SPUTTERED THERMIONIC HEXABORIDE COATINGS

Thermionic coatings based on the hexaborides of rare-earth elements (R eB6) were deposited onto molybdenum and tungsten substrates using DC m agnetron sputtering from LaB6, CeB6, SmB6 and YB6 targets. Films were investigated by means of scanning electron microscopy, electron probe microanalysis and X-ray diffraction. The work function and electron em ission characteristics of the coatings have been studied by the thermi onic emission method using coated tungsten filaments. After optimizati on of the sputtering parameters, extremely fine-columnar coatings cons isting of predominantly (100)-orientated LaB6, CeB6 and SmB6 crystals were obtained. The Y-B films showed nearly amorphous film growth. All coatings were overstochiometric with B:Re atomic ratios ranging from 6 .3 to 7.5. The work function was measured to be in the range 2.6-3.3 e V. Coated cathodes worked with comparable electron emission current at temperatures approximately 1000 degrees C below the operating tempera ture of uncoated tungsten filaments. LaB6 and CeB6 were found to be th e most promising electron emissive coating materials useful as low tem perature emitters. However, satisfactory lifetimes may be only yielded at low operating temperatures and vacuum pressures. (C) 1998 Elsevier Science S.A.