Optimization of the heating of a metal by an electric arc

We investigate and generalize the results of experimental measurements of heat fluxes in a metal anode from a plasma jet of a plasmatron with an external arc (PEA) whose operating conditions vary over a wide range.Notation efficiency of heating of the metal - Q_el heat flux into the calorimeter - I current of the electric arc - U voltage drop across the electric arc - V linear velocity of rotation of the sensor - r_o thermal radius of arc spot - _o time until surface of calorimeter begins to melt - t_p melting point of copper - t_o initial temperature of calorimeter surface - q specific heat flux in the heating spot - a thermal diffusivity - D diameter of rotating calorimeter - t₁ temperature of surrounding medium - t₂ temperature of the surface of the rotating calorimeter - G argon flow rate - h_s specific enthalpy of argon in the region near the anode - h_a specific enthalpy of argon at the surface of the calorimeter - k Boltzmann constant - T_e electron temperature in the anode region - e charge of the electron - U_a potential drop at the anode - _a work function of electrons leaving copper - L length of open part of the arc—distance from the cut in the nozzle of the PEA to the sensor - h depth of cathode insertion into the nozzle—distance from the cut in the PEA nozzle to the cathode - d diameter of the PEA nozzle - U _a ^* effective voltage equivalent Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 687–691, October, 1980.     

Plasma flow crisis and the limiting electron temperature in a vacuum arc in axial magnetic field

The possibility of supersonic motion of cathode plasma in a low-current vacuum arc in an axial magnetic field has been studied. It is shown that an increase in the electron temperature unavoidably leads to a plasma flow crisis, whereby the plasma velocity decreases to the sound velocity. The dependence of the limiting length of a stationary flow on the magnetic field has been studied. The maximum possible electron temperature T _cr in the plasma is determined by the initial ion energy and can be estimated as T _cr ≈ 3T _m, where T _m is the maximum electron temperature in the cathode spot region.     

Thermal and electrical characteristics of a high-frequency electrothermal fluidization bed

Based on an analysis of the thermal and the electrophysical characteristics of a fluidization bed with dielectric clay particles, a method has been developed of distending such particles by means of a high-frequency electric field for the production of ceramic sand.Notation R, r radius of a grain and the radius to any inside point - T(r) temperature inside a grain - T_c temperature at the grain center - T_s temperature at the grain surface - T₀ ambient temperature - time coordinate - ₀ time of temperature leveling inside a grain - _a time of temperature leveling between grain and ambient medium - thermal conductivity of grain material - c specific heat in terms of volume of grain material - _G thermal conductivity of gas - heat transfer coefficient - q volume rate of heat generation - dielectric permittivity of grain material - f frequency of high-frequency electric field - E_m amplitude of high-frequency electric field - tan loss tangent of grain material - C capacitance of effective capacitor with fluidized bed - C_m mean effective capacitance - C₀ capacitance of effective capacitor with stationary bed - D, H diameter and height of reactor - h height of fluidized bed - m=h₀/H relative initial fill of a reactor - w linear velocity of gas stream Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 6, pp. 969–975, June, 1972.     

NO x emission from electric arc furnace in steel industry: contribution from electric arc and co-combustion reactions

Off-gas measurements were conducted at industrial electric arc furnaces (EAF) in Germany in order to investigate the interrelation of NO _x emission with installed plant equipment (e.g. gas burner) and process data (e.g. carbon input). Off-gas data monitor rapid changes in off-gas composition, temperature, and volume flow rates of air into the furnace indicating the transient state batch process of scrap melting. From the measurements two distinct sources of NO _x emission are clearly distinguished: (1) NO _x formation in the electric arc plasma during the start-up period of the melting process in an oxidizing furnace atmosphere after the charging of the furnace. (2) NO _x formation from post-combustion of CO/CO₂/H₂ gas with air inside and outside the furnace. Whereas the contribution from arc ignition is similar for different types of EAFs, other contributions depend on furnace equipment and operation, e.g. gas burners, use of air as carrier medium for carbon or dust injection, air-tightness of furnace, and parameters of off-gas extraction by EAF dedusting system. The positive effect of the minimum volume flow rate of air into the furnace by controlled off-gas extraction to total NO _x emission is shown.     

Volt-ampere characteristics of an ac arc in a transverse gas flow

An analytical dependence of the volt-ampere characteristic of an arc on the velocity of the incoming transverse gas flow is obtained. The stability threshold of arc combustion is determined.Notation h enthalpy - density - t time - v velocity - , thermal conductivity and electrical conductivity - E electric field strength - U potential at the arc - Q radiant flux - T period of oscillation of the current - r radius - l length of electric arc - d, L diameter and length of electrode - temperature - kinematic viscosity - Pr Prandtl number - current function Indices 0 gas parameters far from arc - 1 parameters at arc boundary - x projection on the axis 0x - y projection on the axis 0y - E effective value - e characteristic value Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 133–138, July, 1984.     

Electric current and heat induced acoustic emission in cadmium sulfide

Acoustic emission (AE) induced by the electric current passing through a cadmium sulfide (CdS) single crystal was studied. In the temperature range T=300–450 K, the intensity of the AE signals excited in the CdS crystal exposed to a constant electric field increases with the current density. It is suggested that AE in CdS is due to the dislocations breaking off and moving under the action of direct electric current and thermoelastic stresses. The activation energy for this process (E _a=0.35±0.5 eV) was estimated for a current density in the range of j=(1−7)×10⁵ A/m².     

Heat exchange with different schemes for introducing the heat-exchange agent into a furnace

The effect of the scheme for introducing the heat-exchange agent into a furnace for heat exchange in a selective nonisothermal gaseous medium is investigated taking into account the heat loss due to convection depending on the emissivity of the metal and the furnace lining.Notation T temperature - Er specific resulting radiative flux - q_c convective heat flux density - q_Me overall density of the resulting heat flux to the metal - emissivity - a_g0 absorbing capability of the gas relative to the radiation of the lining - a_g4 same for the metal - a _go /a+b+c absorbing ability of a layer of gas a+b+c... relative to the radiation of the lining - a_gj/ikmnp absorbing ability of the gas layer i+k + m + n + p with radiation from layer j - A emissivity of the walls - l _ef effective length of a ray path - coefficient of convective heat transfer - k heat-exchange coefficient from the internal surface of the lining with temperature T_o in the medium with temperature T_m - B fuel expenditure - Q_H ^P heat of combustion - v volume of the combustion fuel products per 1 m³ of gas - c_g specific heat capacity of the gas - h overall height of the channel - x instantaneous height coordinate in the channel The indices for the temperatures and heat fluxes are as follows: 0, lining; 4, metal; 1, 2, and 3, gas zones; m, media.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 692–698, October, 1980.     

Transmission electron microscopy observations on the microstructure of naturally aged Al–Mg–Si alloy AA6022 processed with an electric field

The influence of an electric field applied during the solution heat treatment and during a short subsequent initial natural aging of AA6022 on the nature of the precipitates which occurred after natural aging for a long time (2–3 years) was determined employing transmission electron microscopy and selected area electron diffraction. The precipitates were spherical in shape ranging from 2 nm to 40 nm in diameter d _p with an average of 7.9 nm. Their average size was larger than that (4.9 nm) in a specimen processed without field and their size distribution had a greater number of precipitates with d _p>∼3 nm. Moreover, their crystal structure differed from that in specimens processed without a field. The increase in yield stress which occurred with the electric field treatments is attributed to an increase in the interaction force between the precipitates and dislocations, which resulted from the larger size and different crystal structure of the precipitates by the field.     

Effect of electric field on the crystallization of lead titanate in a glass

An electric field was applied to a PbO-TiO₂-Al₂O₃-SiO₂ glass during the crystal nucleation treatment at 570°C in order to produce oriented glass-ceramics. After the crystallization treatment at 620°C of the nucleated glass, an enhanced a-axis preferred orientation of the lead titanate cubic perovskite, which is not ferroelectric, was developed in the surface-layer region. However, the a-axis orientation was difficult to develop in the interior region of the sample, possibly due to ion migration during the nucleation treatment. The degree of a-axis orientation on the as-heated surface showed a maximum at an optimum nucleating time t _max and the t _max-value decreased with an increase in the applied electric field strength.     

Arc Erosion Properties of WCu Material Reinforced with Ti3SiC2 under Different Applied Voltages

Introducing reinforcements is a crucial way to modify WCu electric contacts. Herein, the arc erosion performances of WCu/Ti₃SiC₂ under various direct current voltages are investigated. The eroded microstructures are observed by a 3D laser scanning confocal microscopy and a thermal field emission scanning electron microscope. Compositions of the eroded surfaces are analyzed by a Raman spectroscopy. With the elevated voltages, values of breakdown current, arc duration, arc energy, and arc discharge distance increase, while the breakdown strength decreases. Meanwhile, the eroded regions gradually change from circular shape to irregularly curved corrugations under the elevated voltages. Copper and Ti₃SiC₂ can disperse arc during arc discharge, and the liquid and gas generated by the two phases can make differences in protecting WCu/Ti₃SiC₂.     

The Effect of Radiation on the Spatial Distribution of the Temperature of Subsonic Flows of Induction Plasma

Results are given of measurements of the temperature characteristics of subsonic flows of air and oxygen plasma, generated by a VGU-4 100-kW induction plasmatron developed at IPM RAN (Institute of Problems of Mechanics of the Russian Academy of Sciences). The values of absolute intensity of spectral lines of oxygen and nitrogen atoms are used to measure the average (over the plasma jet diameter) temperature of atomic level populations (hereinafter referred to as atomic temperature) as a function of the parameters of the mode of generation, namely, the gas pressure in the range from 25 to 200 GPa and the power of anode supply within 29–72 kW. The distributions of the atomic temperature of plasma (T _a ) along the jet are obtained for some modes of generation. The radial distributions of the atomic temperature are obtained for some modes of generation of air and oxygen plasma. Their comparison with the temperature profiles obtained by the calculation of flows of equilibrium plasma in a discharge channel reveals a divergence for values of temperature on the flow axis exceeding 8000 K.     

Optical properties of (1−<Emphasis Type="Italic">x</Emphasis>)PbZn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>−<Emphasis Type="Italic">x</Emphasis>PbTiO<Subscript>3</Subscript> single crystals

The temperature dependence of the optical transmission and small-angle light scattering with and without applied constant electric field was studied in relaxor single crystals of 0.91PbZn_1/3Nb_2/3O₃-0.09PbTiO₃ (PZN-PT 91/9) and 0.93PbZn_1/3Nb_2/3O₃-0.07PbTiO₃ (PZN-PT 93/7) solid solutions in the region of two phase transitions: (i) from cubic paraelectric to tetragonal ferroelectric phase at T=T _c and (ii) from tetragonal ferroelectric to rhombohedral ferroelectric phase at T=T _rt. In the absence of external electric field, only the phase transition at T _c proceeds in both PZN-PT 91/9 and PZN-PT 93/7 crystals according to a percolation mechanism and is accompanied by the appearance of a sharp maximum in the small-angle light scattering intensity curve. In PZN-PT 93/7 crystals, the application of a relatively weak electric field induces an additional percolation type phase transition at T _rt.     

Nonlinear Features of Phonon–Ripplon Modes in the Electron Crystal Over Liquid Helium

The resonance spectra of coupled phonon–ripplon oscillations in two-dimensional electron crystals over liquid helium are measured at different driving electric potentials. The crystals with surface electron densities n _s = (3.2 − 12) × 10⁸ cm⁻² are studied at holding electric fields E _⊥ = 590−1180 V/cm at temperature T≈ 80 mK. It is found that an increase of the driving potential leads to a change of resonance curves: the curves become more flat, asymmetric, and splintered depending on electron density and holding potential. The data obtained are used to estimate the electron effective mass in the crystal. It is shown for the first time that the effective electron mass in the crystal increases as the driving field increases.     

Influence of the electric field on the soot formation in the flame at a low pressure

We have investigated the influence of an electric field in the form of a gas discharge on the flame pattern, the radiographic parameters of soot particles, and the yield of fullerenes and polycyclic aromatic hydrocarbons. It has been shown that an external electric field permits controlling the processes proceeding in the flame, as well as the soot formation, and even increasing the soot yield under certain conditions. It has been established that the height of the soot packet L _c and the interplanar spacing d ₀₀₂ remain unchanged with increasing voltage, which is confirmed by the absence of graphitization under the action of the electric field. It has been revealed that negative polarity has a stronger effect on the increase in the yield of fullerenes with increasing voltage applied compared to positive polarity.     

Calculation of Laminar Flows of Plasma in the Channel of a Plasmatron with the Self-Adjusting Length of the Electric Arc

A simplified method is suggested for the calculation of the parameters of laminar flows of plasma in the channel of a plasmatron. A new analytical solution of the Elenbaas–Heller equation is derived, which generalizes the channel model of electric arc to the case when the volume radiation makes a significant contribution to the electric arc energy balance. Numerical calculations are performed in order to determine the electric field intensity, the longitudinal pressure gradient, and the heat transfer to the electric arc channel wall depending on the working parameters of the plasmatron in a laminar flow of gas in a stabilized section. In determining the arc length, it is assumed that the electric arc is shifted downstream of the flow and, at the same time, performs random walks over the channel cross section. The walks occur under the effect of vortexes whose characteristic size is of the order of the arc diameter.     

Looking Outside the Square: The Growth,Structure, and Resilient Two-Dimensional Surface Electron Gas of Square SnO2 Nanotubes

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb-doped SnO₂ nanotubes with perfectly-square geometries on Au nanoparticle covered m-plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r- and a-plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X-ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X-ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor-like states produced by the hydroxylation of the surface and is sustained at temperatures above 400 °C by the formation of in-plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO₂ nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.     

Study of the effect of the deposition parameters on the structural, electric and optical characteristics of polymorphous silicon films prepared by low frequency PECVD

In this work we present our results on the deposition and characterization of polymorphous silicon (pm-Si:H) films prepared by low frequency plasma enhanced chemical vapor deposition (LF-PECVD). We have studied the effect of the plasma deposition parameters (as the chamber pressure and gas flow rates of SiH₄ and H₂) on the structural, electric, and optical characteristics of the films.The temperature dependence of conductivity (σ(T)), activation energy (E_a), optical band gap (E_g) and deposition rate (V_d) were extracted for pm-Si:H films deposited at different pressure values and different gas flow rates. We observed that the chamber pressure is an important parameter that has a significant effect on the electric characteristics, and as well on the morphology of the pm-Si:H films (deduced from atomic force microscopy). It was found an optimal pressure range, in order to produce pm-Si:H films with high E_a and room temperature conductivity, σ_RT, which are key parameters for thermal detection applications.     

Carbon Nanotubes Grown on the Surface of Cathode Deposit by Arc Discharge

Abstract

Carbon nanotubes prepared by de arc discharge of graphite electrodes in He and CH₄ gas took markedly different morphology. Thick nanotubes embellished with many carbon nanoparticles were obtained by evaporation under high CH₄ gas pressure and high arc current. Thin and long carbon nanotubes were obtained under a CH₄ gas pressure of 50Torr and an arc current of 20A for the anode with a diameter of 6mm.     

Formation of Nd2Fe14B hydride by milling of anhydride particles in toluene in a closed reactor

When milling micrometer thin Nd₂Fe₁₄B platelets, of an average 1–2 mm diameter, in toluene in a closed reactor, part of the toluene decomposes at the surface of the platelets and yields nascent hydrogen and carbon/low hydrocarbons. The hydrogen diffuses into the Nd₂Fe₁₄B platelets and the carbon forms a thin surface passivation layer of the platelets, forming the stable Nd₂Fe₁₄BH _x ,x ≤ 5, hydride at room temperature. On heating in a calorimeter, the hydrogen desorbs off the sample with a well-defined endotherm between 370 and 425 K. An N₂ gas atmosphere, if used during the heating, facilitates the H-desorption process with the modified kinetic parameters. For example, the enthalpy of the H-desorption ΔH and the related activation energyE _a have the measured values ΔH = 153 J/g andE _a = 58·2 kJ/mol in argon and ΔH = 256 J/g andE _a = 41·6 kJ/mol in N₂. It is argued that N₂ gas has a fast reaction with the H atoms desorbing off the thin sample platelets and forms NH₃ gas with an instantaneous decrease of the total external gas pressure at the sample. This supports the fast desorption of H atoms in the sample with the modified desorption kinetics in N₂ gas.     

The preparation of carbon-coated iron nanocrystals produced from Fe2O3-containmg composite anode in arc discharge

Using an Fe₂O₃-containing composite anode instead of an Fe-containing composite anode in the Kratschmer-Huffman carbon arc method, carbon-coated Fe (not Fe₂O₃) nanocrystals are produced both in the soot on the reactor walls and in the cathode deposits. The encapsulates also contain a little iron carbide, but do not contain iron oxides, as identified by transmission electron microscopy ( TEM ) and X-ray diffraction (XRD). As compared with Ni₂O₃- and Co₂O₃-containing composite anodes, the action of the Fe₂O₃-containing composite anode is unique. When the Fe₂O₃ contents in the composite graphite rod for the carbon arc nanocrystal production are increased in the range 5-33 wt.%, the effect on the structure and diameter distribution of the iron nanocrystals is investigated. The diameter distribution of iron nanocrystals is about 1–30 nm. The effect of helium pressure in the range 80–680 torr on the yield of C_60/70 in carbon soot produced from the composite graphite rod has also been examined. We present a novel result about the relationship of C_60/70 yields and helium gas pressure using a composite anode differing remarkably from the result using a homogeneous graphite anode in arc discharge.