Materials

Filled Skutterudites as Thermoelectric Materials

The highest known thermoelectric figure of merit was observed in one of these materials. Filled skutterudites have shown promise as semiconducting materials with superior thermoelectric properties at temperatures up to at least 650 °C. This finding is a breakthrough in a continuing investigation of the potential utility of skutterudites as thermoelectric materials. Previous results of this investigation were reported in several articles in NASA Tech Briefs; namely, "Skutterudite Compounds for Power Semiconductor Devices" (NPO-19378), NASA Tech Briefs, Vol. 20, No. 3 (March 1996), page 60; "Two Potentially Useful Ternary Skutterudite Compounds" (NPO-19409), NASA Tech Briefs, Vol. 20, No. 11 (November 1996), page 66; and "Preparation and Some Properties of n-Type IrxCo1 — xSb3" (NPO-19852), NASA Tech Briefs, Vol. 20, No. 11 (November 1996), page 94.

Posted in: Materials, Briefs, TSP

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Internal-Combustion Engines With Ringless Carbon Pistons

Efficiencies would be higher and weights lower than those of conventional engines. Internal-combustion engines would be constructed with cylinders and ringless pistons made of lightweight carbon/carbon composite materials, according to a proposal. This proposal is a logical extension of previous research that showed that engines that contain carbon/carbon pistons with conventional metal piston rings running in conventional metal cylinders perform better than do engines with conventional aluminum-alloy pistons. The observed performance improvement (measured as increased piston life during high-performance operation) can be attributed mainly to the low thermal expansion of the carbon-carbon composite. Carbon-carbon pistons can continue to operate under thermal loads that cause aluminum pistons to seize or sustain scuffing damage due to excessive thermal growth and thermal distortion.

Posted in: Materials, Briefs

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Ceramics Made From Wood

Properties can be tailored in many different ways. The term “ecoceramics” (a contraction of “environment-conscious ceramics”) denotes a class of ceramics made partly from wood-based products, which can include natural wood, sawdust, cardboard, and/ or paper. In addition to the environmental advantage of renewability of the carbonaceous ingredients, the concept of ecoceramics offers an advantage of tailorability of the properties of the ceramic end products.

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Annealing for Tailoring Au/GaN Schottky-Barrier Height

It should be possible to make metal/semiconductor contacts more reproducible. Annealing has been found to be an effective means of tailoring the height of a Schottky barrier between gold and gallium nitride. This finding offers promise for the development of improved metal contacts on GaN semiconductors. Heretofore, the commercialization of GaN semiconductor devices has been impeded by difficulties of fabrication and by nonreproducibility of the Schottky-barrier heights and other properties of the metal/GaN interfaces. Now it appears that annealing may be the key to making GaN devices with smaller unit-tounit variations of contact properties and, in particular, tailorability of Schottky-barrier heights over a wide energy range.

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Advanced Cryogenic Insulation System for Graphite/Organic Resin Composite Cryogenic-Tank Structures

Foam-filled honeycomb cores withstand tensile strain test in cryogenic conditions. Engineers at Rockwell International Corporation's Space System Division have developed a new method of insulating composite structural material that will stand up to the harsh environment of space. The new liquid-hydrogen cryogenic tankage proposed for advanced launch systems — such as the Reusable Launch Vehicle and the X-33 — will be made from a graphite/epoxy material. Although this composite material will produce a lighter-weight cryogenic insulation tank, current cryogenic insulation materials did not endure rigorous stress testing.

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Nanorod-Based Fast-Response Pressure-Sensitive Paints

Improved, nanostructured coatings could be used to measure rapid pressure fluctuations. A proposed program of research and development would be devoted to exploitation of nanomaterials in pressuresensitive paints (PSPs), which are used on wind-tunnel models for mapping surface pressures associated with flow fields. Heretofore, some success has been achieved in measuring steady-state pressures by use of PSPs, but success in measuring temporally varying pressures has been elusive because of the inherent slowness of the optical responses of these materials.

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Coating Carbon Fibers With Platinum

Uniform coats are produced relatively inexpensively. A process for coating carbon fibers with platinum has been developed. The process may also be adaptable to coating carbon fibers with other noble and refractory metals, including rhenium and iridium. The coated carbon fibers would be used as ingredients of matrix/fiber composite materials that would resist oxidation at high temperatures. The metal coats would contribute to oxidation resistance by keeping atmospheric oxygen away from fibers when cracks form in the matrices.

Posted in: Materials, Briefs

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