INVESTIGATIONS OF CARBIDES AS CATHODES FOR THERMIONIC SPACE REACTORS. Final Report, May 15, 1961-August 31, 1962

Technical Report · Thu Oct 31 04:00:00 EST 1963

OSTI ID:4751746

Weinberg, A F; Hudson, R G; Yang, L; Carpenter, F D

The physicochemical properties of mixed carbides of U and Zr were measured to determine the usefulness of these materials as nuclear-heated thermionic emitters or as the nuclear fuel for refractory-metal thermionic cells. Property measurements of other carbide systems having potential as thermionic emitters were also performed. Vaporloss data for mixed carbides were obtained in vacuum to determine the effects of temperature, time of exposure, U: Zr ratio, and specimen density. At 1988 to 2083 deg K, the rate of weight loss for specimens of nominal composition 90 mol-% UC-10 mol-% ZrC can be expressed as log w = mg/cm/sup 2//sec and T is the absolute temperature. At 2130 to 2400 deg K, the rate of weight loss for specimens with compositions ranging from 14 mol-% UC-- 86 mol-% ZrC to 19 mol-% UC--81 mol-% ZrC can be expressed as log w- = (0.06 to 0.09 Torr) Cs-vapor environment was found to have no effect on the rate of weight loss. Studies were also conducted on the redistribution of the carbide components during long-time exposures and the relation of this redistribution to the rate of weight loss and the electron-emission characteristics. The rate of weight loss for a specimen of nominal composition 20 mol-% UC-80 mol-% HfC was determined at 2063 deg plus or minus 11 deg K. Vacuum electron-emission measurements were made for a series of mixed carbides of varying U: Zr ratio. It was determined that these carbides behave as dispenser cathodes whose emission is not dependent upon the bulk U content. Methods of disturbing the Umetal monolayer on the specimen surface, which is requisite to good electron emission, were studied, as were methods of reactivating'' the layer after it had been removed. Electron emission from all of the carbides attained the useful level of 5 amp/cm/sup 2/ at temperatures below 2000 deg K. In this temperature range, the materials loss from specimens of high UC content would be less than 3 mils/yr and that from specimens of low UC content less than 1 mil/yr. Materials compatibility as related to the marriage'' of reactor fuels to refractory-metal thermionic emitters was studied. Four metals---W, Mo, Nb, and Ta were studied in combination with UC, UC/sub 2/, and UZrC at 1200 to 2000 deg C for times up to 1000 hr. The tests showed that only two of these materials combinations are compatible at temperatures to 1800 deg C: W with UC (or UZrC mixtures), and Mo with UZrC mixtures of low UC content. Barrier layers of Re, W--25 wt-% Re alloy, and ZrC were also investigated, with only ZrC proving effective for this application. To provide data for the engineering design of thermionic reactors using mixed carbides, measurements were also made of the electrical and thermal conductivities of these materials at temperatures up to reactor operating ranges. (auth)

• Research Organization: General Atomic Div. General Dynamics Corp., San Diego, Calif.
• NSA Number: NSA-17-005577
• OSTI ID: 4751746
• Report Number(s): GA-3523
• Country of Publication: United States
• Language: English

Related Subjects

CARBIDES
CATHODES
CESIUM
CURRENTS
DENSITY
ELECTRIC CONDUCTIVITY
ELECTRONS
ENRICHMENT
ENVIRONMENT
HAFNIUM CARBIDES
HEATING
LAYERS
LOSSES
MATERIALS TESTING
MEASURED VALUES
METALS
MIXING
MOLYBDENUM
NESDPS Office of Nuclear Energy Space and Defense Power Systems
NIOBIUM
PLANNING
PRESSURE
PRODUCTION
REACTOR TECHNOLOGY
REFRACTORIES
RHENIUM
RHENIUM ALLOYS
SPACE FLIGHT
SURFACES
TANTALUM
TEMPERATURE
THERMAL CONDUCTIVITY
THERMIONICS
TUNGSTEN
TUNGSTEN ALLOYS
URANIUM
URANIUM CARBIDES
USES
VACUUM
VAPORS
WEIGHT
ZIRCONIUM CARBIDES