Materials

Ultrahigh-Temperature Ceramics

Materials are being developed to withstand temperatures above 1,650 °C. Figure 1. Examples of UHTC Components areshown that have been tested in the NASA AmesArc Jet facility to evaluate the materialsresponse in a simulated reentry environment.The cone and wedge models are representativeof the scale and geometries anticipated for useon UHTC sharp leading-edge vehicles.Ultrahigh temperature ceramics (UHTCs) are a class of materials that include the diborides of metals such as hafnium and zirconium. The materials are of interest to NASA for their potential utility as sharp leading edges for hypersonic vehicles. Such an application requires that the materials be capable of operating at temperatures, often in excess of 2,000 °C. UHTCs are highly refractory and have high thermal conductivity, an advantage for this application. UHTCs are potentially applicable for other high- temperature processing applications, such as crucibles for molten-metal processing and high-temperature electrodes.

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Rapid Fabrication of Carbide Matrix/Carbon Fiber Composites

Melt infiltration offers advantages over chemical vapor infiltration. Composites of zirconium carbide matrix material reinforced with carbon fibers can be fabricated relatively rapidly in a process that includes a melt infiltration step. Heretofore, these and other ceramic matrix composites have been made in a chemical vapor infiltration (CVI) process that takes months. The finished products of the CVI process are highly porous and cannot withstand temperatures above 3,000 °F (≈1,600 °C). In contrast, the melt-infiltration-based process takes only a few days, and the composite products are more nearly fully dense and have withstood temperatures as high as 4,350 °F (≈2,400 °C) in a highly oxidizing thrust chamber environment. Moreover, because the meltinfiltration- based process takes much less time, the finished products are expected to cost much less.

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Coating Thermoelectric Devices To Suppress Sublimation

Thermoelectric materials are covered with adherent, chemically stable metal outer layers. A technique for suppressing sublimation of key elements from skutterudite compounds in advanced thermoelectric devices has been demonstrated. The essence of the technique is to cover what would otherwise be the exposed skutterudite surface of such a device with a thin, continuous film of a chemically and physically compatible metal. Although similar to other sublimation- suppression techniques, this technique has been specifically tailored for application to skutterudite antimonides.

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Polyaniline Compounds for Protection Against Corrosion

Protective surface layers can be formulated and applied in various ways. Corrosion of iron and steel substrates can be inhibited by coating them with any of the wide variety of compounds denoted generally as polyanilines. A polyaniline suitable for this type of application can be in either an electrically conductive salt (doped) form or an electrically nonconductive base form. Typically, polyaniline is dissolved in an organic solvent and the resulting solution is applied to a substrate by spraying, dipping, or brushing. The solvent is then allowed to evaporate leaving the substrate coated with a solid film of polyaniline, typically 1 to 200µm thick.

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Composite-Matrix Regenerators for Stirling Engines

One can exploit the properties of composites to reduce thermal and flow losses. A prototype Stirling-engine regenerator containing a matrix made of carbon-fiber-based composite materials has been developed. The concept underlying this development effort is one of exploiting the properties of composite materials (e.g., the anisotropy of thermal conductivity of carbon fibers and the tailorability of composite materials and structures) to reduce thermal and flow losses below those of previously developed regenerators containing metal matrices.

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Photocatalytic/Magnetic Composite Particles

Magnetic agitation enhances effectiveness. Photocatalytic/ magnetic composite particles have been invented as improved means of exploiting established methods of photocatalysis for removal of chemical and biological pollutants from air and water. The photocatalytic components of the composite particles are formulated for high levels of photocatalytic activity, while the magnetic components make it possible to control the movements of the particles through the application of magnetic fields. The combination of photocatalytic and magnetic properties can be exploited in designing improved air- and water-treatment reactors.

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Water-Free Proton-Conducting Membranes for Fuel Cells

Fuel cells could be operated at higher temperatures for greater efficiency. Poly-4 -vinylpyridinebisulfate (P4VPBS) is a polymeric salt that has shown promise as a water-free proton-conducting material (solid electrolyte) suitable for use in membrane/electrode assemblies in fuel cells. Heretofore, proton-conducting membranes in fuel cells have been made from perfluorinated ionomers that cannot conduct protons in the absence of water and, consequently, cannot function at temperatures >100 °C. In addition, the stability of perfluorinated ionomers at temperatures >100 °C is questionable. However, the performances of fuel cells of the power systems of which they are parts could be improved if operating temperatures could be raised above 140 °C. What is needed to make this possible is a solidelectrolyte material, such as P4VPBS, that can be cast into membranes and that both retains proton conductivity and remains stable in the desired higher operating temperature range.

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