Turbostratic Carbon Structure Research Articles

Simulations of X‐Ray Scattering on Two‐Dimensional, Graphitic and Turbostratic Carbon Structures

Simulations of scattered intensity distributions from two and three dimensional carbon structures of different shapes and sizes were done using the general Debye scattering equation. The influence of the lattice defects typical for the turbostratic structure, i.e., random fluctuations in the parallel layer spacings, random lateral translations of graphitic layers and mutual disorientations of individual parallel layers around the layers normal direction, on the resulting simulated scattered intensities were studied and discussed. The microstructure‐induced changes in the line broadening, in the shape parameter in the Scherrer formula and in the lattice parameters determined from the positions of the X‐ray diffraction lines are discussed in particular. The set of presented Scherrer parameters allows the calculation of the cluster sizes along and normal to the basal planes from the measured X‐ray scattering. The reliability of the Warren–Bodenstein approach for scattering on turbostratic carbon structures was proven. Intensity distributions simulated using the Warren–Bodenstein approach were compared to those obtained using the general Debye scattering equation. It was confirmed that both approaches yield, for particular cluster size, similar results.

Distribution and stability of carbon nanotubes during multi-pass friction stir processing of carbon nanotube/aluminum composites

Multi-pass friction stir processing was used to fabricate multi-walled carbon nanotube/aluminum composites with a high volume fraction (>50%) of reinforcing material homogeneously dispersed in the matrix. The microhardness of the produced composites were two times higher than the original aluminum alloy. SEM and TEM analyses of the microstructures indicated that the reinforcing phase was uniformly distributed after three friction stir passes but the thermo-mechanical cycles destroyed the tubular structure of the carbon nanotubes. TEM micrographs revealed that carbon nanotubes were mostly transformed to polyaromatic and turbostratic carbon structures along with Al4C3 after 3 passes.

An effective method for manufacturing hollow carbon nanofibers and microstructural analysis

Hollow carbon nanofibers (HCNFs) were successfully manufactured by co-axial (core/shell) electrospinning of poly(styrene-co-acrylonitrile) (SAN) and poly(acrylonitrile) (PAN) solutions. The shell component (PAN) was converted into a turbostratic carbon structure by thermal treatment, whereas the sacrificial core component (SAN) was eliminated. SAN was found to be a very suitable material for the sacrificial core. SAN exhibited excellent co-axial electrospinnability to produce a uniform core/shell nanofiber precursor because of its immiscibility with PAN. Also, SAN had a good thermal sustainability that prevented the PAN shell from shrinking during the stabilization and carbonization processes, thus maintaining the shell structure. These two predominant properties of SAN enabled the manufacturing of uniform HCNFs with controlled inner diameters and wall thickness that ranged from 120–510 nm and 52–145 nm, respectively. The core solution properties, such as solution concentration and flow rate, were mostly effective in controlling both the outer diameters and the wall thicknesses of HCNFs. The microstructure of these HCNFs was investigated using high resolution transmission electron microscopy. The crystallite size and crystallinity of HCNFs were dependent on their wall thicknesses. As the wall thicknesses of HCNFs decreased, they developed smaller crystallites and higher crystallinities.

Anodic properties of hollow carbon nanofibers for Li-ion battery

This work reports on hollow carbon nanofibers (HCNFs) as anode materials for Li-ion batteries. Various HCNFs are synthesized using co-axial electrospinning of styrene- co-acrylonitrile (core) and poly(acrylonitrile) (shell) solutions and subsequent thermal treatments. The microstructures of HCNFs are examined using SEM, Raman spectroscopy, WAXD, and HR-TEM. The effect of the carbonization temperature on their turbostratic carbon structures and electrochemical properties is systematically investigated. As the carbonization temperature increases, both crystallite thickness and length significantly increases while the initial irreversible capacity decreases. These predictable microstructure and electrochemical performance of HCNFs provide important insight for the design of novel nanostructured anode materials such as Si or Sn encapsulated HCNFs.

Homogeneous micro and nanocrystalline diamond coating on reticulated vitreous carbon treated at different temperatures

Diamond film grown on reticulated vitreous carbon (RVC), as a three dimensional composite, was investigated. RVC substrates were obtained at three different heat treatment temperatures (HTT) which promoted different structural organization indexes on their turbostratic carbon (TC) structure. Homogeneous microcrystalline (MCD) and nanocrystalline (NCD) diamond grains were formed and showed that are strongly affected by the substrate HTT. Particularly, the changes in diamond films properties were discussed as a function of the film nucleation and morphology evolution for growth times of 1 and 3 h. Scanning electron microscopy images confirmed the continuous films with high nucleation rate, including on RVC struts edges which actuality demand higher film growth rate. The high nucleation rate, typical of NCD formation in the first stage of film growth, was mainly attributed to the different graphitization indexes of such carbon substrates which were etched by atomic hydrogen on RVC prism or basal planes. Micro-Raman spectra also confirmed the features of NCD coating. The high contribution of NCD population in diamond Raman peak at 1332 cm − 1 promoted its asymmetry with some tailing towards lower wavelength shifts, better observed for diamond grown on RVC treated at 1300 K.

Preparation and gas separation properties of poly(furfuryl alcohol)-based C/CMS composite membranes

C/CMS composite membranes from poly(furfuryl alcohol) for gas separation were successfully prepared, in which porous coal-based carbon tubes with an average pore diameter of 0.11 μm and a porosity of 40.3% were used as support. The tubular support was coated using viscous poly(furfuryl alcohol) liquid to form an organic layer that was transformed into a thin top layer of carbon membrane after pyrolysis. The gas separation performance of the as-prepared carbon membranes was evaluated at 25, 60 and 80 °C by molecular probe method. The morphology and structure changes of C/CMS composite membranes during pyrolysis were examined using SEM, HRTEM, FTIR and XRD techniques. The results show that C/CMS composite membranes with uniform and defect-free thin top layer can be made by one-step coating with viscous poly(furfuryl alcohol) liquid and following pyrolysis. The as-prepared C/CMS composite membranes have excellent gas separation properties for gas pairs such as H 2/N 2, CO 2/N 2, O 2/N 2 and CO 2/CH, and the highest permselectivity at 25 °C can reach up to 465.0, 58.8, 13.2 and 160.5, respectively. It has been found that the permeabilities of the composite membranes decrease while the permselectivities increase as the pyrolysis temperature increases from 300 to 700 °C. During pyrolysis, the polymeric structure of poly(furfuryl alcohol) has been transformed into an amorphous turbostratic carbon structure with ultramicropores. For the as-prepared C/CMS composite membranes, the gas separation performance in this work exhibits great competition respect to other carbon membranes reported. These clearly indicate that the C/CMS composite membranes prepared from furfuryl alcohol polymers is a promising membrane for gas separation.

Changes in the structure and properties of lyocell fibers treated by a catalyst during pyrolysis

Changes in the structure and mechanical properties during the conversion of lyocell precursors to carbon fibers were investigated by scanning electron microscopy, wide angle X-ray diffraction and small angle X-ray scattering when they were treated with a catalyst containing sulfuric acid and carbamide. It was found that the density of fibers first increased with temperature, then decreased when the temperature exceeded 200℃. The strength decreased gradually and was close to zero between 150℃ and 200℃, but then increased with increasing temperature. The degree of polymerization showed similar changes to the strength changes during pyrolysis. The crystalline structure of the cellulose II crystals in lyocell fibers was converted to a 2D turbostratic carbon structure.

Effect of microstructure on the modulus of PAN-based carbon fibers during high temperature treatment and hot stretching graphitization

The changes of microstructure and Young’s modulus of PAN-based carbon fibers during the high temperature treatment (2400–3000 °C, stretching 0%) and hot stretching graphitization (0–5%, 2400 °C) were compared. It was observed that although the Young’s modulus of the fibers could be increased by the two graphitization techniques, the microstructure parameters determined by X-ray diffraction were different for the same value of modulus. The relationship between microstructure and modulus showed that Young’s modulus not only depended on the preferred orientation, but also related to the crystallite size (L c and L a) and shape (L a/L c). On the other hand, it was found that crystallite size of the fibers was mainly affected by heat treatment temperature and the crystallite shape could be altered by hot stretching graphitization. Further investigation indicated that the fibers were composed of turbostratic carbon structure even after heat treated to 3000 °C, which could be detected from the absence of 101 and 112 peaks in X-ray diffraction pattern, and the interlayer spacing (d002) and preferred orientation (Z) were only 0.3430 nm and 14.71°, respectively.

Growth of cone-shaped carbon material inside the cell lumen by heat treatment of wood charcoal

With treatment at 2500°C, wood cell walls partially transform into a turbostratic carbon structure, which has an X-ray diffraction peak corresponding to a layer plane spacing of 0.343 nm. Despite this change, there was no apparent change within the cell wall seen by SEM. Cone-shaped carbon material was formed inside the cell lumen after treatment at 2500°C. This is not a feature originating from any wood cell organism and seems to result from vaporized carbon or pyrolysis gases that originate within the cell wall.

High-purity fibrous carbon deposit on the anode surface in hydrogen DC arc-discharge

A brittle porous deposit consisting of high-purity fibrous carbon products with a diameter of 25–100 nm was obtained on a heated anode surface in hydrogen DC arc-discharge. Hydrogen arc plasma was generated between a graphite cathode and a carbon/metal composite anode consisting of 1.0 at.% Fe, 0.6 at.% Co, 2.4 at.% Ni, and 0.4 at.% FeS. X-ray diffraction analysis revealed that the fibrous products had a turbostratic carbon structure, with an interlayer spacing of 0.346 nm. Elemental analysis showed that the fibrous products were composed of 98.4 mass% of carbon. Three types of nano-structured fibrous products were observed using transmission electron microscopy, (1) with a bamboo structure, (2) with a hollow core, and (3) without a hollow core. The formation of fibrous products was initiated by arc-generated metal particles with a diameter of 5–75 nm, then carbon for further growth was supplied by the decomposition of polycyclic aromatic hydrocarbons that were created by interactions between arc-generated carbon clusters and atomic hydrogen. The nano-structure of the fibrous products is thought to depend on the size and morphology of the catalytic metal particles. The synthesis conditions, microstructural characterization and the growth mechanism of the fibrous carbon products are reported.

Role of Structural Characteristics of Aromatic Polyimides in Carbonization

The influence of the structural characteristics of polyimides belonging to four standard classification groups was analyzed. The main factors responsible for formation of ordered and/or turbostratic carbon structures in the course of carbonization were studied for the example of film and fibrous materials.

Loss of gasification reactivity toward O 2 and CO 2 upon heat treatment of carbons

The effect of heat treatment on the reactivity of different carbons toward oxygen and carbon dioxide was studied for heat-treatment temperatures in the range 500–2000 °C. Results were fitted to a kinetic model of annealing based on an nth order power law reaction kinetics. The sensitivity of the model to the choice of the ultimate char reactivity upon prolonged heat treatment was assessed. The thermodeactivation model was used to fit other experimental data of annealing available in the literature. As far as the loss of oxyreactivity is concerned, different thermodeactivation patterns (reflected by a change of annealing kinetic parameters) are observed at heat-treatment temperatures below and above about 1200°C, depending on the carbon. Activation energy of thermodeactivation toward oxygen is larger under severe heat-treatment conditions than under moderate ones for the coals; it is relatively insensitive to heat-treatment temperature for the petroleum coke. On the contrary, a single set of annealing kinetic parameters fairly well represents the loss of carbon reactivity toward carbon dioxide throughout the temperature range investigated and for all the carbons. It is speculated that changes of the turbostratic carbon structure and modifications of the ash constituents, affecting the heterogeneous reaction of carbon along different pathways for the two oxidants, may both be relevant to the observed behavior.

Controlling Factor of Electrocatalytic Activity of Iron-containing Carbon Materials

Abstract Iron-containing carbons were prepared from mixtures of ferrocene and poly(furfuryl alcohol) by varying their preparation conditions. The electrocatalytic activity of the carbons was assessed with Fe(CN)63−/Fe(CN)64− redox reaction, and was revealed to be strongly affected by the fraction of a turbostratic carbon structure (Ts-phase) in the material determined by X-ray diffraction.

The relevance of thermal annealing to the evolution of coal char gasification reactivity

The present work is focused on the influence of the severity of heat treatment of coals during pyrolysis and/or early stages of carbon oxidation on the reactivity of the resulting char towards further gasification. Thermogravimetric analysis has been directed to characterize the reactivity of heat treated samples of a bituminous coal in atmospheres containing different carbon dioxide partial pressures. Carbonization was carried out at different temperatures and for different times prior to thermogravimetric analysis. Parallel microstructural characterization of the samples was accomplished by XRD analysis of untreated coal and heat treated char. Differences in gasification reactivities of chars prepared at a given temperature for various carbonization times are observed, larger in the early stage of gasification, levelling off at larger burn-off. The apparent reaction order with respect to carbon dioxide partial pressure decreases along burn-off for the gasification of equally heat treated samples. Increasing the heat treatment temperature results in substantial char reactivity reduction, associated with major changes of both the turbostratic carbon structure and the mineralogical constitution of the inorganic matter. Experimental results are analysed in the light of a phenomenological model of combined annealing and char gasification. The model is based on a simple reaction scheme according to which char deactivation due to thermal annealing occurs in series-parallel to gasification. The two processes take place over comparable time scales under the experimental conditions tested. The ability of the model to fairly reproduce the observed phenomenology suggests that annealing can be one key to the explanation of the evolution of char gasification rate with burn-off. The discussion on the relative time scales of annealing versus gasification is further extended by examining a body of literature data on annealing of a variety of carbons at different temperatures.

TEM observations of SiC‐SiC composites with a carbon interphase layer annealed in air at high temperatures

SUMMARYSiC–SiC composites with a carbon interphase layer have been annealed in air at high temperature (800–1400°C) in order to determine the oxidation mechanism. Transmission electron microscopy and high‐resolution electron microscopy observations have been performed on thin slices of oxidized specimens. Observations show that, with oxidation above 800°C, the carbon interphase can be replaced totally by a thick silica layer, in agreement with previous work. However, in many cases the modified interphase is heterogeneous. The turbostratic carbon structure disappears and the carbon interphase layer is partially replaced by an amorphous silica layer found preferentially at the contact zone between the fibres and the carbon layer. A very thin silica layer can also be observed on the matrix side at the matrix‐fibre interfaces. Some decohesion appears in areas where carbon is still present. For short‐duration exposures, decohesion occurs between the fibres and the carbon interphase layer.

Structure of mesophase pitch-based carbon fibres

The structure of five samples of commercially available carbon fibres with ultra-high modulus produced from mesophase pitch was studied by the complementary techniques of high resolution electronmicroscopy, X-ray diffraction and transverse magnetoresistance effect. The fibres with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three-dimensional graphite structure in ultra-high modulus carbon fibres. Transmission electron microscope examination revealed that the mesophase pitch-based fibres with high strength have a basic structure unit with folded sheets arranged nearly parallel to the fibre axis similar to those of high modulus carbon fibres produced from PAN. The present fold structure was suggested to contribute consequently to the lower graphitizability of the fibres and to the strong effects on the fibre strength. By controlling the microstructure, it is expected that the crystallographic as well as the mechanical properties could be improved significantly even from the same kind of precursor materials such as mesophase pitch.