Min-Kang Seo

Surface characteristics of fluorine-modified PAN-based carbon fibers

Different fluorination methods were applied to modify the surface properties of carbon fibers. The relationship between the degree of fluorination and the physicochemical properties of carbon fibers was studied using a combination of mechanical tests, elemental analysis (EA), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). EA and XPS analyses of fluorinated carbon fibers showed that treatment with mixtures of F2/O2 introduced a much higher fluorine concentration than that with F2 only. However, XRD analysis showed that there was no increase in the interlayer distance, due to the mild fluorination condition applied. Consequently, the oxyfluorination was one of the more effective methods to increase surface polarity of carbon fibers, which probably played an important role in improving the tensile properties of the fibers in the epoxy resin system.

The effects of MoSi2 on the oxidation behavior of carbon/carbon composites

Abstract PAN-based carbon fibers impregnated with phenolic resin used as a precursor of carbonized matrix were modified by addition of molybdenum disilicide (MoSi2) in different concentrations i.e., 0, 4, 12 and 20% by weight for improving anti-oxidation properties of the composites. In this work, the oxidation behavior of carbon–carbon composites made with MoSi2 as an oxidation inhibitor in flowing air, has been studied in the temperature range of 600–1000°C. The results showed that the oxidation behavior of the composites made without MoSi2 differs from those of the composites made with MoSi2 and involves a degradation mechanism of its own, resulting in the highest oxidation rate during the initial phase of the oxidation. And the composites made with MoSi2 led to a significantly improved oxidation resistance owing to both the reduction of the crack in composites and the formation of a mobile diffusion barrier for oxygen when compared to those made without MoSi2. Hence, carbon active sites were blocked and oxidation of carbon was limited. This is probably due to the effect of the inherent MoSi2 properties, resulted from a formation of the protective layer against oxygen attack, in the above of 800°C.

Thermal conductivity and mechanical properties of various cross-section types carbon fiber-reinforced composites

In this work, to study the characteristics of carbon fiber-reinforced composites with different fiber cross-section types, such as round, C, and hollow-shape, the thermal conductivity and mechanical properties were investigated and compared. The thermal conductivity was measured by means of steady-state method to the parallel and perpendicular direction of reinforcing fibers. The mechanical properties were evaluated by a variety of test methods i.e., flexural, interlaminar shear strength, and impact strength. As a result, it was found that the thermal conductivity was greatly depended on the cross-section type of the reinforcing fibers, as well as, the reinforcing orientation. Especially, the anisotropy factor (k///k⊥) and the thermal diffusivity factor (α///α⊥) of C and hollow-type carbon fiber-reinforced composites showed about two times higher values than those of round-type one. Also, the mechanical results showed that C and hollow-type carbon fibers-reinforced composites had higher values than those of round-type one in all mechanical tested. These results were probably due to the basic properties of non-circular (C and hollow-type) carbon fiber which can improve interfacial binding forces and widen interfacial contact area between reinforcement and matrix, resulting in effectively transferring the applied stress.

Studies on mechanical interfacial properties of oxy-fluorinated carbon fibers-reinforced composites

Abstract In this work, the effect of oxy-fluorination on physicochemical properties of polyacrylonitrile (PAN)-based carbon fibers has been investigated. The chemical composition of the oxy-fluorinated carbon fibers is determined by X-ray photoelectron spectroscopy (XPS) measurement. Mechanical interfacial properties, such as interlaminar shear strength (ILSS), fracture toughness ( K IC ), work of fracture ( W f ) and fracture energy ( G IC ) of the composites are also studied in terms of oxy-fluorination conditions. From the surface analysis, it is found that oxy-fluorination led to an introducing of the fluorine and oxygen functional groups on carbon fiber surfaces, which are more efficient and reactive to undergo an interfacial reaction to matrix materials. Moreover, the formation of CF x physical bonding of the carbon fibers with fluorine increases the surface polarity of the fibers, resulting in increased ILSS, K IC , W f and G IC of the composites, due to the improvement of interfacial adhesion between fibers and matrix resins.

Surface characteristics of carbon fibers modified by direct oxyfluorination.

The effect of oxyfluorinated conditions on the surface characteristics of carbon fibers was investigated. Infrared (IR) spectroscopy results indicated that the oxyfluorinated carbon fibers showed carboxyl/ester groups (CO) at 1632 cm(-1) and hydroxyl groups (OH) at 3450 cm(-1) and had a higher OH peak intensity than that of the fluorinated ones. X-ray photoelectron spectroscopy (XPS) results for the fibers also showed that oxyfluorination introduced a much higher oxygen concentration onto the fiber surfaces than fluorination with F(2) only. Additionally, contact-angle results showed that the surface was better wetted by following oxyfluorination and that the polarity of the surface was increased by increasing the oxyfluorination temperature.

Isothermal cure kinetics of epoxy/phenol‐novolac resin blend system initiated by cationic latent thermal catalyst

The investigation of the cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)/phenol-novolac blend system with different phenolic contents initiated by a cationic latent thermal catalyst [N-benzylpyrazinium hexafluoroantimonate (BPH)] was performed by means of the analysis of isothermal experiments using a differential scanning calorimetry (DSC). Latent properties were investigated by measuring the conversion as a function of curing temperature using a dynamic DSC method. The results indicated that the BPH in this system for cure is a significant thermal latent initiator and has good latent thermal properties. The cure reaction of the blend system using BPH as a curing agent was strongly dependent on the cure temperature and proceeded through an autocatalytic kinetic mechanism that was accelerated by the hydroxyl group produced through the reaction between DGEBA and BPH. At a specific conversion region, once vitrification took place, the cure reaction of the epoxy/phenol-novolac/BPH blend system was controlled by a diffusion-control cure reaction rather than by an autocatalytic reaction. The kinetic constants k1 and k2 and the cure activation energies E1 and E2 obtained by the Arrhenius temperature dependence equation of the epoxy/phenol-novolac/BPH blend system were mainly discussed as increasing the content of the phenol-novolac resin to the epoxy neat resin.

Effect of fiber shapes on physical characteristics of non-circular carbon fibers-reinforced composites

Round, hollow, and C-shape carbon fibers (CF)-reinforced composites were investigated to study the effect of carbon fiber shapes on physical properties. The damping factor was measured by FFT-analyzer and the mechanical properties were evaluated using the Instron. Also, surface free energy analysis was executed to examine the relationship between the shape of fiber and wettability on composites. As a result, it was found that C-CF/epoxy (EP) composites showed the highest values in mechanical properties, while R-CF/EP composites the lowest values. This was due to the wider interfacial contact area between reinforcement and matrix that could effectively transfer the applied load. C-CF/EP composites had a greater damping factor than round and hollow-shape ones. This meant that C-shape carbon fiber played a more important role in damping of fiber-reinforced composites, owing to have wider contact area. Also, C-shape carbon fiber had the highest surface free energy than that of other carbon fibers. This wettability made it increase in mechanical properties in composites, due to the stronger interfacial binding force.

Studies on epoxy resins cured by cationic latent thermal catalysts: The effect of the catalysts on the thermal, rheological, and mechanical properties

To investigate the effect of catalysts on the thermal, rheological, and mechanical properties of an epoxy system, a resin based on diglycidyl ether of bisphenol-A (DGEBA) was cured by two cationic latent thermal catalysts, N-benzylpyrazinium hexafluoroantimonate (BPH) and N-benzylquinoxalinium hexafluoroantimonate (BQH). Differential scanning calorimetry was used for the thermal characterization of the epoxy systems. Near-infrared spectroscopy was employed to examine the cure reaction between the DGEBA and the latent thermal catalysts used. The rheological properties of the blend systems were investigated under an isothermal condition with a rheometer. To characterize the mechanical properties of the systems, flexure, fracture toughness (KIC), and impact tests were performed. The phase morphology was studied with scanning electron microscopy of the fractured surfaces of mechanical test samples. The conversion and cure activation energy of the DGEBA/BQH system were higher than those of the DGEBA/BPH system. The crosslinking activation energy showed a result similar to that obtained from the cure kinetics of the blend systems. The flexure strength, KIC, and impact properties of the DGEBA/BQH system were also superior to those of the DGEBA/BPH system. This was a result of the substituted benzene group of the BQH catalyst, which increased the crosslink density and structural stability of the epoxy system studied.

Relationship between surface characteristics and interlaminar shear strength of oxyfluorinated carbon fibers in a composite system

In this work, a direct oxyfluorination method was used to study the effect of oxygen content on surface and mechanical interfacial characteristics of oxyfluorinated carbon fibers in an epoxy matrix system. The changes of surface functional groups, chemical compositions, and structures of the carbon fibers were characterized by Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, and X-ray diffraction measurements. Also, the mechanical interfacial properties of the composites were evaluated by means of interlaminar shear strength tests. The results indicated that graphitic carbon was the major carbon functional component on the carbon fiber surfaces and other functional groups were also present, such as Cz.sbnd;O, Cz.dbnd;O, HOz.sbnd;Cz.dbnd;O, Cz.sbnd;F(x), after oxyfluorination of carbon fibers. No large changes of structure were found with the content of oxygen. Consequently, these introductions of oxygen functional groups onto the carbon fiber surfaces led to an improvement of the ILSS of the composites.

Facile Synthesis of Core/Shell-like NiCo2O4-Decorated MWCNTs and its Excellent Electrocatalytic Activity for Methanol Oxidation

The design and development of an economic and highly active non-precious electrocatalyst for methanol electrooxidation is challenging due to expensiveness of the precursors as well as processes and non-ecofriendliness. In this study, a facile preparation of core-shell-like NiCo2O4 decorated MWCNTs based on a dry synthesis technique was proposed. The synthesized NiCo2O4/MWCNTs were characterized by infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and selected area energy dispersive spectrum. The bimetal oxide nanoparticles with an average size of 6 ± 2 nm were homogeneously distributed onto the surface of the MWCNTs to form a core-shell-like nanostructure. The NiCo2O4/MWCNTs exhibited excellent electrocatalytic activity for the oxidation of methanol in an alkaline solution. The NiCo2O4/MWCNTs exhibited remarkably higher current density of 327 mA/cm2 and a lower onset potential of 0.128 V in 1.0 M KOH with as high as 5.0 M methanol. The impressive electrocatalytic activity of the NiCo2O4/MWCNTs is promising for development of direct methanol fuel cell based on non-Pt catalysts.