Phillip Swartzentruber

Magnesium Alloy Precursor Thin Films for Efficient, Practical Fabrication of Nanoporous Metals

An improved approach to fabrication of nanoporous (np) metals is demonstrated for several metallic systems that were successfully created by dealloying magnesium-based precursor alloys (also containing iridium, nickel, gold, or osmium-ruthenium). A significant advantage is that magnesium alloys can be dealloyed effectively using water or, if needed, dilute acetic acid. The crystal structures of magnesium-based precursor films were significantly different from those of alloys commonly used as precursors. This approach should be generally applicable to np metal synthesis.

Correlation between microstructure and thermionic electron emission from Os-Ru thin films on dispenser cathodes

Osmium-ruthenium films with different microstructures were deposited onto dispenser cathodes and subjected to 1000 h of close-spaced diode testing. Tailored microstructures were achieved by applying substrate biasing during deposition, and these were evaluated with scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy before and after close-spaced diode testing. Knee temperatures determined from the close-spaced diode test data were used to evaluate cathode performance. Cathodes with a large {10-11} Os-Ru film texture possessed comparatively low knee temperatures. Furthermore, a low knee temperature correlated with a low effective work function as calculated from the close-spaced diode data. It is proposed that the formation of strong {10-11} texture is responsible for the superior performance of the cathode with a multilayered Os-Ru coating.

6.2: Optimizing osmium-ruthenium films to inhibit tungsten interdiffusion

Osmium-ruthenium thin films were deposited on porous tungsten pellets, at the same time as cathode assemblies, to investigate possibilities for minimizing interdiffusion. Previous studies had identified promising film characteristics for inhibiting tungsten interdiffusion. For example, it was found that a 5W-substrate-biased film of 550 nm thickness exhibited high structural and compositional stability, and several other films exhibited promising properties as well. These films were produced and annealed, then analyzed for composition. Emission tests of M-type cathode assemblies, coated with the same candidate films, were performed to assess the degree of lifetime improvement imparted by the films to the cathodes.

Composition and work function relationship in Os–Ru–W ternary alloys

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings.

Influence of Os-Ru coating on closed-space diode tests of M-Type dispenser cathodes

Life testing of M-Type dispenser cathodes was performed for 1000 hours to help quantify the performance enhancement due to different osmium-ruthenium (Os-Ru) thin film microstructures. Os-Ru film microstructures were selected based on their potential to inhibit tungsten/Os-Ru interdiffusion as identified in previous studies. The results show that the 10W substrate biased 150nm thick film yielded a very stable knee temperature throughout its life. However, the knee temperature was high in comparison to the standard Semicon film.

Microstructural influence of OsRu thin films on dispenser cathodes

Osmium-Ruthenium films were subjected to 1000 hours of close-spaced diode (CSD) testing and the film microstructures were evaluated in light of the CSD test results in order to generate improved film architectures for further CSD testing. Low knee temperatures correlated with a {10-10}/{10-11} film texture of annealed films.

Alternative ceramic potting materials for dispenser cathodes

Dispenser cathodes operate under ultra high vacuum and high temperature conditions. Because of these harsh conditions the tungsten rhenium heater coil is surrounded by a ceramic potting material to maintain a uniform temperature profile. Many cathodes are scrapped due to cracking in the potting which could lead to thermal hot spots. This study explores alternative ceramic potting materials that pass the stringent requirements of high end cathodes including slight expansion upon sintering. Ternary aluminum nitride based ceramics were proposed with calcium oxide, yttria, and alumina as additions. The standard potting material used by Semicon Associates was also profiled. It was found that a ceramic with composition 70.5 wt% Al2O3, 23.5 wt% AlN, and 6 wt% CaO expanded 1.71 volume percent compared to 0.81 percent expansion of the standard potting. This new Al2O3/AlN/CaO material was measured to have a thermal conductivity of 3 W/mK at 1200ºC and the standard potting to have 12 W/mK at 1200ºC. All other compositions considered contracted significantly upon sintering, disqualifying each as a viable alternative potting material. This paper is the result of a student project performed by University of Kentucky MSE480 Senior Design class in coordination with the sponsor Semicon Associates of Lexington, KY.

In-vacuo work function measurement of dispenser cathodes

A novel experimental technique was used for direct measurement of cathode material work function at elevated temperature. Dispenser cathodes were activated in a vacuum chamber and the work function determined from the contact potential difference between the cathode and a precisely calibrated probe tip. The experimental test setup allowed for cathodes to be activated and the work function measured without breaking vacuum. B-type, M-type, and other cathodes were activated and their work function tracked during the initial heating stage and during cooling after activation. The various cathode types showed a smooth increase in work function as the cathode cooled, but the B-type had a lower room temperature work function than other cathodes. It is suggested that residual gas in the chamber may have caused the rise in work function during cooling.

Direct work function measurement of activated M-type dispenser cathodes

M-type dispenser cathodes were activated in a vacuum chamber and the absolute work function determined from the contact potential difference between the cathode and a precisely calibrated probe tip. The experimental test setup allowed for cathodes to be activated and the work function measured without breaking vacuum. In addition to the commercially available Os-Ru coating from Ceradyne, Inc., a 3M Company, the work functions of Os-Ru coatings with engineered microstructures were measured to quantify microstructural influences. The work functions of both pre- and post-activation cathodes are presented and discussed in light of the microstructure of their Os-Ru coatings.

Work function measurements on coated and uncoated tungsten dispenser cathodes using a Kelvin probe

The room temperature work functions of several dispenser cathode configurations under different conditions were evaluated using a Kelvin probe.