Equal Channel Angular Extrusion Technique for Controlling the Texture of n-Type Bi<SUB>2</SUB>Te<SUB>3</SUB> Based Thermoelectric Materials

Abstract

The repetitive equal channel angular extrusion (ECAE) technique was applied to prepare n-type Bi1.9Sb0.1Te2.7Se0.3 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 CY. 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 CY 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 μm 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 4.01×10−3 Wm−1·K−2 was measured mainly due to increased carrier mobility. The Z value of the specimen after 6 passes of ECAE via route CY at 773 K reached 3.04×10−3 K−1. This result indicates that the ECAE technique largely improves thermoelectric properties of bismuth telluride based compounds.