Synthesis And First-Principles Study Of Layered BC_x Compounds

In this dissertation, the experiment and computation of the layered BCx materials were reviewed. Some problems in the research of the graphite-like BCx materials were proposed.Amorphous BCx precursors were synthesized by mechanical ball milling, in which graphite and amorphous boron powder were used as starting materials. Then the obtained precursor was treated at different temperatures and 5.5 GPa. When the temperature lower than 1000℃, some crystalline BCx grains with different compositions were found. However with the temperature higher than 1000℃, only B4C and pure carbon grains were found.A chemical pyrolysis process with boron acid and saccharose as starting materials was used to synthesize BC3 precursors. Then the obtained amorphous BC3 precursor was treated at different temperatures and 5.5 GPa. When the higher the temperature is, the higher the crystalline degree of the sample is, and the smaller the resistivity of the sample is.Turbostratic BCx powders with different compositions were synthesized by hot filament chemical vapor deposition method, in which diborane and methane were used as gas sources. When the boron concentration in the gas source increases from zero to 5.6 at.%, the boron concentration in the obtained sample also increase approximately linearly with the boron concentration in the gas source. And in this situation, the prepared samples only consist of BCx phases. When the boron concentration in the gas source is larger than 5.6 at.%, the amorphous boron phase appears in the sample. At the condition that the boron concentration in the gas source is 5.6 at.%, a turbostratic BC5 compound was obtained. The TG results indicate that the BC5 compound has higher oxidation resistance than carbon.Atomic arrangements in the monolayer, stacking sequences along the c-axis, energy band structures and density of states for BC3, BC5 and BC7 compounds were studied using ab initio pseudopotential density functional method within the local density approximation as implemented in CASTEP code. BC3-sun, BC5-I and BC7-VIII configurations have the lowest total energy in the constructed BC3, BC5 and BC7 structures, respectively. The calculated results of formation energy indicate that all of the BC3, BC5 and BC7 compounds are metastable.Adsorption of atomic oxygen on BC5 surface has been investigated using the CASTEP software. Different adsorption sites on periodic basal and prismatic surfaces have been investigated. The BC-b2 site locating on the seat of armchair surface is the most stable among all the investigated sites. Generally, adsorption of atomic oxygen is most preferable on zigzag surface and then armchair and basal surfaces. For the same kind of surface, O atom prefers to bond with B atom than C atom. The nature of adsorbent–adsorbate interactions is a chemisorption. For armchair surface, the adsorption energy on BC5 is lower than that on graphite. However for zigzag surface, the adsorption energy on BC5 is higher than that on graphite. When the more close to the edge the substituted B atom site is, this effect is more visible.Diffusion of atomic oxygen and desorption of CO molecule on BC5 surface have been investigated using the DMol3 software. The diffusion of atomic oxygen on BC5 basal surface is easier than on graphite basal surface. But for prismatic surface, the diffusion of atomic oxygen on BC5 is more difficult than on graphite basal surface. The desorption of CO molecule on BC5 armchair surface is easier than on graphite armchair surface. Comparing with the desorption energy barrier of CO molecule on graphite zigzag surface, the energy barrier on BC5 zigzag1 surface is larger, but it is smaller for BC5 zigzag2 surface.The role of substitutional boron in carbon oxidation and the oxidation reactivity of different surfaces of BCx materials were studied using the CASTEP software. Armchair surfaces of graphite, BC23 and BC5 are more stable than zigzag surfaces respectively. The oxidation reactivities of zigzag surfaces of graphite, BC23 and BC5 are larger than those of armchair surfaces, respectively. The role of substitutional boron for zigzag surface is inhibiting effect, however for armchair surface it is catalytic effect. The catalytic effect of boron for armchair surface is much larger than the inhibiting effect for zigzag surface.