Molecular Dynamics Study Of The Thermal Conductivity And Mechanical Properties Of Crystalline CoSb₃ With Sb-vacancy Defects

Thermoelectric material as a functional material,is able to convert the heat into electricity directly and vice-versa.It has a promising future in thermoelectric generation and thermoelectric refrigeration.At present,Skutterudite?CoSb₃?material is the most promising thermoelectric material to be put into wide-scale application in the intermediate temperature range,and its maximum working temperature could reach to about 800K.The sublimation of the antimony in the process of preparation and service of CoSb₃ material always lead to Sb vacancy defects at the Sb position in the lattice,resulting in lattice imperfection,which gradually influence the thermal properties and mechanical properties of CoSb₃ material.In this article,the molecular dynamics simulations are conducted to study the influence of the Sb vacancy defects on the lattice thermal conductivity and mechanical properties of CoSb₃ material,and its influencing mechanisms are revealed,to provide guidance for its industrial application.For the discussion of the influence of Sb vacancy defects on lattice thermal conductivity,the non-equilibrium molecular dynamics simulations are conducted,to simulate the lattice thermal conductivity of crystalline CoSb₃ bulk with different Sb vacancy fraction and different vacancy distribution,along with the temperature effects that are also considered.The influencing mechanism is revealed.The results indicate that lattice thermal conductivity is significantly reduced by Sb vacancies.Compared with the lattice thermal conductivity of vacancy-free crystalline CoSb₃bulk,the lattice thermal conductivity of the 4.0%vacancy distributed CoSb₃ bulk decreases significantly from 9.92 W/mK to 3.43 W/mK,and the mechanism is that the vacancy defects disturb the bonding behaviors of antimony rings and increase the scattering of phonons,leading to the decreased thermal conductivity.On the condition that the simulation temperature and vacancy fraction are settled,the better the uniformity of vacancy distribution is,the higher the thermal conductivity is.The lattice thermal conductivity of the system with 4.17%vacancies uniformly distributed is 3.74 W/mK,while a non-uniformly vacancy distributed model presents a lattice thermal conductivity of 2.09 W/mK.The mechanism is that when the vacancies are distributed unevenly,the related phonons are more scattered by defects,and the related vibration modes of Sb atoms are more disturbed,resulting in decreased lattice thermal conductivity.From 300 K to 800 K,lattice thermal conductivity decreases from 6.65 W/mK to 3.68 W/mK for the vacancy-free material,and decreases from3.74 W/mK to 2.51 W/mK for the material with 4.17%vacancies uniformly distributed.The influence of the vacancy defects on lattice thermal conductivity gradually decreases by the temperature increasing.For the discussion of the influence of Sb vacancy defects on mechanical properties,the tensile mechanical properties and shear mechanical properties are studied.The molecular dynamics simulations are also conducted to study the influence of the vacancy fraction,vacancy distribution,simulation temperature on tensile mechanical properties,and the influence of the vacancy fraction,simulation temperature on shear mechanical properties.When the vacancy fraction increases from 0.00%to 4.00%,the tensile ultimate strength of bulk CoSb₃ decreases from 19.39 GPa to 15.65 GPa,and the shear ultimate strength decreases from 17.27 GPa to 13.40 GPa.The elastic module and shear module are also decreased.With the tensile or shear strain increasing,the brittle failure happens for all the vacancy materials.The crack initiation lies in the location of vacancies,because the complete structure of Sb rings is destroyed by the vacancies,and the disappeared or changed Sb-Sb bonds enable the cracks to develop at the location of the vacancy Sb rings.The cracks develop by the increasing strain until the structure failure.The more the vacancies are,the weaker the mechanical strength is.From 300 K to 800 K,the tensile ultimate strength decreases from 19.39 GPa to 18.06 GPa,and the shear ultimate strength decreases from 17.27 GPa to16.23 GPa for the vacancy-free CoSb₃.The temperature effects are not obvious.The influence of Sb vacancy distribution on tensile mechanical properties is significant.The non-uniformity of distributed vacancies leads to the weakened mechanical strength,for the reason that the clustered vacancies at local location enable the crack easier to grow and develop,until the crystal structure failure.