Electron-beam zone melting is the preferred method to produce larger, single crystals of tungsten. A polycrystalline tungsten rod is mounted in a vertical position in a stainless steel bell jar and connected as anode. An electron gun (cathode) traverses along the rod by means of a screw drive mechanism. On melting in a high vacuum, a narrow, liquid zone is formed and held between the two solid rod parts by surface tension forces. As the zone slowly moves through the rod several times (zone floating), impurities are concentrated on both the starting and terminal end of the rod. Since the molten zone is quite narrow and the temperature gradient in its vicinity rather steep, nucleation starts essentially at one point and a single crystal is the result. The concentration of many impurity elements can be decreased considerably by zone melting. Al, Ca, Co, Cu, Fe, Mg, Mn, Ni, Si, Na, Ti, Ta, Re, Mo, and Nb concentrations can be reduced below 1 pg/g Interstitials remain at the following pg/g levels: O2 2, N25, C10, and H1.
Single crystals produced by this technique represent the most ductile tungsten currently available. They are free of grain boundaries, which cause brittleness.
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