A primary difference between terrestrial and remotely located reactors is the ability to periodically recalibrate and replace the instrumentation. Because of this, space reactors place a premium on self‐calibrating, ...
A primary difference between terrestrial and remotely located reactors is the ability to periodically recalibrate and replace the instrumentation. Because of this, space reactors place a premium on self‐calibrating, long‐term reliable instrumentation. The primary temperature measurements for the SP‐100 reactor were to be made using W/W‐Re thermocouples. However, the large gamma and neutron dose expected at the coolant outlet (>1 MGy γ; 3×1015 fast neutron fluence) combined with the high temperature (1375 K nominal; 1650 K maximum) meant that the thermocouples would drift significantly over the lifetime of the reactor. A combined Johnson noise resistance thermometer capable of performing under these extreme conditions was developed by ORNL (Carroll, 1994). Johnson noise is a fundamental representation of temperature—it is the vibration of the electronic field surrounding atoms as they thermally vibrate. Johnson noise, however, is fundamentally a small signal (∼4×10−7 Vrms for a 100 Ω resistor at 300 K, using a 100 kHz bandwidth) spread throughout the frequency spectrum. Creating the electronics and signal processing required to effectively measure and interpret the noise signal remains challenging. ORNL has recently developed closely related Johnson Noise Thermometry (JNT) electronics and signal processing capabilities under a DOE International Nuclear Energy Research Initiative Project with the Korean Atomic Energy Research Institute (U.S. DOE, 2002). An overview of the application of JNT to space nuclear power and the current status of the ORNL JNT capabilities is the subject of this paper.
Stokes and anti‐Stokes Resonance Raman experiments on carbon nanotubes using a tunable laser system are reported. Both arc discharge and HiPco samples are investigated with laser excitation energies varying continuou...
Stokes and anti‐Stokes Resonance Raman experiments on carbon nanotubes using a tunable laser system are reported. Both arc discharge and HiPco samples are investigated with laser excitation energies varying continuously from 1.6eV up to 2.7eV. The usual features, i.e., the RBM, D‐band and G‐band are analyzed, as well as the intermediate frequency modes (IFMs) appearing between the RBM and the D/G band spectral region. The results are analyzed based on electron and phonon structure. Step‐like dispersive behavior is observed for the IFMs and this anomalous result is analyzed based on Raman selection rules plus quantum confinement of electrons and phonons. An experimental determination of the van Hove singularity energies Eii is compared with photoluminescence results. The single versus double resonance nature of the G band SWNT spectra is briefly addressed.
Work towards the development of an innovative, potentially high power density, MEMS loop heat pipe is in progress at the Center for Microelectronic Sensors and M E M S at the University of Cincinnati. The design of th...
Work towards the development of an innovative, potentially high power density, MEMS loop heat pipe is in progress at the Center for Microelectronic Sensors and M E M S at the University of Cincinnati. The design of the loop heat pipe is based upon the very unique coherent porous silicon technology, a technique in which vast arrays of micrometer ‐ sized through ‐ holes are photo ‐ electrochemically etched into a silicon wafer perpendicular to the (100) surface. The initial mathematical model, the design, fabrication and characterization of the device in the open loop configuration were previously reported at this conference, STAIF 2002. This paper begins with a very brief explanation of the device and its theory of operation. The design of the device components and their production utilizing the various techniques of microelectronic and microelectromechanical fabrication are presented. The modifications made to the photon ‐ induced, electrochemical etch process, which significantly increase the etch rate of the pores, are explained. Attention is given to the mathematical model of the planar, MEMS, loop heat pipe with respect to the generation of the dimensions of the components through a summary of the recent advances. The emphasis of this paper is upon the design, construction and the characterization of the evacuated closed loop test cell structure.
To study atmospheric convection and its possible roles in the interaction with larger‐scale atmosphere, various kinds of data from devices such as radiosonde soundings, satellite imagery, and airborne Doppler radars ...
To study atmospheric convection and its possible roles in the interaction with larger‐scale atmosphere, various kinds of data from devices such as radiosonde soundings, satellite imagery, and airborne Doppler radars have been measured. Among them, satellite imagery‐deduced data has been wisely used as indexes for convection. In this paper, we carry out multifractal analysis of the “deep convective index” time series based on the equivalent black body temperature (Itbb) derived from hourly geostationary satellite measurements from 1 November 1992 through 28 February 1993. We find that Itbb data is consistent with the random multiplicative model. Since Itbb strongly correlates with the rainfall during the same period, our result also suggests rainfall may also be consistent with the random multiplicative model. Earlier fractal and multifractal analysis of rainfall has been focused on the mature stage of a storm. The Itbb data analyzed here, however, characterizes a collection of convection systems (i.e., storms) in a four‐month period. Hence the multifractal variations in Itbb reflect the intermittent occurrence and variation of convection (hence storm) systems in the tropical regions. Various implications of this finding are addressed, in particular, the correlation structure in the Itbb and the implication of this correlation to the interpretation of some concepts and techniques in data analysis, such as signal‐to‐noise ratio and smoothing.
There are several existing methods for damage detection based on identifying changes in strain energy mode shapes. Most of these methods require knowing strain energy mode shapes for a structure without damage in orde...
There are several existing methods for damage detection based on identifying changes in strain energy mode shapes. Most of these methods require knowing strain energy mode shapes for a structure without damage in order to establish a baseline for damage detection. Usually, the mode shapes from the structure under test should be compared to the baseline mode shapes to identify and locate damage. However, these methods of damage detection are not very suitable for application on structures where baseline mode shapes cannot be readily obtained, for example, structures with preexisting damage. Conventional methods, like building a finite element model of a structure to be used as a baseline might be an expensive and time‐consuming task that can be impossible for complex structures. A new (non‐baseline) method for the extraction of localized changes (damage peaks) from strain energy mode shapes based on Fourier analysis of the strain energy mode shapes has been developed and analytically proved for the cases of a pinned‐pinned and a free‐free beam. The new method looks for characteristic changes in the power spectrum of the strain energy mode shapes in order to locate and identify damage. The analytical results have been confirmed both by the finite element model and impact testing experiments on a free‐free aluminum beam, including single and multiple damage scenarios. This paper presents results of testing the non‐baseline method on a complex structure — Armored Vehicle Launched Bridge, which consists of loosely coupled hinged beams with variable cross‐section. The results of testing confirm applicability of the non‐baseline method to damage detection in complex structures and highlight certain particularities of its use.
Iowa State University’s pulsed eddy‐current system has been redesigned to be smaller, more sturdy and easier to install and operate. Improvements have been made to the electronic circuits, the computer interface, th...
Iowa State University’s pulsed eddy‐current system has been redesigned to be smaller, more sturdy and easier to install and operate. Improvements have been made to the electronic circuits, the computer interface, the enclosure design and to the software. Changes have resulted in a much more user‐friendly system that it portable and can be installed quickly and easily in the field. As part of an evaluation of the new system, bond‐thickness measurements on specimens supplied by Cessna were carried out. Results were found to be in good agreement with those obtained by direct measurement. Additional experiments were carried out on corrosion specimens from SAIC Ultra Image Inc. Using data from these specimens a calibration‐based approach to flaw quantification is demonstrated.
In order to estimate the amount of plane bending fatigue damage in an austenitic stainless steel (SUS304), we were investigating the relationship between plane bending fatigue damage and the perpendicular residual lea...
In order to estimate the amount of plane bending fatigue damage in an austenitic stainless steel (SUS304), we were investigating the relationship between plane bending fatigue damage and the perpendicular residual leakage magnetic flux density caused by martensitic structure induced by plane bending fatigue. A specimen such as SUS304 had been excited in a constant external magnetic field perpendicularly to measure dependence of the perpendicular residual leakage magnetic flux density on plane bending fatigue damage accurately. The Z component of the magnetic flux density at 1 mm above a specimen is measured by using a thin‐film flux‐gate (FG) magnetic sensor. Residual magnetization is caused by partial martensitic structure in an austenitic stainless steel induced by cyclic bending stress. From our experiments, we can evaluate dependence of the perpendicular residual leakage magnetic flux density on plane bending fatigue damage and know the relationship between growth of a crack and the perpendicular residual leakage magnetic flux density.
This paper presents results of experimental and theoretical research on antimonide‐ based thermophotovoltaic (TPV) materials and cells. The topics discussed include: growth of large diameter ternary GaInSb bulk cryst...
This paper presents results of experimental and theoretical research on antimonide‐ based thermophotovoltaic (TPV) materials and cells. The topics discussed include: growth of large diameter ternary GaInSb bulk crystals, substrate preparation, diffused junction processes, cell fabrication and characterization, and, cell modeling. Ternary GaInSb boules up to 2 inches in diameter have been grown using the vertical Bridgman technique with a novel self solute feeding technique. A single step diffusion process followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency, p‐n junction GaSb and GaInSb thermophotovoltaic cells. The optimum junction depth to obtain the highest quantum efficiency and open circuit voltage has been identified based on diffusion lengths (or minority carrier lifetimes), carrier mobility and experimental diffused impurity profiles. Theoretical assessment of the performance of ternary (GaInSb) and binary (GaSb) cells fabricated by Zn diffusion in bulk substrates has been performed using PC‐1D one‐dimensional computer simulations. Several factors affecting the cell performances such as the effects of emitter doping profile, emitter thickness and recombination mechanisms (Auger, radiative and Shockley‐Read‐Hall), the advantages of surface passivation and the impact of dark current due to the metallic grid will be discussed. The conditions needed for diffused junction cells on ternary and binary substrates to achieve similar performance to the epitaxially grown lattice‐ matched quaternary cells are identified.
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 ...
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.
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