Study On The Synthesis And Properties Of New Superhard Bulk Sp~3 Amorphous Carbon Under High Pressure And High Temperature

Diamond and graphite are typical crystalline carbon allotropes with fully sp³-hybridized and sp²-hybridized C-C bonds,respectively.Different hybridization makes the two materials exhibit quite different properties.The 3D network structure constructed by strong?bonds makes diamond the hardest material in nature,while the weak interlayer interaction of graphite leads to its soft characteristic and good conductive properties.However,when carbon atoms arrange without long-range periodicity,amorphous carbon materials are formed.The structure and properties of amorphous carbon depend on the relative proportion of its internal sp³ and sp²components.Studies have shown that with the increasing concentration of internal sp³-hybridized bonds,amorphous carbon materials exhibit more excellent mechanical,optical and thermal properties.However,the bulk amorphous carbon material,corresponding to diamond,constructed by fully sp³-hybridized bonds has not been realized,which is a scientific challenge that has not been broken through for a long time in the field of carbon materials.As an extreme physical condition,high pressure can change the interaction between atoms,adjust the bonding forms,and effectively trigger the sp² to sp³bonding conversion in carbon materials.In this paper,fullerene C₆₀/C₇₀ and glassy carbon are selected as carbon precursors to study their transformation to amorphous carbon with high sp³ content under ultrahigh pressure using the self-developed technology of large-volume press(LVP).The structure,properties and correlations between them of sp³ amorphous carbon samples were analyzed combined with a variety of advanced testing methods.The following research results were obtained:1.The key technology of ultra-high pressure generation based on LVP is established.On a Walker-type LVP with a maximum press load of 1000 tons,the pressure limit of commercial LVP(?25 GPa)is broken with the optimized design of second-stage anvils and high temperature and high pressure experimental assembly.Pressure generation more than 35 GPa are both reached at room temperature and high temperature with a large sample chamber(diameter and height are both larger than 1.5mm),and pressure of 36?40 GPa can be reached under a higher press load,which provides a technical guarantee for further exploring and synthesis of new materials under ultrahigh pressure.2.Based on the self-developed technology for high temperature and high pressure generation in an LVP,large amounts of exploration and research on the structural transition of fullerene C₆₀ in the range of 20 to 37 GPa were carried out,and the P-T phase diagram of fullerene C₆₀ in this range was given.Millimeter-sized transparent bulk amorphous carbon material with nearly fully sp³-bonded was synthesized under the pressure close to the cage collapse boundary,and the sp³content is more than 95%.Using advanced technology of high-resolution transmission electron microscopy and high-energy synchrotron X-ray diffraction PDF analysis,the microstructure of nearly fully sp³-bonded amorphous carbon was analyzed,and it was found that sp³ amorphous carbon was composed of four-coordinate diamond-like carbon clusters with short/medium-range order randomly distributed in sp³ amorphous carbon network.Physical properties were further characterized based on large-size and high-quality bulk amorphous carbon materials.It was found that the optical band gap of nearly fully sp³ amorphous carbon was as high as 2.7 e V,the Vickers hardness was as high as 102 GPa(under a load of 9.8 N),the elastic modulus was as high as1182 GPa and thermal conductivity was as high as 26 W/m K,it is currently the material with the highest hardness,thermal conductivity and elastic modulus found in any known amorphous materials.Besides,the fine control of the sp³ content in the amorphous carbon samples was achieved by reasonably changing the temperature and pressure conditions,and it was found that the thermal conductivity and bandgap were approximately linear with the sp³ concentration,thus a series of amorphous carbon materials with tunable optical bandgaps(1.85 e V-2.79 e V)were obtained and may open new applications for amorphous solids.3.Using the technology of ultrahigh pressure generation in an LVP,a more homogeneous,highly transparent millimeter-sized bulk amorphous carbon with high thermal conductivity was synthesized from fullerene C₇₀.The sp³ content in the amorphous carbon can be as high as 93.5%.The properties of the amorphous carbon sample were further studied,and it was found that it exhibited higher thermal transport properties with a thermal conductivity of 32 W/m K,while maintaining excellent mechanical properties(Hv,109 GPa)and wide optical bandgap(2.65 e V).The high thermal conductivity may originate from its more homogeneous structure.The result of this reasearch shows that by selecting different carbon sources,the structure and properties of sp³ amorphous carbon samples can be adjusted under high temperature and high pressure,which provides a new idea for the controllable preparation of amorphous carbon materials.4.Using the technology of ultrahigh pressure generation in an LVP,millimeter-sized bulk compressed glassy carbon was synthesized from glassy carbon under 37 GPa and 600°C.The structure of compressed glassy carbon was characterized by XRD and transmission electron microscopy.It was found that the(002)interplanar spacing of the compressed glassy carbon was 2.96(?),which was significantly smaller than that of the initial glassy carbon,indicating more interlayer sp³ bonds formed.The amorphous sample exhibits excellent mechanical properties,with a elastic modulus of 315 GPa and a hardness of over 40 GPa,and is a new type of superhard material.