Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.     

Preparation of carbon fibers from linear low density polyethylene

Carbon fibers were produced from linear low density polyethylene (LLDPE) instead of commonly used precursors, such as viscose rayon, mesophase pitch and polyacrylonitrile (PAN). Cross-linked fibers were produced at various temperatures, times and stress conditions during a sulfuric acid treatment using LLDPE fibers obtained from dry-wet spinning. The effects of cross-linking were analyzed using a range of characterization techniques, such as differential scanning calorimetry, color change, fourier transform infrared spectroscopy, elemental analysis, density, scanning electron microscopy, and single filament mechanical properties. The carbonization process of cross-linked fibers was carried out at 950 °C for 5 min in a nitrogen atmosphere. The carbon fibers with the best mechanical properties were obtained from the cross-linked fiber with the highest tensile modulus. In particular, the carbon fibers with the best mechanical properties (tensile strength and tensile modulus of 1.65 GPa and 110 GPa, respectively), similar to commercial-grade carbon fiber, were obtained from the cross-linked fiber that had undergone a carbonization process with a stress of 0.25 MPa after an acid treatment for 150 min at 140 °C and a stress of 0.26 MPa.     

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.     

High resolution transmission electron microscopy study on polyacrylonitrile/carbon nanotube based carbon fibers and the effect of structure development on the thermal and electrical conductivities

Polyacrylonitrile (PAN) and PAN/carbon nanotube (CNT) based carbon fibers at various CNT content have been processed and their structural development was investigated using high resolution transmission electron microscope (HR-TEM). In CNT containing carbon fibers, the CNTs act as templating agents for the graphitic carbon structure development in their vicinity at the carbonization temperature of 1450 °C, which is far below the graphitization temperature of PAN based carbon fiber (>2200 °C). The addition of 1 wt% CNT in the gel spun precursor fiber results in carbon fibers with a 68% higher thermal conductivity when compared to the control gel spun PAN based carbon fiber, and a 103% and 146% increase over commercially available IM7 and T300 carbon fibers, respectively. The electrical conductivity of the gel spun PAN/CNT based carbon fibers also showed improvement over the investigated commercially available carbon fibers. Increases in thermal and electrical conductivities are attributed to the formation of the highly ordered graphitic structure observed in the HR-TEM images. Direct observation of the graphitic structure, along with improved transport properties in the PAN/CNT based carbon fiber suggest new applications for these materials.     

Effect of composition and heat treatment conditions on the tensile strength and creep resistance of SiC-based fibers

Polymer-derived SiC-based fibers with fine-diameter (∼10–15 μm) and high strength (∼3 GPa) were prepared with carbon-rich and near-stoichiometric compositions. Fiber tensile strengths were determined after heat treatments at temperatures up to 1950 °C in non-oxidizing atmospheres and up to 1250 °C in air. The creep resistance of fibers was assessed using bend stress relaxation measurements. Fibers showed excellent strength retention after heat treatments in non-oxidizing atmospheres at temperatures up to 1700 °C for the carbon-rich fibers and up to 1950 °C for the near-stoichiometric fibers. The near-stoichiometric fibers also showed considerably better strength retention after heat treatments in air. Creep resistance of the as-fabricated fibers was greatly improved by high-temperature heat treatments. Heat-treated near-stoichiometric fibers could be prepared with ∼3 GPa tensile strengths and bend stress relaxation creep behavior which was significantly better than that reported for the Hi-Nicalon™ Type S near-stoichiometric SiC fibers.     

High strength and high modulus carbon fibers

Carbon fibers have been processed from gel spun polyacrylonitrile copolymer on a continuous carbonization line at Georgia Tech (GT) with a tensile strength in the range of 5.5–5.8 GPa, and tensile modulus in the range of 354–375 GPa. This combination of strength and modulus is the highest for any continuous fiber reported to date, and the gel spinning route provides a pathway for further improvements in strength and modulus for mass production of carbon fibers. At short gauge length, fiber tensile strength was as high as 12.1 GPa, which is the highest value ever reported for a PAN based carbon fiber. Structure analysis shows random flaws of about 2 nm size, which results in limiting tensile strength of higher than 20 GPa. Inter-planar turbostratic graphite shear modulus in high strength carbon fibers is 30 GPa, while in graphite the corresponding value is only 4 GPa.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

Synthesis and electrochemical properties of nickel oxide/carbon nanofiber composites

The electrospinning of polyacrylonitrile (PAN) with a polyaniline and graphene sol–gel mixture produced uniform, smooth fibers with an average diameter of 0.3 μm. These electrospun fibers were stabilized for 2 h at 200 °C and then carbonized at 800 °C for 5 h. Composites were prepared by depositing Ni(OH)₂ on the carbon nanofibers (CNFs) and calcining them at different temperatures. The composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of the calcination temperatures on the electrochemical properties was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The specific capacitance (SC) was found to be highest (738 F g⁻¹) at a calcination temperature of 400 °C. The charge transfer resistance (R_p) decreased as the calcination temperature was increased. However, the electrical double layer capacitance (EDLC) increased with an increase in the calcination temperature. The EDLC increased from 0.144 F g⁻¹ at a calcination temperature of 100 °C to 485 F g⁻¹ at a calcination temperature of 500 °C.     

Catalytic graphene formation in coal tar pitch- derived carbon structure in the presence of SiO2 nanoparticles

A simple and effective way to manufacture graphene from a coal tar pitch (CTP) is demonstrated. Silica (SiO₂) nanoparticles were used to modify the CTP as carbon precursor. A silica nanofiller introduced into the CTP matrix underwent carboreduction during heat treatment to 2000 °C, resulting in the formation of silicon carbide. Surfaces of SiC grains were sites for graphene formation. The influence of SiO₂ on the structure and microstructure of CTP- based carbon matrix, after annealing up to 2800 °C, was studied. Carbon samples were analyzed using X- ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman Spectroscopy. Crystallite sizes (L_a, L_c) and interplanar distance (d₀₀₂) were determined. The presence of SiO₂ in CTP carbon precursor favored the crystallites’ growth in the ‘a′ crystallographic graphite direction, and inhibited their growth on the ‘c′ axis. The crystallites composing of graphene layers, were characterized by an elongated dimension in the ‘a′ axis direction. Above 2000 °C silicon carbide decomposed, followed by the sublimation of silicon from the carbon matrix.     

Structure of a new rotationally faulted multi-layer graphene–carbon nanoflower composite

The structure of a new carbon–carbon nanocomposite that consists of thin (<15 layers) multi-layer graphene microsheets and carbon nanoflowers (CNF) was examined by high-resolution transmission electron microscopy combined with selected area electron diffraction (SAED) analysis, and Raman spectroscopy. Both SAED and Raman analyses verified that graphene layers in the sheets were rotated to each other. A typical rotation angle in SAED analysis was 30 ± 2° but also other rotation angles (e.g., 2 ± 1°, 12 ± 2°, 19 ± 2° and 25 ± 2°) were detected. Raman analysis designated the rotation angle of 11–12° which may indicate that this is the predominant rotation angle in the composite. Both folded and free standing, unfolded edges were present in the sheets. The free standing edges were rough and no preferred chirality was found. Overlapping boundary interfaces were dominant between the graphene domains in the sheets. These features may degrade the electronic properties of the composite from the ideal values. However, the interlayer distance in the sheets was increased ∼12% compared to graphite. This, together with the wrinkled network of the sheets and the CNFs that contain nanosize (∼5–10 nm) cavities, may increase, e.g., lithium-ion insertion capacity of the composite.     

Magnetic properties of PAN-based carbon fibres modified with magnetite nanoparticles

The magnetic properties of PAN-based carbon fibres modified with magnetite nanoparticles are presented and analyzed. PAN fibres and PAN-based carbon fibres modified with different amounts of magnetite are characterized by the use of magnetization measurements and Mössbauer spectroscopy techniques. The investigations revealed that magnetite (Fe₃O₄) decomposed into Fe_α, Fe_γ and cementite (Fe₃C). Precise analysis of the phase’s contents for different carbon fibres has been carried out in relation to the initial magnetite compound. Analysis of the Mössbauer spectra allowed the determination of the phase contents in fibres with different initial magnetite concentrations. Partial transformation of magnetite into γ-Fe induces catalytic carbonization and formation of a highly crystalline carbon matrix at 1000 °C. The apparent crystallite size in carbon fibres containing 30% magnetite was almost seven times higher than that found in the pure carbon fibres.     

Thermal chemical vapor deposition grown graphene heat spreader for thermal management of hot spots

Graphene of different layer numbers was fabricated using thermal chemical vapor deposition (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum thermal evaluation chips were used to evaluate the thermal performance of the graphene heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm⁻² was decreased from 121 °C to 108 °C (ΔT ≈ 13 °C) with the insertion of the monolayer graphene heat spreader, compared with the multilayer (n = 6–10) ones’ temperature drop of ∼8 °C. Various parameters affecting the thermal performance of graphene heat spreaders were discussed, e.g. layer numbers of graphene, phonon scattering, thermal boundary resistance. We demonstrate the potentials of using a complementary metal oxide semiconductor compatible TCVD process to utilize graphene as a heat spreader for heat dissipation purposes.     

Fe3O4 nanoparticles encapsulated in electrospun porous carbon fibers with a compact shell as high-performance anode for lithium ion batteries

Fe₃O₄ nanoparticles encapsulated in porous carbon fibers (Fe₃O₄@PCFs) as anode materials in lithium ion batteries are fabricated by a facile single-nozzle electrospinning technique followed by heat treatment. A mixed solution of polyacrylonitrile (PAN) and polystyrene (PS) containing Fe₃O₄ nanoparticles is utilized to prepare hybrid precursor fibers of Fe₃O₄@PS/PAN. The resulted porous Fe₃O₄/carbon hybrid fibers composed of compact carbon shell and Fe₃O₄-embeded honeycomb-like carbon core are formed due to the thermal decomposition of PS and PAN. The Fe₃O₄@PCF composite demonstrates an initial reversible capacity of 1015 mAh g⁻¹ with 84.4% capacity retention after 80 cycles at a current density of 0.2 A g⁻¹. This electrode also exhibits superior rate capability with current density increasing from 0.1 to 2.0 A g⁻¹, and capacity retention of 91% after 200 cycles at 2.0 A g⁻¹. The exceptionally high performances are attributed to the high electric conductivity and structural stability of the porous carbon fibers with unique structure, which not only buffers the volume change of Fe₃O₄ with the internal space, but also acts as high-efficient transport pathways for ions and electrons. Furthermore, the compact carbon shell can promote the formation of stable solid electrolyte interphase on the fiber surface.     

Carbon nanotube and nanofiber syntheses by the decomposition of methane on group 8–10 metal-loaded MgO catalysts

We have found that several precious metal-loaded MgO catalysts are active in the formation of carbon nanotube (CNT) by the chemical vapor deposition (CVD) of methane. The catalysts were prepared with nine metals (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) by impregnation onto a high surface area MgO. CNT synthesis was carried out in the temperature range from 600 °C to 1000 °C after reduction with H₂ at 800 °C.The amount of carbon deposited and crystallinity in the produced CNT on nine metals showed interesting tendencies: (i) The amount of carbon formed increased in the following transition series metals: first < second < third row transition elements, and (ii) the index of crystallinity I_G/I_D in Raman-bands of the CNTs decreased in the following order: 8 > 9 > 10 in the Periodic Table. Group 8 and 9 metals produced tube type fibers composed of the graphite layers arranged parallel to the fiber axis. On the other hand, carbon nanofibers (CNFs) grown on group 10 metals had herringbone type graphene sheets.     

Continuous and rapid stabilization of polyacrylonitrile fiber bundles assisted by atmospheric pressure plasma for fabricating large-tow carbon fibers

A bundle of carbon fibers (CFs) with 24,000 filaments was prepared using atmospheric pressure plasma and heat simultaneously for only 30 min, followed by the carbonization process. Conventional thermal stabilization process takes more than few hours to prevent the ignition of the polyacrylonitrile (PAN) fiber when large-tow PAN fibers are stabilized. The CFs produced using the plasma stabilization process had a tensile strength as high as 2.6 GPa. This strength level, which is slightly higher than that of CFs stabilized by the conventional process for 120 min, satisfies the demands in automobile applications. It is believed that the plasma-based stabilization process provides a potential solution not only for shortening the process time but also for providing continuous stabilization of large-tow carbon fibers.     

Low temperature graphitization of interphase polyacrylonitrile (PAN)

Ordered polyacrylonitrile (PAN) interphase structures were formed in solution-cast PAN/carbon nanotube (CNT) composite films by enhancing polymer crystallization conditions and processing parameters for five types of CNTs. All film samples were heat-treated using similar stabilization and carbonization (up to 1100 °C) processes. Both the precursor and carbonized materials were characterized by electron microscopy and X-ray spectroscopy. Highly ordered graphitic structure was formed predominantly in the carbonized materials at 1100 °C (i.e., ∼1500 °C lower than the temperature used in a commercial graphitization process). The ordering of the graphite structure formed at 1100 °C was further improved by heat treatment up to 2100 °C. Multiple characterization results indicate that the early onset of PAN conversion to graphite is directly related to the polymer interphase formation as well as the CNT type. Based on the stabilization and carbonization parameters used in this study, PAN/single-wall carbon nanotube (SWNT) samples showed more prevalent graphite formation at 1100 °C. This work demonstrates the influence of CNT type regarding interfacial confinement toward this low-temperature polymer-to-graphite conversion process.     

Effect of gamma-irradiation on the mechanical properties of polyacrylonitrile-based carbon fiber

The influence of gamma-irradiation on the structure and mechanical properties of polyacrylonitrile-based carbon fibers was studied. It was observed that the Young’s modulus of the fibers irradiated increased with irradiation dose, increasing to 267 GPa at 300 kGy, a 16.1% improvement compared to that of as-received carbon fiber. However, the tensile strength increased to 5.4 GPa at 30 kGy, i.e., a 17.4% increase, and then dropped to 4.65 GPa at 300 kGy, which is almost the same as for the untreated fiber. Uniform stress model analysis and Raman spectroscopy show that the degree of covalent cross-linking between the graphene planes increased to a maximum at 30 kGy and then remained almost constant with further irradiation. The SEM images show that the degree of surface roughness increased with the increasing irradiation dose. It is believed that when the dose is less than 30 kGy, the increased cross-linking is the dominant effect, and thus improves the tensile strength. On the other hand, further irradiation generates surface flaws, which neutralizes the increase and results in a decrease of tensile strength.     

Graphene synthesis by C implantation into Cu foils

Cu foils of 2 × 2 cm² have been implanted with 70 keV C⁻ ions to nominal fluences of (2–10) × 10¹⁵ cm⁻² at room temperature (RT) and subsequently annealed at 900–1100 °C for 15 min, before being cooled to RT to form graphene layers on the Cu surfaces. Analyses with Raman spectroscopy and atomic force microscopy demonstrate that a continuous film of bi-layer graphene (BG) is produced for implant fluences as low as 2 × 10¹⁵ cm⁻², much less than the carbon content of the BG films. This suggests that the implanted carbon facilitates the nucleation and growth of graphene, with additional carbon supplied by the Cu substrate (0.515 ppm carbon content). No graphene was observed on unimplanted Cu foils subjected to the same thermal treatment. This implantation method provides a novel technique for the selective growth of graphene on Cu surfaces.     

Direct formation of graphene layers on diamond by high-temperature annealing with a Cu catalyst

The formation of high-quality graphene layers on diamond was achieved based on a high-temperature annealing method using a Cu catalyst. Typical features of monolayer graphene were observed in the Raman spectra of layers formed by annealing of Cu/diamond heterostructures at 950 °C for 90 min. The coverage ratio of these graphene layers on diamond was estimated to be on the order of 85% by Raman mapping of the 2D peak. The sheet hole concentration and mobility values of the layers were estimated to be ~ 10¹³ cm^− 2 and ~ 670 cm²/Vs, respectively. These values are comparable to those previously observed for high-quality graphene layers on SiC.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.