Analysis Of Interlaminar Thermal Stress And Associated Delamination In Multilayered Thermoelectric Materials

Thermoelectric material(TEM)is an environment-friendly functional material,which can directly interconvert heat energy and electrical energy on a solid state without moving parts and refrigerant.TEM has significant potential in many engineering applications,such as solar energy harvesting,thermoelectric cooling,waste heat recovery.In addition,employing multilayered TEMs can improve their properties,such as thermostability and thermoelectric converting efficiency.Due to non-homogeneity in nature,multilayered materials may result in obvious interlaminar thermal and residual stresses at the free ends.Furthermore,TEMs are brittle in nature and prone to damage by cracking/delamination.Therefore,understanding the interlaminar thermal stress and delamination properties of multilayered TEMs are important for guiding the reliable design of thermoelectric devices.Based on the theory of elastic and fracture mechanics,this paper investigates the steady-state and transient-state interlaminar thermal stresses of multi-layered thermoelectric materials.The delamination of multi-layered thermoelectric materials with edge/interior crack is also studied,respectively.The main research contents and conclusions are organized as follows:The steady-state and transient-state interlaminar thermal stresses of multi-layered thermoelectric materials are studied.The influences of geometry parameter,material properties of insulating layer and electrical current densit y on the interlaminar peeling stress are investigated.Utilize the energy balance method and the concept of interfacial compliance,analytical solution for temperature field and the associated interlaminar stress of multi-layered thermoelectric materials,which consists a N-type and P-type thermoelectric layer,sandwiched by an insulating layer,are explored for the steady-state and the transient-state.The distributions of interlaminar peeling stress for different geometry parameter,material properties of insulating layer and electrical current density are presented graphically.The results show that(a)there is significant interlaminar stress concentration at the free ends of the multilayered TEM;(b)the distributions of thermal stresses at transient-state are very different fro m these at the steady state.Overall,the region of the interlaminar stress concentration at transient-state is much more serious than that at the steady-state;(c)a multilayered TEM with a thinner insulating layer will result in a smaller interlaminar stress.The interlaminar stress also reduces when the insulting layer has a smaller Young's modulus.For multilayered thermoelectric materials,the interlaminar stress vanishes if the coefficients of thermal expansion of all layers are identical.(d)the applied electric current enhance the interlaminar stress concentration therefore cannot be neglected.Based on the theory of fracture mechanics,the generalized problem of edge delamination for thermoelectric composite structure subjected to temperature load is investigated.Employ the geometry model of multi-layered thermoelectric materials with edge crack,stability analysis of crack propagation is carried out,and the closed form explicit expressions of transient mode-I and mode-II delamination stress intensity factors are derived.It is found that the peak value of mode-I stress intensity factor is considerably higher than that of mode-II.In addition,the length of delamination crack corresponding to the peak value of mode-I stress intensity factor is much smaller than that of mode-II.This means that a pn-junction with an edge crack is prone to damage by delamination rather than slippage.Analysis of interior debonding for multi-layered thermoelectric material is carried out in the last part of this thesis.Employing the equivalent model of multi-layered thermoelectric materials with interior crack and the theory of elastic and fracture mechanics,the approximate stress field and associated energy release rate at the tip of debonding part are presented.It is found that(a)the axial force of debonding part reduces with the increase of time and debonding length.Particularly,the distribution of debonding axial force remains the same as that of the no debonding state when the structure is in steady state.In addition,the axial force in thermoelectric layer is independent of time and debonding length away from the debonding part.(b)The energy release rate increases with the debonding length from zero value,displays a peak value and then decreases as the debonding length continues to increase.Furthermore,the energy release rate increases with time for small debonding length,and the energy release rate reduces with time if the debonding length is big enough.