First-principles Study And Experimental Synthesis Of New B-C Compounds

Composed of neighbor elements in periodic table (namely boron and carbon), B-Ccompounds with B12icosahedron structure possess high hardness, which is about40GPa;this makes them next to those of diamond and cubic BN. Diamond-like B-C compoundsare even harder than that of cubic BN. Moreover, B-C compounds also havesuperconductivity. Therefore, multifunctional B-C materials would have potentialapplications on frontier fields, such as novel electron devices, medical apparatus, andmeasurement equipments.Most B-C compounds synthesized up to now are amorphous with graphite-likestructure. Although some nano-crystalline B-C compounds have been successfullyproduced, characterizations of their crystal structures and physical properties are stillchallenging. Here we used CALYPSO code, combined with hybrid particle swamoptimizations with density functional theory, to predict B-C compounds with variouscompositions. Then we used CASTEP code to verify structural stabilities of thesepredicted B-C compounds. For the stable compounds, we further studied their electricalproperties, namely band structures and density of states (DOS).By calculating elastic constants as implemented in CASTEP code, the effect ofcarbon content on elastic properties of B-C compounds was evaluated. Results show thatB-C compounds with higher carbon content have better abilities of resisting compressionand shear deformation. Among these B-C compounds, the ones with higher bulk moduluswere chosen for further calculations of the Vickers hardness using semi empirical theorydeveloped by our group. These compounds have values of Vickers hardness higher than40GPa; all can be considered as superhard materials. The role of carbon content inaffecting Vickers hardness was also discussed. Besides mechanical properties, we alsostudied superconductive properties of the four compositions, P4/nmm-B₃C,I4/mmm-BC,P42/mmc-BC₄,P-4m2-BC7. We calculated their superconducting transitiontemperatures (STT), and found that the STT does not show a linear trend as a function ofcarbon content. As the carbon content increases, the STT first increases and then decreases。By MW-ECR-CVD, B-C thin films were prepared on (100) silicon substrate frommixture gases of C₂H₄and B₂H₆. Morphologies and structures of these films werecharacterized by FESEM, GIXRD, and XPS. The surfaces of films are densification withuniform grain sizes, and are composed of BC(25) with orthorhombic structure andgraphite-like amorphous materials. We also found that high substrate temperaturepromotes the formation of crystalline B-C compounds.Dynamic high pressure light-gas gun were employed to compress the powerscomposed of nano-boron and amorphous carbon. FESEM, power XRD, and TEM wereused to characterize compositions, morphologies, and structures of these compressedsamples. Plenty of bulk crystalline grains characterized as single crystal B13C2phases canbe observed by FESEM and TEM, and they are embedded among layered structures ofgraphite-like carbon. No new B-C phase was observed.