the plasma ions in the electrostatic waves, which does not affect the dust grains very much. Including ion-neutral collisions tends to suppress ion trapping, which in turn leads to larger wave amplitudes and trapping of the dust, resulting in significant drag on the dust grains. Inclusion of neutral-dust collisions leads to a grain size-dependent result, with the persistence of trapping of, and thus drag on, larger grains only. For the parameters of this study, the wave drag force is much larger than the ion drag usually considered.

Charged particle transport modelling in silane–hydrogen radio-frequency capacitively coupled discharges

This paper reviews the formulation and updates some numerical procedures usually adopted in two-dimensional, time-dependent, fluid models to study the transport of charged particles in radio-frequency capacitively coupled discharges. The model solves the continuity and momentum transfer equations for electrons, SiH 3 +, SiH 2 +, H 3 + and H 2 + positive ions and SiH 3 − negative ions, coupled with Poisson's equation and the electron mean energy transport equation. The electron mean energy transport equations are written as directly obtained from the electron Boltzmann equation. The inertia term in the ion momentum transfer equation is accounted for, by generalising the concept of effective electric field. The model is applied to a cylindrical parallel plate reactor, operating at 13.56 MHz, 100–500 V applied potentials and 68–300 mTorr pressures, for various silane–hydrogen composition mixtures. The effects of silane dilution are especially investigated.

Two-dimensional fluid modelling of charged particle transport in radio-frequency capacitively coupled discharges

This paper reviews the formulation and updates some numerical procedures usually adopted in two-dimensional, time-dependent fluid models to study the transport of charged particles in radio-frequency capacitively coupled discharges. The description of charged particle transport is made by solving the continuity and momentum transfer equations for electrons and ions, coupled with Poisson's equation and the electron mean energy transport equations. Inertia terms are considered in the ion momentum transfer equations, by generalizing the earlier definition of effective electric field. The electron mean energy equations are written using specific energy transport parameters, deduced from integration over the electron energy distribution function (EEDF). The model adopts the local mean energy approximation, i.e. it computes the electron transport parameters as a function of the electron mean energy, using either a homogeneous, two-term Boltzmann equation solver or a Maxwellian EEDF. More appropriate boundary conditions for the electron and ion fluxes are used successfully. The model is solved for a GEC Cell reactor type (with 6.4 cm radius and 3.2 cm interelectrode distance) operating at frequency 13.56 MHz, pressures between 10 mTorr and 10 Torr and applied voltages from 100 to 500 V, in electropositive (helium) and electronegative (silane–hydrogen) gases or gas mixtures. The ion kinetics in silane and hydrogen is updated with respect to previous works, by further considering SiH2+, H+ and H3+ ions. In general, simulation results for some typical electrical parameters are closer to experimental measurements available than calculations reported in previous works.

Ion Energy/Momentum Effects During Ion Assisted Growth of NbXNY Films

ABSTRACTIn a previous paper, it was shown that the tribological properties of NbxNy thin films produced by ion beam assisted deposition (IBAD) depend strongly on the beam energy and the ion-to-atom (R) ratio. This study was designed to separate ion energy vs. ion momentum effects on film stress, crystalline phase, grain size, morphology, and composition, all of which influence the tribological properties of the films. Inert ion beams (Kr, Ar, and Ne) were used in conjunction with a nitrogen gas backfill to independently control ion energy and ion momentum transfer to NbxNy films. The ion species, energies, and R ratios were chosen to create a matrix of coatings that exhibited the same total energy deposition with different momentum transfer or the same momentum transfer but different total energy deposition. The resultant films were characterized using Rutherford Backscattering Spectroscopy (RBS), x-ray diffraction (XRD), atomic force microscopy (AFM), and residual stress analysis. Crystalline phases and texture, as well as residual stress, were more closely correlated with ion momentum transfer to the coating atoms than with overall ion energy input.

Structural and optical modification in hafnium oxide thin films related to the momentum parameter transferred by ion beam assistance

Hafnium oxide (HfO 2) films were deposited on silica and glass substrates by ion (Xe +) assisted deposition with increasing ion momentum transfer to the growing film. The relationship among the ion momentum values, the crystalline phase and the refractive index (packing density) has been worked out by means of X-ray diffraction and spectrophotometric analysis. Compaction of the films by ion beam assistance is clearly evidenced by the changes in their microstructure. A three steps transition from a random monoclinic phase, via amorphous phase, up to an highly phase oriented (fiber texture), as a function of ion momentum, has been found.

Plasma transport around dust agglomerates having complex shapes

Dust particles generated in low temperature plasmas as used for microelectronics fabrication are often agglomerates of smaller monodisperse particles. The transport of these agglomerates, and the subsequent contamination of surfaces, depends on the details of ion-momentum transfer (ion drag) to, and electrostatic forces on, the agglomerate. Given that the charge distribution on the surface of the agglomerate and local electric fields in the vicinity of the agglomerate depend on its shape, the subsequent forces on the agglomerate will also be a function of shape. In this article, we describe results from a simulation in which plasma transport around, and the charging of, agglomerates are investigated. We find that the charge distribution on the agglomerates is generally nonuniform, a consequence of both shadowing and charge depletion. The ion-momentum transfer cross section, calculated using a Monte Carlo simulation, also depends on the shape of the agglomerate.