Physical Sciences

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

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, Physical Sciences, Mirrors, Composite materials, Test equipment and instrumentation

Electrostatic/Electrodynamic Nanoparticle-Capture Vessel

Particles could be sampled under remote or automatic control in harsh environments.

A proposed simple, portable, robust apparatus, capable of automated operation, has been proposed for collecting samples of selected biological or chemical species in harsh environments. The sampled species could range in size from molecules to nanoparticles (that is, particles with dimensions of the order of nanometers). The apparatus would select a biological or chemical species of interest for sampling by utilizing a combination of (1) electrostatic or electrodynamic fields and (2) a sieve containing holes of predetermined size.

Posted in: Briefs, TSP, Physical Sciences, Biological sciences, Containers, Chemicals, Test equipment and instrumentation

Radiometer for Measuring Cirrus-Cloud Ice and Water Vapor

Accurate measurements would contribute to understanding of weather and climate.

An airborne submillimeter-wavelength radiometer, expected to be built and tested in the near future, is designed primarily to yield measurement data that can be processed to quantify the ice contents and mean sizes (and, to some extent, the shapes) of ice crystals in cirrus clouds that range from optically thin to opaque. Secondarily, this radiometer is also designed to enable the characterization of watervapor profiles in the presence of optically thick clouds. The ice and water-vapor data are needed to improve understanding of processes that affect weather and climate.

Posted in: Briefs, TSP, Physical Sciences, Measurements, Data acquisition and handling, Data acquisition and handling, Water, Weather and climate, Test equipment and instrumentation

System For Measuring Osmotic Transport Properties of a Membrane

Membrane-testing operations are performed automatically.

The membrane test cell (MTC) is an automated laboratory apparatus that applies a known osmotic potential across a membrane and measures the kinetics of the resulting transport of solvents across the membrane as a function of time. Data acquired by use of the MTC should prove especially helpful in designing industrial processes that rely on membrane separation techniques. Examples of such processes include desalination, recovery of designated chemicals from process streams, and some recycling operations.

Posted in: Briefs, Physical Sciences, Measurements, Chemicals, Test equipment and instrumentation, Industrial vehicles and equipment

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, Physical Sciences, Finite element analysis, Sensors and actuators, Sensors and actuators, Product development, Conductivity, Gases

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, Physical Sciences, Mathematical analysis, Mathematical models, Thermodynamics, Thermodynamics

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, Physical Sciences, Mathematical models, Computer software / hardware, Computer software and hardware, Electronic equipment, Computer software / hardware, Computer software and hardware, Electronic equipment, Test procedures

Slab-Waveguide Interferometer for Sensing Ammonia in Wet Air

Ammonia changes the pattern of interference between TM0 and TM1 waveguide modes.

The figure depicts a single-arm, slabwave- guide interferometer that has been demonstrated to be useable as a means of sensing ammonia in wet air. The slabwaveguide portion of this device comprises a 2-μm-thick film of poly(methyl methacrylate) [PMMA] on a substrate of fused quartz. The PMMA layer acts as a waveguide because its index of refraction is greater than the indices of refraction of both the fused quartz on one side and the ambient air on the other side. The PMMA film is doped with bromocresol purple — an indicator dye that causes the index of refraction of the film to vary with the amount of ammonia that diffuses into the film from the ambient air. The remaining basic features of design and operation, as described below, are devoted to enhancing and measuring the change in an optical phase difference attributable to the change in the index of refraction and thus to the presence of ammonia.

Posted in: Briefs, TSP, Physical Sciences, Optics, Waveguides, Optics, Waveguides, Materials identification

Aircraft Anti-Icing Heaters Made From Expanded Graphite

These heaters could be lightweight and inexpensive enough to be practical for small aircraft.

Improved electrical resistance heaters for preventing the accum- ulation of ice on aircraft surfaces are undergoing development. The primary intended market for these heaters is that of small single- and twin-engine airplanes and helicopters, most of which have not been equipped with anti-icing heaters because the weights and costs of such heaters have made them impractical. The improved heaters are expected to add very little to the weights of aircraft and, when massproduced, to cost about half as much as do anti-icing systems of prior design. The aircraft could be equipped with high-output alternators to supply the additional electric power needed for the heaters.

Posted in: Briefs, TSP, Physical Sciences, Aircraft deicing, Graphite, Aircraft

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