New Directions for Nanoscale Thermoelectric Materials Research
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New Directions for Nanoscale Thermoelectric Materials Research M. S. Dresselhaus,1 G. Chen,1 M. Y. Tang,1 R. G. Yang,1 H. Lee,1 D.Z. Wang,2 Z. F. Ren,2 J. P. Fleurial,3 and P. Gogna3 1 Massachusetts Institute of Technology, Cambridge, MA 02139 2 Boston College, Chestnut Hill, MA 02467 3 Jet Propulsion Laboratory, Cal Tech, Pasadena, CA 91109 Abstract Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena with both bulk samples containing nanoscale constituents and nanoscale materials exhibiting enhanced thermoelectric performance in their own right. Prior theoretical and experimental proof of principle studies on isolated quantum well and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents. In this review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications. A review of some of the results obtained to date are presented.
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INTRODUCTION
The field of thermoelectrics advanced rapidly in the 1950’s when the basic science of thermoelectric materials became well established, the important role of heavily doped semiconductors as good thermoelectric materials became accepted, and the thermoelectric material bismuth telluride was discovered and developed for commercialization. Thus in the 1950’s the thermoelectrics industry was launched. By that time it was already established that the effectiveness of a thermoelectric material could in an approximate way be described in terms of the dimensionless thermoelectric figure of merit, ZT = S 2 σT /κ, where S, σ T and κ are, respectively, the Seebeck coefficient, the electrical conductivity, the temperature and the thermal conductivity. Over the following 3 decades 1960–1990, only incremental gains were made in increasing ZT , with Bi 2 Te3 remaining the best commercial material today, with ZT ≈ 1. During this 3 decade period, the thermoelectrics field received little attention from the worldwide scientific research community. Nevertheless, the thermoelectrics industry grew slowly but steadily, by finding niche applications for space missions, laboratory equipment, and medical applications, where cost and energy efficiency were not as important as energy availability, reliability, and predictability. In the early 1990s, the US Department of Defense became interested in the potential of thermoelectrics for new types of applications, encouraging the research community to re-examine research opportunities for advancing thermoelectric materials to the point they could be used more competitively for cooling and power conversion applications. This attempt was successful in stimulating the research community to once again become active in this field and to find new research directions that would have an impact on future developments and would lead to thermoelectric materials with better performance. As a result of this stimulation, two different research approaches were taken for developing
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