Graphite-diamond Phase Transformation Mechanism Studied By Md Simulations

Because diamond has excellent high thermal conductivity,high hardness and wear resistance,it is an indispensable high-quality material in industrial manufacturing,and natural diamond resources are scarce and difficult to obtain.Therefore,synthetic diamond has become a hot topic.As an allotrope of diamond,graphite exists in large quantities in nature.Since it was discovered that graphite can be transformed into diamond,the mechanism of graphite-diamond phase transition and nucleation has aroused people’s research interest.At present,the graphite-diamond phase transition path,that is,the crystal orientation relationship,has been widely discussed,but the phase transition mechanism is still unclear.From the perspective of crystal nucleation kinetics,the addition of defects will lower the energy barrier of direct phase transition and promote the phase transition process.However,considering the structural characteristics of graphite and diamond,the effect of defects on diamond nucleation-long large-scale phase transition is not In a word.Therefore,this paper calculates and discusses the differences in the effects of defects.The unique layer structure of graphite makes defects have different characteristics in graphite.For example,edge dislocations do not cause lattice distortion inside the crystal,while prismatic edge dislocations can cause large-scale lattice distortion.The location of point defects in graphite can be divided into intralayer and interlayer.For example,vacancies in layers and heteroatoms between layers.In addition to point defects,graphite also has dislocations and grain boundaries,which can be transformed into each other.According to the principle of energy minimization,the study of the phase transition path from graphite to diamond roughly summarizes three types.One is that rhombohedral graphite transforms into cubic diamond(CD)through the "chair-shaped" displacement of atoms in the layer.Hexagonal graphite changes its stacking method to AA-like stacking through interlayer slippage,and the atoms in the layer pass through the "boat-shaped" phase change path to transform into hexagonal diamond(HD).Third,the layers of hexagonal graphite slip into AA stacks,and the atoms in the layers move through the "extra-shaped chair shape" to form hexagonal diamonds.In this paper,molecular dynamics simulation methods are used to discuss the three paths of graphite-diamond shear-type phase transition,among which the "heteromorphic chair-type" phase transition path is not supported by molecular dynamics.The influence of graphite defects on diamond nucleation along the "chair-shaped" and "boat-shaped" paths during the long and large-scale phase transition of graphite-diamond nucleation is analyzed.The conclusion is that the promotion effect of dislocations on the diamond nucleation-long-major phase transition increases with the increase in length,and the increase in the strength of the dislocations will inhibit the diamond nucleation-long-major phase transition.Therefore,after the length of the dislocation and the Burt vector interact with each other,they show an promoting effect on the phase transformation of hexagonal diamond,and the influence on the phase transformation of cubic diamond is first promoted and then suppressed as the Burst vector increases.As the angle of the grain boundary increases,the grain boundary will inhibit the phase transformation and nucleation of the two diamonds along their respective paths,and the inhibition is affected by the external pressure.