Yousi Chen

Surface structural evolvement in electrochemical oxidation and sizing and its effect on carbon fiber/epoxy composites properties

An evolvement of surface physicochemical structure in the process of electrochemical oxidation and sizing treatment was monitored by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. An effect of this evolvement on the properties of carbon fiber/epoxy composites was also researched. The results showed that the root mean square roughness increased from 4.6 nm for untreated fibers to 9.1 nm for surface-treated fibers, and the root mean square value of sized fibers decreased to 8.5 nm. The relative contents of oxygen and nitrogen atomic on carbon fiber surface increased obviously after electrochemical oxidation. Oxygen atomic concentration took a further improvement after sizing treatment and only hydroxyl functional group was found on its surface. The interfacial strength between carbon fiber and the resin matrix improved after surface electrochemical oxidation and sizing, and the mechanical interlock was considered to be the governing factor in fiber/resin adhesion of carbon fiber/epoxy composites.

The long life-span of a Li-metal anode enabled by a protective layer based on the pyrolyzed N-doped binder network

Attempts to utilize lithium metal in secondary batteries are seriously restricted by its uncontrollable side reactions with the electrolyte solvent. Here we utilize a protective porous structure based on the pyrolyzed PAN binder to stabilize the electrolyte/lithium interface to prolong its working life. With the increase of pyrolysis temperatures, the treated PAN fibers possess two mutational points in mechanical properties located at ∼300 & ∼700 °C, and exhibit carbon-like characteristics at ∼400 °C and higher. Compared to the control electrode, the cyclic life-span of the treated electrodes can increase 1.8 times at the first mutational point, and surprisingly rise to 12 & 7 times for the samples pyrolyzed at 400 & 500 °C, then fall back to 1.6 times at the second mutational point. These results reveal that the stable operation of lithium plating/stripping could be provided by the internal interwoven SEI layer grown on the carbon-like binder network with appropriate rigidity. Among the investigated systems, the protective structure treated at 400 °C can stably operate for 350 cycles with an average coulombic efficiency as high as ∼98%, which is the best efficiency recorded for carbonate-based electrolytes to date.

Effect of Jet Swell and Jet Stretch on the Structure of Wet-Spun Polyacrylonitrile Fiber

The jet swell effect in the wet spinning of polyacrylonitrile (PAN) fiber was studied by optical microscopy and the jet swell ratio was obtained through directly measuring the diameter of the freely extruded fibers. For reflecting the actual drawing situation of the fibers in the coagulation process, the jet stretches were then corrected from the apparent values to the true values, and their effect on the cross-sectional morphology, internal structure, and orientation of the wet-spun PAN fibers was studied by optical microscopy, scanning electron microscopy, and X-ray diffraction, respectively. The results showed that jet stretch plays an important role in eliminating the adverse effects caused by the jet swell effect and affects the fiber structure; PAN fibers of uniform denier, dense and homogenous structure, and high orientation can only be obtained at a suitable jet stretch.

Supramolecular structure of highly oriented wet-spun polyacrylonitrile fibers used in the preparation of high-performance carbon fibers

Highly oriented polyacrylonitrile (PAN) fibers, which are used in the preparation of high-performance carbon fibers, were prepared via a wet spinning process. The supramolecular structure—i.e., the degree of crystallinity, crystal size, and crystallite orientation—of the PAN fibers was characterized by X-ray diffraction (XRD) using fiber and powder techniques. Two equatorial diffraction peaks with 2θ ∼ 17° and 29.5° were observed in the fiber XRD patterns, which confirmed that the crystalline structure of the PAN fibers was pseudohexagonal with only two-dimensional order in the transverse direction. The powder XRD patterns were resolved into three constituent Lorentz peaks to determine the relative degree of crystallinity. In order to increase the reproducibility of the value for the crystallinity of the fibers, the Gupta–Singhal method was modified by assuming that the degree of crystallinity could be determined from the area under peak 1 (with 2θ ∼ 17°), rather than the combined area of peaks 1 and 3. The evolution of the supramolecular structure of the PAN fibers during the spinning process was also investigated. Results indicated that drying and steam stretching play important roles in the formation and growth of the crystalline structure of the PAN fibers, while the orientation of the structure was only strongly related to the degree of stretching. The effects of the supramolecular structure on the thermal properties and mechanical properties of the PAN fibers were also studied. The perfection of the supramolecular structure influenced the feasibility of cyclization reactions within the PAN fibers and the thermal decomposition of those fibers. The mechanical properties of the PAN fibers were significantly enhanced by increasing the perfection of the supramolecular structure.

Fabrication of partially biobased carbon fibers from novel lignosulfonate-acrylonitrile copolymers

Lignin, as an abundant carbon-rich renewable resource, is a promising precursor for carbon fibers. However, due to the poor spinnability of lignin, a great challenge comes from the spinning of precursor fibers. In this work, a series of lignosulfonate–acrylonitrile (LS–AN) copolymers with different LS contents were prepared by a two-step process consisting of esterification and free radical copolymerization. In this strategy, lignin was used as a macromer and chemically bonded to acrylonitrile segmer, resulting in significant improvement of the spinnability. Continuous long precursor fibers with a dense structure were successfully prepared from these copolymers by a wet spinning technique and then converted into carbon fibers by thermal stabilization and carbonization. The LS–AN copolymers were characterized by FTIR, GPC, and rheological method. The results confirmed the macromolecular characteristic of the LS–AN copolymers. A hanging lantern model was proposed to describe the molecular structure of the LS–AN copolymers. Effect of the LS–AN copolymers on the microstructure and mechanical properties of carbon fibers was investigated by SEM, single fiber tensile testing, XRD, and HRTEM. The results demonstrated the feasibility of developing partially biobased carbon fibers from the novel LS–AN copolymers.

Quantitative structure–property relationships of polyacrylonitrile-based graphite fibers revealed by laser confocal Raman spectroscopy

ABSTRACT A series of polyacrylonitrile-based graphite fibers with different tensile modulus and electrical resistivity were characterized by laser confocal Raman spectroscopy. The Raman spectra of the graphite fibers were deconvolved into five constituent bands using Lorentzian peak fitting function. The Raman spectra parameters, including band position, full width at half maximum, and integrated band area, were extracted and correlated with the tensile modulus and electrical resistivity. Most of the Raman parameters changed monotonically with the tensile modulus and electrical resistivity. A good linear relationship between the electrical resistivity and the structural order was found.

A multiscale hydrothermal carbon layer modified carbon fiber for composite fabrication

A novel multiscale hydrothermal carbon layer (MHTCL) for carbon fiber (CF) surface modification was developed. The MHTCL is a multiscale high-disorder amorphous carbon coating with a colored appearance, abundant functional groups, multiscale roughness, a large specific surface area, a high surface energy, and good wetting ability. The O/C atom ratios of the MHTCL-modified CF were in the range of 0.17–0.23, and the functional groups were mainly C–O and CO groups. During the low-concentration glucose hydrothermal treatment with the carbon fibers (CFs), the glucose generates furan derivative intermediates, which adsorb on the surface of the CFs and carbonize continuously, finally forming the MHTCL on the CFs. The fracture and rupture of the MHTCL during the forming process produce new nucleation centers on the CF surface, which result in abundant multiscale irregular particles. The MHTCL is a facile method for the modification of CFs. The fabrication of the CF composites demonstrated that the MHTCL obviously increases the interlaminar shear strength of the CF/polyimide composite and the interfacial interaction of the CF and polyetheretherketone.

Preparation of low-cost carbon fiber precursors from blends of wheat straw lignin and commercial textile-grade polyacrylonitrile (PAN)

Abstract Low-cost precursor fibers (PFs) were prepared from blends of a wheat straw lignin (WSL) and a commercial textile-grade polyacrylonitrile (PAN) by wet spinning, and then the precursors were converted into carbon fibers (CFs) by thermal stabilization and carbonization. The PFs were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The lignin content in the blends was found to play an important role in the PF structure, which was closely related to the change of intermolecular forces in the spinning solution. The lignin, acting as the carboxylic acid comonomer, had significantly promoted the thermal stabilization of the lignin/PAN blend PFs, which helped to further decrease the production cost of CFs. With increasing lignin contents, the carbon content of CFs remained at about 95%. The carbon of lignin could be utilized for the preparation of CFs.