Behavior Study Of Delamination Crack And Energy Conversion Efficiency Of Thermoelectric Device

Thermoelectric(TE)materials can directly convert thermal energy into electric power.Thermoelectric devices made of TE materials have some unique advantages,such as free from maintenance,long lifetime,feasible for miniaturization,high reliability.Due to these excellent properties,TE devices have significant applications in waste heat recovery and on-chip cooling.In the process of fabrication and application of TE devices,microcrack formation is un-avoidable.The propagation of microcrack will further result in delamination,buckling and fatigue failure of TE devices.The mechanical failure certainly reduces performance and even results in lossing the natural funcation of TE devices.However,studies on the mechanical failure of TE devices induced by delamination crack are poor.It is urgent to provide the theoretical guidance for the safety design of T E devices.In addition,the traditional bulk TE devices can’t be used in a large-scale commercial applications due to the poor energy conversion efficiency.Thus,research on the mechanical failure due to microcrack propagation have high theoretical and practical significances for application of TE devices.Based on the thermo-elastic theory and the fracture theory,this thesis studies the energy conversion efficiency and the mechanical failure problem of TE materials and devices.The main research constents and the corresponding conclusions are presented as follows.The delamination behavior of multilayered TE devices is studied.According to the principle of energy conservation,the temperature governing equation of the temperature-dependent TE device is given.The homotopy perturbation method and the finite difference scheme are employed to solve the strong nonlinear thermal-electric-stress coupled field.It is found that the power output and energy conversion efficiency are slightly underestimated if the temperature dependence of material properties is ignored.Influence of the heat loss induced by radiation and convection on the power output and energy conversion efficiency of the micro-TE device is very small and can be neglected in the practical applications.Based on the compatibility equations of deformation,equilibrium equations of axial force and the strain compatibility equation at the interface of the bonding part s are derived.Analytical solutions of the axial force,the interlaminar shear stress and the delamination energy release rate are obtained.The critical temperature difference for unstable delamination propagation is presented.It is found that the critical temperature differences for crack propagation decrease with the length of delamination crack.Taking the temperature dependence of material properties into account,the axial forces and interlaminar stresses in multilayered TE devices are much bigger than the model with constant material properties.The critical temperature of the delamination propagation is greatly overestimated without considering the temperature dependence of material properties.This confirms the significance of considering the temperature dependence of material properties for safe design of TE devices.With the propagation of delamination crack,multilayered TE devices will buck.Thus,behavior of the delamination induced buckling of multilayered TE devices is analyzed.Based on the differential relationship between deflection and the arc length at the buckling region,the total deformation of the bucked TE layer is determined.Then,the axial force,interlaminar shear stress,buckling deflection and the critical buckling temperature are derived based on the compatibility equations of deformation.Results show that either a higher temperature difference or a higher electric current density can result in a larger buckling deflection and a strong axial force.The buckling deflection increases but the axial force decreases with the increase of buckling length.Based on the model of a fixed-fixed thin plate,the electric potential and the corresponding coefficient of cooling performance of the bucked TE device are given.It is found that buckling of the TE device will reduce its cooling performance.To improve the energy conversion efficiency of the TE device,the porous thermoelectric foam is employed.Power output and the thermal shock fracture behavior of a porous annular TEG are analyzed.Based on the thermal non-equilibrium,the energy equations for the porous TEG and t he waste gas are given.Then,the analytical and simplified expressions of power output are derived.The analysis demonstrates that the porous structure can significantly enhance the performance of the TEG with comparison to the bulk TEG.It is found that power output is inversely proportional to the cross-sectional area of the TEG.The optimized porosity of TEG for maximum power output is given.Additionally,the thermal shock fracture behavior of the porous TEG is analyzed based on the energy method.Based on the criterion of energy release rate,a simplified and useful expression of the critical heat flux for crack propagation is identified.It is found that the critical heat flux for crack propagation is inversely proportional to the porosity and the crack length.A more rigid contact between the TEG and the elastic boundary results in a larger thermal stress.When the porosity of the porous TEG is small,results show that the power output increases to a peak value then decreases with the crack length.A longer crack length results in a smaller power output when the porosity of the porous TEG is large.Finally,the segmented phytovoltaic-thermoeelctric(PV-TE)hybrid system is employed.Energy conversion efficiency and fatigue life of the PV-TE hybrid system are evaluated.Based on the photoelectric effect,thermoelectric effect and the classical Fourier heat conduction law,temperature of the PV-TE hybrid system is obtained considering the convection and radiation.Analytical solutions of the energy conversion efficiency and the power output are given.Results demonstrate that the power output and efficiency of the hybrid PV-TE systems with segmented TE model can be greatly enhanced compared to those of the hybrid PV-TE systems with the uniform TE model.For the solar cell with bigger efficiency correction coefficient,a larger combined heat loss results in a higher power output and efficiency.Conversely,the power output and efficiency decrease with the combined heat loss.The optimized height of TE model corresponding to the highest performance decreases with the height ratio of the upper to lower TE leg.Based on the thermoelasticity,the superposition principle and the Paris law,the interlaminar shear stress and the fatigue life of the hybrid PV-TE system are derived.It is found that the combined heat loss by radiation and convection can improve the fatigue life of the hybrid system.The enhancement of fatigue life and total energy converiosn efficiency can be obtained by optimizing the length and cross-sectional area of the TE leg.The optimized length and cross-sectional area of the TE leg are also presented.