【作者】 胡前库；

【导师】 田永君；

【作者基本信息】 燕山大学 ， 材料学， 2008， 博士

【摘要】 本论文综述了近年来层状BCx材料的实验合成和理论计算的研究进展,提出了目前类石墨层状BCx材料研究中存在的问题。以石墨和非晶硼粉为原料,采用机械球磨法合成了非晶结构的BCx前驱物。在5.5 GPa压力下,对该前驱物进行了高温高压处理。当温度小于1000℃时,样品中存在结晶较好的、成分不同的BCx (x≥3)颗粒;当温度高于1000℃时,样品中只含有B4C颗粒和纯C颗粒。以硼酸和蔗糖为原料,采用化合热解法合成了非晶结构的BCx前驱物。在5.5 GPa压力下,对该前驱物进行了高温高压处理。温度越高,样品的晶化程度越高、电阻率则越小。以乙硼烷和甲烷分别作为B源和C源,采用热丝化学气相沉积的方法,制得了成分可调的、乱层石墨结构的BCx粉末。随着气源中B的含量从0增加到5.6 at.%,样品中B的含量也呈近似线性增加趋势,样品中只含有BCx相。当气源中B的含量为5.6 at.%时,合成了乱层石墨结构的BC5化合物。气源中的B含量继续增加,BCx颗粒中的B含量不再增加,而是生成了非晶B相。采用热重法对样品的热氧化行为进行了研究,发现BC5化合物比纯C具有更好的抗氧化性能。采用CASTEP计算软件,研究了三种成分的层状BCx化合物BC3、BC5和BC7单层内的原子排布、c轴方向的堆垛顺序、以及能带结构和态密度。BC3-sun结构、BC5-I结构和BC7-VIII结构分别是三种成分的BCx化合物中总能量最低的。形成能的计算表明,BC3、BC5和BC7化合物都是亚稳相。采用CASTEP计算软件,研究了O原子在BC5化合物表面上的吸附行为。研究结果发现,相对BC5化合物基表面来说,O原子更倾向于吸附在棱柱曲表面上。除了armchair1表面上的BC-b2吸附位以外,zigzag表面比armchair表面具有更负的吸附能。在本文考虑的所有吸附位中,BC-b2位具有最小的吸附能,因此是最稳定的。对于同一种表面来说,O原子倾向吸附于含有B原子的位置上。通过Mulliken布居数和态密度分析,证实了O原子在BC5化合物表面上的吸附是化学吸附。氧原子吸附于BC5化合物的zigzag表面上时的吸附能,高于在石墨zigzag表面上的。而对于armchair表面,结果却完全相反。而且当替代的B原子越靠近表面时,这种趋势越明显。采用DMol3计算软件,研究了O原子在BC5化合物表面上的扩散行为、以及CO分子在BC5化合物表面上的脱附行为。O原子在BC5化合物基表面上的扩散,比在石墨基表面上的扩散更容易。而对于棱柱曲表面,结果却是相反的。CO分子从BC5化合物armchair表面上脱附,要比从石墨armchair表面上更容易一些。相对石墨zigzag表面上的脱附势垒,BC5化合物zigzag1表面上的势垒更大,而zigzag2表面上的势垒则更小。采用CASTEP计算软件,建立了BC5和BC23的zigzag和armchair表面模型,比较了替代的B原子对不同表面氧化活性的影响。石墨、BC23和BC5的armchair形表面都比zigzag形表面更稳定。石墨、BC23和BC5的zigzag形表面的氧化活性都大于armchair形表面的。在棱柱曲表面中,armchair形表面要比zigzag形表面占有更大的比例。随着B含量的增加,armchair形表面所占比例逐渐减少,zigzag形表面所占比例逐渐增大。替代的B原子对zigzag形表面是抑制氧化,对armchair形表面是促进氧化。B原子的替代对armchair形表面的促进氧化作用,要大于对zigzag形表面的抑制氧化作用。更多还原

【Abstract】 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.更多还原