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Fatigue acceptance test limit criterion for larger diameter rolled thread fasteners

Description: This document describes a fatigue lifetime acceptance test criterion by which studs having rolled threads, larger than 1.0 inches in diameter, can be assured to meet minimum quality attributes associated with a controlled rolling process. This criterion is derived from a stress dependent, room temperature air fatigue database for test studs having a 0.625 inch diameter threads of Alloys X-750 HTH and direct aged 625. Anticipated fatigue lives of larger threads are based on thread root elastic stress concentration factors which increase with increasing thread diameters. Over the thread size range of interest, a 30% increase in notch stress is equivalent to a factor of five (5X) reduction in fatigue life. The resulting diameter dependent fatigue acceptance criterion is normalized to the aerospace rolled thread acceptance standards for a 1.0 inch diameter, 0.125 inch pitch, Unified National thread with a controlled Root radius (UNR). Testing was conducted at a stress of 50% of the minimum specified material ultimate strength, 80 Ksi, and at a stress ratio (R) of 0.10. Limited test data for fastener diameters of 1.00 to 2.25 inches are compared to the acceptance criterion. Sensitivity of fatigue life of threads to test nut geometry variables was also shown to be dependent on notch stress conditions. Bearing surface concavity of the compression nuts and thread flank contact mismatch conditions can significantly affect the fastener fatigue life. Without improved controls these conditions could potentially provide misleading acceptance data. Alternate test nut geometry features are described and implemented in the rolled thread stud specification, MIL-DTL-24789(SH), to mitigate the potential effects on fatigue acceptance data.
Date: May 1, 1997
Creator: Kephart, A.R.
Partner: UNT Libraries Government Documents Department

The effect of neutron irradiation on silicon carbide fibers

Description: Nine types of SiC fiber have been exposed to neutron radiation in the Advanced Test Reactor at 250 C for various lengths of time ranging from 83 to 128 days. The effects of these exposures have been initially determined using scanning electron microscopy. The fibers tested were Nicalon{trademark} CG, Tyranno, Hi-Nicalon{trademark}, Dow Corning SiC, Carborundum SiC, Textron SCS-6, polymethysilane (PMS) derived SiC from the University of Michigan, and two types of MER SiC fiber. This covers a range of fibers from widely used commercial fibers to developmental fibers. Consistent with previous radiation experiments, Nicalon fiber was severely degraded by the neutron irradiation. Similarly, Tyranno suffered severe degradation. The more advanced fibers which approach the composition and properties of SiC performed well under irradiation. Of these, the Carborundum SiC fiber appeared to perform the best. The Hi-Nicalon and Dow Corning Fibers exhibited good general stability, but also appear to have some surface roughening. The MER fibers and the Textron SCS-6 fibers both had carbon cores which adversely influenced the overall stability of the fibers.
Date: January 1, 1997
Creator: Newsome, G.A.
Partner: UNT Libraries Government Documents Department

Bridgman growth and characterization of bulk single crystals of Ga{sub 1{minus}x}In{sub x}Sb for thermophotovoltaic applications

Description: Thermophotovoltaic generation of electricity is attracting renewed attention due to recent advances in low bandgap (0.5--0.7 eV) III-V semiconductors. The use of mixed pseudo-binary compounds allows for the tailoring of the lattice parameter and the bandgap of the material. Conventional deposition techniques (i.e., epitaxy) for producing such ternary or quaternary materials are typically slow and expensive. Production of bulk single crystals of ternary materials, for example Ga{sub 1{minus}x}In{sub x}Sb, is expected to dramatically reduce such material costs. Bulk single crystals of Ga{sub 1{minus}x}In{sub x}Sb have been prepared using a Bridgman technique in a two-zone furnace. These crystals are 19 mm in diameter by approximately 50 mm long and were produced using seeds of the same diameter. The effects of growth rate and starting materials on the composition and quality of these crystals will be discussed and compared with other attempts to produce single crystals of this material.
Date: December 1, 1997
Creator: Boyer, J.R. & Haines, W.T.
Partner: UNT Libraries Government Documents Department

{sup 16}O neutron cross section evaluation

Description: This work has resulted from a need to compute more accurately the neutron scattering cross sections and angular distributions for {sup 16}O. Several oxygen evaluations have been performed in the past with R-Matrix theory, including ENDF/B-V and ENDF/B-VI. ENDF/B-VI is an improvement over ENDF/B-V, but still underpredicts in general the forward scattering of neutrons below 2.5 MeV. R-Matrix theory is used in describing cross sections at and near the resonance energies; but may not always be adequate in describing cross sections between resonances, especially when they are widely spaced. The optical (potential well) model of the nucleus is very good in representing cross sections that vary smoothly with energy, but not at describing all of the detailed resonance cross sections. A combination of the potential well model and R-Matrix theory was used for this work to represent cross sections with isolated resonances with large spacings between them. The total neutron cross section of oxygen-16 below 3.0 MeV has widely separated resonances and a dip in the cross section at 2.35 MeV. In the vicinity of resonances, where cross sections vary rapidly with energy, R-Matrix theory has been successful in fitting experimental data. In the region between resonances, an analytical procedure with physical basis is needed that agrees with data over a wide range of energies bracketing regions where experimental measurements are lacking.
Date: June 1, 1998
Creator: Caro, E.
Partner: UNT Libraries Government Documents Department

LDV measurement and Navier-Stokes computation of parallel jet mixing in a rectangular confinement

Description: Laser Doppler Velocimetry (LDV) measurements were taken in a rectangular confinement into which issues a row of parallel jets. Two-component measurements were taken with two optics orientations yielding three mean velocity components and four Reynolds stress components. As observed in isolated three dimensional wall bounded jets, the transverse diffusion of the jets is quite large. The data indicates that this rapid mixing process is due to strong secondary flows, transport of large inlet intensities and Reynolds stress anisotropy effects. Navier-Stokes analyses of this configuration underpredict the rate of transverse jet diffusion. Detailed numerical accuracy studies show that this is attributed to shortcomings in low-Reynolds number two-equation turbulence modelling. A low-Reynolds number full-Reynolds stress model is shown to provide improvement.
Date: June 1, 1995
Creator: Kunz, R.F.; D`Amico, S.W.; Vassallo, P.F.; Zaccaria, M.A.; Aksoy, H. & So, R.M.C.
Partner: UNT Libraries Government Documents Department

Neutron cross sections for uranium-235 (ENDF/B-IV Release 3)

Description: The resonance parameters in ENDF6 (Release 2) U235 were adjusted to make the average capture and fission cross sections below 900 eV agree with selected differential capture and fission measurements. The measurements chosen were the higher of the credible capture measurements and the lower of the fission results, yielding a higher epithermal alpha. In addition, the 2200 m/s cross sections were adjusted to obtain agreement with the integral value of K1. As a result, criticality calculations for thermal benchmarks, and agreement with a variety of integral parameters, are improved.
Date: September 1, 1996
Creator: Lubitz, C. R.
Partner: UNT Libraries Government Documents Department

Performance status of 0.55 eV InGaAs thermophotovoltaic cells

Description: Data on {approximately} 0.55 eV In{sub 0.72}Ga{sub 0.28}As cells with an average open-circuit voltage (Voc) of 298 mV (standard deviation 7 mV) at an average short-circuit current density of 1.16 A/cm{sup 2} (sdev. 0.1 A/cm{sup 2}) and an average fill-factor of 61.6% (sdev. 2.8%) is reported. The absorption coefficient of In{sub 0.72}Ga{sub 0.28}As was measured by a differential transmission technique. The authors use a numerical integration of the absorption data to determine the radiative recombination coefficient for In{sub 0.72}Ga{sub 0.28}As. Using this absorption data and simple one-dimensional analytical formula the above cells are modeled. The models show that the cells may be limited more by Auger recombination rather than Shockley-Read-Hall (SRH) recombination at dislocation centers caused by the 1.3% lattice mismatch of the cell to the host InP wafer.
Date: October 1, 1998
Creator: Wojtczuk, S.; Colter, P.; Charache, G. & DePoy, D.
Partner: UNT Libraries Government Documents Department

Recent progress in GaInAsSb thermophotovoltaics grown by organometallic vapor phase epitaxy

Description: Studies on the materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for thermophotovoltaic (TPV) devices are reviewed. Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, substrate misorientation, and to a lesser extent, growth rate. A mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.5 {micro}m. The crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content, as based on epilayer surface morphology and low temperature PL spectra. A trend of smaller full width at half-maximum for low temperature PL spectra is observed as the growth rate is increased from 1.5 to 2.5 and 5 {micro}m/h. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited overall better material quality than layers grown on the more standard substrate (100)2{degree} toward (110). Consistent growth of high performance lattice-matched GaInAsSb TPV devices is also demonstrated.
Date: June 1, 1998
Creator: Wang, C.A.; Choi, H.K.; Oakley, D.C. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

GaInSb and GaInAsSb thermophotovoltaic device fabrication and characterization

Description: Thermophotovoltaic (TPV) devices have been fabricated using epitaxial ternary and quaternary layers grown on GaSb substrates. The GaInSb layers were grown by organometallic vapor phase epitaxy (OMVPE) and the InGaAsSb lattice-matched layers were grown by liquid phase epitaxy (LPE). Device fabrication steps include unannealed p-type ohmic contacts, annealed Sn/Au n-type ohmic contacts, and a thick Ag top-surface contact using a lift-off process. Devices are characterized primarily by dark I-V, photo I-V, and quantum efficiency measurements, which are correlated to microscopic and macroscopic material properties. Particular emphasis has been on material enhancements to increase quantum efficiency and decrease dark saturation current density. TPV device performance is presently limited by the base diffusion length, typically 1 to 2 microns.
Date: May 1, 1997
Creator: Hitchcock, C.; Gutmann, R.; Borrego, J.; Ehsani, H.; Bhat, I.; Freeman, M. et al.
Partner: UNT Libraries Government Documents Department

Theoretical prediction of the plasma frequency and Moss-Burstein shifts for degenerately doped In{sub x}Ga{sub 1{minus}x}As

Description: Theoretical predictions for the plasma frequency and Moss-Burstein shift (optical band gap) of degenerately doped (n > 10{sup 19} cm{sup {minus}3}) In{sub x}Ga{sub 1{minus}x} As are presented. This system is of interest because it possesses desirable optical properties for thermophotovoltaic (TPV) applications. The studies presented are based on electronic band structures calculated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method which includes non-local screened exchange (sX-LDA) and spin-orbit effects. The plasma frequency and Moss-Burstein shift are calculated vs. doping assuming a rigid band approximation (i.e. conduction band filling of the undoped bands). The doping dependence of the effective mass (band non-parabolicity) plays an important role at the high dopings considered here. This effect leads to a maximum in the plasma frequency vs. doping (2--3 {times} 10{sup 14}/s) and a significant departure from the constant effective mass prediction for the optical band gap vs. doping. These calculations are in good agreement with measurements.
Date: October 1, 1998
Creator: Raynolds, J. E.; Charache, G. W.; Geller, C. B.; Holden, T. & Pollak, F. H.
Partner: UNT Libraries Government Documents Department

Microstructural evaluation of Sb-adjusted Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer layer systems for IR applications

Description: The authors report on a transmission electron microscopy (TEM) study of Sb-adjusted quaternary Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer-layers grown on <001> GaAs substrates. A series of structures were grown by MBE at 470 C that utilize a multilayer grading scheme in which the Sb content of Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer-layers grown on <001> GaAs substrates. A series of structures were grown by MBe at 470 C that utilize a multilayer grading scheme in which the Sb content of Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} is successively increased in a series of 125 nm thick layers. Post growth analysis using conventional bright field and weak beam dark field imaging of these buffer layers in cross-section reveals that the interface misfit dislocations are primarily of the 60{degree} type and are distributed through out the interfaces of the buffer layer. When optimized, the authors have shown, using plan view and cross-sectional TEM, that this approach can reduce the threading defect density to below the detectability limit of TEM (< 10{sup 5}/cm{sup 2}) and preserve growth surface planarity. The Sb-graded approach was used to fabricate two 2.2 {micro}m power converter structures fabricated using InGaAs grown on Sb-based buffer layers on GaAs substrates. A microstructural and electrical characterization was performed on these device structures and the results are contrasted with a sample in which InP was selected as the substrate. Microstructure, defect density and device performance in these not-yet-optimized Sb-based buffer layers compares favorably to equivalent devices fabricated using InP substrates.
Date: June 1, 1998
Creator: Chen, E.; Paine, D.C.; Uppal, P.; Ahearn, J.S.; Nichols, K. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Real time programming environment for Windows

Description: This document provides a description of the Real Time Programming Environment (RTProE). RTProE tools allow a programmer to create soft real time projects under general, multi-purpose operating systems. The basic features necessary for real time applications are provided by RTProE, leaving the programmer free to concentrate efforts on his specific project. The current version supports Microsoft Windows{trademark} 95 and NT. The tasks of real time synchronization and communication with other programs are handled by RTProE. RTProE includes a generic method for connecting a graphical user interface (GUI) to allow real time control and interaction with the programmer`s product. Topics covered in this paper include real time performance issues, portability, details of shared memory management, code scheduling, application control, Operating System specific concerns and the use of Computer Aided Software Engineering (CASE) tools. The development of RTProE is an important step in the expansion of the real time programming community. The financial costs associated with using the system are minimal. All source code for RTProE has been made publicly available. Any person with access to a personal computer, Windows 95 or NT, and C or FORTRAN compilers can quickly enter the world of real time modeling and simulation.
Date: April 1, 1998
Creator: LaBelle, D.R.
Partner: UNT Libraries Government Documents Department

Influence of dissolved hydrogen on nickel alloy SCC in high temperature water

Description: Stress corrosion crack growth rate (SCCGR) tests of nickel alloys were conducted at 338 C and 360 C as a function of the hydrogen concentration in high purity water. Test results identified up to a 7 x effect of hydrogen levels in the water on crack growth rate, where the lowest growth rates were associated with the highest hydrogen levels. At 338 C, the crack growth rate decreased as the hydrogen levels were increased. However, different results were observed for the test conducted at 360 C. As the hydrogen level was increased in the 360 C tests, the crack growth rate initially increased, a maximum was exhibited at a hydrogen level of {approximately} 20 scc/kg, and thereafter the crack growth rate decreased. Based on this testing and a review of the commercial literature, the thermodynamic stability of nickel oxide, not the dissolved hydrogen concentration, was identified as a fundamental parameter influencing the susceptibility of nickel alloys to SCC. These test results are discussed in relation to the accuracy of extrapolating high temperature SCC results to lower temperatures.
Date: March 1, 1999
Creator: Morton, D.S.; Attanasio, S.A.; Fish, J.S. & Schurman, M.K.
Partner: UNT Libraries Government Documents Department

Bulk crystal growth of antimonide based III-V compounds for thermophotovoltaics applications

Description: In this paper, the bulk growth of crack-free GaInSb and single phase GaInAsSb alloys are presented. A new class of III-V quasi-binary [A{sub III}B{sub V}]{sub 12{minus}x}[C{sub III}D{sub V}]{sub x} semiconductor alloys has been synthesized and bulk crystals grown from the melt for the first time. The present investigation is focused on the quasi-binary alloy (GaSb){sub 1{minus}x}(InAs){sub x} (0 < x < 0.05) due to its importance for thermophotovoltaic applications. The structural properties of this melt-grown quasi-binary alloy are found to be significantly different from the conventional quaternary compound Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} with composition x = y. Synthesis and growth procedures are discussed. For the growth of ternary alloys, it was demonstrated that forced convection or mixing in the melt during directional solidification of In{sub x}Ga{sub 1{minus}x}Sb (0 < x < 0.1) significantly reduces cracks in the crystals.
Date: October 1, 1998
Creator: Dutta, P.S.; Ostrogorsky, A.G. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

Neutron detector resolution for scattering

Description: A resolution function has been determined for scattered neutron experiments at Rensselaer Polytechnic Institute (RPI). This function accounts for the shifting and broadening of the resonance peak due to the additional path length, traveled by the neutron after scattering and prior to detection, along with the broadening of the resonance peak due to the bounce target. This resolution function has been parameterized both in neutron energy and size of the sample disk. Monte Carlo Neutron and Photon (MCNP) modeling has been used to determine the shape of the detector resolution function while assuming that the sample nucleus has an infinite mass. The shape of the function for a monoenergetic neutron point source has been compared to the analytical solution. Additionally, the parameterized detector resolution function has been used to broaden the scatter yield calculated from Evaluated Neutron Data File ENDF/B-VI cross section data for {sup 238}U. The target resolution function has been empirically determined by comparison of the broadened scatter yield and the experimental yield for {sup 238}U. The combined resolution function can be inserted into the SAMMY code to allow resonance analysis for scattering measurements.
Date: March 1, 1997
Creator: Kolda, S.A.
Partner: UNT Libraries Government Documents Department

RF/microwave non-destructive measurements of electrical properties of semiconductor wafers for thermophotovoltaic applications

Description: A radio-frequency/microwave measurement system has been designed for non-contacting determination of sheet resistance and excess carrier lifetime of low-bandgap materials and junctions, specifically GaSb-based alloys for thermophotovoltaic (TPV) applications. The design incorporates RF circuitry in the 100--500 MHz frequency range and utilizes a Q-switched YAG laser at 1.32 microns to photo-generate electron-hole pairs and conductivity modulate the material and/or junction under test. Supplementary measurements with a GaAs pulsed diode laser at 904 nm provides a faster transient response with near-surface photogeneration. Initial measurements on GaSb substrates, Zn-diffused materials and epitaxially grown layers are presented and discussed.
Date: May 1, 1997
Creator: Saroop, S.; Borrego, J.M. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

MBE growth of GaInAsSb p/n junction diodes for thermophotovoltaic applications

Description: This paper reports recent progress in the development of quaternary III-V thermophotovoltaic (TPV) devices based on MBE grown Ga{sub x}In{sub 1{minus}x}As{sub y}Sb{sub 1{minus}y}. TPV is of great interest for a variety of applications. The objective of this work is to develop a TPV cell which is tunable to the emission spectrum of a heated blackbody, at temperatures in the range of 1200--1473 K. One aspect of this tuning is to match the band gap, E{sub gap}, of the photovoltaic device to the peak output of the heat source., An advantage of the quarternary III-V semiconductor systems is that devices can be fabricated by molecular beam epitaxy on a suitable binary substrate, such as GaSb or InAs, and the band gap and lattice constant can be adjusted more or less independently, to match requirements. Quarternary cells, with band-gaps in the 0.5 to 0.72 eV range, have been fabricated and tested. For 0.54 eV devices the authors obtained V{sub oc} = 0.3 V and I{sub sc} = 1.5 amperes/cm{sup 2} under infrared illumination of a 1200 K blackbody. Under high illumination levels the V{sub oc} and I{sub sc} ranged from 0.5 V at 3 amperes/cm{sup 2} for 0.72 eV devices to 0.31 V at 1.2 amperes/cm{sup 2} for 0.5 eV devices, indicating good photovoltaic device characteristics over the range of bandgaps. The diode ideality factor for 0.54 eV devices ranged from 2.45 at low illumination indicating tunneling-dominated dark current, to 1.7 at high illumination intensity indicating recombination-generation dominated dark currents.
Date: August 1, 1996
Creator: Uppal, P.N.; Charache, G.; Baldasaro, P.; Campbell, B.; Loughin, S.; Svensson, S. et al.
Partner: UNT Libraries Government Documents Department

Characteristics of indium oxide plasma filters deposited by atmospheric pressure CVD

Description: Thin films of undoped and tin-doped In{sub 2}O{sub 3} are being investigated for use as plasma filters in spectral control applications for thermal photovoltaic cells. These films are required to exhibit high reflectance at wavelengths longer than the plasma wavelength {lambda}{sub p}, high transmittance at wavelengths shorter than {lambda}{sub p} and low absorption throughout the spectrum. Both types of films were grown via atmospheric pressure chemical vapor deposition (APCVD) on Si (100) and fused silica substrates using trimethylindium (TMI), tetraethyltin (TET), and oxygen as the precursors. Fourier Transform InfraRed (FTIR) spectroscopy was used to measure the filter transmittance and reflectance between 1.8--20 {micro}m. Nominal conditions used during the growth of undoped In{sub 2}O{sub 3} were a substrate temperature of 450 C and partial pressures of 1.4 {times} 10{sup {minus}4} atm. and 1 {times} 10{sup {minus}3} atm. for TMI and O{sub 2} respectively. The O{sub 2}/TMI partial pressure ratio and substrate temperature were systematically varied to control the filter characteristics. The plasma wavelength {lambda}{sub p} was found to be a sensitive function of these parameters. Post-growth annealing of the films was done in inert as well as air ambient at elevated temperatures, but was found to have no beneficial effect. Tin-doped In{sub 2}O{sub 3} was grown under similar conditions as above, with a typical TET partial pressure of 4 {times} 10{sup {minus}6} atm. Here also, the material properties and consequently the optical response were found to be strongly dependent on growth conditions such as O{sub 2} and TET partial pressures. Both undoped and tin-doped In{sub 2}O{sub 3} grown on fused silica exhibited enhanced transmittance due to the close matching of refractive indices of In{sub 2}O{sub 3} and silica. X-ray diffractometer measurements indicated that all these films were polycrystalline and highly textured towards the (111) direction. The best undoped and tin-doped In{sub ...
Date: July 1, 1995
Creator: Langlois, E.; Murthy, S.D.; Bhat, I.; Gutmann, R.; Brown, E.; Dziendziel, R. et al.
Partner: UNT Libraries Government Documents Department

Growth and characterization of In{sub 0.2}Ga{sub 0.8}Sb device structures using metalorganic vapor phase epitaxy

Description: In{sub 0.2}Ga{sub 0.8}Sb epitaxial layers and thermophotovoltaic (TPV) device structures have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). Control of the n-type doping up to 1 {times} 10{sup 18} cm{sup {minus}3} was achieved using diethyltellurium (DETE) as the dopant source. A Hall mobility of greater than 8,000 cm{sup 2}/Vs at 77 K was obtained for a 3 {times} 10{sup 17} cm{sup {minus}3} doped In{sub 0.2}Ga{sub 0.8}Sb layer grown on high-resistivity GaSb substrate. The In{sub 0.2}Ga{sub 0.8}Sb epilayers directly grown on GaSb substrates were tilted with respect to the substrates, with the amount of tilt increasing with the layer thickness. Transmission electron microscopy (TEM) studies of the layers showed the presence of dislocation networks across the epilayers parallel to the interface at different distances from the interface, but the layers above this dislocation network were virtually free of dislocations. A strong correlation between epilayer tilt and TPV device properties was found, with layers having more tilt providing better devices. The results suggest that the dislocations moving parallel to the interface cause lattice tilt, and control of this layer tilt may enable the fabrication of better quality device structures.
Date: May 1, 1997
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C.; Gutmann, R.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department

Neutron time-of-flight measurements at the Rensselaer linac

Description: Neutron transmission measurements of Ho, Er, Tm and Au samples have been made from thermal to several hundred eV and the data have been fitted with the SAMMY program. A 16-section NaI multiplicity detector has been used to measure simultaneously capture and scattering partial cross sections. These measurements are used to obtain accurate resonance parameters over this energy range for samples of Mo, Ho, Er, Tm and Au.
Date: April 1, 1997
Creator: Block, R.C.; Danon, Y. & Slovacek, R.E
Partner: UNT Libraries Government Documents Department

Stability analysis of implicit multi-fluid schemes

Description: A new implicit method has been developed for solving the viscous full multi-fluid equations, which incorporate transport and generation of mass and momentum for each component present in a system. This work presents stability analysis and application of the important full multi-fluid system in a fully implicit algorithm. The stability analyses presented demonstrate the performance of several iterative schemes applied to the solution of the linearized systems which arise in the formulation. These include block Jacobi and symmetric block Gauss-Siedel schemes with various preconditioners applied. A hierarchy of increasing physical complexity is pursued, starting with one-dimensional, two-fluid systems with minimum inter-field dynamic coupling and no mass transfer. These analyses are extended to systems employing physically important inter-field forces (drag, turbulence dispersion, virtual mass). The effects of mass transfer, multiple fields (i.e., n{phi} > 2) and multiple dimensions are also considered. A two-fluid Navier-Stokes code has been developed based on this new scheme. Results are presented which verify the validity of the stability analyses presented for the coupled scheme. Multi-phase flows which require full multi-fluid modeling arise in a wide class of engineering problems, where non-equilibrium dynamics and thermodynamics of the interfaces between constituents play important roles in the evolution of the ensemble averaged mean flow. Examples include cyclone separators, two-phase flow in jets and curved ducts and boiling flow in heat exchangers.
Date: June 1, 1997
Creator: Kunz, R.F.; Cope, W.K. & Venkateswaran, S.
Partner: UNT Libraries Government Documents Department

Enhanced electro-magnetic energy transfer between a hot and cold body at close spacing due to evanescent fields

Description: Theoretical studies have demonstrated that the energy transfer between a hot and cold body at close spacing (on the order of the radiation wavelength) can greatly exceed the limit for black body radiation (ie, Power = {sigma}T{sup 4}). This effect, due to the coupling of evanescent fields, presents an attractive option for thermo-photovoltaic (TPV) applications (assuming the considerable technical challenges can be overcome). The magnitude of the enhanced energy transfer depends on the optical properties of the hot and cold bodies as characterized by the dielectric functions of the respective materials. The present study considers five different situations as specified by the materials choices for the hot/cold sides: metal/metal, metal/insulator, metal/semiconductor, insulator/insulator, and semiconductor/semiconductor. For each situation, the dielectric functions are specified by typical models. An increase in energy transfer (relative to the black body law) is found for all situations considered, for separations less than one micron, assuming a temperature difference of 1,000 C. The metal/metal situation has the highest increase vs. separation while the semiconductor/semiconductor has the lowest. Factor-of-ten increases are obtained at roughly 0.1 microns for the metal/metal and roughly 0.02 microns for the metal/semiconductor. These studies are helping to increase the understanding of the close-spaced effect in the context of a radiator/TPV context.
Date: October 1, 1998
Creator: Raynolds, J.E.
Partner: UNT Libraries Government Documents Department

Production data on 0.55 eV InGaAs thermophotovoltaic cells

Description: Low bandgap 0.55 eV (2.25 {micro}m cutoff wavelength) indium gallium arsenide (In{sub 0.72}Ga{sub 0.28}As) thermophotovoltaic (TPV) cells use much more of the long wavelength energy emitted from low temperature (< 1,200 C) thermal sources than either Si or GaSb cells. Data are presented on a statistically significant number (2,500) of these TPV cells, indicating the performance obtainable in large numbers of cells. This data should be useful in the design and modeling of TPV system performance. At 1.2 A/cm{sup 2} short-circuit current, an average open-circuit voltage of 283 mV is obtained with a 60% fill factor. The peak external quantum efficiency for uncoated cells is 65% and is over 50% from 1.1 to 2.2 {micro}m. Internal quantum efficiency is over 76% in this range assuming an estimated 34% reflectance loss.
Date: May 1, 1996
Creator: Wojtzuk, S.; Colter, P.; Charache, G. & Campbell, B.
Partner: UNT Libraries Government Documents Department

Silicon as the P-type dopant in GaSb and Ga{sub 0.8}In{sub 0.2}Sb grown by metalorganic vapor phase epitaxy

Description: P-type GaSb and Ga{sub 0.8}In{sub 0.2}Sb layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using silane as the doping precursor. Hall measurements show that the concentration and mobility of holes in GaSb and Ga{sub 0.8}In{sub 0.2}Sb are higher when the layers are grown on GaSb substrates than when grown on GaAs substrates. Secondary ion mass spectroscopy (SIMS) results show that the incorporation of Si is higher when GaSb substrates are used. The compensation of Si acceptors is negligible in GaSb, but is as high as 25% in Ga{sub 0.8}In{sub 0.2}Sb.
Date: June 1, 1998
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C. Gutmann, R.J.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department