R. J. Munz

Application of transferred arcs to the production of nanoparticles

A brief review is given of the application of transferred arcs to the production of nanoparticles. The advantages and limitations of transferred arc reactors and the concept of the CRTP reactor are discussed. Examples are given of experimental and modelling work carried out at CRTP on the production of fumed silica from quartz and aluminum nitride from the reaction aluminum vapour and ammonia.

The influence of the cathode surface on the movement of magnetically driven electric arcs

The arc movement was examined for tubular copper electrodes using different plasma gases (argon, helium, nitrogen, air, chloride and mixtures of these gases). The normal arc current was 100 A; the arc was moved using an external magnetic field which varied between 10 and 1500 G. The arc velocity followed an aerodynamic type of equation when the cathode surface was contaminated. For clean or heavily contaminated cathode surfaces another opposing force becomes important, the surface drag force. The arc movement was also investigated by using high speed filming and following the arc current fluctuations. The cathode surface was examined using Auger, ESCA and SEM; work function values were obtained using a Kelvin probe. The results were consistent with the model presented for the arc movement.

Effect of the arc velocity on the cathode erosion rate in argon-nitrogen mixtures

Erosion measurements on a copper cathode are reported. The 100 A arc, driven by a magnetic field, runs continuously for up to 30 min between two concentric cylindrical electrodes. Argon-nitrogen gas mixtures in various proportions are blown through the electrode gap. The erosion rate in argon is drastically reduced by the addition of only 1% nitrogen and is further reduced as the nitrogen content increases in the gas mixtures. The decrease in erosion rate is found to be correlated to an increase in arc velocity.

Electrode erosion in plasma torches

Cathode erosion rates are reported (or copper electrodes in a simulated plasma torch operating at atmospheric pressure. The are current was 100A (or most experiments; the magnetic field used to move the arc varied between 0.001 and 0.15 T. Different plasma gases were used (Ar, He, air, N2, CO, and mixtures of the noble gases with O2, N2, CO, CH4, Cl2, and H2S) at flow rates varying between 0.2 and 20 liters/min. Different criteria (arc velocity, arc attachment residence time, arc current density) were used to analyze the erosion results.

Arc velocity and cathode erosion rate in a magnetically driven arc burning in nitrogen

Measurements of the arc velocity and of the erosion rate are reported for a copper cathode. The 100 A arc, burning in nitrogen, is driven by a magnetic field B, varying from 5.1 to 171.0 mT, between concentric copper electrodes having an inter-electrode gap of 4 mm. The arc velocity varied with B0.60 throughout the range investigated. The erosion rates dropped from 9.0 to 1.0 mu g C-1 as the arc velocity was increased from 15 to 135 m s-1.

Cathode erosion in inert gases: the importance of electrode contamination

Experimental results are presented for electrode erosion on copper electrodes in magnetically rotated arcs in argon and helium. Measurements were also made of the arc voltage and velocity. The effects due to the contamination of the electrode surface by either a native contaminant layer (copper oxide and carbon traces) or the continuous injection of very small amounts of various diatomic gases (nitrogen, oxygen, chlorine, and carbon monoxide) into the inert plasma gases were determined. The erosion rates for pure argon were significantly higher than those for pure helium (13.5 μg/C for argon and 1 μg/C for helium) and with both gases, very high arc velocities were measured initially (>60 m/s for argon and >160 m/s for helium) when a natural contaminant layer was still present on the cathode. The removal of this layer resulted in lower velocities (2m/s for argon and 20m/s for helium) and higher erosion rates. The removal of the layer was much faster with argon, due possibly to higher electrode surface current densities for argon arcs.

Vapor phase synthesis of fine particles

Aluminum nitride (AlN) ultrafine powder is synthesized in a new concept plasma reactor whose reaction section is separated from the plasma chamber, Aluminum is evaporated using a transferred-arc in either an Ar or Ar/H/sub 2/ atmosphere. The hot gas carrying metal vapor is mixed with radial jets of NH/sub 3//Ar mixture. A mathematical model of the reaction zone is developed, which includes the calculation of fluid flow, temperature, and concentration fields, followed by the nucleation and growth of fine particles, using the method of moments. The initial results of the experimental and modeling study on the influence of temperature and nitriding jet intensity on particle size and conversion are presented. The powder produced has a specific surface area (SSA) in the range of 40-280 m/sup 2//g.

The effect of low concentrations of a polyatomic gas in argon on erosion on copper cathodes in a magnetically rotated arc

Experimental results are presented for electrode erosion on copper cathodes in magnetically rotated arcs in argon, dry air, nitrogen, ammonia, and carbon monoxide as well mixtures of the above with argon. Water-saturated argon was also used. Erosion rates were determined by weight loss after chemical cleaning, and the runs were sufficiently long (between 5 to 60 min) to represent steady-state operation. Arc currents of 100 A and gas pressures of 1.1 atm. were used. Pure argon gave the highest erosion rates and the lowest arc velocities. Small concentrations of any of the diatomic gases in argon greatly increased the arc velocity and decreased the erosion rates. The results suggest that erosion is primarily a thermal phenomenon but that the surface chemistry can greatly influence erosion rates by modifying arc behavior.

Solid-Phase Synthesis of Calcium Carbide in a Plasma Reactor

A laboratory-scale spout-fluid bed reactor with a dc plasma torch was used to study the solid-phase synthesis of calcium carbide. Calcium oxide powder with a mean particle size of 170 μm was reacted with graphite powder (130 μm). Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reaction took place in the vicinity of the plasma jet and that conversion to calcium carbide increased linearly with reaction time. The rate of conversion increased exponentially with plasma jet temperature, indicating that chemical reaction was the controlling mechanism. Microscopic analysis of the solid product showed that calcium carbide was formed around both reactants, and that the reaction followed a shrinking core model. Although melting and agglomeration of partially reacted particles occurred at high temperature, resulting in instability of the bed and impeding the reaction progress, high conversions are expected in a continuous process with optimized reactor design.

Current distribution of an electric arc at the surface of plasma torch electrodes

Current distributions of the arc foot on the surface of the electrodes in a geometry simulating a plasma torch are reported. Most of the experiments were to determine the arc current distribution at the cathode. The results were obtained using a novel technique and represent experiments using different plasma gases (Ar, He, Ar+0.3% CO, He+0.4% CO, He+0.4% N2, N2, CO) and operating conditions (magnetic field and arc velocity). It is shown that all three, the surface composition due to contamination, the transverse magnetic field used to move the arc, and the arc velocity have a strong influence on the current distribution. Higher erosion rates were found for higher current densities of the arc foot.