Herbert Wilhelmi

Mass spectrometric diagnosis of the surface nitriding mechanism in a d.c. glow discharge

Reactive constituents have been investigated in a molecular beam generated in the cathode surface glow area and surface boundary layer. Mixtures of nitrogen and hydrogen form NHx(x=0–4) compounds, which are of relevance in heterogeneous, plasma vs. metal nitriding reactions. Ammonia decomposition leads to NHx(x=2–4). Strong cataphoretic enrichment of hydrogen has been observed in the cathode glow area. Heterogeneous reactions of NHx with iron lead to the formation of iron nitrides via intermediates such as FeNH2–3. In a pulsed d.c. glow discharge, increased sputtering and decreased hydrogen enrichment have been observed.

Investigations concerning the work function of doped graphite

In order to reduce the burning off of electrodes in electric arc furnaces, it is necesary to decrease the specific thermal load at the tip in the arc attachment area. This can be achieved by minimizing the work function of the electrode materials, whereby the peak temperature is reduced. The work function of doped graphite electrodes was investigated under conditions prevailing during standard industrial practice in order to achieve transferrable results. The measurement equipment developed to meet this requirement is based on a design first proposed by Bade and Yos. By measuring the peak temperature of the electrode and its current density, the value of the work function of the material was calculated with a modified Richardson equation. Starting from materials with well-known values of the work function, to verify, the accuracy of the experimental method and the mathematical model, those of pure and dopedgraphites were determined. The additives, oxides as a rule, result in a significant reduction in electron work function in the case of magnesium, calcium and cerium oxides.

Mass Transfer from Copper Melts by Top-Blowing of an Argon-Hydrogen Plasma Jet

The volatilization of lead, copper, tin, and zinc from copper melts using the technique of top-blowing with an argon-hydrogen plasma jet was experimentally investigated and theoretically evaluated. A plasma burner with 16 kW power was used in the experiments. The mole flow of the plasma gases was 0.017 mol/s (25 liters/min when T = 25°C and PG = 1 bar). The temperature was 1830°C on the surface of the melt and between 1200 and 1500°C in the molten solution. When the zinc concentration is above 2 mole%, supersaturation of zinc occurs on the surface. In this range of concentrations the ratio of dilution of the concentration in relation to time is linear (zeroth-order reaction). When the concentration of zinc is below 2 mole%, the time dependence of volatilization can be described by an exponential law corresponding to a first-order reaction, because in this case the rate-determining step is the mass transport of zinc in the molten copper phase. From the change from zeroth-order to first-order reaction during the volatilization of zinc, the temperature on the surface of the melt can be estimated with a high degree of accuracy. On the other hand, the volatilization of tin and lead is determined by mass transfer in the gas phase, which leads to an exponential law for the whole range of concentrations. Reaction models were set up on the basis of the experimental data. The relationships thereby obtained permit one to evaluate in advance the yield of future industrial volatilization processes with top-blown plasma jets.