CVD制备各向同性热解炭的微观结构表征及沉积机制研究

利用化学气相沉积设备及自行设计的沉积室,以高纯石墨为沉积基体,天然气为前驱体,沉积温度1200℃,在不同气体流量下(0.10,0.15,0.20m3/h)制备出了各向同性热解炭块体材料。利用XRD、Raman、SEM、TEM分析手段对材料的微观结构进行了研究。结果表明:各向同性热解炭由粒径为1.5～2.5μm的颗粒组成;热解炭微晶的Lc和La随着气体流量的增加而增大;TEM中选区电子衍射图谱中d002环为圆环,取向角为180°,定量地揭示制备的热解炭为各向同性。     

各向同性热解炭低温氧化行为研究

利用自行设计的化学气相沉积(CVD)炉,以丙烯为碳源,氢气和氩气为载气,采用固定基体CVD法制备各向同性热解炭块体材料。通过恒温氧化实验对所获得的各向同性热解炭材料的低温(450℃)氧化行为进行研究,并与碳纳米管、炭黑和天然鳞片石墨进行比较。利用拉曼光谱(Raman)、扫描电镜(SEM)和透射电镜(TEM)等分析手段对氧化前后试样的表面形貌和微观结构进行表征。结果表明,炭材料的氧化特征与其内部的缺陷种类和密度密切相关,其中各向同性热解炭的抗氧化性能优于其他3种炭材料,其低温氧化过程服从直线规律,氧化反应主要发生于内部的条带状热解炭结构。     

低压化学气相沉积制备掺硼碳薄膜及其表征

以BCl3和C3H6分别作为低压化学气相沉积制备掺硼碳材料的硼源和碳源,采用热壁化学气相沉积炉,于1 100℃在碳纤维基底上制备了掺硼碳薄膜.采用扫描电镜、X射线衍射和X射线光电子能谱对样品作了表征.结果表明:产物表面光滑,断面呈细密的片层状结构,产物由B4C和石墨化程度较高的热解碳组成.采用掺硼碳薄膜中含有15%(摩尔分数,下同)硼.硼原子化学键结合状态共有5种,分别是:B4C的中的B-C键,硼原子替代固溶在类石墨结构中形成的B-C键,BC2O和BCO2结构中B-C键和B-O键的混合态,以及B2O3中的B-O键.其中超过40%的硼原子以替代固溶的形式存在于热解碳的类石墨结构中.     

硼掺杂石墨烯基介孔碳的制备及性能EI北大核心CSCD

以酚醛树脂为碳前驱体,硼酸为硼源,三嵌段共聚物F127为软模板,氧化石墨烯为改性剂,通过溶剂挥发诱导自组装法制备了硼掺杂介孔碳(BMCs)以及硼掺杂石墨烯基介孔碳材料复合材料(BMC/GOs)。通过扫描电子显微镜、透射电子显微镜、X射线光电子能谱仪等研究硼掺杂及氧化石墨烯复合对介孔碳结构的影响,利用电化学工作站研究硼掺杂及氧化石墨烯复合对介孔碳性能的影响。测试结果表明,所制备的复合材料BMC/GO-40(氧化石墨烯悬浮液质量为40 mg)的比表面积为462.5 m^(2)/g,平均孔径为5.52 nm。硼元素以BC_(3)键和BCO_(2)/BC_(2)O键的形式存在于介孔碳中。电化学测试表明,通过适量硼掺杂以及调整氧化石墨烯复合量可获得最大比电容。在电流密度为0.5 A/g,样品BMC/GO-40容量达167 F/g。     

硼化酚醛树脂炭材料的制备及嵌锂性能

采用化学法制备含B酚醛树脂热解炭材料,比较了不同热解温度及B的掺杂对炭材料微观结构及嵌锂性能的影响。结果表明:H3BO3的加入及热解温度的提高,可提高炭材料的石墨化程度;含B炭材料的首次充放电比容量及充放电效率高于纯炭材料;600℃时含B炭材料循环5次后可逆比容量为423 mAh/g,大于石墨电极的理论比容量372 mAh/g。     

锂离子二次电池用负极炭材料的掺硼改性

掺杂法是进行锂离子二次电池用炭负极材料改性与修饰的方法之一,通过对国内外以硼作为掺杂原子进行炭负极材料改性的方法的总结,举例分析了浸渍法、包埋法和共混法掺硼的特点,并指出了目前掺硼工艺中存在的硼在炭材料中的分散不均匀和硼含量较低的两大问题。此外,总结介绍了前人关于硼与炭材料相互作用的机理,从理论上分析探讨了掺杂硼炭材料的特点以及掺杂硼炭材料用作锂二次电池负极的电化学性能。并且指出添加硼元素后,炭负极材料的各种容量都有所提高,而今后的任务之一就是降低不可逆容量。     

B–Si–Zr掺杂炭材料的制备及其抗氧化性能

以煤沥青甲苯可溶组分、聚碳硅烷、吡啶硼烷和ZrB_2有机前驱体为原料,通过低温裂解制备掺杂沥青,经过不同温度热处理得到B–Si–Zr掺杂炭材料,考察了掺杂炭材料抗氧化性能。用X射线衍射仪、扫描电子显微镜等对B–Si–Zr掺杂炭材料氧化前后的物相组成和微观结构进行表征。结果表明:1600℃热处理得到的B–Si–Zr掺杂炭材料中,ZrB_2陶瓷颗粒逐渐形成,在氧化过程中,SiC和ZrB_2等陶瓷与氧气反应生成SiO_2、B_2O_3和ZO_2,氧化物在炭材料表面形成保护膜,该热处理温度得到的掺杂炭材料抗氧化性较强。     

低温各向同性热解炭的微观结构

采用流化床化学气相沉积工艺可制备低温各向同性热解炭,沉积工艺参数变化导致沉积的炭材料微观结构呈现多样性,不同结构炭的物理和力学性能会有很大差异.本文列举了高密度和低密度低温各向同性热解炭,分别对它们在扫描和透射电镜下的微观结构特征进行了综述.通过归纳分析,对低温各向同性热解炭的微观结构有了较为系统地认识,在此基础上对低温各向同性热解炭的研究发展方向进行了展望.     

锂离子电池LiNi0.6Co0.2Mn0.2O2正极材料的硼元素掺杂改性调控

采用草酸盐共沉淀法结合后续热处理技术制备硼掺杂LiNi0.6Co0.2Mn0.2O2正极材料.研究了不同硼源(B2O3,H3BO3和LiBO2)掺杂对材料形貌、结构和电化学性能的影响.通过X射线衍射仪和Rietveld精修分析证明了硼(B)元素掺杂到材料晶格中.电化学性能研究表明:B2O3掺杂效果最佳,具有优异的倍率性...     

硼掺杂活性炭催化生物质与塑料共热解制芳烃

活性炭（AC）由于其发达的孔隙结构和官能团,被用作生物质和塑料催化裂解的催化剂或催化剂载体。然而,AC催化剂的催化活性较低,需对其进行改性处理以提高催化性能。本文利用固定床反应器探究了掺硼活性炭（BAC）催化剂催化玉米秸秆和高密度聚乙烯共热解过程中硼掺杂量、催化剂/原料质量比、共热解温度对产物产率及分布的影响规律。采用BET、FT-IR、NH₃-TPD测试了AC与BAC催化剂的比表面积、孔容、表面官能团及酸性等性能,并采用XRD和XPS对BAC使用前后硼的晶体结构和存在形态进行了表征。结果表明,随着硼掺杂量的增加,BAC催化剂的比表面积和孔径逐渐降低,表面官能团无明显变化,而强弱酸量显著增加。使用后的BAC催化剂中硼主要以B—O键的形式存在,BC₃衍射峰消失,出现了B—C弱衍射峰。随着硼掺杂质量分数从0.5%增至3.0%,单环芳烃的含量先升高后降低,而多环芳烃的含量呈现出与单环芳烃相反的变化趋势。当硼掺杂量为1.0%、共热解温度为600℃和BAC催化剂/原料质量比为1.25时,单环芳烃含量达到最大值44.18%,此时多环芳烃的含量为19.75%。此外,硼的存在能有效抑制焦炭沉积,提高催化剂的寿命。     

B／P共掺杂呋喃树脂炭用作锂离子电池负极材料的研究

以添加8wt％磷酸的呋喃树脂为前驱体,经固化后混入不同含量的硼酸,升温至8500C炭化制得B／P共掺杂树脂裂解炭。采用热重分析考察固化后树脂的热失重,采用XRD和氮气物理吸附分别考察物相、微晶结构变化以及比表面积和孔结构的变化,同时采用恒流充放电技术对其充放电性能进行了研究。结果表明,B／P共掺杂使裂解炭的比表面积明显下降,孔洞数量减少,且随着B／P的增加,裂解炭中BPO4的含量增加;当B：P（at％：at％）=1．0时,树脂裂解炭的电化学性能得到了有效改善,首次可逆容量高达378．2mAh／g,较磷掺杂树脂裂解炭提高～30mAh／g,且循环50次后可逆容量保持率为86．8％。     

A study of the properties of pyrolytic carbons deposited from propane in a tumbling and stationary bed between 900 and 1230°C

In order to obtain low-temperature-isotropic (LTI) pyrolytic carbons, a new “Tumbling Bed” reactor has been developed. The characteristics of the pyrolytic carbons deposited in this tumbling bed have been studied by varying the gas composition, the total gas flow rate, the weight of bed particles and the rotational speed (RPM) of the reaction tube in the temperature range of 900–1230°C. It was found that all pyrolytic carbons deposited were isotropic in the temperature range of 1050–1230°C. The density of the Isotropie carbon increased slightly with temperature, but it was independent of the other variables with values of 1.9–2.0 g/cm³. The apparent crystallite size, L_e, of the isotropic carbon was about 30 Å regardless of coating conditions. The deposition rate increased with temperature, propane concentration, and total flow rate, showed a minimum with increasing RPM of reaction tube, and decreased with the weight of bed particles. The deposition mechanism of the isotropic pyrolytic carbon was suggested from the results. Additionally, a few experiments were carried out in a stationary bed in order to study the role of the rotating action in the tumbling bed reactor. Columnar, sooty and filamentous carbons were obtained in the stationary bed. From scanning electron micrographs of fracture surfaces of the filamentous carbons, it appeared that they are constructed by spiral growth of carbon flakes.     

A study on the deposition of pyrolytic carbons from hydrocarbons

A new method for forming isotropic, laminar, and columnar pyrolytic carbons is proposed. For this, a low RPM (below 2.4 rpm) tumbling bed has been used to deposit pyrolytic carbons from hydrocarbon gases. All deposits were made on graphite substrates from propane and methane at a constant temperature of 1200°C. The microstructures of the pyrolytic carbons deposited were dependent on the flow pattern of the reactant gas, the rpm of the reactor, the hydrocarbon concentration, the nature of the hydrocarbon, and the geometry of the bed. Isotropic pyrolytic carbon is formed under deposition conditions where homogeneous nucleation occurs in the gas phase and at the gas flow conditions where the gas-borne droplets can collide on the substrate. Laminar carbon is formed under deposition conditions where homogeneous nucleation does not occur in the gas phase and at gas flow conditions where the carbon species existing in the bulk of the gas phase can collide on the substrate. Columnar carbon is formed when any carbon products existing in the bulk of the gas phase cannot collide on the substrate. The suggested deposition mechanism can also be applied to pyrolytic carbons deposited in a fluidized bed or in a stationary bed. In particular, isotropic carbon can be obtained even in a stationary bed if the requirements for the deposition of the isotropic carbon described above are satisfied.     

Influence of the deposition parameters on the texture of pyrolytic carbon layers deposited on planar substrates

Pyrolytic carbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at various residence times, methane pressures and deposition temperatures. The depositions were performed in a cavity oriented perpendicular to the gas flow. The small surface area/reactor volume ratio of the reactor geometry allows depositions in the growth and nucleation mechanism. Transmission electron microscopy was applied to study the texture and microstructure of the carbon layers. A texture transition from medium- to high-textured pyrolytic carbon occurs as a result of increasing residence times, methane pressures and temperatures. Improved textures are generally correlated with increasing deposition rates, which are not necessarily constant during long-term depositions. Lower textures are observed in the vicinity of the substrate interface that are attributed to the influence of the substrate morphology and microstructure.     

液相气化快速致密化工艺研究

对一种新型快速炭／炭复合材料制备工艺－－液相气化快速致密化工艺进行了初步探索。研究表明，采用该工艺，致密化效率可以得到快速提高，数小时内制得密度达1．7g/cm^3以上的炭／炭复合材料，致密速率 达到37g/h。偏光显微镜观察表明，试样中热解炭具有较高的光学活性；束内小孔隙热解炭，绝大多数为光学各向异性组织，但具体归属粗糙组织（RL）还是光滑组织（SL），很难定论；束间大孔隙内的热解炭具有明显的锥状生长结构，是较典型的RL组织；在偏光显微镜下没有观察到试样中有炭黑出现。     

添加硼化合物对含钴沥青活化产物形态结构的影响

含钴沥青添加不同硼化合物进行化学活化,采用XRD和SEM对活化产物的形态、结构进行分析,研究不同硼化合物对活化产物形态、结构的影响。研究结果表明,含钴沥青添加一定比例的硼氢化钠,以氢氧化钾为活化剂,在900℃进行化学活化,活化产物中形成较多的纤维状纳米碳结构,并且具备良好的石墨微晶结构;而添加硼酸和硼酸钠的活化产物中没有发现纳米碳结构的形成。该结果说明,硼氢化钠对活化产物中纳米碳结构形成的促进作用主要是由于其强还原剂的特性,而不是由于硼的催化石墨化作用。     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

Studies on pyrolytic carbons obtained by acetylene pyrolysis at 1273 K—I: Types,properties and structures of carbons deposited

Various types of pyrolytic carbon, i.e. compact, feathery, brittle, spongy and soot-like, were obtained by acetylene pyrolysis in a flow reactor at 1273 K. The effects of the process variables (acetylene concentration in argon and gas flow rate) on both the solids yield and the relative proportions of the various types of carbon were estimated. These carbons, except the soot-like, showed similar elementary composition and structure parameters, but were distinguishable by their microstructures. In general, the compact carbon showed various forms of conical texture; the feathery, brittle and spongy carbons had anisotropic spherulitic textures with characteristic sphere size and packing for each type; and the soot-like carbon had an isotropic texture.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.