Investigation On The Mechanical Properties Of Penta-graphene With Vacancy Defects

Penta-graphene is a new two-dimensional metastable carbon allotrope that is composed entirely of carbon pentagons and resembles the pentagon-based Cairo tiling pattern.First principle calculations have demonstrated that penta-graphene is not only dynamically and mechanically stable,but also can withstand temperatures as high as1000 K.Penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 e V,so it is considered as a quasi-direct-band-gap semiconductor.Due to its unique crystal structure,penta-graphene possesses an unusual negative Poisson ratio and ultrahigh mechanical strength that can outperform that of graphene.Recent years,penta-graphene has attracted great interest in the fields of materials science and condensed-matter physics.Similar to other crystalline materials,structural defects may inevitably exist in penta-graphene.The types of defects in penta-graphene include vacancy defects,Stone-Wales topological defects and adatom defects.Up to now,there are few related reports on the mechanical properties of defective penta-graphene,which stimulates the interest in exploring the mechanical properties of defective pentagraphene and the corresponding tuning mechanism.In this thesis,molecular dynamics simulations were performed to investigate the mechanical properties and deformation mechanism of penta-graphene with vacancy defects under tensile and shear loads.The effects of hydrogenation on the mechanical properties of defective penta-graphene and the tuning mechanism were also studied.The main work is as follows:1.The mechanical behavior of defective penta-graphene under tensile and shear loads and the effects of defect concentration on the mechanical properties were investigated.The structural transformation of defective penta-graphene triggered by rising temperature was also studied.The defects studied include monovacancy at the4-coordinated C1 sites(MVC1),monovacancy at the 3-coordinated C2 sites(MVC2),the divacancy as missing a pair of the 3-coordinated C2 atoms(DV)and mixed vacancy defects with a combination of the above vacancy defects(MIXEDV).The results show that defective penta-graphene also exhibits a plastic behavior similar to pristine pentagraphene,which is caused by the irreversible pentagon-to-polygon structural transformation under external load.The existence of vacancy defects usually leads to the formation of new defects.The Poisson's ratio shows an increasing trend with increasing defect concentration.The Poisson's ratio of penta-graphene with MVC1 and DV changes from negative to positive when the defect concentration rises to about 3%and 6%,respectively,but the Poisson's ratio is always negative for penta-graphene with MVC2 and MIXEDV.All four types of vacancy defect lead to a monotonic decrease of Young's modulus,shear modulus,tensile and shear elastic limits of penta-graphene with increasing defect concentration,while tensile and shear elastic ultimate strains are less sensitive to defect concentration.Temperature can trigger structural reconstruction for free-standing defective penta-graphene.Compared with perfect penta-graphene,the critical transition temperature is increased due to the vacancy defects and the defects can delay the structural transition,but the melting temperature of defective pentagraphene is still very close to that of pristine penta-graphene.2.The effects of hydrogenation on the tensile and shear mechanical properties and deformation mechanism of defective penta-graphene were investigated.The results show that defective penta-graphene with low hydrogen coverages possesses obvious plastic characteristics and structural transformation under tensile and shear loads,while fully hydrogenated defective penta-graphene exhibits a brittle fracture feature without phase transformation.Fully hydrogenation improves the stability of the buckled structure and hinders the structural transition,thereby causing large enhancement in the shear modulus,elastic limit and elastic ultimate strain during tensile and shear deformation.The Young's modulus and tensile elastic limit of defective penta-graphene show a down-and-up trend with increasing hydrogen coverage,while tensile elastic ultimate strain,shear elastic limit and elastic ultimate strain increase monotonically.The increase of defect concentration result in significant reduction of the Young's modulus,shear modulus,tensile and shear elastic limits for fully hydrogenated pentagraphene,while tensile and shear elastic ultimate strains do not appear to be sensitive to defect concentration.