Thermoelectric properties from 353K to 1073Kfor metal-doped b-rhombohedralboron

H. K. Kim¹, T. Nakayama¹,J. Shimizu², K. Kimura^1,2

¹Department of Advanced MaterialsScience, The University of Tokyo, Kiban-toh 502, 5-1-5 Kashiwanoha,Kashiwa-shi,, Chiba 277-8561, Japan

²Department of Materials Science,The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

b-boron is oneof polymorphs of pure boron that consists mainly of B₁₂ icosahedralclusters. In boron-rich icosahedral cluster solids, a slight change of thestructure or environment of B₁₂ clusters can cause metallic-covalentbonding conversion, which can have great advantage for enhancement of the ZT, becausethe electrical conductivity σ and the Seebeck coefficient S can be as high as those ofmetals and semiconductors, respectively. In b-boron, three kinds ofinterstitial sites, A₁, E and D, which have space large enough toaccommodate foreign atoms, are known. To enhance ZT of b-boron, we investigatedthe change of σ, S andκ, from 353Kto 1073K when various metal atoms were doped. V, Co and Zr are successfullydoped and the amounts of second phases are very small. For the V dopedhot-pressed sample, in which V atoms mainly occupy A₁ site of b-boron, s is increasedvery much, S is decreased even to negative value and k is decreased. Themaximum and n-type ZT value is obtained and is approaching to that of B₄C,which is considered to have the largest and p-type ZT value in boron-rich icosahedralcluster solids.