Vapor Grown Carbon Fiber Research Articles

Effects of Gamma Irradiation on the Kinetics of the Adsorption and Desorption of Hydrogen in Carbon Microfibres

In this study, three types of carbon fibres were used, they were ex-polyacrylonitrile carbon fibres with high bulk modulus, ex-polyacrylonitrile fibres with high strength, and vapour grown carbon fibres. All the samples were subjected to a hydrogen adsorption process at room temperature in an over-pressured atmosphere of 25 bars. The adsorption process was monitored through electrical resistivity measurements. As conditioning of the fibres, a chemical activation by acid etching followed by γ-ray irradiation with 60Co radioisotopes was performed. The surface energy was determined by means of the sessile drop test. Both conditioning treatments are supplementary; the chemical activation works on the outer surface and the γ-irradiation works in the bulk material as well. Apparently, the most significant parameter for hydrogen storage is the crystallite size. From this point of view, the most convenient materials are those with small grain size because hydrogen is accumulated mainly in the grain boundaries.

VGCF-core@LiMn0.4Fe0.6PO4-sheath heterostructure nanowire for high rate Li-ion batteries

Vapor Grown Carbon Fiber (VGCF)-core@LiMn0.4Fe0.6PO4-sheath heterostructure nanowire was successfully fabricated by an electrospinning method for high rate Li-ion batteries. The fabricated heterostructure has both the electronic conduction path (VGCF-core) and large reaction surface area with high crystallinity (LiMn0.4Fe0.6PO4-sheath), which enables the high charge–discharge rate capability.

OS0903 VGCF-CNTフィラーを用いたアルミニウム基高熱伝導複合材料の熱および強度特性の温度依存性

Aluminium (Al) based composites containing vapor-growth carbon fibers (VGCF) and carbon nanotubes (CNT) are fabricated by spark plasma sintering (SPS) in this paper. The thermal conductivity of the composite is three times higher than that of Al material. The composite materials will be able to use as a radiation fin of a heat exchanger and a heatsink. To apply the composites to real used conditions, the strengths and thermal conductivities at several temperatures of the composites should be clarified experimentally. In this paper, therefore, pure tensile tests and laser flash methods are conducted to clarify strength and thermal conductivity of the composites at several temperatures. According to the experimental result, the composite has the different thermal effect on the strength from pure Al. And then the composite has better thermal conductivity than pure Al at high temperature.

High-intensity ultrasonication modification of vapor grown carbon fibers with poly(vinyl alcohol) for the preparation of high strength composites by simple water casting

High-intensity ultrasonication was demonstrated to be facile and efficient for the preparation of water-dispersible poly(vinyl alcohol) functionalized vapor-grown carbon fibres (f-VGCFs). In contrast to multiwalled carbon nanotubes, VGCFs are mechanically strong and not observably shortened or damaged by ultrasonication. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy show that the surface of f-VGCFs is decorated with a thin layer of PVA, imparting good dispersibility and suspending stability in water. High-strength PVA–f-VGCF composite films were then successfully obtained by the green and low-cost water casting method, which is totally free of organic solvents. The tensile strength and Young's modulus of the composite films filled with 5 wt% f-VGCFs are remarkably increased to 177.5 ± 9.8 MPa and 4.6 ± 0.4 GPa, respectively, which are about 45.0% and 53.3% higher than those of neat PVA. After being stretched by 5 times their length, the tensile strength and Young's modulus are increased to 681.0 ± 4.3 MPa and 12.1 ± 0.6 GPa, which are 58.8% and 49.4% higher than those of neat PVA stretched by 5 times their length, respectively. The uniform dispersion of f-VGCFs and the effective load transfer between the fillers and the matrix are responsible for the greatly improved mechanical properties.

Variations in the Electrical Resistivity of Vapour Grown Carbon Fibres by Effect of Successive Operations of Intake and Outgassing of Hydrogen

The aim of this work is to study the behaviour of Vapour Growth Carbon Fibres (VGCF) with different treatments after the intake and outgassing of hydrogen takes place. The VGCFs are prepared via catalyzed decomposition of a mixture of hydrogen and methane. The catalyst was minute particles of iron. The fibre thickness was about 10 to 20 microns and they have a trunk-tree inner structure. Two different treatments are tested to improve the properties of the as grown samples: a heat treatment under vacuum conditions and an acid etching with nitric acid. The desorption kinetics of the three type of samples is studied measuring the electrical resistivity during the hydrogen outgassing. As a result, the surface treatment with acid etching seems to be an efficient method to improve the kinetics of the hydrogen sorption in the first hydrogenation cycle.

Synergetic effect of conductive additives on the performance of high power lithium ion batteries

Two commercial conductive additives, carbon black (super P, SP) and vapor grown carbon fibers (VGCFs), were used to construct an effective conducting network in the cathode of commercial LiFePO4 lithium ion batteries (LIBs). Results suggest that the LIB with SP possesses a higher discharge capacity than that with VGCFs with the same mass fraction of the additives. The high-rate capacity of LIB with SP is much higher than that with VGCFs. Furthermore, the LIB with a mixture of these two additives has an apparently improved performance in low and high rate discharge capacity compared with the LIBs with a single component additive with the same mass fraction due to the synergetic effect. The same conclusion can be reached for the larger-capacity batteries (10 Ah or 50 Ah packs). Therefore, the use of two different fillers is important for the high power LIBs used in the electric vehicle and mass energy-storage industry.

Improvement of electrochemical properties of PTMA cathode by using carbon blacks with high specific surface area

A PTMA (poly(4-methacryloyloxy-2,2,6,6-tetramethyl-piperidine-N-oxyl)) electrode with high energy density is prepared with Black Pearl 2000 (BP-2000). For comparisons, vapor grown carbon fiber (VGCF) and acetylene black (AB) are also employed to fabricate the PTMA-electrodes. The electrochemical properties of the electrode are improved obviously by employing BP-2000. The specific capacity of the PTMA-BP electrode based on the mass of PTMA is 26.7% larger than that of the PTMA-VGCF and PTMA-AB electrodes at a 1 C rate. At higher discharge rates, the polarization degree of the Li/PTMA-BP cell is the minimum one. At a discharge rate of 50 C, the specific capacity of the PTMA-BP electrode is 104.9 mA h g−1, and is 27.6 and 16.7% larger than that of the PTMA-VGCF and PTMA-AB electrodes, respectively. Besides, the discharge plateau of the Li/PTMA-BP cell is 3.35 V, and is 0.03 and 0.13 V higher than that of the Li/PTMA-AB and Li/PTMA-VGCF cells, respectively. The larger specific capacity of BP-2000 and the improved electrochemical kinetics of PTMA at the surface of BP carbon, resulted from the larger surface area of BP-2000, are the main factors for improving the capacity and rate capability of the PTMA-electrode. The high specific surface area of BP-2000 is also beneficial to the thorough contact of PTMA with BP carbon, resulting in the improved conductivity of the PTMA-BP composites. The cycling performance of the PTMA-BP electrode is also satisfied.

High capacity carbon anode for dry polymer lithium-ion batteries

A high capacity carbon anode with good cyclability for dry polymer lithium-ion batteries was obtained by the co-addition of vapor grown carbon fiber (VGCF) and carbon nanotubes (CNTs) into a composite carbon electrode of spherical mesocarbon microbeads (MCMB) and a lithium ion conducting binder of polyethylene oxide (PEO) with Li(CF3SO2)2N (LiTFSI). An electrode with MCMB, PEO19LiTFSI and VGCF exhibits high reversible capacity of over 330 mAh g−1, but poor cycling performance; the capacity retention was 71% at the 50th cycle. In contrast, the electrode with MCMB, PEO19LiTFSI, VGCF and CNTs has excellent cycling performance with a reversible capacity of ca. 310 mAh g−1 and reversible capacity retention of 97% at the 50th cycle. The thin and long CNTs could be useful to maintain electrical contact in the cathode matrix during the volume change caused by lithium insertion and extraction into or from the MCMB.

Highly efficient load transfer in polypropylene/vapor grown carbon fiber nanocomposites

The alignment of fillers and the filler–matrix interfacial adhesion are crucial factors for improving the mechanical properties of nanocomposites. This study focuses on these effects in the melt-spun vapor grown carbon fibers and polypropylene (VGCFs/PP) composite fibers. By using different draw ratio, the alignment of VGCFs and the interfacial crystalline structure in the as-spun fibers can be easily controlled. Tensile testing showed that remarkable enhancements in tensile strength and modulus were achieved at high draw ratio. Topographic analysis of the fractured surfaces indicated that the enhancements were directly related to the strong interfacial adhesion and effective stress-transferring effect during the tensile test. Further analysis showed that the strong interfacial adhesion might be the result of stretching-induced transcrystalline structure. Our study sets a good example to realize a remarkable reinforcement in semicrystalline polymer composites by controlling the interfacial adhesion and filler alignment in a simple but efficient way.

Effects of VGCF pretreatment on the characteristics of Fe2O3/VGCF composites as anode materials for Li-ion batteries

Two main topics were handled in this study: loss minimization in the irreversible capacity of vapor-grown carbon fibers (VGCFs) by pretreatment techniques and reversible capacity increase in pretreated VGCFs by the deposition of nano-sized Fe2O3. VGCFs were ball-milled and acid-treated, and nano-sized Fe2O3 particles were supported on the pretreated VGCFs. Their performance as anodes in lithium-ion batteries was characterized using a variety of techniques. Compared to the electrode containing pristine VGCF only, electrodes composed of the pretreated-VGCFs enhanced both charge transfer and discharge capacities due to an increase in lithium-ion mobility. Moreover, the Fe2O3 nanoparticles supported on the pretreated VGCFs could eliminate defects and functional groups on the surface of VGCFs generated by the pretreatment, such that the composite of Fe2O3 and pretreated VGCF showed higher reversible capacity at any rate of charge/discharge than the composite of Fe2O3 and the pristine VGCF. Therefore, the Fe2O3/pretreated VGCF composites could be a good candidate for high capacity anodes.

Ultrasensitive strain sensors made from metal-coated carbon nanofiller/epoxy composites

A set of resistance-type strain sensors has been fabricated from metal-coated carbon nanofiller (CNF)/epoxy composites. Two nanofillers, i.e., multi-walled carbon nanotubes and vapor growth carbon fibers (VGCFs) with nickel, copper and silver coatings were used. The ultrahigh strain sensitivity was observed in these novel sensors as compared to the sensors made from the CNFs without metal-coating, and conventional strain gauges. In terms of gauge factor, the sensor made of VGCFs with silver coating is estimated to be 155, which is around 80 times higher than that in a metal-foil strain gauge. The possible mechanism responsible for the high sensitivity and its dependence with the networks of the CNFs with and without metal-coating and the geometries of the CNFs were thoroughly investigated.

Synthesis of NiS–carbon fiber composites in high-boiling solvent to improve electrochemical performance in all-solid-state lithium secondary batteries

Newly designed composites of nickel sulfide (NiS) and vapor grown carbon fiber (VGCF) for use in all-solid-state lithium secondary batteries were synthesized using thermal decomposition of nickel acetylacetonate and 1-dodecanethiol in a mixture of VGCF and 1-octadecene. X-ray diffraction measurements and transmission electron microscopy revealed that NiS nanoparticles of 50nm diameter were grown on the surface of vapor grown carbon fiber. All-solid-state cells using 80Li2S·20P2S5 (mol%) glass–ceramic were fabricated as a solid electrolyte. The cell using the mixture of the NiS–VGCF composite and the solid electrolyte as a working electrode showed initial discharge capacity of 590mAhg−1 at 1.3mAcm−2, and exhibited better cycle performance than the cell using the mixture of NiS, solid electrolyte, and VGCF. Electrochemical performance in all-solid-state batteries was improved by forming a favorable solid–solid interface between NiS active materials and VGCF as electron conduction paths in the working electrode.

Supercapacitive properties of electrodeposited polypyrrole on acrylonitrile–butadiene rubber as a flexible current collector

Flexible sheets consisting of acrylonitrile–butadiene rubber (NBR) and vapor-grown carbon fiber (VGCF) are newly prepared varying the composition (VGCF 10–30 wt%) for use as a current collector of supercapacitor electrodes. The electrical conductivity of as-prepared VGCF/NBR current collector can be enhanced as the content of VGCF increases. The VGCF/NBR current collector is then electrodeposited with pyrrole using a potentiodynamic cyclic voltammetry to yield a polypyrrole (PPy)/VGCF/NBR composite electrode. Cyclic voltammetry result for the PPy/VGCF/NBR composites shows that the sample with 30 wt% VGCF achieves a maximum specific capacitance (125.8 F g−1) at 5 mV s−1 and reaches a lower specific capacitance at higher scan rates. In addition, the flexibility of supercapacitor electrode of PPy can also be established with a comparable capacitance value by using the NBR-based current collector.

Effect of conductive carbon on capacity of iron phthalocyanine cathodes in primary lithium batteries

The effect of conductive carbon type and loading was examined for the iron phthalocyanine (FePc) cathode in primary lithium (Li) batteries. The use of nano graphene platelets (NGPs) as a conductive carbon was shown to increase the discharge capacity by a factor of at least 2.5 compared to typical carbon blacks and vapor grown carbon fibers. Discharge capacities of up to 2050mAh g−1 FePc, corresponding to the insertion of 43 Li+, were obtained for FePc based cathodes with 25% NGPs. This is the highest cathode capacity reported in literature to our knowledge. Cathodes with 10% NGP still demonstrated reasonable capacities. Electrolyte solutions based on propylene carbonate demonstrated higher discharge voltages than those with the 1.2 M lithium tetrafluoroborate in γ-butyrolactone and 1,2-dimethoxyethane (1:1 by volume).

Mixed modes interlaminar fracture toughness of cfrp laminates toughened with CNF interlayer

In the present paper, the influence of carbon nanofiber on interlaminar fracture toughness of CFRP investigated using MMB(Mixed Mode Bending) tests. Vapor grown carbon fiber VGCF and VGCF-S, and multi-walled carbon nanotube MWNT-7 has been employed for the toughener of the interlayer on the CFRP laminates. In order to evaluate the fracture toughness and mixed mode ratio of it, double cantilever beam (DCB) tests, end notched fracture (ENF) tests and mixed mode bending (MMB) tests have been carried out. Boundary element analysis was applied to the CFRP model to compute the interlaminar fracture toughness, where extrapolation method was used to determine the fracture toughness and mixed mode ratio. The interlaminar fracture toughness and mixed mode ratio can be extrapolated by stress distribution in the vicinity of the crack tip of the CFRP laminate. It was found that the interlaminar fracture toughness of the CFRP laminates was improved inserting the interlayer made by carbon nanofiber especially in the region where shear mode deformation is dominant.

Electrospinning Synthesis of Wire-Structured LiCoO2 for Electrode Materials of High-Power Li-Ion Batteries

An application of the Li-ion batteries to advanced transportation systems essentially requires the enhancement of the rate capability; thus, the fabrication of nanostructured cathode materials with the large surface area and short Li-ion diffusion length is particularly important. In this study, an electrospinning method was adopted for the synthesis of wire-structured LiCoO2. The diameter of the as-spun fiber obtained from the precursor solution with multiwalled carbon nanotubes (vapor-grown carbon fiber, VGCF) was thinner than that of as-spun fiber obtained from the solution without VGCF. After the heat treatment, wire-structured LiCoO2 was successfully obtained regardless of the existence of dispersed VGCF in the precursor solution, although the particle size of LiCoO2 fabricated with VGCF was smaller than that of LiCoO2 fabricated without VGCF. The charge/discharge and rate-capability experiments revealed that both resulting materials show the reversible Li-ion insertion/extraction reaction. However, due to the existence of a small irreversible capacity at the initial cycles, the interfacial resistance increases, resulting in the poor cyclability and lower charge/discharge rate capability, especially for nanowire LiCoO2 fabricated with VGCF.

Carbon anodes for solid polymer electrolyte lithium-ion batteries

A high specific capacity carbon anode for lithium-ion polymer batteries was developed using a mixture of a spherical mesocarbon microbeads (MCMB), vapor grown carbon fiber (VGCF), and a lithium conducting binder of polyethylene oxide (PEO) with Li(CF3SO2)2N (LiTFSI). The reversible specific capacity of the anode was dependent on the thickness of the electrode and the molecular weight (Mw) of PEO in the composite electrode. The specific capacity of a 30 μm thick carbon anode with PEO at Mw = 6 × 105 was 310 mA h g−1, which was decreased to 260 mA h g−1 for a 60 μm thick electrode. A reversible specific capacity of 320 mA h g−1 was observed for the lithium-ion conducting binder of PEO18LiTFSI with a high Mw of 5 × 106 and an electrode thickness of 60 μm. The specific capacity is comparable to that for a carbon anode for lithium-ion batteries using a liquid electrolyte.

Synthesis and supercapacitive properties of electrodeposited polyaniline composite electrode on acrylonitrile-butadiene rubber as a flexible current collector

Flexible sheets consisting of acrylonitrile-butadiene rubber (NBR) and vapor-grown carbon fiber (VGCF) are newly prepared varying the composition (VGCF 20–30wt.%) for use as a current collector of supercapacitor electrodes. The as-prepared VGCF/NBR is then electrodeposited by aniline using a potentiodynamic cyclic voltammetry to yield a polyaniline (PANI)/VGCF/NBR composite electrode. It is confirmed that the electrical conductivity of VGCF/NBR current collector can be enhanced as the content of VGCF increases. Cyclic voltammetry result for the PANI/VGCF/NBR composites shows that the sample with 30wt.% VGCF achieves a maximum specific capacitance (271.8Fg−1) at 5mVs−1 and also reaches a superior specific capacitance at high scan rates. Such supercapacitor performance is possibly originated from the synergistic effect consisting of higher polarity of nitrile groups in NBR, conducting pathway of VGCF, and electroactive property of PANI.

Variation of the Seebeck coefficient with hydrogen content in carbon microfilaments

The Seebeck coefficient of vapour grown carbon fibres and carbon fibres made from polyacryolytrile precursor has been studied as a possible parameter to control hydrogen storage inside. The sign of the Seebeck coefficient gives the sign of the dominant charge carriers in the fibres, and when hydrogen is absorbed by the carbonaceous material, mainly as H+, it acts as a positive charge carrier. A simple two-band electronic model has been considered to explain the influence of hydrogenation on the Seebeck’s coefficient of these carbon microfibres. The most favourable condition for hydrogen adsorption is a moderately low pressure of hydrogen. Furthermore, it was observed that outgassing is more pronounced than expected in some types of fibres, thereby supporting the proposed presence of hydrogen generated during the manufacturing process.

Fabrication of Vapor Grown Carbon Fiber Reinforced Aluminum Composites by Spark Sintering

As vapor grown carbon fiber (VGCF) possesses the good mechanical properties, high thermal conductivity, high electrical conductivity and low thermal expansion, VGCF/ Al composites are expected to be suitable materials for a high performance radiator. In this study, VGCF were dipped and treated with ultrasonic vibration in some kinds of solution at first. Then, the mixed powders including three kind of average particle size of Al (1, 3 and 30µm) was milled by wet process with three kinds of solution, which are acetone, ethanol and butanol. Ethanol is most suitable for mixing solution because of homogeneous distribution of VGCF and Al powders. The mixed powders were spark-sintered in order to obtain dense VGCF/Al composites. The densification mechanism of VGCF/Al composites was divided into the plastic deformation (2nd stage) and creep deformation (3rd stage) after the 1st stage of rectangular wave pulse discharge. The densification rate of VGCF/Al composite powder depended on Al powder for matrix, but independent on VGCF. VGCF are dispersed uniformly in the VGCF/1 µm Al composite. But the aggregations of VGCF exhibited a preferential orientation in VGCF/30 µm Al composite, which results from the deformation of Al powders under the uniaxial pressure during the hot pressing in sintering process.