The anisotropic powder metallurgy of n -type Bi 2 Te 2.85 Se 0.15 thermoelectric material
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I. INTRODUCTION
The alloy (Bi2Te3)r(Bi2Se3)i-;t: has been studied over the last two decades for the preparation of thermoelectric elements.1"3 The methods of fabrication for thermoelectric application can be divided into two categories: zone-melting4 and sintering.3'5 The former is well-known to yield homogeneous and stable materials with well established methods controlling their composition and doping impurities, but it has mechanical brittleness and electrical instabilities due to cleavage during the zone-melting process. The latter process yields materials with inhomogeneous structure, but has the possibility of controlling thermoelectric properties of materials with the added freedom of powder processing. The sintering method can also be divided into hotpressing and cold-pressing methods.2'6 The former has the advantage of attaining ideal density and polycrystalline elements with higher mechanical strength of small particle size under high pressure and temperature.5 The higher hot-pressing pressure and temperature accelerate the contact and material diffusion between particles compared to cold-pressing. On the other hand, the lower sintering temperature of the cold pressing inhibits grain growth.7 Though the elements made by this process tend to exhibit higher anisotropy because of the oriented C-axis of crystal grains, even this latter property may be utilized to achieve higher performance of the elements by, e.g., lowering the resistivity. We have adopted the hot-pressing method to determine the influence of hot-pressing conditions and particle size3 on the thermoelectric properties (Seebeck coefficient a and resistivity p), especially through changes in anisotropy. The orientation factor / of the ceramics estimated from x-ray diffraction was used to characterize the anisotropy. We find that the orientation factor is a 1052
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 5, May 1990
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convenient measure of the anisotropy, which is useful in finding the optimum hot-press condition and the particle size in the anisotropic metallurgy of rc-type (Bi2Te3UBi2Se3)i-,. II. EXPERIMENTAL A. Sample preparation
The Bi2Te2.85Se0.i5 alloys doped with bromine ion 0.05 wt.% were melted in 1 atm Ar atmosphere at 800 °C for 3-5 h. After homogeneous melting, the alloys were cooled slowly and crushed in a stamp mill in an Ar atmosphere. They were then sifted into four classes (50-75,75-90,90-150, and 150-250 /xm) according to the particle diameters. The powders thus prepared for sintering were hot-pressed at 300-500 °C under 100-500 kg/cm2 for an hour. The cross-sectional view of the hot-press chamber is shown in Fig. 1. It is made of a carbon block which has a bore with 60 mm x 500 mm length fitted with pistons to transmit pressure to the powder sample contained inside. The hotpressed samples were cut into the form of a small plate with dimensions 5 x 15 x 3 mm. Measurements were made on these samples. B. Measurements
The Seebeck coefficient a was measured at 300 K by the standard dc tech
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