| Bi2Te3-based alloys are the state-of-the-art thermoelectric materials with a ZT(figure of merit) of about 1 at room temperature. Among them, Bi2-x Sbx Te3 is the best optimized P-type Bi2Te3-based alloy. Although Bi2Te3-based thermoelectric materials have achieved some progress in power generation and semiconductor cooling, it has not realized the large scale applications yet due to the high cost of raw materials, time consumption and not high enough thermoelectric transfer efficiency. Hence, how to realize the preparation of high-performance thermoelecteic materials through the low cost method will have profound meanings for promoting the thermoelectric materials and its devices applications.The three parameters: Seebeck coefficient(α), electrical conductivity(σ) and thermal conductivity(κ) that determine the final ZT values are strongly coupled via band structures and scattering mechanisms. Therefore, how to optimize three paramaters synchronously is the big issue for thermoelectric materials. The key to develop the high-performance thermoelectric materials is to improve both the electrical conductivity and Seebeck coefficient while decreasing the thermal conductivity. In this paper, we first prepare Bi2-x Sbx Te3 powders by low cost hydrothermal method, then the second phase is introduced to realize the improvement in ZT value by enhancing both the electrical conductivity and Seebeck coefficient, at the mean time, reducing or not largely affecting the thermal conductivity.In this study, we introduce MWCNT, Ag NWs and Cu Al O2 to realize the synchronously optimization of these thress parameters to enhance the final ZT values. In this paper, hydrothermal method combined with Spark Plasma Sintering is used to prepare the Bi0.4Sb1.6Te3 matrix. Multi-scale Cu Al O2 powders is prepared by solid reaction sintering method. MWCNT、Ag NWs and Cu Al O2 are separately introduced into Bi0.4Sb1.6Te3 nanopowders to form the composite powders. Then these composite powders were sintered as the dense compacts. Systematic studies are carried on the crystal phase, microstructure and thermoelectric properties. The main studies are as follows:(1)Bi0.4Sb1.6Te3 powders are obtained by hydrothermal method. The obtained powders are nanoscale, ranging from several nanometers to several hundred nanometers, showing a broad size distribution. The powders possess flate-like and rod-like structures. Combined with the consideration of thermoelectric and mechanical properties, the best SPS sintering condition is 400℃,40 MPa.(2)MWCNT and Ag NWs are separately mixed at different proportion through cold grinding, ultrasound and stirring. The as-prepared MWCNT-dispersed and Ag NWs-dispersed Bi0.4Sb1.6Te3 composite powders are sintered by SPS to form the corresponding dense compacts. The influence of different concentration of MWCNT and Ag NWs on the morphology, microstructure, grain size and thermoelectric properties is fully studied. The results show that although the introduction of MWCNT effectively reduce the thermal conductivity of the composites, the electrical conductivity also reduced. However, the enhanced Seebeck coefficient compromise the reduced electrical conductivity, which improves the power factors. When the MWCNT is 5wt%, the composites achieve the highest ZT values 0.22 at 300 K. The highest ZT value is improved by 45.6% compared with that of the pure Bi0.4Sb1.6Te3 compacts. However, it is not realizing the synchronous improvement of the electrical conductivity, Seebeck coefficient and thermal conductivity.The introduction of Ag NWs largely improves the electrical conductivity and Seebeck coefficient at the higher measuring temperature while reducing the lattice thermal conductivity of the composite compacts. When the Ag NWs is 5wt%, the composites reach the highest ZT values 0.54 at 480 K, which enhanced by 260% compared with that of the pure Bi0.4Sb1.6Te3 compacts. It has realized the improvement of electrical conductivity and Seebeck while reducing the thermal conductivity.(3)Although the introduction of Ag NWs can synchronously regulate α, σ and κ, introducing the low cost second phase to regulate the thermoelectric properties is more pracital significant for the applications. Here, we propose a new way to improve the thermoelectric properties: we introduce different particle sizes of Cu Al O2 into the Bi0.4Sb1.6Te3 matrix, the different particle sizes will cause the different chemical stability, which will realize the synchronously regulation of electrical conductivity, Seebeck coefficient and thermal conductivity to largely improve the thermoelectric properties. The muli-scale Cu Al O2 with the combination of small particles(50~200nm) and large particles(1~2μm) is obtained by solid reaction sintering method. The Cu Al O2 powders are first screened and then different particles sizes of Cu Al O2 are mixed with Bi0.4Sb1.6Te3 powders, the as-prepared Cu Al O2-dispersed Bi0.4Sb1.6Te3 composite powders are sintered by SPS next to form the corresponding dense compacts. The influence of different particle sizes and concentration of Cu Al O2 on the morphology, microstructure, grain size and thermoelectric properties are fully studied. The results show that Cu of the small Cu Al O2 particle may diffuse into the Bi0.4Sb1.6Te3 matrix to increase the electrical conductivity while the large Cu Al O2 particles still maintain its constituent and improve the Seebeck coefficient at the higher measuring temperature. The highest Seebeck coefficient of the composites is 7.7% higher that of the pure Bi0.4Sb1.6Te3 compact, showing an effective way to decouple the electrical conductivity and Seebeck coefficient. Furthermore, the lattice thermal conductivity is also reduced by the enhanced phonon scattering which caused by two reasons:(1) the rough, newly formed interfaces between Cu Al O2 and the Bi0.4Sb1.6Te3,(2) the smaller grains of Bi0.4Sb1.6Te3 matrix after the dispersion of Cu Al O2. When the Cu Al O2 is 0.6 vol.%, the composutes achieved the maximum ZT values 0.62, which is 307% higher than that of pure Bi0.4Sb1.6Te3. It has realized the goal of synchronously regulating the electrical conductivity, Seebeck coefficient and thermal conductivity by using the chemical difference between the nanoparticles and microparticles. It also explores a novel method for the low cost regulation of thermoelectric properties. |
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