Showing papers by "Joachim Heberlein published in 1998"

Anode-boundary-layer behaviour in a transferred, high-intensity arc

Abstract: The effects of the plasma's gas-flow rate on phenomena such as the negative anode fall and the heat flux to the anode in high-intensity arcs are discussed on the basis of a numerical model of the anode's boundary layer. The modelling system consists of a rotationally symmetrical argon plasma formed between the outlet of a constrictor tube and a water-cooled flat copper anode perpendicular to the axis of the plasma flow which is directed towards the anode. The arc is operated at atmospheric pressure and at a current level of 200 A. The boundary conditions for the electron temperature and the electron number density at the anode's surface are obtained by solving the electron-energy equation and the diffusion equation at the anode's surface simultaneously with all conservation equations for the calculation domain. A diffuse anode attachment is obtained for a mass flow rate exceeding 0.2 g and a constricted anode attachment is found for rates below 0.02 g . There is a (probably unstable) transition region between these flow rates. The anode boundary layer of the diffuse attachment is strongly affected by the mass-flow rate. Increasing the mass-flow rate shifts the location of the peak of the electrical potential towards the anode while its magnitude decreases and the total heat flux to the anode increases. This is a consequence of the effects of the mass-flow rate on the axial profiles of electron and heavy-particle temperatures and of the electron number density. In contrast, the anode boundary layer of the constricted attachment seems not to be affected by the mass-flow rate.

An experimental investigation of factors affecting arc-cathode erosion

Abstract: A specially designed thermal plasma reactor system for the investigation of arc-cathode erosion has been set up. By using an OMA-spectrometer system, emission spectroscopic measurements of electron temperature and electron number density in the cathode region have been performed, together with single-colour and two-colour pyrometry of cathode temperature distributions. Observation of cathode spot behaviour has been carried out simultaneously by employing a telemicroscope and a high-speed vision system. Cathodes have been examined by SEM and EDX after arcing. For pure tungsten cathodes, the initial cathode geometry has almost no effect on the cathode spot's behaviour due to the molten state of the cathode spot. The major erosion mechanism is the ejection of liquid droplets from the cathode spot. However, the initial cathode geometry has a certain influence on the cathode's erosion for 2% thoriated tungsten cathodes. A highly non-uniform erosion pattern will occur if the cathode is overcooled, probably due to ion bombardment in the low-temperature regions of the arc-attachment spot.

Deposition of boron carbide thin film by supersonic plasma jet CVD with secondary discharge

Abstract: Boron carbide thin films have been deposited on Si(100) using supersonic plasma jet chemical vapor deposition from boron trichloride and methane. Vickers hardnesses have been measured around 30 GPa and have been found to be dependent on the substrate temperature and the deposition rate. Growth rates as high as 80 μm/h have been achieved and their dependence on substrate temperature is shown. The films are composed solely of pure boron carbide and are not contaminated with chlorine or hydrogen, as found by FTIR and XPS analysis. Micro X-ray diffraction studies indicated that the films were microcrystalline. The influence of a positive bias on the growth rate and composition of boron carbide has also been investigated. We have found that the minimum temperature at which films can be grown could be lowered from 550 to 500°C and that the growth rate was significantly enhanced at any substrate temperature.

Theoretical radiative transport results for a free-burning arc using a line-by-line technique

Abstract: Many thermal radiation results for a two-dimensional, axisymmetric, atmospheric, free-burning arc for several different wavelength intervals, as well as the entire thermal spectrum, are presented. Quantities such as the emission, average intensity, radiative source term and radial flux are calculated from a theoretically rigorous technique for handling radiative transport in a medium such as a plasma in which the absorption coefficient is a strong function of the wavelength. An S-N discrete ordinates method is used on a line-by-line basis to perform this task. Most of the results presented are for a pure-argon arc; however, results indicating the effect of copper contamination on the radiative quantities are also given. Comparisons between the pure-argon and copper-contaminated arcs show that a small amount of copper greatly affects the radiative characteristics of the plasma.

A three-dimensional two-phase model for thermal plasma chemical vapor deposition with liquid feedstock injection

Abstract: In this paper, a comprehensive model for thermal plasma chemical vapor deposition (TPCVD) with liquid feedstock injection is documented The gas flow is assumed to be steady, of a single temperature Radiation and charged species contributions are excluded, but extensive homogeneous and heterogeneous chemistry is included The liquid phase is traced by considering individual droplets Discussion on the model's application to diamond production from acetone in a hydrogen–argon plasma is included The major conclusions are: (1) Liquid injection possesses a capability to deliver the hydrocarbon precursor directly onto the deposition target (2) For the case of complete evaporation of the droplet before reaching the substrate, the deposition rate is similar to that obtained with gaseous precursors (3) The computational results compare well with experimental data The modeling results can be used to optimize the injection parameters with regard to the deposition rate

Diamond Deposition on Substrates with Different Geometries in a Thermal Plasma Reactor

Abstract: The effects of process parameters on diamond film deposition have been considered in an atmospheric-pressure dc thermal plasma jet reactor. Two different precursor injection systems have been evaluated, counterflow and side injection. The precursor flow rate using ethanol has been found to strongly affect crystal size as well as orientation of crystal growth planes. Further, crystal size on sharp edges has been found to be up to five times larger than on planar surfaces. The effects of substrate geometry on the morphology and area of deposited diamond have been investigated as well. The results of this study show that dc thermal plasma jets can provide high diamond deposition rates, for example on wires and drills, although crystal size and film thickness show substantial variation.