Microstructures of chemical vapor deposited high-purity tungsten achieved by two different precursors

Chemical vapor deposited (CVD) high-purity tungsten can be manufactured as electron emitters in thermionic fuel elements. Exposed surface planes of the emitter emit electrons and affect the efficiency and load capacity of the thermionic fuel element. In order to determine the electron emission planes of the emitter, the metallurgic structures, the preferred orientation and the as-deposited surface morphology of CVD tungsten have been investigated. In this work, high-purity (over 99.99%) tungsten was achieved by two different precursors: hydrogen reduction of tungsten hexafluoride (WF6) and thermolysis of tungsten hexachloride (WCl6). The microstructures of CVD tungsten were characterized by metallography analysis, X-ray diffraction (XRD), electron backscattered diffraction (EBSD), and white light interference (WLI). Tungsten fabricated by hydrogen reduction of WF6 and thermolysis of WCl6 has ⟨100⟩-preferred columnar microstructures. However, the latter exhibits larger columnar grains and preferred ⟨100⟩ less. The preferred orientation is caused by the higher growth rate of the ⟨100⟩ orientation and competition between the ⟨100⟩ orientations of different grains. The as-deposited surface consists of tiny pyramids, with ⟨100⟩ axis and {111} side faces. In contrast to tungsten produced by hydrogen reduction of WF6, the pyramid on tungsten synthesized via thermolysis of WCl6 contains small {110} facets on the four edges. Therefore, tungsten made via thermolysis of WCl6 is a better candidate for thermionic fuel elements, because the higher work function of {110} planes enables better output efficiency.