Porous tungsten

Parts with an open coherent porosity of 20% are used in vacuum tubes and aeronautics because of refractoriness, shape stability, and chemical resistance. Examples are porous cathodes impregnated with alkaline earth oxides as electron-emitting sources in special tubes, thermoionic converters heated by nuclear energy, and ionic propulsion units with porous tungsten plates as ionic sources for Cs vapor of high temperature. Those emitter plates are made of globular tungsten powder (7 pm) density 75-85%, pore size 2-30 pm, and pore number 1.4 x 10 -8 x 10 cm .  

Porous tungsten can be infiltrated either to machine it to rigid dimensional tolerances, or to fabricate composite materials. Copper, which is used as infiltrant for machining, can be completely removed afterward by heating in vacuum by evaporation. 

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Space Applications. The growth of powder metahurgy in space technology has arisen from the difficulty of handling many materials in conventional fusion-metahurgy techniques, the need for controhed porosity, and the requirement of many special and unique properties (60,61). Powder metahurgy is applied in low density components with emphasis on porous tungsten for W—Ag stmctures, beryhium compounds, titanium and... 

Ion engines are used in sateUites for orientation control. Cesium is vaporized in a vacuum and ionized as it passes through a heated porous tungsten disk, the ions are accelerated by an electric field to about 135 km/s and are neutralized by the injection of electrons and exhausted from the thmster. However, mercury, xenon, and argon-based ion engines are preferred. 

Method of Producing Porous Tungsten for Ion Rocket Engines , NASA TND 864 (1961) (AD 258265) 7) Anon, EngrgDesignHndbk,... 

Substitution of poly-Si by tungsten by a WF6 exposure at 300°C for 30 min. This step is possible because the volume of the replaced Si is about twice that of the formed W. In fact porous tungsten is formed, as can be inferred from the low density mentioned above. 

BaO- resp. BaCOs-doped porous tungsten play an important role as cathodes in high-energy electron tubes. The emitting element is metallic Ba, which forms upon heating above 1000 °C. 

FIGURE 7.7. Porous tungsten cathodes with large surfaces for high-power microwave tubes. By courtesy of Plansee AG, Austria. 

Ticknor, B. W., Anderson, J. K., Fritz, B. A. Chiu, Y.-H. (2010). Effect of Asp>ect Ratio on the Wettability and Electrospray Properties of Porous Tungsten Emitters with the Ionic Liquid [Emim] [Im], AIAA-Paper 2010-6618, ISSN 0146-3705 Wingborg, N., Eldsater, C. Henrik, S. (2004). Formulation and characterization of ADN-based liquid monopropellants, European Space Agency, SP-557, ISBN 978-92-909286-6-9, pp. 101-106... 

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