Modeling elastic and viscous effects on the texture of ribbon-shaped carbonaceous mesophase fibers

Carbonaceous mesophases are discotic nematic liquid crystals that are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a wide range of different fiber textures and cross-sectional shapes. Circular planar polar (PP), circular planar radial (PR), ribbon planar radial (RPR), and ribbon planar line (RPL) textures are ubiquitous ones. This paper presents, solves, and validates a model of mesophase fiber texture formation based on the classical Landau-de Gennes theory of liquid crystals, adapted here to carbonaceous mesophases. The effects of fiber cross-sectional shape and elongational flow on texture formation are characterized. Emphasis is on qualitative model validation using existing experimental data [1]. The role of elasticity and flow-induced orientation on texture selection mechanism on ribbon-shaped mesophase fibers is characterized. The model is able to predict the formation of the commonly observed line texture, and the fine structure of the line is reproduced and explained in terms of classical liquid crystal defect physics. The results provide additional knowledge on how to optimize and control mesophase fiber textures.     

Computational modeling in processing flows of carbonaceous mesophases

Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow sequence consisting of capillary, diverging, porous media, converging, and extensional flows that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystals and computer simulations of structure formation for pressure-driven capillary flow of carbonaceous mesophase precursors used in the industrial carbon fiber spinning process. The simulation results capture the non-Newtonian rheology of mesophase and the formation of characteristic fiber macro-textures.     

纺丝温度对中间相沥青炭纤维结构性能的影响

以萘催化合成中间相沥青(MP-1)和热缩聚法制备中间相沥青(MP-2)两种中间相沥青为原料,对其进行簇组成分析、偏光显微镜观察及红外光谱分析,研究其组成及结构.采用实验室气压式单孔纺丝装置在不同温度下对两种中间相沥青进行熔融纺丝,探讨纺丝温度对炭纤维结构及性能的影响.研究表明:MP-1低温获得无规结构,高温出现中心放射状边缘洋葱皮混合结构;MP-2随纺丝温度升高依次出现无规结构、准洋葱皮结构和洋葱皮结构.中间相沥青原料的性质影响着纤维截面结构随纺丝温度变化的规律.     

Disclination structures of carbonaceous mesophase spherules

The permissible disclination structures of carbonaceous mesophase spherules with tangential boundary conditions were analysed theoretically. Besides the well-known Brooks-Taylor spherules, we observed during experiment the equilibrium carbonaceous mesophase spherules with symmetric configurations of 5 and 6 disclinations which are in close agreement with theoretical results. These spherules have been found to be formed and stabilized in coalescence of two or more mesophase spherules with a Brooks-Taylor structure. A thorough study has been made of the dynamics of the coalescence of carbonaceous mesophase spherules and disclination reactions in them, including the annihilation of disclinations of different sign. The rate of change of distance r between the disclination nuclei was shown to be r² = a (t−t_c), where t_c is the time of disclination annihilation. It is known that the bulk carbonaceous mesophase contains primarily wedge disclinations of strength . We have found that the disclinations of greater strengths, namely, and +2 may also exist. The effect of small impurity particles and gaseous bubbles on the stabilization of these possible but energetically less favorable disclinations in the carbonaceous mesophase is discussed.     

影响炭质中间相形成和发展的因素——Ⅰ.原料和热缩聚条件

炭质中间相是制备炭材料,尤其是高附加值炭材料的优质前驱体。它在高性能炭纤维、超高功率电极用针状焦、锂离子二次电池负极、超高比表面积活性炭及催化剂载体、泡沫炭等领域得到了较好的应用。综述了原料和热缩聚条件对炭质中间相形成和发展的影响,这些影响因素包括原料的种类、热缩聚温度、反应压力和保温时间等。通过讨论指出,由于炭质中间相涉及的原料种类多样,热缩聚制备过程中涉及的反应、物相复杂,因此,在制备特定性能的产品时,应针对产品的特定性能要求来选择原料、设定热缩聚条件,以制得具有最佳结构和性能的产品。     

Microscopic investigation of carbonaceous deposits formed in a solvent-treated coal reactor

The mode of occurrence and optical characteristics of the carbonaceous deposits from a 3 t/d process development unit solvent-treated coal reactor were investigated by optical microscopy to increase understanding of the formation and development of reactor blockage materials such as mesophase and semicoke. The solid deposits showed a layered texture consisting of mesophase matrix as a binder, particles of non-plastic coal, pyrolytic carbon and mineral matter. The layered appearance of the deposits was due to variations in the proportions of the microscopic constituents and pores. The variations in reflectance values of the carbonaceous deposits from the centre of the reactor to the wall provided evidence of the thermal alteration after deposition. Based on these results, the formation and sedimentation of carbonaceous deposits and subsequent secondary alteration processes are discussed.     

沥青基中空碳纤维的制备及其吸波性能

以中间相沥青为原料,采用氮压式纺丝机制备了不同炭化温度的中空截面沥青基碳纤维,通过SEM表征了其断面形貌,利用矢量网络分析仪研究了炭化温度对中空碳纤维电磁损耗性能的影响。结果显示,最佳的炭化温度为900 ℃时,中空碳纤维具有最大电磁损耗;研究了铺层方式对复合材料吸波性能影响,利用弓形法对复合材料的反射率进行了测试,结果显示,碳纤维交叉铺层时,吸波合格带宽为10 GHz,最大吸收峰在25 dB。     

Crystallization and chain adsorption of poly(etheretherketone) in discontinuous pitch-derived carbon fiber composites

The crystallization kinetics of poly(etherether ketone) (PEEK) in chopped mesophase pitch-derived carbon fiber/PEEK composites have been studied. Various processing techniques are used in order to obtain controlled fiber length in the composites. Scanning electron microscopy performed on properly etched long fiber composite samples reveals that the nucleating sites density is low at the carbon fiber surface: transcrystalline layers are rarely observed. This is confirmed by differential scanning calorimetry. However, samples processed by mixing carbon fibers and molten polymer in a high temperature mixer have a widely different behavior: the nucleation density and the crystallization rate increase, the glass transition of these samples is displaced towards higher temperatures, and the solubility is dramatically lowered. We ascribe these phenomena to the adsorption of the polymer chains on carbon particles created by attrition during the mixting.     

热解炭微观结构对炭/炭复合材料力学性能的影响

以PAN基针刺纤维毡为基体,采用等温化学气相渗透技术,在温度1000℃、压力5.0~20.0 kPa条件下制备了2种具有不同微观结构热解炭的炭/炭复合材料,研究了其力学性能与热解炭微观结构的关系. 结果表明,压力8.0 kPa下得到的具有单一低织构热解炭的炭/炭复合材料的断裂强度较高,为86±3 MPa,热解炭与炭纤维间界面结合紧密,加载过程中二者同时断裂,呈现明显的脆性断裂行为;压力10.0~20.0 kPa下得到的具有中织构-高织构-中织构热解炭的炭/炭复合材料的断裂强度稍低,为82±4 MPa,加载过程中材料内部不同织构热解炭间多层次界面通过改变裂纹扩展路径而延缓其扩展速度,断口形貌呈现锯齿状,表现出假塑性断裂特征.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part I. Injection process

The fabrication of carbon-carbon composites by injection of low viscosity mesophase pitch through a fiber preform followed by stabilization and carbonization was examined. The fully transformed mesophase MOMP and AR pitches were injected through either soft or rigidized disk preforms 35 mm thick and 68 mm in diameter. Injection provided good even filling of major flow channels and fiber bundles. Flow-induced fibrous microstructures were retained by quenching and preserved by stabilization upon carbonization. A second injection cycle was effective in filling voidage created by thermal densification. A third cycle was applied, but required severe injection conditions and provided only incremental improvement. The carbon-carbon composite reached a density of 1.8 g/cm³ after three injection cycles.     

Disclinations in the optical texture of a high-rank vitrinite coke and a petroleum needle-coke

The microstructure of coke formed on carbonization of a low-volatile bituminous rank vitrinite (NCB Classification 204), at a heating rate of 60 °C min^?1, was found to consist largely of coarse-grained mozaic and domain type optical texture. The domain optical texture possessed a variety of optical structures, e.g. $\text{‘X’}$ and $\text{‘Y’-}\text{type}$ crosses and nodes, characteristic of it having passed through a nematic liquid crystal stage (mesophase) during carbonization. These structures are compared with, and found equivalent to, disclination structures found in the domains of a petroleum needle-coke, the surface of which had been gasified to reveal the underlying graphitic structure. The formation of disclination structures in the domains of the coal-coke is strong evidence for the formation and coalescence of mesophase during its carbonization as a result of the very high heating rate used.     

Twist disclinations in the carbonaceous mesophase

Disclinations are prominent features of the microstructures of coke and graphite because their lamelliform structures are formed via a liquid crystal, the carbonaceous mesophase. Polarized-light techniques have been applied to study disclination structures in the fibrous and lamellar microconstituents which result from uniaxial and biaxial extension, respectively, of the plastic mesophase. The strong preferred orientations of these characteristic microconstituents of needle cóke aid in resolving their disclination structures. The fibrous microconstituent contains primarily wedge disclinations in parallel, with rotations of ±π and ±2π. Disclinations with appreciable twist character have been identified at the boundaries of the folds which characterize the lamellar microconstituent. The twist disclinations are not rare, but their inherent structure makes identification difficult on a random plane of section. The folded regions constitute disclination loops of rotation π. Vestiges of the doubly curved surfaces of undeformed mesophase are found in the double curvature of some folded regions; the rotation vector along the disclination loop bounding such folds is conserved in magnitude but not in direction.     

Development and high stress abrasive wear behavior of milled carbon fiber‐reinforced epoxy gradient composites

Milled carbon fiber‐reinforced polysulfide‐modified epoxy gradient composites have been developed. Density and hardness increases with the increase of carbon fiber content in the direction of centrifugal force, which shows the formation of gradient structure in the composite. High stress abrasive wear test was conducted on the gradient composites by using a Suga Abrasion Wear Tester. Abrasive wear rate reduced on increase of milled carbon fiber content from 0.15 to 1.66 vol%. Reduction in abrasive wear rate in milled carbon fiber‐reinforced epoxy gradient composites has been attributed to the increase of hardness, presence of random milled fibers, and debris of composite materials, which gave resistance and reduced wear rate. There is a small decrease in specific wear rate on adding 0.15 vol% milled carbon fibers. Further decrease of specific wear rate is observed on adding 0.45 vol% milled carbon fibers. After 3 N load, there is a decrease in specific wear rate behavior on adding 0.45 vol% carbon fibers, which further decreases on adding 0.60 vol% of carbon fibers. There is a remarkable decrease in specific wear rate up to 5 N load for 1.66 vol% milled carbon fiber‐reinforced composite. Reduction in specific wear rate on adding milled carbon fibers is based on the formation of debris, which remained intact in their respective positions due to the interfacial adhesion between milled carbon fibers and epoxy resin. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Carbon black modification of mesophase pitch-based carbon fibers

The effects of carbon black (CB) on the microstructural development within mesophase pitch-based carbon fibers are reported. Unlike carbon nanotubes, which were previously studied for modifying fiber microstructure, CB represents a significantly lower cost alternative making it more favorable for industrial application. Additionally, the aspect ratio of CB is much closer to unity than carbon nanotubes, which have an aspect ratio ∼100 to 1000. Fibers were produced by first dispersing CB into a synthetic mesophase pitch at a dilute concentration of 0.3 wt%. These precursor materials were spun and then processed into carbon fibers. Unmodified (0 wt%) carbon fibers exhibited a severe radial texture with increasing orientation of graphitic pleats away from the fiber core. Carbon fibers modified with CB showed a strong flat layer structure, with only a slight increase in pleat folding away from the core. This difference in structure was also accompanied by a decrease in the number of fibers that exhibited “pac-man” splitting. No discernable reduction was observed in the graphitic crystallinity (interplanar spacing and crystallite size) or axial orientation of crystallites within the fiber, as a result of nanomodification.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: Part II. Stabilization process

In the fabrication of carbon-carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.     

影响炭质中间相形成和发展的因素——I．原料和热缩聚条件

炭质中间相是制备炭材料，尤其是高附加值炭材料的优质前驱体。它在高性能炭纤维、超高功率电极用针状焦、锂离子二次电池负极、超高比表面积活性炭及催化剂载体、泡沫炭等领域得到了较好的应用。综述了原料和热缩聚条件对炭质中间相形成和发展的影响，这些影响因素包括原料的种类、热缩聚温度、反应压力和保温时间等。通过讨论指出，由于炭质中间相涉及的原料种类多样，热缩聚制备过程中涉及的反应、物相复杂，因此，在制备特定性能的产品时，应针对产品的特定性能要求来选择原料、设定热缩聚条件，以制得具有最佳结构和性能的产品。     

Microscopical study of carbon/carbon composites obtained by chemical vapor infiltration of 0°/0°/90°/90° carbon fiber preforms

The microstructure of carbon/carbon composites obtained by isothermal, isobaric chemical vapor infiltration (CVI) of carbon fiber preforms consisting of aligned fiber bundles separated by fiber fleeces was studied comparatively by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED). Deposition rate as well as matrix microstructure do not differ in the aligned fiber bundles and fiber fleeces exhibiting different local surface area/volume ratios. The matrices which are homogeneously textured according to PLM exhibit pronounced spatial texture gradients at the sub-μm-scale if investigated by SAED. The texture gradients appear to be independent on the infiltration time, distance between fibers but evidently depend on the total methane pressure. TEM and SEM observations show a thin high-textured layer between the fiber and the medium-textured transitional layer below the high-textured matrix layer containing columnar grains. This thin layer replicates the surface unevenness of the fiber surface while it is absent at the initial carbon fiber surface before infiltration.     

Preparation and microstructure of hollow mesophase pitch-based carbon fibers

Hollow mesophase pitch fibers with rather thin diameter were successfully prepared by spinning through a C-shaped capillary. Die-swell was found to be the main factor affecting the formation of hollow fiber, and this was controlled by varying the spinning temperature. After carbonization at 1000°C, the hollow fibers possess a relatively small outer diameter of 21 μm and an inner diameter of 6 μm, and show better mechanical properties than solid fibers with similar outer diameter. The higher mechanical properties are attributed to the orientation of mesophase molecules which is related to the shape and dimension of the spinneret. The transverse microstructure of hollow carbon fibers is illustrated by scanning electron microscopy (SEM) observations on fracture sections.     

Effect of oxidation inhibitor on the low energy tribological behavior of carbon-carbon composites

In this study, the tribological performance of carbon-carbon composites impregnated with different amounts of MoSi₂ as an oxidation inhibitor were investigated. The results of the friction tests indicated that the carbon-carbon composites underwent an abrupt transition of the coefficient of friction at the frictional temperature range of 150-180°C. And the composites made with MoSi₂, exhibited lower frictional coefficient and wear rate in comparison with the composites made without MoSi₂. The composites made with 4 wt% MoSi₂ showed an improvement in activation energies for wear resistance when compared with the other composites under the present condition. These results were probably due to the consequence that the friction and wear properties of carbon-carbon composites are sensitive to the friction temperature and can be largely dependent on the adhering force between fibers and matrix-MoSi₂, the reduction of porosity, and the formation of a lubricative, powdery, debris film, formed on the friction surfaces of the carbon samples.     

Mesophase formation in defined mixtures of coal tar pitch fractions

A QI-free coal tar pitch is separated into a toluene-soluble (TS) and a toluene-insoluble (TI) fraction by solvent extraction. The effect of controlled mixing of both fractions on mesophase formation is studied. The formation of mesophase is correlated with the carbon and hydrogen contents of the sample materials. The influence of TI content on the viscosity and thus on the mesophase formation is measured in order to obtain viscosity data as a function of pitch composition for needle coke, carbon fiber, and carbon electrode manufacture.