Structure Design And Property Prediction Of Novel Superhardness Carbon Materials

Superhard materials refer to a kind of structural materials with Vickers hardness greater than 40 GPa.Among them,diamond is the first superhard material studied,and it is also a kind of material with the largest hardness known so far.It is generally speaking that the properties of materials are determined by their structures.Superhard materials also belong to functional materials and have been widely used in industrial tools,military,civil,scientific research,mining,national defense,electronic information,aerospace and other fields.To improve the hardness of hard materials and to look for or design new superhard materials with other unique properties is an important topic in the field of material research.In this paper,several new superhard carbon materials are obtained by manual construction and structure prediction software,and their properties are characterized in detail by the first principle method.The main conclusions are as follows:?1?A new type of orthorhombic carbon crystal structure with space group of PMMA?named o-C8?is found by particle swarm optimization algorithm.The density of the sp³-hybridized bonds carbon phases is 2.993 g/cm³.Density functional theory calculations shows that the elastic constant of the crystal satisfies the criterion of thermodynamic stability of orthorhombic crystal,and the phonon dispersion spectrum has no imaginary frequency in the whole Brillouin zone,which proves that the crystal structure is dynamically stable.The structure is a semiconductor material with an indirect band gap of 2.267 eV.The results of hydrostatic pressure calculation and tensile stress-strain show that the carbon crystal is a incompressible material.The calculated results of bulk modulus,shear modulus and Young's modulus are 298.6 GPa,274.1 GPa and 629.6 GPa respectively,and the Vickers hardness based on the Gao's model is greater than 82.4 GPa,indicating that the crystal o-C₈ is a new superhard and low density carbon material.There is a certain proportion of four-ring in the structure,that is,there will be?-?repulsion between adjacent bonds.The structural analysis and comparison with the known crystals with four-membered ring,the minimum value of energy per atom decreases with the decrease of the proportion of four-membered ring in the structure,so the stability of carbon crystal can be simply judged by the proportion of four-membered ring in the structure.?2?To study the relationship between microstructure and properties,a series of new orthorhombic carbon allotropes(oC₂₄?oC₃₂?oC₄₀0 and oC₄₈)were designed by inserting eight-atom fragments between the four-and eight-membered rings of bct-Carbon and oC₁₆-?.The first principle calculations show that these carbon crystals are stable in thermodynamics,mechanics and dynamics.With the increase of the proportion of six membered rings in the structure,the density will gradually increase,ranging from 3.332to 3.481 g/cm3,which tends to be close to that of diamond?3.521 g/cm3?.The calculation of electronic properties shows that this carbon phases have indirect semiconductor with a band gap of 3.280-3.348 eV.The tensile stress-strain shows that this carbon crystals have the weakest tensile strength along[010]direction,and the peak tensile strength is range from 69.74 GPa to 76.87 GPa,which is weaker than that of diamond along[111]direction.The anisotropy ratio of ideal tensile strength will decrease,with the increase of the six membered ring in this structure.The results also show that the anisotropy of carbon crystals increases with the increase of the ratio of six membered rings.The short and strong sp³ covalent bond in the structure is similar to that of diamond,so this carbon phases have better deformation resistance and the calculated Debye temperature are almost similar to that of diamond.Among them,the new designed oC₄₀0 and oC₄₈8 possess a higher higher bulk modulus,shear modulus,Young's modulus and Vickers hardness than the theoretically proposed oC₃₂2 and M-carbon.The calculated relative enthalpy shows that oC₄₀0 and oC₄₈8 can be transformed from graphite under a certain pressure,and this pressure value are lower than the pressure required to obtain bct-Carbon and o-C₁₆-?from cold compressed graphite.A regular result is that the stability,bandgaps,bulk modulus,shear modulus,Young's modulus,incompressibility,ideal tensile strength and Vickers hardness of these carbon crystals increase with increasing proportion of six-membered rings in the structure,which provides a clue for researchers to design new functional materials.?3?Three novel and similar orthorhombic carbon crystals were designed,and their stability,sound velocity,thermal conductivity and dynamic properties were investigated.The calculated results show that the three carbon crystals are energetically stable with cohesive energies less than-8.94 eV and their elastic constants C_ijj meet the mechanical stability criterion of the orthorhombic system under pressure of 0-100 GPa.The transverse and longitudinal sound velocity of carbon crystal along the x,y and z axes is related to the elastic constants C₁₁,C₂₂2 and C₃₃.Due to C₁₁?C₂₂?C₃₃,the sound velocity of carbon crystals are anisotropic.Based on the elastic constants,the type of carbon crystals have higher modulus,which only slightly smaller than that of diamond.The order of modulus is PM3>PM2>PM1,which is consistent with the order of the number of atoms in the crystal cell,that is,this crystals have higher shear deformation resistance.By calculating the anisotropy index Au,the percentage of anisotropy and the shear anisotropy factor,it can be concluded that this type of carbon phases is anisotropic,and the order of anisotropy is PM1>PM2>PM3.The XRD simulation of the three orthorhombic carbon crystals shows that there is a main peak at the diffraction angle of41.7°,while the other peaks are different due to the different structures.The calculated values of Vickers hardness based on different models are all over 79 GPa,which proves that the three carbon crystals are superhard materials.The Debye temperatures of the three new materials are all over 2100 K,and they have different thermal conductivity in different directions.The results of relative enthalpies show that they can be experimentally synthesized from cold-compressed graphite under pressure of less than20 GPa.