Phase Evolution Under High Pressure And High Temperature And Properties Of B-C-N Compounds

The light-element ternary metastable B-C-N compounds have become the front subject of material science due to their expected intermediate properties between carbon and boron nitride.In this paper, a turbostratic B-C-N precursor which was prepared by chemical process was directly compressed at 5.5 GPa and 800¹500 oC. X-ray diffraction (XRD), energy dispersive spectrum (EDS), transmission electron microscopy (TEM) and electronic energy loss spectroscopy (EELS) were used to characterize the structure and composition of the samples obtained under high pressure and high temperature. The results show that the prepared B-C-N compounds are metastable and the compositions of the obtained B-C-N compounds are influenced by the temperature. At 5.5 GPa, the turbostratic B-C-N precursor transforms to hexagonal B-C-N compounds with small crystalline grains at 1200 oC, meanwhile a little of hexagonal BN (h-BN) and amorphous C phases are also observed at this condition. Above 1400 oC, the B-C-N compounds completely decompose into h-BN and graphite.The hexagonal BC2N (h-BC2N) compound was synthesized using a turbostratic B-C-N compound as precursor and Fe as catalyst at 5.5 GPa and 1500¹600 oC. The lattice parameters of the h-BC2N are determined to be a=0.2487 nm, c=0.6637 nm. The possible structures were calculated using the CASTEP calculation module, which is a subunit of Materials Studio software. The calculated results show that h-BC2N compounds possibly stack as graphite structure with N atoms lying directly on the centers of the hexagonal rings of the neighboring layers, especially with B atoms facing directly to the B atoms of the neighboring layers, but do not possibly stack as h-BN structure. High-temperature X-ray diffraction was used to measure the average lattice liner thermal expansion of the h-BC2N. The results show that in the a-b basal plane, h-BC2N displays slight linear contraction up to 750°C with a contraction coefficient of -8.76×10-7 K-1, but above 750°C a linear expansion is observed with a larger expansion coefficient of 1.52×10-6 K-1, while the thermal expansion in the c direction is large and linear with an expansion coefficient of 35.86×10-6 K-1 up to 1000°C. Thermal oxidation behavior ofh-BC₂N was studied by means of simultaneous non-isothermal thermogravimetry (TG) and differential scanning calorimetry (DSC) thermoanalyses. Under dry air atmosphere, h-BC₂N has higher thermal oxidation resistance than carbon. The apparent activation energy Ea for the oxidation of h-BC₂N is determined to be about 223.57 and 231.48 kJ/mol according to the Kissinger and Ozawa methods respectively.The possible B₂CN configurations with layered and wurtzite structures were studied by first principles calculation. The calculated results of total energies show that h-B₂CN-IV with AA stacking sequence and w-BB₂CN-I are the most stable structures in the layered and wurtzite structures respectively. The calculated electronic energy band and density of states indicate that the layered and wurtzite B₂CN show metallicity, which mainly comes from -B1-C-B1-C- chains. The structural transformation of layered B₂CN with AA stacking sequence under high pressure was studied by first principles calculation. The calculated results show that the transformation from layered B₂CN with AA stacking sequence to dense phase needs very high pressure (more than 500 GPa). But it is hard to synthesize dense phase B₂CN without high temperature and the assist of appropriate catalyst.