Short residence time graphitization of mesophase pitch-based carbon fibers

The effects of graphitization time and temperature on the properties of three mesophase pitch-based carbon fibers have been characterized. Graphitization temperatures studied were 2400, 2700, and 3000 °C and residence times ranged from 0.7 to 3600 s. Helium pycnometry, measurements of fiber tow resistance, and X-ray diffraction were employed to study fiber properties. As anticipated, substantial variations in fiber properties were noted for the range of graphitization conditions studied and among the three fiber types. Significant structural evolution and property development occurred even at the shortest furnace residence times. For example, for one of the fibers, a furnace residence time of 0.7 s at 3000 °C resulted in a degree of graphitization value of ∼50%, a density of 1.98 g/cm³, and an electrical resistivity of 6.3 μΩ m (corresponding thermal conductivity ∼200 W m⁻¹ K⁻¹). A simple energy consumption analysis suggests that short residence time graphitization at high temperature may result in both lower costs and substantially higher production rates for fibers prepared from mesophase pitch.     

Isotropic carbon and graphite fibers from chemically modified coal-tar pitch

A precursor for a general purpose carbon fiber was prepared from coal tar pitch (CP) modified with 10 % p-benzoquinone (BQ) at 380 ?C for 3 hours. Such a modification raised the softening of the pitch from 85 ?C to 271 ?C at a yield of 43 %. The modified pitch was spun smoothly at a rate of 480 m/min into a fiber of 20 Μm diameter. The fiber was stepwise stabilized at 236 ?C (5 ?C/min) and 312 ?C (1 ?C/min) for 3 hours at each temperature. Successively,carbonization and graphitization were performed at 1,000 ?C and 2,400 ?C, respectively, for one hour. Both the carbonized and graphitized fibers exhibited tensile strength of 570 MPa. The structural parameters of carbon and graphite fibers were their orientation values of 56.2 and 58.1 %, relatively low Lc(002) of 11.24 and 25 å, and large interlayer spacing (d₀₀₂) of 3.86 and 3.49 å, respectively.     

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.     

The effect of magnetic field on the catalytic graphitization of phenolic resin in the presence of Fe-Ni

The effect of an externally applied magnetic field on the Fe-Ni catalyzed graphitization of phenolic resin was investigated. The Fe and Ni doped phenolic resin was first carbonized at 800 °C and then graphitized at different temperatures (800-1200 °C). Both the carbonization and graphitization were carried out in a magnetic field and the crystal structure was characterized by X-ray diffraction and transmission electron microscopy. The externally applied magnetic field was found to promote the graphitization and to improve the orientation of the hexagonal carbon layers. In the presence of Fe-Ni, a high degree of graphitization could be achieved by applying a magnetic field. This resulted in a d₀₀₂ of 0.3355 nm and full-width at half maximum (FWHM) value of 0.103° after a 1200 °C heat treatment. In comparison, the absence of a magnetic field resulted in a d₀₀₂ of 0.3358 nm and FWHM of 0.305°.     

沥青基短切炭纤维/石墨纤维表面镀镍及其吸波性能

以中间相沥青为原料,采用熔融纺丝法制备出中间相沥青纤维,再经炭化、石墨化后进行短切,化学法镀镍处理,最终获得短切镀镍纤维制品.采用扫描电镜(SEM)、电子探针能谱(EDS)、矢量分析仪等分析测试方法对比研究了短切炭(石墨)纤维、镀镍炭(石墨)纤维的镀镍效果、电磁损耗性能及其树脂基复合材料的吸波性能.测试结果表明:镀镍短...     

Single domain oval carbon nanofiber prepared from a polymer blend process

Single domain graphite fibers of 50–500 nm in diameter were prepared from a mesophase pitch mixed with a matrix polymer to form a polymer alloy. The alloy was then spun, stabilized, carbonized, and graphitized at 2900 °C. The carbonized fiber had a circular or Y-shaped cross-section, but all of the graphitized carbon nanofiber (CNF) changed into an elliptical cross-section with an aspect ratio (major axis/minor axis) of 2.47. The CNF consisted of well-developed graphite layers ordered throughout the cross-section, as observed in a single crystal and the layer edges formed loops consisting of 5–10 layers.     

Novel carbon nanofibers of high graphitization as anodic materials for lithium ion secondary batteries

Carbon nanofibers (CNFs) of high graphitization degree were prepared by a CVD process at 550-700 °C. They showed different structures according to catalyst and preparation temperatures. The structure of CNF prepared from CO/H₂ over an iron catalyst was controlled from platelet (P) to tubular (T) by raising the decomposition temperature from 550 to 700 °C. The CNFs prepared over a copper-nickel catalyst from C₂H₄/H₂ showed the typical herringbone (HB) structure regardless of the reaction temperatures. The CNFs prepared over Fe showed d₀₀₂ of 0.3363-0.3381 nm, similar to that of graphite, indicating very high graphitization degree in spite of the low preparation temperature. Such CNFs of high graphitization degree showed high capacity of 297-431 mA h/g, especially in the low potential region. However, low first cycle coulombic efficiency of ≈60% is a problem to be solved. The graphitization of the CNF preserved the platelet texture, however, and formed the loops to connect the edges of the graphene sheets. Higher graphitization temperatures made the loop more definite. The graphitized CNF showed high capacity (367 mA h/g); however, its coulombic efficiency was not so large despite its modified edges by graphitization, indicating that the graphene edges were not so influential for the irreversible reaction of Li ion battery.     

Overcoming the barrier to graphitization in a polymer-derived nanoporous carbon

A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures has been discovered by annealing at 2000 °C a CO₂ activated, non-graphitizing, nanoporous carbon originally derived from polyfurfuryl alcohol. The activation process with CO₂ overcomes the barrier to graphitization normally present in this carbon even when treated at high temperature. Gas adsorption analysis, skeletal density measurements, X-ray diffraction, and transmission electron microscopy are utilized to probe the structure of both the non-activated and the activated carbons at 800, 1200, 1800, and 2000 °C. The influence of activation time is also examined. Prior to activation the nanopore walls are comprised of several layers of disordered graphenes. Activation eliminates the barrier to graphitization by reducing the number of layers below the limit of detection and by removing carbon material highly susceptible to oxidation. Annealing at 2000 °C of the carbon activated to 84% burnoff induces the formation of pre-graphitic domains amongst the nanoporous carbon. The (0 0 2) bands corresponding to 2θ = 24.3°, 26°, and 26.5° are identified and assigned to amorphous, turbostratic, and graphitic morphologies. A pore volume of 0.50 cm³ g⁻¹ localized in pores below 2 nm in size is preserved after annealing.     

Preparation of activated carbon fibers from polyvinyl chloride

Waste polyvinyl chloride (PVC) contains high content of chlorine, which is believed to liberate dioxine at its combustion. Efficient removal of chlorine from PVC achieved by selecting the heat-treatment conditions provided free-chlorine PVC based pitch by a two-stage heat-treatment process. The obtained pitch (softening point: 218 °C) was spun, stabilized, carbonized and activated to prepare activated carbon fibers (ACF) whose DeSO_x activity was tested preliminarily and found comparable to other ACF.     

Structural evolution in graphitization of nanofibers and mats from electrospun polyimide–mesophase pitch blends

The evolution of structure in multi-step thermal treatment of polyimide–mesophase pitch (PI–pitch) blend nanofiber mats obtained by an electrospinning process is described. The mats were thermally treated at a series of stages up to 3000 °C. The structural transformation of the nanofiber mats consisted of three regimes. First regime corresponds to the removal of the majority of non-carbon elements and the formation of initial residual carbon. Second regime involves slow growth of the graphitic layers and slow improvement of their stacking order. Progressive graphitization occurs in regime three when the fibers become highly graphitic. The addition of pitch was found to give rise to overall enhanced graphitic order in the PI–pitch blend nanofibers as reflected in the smaller inter-layer spacing d₀₀₂ approaching that of the perfect graphite crystal, and the larger crystal sizes, L_c and L_a, confirmed by XRD analysis, as well as the higher ratio of graphitic structure revealed by Raman spectroscopy. Development of highly localized oriented domains in these nanofibers were observed by dark field TEM. The addition of pitch led to enhancement of both electrical and thermal conductivity.     

Microstructural analysis of in situ mesophase transformation in the fabrication of carbon-carbon composites

In this work, we examined the microstructures formed during the pyrolysis of naphthalene mixed with AlCl₃ catalyst, in the critical temperature range of 300-500 °C and at varying pressures. In addition, non-rigidized preforms were densified by multiple cycle in situ transformation and compared the process with impregnation using fully transformed AR mesophase pitch under similar conditions. The process of mesophase formation in the bulk phase and within tightly packed fiber bundles was observed to be similar: spherule nucleation from the isotropic phase, coalescence of spherules forming bulk mesophase, and mesophase flow before hardening. The hardened mesophase displays the coarse, fibrous, and lamellar microstructure observed in needle cokes. The molten naphthalene was observed to evenly penetrate in-depth the large void spaces and fiber bundles. After two in situ cycles, the fiber bundles and the inter-fiber bundle regions were well filled with transformed mesophase. The incremental filling of the larger void spaces reduced the calculated filling efficiencies from 47% in the first cycle to below 15% in the third through fifth cycle. An 8% improvement in densification efficiencies was achieved by applying modest pressures during the pyrolysis. The extent of mesophase penetration with AR mesophase was observed to decrease from the outer to the inner regions of the preform. The results suggest impregnation with naphthalene catalyst mixture is efficient in filling tightly packed fiber bundles but not large void spaces. Multiple cycles are required in order to fill the large void spaces.     

Structure and property studies of bioabsorbable poly(glycolide-co-lactide) fiber during processing and in vitro degradation

Structure and properties of a bioabsorbable poly(glycolide-co-lactide) (PGA-co-PLA) fiber during several processing stages and the final in vitro degradation stage were investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis and mechanical property tests. In the orientation stage, an increase in the temperature of the first encountered orientation roll resulted in a lower level of crystallinity and larger crystallites. The temperature of the second encountered pre-annealing roll (PR) imposed a smaller effect on the structure. In the hot-stretching stage after fibers were braided, the maximum crystallinity was achieved at around 126 °C. Higher hot-stretching temperatures increased the crystal size, glass transition temperature (T_g) and tensile strength, but decreased the elongation at break and the heat shrinkage near T_g. In the post-annealing stage, it was found that crystallinity, T_g and tensile strength all increased significantly while the heat shrinkage near T_g sharply decreased after annealing. This suggests that the internal stress accumulated in the orientation and hot-stretching stages can be effectively reduced by post-annealing. During in vitro degradation, crystallinity was found to increase with time while the heat shrinkage near T_g and in the supercooling region (T_g<T<T_m) was greatly reduced. These results support the process of cleavage-induced crystallization.     

A photocrosslinkable melt processible acrylonitrile terpolymer as carbon fiber precursor

A novel photocrosslinkable and melt processible terpolymer precursor for carbon fiber has been successfully synthesized and characterized. The terpolymer was synthesized by an efficient emulsion polymerization route and has a typical composition of acrylonitrile/methyl acrylate/acryloyl benzophenone in the mole ratio, 85/14/1. It has been characterized by FTIR, NMR, intrinsic viscosity and GPC molecular weights. The composition of the monomer repeat units in the terpolymer was determined by NMR, and was almost identical to the molar feed ratios of the monomers used for polymerization. The T_g of the terpolymers, were somewhat a function of molecular weight, but were in the range 77-91 °C. The fibers were spun from the terpolymer melts unlike the conventional solution spinning method. The terpolymers when stabilized with boric acid afforded a stable melt for about 30 min at 200-220 °C, which was empirically found to be sufficiently long to spin fibers. The terpolymer with the highest molecular weight (M_n, ∼48,000) was not melt processible, apparently because the melt viscosity was very high and the terpolymer degraded fast. However, terpolymers, which had an intrinsic viscosity <0.6 dL/g (NMP, 25 °C) were invariably melt processible. The initial carbon fibers produced from these terpolymer fibers upon complete carbonization exhibited good mechanical properties for proposed automotive applications; the tensile strength of the best fibers generated thus far was in the range 450-700 MPa with a strain to failure of ∼0.4%. The diameter of the carbon fibers was of the order of 7 μm.     

沥青基碳纤维的研发及产业化

沥青基碳纤维是碳纤维的一个重要品种,但我国在沥青基碳纤维的研发和生产较国外还有很大差距。介绍了我国沥青基碳纤维研发和产业化现状,就其中的关键工艺(纺丝沥青调制和熔融纺丝)进行了综合分析。通用级沥青基碳纤维在国内已有一定的科研和生产基础,近期可望完成自主技术工业化装备的建立和生产;高性能沥青基碳纤维的研发虽然较为充分,但用于纺丝的中间相沥青的制备和连续长丝工艺的开发还要经过努力才能实现产业化,以摆脱美日的技术和产品的封锁。     

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.     

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%.     

Synergistic catalytic effect of Ti-B on the graphitization of polyacrylonitrile-based carbon fibers

A novel carbon fiber pretreatment is proposed. PAN-based carbon fibers are first anodized in a H₃PO₄ electrolyte to achieve an active surface, and then coated with Ti-B catalyst by immersion of the carbon fibers in a uniformly dispersed H₃BO₃-doped TiO₂ sol. The as-treated carbon fibers are then graphitized at 2400 °C for 2 h. The effects of the anodization and the Ti-B catalyst on the graphitization are investigated.     

Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

The polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high crystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to ∼1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5-40 μm single crystals. The p,T values of diamond formation here obtained are significantly lower than those previously known for direct graphite-diamond transformation.     

The effect of graphitization temperature on the structure of helical-ribbon carbon nanofibers

Structural rearrangement of helical-ribbon carbon nanofibers (CNFs) was studied as a function of graphitization temperature. The as-produced nanofibers are composed of a helical ribbon of graphene spiralled about and angled to the fiber axis. The discrete layers of graphene ribbon overlap each other forming the helical-ribbon in contrast to the discontinuous cones of the more common stacked-cup CNF morphology. After heat treatment to 2400 °C and above, the CNFs were completely free of residual metal catalyst inclusions, principally nickel used in their synthesis, and other functionalities. The formation of loops at the graphene edges was also observed. Heat treatment through the temperature range 1500-2800 °C resulted in a relatively minor contraction in interlayer spacing d₀₀₂ from 0.3381 to 0.3363 nm. This was attributed to the highly graphitic character of the as-produced CNFs. However, there were significant increases in the crystallite thickness L_c through this temperature range. In addition, heat treatment above 2400 °C induced a marked change of the nanofiber morphology from circular to faceted polygonal cross-section resulting from the re-ordering of the turbostratic, curved graphene layers to regions of planar graphene layers with 3-dimensional graphitic structure (AB stacking).     

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.