Thomas John Balk

Improved Product Quality and Resource Efficiency in Porous Tungsten Machining for Dispenser Cathode Application by Elimination of the Infiltration Process

Porous tungsten is a difficult-to-machine material commonly used in the manufacture of high-performance dispenser cathodes [1]. The functional performance of such cathodes is directly linked to the surface porosity of the machined porous tungsten workpiece. Conventional machining leads to smearing of surface pores [1]. Current industry practice involves the use of a plastic infiltrant that stabilizes the pores during machining [2]. In order to increase the sustainability of the dispenser cathode manufacturing process, we propose a materials-science driven approach to machining porous tungsten. Heated and cryogenic machining are compared to determine an effective method to increase surface porosity. The ductile/brittle transition of the body-centered-cubic (BCC) refractory metal tungsten is exploited to alter the cutting mode between shear cutting (heated machining) and controlled brittle fracture (cryogenic machining). We conclude that cryogenic machining via controlled brittle fracture using a fine-grained PCD tool is the most effective approach to infiltrantfree machining of porous tungsten for dispenser cathode applications.

Effects of Substrate Bias on Microstructure of Osmium-Ruthenium Coatings for Porous Tungsten Dispenser Cathodes

Osmium-ruthenium (Os-Ru) films used as coatings for porous tungsten (W) dispenser cathodes were investigated in this study. By applying different levels of substrate biasing power during sputtering, the texture of deposited Os-Ru films varied between {0002}, {10-10}, and {10-11} in this hexagonal close-packed alloy. Furthermore, film texture changed from one preferred orientation to other preferred orientation(s) during annealing (at 1050degCB for 10 min), during which the microstructure of certain Os-Ru films changed from a columnar to an equiaxed grain morphology. Due to a low degree of texture transition during annealing and due to the high compositional and structural stability of the Os-Ru grains, a 5 W bias power appears to be best for film deposition. In addition, the 5 W biased Os-Ru films transformed completely into a basal plane texture, which has the highest planar density, and grain size increased significantly (from ~20 to ~100 nm) during annealing, all while maintaining a columnar grain structure. These characteristics are believed to be helpful in inhibiting interdiffusion between the W substrate and the Os-Ru film. Preventing or slowing the interdiffusion of W atoms can improve the lifetime and even the performance of dispenser cathodes.

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.

Characterization of osmium-ruthenium coatings for porous tungsten dispenser cathodes

Dispenser cathodes serve as electron sources in numerous vacuum devices, including traveling wave tubes and cathode ray tubes. These devices find use in commercial, military and space applications, requiring a long and reliable operating lifetime, especially for space-based operation. Semicon Associates, in Lexington, KY, leads this market, producing over 20,000 dispenser cathodes annually. The cathodes comprise several materials, including platinum group metal coatings that add significant production cost. It is not yet understood how the microstructure of these precious metal films affects cathode performance. Fundamental understanding of microstructure-property relationships in the coating could improve device performance and allow more economical use of the precious metals.

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.

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.

Effects of annealing on microstructure of osmium-ruthenium thin films

Osmium-ruthenium (OsRu) thin films of different thickness were deposited on porous tungsten (W) pellets in a configuration similar to dispenser cathodes, using two separate sputtering systems. In order to assess which films may best inhibit interdiffusion between the OsRu film and the W substrate during annealing, the grain structure and texture of OsRu films were characterized by x-ray diffraction and scanning electron microscopy.

Interplay of composition, structure, and electron density of states in W-Os cathode materials and relationship with thermionic emission

The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungsten dispenser cathodes.The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungste...