Thermal Conductivity and Thermoelectric Properties of Novel Rare Earth Boron-Rich Cluster Compounds; Discovery of First Undoped n-type B_(12) Icosahedral Compound

摘要

Novel rare earth boron icosahedral compounds are investigated as potential high temperature thermoelectric materials. REB_(50)-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 REB_(50)-type compounds. Strikingly, n-type behavior was observed for REB_(22)C_2N and REB_(17)CN. Up to now, non-doped B_(12) 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 ? < 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 B_6 octahedra layers separated by an increasing number of B_(12) icosahedra layers, we observe that the thermal conductivity actually increases as the number of boron cluster layers increases. We find that the rare earth B_(12) 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.