Mesophase Carbon Fibers Research Articles

Impedance measurements of low temperature mesophase carbon fibers during lithium insertion/extraction

Surface modifications with three different procedures were performed on an integrated felt of mesophase carbon fiber (MCF) fired at 600 °C. The surface modification procedures were (1) heating in acetylene black powder under a low pressure of oxygen (MOT), (2) vacuum deposition of Ag film on the fiber surface and (3) vacuum deposition of Ag film on the fiber, followed by heating under low pressure of oxygen. The performance of Li insertion/extraction reaction was found improved by any of these modification treatments. The cause of the improvement was examined mainly with impedance spectroscopy analysis (EIS). EIS spectra revealed that both the film resistance and the charge-transfer resistance were decreased remarkably by these surface modifications. Among the three modification procedures, MOT revealed the best improving effect. By comparing the EIS data with the results obtained from the double layer capacity measurement, we concluded that the major cause of the increase in the reaction rate is the activation of the reaction site.

Structural Changes during Oxidation Process of Anisotopic Mesophase Carbon Fibers(II)-Surface Texture Observation by Scanning Electron Microscopy

Anisotropic mesophase carbon fiber(AMCFs) was exposed to isothermal oxidation in air and atmosphere, and burn-off rates have measured by TGA. The microstructure changes of oxidized carbon fibers, were observed by SEM. It was observed that oxidation rate in the air is over 100 times faster than that in atmosphere. The activation energy obtained in air was about 43.4 Kcal/mole in the temperature range of , and in was about 55.2 Kcal/mole in the temperature range of . Therefore, the oxidation reaction in both atmospheres was under chemical reaction regime in the above temperature ranges. It was shown that the oxidation of the AMCFs is initiated at the end of fibers at high temperature() with developing the large pores, and the small pores are developed on the fiber surface at low temperature(). In conclusion, the oxidation of the AMCFs is progressed through the imperfection.

Structural Changes during Oxidation Process of Anisotopic Mesophase Carbon fibers(I) - TEM and XRD Study

Structural changes during oxidation of anisotopic mesophase carbon fiber(AMCFs) have been observed with TEM and XRD. It was shown that the AMCFs are three dimensionally stacked structure of turbostratic layers, and are non-homogeneous structure which consist of the highly ordered areas and the random areas. The <TEX>$d_{ 002}$</TEX>, Lc, and La of AMCFs which were measured by XRD were 3.44, 146, and <TEX>$135\AA$</TEX>, respectively. It was observed that the oxidation initiated at the random areas, because the <TEX>$d_{002}$</TEX> / decreased to the value of 3.41 <TEX>$\AA$</TEX> during initial oxidation stage. It was also observed that the La of the oxidized AMCFs increased up to 182 <TEX>$\AA$</TEX> during the whole oxidation process, and the <TEX>$d_{002}$</TEX> of that increases up to 9.44 <TEX>$\AA$</TEX> when the burn-off is over the degree of 20%. Therefore, it was suggested that the micro-crystalline grew up by heat treatment effects during the fibers were oxidized, In addition, it was shown that there was difference in the measured value of La by XRD and TEM, in case of 39% oxidized fibers for example, the measured La was <TEX>$ 165\AA$</TEX> by XRD and in the range of 180∼220 <TEX>$\AA$</TEX> by TEM.

Computational modeling of ring textures in mesophase carbon fibers

Carbon fibers are widely used in many industrial applications due the fact of their excellent properties. Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow cascade consisting of pressure driven flow-converging die flow-free surface extensional spinline flow that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystalsand computer simulations of structure formation for pressure-driven flow of carbonaceous liquid crystalline precursors used in the industrial carbon fiber spinning process. The simulations results capture the formation of characteristic fiber macro-textures and provide new knowledge on the role of viscous and elastic effects in the spinning process.

Influence of surface treatment on the Li doping/undoping reaction at a mesophase low temperature carbon fiber

In an attempt to improve the Li insertion/extraction rates of a low-temperature disordered carbon having a very high Li loading capacity, several surface modification methods were examined for a mesophase carbon fiber prepared at 800 °C. The electrochemical reaction rates were evaluated by cyclic voltammetry and potential step chronoamperometry (PSCA) in an electrolyte of propylene carbonate (PC) containing 1 M LiClO 4 at an ambient temperature in a glove box filled with dried argon. In contrast to a well-graphitized carbon fiber, the vacuum deposition of a silver film on the 800 °C fiber surface revealed no appreciable rate-enhancing effect except at the lowest potential limit. Successive heating of the silver-deposited fiber in low-pressure oxygen, however, resulted in a remarkable rate enhancement. Using a single carbon fiber (10 μm in diameter), the chemical diffusion coefficient ( D chem) was evaluated by PSCA. The D chem value was found to be increased up to one order of magnitude after the treatment, by which we could show that the low-temperature carbon has a capability to be put to practical use for the next generation of Li-ion batteries having enhanced capacity and power.

Lithium Storage Mechanism of Disordered Mesophase Carbon Fibers Studied by [sup 7]Li-Nuclear Magnetic Resonance

To clarify the lithium storage states in disordered carbon materials for Li-ion secondary batteries 7 Li-nuclear magnetic resonance (NMR) spectra and cyclic voltammograms (CVs) of mesophase carbon fibers were measured. The CV obtained with mesophase carbon fibers fired at 800°C showed two peaks positioned in different potential regions. A sharp peak appearing at about 0.03-0.2 V vs. Li/Li + is attributed to lithium accommodation in the crystallites having a graphite structure and a broad one in the range of 0.2 to 1.2 V is attributed to the disorder site. In contrast, 7 Li-NMR spectra of the 800°C sample gave only a single peak, which shifted from 0 ppm at 0.8 V to 16 ppm at 30 mV depending on the charging level of lithium. Similar aspects of the spectra shift were also observed in a series of fully charged mesophase carbon fibers prepared at different firing temperatures. These results indicate that the rate of the exchange of lithium between the two different sites, the edge/surface site and intercalated site, is too fast to differentiate the different states. This suggests that there is a possibility of realizing a high capacity and high power anode for Li-ion secondary batteries based on a low-temperature mesophase carbon.

Theory and simulation of texture formation in mesophase carbon fibers

Carbonaceous mesophase precursors are spun into high-performance commercial carbon fibers using the standard melt spinning process. The spinning process produces a wide range of cross-sectional fiber textures whose origins are not currently well understood. The planar polar (PP) and planar radial (PR) textures are two frequently observed textures. This paper presents theory and simulations of the elasticity-driven formation process of the PP texture using the classical Landau–de Gennes mesoscopic theory for discotic liquid crystals, including defect nucleation, defect migration, and overall texture geometry. The main characteristic of the real PP texture is the presence of a pair of defects equidistant from the fiber axis. In this research it is analytically and numerically found that, under elastic isotropy, the ratio of the equilibrium defect–defect separation distance to the fiber diameter is always equal to 1/ 5 4 . The computed PP and PR textures phase diagram, given in terms of temperature and fiber radius, is used to establish the processing conditions and geometric factors that lead to the selection of these two textures.

Morphological effect on the electrochemical behavior of electric double-layer capacitors

Activated milled mesophase carbon fibers (AC-mMPCF, MP-series) show a higher specific capacitance in spite of a smaller specific surface area than those of powder-type activated carbons (AC-series). This phenomenon can be interpreted to mean that it is difficult to predict the capacitance of an electric double-layer capacitor (EDLC) knowing just the surface area and the pore size. More information is needed about other inherent characteristics of the samples, for example, the equivalent series resistance (ESR), shape of the pores, etc. We investigate here other characteristics of the samples. Consequently, it was deduced that the MP-series of EDLCs have a slit pore shape, which affects the accessibility of the electrolyte ion onto the electrode surface. Moreover, the MP-series of materials have suitable ESR values, and these material properties themselves should be considered as factors that affects the deterioration of the specific capacitance of EDLCs.

Theory and Simulation of Texture Formation in Mesophase Carbon Fibers

ABSTRACTCarbonaceous mesophases are spun into high performance carbon fibers using the melt spinning process. The spinning process produces a range of fiber textures whose origins are not well understood. Planar polar (PP) and planar radial (PR) textures are two ubiquitous ones. A model that describes the formation process of the PP texture based on the Landau-de Gennes mesoscopic theory for discotic liquid crystals, including defect nucleation, defect migration, and overall texture geometry, is presented, solved, and validated. The computed PP and PR textures phase diagram, given in terms of temperature and fiber radius, is presented to establish the processing conditions and geometric factors that lead to the selection of these textures. The influence of elastic anisotropy to the textures formation and structure is also characterized.

On the improvement of Li charge/discharge cyclability of carbon fibers by making a C/C composite with thermosetting resins

The charging capacity of the anode of Li-ion secondary batteries is based on the insertion of Li ions into the internal structure of carbonaceous materials. Low temperature mesophase carbon gives rise to an extraordinary capacity up to 900 mAh/g which is over two times that of graphitic carbon now utilized. However, the serious issue of its low cyclability prevents its actual application. In an attempt to improve the low cyclability of a mesophase carbon fiber, C/C composites have been prepared with an epoxy or a phenolic resin. The cyclability was found to be considerably improved by making the fiber felt into a C/C composite. The cyclability is dependent on how each individual fiber is bound together by the binder carbon, as well as how the electrical conductivity of the consisting material is increased. Epoxy resin created difficulty in forming a smooth binding but it still gave an improved cyclability as compared to the unbound fiber. Phenolic resin of resol type showed a very smooth binding, giving rise to a more stabilized cyclability. The importance of the binding of individual active material particles with an electroconductive material for obtaining a long cycle life has been shown.

Li-ion polymer battery based on phase-separated gel polymer electrolyte

The characteristics of the Li ion battery based on poly(vinyl chloride)/poly(methyl methacrylate) (PVC/PMMA) polymer blend electrolyte are reported. The polymer blend electrolytes exhibited phase-separated structure which consists of a PVC-rich phase and an electrolyte-rich phase. At the constant blend ratio of PVC/PMMA=6/4, mechanical property and ionic conductivity increase with the increase of the polarity of the liquid electrolyte together with the decrease of domain size of the electrolyte-rich phase. However, these merits obtained by using the liquid electrolyte of high polarity were somewhat contrasted by the low adhesion of the corresponding polymer electrolyte toward electrode. The prototype battery cell, which consists of LiCoO 2 cathode, meso-phase carbon fiber anode and the polymer electrolyte based on PVC/PMMA=6/4 blend and 1M LiPF 6 in EC/DMC=1/1 mixture, showed the discharge capacity of 111 mAh/g (per cathode material) at charge/discharge rate of 2 C and 92% of the initial discharge capacity at 100 cycle upon the repeated charge/discharge at 0.5 C rate (0.5×complete charge/discharge in 1 h).

Charge/discharge efficiency improvement by the incorporation of conductive carbons in the carbon anode of Li-ion batteries

Influence of the conductive additive loading on the anode performance of Li-ion battery was investigated for the coated anode where graphitized mesophase carbon fiber (MCF) was used as an active material, and loaded with natural graphite (NG), acetylene black (AB) or Ketjen black (KB). Incorporation of the conductive additive was quite effective to improve not only the cycle life, but also the rate of the electrochemical reaction. In addition, we found that the initial irreversible charging capacity (IICC) of MCF was suppressed to a great extent. The effectiveness was dependent on the kind of material loaded. On the other hand, making a homogeneous slurry for coating was found to be a key factor for the performance improvement. The most preferable choice of the preparation condition gave rise to a dramatic improvement. In contrast to these advantages, the carbon blacks showed a large IICC inherent to the materials which is an issue to be solved for actual use.

Elasticity-driven texture selection mechanism in mesophase carbon fibers.

In this Brief Report, we identify the parametric conditions and elasticity-driven mechanism that give rise to a characteristic texture commonly found during the fiber melt spinning of carbonaceous mesophase precursors; this mesophase consists of a biphasic isotropic-uniaxial discotic nematic liquid crystal mixture. The essential feature of the characteristic texture consists of a radial zig-zag molecular arrangement on a cross section of the mesophase carbon fiber. Using the Williams elastic-driven mechanism [D.R.M. Williams, Phys. Rev. E 50, 1686 (1994)] that gives rise to periodic director oscillations in cylindrical geometries, here adapted to discotic nematic fibers subjected to extensional flow, we show that when the bend ${\mathit{K}}_{3}$ constant becomes larger than the splay ${\mathit{K}}_{1}$ constant, the radial trajectories of the molecular planes become unstable in the presence of spatially periodic perturbations, leading to the experimentally observed radial zig-zag texture.

Membrane with Straight Microholes Prepared by Electrochemical Oxidation of Carbon Fiber-Epoxy Composites

An epoxy resin membrane in which carbon fibers are vertically arranged in array format was electrochemically oxidized to remove carbon fibers and to create microholes of diameter of the carbon fiber (4-7μm). Carbon fiber tips remained at one end of the holes. The microhole membrane possesses an acidic function at the surface of carbon fibers and hence is a charged membrane. The flow flux of water depended on the acidity of an aqueous solution and it increased when the solution was made an acidic.Etching rate of PAN-based carbon fibers was much faster than that of mesophase carbon fibers.

Tensile properties of intercalated pitch mesophase carbon fibers

The effects of intercalation with Br (residue) or CuCl 2 on the diameters d, microstructures, Young's moduli E, tensile strengths S and electrical conductivities of P-100 and P-120 fibers were determined using single filament methods. Circular cross section P-100, with an uncracked radial transverse texture, developed small axial cavities and longitudinal surface cracks on CuCl 2 intercalation, whereas the elliptical P-120 fibers with a “Pan Am” texture became more circular without evident damage on bromination. E decreases of 18% (P-120/Br) and 39% (P-100/CuCl 2) resulted from mean d increases of ≤ 11% and 28% respectively, which also contributed to a ∼ 38% decrease in mean S at 1 cm gage length in both cases. The gage length dependence of S increased for P-100 and decreased for P-120 due to changes in flaw distribution which depended on fiber and intercalant type.