The production of structural parts from porous tungsten affords special technological procedures.
Starting materials are mainly coarse or globular powders, which are compressed by special methods, sometimes with the addition of pore-forming materials which evaporate during sintering. Also in sintering, special procedures such as infiltration, evaporation, surface treatment, etc. are necessary. Powder grain size, compacting pressure, sintering temperature, and time determine pore size and permeability. At densities between ’50 and 78% all pores are open; at >90% density, all are closed.
Parts with ran 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 (7µm) density 75-85%, pore size 2-30 µm and pore number l.4 x 106-8 x106 •cm-3.
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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