Theoretical And Experimental Study On The Formation Of Superhard Carbon By Phase Transformation Of C₆₀ At High Pressure And High Temperature

Carbon is one of the most important elements that constitute space and the substance,and it exists in various forms in the earth or even in the universe.The exploration of new forms of carbon has always been the eternal theme of contemporary scientific research.In this work,high pressure and high temperature experiments and molecular dynamics simulation calculations are carried out for the abundant phase transition behavior of fullerene C₆₀under high pressure and high temperature.The semiconducting amorphous carbon having bandgaps of 0.1-0.3 e V and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses are produced from fullerene at high pressure of 15 GPa and moderate temperatures.A systematic investigation of the structure and bonding evolution helps to build a transformation model from C₆₀to amorphous carbon that can be used in further synthesis of C₆₀under high pressure and high temperature for advanced applications.The produced amorphous carbon materials have the potential of demanding optoelectronic applications that diamond and graphene cannot achieve.A series of amorphous carbon materials containing high fraction of sp³are recovered from compressing C₆₀under 25 GPa and high temperatures.The results show that they are semiconductors with bandgaps of 1.5-2.2 e V,comparable to that of amorphous silicon.The synthesized AM-III sample is the hardest and strongest amorphous material known to date,with the Vickers hardness of 113 GPa and the Knoop hardness of 72 GPa,which can scratch the（001）face of single-crystal diamond and exceeds the record of 80.2 GPa held by the diamond-like amorphous carbon film;the compressive strength of its micropillars is up to 70 GPa,which is comparable to that of diamond.The combination of outstanding mechanical and electronic properties makes these amorphous carbon an excellent candidate for photovoltaic applications demanding ultrahigh strength and wear resistance.According to the structural characteristics and performance differences of amorphous carbon,the amorphous carbon materials are divided into five categories:i)sp²bonding+obvious graphite-like fragments or multilayer graphene;ii)low amount of sp³bonding+ disordered multilayer graphene fragments;iii)sp²bonding+disordered graphite-like fragments;iv)sp³bonding dominant dense disordered structure;v)100%sp³amorphous diamond.The definition and classification of amorphous carbon is of have far-reaching significance for the future research and application of amorphous carbon.The transformations from C₆₀to amorphous carbon are simulated at high pressure and high temperature conditions by large-scale molecular dynamics simulation.It is revealed that there are three stages,that is,i)P/T leads to the fracture of some sp³bonds and the collapse of C₆₀buckyballs,forming the"three-dimensional polymerized C₆₀+amorphous carbon"composite;ii)the increased P/T leads to the complete collapse of C₆₀buckyballs,forming the sp²-dominated amorphous carbon;iii)the higher P/T leads to the gradual transformation of sp²into sp³carbon,and promotes the gradual connection of sp³carbon clusters into a volume,and finally becomes the amorphous carbon system dominated by sp³carbon.The irregular polymerized polyhedron composed of sp²+sp³-hybridized carbon atoms are distinguished from the amorphous carbon systems as the basic structural units.The structure formation and transformation mechanism of“C₆₀→amorphous carbon”and an ideal structure model of amorphous carbon are obtained.A dense,light-yellow transparent polycrystalline diamond sample is obtained by quenching fullerene C₆₀from 25 GPa and 2000℃.The sample is composed of various nano-grains of with average grain size of～171 nm,and there are a lager number of twins and high concentrations of stacking faults within the grain.A reconnaissance study shows that the sample has an ultra-high Knoop hardness of 137 GPa,which is comparable to that of the the well-sintered nano-polycrystalline diamond from compressing graphite.Further investigation of the dependence of uniaxial compressive strength on various defect structures of polycrystalline diamond by large-scale molecular dynamics simulation provides a possible explanation for the mechanism of its ultrahigh hardness.