Materials

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.

Posted in: Materials, Briefs, TSP

Read More >>

SiC Composite Turbine Vanes

Y-cloth was conceived to provide fiber reinforcement for sharp trailing edges. Turbine inlet guide vanes have been fabricated from composites of silicon carbide fibers in silicon carbide matrices. A unique design for a cloth made from SiC fibers makes it possible to realize the geometric features necessary to form these vanes in the same airfoil shapes as those of prior metal vanes.

Posted in: Materials, Briefs, TSP

Read More >>

Polymer Electrolytes for Rechargeable Lithium Batteries

Cyanoresins would be blended and complexed with Li salts. Polymeric electrolytes for rechargeable lithium-based electrochemical cells and batteries would be made by blending and complexing cyanoresins with lithium salts, according to a proposal. In particular, polymeric electrolytes for separators, carbon-composite anodes, and cathodes would be formulated from appropriate blends of different polymers that are mutually insoluble and do not chemically react with each other. As a result, each polymeric component would retain its specific desired characteristics in high-energy-density batteries that would be capable of long cycle lives and high charge/discharge rates. For example, one polymeric component could provide high ionic conductivity and charge-carrier concentration while another polymeric component would provide structural integrity. Conceivably, a lithium battery made with such materials could exhibit an energy density of 80 W×h/lb for more than 1,000 charge/discharge cycles. Batteries like this could be used in applications ranging from geosynchronous satellites to electric vehicles to small consumer electronic equipment.

Posted in: Materials, Briefs, TSP

Read More >>

Reactive Additives for Phenylethynyl-Containing Resins

Processability is improved. Phenylethynyl containing reactive additive (PERA) compounds and mixtures have been found to be useful for improving the processability of oligomers, polymers, cooligomers, and copolymers that contain phenylethynyl groups. The additives can be incorporated in different forms:

Posted in: Materials, Briefs

Read More >>

Multicomponent, Rare-Earth-Doped Thermal-Barrier Coatings

Thermal conductivities are reduced while maximum use temperatures are increased. Multicomponent, rare-earth-doped, perovskite-type thermal-barrier coating materials have been developed in an effort to obtain lower thermal conductivity, greater phase stability, and greater high-temperature capability, relative to those of the prior thermal-barrier coating material of choice, which is yttriapartially stabilized zirconia. As used here, “thermal-barrier coatings” (TBCs) denotes thin ceramic layers used to insulate air-cooled metallic components of heat engines (e.g., gas turbines) from hot gases. These layers are generally fabricated by plasma spraying or physical vapor deposition of the TBC materials onto the metal components.

Posted in: Materials, Briefs

Read More >>

Development of Carbon-Nanotube/Polymer Composites

A report presents a short discussion of one company’s effort to develop composites of carbon nanotubes in epoxy and other polymer matrices.

Posted in: Materials, Briefs

Read More >>

Modifications of a Composite-Material Combustion Chamber

Two short reports discuss modifications of a small, lightweight combustion chamber that comprises a carbon/carbon composite outer shell and an iridium/rhenium inner liner. The first report discusses chamber design modifications made as results of hot-fire tests and post-test characterization. The modifications were intended to serve a variety of purposes, including improving fabrication, reducing thermal-expansion mismatch stresses, increasing strength-to-weight ratios of some components, and improving cooling of some components. The second report discusses (1) the origin of stress in the mismatch between the thermal expansions of the Ir/Re liner and a niobium sleeve and flange attached to the carbon/carbon shell and (2) a modification intended to relieve the stress. The modification involves the redesign of an inlet connection to incorporate a compressible seal between the Ir/Re liner and the Nb flange. A nickel alloy was selected as the seal material on the basis of its thermal-expansion properties and its ability to withstand the anticipated stresses, including the greatest stresses caused by the high temperatures to be used in brazing during fabrication.

Posted in: Materials, Briefs

Read More >>