Study On The High Pressure-induced Structure Transitions Of Graphite And Glassy Carbon

Because of the flexible bonding ability,carbon materials own an extended family,from traditional graphite,diamond to recently developed fullerene,carbon nanotube and graphene,which play important roles in scientific development and life/industrial fields.Searching for carbon materials possessing excellent properties and novel structures is always the hot topic in Material science,Physics and Chemistry fields.As an extremely physical condition,high pressure can shorten the distances among atoms,change their interaction and even transform their bonding styles.Carrying out high pressure study on carbon materials has become an important approach to search for novel carbon materials and explore new properties.People found that the sp~2-sp~3 bonding conversion usually occurs in sp~2 carbon materials and induces some novel sp~3 carbon structures under high pressure.However,until now,the mechanisms of pressure induced carbon bonding transition are still not very clear.There are still some controversies of pressure induced structure transition in sp~2 carbon materials,which hinders the well-controlled carbon structure transitions and synthesis of novel carbon materials under high pressure.As two typical sp~2 carbon materials,ordered graphite and disordered glassy carbon also have these controversies under pressure.In this paper,graphite and glassy carbon were selected as the study templates.High pressure studies under different pressure conditions were carried out,including shear high pressure,ultrahigh hydrostatic pressure,high pressure and high temperature etc.,to further investigate the high pressure induced the structure transitions of sp~2 carbon materials.Here are the results of our paper:1.Theoretical simulations and high pressure experiments were carried out to investigate the structure transitions of graphite under pressure with shear stress at room temperature,proposing a new transformation mechanism of graphite to diamond.Based on results of molecular dynamics simulation of graphite under pressure with shear,graphite transformed into various“layered diamond”and fully sp~3 carbon structures whose structures depend on the degree of shear stress.To our surprise,these high pressure carbon structures transformed into either diamond or graphite upon decompression,suggesting a new pathway of diamond formation under pressure with stress at room temperature.In order to prove this pathway of diamond formation,a well-controlled uniaxial compression experiment was carried out on graphite microspheres.The experimental results revealed that when compressed graphite was submitted to large shear stress by uniaxial compression at high pressure,ultra-strong,sp~3-rich carbon phases(yield strength could reach 150GPa at a confining pressure of 52 GPa)were formed.When released to atmospheric condition,based on the characterizations by Raman spectra,HRTEM and EELS,the sp~3 carbon phase transformed into a mixture of diamond and graphite,which was consistent with the results of theoretical simulations.Our results explain several recent experiment observations of low-temperature diamond formation.They also emphasize the importance of shear stress for diamond formation,providing new insight into the graphite-diamond transformation mechanism,addresses a long-standing problem of apparent formation of diamond and diamond-like phases in high pressure experiments at room temperature.2.Ultrahigh pressure Raman spectra studies were carried out on graphite and glassy carbon to analyze and discuss their bonding change behaviors under(quasi-)hydrostatic pressure.During high pressure experiments,the small particle samples were employed to decrease the shear stress in chamber as much as possible and realized good quasi-hydrostatic pressure conditions.Base on in situ high pressure Raman spectra of graphite,the G band of carbon-carbon double bond vibration showed a monotonous upshift as the pressure increasing and kept to the highest experimental pressure 108 GPa,suggesting no obvious bonding change in graphite under quasi-hydrostatic pressure condition.Interestingly,during compress to 107 GPa,the amorphous glassy carbon showed similar Raman behaviors with graphite,in which G band always existed and monotonously upshifted.The Raman spectra suggest that the disordered degree of sp~2 carbon materials shows little effect on their bonding transition under pressure.Differ from formation ultra-strong sp~3carbon phases under pressure with shear stress,there is no obvious bonding transition in both ordered graphite and disordered glassy carbon under hydrostatic pressure condition,consisting with the theoretical prediction that the interlayer bonding in graphite is difficult under hydrostatic pressure.Our results help to explain many controversies about structure transformation of sp~2 carbon materials under pressure,which are significant for deeper understanding the structure transformation of carbon materials under pressure.3.High pressure and high temperature study was carried out on glassy carbon nanofilm(GCF,thickness about 200-300 nm)by laser heating,in which freestanding,transparent aerogel-like diamond nanofilm(ADF)was prepared firstly.The GCF was compressed to 20 GPa by using sodium chloride as pressure medium and heated to 2300 K.The Raman spectra,SEM,TEM were employed to character the structure of GCF after laser heating.The ADF prepared from GCF by high pressure and high temperature was constructed by interconnected ultra-small diamond nanograins and possesses a porous aerogel-like microstructure,while inherited a film morphology of the parent glassy carbon.Further discussions found that the low density,low thickness of GCF precursor and suitable high pressure and high temperature conditions played important roles for the aerogel-like porous structure formation.Our research breaks previous strategy in the synthesis of diamond aerogel by using porous aerogel carbon as precursor under high pressure and high temperature.The prepared freestanding,transparent aerogel-like diamond nanofilm adds a new member into diamond film family and may expand the applications of this type of porous nanofilm.Moreover,our results reveal the structural transformation behaviors of nano-glassy carbon under the fast high pressure and high temperature condition by laser heating,providing new ideas for constructing and preparing new functional carbon materials in the future.