Jean-Luc Meunier

Thermo-field emission: a comparative study

Thermo-field emission current densities calculated by the commonly used Richardson - Dushman equation corrected for the Schottky effect are compared with those calculated using the more accurate treatment of Murphy and Good for a wide range of temperatures (1000 - 5000 K), electric field strengths () and work functions (2 - 5 eV) encountered in numerical modelling of the thermal arc-cathode interactions. Results show that the emission current densities predicted by the Richardson - Dushman equation are always lower than those predicted by the more accurate treatment. The disagreement between the two predictions is in the range 20 - 30% for the conditions encountered in numerical modelling of the attachment of a thermal arc to a hot tungsten cathode whereas it is much larger when a cold copper cathode is considered (>175%). It is suggested that the more accurate treatment of Murphy and Good should be used in order to increase the accuracy of prediction of the numerical models.

Experimental Study of the Effect of Gas Pressure on Arc Cathode Erosion and Redeposition in He, Ar, and SF6 from Vacuum to Atmospheric Pressure

Experimental data on the erosion rates of a copper cathode in He, Ar, and SF6 from 10-6 to 760 torr are presented. The work performed by the cathodic-erosion plasma expanding against the gas is found experimentally to be constant, the volume of the expanding plasma cloud being linked to the gas pressure by the relation R3p = constant. These data agree with a redeposition model based on condensation of the metallic vapor produced by the arc on the cathode surface. The redeposited mass on the cathode is found to be proportional to the cube root of the gas mass density.

Arc-cold cathode interactions: parametric dependence on local pressure

A numerical model describing the attachment of an electric arc on a vaporizing non-refractory cathode is developed and applied to a Cu cathode. The model describes the arc - cathode interaction zone by a combination of a quasi-stationary vacuum arc cathode spot model with a collisionless cathode sheath model for the current transfer in the cathode region. The conditions of pressure and electron temperature within the cathode spot plasma necessary to account for current densities ranging from A (upper limit for non-vaporizing cathode models) to A are presented. Results show that current densities higher than A can only be accounted for with metallic plasma pressures exceeding 35 atm and electron temperatures ranging from 1 to 2 eV within the cathode spots. The current transfer to the cathode is mainly assumed by the ions at low current densities ( A ) and by the thermo-field electrons for higher current densities. The heat flux to the cathode surface under the spots is mainly due to the flux of returning ions and ranges from to W for current densities ranging from to A . At low current densities (), the main heat loss is by conduction through the cathode while at high current densities, the Nottingham cooling associated with the thermo-field emission of electrons dominates. The model allowed us to define the upper and lower limits for the vacuum erosion rate by vaporization of the cathode. It is shown that the experimentally obtained vacuum erosion rate value for Cu falls between both limits for an electron temperature within the cathode spot of 1 to 2 eV.

A comparison of electron-emission equations used in arc - cathode interaction calculations

The predictions of a numerical model of the cathode-sheath region of high-pressure arcs using three different equations for the cathodic electron-emission current density are compared for two typical arc - cathode systems. These equations for cathodic electron emission include: (i) the field-enhanced thermionic emission (FEE) equation representative of the electron emission for high temperatures and moderate electric field strengths ; (ii) the Murphy and Good (MG) equation for thermo-field (T-F) emission valid for high temperatures and high surface electric field strengths in the range about - ; and (iii) the Murphy and Good equation for T-F emission enhanced by the presence of a high density of slowly moving ions in the cathode region (the MG+I equation). Results show that, in the case of a metal vapour arc (representative of vacuum arcs and high-pressure arcs on non-refractory cathodes) the use of the MG+I equation is always prescribed due to the formation of the high-local-pressure cathode-spot plasma. For high-pressure arcs on refractory cathodes the results show that, at atmospheric pressure, the generally used FEE equation introduces an underestimation by at least around 20% of the cathodic electron current density compared with the MG+I equation. The same comparison but for an ambient pressure of 50 atm shows that this underestimation becomes considerable.

Study of microdroplet generation from vacuum arcs on graphite cathodes

The emission of microdroplets from the cathode surface in the vacuum arc ion plating deposition technique is the major drawback to the technique’s industrial use. The generation of these particles from graphite cathodes is studied in this article and correlated to the local thermal load in the cathode spot area. A pulse discharge was used for a precise control of this load. Increases in the arc current level, arc duration time, and, more generally, the local temperature of the cathode were found to increase the number and the average size of the emitted particles. Particles under these conditions also show an increase in the width of their size distributions. Increasing the distance between cathode and substrate was found to decrease the number density of particles observed on the substrate according to the solid angle covered. The microdroplets show a graphite structure and diameters between 0.2 and 2.0 μm. Conditions needed to decrease the number of particles emitted to the substrate are given.

Pressure limits for the vacuum arc deposition technique

Observations of the cathodic copper plasma expansion at low pressures of He, Ar, and SF/sub 6/ showed that, for background gas mass densities of rho /sub g/=1 to 4*10/sup -4/ kg/m/sup 3/ and higher, the plasma and gas are separated into two volumes. A shock wave acts as a boundary between the two volumes. The boundary attains a stationary position once its expansion velocity decreases to the velocity of sound in the background gas. This position corresponds to a distance R/sub c/ to the cathode that agrees with a snowplow expansion model, giving R/sub c//sup beta /f=E/sub r/, where f is a function of the arc current and background gas characteristics, E/sub r/ is the erosion rate of the cathode, and beta varies between 2.1 and 2.5. The interaction model is based on kinetic energy exchanges between two gas-like volumes without other energy losses. A maximum pressure limit for vacuum arc deposition is set for rho /sub g//I=2 to 9*10/sup -6/ kg/m/sup 3/ A. >

Monitoring and control of RF thermal plasma diamond deposition via substrate biasing

In a RF induction thermal plasma chemical vapour deposition system the substrate is used as an electrical probe to monitor the diamond film evolution in situ. The evolving electron emission current allows us to identify the transition from the initial nucleation stage to the diamond growth stage. A direct-current bias voltage is applied to the substrate, and the polarity is adjusted in situ according to the changing growth requirements, providing a tool for controlling the diamond formation independent of the plasma source.

Importance of high local cathode spot pressure on the attachment of thermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of /spl sim/1 eV are needed to account for current densities of 10/sup 9/ and 10/sup 10/ A/spl middot/m/sup -2/, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j/sub tot/=10/sup 9/ A/spl middot/m/sup -2/, the current to the cathode surface is mainly transported by the ions (76-91% of j/sub tot/ while for a current density j/sub tot/= 10/sup 10/ A/spl middot/m/sup -2/, the thermo-field electrons become the main current carriers (61-72% of j/sub tot/). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed.

SYNTHESIS OF FULLERENES VIA THE THERMAL PLASMA DISSOCIATION OF HYDROCARBONS

A thermal plasma process is described for the synthesis of fullerenes via the dissociation of hydrocarbons. The plasma reactor is equipped with a nontransferred dc plasma torch which was used to dissociate hydrocarbons. The hydrocarbons investigated include CH4, C2H2, CBrF3, CCl2F2, and C2Cl4. The best results are obtained with C2Cl4. The collection temperature of the fullerene soot in the process was found to play a critical role in the collection rate of fullerenes.

Experimental study of the effect of nitrogen on titanium-arc cathode erosion

Experimental data on the dependence of titanium erosion rate on cathode temperature, Ar, and N/sub 2/ pressure from vacuum to 1 torr and arc motion are presented. Erosion rate is found to decrease with conditions that promote cathode poisoning/contamination. Higher cathode temperatures result in enhanced nitriding (poisoning), leading to a reduced erosion rate. A critical nitrogen pressure (0.001 torr) exists where a sharp drop in erosion is measured. Steered arcs show lower erosion rate values of 38 and 15 /spl mu/g/C for argon and nitrogen, when compared to random arc values of 45 and 35 /spl mu/g/C. Erosion rate studies on TiN-coated cathodes show a low value of around 22 /spl mu/g/C.