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Implementation of Chord Length Sampling for Transport Through a Binary Stochastic Mixture

Description: Neutron transport through a special case stochastic mixture is examined, in which spheres of constant radius are uniformly mixed in a matrix material. A Monte Carlo algorithm previously proposed and examined in 2-D has been implemented in a test version of MCNP. The Limited Chord Length Sampling (LCLS) technique provides a means for modeling a binary stochastic mixture as a cell in MCNP. When inside a matrix cell, LCLS uses chord-length sampling to sample the distance to the next stochastic sphere. After a surface crossing into a stochastic sphere, transport is treated explicitly until the particle exits or is killed. Results were computed for a simple model with two different fixed neutron source distributions and three sets of material number densities. Stochastic spheres were modeled as black absorbers and varying degrees of scattering were introduced in the matrix material. Tallies were computed using the LCLS capability and by averaging results obtained from multiple realizations of the random geometry. Results were compared for accuracy and figures of merit were compared to indicate the efficiency gain of the LCLS method over the benchmark method. Results show that LCLS provides very good accuracy if the scattering optical thickness of the matrix is small ({le} 1). Comparisons of figures of merit show an advantage to LCLS varying between factors of 141 and 5. LCLS efficiency and accuracy relative to the benchmark both decrease as scattering is increased in the matrix.
Date: November 18, 2002
Creator: Donovan, T.J.; Sutton, T.M. & Danon, Y.
Partner: UNT Libraries Government Documents Department

A Simple Single Step diffusion and Emitter Etching Process for High Efficiency Gallium Antimonide Thermophotovoltaic Devices

Description: A single step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency GaSb thermophotovoltaic cells. The junction depth was controlled through monitoring of light current-voltage (I-V) curves (photovoltaic response) during the post diffusion emitter etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies and the open circuit voltages in the fabricated devices. An optimum junction depth for obtaining highest quantum efficiency and open circuit voltage is presented based on diffusion lengths (or monitoring carrier lifetimes), carrier mobility and typical diffused impurity profile in GaSb.
Date: August 29, 2003
Creator: Rajagopalan, G.; Reddy, N.S.; Ehsani, E.; Bhat, I.B.; Dutta, P.S.; Gutmann, R.J. et al.
Partner: UNT Libraries Government Documents Department

Performance Limits of Low Bandgap Thermophotovoltaic Antimonide-Based Cells for Low Temperature Radiators

Description: This paper assesses the performance of antimonide-based thermophotovoltaic cells fabricated by different technologies. In particular, the paper compares the performance of lattice matched quaternary (GaInAsSb) cells epitaxially grown on GaSb substrates to the performance of ternary (GaInSb) and binary (GaSb) cells fabricated by Zn diffusion on bulk substrates. The focus of the paper is to delineate the key performance advantages of the highest performance-to-date of the quaternary cells to the performance of the alternative ternary and binary antimonide-based diffusion technology. The performance characteristics of the cells considered are obtained from PC-1D simulations using appropriate material parameters.
Date: August 29, 2000
Creator: Borrego, J.M.; Wang, C.A.; Dutta, P.S.; rajagopalan, G.; Bhat, I.B.; Gutmann, R.J. et al.
Partner: UNT Libraries Government Documents Department

Improved Thermophotovoltaic (TPV) Performance Using Dielectric Photon Concentrations (DPC)

Description: This report presents theoretical and experimental results, which demonstrate the feasibility of a new class of thermophotovoltaic (TPV) energy converters with greatly improved power density and efficiency. Performance improvements are based on the utilization of the enhanced photon concentrations within high refractive index materials. Analysis demonstrates that the maximum achievable photon flux for TPV applications is limited by the lowest index in the photonic cavity, and scales as the minimum refraction index squared, n{sup 2}. Utilization of the increased photon levels within high index materials greatly expands the design space limits of TPV systems, including: a 10x increase in power density, a 50% fractional increase in conversion efficiency, or alternatively reduced radiator temperature requirements to as low as {approx} 1000 F.
Date: January 3, 2003
Creator: Baldasaro, P.F. & Fourspring, P.M.
Partner: UNT Libraries Government Documents Department

Recombination Parameters for Antimonide-Based Semiconductors using RF Photoreflection Techniques

Description: RF photoreflection measurements and PC-1D simulations have been used to evaluate bulk and surface recombination parameters in antimonide-based materials. PC-1D is used to simulate the photoconductivity response of antimonide-based substrates and doubly-capped epitaxial layers and also to determine how to extract the recombination parameters using experimental results. Excellent agreement has been obtained with a first-order model and test structure simulation when Shockley-Reed-Hall (SRH) recombination is the bulk recombination process. When radiative, Auger and surface recombination are included, the simulation results show good agreement with the model. RF photoreflection measurements and simulations using PC-1D are compatible with a radiative recombination coefficient (B) of approximately 5 x 10{sup -11} cm{sup 3}/s, Auger coefficient (C) {approx} 1.0 x 10{sup -28} cm{sup 6}/s and surface recombination velocity (SRV) {approx} 600 cm/s for 0.50-0.55 eV doubly-capped InGaAsSb material with GaSb capping layers using the experimentally determined active layer doping of 2 x 10{sup 17} cm{sup -3}. Photon recycling, neglected in the analysis and simulations presented, will affect the extracted recombination parameters to some extent.
Date: October 10, 2002
Creator: Kumar, R.J.; Borrego, J.M.; Dutta, P.S.; Gutmann, R.J.; Wang, C.A.; Martinelli, R.U. et al.
Partner: UNT Libraries Government Documents Department

Factors Affecting the Hydrogen Environment Assisted Cracking Resistance of an AL-Zn-Mg-(Cu) Alloy

Description: Precipitation hardenable Al-Zn-Mg alloys are susceptible to hydrogen environment assisted cracking (HEAC) when exposed to aqueous environments. In Al-Zn-Mg-Cu alloys, overaged tempers are used to increase HEAC resistance at the expense of strength but overaging has little benefit in low copper alloys. However, the mechanism or mechanisms by which overaging imparts HEAC resistance is poorly understood. The present research investigated hydrogen uptake, diffusion, and crack growth rate in 90% relative humidity (RH) air for both a commercial copper bearing Al-Zn-Mg-Cu alloy (AA 7050) and a low copper variant of this alloy in order to better understand the factors which affect HEAC resistance. Experimental methods used to evaluate hydrogen concentrations local to a surface and near a crack tip include nuclear reaction analysis (NRA), focused ion beam, secondary ion mass spectroscopy (FIB/SIMS) and thermal desorption spectroscopy (TDS). Results show that overaging the copper bearing alloys both inhibits hydrogen ingress from oxide covered surfaces and decreases the apparent hydrogen diffusion rates in the metal.
Date: April 9, 2002
Creator: Young, G.A. & Scully, J.R.
Partner: UNT Libraries Government Documents Department