| Filled skutterudites(SKD)is one of the most promising candidates for thermoelectric(TE)power generation applications in the intermediate temperature region.The skutterudites have cagelike voids in their crystal structures which can be filled with different kinds of guest atoms,such as the rare-earth,alkaline-earth,and alkali-metals.Filling different foreign atoms is a common method to improve the thermoelectric performance due to the filled foreign atoms both improve the carrier transport properties and strongly scatter the phonons to suppress the lattice thermal conductivity.Although the multi-filled SKD owning the higher performance,the single-filled SKD is more preferable for the preparation of TE devices for their simple composition,better controllability of the synthesis process,more stable performance,the potential for scaling-up and cost-down in industrial fabrication.Therefore,it is significantly essential to improve the performance of single-filled SKD.On the premise of constant composition,it is a feasible way to improve the properties by controlling the microstructure.In this paper,the technology of ultra-low temperature cryogenic grinding(CG)was first introduced to prepare the high performance Yb single-filled Co Sb3 skutterudites thermoelectric powders.A series of in-depth researches have been carried out by controling the cryogenic grinding & SPS sintering(CG+SPS)process and the second nanophase composite process.At last,we successfully fabricated the multiscaled nano/microstructured single-filled skutterudites with the highest reported thermoelectric performance,and realize the synergistic improvement of its thermoelectric properties and mechanical properties.The main research results are as follows:1.We use the melt-quenching-annealing method to fabricate the n type Yb singlefilled skutterudites Ybx Co4Sb12.After researching the filled fraction,we got the best performance compounds Yb0.3Co4Sb12,which owning the highet ZT value about 1.14 at 800 K.And this laid the foundation for our later research.2.Based on previous work,we use the large-scaled synthetic bulk Yb0.3Co4Sb12 as the starting material.We successfully fabricated the single-filled skutterudites powders with different sizes and distributions by using the cryogenic grinding(CG)techniques.Through a series of experiments on the technological processes such as cryogenic grinding,SPS sintering and annealing process,various technical parameters were optimized.Finally,the multi-scaled micro/nano structured Yb0.3Co4Sb12 bulk material with high thermoelectric performance was obtained.These the multi-scaled micro/nanostructure can not only decrease the lattice thermal conductivity by increasing the phonon scatting at micro/nano broad scale,but also increase the Seebeck coefficient by the energy filter effects.Finally,the large scale produced samples' ZT value reached 1.39 at 800 K,which lays a foundation for our further researches on the second phase composites.3.In order to further investigate the influence of CG process on filled skutterudites materials,we synthesized a series of filled skutterudites thermoelectric materials by controlling the initial amount of different element,especially Yb content,to fabricated in-suit second phase composites with multi-scale microstructured bulk materials through CG+SPS process regulation.Optimization of both compositions from optimizd Yb contents and microstructure through cryogenic grinding(CG)process can simultaneously tune the electrical,thermal,and mechanical properties of single-filled skutterudite.When the contents of Yb is over 0.30,Yb2O3 nanoprecipitates were successful grown on the grain surface in situ.When the content of Yb was 0.35,a peak ZT of 1.55 at 825 K is realized which is the highest value among all reported single-element-filled Co Sb3 materials,and an average ZT of around 1.09 from 300 to 875 K,comparable to the best result of the multiplefilled skutterudite.Simultaneously,the Vickers hardness,fracture toughness and flexural strength are improved.Furthermore,the high-performance material could be synthesized reproducibly under the premise of the strict control of process parameters.4.So far,one effective strategy to improve the figure of merit is incorporating nanosized particles into the matrix to reduce the lattice thermal conductivity and improve the mechanical properties.Based on the previous study about CG+SPS process,the ?-Si C /Yb0.3Co4Sb12 nanocomposites were fabricated from Yb0.3Co4Sb12 and ?-Si C mixed powders via CG+SPS process.The homogeneously dispersed ?-Si C particles leads to increased Seebeck coefficient and power factor of the SKD composites,probably due to the energy filtering effect at the intimate and clean interface between Si C and Yb0.3Co4Sb12 matrix.Meanwhile,the thermal conductivity of Yb0.3Co4Sb12/Si C nanocomposites is significantly decreased due to the increase of phonon scattering at Si C/ Yb0.3Co4Sb12 interfaces and grain boundaries.A maximum ZT value of 1.42 in the Si C/Yb0.3Co4Sb12 composite with 0.5 vol.% filler content has been achieved,which is among the best results of single-filled SKD based TE materials.Besides,the mechanical properties including Vickers hardness,fracture toughness and flexural strength were also improved by adding Si C nanoparticles,which makes the composite very attractive for TE device with high performance and stability.5.Based on our previous work,the Yb0.3Co4Sb12 / SWCNT and Yb0.3Co4Sb12 / MWCNT nanocomposites were synthesized using single-walled carbon nanotubes(SWCNTs)and multi-walled carbon nanotubes(MWCNTs)as nano-reinforcing phases.From Yb0.3Co4Sb12 and ?-Si C mixed powders via CG+SPS process that Optimized on the previous study.The dispersed CNTs significantly enhance both thermoelectric and mechanical properties of the n-type Yb-filled Co Sb3.The concurrent optimization of the electrical power factor and significant suppression of the thermal conductivity provide a maximum ZT up to 1.44 at 875 K via introducing 0.5 vol.% MWCNTs.The trace introduction of CNTs,especially MWCNTs,is very effective at improving the Vickers hardness and fracture toughness of the matrix,which grants benefits for TE material processing and device integration.To demonstrate the utility and efficiency of this material,an excellent performance prototype TE module made of n-type Yb0.3Co4Sb12/MWCNTs and p-type Ce0.9Fe3 Co Sb12 was fabricated and tested.A thermal-to-electric generation efficiency of 9.3% is obtained at a temperature of 600 K,which is 33% higher than the traditional SKD module,and our module is the highest performance yet observed among the SKD-based TE modules.6.In order to further investigate the influence of the second phase composite on the thermoelectric properties of filled skutterudites,different bandgap semiconductors and conductors like nano Te,nano Ba Ti O3,nano Ti O2 and Ag nanowires were selected for the composites via a CG+SPS composite technique.We systematically studied the influence of these second nano phases on the thermoelectric properties of filled SKD separately.The nano Ba Ti O3 and nano Ti O2 have significantly reduced the electric conductivity and power factor that decrease the ZT value.Besides,the nano Te is beneficial to the electrical transport performance that Yb0.3Co4Sb12/Te has a higher ZT value in the low temperature region.Similarly,the composite Ag NWs can effectively improve the electrical transport performance of the material and improve the ZT value.In this paper,a new process called ultra-low temperature cryogenic grinding combined with SPS sintering(CG+SPS)is proposed,which is suitable for large scale preparation of high performance Yb-filled skutterudites thermoelectric materials.By controlling the Yb amount,cryogenic grinding process and SPS sintering process,we successfully synthesize the multi-scale microstructured bulk thermoelectric materials and uniformly dispersed second phase composites,which greatly improve the thermoelectric and mechanical performance of single filled skutterudites materials.Finally,we obtain the best performanced single filled skutterudites thermoelectric materials and successfully fabricated the thermoelectric device with excellent performance. |
PDF Full Text Request