Preparation And Property Optimization Of Bi-Te-Se Thermoelectric Materials

Thermoelectric materials interconvert heat and electricity directly, which are used in power generation and cooling. Due to a good combination of high electrical performances and low thermal conductivities, bismuth telluride (Bi2Te3) based compounds are the most widely used materials at room temperature. State-of-art commercial Bi2Te3 based materials are highly oriented polycrystalline materials prepared by the zone melting(ZM) process. This preparation method induces big crystal grain size, leading to severe cleavage problem and high production cost. Here two new methods are developed to prepare polycrystalline N-type Bi2Te3 based materials and both are beneficial in improving thermoelectric performances.The first method we used is rapid solidification followed by hot pressing in the fabrication of Bi2Te3-xSex(x=0-1.05) compounds. In addition to grain refinement, rapid solidification technique allows Bi2Te3 based alloys to be more homogeneous without incongruent melting compounds such as BiTe and Bi2Te by suppressing the peritectic reaction during solidification. X-ray diffraction (XRD) measurements on hot pressed bulk samples indicate there are no second phases and preferred orientations. The Seebeck coefficient of the rapid solidified alloys is improved, while thermal conductivity lowered. As a result, a maximum ZT of 0.95 is achieved in Bi2Te2.1Seo.9 bulk ingot prepared by quenching in liquid nitrogen and in Bi2Te2.25Seo.75 bulk prepared by melt spinning. From Field Emission Scanning Electron Microscopy (FESEM) it is found tOhat all the particles are under 3μm, compared with that of furnace cool samples-10μm.Secondly, a simple hot forging process is applied to Bi2Te2.1Se0.9 compounds to adjust the microstructure and thermoelectric properties of hot pressed polycrystalline alloys. As indicated from previous work, thermoelectric properties of Bi2Te3 based compounds are related with the level of texture as well as grain size. In this work, the as-solidified Bi2Te2.1Se0.9 ingots were pulverized and hot pressed in aΦ10 mm graphite die at 673 K for 30 min, resulting in the disk-shaped initial bulk samples. Then a hot forging process was performed on the hot pressed bulk samples at 823 K in a larger graphite die ofΦ12.7 mm. XRD peaks of (00l) planes in as-forged bulk become stronger than the as-pressed bulk, indicating the hot forged samples are more oriented along (00l) planes. To investigate the degree of preferred orientation of (00l) planes, a quantitative parameter, F was used to identify the texture evolution through the XRD results. The orientation factor F of the bulk prepared by two pass hot forging is 0.52, compared with F factor～0.18 in the as-pressed sample. The FESEM observations of the fractured surface of the two pass hot forged bulk show highly textured microstructure with higher laminar realigned particles, in agreement with XRD results. After two pass forging an obvious increase in electrical conductivity is observed. This is attributed to a significant increase in carrier concentration and a slight decrease in mobility. The carrier concentration is enhanced by a donor-like effect as several kinds of point defects (vacancies and antisite defects) and their interaction come into form.In order to study the effect of anisotropy on thermoelectric properties in Bi2Te3-xSex materials, a big disk of 30 mm in diameter and 10 mm in length was fabricated by hot forging by spark plasma sintering method. In XRD profiles, the relative intensities of (006), (0015), and (0018) planes in radial direction perpendicular to the compressing direction are significantly stronger than peaks from axial direction parallel to the compressing direction, indicating a preferred orientation occurring in agreement with the morphological anisotropy. A narrow orientation distribution will yield strongly anisotropic thermoelectricity, because thermoelectric properties are dependent on the measurement direction. Both Seebeck coefficient and electrical conductivity are improved and the thermal conductivity is decreased in the radial direction compared with those in the axial direction. Finally a maximum ZT of 0.90 at 375 K is obtained in radial direction which is much higher than that in axial direction.Besides, several single-stage thermoelectric refrigeration devices were also made from polycrystalline based Bi2Te3 compounds by substituting ZM materials.