Modeling And Stress Simulation Of Skutterudite-based Thermoelectric Joints

Thermoelectric?TE?technology can realize direct conversion between heat and electricity,having advantages of small volume,no moving parts,extremely long service life and environment-friendly.Thermoelectric conversion efficiency depends on the thermoelectric figure of merits of the material,zT.Over the past years,searching high performance thermoelectric materials has received continuously increased attention,and recorded zT values of more than 2 have been realized in lots of materials.Filled skutterudites?SKD?are one of the promising thermoelectric materials for power generation in medium-temperature range,which had been thorough studied because of its low cost and excellent performance.However,on the other hand,studies on the design and integration technology of thermoelectric device fell behind,which has become an obstacle to the industrial application of TE technology.A thermoelectric device is constructed from multiple pairs of unicouples,which is composed of an n-leg and a p-leg elements.In elevated temperatures,most thermoelectric materials would react with electrodes or solder,causing the performance degradation or failure.To avoid this,diffusion barrier layers?DBL?are usually introduced between thermoelectric materials and electrodes.Generally,ones evaluate service behavior of TE device by static approaches,such as accelerated aging,observation of interfacial structures and measurement of contact electric resistivity.However,it's hard to evaluate the practical behavior of devices under service condition by using such static research approaches.This work focuses on the reliability of practical SKD device under service condition,and studies the evolution of the interfacial structure and mechanical properties during aging.A corresponding finite element model is established,and calculation results reveals the influence of microstructure evolution on the interfacial reliability.Results and conclusions are listed as follows:1.A model for predicting thermal stress of the thermoelectric element consisting of thermoelectric material and DBL is built.Relationship between interfacial stress and DBL material properties is established.According to the principle of minimized stress,selecting materials possessing smaller Young's modulus and similar coefficients of thermal expansion with SKD as diffusion barrier layer is favorable to improve interfacial reliability.2.Based on the structural feature of aged n-SKD/Nb/n-SKD joints,a double-reaction-layered model taking micropores into account is proposed.It is found that the maximum stress appears at CoSb₂/NbSb₂ interface,which is also the location of micropores.The stress increases with increasing aging period and aging temperature.Tensile test results showed that bonding strength fell down after aging,and the failure surfaces locate at CoSb₂/NbSb₂ interface,which coincides with the prediction by simulation model.The double-reaction-layered model is also applied to the n-SKD/Zr/n-SKD joint and familiar conclusions to n-SKD/Nb/n-SKD joints has been achieved.3.A multi-reaction-layered model is further built for aged n-SKD/Ti-Al/n-SKD joints according to the microstructural evolution.The simulation reveals that n-SKD/AlCo-TiCoSb interface is the weakest due to the existence of micropores,and the stress also increases with aging period and aging temperature.The fracture after tensile test tends to occur at the n-SKD/AlCo-TiCoSb interface and is consistent with prediction by simulation.In addition,a gradual decrease in bonding strength is observed after aged at 600?for less than 25 days,which is principally consistent with the prediction of simulation.