Materials

Molybdate Coatings for Protecting Aluminum Against Corrosion

These coatings show promise, but further development is needed. Conversion coatings that comprise mixtures of molybdates and several additives have been subjected to a variety of tests to evaluate their effectiveness in protecting aluminum and alloys of aluminum against corrosion. Molybdate conversion coatings are under consideration as replacements for chromate conversion coatings, which have been used for more than 70 years. The chromate coatings are highly effective in protecting aluminum and its alloys against corrosion but are also toxic and carcinogenic. Hexavalent molybdenum and, hence, molybdates containing hexavalent molybdenum, have received attention recently as replacements for chromates because molybdates mimic chromates in a variety of applications but exhibit significantly lower toxicity.

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Single-Wall Carbon Nanotube Anodes for Lithium Cells

Capacities are greater than those of graphite anodes. In recent experiments, highly purified batches of single-wall carbon nanotubes (SWCNTs) have shown promise as superior alternatives to the graphitic carbon-black anode materials heretofore used in rechargeable thin-film lithium power cells. The basic idea underlying the experiments is that relative to a given mass of graphitic carbon-black anode material, an equal mass of SWCNTs can be expected to have greater lithium-storage and charge/discharge capacities. The reason for this expectation is that whereas the microstructure and nanostructure of a graphitic carbon black is such as to make most of the interior of the material inaccessible for intercalation of lithium, a batch of SWCNTs can be made to have a much more open microstructure and nanostructure, such that most of the interior of the material is accessible for intercalation of lithium. Moreover, the greater accessibility of SWCNT structures can be expected to translate to greater mobilities for ion-exchange processes and, hence, an ability to sustain greater charge and discharge current densities.

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Microsphere Insulation Panels

Thermal performance and lifetime exceed those of foam insulation. Microsphere insulation panels (MIPs) have been developed as lightweight, long lasting replacements for the foam and vacuum-jacketed systems heretofore used for thermally insulating cryogenic vessels and transfer ducts. Whether preformed or applied in place, foam insulation deteriorates fairly rapidly: on cryogenic transfer lines, it has a life expectancy of about three years. Vacuum-jacketed insulation is expensive and heavy. For both foam and vacuum-jacketed insulation, intensive maintenance is necessary to keep performance at or near its original level. Relative to a polyurethane foam insulation panel, a comparable MIP offers greater thermal performance and longer service life at approximately the same initial cost.

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Tantalum-Based Ceramics for Refractory Composites

Compositions can be graded from porous substrates to impervious outer layers. A family of tantalum-based ceramics has been invented as ingredients of high-temperature composite insulating tiles. These materials are suitable for coating and/or permeating the outer layers of rigid porous (foamlike or fibrous) ceramic substrates to (1) render the resulting composite ceramic tiles impervious to hot gases and (2) enable the tiles to survive high heat fluxes at temperatures that can exceed 3,000 °F (≈1,600 °C). Originally intended for use on the future space exploration vehicles, insulating tiles made with these materials may also be useful in terrestrial applications (e.g., some industrial processes) in which there are requirements to protect against flows of hot, oxidizing gases.

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Wire-Mesh-Based Sorber for Removing Contaminants From Air

A paper discusses an experimental regenerable sorber for removing CO2 and trace components — principally, volatile organic compounds, halocarbons, and NH3 — from spacecraft cabin air. This regenerable sorber is a prototype of what is intended to be a lightweight alternative to activated-carbon and zeolite- pellet sorbent beds now in use. The regenerable sorber consists mainly of an assembly of commercially available meshes that have been coated with a specially formulated washcoat containing zeolites. The zeolites act as the sorbents while the meshes support the zeolite-containing washcoat in a configuration that affords highly effective surface area for exposing the sorbents to flowing air. The meshes also define flow paths characterized by short channel lengths to prevent excessive buildup of flow boundary layers. Flow boundary layer resistance is undesired because it can impede mass and heat transfer. The total weight and volume comparison versus the atmosphere revitalization equipment used onboard the International Space Station for CO2 and trace-component removal will depend upon the design details of the final embodiment. However, the integrated mesh-based CO2 and trace-contaminant removal system is expected to provide overall weight and volume savings by eliminating most of the trace-contaminant control equipment presently used in parallel processing schemes traditionally used for spacecraft. The mesh-based sorbent media enables integrating the two processes within a compact package. For the purpose of regeneration, the sorber can be heated by passing electric currents through the metallic meshes combined with exposure to space vacuum. The minimal thermal mass of the meshes offers the potential for reduced regeneration- power requirements and cycle time required for regeneration compared to regenerable sorption processes now in use.

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Low-Density, Creep Resistant Single Crystal Superalloys

Weights of aircraft turbine rotors could be reduced significantly. Several recently formulated nickelbase superalloys have been developed with excellent high-temperature creep resistance, at lower densities than those of currently used nickel-base superalloys. These alloys are the latest products of a continuing effort to develop alloys that have even greater strength-toweight ratios, suitable for use in turbine blades of aircraft engines. Mass densities of turbine blades exert a significant effect on the overall weight of aircraft. For a given aircraft, a reduction in the density of turbine blades enables design reductions in the weight of other parts throughout the turbine rotor, including the disk, hub, and shaft, as well as supporting structures in the engine. The resulting total reduction in weight can be 8 to 10 times that of the reduction in weight of the turbine blades.

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Aerogels for Thermal Insulation of Thermoelectric Devices

Energy-conversion efficiencies would be increased and operational lifetimes prolonged. Silica aerogels have been shown to be attractive for use as thermal insulation materials for thermoelectric devices. It is desirable to thermally insulate the legs of thermoelectric devices to suppress lateral heat leaks that degrade thermal efficiency. Aerogels offer not only high thermal insulation effectiveness, but also a combination of other properties that are especially advantageous in thermoelectric device applications.

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