Erosion and cathodic arc plasma of Nb–Al cathodes: composite versus intermetallic

Abstract

Many properties of cathodic arcs from single-element cathodes show a correlation to the cohesive energy of the cathode material. For example, the burning voltage, the erosion rate, or, to a lesser extent, plasma properties like electron temperatures or average ion energy and charge states. For multi-element cathodes, various phases with different cohesive energies can initially be present in the cathode, or form due to arc exposure, complicating the evaluation of such correlations. To test the influence of morphology and phase composition of multi-element cathodes on cathodic arc properties, a Nb–Al cathode model system was used that includes: pure Nb and Al cathodes; intermetallic Nb3Al, Nb2Al and NbAl3 cathodes; and three composite Nb–Al cathodes with atomic ratios corresponding to the stoichiometric ratios of the intermetallic phases. Pulsed cathodic arc plasmas from these cathodes were examined using a mass-per-charge and energy-per-charge analyzer, showing that charge-state-resolved ion energy distributions of plasmas from the intermetallic and corresponding composite cathodes are nearly identical. An examination of converted layers of eroded cathodes using x-ray diffraction and scanning electron microscopy indicates the formation of a surface layer with similar phase composition for intermetallic and their corresponding composite cathode types. The average arc voltages do not follow the trend of cohesive energies of Nb, Al and intermetallic Nb–Al phases, which have been calculated using density functional theory. Possible reasons for this effect are discussed based on the current knowledge of multi-element arc cathodes and their arc plasma available in literature.

Determination of the critical temperature of Nb–Al alloys from the plasma parameters of a vacuum arc cathode spot

“…The application of this approach to determine the parameters of new complex materials, such as surfaces with a nanostructure (W-fuzz) [26] and multicomponent alloys [27] is of current interest.…”

“…In particular, a deviation from the cohesive energy rule [10]-the well-known empirical relation-was obtained in [27], according to which the arc discharge burning voltage depends on the cohesive energy E coh as…”

“…In addition to detailed measurements of the plasma parameters, i.e. the charge spectra and kinetic energy of ions, the cohesive energy E coh was calculated in [27] using the density functional theory (a) and the USPEX algorithm (b). Both techniques showed an approximately linear dependence that is inconsistent with the cathode fall measurements and cohesive energy rule.…”

“…Let us estimate the parameters of the matter, i.e. the critical temperature and burning voltage, from the plasma parameters measured in [27] in accordance with our model of an explosive emission pulse [18][19][20]. Note that three important plasma parameters have been measured in [27] for various Nb-Al cathodes-the kinetic energy of ions, their average charge and cathode potential fall.…”

“…From the spectra of the kinetic energy and charge state of Nb and Al ions measured in [27], one can estimate the average charge and average kinetic energy of the ions, and obtain the corresponding estimates of the critical temperature from equations ( 3) and ( 4) for Nb and Al, see figure 2.…”