Polyacrylonitrile-based Carbon Fibers Research Articles

Stand-up structure of graphene-like carbon nanowalls on CNT directly grown on polyacrylonitrile-based carbon fiber paper as supercapacitor

This work demonstrates the directly grown graphene-like carbon nanowall (GNW)–carbon nanotube (CNT)–polyacrylonitrile-based carbon fiber (CF) paper structure for use as a highly efficient supercapacitor. The CF was prepared by electrospinning, and was then treated by carbonization. The CNTs were directly grown on the CF paper by microwave plasma-enhanced chemical vapor deposition using CH4/H2 precursors at 800°C. The GNW was further reconstructed onto the CNT surface by microwave plasma-enhanced chemical vapor deposition using CH4/H2 precursors at 1500°C (GNW–CNT/CF). Electrochemical measurements demonstrate that the capacitance of the GNW–CNT/CF electrode is around 176Fg−1 at a charging/discharging current density of 0.5mA/cm2. The stand-up structure of GNW–CNT/CF has a high capacitance, attributable to its large surface area, high electrical conductivity and direct growth with low energy-loss. This novel stand-up structure of GNW–CNT/CF with a high surface area and low electron-transfer resistance has great potential for developing a revolutionary new class of nanostructured electrodes in supercapacitors or other energy-conversion applications.

Relationship between axial compression strength and longitudinal microvoid size for PAN-based carbon fibers

Single fiber axial compression strength has been determined on a series of polyacrylonitrile-based carbon fibers using micro compression test at a gage length of about 10μm and a detailed analysis has been made on the distribution and the fiber diameter dependence of the axial compression strength. The length of the unsupported region of carbon layer stack calculated from the axial compression strength was well comparable with the longitudinal length of the microvoids determined with small-angle X-ray scattering. This indicated that the fracture of carbon fibers is initiated from the buckling of the unsupported region of carbon layer stack which is formed where the microvoid adjoins it. The tensile and the axial compression strengths of the carbon fibers showed different relationships between the coefficient of variation and the average. This indicated that the tensile and the axial compression strengths are governed by different factors although they showed a correlation. Comparison has also been made between the longitudinal length of the microvoid and the microvoid sizes perpendicular to the fiber axis.

Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.

Microstructural evolution during oxidative ablation in air for polyacrylonitrile based carbon fibers with different graphite degrees

Polyacrylonitrile‐based carbon fibers with different graphite degrees were oxidative ablated at 500 and 600 °C in air. By Thermal gravimetric (TG), Raman spectroscopy, X‐ray diffraction, and SEM, the mass loss, microstructure, and surface morphology of carbon fibers were investigated. The mass loss of carbon fiber increases linearly with increasing oxidative ablated time under 500 and 600 °C. The carbon fiber with higher graphite degree shows higher oxidative resistance, and the surface roughness increases gradually because of chemical ablation during the whole oxidation. A gloss morphology appears on the surface primarily because of physical denudation for carbon fibers with lower graphite degree and then burn off according to carbon and oxygen reaction. The crystallite size (La) decreases significantly, while interlayer spacing(d002) remains nearly unchanged. SEM observation suggests the two kinds of ablation mechanisms for carbon fibers with different graphite degrees indicating that CC band in sp3 hybridization prefers to be attacked by oxygen molecule more than that in sp2 hybridization during oxidation ablation in air. Copyright © 2012 John Wiley & Sons, Ltd.

Preparation and Photocatalytic Performances of TiO<sub>2</sub>/SnO<sub>2</sub> Composite Films Supported on Carbon Fiber

The compound photocatalytic materials of TiO2/SnO2 films with bilayer structure supported on polyacrylonitrile based carbon fiber (PAN-CF) substrates were prepared via the dip-coating technology, and Titanium dioxide (TiO2) and tin dioxide (SnO2) precursory sol were prepared by sol-gel method. Field emission scanning electron microscopy (FE-SEM), X-ray energy spectrum (EDS) and X-ray diffraction (XRD) were used to characterize the structure of materials, and methyl orange (MO) with concentration of 80mg/L as the target degradants was used to investigate the photocatalytic property of the composites under UV irradiation. The results revealed that the photocatalytic activity of composite is effectively enhanced result from the application of TiO2/SnO2 bilayer structure films, which can be considered as that the recombination of photo-induced electrons is restrained by SnO2 film effectively, and the 3#-TiO2/SnO2/CF exhibited optimum catalytic performance. In addition, the superiority of carbon fiber as carrier was played fully due to the generation of porous films, which is favorable to capture the intermediate products.

Preparation and Photocatalytic Performances of TiO<sub>2</sub>/SnO<sub>2</sub> Composite Films Supported on Carbon Fiber

The compound photocatalytic materials of TiO2/SnO2 films with bilayer structure supported on polyacrylonitrile based carbon fiber (PAN-CF) substrates were prepared via the dip-coating technology, and Titanium dioxide (TiO2) and tin dioxide (SnO2) precursory sol were prepared by sol-gel method. Field emission scanning electron microscopy (FE-SEM), X-ray energy spectrum (EDS) and X-ray diffraction (XRD) were used to characterize the structure of materials, and methyl orange (MO) with concentration of 80mg/L as the target degradants was used to investigate the photocatalytic property of the composites under UV irradiation. The results revealed that the photocatalytic activity of composite is effectively enhanced result from the application of TiO2/SnO2 bilayer structure films, which can be considered as that the recombination of photo-induced electrons is restrained by SnO2 film effectively, and the 3#-TiO2/SnO2/CF exhibited optimum catalytic performance. In addition, the superiority of carbon fiber as carrier was played fully due to the generation of porous films, which is favorable to capture the intermediate products.

Numerical investigation and experimental verification of the Joule heating effect of polyacrylonitrile-based carbon fiber tows under high vacuum conditions

Joule’s first-law dictates that when electric current passes through a conductor heat is generated. The scope of this work is to investigate the Joule heating effect of two types of carbon fiber tows in dry form. The tows are used as primary heating elements on various preform structures that require accurate and uniform temperature control, using a DC power supply. The investigated temperatures, ranged from 300 K up to 650 K. A finite difference scheme was developed, in order to predict and quantify the transient phenomenon and is verified against experimental results. In order to exploit in-depth the phenomenon and achieve very good agreement between numerical and experimental results, temperature dependence of thermal and electrical properties (specific heat capacity, total hemispherical emissivity, and volume resistivity) was examined under controlled vacuum environment, ensuring no fiber oxidation.

Further investigation on boric acid catalytic graphitization of polyacrylonitrile carbon fibers: Mechanism and mechanical properties

Catalytic graphitization of polyacrylonitrile based carbon fibers by boric acid doping was studied and the dependence of fiber tensile strength on the boron content and temperature was discussed. It was found that there existed a key temperature point for the boron to take effect. When the fibers were modified with 7.0wt.% boric acid solution, with increasing temperature, the tensile strength was lower than that of the unmodified ones below 2300°C, but a reverse thing happened above 2300°C. Moreover, when being heated at 2500°C, the modified fibers showed an increasing tensile modulus and strength with increasing boron content till maximums of 404GPa and 2.46GPa, 26% and 16% higher than those of unmodified ones. The mechanical properties of the fibers were affected by the interaction of carbon and boron, and also related with boron states. The decomposition of boron acid and its interaction with carbon brought defects on fiber surface, degrading the mechanical properties below 1300°C. With further heat treatment, the boron diffused into the fibers and divided into two states: substitutional and interstitial. At a temperature over 2300°C with an appreciate boron content, the substitutional would be formed predominantly, which removed the structural defects and relaxed the distortions, so as to benefit the mechanical properties.

Carbon layer structures and thermal conductivity of graphitized carbon fibers

The results of X-ray diffraction on polyacrylonitrile-based carbon fibers graphitized at temperatures from 1800 to 2500 A degrees C are reported. The carbon crystallites consist of graphene-layer stacks. A Hirsch analysis showed that the average graphene stack height increased with temperature up to similar to 2000 A degrees C, reaching a relatively stable stack height thereafter. The average lateral layer size increased from 0.88 nm for the as-received fibers to 3.37 nm at 2500 A degrees C. With increasing temperature the formation of microfibrillar structures was observed, accompanied by the flattening of carbon layers. The changes in electrical and thermal properties were in good agreement with the aspect ratio of carbon layers.

MODIFICATION OF POLYACRYLONITRILE-BASED CARBON FIBER WITH CARBON NANOTUBES TREATED BY CHITOSAN DERIVATIVES

A polyacrylonitrile-based carbon fiber modified with multi-walled carbon nanotubes(MWNTs) was prepared in this paper.Three chitosan derivatives,phthaloyl chitosan(PhthCS),naphthoyl chitosan and carboxymethyl chitosan,were used for surface treatments of MWNTs in order to improve their dispersion and stability.Acidizing MWNTs and original MWNTs were also used for comparison with the MWNTs treated with chitosan derivatives.Results indicate that the dispersion and stability of PhthCS-MWNTs were improved significantly in those solvents possessed of polarity from 6 to 10.The effects of molecular weight and concentration of PhthCS on the dispersion stability of the MWNTs were evaluated.Results of transmission electron microscopy and element analysis of the MWNTs reveal that part of MWNTs' surfaces are coated by a layer of PhthCS with the thickness of 5 ~ 10 nm,which changes the surface properties and dispersion stability of the MWNTs.A composite PhthCS-MWNTs/carbon fiber was fabricated through gel-spinning,preoxidation,low-temperature carbonization and high-temperature carbonization.SEM results exhibit that the PhthCS-MWNTs dispersed uniformly and oriented preferentially within the composite fibers,which led to improved mechanical properties of the composite carbon fiber.The tensile strength of the composite carbon fiber with 0.5% PhthCS-MWNTs is 19.8% higher than that of the carbon fiber without MWNTs.And the Young's modulus of the composite carbon fiber containing 3% PhthCS-MWNTs is 13.86 GPa,which is 122% higher than that of the carbon fiber without MWNTs.However,the elongation at break of the composite carbon fiber changed scarcely by adding the PhthCS-MWNTs.

Structural features of polyacrylonitrile-based carbon fibers

The structural changes as functions of spinning conditions and heat treatments were investigated with respect to the structural feature of PAN-based carbon fibers by scanning tunneling microscopy (STM). The distinct granule structure on the cross section of both high tensile strength and high modulus carbon fibers was observed by SEM, while slender granule-shape domain on the longitudinal surface was revealed by STM. A structure model was proposed, which depicted that the PAN-based carbon fiber was a heterogeneous structure composed of aggregated mesostructural domains. These domains were closely arranged into spiral form along fiber axis, allowing the fibers have high strength and good elongation. The initial shape and size of domains was determined by the precursor composition and spinning conditions and also strongly depended on the heat-treated temperature and stretching conditions. The smaller or slender domain, the higher tensile strength obtained for fibers. We expect that the PAN-based carbon fiber with better performance should be produced by optimizing the size and shape of these domains.

Densification mechanism of polyacrylonitrile-based carbon fiber during heat treatment

Polyacrylonitrile (PAN)-based carbon fibers were heat treated at various temperatures for varying durations to simulate the graphitization process in the manufacture of C/C composites. Densification of the resulting fibers was confirmed by density measurement. The composition and structure of the fibers were investigated by means of elemental analysis, X-ray diffraction and Raman spectroscopy. For specified isothermal heat treatment time, the structural parameters depended strongly on heat treatment temperature. The nitrogen content decreased with increased heat treatment temperature and extended time at constant temperature. Nitrogen loss was complete at temperatures above 1900 °C. The graphite crystallite size increased rapidly with increasing heat treatment temperature, and slowly with extended isothermal heat treatment time. At 2100 °C a more ordered graphitic structure appeared. Denitrogenation induced “puffing”, which made the fibers expand. Decrease in density in the heat treatment temperature range 1500–1900 °C originated from the abrupt evolution of nitrogen, and above 1900 °C the graphitization transition induced steadily increasing density. Densification of the carbon fibers was determined both by the rate of denitrogenation and the rearrangement of carbon atoms.

Effect of an ultraviolet/ozone treatment on the surface texture and functional groups on polyacrylonitrile carbon fibres

The effect of an ultraviolet generated ozone treatment (UV/O 3), on the surface and near surface functionality and structure of polyacrylonitrile based carbon fibres has been studied using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and Raman spectroscopy. Results were compared to electrochemically treated fibres. The UV/O 3 treated fibres showed increased levels of oxygen functionalities. Levels of oxygen comparable to a high level electrochemical shear treatment were achieved within 5 min of treatment (O/C 0.11 ± 0.03 for both treatments). XPS O1s/C1s ratios as high as 0.3 were produced, with saturation occurring at approximately 40 min exposure. The main functional groups introduced were, in addition to hydroxyl species, alkoxides (ca. 286.5 eV), carbonyl (288.0 eV), and carboxyl (289.5 eV). Examination of the full width half maximum of the graphite peak from XPS C1s showed some disorder was introduced to the first few layers of the fibre with treatment but the effect was not evident in the Raman, i.e. in the bulk of the fibre.

Preparation of Polyacrylonitrile High Modulus Carbon Fibers by Catalytic Graphitization Using Boron

Catalytic graphitization of polyacrylonitrile-based carbon fiber by doping boric acid was reported in this paper. The microstructure and mechanical properties of polyacrylonitrile-based carbon fibers with and without doping boric acid after heat treatment of 1300°C,1500°C,1800°C, 2100°C,2300°C,2400°Cand 2500°Cwas investigated by X-ray diffraction (XRD) and mechanical testing. The results showed that the tensile modulus of the carbon fibers either boron modified or not, increased obviously with increasing temperatures, and that of the modified carbon fibers was much higher than the unmodified fibers at all temperatures, reaching 404Gpa when the fiber was graphitized at 2500°C. The tensile strength of the modified carbon fibers was lower than the unmodified ones after being graphitized at temperatures below 2300°C, but increased to 2.69 GPa and 2.46 GPa respectively after the fibers were treated at 2300°C and 2500°C, which were higher than that of unmodified fibers treated under the same conditions, indicatinging that the mechanism of boron catalytic graphitization changed at the temperatures higher than 2300°C. It also showed that the interlayer spacing (d002) decreased, while the crystallite size (Lc) and the orientation increased with increasing temperatures.

High-temperature vapor deposition polymerization polyimide coating for elimination of surface nano-flaws in high-strength carbon fiber

The effect of polyimide coatings on the filament tensile strength of high-strength polyacrylonitrile-based carbon fiber was studied by using dip and high-temperature vapor deposition polymerization (VDP) coating processes. Unlike a VDP on a cold substrate, high-temperature VDP has the potential to directly synthesize and isotropically deposit a polyimide, from diamine and dihydride monomers without any by-products, on a substrate heated up to 200 °C. The average filament tensile strength of the flaw-sensitive carbon fiber improved with all the polyimide coatings used. Nevertheless, for the same monomers, the high-temperature VDP coating process was advantageous for high-efficiency surface flaw healing compared to the dip-coating process, resulting in a 25% increase in the average tensile strength of the carbon fiber. These results were evident not only for the carbon fibers without artificial nano-notches but also for those with artificial notches less than 30 nm in depth. Thus, we clearly showed the potential for the VDP polyimide coating to heal surface nano-flaws of the carbon fiber. The different infiltrations of the coating into nano-notches and its effect on the filament tensile properties were characterized, as well as discussing the impact of the VDP coating with an interlayer between the coating and the fiber.

Oxidation kinetics of polyacrylonitrile-based carbon fibers in air and the effect on their tensile properties

Polyacrylonitrile-based carbon fibers are oxidized by air at temperatures from 400 to 700 °C. The mass loss, morphology, graphitic structure and residual properties are investigated. The activation energy is about 111 kJ/mol and is independent of carbonization temperature. With increasing oxidation temperature, the crystallite height decreases gradually, while the d 002 spacing increases presumably due to combination of oxygen with carbon. The tensile strength decreases significantly, while the tensile modulus remains nearly unchanged. The increase in carbon fiber density after oxidation suggests that glass-like carbon in the fiber is easier to burn off than the more graphitic carbon.

Electromagnetic interference shielding with Portland cement paste containing carbon materials and processed fly ash

The study described in this article explored the effect of adding different types of carbon materials (graphite powder and three types of carbon fibre), fly ash (with 5.6%, 15.9% and 24.3% Fe 2 O 3 ), and a mix of both on electromagnetic interference (EMI) shielding in Portland cement pastes. The parameters studied included the type and aspect ratio of the carbonic material, composite material thickness, the frequency of the incident electromagnetic radiation and the percentage of the magnetic fraction in the fly ash. The findings showed that the polyacrylonitrile-based carbon fibres, which had the highest aspect ratio, provided more effective shielding than any of the other carbon materials studied. Shielding was more effective in thicker specimens and at higher radiation frequencies. Raising the magnetic fraction of the fly ash, in turn, also enhanced paste shielding performance. Finally, adding both carbon fibre and fly ash to the paste resulted in the most effective EMI shielding as a result of the synergies generated.

The Influence of Sample Preparation on the Strength Results ofa Pan-based Carbon Fiber

In this work the influence of different configurations in the sample preparation process on commercial polyacrylonitrile-based carbon fibers mechanical tests were studied. Mechanical properties, such as tensile strength, Young’s modulus, elongation and Weibull modulus, were evaluated. The results showed that all sample preparation steps may have strong influence on the results.

Synthesis of the Tube-Brush-Shaped SiC Nanowire Array on Carbon Fiber and Its Photoluminescence Properties

Tube-brush-shaped nanostructure of SiC nanowires was synthesized on polyacrylonitrile-based carbon fibers. The morphology and microstructure of the nanowires were characterized by X-ray powder diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. A quasi-periodically twin structure with (111) plane as the boundary along the SiC nanowires was observed. The vapor-solid growth mechanism of the SiC nanowire brush is also discussed. Moreover, some separated blue-shifted photoluminescence peaks around 469 nm were measured. The separated blue-shifted emission peaks are attributed to the quantum confinement of nanoscaled twin segments along each nanowire rather than the apparent diameters of the nanowires. The SiC nanowire brushes hopefully can find potential applications in nanotechnology.

Effects of plasma and nitric acid treatment of carbon fibers on the mechanical properties of thermoplastic polymer composites

Polyacrylonitrile-based carbon fibers were submitted to nitric acid oxidation and a dielectric barrier discharge treatment to improve the interfacial adhesion in carbon-fiber-reinforced thermoplastic polymer composites. The mechanical properties of the composites were investigated. The results obtained showed that the tensile strength of the composites improved considerably due to the treatments.