The high-temperature electron emission and vaporization of tungsten-osmium alloys
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I.
INTRODUCTION
THERMIONIC energy conversion has been shown to be a viable method o f direct energy conversion, tl,2j particularly for use in extraterrestrial applications. The development o f tungsten-base alloys that exhibit superior physical and mechanical properties at elevated temperatures is imperative for high-efficiency thermionic energy conversion to be realized. Important aspects in the physical characterization o f these materials include the effective work function behavior and vaporization mechanisms. Because o f the complexity o f electron emission and surface atomic l a y e r sublimation, coupled with a lack o f data, a theoretical consideration is not adequate to predict emission phenomena. H e n c e , empirical data are needed to screen candidate alloys. This treatise presents the findings f o r a series o f binary alloys o f o s m i u m in tungsten. The advantages o f alloying tungsten with rhenium are well documented in the literature, t3,41 A recent investigation o f tungsten-iridium alloys produced similar results.t5] All three metals (i.e., osmium, rhenium, and iridium), when added to tungsten in dilute solution quantities, produce the favorable "rhenium effect" with respect to mechanical properties. [6,7,8] Osmium fails between r h e n i u m and iridium in the periodic table and is similar to both in several o f its characteristics. Osmium, rhenium, and iridium have the highest melting points o f all metals (except tungsten) with corresponding l o w vapor pressures. H e n c e , it was expected that tungsten-osmium alloys would exhibit enhanced work functions, as was observed for the tungsteniridium and tungsten-rhenium systems. In f a c t , an earlier RALPH N. WALL, Staff Engineer, is with the IBM East Fishkill Facility, Hopewell Junction, NY 12533. DEAN L. JACOBSON, Professor of Engineering, and DAVID R. BOSCH, Research Associate, are with the Department of Chemical, Biological, and Materials Engineering, Arizona State University, Tempe, AZ 85281-6006. Manuscript submitted April 1 0 , 1992. METALLURGICAL TRANSACTIONS A
study o f two tungsten-osmium samples yielded significantly enhanced effective work functions as compared to pure tungsten,t9~ The objective o f the present effort was to confirm an increased work function and to reveal the work function behavior as a function o f temperature, bulk composition, time, and surface o s m i u m and surface contaminate concentration. II.
METHODS
A detailed description o f the procedures followed in this work are given elsewhere, t51 An abbreviated m e t h odology is described below. Work functions were obtained from high-temperature electron emission measurements with a guard-ringed vacuum emission vehicle (VEV) designed to use a Schottky technique. The Richardson-Dushman equation was employed to calculate the work function,
Jo=AT2exp
~
[11
where J0 = zero-field saturation current density (A/cm2); A = 120.4 ( A / c m2 K2); T = temperature (K); k = Boltzmann's constant (0.861 × 10-4 eV/K); and the = effective work function (eV). Experimenta
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