Plasma processing is being used more frequently to develop new materials processing technologies and improve existing ones. Process models may be used as design tools to simulate complex phenomena such as magneto-fluid-dynamic interactions, turbulence, and particle breakup and transport that take place in processes such as wire-arc spraying, plasma deposition, and plasma spraying.
A computer code for the accurate calculation of thermodynamic and transport properties of different plasma mixtures has been developed and is currently being modified. These transport properties help simulate the various processes. All of the projects being worked on involve the iterative or transient solution of a large set of strongly coupled non-linear equations, often with a very fine grid resolution in a two- or three-dimensional space.
One project is dealing with the numerical simulation of the wire-arc spray process. Wire-arc spraying is becoming a popular process to apply metallic coatings to parts with a large number of applications in mechanical, automotive, biomedical, chemical, and electronics engineering. To improve the quality of the coatings, computer models may be used as design tools to simulate the complex interaction between magneto-fluid dynamic phenomena, turbulent transport, and molten droplet breakup. A three-dimensional code has been developed to simulate the wire-arc spray process. This code calculates the compressible flow and temperature fields in the nozzle, torch, and jet of the system. A Lagrangian model is used to predict the trajectories, velocities, and temperatures of the molten droplets of the metallic wire as they travel from the electrode to the substrate. A sub-model is used to predict the breakup and resulting size distribution of the molten droplets due to the action of high-velocity and high-temperature plasma.
Marcus Asmann, Graduate Student Researcher
Marwane Berrada, Undergraduate Student Researcher
Gianluca Gregori, Graduate Student Researcher
Nakhleh A. Hussary, Graduate Student Researcher
Joon Park, Graduate Student Researcher
Shiwei Zhu, Graduate Student Researcher
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"Theoretical Radiative Transport Results for a Free-Burning Arc Using a Line-by-Line Technique," J.A. Menart, J. Heberlein, and E. Pfender, Journal of Physics D: Applied Physics, 32, p. 55 (1999). |
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"Thermal Plasma Deposition of Nanostructured Films," A. Neuman, J. Blum, N. Tymiak, Z. Wong, N.P. Rao, W.W. Gerberich, P.H. McMurry, J. Heberlein, and S.L. Girshick, IEEE Transactions on Plasma Science, 27, p. 46 (1999). |
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"Hypersonic Plasma Particle Deposition of Nanostructured Silicon Carbide Films," N. Tymiak, D.I. Iordanoglou, D. Neumann, A. Gidwani, F. Di Fonzo, M.H. Fan, N.P. Rao, W.W. Gerberich, P.H. McMurry, J. Heberlein, and S.L. Girshick, Proceedings of the 14th International Syumposium on Plasma Chemistry, 4, p. 1989 (1999). |
Further work is being done on computation of thermodynamic and transport properties of plasmas. The accurate calculation of thermodynamic properties such as enthalpy, specific heat, and density, and transport properties such as viscosity, thermal, and electronical conductivities are required as inputs for a large number of computational and experimental calculations. In particular, for novel plasma processing applications such as reactive plasma spraying, accurate thermo-physical and thermo-chemical data of mixtures of plasma gases is necessary. A computer code for the calculation of these properties has been developed and is currently being modified. For the calculation of chemical compositions and thermodynamic properties, an algorithm for the minimization of the Gibbs free energy is used, while the calculation of transport properties is based on the Chapman-Enskog method based on solving the collision integrals.
Another project involves the modeling of r.f. plasma deposition process on a preform. The temperature and flow fields in the r.f. plasma torch and various characteristics of the particles of actual size distributions injected into the plasma plume have to be modeled to simulate the actual deposition process. Loading effect due to the interaction between the plasma and particles, the radiation losses from particles, and the carrier gas cooling effect are also modeled. Sensitivity analysis of some factors influencing the deposition rate is conducted to suggest possible means of improving the process with a better efficiency.
Additional work is pursuing three-dimensional numerical modeling of an arc-anode attachment. A plasma jet issuing from a d.c. plasma torch shows strong temporal fluctuations due to time-dependent three-dimensional motion of an arc root along the anode surface inside the plasma torch. The complex behavior of the arc is caused by combined effects of fluid-dynamic drag and Lorentz force acting on the arc-anode column and plasma flow. Even though many numerical models have been proposed to simulate the arc behavior inside the plasma torch, most of them have considered the arc-anode attachment assuming the axisymmetry assumption in two-dimensional geometries. Therefore, those conventional models have failed to exactly describe the transient and three-dimensional motion of the arc root. The development of a transient three-dimensional numerical code is attempted in this project with the recent progresses of parallel algorithms. Message Passing Interface libraries are used to realize the parallel computer code.
A final project is dealing with molecular dynamics simulations of the dynamic structure factor. The experimental investigation of the plasma properties by scattering techniques requires the accurate knowledge of the dynamical properties of particle system. In particular, Thomson scattering of laser light by plasma electrons is frequently used to measure electron temperatures and densities in weakly non-ideal plasmas. However, the precise determination of these properties is subjected to several analytical assumptions on the particular form of the fluctuation spectrum of the electrons. The space-time Fourier transform of such fluctuation spectrum is the dynamic structure factor, and it represents the effect of the inter-particle correlation in the plasma ensemble. A molecular dynamics computer code can provide an efficient mean to probe the dynamic structure factor from general principles. Results from these simulations could then be used against proposed theoretical models in order to validate the approximations used for the temperatures and density calculations in Thomson scattering experiments. Non-ideal and strongly coupled plasma system could be investigated using multi-component (ions and electrons) molecular dynamics simulations. Non-uniform and non-translationally invariant plasma system may also be studied under various conditions in order to reproduce more accurate experimental conditions.
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