Materials & Coatings

Ceramic Fiber Structures for Cryogenic Load-Bearing Applications

Woven or braided fibers resist embrittlement under cryogenic conditions, enabling ultralow-temperature applications.

This invention is intended for use as a load-bearing device under cryogenic temperatures and/or abrasive conditions (i.e., during missions to the Moon). The innovation consists of small-diameter, ceramic fibers that are woven or braided into devices like ropes, belts, tracks, or cables. The fibers can be formed from a variety of ceramic materials like silicon carbide, carbon, aluminosilicate, or aluminum oxide. The fiber architecture of the weave or braid is determined by both the fiber properties and the mechanical requirements of the application. A variety of weave or braid architectures is possible for this application. Thickness of load-bearing devices can be achieved by using either a 3D woven structure, or a layered, 2D structure. For the prototype device, a belt approximately 0.10 in. (0.25 cm) thick, and 3.0 in. (7.6 cm) wide was formed by layering and stitching a 2D aluminosilicate fiber weave. The circumferential length of the 2D, layered belt was approximately 36 in. (91 cm).

Posted in: Briefs, TSP, Materials, Ceramics, Fibers, Materials properties, Spacecraft

Elastomer Reinforced With Carbon Nanotubes

Elastomers are reinforced with functionalized, single-walled carbon nanotubes (SWNTs) giving them high- breaking strain levels and low densities. Cross-linked elastomers are prepared using amine-terminated, poly(dimethylsiloxane) (PDMS), with an average molecular weight of 5,000 daltons, and a functionalized SWNT.

Posted in: Briefs, Materials

Improving Heat Flux Performance of Flat Surface in Spray-Cooling Systems

A method has been developed for improving heat flux performance relative to flat surfaces in spray-cooling systems. Similar enhancement techniques have been used for convective heat transfer, but, to the best knowledge at the time of this reporting, never spray cooling of foam. Previous studies have shown that spray-cooling heat flux enhancements may be attained using enhanced surfaces. However, most enhanced surface spray-cooling studies have been limited to extended and/or embedded surface structures. This study investigates the effect of foam on spraycooling heat flux.

Posted in: Briefs, Materials, Thermal management, Cooling, Foams

Treating Fibrous Insulation To Reduce Thermal Conductivity

A chemical treatment reduces the convective and radiative contributions to the effective thermal conductivity of porous fibrous thermal-insulation tile. The net effect of the treatment is to coat the surfaces of fibers with a mixture of transition-metal oxides (TMOs) without filling the pores. The TMO coats reduce the cross-sectional areas available for convection while absorbing and scattering thermal radiation in the pores, thereby rendering the tile largely opaque to thermal radiation.

Posted in: Briefs, TSP, Materials

Silica-Aerogel Composites Opacified With La0.7Sr0.3MnO3

Sizes of La0.7Sr0.3MnO3 particles affect their effectiveness as opacifiers.

As part of an effort to develop improved lightweight thermal-insulation tiles to withstand temperatures up to 1,000 °C, silica aerogel/fused-quartz- fiber composite materials containing La0.7Sr0.3MnO3 particles as opacifiers have been investigated as potentially offering thermal conductivities lower than those of the otherwise equivalent silica-aerogel composite materials not containing La0.7Sr0.3MnO3 particles. The basic idea of incorporating opacifying particles into silica-aerogels composite to reduce infrared radiative contributions to thermal conductivities at high temperatures is not new: it has been reported in a number of previous NASA Tech Briefs articles. What is new here is the selection of La0.7Sr0.3MnO3 particles as candidate opacifiers that, in comparison with some prior opacifiers (carbon black and metal nanoparticles), are more thermally stable.

Posted in: Briefs, Materials, Composite materials, Insulation, Materials properties, Silicon alloys

Biologically Inspired Purification and Dispersion of SWCNTs

A biologically inspired method has been developed for (1) separating single-wall carbon nanotubes (SWCNTs) from other materials (principally, amorphous carbon and metal catalysts) in raw production batches and (2) dispersing the SWCNTs as individual particles (in contradistinction to ropes and bundles) in suspension, as required for a number of applications. Prior methods of purification and dispersal of SWCNTs involve, variously, harsh physical processes (e.g., sonication) or harsh chemical processes (e.g., acid reflux). These processes do not completely remove the undesired materials and do not disperse bundles and ropes into individual suspended SWCNTs. Moreover, these processes cut long SWCNTs into shorter pieces, yielding typical nanotube lengths between 150 and 250 nm.

Posted in: Briefs, TSP, Materials

Quantifying Airborne Hydrogen in Nearly Real Time

An indirect method of measuring small concentrations of hydrogen gas in air in nearly real time has been devised to circumvent the difficulty of performing such measurements directly. In this method, a sample of air suspected of containing hydrogen is first enclosed in a suitable container, and its humidity is measured. The enclosed sample is then exposed to ultraviolet light (typically at a wavelength of 254 nm), which photolyzes the hydrogen to water vapor. The exposure time needed for photolysis is of the order of minutes, the exact value depending on the shape and size of the sample container.

Posted in: Briefs, TSP, Materials

Silicon Nanowire Growth at Chosen Positions and Orientations

There are numerous potential applications in highly miniaturized sensors and electronic devices.

It is now possible to grow silicon nanowires at chosen positions and orientations by a method that involves a combination of standard microfabrication processes. Because their positions and orientations can be chosen with unprecedented precision, the nanowires can be utilized as integral parts of individually electronically addressable devices in dense arrays.

Posted in: Briefs, TSP, Materials, Fabrication, Nanotechnology, Semiconductors, Silicon alloys

Detecting Airborne Mercury by Use of Palladium Chloride

These sensors can be regenerated under relatively mild conditions.

Palladium chloride films have been found to be useful as alternatives to the gold films heretofore used to detect airborne elemental mercury at concentrations of the order of parts per billion (ppb). Somewhat more specifically, when suitably prepared palladium chloride films are exposed to parts-per-billion or larger concentrations of airborne mercury, their electrical resistances change by amounts large enough to be easily measurable. Because airborne mercury adversely affects health, it is desirable to be able to detect it with high sensitivity, especially in enclosed environments in which there is a risk of leakage of mercury from lamps or other equipment.

Posted in: Briefs, TSP, Materials

Detecting Airborne Mercury by Use of Gold Nanowires

Mercury has been detected at concentrations as low as 2 ppb.

Like the palladium chloride (PdCl2) films described in the immediately preceding article, gold nanowire sensors have been found to be useful for detecting airborne elemental mercury at concentrations on the order of parts per billion (ppb). Also like the PdCl2 films, gold nanowire sensors can be regenerated under conditions much milder than those necessary for regeneration of gold films that have been used as airborne-Hg sensors. The interest in nanowire sensors in general is prompted by the expectation that nanowires of a given material covering a given surface may exhibit greater sensitivity than does a film of the same material because nanowires have a greater surface area.

Posted in: Briefs, TSP, Materials, Sensors and actuators, Air pollution, Metals, Nanotechnology, Hazardous materials

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