Novel Thermal Transport in Stable Binary Cd 5.7 Yb Quasicrystals
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Novel Thermal Transport in Stable Binary Cd5.7Yb Quasicrystals A.L. Pope and T.M. Tritt Dept. of Physics and Astronomy Clemson University, Clemson, SC 29634 USA R. Gagnon and J. Strom-Olsen Dept. of Physics McGill University, Montreal, Canada ABSTRACT Quasicrystalline materials have been investigated for application as thermoelectric materials due to their inherently low thermal conductivity. With the discovery of a new stable, binary Cd5.7Yb quasicrystal, thermal and electrical transport measurements have been performed on these materials. It is found that the electronic contribution to the thermal conductivity calculated from the Wiedemann-Franz relationship is comparable to or greater than the total measured thermal conductivity, leaving the appearance of a “negligible lattice contribution.” In addition, no semblance of the lattice contribution appears in the temperature dependence of the thermal conductivity. The thermal conductivity increases linearly with temperature above 75K and proportional to T3/4 between 2 K and 75 K.
INTRODUCTION Schlectmann and co-workers first discovered quasicrystals in the early 1980’s since which time these materials have been investigated for many applications. [1,2] Quasicrystals behave electronically in a manner similar to that of a semimetal while exhibiting thermal properties like a glass. The structure of these materials has been extensively studied and only recently have large, stable quasicrystals been synthesized. With the synthesis of large stable quasicrystals, thermal and electrical transport of this unique class of materials has been studied. It is found that the thermal conductivity in these materials is similar to that of a glass (on the order of 1 W/m-K at room temperature), and is observed to remain low despite changes in composition or growth conditions. With inherently low thermal conductivity and favorable electrical transport, quasicrystals have the attributes of Glen Slack’s “phonon-glass, electroncrystal” description of a good thermoelectric material. [3] Recently Macià has theoretically evaluated the potential of quasicrystals for thermoelectric applications and has found the “spiky” features around the density of states (DOS) near the Fermi level. Large variation of the DOS, g(E), leads to a large thermopower. Macia has predicted that a figure of merit of ZT ≈ 1.6 at room temperature is possible for some quasicrystals. [4] Many quasicrystalline materials have been investigated, yet materials suitable for thermoelectric applications have not yet been discovered. The largest figure of merit to date in quasicrystals is ZT = 0.25 at T ≈ 550K (or ZT = 0.08 at T ≈ 300K, for the same sample) and agree with Macià’s theory for the AlPdMn system. [5] Recently, Tsai et. al. discovered a stable binary icosahedral Cd-Yb quasicrystal that grows in a very narrow region of the phase diagram, with composition Cd5.7Yb. [6,7] This binary system is conceptually simpler than its ternary quasicrystalline counterparts, making it interesting to investigate the basic electrical and
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