Physical Sciences

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

Read More >>

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

Read More >>

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

Read More >>

Lightweight Mirrors for Orbiting Earth-Observing Instruments

A report discusses selected aspects of a continuing program to develop thermally stable, lightweight mirrors for planned Earth-observing spaceborne instruments. These mirrors are required to retain precise concave or convex surface figures required for diffraction-limited optical performance, even in the presence of transient, asymmetric thermal loads, which include solar heating and radiational cooling. In the first phase of the program, preliminary analyses were performed to select one of three types of mirror structures: one made of SiC, one made of Be, and a hybrid comprising a lightweight composite-material substructure supporting a glass face sheet that would be a substrate for the required precise optical surface. The hybrid structure was selected for further development because it would offer a combination of high stiffness and low mass and because, relative to the Be and SiC structures, (1) the coefficients of thermal expansion of its constituent materials and the resulting wavefront error would be smaller, and (2) it could be fabricated at lower cost. A prototype hybrid structure with an aperture diameter of 0.3 m was fabricated. Planned efforts in the next phase of the program include optical polishing of the glass face sheet and testing.

Posted in: Briefs, TSP

Read More >>

Assessing Energetic-Ion Effects Using Energetic Protons Only

Two reports describe a method of assessing the susceptibility of digital electronic equipment to upsets (bit errors) caused by impingement of energetic ions (both protons and heavier ions) in outer space. The method, which is applicable at the single-component, circuit-board, box, and totalsystem levels, involves terrestrial testing by use of 200-MeV protons only. Unlike in a prior method that involves lower-energy heavier ions, one need not place a test article in a vacuum or remove it from its normal packaging. One of the reports discusses the origin of the present method, describes the procedure for exposing various parts of a test article to an energetic-proton beam and analyzing the resulting test data to obtain radiation susceptibilities, and summarizes the experience gained by use of the method since its inception in the year 1995. The other report discusses mathematical modeling and development of software to estimate the effects of energetic heavier ions on the basis of testing by use of energetic protons only. Among the conclusions reached in this development is that the heavy-ion error rate can be estimated as an orbital-altitudedependent fraction of the proton error rate.

Posted in: Briefs

Read More >>

Quasi-Decoupling of Shvab-Zel’dovich Variables

A paper presents some novel conclusions concerning the Shvab-Zel’dovich (SZ) vari- ables, which are linear combinations of dep- endent variables in mathematical models of multicomponent, chemically reacting flows. The SZ variables represent scalar quantities that are conserved, that is, are not affected by chem- istry. The role of SZ variables is to decouple the conservation equations and make it simpler to solve them. However, SZ variables that entirely decouple the system of equations are generally defined only under the restrictive assumption that all Lewis numbers are unity (ALeU). Each Lewis number represents the ratio of a single species mass-diffusion characteristic time to the thermal conduction characteristic time. The present paper discusses the foregoing issues and further presents a mathematical analysis addressing the question of whether the ALeU assumption is a necessary condition for such decoupling. The conclusion reached in the analysis is that the ALeU assumption is sufficient but not necessary and that quasi-decoupling (that is partial decoupling) may be performed in the absence of thermal diffusion. When thermal diffusion is present, quasi-decoupling may still be performed subject to a controllable error.

Posted in: Briefs, TSP

Read More >>

Vacuum Leak Detection Using Piezoelectric Film

A technique for detecting a small leak of gas into a vacuum involves the use of a diaphragm made of a thin film of poly (vinylidene fluoride) [PVDF]. To exploit the piezoelectricity of PVDF for this purpose, both sides of the film are coated with thin, electrically conductive layers that serve as electrodes. Wires connect the electrodes to the input terminals of a buffer amplifier and associated circuitry that measures the voltage induced between the electrodes by the piezoelectric effect in the film. In operation, the diaphragm is moved around in the vacuum in the vicinity of, and facing toward, a suspected leak. When the diaphragm crosses the stream of leaking gas, the pressure of the gas impinging on the diaphragm bends the diaphragm, thereby inducing a voltage. In an experiment, a prototype sensor based on this concept generated a signal of about 60 mV from air leaking into a vacuum through an orifice 10 μm wide at a rate of 0.017 standard cm3/s. The noise floor of the sensor was found to be about 5 mV. It was concluded that even this initial unoptimized sensor should be able to detect leaks somewhat smaller than 0.01 standard cm3/s.

Posted in: Briefs

Read More >>

The U.S. Government does not endorse any commercial product, process, or activity identified on this web site.