Thermal Conductiviy of type I and II Clathrate Compounds
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THERMAL CONDUCTIVIY OF TYPE I AND II CLATHRATE COMPOUNDS G.S. Nolas1, J.L. Cohn2, M. Kaeser3 and T.M. Tritt3 R&D Division, Marlow Industries, Inc., 10451 Vista Park Road, Dallas, Texas 75238 2 Department of Physics, University of Miami, Coral Gables, Florida 33124 3 Department of Physics, Clemson University, Clemson, South Carolina 29634 1
ABSTRACT Compounds with clathrate-hydrate type crystal lattice structures are currently of interest in thermoelectric materials research. This is due to the fact that semiconducting compounds can be synthesized with varying doping levels while possessing low, even ‘glass-like’, thermal conductivity. Up to now most of the work has focused on type I Si and Ge clathrates. Snclathrates however are viewed as having the greatest potential for thermoelectric cooling applications due to the larger mass of Sn and the expected small band-gap, as compared to Si and Ge clathrates. Transport properties on type I Sn-clathrates has only recently been reported [1-3]. In this report we present ongoing experimental research on both type I and II clathrates with an emphasis on the thermal transport of these novel materials. We present thermal conductivity data Si-Ge and Ge-Sn alloys as well as on a type II Ge clathrate for the first time, and compare these data to that of other clathrate compounds. INTRODUCTION The recent interest in thermoelectrics is fueled by new and novel materials and synthesis techniques presently under development in an effort to improve the efficiency of thermoelectric devices. One approach that has received enormous attention in the past few years is the ‘phononglass-electron-crystal’ (PGEC) approach introduced by Slack [4]. In such an approach the best thermoelectric material would be ‘engineered’ to have thermal properties similar to those of an amorphous material, ‘a phonon-glass’, and electronic properties similar to that of an ordered, highly covalent single crystal, ‘an electron single crystal’. The importance of this approach emerges very clearly from the definition of the dimensionless thermoelectric figure of merit, ZT, with ZT=TS2 ,QWKLVHTXDWLRQ6LVWKH6HHEHFNFRHIILFLHQW WKHHOHFWULFDOFRQGXFWLYLW\ WKH thermal conductivity and T the absolute temperature. In practice, however, the development of such a material system is not straightforward. Slack[4] has identified a series of crystal structures that can potentially be ‘engineered’ for PGEC properties. These are crystal systems with an ‘open structure’, i.e. with high coordination number, large voids in their lattice wherein loosely-bound ‘guest’ atoms may be inserted to create SKRQRQVFDWWHULQJFHQWHUVWKDWVXEVWDQWLDOO\VXSSUHVV 0XFKRIWKHZRUNRQWKLVDSSURDFKKDV
been dominated by the skutterudite family of compounds [5] however most recently Nolas and coworkers have presented evidence that certain compounds with the clathrate-hydrate crystal structure possess PGEC properties.[1-3, 6-12] The semiconductor clathrate compounds have crystal structures that are formed by fullerene-like poly
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