Experimental Synthesis Of Novel B-C-N Materials And First-Principles Study Of Their Related Materials

In this thesis, the structures, properties and fabrication methods of boron, carbon and nitrogen elements and their compounds in B-C-N system have been summarized, and some scientific issues in the system were proposed. A turbostratic B2CN compound was prepared using melamine and boric acid by chemical combination and pyrolysis process. It was found that the carbon content in the compound would decrease when oxygen exists in the synthesizing atmosphere and a crystalline (B0.82C0.18) N compound was formed. Using turbostratic B2CN as a precursor, a hexagonal B2CN crystal was obtained under high pressure and temperature. Using Ca3B2N4 as catalyst, the P-T phase diagram of the B(CxN1-x) compounds was measured at high pressure of 3-6 GPa and at temperature of 800-1600℃, and the cubic B(CxN1-x) (x=0.1-0.4) compounds were obtained. The formation temperature of the cubic B(CxN1-x) is about 200 ℃ lower than that of c-BN. A dominant B2CN crystal with orthorhombic structure was synthesized using deliquescent Ca3B2N4 catalyst by high pressure and high temperature. This crystal is a white photoluminescence material with blue-violet luminescence characteristic. A eutectic product of boron carbide and graphite was synthesized when turbostratic B2CN was melted in graphite crucible at about 2600 ℃. It was found that the carbon content in the boron carbide phase is higher than B4C. The carbon content of a lamellar boron carbon carbide crystal with uniform thickness changes from 10 at% to 61 at %. The results of first principles calculations show that the carbon content limit in boron carbide crystal is 53 at%, which is consistent with the experimental value. The possible B2CN crystal structures with hexagonal and heterodiamond structures were studied by first principles calculation. Although boron nitride crystals are insulators, B2CN crystals are conductors with metallicity. The bond types in α-B crystal and the relative ionicity in B12 icasohedra were investigated. A new expression of ionicity hi based on Mulliken population was proposed. The relation between Mulliken population and ionicity was set up. The hardness of typical boron-rich solids such as α-B,B13C2,B6O and B13N2 was calculated, the calculated hardness is consistent with measured values. A conducting diamond-like superhard BC3 crystal was designed. The hardness of t-BC3 was predicted to be 79.0 GPa exceeding that of cubic BN. The alternate stacking sequence of conducting and insulating layers blocks in t-BC3 crystal constructs a sandwichlike lattice at an atomic level; which will bring some potential applications expected for novel electronic devices.