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We calculated the electronic structures and the thermoelectric properties for type-I and type-VIII Ba8Ga16Sn30 (BGS) clathrates. The band structures show that type-I and type-VIII BGS are indirect semiconductors with band gaps of 0.51 eV and 0.32 eV, respectively. The calculated Seebeck coefficient of n-type type-I BGS is higher than that of n-type type-VIII BGS because of the larger density of states in type-I at the bottom of the conduction band. This is in good agreement with the experimental results. We also calculated the electrical conductivity and thermal conductivity due to charge carriers. Estimated thermoelectric figure of merit, ZT, exceeds 1.0 for both types. [ABSTRACT FROM AUTHOR]

A limiting mean free path was considered in order to better understand the temperature and wire diameter dependence of the resistivity and Seebeck coefficient of bismuth microwire and nanowire samples. The mean free path limited mobility was numerically calculated from experimentally measured mobility in a bulk bismuth sample, and the electron and hole mobilities were dramatically decreased to a 10 μm mean free path. Therefore, the temperature dependence of resistivity in very thin wire was quite different from that of a bulk sample, which had a positive temperature coefficient. The calculations showed that the temperature coefficient decreased gradually with decreasing mean free path, and the coefficient became negative for a mean free path of less than 1 μm at about 150 K. The Seebeck coefficient was also calculated, but showed only a weak dependence on mean free path compared with the resistivity. Experimental comparisons were made to previous measurements of bismuth microwire or nanowire samples, and the temperature and wire diameter dependencies of the resistivity and Seebeck coefficient were qualitatively and quantitatively in very good agreement. Therefore, the temperature dependencies of nanowire samples over 850 nm in diameter were well described using the mean free path limitation. [ABSTRACT FROM AUTHOR]

Wire-diameter dependencies were investigated using microwire sample arrays with diameters from 2 to 25 ¿m and polycrystalline bulk bismuth. Seebeck coefficient and resistivity were measured from 50 to 300 K. Although the temperature dependence of the Seebeck coefficients of all samples were similar to that of the bulk, the resistivity dependence of the 2-¿m-diameter microwire array sample had a small positive temperature coefficient, and was quite different from the other samples. Mobility was estimated using a simple model to identify the dependences on temperature and wire diameter. The ratio of electron mobility divided by hole mobility gradually decreased with decreasing temperature for samples with wire diameters below 6 ¿m, and electron mobility was suppressed at low temperatures, explaining the promotion of the boundary scattering. Therefore, small positive temperature coefficients of resistivity were obtained for very small diameter microwire array samples. To compare with the temperature dependence of the nanowire array samples that typically have negative temperature coefficient reported, a model for mobility was developed to represent the temperature dependence of resistivity. [ABSTRACT FROM AUTHOR]

Bismuth nanowires with lengths of over 1 mm length and diameters of the order of nanometers have been fabricated by high-pressure injection into a quartz template. The temperature dependences of the Seebeck coefficient and resistivity were simultaneously measured over the temperature range of 77–300 K. The Seebeck coefficient and resistivity at 300 K were estimated to be approximately -57 μV/K and 1.31 μΩ m, respectively. We also estimated the mobilities of electrons and holes to determine their temperature dependences. We found that the temperature dependence of the resistivity can be accounted for by the reduction in the mobility. [ABSTRACT FROM AUTHOR]