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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Closed loop dynamics of in-core thermionic reactor systems.

Use of a point model of an in-core thermionic converter to investigate alternative schemes for providing closed-loop reactor control. It was found that schemes based on variable-gain power regulation buffers which use the reactor current as the control variable provide complete protection from thermionic burnout and also provide a virtually constant voltage to the user. A side benefit is that the emitter temperature transients are small; even for a complete electric load drop the emitter temperature transient is less than 100 K. The current regulation scheme was selected for further study with a distributed parameter model which was developed to account for variations in thermionic and heat transfer properties along the length of a cylindrical converter. It was found that, even though the emitter temperature distribution is about 200 K along the converter length, the dynamic properties are unchanged when using the current control scheme.

Sawyer, C. D.↗
NASA NTRS1972

Development of techniques and associated instrumentation for high temperature emissivity measurements

The calibrating and testing procedures for the radiometric emittance measurement apparatus are described. Test results are given for the NBS platinum-rhodium standard (SRM No. 1409), the NBS oxidized Kanthal standard, and the oxidized Rene 41 materials. Results indicate that with the platinum-rhodium standard the measurement system compared with the NBS data to within 0.02 for both total and spectral normal emitttance. Data from the Kanthal strip was consistently higher than NBS values. For oxidized Rene 41, no significant changes in as prepared emittance was observed. The emittance was stable for temperatures up to 1150 K.

Cunnington, G. R.↗
NASA NTRS1973

Fused Silica Surface Coating for a Flexible Silica Mat Insulation System

Fused silica insulation coatings have been developed for application to a flexible mat insulation system. Based on crystalline phase nucleation and growth kinetics, a 99+% SiO2 glass was selected as the base composition. A coating was developed that incorporated the high emissivity phase NiCr2O4 as a two phase coating with goals of high emittance and minimum change in thermal expansion. A second major coating classification has a plasma sprayed emittance coating over a sealed pure amorphous SiO2 layer. A third area of development centered on extremely thin amorphous SiO2 coatings deposited by chemical vapor deposition. The coating characterization studies presented are mechanical testing of thin specimens extracted from the coatings, cyclic arc exposures, and emittance measurements before and after arc exposures.

Rhodes, W. H.↗
NASA NTRS1974

Atmospheric effects on remote sensing of non-uniform temperature sources

The effects are considered of an absorbing, emitting, and scattering atmosphere upon the remote sensing of surface areas having non-uniform intensity. These atmospheric effects may be significant in determination, by remote sensing, of non-uniform surface temperature distributions, and the results of the investigation are applicable in such cases. Analytical methods and a digital computational program are presented, expressing the results in terms of contrast and contrast transmittance between two adjacent emitting areas having unequal intensities, in the presence of a additional disturbing emitters. In the computational procedure, emitting areas are replaced by point-source emitters, each assigned and effective intensity based upon the intensity of the area it replaces. Absorbing, emitting, and scattering behavior of the atmosphere may be specified in the computational procedure either by means of analytical atmospheric models or by means of calibrating ground level emitters.

Mcneill, W. A.↗
NASA NTRS1973

RFI analysis applied to the TDRSS system

The effect of radio frequency interference (RFI) on the proposed Tracking and Data Relay Satellite System (TDRSS) was assessed. The method of assessing RFI was to create a discrete emitter listing containing all the required parameters of transmitters in the applicable VHF and UHF frequency bands. The transmitter and spacecraft receiver characteristics were used to calculate the RFI contribution due to each emitter. The individual contributions were summed to obtain the total impact in the operational bandwidth. Using an as yet incomplete emitter base, it is concluded that the 136- to 137-MHz band should be used by TDRSS rather than the whole 136- to 138-MHz band because of the higher interference levels in the 137- to 138 MHz band. Even when restricting the link to 136 to 137 MHz, the existing link design is marginal, and it is recommended that interference reduction units, such as the adaptive digital filter, be incorporated in the TDRSS ground station.

Jenny, J. A.↗
NASA NTRS1975

Spectral reflectance properties of electroplated and converted zinc for use as a solar selective coating

The spectral reflectance properties of electroplated and chemically converted zinc were measured for both chromate and chloride conversion coatings. The reflectance properties were measured for various times of conversion and for conversion at various chromate concentrations. The values of absorptance, integrated over the solar spectrum, and of infrared emittance, integrated over black body radiation at 250 F were then calculated from the measured reflectance values. The interdependent variations of absorptance and infrared emittance were plotted. The results indicate that the optimum combination of the highest absorptance in the solar spectrum and the lowest emittance in the infrared of the converted electroplated zinc is produced by chromate conversion at 1/2 concentration of the standard NEOSTAR chromate black solution for 0.50 minute or by chloride conversion for 0.50 minute.

Mcdonald, G. E.↗
NASA NTRS1976

Argon hollow cathode

An interest in alternate propellants for ion-bombardment thrusters, together with ground applications of this technology, has prompted consideration of argon. Several variations of conventional hollow cathode designs were tried, but the bulk of the testing used a hollow tube with an internal tungsten emitter and an orifice at one end. The optimum cathode tube diameter was found to be in the range of 1.0-2.5 cm, somewhat larger than those used for cesium and mercury. Optimum orifice diameter depended on operating conditions, and varied from 0.5 to 5 mm. Biasing the internal emitter negative relative to the cathode chamber reduced the external coupling voltage and should therefore improve orifice lifetime. The expected effect of this bias on emitter lifetime was less clear. Lifetime tests were not conducted as part of this investigation, but several designs show promise of long lifetime in specific applications.

Rehn, L. A.↗
NASA NTRS1977

Optical properties of ion beam textured metals

Copper, silicon, aluminum, titanium and 316 stainless steel were textured by 1000 eV xenon ions from an 8-cm diameter electron bombardment ion source. Copper, silicon, and aluminum developed a cone structure with an average peak-to-peak distance ranging from 1 micron for silicon to 6 microns for aluminum. Titanium and 316 stainless steel developed a serpentine ridge structure. The average peak-to-peak distance for both of these materials was 0.5 micron. Total reflectance for air mass 0 and 2, solar absorptance, and total emittance, normalized for a 425 K black body were calculated from the reflectance measurements. All materials had solar absorptances between 0.93 (stainless steel) and 0.97 (silicon and aluminum). Total emittance values of 0.75, 0.80, and 0.85 for Si, Al, and Cu, respectively, resulted in a solar absorptance/total emittance ratio of approximately 1.2. Titanium and 316 stainless steel had values of 2.6 and 2.9, respectively, for this ratio.

Hudson, W. R.↗
NASA NTRS1977

Thermionic converter performance with oxide collectors

Thermionic converters using a variety of metal oxide collector surfaces have been fabricated and tested. Both work function and power output data are presented and evaluated. Oxides of barium, strontium, zinc, tungsten and titanium have been incorporated into a variable spacing converter. Tungsten oxide was found to give the highest converter performance and to furnish oxygen for the emitter at the same time. Oxygenated emitters operate at reduced cesium pressure with an increase in electrode spacing. Electron spectroscopy for chemical analysis (ESCA) performed on several tungsten oxide collectors showed cesium penetration of the oxide layer, possibly forming a cesium tungstate bronze. Titanium oxide showed high performance but did not furnish oxygen for the emitter; strontium oxide, in the form of a sprayed layer, appeared to dissociate in the presence of cesium. Sprayed coatings of barium and zinc oxides produced collector work functions of about 1.3 eV, but had excessive series resistance. Lanthanum hexaboride, in combination with oxygen introduced through a silver tube, and cesium produced a low work function collector and better than average performance.

Lieb, D.↗
NASA NTRS1977

Low arc drop hybrid mode thermionic converter

The hybrid mode operation for the reduction of plasma drops is being investigated. This report discusses the results obtained from two molybdenum emitter converters. One converter had a molybdenum collector and the other a nickel collector. The molybdenum collector converter was operated in a hybrid mode (at an interelectrode distance of 1.7 mm) and produced a minimum barrier index of 1.96 eV at an emitter temperature of 1500 K. The arc drop was calculated to be 0.14 eV, using the published results for a molybdenum collector. On the other hand, the nickel collector converter was operated in a conventional ignited mode (at an interelectrode distance of 0.5 mm) and produced a minimum barrier index of 2.1 eV at an emitter temperature of 1700 K. It is tentatively concluded that a large-gap operation of the hybrid mode converter permits the diffusion of cesium ions to a distance in the order of one millimeter for an effective neutralization of electron space charge. By employing a low work function collector (1.55 eV) in a hybrid mode converter with an arc drop of 0.14 eV, it appears that a barrier index as low as 1.69 eV could be achieved.

Shimada, K.↗
NASA NTRS1977

High-temperature, high-power-density thermionic energy conversion for space

Theoretic converter outputs and efficiencies indicate the need to consider thermionic energy conversion (TEC) with greater power densities and higher temperatures within reasonable limits for space missions. Converter-output power density, voltage, and efficiency as functions of current density were determined for 1400-to-2000 K emitters with 725-to-1000 K collectors. The results encourage utilization of TEC with hotter-than-1650 K emitters and greater-than-6W sq cm outputs to attain better efficiencies, greater voltages, and higher waste-heat-rejection temperatures for multihundred-kilowatt space-power applications. For example, 1800 K, 30 A sq cm TEC operation for NEP compared with the 1650 K, 5 A/sq cm case should allow much lower radiation weights, substantially fewer and/or smaller emitter heat pipes, significantly reduced reactor and shield-related weights, many fewer converters and associated current-collecting bus bars, less power conditioning, and lower transmission losses. Integration of these effects should yield considerably reduced NEP specific weights.

Morris, J. F.↗
NASA NTRS1978

Diminiode thermionic energy conversion with lanthanum-hexaboride electrodes

Thermionic conversion data obtained from a variable gap cesium diminiode with a hot pressed, sintered lanthanum hexaboride emitter and an arc melted lanthanum hexaboride collector are presented. Performance curves cover a range of temperatures: emitter 1500 to 1700 K, collector 750 to 1000 K, and cesium reservoir 370 to 510 K. Calculated values of emitter and collector work functions and barrier index are also given.

Kroeger, E. W.↗
NASA NTRS1978

Lithium and potassium heat pipes for thermionic converters

A prototypic heat pipe system for an out-of-core thermionic reactor was built and tested. The emitter of the concentric thermionic converter consists of the condenser of a tungsten heat pipe utilizing a lithium working fluid. The evaporator section of the emitter heat pipe is radiation heated to simulate the thermal input from the nuclear reactor. The emitter heat pipe thermal transport is matched to the thermionic converter input requirement. The collector heat pipe of niobium, 1% zirconium alloy uses potassium as the working fluid. The thermionic collector is coupled to the heat pipe by a tapered conical joint designed to minimize the temperature drop. The collector heat flux matches the design requirements of the thermionic converter.

Miskolczy, G.↗
NASA NTRS1978

ISEE-C solar wind plasma experiment

Two 135 deg spherical section electrostatic analyzers furnish electron and ion measurements of the solar wind on ISEE-C. Each of these instruments utilizes a divided secondary emitter system to intercept the analyzed particles. Secondary electrons selected from all of the emitters simultaneously provide fast two-dimensional measurements of the particle fluxes integrated over polar angle; at a slower rate secondary electrons are successively selected from individual emitters to provide three-dimensional measurements. Speed of the ion measurements is increased by a factor of two by using an active proton peak tracking system to reduce the total range of energy per charge which has to be covered.

Bame, S. J.↗
NASA NTRS1977

Studies of silicon pn junction solar cells

Modifications of the basic Shockley equations that result from the random and nonrandom spatial variations of the chemical composition of a semiconductor were developed. These modifications underlie the existence of the extensive emitter recombination current that limits the voltage over the open circuit of solar cells. The measurement of parameters, series resistance and the base diffusion length is discussed. Two methods are presented for establishing the energy bandgap narrowing in the heavily-doped emitter region. Corrections that can be important in the application of one of these methods to small test cells are examined. Oxide-charge-induced high-low-junction emitter (OCI-HLE) test cells which exhibit considerably higher voltage over the open circuit than was previously seen in n-on-p solar cells are described.

Lindholm, F. A.↗
NASA NTRS1979

Open-circuit voltage improvements in low-resistivity solar cells

Mechanisms limiting the open-circuit voltage in 0.1 ohm-cm solar cells were investigated. It was found that a rather complicated multistep diffusion process could produce cells with significantly improved voltages. The voltage capabilities of various laboratory cells were compared independent of their absorption and collection efficiencies. This was accomplished by comparing the cells on the basis of their saturation currents or, equivalently, comparing their voltage outputs at a constant current-density level. The results show that for both the Lewis diffused emitter cell and the Spire ion-implanted emitter cell the base component of the saturation current is voltage controlling. The evidence for the University of Florida cells, although not very conclusive, suggests emitter control of the voltage in this device. The data suggest further that the critical voltage-limiting parameter for the Lewis cell is the electron mobility in the cell base.

Godlewski, M. P.↗
NASA NTRS1979

Multi-channel rotating optical interface for data transmission

Apparatus for transmitting multiple channels of data across a rotating interface, such as between an antenna that rotates with respect to a platform, is described. Features of the apparatus include: (1) light emitter elements and light detector elements located on the two bodies that rotate relative to each other; (2) a lens for focusing light from each emitter element onto a corresponding detector element; and (3) an image rotating means which is turned as one of the objects rotates, to derotate the images of the emitter elements that are to be focused on the detector elements.

Ivie, C. V.↗
NASA NTRS1978

Lithium and potassium heat pipes for thermionic converters

A prototypic heat pipe system for an out-of-core thermionic reactor has been built and tested. The emitter of the concentric thermionic converter consists of the condenser of a tungsten heat pipe utilizing a lithium working fluid. The evaporator section of the emitter heat pipe is radiation heated to simulate the thermal input from the nuclear reactor. The emitter heat pipe thermal transport is matched to the thermionic converter input requirement. The collector heat pipe of niobium, 1%-zirconium alloy uses potassium as the working fluid. The thermionic collector is coupled to the heat pipe by a tapered conical joint designed to minimize the temperature drop. The area ratio of the evaporator to condenser is 16:1, which increases the radiation area. The composite wick structure consists of seven arteries and cylindrical wraps. The collector heat flux matches the design requirements of the thermionic converter.

Miskolczy, G.↗