How Hydrogen Admixture Changes Plasma Jet Characteristics in Spray Processes at Low Pressure

In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions.

Synthesis of Nanocarbons Using a Large Volume AC Plasma Reactor

ABSTRACTThere is an opportunity for scaling up, optimizing, and controlling the process of production of nanoparticles due to their numerous diverse applications. We present a system for continuous, high rate production of nanoparticles, particularly those of carbon, using large volume thermal plasma based on a three-phase diverging electrode configuration. The goal of using this 3-phase plasma reactor is to have a plasma arc that is scalable, self-stabilizing, and low maintenance, with sufficient plasma volume to maximize residence time of feed materials for evaporation to atomic species. Plasma carrier gas, typically inert gas such as helium, is injected into the reactor allowing the vaporization of any feedstock due to plasma temperatures >5000 °C. Controlling plasma enthalpy, diffusion/temperature gradients and carbon feed rates allow the controlled growth of clusters leading to nanoparticles less than 100 nm. Once the desired size is achieved the gas stream is expanded to reduce the reaction rate and quenched by natural cooling to chamber walls or injection of a cooling gas stream, preferably of the same composition as plasma carrier gas. Recoverable yields in the nanoparticle-laden gas stream are then isolated by standard means (filtration, cyclone separation, electrostatic precipitation), and the plasma gas and unreacted feedstock are routed to the plasma reactor for recycling. Computational Fluid Dynamics (CFD) is employed to measure and predict fluid flow, energy/temperature, and other species distributions in the plasma process.

Modeling of Surface Roughness and the Role of Debris in Micro-EDM

Surface roughness is one of the important quality characteristic of a micromachined component. This paper presents a model to predict surface roughness of micro-EDmachined surfaces. The model is based on the configuration of a single-spark cavity formed as a function of process parameters. Assuming the normal distribution of surface heights, the μ and σ(Rq) of the surface profile are evaluated after every spark. The model was further extended to capture the role of debris in micro-EDM in changing electric potential at the micropeaks on the cathode surfaces. The chemical kinetics approach was used to evaluate the change in plasma enthalpy and composition as a result of debris inclusion in the dielectric. The corresponding energy distribution between the electrodes was used to predict configuration of the single-spark cavity and the consequent surface roughness using the earlier surface roughness model. The modeling results were found to agree well with the micro-EDM validation experiments performed without and with the inclusion of artificial debris (iron particles) in the dielectric.

The Effects of Gas Composition and Pressure on RF Plasma Sintering of MgO

ABSTRACTRapid densification of MgO in an rf plasma without sintering aids is demonstrated. The sintered densities of MgO are sensitive to the plasma composition and gas pressure. In general, specimens are sintered to higher densities in Ar/O2 and Ar/H2O plasmas than in pure Ar plasmas. This may be explained by the effects of plasma enthalpy and surface recombination of charged particles and of atomic species. Analyses show that the evolution of the microstructures of the sintered specimens remains similar, regardless of the plasma conditions. No substantial grain growth is observed.