Jianhui Li

Measurements of the Seebeck coefficient of thermoelectric materials by an ac method

An ac method for measurement of the Seebeck coefficient was developed. Specimens were heated periodically at frequencies in the range 0.2 10 Hz using a semiconductor laser. The small temperature increase and the resultant thermoelectric power were measured with a Pt Pt 13% Rh thermocouple (25 μm in diameter) through a lock-in amplifier. The Seebeck coefficient of a Pt90Rh10 foil measured by the ac method was in agreement with that obtained from the standard table. The optimum frequency and specimen thickness for the ac method were 0.2 Hz and 0.1 0.2 mm, respectively. The Seebeck coefficients of silicon single crystal and several thermoelectric semiconductors (Si80Ge20, PbTe, FeSi2, SiB14) measured by the ac method agreed with those measured by a conventional de method in the temperature range between room temperature and 1200 K. The time needed for each measurement was less than a few tens of minutes, significantly shorter than that for a conventional de method.

Microstructure and thermoelectric properties of B-C-Y system composites

B-C-Y system composites were prepared by arc-melting using B/sub 4/C, YB/sub 6/ and B powders. The two-phase region of B/sub 4/C+YB/sub 6/ was a quasi-binary eutectic system whose eutectic composition was about 40 mol% YB/sub 6/. In the B/sub 4/C-YB/sub 6/ system composites, with increasing YB/sub 6/ content, the electrical conductivity and thermal conductivity increased, but the Seebeck coefficients and dimensionless figure-of-merit (ZT) decreased. The B/sub 4/C-YB/sub 6/ composites containing a small amount of YB/sub 6/ had larger /spl alpha/ and ZT values than those of B/sub 4/C. The greatest ZT value obtained in the B-C-Y system was 0.54 at T=1100 K.

Thermal and electrical properties of Czochralski grown GeSi alloys

The thermoelectric parameters, the thermal conductivity, electrical conductivity and Seebeck coefficient were evaluated on high purity crystals of Ge/sub 1-x/Si/sub x/ alloys in the whole composition 0<x<1 at elevated temperatures up to 800/spl deg/C. The thermal resistivity showed a maximum at x/spl ap/0.5 due to the phonon scattering. The electrical conductivity decreased with increasing x, relating to the intrinsic carrier concentration determined by the band gap energy at high temperatures. The Seebeck coefficient was extremely low in the composition 0.1<x<0.5 due to the small difference of the electron and hole mobilities. The large magnitude of the Seebeck coefficient was obtained in the GeSi with x=0.8 at elevated temperatures.

Microstructure and thermoelectric properties of arc-melted silicon borides

Silicon borides in a boron content range of 80 to 94 mol% were prepared by arc-melting. As-melted specimens consisted of SiB/sub n/ (hexagonal, n=14-49) and free silicon. The free silicon content decreased with increasing boron content in raw materials. The arc-melted specimens were annealed in an argon atmosphere at 1663 K. By annealing for 1.8 ks, SiB/sub 4/ phase formed at the Si-SiB/sub n/ boundary. By annealing for more than 5.4 ks, SiB/sup 6/ phase formed and SiB/sub 4/ phase disappeared. SiB/sub n/ content increased with increasing annealing time. The annealing for 5.4 ks caused a great increase of the Seebeck coefficient.

Thermoelectric properties of boron-rich boride composites prepared through eutectic and peritectic reactions

Boron-rich boride composites were prepared by arc-melting in an argon atmosphere, and their microstructures and thermoelectric properties were studied. B/sub 4/C-TiB/sub 2/ system was quasi-binary, and the typical lamellar structures indicating a eutectic reaction were observed. The eutectic composition was 25 mol% TiB/sub 2/, and a 6 mol% TiB/sub 2/-B/sub 4/C composite showed the greatest ZT values of 0.55 at 1100 K. The boron-rich region of Si-B-C system contained a peritectic reaction. The specimens consisting of SiB/sub 14/, B/sub 4/C and free-Si were prepared. The greatest ZT value of this system was 0.4 at 1100 K for a specimen in which SiB/sub 14/ is the main phase with several 10 mol% of B/sub 4/C and free-Si.