Mesophase Spheres Research Articles

Structural and electrochemical characterization of carbonaceous mesophase spherule anode material for rechargeable lithium batteries

Structural and electrochemical characterization of carbonaceous mesophase spherule (CMS) anode material for rechargeable lithium batteries are reported. The particles of CMS material have a spherical shape with a size range 6–20 μm . According to X-ray diffraction (XRD) studies, a typical ordered crystal structure of hexagonal graphite was observed, and two diffraction peaks, d 001 and d 002, formed after charge. The charge and discharge capacities for the CMS anode were 320 and 282 mAh g −1 , respectively, in the first cycle, and the coulomb efficiency of the CMS anode is 88%. The efficiency is close to 100% after the second cycle. CMS material shows good cycling performance as an anode material for rechargeable lithium batteries. The mechanism of the electrochemical intercalation of lithium ion in CMS anode was studied by cyclic voltammetry (CV) experiments. Four insertion steps were observed in the CMS anode for the electrochemical intercalation of lithium ions. These insertion steps correspond to the stage formation during the insertion of lithium ions into the graphite anode for rechargeable lithium batteries.

Primary carbonization of an anisotropic ‘mesophase’ pitch compared to conventional isotropic pitch

The evolution during carbonization treatments of a 100% anisotropic pitch (pitch C) was compared to that of Ashland 240 (100% γ resins). The anisotropic pitch C results from a gas-sparge preparation leading to a composition of 93% β resins (QS-TI) and 7% γ resins (QS). It is made of a major component (QS-TI), in which droplets (100–300 nm in size) partially toluene soluble are distributed. The physicochemical, textural and microtextural evolutions of the two pitches were studied. During pitch C primary carbonization, anisotropic droplets grow by coalescence, then decompose into Brooks and Taylor mesophase spheres suspended in isotropic drops. These drops develop at the expense of the anisotropic matrix by a continuous regeneration of the small anisotropic droplets which feed the isotropic drops by diffusion process. Then inside these drops, mesophase spheres grow then coalesce and the behaviour of a conventional pitch is restored. These various molecular associations are due to absence of chemical events below 450 °C, leading to the global mass spectrum being constant. At 500 °C the material is homogeneously anisotropic though plastic, the metastable system is destroyed and the evolution of conventional pitches is recovered, i.e. above 550 °C macropores develop up to solidification at 600 °C (semi-coke stage).

Advanced Sn/C composite anodes for lithium ion batteries

Metallic tin was deposited in fine particulate form on the surface of carbonaceous mesophase spherules (CMS) and in the pores of porous carbon by the decomposition and reduction of tin(II) 2-ethylhexanoate at 450 °C. The Sn/C composite powders obtained were used as anode materials for lithium ion cells. Electrochemical cycling tests of coin cells show that the dispersion of tin into the carbonaceous materials enhances the reversible capacity of the electrodes. The capacity retention at the 50th cycle is 91 % for Sn/CMS composite containing 22% tin, against 428 mAh g−1 at the first cycle. With further increase in tin content, the capacity fade upon cycling is more rapid.

Mesophase development in coal-tar pitch modified with various polymers

The effect of 10% addition of various polymers to coal-tar pitch on its conversion into mesophase during isothermal treatment at 450 °C has been studied using polarized light optical microscopy. The additives appear to modify to a different extent the kinetics of the conversion and morphology of the developed mesophase. A common effect is an acceleration of mesophase unit growth in the early stage of transformation, the influence decreasing from polystyrene to cumarone–indene resin, and, at the advanced stage, an increase of proportion of areas of isotropic appearance in the optical texture. The addition of poly(vinyl chloride) leads to non homogeneous texture of mesophase pitch due to early separation of particulate matter and growth of mesophase in a way typical of pitch matrix which is free from quinoline insolubles. Poly(ethylene glycol) and unsaturated polyester resin tend to hinder the ability of mesophase to coalescence giving products of enhanced proportion of spherical mesophase. The cumarone–indene resin is the only additive that distinctly increases residue yield but does not affect its optical texture. Based on the results the behaviour of polymers within pitch matrix under carbonization conditions and possible interactions of polymer degradation products with pitch constituents are discussed.

Formation of mesophase spheres from a coal tar pitch upon heating and subsequent cooling observed by an in situ 1H-NMR

Coal tar pitch was heated at a rate of 10 K/min up to 723 K in an atmospheric flow of nitrogen gas, soaked at the temperature for time, t H , ranging from 0 to 480 min. A microscopic observation revealed that soaking for t H shorter than 120 min produced dispersed mesophase spheres, while longer soaking resulted in a flow-textured mesophase due to a progress of coalescence of the spheres. Diameter and yield of the dispersed spheres depended on the rate of cooling, suggesting that the spheres formed upon cooling. In situ proton nuclear magnetic resonance ( 1 H-NMR) detected three components, G1, G2 and W having different transverse magnetization decays, in the pitch upon soaking. Magnetization decays of G1 and W agreed well with those of meso-carbon microbeads (MCMB) and fully fused isotropic pitch, respectively. When t H was shorter than 60 min, no G1 was detected while spheres were observed in the pitch after cooling, confirming mesophase formation upon cooling after soaking. It was also found that the formation of spheres upon cooling accompanies the complete conversion of G2 into G1 and that the G1 formed hardly fuses while reheated to 723 K.

Observation of Formation and Coalescence of Mesophase Spheres by Ultrasonic Method

The authors introduced an ultrasonic technique as a method to investigate the process and/or rate of growing and developing of mesophase with heating. Two kinds of pitches were chosen for this purpose i.e., synthetic naphthalene pitch (MC) and hydrogenated coal-tar pitch (HC). In addition, solvent extraction analysis, polarizing optical microscope observation and rheological data were also used to help understand the ultrasonic data. For pitches of MC and HC, two peaks on the curve of the attenuation coefficient related to appearance and development of mesophase were observed against heating time at 330°C and 380°C, respectively. Thus, it was found that the investigation of pitches by means of their ultrasonic observation leads to a good description of appearance and development of mesophase during heating treatment. Consequently, it is expected that the ultrasonic method will be useful in examining the in situ observation for appearance and development of meso-carbon microbeads.

Effect of Graphite Surface Structure on Initial Irreversible Reaction in Graphite Anodes

The initial irreversible reaction that occurs in graphite anodes during the first lithium intercalation in lithium rechargeable batteries was studied in view of graphite surface structure. Graphitized mesophase spheres and pitch‐based carbon fibers, which show low irreversible capacity, were shown to have turbostratic surface regions and highly graphitized cores using Ar‐ion laser Raman spectroscopy. Burning off these surface regions resulted in remarkable increases of initial irreversible capacity. Those results can be explained by a proposed model that a turbostratic structure of the graphite surface region resists drastic swelling of interlayer spaces arising from cointercalation of solvated ions and depresses the side reaction. © 1999 The Electrochemical Society. All rights reserved.

Influence of pressure variations on the formation and development of mesophase in a petroleum residue

The objective of this study is to search for a method, involving control over the pressures within a pyrolyzing system, to produce non-coalesced mesophase spheres of required sizes. Semicokes were studied from pyrolyses of a petroleum residue, R1, carried out in a tube reactor, 440–445°C, 1.0 MPa pressure, and soak times 1.0–2.0 h. In some pyrolyses, pressures were reduced from 1.0 to 0.1 MPa and the pyrolysis continued at the same reaction temperature at 0.1 MPa. These experiments are termed depressurizations. Semicokes were examined by optical microscopy to measure mesophase content as well as distributions of size of mesophase spherules and sphere concentration (number of spheres mm−2). High-pressure (1.0 MPa) pyrolyses favour the coalescence of mesophase spheres to form bulk mesophase. Initially formed spheres do not grow beyond a certain size indicating that rates of coalescence exceed rates of growth of the spheres. Depressurization causes a loss of volatile matter with a proposed resultant increase in viscosity of the remaining isotropic phase. Some volatile material may also be removed from the mesophase spheres themselves, again promoting a viscosity increase. As a result of this, the spheres increase their diameter and there is less production of bulk mesophase. With depressurization, coalescence is not stopped entirely but is reduced considerably in extent.

Pitch-based carbon fiber of high compressive strength prepared from synthetic isotropic pitch containing mesophase spheres

Two pitches containing mesophase spheres were prepared from a synthetic isotropic naphthalene pitch at 360°C and 375°C so as to contain spheres of different diameters. Both pitches were smoothly spun into thin fibers, which were converted into carbon fibers through stabilization, carbonization and graphitization. The fiber from the pitch with finer spheres exhibited a higher compressive strength, which reflected smaller microdomains, smaller, and thinner fibrils, being different from both isotropic and anisotropic pitch fibers. The isotropic fraction of the pitch appears to give areas of smaller microdomains. The fiber exhibited intermediate values in graphitic parameters between those of isotropic and anisotropic pitch-based fibers.

Preparation of Mesocarbon Microbeads from Coal Tar

Mesocarbon microbeads (MCMB) was prepared from coal tars with various primary pyridine insoluble fraction (PI) contents from 0 to 3.7 wt %, through heat treatment at 430–480°C for different time under autoginious pressure. The heat treated coal tars were filtered under 150°C and the residue was rinsed with pyridine. MCMB was obtained as the pyridine insoluble fraction. Both yield and diameter of MCMB increased with the increasing of reaction temperature and holding time. It was found that primary PI also had strong influence on the formation of MCMB, which restricting coalescence between mesophase spheres by sticking on the surface of mesophase spheres or dispersing in the matrix. Low temperature, short reaction time and high PI content resulted in the MCMB with small diameter and narrow size distribution. Carbon block with 1.75 g/cm3 density could be easily obtained from the MCMB after molded under 100 MPa and carbonized at 1000°C.

Carbonization of various precursors. Effect of heating rate: Part II: Transmission electron microscopy and physicochemical studies

During primary carbonization, various types of liquid crystal phases can demix depending on the elemental composition of the material. Their occurrence temperature was determined by TEM and elemental analyses (inflection point of (H/C)at. versus HTT). As the factor FLMO=(O+SR/H) measured at anisotropy occurrence increases, (1) the size and ordering of mesophases decrease from spherical Brooks and Taylor mesophase spheres to other mesophases with a different microtexture (1000–150 nm in size), down to volumes with digitized contours (100 nm in size); (2) the temperature of mesophase formation increases and that of final local molecular orientation (LMO) domains decreases, so that the gap of temperature allowing the liquid crystal phase development narrows. At solidification, determined by Vickers microhardness, the anisotropic domain (final LMO) size decreases from micrometric mosaics down to nanometric final LMO (200 nm). It results in a good correlation between final LMO size and the factor FLMO measured at anisotropy occurrence. Above solidification, perfect aromatic layer stacks (grains) derive from final LMO with the same diameter. The smaller the grains, the lower the graphitability. If during primary carbonization an excess of H coincides with 100% of anisotropy before solidification (Ashland 240), macroporosity develops under gas-pressure. The pore walls become oriented mosaics inside which stresses provide disclinations themselves preventing the formation of perfect grains and high graphitizability.

98/03798 Preparation of carbon fiber from isotropic pitch containing mesophase spheres

98/03798 Preparation of carbon fiber from isotropic pitch containing mesophase spheres

Pyrolysis of coal tar pitch mixed in the presence of a graphite intercalation compound: A kinetic study

A kinetic study has been carried out to determine the influence of the addition of first stage FeCl 3-graphite intercalation compound (GIC) to coal tar pitch (CTP) on the formation and the development of the mesophase. The mesophase formation was followed by the determination of the tetrahydrofuran insoluble fraction (THFI) of the solid residues, which were further characterized by polarized light microscopy. The presence of the Lewis acid (desorbed from the graphite layers) has a great influence during the mesophase spherule formation and development. The activation energy of the formation and the development of the mesophase (E a) is equal to 159 and 204 kJ/mol in the case of the CTP-GIC 8% mixture and the CTP respectively. This corresponds to a decrease of 22% of E a when the CTP is mixed with the GIC. The authors want to highlight the difficulties and mainly the limitations of the method used to calculate this activation energy by THFI measurements. However, we assume that the mesophase formation and growth rates are accelerated in the presence of compounds such as GIC for two reasons: the absence of QI particles on the surface of the mesophase spherules and the catalytic activity of the Lewis acid.

Silica Coating on Carbonaceous Mesophase Spherules

Carbonaceous mesophase spherules (MS), prepared from coal-tar pitch, have a layer structure exposing the edgesof constituent aromatic molecules to the surface. Oxidation with nitric acid converted MS from hydrophobic to hydrophilicnature due to introduction of acidic functional groups to the surface. A water thin layer was formed on MS surface bysuspending oxidized MS in water followed by in benzene, i.e., W/O emulsion was formed. Na2O·nSiO2(n: 2.18-3.14) dissolved in the water layer was allowed to react with (NH4) 2SO4 to coat MS surface with silica. Detailed reaction conditions were clarified to successfully coat MS by silica. Silica-coated MS showed high BET surface area (as much as 200 times) compared with oxidized MS.

Characterization of coal-derived pitches as precursors for advanced carbon materials by chromatographic and related techniques

Most applications of pitches involve a thermal treatment with formation of an intermediate liquid crystal phase (mesophase). The nature of the raw material is a crucial factor in the formation, growth and development of mesophase spheres. Hence, better understanding of the pitch constitution is of primary importance for both determining the suitability of a given pitch for a specific application and improving the carbonization behaviour. Differences in chemical composition of two selected pitches derived from coal (coal-tar and coal-hydrogenation pitches) have been evaluated by gas chromatography and probe-mass spectrometry of the extrography fractions. Coal hydrogenation pitch is constituted by a higher amount of neutral polycyclic aromatic hydrocarbons with lower molecular mass and polynuclear aromatic compounds of higher polarity than the coal-tar pitch. These compositional differences can explain the lower residue yield, the higher mesophase formation and the polydispersive size of mesophase spheres as compared to the coal-tar pitch.

97/02819 Potential use of fly ash in Colombian blended cements

Iron benzoate and naphthoate were studied as catalyst precursors of an iron-catalyzed pyrolysis of a coal-tar pitch as a technical hydrocarbon mixture and of anthracene as an aromatic model compound. The results are compared with those obtained with ferrocene, which has been studied previously. Both iron compounds are decomposed by formation of iron clusters at a much lower temperature than ferrocene. For this reason the catalytic effect of iron is found at shorter reaction times and particularly at lower temperatures. Anthracene is converted to a semi-coke at temperatures as low as 444 °C. In pitch pyrolysis, the catalytic activity of iron is limited, because the iron is poisoned by formation of iron sulfides. With iron benzoate, but not with iron naphthoate, the nucleation and growth of mesophase spheres is strongly influenced. Spherulites are nearly equal in size and have a reduced tendency to coalesce up to very high spherulite content. The surface of the spheres is disturbed by segregated material containing iron sulfide. Similar results were also found with ferrocene.

Preparation of carbon fiber from isotropic pitch containing mesophase spheres

Naphthalene based synthetic isotropic pitch was found to give spinnable precursors which carried 10–100 μm anisotropic spheres of 45 and 50 vol% in the isotropic matrix, when the pitch was heat-treated at 375 and 380 °C for 20 and 23 hours, respectively. The spun fiber was stabilized and carbonized into a carbon fiber, which showed anisotropic belts running along the fiber axis, suggesting that both anisotropic spheres and isotropic matrix are deformed to align within the nozzle. The structure and mechanical properties of the carbon fibers, thus prepared, were examined under microscopes and by mechanical testing.

Raman study of the effect of intense laser irradiation on graphitized mesophase spherules

We have found that the Raman spectra of graphitized mesophase spherules, heat treated at 2800°C, show interesting change under intense laser irradiation. When they are irradiated with an intense laser beam, the width of the E 2 g band at 1580 cm −1 decreases while the intensity of the D band at 1350 cm −1 increases, i.e. the correlation between these quantities is negative. This is an interesting finding because a positive correlation between these quantities has been reported in the case of “thermal treatment” in the graphitization process of this material. We have also observed luminescence from graphitized mesophase spherules under intense laser irradiation. The intensity of the luminescence is correlated with the change of the Raman spectra. In order to interpret these phenomena, we suggest a model in which the bridge bonds between aromatic molecules in the graphitized mesophase spherules are broken by intense laser irradiation.

Anisotropic crystalline growth developed in Bangladeshi coking coal during mesophase transformation

Abstract The anisotropy of the mesophase spherules is developed during the carbonization of extracted coking coal in the temperature range 430 to 450°C. Mesophase transition is observed by the differential scanning calorimetric study. Enthalpy ΔH involved in the mesophase transformation is found to be 46.26 J/g. The growth of mesophase spherules, their coalesced structures, mosaic and flow type mosaic are observed by the polarized light technique as the evidence of graphitizing carbon. The solvent extraction of coking coal is carried out using benzene as a solvent at atmospheric pressure by Soxhlet apparatus.

Electroless metal plating on carbonaceous mesophase spherules

Carbonaceous mesophase spherules prepared from coal-tar pitch have a layer structure exposing the edges of aromatic molecules on the surface. Oxidation by nitric acid introduced many acidic groups to carbonaceous mesophase spherules due to their layer structures. Tetraamminepalladium ions (Pd(NH 3) 4 2+) were electrostatically immobilized on the oxidized carbonaceous mesophase spherules at pH 11, whereas PdCl 4 2− ions could not be immobilized. Electroless nickel and copper plating on the spherules after immobilization of 30 wt% Pd(NH 3) 4 2+ on the oxidized mesophase spherules was successfully conducted. With nickel plating, spherules with 6 μm diameter were coated uniformly with nickel 0.8 μm thick at pH 7 and 2.0 μm thick at pH 8. However, a uniform coat of nickel on mesophase spherules heat-treated at 2800 °C and carbon blacks was in failure, because small amounts of acidic groups was introduced after nitric acid oxidation. The oxidized mesophase spherules was paramagnetic, while the oxidized spherules plated by nickel was ferromagnetic and its value of saturation magnetization was estimated as ca. 27 emu g −1, corresponding to about a half of that of bulk nickel, 58.6 emu g −1.