Glass-like Carbons Research Articles

Elasto-plastic deformation of glass-like carbons heat-treated at different temperatures

Nano-indentation of glass-like carbons (GCs) heat-treated at different temperatures was carried out with a 3 μm radius spherical tipped indenter. The effect of crystal structure and micro-texture on elastic and elasto-plastic deformation resulting from the indentation was observed. The elastic modulus and yield stress of GCs was found to reduce with elevation of heat treatment temperature (HTT). The power law fitting constant k for the elasto-plastic deformation of the loading stress–strain curves by the indentation and the indentation elasticity index k/ E* reduced with development of the graphitic structure of GC, so that ‘elasticity’ of GC decreased with the elevation of HTT of GCs. Hysteresis loops were observed on the indentation force–displacement curve during the loading–unloading cycle. Hysteresis energy loss, which corresponds to the area of the hysteresis loops, increased with the elevation of HTT for the same terminal load. The indentation ductility index D, the ratio of hysteresis energy loss to total indentation energy, was linearly dependent on indentation elasto-plastic deformation strain ε ep. The indentation elasto-plastic modulus m, which is obtained from the slope of the plots of D versus ε ep, reduced with the elevation of HTT of GCs. From the reduction of m, it was concluded that ‘plasticity’ (or ‘ductility’) of GC increased with the elevation of HTT of GCs.

Carbonization of ferrocene containing polymers and their electrochemical properties

Two iron-carbon composites were prepared from ferrocene-polymer mixtures with resol resin (Fe-RESOL) or epoxy resin (Fe-EPOXY), and they were compared with the previous composite (Fe-PFA) made from a ferrocene-poly(furfuryl alcohol) mixture from structural and the electrochemical viewpoints. The features of the composites were divided into two groups, that is, a group consists of Fe-PFA and Fe-RESOL and another of Fe-EPOXY. The (002) diffraction by X-ray diffraction of Fe-PFA and Fe-RESOL commonly showed defined peaks at ca 2θ = 26 ° which is characteristic to turbostratic structure, while only amorphous peak was observed for Fe-EPOXY. The electron microscopic studies revealed that both Fe-PFA and Fe-RESOL have fur-like surfaces by scanning electron microscopy and round-shaped crystalline by transmission electron microscopy, in contrast to the characteristics of Fe-EPOXY showing typical features to glass-like carbons. The electrochemical properties of the iron-carbon composites as working electrodes were studied by using cyclicvoltammetry of ferricyanide. Fe-RESOL showed narrow peak potential separation in the voltammogram as found for Fe-PFA previously, indicating the fast electron transfer at the surfaces of the electrodes, while Fe-EPOXY showed only a narrow voltammogram. It was inferred that turbostratic structure would be responsible for the higher electron transfer rate found for Fe-PFA and Fe-RESOL.

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.

Magnetoresistance and microtexture of carbon films prepared from polyimide film LARC-TPI

Structural changes in carbon films prepared from a commercially available polyimide, LARC-TPI, of 50 μm thickness, with heat-treatment temperatures between 2000° and 2800°C were investigated by measuring magnetoresistance at liquid nitrogen temperature and by scannign electron microscopy. All the specimens exhibited granular microtextures, with mean grain diameters spanning 10–12 nm, similar to those of bulk glasslike carbons. The films heat treated between 2000° and 2600°C showed a small negative magnetoresistance characteristic of bulk glasslike carbon. The carbon films heat treated at 2750° and 2800°C gave a composite magnetoresistance attributed to granular microtexture (negative, isotropic) with graphite layer skins (positive, anisotropic). The fractured cross-sections of these films indicated layered skins about 100 nm thick. These graphitizable skins may be attributed to constraints that influenced the orientation of the molecules near the surfaces of the film when they were formed.

Microtexture and magnetoresistance of glass-like carbons

Microtextures of glass-like carbons commercially available were investigated through scanning electron microscopy and measurement of their magnetoresistance. A field-emission electron gun type SEM, which is characterized by a specimen chamber equipped in its objective lens, was operated with the acceleration voltage of 2 kV and high fidelity observations were thereby achieved with a resolving power of 2 nm. All the specimens exhibit granular microtextures with the mean grain diameter spanning 6–13 nm. The mean grain diameter correlates to the mean crystallite size L a (110) determined by the X-ray powder diffractometry. The density of each sample calculated by using the mean diameter and the X-ray parameters reproduces well its catalog value. Negative magnetoresistance was observed just for the specimens whose average grain sizes are not smaller than 7.3 nm. The crystallite size L a(110) of the specimen whose grain size shows the minimum value for appearance of negative magnetoresistance is estimated to be 2.7 nm. The absolute value of negative magnetoresistance under a constant magnetic field increases with increasing the average grain size.

Development of glass-like carbon from phenol formaldehyde resins employing monohydric and dihydric phenols

A study on the development of glass-like carbon from phenol formaldehyde resins employing monohydric (simple) and dihydric phenols (resorcinol and catechol) has been made. It is revealed that to obtain a good glass-like carbon, the optimum molar ratio of formaldehyde to resorcinol in the resorcinol formaldehyde resin is 1.5, as was found earlier in the case of a simple phenol formaldehyde resin, whereas for catechol formaldehyde resin, the optimum molar ratio of formaldehyde to catechol is found to be 2.0. Further, it is observed that the three types of resins lead to glass-like carbons of essentially the similar characteristics, except that the catechol formaldehyde based carbons possess the highest strength of 326 M Pa. A mechanism has been proposed on the basis of three types of phenolic rings designated as attached, bridging and cross-linking, in the structure of the three cured phenol formaldehyde resins to explain the respective optimum molar ratio of formaldehyde to type of phenol.

Nonkinetic changes in heat-treated glasslike carbons part I: Pore size and bulk dimensions

Small-angle X-ray scattering measurements have been used to characterize the radius of gyration R g of the pores in a glasslike carbon heat treated between 1000 and 2800°C for periods up to 150 hours. Measurements were also made of the irreversible bulk dimensional changes Δl. In both cases, a nonkinetic increase occurs when the heat treatment temperature is increased stepwise to a higher value. The relative changes ΔR g / R g and Δl/ l exhibit a similar three-region behavior: an initial increase with the annealing temperature up to 1600°C, a plateau up to about 2200°C, and thereafter an increase again. However, the maximum dilatation Δl/ l is scarcely 3% whereas the increase in ΔR g / R g is twofold. The first region is attributed to a gas pressure mechanism, and the highest-temperature region results from strain relief due to the mismatch of the thermal expansion coefficients in the directions perpendicular and parallel to the layers in graphitic materials. The intermediate region is explained by a combination of both mechanisms.

Pore growth kinetics in heat-treated glass-like carbons

The kinetics of changes in void size during annealing of glass-like carbons in the temperature range 1000°−2800 °C for times up to 150 h were studied using small-angle x-ray scattering determinations of the radius of gyration Rg. The results show that Rg ranged from 9 Å at 1000°C to about 24 Å at 2800 °C. A pore coarsening analysis and a superimposition kinetic analysis applied to Rg gave activation energies of 76 ± 4 kcal/mole and 74 ± 9 kcal/mole, respectively, which are associated with migration of vacancies within graphitic layers in the matrix material.

ESR Study of Glass-Like Carbons

Abstract ESR studies were carried out on a variety of glass-like carbons made from different precursors heat-treated in the range 300–2500 °C. It was found that the behaviors of unpaired electrons were nearly identical. The temperature dependence of ESR spectrum showed that the behavior of localized unpaired electrons in the glass-like carbons changed to that of delocalized ones at HTT about 2500 °C. The ESR spectrum of the glass-like carbons heat-treated at 800 °C consisted of a single line under a pressure of above 10−4 Torr but split into two components when air was admitted. This strong interaction with unpaired electrons and air (oxygen molecules) is caused by the active structure related to micropores in the glass-like carbons. We suppose that the polyene type radicals are the most suitable as the active structure to interpret the behavior of unpaired electrons in glass-like carbons heat-treated at 800 °C. As another major point in this study, it was found that there is a correlation between unpaired electron concentration and the g value.

Temperature dependence of Raman scattering by disordered carbon materials

Raman spectra of an as-deposited (2100 C) pyrolytic carbon and two glass-like carbons (heat treatment temperatures of 1800 and 2400 C) were determined as a function of temperature T over the range 90–310 K. Both the widths and the intensities of the bands at ~1350 (D). ~ 1585 (G) and ~ 2710 (G') cm −1 were found to be substantially independent of T. The frequency of the G band decreased slightly with increasing T (by ~ 0.013 cm −1/K), in agreement with reported results for highly oriented pyrolytic carbon (HOPG). However, the T dependence of the G' frequency for these carbons appears to be comparable to or less than that of G, unlike HOPG where the reported dependence is higher. Of most significance, the frequency of the disorder band D, which does not occur in graphites, was found to be substantially temperature independent in this range.

Conversion of phenol formaldehyde resin to glass-like carbon

Glass-like carbons have been produced for many years by the careful carbonization of a variety of starting materials such as cellulose, phenol formaldehyde, polyfurfuryl alcohol, acetone furfuryldehyde and other suitable thermosetting resins. A number of publications have appeared in the literature concerning their characteristics and applications. However, little is known about the preparation procedure and its influence on the development of this form of carbon. The present paper incorporates the studies on the carbonization behaviour of a suitable phenol formaldehyde resin and on the optimization of the phenol to formaldehyde molar ratio in the resin, carried out with respect to the various physical characteristics of the resins and the resulting carbons. The implications of these studies have been disucussed in detail.

64. Comparative profile analysis of glass-like carbons using X-ray and neutron diffraction

64. Comparative profile analysis of glass-like carbons using X-ray and neutron diffraction

209. Thermal expansion and density decrease of glass-like carbons with heat treatment

209. Thermal expansion and density decrease of glass-like carbons with heat treatment

Magnetic susceptibility and kinetics of “graphitization” of glass-like carbons

The room temperature magnetic susceptibility ( X) of glass-like carbons (GLC) from several sources has been determined as a function of heat treatment temperature (HTT) over the range 1000–3000°C. Effects of dilute ferromagnetic impurities were observed for HTT < 1500°C. The impurity (probably Fe) exists in a non-magnetic form in the carbonized GLC; transforms to a ferromagnetic state with 1000 ⩽ HTT ⩽ 1400°C; then disappears at higher HTT. The dependence of the diamagnetism of pure GLC on HTT is characterized by a slope decrease and inflection near 1500°C and increases smoothly thereafter. X ∼- 5.2 × 10 −6emu/g and is still increasing at HTT = 3000°C, and the values for different GLCs with the same HTT differ by < 10%. The evolution of X as a function of isothermal residence time (HTt) over the range 2400–3000°C for three GLCs was analyzed by the superposition method. Very high effective activation energies, 360–420 kcal/mole (~ 1500–1750 kJ/mole) were obtained and attributed to successive-activation processes of structural development required by the microstructural constraints inherent to difficult-to-graphitize carbons. Evidence was found for a small Xincrease due to the plastic volume dilation (density decrease) processes that occur in some GLCs at high HTT.

245. Experimental determination of compton scattering from pyrolytic graphite and glass-like carbons

In pyrolytic graphite, PG, the X-ray scattering from a Bragg reflection is all Compton modified radiation. A comparison of direct measurements shows that the Compton fraction in the scattering from disordered carbons, e.g., glassy carbon, can be calculated from measurements on PG and the adsorption coefficient of the disordered carbon.

Novel Applications of organometals: Magnetic, etc. properties of iron dispersions (I) in zeolites and (II) in amorphous glass-like carbons

The first part describes a novel technique for dispersing ultrafine species of iron oxide of controlled particle size (radii < 150 ») within the cage structure of a zeolite matrix, with the aid of organometals such as ferrocene and iron pentacarbonyl. The particle size and magnetic ordering within the iron clusters are elucidated via 57Fe Mössbauer spectrocopy and via their superparamagnetic behavior displayed by magnetization curves obtained as a function of the magnetic field and temperature. The second part of the paper describes the magnetic properties of iron species (Fe 3C, Fe 3O 4, etc.) dispersed in amorphous glass-like carbons. These are obtained by the polymerization of furfuryl alcohol-containg ferrocene derivatives and by the subsequent pyrolysis of the polyfurfuryl alcohol. The technological relevance and applications of the materials studied in Parts I and II of the paper are pointed out.

Crystallite orientation in polycrystalline graphites made from glass-like carbons under high pressure

The preferred orientation of crystallites was investigated on polycrystalline graphites made from the glass-like carbons with spherical particles under a pressure of 5 kbar at 1300 to 2000‡ C. The glassy carbon spheres 40 to 70 Μm diameter which were pre-heated at 1000‡ C under normal pressure, gave sintered discs with a bulk density of 1.5 g cm−3 and a relatively high degree of orientation. However, spheres of the same type pre-heated at 2000‡ C gave the high bulk density of 1.9 g cm−3 and a very low degree of orientation. The carbon beads 40 to 70 Μm diameter gave a relatively low degree of orientation and pre-heating at 2000‡ C reduced the degree very remarkably. Carbon beads with a smaller particle size, <20 Μm, gave a lower degree of orientation. Pre-heating of the initial carbon of small particle size at temperatures as high as 2000‡ C resulted in a low degree of preferred orientation of the crystallites in the sintered discs, the temperature dependence being the strongest.

Chemical reactivity of glass-like carbons

Chemical reactivity of glass-like carbons

The effect of crosslinking on the formation of glasslike carbons from thermosetting resins

The pyrolysis of polyfurfuryl alcohol and phenolic polymers leading to glasslike carbon is studied in detail. The effect of crosslinking of the polymers is shown by the modification of polyfurfuryl alcohol with formaldehyde as well as by comparison with oxidized polyphenylene oxide. Analogous shrinkage behavior independent of the starting materials leads to a comparable porosity of the glasslike carbons of about 30 percent. The pores are of uniform size, having a diameter of 25 Å after heat treatment to 2200°C and of 35 Å after heat treatment to 3000°C. Most of the pore entrances lie between 4 and 5 Å. The strength characteristics of glasslike carbon in dependence upon the heat treatment temperature exhibit maximum values after heat treatment to 2000°C, the resulting room temperature values being 15,000 psi for the flexural strength and 4.5−5 × 10 6 psi for Young's modulus.

Glass-like carbons

The preparation, structure, properties and some applications of glass-like carbons are outlined by reviewing the published studies made on these materials.