Pre-oxidized Fibers Research Articles

Radial Distribution of Functional Groups in Polyacrylonitrile Pre-oxidized Fibers

Radial Distribution of Functional Groups in Polyacrylonitrile Pre-oxidized Fibers

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.

Preparation of needled nonwoven enhanced silica aerogel for thermal insulation

Benefiting from the advantages of light weight, porosity, high specific surface area and ultra-low thermal conductivity, silica aerogel (SiAG) has been regarded as ideal thermal insulation material. However, the low strength and fragility limited its application. In this paper, the needled nonwoven (NNW) composed of polyphenylene sulfide fibers and pre-oxidized fibers was applied as substrate for incorporating with silica aerogel (SiAG/NNW) to coordinate the heat preservation. Various characterizations were carried out to discuss thermal insulation property. The results showed that SiAG/NNW can last for 4min under butane flame with a certain impact force at about 1300 °C. According to TG analysis, the weight loss of the SiAG/NNW is 5% at 464 °C, which is much higher than individual NNW (327 °C) or SiAG (456 °C). Moreover, the carbon residue rate of the nonwoven was raised from 54.3% to 73.7% at 800 °C after SiAG incorporation. The fiber morphology can still be observed at the flame perforation of SiAG/NNW with low carbonization degree and no obvious decomposition to SiAG. Significantly, thermal protective performance (TPP) values of the SiAG/NNW can reach 1272 kW s/m2 which is twice of the NNW. In conclusion, the prepared SiAG/NNW showed high temperature resistance and excellent thermal insulation, the results also provides a technical scheme for future development of thermal insulation materials at high temperature.

The effect of the molecular structure of naphthalene-based mesophase pitch on the properties of carbon fibers derived from it

AbstractMesophase pitch-based carbon fibers (MPCFs) have a high modulus of elasticity, low electrical resistivity and high thermal conductivity, so can be used in many fields. Carbon fibers were prepared from two naphthalene-based mesophase pitches, one synthesized by a HF/BF3 catalytic one-step method (AR-MP) and the other by an AlCl3 catalytic two-step method (N-MP). The mesophase pitches, spun pitch fibers, pre-oxidized fibers, carbonized fibers and graphitized fibers produced from them were characterized by TG-MS, FT-IR, 13C-NMR, MALDI-TOF-MS, XRD, SEM and elemental analysis. The molecular structures and properties of mesophase pitches were compared, and the effects of molecular structures on the structures and properties of the carbon fibers produced from them were measured. In comparison to N-MP, AR-MP has a rod-like semi-rigid molecular configuration containing more naphthenic structures and methyl side chains. The pre-oxidized fibers derived from AR-MP have a better carbon layer orientation, so that their graphitized fibers have a higher thermal conductivity of 716 W/(m·K). N-MP has a higher aromaticity with a disc-like rigid molecular configuration, so that the graphitized fibers prepared from it have a higher tensile strength of 3.47 GPa due to fewer defects being formed during preparation. The molecular structures of AR-MP and N-MP have an obvious influence on the structures and properties of their graphitized fibers.

Influence of fiber/matrix interface on the texture evolution and fracture toughness of C/C

This study was conducted to obtain a more in-depth insight into the role of fiber/matrix interface on the texture and mechanical performance of carbon fiber reinforced carbon composites (C/C). In this study, a series of isothermal chemical vapor infiltration (ICVI) experiments were performed on the deposition of pyrocarbon (PyC) onto carbon fibers (CFs) and pre-oxidized fibers (PFs). The results showed that the graphitization degree and orientation angle (OA) of the fiber and matrix of the PF-SL were relatively low compared with the CF-SL. Most importantly, the OA of CF-SL between the interfaces was much higher than that of PF-SL. Besides, a low texture transition layer with a width of tens of nanometers was observed between the fiber/matrix interfaces, which may reduce the lattice distortion and interfacial energy. In addition, the fracture toughness of CF-SL was higher than that of PF-SL, which was attributed to interfacial differences. Further, PF-SL had a coarser interface than CF-SL due to the carbonization reaction during ICVI, which induced gas escape and PF shrinkage and bulges on the surface.

Evolution and Regulation of Radial Structure of PAN Pre-Oxidized Fiber Based on the Fine Denier Model.

Affected by ambient oxygen and thermal diffusion mechanism, the radial structural distribution of polyacrylonitrile (PAN) fiber during the pre-oxidation process will be inherited to the carbon fiber, which had a remarkable effect on the mechanical properties of carbon fiber. It is important to understand the evolution mechanism of radial structure evolution of PAN fiber during the pre-oxidation process to manufacture the high-performance carbon fiber. In this paper, a series of fine denier model fibers were prepared by adjusting the oxygen concentration to describe the structural characteristics at different radial regions of pre-oxidized fibers. The evolution mechanism of the radial structure of pre-oxidized fiber, with the increase of heat treatment temperature, was studied by the methods of optical microscope, C13 nuclear magnetic resonance (13C-NMR), and thermogravimetric analyzer (TGA). The results showed that along the radial direction of pre-oxidized fiber from skin to core layer, the degree of cyclization changed little while the dehydrogenation and oxygen-containing structure gradually decreased. Specifically, the oxygen-containing functional groups in the core decreased to the lowest level or even disappeared. A moderate increase of temperature in the initial and middle pre-oxidation processes could effectively promote the formation of cyclized structure and stabilize cross-linked ladder structure in the skin region of the fiber. With it, the thermal stability of obtained pre-oxidation fiber was improved.

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.

Study on Structure Evolution and Reaction Mechanism in Microwave Pre-oxidation

Microwave pre-oxidation was compared with conventional pre-oxidation process in 180–280 °C, and the mechanism of microwave pre-oxidation in different temperature was proposed. The crystalline structure (WAXD), chemical structure (FTIR), microstructure (SEM), defect degree (RAMAN) and bulk density were studied. The results showed that the pre-oxidized fibers first undergo crystallization and then undergo a process of amorphous transformation by microwave heating. And at 220 °C, the pre-oxidized fibers under microwave heating show a diffraction peak of 25° (002 diffraction surface), which indicates that a new disordered structure (ladder structure) is generated inside the pre-oxidized fibers. The dehydrogenation reaction runs through the entire pre-oxidation process and is violent at 280 °C. The cyclization reaction is violent at 220 °C and the oxidation reaction mainly occurs at 240 °C.

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

High-performance polyacrylonitrile-based pre-oxidized fibers fabricated through strategy with chemical pretreatment, layer-by-layer assembly, and stabilization techniques

Novel high-performance polyacrylonitrile (PAN)-based pre-oxidized fibers (i.e. OPFHA-MEA-L) with improved thermal stability and flame-retardant and mechanical properties were designed and made from the pristine PAN fibers through chemical pretreatment with hydroxylamine hydrochloride (HA) and monoethanolamine (MEA) aqueous solutions, then coated with chitosan (CS) and sodium tripolyphosphate (STPP) via layer-by-layer (LbL) assembly, and finally followed by stabilization in the air. The morphological structure, flammability, and thermal and mechanical properties of fabricated OPFs were systemically investigated. The results indicated that the PAN fibers after chemical pretreatment with HA and MEA had a large amount of hydrophilic groups. It would facilitate the increase of pre-oxidation degree for PAN fibers during stabilization and the deposition of positively and negatively charged CS-STPP flame-retardant coating. The fabricated OPFs (i.e. OPFHA-MEA-10) demonstrated superior comprehensive properties with charred residue of about 68.2%, breaking strength of about 295.1 N, breaking elongation of 12.6%, and limiting oxygen index value of about 41.5%, respectively, contributing to the improved thermal stability and flame-retardant and mechanical properties. It is envisioned that this innovative type of high-performance OPFs could be utilized for potential applications as flame retardant and in high temperature filtration.

Effects of chemical pre-treatment on structure and property of polyacrylonitrile based pre-oxidized fibers

Polyacrylonitrile-based pre-oxidized fibers with improved thermal stability, flame retardant, and mechanical properties were made from the pristine polyacrylonitrile fibers through chemical pretreatment followed by pre-oxidation in air. The morphological structure of the polyacrylonitrile-based pre-oxidized fibers was investigated by Fourier transfer infrared spectra, X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectrometer. The changes of characteristic functional groups and chemical compositions confirmed the successful modification of the polyacrylonitrile fibers during pre-treatment. The grooves and cracks on the surface of polyacrylonitrile-based pre-oxidized fibers were remarkably decreased in comparison with that of pristine polyacrylonitrile fibers. The evolution of crystalline structure of the polyacrylonitrile fibers proved the occurrence of cyclization reactions during pre-oxidation. Meanwhile, thermal stability, flame retardant, and mechanical properties of polyacrylonitrile-based pre-oxidized fibers were also investigated by thermogravimetric analyzer, oxygen index meter, micro combustion calorimeter, and single fiber tensile tester, respectively. The results demonstrated that the polyacrylonitrile-based pre-oxidized fibers initially pre-treated by hydroxylamine hydrochloride, followed by monoethanolamine, had a high limiting oxygen index of 40.1 and breaking strength of 2.03 cN/dtex. The peak of heat release rate and total heat release of polyacrylonitrile-based pre-oxidized fibers decreased significantly while its charred residues increased, contributing to the improved flame retardant property.

Evaluation of composite PAN fibers incorporated with carbon nanotubes and titania and their performance during the microwave-induced pre-oxidation

The composite PAN fibers which incorporated with CNTs and Titania were prepared by mean of wet spinning. These fibers were then pre-oxidized with microwave heating in an air atmosphere. A combination of characterizations was carried out to study the impact of nanoparticles fillers on the properties of as-spun fibers and their performance during the microwave pre-oxidation. The addition of an equal amount of fillers made obvious changes in the chemical and crystalline structure, consequently improves the strength, and this could lower the capability to creep over a wide range of temperatures in the subsequent processes. FTIR and NMR analyses results of the pre-oxidized fibers exhibited clear changes in the PAN structure, where the dehydrogenation reaction and the degree of cyclization were investigated. Additional confirmation of the occurrence of cyclization reaction was achieved by XRD and thermal analysis. According to the TGA results, the pre-oxidized CNT1/Ti-PAN fibers exhibit greater thermal stability suggesting high carbon content and good quality could result in the dependent carbon fibers.

A new perspective on the internal structure of polyacrylonitrile-based preoxidized fibers through ultrathin sections

The dry-jet wet spinning polyacrylonitrile fibers (PAN fibers) were preoxidized in six gradient-elevated temperature zones and the internal structure changed greatly. Ultrathin sections for electron microscopy were used to study the internal structure of the fibers, and the fibril structure in the fibers was first observed by ultrasonic etching under a high-resolution transmission microscope. The fibers were ultrasonically etched in dimethyl sulfoxide (DMSO) solution to separate the fibril structure. The fibril structure is parallel to the fiber axis and the lamellar-plate structure is arranged perpendicular to the fiber axis.

Single and hybrid electromagnetic absorbing coatings on polyacrylonitrile precursor to motivate the microwave pre-oxidation

Nano silicon carbide was used separately and as an auxiliary absorber for the modification of the PAN precursor fibers to upgrade their microwave absorbability in order to undergo the subsequent pre-oxidization by using microwave. Surface coatings were successfully applied to the PAN precursor and the coated fibers were pre-oxidized over the microwave furnace for 60 min. To study the effect of the coatings on the pre-oxidized fibers, a series of characterizations were carried out. FTIR analysis results showed clear changes in the basic chemical structure of pre-oxidized fibers, which approved the occurrence of the cyclization and crosslinking reactions. Furthermore, the cyclization was confirmed by the thermal analysis results. According to the listed oxygen values, it can be suggested that the pre-oxidized fibers can resist the high temperatures in the following stage (carbonization). In addition, the crystalline analysis revealed a reduction in crystallinity, while the mechanical analysis demonstrated a reduction in tensile strength and elongation at break of pre-oxidized fibers.

Microwave treatment of pre-oxidized fibers for improving their structure and mechanical properties

The influence of microwave heating up to 1000 °C on the structure and mechanical properties of pre-oxidized fibers (POFs) was investigated. Heating curves, changes in length, and shrinkage rates (SRs) were studied. The microstructures were analysed by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and Raman spectroscopy. The mechanical properties were investigated by analysing the fineness, breaking strength, elongation at break, and initial modulus. Results show that as the temperature increased, the trend of the heating curve, change in length, and SR (up to 23%) tended to be flat, and the degree of carbonization increased. Microwave could effectively reduce the surface defects of the carbonized fiber. RI (ID/IG) at 1000 °C was 1.23. Fineness was reduced from 0.66 to 0.32. Breaking strength decreased from 7.94 dtex to 4.32 dtex. Elongation at break decreased from 2.17% to 0.99%. Initial modulus increased from 304.91 dtex to 654.72 dtex. These characteristics endowed the fibers with enhanced mechanical property.

Dynamic oxidation and protection of the PAN pre-oxidized fiber C/C composites

Pre-oxidized fibers as reinforcement are candidates for reducing the overall cost of C/C composites with superior properties. This study investigated the dynamic oxidation and protection of the pre-oxidized fiber C/C composites (Pr-Ox-C-C). According to the Arrhenius equation, the oxidation kinetics of the Pr-Ox-C-C consisted of two different oxidation mechanism with the transition point was at about 700°C. Scanning electron microscopy investigation showed that oxidation initiated from the fiber/matrix interface of composites, whereas the matrix carbon was easily oxidized. To improve the anti-oxidant properties of Pr-Ox-C-C, a ceramic powder-modified organic silicone resin/ZrB2-SiC coating was prepared by the slurry method. The coated samples were subjected to isothermal oxidation for 320h at 700°C, 800°C, 900°C, 1000°C and 1100°C with incurred weight losses of − 1.6%, 0.77%, − 1.28%, 0.68% and 1.19%, respectively. After 110 cycles of thermal shock between 1100°C and room temperature, a weight loss of 1.30% was obtained. The Arrhenius curve presented four different phases and mechanisms for coating oxidation kinetics. The excellent oxidation resistance properties of the prepared coating could be attributed to the inner layer which was able to form B2O3-Cr2O3-SiO2 glass to cure cracks, and the ZrB2-SiC outer layer that could provide protective oxides to reduce oxygen infiltration and to seal bubbles.

Effect of carbonization temperature on microwave absorbing properties of polyacrylonitrile-based carbon fibers

ABSTRACTThe pre-oxidized fibers were carbonized at the temperature ranging from 400 to 1300 °C for 1 h. The microwave absorption properties of carbon fibers (CFs) were examined in the frequency range of 2–18 GHz. It is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. At a thickness of 2 mm, the CFs obtained at 710 °C exhibit the best microwave absorbing ability with a maximum reflection loss of −22.9 dB at 15 GHz, and a bandwidth exceeding −10 dB in the range 12.4–18 GHz. Results indicate that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of CFs. Low temperature makes ϵ′ and ϵ″ too small to consume the energy of microwave, while over high temperature makes ϵ′ and ϵ″ too large to transmit the microwave into the CFs.

Comparison of microwave and conventional heating methods in carbonization of polyacrylonitrile-based stabilized fibers at different temperature measured by an in-situ process temperature control ring

The pre-oxidized fibers were carbonized by microwave heating and conventional heating, respectively. The temperature in the two processes was calibrated by a process temperature control ring that was wrapped by the fibers. The microwave carbonized fibers (MCFs) fabricated at 750–1000 °C possess the higher carbon content, larger average crystallite dimensions (La and Lc), higher porosity, and more homogeneous structure along cross-section than those of conventionally carbonized fibers (CCFs) obtained at the same temperature. However, when the temperature reaches 1000 °C and above, the crystallite sizes of MCFs are smaller than those of CCFs fabricated at the same temperature, and the difference between the skin and core structure becomes larger as the temperature increases for MCFs. The tensile strength and modulus of MCFs are higher than those of CCFs obtained at 750–1000 °C, while the mechanical properties for MCFs and CCFs obtained at temperature above 1000 °C is close. This phenomenon could be attributed to the temperature dependence of microwave adsorption ability of the as-obtained CFs.

Analysis and characterization of orientation structure of pre-oxidized PAN fibers in high magnetic fields

PAN fibers pre-oxidized at 240 °C, 260 °C, 280 °C, 300 °C were treated in 8 T, 12 T, 16 T high magnetic fields, respectively. The experimental result implied that there were two kinds of magnetic units cyan and carbon-nitrogen heterocycle in pre-oxidized PAN fibers, and the movement of magnetic units resulted in changes of orientation structure of pre-oxidized PAN fibers in high magnetic fields. Overall orientation increased with increase of magnetic field intensity and extension of processing time, whereas change of crystalline orientation depended on magnetic field intensity and content of carbon-nitrogen heterocycle due to the competition of two magnetic units. Furthermore, magnetic fields induced the conversion from amorphous region to crystal region and improved crystalline properties of pre-oxidized PAN fibers.

2D SAXS/WAXD analysis of pan carbon fiber microstructure in organic/inorganic transformation

Structure of PAN fibers during pre-oxidation and carbonization was studied using two dimensional small angle X-ray scattering/wide angle X-ray diffraction (2D SAXS/WAXD). The SAXS results show that during pre-oxidation between 180 °C and 275 °C, the volume content of microvoids increases with the temperature increasing, which may be one of reasons for the decrease of tensile strength of pre-oxidized fibers. 253 °C was the critical transition temperature, the length, diameter, aspect ratio and orientation distribution of microvoids increased with temperature before this temperature and decreased after this temperature. After the high temperature carbonization, lots of spindly microvoids formed. WAXD patterns demonstrate that the crystallite size of PAN fibers first increased before 230 °C and then decreased with the increase of temperature during the pre-oxidation. The diffraction peak of PAN fibers at 2θ ≈ 17° almost disappeared at the end of preoxidation while the diffraction peak of aromatic structure at 2θ ≈ 25° appeared at 253 °C. During carbonization, the peak intensity at 2θ ≈ 25° increased apparently due to the formation of graphite structure. The results obtained give a deep understanding of the microstructure development in the PAN fibers during pre-oxidation and carbonization, which is important for the preparation of high performance carbon fibers.