| Recently, the human life and social development have been restricted and even threatened by the increasing of environmental pollution and energy crisis. As a consequence, it has caught worldwide attention to develop clean energy and new energy technology, among which thermoelectric(TE) conversion emerges as an environmentally friendly technology. It can directly convert heat into electricity and vice versa based on Seebeck and Peltier effects. Bismuth telluride-based alloys, as one of the most mature and widely used TE materials, have great potential in the field of TE refrigeration and power generation from waste heat. Nowadays, zone melting(ZM) has been adopted as the most common method to fabricate both n- and p-type Bi2Te3-based compounds with the maximum ZT around 1.0, due to its adaptabilities of mass production and stable performance. The obtained ZM ingots are highly textured with large crystals and preferential orientation, which, however, lead to the easy cleavage and thus weak mechanical strength and machinability. In addition, TE materials may suffer from substantial and long-period thermal cycling and also vibrational stress while in power generation. Therefore, it's essential to improve the mechanical performance of ZM materials, which can enhance the stability and extend the application of TE devices.The target of this study is p-type Bi0.5Sb1.5Te3 alloys. Aiming at the drawbacks of ZM ingots, melt spinning combined with plasma activated sintering(MS-PAS) method is employed in the current research to optimize the TE and mechanical properties of the corresponding ZM materials. First of all, the MS parameters, such as ingot mass, chamber pressure and roller speed, were optimized to improve the yield of MS ribbons and TE and mechanical performance of bulk materials. The fracture mechanisms of MS-PAS samples were systematically investigated in comparison to ZM ingots. In order to simulate the sample status under service environment, we conducted experiments on fatigue tests and thermal stability analysis, and studied the fatigue crack propagation and the evolution of structural defects. The main contents and results are listed as follows.P-type Bi0.5Sb1.5Te3 ZM ingots were employed as the starting materials. The influences of MS parameters on the phase composition, microstructures and yield of MS ribbons, and also the TE performance of bulk materials were systematically investigated. The results show that the phase compositions of all samples exhibit as a single phase of Bi0.5Sb1.5Te3, which indicates that change in the MS parameters didn't affect the composition of ribbons. The free surface and contact surface of MS ribbons display typical dendritic crystals and nanograins, respectively. With the increasing speed of copper roller, the nanocrystals on the contact surface become smaller. Samples after PAS sintering exhibit random orientation of grains, and lots of nanoprecipitates with the size of ~50 nm can be found at the grain boundaries. Ascribing to the hierarchical structures in samples with the roller speed of 10 m/s, heat-carrying phonons are strongly scattered, thus leading to a significant decrease of lattice thermal conductivity. A maximum ZT of 1.22 can be obtained at 340 K, which is about 40% improvement over that of ZM ingots.The effects of roller speed on the static mechanical properties of MS-PAS samples have been studied carefully. Combined with the analysis of microstructures, the fracture mechanisms of ZM and MS-PAS samples after mechanical tests are discussed in details. Since the lamellar structures of ZM ingots are combined by weak van der Waals bonds, ZM ingots can easily cleavage along these planes and thus exhibit weak mechanical strength. MS-PAS technique can greatly enhance the mechanical properties and machinability, resulting in the significant improvement for MS-PAS samples of Vickers hardness, fracture toughness, as well as compressive strength and bending strength in comparison to ZM ingots. The enhancement of mechanical strength of MS-PAS samples can be attributed to the hierarchical structures and nanoprecipitates generated in the MS-PAS process, which dissipate the crack propagation energy by introducing such toughening mechanisms as crack deflection, crack bridging, and pull-out. Therefore, the resultant MS-PAS samples with both superior mechanical and thermoelectric performances show great potential in the commercial application of bismuth telluride-based materials.Compressive fatigue experiments were conducted on the MS10 samples(i.e. MS-PAS samples with the roller speed of 10 m/s) with both excellent TE and mechanical properties. The influence of stress ratio on the fatigue life of MS10 specimens was investigated. The stress-fatigue life curve(S-N curve) was plotted. The results show that with the increasing stress ratio, the fatigue life just decreases. When the ratio decreases to 60%, the fatigue life can reach 9.0×105. Moreover, the typical features of fatigue cracks and fringes can be found in all failure samples. And substantial dislocations and crystal distortions formed in these specimens increase with the improving stress ratio. Due to the cyclic loading, most of the dislocations accumulate at grain boundaries, leading to the obvious phenomenon of dislocation pile-up.The high temperature mechanical properties and fracture mechanisms are illustrated in the study. The results indicate that with the increasing temperature, the bending strength of all samples first increases and then decreases, which exhibits the maximum value at 373 K. This is mainly due to the following two reasons:(a) the thermal expansion mismatch between the randomly orientated grains may lead to microcracks, which can deflect the main crack and dissipate crack propagation energy, thus improving the strength.(b) high temperature can weaken the grain bonding, resulting in the crack propagation along grain boundaries. These two factors coexist and compete each other. With the further increase of temperature up to 473 K, both the bending and compressive strength of ZM and MS-PAS samples decrease gradually.The thermal stability of both TE and mechanical properties are systematically studied for ZM and MS-PAS samples. The results show that annealing at 473 K for a week doesn't change the phase composition and microstructures. Both the ZM and MS-PAS materials exhibit stable TE and mechanical properties. However, when annealed at 573 K for a week, MS10 specimens demonstrate dramatical deterioration in TE performance. The room temperature carrier concentration is 7×1018 cm-3, about 60% decrease compared to that of unannealed sample, which results in the significant reduction in electrical transport properties. The maximum power factor and ZT value of samples annealed at 573 K can reach 2.2×10-3 Wm-1K-2 and 0.9 at room temperature, respectively. It can be concluded that MS-PAS samples display excellent stability in TE and mechanical properties below 473 K, which is of guiding significance for the commercial application. |
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