Electrical conductivity of carbon blacks under compression

A study was made of the electrical conductivity of six commercial carbon blacks under compression. The influence of the applied pressure, sample volume, mechanical work, and density of the carbons was investigated. The conductivity was measured at room temperature by impedance spectroscopy over the frequency range 20-10⁶ Hz with a voltage of 1 V. The decrease in volume under compression depended significantly on each particular carbon and on the applied pressure, and for a larger number of carbons it was only important at low pressures. The patterns of variation of the conductivity with pressure and mechanical work were similar. Of the pressure and volume effects comprised in the influence of mechanical work on the conductivity, that of the pressure appeared to predominate. The increase in conductivity with decreasing sample volume was greater for the carbons in which the volume under compression was smaller. The total increase in conductivity was greater for the densest carbons and least for the lightest one. Both the conductivity and its variation under compression were positively related to the density of the carbons.     

Filler-elastomer interactions: influence of oxygen plasma treatment on surface and mechanical properties of carbon black/rubber composites

In this work, the influence of oxygen plasma treatment on the surface and adsorption properties of carbon blacks was investigated using X-ray photoelectron spectroscopy (XPS), ζ-potential, and BET isotherms. Then the mechanical properties [tensile strength and tearing energy (G_IIIC)] of carbon black/acrylonitrile butadiene rubber (NBR) composites were measured. As a result, it was found that oxygen plasma treatment generated oxygen-containing functional groups, such as, carboxyl, hydroxyl, lactone, and carbonyl groups, on the carbon black surfaces, resulting in a decrease in the equilibrium spreading pressure or London dispersive component of surface free energy. The tearing energy of the carbon black/NBR composites improved as the oxygen-containing functional groups on the carbon black surfaces increased. This revealed that there is a relatively high degree of interaction between the polar NBR and the oxygen-functional groups of carbon blacks.     

Observation of single-walled carbon nanotubes by photoemission microscopy

Single-walled carbon nanotube networks grown on SiO₂ pillars were studied by means of scanning photoemission microscopy. The individual nanotubes or nanotube bundles growing from the pillar tops were observed in C 1s images. Band bending near catalytic Fe/nanotube contacts in an end-bonded configuration was studied by measuring C 1s spectra along the tube axes. Within our experimental resolution, no band bending was observed. This implies that the depletion width is less than the spatial resolution of the scanning photoemission microscope (90 nm) or that the amount of the band bending is less than 0.1 eV.     

Microporosity in carbon blacks

Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO₂ adsorption isotherm at 273 K, combined with computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N₂ (77 K), CO₂ (273 K) and C₆H₆ (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.     

Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study: IV. Reactivity of different carbon blacks in CF4 radiofrequency plasma

The effect of CF₄-plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF_x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp² character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp³C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp³C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF₂ and CF₃ groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.     

Experimental comparison of N(1s) X-ray photoelectron spectroscopy binding energies of hard and elastic amorphous carbon nitride films with reference organic compounds

In this work, hard and elastic amorphous carbon nitride (a-CN_x) films were deposited by DC magnetron sputtering on heated Si(001) substrates at 400 °C. Nanoindentation results confirmed that the films were highly compliant and had high elastic recovery. X-ray photoelectron spectroscopy (XPS) was used to investigate nitrogen bonding by directly comparing the N(1s) spectra of a-CN_x with the N(1s) peak positions of a variety of organic compounds that were characterized in the same XPS system. The N(1s) XPS spectra of hard and elastic a-CN_x is resolved into two dominant intensity contributions at 398.5 and 400.6 eV. We show that the N(1s) spectra of a-CN_x do not conclusively support a film-structure model with nitrogens bonded to sp³ carbons. We offer an alternate interpretation based on the presented data and previous XPS, nuclear magnetic resonance (NMR), and computational work. Together, the data suggest that hard and elastic a-CN_x consists of an sp² carbon network and that single-atom vacancy defects, as found in a graphite layer, may be present in the material. This implies that the low binding energy N(1s) component at 398.5 eV may be due to pyridine-like nitrogen bonded at the perimeter of a vacancy defect.     

Surface oxides on carbon and their analysis: a critical assessment

The methods for the determination of various types of oxygen surface functions on carbon materials are briefly described, and their relative advantages and problems that may arise are discussed. Acidimetric titration techniques, IR spectroscopy, XPS, thermal desorption spectroscopy, and electrokinetic measurements are described.     

Fluorinated carbon blacks: influence of the morphology of the starting material on the fluorination mechanism

The effect of fluorination, using CF₄ r.f. plasmas, has been studied on three different types of carbon blacks: a thermal black, a furnace black and a high electrical conducting black. The influences of the morphology and structure of the three blacks on the fluorination mechanism have been investigated. In particular, the ratio Type I/Type II structures (i.e., surface (CF) and border groups of graphitic domains with sp²C/polyalicyclic perfluorinated structures with sp³C), has been correlated to the microstructural organisation. The transformation into Type II structures is more easily achieved in highly accessible XE2 blacks, whereas in materials with lower crystallinity (MT), the presence of numerous defects leads preferentially to surface (CF_x) perfluorinated groups.     

Microstructure of carbon blacks determined by X-ray diffraction profile analysis

The microstructure of carbon blacks is investigated by X-ray diffraction peak profile analysis. Strain anisotropy is accounted for by the dislocation model of the mean square strain in terms of average dislocation contrast factors. Crystallite shape anisotropy is modeled by ellipsoids incorporated into the size profile function. Different grades of carbon blacks, N990, N774 and N134, untreated, heat-treated and compressed at 2.5 GPa have been investigated. The microstructure is characterized in terms of crystallite size-distribution, dislocation density and crystallite shape anisotropy. Heat treatment results in increased vertical and lateral sizes of graphitic crystallites. Postproduction pressure treatment has little effect on the average sizes of the crystallites, however, it affects the crystallite size distribution function. The average sizes of the crystallites obtained by X-ray diffraction agree with those estimated from Raman spectra. Applied pressure affects the magnitude of strain within the crystallites.     

Surface characteristics of fluorine-modified PAN-based carbon fibers

Different fluorination methods were applied to modify the surface properties of carbon fibers. The relationship between the degree of fluorination and the physicochemical properties of carbon fibers was studied using a combination of mechanical tests, elemental analysis (EA), X-ray photoelectron spectrometry (XPS), and X-ray diffraction (XRD). EA and XPS analyses of fluorinated carbon fibers showed that treatment with mixtures of F₂/O₂ introduced a much higher fluorine concentration than that with F₂ only. However, XRD analysis showed that there was no increase in the interlayer distance, due to the mild fluorination condition applied. Consequently, the oxyfluorination was one of the more effective methods to increase surface polarity of carbon fibers, which probably played an important role in improving the tensile properties of the fibers in the epoxy resin system.     

A comparison of surface morphology and chemistry of pyrolytic carbon blacks with commercial carbon blacks

The surface morphology and chemistry of CBp obtained by pyrolysis of waste tyres at 500 and 700 °C, respectively was studied compared with a commercial tyre carbon blacks by laser particle size analyzer, X-ray diffractogram (XRD) and electron spectroscopy for chemical analysis (ESCA). The distribution of CBp aggregates was the mixed particle distribution of commercial carbon blacks added to tyres in fabrication. The concentration of inorganic compounds and carbonaceous deposits (the organic compounds deposited on the surface of the CBp) depends on the pyrolysis temperature. The chemical nature of the CBp from pyrolysis at 700 °C was found to be closer to the commercial tyre carbon blacks than the CBp from pyrolysis at 500 °C.     

Oxygen plasma modification of pitch-based isotropic carbon fibres

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N₂/CO₂ adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did).     

Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites

The effect of an oxygen plasma treatment upon the structural and morphological properties of high-modulus carbon fibres has been studied by means of several characterisation techniques. Scanning electron microscopy showed that there were only minor changes of the morphology of the fibres following treatment. X-ray diffraction traces revealed that there were differences in structural parameters between the untreated fibres but no further modifications in the crystalline structure were detected after the plasma oxidation. Raman spectroscopy was used to follow the changes on the fibre surface structure following treatment. The peak positions and widths of the four main Raman bands (D, G, D′ and G′) were determined, with no significant changes observed after the surface treatment. A relationship between the width of the G band and the crystal parameter d₀₀₂ was found, with the magnitudes of both decreasing as the fibre modulus increased. A reference order parameter I_D/(I_D+I_G) ratio was calculated from the intensities of D and G bands. The treated fibres exhibited a more highly disordered surface structure that the untreated ones, as revealed by the increase of I_D/(I_D+I_G) after the plasma oxidation.     

Surface energy distribution of carbon black measured by static gas adsorption

The energetic surface heterogeneity of various carbon blacks has been investigated by means of volumetric gas adsorption techniques. From the adsorption isotherms of ethene at a coverage of about 0.001 up to 1 monolayer the energy distribution functions of adsorption sites are evaluated. Four discrete species of energetic adsorption sites are identified on all carbon black surfaces irrespective of morphology. The fraction of the different adsorption sites is found to depend significantly on the microstructure and primary particle size and on manufacturing conditions as well.     

On the nature of basic sites on carbon surfaces: an overview

The nature of basic sites on carbon surfaces is a controversial issue within Carbon Science. Traditionally, oxygen-containing groups like: γ pyrone-type, chromene, diketone or quinone groups, on the one hand, and delocalized π-electrons of the basal planes, on the other, are assumed to have a basic nature. However, there is no consensus about neither the strength as bases of these sites, nor the extent of their contribution to the overall carbon basicity. Researchers are divided between those that support basic-oxygen-containing groups as main responsible of carbon basicity and those that attribute the overall basicity of a carbon to the delocalized π-electrons of the basal planes. In this work we will discuss the virtues and drawbacks of the different hypotheses about basic carbon sites in view of experimental and theoretical results.     

Characterisation of surface oxides on carbon and their influence on dynamic adsorption

The aim of this work is to gain a better fundamental understanding of the nature of surface oxide sites present on carbon surfaces, and their role in the adsorption process. A number of model carbon substrates with different degrees of surface oxidation and similar textural properties were prepared using a wide range of solution and gaseous phase oxidation techniques. Some of the carbons were characterised using established techniques including flow microcalorimetry, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The results showed that both carboxylic acid (-CO₂H) and ether/hydroxyl (C-O) surface oxygen complexes were introduced to all of these carbons as a result of the oxidation processes. The number, strength and thermal stability of the surface groups formed were dependent on the nature of the base material and the oxidation conditions employed. The dynamic adsorption performance of the carbons against hexane, under humid conditions, was found to be mainly determined by the quantity of acidic surface functional groups. However, the location, strength of interaction and availability of the surface oxygen complexes to the adsorbate molecules, are also thought to affect the breakthrough characteristics of the carbons used in this work.     

Bulk and surface chemical functionalities of type III PAN-based carbon fibres

PAN-based carbon fibres carbonised at relatively low temperature, i.e. type III carbon fibres, were submitted to heat treatment at 2300 °C (GR) or oxidation in nitric acid. The samples were characterised by XPS, FTIR, wetting measurements, gas adsorption, elemental analysis and acid/base titration. While oxidation only slightly affects the nitrogen concentration, it produces an appreciable change in the nature of the chemical functions, namely the conversion of pyridine-type nitrogen and quaternary nitrogen into aliphatic functions. Oxidation treatment modifies all the material constituting the fibre, the oxygen concentration being about 1.5 times higher at the fibre external surface compared with the whole material. Three components (531.2, 532.6 and 533.8 eV) are clearly identified in the oxygen XPS peak, allowing a comparison to be made between the whole material and the external surface regarding chemical species. The acidic groups are mainly carboxyl. Fibres submitted to extensive oxidation also show a high basicity, attributed mainly to calcium carboxylate. Although the acidic and basic groups present in the whole material can be titrated with aqueous solutions, the fibres develop only a very small surface area and no microporosity as determined by krypton adsorption. The material may be viewed as a sponge, collapsed when dry but able to swell in water and developing a high cation-exchange capacity.