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.
Highlights
IntroductionFor cathodic arcs of single-element cathodes in high vacuum, experimental and calculated data for the arc plasma properties covering most conducting elements in the periodic table is available in literature [2,3,4,5,6]
This includes for example, average burning voltages, electron temperatures, ion charge state distributions (ICSDs), or ion energy and velocity distributions
Backscattered electron (BSE) images were recorded to visualize the elemental contrast between Nb and Al
Summary
For cathodic arcs of single-element cathodes in high vacuum, experimental and calculated data for the arc plasma properties covering most conducting elements in the periodic table is available in literature [2,3,4,5,6]. This includes for example, average burning voltages, electron temperatures, ion charge state distributions (ICSDs), or ion energy and velocity distributions. Concepts like the correlation of the cohesive energy of the cathode material to burning voltage, energy input and erosion rate [7], or the independence or weak dependence of most likely ion velocities on ion charge state [8] show a good agreement to these experimental data sets
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