Thermoelectric Properties of n-type Bismuth Telluride Based Alloys Prepared by Hot Pressing and Zone Melting Method

Alloys of Bi2Te3 rich side of Bi2Te3-Bi2Se3 were prepared by the zone melting method and the hot pressing method in order to compare their thermoelectric properties. When specimens were hot pressed thermoelectric properties changed as a function of particle size, pressing time and hot pressing temperature. The reasons for the variation of the thermoelectric properties were investigated by examining influences of following parameters; oxidation, mechanical deformation during pulverization and the hot pressing temperature. Thermoelectric properties of zone melted ingot were largely affected by dopants when composition is fixed, whereas thermoelectric properties of hot pressed material were mainly related with the variation of the carrier concentrations caused by the generation of electrically active defects from many sources. Defects induced by the mechanical deformation and oxygen causes generation of donors. Defect concentration is also altered with different hot pressing temperatures depending on the amount of previously received mechanical deformation

Effect of Mechanical Deformation on Thermoelectric Properties of p-Type (

The effect of mechanical deformation and annealing on thermoelectric properties of p-type (Bi0.225Sb0.775)Te3 was performed. The ingots were prepared by melting, followed by quenching method using source materials with compositions of (Bi0.225Sb0.775)2Te3. Rectangular shaped specimens (5×5×12 mm3) were cut from ingots and then cold-pressed at 700 MPa for 2 to 20 times by changing the press direction perpendicular to previous one. The cold-pressed samples have been annealed in a quartz ampoule at 573 K. The grain size of the samples was controlled by the number of cold-pressing process and annealing time. Fine grain structure with a grain size of not more than 10 μm is obtained in highly deformed samples. The Seebeck coefficient of the deformed samples were gradually increased with annealing and converged to the similar value of about 225 μV/K after 30 hrs. The small grain size in highly deformed sample enables a rapid increase of Seebeck coefficient with annealing time (~2 hrs.), indicating that the thermal energy needed to recrystallize in highly deformed specimens is lower than that in low deformed specimens. Z values are rapidly increased with annealing time especially in highly deformed alloys, and converge to about 3.0×10−3/K at room temperature. A higher thermoelectric performance could be expected by the optimization of composition and microstructural adjustment. The present study experimentally demonstrates a simple and cost-effective method for fabricating Bi-Te-based alloys with higher thermoelectric performance

Effect of spark plasma sintering conditions on the thermoelectric properties of (Bi0.25Sb0.75)2Te3 alloys

As a field-assisted technique, spark plasma sintering (SPS) enables densification of specimens in a very short period of time compared to other sintering techniques. For high performance thermoelectric material synthesis, SPS is widely used to fabricate nanograin-structured thermoelectric materials by rapidly densifying the nanopowders suppressing grain growth. However, the microstructural evolution behavior of thermoelectric materials by SPS, another important process during sintering, has been rarely studied. Here, we explore SPS as a tool to control the microstructure by long-time SPS. Using p-type (Bi0.25Sb0.75)2Te3 thermoelectric materials as a model system, we systematically vary SPS temperature and time to understand the correlations between SPS conditions, microstructural evolution, and the thermoelectric properties. Our results show that the relatively low eutectic temperature (???420??C) and the existence of volatile tellurium (Te) are critical factors to determine both microstructure and thermoelectric property. In the liquid-phase sintering regime, rapid evaporation of Te leads to a strong dependence of thermoelectric property on SPS time. On the other hand, in the solid-phase sintering regime, there is a weak dependence on SPS time. The optimum thermoelectric figure-of-merit (Z) of 2.93 ?? 10-3/K is achieved by SPS at 500??C for 30 min. Our results will provide an insight on the optimization of SPS conditions for materials containing volatile elements with low eutectic temperature.clos

Hardening of Bi-Te based alloys by dispersing B4C nanoparticles

Thermoelectric devices have attracted a great attention for renewable energy harvesters and solid-state coolers. For practical applications, the mechanical properties of thermoelectric materials become critical for the device reliability, a persistent performance with a long time and high operation cycles. Bi-Te based single-crystals, mostly used in commercial thermoelectric devices, are intrinsically brittle with weak van der Waals bonding, often leading to device failures such as crack and debonding during fabrication and operation. Thus, it is highly desirable to enhance the mechanical property of Bi-Te based alloys as well as the thermoelectric property. Here, we investigate the effect of B4C nanoparticles (less than 0.5 wt%) dispersed in p-type Bi0.4Sb1.6Te3 matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B4C-dispersed Bi0.4Sb1.6Te3 has a single phase. We observe that the grain size of Bi0.4Sb1.6Te3 becomes decreased with the B4C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young's modulus, and flexural strength of B4C-dispersed Bi0.4Sb1.6Te3 are enhanced, compared to the B4C-free Bi0.4Sb1.6Te3 polycrystals. On the other hand, the thermoelectric figure-of-merit of B4C-dispersed Bi0.4Sb1.6Te3 is almost identical to that of the pure Bi0.4Sb1.6Te3. Such enhancements of the mechanical properties of the B4C-dispersed Bi0.4Sb1.6Te3 are attributed to the grain boundary hardening and second-phase hardening. Beyond thermoelectric materials, our result implies that the grain refinement by nanoparticle dispersion is a simple and promising way to strengthen the mechanical properties of other brittle materials with layered structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reservedclose0