Effective Collision Frequency Research Articles

Collision theory and membrane photocatalysis: Steroid hormones meet singlet oxygen in palladium-porphyrin-coated polytetrafluoroethylene membranes

Photocatalytic membrane (PCM) reactors are an emerging technology for the continuous elimination of micropollutants from water. PCM materials and process properties define performance limitations. A collision theory framework was established to elucidate the limiting factors of steroid hormone micropollutant photodegradation inside the pores (200 nm) of a palladium-porphyrin-coated polytetrafluoroethylene (PTFE) PCM under simulated sunlight. The collision theory can describe the degree of photodegradation of 17β-estradiol (E2). The production of singlet oxygen reactive species was limited by light intensity (up to 14 mW cm−2), porphyrin loading (up to 50 μmol/L), and membrane layer thickness (46 μm, achieved via stacking thin membranes). Further increases in these parameters did not significantly enhance the removal of E2, because the quantity of singlet oxygen generated, and consequently the collision frequency, levelled off. Not all collisions result in photodegradation reaction. By reducing the reaction time via increasing the E2 molar flux, the rate of disappearance reached a threshold of 7 ± 2 nmol/L s−1. This is identified as the maximum effective collision frequency, implying that 11 % of the total collisions (64 ± 5 nmol/L s−1) resulted in successful reaction. The study presents a novel framework based on collision theory to predict the fundamental mechanisms of and limitations to photodegradation in porphyrin-PTFE membranes, providing unprecedented insight into the performance limitations of diverse membrane reactors for advances in materials and process engineering.

Power transfer efficiency in an air-breathing radio frequency ion thruster

Abstract Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas composition is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N2 discharge, with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N2/O2 discharge, increasing the N2 content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Determination of the time-dependent effective ion collision frequency from an integral observation

Abstract Identification of physical properties of materials is very important because they are in general unknown. Furthermore, their direct experimental measurement could be costly and inaccurate. In such a situation, a cheap and efficient alternative is to mathematically formulate an inverse, but difficult, problem that can be solved, in general, numerically; the challenge being that the problem is, in general, nonlinear and ill-posed. In this paper, the reconstruction of a lower-order unknown time-dependent coefficient in a Cahn–Hilliard-type fourth-order equation from an additional integral observation, which has application to characterizing the nonlinear saturation of the collisional trapped-ion mode in a tokamak, is investigated. The local existence and uniqueness of the solution to such inverse problem is established by utilizing the Rothe method. Moreover, the continuous dependence of the unknown coefficient upon the measured data is derived. Next, the Tikhonov regularization method is applied to recover the unknown coefficient from noisy measurements. The stability estimate of the minimizer is derived by investigating an auxiliary linear fourth-order inverse source problem. Henceforth, the variational source condition can be verified. Then the convergence rate is obtained under such source condition.

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Collisions between electrons and neutral molecules are of special interest for the physics of the Earth’s ionosphere, in particular, for determining the ionospheric conductivity and current systems in the lower ionosphere of the planet, as well as elucidating the role they play in attenuating radio waves propagating inside the D and E regions of the ionosphere. The effective collision frequency of electrons can be estimated from laboratory studies of electron mobility in atmospheric gases in combination with rocket measurements of temperature and particle density in the Earth’s upper atmosphere, or it can be determined independently from analysis of radio occultation data. We have developed a method for reconstructing the vertical profiles of the absorption coefficient of decimeter (wavelength ~19 cm) radio waves by solving the inverse problem of signal absorption in the D and E regions of the Earth’s ionosphere. Based on the analysis of radio occultation data from the FORMOSAT-3/COSMIC satellites, the altitude profiles of the absorption coefficient of decimeter (DM) radio waves in the planet’s ionosphere during the geomagnetic storm on June 22–23, 2015, were determined. It is known that the absorption coefficient at a given fixed frequency is directly proportional to both the electron density and the collision frequency of electrons with ions and neutrals. Using the data on the vertical profiles of the absorption coefficient of DM radio waves and the electron density reconstructed from the analysis of FORMOSAT-3/COSMIC radio occultations, we estimated the effective collision frequency of electrons in the D and E regions of the Earth’s high-latitude ionosphere. The practical significance of studying the frequency of electron collisions and the effects of radio wave absorption in the D and E regions of the planet’s ionosphere is associated with maintaining the uninterrupted operation of space radio communication and navigation systems.

Influence of impact medium particle size on mechanical deposition layer formation process based on discrete element method

Based on the discrete element method, this study has simulated the movement of materials in the plated barrel using EDEM software to investigate the effect of the impact medium with different particle sizes on the mechanical deposition plating. The simulation analyzed the interaction between five impact mediums with different particle sizes (2, 3, 4, 5, and 6 mm) and the base body while determining the particle size ratio of the impact medium. The rationality of the particle size ratio was verified by the simulation analysis of the interreaction of two groups of mixed particle size impact medium and the base body. The samples of mechanically deposited zinc coating were prepared. The average thickness, relative density, and average deviation of the coating were measured. The results show that When the particle size is 2 and 3 mm, the effective collision frequency of the materials is higher. When it is 3, 4, and 5 mm, the kinetic energy of collision between materials fluctuates evenly. When it is 4 and 5 mm, the collision contact force of the materials changes regularly. Consequently, the obtained particle size ratio (second group) is 2 mm 23%; 3 mm 23%; 4 mm 23%; 5 mm 27%; and 6 mm 4%. Compared with mixed impact media with 20% of each of the five particle sizes (first group), in the second group of assorted particle size simulation test, the collision frequency between materials is high. The contact force changes more regularly between materials. The average thickness, relative density, and average deviation of the mechanically deposited galvanized layer are consistent with the simulation results of the two groups of mixed particle sizes. The average thickness and relative density of the galvanized layer in the second group are higher, the average deviation is smaller, and the compactness and uniformity are the best.

Microwave-intensified hydrolysis for high efficiency hydrogen generation over magnetically separable Co3O4-carbon nanotube composites

Microwave (MW)-assisted process intensification technology has the potential to improve catalytic conversion under mild reaction conditions. In this study, a series of MW-sensitive CNTs-Co3O4 composites were designed and synthesized as catalysts for the conversion of NaBH4 to H2. It was found that MW irradiation leads to a 19.6 (35 °C) to 49.5% (55 °C) increase in the hydrogen generation rate (HGR) of NaBH4 hydrolysis catalyzed by the CC14, which is attributed to the combined contribution of thermal and non-thermal effects of microwave irradiation. Moreover, the NaBH4 hydrolysis catalyzed by CC14 catalyst at different heating modes followed the zero-order kinetic model, and the pre-exponential factor of NaBH4 hydrolysis catalyzed by CC14 catalyst under MW irradiation was approximately 157 times higher than under conventional heating, indicating that microwave irradiation increased the effective collision frequency at the reaction interface of CC14 catalyst. In addition, the selective and instantaneous heating of microwave irradiation was demonstrated by using a specially designed pseudo-homogeneous low temperature reaction system. Moreover, DFT calculations show that the rate-determining step of NaBH4 hydrolysis is H2O dissociation.

Influence of matching network and frequency on the effective quality factor and performance of a miniature radio frequency ion thruster

The radio frequency ion thruster (RIT) is an electric propulsion device that utilizes radio frequency (RF) power to ionize propellants; thus, its performance is influenced by the frequency and matching network of the RF. In this study, an L&T type wide-range matching network was employed to experimentally investigate the performance of a miniature RIT with 4 cm diameter at different frequencies. The experimental results show that changing the matching parameters of the L&T network at the same frequency leads to changes in the effective quality factor and varying performances of the thruster; under similar effective quality factors, changing the frequency does not significantly affect the performance of the thruster; however, the plasma resistance exhibits frequency dependence. As the frequency increases, the effective collision frequency increases, leading to an increase in plasma resistance, which is beneficial for improving the performance of the thruster at high frequencies. Finally, a set of methods for quickly comparing the performance of RITs was summarized based on the effective quality factor. These research results are also instructive for improving the ionization of induction-coupled discharge devices of other sizes.

Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Particle pump-out effects induced by low-frequency (<200 kHz) density fluctuations were observed in solely electron cyclotron wave (ECW)-heated plasmas on the spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increases with increasing ECW power and decays when the ECW is turned off while sustaining the plasma current. The electron densities are maintained relatively high and steady when the density fluctuations are completely absent, indicating that the outward transport of electrons is dominated by the particle pump-out effect of the ECW. The density fluctuations are modulated by a supersonic molecular beam injection pulse and the modulation amplitude decreases with increasing electron density at the same ECW injection power and decreasing ECW power at the same electron density, respectively. Analysis revealed that a critical value of electron temperature gradient (ETG) triggers the density fluctuations, and the intensity of the relative density fluctuations is positively correlated with the ETG and approximately inversely proportional to the effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations demonstrate that trapped electron mode (TEM) turbulence can be excited by ETG higher than the critical value observed in the experiment. In addition, the dependence of the mode growth rate (supposed to be proportional to the saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuations is comparable. The simulated outward particle flux integrated over the poloidal wave number spectrum is significant and proportional to ETG. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by ETG and induces particle pump-out when the electron density/effective electron collision frequency is low. The potential relevance of this work with the controls of plasma profiles, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is also discussed.

Enhanced Single-Particle Collision Electrochemistry at Polysulfide-Functionalized Microelectrodes for SARS-CoV-2 Detection.

Single-particle collision electrochemistry (SPCE) has shown great promise in biosensing applications due to its high sensitivity, high flux, and fast response. However, a low effective collision frequency and a large number of interfering substances in complex matrices limit its broad application in clinical samples. Herein, a novel and universal SPCE biosensor was proposed to realize sensitive detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) based on the collision and oxidation of single silver nanoparticles (Ag NPs) on polysulfide-functionalized gold ultramicroelectrodes (Ps-Au UMEs). Taking advantage of the strong interaction of the Ag-S bond, collision and oxidation of Ag NPs on the Ps-Au UME surface could be greatly promoted to generate enhanced Faraday currents. Compared with bare Au UMEs, the collision frequency of Ps-Au UMEs was increased by 15-fold, which vastly improved the detection sensitivity and practicability of SPCE in biosensing. By combining magnetic separation, liposome encapsulation release, and DNAzyme-assisted signal amplification, the SPCE biosensor provided a dynamic range of 5 orders of magnitude for spike proteins with a detection limit of 6.78 fg/mL and a detection limit of 21 TCID50/mL for SARS-CoV-2. Furthermore, SARS-CoV-2 detection in nasopharyngeal swab samples of infected patients was successfully conducted, indicating the potential of the SPCE biosensor for use in clinically relevant diagnosis.

Terahertz sound generation at the effect of a femtosecond pulse of laser radiation on a metal.

Sound generation by a femtosecond laser pulse in a metal layer on a dielectric substrate is studied. The excitation of sound caused by the effect of the ponderomotive force, temperature gradients of electrons, and lattice is considered. A comparison is made of these generation mechanisms for various excitation conditions and frequencies of generated sound. It is shown that in the case when effective collision frequencies in the metal are low, sound generation dominates in the terahertz frequency range due to the ponderomotive effect of the laser pulse.

Challenges with the self-consistent implementation of closure models for anomalous electron transport in fluid simulations of Hall thrusters

The performance of closure models for the anomalous electron transport when self-consistently implemented in a fluid model for a Hall effect thruster is investigated. This cross-field transport, which is orders of magnitude higher than classical collisional transport, is represented as an effective collision frequency. The proposed closure models relate this transport coefficient to local fluid properties of the plasma. Before implementation, the models are calibrated against values of the collision frequency inferred empirically from a 9 kW Hall thruster at 300 V and 15 A. It is found that even though closure models match the empirical collision frequency values, they diverge from these values when implemented self-consistently in a Hall thruster code. Possible drivers of this behavior are examined, including the role of non-linearity in the governing equations of the Hall thruster fluid model, artifacts from using time-averaged calibration data, and the non-uniqueness of the empirically-inferred collision frequencies. These results are discussed in the context of their implications for discovering and validating new closures necessary for enabling fully-predictive Hall thruster models.

ELECTROMAGNETIC DIPOLAR MODES IN MAGNETIZED NONUNIFORM PLASMA-VACUUM-METAL WAVEGUIDE

This article presents the results of theoretical study of the phase and attenuation properties of the dipolar electromagnetic modes in cylindrical plasma-vacuum-metal waveguide structure. Plasma is described in hydrodynamic approach and is supposed to be slightly non-uniform in axial direction and strongly non-uniform in radial direction. It was studied the influence of the external magnetic field value, the electron effective collision frequency and other waveguide parameters on the phase and attenuation properties of the considered waves.

Study on the Absorption Properties of Terahertz Circularly Polarized Wave in FPL Model Fully Ionized Dusty Plasma

In this article, Boltzmann’s general plasma kinetics and the Fokker–Planck–Landau (FPL) collision model of dusty plasma are used to obtain the expression of the dielectric coefficient of dusty plasma under the influence of an external magnetic field and the collision condition of the fully ionized FPL model. The charging effect of dusty plasma is fully considered. The absorption characteristics of circularly polarized waves in the terahertz band under different plasma parameters are calculated and analyzed by using the transfer matrix method. Results show that the intensity of the external magnetic field, the size and concentration of dust particles, the effective collision frequency and thickness of dusty plasma, the incidence angle of the electromagnetic waves, and the electron temperature greatly influence the absorption characteristics of circularly polarized electromagnetic waves. The propagation characteristics of circularly polarized waves caused by different external magnetic fields are evaluated. An absorption band is formed near the cyclotron frequency when the external magnetic field increases to a certain extent, and the direction of the electron cyclotron and the rotation direction of the polarized wave are consistent. The position and frequency band of dusty plasma can be controlled by artificially changing the magnitude of the external magnetic field. These results provide a theoretical basis for the study of electromagnetic wave propagation in weakly colliding, fully ionized dusty plasma and the propagation mechanism of an electromagnetic wave in the local high-temperature area of the hypersonic target.

Microwave-accelerated hydrolysis for hydrogen production over a cobalt-loaded multi-walled carbon nanotube-magnetite composite catalyst

Microwave (MW) irradiation is a promising option for the intensification of chemical reaction processes and has been applied in the promotion of many catalytic reactions. Herein, the (CNTs-Fe3O4)-Co nanocomposites were designed as microwave-responsive catalysts and was fabricated under a controlled manner. The experimental results showed that MW irradiation can lead to the hydrogen generation rate (HGR) of the (CNTs-Fe3O4)(1:4)-Co (10 wt%) catalyst being boosted from 75.0 to 95.4% as compared with conventional heating under the temperature range of 40 to 60℃. Moreover, the evaluation of the catalytic performance of the (CNTs-Fe3O4)(1:4) composite with different Co loadings and DFT calculations were carried out to verify the synergistic effect of cobalt and Fe3O4 sites of the (CNTs-Fe3O4)(1:4)-Co (10 wt%) catalyst. Furthermore, the pre-exponential factor (A) of NaBH4 hydrolysis under MW heating was found to be approximately 15 times higher than that of conventional heating, implying that MW irradiation significantly improved the effective collision frequency of the atoms at the reaction interface of the catalyst, resulting in a higher number of active sites on the surface of the (CNTs-Fe3O4)(1:4)-Co (10 wt%) catalyst. Additionally, the existence of the non-thermal effect of MW irradiation was studied by using a specially designed experimental set-up. The results showed that MW thermal and non-thermal effects contributed to the enhancement of HGR.

Neoclassical toroidal plasma viscosity in bounce-transit and drift resonance regimes in tokamaks

Neoclassical toroidal plasma viscosity in the bounce-transit and drift resonance regimes is calculated using a version of the drift kinetic equation that encompasses the physics of the nonlinear trapping and quasilinear plateau regimes in tokamaks. It is demonstrated that the mirror-force like term controls the transition between these two regimes. When the effective collision frequency is larger than the mirroring or the nonlinear bounce frequency, the quasilinear regime prevails; otherwise, the nonlinear trapping regime reigns. The demonstration is accomplished by using the Eulerian approach and is beyond the grasp of the method of the integration along the unperturbed orbit in solving the drift kinetic equation. The neoclassical toroidal plasma viscosity in the quasilinear plateau regime is calculated. Approximate analytic expressions for the neoclassical toroidal plasma viscosity that include the asymptotic limits of the nonlinear trapping and quasilinear regimes are presented to facilitate thermal and energetic alpha particle transport modeling in tokamaks.

Power transfer efficiency and the power threshold for E–H mode transition in inductively coupled plasmas

The effects of gas pressure and gas component on the power transfer efficiency [Formula: see text] and transition power threshold [Formula: see text] during the E–H mode transition in inductively coupled plasmas are studied. The evolutions of [Formula: see text] during the E–H mode transition in pure Ar and Ar/O2 discharges are similar, i.e., in E-mode discharge, [Formula: see text] slightly increases with raising the applied power ([Formula: see text] is below 30%), whereas, an abruptly upward jump of nearly two to four times for [Formula: see text] occurs when the discharge transits to H-mode, and then [Formula: see text] monotonously increases with increasing the applied power. In addition, as the pressure rises, [Formula: see text] increases rapidly first and then slowly in pure Ar discharge. However, [Formula: see text] in Ar/O2 mixture gases discharge shows a non-monotonic behavior. It reveals a tendency of first increase at low pressures and then decreases at high pressures, which is because the resistive component of the plasma varies with the electron density [Formula: see text] and the electron effective collision frequency [Formula: see text]. Furthermore, a non-monotonic behavior of transition power threshold [Formula: see text] with the pressure is found, i.e., with increasing the pressure, [Formula: see text] first decreases and then increases. It may be caused by the threshold electron density, which is constant at low pressures [Formula: see text] but increases at high pressures [Formula: see text].

Which factor determines the optical losses in refractory tungsten thin films at high temperatures?

Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering.Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.

Laser-induced photodetachment of negative oxygen ions in the spatial afterglow of an atmospheric pressure plasma jet

Negative ions are an important constituent of the spatial afterglow of atmospheric pressure plasmas, where the fundamental plasma-substrate interactions take place that are vital for applications such as biomedicine, material synthesis, and ambient air treatment. In this work, we use laser-induced photodetachment to liberate electrons from negative ions in the afterglow region of an atmospheric pressure plasma jet interacting with an argon-oxygen mixture, and microwave cavity resonance spectroscopy to detect the photodetached electrons. This diagnostic technique allows for the determination of the electron density and the effective collision frequency before, during and after the laser pulse was shot through the measurement volume with nanosecond time resolution. From a laser saturation study, it is concluded that O− is the dominant negative ion in the afterglow. Moreover, the decay of the photodetached electron density is found to be dominantly driven by the (re)formation of O− by dissociative attachment of electrons with O2. As a consequence, we identified the species and process responsible for the formation of negative ions in the spatial afterglow in our experiment.

The diffusion and anomalous diffusion of charged particles in the plateau regime of toroidal plasma

The diffusions and anomalous diffusions of charged particles in the plateau regime of toroidal plasma with a Maxwellian velocity distribution are studied. The transport theory of plasma is discussed under the toroidal coordinate system and the magnetic field is assumed to be axisymmetric. Based on the diffusion flux obtained in the toroidal plasma, we derive the diffusion coefficient tensor and thermal diffusion coefficient tensor, where two angular diffusion coefficients are the anomalous diffusions, depending on the radial position and having nothing to do with the magnetic field. All the radial transport coefficients and the normal angular coefficients are inversely proportional to square of the gyrofrequency and so to square of the magnetic induction. The numerical analyses show clearly dependences of these complex transport coefficients on the temperature, the magnetic fluid stability safe factor and the effective collision frequency, respectively.