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# Silicon Nanowire Growth at Chosen Positions and Orientations

### There are numerous potential applications in highly miniaturized sensors and electronic devices.

It is now possible to grow silicon nanowires at chosen positions and orientations by a method that involves a combination of standard microfabrication processes. Because their positions and orientations can be chosen with unprecedented precision, the nanowires can be utilized as integral parts of individually electronically addressable devices in dense arrays.

Posted in: Briefs, TSP, Materials, Fabrication, Nanotechnology, Semiconductors, Silicon alloys

# Detecting Airborne Mercury by Use of Palladium Chloride

### These sensors can be regenerated under relatively mild conditions.

Palladium chloride films have been found to be useful as alternatives to the gold films heretofore used to detect airborne elemental mercury at concentrations of the order of parts per billion (ppb). Somewhat more specifically, when suitably prepared palladium chloride films are exposed to parts-per-billion or larger concentrations of airborne mercury, their electrical resistances change by amounts large enough to be easily measurable. Because airborne mercury adversely affects health, it is desirable to be able to detect it with high sensitivity, especially in enclosed environments in which there is a risk of leakage of mercury from lamps or other equipment.

Posted in: Briefs, TSP, Materials

# Detecting Airborne Mercury by Use of Gold Nanowires

### Mercury has been detected at concentrations as low as 2 ppb.

Like the palladium chloride (PdCl2) films described in the immediately preceding article, gold nanowire sensors have been found to be useful for detecting airborne elemental mercury at concentrations on the order of parts per billion (ppb). Also like the PdCl2 films, gold nanowire sensors can be regenerated under conditions much milder than those necessary for regeneration of gold films that have been used as airborne-Hg sensors. The interest in nanowire sensors in general is prompted by the expectation that nanowires of a given material covering a given surface may exhibit greater sensitivity than does a film of the same material because nanowires have a greater surface area.

Posted in: Briefs, TSP, Materials, Sensors and actuators, Air pollution, Metals, Nanotechnology, Hazardous materials

# Electrolytes for Low-Temperature Operation of Li-CFₓ Cells

A report describes a study of electrolyte compositions selected as candidates for improving the low-temperature performances of primary electrochemical cells that contain lithium anodes and fluorinated carbonaceous (CFx) cathodes. This study complements the developments reported in “Additive for Low-Temperature Operation of Li-(CF)n Cells” (NPO-43579) and Li/CFx Cells Optimized for Low-Temperature Operation (NPO-43585), which appear elsewhere in this issue of NASA Tech Briefs.

Posted in: Briefs, TSP, Materials

# Polyimide-Foam/Aerogel Composites for Thermal Insulation

### These composites may also afford enhanced acoustic attenuation.

Composites of specific types of polymer foams and aerogel particles or blankets have been proposed to obtain thermal insulation performance superior to those of the neat polyimide foams. These composites have potential to also provide enhanced properties for vibration dampening or acoustic attenuation. The specific type of polymer foam is denoted “TEEK-H,” signifying a series, denoted “H,” within a family of polyimide foams that were developed at NASA’s Langley Research Center and are collectively denoted “TEEK” (an acronym of the inventors’ names).

Posted in: Briefs, Materials, Thermal management, Performance upgrades, Composite materials, Foams, Insulation

# Yb₁₄MnSb₁₁ as a High- Efficiency Thermoelectric Material

### This material could supplant the state-of-theart material, SiGe.

Yb14MnSb11 has been found to be well-suited for use as a p-type thermoelectric material in applications that involve hot-side temperatures in the approximate range of 1,200 to 1,300 K. The figure of merit that characterizes the thermal-to-electric power-conversion efficiency is greater for this material than for SiGe, which, until now, has been regarded as the state-of-the art high-temperature p-type thermoelectric material. Moreover, relative to SiGe, Yb14MnSb11 is better suited to incorporation into a segmented thermoelectric leg that includes the moderate-temperature p-type thermoelectric material CeFe4Sb12 and possibly other, lower-temperature p-type thermoelectric materials.

Posted in: Briefs, Materials

# Room-Temperature Ionic Liquids for Electrochemical Capacitors

A document discusses room-temperature ionic liquids (RTILs) used as electrolytes in carbon-nanotube-based, electrochemical, double-layer capacitors. Unlike the previous electrolyte (EtNB4 in acetonitrile), the RTIL used here does not produce cyanide upon thermal decomposition and does not have a moisture sensitivity.

Posted in: Briefs, TSP, Materials, Capacitors, Electrolytes, Nanotechnology

# Ultra-Lightweight Self- Deployable Nanocomposite Structure for Habitat Applications

A document discusses self-deployable, rigidized structures that are ultra-lightweight and have gas barrier properties, space durability, and high impact resistance. Developed here are microcellular-foamed sandwich structures made from nanocomposite shape memory polymers (SMPs) with Cold-Hibernated Elastic Memory (CHEM) deployed technique for space structural components including space habitats. This type of foam sandwich also does not suffer from the toxicity problems of conventional foams, and has higher mechanical properties than those processed with conventional techniques. This design can be compacted into a very small volume for launch. Once deployed, the microcellular structure can use the heat from the Sun to recover 98 to 100 percent of its shape.

Posted in: Briefs, TSP, Materials

# Room-Temperature-Cured Copolymers for Lithium Battery Gel Electrolytes

### Room-temperature curing offers an important advantage in room-temperature functionality.

Polyimide-PEO copolymers (“PEO” signifies polyethylene oxide) that have branched rod-coil molecular structures and that can be cured into film form at room temperature have been invented for use as gel electrolytes for lithium-ion electric- power cells. These copolymers offer an alternative to previously patented branched rod-coil polyimides that have been considered for use as polymer electrolytes and that must be cured at a temperature of 200 °C. In order to obtain sufficient conductivity for lithium ions in practical applications at and below room temperature, it is necessary to imbibe such a polymer with a suitable carbonate solvent or ionic liquid, but the high-temperature cure makes it impossible to incorporate and retain such a liquid within the polymer molecular framework. By eliminating the high-temperature cure, the present invention makes it possible to incorporate the required liquid.

Posted in: Briefs, TSP, Materials, Lithium-ion batteries, Conductivity, Materials properties, Polymers
Several metal alloys have shown promise as improved catalysts for catalytic thermal decomposition of hydrocarbon gases to produce carbon nanotubes (CNTs). Heretofore almost every experiment on the production of carbon nanotubes by this method has involved the use of iron, nickel, or cobalt as the catalyst. However, the catalytic-conversion efficiencies of these metals have been observed to be limited. The identification of better catalysts is part of a continuing program to develop means of mass production of high-quality carbon nanotubes at costs lower than those achieved thus far (as much as $100/g for purified multi-wall CNTs or$1,000/g for single-wall CNTs in year 2002).