TEM and electron tomography studies of carbon nanospheres for lithium secondary batteries

The structure of carbon nanospheres of 100-200 nm diameter, which showed superior high-speed charge-discharge behavior as the negative electrode in a lithium ion battery, was investigated with XRD, SEM and TEM with an electron tomography attachment. Observation of carbon 0 0 2 lattice images, as well as electron diffraction patterns, illustrated that heterogeneous microtexture was formed as the polyhedronization of the particle proceeded with heat-treatment. The outside region of the particle heat-treated at 2800 °C has stacking structure of aromatic layers with some distribution of d₀₀₂, while the center region consisted of non-graphitic. Structure defects seemed to be concentrated along the ridgelines of the polyhedronized particles after heat-treatment. The electron tomography technique clarified the morphology of the graphitized particles, although the images should be understood with other crystallographic measurements. A slice image computed in the 3D-reconstruction process showed the inner texture of the graphitized particles more clearly than the conventional TEM bright-field image.     

Microstructure difference between core and skin of T700 carbon fibers in heat-treated carbon/carbon composites

The core and skin microstructure of T700 carbon fibers in carbon/carbon composites, prepared with chemical vapor infiltration (CVI) at 1000 °C and heat treated at 2300 and 2800 °C, have been studied by means of high-resolution transmission electron microscopy (HRTEM). The orientation angles of the graphitic basal planes obtained from selected-area electron-diffraction patterns show that the structure difference between the core and skin is a change of the degree of preferred orientation of the graphitic basal planes which decreases gradually from the skin region to the core region after CVI at 1000 °C. With increasing heat-treatment temperature, the basal planes orient parallel to the fiber axis, first in the skin region and then in the core region. In addition, the diameter of the core region decreases considerably from about 3.3 to 2.2 μm after heat treatment at 2800 °C. HRTEM lattice-fringe images show that the graphitic crystallite size increases significantly both in the core and skin, but more in the skin. Moreover, with increasing crystallite size, pores of nanometer scale start to form in the fiber.     

Carbons from furan-polymers prepared in the presence of a double-chain amphiphile

Furfuryl alcohol was polymerized in the presence of a double-chain amphiphile using malonic acid or phosphoric acid as the catalyst, leading to an aggregate of spherical particles with a long period of 2.6 nm. On calcination at 1000 °C in nitrogen gas, the polymer particles were converted into lamella-patterned carbons with a long spacing of some micrometers that are composed of carbon layers 0.4 nm in spacing. The lamella-patterned carbon particles were further developed into a highly ordered structure in an appreciable portion on calcination at 2800 °C in argon gas. The present results demonstrate that the presence of a double-chain amphiphile in the polymerization process is effective for the synthesis of such a structurally modified carbon from non-graphitizable furan polymers.     

Morphology of heat-treated tunsgten doped monolithic carbon aerogels

The morphology of a tungsten-doped monolithic organic aerogel, prepared by the sol-gel method from the polymerisation of a resorcinol, formaldehyde and ammonium tungstate mixture, and of its carbonized derivatives at 500 and 1000 °C was studied by scanning and high-resolution electron microscopy. Tungsten influenced the surface morphology of the carbon aerogels. The tungsten-containing phase was homogeneously distributed in the organic aerogel and its heat treatment produced changes in the metal phase distribution throughout the pellets. Tungsten oxide particles of needle-like structure similar to hollow tubules were detected after heat treatments at different temperatures. In addition, particles of dendritic appearance, formed by tungsten carbide and an intermediate Magnelli phase, appeared when the heat treatment was carried out at 1000 °C.     

Transmission electron microscopy studies on carbon materials prepared by mechanical milling

The effects of mechanical milling on the multiscale organization (structure and microtexture) of various carbon materials were investigated by means of Transmission Electron Microscopy. We show that mechanical grinding generates an increasing amount of disordered carbon at a rate depending on the type of grinding mode used (shear- or shock-type grinding). When the shock-type grinding is used, the triperiodic structure and the lamellar microtexture of the graphite completely break down to give microporous and turbostratic carbons made of misoriented nanometric Basic Structural Units (BSUs). Graphite grinding permits the elaboration of disordered carbons. The involved mechanism is different from a simple reverse graphitization, since not only structure but also microtexture are strongly modified by the grinding. After heat treatment at 2800°C, the graphite organization is not recovered, and a mesoporous turbostratic carbon is mainly obtained. All the carbon precursors studied, submitted to strong grinding, leads to similar microporous carbons. Shear grinding is less effective since remnants of graphitic carbon are still present within the disordered carbon.     

The porous network in carbon aerogels investigated by small angle neutron scattering

Small angle neutron scattering treated with the Porod approach has been applied to compare the influence of catalysts (C = NaOH, Na₂CO₃ and Ca(OH)₂) on the porous structure of resorcinol-formaldehyde (RF) carbon aerogels. Investigated parameters are the molar ratio (R/C varies from 10 to 800 mol/mol) and the pyrolysis temperature (1050 °C, 1700 °C and 2600 °C).At 1050 °C, carbon aerogels based on NaOH and Na₂CO₃ catalysts provide denser materials than with Ca(OH)₂-based one, due to a three-dimensional network of smaller particles. The density of particles decreases with the amount of catalyst. At 2600 °C the development of an intraparticle microporosity, which is quantified, leads to a slight decrease of the interparticle mesoporosity noticed at 1050 °C. This effect is induced by a stiffness of carbon layers in polyhedral pore walls as illustrated by the feature of the chords length distribution g(r) and TEM micrographs.     

EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. After electrochemical interaction of cokes with lithium, the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity. Localised paramagnetic centres interact with Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with ‘near-metallic’ lithium, which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 °C, where a higher hysteresis in the voltage versus composition curve is observed.     

The key role of microtexture in the graphitisation of PBO fibre chars as seen by X-ray scattering and transmission electron microscopy

Poly(p-phenylene benzobisoxazole) (PBO) fibres, heat-treated between 900 and 2700 °C, were studied by both small- and wide-angle X-ray scattering, and by HRTEM. As directly imaged by HRTEM, the material treated at 900 °C consists of nanometre-sized graphitic domains with a microtexture that resembles non-graphitisable carbon. However, above 2000 °C, these elongated structures coalesce, yielding highly graphitised lamellar carbon. This graphitisation mode is very different from the conventional progressive graphitisation of lamellar carbon. It is similar to that of polyimide films, and is characterized by a preferential planar orientation of the polyaromatic structural units inherited from the pristine fibre microtexture.     

Carbon nanotubes and onions from carbon monoxide using Ni(acac)2 and Cu(acac)2 as catalyst precursors

New catalyst precursors (copper and nickel acetylacetonates) have been used successfully for the synthesis of carbon nanotubes and onion particles from carbon monoxide. Catalyst nanoparticles and carbon products were produced by metal-organic precursor vapour decomposition and catalytic disproportionation of carbon monoxide in a laminar flow reactor at temperatures between 705 and 1216 °C. Carbon nanotubes (CNTs) were formed in the presence of nickel particles at 923-1216 °C. The CNTs were single-walled, 1-3 nm in diameter and up to 90 nm long. Hollow carbon onion particles (COPs) were produced in the presence of copper particles at 1216 °C. The COPs were from 5 to 30 nm in diameter and consisted of several concentric carbon layers surrounding a hollow core. The results of computational fluid dynamics calculations to determine the temperature and velocity profiles and mixing conditions of the species in the reactor are presented. The mechanisms for the formation of both CNTs and COPs are discussed on the basis of the experimental and computational results.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Carbon coating of anatase-type TiO2 through their precipitation in PVA aqueous solution

Fine particles of photocatalytic anatase-type TiO₂ prepared through hydrolysis of titanium-tetraisopropoxide were coated by carbon through their precipitation in poly(vinyl alcohol) (PVA) aqueous solution, followed by heat treatment at high temperatures of 400-1000 °C in a flow of high purity Ar. Without carbon coating, the phase transformation from anatase to rutile started above 600 °C, but it was suppressed up to 800 °C with carbon coating. Suppression of the phase transformation depended on the amount of carbon coated, apparent suppression being observed with carbon content above 5 mass%. The amount of carbon coated on anatase was controlled by changing the concentration of PVA in the solution. In order to have a carbon content of about 5 mass%, a PVA solution with more than 2 mass% had to be used.     

Chemical vapor infiltration of carbon fiber felt: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 °C and methane pressures from 5 to 30 kPa to confirm the inside-outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside-outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 °C and up to 13.5 kPa at 1095 °C, leading to bulk densities above 1.9 g/cm³. A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 °C. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures.     

Preparation of activated carbon fibers from polyvinyl chloride

Waste polyvinyl chloride (PVC) contains high content of chlorine, which is believed to liberate dioxine at its combustion. Efficient removal of chlorine from PVC achieved by selecting the heat-treatment conditions provided free-chlorine PVC based pitch by a two-stage heat-treatment process. The obtained pitch (softening point: 218 °C) was spun, stabilized, carbonized and activated to prepare activated carbon fibers (ACF) whose DeSO_x activity was tested preliminarily and found comparable to other ACF.     

Effect of silicon and heat-treatment temperature on the morphology and mechanical properties of silicon - substituted hydroxyapatite

The precipitation method was used to synthesize silicon-substituted hydroxyapatite with different Si contents of 0.4, 0.8 and 1.6 wt.% (0.4, 0.8 and 1.6Si-HA) using silicon acetate [Si(OCOCH₃)₄] as a Si source. As-synthesized hydroxyapatite (HA) and Si-HA powders/bulks were heat-treated at different temperatures of 1150, 1200 and 1250 °C for 1 h. Pure 0.4Si-HA and 1.6Si-HA were obtained after heat-treatment at all temperatures, whilst α-TCP phase was formed in the 0.8Si-HA sample after heat-treatment at 1250 °C. SEM observation clearly showed that the substitution of Si in HA inhibited the grain growth of Si-HA even at high heat-treatment temperatures (1200 or 1250 °C). The highest diametral tensile strength (DTS) of 15.93 MPa was obtained in the 1.6Si-HA sample after heat-treatment at 1250 °C.     

Resistivity, Hall coefficient, magnetoresistance, and microtexture of cellulose carbon films

Dense carbon films of about 20 μm in thickness were prepared from a commercially available cellulose film of 40 μm thick by heat treatment up to 900°C. Carbon films thus obtained were heat treated at temperatures between 1800 and 3000°C for 30–60 min. The heat-treated carbon films were investigated by the measurements of resistivity, Hall coefficient and magnetoresistance at liquid nitrogen temperature. Reflected and transmitted X-ray diffraction experiments and observations by a high-resolution scanning electron microscope (SEM) were also carried out. The results of the resistivity and Hall coefficient for high-temperature-treated specimens indicated that the carbonized film was more graphitizable than bulk glass-like carbons. The magnetoresistance and X-ray diffraction profiles for the high-temperature-treated specimens suggested that each of these specimens substantially consisted of the matrix of granular microtexture and the graphite layer skin. The skin was clearly observed by SEM for the specimens heat-treated at 2800 and 3000°C and their thickness was 100–300 nm.     

X-ray structure analysis and elastic properties of a fabric reinforced carbon/carbon composite

The effect of heat treatment on microstructure of a plain-weave carbon fabric reinforced carbon-carbon composite with phenolformaldehyde-derived carbon matrix was investigated by X-ray diffraction. The diffraction patterns were analysed by the least-square fitting program Carbonxs. After heat treatment from 1000 to 2800 °C the interplanar distance of (002) planes decreased from 3.488 to 3.420 Å and the lattice parameter in basal plane increased from 2.440 to 2.464 Å, respectively. Simultaneously, the coherent block size in the basal plane directions increased from 18 to 54 Å, which was accompanied by an increase of the fraction of organised carbon atoms from 0.50 to 0.85. The 002 diffraction profile of the composite was much narrower than the sum of peaks of the matrix and fabric alone. This can probably be caused by a better crystallographic ordering (or by a partial graphitisation) of the matrix in the composite. On the other hand, the composite Young’s modulus slightly decreased with the treatment temperature increasing from 2200 to 2800 °C in spite of the established strong improvement of fibre crystallinity and, therefore, fibre modulus. The mechanisms diminishing the modulus of composite (e.g. partial matrix graphitisation at the fibre/matrix interface and decreasing fibre/matrix contact area) probably prevailed over the increasing contribution of the fibre modulus.     

Carbon aerogels derived from cresol–resorcinol–formaldehyde for supercapacitors

The objective of the present paper is to demonstrate the possibility to synthesize mixed carbon aerogels (denoted C_mRF) from cresol (C_m), resorcinol (R) and formaldehyde (F), as an alternative economic route to the classical RF synthesis. These porous carbon aerogels can be used as electrode materials for supercapacitors with a high volume-specific capacitance. Organic precursor gels were synthesized via polycondensation of a mixture of resorcinol and cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation and aging the solvent was removed via drying at ambient pressure to produce organic aerogels. Upon pyrolysis of the organic aerogels at 1173 K, monolithic carbon aerogels can be obtained. By controlling the catalyst (Cat) molar ratio (C_m+R/Cat) in the range 200–500, up to 70% of the resorcinol can be replaced with the cheap cresol. The resulting homogeneous organic aerogels exhibit a drying shrinkage below 15% (linear). The shrinkage and mass loss upon pyrolysis of the mixed aerogels increase with increasing cresol content. Nitrogen adsorption at 77 K was employed to characterize the microstructure of the carbon aerogels. The data show that the porous structure of mixed carbon aerogels is similar to that of RF carbon aerogels. Cyclic voltammetry measurements show that the as-prepared C_mRF carbon aerogels exhibit a high volume-specific capacitance of up to 77 F/cm³.     

Isochronal annealing kinetics and neutron-induced dimensional changes of titanium-doped pyrolytic carbons

A series of isochronal 1 hr anneals was conducted on a laminar pyrolytic carbon deposited at 1225°C containing 6.7 wt.% titanium, an isotropic pyrolytic carbon deposited at 2200°C containing 9.7 wt.% titanium, and titanium-free companion carbons. Changes in layer spacing, apparent crystallite height (L_c), density, electrical resistivity, and microhardness were followed. Heterogeneous recrystallization to graphite started in the titanium-doped isotropic carbon at 2300°C and was completed at 2500°C. The titanium-doped laminar carbon did not show catalytic recrystallization. The undoped isotropic carbon was not responsive to annealing. The titanium-doped and titanium-free laminar carbons showed a steady increase in L_c and density and a steady reduction in layer spacing and microhardness over a temperature range from 1400° to 2800°C, but significant reduction in electrical resistivity did not begin until ~ 1800°C. The dimensional changes induced by fast-neutron exposures up to ~ 8 × 10²¹n/cm² at ~ 600°C to 1250°C were measured and found to be lower by a factor of 3–10 in a catalytically graphitized pyrolytic carbon than in an as-deposited carbon with the same degree of preferred orientation.     

Surface characteristics and electrochemical capacitances of carbon aerogels obtained from resorcinol and pyrocatechol using boric and oxalic acids as polymerization catalysts

Carbon aerogels were prepared by carbonizing (at 500–1500 °C) organic aerogels obtained from the polymerization reaction of resorcinol and/or pyrocatechol with formaldehyde using boric and oxalic acids as polymerization catalysts. Prepared samples were characterized by different techniques to ascertain their composition, surface chemistry, morphology, and surface physics, determining their electrochemical capacitances in acidic medium. The use of pyrocatechol yielded carbon aerogels that were micro–mesoporous, showing Type IV N₂ adsorption isotherms with Type H2 hysteresis cycles. The volume and size of mesopores depended on the acid catalyst used and the temperature at which the carbon aerogel was obtained. Conversely, the sample prepared with resorcinol and boric acid as catalyst was micro–macroporous and that obtained with a resorcinol–pyrocatechol mixture was micro–mesoporous but with large mesopores. Most of the boric acid used was lost during the exchange of water with acetone in the organic hydrogels before their supercritical CO₂ drying. Carbon aerogels obtained at 900 °C and using boric acid as polymerization catalyst showed a capacitance between 17 and 24 μF/cm². Boron influenced the capacitance because it increased the oxygen content. Sample synthesized using pyrocatechol, formaldehyde, and oxalic acid and heat-treated at 900 °C had the highest capacitance, 34 μF/cm².     

The effect of heat-treatment on the properties of activated carbon fibre cloth polarizable electrodes

The effect of heat-treatment on the properties of activated carbon fibre cloths (ACFC) as polarizable electrodes has been investigated. The heat-treated ACFC was prepared by heating phenolic resinbased activated carbon fibre cloths at temperatures higher than 1000° C in an inert gas atmosphere. The electrical resistance of ACFC and the specific surface area began to decrease at 1000° C and 1500° C, respectively. The ACFC maintained amorphous structure even when it was treated at 2000° C. The cyclic voltammogram of ACFC heat-treated at 1000° C showed a stable electric double layer in organic electrolytes in the range between–1.5 and 1.5V with respect to SCE. In accordance with the lower resistance of heat-treated ACFC, electric double layer capacitors with ACFC heat-treated at 1000° C showed lower d.c. resistance in comparison with capacitors without heat-treated ACFC.