Glass-like Carbons Research Articles

Continuous-wave and pulsed EPR studies of glass-like carbon with high spin concentration: Evidence for triplet states

Glass-like carbons (GLCs) are a large group of disordered carbon materials which have applications in various fields, but not all their properties are well characterized. In particular, they have high concentration of paramagnetic centers (PCs) which are easily observed by electron paramagnetic resonance (EPR). However, an apparently simple EPR spectrum, together with its peculiarities caused by high electrical conductivity, make it difficult to identify the PCs and their origin remains unknown. To resolve these unsettled questions, in this work we use the vacuum Rabi splitting and Rabi oscillations which so far have not been applied to study these materials by the continuous wave (CW) and pulsed EPR. GLC produced by pyrolysis of phenol-formaldehyde resin at 1120 K was studied in the temperature range of 4.3–300 K. Using the Rabi splitting we found conditions for undistorted registration of CW EPR spectra and determined spin concentration (1×1020 spin/g). The temperature dependence of the EPR susceptibility shows the coexistence of Pauli paramagnetism due to conduction electrons and the Curie-type paramagnetism of thermally activated PCs with the activation energy of 5.7 cm-1. These PCs may be excited triplet states, the presence of which is revealed by Rabi oscillations in the pulsed EPR.

On the Highly Ordered Graphene Structure of Non-Graphitic Carbons (NGCs)—A Wide-Angle Neutron Scattering (WANS) Study

Non-graphitic carbons (NGCs), such as glass-like carbons, pitch cokes, and activated carbon consist of small graphene layer building stacks arranged in a turbostratic order. Both structure features, including the single graphene sheets as well as the stacks, possess structural disorder, which can be determined using wide-angle X-ray or neutron scattering (WAXS/WANS). Even if WANS data of NGCs have already been extensively reported and evaluated in different studies, there are still open questions with regard to their validation with WAXS, which is usually used for routine characterization. In particular, using WAXS for the damping of the atomic form factor and the limited measured range prevent the analysis of higher-ordered reflections, which are crucial for determining the stack/layer size (La, Lc) and disorder (σ1, σ3) based on the reflection widths. Therefore, in this study, powder WANS was performed on three types of carbon materials (glass-like carbon made out of a phenol-formaldehyde resin (PF-R), a mesophase pitch (MP), and a low softening-point pitch (LSPP)) using a beamline at ILL in Grenoble, providing a small wavelength and thus generating WANS data covering a large range of scattering vectors (0.052 Å−1 < s < 3.76 Å−1). Merging these WANS data with WANS data from previous studies, possessing high resolution in the small s range, on the same materials allowed us to determine both the interlayer and the interlayer structure as accurately as possible. As a main conclusion, we found that the structural disorder of the graphene layers themselves was significantly smaller than previously assumed.

Structure of Carbon Materials Explored by Local Transmission Electron Microscopy and Global Powder Diffraction Probes

Transmission electron microscopy and neutron or X-ray diffraction are powerful techniques available today for characterization of the structure of various carbon materials at nano and atomic levels. They provide complementary information but each one has advantages and limitations. Powder X-ray or neutron diffraction measurements provide structural information representative for the whole volume of a material under probe but features of singular nano-objects cannot be identified. Transmission electron microscopy, in turn, is able to probe single nanoscale objects. In this review, it is demonstrated how transmission electron microscopy and powder X-ray and neutron diffraction methods complement each other by providing consistent structural models for different types of carbons such as carbon blacks, glass-like carbons, graphene, nanotubes, nanodiamonds, and nanoonions.

Peering into the structural evolution of glass-like carbons derived from phenolic resin by combining small-angle neutron scattering with an advanced evaluation method for wide-angle X-ray scattering

The structural evolution of two non-graphitizing glass-like carbons derived from a liquid resole and a solid novolac-type phenolic resin was quantitatively characterized by combining small-angle neutron scattering (SANS) with an advanced evaluation for wide-angle X-ray scattering (WAXS) data. Utilizing these two methods allowed for studying the microstructure on the Ångstrom level (graphene stacks, WAXS) and the inaccessible microporosity (SANS). The applied WAXS analysis provided quantitative structural parameters for both, size and disorder in the polyaromatic sp2 microstructure. Hence, the combined SANS-WAXS analysis yielded comprehensive insights into the relation between the graphene microstructure and the inaccessible porosity upon heat treatment for glass-like carbons, i.e. a non-graphitizing class of carbon. In particular, the analogue investigation of a graphitizing mesophase pitch demonstrates the major impact of the chemical composition of the utilized carbon precursor. For the glass-like carbons the results revealed different growth rates for the lateral extent of the basic structural units (La) depending on the temperature range, finally reaching 12 nm, whereas the stack height (Lc) exhibiting 2.2 nm is hardly affected by the thermal processing up to 3000 °C. As a major finding our study thus relates the evolution of microstructure and porosity to changes in chemical composition.

Modelling of glass-like carbon structure and its experimental verification by neutron and X-ray diffraction

Glass-like carbon is a well known carbon form that still poses many challenges for structural characterization owing to a very complex internal atomic organization. Recent research suggests that glassy carbon has a fullerene-related structure that evolves with the synthesis temperature. This article reports on direct evidence of curved planes in glassy carbons using neutron and X-ray diffraction measurements and their analysis in real space using the atomic pair distribution function formalism. Changes in the structure including the degree of curvature of the non-graphitizing glassy carbons as a function of the pyrolysis temperature in the range 800–2500°C (1073–2773 K) are studied using optimized models of the atomic structure. Averaged models of single coherent scattering domains as well as larger structural fragments consisting of thousands of atoms were relaxed using classical molecular dynamics. For such models the diffraction intensities and the pair distribution functions were computed. The compatibility of the computer-generated models was verified by comparison of the simulations with the experimental diffraction data in both reciprocal and real spaces. On the basis of features of the developed structural models for glass-like carbons, the origin of the properties such as high strength and hardness and low gas permeability can be better understood.

Magnetic properties of magnetic glass-like carbon prepared from furan resin alloyed with magnetic fluid

Magnetic glass-like carbons that were heat-treated at different temperatures or were filled with different magnetic nanoparticle contents were prepared from furan resin alloyed with magnetic fluid (MF) or Fe3O4 powder in their liquid-phase states during mixing. Compared to the Fe3O4 powder-alloyed carbon, the MF-alloyed carbon has highly dispersed the nanoparticles, and has the excellent saturation magnetization and coercivity. It is implied that saturation magnetizations are related to changes in the types of phases for the nanoparticles and the relative intensities of X-ray diffraction peaks for iron and iron-containing compounds in the carbons. Additionally, the coercivities are possibly affected by the size and crystallinity of the nanoparticles, the relative amounts of iron, and the existence of amorphous compounds on the carbon surfaces.

Controlled synthesis of nanocrystalline glass-like carbon thin films with tuneable electrical and optical properties

Graphene is an emerging electronic material but expensive and difficult to produce in pure, large-area thin films. Organic electronic applications such as flexible devices and energy applications would benefit from low-cost alternatives to graphene. The controlled synthesis of nanometer-thin carbon films by atmospheric pressure chemical vapor deposition on copper foils is presented. These nanostructured carbon thin films (5–237nm of thickness) are composed of curved graphene fullerene-like fragments of ca. 3nm average size, which replicate the structure of widely used glass-like carbons. The optical and electrical properties of these nanostructured carbon thin films are defined by the thickness of these films; high transparency (86%) and moderate high electrical conductivity (7.8kΩ/□) are achieved for the thinnest samples (5nm). Although these values are in the range of other thin films prepared with graphene, these films are fundamentally different since they are composed entirely of graphene flakes joined by a carbon matrix, which presents a high density of defects; thus they are also interesting candidates for flexible and transparent electronics, especially when biocompatibility, friction, high temperature, UV radiation, and corrosion resistance are also needed.

Surface oxidation and/or corrosion behavior of glass-like carbon in sulfuric and nitric acids, and in aqueous hydrogen peroxide

Surface oxidation and/or corrosion behavior of glass-like carbons (GLCs) in concentrated H 2SO 4, HNO 3 and H 2O 2 aq. were investigated with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and by weight changes. With GLCs treated in H 2SO 4, formation of sulfur-containing chemical species was observed, with some weight gain. However, no change was observed in morphology. With the samples treated in HNO 3, the weight initially increased and, then, decreased. No nitrogen-containing chemical species were observed. Crack formation and scaling off of the upper layers were observed. With the samples treated in H 2O 2 aq., rapid weight loss and void formation were observed.

Effects of heat treatment temperature on oxidation behavior of glass-like carbon derived from acetone-furfural resin

Glass-like carbons (GCs) were prepared by carbonization of acetone-furfural resin in nitrogen atmosphere at 850 °C, followed by heat treatment over a range of 1 200-2 500 °C in inert atmosphere. The effect of heat treatment temperature (HTT) on the oxidation behavior was investigated by dynamic and isothermal thermogravimetric analyses. The structure of GC was examined by X-ray diffractometry (XRD) and the morphologies of GC before and after oxidation were examined by scanning electron microscopy (SEM). It is shown that the GC samples present peculiar oxidation behavior. The anti-oxidation behavior increases with increasing the HTT to 1 600 °C, whereas decreases gradually thereafter. GC sample heat treated at 1 600 °C obtains relatively optimal anti-oxidation properties under this condition. During the oxidation, this material produces grid network matrix surface and numerous nodular residues on the surface, resulting in excellent resistance to the attack of oxygen atoms.

Analysis of surface oxidation behavior at 500 °C under dry air of glass-like carbon heat-treated from 1200 to 3000 °C

The Surface oxidation behavior at 500 °C under dry air of glass-like carbons heat-treated from 1200 to 3000 °C was investigated by thermogravimetric analysis (TGA), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that the oxidation rate of specimens heat-treated above 2000 °C was much slower than the rate of those heat-treated below 1500 °C and varies with the heat treatment temperature of the carbons. Raman spectroscopy revealed that the developed graphite structure of the carbons did not dominate the rates. It was inferred by XPS that the carbon bonded to oxygen in the internal region as well as that on the external surface was easily oxidized. The heat treatment temperature for preventing oxidation was above 2000 °C.

Interior graphitization of furan resin-derived carbon by hot isostatic pressing

This paper reports the investigation of morphological, textural and structural changes for furan resin-derived glass-like carbons, and the formation of spherical graphite in the carbon by hot isotropic pressing (HIP) at a temperature 2500 °C and under pressure 200 MPa even though the changed area took place at interior parts. We characterized the changed area by using Raman spectroscopy and XRD. It was found that the interior changes into graphite after HIP. From the scanning electron microscopy (SEM) images the interior part consisted of spheres, whose dimensions were about 20–150 μm. This graphitization is far from the previously reported ones on hard carbons even though the changed area was limited to interior.

Air Oxidation of Carbon Spheres. II. Micropore Development

The formation of micropores at the start of the oxidation of carbon spheres was studied by N2 gas adsorption, small-angle X-ray scattering and transmission electron microscopy with the aid of image processing. On the basis of a detailed analysis of the measurement results, it was suggested that the principal process in the formation of micropores was the opening of closed pores initially present in the carbon matrix of the spheres. The fact that these closed pores were formed during carbonization as the nano-texture of glass-like carbons suggests that the carbonization process may govern the size of the micropores formed at the start of the oxidation process employed for activating such activated carbon precursors.

Analysis of the oxidation behavior of neat and Ta-alloyed glass-like carbons heat-treated at 1200 and 3000 °C by nitric, sulfuric and hydrofluoric acid

Surface oxidation and/or corrosion behavior of furan-resin-derived carbons in nitric, sulfuric and hydrofluoric acids were investigated by the relative weight changes and X-ray pohotoelectron spectroscopy (XPS). The behavior of the neat and Ta-alloyed furan-resin-derived carbons heat-treated at 1200 and 3000 °C were compared in this analysis. The weights of the neat specimens heat-treated at 1200 °C obviously decreased after the acidic treatments. The spectroscopic analysis found that oxygen in the internal area as well as on the external surface were increased. The Ta-alloyed specimens heat-treated at 1200 °C after treated by nitric and sulfuric acid have lesser amounts of oxygen in the internal area than the neat specimens. We, therefore, imply in this report that Ta prevents oxygen from penetrating into the depths of the carbons.

Pulsed laser deposition of glass-like cluster assembled carbon films

Carbon films have been synthesized at room temperature in helium atmosphere, at high pressure, on (1 0 0) Si substrates by pulsed KrF excimer laser ablation of highly oriented pyrolitic graphite. By changing laser power density (from 8.5 to 19 MW mm −2) and gas pressure (from 0.6 Pa to 2 kPa), nanometer sized cluster assembled films were obtained. Film morphology, as studied by scanning electron microscopy, changes with increasing helium pressure, from dense columns, to node-like morphology, then to an open dendritic structure. Carbon coordination was studied by visible Raman spectroscopy in all films. They are structurally disordered, sp 2 coordinated and belong to the family of glass-like carbons. The deduced film coherence length agrees with the average size of carbon aggregates that build up the films, as measured by transmission electron microscopy in representative samples. The average number of carbon atoms per cluster, that depends on helium (high) pressure, was obtained by a simple model.

Structural and property changes in glass-like carbons formed by heat treatment and addition of filler

Glass-like carbon precursors shrink significantly during curing and carbonization, which leads to crack formation and bending. Cured furan resin powder and ethanol were added to furan resin to diminish the weight loss, to suppress the shrinkage and bending, and to readily release the gases evolved during polymerization and curing. Curing and carbonization were controlled by pressure and slow heating to avoid damage to the samples. The effect of the filler and ethanol on the fabrication process was examined by measuring the properties of the glass-like carbon, such as the specific gravity, bending strength, electrical resistivity, and microstructural change. The specific gravities of the filler-added glass-like carbons were higher than those of the ethanol-added samples because of the formation of macropores from the vaporization of ethanol during the curing and polymerization processes. Although the ethanol-added glass-like carbons exhibited lower bending strengths after carbonization than did the filler-added samples, the opposite result was observed after aging at 2,600 oC. We found that the macropores created from ethanol were contracted and removed upon heat treatment. The electrical resistivity of the glass-like carbon aged at 2,600°C was lower than those of the samples carbonized at 1,000 oC. We attribute this phenomenon to the fact that aging at high temperature led to well-developed microstructures, the removal of macropores, and the reduction of the surface area.

Focused Ion Beam Machining of Precision Die for Micro Optical Lens Molding

Glass-like carbons (GCs) for press molding die of micro optical glass lens and electroless Ni-P alloy films for injection molding die of micro optical plastic lens were machined by focused ion beam (FIB) and their dimensional accuracy, surface roughness and transcriptional performance were investigated. The GCs completed a fine surface roughness of Ra=1nm after FIB machining, although their FIB etching rate was decreased to a half rate of Si(100) single crystal. On the other hand, amorphous Ni-P alloy films containing phosphorus more than 7wt% possessed higher etching rate than Si(100) crystal, as well as fine surface roughness less than 8nm. Precision GCs and Ni-P alloy films molding dies with 3-D microlens array of 7μm square were fabricated by FIB. Subsequently, transcriptional performance to micro optical lens was evaluated on precision GC dies.

Effects of filler and ethanol on mechanical properties of glass-like carbon

The effects of filler and ethanol on the mechanical properties of glass-like carbon were studied. The specific gravities of glass-like carbons with the filler were higher than those of alcohol added glass-like carbons due to the formation of pores and bubbles as the alcohol was vaporized in the curing and polymerization stages. The ethanol added glass-like carbon samples also had lower bending strength than the filler added glass-like carbon samples after carbonization; a contrary result was observed after heat treatment at 2600°C. This was due to the pores that were made with ethanol which plays the role of pore former during carbonization. The fabricated glass-like carbons are quite brittle material. The shore hardness of the glass-like carbon was observed not to be affected by the addition of filler or ethanol, but increased a lot after carbonization.

Chemical and micro-structural changes in glass-like carbon during high temperature heat treatment

A glass-like carbon was fabricated using furan resin. The influence of heat treatment temperature during fabrication process on the chemical and micro-structural changes was studied by various analytical and spectroscopic methods including TGA, FT-IR, CHN, TEM and XRD. The chemical resistance properties of the fabricated glasslike carbon were also investigated. It has been found that the heat-treated samples at higher temperature up to 2600 °C in N2 atmosphere had little weight loss, small amounts of functional groups, and high carbon content. The fabricated glass-like carbons upon heat treatment at 2600 °C showed an amorphous stage without any grain growth and/or reconstruction of structure. The glass-like carbon had much better chemical resistance than the artificial graphite, and exhibited a high chemical resistance due to its low surface areas, minimum impurities, and low graphite crystallites.

A new structural model of glass-like carbon

The results of an experimental study of X-ray diffraction, diamagnetic susceptibility, thermoelectricity and electron emission are presented for glass-like carbons. They lead to a new model of glass-like carbon structure and its modification due to heat treatment. At the initial stage of pyrolysis the stacks of narrow graphite-like sheets are formed being surrounded by a chain-like matrix. The main factor determining such system behavior under heat treatment is mutual parallelism of layer borders and adjacent chains. The most important feature of the model is the destruction of chain-like fragments with simultaneous formation of interstitials above 2000°C. The proposed model is consistent with many other properties of the material.

Investigation of the pore structure in glass-like carbon prepared from furan resin

Small-angle X-ray scattering (SAXS) measurements were carried out on glass-like carbons (GCs) prepared from furan resin at 950°C and thereafter treated at various temperatures. The scattering intensities grow gradually with increasing heat-treatment temperature (HTT) up to 1600–1800°C, and then the intensities increase abruptly at HTT higher than 1800°C. HTT dependence of the reciprocal width for the 002 peaks shows a similar behavior. Distance distribution functions (DDFs) were obtained by the Fourier transform of the SAXS intensities, from which the changes of the gyration radius and the shape of pores in the GC with increase of HTT are discussed. The HTT dependence of the structural change of the GCs from furan resin is almost the same as that of GCs from phenolic resin. However, it was found that the GC prepared at 1200°C from furan resin shows a peculiar behavior, namely, the largest interlayer spacing in the carbon matrix and the smallest value of the gyration radius for pores.