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Scandia-and-Yttria-Stabilized Zirconia for Thermal Barriers

These compositions offer thermal stability greater than that of yttria-stabilized zirconia. Zirconia stabilized with both scandia and yttria in suitable proportions has shown promise of being a superior thermal-barrier coating (TBC) material, relative to zirconia stabilized with yttria only. More specifically, a range of compositions in the zirconia/scandia/yttria material system has been found to afford increased resistance to deleterious phase transformations at temperatures high enough to cause deterioration of yttria- stabilized zirconia.

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Switching and Rectification in Carbon-Nanotube Junctions

Research shows promise for nanoscale electronic devices. Figure 1. Symmetric and Asymmetric Carbon-Nanotube Multiterminal carbon-nanotube junctions are under investigation as candidate components of nanoscale electronic devices and circuits. Three-terminal "Y" junctions of carbon nanotubes (see Figure 1) have proven to be especially interesting because (1) it is now possible to synthesize them in high yield in a controlled manner and (2) results of preliminary experimental and theoretical studies suggest that such junctions could exhibit switching and rectification properties.

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Environmentally Safer, Less Toxic Fire-Extinguishing Agents

Water droplets would be microencapsulated in flame-retardant polymers. Fire-extinguishing agents comprising microscopic drops of water micro- encapsulated in flame-retardant polymers have been proposed as effective, less toxic, non-ozone- depleting, non-global- warming alternatives to prior fire-extinguishing agents. Among the prior fire-extinguishing agents are halons (various halocarbon fluids), which are toxic and contribute both to depletion of upper- atmospheric ozone and to global warming. Other prior fire-extinguishing agents are less toxic and less environmentally harmful but, in comparison with halons, are significantly less effective in extinguishing fires.

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Multiaxial Temperature- and Time-Dependent Failure Model

This model should be applicable to a variety of materials. A temperature- and time-dependent mathematical model predicts the conditions for failure of a material subjected to multiaxial stress. The model was initially applied to a filled epoxy below its glass-transition temperature, and is expected to be applicable to other materials, at least below their glass-transition temperatures. The model is justified simply by the fact that it closely approximates the experimentally observed failure behavior of this material: The multiaxiality of the model has been confirmed (see figure) and the model has been shown to be applicable at temperatures from —20 to 115 °F (–29 to 46 °C) and to predict tensile failures of constant-load and constant-load-rate specimens with failure times ranging from minutes to months.

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Oxide Protective Coats for Ir/Re Rocket Combustion Chambers

An improved material system has been developed for rocket engine combustion chambers for burning oxygen/hydrogen mixtures or novel monopropellants, which are highly oxidizing at operating temperatures. The baseline for developing the improved material system is a prior iridium/rhenium system for chambers burning nitrogen tetroxide/monomethyl hydrazine mixtures, which are less oxidizing. The baseline combustion chamber comprises an outer layer of rhenium that provides structural support, plus an inner layer of iridium that acts as a barrier to oxidation of the rhenium. In the improved material system, the layer of iridium is thin and is coated with a thermal fatigue-resistant refractory oxide (specifically, hafnium oxide) that serves partly as a thermal barrier to decrease the temperature and thus the rate of oxidation of the rhenium. The oxide layer also acts as a barrier against the transport of oxidizing species to the surface of the iridium. Tests in which various oxygen/hydrogen mixtures were burned in iridium/rhenium combustion chambers lined with hafnium oxide showed that the operational lifetimes of combustion chambers of the improved material system are an order of magnitude greater than those of the baseline combustion chambers.

Posted in: Materials, Briefs, TSP

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Simplified Waterproofing of Aerogels

Silanization is performed in a single treatment at moderate temperature and pressure. A relatively simple silanization process has been developed for waterproofing or rewaterproofing aerogels, xerogels, and aerogel/tile composites, and other, similar low-density, highly microporous materials. Such materials are potentially attractive for a variety of applications — especially for thermal-insulation panels that are required to be thin and lightweight. Unfortunately, such materials are also hydrophilic and tend to collapse after adsorbing water from the air. Hence, an effective means of waterproofing is necessary to enable practical exploitation of aerogels and the like.

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Improved Thermal-Insulation Systems for Low Temperatures

Efficient, robust insulation for soft vacuum. Improved thermal- insulation materials and structures and the techniques for manufacturing them are undergoing development for use in low-temperature applications. Examples of low- temperature equipment for which these thermal insulation systems could provide improved energy efficiency include storage tanks for cryogens, superconducting electric- power- transmission equipment, containers for transport of food and other perishable commodities, and cold boxes for low-temperature industrial processes. These systems could also be used to insulate piping used to transfer cryogens and other fluids, such as liquefied natural gas, refrigerants, chilled water, crude oil, or low-pressure steam.

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