Materials

Alkaline Capacitors Based on Nitride Nanoparticles

One key to success is an oxygen-free, plasma-assisted nitride-synthesis process. High-energy-density alkaline electrochemical capacitors based on electrodes made of transition-metal nitride nanoparticles are undergoing development. Transition-metal nitrides (in particular, Fe3N and TiN) offer a desirable combination of high electrical conductivity and electrochemical stability in aqueous alkaline electrolytes like KOH. The high energy densities of these capacitors are attributable mainly to their high capacitance densities, which, in turn, are attributable mainly to the large specific surface areas of the electrode nanoparticles. Capacitors of this type could be useful as energy-storage components in such diverse equipment as digital communication systems, implanted medical devices, computers, portable consumer electronic devices, and electric vehicles.

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Low-EC-Content Electrolytes for Low-Temperature Li-Ion Cells

Electrolytes comprising LiPF6 dissolved at a concentration of 1.0 M in three different mixtures of alkyl carbonates have been found well suited for use in rechargeable lithium-ion electrochemical cells at low temperatures. These and other electrolytes have been investigated in continuing research directed toward extending the lower limit of practical operating temperatures of Li-ion cells down to –60 °C. This research at earlier stages was reported in numerous previous NASA Tech Briefs articles, the three most recent being "Ethyl Methyl Carbonate as a Cosolvent for Lithium-Ion Cells" (NPO-20605), Vol. 25, No. 6 (June 2001), page 53; "Alkyl Pyrocarbonate Electrolyte Additives for Li-Ion Cells" (NPO-20775), Vol. 26, No. 5 (May 2002), page 37; and "Fluorinated Alkyl Carbonates as Cosolvents in Li-Ion Cells (NPO-21076), Vol. 26, No. 05 (May 2002), page 38. The present solvent mixtures, in terms of volume proportions of their ingredients, are 1 ethylene carbonate (EC) + 1 diethyl carbonate (DEC) + 1 dimethyl carbonate (DMC) + 3 ethyl methyl carbonate (EMC); 3EC + 3DMC + 14EMC; and 1EC + 1DEC + 1DMC + 4EMC. Relative to similar mixtures reported previously, the present mixtures, which contain smaller proportions of EC, have been found to afford better performance in experimental Li-ion cells at temperatures <–20 °C.

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Advancements in Technology for Controlling Fiber Orientation in Composite Parts

The performance of a composite part is primarily determined by the orientation of fibers in the plies. Designers wishing to exploit the full potential of composite materials, while avoiding manufacturing problems and part failures, must define and control fiber orientation. Anticipating true fiber orientation for a single ply is seldom intuitive, and predicting the behavior of an entire laminate made of tens or hundreds of plies is nearly impossible.

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Glass/Ceramic Composites for Sealing Solid Oxide Fuel Cells

Ceramic fillers in a glass contribute to strength and fracture toughness. A family of glass/ceramic composite materials has been investigated for use as sealants in planar solid oxide fuel cells. These materials are modified versions of a barium calcium aluminosilicate glass developed previously for the same purpose. The composition of the glass in mole percentages is 35BaO + 15CaO + 5Al2O3 + 10B2O3 + 35SiO2. The glass seal was found to be susceptible to cracking during thermal cycling of the fuel cells.

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Composite Elastic Skins for Shape-Changing Structures

Anisotropic stiffness properties can be tailored for specific applications. Composite elastic skins having tailorable mechanical properties have been invented for covering shape-changing (“morphable”) structures. These skins are intended especially for use on advanced aircraft that change shapes in order to assume different aerodynamic properties.

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Oxygen-Permeable, Hydrophobic Membranes of Silanized α-Al2O3

These membranes perform better than do organic polymer oxygen-diffusion membranes. Membranes made of silanized alumina have been prepared and tested as prototypes of derivatized ceramic membranes that are both highly permeable to oxygen and hydrophobic. Improved oxygen-permeable, hydrophobic membranes would be attractive for use in several technological disciplines, including supporting high-temperature aqueous-phase oxidation in industrial production of chemicals, oxygenation of aqueous streams for bioreactors, and oxygenation of blood during open-heart surgery and in cases of extreme pulmonary dysfunction. In comparison with organic polymeric oxygen-permeable membranes now commercially available, the derivatized ceramic membranes are more chemically robust, are capable of withstanding higher temperatures, and exhibit higher oxygen-diffusion coefficients.

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Thermoelectric Inhomogeneities in (Ag1-SbTe2)x(PbTe)1-x

A document presents a study of why materials of composition (Ag1–ySbTe2)0.05 (PbTe)0.95 [0=y=1] were previously reported to have values of the thermoelectric figure of merit [ZT (where Z = a2/??, a is the Seebeck coefficient, ? is electrical resistivity, ? is thermal conductivity, and T is absolute temperature)] ranging from 2. In the study, samples of (AgSbTe2)0.05(PbTe)0.95, (Ag0.67SbTe2)0.05 (PbTe)0.95, and (Ag0.55SbTe2)0.05(PbTe)0.95 were prepared by melting followed, variously, by slow or rapid cooling. Analyses of these samples by x-ray diffraction, electron microscopy, and scanning microprobe measurements of the Seebeck coefficient led to the conclusion that these materials have a multiphase character on a scale of the order of millimeters, even though they appear homogeneous in x-ray diffraction and electron microscopy. The Seebeck measurements showed significant variations, including both n-type and p-type behavior in the same sample. These variations were found to be consistent with observed variations of ZT. The rapidly quenched samples were found to be less inhomogeneous than were the furnace-cooled ones; hence, rapid quenching was suggested as a basis of research on synthesizing more nearly uniform high-ZT samples.

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