Molecular Dynamic Simulation Of Fracture In Silicon

With the rapid development of the global information industry,a variety of electronic products which relied on the Lithium ion batteries require the higher performance,especially the capacity of the battery.Silicon based anode materials has attracted widely attention from the researchers of Li ion batteries with high theoretical Li storage capacity.But with the process of forming the Li-Si alloy,a high level of cubical dilatation occured.And a lot of cracks appeared in the Si cathode,which cause the electrode spalling invalidated and limit the batteries' commercial application.In this paper,molecular dynamics method is used to study the propagation of crack in Si crystal in the micro scale.The process of crack propagation and the effect of dislocation on crack propagation are explained in atomic scale.By comparing with the experimental results of some scholars,it is shown that the accuracy of the molecular dynamics method in the study of crack propagation in Si crystal.In this paper,the molecular dynamics method is used to calculate the surface energy of(111),(110)and(100)surface of Si crystal by using COMB potential function.We find that(111)shuffle surface's surface energy(1.2J?m-2)is the lowest in all energies of the calculated results,so the(111)shuffle surface is the first cleavage plane in Si crystal.In addition,(111)glide surface's surface energy(4.14J?m-2)is the highest in all the calculation results,which means that the most difficult place where crack propagation occurs.Due to the characteristics of crystal structure,there are no shuffle planes and glide planes in(110)surface,which is the second cleavage plane of Si crystal.The calculation result of the(110)surface energy is 1.523J?m-2,which is higher than(111)surface.The results of(100)shuffle plane and(100)glide plane are equle to each other,where both of them is 2.27J?m-2.Compared to the other two kinds of surfaces,the possibility of crack propagation is smaller.The conclusion above is consistent with the results which calculated with S-W and Tersoff potential function,and is agree with other scholars' experimental results,which verify the method used in the study of crack propagation in Si crystal in this paper is correct.Secondly,the atomic process of crack propagation and deflection in(111)and(110)surface of Si crystal under the different temperature and loading velocity conditions is studied.Also,we discuss the principle of crack propagation process at molecular scale and find that the stress concentration at the crack tip makes the maximum stress atoms appear at the crack tip.When the stress reaches the critical stress of the fracture propagation,the crack extends a lattice and forms a new crack tip and stress concentration.Besides,the velocities of the crack propagating in Si crystal are calculated.We find that with the increase of loading velocity,the crack propagation's speed approximately straight increase under the same temperature.And under the condition of constant loading velocity,the crack propagation's speed will be faster along with the increase of temperature.What's more,the crack propagation becomes more complicated.Through comparing the cracks propagation in(111)and(110)surface,we find that the crack propagation occurs along a straight line on(111)surface who has smallest surface energy.And when the crack propagation occurs on(110)surface,it will deflect to(111)surface.This is consistent with the theoretical analysis of crack propagation.Finally,this paper studies the effects of reconstruction defect,30 angle partial dislocation and 90 angle partial dislocation on the crack propagation in Si crystal by using molecular dynamics method respectively.It is found that the existence of the reconstruction defect changes the position of the stress concentration atom in the crack tip,which makes the crack deflect to the lower(111)surface layer of the reconstruction defect.In addition,the atomic bond reconstruction is caused by the 30 angle and 90 angle partial dislocation existing,where the stress field different from perfect crystal exists,which makes the original crack along a straight line deflected near the dislocation stress field.The difference is that the 30 angle partial dislocation leads to the atomic bond of the atomic layer where dislocation exist become weak,and the 90 angle partial dislocation leads to the atomic bond of the next-door atomic layer where dislocation exist become weak.Effects of two kinds of dislocation on the crack propagation exists in the critical distance,beyond what the dislocation will not affect the crack propagation.What's more,the loading velocity has influence on the crack propagation based on 30 angle partial dislocation The larger the loading velocity,the smaller that the range of influence of dislocation on crack propagation is,and 90 angle partial dislocation have no influence on crack propagation.