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Prasad N.S.,NASA | Taylor P.,U.S. Army | Nemir D.,TXL Group, Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

Nanotechnology based thermoelectric materials are considered attractive for developing highly efficient thermoelectric devices. Nano-structured thermoelectric materials are predicted to offer higher ZT over bulk materials by reducing thermal conductivity and increasing electrical conductivity. Consolidation of nano-structured powders into dense materials without losing nanostructure is essential towards practical device development. Using the gas atomization process, amorphous nano-structured powders were produced. Shockwave consolidation is accomplished by surrounding the nanopowder-containing tube with explosives and then detonating. The resulting shock wave causes rapid fusing of the powders without the melt and subsequent grain growth. We have been successful in generating consolidated nanostructured bismuth telluride alloy powders by using the shockwave technique. Using these consolidated materials, several types of thermoelectric power generating devices have been developed. Shockwave consolidation is anticipated to generate large quantities of nanostructred materials expeditiously and cost effectively. In this paper, the technique of shockwave consolidation will be presented followed by Seebeck Coefficient and thermal conductivity measurements of consolidated materials. Preliminary results indicate a substantial increase in electrical conductivity due to shockwave consolidation technique. © 2014 SPIE.

Delgado A.,University of Texas at El Paso | Cordova S.,University of Texas at El Paso | Lopez I.,University of Texas at El Paso | Nemir D.,TXL Group, Inc. | Shafirovich E.,University of Texas at El Paso
Journal of Alloys and Compounds | Year: 2016

Magnesium silicide (Mg2Si) is a promising intermetallic compound for applications such as light-weight composite materials and thermoelectric energy conversion. It is difficult, however, to synthesize high-quality Mg2Si on a large scale. Self-propagating high-temperature synthesis (SHS) is an attractive pathway, but it is difficult to ignite the low-exothermic Mg/Si mixture and achieve a self-sustained propagation of the combustion wave. In the present paper, mechanical activation was used to facilitate the ignition. Magnesium and silicon powders were mixed and then milled in a planetary ball mill in an argon environment. The mixtures were compacted into pellets and ignited at the top in a reaction chamber filled with argon. Depending on the pellet dimensions and diameter-to-height ratio, two modes of combustion synthesis, viz., thermal explosion and SHS, were observed. In both modes, Mg2Si product was obtained. Thermocouple measurements have revealed that the exothermic reaction stages include two self-heating events separated by a long period of relatively slow interaction. To clarify the reaction mechanisms, differential scanning calorimetry was used, which also revealed two peaks of exothermic reaction in the milled Mg/Si mixture. The first peak is explained by the effect of mechanical activation. Explosive-based shockwave consolidation was used to increase the product density. Thermophysical properties of the obtained material were determined using a laser flash apparatus. © 2015 Elsevier B.V. All rights reserved.

Beck J.,TXL Group, Inc. | Alvarado M.,TXL Group, Inc. | Nemir D.,TXL Group, Inc. | Nowell M.,EDAX Inc. | And 2 more authors.
Journal of Electronic Materials | Year: 2012

Nanostructured thermoelectric powders can be produced using a variety of techniques. However, it is very challenging to build a bulk material from these nanopowders without losing the nanostructure. In the present work, nanostructured powders of the bismuth telluride alloy system are obtained in kilogram quantities via a gas atomization process. These powders are characterized using a variety of methods including scanning electron microscopy, transition electron microscopy, and x-ray diffraction analysis. Then the powders are consolidated into a dense bulk material using a shock-wave consolidation technique whereby a nanopowder-containing tube is surrounded by explosives and then detonated. The resulting shock wave causes rapid fusing of the powders without the melt and subsequent grain growth of other techniques. We describe the test setup and consolidation results. © 2012 TMS.

Agency: National Aeronautics and Space Administration | Branch: | Program: STTR | Phase: Phase I | Award Amount: 100.00K | Year: 2009

Thermoelectric (TE) generators/refrigerators have the advantages of lack of moving parts, quiet operation, and flexibility in deployment, but their use has been limited because of their relatively low conversion efficiency. Two major loss components are conductive (phonon) heat transfer through the TE lattice and parasitic losses at fabrication interfaces. Shock wave consolidation of thermoelectric nanopowders to produce TE devices will reduce both loss sources, leading to enhanced efficiency devices. The conversion efficiency of a TE device will always be thermodynamically limited by the Carnot ratio of (Th-Tc)/Th, where Th and Tc are the temperatures of the hot and cold junctions. Present technology thermoelectric devices can provide conversion efficiencies up to a third of the Carnot limit. With the restrictions on phonon transport acruing from nanopowder consolidation, conversion efficiencies of over 50% of the Carnot limit should be possible.

Agency: National Aeronautics and Space Administration | Branch: | Program: STTR | Phase: Phase II | Award Amount: 600.00K | Year: 2010

Thermoelectric (TE) generators have the advantages of no moving parts and flexibility in deployment but suffer from low heat to electricity conversion efficiencies, with a major loss component being conductive (phonon) heat transfer through the TE lattice. By using a high pressure shockwave consolidation, nanopowders can be fused into a solid bulk TE material while preserving the nanostructure. The high density of grain boundaries and lattice defects impedes phonon transport while allowing electron flow. Specific Phase 2 research thrusts will be directed at transitioning laboratory fabrication into volume manufacturing, at producing a graded thermoelectric that is optimized for different temperature ranges over the length of the element, and at preparing bulk thermoelectric material from transition metal trichalcogenides that are not appropriate for melt or powder sintered fabrication. The overall conversion efficiency of a TE device will always be limited by the Carnot ratio of (Th-Tc)/Th, where Th and Tc are the temperatures of the hot and cold junctions. With the restrictions on phonon transport accruing from nanopowder consolidation, conversion efficiencies in excess of 30% of the Carnot limit are reasonable.

Nemir D.,TXL Group, Inc. | Beck J.,TXL Group, Inc.
Journal of Electronic Materials | Year: 2010

When the electrical conductivity, σ, thermal conductivity, λ, and thermopower, S, of a material are all assumed to be constant over the temperature range of interest, then the well-known thermoelectric (TE) figure of merit, Z = σS 2/λ, arises as part of the derivation of conversion efficiency in a TE generator. However, there are an infinite number of parameter sets (σ, λ, S) that yield any given Z. So, are they truly equivalent? This paper reviews the historical basis for Z as a metric for TE quality and discusses results of simulations on three systems having different parameter sets but the same Z. The three systems exhibit different power generation capabilities, illustrating that Z is not sufficient to specify the likely performance of a TE material in a system. Instead, a systems analysis is required that incorporates, at a minimum, source and sink temperatures and thermal resistances. © 2010 TMS.

TXL Group, Inc. | Date: 2012-04-06

The explosive consolidation of semiconductor powders results in thermoelectric materials having reduced thermal conductivity without a concurrent reduction in electrical conductivity and thereby allows the construction of thermoelectric generators having improved conversion efficiencies of heat energy to electrical energy.

TXL Group, Inc. | Date: 2010-08-03

Thermoelectric generating apparatus used for converting heat energy to electrical energy, namely, thermoelectric generation module.

TXL Group, Inc. | Date: 2015-06-03

Systems and methods for producing a dense, well bonded solid material from a powder may include consolidating the powder utilizing any suitable consolidation method, such as explosive shockwave consolidation. The systems and methods may also include a post-processing thermal treatment that exploits a mismatch between the coefficients of thermal expansion between the consolidated material and the container. Due to the mismatch in the coefficients, internal pressure on the consolidated material during the heat treatment may be increased.

Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 150.00K | Year: 2015

ABSTRACT: Phase 1 research will validate an approach for material synthesis that combines mechanical alloying with explosive powder consolidation to produce bulk materials with novel properties. The main advantage to the approach is that it can be applied to diverse material systems, including the creation of alloys with otherwise insoluble constituents. An additional advantage is that it has a straightforward path for scale up to industrial levels of production. In Phase 1, elemental powders of bismuth, antimony and tellurium will be mechanically alloyed, both with and without inert nanoinclusions, to produce a powder with particles that are in a highly non-equilibrium state. Explosive powder consolidation will allow the imposition of dynamic pressures in excess of 10 GPa to accomplish densification and interparticle bonding without the attendant grain growth and loss of non-equilibrium features that occur with other powder metallurgy approaches. A series of post-consolidation heat treatments for the step restoration of the material to the equilibrium state allows a characterization of bulk properties as a function of departure from equilibrium. The specific application addressed in Phase 1 is the development of high performance p-type thermoelectric material, a choice that not only has immediate market potential but that also allows microstructural assessment through measurements of bulk quantities such as thermopower, electrical conductivity and thermal conductivity. BENEFIT: Non-equilibrium materials produced by shockwave synthesis may exhibit unique properties such as increased ductility, higher strength and higher melting point. A specific application that will benefit is in the production of thermoelectric materials where the high incidence of grain boundaries and lattice defects will reduce thermal conductivity and increase the figure-of-merit, Z. A higher Z thermoelectric material has an immediate home in generation and Peltier heat pumping applications.

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