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P-type Bi 0:4 Sb 1:6 Te 3 thermoelectric materials were prepared by an angular extrusion technique with rapidly solidified and stacked foils in a temperature range from 643 K to 838 K. Textures of the angular-extruded specimens were observed by Orientation Imaging Microscopy (OIM). The average grain size was monotonously enlarged from 4.7 to 16.1 mm while increasing the extrusion temperature. The texture of the angularextruded specimens shows that the basal planes are preferably aligned along the extrusion direction. Strong textures were observed in specimens extruded at a temperature range from 683 K to 803 K. The carrier mobility of the extruded specimens depends strongly on both the texture strength and grain size. As the extrusion temperature rises, the bending strength decreases. This change in bending strength is in good agreement with the Hall-Petch relationship. A maximum Z value of 3:33 Â 10 À3 K À1 and bending strength of 80.3 MPa were achieved in a specimen angular-extruded at 773 K. The Z value and the bending strength were sufficiently high compared with conventional hot-extruded or hot-pressed specimens. These results indicate that the angular extrusion technique is effective in improving the thermoelectric and mechanical properties of bismuth-telluride based thermoelectric materials.

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The repetitive equal channel angular extrusion (ECAE) technique was applied to prepare n-type Bi 1:9 Sb 0:1 Te 2:7 Se 0:3 thermoelectric materials prepared from rapidly solidified and stacked foils. Each ECAE process was performed at temperatures ranging from 693 to 773 K in an argon atmosphere. The number of passes in the ECAE process was varied up to 6 passes via route C Y . The texture of the specimens after ECAE was observed using an Orientation Imaging Microscopy (OIM). Observing the texture revealed that basal planes are satisfactory aligned, and that the direction of the basal planes tends to rotate in the extrusion direction with an increasing number of passes. A highly orientated texture was gradually formed by repetitive ECAE processing. Both the number of passes and extrusion temperature were found to affect the texture characteristics. Formation of twin boundaries in the initial stage of the repetitive ECAE plays a key role in releasing the shear strain imposed during ECAE operation. A strongly orientated texture was observed in the specimen after 6 passes of ECAE via route C Y extruded at 773 K. The grain size of the specimen after ECAE, however, was found to be unrelated to the number of passes. The average grain size was within the range 9.0 to 19.2 mm in specimens extruded at 693 to 773 K. Measurements of thermoelectric properties revealed that carrier mobility was strongly dependent on the degree of orientation. Results also showed that a higher power factor of 4:01 Â 10 À3 Wm À1 ÁK À2 was measured mainly due to increased carrier mobility. The Z value of the specimen after 6 passes of ECAE via route C Y at 773 K reached 3:04 Â 10 À3 K À1 . This result indicates that the ECAE technique largely improves thermoelectric properties of bismuth telluride based compounds.

A rapid solidification process was found to form unidirectional crystal structures in (Bi,Sb) 2 Te 3 and (Bi,Sb) 2 (Te,Se) 3 based thermoelectric alloys. This fine microstructure with its unidirectional crystal orientation is expected to yield high value thermoelectric properties because the fine grain size is extremely effective in decreasing thermal conductivity by boundary scattering of phonons and because this crystal orientation is extremely effective in decreasing the electrical resistivity (increasing the carrier mobility). The rapidly solidified alloys were prepared by a single-roller liquid quenching method. These alloys were shaped as a thin foil with thicknesses ranging from 5 to 20 mm. The crystal orientation was analyzed by the x-ray diffraction method and the microstructure was observed by optical microscopy and SEM of the thin foil samples. The crystal grains of the rapidly solidified foils were very fine and highly oriented. The thermoelectric properties were measured for p-type Bi 0:4 Sb 1:6 Te 3 and the n-type Bi 1:9 Sb 0:1 Te 2:6 Se 0:4 compacted alloys. The figures of merit of 3:5 Â 10 À3 K À1 for p-type alloy and 3:3 Â 10 À3 K À1for n-type alloy were obtains. It is proposed that the rapid solidification is very useful technique for the improvement of thermoelectric properties.