Cathode spots on electrode slag films in an open-cycle MHD generator

A study has been made of the function of the electrodes (cathodes) in an open-cycle MHD generator for several different reasons [1–3], because the electrode processes have marked effects on the erosion and electrical characteristics of the electrodes. The specific features of the conditions in an MHD generator channel include, particularly, the high-temperature plasma composed of combustion products together with the deposition of potassium salts on the electrodes. These factors have a marked effect on the behavior of the cathode spots. In the case of an MHD generator fueled by coal, the plasma contains the incombustible mineral part of the fuel (ash). Therefore, the electrode surfaces receive not only potash salts, but also slag, which consists of various refractory oxides that differ from the potassium compounds in electrical conductivity, thermal conductivity, and emissivity. These films may substantially affect the parameters of the cathode spots, and hence the erosion, and the values may differ substantially from those given in [3]. We have examined the major features of the cathode spot behavior for an open-cycle MHD generator fueled by coal.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 29–33, July–August, 1976.     

Quantification of the effect of surface heating on shock wave modification by a plasma actuator in a low-density supersonic flow over a flat plate

This paper describes experimental and numerical investigations focused on the shock wave modification induced by a dc glow discharge. The model is a flat plate in a Mach 2 air flow, equipped with a plasma actuator composed of two electrodes. A weakly ionized plasma was created above the plate by generating a glow discharge with a negative dc potential applied to the upstream electrode. The natural flow exhibited a shock wave with a hyperbolic shape. Pitot measurements and ICCD images of the modified flow revealed that when the discharge was ignited, the shock wave angle increased with the discharge current. The spatial distribution of the surface temperature was measured with an IR camera. The surface temperature increased with the current and decreased along the model. The temperature distribution was reproduced experimentally by placing a heating element instead of the active electrode, and numerically by modifying the boundary condition at the model surface. For the same surface temperature, experimental investigations showed that the shock wave angle was lower with the heating element than for the case with the discharge switched on. The results show that surface heating is responsible for roughly 50 % of the shock wave angle increase, meaning that purely plasma effects must also be considered to fully explain the flow modifications observed.     

The microarc operating regime of MHD generator electrodes

The preliminary results of an investigation of the operation of MHD generator electrodes at relatively high current densities are reported. The experiments were conducted in the channel of a MHD generator, driven by combustion products, with both cooled metal and silicon carbide electrodes. Observation and photographs of the electrodes revealed that at sufficiently high currents microarcs appear at the electrode surface. The phenomenological aspects of arc behavior under conditions characteristic of MHD generator operation are examined. The electrode-insulator interface has an important influence on arc behavior, as does the film of potassium compounds deposited on the electrode surface. These characteristics of the microarcs may be of considerable significance in relation to electrode erosion processes.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, Vol. 11, No. 1, pp. 130–134, January–February, 1970.     

Effect of electrical conductivity of flow on current distribution in a magnetogasdynamic channel with allowance for the Hall effect

The effect of a magnetic field on the current distribution on a plane continuous anode situated opposite the cathode in a rectangular magnetogasdynamic channel with an external magnetic field was experimentally investigated. The distributions of the charged-particle density and the electron temperature near the outlet end of the electrodes were measured. The distribution of electrical conductivity in the flow was calculated. The electron density distribution along the channel is attributed to ambipolar diffusion of plasma to the walls. For an interpretation of the current distribution results, the method of integral relations in a linear approximation was used to solve the problem of a constant-velocity flow of a gas with variable electrical conductivity across a magnetic field in a plane magnetogasdynamic channel of constant cross section formed by electrodes of finite length and insulators. The Hall effect was taken into account. Experiments in which the effect of an external magnetic field on the current distribution on plane sectioned short electrodes in a magnetogasdynamic accelerator was investigated were described in [1]. In the present investigation, continuous long electrodes were used. These electrodes prevented the side effects due to coupling of the current to the ends of the electrode sections and helped to reveal some features of the current density distribution on the anode.     

Current distribution of permeable electrodes in the presence of the hall effect in a stream of electrically conducting medium

The two-dimensional problem of the current distribution on the surface of permeable electrodes contiguous with a stream of incompressible medium with Hall effect is considered. An electrically conducting medium with the same physical properties as those of the main stream is pumped in (out) through the electrodes.This problem was solved in [1] for one particular case when the electrodes are impermeable. It was established that due to the Hall effect in magnetohydrodynamic channels the current is distributed non-uniformly on the electrodes; for values of the Hall parameter of the order of several units or greater, the current flows into an isotropically conducting medium mainly from a small portion on the edge of the electrode. It was also noted that this phenomenon creates unfavorable conditions for the operation of electrodes in magnetohydrodynamic devices.It is shown in what follows that the current distribution on the electrodes may be controlled, and in particular made more uniform, by injecting an electrically conducting medium.     

Experimental comparison between the effect of electrical discharge and the effect of surface heating on Mach 2 rarefied airflow over a flat plate

This study describes the experimental investigation of the effect of a negative DC glow discharge on a Mach 2 rarefied airflow around a flat plate. More precisely, we will show a comparison between two experiments. In the first one we will observe the effect of discharge by Pitot probe measurement. This discharge is created by applying negative DC potential difference between two electrodes flush mounted on the surface of a quartz flat plate placed in Mach 2 rarefied airflow. The electrodes are arranged in the spanwise direction. In the second experiment, electrodes are removed and replaced with a surface heater. The pressure profiles obtained by a glass Pitot tube are presented, and a comparison between the plasma effect and the surface heater effect is made, for the same surface temperature and in thermal equilibrium, with the aim of identifying the origin of the observed effect. For both experiments, surface heating causes a decrease in the boundary layer stagnation pressure, while increasing the boundary layer thickness, with the effects becoming larger for higher mean surface temperature. The effects due to the plasma actuator seem to be larger over the active electrode.     

Calculation of the thermally nonequilibrium near-electrode region in an alkali-metal-seeded plasma

Distribution of parameters in the region of disturbance of a plasma near the surface of an electrode is considered based on diffusion equations. Thermoelectronic and thermionic emission from the electrode surface, the Schottky effect, and volume ionization and recombination are borne in mind. Two regions are assumed in the solution, namely, the region of ambipolar diffusion and the region of the space charge. A comparatively simple geometry for the discharge gap, given in the form of two infinite plane-parallel electrodes, is considered. A comparison is carried out with calculations for a thermally balanced region of a plasma disturbance.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 13–17, January–February, 1976.     

High-frequency plasma oscillations in junction diodes

Longitudinal high-frequency plasma oscillations in junction diodes are examined by the kinetic equation method with allowance for the asymmetry of the boundary conditions at the boundaries between the plasma and the electrode barriers. It is shown that when the time taken by the wave to travel the distance between the electrodes is half the wave attenuation time in the unbounded plasma, an undamped wave may occur as a result of the superposition of waves reflected from the electrodes on the perturbation wave. In many gas-discharge and semiconductor diodes there are regions of quasineutral plasma bounded by potential barriers which create favorable conditions for the formation of standing waves in the plasma. Studies [1–5] are devoted to the investigation of these waves in an electron plasma between plane electrodes. In all these instances, however, the conditions at the two boundaries were assumed to be the same. In fact, in a glow or arc-discharge plasma, and also in semiconductor diodes when current passes through the diode, the conditions at the boundaries between the plasma and the space-charge regions are not the same. For example, in a p-i-n diode in the forward-current, mode holes and electrons from the i region that reach the boundary between the i region and the n and p regions behave differently. Holes are reflected from the boundary between the i and the p regions and readily pass into the n region, where they recombine; electrons, on the other hand, readily pass into the p region, where they recombine, but are reflected from the boundary between the i and the n regions. A similar asymmetry of the boundary conditions occurs at the boundaries between the positive column and the electrode barriers in glow and arc discharges, as well as in a cesium diode plasma. This paper examines the longitudinal high-frequency plasma oscillations in junction diodes by the kinetic equation method with allowance for the asymmetry of the boundary conditions.     

Effect of surface and volume ionization on the electrode potential drop

We consider the change in the potential of the electric field in the free fall layer at the electrodes as a function of the characteristics of the surface and the volume ionization. Systematic calculations are made of the electrode potential drop for a tungsten cathode and anode in a lithium and cesium plasma. The potential of the electric field is obtained as a function of the plasma pressure, the degree of volume ionization, the electron temperature, the electrode temperature, and the current density.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 137–139, July–August, 1970.     

Stochastic many-particle model for LFP electrodes

In the framework of non-equilibrium thermodynamics, we derive a new model for many-particle electrodes. The model is applied to \(\text {LiFePO}_{4}\) (LFP) electrodes consisting of many LFP particles of nanometer size. The phase transition from a lithium-poor to a lithium-rich phase within LFP electrodes is controlled by both different particle sizes and surface fluctuations leading to a system of stochastic differential equations. An explicit relation between battery voltage and current controlled by the thermodynamic state variables is derived. This voltage–current relation reveals that in thin LFP electrodes lithium intercalation from the particle surfaces into the LFP particles is the principal rate-limiting process. There are only two constant kinetic parameters in the model describing the intercalation rate and the fluctuation strength, respectively. The model correctly predicts several features of LFP electrodes, viz. the phase transition, the observed voltage plateaus, hysteresis and the rate-limiting capacity. Moreover we study the impact of both the particle size distribution and the active surface area on the voltage–charge characteristics of the electrode. Finally we carefully discuss the phase transition for varying charging/discharging rates.     

Electroelastic analysis for a piezoelectric layer with surface electrodes

An electroelastic analysis of a transverse isotropic piezoelectric layer with surface electrodes is made. The piezoelectric layer is infinite long along the poling direction, and the top surface is perfectly bonded to a rigid electrode. The problem is solved via the conformal mapping technique for two cases of elastic boundary conditions on the bottom surface with two spaced electrodes, and the distribution of the electrostatic field in the entire piezoelectric layer is determined in an explicit analytic form, respectively. It is found that for the bottom surface electrodes with vanishing stiffness, the induced strain is singular, but no stress. Instead, for the bottom electrodes with stiffness as infinity, the induced stress is singular, but no strain.     

Erratum to: Active thermal management of on-chip hot spots using EWOD-driven droplet microfluidics

In response to the rapid advances in microelectronics, novel cooling technologies are needed to meet increasing cooling requirements. As a paradigm-shifting technique, electrowetting-on-dielectric (EWOD) uses electric potential to control the movement of a liquid droplet on a dielectric surface. In this work, we developed an EWOD-based microfluidic technique for active and adaptive thermal management of on-chip hot spots. A two-dimensional array of control electrodes was patterned on the chip surface for EWOD operations. By applying DC or AC voltages with appropriate sequence and timing to the electrode units, we were able to transport microdroplets of tens of μL along a programmable path. Without the need of external pumps and valves, the droplets were precisely delivered to cooling targets. With the driving voltage as low as 40 V_AC, we demonstrate high heat flux (7.6 W/cm²) cooling on a hot spot. The EWOD-induced internal circulation within the droplets led to a time-averaged Nusselt number of ~45.     

Active thermal management of on-chip hot spots using EWOD-driven droplet microfluidics

In response to the rapid advances in microelectronics, novel cooling technologies are needed to meet increasing cooling requirements. As a paradigm-shifting technique, electrowetting-on-dielectric (EWOD) uses electric potential to control the movement of a liquid droplet on a dielectric surface. In this work, we developed an EWOD-based microfluidic technique for active and adaptive thermal management of on-chip hot spots. A two-dimensional array of control electrodes was patterned on the chip surface for EWOD operations. By applying DC or AC voltages with appropriate sequence and timing to the electrode units, we were able to transport microdroplets of tens of μL along a programmable path. Without the need of external pumps and valves, the droplets were precisely delivered to cooling targets. With the driving voltage as low as 40 V_AC, we demonstrate high heat flux (7.6 W/cm²) cooling on a hot spot. The EWOD-induced internal circulation within the droplets led to a time-averaged Nusselt number of ~45.     

柱状电极屈曲或裂纹扩展的临界力学参数研究

柱形结构电极是近年来使用最为广泛的锂电池电极结构之一．本文以硅材料细长柱形电极为例，研究了充电电流大小、电极长径比、初始裂纹长度以及断裂韧性对于电极的屈曲现象和裂纹扩展现象发生时间的影响．计算结果表明，屈曲与裂纹扩展现象出现的先后顺序与充电电流大小无关；具有小的长径比，大的初始裂纹长度以及较小断裂韧性的电极，裂纹扩展比屈曲现象更早发生．对于硅材料，不同长径比的电极具有不同临界断裂韧性值，当材料的断裂韧性小于该临界值，在锂化过程中裂纹扩展会先于屈曲现象发生；该临界断裂韧性值随初始裂纹长度的增加而增加．本文的结论对于电极的结构设计以及材料选取具有一定指导意义．     

Conditions near the electrode in a plasma with an alkali admixture

An analysis is made of the effects of certain processes in the interior of the gas and at the electrode surface on the potential drop near the electrode in a discharge in a dense, slightly ionized gas. Thermionic emission from the electrode, the Schottky effect, diffusion, and volume and surface ionization and recombination are taken into account. The analysis is carried out for a simple discharge-gap geometry: two infinite, plane-parallel electrodes. Relations are found for the potential drop near the electrode in a two-temperature plasma as a function of the discharge parameters and emission characteristics of the material. The calculated results are compared with experiment.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 3–12, July–August, 1971.The authors thank G. A. Lyubimov for interest in the study and for discussion of the results, and B. V. Parfenov for graciously furnishing the necessary experimental data from [13].     

Influence of the hall effect on the characteristics of a MHD generator with two pairs of electrodes

The characteristics of segmented-electrode MHD generators with Hall currents are at present the object of considerable interest. Various types of electrode connections ate being examined: ordinary segmented-electrode generators, Hall generators, mixed-type generators, and Montardy generators. Research is being pushed in several directions. In some cases infinitely fine segmentation is assumed [1--4]. In these studies it is considered that the current density j in the duct is everywhere uniform; the net characteristics or the most favorable electrode connection angle are determined. In another group of studies periodic problems are solved, it being assumed that the processes taking place in a single elementary celi are repeated in the other ceils; fringe effects are not taken into account. In this case it is usually assumed that the lengths of the electrodes and insulators are finite, but small as compared with the duct height [5–7 ]. Finally, in a last group of studies nonperiodic problems are considered. In [3] Vatazhin solves the problem of the current distribution in a duct with a single pair of electrodes between infinitely long insulators. In [8] a general expression is found for the current density function in the case of an arbitrary number of electrode pairs and for any scheme of electrode connection at finite electrode and insulator dimensions. However, numerical calculations are made only for the periodic problem, whose solution is also obtained in [8]; the effect of segment pitch on the characteristics of a Montardy generator is studied.The present author has investigated the influence of the Hall effect on the characteristics of a MHD generator having two pairs of electrodes with symmetrical and crossed electrode connections. Although it is obvious that in practice only multisegment ducts will be employed, the examination of a generator with two pairs of electrodes makes possible the qualitative anatysis of the various effects observed in segmented-electrode ducts in which the electrodes are connected in different ways. Numerical calculations, based on formulas obtained by solving the corresponding problems, have been made on a M-20 computer. Integrated characteristics of the various generator systems have been obtained as a function of electrode and insulator length, external loads, and Hall parameter w.In conclusion, the author thanks A. B. Vatazhin and A. N. Kraiko for their helpful advice.     

An experimental investigation of the effect of oxygen on the erosion of a multichannel tungsten cathode

The problem of the preservation of electrodes and the reduction of their depletion to a minimum has decisive significance among the practical questions of maintaining the efficiency of steady sources and plasma accelerators during an extended period of operation. For this purpose electrodes are made, as a rule, out of the refractory metals in familiar designs. In particular, tungsten, which has the highest phase-transition; temperatures among all the metals, thus permitting an appreciable reduction in the evaporation rate of the material under the thermally stressed operating conditions of these devices, is attractive. However, there exist other factors besides evaporation which affect the integrity of hot tungsten electrodes and their ablation (chemical processes, erosion in microarcs, ion bombardment, and so on). One of these factors requiring special investigation is the chemical interaction of tungsten with the oxygen contained in the working medium as an industrial impurity. Such an impurity can, for example, be present in industrially pure alkali and alkali-earth metals used as the working media. In addition, these metals can be contaminated by the oxygen of the air in the process of different industrial operations, in particular, in connection with the servicing of the supply system. The entrance into the working material of oxygen liberated from the structural elements of the supply route and the operating cavities of the plasma source is not excluded. In practice it is impossible to eliminate oxygen, even when using contemporary methods of deep cleaning of lithium, due to its great affinity for oxygen. The actual role of the oxygen impurity and the necessity for taking it into account in connection with the creation and operation of plasma devices can only be revealed as a result of direct experimental investigation on samples having an electrode. geometry similar to the class of sources or accelerators in question. Results are presented in this article of an experimental investigation of the erosion of a multichannel tungsten electrode [1–4] and a coaxial source of lithium plasma [5]. The use of lithium as the main plasma-forming material ensured the maintainance of a discharge distributed over the electrodes and having a small value of the potential drop next to the cathode, which lies below the threshold of cathode erosion upon bombardment of the surface of polycrystalline tungsten by lithium ions [6]. Thus, the phenomena of cathode destruction associated with microarcs and ion bombardment could be exluded from consideration in practice. Thus evaporation and chemical ablation due to interaction with the oxygen of the working medium remain the principal, competing processes contributing to the erosion of the cathode mass. Therefore, in order to obtain reliable quantitative relationships which characterize differential erosion due to oxidation and evaporation, the oxygen content in the working medium was varied in the experiments described within limits extending beyond the framework of the usual industrial impurity, which does not exceed tenths of a percent. One should note that in the process of investigating the role of an industrial oxygen impurity in causing the erosion of a tungsten cathode in a lithium plasma its direct effect was revealed on the anomalous current of a hollow cathode, which is characterized by the recorded average densities of the discharge current exceeding significantly the values of the emission current given at the same surface temperature by the Richardson-Schottky equation [7–9].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 67–73, May–June, 1979.The authors express their gratitude to S. S. Kellin, N. P. Mezhevov, and V. N. Belinskii for their participation in the preparation and performance of the experiment.     

Mesoscopic numerical computation model of air-diffusion electrode of metal/air batteries

This work creates a droplet battery model based on the electrolyte performance in the porous electrode, studies the current density on the mesoscopic scale, and explains how the mesoscopic structure of the porous electrode influences the current density on the air-diffusion electrode. Near the three-phase line, there is a strong band containing nearly 80% current. For porous electrodes, the total current is proportional to the length of the strong band. Thus, it can be inferred that on the macroscopic scale, the longer the total length of the strong band on unit area is, the larger the current density is.     

In Situ Measurements of Strains in Composite Battery Electrodes during Electrochemical Cycling

The cyclic stress in lithium-ion battery electrodes induced by repeated charge and discharge cycles causes electrode degradation and fracture, resulting in reduced battery performance and lifetime. To investigate electrode mechanics as a function of electrochemical cycling, we utilize digital image correlation (DIC) to measure the strains that develop in lithium-ion battery electrodes during lithiation and delithiation processes. A composite graphite electrode is cycled galvanostatically (with constant current) in a custom battery cell while optical images of the electrode surface are captured in situ. The strain in the electrode is computed using an in-house DIC code. On average, an unconstrained composite graphite electrode expands 1.41 % during lithiation and contracts 1.33 % during delithiation. These strain values compare favorably with predictions based on the elastic properties of the composite electrode and the expansion of graphite-lithium intercalation compounds (G-LICs). The establishment of this experimental protocol will enable future studies of the relationship between electrode mechanics and battery performance.     

Aerodynamic modification of flow over bluff objects by plasma actuation

Particle image velocimetry and smoke visualization are used to study the alteration of the flow field in the wake of a bluff body by use of an alternating current (AC) surface dielectric barrier discharge. Staggered, surface, and buried electrodes were positioned on the downstream side of circular cylinders at conditions of Re _D = 1 × 10⁴−4 × 10⁴ configured to impose a force due to the ion drift that is either along or counter to the free-stream flow direction. Smoke visualization and Particle Image Velocimetry (PIV) in the wake of the flow confirms that the configuration of the surface electrodes and operation of the discharge significantly alters the location of the flow separation point and the time-averaged velocity profiles in the near and distant wake. Measurements of the vibrational and the rotational temperature using optical emission spectroscopy on the N₂ second positive system (C³Π_u–B³Π_g) indicates that the resulting plasma is highly non-equilibrium and discounts the possibility of a thermal effect on the flow separation process. The mechanism responsible for reduction or enhancement of flow separation is attributed to the streamwise force generated by the asymmetric ion wind—the direction of which is established by the electrode geometry and the local surface charge accumulated on AC cycles.