Materials

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.

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Improved C/SiC Ceramic Composites Made Using PIP

These materials are expected to remain strong for longer times at high temperatures. Improved carbon-fiber- reinforced SiC ceramic-matrix composite (C/SiC CMC) materials, suitable for fabrication of thick-section structural components, are producible by use of a combination of raw materials and processing conditions different from such combinations used in the prior art. In comparison with prior C/SiC CMC materials, these materials have more nearly uniform density, less porosity, and greater strength. The majority of raw-material/processing-condition combinations used in the prior art involve the use of chemical vapor infiltration (CVI) for densifying the matrix.

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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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