Electron microscope study of the formation of graphitic nanostructures in nickel-loaded wood char

We investigated the graphitization of carbonized larch wood chars impregnated with aqueous solution of nickel acetate, using scanning electron microscopy (SEM), both in secondary and transmission modes, and high-resolution transmission electron microscopy (HRTEM). Heat treatment of the chars at 500 °C brought about homogeneous distribution of metallic Ni particles about 5 nm in size in the amorphous carbon matrix. Graphitization sporadically started at this temperature, and some of the Ni particles are aggregated. SEM observations on chars heat-treated at 900 °C suggested that graphitic nanoshells about 50–200 nm in diameter, formed by catalytic effects of the Ni particles, grow in a “meandering” manner inside the amorphous carbon matrix. Some graphitic protrusions are found to grow outwards. Upon removal of the residual amorphous carbon matrix, long chains of the graphitic nanoshells exhibited a three-dimensionally intertwined structure, while transmission SEM showed that the interior of the shells is empty. HRTEM images exhibited not only stacked graphitic layers, but also cross-sectional contrasts expected from the hexagonal net of the graphite structure. These findings are discussed from the viewpoints of processing parameters, such as the use of aqueous solutions and atmosphere, specific to the catalytic graphitization of lignocellulosic materials.     

Tailored production of nanostructured metal/carbon foam by laser ablation of selected organometallic precursors

Aromatic, triphenylphosphine-containing organometallic compounds, as well as non-aromatic organometallic targets, were irradiated with a Nd:YAG laser to study the role of the metals and ligands used on the structure of the resulting materials. Characterization shows that the composition, metal nanoparticle dilution and crystallite size, and structure of the carbon foams (CFs) produced can be tailored by choosing the metals and ligands of the irradiated targets. The results indicate that precursors containing triphenylphosphine ligands tend to yield larger amounts of ablation products in which graphitic structures are observed in appreciably larger quantities. Electron microscopy reveals that these CFs are multi-component materials that consist of metal nanoparticles embedded in amorphous carbon aggregates, amorphous carbon nanoparticles, and graphitic nanostructures, which can be eventually observed as independent, separate components in the produced soots. The graphitic structures observed can also be produced by laser ablation of triphenylphosphine, indicating that their growth is not promoted by the metals of the chosen aromatic organometallic compounds.     

Graphitic carbon growth on crystalline and amorphous oxide substrates using molecular beam epitaxy

We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp² amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.PACS: 81.05.uf; 81.15.Hi; 78.30.Ly.     

Graphitized boron-doped carbon foams: Performance as anodes in lithium-ion batteries

The electrochemical performance as potential anodes in lithium-ion batteries of several boron-doped and non-doped graphitic foams with different degree of structural order was investigated by galvanostatic cycling. The boron-doped foams were prepared by the co-pyrolysis of a coal and two boron sources (boron oxide and a borane–pyridine complex), followed by heat treatment in the 2400–2800 °C temperature interval. The extent of the graphitization process of the carbon foams depends on boron concentration and source. Because of the catalytic effect of boron, lightweight graphite-like foams were prepared. Boron in the foams was found to be present as carbide (B₄C), in substitutional positions in the carbon lattice (B–C), bonded to nitrogen (B–N) and forming clusters. Larger reversible lithium storage capacities with values up to ∼310 mA h g⁻¹ were achieved by using the boron oxide-based carbon foams. Moreover, since the electrochemical anodic performance of these boron-doped foams with different degree of structural order is similar, the beneficial effect of the presence of the B–C boron phase was inferred. However, the bonding of boron with nitrogen in the pyridine borane-based has a negative effect on lithium intercalation.     

Thermal conductivity of nanocrystalline carbon films studied by pulsed photothermal reflectance

The effect of nanocrystals with preferred orientation on the thermal conductivity of carbon films is studied. During graphitization, the presence of biaxial compressive stress results in the formation of preferred orientation in the microstructure of graphitic nanocrystals if the corresponding activation energy is supplied. This formation of preferred orientation leads to the orientation of graphitic basal planes perpendicular to the substrate. Due to the high thermal conductivity of graphite in the basal planes, there is a significant increase in thermal conductivity of textured nanocrystalline films compared to amorphous film.     

Formation of graphitic rods in carbon/carbon composites reinforced with carbon nanotubes

The formation of graphitic rods with a carbon nanotube (CNT) in the center was observed in CNT-reinforced phenolic resin-based carbon/carbon composites heat treated at 2000 °C. TEM characterization indicated that the carbon surrounding the CNT has a much better degree of graphitization compared to the carbon in most of the matrix. The formation temperature (2000 °C) of the graphitic rod is lower than for stress graphitization and normal graphitization of phenolic resin.     

Nanoscopic observations of stress-induced formation of graphitic nanocrystallites at amorphous carbon surfaces

The formation of graphitic nanocrystallites at the surface of amorphous carbon under large mechanical stresses was examined by using micro-Raman spectrometry, transmission electron microscopy and in-situ compressions. In the Raman analyses of severely deformed (above a strain energy density criterion of 5.9 J/m²) surface regions of nanoscratched and nanoindented amorphous carbon films, two additional sharp and narrow peaks, D_Gr and G_Gr at 1330 and 1580 cm⁻¹, appeared from the main unchanged broad spectra, revealing the transformation of some small-range amorphous carbon to nanocrystalline graphite. Transmission electron microscopic images presented the formation of surface shear layer within which dispersed graphitic nanocrystallites (a size of about 3 nm) were formed in the remaining amorphous matrix. The in-situ nanoscopic observation of amorphous carbon nanopillars under compressions confirmed the formation of graphitic nanocrystallites at pillar edge surfaces. The formed graphite (0 0 1) and (1 0 0) lattices were well oriented along maximum resolved shear stresses, being an evidence of lattice reconstruction and suggesting a possibility of stress-induced graphitization of amorphous carbon in the absence of heat.     

In situ hydrothermal deposition as an efficient catalyst supporting method towards low-temperature graphitization of amorphous carbon

In view of the fact that the catalytic activity of supported catalyst is greatly influenced by its impregnation method, an in situ hydrothermal deposition method was explored and applied to load iron catalyst onto the amorphous carbon matrix, which derived from a soft-templating method with sucrose as precursor, to prepare nanoporous graphitic carbon at low temperature. Compared to traditional impregnation method, this hydrothermal loading method could realize the phase transformation from amorphous carbon to graphitic carbon at a temperature as low as 650 °C. Systematical studies on the hydrothermal deposition and catalytic graphitization were made by X-ray diffraction and Raman spectrum, indicating that hydrothermal impregnation temperature and duration are the two crucial factors to the catalytic performance of loaded iron oxide. The obtained graphitic carbon by this approach possesses a nanoporous structure with surface area up to 329 m²/g, pore volume of 0.39 cm³/g and achieves a capacitance per unit surface area of 39.5 μF/cm², much higher than that of the carbon derived from traditional impregnation method (24.6 μF/cm²).     

A simple approach for fabrication of interconnected graphitized macroporous carbon foam with uniform mesopore walls by using hydrothermal method

Three-dimensional (3D) highly interconnected graphitized macroporous carbon foam with uniform mesopore walls has been successfully fabricated by a simple and efficient hydrothermal approach using resorcinol and formaldehyde as carbon precursors. The commercially available cheap polyurethane (PU) foam and Pluronic F127 were used as a sacrificial polymer and mesoporous structure-directing templates, respectively. The graphitic structure of carbon foam was obtained by catalytic graphitization method using iron as catalyst. Three different carbon foams such as graphitized macro-mesoporous carbon (GMMC) foam, amorphous macro-mesoporous carbon (AMMC) foam and graphitized macroporous carbon (GMC) foam were fabricated and their physicochemical and mechanical properties were systematically measured and compared. It was found that GMMC possess well interconnected macroporous structure with uniform mesopores located in the macroporous skeletal walls of continuous framework. Besides, GMMC foam possesses a well-defined graphitic framework with high surface area (445 m²/g), high pore volume (0.35 cm³/g), uniform mesopores (3.87 nm), high open porosity (90%), low density (0.30 g/cm³) with good mechanical strength (1.25 MPa) and high electrical conductivity (11 S/cm) which makes it a promising material for many potential applications.     

The use of organometallic additives to promote graphitization of carbons derived from furfuryl alcohol resins

The promotion of graphitization of carbons derived from furfuryl alcohol resins and phenolic resins, which had been modified by the addition of various organometallic (OM) compounds, was investigated. Of 15 different OM additives evaluated, the most effective at promoting graphitization were those which contained Ti, V or Zr. Resins doped with these additives yielded carbons whose L_c values were between 150 and 250 Å, as compared to values of less than 30 Å for control specimens. The OM compounds of Co, Fe, and Ni yielded carbons with L_c values of approximately 80 Å; the remaining additives had little, if any, effect. Because of the efficient dispersal of the dissolved OM compounds, additions representing as little as 0·1% metal in the precursor resin were usually sufficient to promote sample graphitization. Also investigated were mixtures containing OM-doped furfuryl alcohol resins and glassy carbon filler. Electron and optical microscopy revealed that reorganization of the amorphous filler particles takes place in preference to the moderately graphitic binder residue. The experimental data suggest that the promotion of graphitization is not the result of low-temperature structural modification to the precursor resins during crosslinking or carbonization, but that promotion occurs at higher temperatures and is consistent with the mechanism of catalytic graphitization of amorphous carbons by metals or metal carbides as proposed by Gillot et al. and Fitzer and Kegel.     

Co-gelation synthesis of porous graphitic carbons with high surface area and their applications

An easy co-gelation route has been developed to synthesize porous graphitic carbons with high surface areas by using teraethylorthosilicate (TEOS), furfuryl alcohol (FA), and metal nitrates as precursors. Using a one-pot co-gelation process, a polyfurfuryl alcohol–silica interpenetrating framework with metal ions uniformly dispersed was formed during the polymerization of FA and the hydrolysis of TEOS within an ethanol solution of the three precursors. This synthesis process is simple and time-saving in comparison with the conventional preparation methods. During the heat treatment, Fe₇Co₃ alloy nanoparticles were produced by carbothermal reduction and they then catalyzed the graphitization of the amorphous carbon. The graphitic carbons obtained have a high crystallinity as shown by X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy analysis. The degree of graphitization can be controlled by the varying the loading amount of catalyst. The porous texture of the carbons combines miropores and bimodal mesopores, mainly originating from the silica template formed with different sizes and the loose packing of the graphite sheets. The carbons have large surface areas (up to 909 m²/g) and exhibit excellent electrochemical performance.     

Carbon foams with high compressive strength derived from mixtures of mesocarbon microbeads and mesophase pitch

Carbon foam with relatively high compressive strength and suitable thermal conductivity was prepared from mixtures of mesocarbon microbeads (MCMBs) and mesophase pitch, followed by foaming, carbonization and graphitization. The influence of addition amount of MCMB on the properties of as-prepared carbon foams was investigated in detail. Results showed that addition of MCMBs into mesophase pitch could significantly reduce the amount and length of cracks in carbon foams, which results in increase of compressive strength of carbon foams. Carbon foam with high compressive strength of 23.7 MPa and suitable thermal conductivity of 43.7 W/mK, was obtained by adding 50% MCMBs into mesophase pitch, followed by foaming, carbonization and graphitization.     

Adhesion enhancement of diamond coating on minor Al-modified copper substrate

We report on the enhanced interfacial adhesion of diamond coating on copper substrate modified by a small fraction of Al. For pure copper substrate, the diamond coating formed tends to crack and delaminate, primarily caused by a slight accumulation of detrimental graphite intermediate layer and thermal stress induced by mismatch of the coefficients of thermal expansion. Additions of 1 and 3 at.% Al to the copper substrate gradually decrease the intermediate graphitic phase. At the higher Al concentration, an aluminium oxide forms at the coating–substrate interface, and graphitic/amorphous carbon is completely inhibited, leading to significantly enhanced interfacial adhesion of diamond coating. The electron structure of copper is not observed to significantly alter on this Cu–Al dilute alloy. The alumina barrier layer preferentially formed on copper surface is believed to play a key role in preventing graphitization and adhesion enhancement.     

Influences of particle size of metal on catalytic graphitization of non-graphitizing carbons

It is known that addition of certain metals or inorganic compounds into carbon accelerates the graphitization process at elevated temperature through formation of graphitic carbon. Recently, however, it became apparent that some other kinds of catalytic graphitization effects result from varying the particle size of metal catalysts. Studies on this subject are summarized.     

The effect of pyrocarbon deposition on the microstructure of graphitic foam

Mesophase-pitch-based graphitic foams (MPGFs) were derived. Pyrocarbon (PyC) has been deposited on the inner surface of some MPGFs. Microstructures in the ligaments and nodes of MPGF, morphology of PyC and the boundary between MPGF and PyC have been investigated by transmission electron microscopy and related techniques. In MPGF, straight and compact graphitic crystallites within the long and thin ligaments extend along the bubble walls, while graphite sheets within the nodes are more disordered and less compact with many defects. A PyC reinforcement uniformly covered the inner surface and filled microcracks between graphitic sheets, and was almost perfectly bonded to the MPGF substrate even at the nanoscale. On the vertical side of MPGF, whose surface is perpendicular to (0 0 0 2) planes of graphite layers, some amorphous carbon was formed between MPGF and PyC on the boundary. On the parallel side with the surface parallel to (0 0 0 2) planes, PyC combines with MPGF surface directly and closely without any amorphous carbon.     

Catalytic graphitization of carbons by various metals

A study was made of the catalytic graphitization of carbons by 22 kinds of metals. Heat treatments were carried out at 2600°C for 1 hr and 3000°C for 10 min under argon atmosphere. In graphitizing 3,5-dimethylphenol-formaldehyde resin carbon powder with which 20w/o metal powder (Al, Cr, Mn, Fe, Co, Ni, Ca, Ti, V, Mo and W) was mixed, graphitic carbon was catalytically formed. The first six metals, which belong to the carbon dissolution-precipitation mechanism, gave large graphitic crystal flakes at an early stage of the reaction; the other metals resulted in fine crystals through the carbide formation-decomposition mechanism. For the non-graphitizing phenol formaldehyde resin carbon in which 10w/o metal powder was dispersed, Mg, Si, Ca, Cu and Ge catalyzed formation of only graphitic carbon; and Al, Ti, V, Cr, Mn, Fe, Co, Ni, Mo and W formed both graphitic and turbostratic carbons. Except for Al and Cu, the former effect was exerted by non-transition metals and the latter effect by transition metals. Boron alone markedly accelerated homogeneous graphitization of both kinds of carbon; and Zn, Sn, Sb, Pb and Bi had no catalytic effect. On the basis of these results, the relationships between some properties of the metals, their catalytic abilities and the kind of catalytic effects are discussed.     

Highly porous graphitic materials prepared by catalytic graphitization

Ultrathin graphitic nanostructures are grown inside solid activated carbon particles by catalytic graphitization method with the aid of Ni. The graphitic nanostructures consist of 3–8 graphitic layers, forming a highly conductive network on the surface of disordered carbon frameworks. Owing to the ultrathin characteristic of the produced graphitic nanostructures, the resulted porous graphitic carbons show a high specific surface area up to 1622 m²/g. A detailed investigation reveals that the features of the growing graphitic nanostructures are strongly associated with the catalytic temperature as well as the state of Ni nanoparticles. Some well-dispersed fine Ni particles with diameter below 15 nm are found to be the key to form the ultrathin graphitic nanostructures at appropriate catalytic temperature. Also, a novel mechanism is proposed for the catalytic formation of the ultrathin graphitic nanostructures. As the electrode material of electrochemical capacitors, the porous graphitic carbon exhibits much higher high-rate capacitive performance compared to its activated carbon precursor.     

Effects of pitch fluoride on the thermal conductivity of carbon foam derived from mesophase pitch

Carbon foams with high thermal conductivity were obtained from mixtures of mesophase pitch and pitch fluoride. The addition of pitch fluoride in mesophase pitch could significantly increase the specific thermal conductivity of as-prepared carbon foams. After graphitization at 2873 K, the specific thermal conductivity of carbon foams increased from 82 up to 155.4 (W/mK)/(g/cm³) when the content of pitch fluoride was 3% in the raw material.     

Atomistic process of electron-stimulated structural ordering in tetrahedral amorphous carbon

To have insight to the atomistic process of graphitic ordering in tetrahedral amorphous carbon films which is induced by irradiation of high-energy electrons, the role of temperature in the graphitization was experimentally studied. The change of electron diffraction patterns before and after irradiation at room temperature indicated graphitic ordering, but irradiation at − 170 °C resulted in disordering. Systematic measurements of the temporal evolution of electron energy loss spectra with irradiation at various elevated temperatures and data analysis based on the Johnson–Mehl–Avrami model yielded the activation energy for electron-stimulated ordering of as small as ∼ 0.09 ± 0.01 eV. The most plausible model to account for all the experimental facts is the dissociative diffusion that is triggered by the electron-stimulated displacement of carbon atoms at the initial sites.     

Near net-shape fabrication and properties of graphitic carbon foam with a continuous network of graphite nanosheets

A near net-shape graphitic carbon foam (GCF) with a continuous network of graphite nanosheets was prepared by direct carbonization of epoxy resin filled with nano-Al₂O₃. The effects of carbonization temperature on the properties of the resulting carbon foams were investigated by SEM, TEM, XRD, Raman, thermal conductivity and compression strength test. The results show that the as-prepared GCF can maintain well dimensional stability upon carbonization. The carbothermal reaction between the nano-Al₂O₃ and carbon foam matrix greatly influences the microstructure of carbon foam and promotes its growth of the continuous network of graphite nanosheets. In addition, the GCF prepared at 1700 °C possesses a compressive strength of 2.34 MPa with a bulk density of 0.19 g cm^-3, and meanwhile presents a high graphitization degree of 65.12% and a thermal conductivity of 2.02 W/mK. The continuous network of graphite nanosheets favors the enhancement of thermal conductivity of carbon foam and simultaneously prevents the decline of compressive strength further.