The effect of surface depletion on the work function of arc-melted dilute solution tungsten-iridium alloys

Journal Article · Thu Aug 01 04:00:00 EDT 1991 · High Temperature Science (An International Journal); (United States)

OSTI ID:6218216

D'Cruz, L A; Bosch, D R; Jacobson, D L ^[1]

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1. Arizona State Univ., Tempe, AZ (United States). Dept. of Chemical, Bio and Materials Engineering

The requirements of thermionic electrode materials have emphasized the need for substantial improvements in microstructural stability, strength, and creep resistance at service temperature in excess of 2,500K. The present work extends an earlier study of the effective work function trends of a series of dilute solution tungsten, iridium alloys with iridium contents of 1, 3, and 5 wt%. Since the lifetime of candidate electrode materials is an important consideration, the present work attempts to evaluate the repeatability of the work function trends in these alloys. The effective work function was obtained from measurements of the current emitted from the electrode surface under UHV conditions in the temperature range of 1,800-2,500K using a Vacuum Emission Vehicle (VEV). The data generated in this work have been compared with data obtained in earlier studies performed on these alloys. It was found that the magnitude of the effective work function of these alloys was affected by changes in the subsurface iridium concentration. Furthermore, these alloys exhibited a dependence of the work function on temperature, after prolonged exposure to elevated temperatures. Such a temperature dependence can be explained by diffusion-controlled changes in the coverage of an iridium monolayer on the surface. It is proposed that the significant difference in effective work function trends obtained after prolonged exposure to elevated temperatures is a direct consequence of changes in the coverage of an iridium-rich monolayer on the electrode surface. The constitution of such a surface layer, however, would be governed by composition changes in the subsurface regions of the electrode caused thermally-activated transport processes.

• OSTI ID: 6218216
• Journal Information: High Temperature Science (An International Journal); (United States), Journal Name: High Temperature Science (An International Journal); (United States) Vol. 32:1; ISSN HITSAC; ISSN 0018-1536
• Country of Publication: United States
• Language: English

Related Subjects

30 DIRECT ENERGY CONVERSION
300403* -- Thermionic Converters-- Materials
Components
& Auxiliaries
36 MATERIALS SCIENCE
360102 -- Metals & Alloys-- Structure & Phase Studies
ALLOYS
CONCENTRATION RATIO
CORRELATIONS
DATA
DIRECT ENERGY CONVERTERS
ELECTRODES
EXPERIMENTAL DATA
FUNCTIONS
INFORMATION
IRIDIUM ADDITIONS
IRIDIUM ALLOYS
LIFETIME
METALLURGICAL EFFECTS
MICROSTRUCTURE
NESDPS Office of Nuclear Energy Space and Defense Power Systems
NUMERICAL DATA
PLATINUM METAL ALLOYS
SERVICE LIFE
SURFACE PROPERTIES
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE
TEMPERATURE RANGE 1000-4000 K
THERMIONIC CONVERTERS
TUNGSTEN ALLOYS
WORK FUNCTIONS