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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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Lithium Alkoxide Electrolyte Additives for Lithium-Ion Cells

These additives help to extend operating temperatures to as low as –40 °C. Alkoxides of lithium have been found to be useful as electrolyte additives to improve the low-temperature performance of rechargeable lithium-ion electrochemical cells. As explained below, an additive of this type exerts beneficial electrochemical effects both within the bulk of the electrolyte and on the surface of the carbon anode, such that the low-temperature electrical characteristics of the cell are improved.

Posted in: Materials, Briefs, TSP

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Aliphatic Ester Electrolyte Additives for Lithium-Ion Cells

Higher-molecular-weight esters show promise for extending lower temperature limits. Aliphatic esters have been found to be useful as electrolyte additives for improving the low-temperature performances of rechargeable lithium-ion electrochemical cells. The discovery of the beneficial effects of these additives was made during continuing research directed toward extending the lower limit of operating temperatures of these cells. Other aspects of this research have been described in the immediately preceding article and in prior NASA Tech Briefs articles referenced therein.

Posted in: Materials, Briefs, TSP

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Ethyl Methyl Carbonate as a Cosolvent for Lithium-Ion Cells

A low freezing temperature and low viscosity contribute to low-temperature performance. Ethyl methyl carbonate (EMC) has been found to be a suitable cosolvent, along with three other carbonate solvents, for incorporation into electrolytes to enhance the low-temperature performance of rechargeable lithium-ion electrochemical cells. EMC is an asymmetric aliphatic carbonate, and, as noted in the first of the two immediately preceding articles, asymmetric carbonates confer certain benefits. In the research described in that article, the asymmetric carbonates were formed in situ, in reactions catalyzed by lithium alkoxide additives. In contrast, the present finding that EMC is a suitable cosolvent was made by following a different approach; namely, formulating the electrolyte solvents to include an asymmetric aliphatic carbonate — EMC — in the first place.

Posted in: Materials, Briefs, TSP

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Rechargeable Li-Ion Cells Containing TiS2 Anodes

Low-temperature performances exceed those of cells containing graphite anodes. Titanium disulfide has been found to be attractive as an alternative to graphite as the anode material in rechargeable lithium-ion electrochemical cells that are required to operate at temperatures below -20 °C. By using TiS2 as the anode material, LiCoO2 as the cathode material, and a suitable low-temperature electrolyte described below, it is possible to construct cells that exhibit superior low-temperature characteristics, including relatively high charge/discharge capacities, capabilities for charging and discharging at relatively high rates, and excellent retention of capacity after repeated charge/discharge cycling.

Posted in: Materials, Briefs, TSP

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Polyorganosiloxanes for Coating Porous Ceramic Insulation

Protective coating and repairs can be performed relatively easily. Liquid and paste polyorganosiloxane formulations (which become high-temperature- and oxidation-resistant organosilicone-based ceramics upon curing) have been invented for use in (1) protecting porous, lightweight ceramic thermal insulation materials against aeroconvective thermal degradation and (2) repairing and bonding such materials. These formulations were originally intended especially for application to the fibrous refractory composite insulation (FRCI) tiles that protect parts of the space shuttles during re-entry into the terrestrial atmosphere; they may also be suitable for application to similar insulating tiles in laboratory and industrial furnaces.

Posted in: Materials, Briefs

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Polyorganosiloxane Waterproofing for Porous Ceramics

Relatively nontoxic coating materials can be applied and cured easily. Liquid waterproofing agents based on polyorganosiloxanes have been invented for use in treating porous, lightweight, fibrous ceramic thermal-insulation materials in both tile (rigid) and blanket (flexible) forms. Whereas silane-based waterproofing materials developed previously for this purpose are toxic and volatile and must be applied in tedious procedures (involving repeated injection at multiple locations by use of syringes), the present formulations are nontoxic and nonvolatile and can be applied by ordinary coating procedures.

Posted in: Materials, Briefs

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