Molecular Dynamics Study On Fracture Mechanical Properties Of The Thermoelectric Material CoSb₃

Thermoelectric material,a kind of green energy material,can directly convert heat and electric energy into each other,which has a wide application prospect in the thermoelectric power generation field.Skutterudite,represented by Co Sb₃,is a kind of thermoelectric material in the middle temperature（500～900 K）,which has drawn immense attention.The thermoelectric performance has been greatly improved by introducing various micro-nano structure control methods.Due to the harsh service conditions and various micro-nano structural defects,the study of the ultimate bearing capacity of Co Sb₃ is of great significance for the long-term stable use of the corresponding thermoelectric power generation system.Crack is a common defect,in which the nano-scale microcrack is essentially caused by the fracture of atomic bonds.Therefore,the single crystal block Co Sb₃ with mode-I central penetrating crack was studied by molecular dynamics simulation method in this paper.The effects of initial crack length（2a）and local atomic configuration of crack on mechanical properties of single crystal block Co Sb₃ were investigated.In this paper,the molecular dynamics calculation method and the model of single crystal block Co Sb₃ with mode-I central penetrating crack were established.It was verified that the setting of time step and strain rate was reasonable.Through the comparative analysis of a series of simulation results,the appropriate range of the size and proportion of the simulated box and the initial crack was determined.The preliminary calculation results show that the theoretical formula for calculating fracture toughness K_IC in classical fracture mechanics is still applicable at nanometer scale.The effect of the initial crack length on the fracture stressσ_f and fracture toughness K_IC of single crystal block Co Sb₃ was obtained under the condition that the local atomic configuration of the crack front was same.With the increase of the initial crack length,the fracture stress of Co Sb₃ decreased.The fitting results show that the relationship between fracture stressσ_f and（2a）^-1/2 is a quadratic function.The fracture toughness K_IC of single crystal block Co Sb₃ under different initial crack length was calculated by combining the fracture stress results obtained from molecular dynamics simulation and the classical fracture mechanics theory.The variation of fracture toughness with initial crack length was fitted by appropriate function.It is found that the fracture toughness of the model tends to a constant value when the initial crack length reaches the macroscopic scale,which is completely consistent with the theory of classical fracture mechanics.When the initial crack length is less than 1 nm,the predicted results are also reasonable.Under the condition that the model and crack size were identical,two groups of crack models with different local atomic configuration were obtained by moving the crack location from the center of the model along the crystal direction of[100]and[01 0]by different distances respectively.The results show that the fracture stress changes irregularly with different crack configuration.However,the distribution of fracture stress values of the two group models is symmetric with respect to the moving distance of 0.5a₀.The local atomic configuration and bonding characteristics of the crack front have obvious influence on the loading capacity of the whole structure,but do not affect the whole propagation mode of crack（100）[001].The crack propagation direction is still perpendicular to the loading direction,forming a flat fracture surface.The speed of crack propagation,whose limit value is not more than Rayleigh wave velocity,decreases with the increase of initial crack length.