Investigation on bolt-cathodes with floating zone melted polycrystalline and monocrystalline LaB6 emitters

The LaB6 samples were prepared by floating-zone melting in a special electron beam melting device at nitrogen gas pressure of about 20 Torr. In this way it is possible to grow LaB6 single crystals of about 4 mm diameter and 25 mm length. From this material emitters of 1 and 2 mm diameter were fabricated for a bolt-cathode system. The thermionic characteristics of (100)-, (110)-, and (111)-oriented single crystal emitters as well as of polycrystalline emitters were investigated at uhv conditions in an automated Richardson-plot measuring device. The highest thermionic emission is observed with (100)-oriented emitters (1 A/cm2 at approximately 1600 K and 10 A/cm2 at approximately 1790 K). The 1 mm diameter polycrystalline cathodes (zone melted material) were life tested in a 5 kV-electron gun at constant emission current, with the initial current densities of 10 A/cm2 (approximately 1880 K) and 5 A/cm2 (approximately 1810 K). Mass erosion rates of 2 x 10-8 g/cm2.s and 1.5 x 10-8 g/cm2.s were found. On the basis of these results and the investigation of oxygen--LaB6 interaction at high temperatures with Auger electron spectroscopy, an erosion mechanism by oxidation is proposed. 10 figures