Structural Studies of GeTe-AgSbTe2 Alloys
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Structural Studies of GeTe-AgSbTe2 Alloys Alan Thompson1, Jeff Sharp1, C.J Rawn2, and B.C. Chackoumakos2 1 Materials Research and Development, Marlow Industries, 10451 Vista Park Rd., Dallas, TX, 75238 2 Materials Science and Technology, Oak Ridge National Laboroatory, Oak Ridge, TX, 37831 ABSTRACT GeTe, a small bandgap semiconductor that has native p-type defects due to Ge vacancies, is an important constituent in the thermoelectric material known as “TAGS” [1]. TAGS is an acronym for alloys of GeTe with AgSbTe2, and compositions are normally designated as TAGSx, where x is the fraction of GeTe. TAGS-85 is the most important with regard to applications, and there also is commercial interest in TAGS-80. The crystal structure of GeTe1+δ has a composition-dependent phase transformation at a temperature ranging from 430°C (δ = 0) to ~ 400°C (δ = 0.02) [2]. The high temperature form is cubic. The low temperature form is rhombohedral for δ < 0.01, as is the case for good thermoelectric performance. Addition of AgSbTe2 shifts the phase transformation to lower temperatures, and one of the goals of this work is a systematic study of the dependence of transformation temperature on the parameter x. We present results on phase transformations and associated instabilities in TAGS compositions in the range of 70-85 at.% GeTe.
INTRODUCTION Alloying silver antimony telluride, AgSbTe2, with germanium telluride, GeTe produces a pseudobinary semiconductor, (GeTe)1-y (AgSbTe2)y , (TAGS-x), where x represents the % GeTe. TAGS-x is a very good performing p-type power generation material over the temperature range of 500K to 720K. Successful utilization of TAGS-85 in radioisotope thermoelectric generator applications has covered four decades. More recent applications of TAGS in segmented power generation devices have reignited efforts to make improvements to the thermoelectric and mechanical properties. The dimensionless figure-of-merit, ZT, for TAGS85 reaches a maximum of 1.4 at ~750K. [1] The TAGS-80 composition has larger ZT, 1.5 at 750K, but an associated mechanical instability has eliminated the material from commercial use. [1] Christakudis et al have reported a TAGS-80 ZT as high as1.7. [8] A well known polymorphic phase transformation from a high temperature cubic (Fm-3m) phase to a low temperature rhombohedral (R3m) structure occurs in TAGS. The transition temperature and rhombohedral distortion is dependent on mole % GeTe, decreasing with decreasing mole % GeTe. [4, 8] It has been proposed that the inherent mechanical instability is due to the polymorphic phase transformation and the associated rhombohedral distortion. [4] In this work we report microstructural observations and thermoelectric properties of thermally cycled TAGS with compositions that have a transition temperature range near the intended device application temperature (see Table I). Temperature dependent phase transition data are also presented.
Table I. TAGS Compositions Studied Values of x in (GeTe)1-y(AgSbTe2)y TAGS-x y TAGS-70 0.30 TAGS-75 0.
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