The Mechanism Of Lattice Defects On The Shear Mechanical Properties Of Layered Thermoelectric Materials

Thermoelectric materials are functional materials that can realize the mutual conversion of thermal energy and electrical energy.Layered Bi₂Te₃ is currently the only commercially available room temperature thermoelectric materials.However,its weak mechanical properties lead to a very low micromachining yield（less than 30%）of the material,which seriously restricts the application of Bi₂Te₃ microdevices in 5G communication and wearable fields.Therefore,it is of great significance to improve the mechanical properties of layered Bi₂Te₃.This work reveals the influence of stacking faults and nanotwins on the shear mechanical properties of layered Bi₂Te₃ materials at the molecular and atomic scales through molecular dynamics and first-principles methods.The microstructure dynamic evolution of these defects under the stress field is clarified.In addition,this work also investigated the effect of element doping on the shear mechanical properties of layered In₄Se₃ thermoelectric materials.A lattice defect strategy is proposed to improve weak van der Waals interlayer interactions to enhance the mechanical properties of layered thermoelectric materials,providing theoretical guidance for the development of layered thermoelectric materials with high mechanical properties.The main research work and results of this paper are as follows:（1）The influence of the relative density of stacking faults on the shear properties of layered Bi₂Te₃ thermoelectric materials was studied by molecular dynamics method.The introduction of initial stacking faults in Bi₂Te₃ can reduce the van der Waals interlayer spacing between substructures,thereby enhancing the interlayer strength and improving the shear modulus of the material.It is found that in the initial stacking fault structure with high relative density（>70%）,all van der Waals layer structures will gradually and dynamically reorganize into a full stacking fault structure with shorter interlayer spacing under the action of the stress field.Thereby,the van der Waals interlayer interactions are significantly enhanced,resulting in an increase of its ultimate shear strength（～3.0 GPa）up to 3 times that of a defect-free single crystal（0.97 GPa）.On this basis,the paper uses first principles to reveal the relative relationship between the formation energy and cleavage energy of stacking faults,and explains the deformation mechanism of stacking fault structures with different relative densities from the perspective of energy.（2）The thermodynamically stable Bi2Te3 nanotwinned structure was obtained by first-principles method,and the effect of nanotwins on the shear properties of layered Bi2Te3 thermoelectric materials was investigated by molecular dynamics method.The introduction of nanotwin boundaries reduces the van der Waals interlayer spacing at the interface,which can effectively enhance the van der Waals interlayer interaction,thereby improving the shear mechanical properties of layered Bi2Te3 thermoelectric materials.The paper also further studied the effect of nanotwin thickness on the shear properties of Bi2Te3 layered materials,and found that Bi2Te3 shear strength gradually decreased with the increase of nanotwin thickness until it reached a stable value（1.11 GPa）.It was found that the nanotwinned Bi2Te3 with nanotwinned thickness of 4.83 nm has the highest shear strength（1.42 GPa）.（3）The effects of element doping on the shear mechanical properties of layered In4Se3 thermoelectric materials were investigated by first-principles methods.The results show that element doping（such as Ca,Ag,Yb,Pb,Zn,I,and Br）can improve the van der Waals interlayer interaction in In4Se3 and tune the shear mechanical properties of layered In4Se3 thermoelectric materials.In addition,Ca-doping can increase the shear strength of In4Se3 by 14.4%,and Yb-doping can increase the shear modulus of In4Se3 by 9.3%.However,the element doping strategy does not change the failure mechanism of van der Waals interlayer slip in layered In4Se3.