Effects of crystallinity of starting carbons on diamond formation in presence of nickel under high pressure and high temperature condition

The processes of graphitization and diamond formation of several carbons in the presence of nickel were investigated under 8 GPa at temperatures up to 1800° C. Diamond was formed easily from graphitized pitch coke which had a well-developed graphitic structure and in less amount from glassy carbon preheated at about 3000° C which was partly graphitized. On the other hand, pitch coke and glassy carbon, preheated at about 2000° C and not graphitized, did not transform to diamond but remained graphitized even in the diamond stable region. Diamond from graphitized pitch coke and glassy carbon preheated at about 3000° C grew to form by direct bonding.     

Structure and properties of mesophase pitch carbon fibre with a skin-core structure carbonized under strain

Coal tar mesophase pitch fibres stabilized at 270° C to different extents were carbonized under strain by the constant load or constant length, using different heating rates, and further graphitized at 2500° C. Shallow and moderate stabilization provided a skin-core structure in the resultant fibres which exhibited higher orientation, tensile modulus, and better graphitizability after calcination at 1300° C and graphitization at 2500° C than deep stabilization. The tensile strength and modulus of the graphitized fibre was significantly improved through the strained carbonization when the stabilization was performed to a moderate extent. The strain tended to give an onion-like alignment in the fibre to improve the preferred orientation of carbon planes. Larger load and more rapid heating during carbonization modified the structure and properties of resultant fibres through a significant longitudinal elongation. The stabilization extent of pitch fibres governs the mobility or fusibility of mesogen molecules at the carbonization which allows their better alignment by the strain.     

Graphitization behaviour of carbon fibre-glassy carbon composites

Carbon fibre-carbon composites were fabricated by aligning PAN-based carbon fibre unidirectionally in furfuryl alcohol resin char. The graphitization behaviour was investigated by an X-ray diffraction technique and by the measurement of magnetoresistance. The time-temperature superimposition study for interlayer spacing resulted in an activation energy of 242±35 kcal mol⁻¹. The kinetic study on magnetoresistance agreed with the result of X-ray measurement. The activation energy is that for the graphitization of the layer structure formed in the glassy carbon matrix of the composites. The graphitization mechanism of the layer structure is the same as that of soft carbons.     

不同纤维体积分数CVI 炭/ 炭复合材料的石墨化度

为确定不同纤维体积分数的化学气相浸渗(CVI) C/ C 复合材料的最佳热处理工艺, 以40 %、30 %、25 %三种不同纤维体积分数的针刺整体毡为坯体, 经三次CVI 后制得C/ C 复合材料, 采用X射线衍射和拉曼光谱微区分析测试了三种不同纤维体积分数的CVI C/ C 复合材料试样未经热处理及经2200 ℃、2400 ℃热处理下宏观和微区石墨化度。结果表明: 三次CVI 热解炭均为光滑层结构, 且纤维体积分数越高, C/ C 复合材料的石墨化度也越高;纤维与光滑层热解炭界面及两种不同热解炭界面在高温热处理时会发生应力石墨化, 应力石墨化程度前者大于后者, 这是纤维体积分数高的C/ C 复合材料石墨化度高的原因; 热处理温度越高, 应力石墨化程度越大。      

Effect of metal diffusion barrier on thermal stability of metal-coated carbon fibers

The thermal stability of carbon fibers coated with different metals was investigated. The fracture strength of metal-coated carbon fibers was measured at room temperature as a function of heat-treatment temperature. It was demonstrated that the fracture strength of the copper-coated carbon fibers (C/Cu) was not affected by heat-treatment at temperatures up to 900°C because of the inert of copper. However, the fracture strength of the carbon fibers coated with an active metal (nickel, iron and chromium) were found to decrease significantly after heat-treatment at temperatures >700°C, owing to graphitization (induced by nickel) of the carbon fibers or a chemical reaction (between fibers and iron or chromium) at the interface. A diffusion barrier layer between the active metal coating and the fibers can reduce the strength loss of the carbon fibers, but the effectiveness of a metal diffusion barrier differed depending significantly on the nature of the introduced metals. It was found that if the diffusing active metal had an adequate solubility in a metal, the latter can effectively hinder the diffusion of the former; if not, the latter can not effectively hinder the diffusion of the former.     

Some microstructural aspects of vapour-grown carbon fibres to disclose their failure mechanisms

The microstructure of vapour-grown carbon fibres has been studied by an SEM examination of the transverse section. In this way the duplex structure, of catalytic and pyrolytic carbon, can be differentiated by the fracture of each phase; pyrolytic carbon shows concentric circles, termed tree trunk structure, while a glassy appearance characterizes the pyrolytic phase. It was observed that fracture was strongly influenced by the breaking mode of the fibre, because in tensile failure of a thick fibre, fracture similar to the tree trunk appearance can be formed in the outer layer of the pyrolytic phase. Thus it is necessary to study the transverse microstructure of vapour-grown carbon fibres without any failure process. Using a preparation of fibre samples, in slides as thin as necessary for TEM study, the internal structure was disclosed. The pyrolytic phase was constituted of randomly oriented small crystals, while the tree trunk structure was really formed by very elongated crystals with preferential orientation. Electron diffraction of both phases shows a different degree of texture according to the structures. In addition to crystals, TEM examination showed the existence of hollow cavities, that have a clear influence on the failure mechanism. Owing to these faults, the failure process forms parallel grooves, that constitute the tree trunk appearance.     

The oxidation behaviour of carbon fibres

The oxidation behaviour of carbon fibres has been studied in air at different temperatures from 550 to 860 °C. A linear relationship has been observed between the carbon fibre size and oxidation time. Experimental results show that the oxidation process is in a mixed control zone, i.e. controlled both by diffusion of the gaseous reactant and product within the boundary layer and by the chemical reaction. The activation energy of carbon burn-off is 140±5 kJ mol^–1. The uniform nature of the surface oxidation makes it possible to re-size the carbon fibres to smaller diameters.     

Effect of boron-doping on structure and some properties of carbon-carbon composite

Boron-doped carbon-carbon composites with boron concentration around 11–15 mass % were prepared from a carbon fibre felt with dispersed boron carbide powder by infiltration of pyrolytic carbon. The composite was heat treated at several different temperatures from 2000–2800 °C. The highest bending strength was obtained for the composite at a heat treatment temperature (HTT) of 2200 °C. Carbon fibre began to be destroyed after heat treatment at 2400 °C and the structure of the composite was drastically changed above 2600 °C where the anisotropy of the composite originally existing in the thermal expansion coefficient and the thermal conductivity has been faded away. X-ray diffraction measurement indicated that graphitization of the composite was enhanced by boron doping. At HTTs above 2400 °C, the composite became graphitic, the crystallite sizes of which were more than 100 nm in L_c (004) and L_a (110). It was shown that boron was uniformly distributed in the composite at an HTT of 2400 °C and also that heat treatment at higher temperatures, such as 2600 °C, incurred condensation of boron. Air-oxidation loss at 800 °C appeared to be the lowest for the composite with an HTT of 2400 °C and the rate of oxidation loss was 22 times lower than that of the non-boron-doped composite.     

Structure of round-shaped methylnaphthalene-derived mesophase pitch-based carbon fibres prepared by spinning through a Y-shaped die hole

Structures of as-spun, stabilized, carbonized and graphitized fibres prepared by spinning a methylnaphthalene-derived mesophase pitch through a Y-shaped die hole at 295 °C, was examined by combining optical, scanning electron and transmission electron microscopy from the macro- and microscopic view points. The prepared round-shaped fibre spun through a Y-shaped spinning die hole at 295 °C exhibited excellent tensile and compressive strengths of 410 and 70 Kg mm^–2, respectively, after graphitization at 2500 °C. The stabilized fibre consisted of densely packed anisotropic domains in very random alignment, of which transverse domains and longitudinal features appeared as bent, multi-bent and looped, and endless thin stripes, respectively. The size of domain in the transverse section ranged above 100 nm in length and below 100 nm in thickness, respectively. Further heat treatment (carbonization and graphitization) slightly reduced the dimension and deformed the shape of domains to shrink and to have more sharp edges at their bends according to the graphitic growth; however, the shapes and distribution of domains in transverse section were basically unchanged. High-resolution SEM and TEM observations of the domain confirmed the existence of smaller units of graphitic layers in their assemblies which were more closely arranged in the domain. Such a sub-unit was defined as a micro-domain. TEM revealed that the micro-domain was composed of more than one unit of graphitic layers in the graphitized fibre. Most of them were around 10 nm thick and 10–100 nm long. The thickness of micro-domains was observed to be smaller than the value ofL _c (002), 23 nm, in the same graphitized fibre. Micro-domains have not yet been identified in the stabilized fibre, while TEM suggested some stackings of hexagonal planes. A number of voids (micro- and meso-voids) up to 40 nm diameter were formed at the intra- or inter-domain locations, due to the graphitic shrinkage and evolution of volatile matter by the heat treatments. Micro-voids of around 5 nm diameter were formed within a domain. The better mechanical performances of the present fibre spun through a Y-shaped die hole were ascribed to the homogeneous distribution of looped or bent domains in the transverse section (random nature of transverse alignment). Such a random alignment may also lead to the least number of macro-voids and cracks in the fibril.     

Influence of carbide formation on the strength of carbon fibres on which silicon and titanium have been deposited

Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10^–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation _m d ^–1/2, where d is the thickness of the TiC layer.     

Study on silicon carbide nanowires produced from carbon blacks and structure of carbon blacks

Silicon carbide nanowires were produced from carbon blacks at 1473 K and their microstructure was characterized by TEM, X-ray diffraction, FTIR and Raman spectroscopy. Nanowires of uniform diameters, the smallest averaging 10 nm, and narrow size distribution were obtained from graphitized carbon blacks, and their morphology depends on the properties of carbon black pecursors. High concentration of stacking faults and twins was detected. In addition to silicon carbide nanowires, a silicon carbide layer, about 20 nm thick, was formed on the surface of carbon black aggregates. The interior of the aggregates did not react and analysis of the data showed that it is composed of a mixture of amorphous carbon and small graphitic crystallites. The small lateral sizes of these crystallites remain unchanged during the graphitization process which is limited to the outer layer of the aggregates.     

石墨化对微螺旋炭纤维热氧化动力学的影响

以乙炔为碳源,镍粉为催化剂,噻吩为助催化剂,采用化学气相沉积法制备微螺旋炭纤维;在氩气气氛中,2500℃下对所制微螺旋炭纤维进行石墨化处理.通过扫描电子显微镜观察微螺旋炭纤维的螺旋形貌和微观结构,用热重法研究微螺旋炭纤维的耐氧化性能,并探讨了微螺旋炭纤维的氧化动力学行为.结果表明:石墨化处理对微螺旋炭纤维具有显著的纯化作用,其螺旋形貌基本保持不变.微螺旋炭纤维的氧化反应较好地服从一级反应.微螺旋炭纤维石墨化前后的氧化反应活化能分别为263.004kJ/mol和297.191kJ/mol.石墨化处理明显提了微螺旋炭纤维的抗氧化性能.     

C/C复合材料结构显微激光喇曼光谱研究

采用显微激光喇曼光谱,以增强体为薄毡叠层、基体分别为粗糙层及光滑层结构热解炭的两种C/C复合材料为研究对象,分析、表征了两种材料炭结构的微观分布特征及其在石墨化过程中的变化状况。结果表明,不仅复合材料中不同组元,而且同一组元不同部位石墨微晶的完整度不同。在石墨化过程中,各自的石墨化进程及可石墨化能力存在差异:炭纤维体积含量较高的炭布层中的热解炭,与网胎层中的热解炭相比,石墨微晶的完整度较好,石墨化进程较快;在炭纤维体积含量较低的网胎层中,炭纤维及热解炭在其界面部位的石墨化进程较快;粗糙层结构热解炭比光滑层结构热解炭容易石墨化。借助激光喇曼光谱微区分析手段,有可能实现对复合材料中石墨化程度微观分布状态的调整和控制。     

Experiments and modeling for thermal conductivity of graphite nanoplatelets/carbon composites

In this work, the graphite nanoplatelets/carbon composites were fabricated from graphite nanoplatelets and pitch powders by a hot-pressing technology followed by carbonization and graphitization. The XRD and pole figure results show that the incorporation of pitch induces the decrease of size (L_a) and orientation degree of graphitic crystallites, while the in-plane thermal conductivity of graphitized sample is increased with the increasing pitch content up to 6 wt.%, achieving a maximum value of 405 W/m K. The pitch binders are filled into the voids to bridge two or more graphite nanoplatelets particles together to form extra thermal paths, which makes a great contribution to the enhancement of thermal conductivity. A thermal conductivity model for the graphitized composites is constructed based on a bridging mechanism, and the predicted results fit well with the experimental results.     

Fe2O3对聚芳基乙炔树脂石墨化的影响研究

用不同催化剂催化聚芳基乙炔树脂石墨化,重点研究了Fe2O3含量和热处理温度对PAA石墨化的影响。通过XRD、Raman、SEM和HRTEM分析了PAA热处理温前后的结构和形貌变化。实验结果显示：Fe2O3在热处理过程中转换成铁单质,有效地促进了PAA树脂的石墨化;Fe203含量的增加和热处理温度的升高均可促使石墨结构形...     

Capillary Evaporation on High-Dense Conductive Ramie Carbon for Assisting Highly Volumetric-Performance Supercapacitors

Conductive biomass carbon possesses unique properties of excellent conductivity and outstanding thermal stability, which can be widely used as conductive additive. However, building the high-dense conductive biomass carbon with highly graphitized microcrystals at a lower carbonization temperature is still a major challenge because of structural disorder and low crystallinity of source material. Herein, a simple capillary evaporation method to efficiently build the high-dense conductive ramie carbon (hd-CRC) with the higher tap density of 0.47 cm³ g⁻¹ than commercialized Super-C45 (0.16 cm³ g⁻¹) is reported. Such highly graphitized microcrystals of hd-CRC can achieve the high electrical conductivity of 94.55 S cm⁻¹ at the yield strength of 92.04 MPa , which is higher than commercialized Super-C45 (83.92 S cm⁻¹ at 92.04 MPa). As a demonstration, hd-CRC based symmetrical supercapacitors possess a highly volumetric energy density of 9.01 Wh L⁻¹ at 25.87 kW L⁻¹, much more than those of commercialized Super-C45 (5.06 Wh L⁻¹ and 19.30 kW L⁻¹). Remarkably, the flexible package supercapacitor remarkably presents a low leakage current of 10.27 mA and low equivalent series resistance of 3.93 mΩ. Evidently, this work is a meaningful step toward high-dense conductive biomass carbon from traditional biomass graphite carbon, greatly promoting the highly-volumetric–performance supercapacitors.     

Some factors for the high performances of mesophase pitch based carbon fibre

The tensile strength and Young's modulus of the carbon fibres prepared from naphthalene derived mesophase pitch were studied by varying the spinning temperature, heating rate and final temperature of the stabilization and graphitization temperatures to find the best properties obtainable from the particular pitch. The heating rate was very influential on the tensile strength of the fibre; a slow heating of 0.5° C min^–1, provided the highest strength, as high as 5 GPa at the optimum final temperature of stabilization. A higher or lower final temperature reduced the strength. Insufficient oxygen uptake or decomposition of oxygen groups at the surface of the fibre could induce defects, reducing the strength. In contrast, Young's modulus of the fibres was rather insensitive to preparation conditions. The carbonization and graphitization temperature influenced the mechanical properties as follows: the strength increased stepwise with the carbonization temperature, whereas the modulus increased sharply with the graphitization temperature. The structural factors most influential differed with these properties.     

Electron microscopic study on graphitization of bulk mesophases

Two kinds of bulk mesophases separated by quinoline from Polyvinylchloride (PVC) pitch and from 3, 5-xylenol-formaldehyde resin (XF) pitch were studied by high resolution electron microscopy. The ease of graphitization of both bulk mesophases has been examined by heat-treating them up to 1080 and 2600° C. PVC bulk mesophase mostly shows preferred long range orientation (aromatic carbon layers roughly parallel to each other), but also contains minor components where the molecular orientation occurs only over small distances, or not at all. When heat-treated at high temperature PVC bulk mesophase changes mainly into highly graphitized lamellar crystals. Similarly treated XF bulk mesophase does not show any preferred orientation and leads to porous nongraphitizable material.     

R.f.-sputtered SiC coatings on carbon fibres

R.f. sputtering as a new method for the deposition of SiC layers onto carbon fibre substrates was applied at temperatures below 400°C and deposition rates higher than 1 μm h^-1. Optimum conditions for high quality SiC films were selected by variation in the r.f. power, r.f. peak voltage, substrate temperature and gas pressure. The SiC layers were characterized using electron probe microanalysis, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, transmission high energy electron diffraction, IR and UV transmission spectroscopy as well as Vickers' hardness measurements. Amorphous SiC layers were obtained. In the case of discontinuous SiC deposition onto carbon monofilaments the film thickness precalculated from the sputtering parameters was achieved, whereas the continuous process results in film thickness of about one-third of the precalculated value. When fibre bundles with different numbers of monofilaments were used no influence on the resulting SiC layer thickness could be observed. Increasing the SiC film thickness, however, led to a strong decrease in the fibre tensile strength for layers more than 50 nm thick.     

High-temperature compatibility of carbon fibres with nickel

A study has been made of the elevated temperature degradation of a number of carbon fibre types coated with nickel by a variety of methods (electroless, electrolytic, carbonyl and physical vapour deposition). At high temperatures, Ni-coated fibres undergo a transformation of structure to crystalline graphite with a consequent loss of strength and elastic modulus. Resistance to this recrystallization is related to the fibre type and structure and increases in the order HTS PAN-based, HM PAN-based, HM rayon-based. For PAN-based fibres the resistance increases with the degree of structural order and orientation. The recrystallization of HTS fibres is consistent with a simple model of dissolution and reprecipitation controlled by diffusion of carbon in nickel. To explain the higher stability of HM fibres an additional factor must be introduced. For example, their behaviour can be explained in terms of a highly stable surface layer between about 0.1 and 0.5m thick. Rapid recrystallization occurs when the nickel breaks through this layer e.g. by dissolution. The recrystallization was not greatly affected by the type of nickel coating but the recrystallization temperature of HM fibres was considerably reduced by a small proportion of air in the heat-treatment atmosphere. HTS fibres were not affected in this way but the fibres were severely weakened through surface attack by both air and hydrogen at temperatures well below the recrystallization temperature.