Nano-Cage Structured Materials: Clathrates

Abstract

Intermetallic clathrates are the representative of nano-cage structured material. A dimorphic clathrate Ba 8Ga 16Sn 30 possesses type-I and type-VIII structures. The type-I compound displays the glasslike thermal conductivity with a plateau at 4–12 K, which is ascribed to the strong interaction of acoustic phonons with the low-energy off-center rattling mode. In the type-VIII compound, the Ba guest is vibrating nearly on center of the distorted dodecahedron. Nevertheless, the thermal conductivity is as low as 0.7 W/Km for T> 300 K. The carrier type and its density in single crystals can be tuned by controlling the initial amount of flux, Ga or Sn. Further optimization of carrier density by alloying has led to high ZT of 1.0 and 1.45 for p- and n-type, respectively, at around 500 K, where the conventional materials based on Bi-Te and Pb-Te had a valley in ZT. By assembling p- and n-type legs made of Ba 8Ga 16Sn 30, a module was prepared with the segments of Bi-Te legs. The conversion efficiency reached 7.5 % at the temperature difference of 390 K.