Engineering topics
Williams, J. R.
Publications and source records attributed to Williams, J. R..
Crystalline materials for quantum computing: Semiconductor heterostructures and topological insulators exemplars
Abstract High-purity crystalline solid-state materials play an essential role in various technologies for quantum information processing, from qubits based on spins to topological states. New and improved crystalline materials emerge each year and continue to drive new results in experimental quantum science. This article summarizes the opportunities for a selected class of crystalline materials for qubit technologies based on spins and topological states and the challenges associated with their fabrication. We start by describing semiconductor heterostructures for spin qubits in gate-defined quantum dots and benchmark GaAs, Si, and Ge, the three platforms that demonstrated two-qubit logic. We then examine novel topologically nontrivial materials and structures that might be incorporated into superconducting devices to create topological qubits. We review topological insulator thin films and move onto topological crystalline materials, such as PbSnTe, and its integration with Josephson junctions. We discuss advances in novel and specialized fabrication and characterization techniques to enable these. We conclude by identifying the most promising directions where advances in these material systems will enable progress in qubit technology.
Josephson detection of time-reversal symmetry broken superconductivity in SnTe nanowires
A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. In this work, we present such a case: we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in JJs and observe a deviation from the standard CPR. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. The analysis examines how multiband effects and the experimentally visualized ferroelectric domain walls give rise to this behavior, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.
Relative biological effectiveness of light ions in human tumoural cell lines: role of protein p53
Protons and alpha particles of high linear energy transfer (LET) have shown an increased relative biological effectiveness (RBE) with respect to X/gamma rays for several cellular and molecular endpoints in different in vitro cell systems. To contribute to understanding the biochemical mechanisms involved in the increased effectiveness of high LET radiation, an extensive study has been designed. The present work reports the preliminary result of this study on two human tumoural cell lines, DLD1 and HCT116, (with different p53 status), which indicate that for these cell lines, p53 does not appear to take a part in the response to radiation induced DNA damage, suggesting an alternative p53-independent pathway and a cell biochemical mechanism dependent on the cell type.
Analysis of MIR-18 results for physical and biological dosimetry: radiation shielding effectiveness in LEO
We compare models of radiation transport and biological response to physical and biological dosimetry results from astronauts on the Mir space station. Transport models are shown to be in good agreement with physical measurements and indicate that the ratio of equivalent dose from the Galactic Cosmic Rays (GCR) to protons is about 3/2:1 and that this ratio will increase for exposures to internal organs. Two biological response models are used to compare to the Mir biodosimetry for chromosome aberration in lymphocyte cells; a track-structure model and the linear-quadratic model with linear energy transfer (LET) dependent weighting coefficients. These models are fit to in vitro data for aberration formation in human lymphocytes by photons and charged particles. Both models are found to be in reasonable agreement with data for aberrations in lymphocytes of Mir crew members: however there are differences between the use of LET dependent weighting factors and track structure models for assigning radiation quality factors. The major difference in the models is the increased effectiveness predicted by the track model for low charge and energy ions with LET near 10 keV/micrometers. The results of our calculations indicate that aluminum shielding, although providing important mitigation of the effects of trapped radiation, provides no protective effect from the galactic cosmic rays (GCR) in low-earth orbit (LEO) using either equivalent dose or the number of chromosome aberrations as a measure until about 100 g/cm 2 of material is used.
Predicting cancer rates in astronauts from animal carcinogenesis studies and cellular markers
The radiation space environment includes particles such as protons and multiple species of heavy ions, with much of the exposure to these radiations occurring at extremely low average dose-rates. Limitations in databases needed to predict cancer hazards in human beings from such radiations are significant and currently do not provide confidence that such predictions are acceptably precise or accurate. In this article, we outline the need for animal carcinogenesis data based on a more sophisticated understanding of the dose-response relationship for induction of cancer and correlative cellular endpoints by representative space radiations. We stress the need for a model that can interrelate human and animal carcinogenesis data with cellular mechanisms. Using a broad model for dose-response patterns which we term the "subalpha-alpha-omega (SAO) model", we explore examples in the literature for radiation-induced cancer and for radiation-induced cellular events to illustrate the need for data that define the dose-response patterns more precisely over specific dose ranges, with special attention to low dose, low dose-rate exposure. We present data for multiple endpoints in cells, which vary in their radiosensitivity, that also support the proposed model. We have measured induction of complex chromosome aberrations in multiple cell types by two space radiations, Fe-ions and protons, and compared these to photons delivered at high dose-rate or low dose-rate. Our data demonstrate that at least three factors modulate the relative efficacy of Fe-ions compared to photons: (i) intrinsic radiosensitivity of irradiated cells; (ii) dose-rate; and (iii) another unspecified effect perhaps related to reparability of DNA lesions. These factors can produce respectively up to at least 7-, 6- and 3-fold variability. These data demonstrate the need to understand better the role of intrinsic radiosensitivity and dose-rate effects in mammalian cell response to ionizing radiation. Such understanding is critical in extrapolating databases between cellular response, animal carcinogenesis and human carcinogenesis, and we suggest that the SAO model is a useful tool for such extrapolation.
NSBRI Radiation Effects: Carcinogenesis in Sprague-Dawley Rats Irradiated with Iron Ions, Protons, or Photons
Our ability to confidently develop appropriate countermeasures for radiations in space in terms of shielding and design of a spacecraft, the mission scenario, or chemoprevention is severely limited by the uncertainties in both the risk itself and the change in that risk with intervention. Despite the fact that the risk of carcinogenesis from exposures of personnel to radiations on long-term missions is considered one of the worst hazards in space, only a limited amount of in-vivo data exist for tumor induction from exposures to protons or energetic heavy ions (HZEs) at lower doses. The most extensive work remains the landmark study. for tumor development in the harderian gland of the mouse. The objective of this study is to characterize the level of risk for tumor induction in another relevant animal model. Subsequent experiments are designed to test the hypothesis that the level of risk can be reduced by pharmaceutical intervention in the promoting and progressing stages of the disease rather than in the initiating stage. The work presented here results from a cooperative effort on the part of investigators from two projects of the Radiation-Effects Team of the National Space Biomedical Research Institute (NSBRI). The collaborating projects are the Core Project which is investigating the risk of carcinogenesis in Sprague-Dawley rats and the Chemoprevention Project which is investigating the ability of Tamoxifen to reduce the number of malignant tumors in the irradiated animals. Research at the cellular and subcellular levels is being conducted in two other projects of the Radiation-Effects Team, Cytogenetics with J. R. Williams as Principal Investigator and Mutations from Repeated DNA Sequences. Results for these other projects also are being presented at this Workshop.
High-temperature ohmic contact to n-type 6H-SiC using nickel
Specific contact resistances measured at elevated temperatures for Ni ohmic contacts to 6H-SiC were reported. The specific contact resistances were measured with the linear transmission line method at both room temperature and at 500 C and yielded values less than 5 x 10(exp -6) ohm sq cm at both temperatures. The trend shows a decreasing contact resistance at higher temperature. The annealed metal film is a nickel silicide with substantial mixing of C throughout the silicide layer.
X-ray Photoelectron Spectroscopy (XPS), Rutherford Back Scattering (RBS) studies
X-ray photoelectron spectroscopy (XPS), Rutherford Back Scattering (RBS) studies of each of sample received were completed. Since low angle X-ray could not be performed because of instrumentation problems, Auger spectrometry was employed instead. The results of these measurements for each of the samples is discussed in turn.
The interaction of atomic oxygen with thin copper films
A source of thermal, ground-state atomic oxygen has been used to expose thin copper films at a flux of 1.4 x 10 exp 17 atoms/sq cm s for times up to 50 min for each of five temperatures between 140 and 200 C. Rutherford backscattering spectroscopy was used to characterize the oxide formed during exposure. The observations are consistent with the oxide phase Cu2O. The time dependence and the temperature dependence of the oxide layer thickness can be described using oxide film growth theory based on rate limitation by diffusion. Within the time and temperature ranges of this study, the growth of the oxide layers is well described by the equation L(T,t) = 3.6 x 10 to 8th exp(- 1.1/2k sub B T)t exp 1/2, where L,T, and t are measured in angstroms, degrees Kelvin, and minutes, respectively. The deduced activation energy is 1.10 +/- 0.15 eV, with the attendant oxidation rate being greater than that for the corresponding reaction in molecular oxygen.
Chemical reaction of atomic oxygen with evaporated films of copper, part 4
Evaporated copper films were exposed to an atomic oxygen flux of 1.4 x 10(exp 17) atoms/sq cm per sec at temperatures in the range 285 to 375 F (140 to 191 C) for time intervals between 2 and 50 minutes. Rutherford backscattering spectroscopy (RBS) was used to determine the thickness of the oxide layers formed and the ratio of the number of copper to oxygen atoms in the layers. Oxide film thicknesses ranged from 50 to 3000 A (0.005 to 0.3 microns, or equivalently, 5 x 10(exp -9) to 3 x 10(exp -7); it was determined that the primary oxide phase was Cu2O. The growth law was found to be parabolic (L(t) varies as t(exp 1/2)), in which the oxide thickness L(t) increases as the square root of the exposure time t. The analysis of the data is consistent with either of the two parabolic growth laws. (The thin-film parabolic growth law is based on the assumption that the process is diffusion controlled, with the space charge within the growing oxide layer being negligible. The thick-film parabolic growth law is also based on a diffusion controlled process, but space-charge neutrality prevails locally within very thick oxides.) In the absence of a voltage measurement across the growing oxide, a distinction between the two mechanisms cannot be made, nor can growth by the diffusion of neutral atomic oxygen be entirely ruled out. The activation energy for the reaction is on the order of 1.1 eV (1.76 x 10(exp -19) joule, or equivalently, 25.3 kcal/mole).
Commerce Lab - A program of commercial flight opportunities
Commerce Lab is conceived as an adjunct to the National Space Transportation System (NSTS) by providing a focal point for commercial missions which could utilize existing NSTS carrier and resource capabilities for on-orbit experimentation in the microgravity sciences. In this context, the Commerce Lab program provides mission planning for private sector involvement in the space program, in general, and the commercial exploitation of the microgravity environment for materials processing research and development. It is expected that Commerce Lab will provide a logical transition between currently planned NSTS missions and future microgravity science and commercial R&D missions centered around the Space Station. The present study identifies candidate Commerce Lab flight experiments and their development status and projects a mission traffic model that can be used in commercial mission planning.
Overview of materials processing in space activity at Marshall Space Flight Center
An overview of activities involving the Space Transportation System (STS), now in the operational phase, and results of some of the current space experiments, as well as future research opportunities in microgravity environment, are presented. The experiments of the Materials Processing in Space Program flown on the STS, such as bioseparation processes, isoelectric focusing, solidification and crystal growth processes, containerless processes, and the Materials Experiment Assembly experiments are discussed. Special consideration is given to the experiments to be flown aboard the Spacelab 3 module, the Fluids Experiments System, and the Vapor Crystal Growth System. Ground-based test facilities and planned space research facilities, as well as the nature of the commercialization activities, are briefly explained.
Combustor development for automotive gas turbines
Previously cited in issue 17, p. 2742, Accession no. A82-35062
Small Gas Turbine Combustor Primary Zone Study
A development process is described which consists of design, fabrication, and preliminary test evaluations of three approaches to internal aerodynamic primary zone flow patterns: (1) conventional double vortex swirl stabilization; (2) reverse flow swirl stabilization; and (3) large single vortex flow system. Each concept incorporates special design features aimed at extending the performance capability of the small engine combustor. Since inherent geometry of these combustors result in small combustion zone height and high surface area to volume ratio, design features focus on internal aerodynamics, fuel placement, and advanced cooling. The combustors are evaluated on a full scale annular combustor rig. A correlation of the primary zone performance with the overall performance is accomplished using three intrusion type gas sampling probes located at the exit of the primary zone section. Empirical and numerical methods are used for designing and predicting the performance of the three combustor concepts and their subsequent modifications. The calibration of analytical procedures with actual test results permits an updating of the analytical design techniques applicable to small reverse flow annular combustors.
Combustor development for automotive gas turbines
The development of a combustion system for the AGT 100 automotive gas turbine engine is described. A maximum turbine inlet temperature of 1288 C is reached during the regenerative cycle, and air up to 1024 C is supplied to the combustor inlet. A premix/prevaporization ceramic combustor employing variable geometry to control burning zone temperature was developed and tested. Tests on both metal and ceramic combustors showed that emissions were a function of burner inlet temperature (BIT). At 999 C BIT, NO(x) emissions were two orders of magnitude below program goals, and at the same temperature but at a different variable geometry position, the CO was 30 times below program goal. Tests to evaluate the durability of the ceramic materials showed no failures during steady-state operation; however, some cracks developed in the dome during extended transient operation.
Beam builder for space fabrication
Construction of a beam builder for producing triangular truss aluminum beams for use on large space structures is described. The machine will fabricate 0.04 cm thick 2024-T3 aluminum alloy into beams weighing 1.265 kg-m with side braces spot-welded to end caps to fabricate a triangular truss configuration up to 305 m long. The design characteristics, the electrical subsystems, the power requirements of the spot welding equipment, and the computer system for coordinating the motion of the Yoder rolling mills for forming caps from aluminum coils are described. Spot welding was compared with ultrasonic welding, stapling, wire stitching, riveting, grommets, stamp-lok, and pierce and fold joining methods with respect to ease of operation, safety, practicality, and ease of energy consumption. The beam builder will be evaluated to identify design requirements which can be utilized for the space flight version of its concept.
TCV software test and validation tools and technique
The paper describes techniques for testing and validating software for the TCV (Terminal Configured Vehicle) program which is intended to solve problems associated with operating a commercial transport aircraft in the terminal area. The TCV research test bed is a Boeing 737 specially configured with digital computer systems to carry out automatic navigation, guidance, flight controls, and electronic displays research. The techniques developed for time and cost reduction include automatic documentation aids, an automatic software configuration, and an all software generation and validation system.