Study On The Fracture Behavior And Thermoelectric Properties Of Thermoelectric Materials Containing Defects

Nowadays,the fossil fule on earth can only be used for the next 60 years and it is urgent to develop the renewable energy technologies for the upcoming energy shortages.Thermoelectric materials can directly convert heat into electricity and vice versa,which serve as a promising candidate for the new renewable energy technology.However,the thermoelectric materials are ususally brittle materials,the defects are easy to be formed under in-service conditions and cause the thermoelectric concentration and thermal stress concentration at the crack tip.At the same time,the defects often have poor electrical conductivity and thermal conductivity,which increase the electrical and thermal interface resistances,resulting in a significant decrease in the thermoelectric efficiency.Therefore,in order to provide reference for the reliability evaluation of engineering thermoelectric devices and the design of high-performance thermoelectric devices,it is essential to analyze the fracture behavior and effective thermoelectric properties of thermoelectric materials containing defects(including cracks,holes or inclusions).Firstly,this thesis studies the problem of thermoelectric materials with cracks or holes.Based on the complex variable method,the analytical solutions of the electric field,thermal field and thermal stress field around the elliptical hole are derived and the heat flow,current and thermal stress intensity factors at the crack tip are given.The results show that the maximum stress concentration always occurs at the end of the major axis of the elliptical hole,which is also the most valunable point for crack propogation.The fracture toughness of popular thermoelectric material Bi2Te3 is measured by experiment and the fracture criterion is established for engineering applications.Furthermore,the effective thermoelectric properties of cracked thermoelectric materials are derived and it is found that in some cases,cracks or holes can improve the thermoelectric conversion efficiency.Then the hole problem is further generalized to the inclusion problem,the thermoelectric field and thermal stress field inside and outside the elliptical inclusion are derived.In addition,the coupled thermoelectric constitutive equations are completely decoupled into a linearized system by introducing the concept of effective electric potential and effective temperature,based on which the solution of the three-dimensional inclusion problem is obtained by using the Green's function method.The results show that when the applied current and the heat flow are uniform,the current and heat flow inside the inclusion are also uniform.By inserting specific inclusions inside the thermoelectric material,the energy conversion efficiency can be improved.This provides a new approach for the design of new thermoelectric materials.In addition,the transient response of cracked thermoelectric materials under thermoelectric shock is studied.Based on the singular integral equation method,the solution of transient thermoelectric field is solved and the dynamic intensity factors at the crack tip are given.Also,the transient effective thermoelectric properties of the cracked thermoelectric materials are also derived and the effect of crack location on energy conversion efficiency is discussed.The results show that in transient loading state,the energy conversion efficiency has a significant peak value greater than the steady state efficiency,which means the transient loading can make the thermoelectric material more efficient.Finally,an engineering problem is analyzed that a large number of defects are generated on the interface inside the thermoelectric device and causes the decrease of the thermoelectric performance under thermal cycling.Based on the perturbation method,the temperature distribution,thermoelectric conversion efficiency and output power are solved considering the temperature dependent material properties.Furthermore,the interface stresses are derived,the interface damage evolution is given,and the lifetime of thermoelectric devices is predicted.The results are verified by either the finite element analysis(ANSYS 18.0)or the experiment.The numerical results show that when the ratio of the interface thermal conductivity to the electrode thermal conductivity is greater than 0.1,the influence of the interface thermal resistance can be neglected;however,when the ratio is less than 0.01 The interface contact thermal resistance will greatly reduce the performance of the thermoelectric device.The research in this thsis can provide theoretical support for the reliability evaluation of thermoelectric devices,and also provide reference for the design of high-performance thermoelectric devices.