The high-temperature work function behavior of polycrystalline osmium
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I.
INTRODUCTION
T H E thermionic work function of polycrystalline osmium was earlier measured by Wilson tu and Houston. ~2~ Values reported by Wilson were considerably lower than those offered by Houston. The intent of this effort was to resolve the apparent disagreement in these two earlier studies. From a practical consideration, the thermionic emission behavior, as characterized by the vacuum electron work function, is an important material property in the design of a thermionic energy converter. Such a direct energy conversion device is perhaps the leading candidate for high-power space-based energy needs. Presently, the preferred emitter materials being evaluated are rhenium and binary alloys of tungsten and rhenium. Recently, however, tungsten, osmium alloys have also received favorable attention, t3j Considering osm i u m ' s proximity to rhenium, it is only logical that a better understanding of the thermionic characteristics of osmium be sought. II.
METHODS
The thermionic work function was obtained from hightemperature electron emission current measurements with a guard-ringed vacuum emission vehicle (VEV). The VEV has been described elsewhere, t4t A simple schematic is shown in Figure 1. This configuration allows accurate temperature measurement, sensitive temperature control, very high-temperature operation (to 2900 K), and a direct determination of collector current density (Figure 2). A simple form of the Richardson-Dushman equation was used to calculate the effective work function: tSj
Jo = AT2 exp (---~-)the
[1]
where Jo is the zero-field saturation current density (A/cm2), A is a constant whose theoretical value equals RALPH N. WALL, Staff Engineer, is with the Advanced Semiconductor Technology Center, IBM East Fishkill Facility, Hopewell Jct., NY 12533. DEAN L. JACOBSON, Professor of Engineering, is with the Department of Chemical, Biological, and Materials Engineering, Arizona State University, Tempe, AZ 85287. Manuscript submitted September 20, 1990. METALLURGICAL TRANSACTIONS A
120.4 A / c m 2 K 2, T is the temperature, k is Boltzmann's constant, and ~be is the effective work function (eV). By employing the appropriate form of the Schottky equation, t61
[EO.5\ l n J , = lnJ0 + 4.403~--~-)
[2]
where E is given in V / c m . The Richardson-Dushman formula may be modified to account for the experimentally applied field, E. That is, once it was determined that E was within the range for Eq. [2] to be valid, ~4~ Eqs. [1] and [2] were combined to yield the = k T l n
(AT2~ + 3 . 7 9 • 10-4(E ~ \J~/
[3]
where Js is the electron saturation current density (A/cm2). The immediate benefit of Eq. [3] was that data were acquired as a function of time and temperature with the field held constant. This proved to be of particular advantage in studying the emission behavior of osmium. An individual test consisted of holding the temperature constant and monitoring the with time. This procedure was repeated at different temperatures ranging from 1800 to 2600 K. Typically, a single test run lasted 24 ho
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