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Automated Serial Sectioning for 3D Reconstruction

Automation increases speed and reduces uncertainty in alignment. Figure 1 depicts some aspects of an apparatus and method for automated serial sectioning of a specimen of a solder, aluminum, or other relatively soft opaque material. The apparatus includes a small milling machine (micromiller) that takes precise, shallow cuts (increments of depth as small as 1 μm) to expose successive sections. A microscope equipped with an electronic camera, mounted in a fixed position on the micromiller, takes pictures of the newly exposed specimen surface at each increment of depth. The images are digitized, and the resulting data are subsequently processed to reconstruct three-dimensional (3D) features of the specimen.

Posted in: Briefs, TSP

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Magnetic Field Would Reduce Electron Backstreaming in Ion Thrusters

Erosion of accelerator grid could also be reduced. The imposition of a magnetic field has been proposed as a means of reducing the electron backstreaming problem in ion thrusters. Electron backstreaming refers to the backflow of electrons into the ion thruster. Back- streaming electrons are accelerated by the large potential difference that exists between the ion- thruster acceleration electrodes, which otherwise accelerates positive ions out of the engine to develop thrust. The energetic beam formed by the backstreaming electrons can damage the discharge cathode, as well as other discharge surfaces upstream of the acceleration electrodes. The electron-backstreaming condition occurs when the center potential of the ion accelerator grid is no longer sufficiently negative to prevent electron diffusion back into the ion thruster. This typically occurs over extended periods of operation as accelerator-grid apertures enlarge due to erosion. As a result, ion thrusters are required to operate at increasingly negative accelerator-grid voltages in order to prevent electron backstreaming. These larger negative voltages give rise to higher accelerator-grid erosion rates, which in turn accelerates aperture enlargement. Electron backstreaming due to accelerator-grid-hole enlargement has been identified as a failure mechanism that will limit ion-thruster service lifetime.

Posted in: Briefs, TSP

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Alternative Electrochemical Systems for Ozonation of Water

Hydrogen gas, ozone gas, and ozonated water can be delivered under pressure. Electrochemical systems that are especially well suited for the small-scale generation of ozone and ozonated water for local use have been invented. These systems can operate with very little maintenance, and the only inputs needed during operation are electric power and water. These systems are closely related to the ones described in "Electrochemical Systems Generate Ozone and Ozonated Water" (MSC-23046), NASA Tech Briefs, Vol. 26, No. 3 (March 2002), page 68. Ozonated water produced by these systems can be used in diverse industrial applications: A few examples include sterilization in the brewing industry, general disinfection, and treatment of sewage and recycled water.

Posted in: Briefs

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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.

Posted in: Briefs, TSP

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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.

Posted in: Briefs, TSP

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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.

Posted in: Briefs

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