Thermoelectric Performance Optimization Of Filled Skutterudites And Diffusion Welding Mechanism Of Skutterudites To CuW Alloys

Skutterudites are one of the best thermoelectric materials for medium-range applications.Due to their excellent thermoelectric and mechanical properties,skutterudites have successful applications in the automotive applications and NASA's enhanced multi-mission radioisotope generator.Thermoelectric materials need to joint with the conductive electrodes to form a thermoelectric generator.Thus,the key challenges to achieve a high-performance and high-stability thermoelectric generator are to optimize the thermoelectric transport and realize a reliable joint of thermoelectric materials/electrodes with low contact resistivity and high thermostability.To solve the main challenge of realizing the high-performance skutterudites-based thermoelectric generator,we firstly explored the physical mechanism of the thermoelectric transport and optimize the thermoelectric performance in the p-type La-filled La_xFe_yCo_4-ySb₁₂system and n-type Yb-filled Yb_xCo₄Sb₁₂system.Then,we designed a Co-based alloy layer that acts as the diffusion barrier and bonding layer to join the Cu W and skutterudites.We explored the thermostability of the joints and their influence on the contact resistivity of joints and the maximum conversion efficiency of skutterudites-based thermoelectric generators.We calculated the free energy change of the unfilled Fe_xCo_1-xSb₃,La_yFe₄Sb_12,and La_z(Fe_zCo_1-z)₄Sb₁₂ system based on first-principles calculations.It can be confirmed that there is an embossment on the three-dimension free energy surface of the La_y(Fe_xCo_1-x)₄Sb₁₂ system,indicating that a miscibility gap exists.It is then experimentally confirmed that La filling-fraction fluctuates in the La_yFe₄Sb₁₂ system.The lattice thermoelectric conductivity of the La_yFe₄Sb₁₂ system reaches a minimum value of 0.33 W m^-1 K^-1 at 723 K.This value is one of the lowest lattice thermoelectric conductivity for skutterudites to our knowledge and is close to the theoretical thermal conductivity of a glass estimated by Cahill's model.The glass-like ultralow thermal conductivity of our system can be mainly attributed to the multiple effects of La filling atoms on a wide frequency spectrum phonons scattering.It is experimentally confirmed that the La filling-fraction and Fe content per unit cell are both the chemical driving force to the compositional fluctuations in the La_y(Fe_xCo_1-x)₄Sb₁₂skutterudites.The miscibility gap and the direction of compositional fluctuations are determined.We then calculated the electronic structure of La_yFe_xCo_4-xSb₁₂skutterudites by DFT.It is theoretically revealed that the conduction band maximum fluctuation resulted from the cation fluctuation and strain field fluctuation may lead to a significant suppressed bipolar effect.Due to the suppressed bipolar effect and enhanced carrier concentration,compared with the annealed sample,the power factor and ZT value of the melt-spinning sample increase by about 38%and 67%at 723K.The maximum ZT value reaches to about 1.4 at 723 K.Impressively,the remarkable calculated efficiency of 13.3%arrives at the highest records in the p-type lead-free thermoelectric materials,when the cold side is set as 323 K and the hot side are the maximum operable temperature.We prepared Yb_xCo₄Sb₁₂ skutterudites with a record-high filling fraction of 0.63by the extremely rapid solidification of the melt spinning method.The ordered superlattice model of Yb-filled Yb_xCo₄Sb₁₂ skutterudites was constructed based on the simulation of the electron diffraction pattern.The coexistence of ordered and nano(?5 nm)and sub-nano(<1 nm)modulated filling atoms was for the first time revealed in the experiment.The filling fraction limit of Yb_xCo₄Sb₁₂ was up to a record-high value of 0.63 and much higher than the theoretical prediction(?0.3),leading to one of the highest carrier concentrations(1.82×10²¹ cm^-3)in filled skutterudites.We calculated the temperature dependence of the Lorenz factor of Yb_xCo₄Sb₁₂ and actual filling fraction dependences of the Lorenz factor of rare-earth atoms filled skutterudites in this work and the earlier literature using the single parabolic band model.There is a well linear relationship between the L and the actual filling fraction both at 300 K and 800 K.The lattice thermoelectric conductivity of the Yb_xCo₄Sb₁₂ system reaches a minimum value of 0.63 W m^-1 K^-1 at 823 K.Such ultralow?_lat of our system can be mainly attributed to the ultra-high filling fraction,nanostructures,and high-density lattice strain arising from the ordered superlattice and nanoscale modulated Yb fillers.The enhanced scattering from the rattling effect mainly targets the low to mid-frequency phonons(2.5?15 THz).The strain effect and nanostructures provide significant extra scattering to the mid-high frequency(>15 THz)and low frequency(<2.5 THz)phonons,respectively.A high ZT value of about 1.3 was achieved at 823 K for the nominal composition Yb_0.4Co₄Sb₁₂.Laboratory setup for the contact resistivity testing was designed and fabricated based on the two-probe method.The tested electrical resistivity of the constantan guide sample deviated less than 0.5%with the calibration value.The tested electrical resistivity of typical thermoelectric materials and alloys which is from 10^-5to10^-8?m deviated less than 1%with the test value from the commercial equipment.A novel Co/CoW alloy layer was used as the diffusion barrier and bonding layer to successfully join the Cu W with p-type skutterudites by the diffusion bonding method.The typical microstructure was Cu W/Co/CoW/(Fe,Co)Sb/(Fe,Co)Sb₂/p-tye skutterudites.When joining at the optimum process parameters of 873K,10min,20MPa,the contact resistivity and shear strength of the joint were 1.12??cm² and17 MPa,respectively.A novel Co/CoMo alloy layer was used as the diffusion barrier and bonding layer to simultaneously join the Cu W with n-type and p-type skutterudites by the diffusion bonding method,due to its appropriate coefficient of thermal expansion for both of the n-type and p-type skutterudites.The typical microstructure was Cu W/Co/CoMo/CoSb/CoSb₂+Yb Sb₂/n-type skutterudites.When joining at the optimum process parameters of 873K,10min,20MPa,the contact resistivity and shear strength of the joint were 1.20??cm² and 22MPa,respectively.After annealing at 823K for 600 h,the contact resistivity and shear strength of the joint were 2.67??cm² and 20MPa,respectively.The growth of the CoSb and CoSb₂interface reaction layers can be explained by the reaction-diffusion theory.A prediction model of the thickness of the CoSb and CoSb₂ interface reaction layers was constructed by Deal and Grove's relation.After annealing at 823K for 100 days,the model predicted the thickness of 23.7?m and 43.8?m for the CoSb and CoSb₂interface reaction layers,respectively.We also constructed the prediction models for the contact resistivity and the output performance of the skutterudites-based thermoelectric generator.After annealing at 823K for one year,the contact resistivity of joints was only 6.9??cm².When operating for one year at a hot-side temperature of 823K and a cold side temperature of 323K,the maximum conversion efficiency and output power of the skutterudites-based thermoelectric generator decreased only 1.25% and 1.63%,respectively.