Preparation and characterization of ultrafine electrospun polyacrylonitrile fibers and their subsequent pyrolysis to carbon fibers

The present contribution reports the fabrication and characterization of ultrafine polyacrylonitrile (PAN) fibers by electrospinning and further development of the as‐spun PAN fibers into ultrafine carbon fibers. The effects of solution conditions (i.e., solution concentration, viscosity, conductivity, and surface tension) and process parameters (i.e., applied electrostatic field strength, emitting electrode polarity, nozzle diameter, and take‐up speed of a rotating‐drum collector) on morphological appearance and average diameter of the as‐spun PAN fibers were investigated by optical scanning (OS) and scanning electron microscopy (SEM). The concentration, and hence the viscosity, of the spinning solutions significantly affected the morphology and diameters of the as‐spun PAN fibers. The applied electrostatic field strength and nozzle diameter slightly affected the diameters of the as‐spun fibers, while the emitting electrode polarity did not show any influence over the morphology and size of the as‐spun fibers. Utilization of the rotating‐drum collector enhanced the alignment of the as‐spun fibers. Within the investigated concentration range, the average diameter of the fibers ranged between 80 and 725 nm. Finally, heat treatment of the as‐spun fibers with their average diameter of about 450 nm was carried out at 230 and 1000 °C, respectively. Various characterization techniques revealed successful conversion into carbon fibers with an average diameter of about 250 nm. Copyright © 2006 Society of Chemical Industry     

Combustion synthesis of hollow carbon fibers

Mixtures of calcium carbide with different oxidizers, such as C₂Cl₆, C₆Cl₆, (C₂F₄) _n , and (CF) _n , were investigated. Reactions between these substrates in the presence of sodium azide are exothermic enough to proceed in a high-temperature self-sustaining regime. Combustion of tested mixtures was performed in the presence/absence of ferrocene (Fc) as an agent catalyzing the growth of nanostructures. Heat effects accompanying the reactions were measured and solid reaction products were analyzed. SEM and TEM observations revealed the presence of multi-walled hollow carbon fibers in combustion products formed in the CaC₂/C₂Cl₆/NaN₃/Fc mixture. Exfoliated graphite was observed in solid combustion products when (CF) _n was used as an oxidizer. In others systems, a soot-like morphology was found to be predominant.      

Electrospinning of gelatin fibers using solutions with low acetic acid concentration: Effect of solvent composition on both diameter of electrospun fibers and cytotoxicity

Gelatin fibers were prepared by electrospinning of gelatin/acetic acid/water ternary mixtures with the aim of studying the feasibility of fabricating gelatin nanofiber mats at room temperature using an alternative benign solvent by significantly reducing the acetic acid concentration. The results showed that gelatin nanofibers can be optimally electrospun with low acetic acid concentration (25%, v/v) combined with gelatin concentrations higher than 300 mg/mL. Both gelatin solutions and electrospun gelatin mats (prepared with different acetic acid aqueous solutions) were analyzed by Fourier transform infrared spectroscopy and differential scanning calorimetry techniques to determine the chemical and structural changes of the polymer. The electrospun gelatin mats fabricated from solutions with low acetic acid content showed some advantages as the maintenance of the decomposition temperature of the pure gelatin (～ 230°C) and the reduction of the acid content on electrospun mats, which allowed to reach a cell viability upper than 90% (analyzed by cell viability test using human dermal fibroblast and embryonic kidney cells). This study has also analyzed the influence of gelatin and acetic acid concentration both on the solution viscosity and the electrospun fiber diameter, obtaining a clear relationship between these parameters. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42115.     

Comparison about the structure and properties of PAN‐based activated carbon hollow fibers pretreated with different compounds containing phosphorus

Polyacrylonitrile (PAN) hollow fibers were pretreated with five different compounds containing phosphorus, including ammonium dibasic phosphate, ammonium dihydrogen phosphate, triammonium phosphate, phosphoric acid, and metaphosphoric acid, and then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of different compounds containing phosphorus as pretreating agents on the properties and structure of the resultant oxidized hollow fibers, carbon hollow fibers, and activated carbon hollow fibers are discussed. Comparing the Brunaner‐Emmett‐Teller (BET) surface area of PAN‐activated carbon hollow fibers (ACHF) pretreated with five different compounds, ammonium dibasic phosphate > triammonium phosphate > ammonium dihydrogen phosphate > phosphoric acid > metaphosphoric acid, and the surface area of mesopores in PAN‐ACHF pretreated with ammonium dibasic phosphate reaches maximum, 174 m² g^?1. The adsorption ratio to mesomolecule adsorbate, VB₁₂, of PAN‐ACHF pretreated with ammonium dibasic phosphate also reaches maximum, 97.7 wt %. Moreover, the dominant pore sizes of PAN‐ACHF range from 2 to 5 nm in diameter. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 294–300, 2005     

Preparation of carbon hollow fiber membranes by pyrolysis of polyetherimide

The preparation of carbon membranes by pyrolysis of polyetherimide hollow fibers and the influence of process variables on the final membrane morphology using a statistical experimental design are described in this work. The characterization of polymers and membranes was carried out by thermal analysis and scanning electron microscopy (SEM). The carbonization process was accompanied by mass spectroscopy to monitor the products formed. Similar to carbonization of others polymers, H₂O, CO₂ and CO evolution from 420 to 680 °C, and hydrogen evolution from 450 to 800 °C, indicate the formation of crosslinking of polymeric chains and formation of a graphite-like structure. These experiments permitted the production of thermostable carbon hollow fibers and selection of best treatment conditions. The extent of membrane exposure under oxidizing atmosphere and the maximum temperature of stabilization were decisive in the final membrane morphologic characteristics and properties. When the stabilization temperature was above 500 °C an intensive degradation of the fiber was observed. An initial exposure to an oxidizing atmosphere seems to be fundamental in order to control the final membrane properties. In this atmosphere, heating rates as low as 1 °C min⁻¹ during stabilization reduce cracks in the surface of final membranes.     

Large-scale synthesis and characterization of carbon spheres prepared by direct pyrolysis of hydrocarbons

Large-scale production of pure carbon spheres, with diameters from 50 nm to 1 μm, has been achieved via direct pyrolysis of a wide range of hydrocarbons, including styrene, toluene, benzene, hexane, cyclohexane and ethene, in the absence of catalyst. Specific systematic studies using styrene as the feedstock indicate that the sizes of the resulting of carbon nanospheres can be controlled quite well by adjusting the experimental conditions. The resulting materials have been fully characterized using SEM, TEM, AFM, HRTEM, EDX, elemental analysis, density measurement, XPS, FTIR, XRD, Raman, and TGA. The results show that the spheres, which are 99% carbon, consist of concentric incompletely closed graphitic shells. The dangling bonds on the edges of the shells result in high chemical reactivity.     

Enhancement of electrosorption capacity of activated carbon fibers by grafting with carbon nanofibers

The composite films of activated carbon fibers (ACFs) and carbon nanofibers (CNFs) are prepared via chemical vapor deposition of CNFs onto ACFs in different times from 0.5 to 2 h and their electrosorption behaviors in NaCl solution are investigated. The morphology, structure, porous and electrochemical properties are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, N₂ adsorption at 77 K, contact angle goniometer and electrochemical workstation, respectively. The results show that CNFs have been hierarchically grown on the surface of ACFs and the as grown ACF/CNF composite films have less defects, higher specific capacitances, more suitable mesoporous structure and more hydrophilic surface than the pristine ACFs, which is beneficial to their electrosorption performance. The ACFs/CNFs with CNFs deposited in 1 h exhibit an optimized NaCl removal ratio of 80%, 55% higher than that of ACFs and the NaCl electrosorption follows a Langmuir isotherm with a maximum electrosorption capacity of 17.19 mg/g.     

Novel silicon-contained lignin-based carbon fibers derived from bamboo pulping black liquor with improved electrochemical performance for supercapacitors

Carbon fibers (CFs) are a promising candidate as electrode materials for flexible supercapacitors given its light weight and moderate cost. In this study, the lignin used was partially separated from kraft bamboo pulping black liquor and the higher molecular weight fraction, unavoidably contains a small amount of silicon compounds, so named silicon-contained lignin. Novel CFs were prepared using commercial polyacrylonitrile (PAN) and the lignin by electrospinning and further carbonization. Even in the presence of silicon compounds, the fibrous morphology of precursor fibers was significantly good, and the CFs with uniform fiber diameter and high specific surface area up to 182 m²/g were obtained with an increase in silicon-contained lignin. The CFs fabricated from silicon-contained lignin and commercial PAN had higher specific capacitance (22.20 mF/cm² at 10 mA/cm²) and superb cycling stability (94.21%) than that from silicon-free lignin or pure PAN separately.     

Polyimide‐based porous hollow carbon nanofibers for supercapacitor electrode

Porous hollow carbon nanofibers (PHCNFs) using styrene‐acrylonitrile copolymer (SAN) solution as core and polyacrylic acid (PAA) as shell were manufactured by co‐axial electrospinning technique, taking polyvinyl pyrrolidone (PVP) as a pore inducer additive in the shell. The shell thickness of PHCNFs could be adjusted by controlling flow rates of core and shell fluids. The prepared PHCNFs showed excellent electrochemical properties with the high specific capacitance of 221 F g^?1 and superior cycling stability, remaining a capacitance retention of 95% after 5000 cycles under a scan rate of 0.1 V s^?1. In this system, hollow structures bring a 20% capacitance improvement, while the porous morphology brings a 47% capacitance improvement. The attractive performances exhibited by these sponge supercapacitors make them potentially promising candidates for future energy storage systems. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43397.     

聚丙烯腈基碳纤维的新进展

本文主要从聚丙烯腈基碳纤维的国内外发展现状,工艺技术概况和碳纤维贴片在建筑结构物的补强应用3个方面介绍了聚丙烯腈基碳纤维的新进展。     

Melt mixing of carbon fibers and carbon nanotubes incorporated polyurethanes

Polyurethane composites filled with carbon fibers (CF) and carbon nanotubes (CNT) were prepared by mixing and injection molding, and its mechanical as well as their thermal properties were investigated. Dynamic mechanical analysis (DMA), thermogravimetry analysis (TGA), and thermal conductivity tests were done, and the properties were evaluated as a function of the filler concentration. The storage modulus of the composites increased with fillers concentration, which also mean the increase of the stiffness, suggest a good adhesion between the polyurethane matrix and the fillers. Addition of more CF and CNT to the composites broadened and lowered the peak of tan δ specifies that the polyurethane composite became more elastic because there is a good adhesion between the fillers and the matrix. The addition of carbon fillers improves the thermal stability of the polyurethane. The inclusions of CNT show a better thermal stability when compared with CF. The addition of carbon fillers also increased the thermal conductivity of the polyurethane composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Self-healing interfaces of poly(methyl methacrylate) reinforced with carbon fibers decorated with carbon quantum dots

Fiber-polymer matrix interfacial debonding is often observed when mechanical loads are applied to fiber reinforced polymer composites. These defects usually end-up leading to a catastrophic fracture of the composites. In this work, carbon quantum dots (CDs) were incorporated on the surface of carbon fibers (CF), and poly(methyl methacrylate) (PMMA) composites with these modified fibers were able to restore their original properties after been previously damaged. To this end, CDs were synthesized and used to decorate the surface of CF. These decorated CF were then incorporated into PMMA by using a high intensity mixer. The prepared composites were submitted to dynamic mechanical, three-point bending and self-healing tests. Fluorescent CDs with diameters of 10 nm and functional groups, such as amine and carboxylic groups were successfully synthesized by the microwave pyrolysis method. The deposition of CDs on the surface of CF was evaluated and quantified by UV–vis spectroscopy and 1.2 wt.% of CDs on CF was determined. Composites with different surface treatments (including the presence of CD) did not show significant differences in strength, stiffness and damping, suggesting that the surface treatments on CF did not lead to major changes in the degree of interfacial interaction. Self-healing tests showed that damaged composites with CD decorated CF were able to restore their original properties, while no self-healing effect was noted in composites with no CD on CF. The observed self-healing behavior between PMMA and CF decorated with CD is due to the interactions between chemical groups on the surface of the CD and PMMA. Thus, damages related to fiber-matrix interfacial detachments can be repaired through reversible interactions based on CD.     

Nanoporous hollow fibers as a phantom material for the validation of diffusion magnetic resonance imaging

Diffusion-weighted magnetic resonance imaging (MRI) is an emerging noninvasive imaging modality. In this study, highly aligned, uniform, nanoporous, hollow polycaprolactone fibers were successfully synthesized in a single step to mimic the axon bundle structure in human white matter. Their porous nature, morphology, and physicochemical properties were carefully studied with respect to their suitability as a phantom material for brain imaging. The aligned fibrous bundles were then arranged into specific angles (30 and 90°), scanned, and evaluated with high-resolution MRI fiber tractography. Diffusion tensor imaging and the tractography of fibers of five different structures at three temperatures were acquired and compared. Furthermore, an integrated brain phantom created from a combination of agar gel and aligned fibrous bundles was also fabricated and analyzed. The results demonstrate the excellent ability of the fibers to mimic the axonal bundles of brain white matter. The fibrous bundles were well mixed in the common agar phantom while retaining their fibrous configuration; this demonstrated their potential as brain white matter phantoms. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47617.     

Polyvinylbutyral‐assisted synthesis and characterization of SnS mesoporous fibers via electrospinning process

Tin sulfide (SnS) has the potential to be used as a low‐cost absorber material for applications in thin film photovoltaic solar cells. In this study, polyvinylbutyral/SnS (PVB/SnS) composite fibers were synthesized through a relatively simple electrospinning process. SnS mesoporous fibers were obtained from PVB/SnS composite fibers after sintering treatment at 500°C for 1 h in N₂ atmosphere. The SnS mesoporous fibers were then characterized using X‐ray powder diffraction, scanning electron microscopy, thermogravimetric analysis, and transmission electron microscopy. The optical properties of SnS mesoporous fibers were also recorded by UV–vis absorption spectroscopy. The results showed that the synthesized SnS mesoporous fibers exhibited a single phase, stoichiometric composition, with good crystallinity, a size ranging from 100 to 200 nm, and a band gap of 1.49 eV. The as‐prepared SnS mesoporous fibers are thus a suitable material to achieve visible light absorption in a thin film solar cell. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42388.     

HEMP-derived activated carbon fibers by chemical activation with phosphoric acid

Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m²/g and mesopore volume of 1.25 cm³/g were obtained at 550 °C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO₃ and mainly C-PO₃ and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction.     

Dual-layer hollow fibers for gas separation processes produced by quadruple spinning

A novel quadruple spinneret to produce dual-layer hollow fiber membranes by simultaneous spinning of two polymer solutions, using the dual precipitation bath technique is proposed. Hollow fibers aimed at gas separation processes were prepared in extrusion system specifically designed and built for this purpose. A polyurethane polymer was selected as the selective layer (outer-layer), while polyethersulfone was defined as the support (inner-layer). Activated carbon powder was added into the PU solution for further improvement of the transport properties. The hollow fibers showed good adhesion between the polymer layers and a defect-free selective layer. Representative results include a CO₂/N₂ selectivity of 43.     

Capacitance behavior of activated carbon fibers with oxygen-plasma treatment

Modified activated carbon fibers (ACFs) were used as the electrodes of an electric double-layer capacitor and showed an enhanced capacitance effect after a RF-plasma treatment. The capacitance and the surface functional groups of the ACFs were studied. For the plasma-treated ACFs having a specific surface area of 1500 m² g⁻¹, the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142 F g⁻¹ was achieved with an oxygen feed concentration of 10 vol.%. The Brunauer-Emmett-Teller (BET) surface area was 2103 m² g⁻¹, which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1 cm³ g⁻¹ STP, and from the pore distribution plot, quantities of mesopores of 10 nm or less and micropores also increased. However, in order to enhance the capacitance, the quinone functional group had a significant influence in addition to the BET surface area. The correlation between the capacitance and the number of quinone functional groups was confirmed because quinone is an electron acceptor.     

酚醛基活性碳纤维

酚醛树脂属于难石墨化碳，适宜制造活性碳纤维(ACF)，其特点是碳化活化收率高，制品强度高，比表面积大，孔径分布属于单峰形，吸附容量大；细孔入口形状属于狭缝形，吸脱速度快；制品柔软，深加工性好，它除用于治理环境污染外，还可用于制作防化防毒服和制造超级电容器等。     

Effect of applied voltage on diameter and morphology of ultrafine fibers in bubble electrospinning

A novel electrospinning process, bubble electrospinning, was used to produce porous nonwoven fibrous mats, of which the fiber diameter can range from nano‐ to microscales. The deformation of a charged bubble, from which multiple jets were ejected, was observed using a high‐speed motion camera. The effects of different applied voltages on diameter, morphology, and structure of bubble‐electrospun ultrafine fibers were theoretically analyzed and then experimentally validated by scanning electron microscopy and atomic force microscopy. The results showed that the average diameter of fibers increased with the increase of the applied voltage in bubble electrospinning, which is quite different from that in traditional electrospinning process under the similar conditions. The number of beaded fibers decreased with increasing applied voltage. Additionally, the crystallinities of polyvinylpyrrolidone ultrafine fibers obtained in this process were higher than that of polyvinylpyrrolidone powders. The production rate of bubble electrospinning was higher than that of the traditional electrospinning. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011