Processing, Characterization, and Measurement of the Seebeck Coefficient of Bismuth Microwire Array Composites
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Processing, characterization, and measurement of the Seebeck coefficient of Bismuth microwire array composites
T.E. Huber and P. Constant Laser Research Laboratory, Howard University, Washington, DC 20059, USA.
ABSTRACT
We have fabricated Bi microwire array composites ranging in diameter from 10 to 50 micrometer using the method of high-pressure-injection (HPI) of the Bi melt into microchannel arrays (MCA) templates. The composites are dense, with Bi volume fraction in excess of 70 %. The parallel Bi nanowires, whose length appears to be limited only by the thickness of the host template (up to 2 mm), terminate at both sides of the composite in the Bi bulk. The individual Bi microwire crystal structure is rhombohedral, with the same lattice parameters as that of bulk Bi; the wires crystalline orientation is predominantly perpendicular to the (113) lattice plane. The transversal magnetoresistance and Seebeck effect of the wires has been measured in magnetic fields up to 0.8 Tesla and for temperatures ranging between 77 K and room temperature. INTRODUCTION
The suitability of a material for thermoelectric applications is measured by the thermoelectric figure of merit Z = S2σ/κ where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity. Bulk Bi, a semimetal, and Bi1-xSbx, have the highest Z at 100 K [1]. The changes in the thermoelectric properties of bismuth-antimony alloys as a function of magnetic field and various crystalline orientations has been studied extensively. The thermo magnetic figure of merit ZM, which measures the cooling and heating efficiency of a given material when aided by an external magnetic field is as much as 0.5 for both pure Bi and some of its alloys with Sb at 0.75 Tesla and 77 K [2,3]. The properties of Bi and Bi-Sb are strongly anisotropic. For example the thermal conductivity λ of Bi at 100 K is 18 Wm-1K-along the trigonal direction and 13 Wm-1K-1 normal to
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the trigonal direction. The best thermoelectric performance is observed when the electrical current flows along the trigonal crystal axis. Since their lamellar structure confers to the single crystals an aptitude to cleavage along the trigonal planes. This lack of strength has largely prevented the use of these materials in thermoelectric devices [4]. Powder metallurgy may offer the opportunity to increase the mechanical strength of Bi and Bi-Sb alloys. Microengineering Bi and Bi-Sb into composites may lead to a significant improvement in their thermoelectric performance, because of the reduction of phonon thermal conductivity from phonon scattering at the grain boundaries and interfaces. Devaux, Brochin, Lenoir, MartinLopez, Scherrer and Scherrer measured the figure of merit of pellets of polycrystalline materials, as a function of grain size
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