Pre-oxidized Polyacrylonitrile Research Articles

Cost-effective preparation of pre-oxygenated PAN fiber composite SiO2 aerogel felt with excellent mechanical and thermal insulation properties

Silica aerogel is an attractive thermal insulation material due to its low thermal conductivity, light weight and non-combustible inorganic properties, but its inherent brittleness and high production cost hinder its practical application on a large scale. Herein, the high-performance pre-oxidized polyacrylonitrile fiber composite silica aerogel felt (PPAF-SiO2) was successfully prepared by a cost-effective combustion drying technique for the first time. The results show that the PPAF-SiO2 has the feature of lightweight, good flexibility, and its heat resistance, hydrophobicity, and specific surface area are significantly improved. Due to the further polymerization of fiber structure molecules caused by combustion, the tensile strength of the PPAF-SiO2 is increased by 81.9% to 1.71MPa. More importantly, due to the high proportion of silica aerogel in felt, its thermal conductivity can be reduced to 0.02W/(m·K), which is lower than most aerogel composite materials. Moreover, the thermal insulation performance of the PPAF-SiO2 was further confirmed by the infrared thermal imager from room temperature to 300 ℃. In addition, the innovative drying mechanism based on the combustion flame model was proposed. Finally, compared with traditional physical drying technologies in terms of drying time and drying conditions, the combustion drying technology shows great advantages in drying efficiency, energy consumption and equipment investment, which greatly reduces production costs. The combustion drying technology opens up a promisingly method for high efficiency, low energy consumption, and low cost preparation of silica aerogel composites.

Microstructure Evolution of Polyacrylonitrile-Based Fibers during Thermal Pre-Oxidation

In this work, pre-oxidized polyacrylonitrile fibers are treated with ultrasonic etching and solution etching to produce ultra-thin sections. The evolution of the fibers’ microstructure in the pre-oxidation process is observed, and the transformation model of the microstructure of the pre-oxidized fibers is proposed. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the microstructure changes of the fibers. Fourier transform infrared spectroscopy and X-ray diffraction were used to observe the chemical structure transformation and crystallization degree of the fibers in the pre-oxidation process. The results revealed that pre-oxidized fibers exhibited a smooth surface, while their interior consisted of fibrils. The longitudinal microfibrils were connected by the transverse microfibrils and amorphous regions. The fracture morphology of the fibers shifted from ductile to brittle, and the cross-section gradually became smoother. The linear molecular chain of PAN transformed into a ring structure as pre-oxidation progressed, subsequently leading to the cross-linking of this ring structure into an orderly trapezoidal configuration. The connection between the fibrils was enhanced, and the fiber structure became more compact and stable.

Transformation of traditional carbon fibers from microwaves reflection to efficient absorption via carbon fiber microstructure modulation

The high conductivity of commercial carbon fibers leads to a noticeable skinning effect and weak microwave impedance matching. This results in insufficient effective absorption bandwidth and less than ideal overall wave absorption performance, mainly reflecting electromagnetic waves. These limitations restrict the application of carbon fibers. In this study, we successfully prepared carbon fiber/epoxy composites with adjustable electrical conductivity by precisely controlling the temperature and time of pre-oxidized polyacrylonitrile (PAN) fibers during the low-temperature carbonization stage. This allowed us to regulate the components and microstructure of conductive carbon fibers. The carbon fiber/epoxy composites demonstrated exceptional microwave absorption performance due to their various loss mechanisms. The impedance matching conditions and microwave attenuation capability were also quantitatively evaluated. For 850-9 samples, an effective bandwidth of approximately 7 GHz (11 GHz–18 GHz) was achieved with a thickness of 2.5 mm. For 900-9 samples, the best reflection loss achieved is −39.9 dB at 5.8 GHz, which corresponds to a thickness of 3 mm. The carbon fibers with different electrical conductivity can be obtained by modulating the microstructure of carbon fibers, which can improve the impedance matching performance of conductive carbon fibers and reduce their reflection of electromagnetic waves to improve the microwave absorption performance, achieving the transformation of traditional carbon fibers from electromagnetic shielding to effective microwave absorption. This simple and efficient preparation method has yielded promising results and offers a new avenue for the advancement of high-performance carbon fiber microwave absorbing materials.

Modification of pre-oxidized polyacrylonitrile fibers as chelating sorbents for removal of Copper(II) ions

Pre-oxidized polyacrylonitrile (PAN) fibers were chemically modified by sulfuric acid. In the process, conjugated imine sequences were retained and amide groups were transformed from unreacted nitrile groups. The adsorption experiments of Cu(II) ions by modified PAN fibers indicated that the adsorption capacities were significantly improved by increasing initial concentrations of metal ions, reaction time and temperature. The adsorption equilibriums followed Langmuir isotherm with correlation coefficients greater than 0.99. Kinetics analysis demonstrated that the adsorption of copper ions on the modified fiber was fitted well with pseudo-second-order kinetics equation. Indicated by energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) results, channels for metal ions diffusing from the surface to the inner of the fiber were generated by the strong etching of sulfuric acid. The active sites of conjugated imine sequences and amides in the whole modified fiber can thus be easily accessed by the metal ions.

Design and construction of PTFE porous membranes with high modulus and structural stability via in-situ interlocking and anchoring strategy

PTFE nanofiber membrane, as an effective supplement to biaxial stretched PTFE membrane, suffers from low modulus and poor structural stability. In this study, the PTFE porous membrane with high modulus and structural stability was designed and achieved via the in-situ interlocking and anchoring strategy by co-electrospinning polyacrylonitrile (PAN) into PTFE matrix. During the sintering process, the continuous flexible PTFE nanofibers formed, while PAN nanofibers was pre-oxidized to create a stable trapezoidal structure. The effects of pre-oxidized PAN (OPAN) nanofibers' content and diameter on the morphology and structure of the PTFE/OPAN composite nanofiber membrane were investigated in detail. Results showed that the introduction of OPAN nanofiber effectively improve the modulus and structural stability of PTFE nanofiber membrane. The Young's modulus and tensile strength of the composite nanofiber membrane were over 50 MPa and 2 MPa, which were 1166% and 80% higher than that of pure PTFE nanofiber membrane, respectively. These results of this work would contribute to solve the defects of PTFE nanofiber membrane's low modulus and poor structural stability, and then expand its application scope.

Preparation of TiO2/PAN-based activated carbon nanofibers assisted by supercritical CO2 fluid

Polyacrylonitrile (PAN)-based activated carbon nanofibers were used as a photocatalyst carrier and framework due to its large specific area and can adsorb organic pollutants. In this paper, the supercritical carbon dioxide was used to deposit TiO2 on the surface of pre-oxidized PAN nanofibers, and the fibers were activated by heat treatment to prepare activated carbon nanofibers-TiO2 (TiO2/ACNF). The morphology, structure and specific surface area of the modified nanofibers were analyzed, and the degradation test of methyl orange was carried out. The results showed that TiO2 nanoparticles can be uniformly loaded on the surface of pre-oxidized PAN nanofibers after supercritical treatment. The specific surface area of the TiO2/ACNF after activated heat treatment can reach 422.8 m2/g, having higher adsorption and photocatalytic degradation performance.

Evolution of microstructure within carbon fiber during pre-carbonization revealed by mean field theory

Microstructure dominating the conductivity in carbon fiber (CF) was investigated to construct an initial model for the interpretation of the sudden shift in conductivity of pre-oxidized polyacrylonitrile precursor fibers (POF) during pre-carbonization. The electrical conductivity of POF treated at different pre-carbonization temperatures was measured by four-point probe meter. According to the result of high-resolution transmission electron microscopy and X-ray diffraction, the microstructure of CF was approximately regarded as nanocomposite consisting of pseudo-graphite sheet (PGS) and an amorphous carbon matrix. Based on the analysing of this nanocomposite microstructure model with mean-field theory and applying effective-medium approximation, the evolution of electrical conductivity and microstructure of POF was investigated. The microstructure of CF during pre-carbonization process, including the volume fraction and geometric factors of PGS, was determined by X-ray diffraction and Raman spectroscopy. The existence of percolation effect was determined by applying a combination of simulation calculating and experiment. The results indicated that a carbon skeleton consisting of connected PGSs was formed within CF during the pre-carbonization process. In addition, the carbon skeleton provides efficient pathways for the lattice vibrations and phonon propagation required for heat conduction and has a significant impact on the subsequent crystallization of the final CF and its properties.

Effect of beating degree of fiber on the development of porosity in polyacrylonitrile-based activated carbon fiber paper

Activated carbon fiber paper (ACFP) has the potential for adsorption due to their high surface area and low through resistance. In this work, free-standing ACFP was developed from fibrillated polyacrylonitrile (PAN) fiber through wet forming process combined with subsequent pre-oxidation, carbonization and CO2 activation. A PFI refiner was used to prepare fibrillated PAN fibers with a beating degree of 7, 11, 25 and 60 oSR, respectively. As expected, PAN fibers with high beating degree improved surface area and fine fiber content, which resulted in the dense structure and low porosity of paper, thus influencing later stabilization and pyrolysis, as well as final properties of ACFP. With the increase of beating degree, the oxygen content of pre-oxidized PAN paper increased, but the cyclization index first decreased and then increased. ACFP with higher beating degree possessed higher BET surface area, pore volume and CO2 uptake capacity. ACFP, prepared from freeze-dried PAN paper, performed better in physicochemical properties than that from plate-press drier under the same beating degree. This study exhibited that ACFP can be a special material featured by scale production, lightweight, and high microporosity of chemical adsorption and separation.

Effect of ambient atmosphere on the formation and evolution of conjugated structure of pre-oxidized polyacrylonitrile

The ambient atmosphere has the remarkable effect on the chemical reaction mechanism and the formation of characteristic chemical structures and conjugated structures for pre-oxidized polyacrylonitrile (PAN). In the present paper, fourier transform infrared spectroscopy (FT-IR) and ultraviolet visible spectroscopy (UV–vis) were used to characterize the variations of chemical reaction characteristic structures and conjugate structures of copolymerized PAN in nitrogen and air atmosphere, respectively. Combined with the multivariate reaction model proposed by the predecessors, it can be found that two main conjugated structures (conjugated polyene and conjugated aromatic heterocycle) formed during the pre-oxidation process of copolymerized PAN. In nitrogen atmosphere, the formed main structure units were the conjugated diene and conjugated double ring with 1–2 double bonds. By contrast, the formed main units in air were the conjugated diene and conjugated tricyclic with a double bond. During the whole pre-oxidation process, the maximum unit numbers of conjugated double bonds and conjugated rings were no more than four and six, respectively. Increasing the heat treatment temperature favored to form more conjugate structures than prolonging the heat treatment time. Furthermore, with the increase of heat treatment temperature or time in air, the main conjugate unit numbers had no obvious change, while the maximum unit numbers increased slightly. For the exposure in nitrogen atmosphere, the main conjugated polycyclic structure was transformed into conjugated tricyclic structure after the heat treatment at 250 °C for more than 3 h. In addition, when the heat treatment temperature and time were 280 °C for more than 4 h, the main conjugated polyene structure was transformed into conjugated triene, and the maximum conjugated unit numbers increased slightly.

In Situ Transformed Carbon Fibers/Al2O3 Nanocomposites Using Cao–MgO–SiO2 Sintering Agent

In Situ transformed carbon fibers/Al2O3 ceramic matrix nanocomposites with Cao–MgO–SiO2 sintering agent were prepared by hot-pressed sintering technology in vacuum. In the sintering process, pre-oxidized polyacrylonitrile fibers (below named as pre-oxidized PAN fibers) were used as the precursors of In Situ transformed carbon fibers. The micro/nanostructure of composites and interface between In Situ transformed carbon fibers and matrix were investigated, as well as the properties of composites. The results showed that the composites could be sintered well at a relatively low temperature of 1650 °C. During the sintering, the precursors, pre-oxidized PAN fibers, were In Situ transformed into carbon fibers, and the In Situ transformed carbon fibers had the graphitelike structure along the fiber axial direction. The carbon atoms arrangement in the surface layer of the fiber was more orderly than the core. A typical diffraction peak of carbon fiber at 26°, which corresponded to the (002) crystal plane, was observed, and the inter-planar spacing was approximately 0.34 nm. The CaO–MgO–SiO2 sintering agent formed MgAl2O4 and CaAl2Si2O8 phases in the interface between In Situ transformed carbon fibers and matrix, therefore improving the interface bonding, and thereby modifying the mechanical properties of the composites.

NH2-UiO-66 coated fibers to balance the excellent proton conduction efficiency and significant dimensional stability of proton exchange membrane

The proton exchange membrane (PEM) with high sulfonation degree (DS) exhibits high proton conductivity but poor dimensional stability and mechanical properties. In order to improve dimensional stability and strength of PEM as well as maintaining excellent proton conductivity, we proposed to modify pre-oxidized polyacrylonitrile nanofibers (PPNF) through amino functionalization with NH2-UiO-66 (NU6). Compared with primary sulfonated poly (ether ether ketone) (SPEEK), the dimensional stability of 1.3% NU6 @PPNF-SPEEK composite membrane increase 30% due to the chemical crosslinking structure of PPNF. The acid-enriched layers, which provide an efficient proton transport channel, form along the fibers because of interaction between NU6 and -SO3H, since then composite membranes exhibit higher proton conductivity. Most importantly, this study firstly revealed the mechanism of formation process of sulfuric acid-rich layer through FTIR and EDS, which could provide a strategy for constructing and adjusting the proton transport channel efficiently. In summary, NU6@PPNF-SPEEK with excellent proton conductivity, outstanding dimensional stability and great mechanical properties in hydrated condition has been successfully fabricated.

Boosting dimensional stability and proton conductive of proton exchange membrane by introducing SO3H-UiO-66 coated nanofibers

In this work, we fabricated cross-linked pre-oxidized polyacrylonitrile nanofibers (PPNF) coating SO3H-UiO-66 coated (named SU6@PPNF), which was introduced into sulfonated poly (ether ether ketone) SPEEK to produce membrane with superior proton conductivity, dimensional stability and mechanical properties under hydrated condition. The membrane displays a maximum power density of 172.1 mW cm-2 under 100% RH and 60 °C, which is an increase of 12.2% compared with primary membrane.

Reactive molecular dynamics simulation of the laser-induced carbonization of pre-oxidized polyacrylonitrile fibers

Reactive molecular dynamics simulation of the laser-induced carbonization of pre-oxidized polyacrylonitrile fibers

Mechanical and friction-wear properties of two C/C composites using pre-oxidized polyacrylonitrile fibers and carbon fibers as the reinforcements

Mechanical and friction-wear properties of two C/C composites using pre-oxidized polyacrylonitrile fibers and carbon fibers as the reinforcements

Metal W connected Z-scheme C fibers@WO3-x core-shell composites with highly efficient solar-driven photocatalystic activity

A series of novel metal W connected Z-scheme C fibers@WO3-x (0 ≤ x < 3) core-shell composites were developed by simply pyrolyzing preoxidized polyacrylonitrile fibers impregnated with slurries of WO3 nanopowder in absolute ethanol in a vacuum furnace. Such structured composites are a kind of more excellent solar-driven photocatalyst for degrading various aqueous organic and inorganic environmental pollutants, compared with pure tungsten oxides catalysts under the same condition. The optimally selected composite catalyst (x = 2.01) was also applied in photocatalytically purifying waste water from paper mills under sunlight, presenting high efficiency in reducing chemical oxygen demand, biology oxygen demand and heavy metal Cr content, displaying great prospect in environmental remediation through removing various toxic industrial chemicals. The greatly enhanced photocatalytic performance could be mainly ascribed to the C fiber core and metal W connection layer between the tungsten oxides and C core in the composites, which can promptly conduct electrons and promote the separation of photogenerated electrons and holes. Moreover, this work provides a simple, eco-friendly and very efficient synthesis approach, which will be of much help to design diversified semiconductors composite structures, and suitable for large-scale production and thus practical industrial application.

In situ generation of carbonized polyaniline nanowires on thermally-treated and electrochemically-etched carbon fiber cloth for high efficient solar seawater desalination

Utilizing solar energy to evaporate water is an environment-friendly and promising approach for various applications such as seawater desalination. Various photothermal materials can be developed for evaporating water, however, there are still some drawbacks such as complicated synthesis processes, weak/narrow absorbance, bulkiness, and low evaporation rate, which severely limit their commercial applications. Herein, we reported the preparation of electrochemically-etched carbon fiber cloth with surface-coated carbonized polyaniline nanowires (ECFC/CPANW) based on thermally-treated pre-oxidized polyacrylonitrile fiber cloth for highly efficient solar steam generation. This ECFC/CPANW composite has the advantages of low thermal conductivity, effective broadband solar absorption, good hydrophilicity and water transfer ability. When ECFC/CPANW and polyurethane foam are bound together as a solar evaporator, ECFC/CPANW achieved the highest vaporization efficiency of up to 93.7% under 1 sun illumination, which is more efficient than those of most materials previously reported. Therefore, this kind of photo-thermal conversion material with low cost, good stability and environmental friendliness could envision promising practical application for water treatment.

Predicting cost-effective carbon fiber precursors: Unraveling the functionalities of oxygen and nitrogen-containing groups during carbonization from ReaxFF simulations

Blends of polyacrylonitrile (PAN) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) as precursors may offer an opportunity to reduce the cost of carbon fiber (CF) production. The all-carbon ring formations and volatile gas production during the heating and carbonization for 9 PAN/PBO blend precursors with varying mixing ratios are studied via the ReaxFF reactive molecular dynamics simulations. Evolutions of oxygen-containing (O-containing) and nitrogen-containing (N-containing) groups are detailed, in order to reveal the reaction mechanisms of the O and N-based CF precursors. Particularly, the O-containing groups are identified to be more efficient for initiating the carbonization, whereas N-containing groups are far longer retained in the graphitic materials and play a key role in capturing and converting carbon radical species into the graphitic networks. Additionally, the PAN/PBO blend precursor with a mole ratio of 1:1 is compared with the pre-oxidized PAN, PAN, and PBO. It is found that the PAN/PBO blends could be a promising alternative for the cost-effective PAN-based CF precursors, since they can decrease the cost of the CF production by means of: (a) eliminating the pre-oxidation process, (b) having considerable all-carbon ring formations within a short period, and (c) having a relatively fast conversion rate, reaching 95% 6-membered carbon ring formation.

Thermally-treated and acid-etched carbon fiber cloth based on pre-oxidized polyacrylonitrile as self-standing and high area-capacitance electrodes for flexible supercapacitors

The fast development of portable devices has greatly stimulated the demand for flexible supercapacitors (FSCs). The key to assembling FSCs lies in the development of electrodes with high-performance and good flexibility. In this work, oxygeneous groups modified nitrogen-doped carbon fibers cloth (ONCC) was obtained through annealing the pre-oxidized polyacrylonitrile cloth (PPC), followed by a facile oxidation process using a mixture of concentrated sulfuric acid and concentrated nitric acid. By changing annealing temperature, the nitrogen content can be controlled in the obtained ONCC. It was found that the PPC annealed at 950 °C followed by acid etching for 60 min could deliver the excellent areal and gravimetric specific capacitance of 1385 mF cm−2 and 294.7 F g−1 at the current density of 1 mA cm−2, outstanding rate performance and superb cycling stability. Furthermore, the assembled FSC delivered a high energy density of 19.8 μWh cm−2 (4.03 Wh kg−1) at a power density of 50.1 μW cm−2 (10.2 W kg−1) and good flexibility under different bending conditions. This work provides a facile and efficient route to prepare high-performance carbon fiber based electrode materials in large scale for flexible all-solid-state energy storage devices.

Tribological performance of in-situ transformed Cf/Al2O3 self-lubricating composite

Friction and wear behaviors of in-situ Cf/Al2O3, Cf/Al2O3, and Al2O3 were examined under dry sliding, water, PAO oil and PAO+carbon nanotube (CNT) grease at applied load of 6.86N by ball-on-disk sliding tribotester. Both in-situ Cf/Al2O3 and Cf/Al2O3 were fabricated by hot-press sintering in vacuum. In-situ Cf/Al2O3 used cheap and flexible pre-oxidized polyacrylonitrile fibers to in-situ transform into Cf during sintering and that Cf/Al2O3 used commercial Cf directly. The experimental results indicate that in-situ Cf/Al2O3 has comparable friction-reduction ability with Cf/Al2O3 under dry and water lubrication. Specifically, friction coefficient μ of in-situ Cf/Al2O3 reduces from 0.45 to 0.39 with time and fluctuates around 0.17 under dry and water lubrication respectively, while μ of Al2O3 is about 0.52 and varies around 0.35. Meanwhile, in-situ Cf/Al2O3 shows best wear-resistance among the three specimens with slightest spalling under dry and water lubrication because in-situ Cf has good interfacial bonding with matrix. However, lubrication property of carbon fibers are hidden by good lubricants (PAO and PAO+CNT grease), the worn surfaces of all of three specimens look like polished. All these results indicate that in-situ Cf/Al2O3 should be a better and cost-effective self-lubricated composite.

Unusual pre-oxidized polyacrylonitrile fibres behaviour against their activation with CO2: carbonization effect

The CF-PAN activation process with CO2 has been analysed. Activation of CF-PAN with CO2 leads to unusual results both, activation percentage evolution over the time and generated porosity development. In the explanation has been highlighted the role of the carbonization step in the CF-PAN activation process. The analysed results point to the fact that part of the released nitrogen-containing compounds during PANOX fibres carbonization are deposited on the CF surface, which affects to carbonized PAN fibres, decreasing actives sites, or acting as inhibitors of the gasification reaction.