Effects of carbon nanofibers on cell morphology, thermal conductivity and crush strength of carbon foam

The effects of low volume fractions of carbon nanofibers on the structure, thermal conductivity and crush strength of carbon foam were examined. Bulk density of the foam increased linearly with the fiber fraction reflecting the morphological changes in the cells. Thermal conductivity increased at low fiber fractions, but dropped at higher fiber fractions. Crush strength increased linearly with fiber fraction for short length fibers, but decreased for the longer length fibers. Scanning electron microscopy, petrography, and X-ray diffraction illustrated the complex effects of the carbon nanofibers on the foam. Available models for thermal conductivity and crush strength have been extended to accommodate these effects incorporating cell structure and morphology (macroeffect), presence of fibers (microeffect), and graphite crystal d-spacing (nanoeffect). This research has shown that the nanofibers have a complex role in the macro, micro, and nanoproperties of the composite foam.     

Raman spectroscopic study of effect of steam and carbon dioxide activation on microstructure of polyacrylonitrile‐based activated carbon fabrics

This work presents the different effects of steam and carbon dioxide activation on the microstructure of an oxidized polyacrylonitrile (PAN) fabric. An investigation was conducted on a series of carbonized fabrics and two series of activated carbon fabrics. The fabrics were activated by steam and carbon dioxide using heat‐treatment temperatures of 900–1100°C. Steam and carbon dioxide developed the microstructure initially present in the PAN‐based activated carbon fabrics, but with different effects. These fabrics in the form of fabric and powder were examined by X‐ray diffraction and Raman spectrometry. This study indicated that carbon dioxide only reacted with the crystalline edges or the irregular carbon on the fiber surface and that the inside structure of the fibers was not greatly affected. When the fabrics were activated using steam, water molecules reacted not only on the fiber surface but also with the carbon at the crystal edge and/or the nonregular carbon in the fibers, which led to communicating pore structures on the surface and in the inner portions of the fiber. This activation also promoted the denitrogenation reactions. Because of these structures and reactions, the activated carbon fabrics, which were activated by steam, had the highest stacking height for carbon layer planes (L_c), the highest number of layer planes (L_c/d₀₀₂), the highest oxygen content, the largest crystal size (L_a), and the highest density over the other samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1090–1099, 2001     

Synthesis of carbon nanocoils on substrates made of plant fibers

Carbon nanocoils (CNCs) with different shapes and coil diameters have been synthesized on three kinds of substrates made of plant fibers, i.e. tissue, cotton cloth and bamboo fiber cloth, using Fe₂ (SO₄)₃/SnCl₂ catalyst by a thermal chemical vapor deposition method. The average coil diameters of the CNCs on the tissue, cotton cloth and bamboo fiber cloth substrates are 560, 183, and 510 nm, respectively. It is found that the organization difference in the plant fiber substrates results in the difference in the aggregation states of catalyst particles on the fiber surfaces, which has a crucial effect on the morphology and production of the grown CNCs. The tight organization of the carbon fibers in the tissue and cotton cloth substrates can promote the catalyst aggregations to fabricate high yield CNCs. For the bamboo fiber cloth substrate, a relatively small number of catalyst particles are deposited on the surface and tend to be isolated, leading to the growth of a certain amount of the carbon nanofibers and carbon nanotubes. In addition, the catalyst adsorption ability of the bamboo fiber can be improved by coating calcium chloride particles to achieve high production of the regular CNCs.     

碳基吸附储氢材料

本文综述了活性炭、活性碳纤维、碳纳米纤维和碳纳米管的结构与储氢性能．活性炭只是在低温下才有好的吸附储氢性能,碳纳米材料吸附储氢对于工业应用还不成熟．活性碳纤维是一种可大规模生产且成本较低的微孔吸附材料,其作为储氢材料具有一定的工业前景．     

Electrophoretic deposition of different carbon nanoscale reinforcements on carbon fiber fabrics and mechanical properties of the resulting hybrid multi‐scale epoxy composites

Three types of carbon nanoscale reinforcements (CNRs) including the shortened electrospun carbon nanofibers (ECNFs, with diameters and lengths of ∼200 nm and ∼15 µm, respectively), carbon nanofibers (CNFs), and graphite nanofibers (GNFs) were electrophoretically deposited on carbon fiber (CF) fabrics for the fabrication of hybrid multi‐scale epoxy composites. The results indicated that the electrophoretic deposition (EPD) of CNRs onto CF fabrics led to substantial improvements on mechanical properties of hybrid multi‐scale epoxy composites; in particular, the hybrid multi‐scale epoxy composite containing surface‐functionalized ECNFs (with amino groups) exhibited the highest mechanical properties. The study also indicated that some agglomerates of CNRs (particularly GNFs) could form during the EPD process, which would decrease mechanical properties of the resulting composites. Additionally, the reinforcement mechanisms were investigated, and the results suggested that continuous (or long) ECNFs would outperform short ECNFs on the reinforcement of resin‐rich interlaminar regions in the composites. POLYM. COMPOS., 35:1229–1237, 2014. © 2013 Society of Plastics Engineers     

Densification kinetics and matrix microstructure of carbon fiber/carbon nanofiber/pyrocarbon composites prepared by electrophoresis and thermal gradient chemical vapor infiltration

Preforms were fabricated by the application of direct current fields for the alignment and network formation of carbon nanofibres in the needle-punched carbon fiber felts, and infiltrated by using the thermal gradient chemical vapor infiltration at the temperature of 1000 °C under the total pressure of 5 kPa. The voltage had a strong influence on the carbon nanofiber weight obtained in the preform. With the increase of the voltage, the carbon nanofiber content increased. The carbon nanofibers formed networks on the carbon fibers. When the voltage remained at 30 V, the carbon nanofibers were dispersed uniformly on the carbon fibers. However, when the voltage was larger than 60 V, the carbon nanofibers agglomerated themselves and coated the carbon fibers. The carbon nanofiber content has a strong influence on the temperature distribution and on the densification front existence, velocity and width. The achievable degree of pore filling in the carbon nanofiber-added preform at 30 V was the highest, while the carbon nanofiber-added preforms at 60 and 90 V could not be densified efficiently. The microstructure of pyrocarbon at different positions is discussed.     

Index to Volume 31

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (?200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Process development and characterization of spraying carbon nanofibers over fabrics for reinforcing polymer composites

Process development and characterization of spraying carbon nanofibers (CNF) over carbon fiber fabrics for reinforcing polymer composites are presented in this study. The molded composite structure consists of a high‐temperature polymer reinforced with carbon fiber fabrics sprayed with different dosages of carbon nanofibers. The materials were molded using vacuum assisted resin transfer molding process. Tensile testing and scanning electron microscopy (SEM) were used to characterize the molded materials. The results show that the tensile strength and modulus were both improved over the molded materials without CNF. Spraying CNF with a dosage of an 8 µg/mm² of the used fabrics helped to increase the tensile strength by 12%. The tensile modulus increased by 28% with a CNF dosage of 16 µg/mm². Uniform distribution of CNF was observed under SEM in the molded composites. POLYM. COMPOS., 35:1629–1635, 2014. © 2013 Society of Plastics Engineers     

Carbon Nanofibers: Catalytic Synthesis and Applications

Carbon nanofibers (diameter range, 3-100 nm; length range, 0.1-1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (~200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Carbon induced phase transformation in electrospun TiO2/multiwall carbon nanotube nanofibers

Nanofibers of titania and composite nanofibers of titania and multiwall carbon nanotubes were synthesized by electrospinning using a sol-gel process combined with activated carbon nanotubes. The relationships of treatment temperature, carbon nanotube content on the crystal phase, fiber morphology, and electric properties are reported. It is found that the rutile phase becomes more prominent at low heat treatment temperatures with an increase of carbon content in nanofibers, be it for higher amount of carbon due to reducing atmosphere or due to an increase in MWCNT. Atmospheric control and lower heat treatment temperatures enable crystalline nanocomposite fibers of anatase where the level of rutile is below the detection limit of XRD or Raman spectroscopy. This work provides a new path to fabricate electrospun TiO₂/MWCNT nanocomposite nanofibers with limited C-induced rutile phase.     

Structural and physicochemical characterization of carbon nanofibers by the COMPAS method

The results of studies on the adaptation of the complex analysis of soot (COMPAS) method to the characterization of carbon nanofibers are presented. It was found that, with consideration for the morphological characteristics of carbon nanofibers, this method determines the specific adsorption surface area, the average diameter of fibers, and the specific pore and micropore volumes and evaluates pore diameters and the structuring of the material. The method provides an opportunity to determine the ratio between the carbon and catalyst components of fibers and to estimate the degree of graphitization of the fiber. Differences between analytical procedures for soots and carbon nanofibers are specified. Calculation equations for the main characteristics of the test material are given.     

Properties of carbon nanofibers prepared from electrospun polyimide

A solution of a polyimide (PI, Matrimid® 5218) in dimethylacetamide was electrospun, and carbonization of the electrospun nonwoven fabrics produced carbon nanofiber fabrics. The effects of iron(III) acetylacetonate (AAI) on carbonization and the resulting morphology were also investigated. The carbonization behavior of the nonwoven fabrics was examined by X‐ray diffraction and Raman spectroscopy. AAI promoted carbonization of the nonwoven fabrics and increased the carbon yield. Addition of 3 wt % AAI increased the crystal dimension of electrospun PI nanofibers from 1.06 to 4.18 nm and decreased the integrated intensity ratio from 3.37 to 1.83 when heat treated at 1200°C. Scanning electron microscopy images of the carbonized nonwoven fabrics showed that AAI remained as particles within the fibers after carbonization. In addition, transmission electron microscopy observations revealed that turbostratic‐oriented graphite layers were observed around the particles even at 1200°C, which have been reported only on carbonization of rigid‐chain solvent insoluble PI materials under tension. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97:165–170, 2005     

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.     

碳纤维石墨化技术研究进展

碳纤维石墨化可以使其结构趋向于理想石墨结构，拉伸模量大幅提升，因此石墨化碳纤维广泛应用在航空航天等尖端技术领域。本文对比分析了碳纤维石墨化设备优缺点，详细介绍了激光超高温加热等新式石墨化方法及促进石墨化进程的相关工艺，进一步从微观结构层面分析影响力学性能的因素，为高模量碳纤维制备技术的研究提供理论及实践参考。指出目前主流的石墨体间接加热技术由于温度限制阻滞了碳纤维模量的进一步提升，克服高温限制且高效高质量、节能环保的石墨化技术是未来的发展趋势；应从组成碳纤维的分子层面去分析把握碳纤维的结构演变，进而优化控制石墨化工艺及设计相关石墨化设备，不断改善碳纤维石墨化结构，逐步趋向于力学性能的理论值。     

Direct electrochemical attachment of carbon nanotubes to carbon fiber surfaces

A novel electrochemical grafting of carbon nanotubes (CNTs) on the surfaces of carbon fibers using water as dispersive medium was achieved by the electrolysis of carboxylic acid-functionalized CNTs. The resulting CNT-hybridized carbon fibers showed a selective distribution of CNTs at active carbon sites on the fibers associated with the edge graphite layers and defects, without destroying the crystalline structure of carbon fibers. Such hybridized fibers should provide a potential for improving the mechanical properties of advanced composites by increasing the load transfer at fiber/matrix interfaces.     

The effect of substituted alkynes on nickel catalyst morphology and carbon fiber growth

The growth of shaped carbon nanomaterials from a range of substituted alkynes over a NiO catalyst was investigated. It was found that the structure of the substituted alkyne affected both catalyst morphology and carbon fiber growth. For linear alkynes (1-pentyne to 1-octyne) the fiber morphology and yield varied with the type of alkyne used. It was also found that hetero-atoms (Cl, Br, OH and NH₂) greatly impacted carbon fiber growth and structure. An analysis of the catalyst particles associated with the carbon fibers grown from various alkynes showed that different alkynes gave differently shaped Ni catalyst particles. It was found that pre-treatment of the catalyst with an alkyne such as trimethylsilyl acetylene or ethynyl aniline (that did not give fiber growth), followed by treatment with acetylene initiated fiber growth morphologies (Y-junction, helical or straight fibers) different from that observed after direct treatment with acetylene. Further, sequential fiber growth from two alkynes that were both capable of producing fibers (e.g. methyl prop-2-ynoate followed by prop-2-yn-1-amine) resulted in ‘co-block’ fiber growth. These results highlight the dynamic relationships that exists between carbon source, catalyst morphology and carbon nanomaterial growth.     

Preparation of in situ grown silicon carbide nanofibers radially onto carbon fibers and their effects on the microstructure and flexural properties of carbon/carbon composites

Silicon carbide nanofibers (SiCNFs) used as the second reinforcements of carbon/carbon composites were grown radially on the carbon fiber surface. The microstructure of SiCNFs and their effects on the microstructure and flexural properties of C/C composites were investigated. Results show that there are many defects such as twin crystals and stacking faults in SiCNFs which were grown by catalytic chemical vapor deposition. During the same process, the skin region of carbon fiber has changed. Several SiC layers are formed and the arrangement of the graphite layers around SiC layers is more orderly. In the next chemical vapor infiltration, due to the induction of SiCNFs, the middle textural pyrocarbon were formed firstly and then is the high textural pyrocarbon. The existence of SiCNFs also contributes to the three-phase interface between pyrocarbon, SiCNFs and carbon fibers, resulting in a good bond between carbon fiber and matrix. Those structural changes lead the better flexural properties of SiCNF–C/C composites compared with C/C composites.     

Heterogeneities in carbon fibers

Acrylic fibers, stabilized acrylic fibers, and graphite fibers have been selectively etched by ion bombardment. After ion etching, the fibers are characterized by structures oriented transverse to the fiber axis with an average spacing ranging from 500 to 3000 Å. These transverse structures are considered to be representative of structural inhomogeneities in the fibers, which are transmitted from the precursor fiber through the stabilization treatment to the final carbon fibers. The relation between these heterogeneities and the standard microstructura! models of carbon fibers remains to be elucidated satisfactorily.     

碳纤维高温热处理技术进展

从高温热处理技术方面介绍了碳纤维在石墨化过程中微观结构的变化和宏观力学性能的改变。综述了高温、热牵伸、催化、压力、渗碳、外加磁场等条件在碳纤维石墨化过程中的研究与进展,并对高性能石墨纤维的制造技术和研究发展进行了展望。指出优化石墨化条件、创新制造工艺、降低生产成本、进一步提高石墨纤维的性能将成为今后研究的重点和发展方向。     

Heavy oil sorption and recovery by using carbon fiber felts

On fibrous carbon materials, including activated carbon fibers, sorption capacity for heavy oils, less viscous A-grade and more viscous C-grade, was determined. Sorption capacity depended strongly on their bulk density; the correlation was the same as that found previously on exfoliated graphite and carbonized fir fibers. On carbon fiber felts, excellent recycling performance was observed, though sorption capacity was not so high as on exfoliated graphite and carbonized fir fibers. By filtration under suction, about 90% of sorbed A-grade heavy oil could be recovered and no decrease in sorption capacity was detected even after eight cycles. By washing with solvents, n-hexane for A- and C-grade oils and A-grade oil for C-grade oil, almost 100% recovery with no marked reduction in sorption capacity was found for each cycle. For the felts of PAN-based carbon fibers, rather severe operations for oil recovery, centrifugation and squeezing with twisting, could be applied without pronounced decreases in sorption capacity and recovery ratio.