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At least 379 records · Page 21

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NASA NTRS1980

Development of a preprototype thermoelectric integrated membrane evaporation subsystem for water recovery

A three-man urine water recovery preprototype subsystem using a new concept to provide efficient potable water recovery from waste fluids on extended duration space flights has been designed, fabricated, and tested. Low power, compactness, and gravity insensitive operation are featured in this vacuum distillation subsystem that combines a hollow fiber polysulfone membrane evaporator with a thermoelectric heat pump. Application and integration of these key elements have solved problems inherent in previous reclamation subsystem designs. The hollow fiber elements provide positive liquid/gas phase control with no moving parts other than a waste liquid recirculation pump and a product water withdrawal pump. Tubular membranes provide structural integrity, improving on previous flat sheet membrane designs. A thermoelectric heat pump provides latent energy recovery.

Winkler, H. E.↗

NASA NTRS1980

Electric energy production by particle thermionic-thermoelectric power generators

Thermionic-thermoelectric power generators, composed of a thin layer of porous, low work function material separating a heated emitter electrode and a cooler collector electrode, have extremely large Seebeck coefficients of over 2 mV/K and can provide significant output power. Preliminary experiments with 20-micron thick (Ba Sr Ca)O coatings, limited by evaporative loss to temperatures below 1400 K, have yielded short circuit current densities of 500 mA/sq cm and power densities of 60 mW/ sq cm. Substantially more output is expected with cesium-coated refractory oxide particle coatings operating at higher temperatures. Practical generators will have thermal-to-electrical efficiencies of 10 to 20%. Further increases can be gained by cascading these high-temperature devices with lower temperature conventional thermoelectric generators.

Oettinger, P. E.↗

NASA NTRS1981

Advanced high temperature thermoelectrics for space power

Preliminary results from a spacecraft system study show that an optimum hot junction temperature is in the range of 1500 K for advanced nuclear reactor technology combined with thermoelectric conversion. Advanced silicon germanium thermoelectric conversion is feasible if hot junction temperatures can be raised roughly 100 C or if gallium phosphide can be used to improve the figure of merit, but the performance is marginal. Two new classes of refractory materials, rare earth sulfides and boron-carbon alloys, are being investigated to improve the specific weight of the generator system. Preliminary data on the sulfides have shown very high figures of merit over short temperature ranges. Both n- and p-type doping have been obtained. Pure boron-carbide may extrapolate to high figure of merit at temperatures well above 1500 K but not lower temperature; n-type conduction has been reported by others, but not yet observed in the JPL program. Inadvertant impurity doping may explain the divergence of results reported.

Lockwood, A.↗

NASA NTRS1982

Thermoelectric integrated membrane evaporation water recovery technology

The recently developed Thermoelectric Integrated Membrane Evaporation Subsystem (TIMES) offers a highly competitive approach to water recovery from waste fluids for future on-orbit stations such as the Space Operations Center. Low power, compactness and gravity insensitive operation are featured in this vacuum distillation subsystem that combines a hollow fiber membrane evaporator with a thermoelectric heat pump. The hollow fiber elements provide positive liquid/gas phase control with no moving parts other than pumps and an accumulator, thus solving problems inherent in other reclamation subsystem designs. In an extensive test program, over 850 hours of operation were accumulated during which time high quality product water was recovered from both urine and wash water at an average steady state production rate of 2.2 pounds per hour.

Roebelen, G. J., Jr.↗

NASA NTRS1984

Thermoelectric temperature control system for the pushbroom microwave radiometer (PBMR)

A closed loop thermoelectric temperature control system is developed for stabilizing sensitive RF integrated circuits within a microwave radiometer to an accuracy of + or - 0.1 C over a range of ambient conditions from -20 C to +45 C. The dual mode (heating and cooling) control concept utilizes partial thermal isolation of the RF units from an instrument deck which is thermally controlled by thermoelectric coolers and thin film heaters. The temperature control concept is simulated with a thermal analyzer program (MITAS) which consists of 37 nodes and 61 conductors. A full scale thermal mockup is tested in the laboratory at temperatures of 0 C, 21 C, and 45 C to confirm the validity of the control concept. A flight radiometer and temperature control system is successfully flight tested on the NASA Skyvan aircraft.

Dillon-Townes, L. A.↗

NASA NTRS1984

Materials for high-temperature thermoelectric conversion

The development of materials for high temperature thermoelectric energy conversion devices was investigated. The development of new criteria for the selection of materials which is based on understanding of the fundamental principles governing the behavior of high temperature thermoelectric materials is discussed. The synthesis and characterization of promising new materials and the growth of single crystals to eliminate possible problems associated with grain boundaries and other defects in polycrystalline materials are outlined.

Feigelson, R. S.↗

NASA NTRS1983

Present state of boron-carbon thermoelectric materials

Boron-carbon p-type thermoelectric materials show promise for use in advanced thermal-to-electric space power conversion systems. Here, recent data on the thermoelectric properties of boron-carbon materials, such as B9C, B13C2, B15C2, and B4C, are reviewed. In particular, attention is given to the effect of the compositional homogeneity and residual impurity content on the Seeback coefficient, electrical resistivity, and thermal conductivity of these materials. The effect of carbon content for a given level of impurity and degree of homogeneity is also discussed.

Elsner, N. B.↗

NASA NTRS1983

Sublimation measurements and analysis of high temperature thermoelectric materials and devices

High temperature thermoelectric device sublimation effects are compared for rare earth sulfides, selenides, and state-of-the-art Si-Ge alloys. Although rare earth calcogenides can potentially exhibit superior sublimation characteristics, the state-of-the-art Si-Ge alloy with silicon nitride sublimation-inhibitive coating has been tested to 1000 C. Attention is given to the ceramic electrolyte cells, forming within electrical and thermal insulation, which affect leakage conductance measurements in Si-Ge thermoelectric generators.

Shields, V.↗

NASA NTRS1983

Thermoelectric properties of non-stoichiometric lanthanum sulfides

The lanthanum sulfides are promising candidate materials for high-efficiency thermoelectric applications at temperatures up to 1300 C. The non-stoichiometric lanthanum sulfides (LaS(x), where x is in the range 1.33-1.50) appear to possess the most favorable thermoelectric properties. The Seebeck coefficient and resistivity vary significantly with composition, so that an optimum value of alpha sq/rho (where alpha is the Seebeck coefficient and rho is the resistivity) can be chosen. The thermal conductivity remains approximately constant with stoichiometry, so a material with an optimum value of alpha sq/rho should possess the optimum figure-of-merit. Data for the Seebeck coefficient and electrical resistivity of non-stoichiometric lanthanum sulfides will be pressed, together with structural properties of these materials.

Shapiro, E.↗

NASA NTRS1984

Thermoelectric Integrated Membrane Evaporation Subsystem operational improvements

A three-man preprototype Thermoelectric Integrated Membrane Evaporation Subsystem (TIMES) has been developed to provide high quality water recovery from waste fluids on extended duration space flights. In the most recent effort, a number of improvements have been made to simplify subsystem operation and increase performance. These modifications include changes to the hollow fiber membrane evaporator, the condensing section of the thermoelectric heat pump, and the electronic controller logic and display. This paper describes the results of the test program that was conducted to evaluate the implemented improvements. In addition, an advanced design concept is discussed that will provide lower electrical power consumption, greater water production capacity, lower weight, and a smaller package than the present subsystem configuration.

Dehner, G. F.↗

NASA NTRS1985

Thermoelectric properties of rare earth chalcogenides

The rare earth chalcogenides are important thermoelectric materials due to their high melting points, self-doping capabilities, and low thermal conductivities. Lanthanum sulfides and lanthanum tellurides have been synthesized in quartz ampules, hot-pressed into samples, and measured. The n-type Seebeck coefficients, electrical resistivities, and power factors generally all increased as the temperature increased from 200 to 1000 C. The figure-of-merit for nonstoichiometric lanthanum telluride was 0.001/deg C at 1000 C, considerably higher than for silicon-germanium. Thermoelectric measurements were made for LaTe(2) and YbS(1.4), and p-type behavior was observed for these compounds from 300 to 1100 C.

Danielson, L. R.↗

NASA NTRS1987

High-Temperature Thermoelectric Energy Conversion

Theory of thermoelectric energy conversion at high temperatures and status of research on conversion materials reviewed in report. Shows highest values of thermoelectric figure of merit, Z, found in semiconductor materials. Semiconductors keep wide choice of elements and compounds. Electrical properties tailored to particular application by impurity doping and control of stoichiometry. Report develops definition of Z useful for comparing materials and uses it to evaluate potentials of different classes of materialsmetals, semiconductors, and insulators.

Wood, C.↗

NASA NTRS1987

Advances In Thermoelectric Materials

Rare-earth chalcogenides and boron-rich borides show promise. Report discusses search for materials performing more efficiently in high-temperature thermoelectric conversion. Begins with summary of developments during past three decades. Followed by brief introduction to theory of thermoelectric energy conversion, then by experimental findings and by some additional theory on specific materials.

Wood, Charles↗

NASA NTRS1986

Thermoelectric converter for SP-100 space reactor power system

Conductively coupling the thermoelectric converter to the heat source and the radiator maximizes the utilization of the reactor and radiator temperatures and thereby minimizes the power system weight. This paper presents the design for the converter and the individual thermoelectric cells that are the building block modules for the converter. It also summarizes progress on the fabrication of initial cells and the results obtained from the preparation of a manufacturing plan. The design developed for the SP-100 system utilizes thermally conductive compliant pads that can absorb the displacement and distortion caused by the combinations of temperatures and thermal expansion coefficients. The converter and cell designs provided a 100 kWe system which met the system requirements. Initial cells were fabricated and tested.

Terrill, W. R.↗

NASA NTRS1988

Materials for thermoelectric energy conversion

The field of thermoelectric energy conversion is reviewed from both a theoretical and an experimental standpoint. The basic theory is introduced and the thermodynamic and solid state views are compared. An overview of the development of thermoelectric materials is presented with particular emphasis being placed on the most recent developments in high-temperature semiconductors. A number of possible device applications are discussed and the successful use and suitability of these devices for space power is manifest.

Wood, C.↗

NASA NTRS1991

Thermal Strap And Cushion For Thermoelectric Cooler

Inexpensive cushioning strap proposed for use as thermal contact between thermoelectric cooler and device to be cooled, such as laser diode, infrared detector, or charge-coupled device for imaging. Provides high thermal conductance while minimizing thermal and mechanical stresses on thermoelectric cooler. Used as alternative to flexible thermal strap made of silver.

Jones, Jack A.↗

NASA NTRS1990

High figure of merit thermoelectrics - Theoretical considerations

The thermoelectric figure of merit of a semiconductor, ZT, can be calculated from a small number of microscopic material parameters, the material composition, the doping level, and the temperature. The functional dependence of ZT on these parameters has been studied for a range of material parameters using a recently developed model which accurately and self-consistently describes the thermoelectric properties of n-type silicon-germanium alloys. ZT values several times larger than current state-of-the-art values of ZT of about 1 are shown to be entirely consistent with existing theory, even using material parameters already observed. A search for materials with much higher figure of merit values therefore remains of interest, in spite of several decades of relatively slow progress in this area.

Vining, Cronin B.↗

NASA NTRS1990

Effect of neutron irradiation on the thermoelectric properties of SiGe alloys

Zone-leveled and hot-pressed n- and p-type Si80Ge20 alloys were irradiated with neutrons to a fluence of 4 x 1018 n/sq cm and to a fluence of 5.4 x 1019 n/sq cm at a temperature of approximately 200-300 C. The effect of neutron irradiation on the thermoelectric properties of these alloys was evaluated. The carrier concentration and mobility (and hence the resistivity) were measured at room temperature while the thermal diffusivity was measured at 177-192 C both before and after the irradiation and after each subsequent 2-h heat treatment at 350 C, 600, and 1000 C. The irradiation increased the resistivity significantly, but the thermal conductivity decreased only by about 10-15 percent. This tends to indicate that the radiation produced only small defects (single pairs and small vacancy chains). The samples all returned to almost exactly their preirradiation state after the 1000 C anneal. This indicates that SiGe alloys can be operated in this neutron fluence at high temperatures without a degradation of thermoelectric properties.

Vandersande, Jan W.↗