Thermoelectric Properties Optimization Of N-type Skutterudites And Diffusion Bonding Mechanism Of Skutterudites To CuW Alloys

Most of the usable energy in human industrial activities is wasted in the form of waste heat.Thermoelectric devices can be used for thermoelectric power generation,thereby realizing the reuse of waste heat.Adjusting the acoustic and electric transport properties of thermoelectric materials to optimize thermoelectric performance and designing a low resistivity and high strength electrode/thermoelectric material joint interface microstructure to obtain a high thermal stability joint are key challenges to achieve thermoelectric generators with high energy conversion efficiency and high reliability.Considering the thermoelectric properties,mechanical properties,and high-temperature thermal stability,skutterudite thermoelectric materials have great advantages and potentials.This paper carried out related research from three aspects:material preparation,welding,and device design.Firstly,the filling process of n-type skutterudites,the microstructure characteristics brought by the simultaneous filling of five types of atoms,and the optimization mechanism of the microstructure characteristics on the thermoelectric performance were investigated.And then,a new design idea of using a multi-principal element alloy as the intermediate layer was proposed,and a joint between the CuW alloy electrode and skutterudites was achieved by the diffusion bonding method.The diffusion bonding mechanism and the high-temperature thermal stability of the joint were studied.Finally,the finite element method was used to optimize the device structure and to reasonably evaluate and predict its output performance.The diffusion activation energy of Yb,Ca,Al,Ga,and In filling atoms in the CoSb₃ lattice was calculated based on the density functional theory.An extraordinarily large energy barrier over 3 e V exists in the solid solution process of filling atoms.The diffusion coefficient of the filling atoms decreases with the introduction of other fillers.The un-filled CoSb₃,single-filled Yb_0.3Co₄Sb₁₂,and the multiple-filled Yb_0.3Ca_0.1Al_0.1Ga_0.1In_0.1Co_3.75Fe_0.25Sb₁₂ samples were synthesized using an equilibrium preparation method of smelting-quenching-annealing-spark plasma sintering.Different annealing times were set to discretize the reaction process.It is revealed that the introduction of filling atoms significantly slows down the reaction rate,and the multiple-filled system requires a longer annealing time to complete the reaction compared with the single-filled system.The filling process was further quantified by the measurement of the total filling fraction of each reaction stage.The total filling fraction shows an“increasing saturation”relationship with the annealing time.According to the above theoretical analysis and experimental verification,it is clarified that the reaction process is mainly controlled by the diffusion behavior of filling atoms in skutterudites.Completing the multi-stage peritectic reaction is essential to achieve pure phase microstructure and high thermoelectric performance.As a result,a peak ZT value of 1.33 at 773 K was obtained in the multiple-filled Yb_0.3Ca_0.1Al_0.1Ga_0.1In_0.1Co_3.75Fe_0.25Sb₁₂ sample.A variety of n-type filled skutterudite samples were synthesized using a non-equilibrium preparation method of smelting-quenching-melt spinning-spark plasma sintering.The microstructure characteristics（nanoscale grain size,defects,and well-distributed intermediate phases）of melt-spinning ribbons are an essential prerequisite,which can improve the reaction process efficiency.A high total filling fraction,dense dislocations,and fine nanostructures in the multiple-filled Yb_0.3Ca_0.1Al_0.1Ga_0.1In_0.1Co_3.75Fe_0.25Sb₁₂ sample were revealed by transmission electron microscope.To investigate the origin of the Seebeck coefficient of the multiple-filled sample being higher than the trendline of（Co,Fe）Sb₃-based skutterudites,the electronic band structures were calculated by the density functional theory.Band convergence occurs in the multiple-filled sample due to its high total filling fraction.Phonon transport calculations were performed based on the Callaway model.Dense dislocations and fine nanostructures provide significant extra scattering for low-frequency（<2 THz）and mid-to high-frequency（>16 THz）phonons,thereby realizing the full-spectrum phonon scattering.The lattice thermal conductivity of the multiple-filled sample decreases to a minimum value of 0.66W·m^-1·K^-1 at 723 K,which is closed to the glass limit.According to theoretical analysis and experimental verification,it is revealed that the combination of five-element filling and extreme non-equilibrium solidification is the key point for the simultaneous realization of the full-spectrum phonon scattering and band convergence.As a result,a peak ZT value of 1.66 at 823 K was obtained in the multiple-filled Yb_0.3Ca_0.1Al_0.1Ga_0.1In_0.1Co_3.75Fe_0.25Sb₁₂ sample,which is one of the highest ZT values in reported skutterudites.Moreover,the multiple-filled sample also possesses excellent structural and thermal stability.The FeCoNiMo multi-element alloy intermediate layer was designed and then prepared.The CuW electrode,FeCoNiMo intermediate layer,and n/p-type skutterudites have well-matched thermal expansion coefficients in the temperature range of 323-823 K.The diffusion bonding of CuW electrode（Fe plated on the surface）and n/p-type skutterudites was realized using the FeCoNiMo intermediate layer.The nominal compositions of n and p-type skutterudites are Yb0.3Ca0.1Al0.1Ga0.1In0.1Co3.75Fe0.25Sb12andLa0.8Ti0.1Ga0.1Fe3.3Co0.7Sb12,respectively.The typical interface microstructures of joints are CuW/Fe/FeCoNiMo/Ga Mo₃+（Fe,Co,Ni）Sb/（Fe,Co,Ni）Sb+Yb Sb₂/n-type skutterudites and CuW/Fe/FeCoNiMo/Ga Mo₃+（Fe,Co,Ni）Sb/（Fe,Co,Ni）Sb+La Sb₂/p-type skutterudites,respectively.When joining under the optimum technology parameters of 873 K,10 min,and 40 MPa,the shear strengths of n and p-type skutterudite joints are 21 MPa and 18 MPa,respectively;the joint resistivities of n and p-type skutterudite joints are 2.48μΩ·cm² and 2.51μΩ·cm²,respectively.After holding for 600 h at 823 K,the shear strengths are 18 MPa and 16 MPa,respectively;the joint resistivities are 3.17μΩ·cm² and 3.22μΩ·cm²,respectively.The growth of the reaction layer was explained using the reaction-diffusion theory and a prediction model for joint resistivities was established.After holding for 5 years at 823 K,the joint resistivity of the n-type skutterudite joint is predicted to be 8.38μΩ·cm².By developing a simulation model containing the joint resistance and heat loss based on the finite element method,it is predicted that the maximum energy conversion efficiency of the skutterudite thermoelectric module will reach 13.96%under the temperature difference of 823 K/323 K.After operating for 5 years,the maximum energy conversion efficiency is only reduced by 1.66%.