The Preparations And Thermoelectric Properties Of P-and N-type Materials For400-650K Power Generations

Thermoelectric (TE) materials, which can realize direct conversion between heat and electricity based on the Seebeck or Peltier effect, have shown great significance in waste heat recoveries and TE refrigeration. The total energy of waste heat exceeds1013kW-h which is more than60%of the energy consumption per year in China, and most of low-grade waste heat with a temperature range of300-900K is seldom recovered which is highly suitable for TE power generations. Up to now, the TE performances of high temperature materials are significantly improved which can be used to recover650-900K waste heat, however, the400-650K waste heat is seldom considered due to the lack of high efficient p or n-type materials, though which is feasible and of great significance from practical point of view.Herein, in this study, focusing on the demands of high efficient p and n-type materials for400-650K waste heat recovery, we try to improve the TE properties, thermal stabilities and mechanical properties of p-type β-Zn4Sb3and n-type Bi2Te3-based materials, making them for400-650K energy applications through adjusting the compositions, carrier density, electronic structures coupled with the nanostructuring by a melt spinning-spark plasma sintering technique (MS-SPS). The main content and results are listed as follows.Sight Cd-doped nanostructured (Zn1-xCdx)4Sb3(x=0.0,0.005,0.01and0.015) compounds were prepared by a MS-SPS technique, and the phase compositions, microstructures, TE properties and thermal stability were systematically studied. MS-SPS technique introduces multi-scale refined nanostructures in the bulk materials, especially a mass of5-30nm ZnSb nano-dots which significantly block the propagations of heat-carrying phonons. Meanwhile, Cd-doping effectively optimizes the electronic structures and increases the effective mass, giving rise to an improved power factor. In combination of the improved electrical properties with the very low lattice thermal conductivity,1%Cd-doped MS-SPS sample obtains a highest ZT of1.30at700K, which is～65%improvement over the undoped ingot. Furthermore, Cd-doping significantly suppresses the low temperature α-β phase transition and improves the high temperature thermal stability, which makes β-Zn4Sb3material a promising candidate for400-650K power generations combined with the high mechanical properties originated from the refined microstructures by the MS-SPS technique.Minute Ge-doped β-Zn4Sb3compounds were prepared by the melting-slow cooling and MS-SPS technique, respectively. The transport properties measurements and first-principle calculations definitely suggest that Ge-doping favorably modifies the band structures and introduces a remarkable increase in density of state effective mass in β-Zn4Sb5compound, in a manner of adding a shape peak below valence band edge and giving rise to a significant enhancement in power factor which is similar to the case of T1-doped PbTe. As a consequence, the high power factor exceeding1.4mW.m-1.K-2, coupled with the intrinsic very low thermal conductivity originated from complex crystal structures and high degree of disorder, results in a maximum figure of merit of-1.35at680K, as well as an improved average ZT of-1.06between400and650K for the0.25%Ge-doped sample which is-25%enhancement compared with that of the undoped sample, respectively. Furthermore, the combination of high thermoelectric performance and improvement in thermodynamic properties makes this natural-abundant,"non-toxic" and cheap Ge-doped β-Zn4Sb5compound a very promising candidate for400-650K TE energy applications.Bi2(Te,Se)3and Bi2(Te,S)3solid solutions were prepared by a commercialized zone melting method (ZM), and the impacts of Se or S alloying on the phase compositions, microstructures and TE properties were systematically studied. Se completely enters the sub-lattice of Te to form solid solution in entire range. The increased band-gap by Se-alloying effectively suppresses the intrinsic excitation at elevated temperature, refraining the "turn-over" of Seebeck coefficient and appearance of apparent bipolar thermal conductivity which give rise to a high ZT at500-600K. The un-optimized Bi2Te1.5Se1.5sample shows a highest ZT of0.8at560K and an average ZT of0.7between400and600K, displaying some application potentials in low temperature power generations. The band-gap can be increased with the incorporation of S in Te sites, whereas the TE properties of Bi2(Te,S)3compound are much lower than its Se analogue due to the relatively low alloying limit and apparent decrease in electron effective mass.The electron density of Bi2Te1.5Se1.5compound is well adjusted through the addition of iodine as donors. While the electron density locates between3×1019cm-3and4.5×1019cm-3, the samples display high figure of merit ZT. The sample with a electron density of4×1019cm-3shows a highest ZT of0.86at600K and an average ZT400-640K of0.8, and the ideal TE conversion efficiency reaches6.6%between400and640K if adopting a single-leg module, making it competitive in low-temperature power generations. The lattice thermal conductivity is significantly decreased through nanostructuring by the MS-SPS technique or Sb-alloying, however, the remarkable loss of mobility deteriorates the TE properties. Furthermore, the significant improvement in mechanical properties of the MS-SPS samples in comparison to ZM samples would benefit their commercial applications, especially at elevated temperature at which the mechanical performances is one of the major concern.Te-free (Bi,Sb)2Se3compounds were prepared by ZM and MS-SPS techniques, respectively.(Bi,Sb)2Se3compound retains a hexagonal structure while Sb-content is lower than30%, whereas shows a two-phase composite including both hexagonal and rhombohedron compounds while Sb-content exceeding30%. With increasing the Sb-content, electron density and electrical conductivity decreases gradually, and the Seebeck coefficient displays an opposite variation trend. Benefited from the suppression of intrinsic conduction and the significant decrease in lattice thermal conductivity due to strong scattering of phonons by nano structures, point defects, strain fields etc.,20%Sb-alloyed ZM sample and30%Sb-alloyed MS-SPS sample achieve highest ZTs of0.45and0.5at680K and640K, respectively, which are among the best results reported for Bi2Se3compound. Furthermore, the30%Sb-alloyed sample shows average ZT values0.46at the temperature range400-680K, which is about100%improvement over the un-alloyed sample.