Thermal Conductivity and Thermoelectric Properties of Novel Rare Earth Boron-Rich Cluster Compounds; Discovery of first
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Thermal Conductivity and Thermoelectric Properties of Novel Rare Earth Boron-Rich Cluster Compounds; Discovery of first undoped n-type B12 icosahedral compound Takao Mori National Institute for Materials Science, Namiki 1-1, Tsukuba, Japan 305-0044 ABSTRACT Novel rare earth boron icosahedral compounds are investigated as potential high temperature thermoelectric materials. REB50-type compounds and a homologous series of RE-B-C(N) compounds were synthesized and the thermal conductivity and thermoelectric properties measured. Seebeck coefficients in excess of 200 µV/K are observed at temperatures above 1000 K for the REB50-type compounds. Strikingly, n-type behavior was observed for REB22C2N and REB17CN. Up to now, non-doped B12 icosahedral compounds like boron carbide have all been p-type. The discovery of an n-type compound is extremely important in terms of the potential development of this class of compounds as viable thermoelectric materials. Low thermal conductivities of κ < 0.03 W/cm/K at room temperature was observed for these rare earth boron cluster compounds. In comparison among the homologous series in which there are rare earth and B6 octahedra layers separated by an increasing number of B12 icosahedra layers, we observe that the thermal conductivity actually increases as the number of boron cluster layers increases. We find that the rare earth B12 icosahedral cluster compounds in which RE atoms occupy voids among the clusters generally appear to have lower thermal conductivity than boron cluster compounds which do not contain RE atoms. INTRODUCTION The search for thermoelectric materials is being carried out with great intensity because of the huge possibilities for useful energy conversion of waste heat, for example, and the needs of modern society where the limits of classical energy resources are rapidly approaching. There is obviously a particular need to develop materials which can function at high temperatures. Boron-rich cluster compounds are attractive materials for their stability under high temperature, typically exhibiting melting points of above 2300 K. Magnetic properties of some novel rare earth boron cluster compounds have recently attracted increasing interest, being magnetically dilute, insulating/semiconducting materials but displaying a wide range of properties with relatively strong magnetic coupling [1-4]. Interestingly, it has been indicated that the B12 icosahedral clusters play an important role in mediating the magnetic interaction which is a novel phenomenon. Previously, boron-rich compounds such as boron carbide “B4C” and doped β-boron have been investigated as possible thermoelectric materials [5-7]. We note that boron carbide especially, as first discovered by Emin et. al [5], has been found to be one of the best p-type materials able to function at extremely high temperatures (∼1500 K). We are interested in rare earth boron icosahedral compounds as possible new high temperature thermoelectric materials because it has been found for the REB66 compound that they
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