Molecular Dynamics Simulation On Thermo-mechanical Properties Of Crystalline Mg₂Si:Nanopores Effect

Mg₂Si thermoelectric material with wide raw material sources、low price,which can implement direct conversion of heat and electricity in the temperature range from500K to 800 K,has wide application prospects in the areas of solar power generation、recovery of industrial waste heat and refrigeration.Experimental studies have shown that,The introduction of nanopores in thermoelectric materials can effectively reduce the thermal conductivity of materials so as to improve the thermoelectric conversion performance.At present,influence law on the thermal conductivity of the size and distribution of the nanopores is still unkown,At the same time,regular cognition of influence nanopores to mechanical properties of meterial is also lacking.The paper witch was supported by national 973 project,used molecular dynamics method,systematically studied thermal conduction properties and mechanical properties of Mg₂Si thermoelectric materials with nanopores,tried to reveal influence law of nanopores on thermal conductivity and mechanical properties of the material.The paper has used non-equilibrium molecular dynamics Muller-plathe method systematically study the influences of nanopores on thermal conduction properties of the material,the results showed that:phonon scattering effects on the surface of the nanopores,significant impacted thermal transfer behavior in the nanopore material,greatly reduced the thermal conductivity of the material,when introducing nanopores（diameter 0.95 nm,uniform distribution）of 1.77 percent,the thermal conductivity of the material was 35%lower than that of the material without nanopores;when the porosity was 9.62%,the thermal conductivity is about 80%lower than the material without nanopores.When the porosity is certain,the thermal conductivity of the material decreased with the increase of the free surface area,when porosity is 10%,thermal conductivity almost decreased linearly from 9.36 W/mk to4.97 W/mk when the specific surface area increased from 0.047?^-11 to 0.129?^-1,the reason is that the phonon scattering scale mainly associated with the specific surface area in the material with nanopores,the bigger the specific surface area of the material,the stronger the effect of phonon scattering.Further research showed that,when porosity and diameter of nanopores in the material is certain,the distribution of the nanopores also has a significant influence on the material’s thermal conductivity,the Thermal conductivity of Mg₂Si thermoelectric materials with uneven distribution nanopores is lower than the Thermal conductivity of Mg₂Si thermoelectric material with even distribution nanopores,the higher the degree of distributional segregation（the distribution is more uneven）,the lower the material’s thermal conductivity.The simulation results of the tensile mechanical behavior of Mg₂Si thermoelectric material showed that,the introduction of the nanopores led to a significant reduction of the material strength,when porosity was 1.77%,ultimate strength was 12.3 Gpa,about 30 percent lower than 17.76 GPa of the material without nanopores.When the porosity was increased to 9.62 percent,the material’s ultimate strength was only 9.12 GPa,decreased about 49 percent;The results of atomic energy analysis showed that,the atomic energy was much higher than the average atomic energy at the nanopores’edge perpendicular to the loading direction,which was similar to hole edge stress concentration in macro scale;by the analysis of bonds between atoms in the Mg₂Si crystal and bond strength,the separation surface of45°needs the minimal energy,therefore the destruction of the material with nanopores originated from the endpoint of the nanopores diameter perpendicular to the loading direction and extend along 45°direction.The introduction of nanopores also had significant influence on the elastic modulus,but witch was relatively less compared to ultimate strength.when the diameter of uniform distribution nanopores is certain,the ultimate stress and elastic modulus linear decreased with the increase of porosity,when porosity was certain and less than 10%,the change of the nanopores diameter had little effects on the elastic modulus and ultimate stress of the material.By examination of influence of nanopores distribution on mechanical properties,we found that nanopores distribution had a huge impact on ultimate stress of the material,at the porosity of 10%,when degrees of nanopores distribution segregation were 0.37、0.39 and 0.41,the corresponding Strength were 5.04 GPa、4.45 GPa and 3.94 GPa.