Mesophase Carbon Research Articles

Structural aspects of lithiated carbon

In this work, X-ray diffraction (XRD), coulometry titration, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) were used to study the structure of lithium-intercalated pyrolytic poly (furfural alcohol) (PFA) and mesophase carbons formed at 1000 °C (MCMB1000) and 2800 °C (MCMB2800), respectively. The effect of phosphorus doping on the intercalation capacity was identified through a comparison between PFA and MCMB 1000. It was further verified by a separate experiment, in which MCMB 1000 was treated with phosphoric acid followed by heat treatment at 1000 °C. The maximum lithium intercalation concentration x ( x in Li x C 6) increased sharply with the introduction of phosphorus, whereas it was not reflected in XRD. Although there were two turning points in the open circuit voltage E-x curve of PFA, no staging phenomenon could be observed. A simple model based on mean field theory was used to investigate the effect of the attractive interaction between intercalates on the formation of lithium ordering in the structure of the intercalation compound. It was inferred from the XPS spectra that PFA showed a better ability to accept electrons than MCMB1000, although they have similar crystalline structures. The XPS spectra of Li-PFA also indicated partial covalent property of the Li-PFA interaction, which was further confirmed by ESR data. The influence of crystalline and electronic structures of carbon on lithium intercalation capacity was demonstrated.

Pattern selection mechanism in mesophase carbon fibres

Carbonaceous mesophases are spun into high performance carbon fibres using the melt spinning process. The spinning process produces a wide range of different fibre textures whose origins are not well understood. The cross section fibre texture is a strong function of temperature, such that at lower temperatures the selected pattern is known as radial, while at higher temperatures it is known as onion. More intriguing and unexpected is the random pattern that is found in the intermediate temperature range. This paper uses theory and simulation to reproduce the fibre textures, and to identify the selection mechanisms that explain the origin of the observed patterns.

Improved Carbon Anode Materials for Lithium-Ion Cells

ABSTRACTSeveral carbon materials have been studied for suitability as anode materials in lithium-ion cells. Carbons that have been included in this evaluation are three grades of commercially available mesophase carbon microbeads (MCMB) 6–28, 10–28 and 25–28, two specially prepared mesophase fibers (Amoco), a foreign mesophase fiber and KS-15 graphite (Lonza). Differences in cycling behavior between the three types of MCMB material are shown. Data of full lithium-ion cells demonstrate the effect that the choice of carbon material has on the cell discharge voltage and capacity. Lithium reference electrode experiments in full cells (3.0–4.0Ah capacity), elucidate the dynamics under several charge/discharge regimes and provide a comparison between the performance of carbon fiber and graphite anode materials. These test results indicate that the fibers can be charged at significantly higher rates than graphite without showing polarization at the anode. Full and half cell data also demonstrates the high coulombic efficiencies of the mesophase materials and first cycle efficiencies as compared to graphite. A comparison of two mesophase materials with different textures in full cells under strenuous cycling conditions shows significant differences in capacity retention. SEM photos of fibers showing the different textures are also presented.

Diffusion of lithium in carbon

To estimate the solid state diffusion coefficient of lithium in lithium intercalated carbon electrodes, a new model based on the GITT (galvanostatic intermittent titration technique) method was developed after a careful consideration of the models and methods used in the literature. The concept of CPE (constant phase element) which is often used in the interpretation of ac impedance spectroscopy was introduced to account for the roughness of the electrode, leading to a model closer to the powder pellet electrode often used in lithium batteries. Diffusion coefficient of lithium in three kinds of carbon, i.e. pyrolytic poly(furfural alcohol) (PFA), coke and MCMB (mesophase carbon, 1000 °C) were calculated by fitting the voltage response due to a constant low current perturbation with the new proposed model. The results were compared with those obtained according to an old model, which involved an ideal planar electrode. Introduction of the CPE element led to an increase of two orders of magnitude in the D values. The D ~ x ( x in Li x C 6) and diffusion activation energy E a ~ x relationships were also obtained and were interpreted in terms of the structures of the carbons.

96/06435 Mesophase and pyrolytic carbon formation in aircraft fuel lines

96/06435 Mesophase and pyrolytic carbon formation in aircraft fuel lines

Bifurcational analysis of the isotropic-discotic nematic phase transition in the presence of extensional flow

Abstract A bifurcational analysis is performed on a version of Doi's equation of nematodynamics that describes the non-equilibrium isotropic-discotic nematic phase transition in the presence of steady uniaxial extensional flow. The disc-like molecular geometry and the degenerate extensional flow-induced orientation are shown to be the source of a complex bifurcation and multistability behaviour involving two physically equivalent biaxial nematic phases, one uniaxial nematic phase and one uniaxial paranematic phase. Depending on the temperature and the extension rate, the isotropic-discotic nematic transition, involving the two biaxial nematic phases and the uniaxial paranematic phase, may be continuous (2nd order), discontinuous (1st order), or it may exhibit a tricritical non-equilibrium phase transition point. A validation procedure on the validity of the predictions is implemented. The predictions presented here find practical applications in the industrial spinning of mesophase carbon fibres, and also...

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.

Carbon electrode materials for lithium-ion batteries

Five kinds of carbonaceous materials: graphite, carbon black, coke, petroleum coke, mesophase carbon microbeads in terms of the cell carbon|EC-DEC-LiPF 6|Li, were examined by X-ray diffractometry and impedance spectrum.

Small Angle X-Ray Scattering Study of the Structure of Mesophase Carbon Powders

Small Angle X-Ray Scattering Study of the Structure of Mesophase Carbon Powders

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.

Superhigh surface area determination of microporous solids

A new method is proposed for the measurement of specific surface area of microporous carbons of superhigh BET surface area. The method (designated the subtracting pore effects (SPE) method) is based on the analysis of two upward swings from linearity, due to the pore effects, in the high resolution α s plot. The measurement of slope of the linear α s plot after subtraction of the swings gives a definite surface area, even in a microporous system. The surface area of activated mesophase carbon microbeads with a BET surface area of 3000–4000 m 2 g −1 was determined by N 2 adsorption using the SPE method. The BET surface area obtained in the usual manner was an overestimate by about 40%. The overestimation was not attributed to enhanced micropore filling but to capillary condensation.

Formulation of a Mathematical Process Model for the Foaming of a Mesophase Carbon Precursor

ABSTRACTMathematical equations have been formulated to guide an experimental effort to produce an open-celled mesophase pitch foam. The formulation provides an analytical description of homogeneous bubble nucleation and growth, diffusion of the blowing gas through the liquid to the bubble surface, and the average material thickness between bubbles. Implications of the formulation for the experimental production of mesophase pitch foam are discussed.

Active surface area of microporous carbons

Three different types of carbon precursor—mesophase microbeads, preoxidized polyacrylonitrile (PAN) fibres, and phenolic fibres—have been carbonized in argon at 900°C and thereafter activated with carbon dioxide at the same temperature. Physical adsorption of nitrogen at −196°C and of carbon dioxide at 0°C has been carried out on the activated samples. Active surface area of carbons was determined using oxygen chemisorption at 300°C, followed by thermal programmed desorption of oxygen complexes. Micropore volume and mean pore width have been determined using the Dubinin-Radushkevich (DR) equation and the α s method. A relationship between V DR and active surface area ( ASA) is found during activation of mesophase carbon and PAN carbon fibres that are not microporous before activation. The ratio ASA to the micropore surface area remains almost constant during activation and has nearly the same value for all three samples. Changes in active surface area and in micropore volume are discussed in terms of microtexture development in carbons during gasification. The results emphasize the importance of the carbon precursor on the development of microporosity during activation.

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.

Carbonaceous mesophase: History and prospects

This review summarizes the historical development of mesophase, the types of mesophase that can be produced and their structures and properties. Some current and potential uses of mesophase carbons are mentioned, particularly for carbon fiber production and as fillers in liquid chromatography.

Fluorine intercalated graphites and battery applications

A battery based on the fluorine-lithium system gives the highest theoretical energy density for both equivalent weight and volume because of the maximum electronegativity diference between the cell reactant. This possibility has been realized by the use of graphite fluoride, (C xF) n as a cathode material for lithium battery in nonaqueous electrolyte systems. Detailed investigations on electrochemical characterisitics and discharge reaction mechanism of the batteries have shown that structural properties of a host material graphite has important influences on fluorination reaction and physical properties of resulting compounds, particularly electrochemical behaviours. Accordingly, novel fluorine intercalated graphites, (C xF) n have been prepared by the reaction of elemental fluorine with recently developed new forms of carbons; vapour grown carbon fibers from benzene-derived precursor materials, activated carbon fibers derived from rayon and mesophase carbons. An attractive feature of these as a host material is that they can easily take up fluorine at a fairly low temperature range of 20–380°C within very short reaction times. The nature of CF bonds vary from chemisorption through charge transfer to covalent bond with rising reaction temperature, which is reflected in the strong colors from black through brown, yellow and white. Electrochemical characteristics of the cells based on (C xF) n/Li in a nonaqueous electrolyte are associated with stoichiometry, structure and nature of CF bond. Fluorine atoms fixed in carbon matrix at lower temperatures have higher activities and OCV's are a function of the formation temperature; decrease with increasing temperature from ca. 3.8 to 3.2 V. The discharge characteristics strongly depend on structural features of (C xF) n; the size and ordering of crstallites, and the amount of defects play important role for potential and stability of discharge, and cathode utility. A certain three dimensional structure of (C xF) n is required to be able to discharge. Solid state batteries based on graphite intercalation compounds, (C xF) n, C x(SbF − 6) y(SbF 5) 1−y/solide electrolyte, poly(ethylene oxide)-LiClO 4 or PbSnF 4 are also shown.