Physical Sciences

Evaluation of Charge Storage and Decay in Spacecraft Insulators

Two reports discuss methods for evaluating the magnitude of electrostatic charging that occurs in spacecraft dielectric materials (in particular, polyimides) during prolonged exposure to radiation in outer space. The reports describe experiments on the electrical resistivities and charge-storage properties of polyimide specimens in a dark, evacuated environment, both before and after 5-megarad exposures to Υ rays from cobalt-60. The experiments were designed to measure these properties not under standard conditions prescribed for testing dielectrics in air but, rather, under conditions approximating those in the intended spacecraft applications. The results of the experiments showed that the electrical resistivities of the insulations as determined under these conditions are greater, by a factor of roughly a thousand, than those determined under the standard conditions and that the g irradiation reduced resistivities marginally.

Posted in: Briefs, TSP, Physical Sciences, Insulation, Radiation protection, Test procedures, Spacecraft


Nanoscale Hot-Wire Probes for Boundary-Layer Flows

Flow parameters near walls would be measured with unprecedented resolution. Hot-wire probes having dimensions of the order of nanometers have been proposed for measuring temperatures (and possibly velocities) in boundary-layer flows at spatial resolutions much finer and distances from walls much smaller than have been possible heretofore. The achievable resolutions and minimum distances are expected to be of the order of tens of nanometers — much less than a typical mean free path of a molecule and much less than the thickness of a typical flow boundary layer in air at standard temperature and pressure. An additional benefit of the small scale of these probes is that they would perturb the measured flows less than do larger probes.

Posted in: Briefs, TSP, Physical Sciences, Measurements, Sensors and actuators, Nanotechnology, Thermal testing


Silicon Membrane Mirrors With Electrostatic Shape Actuators

Precise shapes could be maintained over a wide temperature range. Efforts are under way to develop deformable mirrors equipped with microscopic electrostatic actuators that would be used to maintain their reflective surfaces in precise shapes required for their intended applications. Unlike actuators that depend on properties of materials (e.g., piezoelectric and electrostrictive actuators), electrostatic actuators are effective over a wide temperature range. A mirror of the present type would be denoted a MEMS-DM (for microelectromechanical system deformable mirror). The reflective surface of such a mirror would be formed on a single-crystal silicon membrane that would be attached by posts to a silicon actuator membrane that would, in turn, be attached by posts to a rigid silicon base (see figure).

Posted in: Briefs, TSP, Physical Sciences, Mirrors, Capacitors, Microelectromechanical devices, Sensors and actuators, Silicon alloys, Mountings


Fiber-Optic Ammonia Sensors

Reversible, colorimetric fiber-optic sensors are undergoing development for use in measuring concentrations of ammonia in air at levels relevant to human health [0 to 50 parts per million (ppm)]. A sensor of this type includes an optical fiber that has been modified by replacing a portion of its cladding with a polymer coat that contains a dye that reacts reversibly with ammonia and changes color when it does so. The change in color is measured as a change in the amount of light transmitted from one end of the fiber to the other. Responses are reversible and proportional to the concentration of ammonia over the range from 9 to 175 ppm and in some cases the range of reversibility extends up to 270 ppm. The characteristic time for the response of a sensor to rise from 10 to 90 percent of full scale is about 25 seconds. These sensors are fully operational in pure carbon dioxide and are not adversely affected by humidity.

Posted in: Briefs, Physical Sciences, Fiber optics, Sensors and actuators, Chemicals


Compact System Detects Potentially Explosive Gas Mixtures

This system can be used in environments too severe for conventional leak detectors. The figure depicts selected aspects of a "smart" microelectronic-based hazardous-gas-detection system that simultaneously measures concentrations of hydrogen and oxygen. Unlike conventional gas-leak-detection systems built around mass spectrometers, this system is not restricted to operation in relatively mild and controlled laboratory or shop environments; instead, this system can operate over a range of temperatures and pressures. Also, in comparison with conventional mass-spectrometer-based leak-detection systems, this system is more robust and compact, weighs less, and consumes less power.

Posted in: Briefs, TSP, Physical Sciences, Gases, Hazardous materials


Scanning Thermography

Large objects can be scanned fairly quickly. Scanning thermography is a noncontact, nondestructive technique that makes it possible to find defects hidden inside structural components in a variety of settings. Scanning thermography can be used to perform inspections of objects that may have large areas and a variety of shapes and that are found in a variety of settings that include, but are not limited to, production lines, industrial tanks and pipes, aircraft, power plants, and bridges. Scanning thermography is applicable to diverse structural materials, including metals, plastics, laminated polymer-matrix composites, and bonded aluminum composites, to name a few. Defects that can be detected by scanning thermography include cracks, disbonds (delaminations), corrosion, and wear.

Posted in: Briefs, Physical Sciences, Composite materials, Metals, Plastics, Inspections, Non-destructive tests, Thermal testing


Ultrahigh-Vacuum Arc-Jet Source of Nitrogen for Epitaxy

Electron-excitation and translational energies can be selected. An arc-jet source of chemically active nitrogen atoms has been developed for use in molecular-beam epitaxy (MBE) to grow such III-V semiconductors as nitrides of gallium, aluminum, and indium. This apparatus utilizes a confined arc to thermally excite N2 and to dissociate N2 into N atoms. This apparatus is compatible with other, ultrahigh-vacuum MBE equipment commonly used in growing such materials.

Posted in: Briefs, TSP, Physical Sciences, Gases, Semiconductors


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