Research On The Structure Design And Performance Of Compressed Glassy Carbon And Carbon-rich Carbon Nitride

In this thesis,the experimental synthesis and theoretical structure design of new functional materials in carbon-nitrogen system are studied.On the experimental side,a kind of sp²-sp³ hybridized amorphous carbon,namely compressed glassy carbon,was synthesized by high temperature and high pressure technology,and its mechanical behavior at micro-and nano-scale was studied;a new type of compressed glassy carbon/nanocrystalline diamond composite was synthesized by adjusting the temperature and pressure conditions,and its physical properties were tested and analyzed.On the theoretical side,by using the first-principles based structure search software CALYPSO and atom substitution method to predict and construct new carbon-rich carbon nitride structures,calculate their possible electronic properties and mechanical properties,so as to explain the experimental phenomena.Using glassy carbon as raw material,compressed glassy carbon was synthesized at high pressure(25 GPa)and high temperature(800?).The mechanical properties and deformation behavior of compressed glassy carbon at micro/nano scale were studied by ex-situ uniaxial compression experiments.According to the statistics of compressive strength and deformation behavior of compressed glassy carbon micro-nano-pillars with different sizes,it can be found that compressed glassy carbon micro-nano-pillars have very high compressive strength,and the strength increases with the decrease of size.The critical size for the change of the deformation mode is 614nm in diameter.The micro-nano-pillars with diameters higher than the critical size exhibit brittle fracture,while the nano-pillars with diameter less than the critical size exhibit plastic deformation.In addition,nano-pillars with D<614 nm have ultra-high energy dissipation density,which are 1-3 orders of magnitude higher than many bulk or nano-scale structural materials.A new type of carbon/carbon composites with high strength and hardness,as well as excellent elastic recovery and electrical conductivity were synthesized from glassy carbon at high pressure(25 GPa)and high temperature(1050-1150?).The microstructure of the compressed glassy carbon/nanocrystalline diamond composite is that the diamond nanoparticles in the composite are dispersed in the disordered compressed glassy carbon matrix and these two phases are combined in the form of coherent or semi-coherent.Its performance is significantly better than the current commonly used conductive ceramic materials,and it has the potential to be used in the field of static dissipation,such as making static-free bearings,anti-static substrates and components.Three kinds of carbon-rich carbon nitride structures were predicted.The first type is the high-pressure phase structures of C₅N with a stoichiometric ratio of 5:1.The hardness calculation results show that all the carbon nitride structures except I4₁-C₅N are potential superhard materials.The calculation of band structure shows that except I4₁-C₅N,the other five C₅N structures are all semiconductors.The second type is carbon nitride with pentadiamond-like structures,namely C₁₀N and C₁₉N₃.The metallicity of these two carbon nitride structures is due to the partial substitution of the electron-rich N atom for the C atom,which causes the Fermi energy level to rise and cross the conduction band.In addition,the calculation results of the partial density of states and the electron orbits show that C₁₀N is isotropic and three-dimensional conductive,while C₁₉N₃ conducts mainly along the plane.The third type is made up of cage-like structures.By simulating the X-ray diffraction(XRD)of these structures,it is found that they may exist in the products of the mixture of shock compressed carbon black and tetrachyanoethylene.Calculations of formation enthalpy using experimental materials as precursors show that these carbon nitride structures are easier to synthesize than the carbon structures previously used to explain the unknown phase.the results of hardness calculation show that they are potential superhard materials.The band structure calculation results show that only sc-C₄N is metallic,and the other three C₁₇N₄ structures are direct or quasi direct band gap wide band gap semiconductors,which have high efficiency of light absorption and emission,indicating their potential applications as ultraviolet light emitting devices.A novel cage-like C₆₀ carbon clathrate structure was theoretically predicted by using the structure searching program.This structure is formed by coplanar connection of small C₂₄ cage and flat C₁₈ drum.All atoms are sp³ hybridized.The carbon clathrate structure has the highest density among the carbon clathrate structures known.Moreover,it also has extremely high hardness,tensile and shear strength.Band structure calculation shows that the structure is a rare direct band gap semiconductor in all sp³ hybridized carbon,which has potential applications in photovoltaic devices.In addition,the larger gap in the polyhedron can accommodate other metal atoms,and it is possible to try to regulate its conductivity by filling it with different atoms.