Study On Stress Concentration Of Segmented Thermoelectric Thin Films And Optimization Of Thermoelectric Properties

Thermoelectric materials are functional materials that enable the direct conversion of thermal and electrical energy.Thermoelectric devices made from them have the advantages of small size,lightweight,no moving parts,and no emissions,and their development in the energy field has received widespread attention.Recent studies have shown that segmented thermoelectric films have higher thermoelectric properties than uniform thermoelectric films,and research in this field has further promoted the development of thermoelectric devices.However,segmented thermoelectric films are generally affixed on top of a substrate during operation,and there is a stress concentration problem under the action of thermal-electrical-mechanical coupling.In addition,clarifying the influence law of multi-field coupling behavior on thermoelectric conversion efficiency can effectively guide the optimal design of thermoelectric devices.In this thesis,the stress concentration problem of segmented thermoelectric films is studied theoretically based on the singular integral equation method and the separation variable method,and the theoretical study is verified and extended by the finite element method;and the thermoelectric conversion efficiency of composite thermoelectric films is studied based on the principle of energy conservation.The main research contents are as follows:(1)The equilibrium condition of the interlaminar shear stress of the film/substrate with the axial stress of the film is considered,and the singular integral equation on the interlaminar stress is obtained by combining the boundary conditions and applying the Chebyshev polynomial to solve it,get the interlaminar stress distribution of segmented thermoelectric films,application of description and analysis of thin film stress intensity factor of edge stress concentration phenomenon.The results show that the size of the segmented thermoelectric film significantly affects the interlayer stress distribution.The larger the thickness of the segmented thermoelectric film,the stronger the stress concentration at the edge.In addition,the leakage behavior at the film/substrate interface reduces the stress concentration at the edge,but the leakage behavior also leads to energy loss and reduces the thermoelectric conversion efficiency of the film.Therefore,the size and performance of the materials should be considered comprehensively according to the working environment when designing thermoelectric thin film devices.(2)The transient temperature distribution of segmented thermoelectric thin films under thermal shock is obtained by using the separation variable method,and the stress intensity factor under thermal shock is obtained according to the method in the previous chapter.The results show that the material ratio of the segmented thermoelectric film can significantly affect the stress intensity factor,and the thermoelectric material with a higher Seebeck coefficient,lower thermal conductivity,and lower electrical conductivity at the high-temperature edge can reduce the stress concentration phenomenon.In addition,the larger temperature difference between the two sides of the film,the larger the stress intensity factor,and the longer the time required to reach the steady state.At the same time,the material parameters of the substrate and the ratio of the length of the two films also affect the stress intensity factor.When the material parameters of the substrate take a certain value,the stress intensity factor increases first and then decreases with the increase of the proportion of the film length of the low-temperature segment.(3)The temperature distribution and thermal stress of the segmented thermoelectric film were numerically simulated by finite element software,and the temperature,deformation and equivalent force distributions were obtained for the two cases of fixed cold end and fixed bottom surface of the film.It is also found that the maximum Von-Mises stress of the film increases with the increase of its thickness when the bottom surface is fixed.(4)The segmented thermoelectric thin film is obtained by energy conservation principle,the expression of the thermoelectric conversion efficiency,analysis the influence of Thomson effect on the thermoelectric conversion efficiency: when ignore the Thomson effect on the high end choose low seebeck coefficient,heat conductivity and low electrical conductivity of thermoelectric materials,can effectively improve the thermoelectric conversion efficiency of compound of thermoelectric film;The thermoelectric conversion efficiency of segmented thermoelectric films is higher when Thomson effect is considered,which indicates that neglecting the effect of Thomson effect may lead to underestimation of the thermoelectric conversion efficiency of thermoelectric devices.These conclusions provide theoretical references for structure design and performance optimization of segmented thermoelectric thin film devices.