Craters formation in a graphite cathode produced by pulsed arc at low pressure

DC pulsed arcs, between face to face opposite electrodes of graphite and stainless steel at pressures varying from 50 to 500 Pa into nitrogen environment, with a current and voltage of 300 A and 167 V, respectively, and a pulsed arc time of 30 ms were produced.Several consecutive craters, created by explosive way and by inclusions over the graphite cathode were found. These are located principally at the outskirts of the center and at the half of the graphite cathode radius; they are originated because of the cathode spots chaotic movement. The most representative craters have sizes over than 300 μm and some melted sectors were observed by means of the SEM technique, corroborating that there are temperatures higher or equal than the melting point of the graphite. The film surface is composed by small spheres with structures similar to the ones in the cauliflower diamond. Craters in some melted sectors on the anode surface were observed.     

Cluster structure of fullerene-containing soot and C60 fullerene powder

Electron microscopy and electron diffraction methods were employed in a study of the structure of a fullerene-containing soot produced in gas discharge and of a C₆₀ fullerene powder. The data obtained were analyzed with the use of fractal geometry concepts. It has been shown that, in the structure of the objects studied, several spatial scales can be identified. The effective radius of the structure’s “elementary particles” calculated using scaling relationships is equal to 6 Å for the soot and 4.5 Å for the C₆₀ fullerene. The “elementary particles” combine into associates. The number of particles in an associate in both the soot and the C₆₀ powder is not large (about 10). The associates form fractal nanoclusters 30–80 nm in size having a fractal dimension of 1.60±0.05 in the soot and of 1.8±0.05 in the C₆₀ fullerene. The structure of the soot nanocluster is unstable and can be significantly modified by externally applied factors (e.g., as a result of treatment with toluene). The nanoclusters combine into aggregates having the form of branching cross-linked filaments. Eventually, these aggregates combine to form macroparticles of soot.     

Laser vaporization synthesis of polyhedral graphite

Polyhedral graphite (PG) particles have been synthesized by CO₂ laser vaporization of graphite in high-pressure Ar gas (8×10⁵ Pa). Faceted PG particles, ranging in size from 110 to 500 nm, have a turbostratic structure. The yield of PG is more than 90%. This synthesis is based on the condensation of hot carbon species confined by an Ar-gas atmosphere. Received: 22 January 2003 / Accepted: 24 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-59/231-9471, E-mail: kokai@chem.mie-u.ac.jp Permanent address: Chemistry Department for Materials, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan     

Effect of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge

Single-walled carbon nanotubes (SWCNTs) have been synthesized in high yield by the dc arc discharge technique under heat-pretreatment of the graphite rod conditions. Before executing arc discharge, the graphite rods containing the catalysts were heat treated at 600, 700, 800 and 900 °C for 1–3 h, respectively. Effects of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge were investigated. The heat-treatment temperature and time were found to be crucial for a high yield of high-purity SWCNTs. Optimum parameter was found to be at the heat-treatment temperature of 800 °C for 2 h. The SWCNTs synthesized under the optimum condition have better field-emission characteristics. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.9 V/μm and the threshold field where current density reaches 10 mA/cm² is 3.9 V/μm.     

Gas formation in liquid in initial stages of a pulsed discharge

Gas formation in electrolytes with=10^–2–10^–4–1cm^–1, distilled water = 10^–5-1.5 ·10^{–6 –1}cm^–1, and chemically pure n-hexane in the initial stages of formation of discharge with rectangular voltage pulses of 0.67 and 1.85 sec duration is investigated. The experimental results are compared with the results of approximate calculations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 42–47, November, 1972.The authors thank V. V. Ryumin for taking part in the discussion of the results and V. V. Lopatin for participation in the experiments with electron-optical light amplifier.     

The influence of vaporization upon the roots of a high current arc. I. Different forms of vaporization in the arc roots

Arc roots at which marked vaporization occur are unstable and therefore in general non-stationary. High speed photography of arc roots using various electrode materials in Ar, SF₆, and N₂ shows that the appearance of the arc roots results from the superposition of several phenomena. On graphite cathodes in Ar it was possible to observe these phenomena separately by varying the cooling of the electrodes. It is shown that distinctions must be drawn between the real current transfer region with relatively weak vaporization, vaporization from the electrode surface outside this region and vaporization from within the body of the electrode. The energy-balance determined experimentally confirms the theoretical prediction that the energy for strong vaporization of the cathode is supplied by an ioncurrent.     

Structure of the direct-current magnetron discharge in neon at different polarities of the electrode system

A dc magnetron discharge in neon is studied at different polarities of the electrode system. It is found that the electron energy distribution function is composed of three groups of electrons: fast electrons accelerated by the strong field of a cathode sheath, slow electrons confined in a potential well due to the space-charge field, and intermediate-energy electrons. It is shown that the energy distribution of the confined electrons is a Maxwell-Boltzmann distribution function, whereas the energy distribution of the intermediate electrons is typical of electron diffusion at a constant total energy. The measured values of the cathode sheath thickness depend on the source polarity.     

Dynamic behaviour of alternating current arc discharge in a multi‐arc generator at atmospheric pressure

In this letter, a multi‐arc generator with three high‐voltage electrodes and a common grounded one was developed for the purpose of obtaining large area and steady arc plasma at atmospheric pressure. Three typical discharge states were found in the multi‐arc generator: independent movement of three arc columns, confluence of two arc columns, and confluence of three arc columns. The three discharge states cyclically occur on the evolution of the arc discharge and their duration is influenced by the power dissipation and plasma working gas flow rate. With an increase of discharge power and a decrease of the gas flow rate, the duration of multiple arc confluence increases, which contributes to the suppression of the fluctuation amplitude of each arc. Frequency domain analysis of the arc voltage envelope shows that the frequency of arc fluctuation increases in the multi‐arc mode in the multi‐arc generator compared to that in the single arc mode.     

Structure modification of highly ordered pyrolytic graphite by Ar plasma beam scanning at different incident angles

The surface of highly ordered pyrolytic graphite (HOPG) was modified by Ar plasma beam scanning at a controllable angle of incidence. The characteristics of plasma modified HOPG were investigated by atomic force microscope (AFM), micro-Raman, X-ray photoemission spectroscopy (XPS), and grazing incident angle of X-ray diffraction (GIAXRD). A smooth surface of HOPG can be obtained by adjusting the incident angles of Ar plasma beam scanning. The surfaces of HOPG become smoother with increasing angle of incidence after Ar plasma beam scanning. Raman spectra indicate that the plasma beam scanning breaks the hexagonal structures of sp² C=C bonds near the surface of HOPG. The broken hexagonal network structures can form C–O bonds that increase the amount of oxygen on the surface of HOPG, supported by C_1s and O_1s XPS spectra. GIAXRD data support that the co-existence of both crystalline structures of 2H and 3R in HOPG. The carbon bond breaking in 2H and 3R is different and depends on the angle of incidence. Most broken carbon bonds form damaged aromatic rings near the surface of HOPG.     

Positioning of the arc discharge channel in vacuum arc facilities

The behavior of an electric arc in a magnetic field is studied theoretically and experimentally. It is found that the arc behavior can be governed by the ponderomotive interaction of the arc with current-carrying elements. In a nonuniform magnetic field, the behavior of the arc depends on the Hall currents and the diamagnetic properties of its plasma. It is shown that the position of the arc channel between the end faces of cylindrical electrodes can be controlled by nonuniform magnetic fields. The methods and devices considered in this paper allow one, in particular, to control arc heat sources used in the heat treatment of metals.     

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研究了滑动弧放电过程中电参数的变化,并对滑动弧等离子体中的非平衡度和各个参数之间的关系进行了讨论。应用双通道电弧模型,对电弧在气流作用下运动规律进行了数值模拟,模拟所得的结果有助于分析滑动弧非平衡等离子体的产生机理。     

含氢电极脉冲放电等离子体特性诊断

利用飞行时间质谱法诊断了含氢电极脉冲真空弧离子源放电等离子体成分、离子电荷状态及离子扩散速度等特性.实验结果表明,含氢电极脉冲真空弧离子源放电等离子体的离子成分主要由H+,Ti+,Ti2+和Ti3+组成,其中Ti2+占主要部分.当放电电流为40～80 A时,Ti离子的平均电荷数在1.95～2.13之间,随着放电电流的增...     

含氢电极脉冲放电等离子体特性诊断

利用飞行时间质谱法诊断了含氢电极脉冲真空弧离子源放电等离子体成分、离子电荷状态及离子扩散速度等特性。实验结果表明,含氢电极脉冲真空弧离子源放电等离子体的离子成分主要由H+,Ti+,Ti2+和Ti3+组成,其中Ti2+占主要部分。当放电电流为40～80 A时,Ti离子的平均电荷数在1.95～2.13之间,随着放电电流的增大,平均电荷数也会增加。同时诊断了不同离子的扩散速度,其值均在104 m/s量级,但不同离子的扩散速度有所不同。     

Dynamic behaviour of electric arc gas discharge

The time-dependent energy and circuit equations are solved numerically to obtain temperature profiles, current-voltage characteristics and electric field strength vs axial temperature diagrams in the asymptotic region of a wall-stabilized electric arc operated in nitrogen. Dynamic current-voltage characteristics and transient phenomena of the formation of the stationary state during the spark ignition phase are studied. The arc timeconstant for free decay is computed and compared with approximate analytical results. It is found that for fixed initial conditions the arc response to the variable applied voltage is several times longer than the arc time-constant for free decay.The author is grateful to Dr. Z. Sedláek, Dr. M. Hrabovský and Dr. R. Klíma for their stimulating advices and helpful comments.     

Investigation of soot sensitivity to strain rate in ethylene counterflow soot formation oxidation flames

Soot sensitivity to strain rate is mainly responsible for soot formation intermittence in practical combustion devices. This work provides a fundamental study on soot formation in Soot Formation Oxidation (SFO) counterflow flames at varying strain rates. While the problem has been extensively studied in Soot Formation (SF) configurations, where the dominant process is nucleation, investigations remain scarce in the corresponding SFO cases. In the latter, the high temperatures and strong oxidative environments make the surface reactions prevail over nucleation. The work provides a new dataset for ethylene SFO flames in a wide range of strain rates and sheds light on the main processes concurring in determining soot strain rate sensitivity in such conditions. In particular, the peak of soot volume fraction (SVF) is primarily controlled by surface growth and oxidation. The latter becomes progressively more dominant on the side of the SVF distribution toward the oxidizer nozzle, where the presence of oxidizing agents is significant. The soot mechanism adopted predicts a SVF distribution and sensitivity to strain rate in agreement with experimental data. The latter is found similar to corresponding SF cases, although soot loads in the two configurations differ by almost an order magnitude, and the SVF sensitivity is known to be more accentuated for lower soot loads. A deeper investigation revealed that the nucleation process through dimerizations primarily controls the SVF sensitivity, providing the onset of soot necessary for further growth. Then, the latter tends to reduce SVF sensitivity depending on its impact. PAH sensitivities mostly agree with theoretical observation even though further validations on the kinetic mechanism are needed to improve its predictions in lean conditions. The simplistic yet effective model based on the hybrid method of moments and the employment of a reduced kinetic mechanism makes the approach amenable for turbulent computational fluid dynamic (CFD) simulations.