Physical Sciences

Study of Dislocation-Ordered InxGa1–xAs/GaAs Quantum Dots

A report describes an experimental study of dislocation-induced spatial ordering of quantum dots (QDs) comprising nanometer-sized InxGa1–xAs islands surrounded by GaAs. Metastable hetero-epitaxial structures were grown by molecular-beam epitaxy of InxGa1–xAs onto n+ GaAs and semi-insulating GaAs substrates. Then the structures were relaxed during a post-growth annealing/self-organizing process leading to the formation of surface undulations that acted as preferential sites for the nucleation of QDs. Structural effects of annealing times and temperatures on the strain-relaxed InxGa1–xAs/GaAs and the subsequent spatial ordering of the QDs were analyzed by atomic-force microscopy and transmission electron microscopy. Continuous-wave spectral and time-resolved photoluminescence (PL) measurements were performed to study the effects, upon optical properties, of increased QD positional ordering, increased QD uniformity, and proximity of QDs to arrays of dislocations. PL spectral peaks of ordered QD structures formed on strain-relaxed InxGa1–xAs/GaAs layers were found to be narrower than those of structures not so formed and ordered. Rise and decay times of time-resolved PL were found to be lower at lower temperatures — apparently as a consequence of decreased carrier-transport times within the barriers surrounding the QDs.

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Tilt-Sensitivity Analysis for Space Telescopes

A report discusses a computational- simulation study of phase- front propagation in the Laser Interferometer Space Antenna (LISA), in which space telescopes would transmit and receive metrological laser beams along 5-Gm interferometer arms. The main objective of the study was to determine the sensitivity of the average phase of a beam with respect to fluctuations in pointing of the beam. The simulations account for the effects of obscurations by a secondary mirror and its supporting struts in a telescope, and for the effects of optical imperfections (especially tilt) of a telescope. A significant innovation introduced in this study is a methodology, applicable to space telescopes in general, for predicting the effects of optical imperfections. This methodology involves a Monte Carlo simulation in which one generates many random wavefront distortions and studies their effects through computational simulations of propagation. Then one performs a statistical analysis of the results of the simulations and computes the functional relations among such important design parameters as the sizes of distortions and the mean value and the variance of the loss of performance. These functional relations provide information regarding position and orientation tolerances relevant to design and operation.

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Nulling Infrared Radiometer for Measuring Temperature

A microwave-radiometer self-calibration principle would be adapted to measurement of infrared. A nulling, self-calibrating infrared radiometer is being developed for use in noncontact measurement of temperature in any of a variety of industrial and scientific applications. This instrument is expected to be especially well-suited to measurement of ambient or near-ambient temperature and, even more specifically, for measuring the surface temperature of a natural body of water. Although this radiometer would utilize the long-wavelength infrared (LWIR) portion of the spectrum (wavelengths of 8 to 12 µm), its basic principle of operation could also be applied to other spectral bands (corresponding to other temperature ranges) in which the atmosphere is transparent and in which design requirements for sensitivity and temperature-measurement accuracy could be satisfied.

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Hot Films on Ceramic Substrates for Measuring Skin Friction

Low-thermal-conductivity ceramic substrates, based on Space Shuttle tile technology, serve to increase sensitivity. Hot-film sensors, consisting of a metallic film on an electrically nonconductive substrate, have been used to measure skin friction as far back as 1931. A hot film is maintained at an elevated temperature relative to the local flow by passing an electrical current through it. The power required to maintain the specified temperature depends on the rate at which heat is transferred to the flow. The heat-transfer rate correlates to the velocity gradient at the surface, and hence, with skin friction. The hot-film skin friction measurement method is most thoroughly developed for steady-state conditions, but additional issues arise under transient conditions.

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Detecting Conductive Liquid Leaking From Nonconductive Pipe

A capacitive detector is scanned over the ground above the pipe. A method that can be implemented with relatively simple electronic circuitry provides a capability for detecting leakage of an electrically conductive liquid from an electrically nonconductive underground pipe. Alternatively or in addition, the method can be applied to locate the pipe, whether or not there is a leak. Although the method is subject to limitations (some of which are described below), it is still attractive as an additional option for detecting leaks and locating pipes without need for extensive digging.

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Predicting and Preventing Incipient Flameout in Combustors

Increases in acoustic signals could trigger rapid adjustments to prevent flameouts. A method of predicting and preventing incipient flameout in a combustor has been proposed. The method should be applicable to a variety of liquid- and gas-fueled combustors in furnaces and turbine engines. Until now, there have been methods of detecting flameouts after they have occurred, but there has been no way of predicting incipient flameouts and, hence, no way of acting in time to prevent them. Prevention of flameout could not only prevent damage to equipment but, in the case of aircraft turbine engines, could also save lives.

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Using Diffusion Bonding in Making Piezoelectric Actuators

Fabrication is simplified and performance improved. A technique for the fabrication of piezoelectric actuators that generate acceptably large forces and deflections at relatively low applied voltages involves the stacking and diffusion bonding of multiple thin piezoelectric layers coated with film electrodes. The present technique stands in contrast to an older technique in which the layers are bonded chemically, by use of urethane or epoxy agents.

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