Spectral normal emittance of single crystals.

Abstract

The spec t ra l normal emittances of molybdenum, tantalum. and tungsten c rys ta l s were measured normal to low Miller index planes at 2000-F, 3000'F. and 4000-F in Vacuum. The levels of spectral normal emittance obtained in this investigation were in good agrgemen: with previously published values for polished surfaces of the metals investigated. One tantalum sample recrystall ized in a manner Such that the emittance normal to a (2yl ) and a (321) plane could bp measured a t essentially the same time. ments indicated that a very small dilference in emittance did exist at approximately 0. 5 microns with the (211) plane having a higher emittance. of the ( 2 1 1 ) plane to the (100) plane of molybdenum, the (110) plane to the ( I O O l plane of tungsten, and the (110) plane to the (211) and ( 3 2 1 ) planes of tantalum showed no significant differences between planes. This se t of measureComparison of the emittance The emittance tended to decrease with successive measurement cycles because the samples became more highly polished. This phenomenon may have obscured emittance differences between planes, but the agreement between measurements for diiferent * Presently employed at Lewis Research Center , NASA planes was remarkably good. than 10 per cent in the in3irarrd region and no l a r g e r than 4 per cen t in the visible region. The results of this investigation indicate that differences in polished surfaces have a grea ter e f fec t On the Spectral normal emittance of the re f rac tory metals than the c rys ta l lographx orientation of the emitting sur face . If differences did exist, they are believed to be no larger INTRODUCTION Work performed in the las t decade' , ' has established that the thermionic emission f rom tungsten and other re f rac tory metals varies considerably with the crystallographic orientation of the emitting surface. phenomenon to improve the operating characterist ics of thermionic conversion devices. Because the efficiency of diodes is affected by the thermal emittance of the cathode, It is important to know if the thermal emittance of materials is b function of crystallographic orientation. crystallographic orientation on the thermal emittance of tungsten, tantalum. and molybdenum by measuring the spec t ra l normal emittance oi low ~ i i l e r Index planes. Recent studies have been d i rec ted toward using this The objective of this program was to empirically de te rmine the effect of 1. M. K. Wilkinson. Crystallographic Variations of F ie ld Emiss ion From Tungsten, ' ' Journal of Applied physics, XXIV (September 1953), 1203-1209. 2. E. W. Muller, 'Work Function of Tungsten Single Crys ta l Planes Measured by the Field Emission Microscope, Journal of Applied Physics, XXVI (June 1955), 732-737. 3. Single-Crystal Thermionic. Converter Studies, AD 407826 (USAF Cambridge R e s e a r c h Laboratorics. 1963).