Growth of buckytubes on the anode at are discharge

It is shown that for an arc discharge between carbon electrodes in an inert gas atmosphere the temperatures at the electrode surfaces play a key role in determining the structure and the electrode on which a deposit can grow. The heat balance equations determine that the anode temperature is higher due to the energy carried by the electrons. This leads to anode sublimation and deposition on the cathode. It is shown that by cathode heating, by anode cooling or by a combination of these, a deposit may be obtained on the anode due to cathode erosion. The deposit grown by the “inverse” method is compared with a deposit obtained on the cathode under the same conditions but at reverse supply voltage polarity. The material from both deposits, studied by TEM, shows that there are graphite crystals within the anode deposit, and that the carbon forms within have a relatively small number of structural defects while the buckytubes are greater in length than those within the cathode deposit. The reasons for these differences are discussed. In the “inverse” method, the constant decrease in cooling of the anode surface leads to an equalization of the anode and cathode temperatures. This creates conditions that favor buckytube growth.     

Cathode overpotential investigation by means of “Built-in” potential electrode

The objective of the present work is the development of a “built-in” potential electrode method for direct measurements of the cathode and anode overpotentials and the corresponding interface resistances of solid oxide fuel cells (SOFC). The studies were performed on a yttria-stabilised zirconia (YSZ) electrolyte-supported SOFC using La_0.8Sr_0.2MnO₃ as cathode, GDC as protecting layer and Ni-ScSZ cermet as anode. The mesh potential electrode was placed inside the YSZ membrane near the cathode side. Using the combination of the I-U and the impedance measurements with the built-in potential electrode technique, the temperature dependencies of the electrodes and electrolyte contributions to the total cell resistance were determined.     

The influence of electron emission from the collector on the potential distribution within a Knudsen diode with surface ionization in the underneutralized regime

This paper gives a theoretical treatment of the distribution of potentials within a Knudsen diode with surface ionization in the underneutralized regime in the presence of electron emission from the collector surface. A method is derived to calculate the potential distribution. It is shown that if the emission from the collector is sufficiently strong, spatially oscillatory (“wavelike”) potential distributions do not form; instead, a continuous transition occurs from a distribution with a virtual anode to one with a virtual cathode. Particular attention is focused on the neighborhood of the transition point from one of these distributions to the other. Zh. Tekh. Fiz. 68, 37–47 (April 1998)     

First demonstration of “white-light” laser cooling of a stored ion beam

“White-light” cooling of an ion beam confined in a storage ring has been demonstrated at Test Storage Ring in Heidelberg. Measurements aimed at comparing “white-light” with single-mode laser cooling show that “white-light” cooling gives lower temperatures at higher ion densities both in a coasting and in a bunched beam. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental study of the influence of hydrogenation on synthesis of carbon nanostructures in a reactor with activated hydrogen

The role of backward pyrolysis of carbon during hydrogenation of unstructured carbon black and nanotube growth in a reactor with hydrogen activated by diffusion through a heated metallic wall is studied. If the wall is heated nonuniformly, the amorphous unstructured carbon deposit “climbs” in the course of hydrogenation by active hydrogen and deposits again when falling on the heated metal surface, self-organizing into more ordered nanofibers and nanotubes as a result of forward pyrolysis. It is shown that this effect can be used for raising the concentration of carbon nanotubes growing on catalyst particles in the deposited layer.     

The influence of vaporization upon the roots of a high current arc. II

The root of an argon are was stabilized by strong cooling of the graphite cathode; both the composition and the temperature of the plasma at the arc root were determined spectroscopically. The measurements of the absolute intensities of two CI and two CII lines revealed that the plasma is composed almost entirely of atomic carbon. Immediately in front of the cathode a temperature of 12000 K was measured and the degree to which the gas was ionized was found to be 30%. The velocity of the plasma was 300 m/s. This work is based upon material presented in the final report “Basic research programme for plasma technology, high-pressure arcs in SF₆” to the Federal Department of Research and Technology, Fed. Rep. Germany.     

Peculiarities of grid control over the current in a discharge with a cathode spot on the cesium liquid-metal cathode

We report on the results of successful implementation of full grid control in a cesium discharge with a cathode spot. The discharge is quenched by a negative grid pulse for a discharge current density up to 75 A/cm² in the grid plane for voltage switching up to 100 V in the pressure range 0.5–1.5 Pa for a voltage drop of 5–6 V in the discharge. It is shown that quenching is the break-off type. The discovered effect of “evacuation” of the heavy component (ions) from the grid-anode gap to the cathode region leads to an unusually long (hundreds of microseconds) time of stabilization of the steady state in the discharge under the experimental conditions.     

Atom as a “dressed” nucleus

We show that the electrostatic potential of an atomic nucleus “seen” by a fast charged projectile at short distances is quantum mechanically smeared due to nucleus motion around the atomic center of inertia. For example, the size of the “positive charge cloud” in the Hydrogen ground state is much larger than the proper proton size. For target atoms in excited initial states, the effect is even larger. The elastic scattering at large angles is generally weaker than the Rutherford scattering since the effective potential at short distances is softer than the Colombian one due to a natural “cutoff”. In addition, the large-angle scattering leads to target atom excitations due to pushing the nucleus (⇒ inelastic processes). The Rutherford cross section is in fact inclusive rather than elastic. These results are analogous to those from QED. Non-relativistic atomic calculations are presented. The difference and the value of these calculations arise from nonperturbatively (exact) nucleus “dressing” that immediately leads to correct physical results and to significant technical simplifications. In these respects a nucleus bound in an atom is a simple but rather realistic model of a “dressed” charge in the QFT. This idea is briefly demonstrated on a real electron model (electronium) which is free from infinities.      

Microhollow cathode sustained discharges: comparative studies in micro- and equivalent macro-cell geometries

We present an experimental study of discharge initiation in a three-electrode configuration consisting of a microhollow cathode discharge (MHCD) and a third planar electrode, biased positively and placed some distance away. This work is based on the microcathode sustained (MCS) configuration where the MHCD acts as a plasma cathode and enables the generation of a stable, non-equilibrium plasma at high pressure in the volume between the MHCD and the third electrode. Our experiments were carried out in two different set-ups, one using a MHCD as a cathode and another in an “equivalent” macrocell geometry, easier to implement and operating at lower pressure in which the same phenomena are observed. Consistent with previous modeling results, we find that the plasma column in the volume between the MHCD and the third electrode is characterized by a low reduced electric field, with values similar to those expected for a positive column. The ignition voltage of the plasma column depends on the voltage difference between the MHCD and the third electrode, the MHCD current, and the gas pressure and gap spacing.     

On the mechanism of occurrence of chaotic dynamics in a vacuum microwave generator with a virtual cathode

The nonlinear dynamics of an electron beam with a virtual cathode in a transit-current vircator system is numerically studied. The existence of complex modes, including chaotic ones, is shown. The physical processes in the electron beam are examined. It is found that the chaotic dynamics in an electron beam with a virtual cathode is due to the nonlinear interaction between the structures organized in it. “College,” State Education and Scientific Center of the State University, Saratov, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 9, pp. 1137–1146, September, 1998.     

Mesoscopic and macroscopic dipole clusters: Structure and phase transitions

A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic” clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms of clusters is considered.     

Electrochemical cell with two layers cathode for NO decomposition

An electrochemical cell composed of an yttria-stabilized zirconia disk and two layers cathode was used for nitrogen monoxide decomposition. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the performance of the electrochemical cell for NO_x decomposition in the presence of excess oxygen. The decomposition activity was measured for the one-compartment cell oxide|(cathode)|YSZ|(anode) by applying a DC voltage lower than 3.7 V in the temperature range 550–700 °C. The microstructure of the YSZ-NiO mixed oxide electrodes was investigated in dependence of the cell operating condition and the working electrode sintering temperature. The correlation between the microstructure of the mixed oxide electrode and conversion rate of NO was studied. The phenomenon of self-optimization of the microstructure of the NiO-YSZ working electrode during the cell operation was observed and investigated. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Plasma torches and electrophysical parameters of the diaphragm discharge in water

This work is aimed at studying plasma torch formation, voltages, currents, power distribution, and resistance variation in the case of the little understood low-energy (≤10 J) diaphragm discharge initiated in water and salt solutions. Plasma torches associated with this type of discharge differ considerably from those of other discharges. Different shapes of anode and cathode torches, breakdowns between the diaphragm and one of the electrodes, and a specific form of cathode breakdown are observed. It is found that the electrical conductivity of the medium influences the parameters of the plasma formations, current pulses, dielectric strength of the medium, current pulse variation at “semibreakdowns,” etc. Based on the experimental data, it is concluded that the process of discharge initiation differs from that described in the literature.     

约束阴极微弧氧化放电特性研究

利用Na₂SiO₃-KOH溶液体系,以工业纯铝为基体材料对约束阴极微弧氧化的放电特性进行了研究.考察了恒压模式下电极距离对氧化电流、电位分布及起弧电压的影响,并对电极距离与微弧氧化电能利用率间的关系进行了分析. 结果表明:对于阴阳极等约束条件下,随阴阳极距离加大,工作电流逐渐减小. 而对于仅约束阴极情况,工作电流随着阴阳极间距增加而增大. 这是由于增加阴阳极间距时,虽然约束阴极正下方试样表面的电场强度降低,工作电流减小,但远离约束电极处,阳极表面电场强度却增加,工作电流增大. 起弧电压随电极间距离的增大而升高,但阳极表面电场强度几乎保持不变. 微弧氧化陶瓷层厚度由处理中心沿半径向外逐渐变薄,且中心处陶瓷膜厚度随电极距离的增大迅速减小,电能利用率随之降低. 关键词： 微弧氧化 约束阴极 放电特性 电极间距     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Hexagonal structures of current in a “semiconductor-gas-discharge gap” system

The results of experimental investigation of the stability boundary for a spatially homogeneous state of a discharge in the planar gap of a “semiconductor-gas-discharge” cryogenic system filled with nitrogen are considered. The semiconductor cathode was prepared from single-crystalline silicon doped with a deep-lying impurity. Quantitative data are obtained for the conditions of formation of a hexagonal dissipative structure in the current distribution for two values of the discharge gap length upon a change in the gas pressure and in the conductivity of the cathode. It is found that for a fixed gap length, the formation of the critical state can be described approximately by a universal function of the electrode conductivity and gas pressure.     

Graphene-graphite phase transition at the surface of a carbonized metal

A phase transition leading to the transformation of a graphene layer into a multilayer graphite film at the surface of a carbonized metal has been experimentally studied on the atomic level under ultrahigh-vacuum conditions. It has been shown that this process is governed by dynamic equilibrium between edge atoms of graphene islands and a chemisorbed carbon phase, two-dimensional carbon “gas,” and is observed in the temperature range of 1000–1800 K. The features of the phase transition at the surfaces Ni(111), Rh(111), and Re(10-10) are similar, although the specific kinetic characteristics of the process depend on the properties of the substrate. It has been shown that change in the emissivity of the substrate after the formation of a multilayer graphite film increases the rate of the phase transition and leads to a temperature hysteresis.     

Similarity of Co-Species in Co and CoMo-sulfide catalysts supported on carbon and alumina. A Mössbauer emission spectroscopic study

It is shown that, irrespective of the application of carbon or alumina as a support, the local structure of the “Co-sulfide” phase formed during sulfidation of Co-and CoMo-catalysts is the same. A relation is found between the quadrupole splitting (Q.S. value) of the “Co-sulfide” phase and its dispersion. The higher the dispersion, the larger the Q.S. value. The so-called “Co-Mo-S” doublet is observed in all cases and it turns out to be related to a highly dispersed “Co-sulfide” phase instead of a Co, Mo and S containing phase.     

Unsteady film cooling with imposed nonuniform pulsations of the main flow

Effects of the main flow pulsations on the unsteady adiabatic film cooling efficiency were investigated. The possibility of using the critical value of the modified Strouhal number for the single-row perforation to identify the quasi-steady flow in the double-row perforation was proved. The penetration of disturbances into the perforation channels due to “plunger” effect was observed. The influence of the imposed pulsations on the adiabatic film cooling efficiency was shown to be weaker for the double-row perforation as compared to the single-row perforation.     

Nonequilibrium phase transitions in ferromagnetic semiconductors (EuO<Subscript>1–δ</Subscript>)

The features of metal‒semiconductor kinetic phase transformation in ferromagnetic semiconductors are studied. It is shown that heat release caused by current results in a positive feedback between the current density and the sample temperature, magnetization, and thermal-spin fluctuation amplitude. The Joule heating of the sample leads to disappearance of magnetization and, as a result, to restoration of the forbidden band between the conduction and valence bands. However, the energy gap width continues to change due to an increase in the fluctuations of internal exchange fields splitting the electron states. Within the framework of the developed model, an S-shaped volt-ampere characteristic is obtained for a EuO_1–δ, ferromagnetic semiconductor. The lower branch of the characteristic corresponds to a ferromagnetic metal state (a “cold” phase) and the upper one is due to a semiconductor paramagnetic state (a “hot” phase).