A variety of activated carbons (ACs) with different burnoff degrees (0-60%) and polystyrene cross-linked with divinylbenzene (PSDVB) were studied using small-angle X-ray scattering (SAXS) and nitrogen adsorption methods. The ACs demonstrate increased deviation of the pore shape from the model of slit-shaped pores with increasing burnoff degree and parallel increasing contributions of pores in the 0.3-50 nm range. The pore size distributions (PSDs) calculated using SAXS and density functional theory (DFT) methods have similar shapes but a more detailed picture for broad pores with SAXS PSDs. The PSDs and chord length distributions of ACs and PSDVB adsorbents have certain close features depending on the specific surface area because contributions of narrower pores and thinner pore walls increase with increasing specific surface area practically independent of the material type.

Three sets of activated carbons (ACs) were prepared with the same precursor but activated differently (by CO2 or water vapour) with various burn-off levels. The ACs demonstrate increased deviation of the pore shape from the slitshaped model with increasing burn-off and contributions of pores of different sizes depending on the activation type. Significant re-arrangement of adsorption complexes, especially of the Van der Waals type characteristic for nonpolar or weakly polar adsorbates (H2, CH4, CH2Cl2, CHCl3), occurs in both micropores and mesopores of ACs with decreasing temperature. The behaviour of their mixtures with water and DMSO can strongly differ from that of individual adsorbates.

In a coherent X-ray small-angle experiment, heterodyning between the scattering amplitudes of two samples is obtained by stacking a static reference and a fluctuating sample. Results of homodyne and heterodyne measurements are compared in the case of 98 nm diameter latex particles in glycerol. The method is also used for the study of the slow relaxation process of carbon-black-filled ethylene–propylene elastomers corresponding to the relaxation of the carbon black skeleton after a 100% elongation. On the scale of the 10 µm coherent beam, heterodyning is used to separate fluctuations from long-term flowing of the sample. We show that this flow can be observed for about 10 h, with velocities of the order of nanometres per second. Random fluctuations are dominant in the speckle changes only for large q values (q > 2 × 10−2 A−1) and after a long relaxation time.

A straightforward way of measuring X-ray intensity fluctuation spectroscopy in a small-angle X-ray scattering configuration is demonstrated using heterodyne techniques. Two examples are presented: the Brownian motion of latex spheres in glycerol, and a Doppler velocity experiment demonstrating the motion and the relaxation of carbon-black-filled elastomers after uniaxial stretching. In the latter case the effects of mechanical relaxation can be separated from those of aggregate diffusion. The results suggest that the dynamics of these filled elastomers are similar to the universal features observed in disordered jammed systems.

A carbon aerogel was obtained by carbonization of an organic aerogel prepared by sol-gel polymerization of resorcinol and formaldehyde in water. The carbon aerogel was then CO(2) activated at 800 degrees C to increase its surface area and widen its microporosity. Evolution of these parameters was followed by gas adsorption and small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) with contrast variation by using dry and wet (immersion in benzene and m-xylene) samples. For the original carbon aerogel, the surface area, S(SAXS), obtained by SAXS, is larger than that obtained by gas adsorption (S(ads)). The values become nearly the same as the degree of activation of the carbon aerogel increases. This feature is due to the widening of the narrow microporosity in the carbon aerogel as the degree of activation is increased. In addition, WAXS results show that the short-range spatial correlations into the assemblies of hydrocarbon molecules confined inside the micropores are different from those existing in the liquid phase.

a Laboratoire de Spectrometrie Physique, UMR 5588 CNRS-UJF, Domaine Universitaire, 38402 Saint Martin d’Heres Cedex, France b Departamento de Quimica Inorganica, Universidad de Granada, 18071 Granada, Spain c Centre Energetique et Procedes, FRE 2861 CNRS-EMP, 06904 Sophia-Antipolis Cedex, France d Laboratoire de Thermodynamique et Physico-Chimie Metallurgique, UMR 5614 CNRS-UJF-INPG, Domaine Universitaire, 38402 Saint Martin d’Heres Cedex, France

The subject of this paper is the investigation of the multiscale structure of a new series of carbon aerogels. These carbon aerogels are obtained by resorcinol resorcinol–formaldehyde sol–gel reactions in acetone in a single-step base catalysis (AB) or a double-step base–acid catalysis (ABA) followed by supercritical drying and pyrolysis at 1050°C under nitrogen flow. Two complementary techniques were used: small-angle and wide-angle X-ray scattering (SAXS and WAXS) and nitrogen adsorption. Carbon aerogels AB and ABA are distinguished by a high surface area (>600m2g−1) and by multiscale structural features that are very different from those of carbon aerogels resulting from gels prepared under the same conditions but in an aqueous solution (WB). The anisotropy observed at the mesoscale for aerogels obtained in acetone is attributed to strain occurring in the extreme operating conditions near the critical gelation concentration limit. The SAXS measurements on AB carbon aerogel reveal the presence of a high surface area with the main pore size contribution being at the nanoscale, resulting from micropores not accessible to nitrogen at −196°C. By combining SAXS and WAXS measurements, it is suggested that all carbon aerogels display structural narrow micropores.

Complementary techniques, including low-temperature nitrogen adsorption and small-angle X-ray scattering (SAXS), are applied to detect the effects of surface functionalization on the morphology of activated carbon derived from poly(ethylene terephthalate) (PET). Scanning electron microscopy (SEM) is also employed as an auxiliary method to visualize the surface below the micron scale. The SEM images reveal a micron-sized ridgelike texture. Room temperature acid treatment makes the ridges become more pronounced, while treatment with boiling acid uncovers fiberlike structures of roughly 1 microm diameter. All samples display an apparent surface fractal dimension of Ds = 2.4 in the wave vector range 0.001-0.02 A(-1). Nitric acid at room temperature increases the surface oxygen content only by 3 at. %, while all the adsorption properties and structural parameters reported in this paper are virtually unaffected. Significant differences in the morphology at submicron scales appear only after boiling acid treatment. The resulting carbon remains highly microporous, but the loss of Brunauer-Emmett-Teller (BET) surface area from about 1150 to 304 m2/g is approximately 75%. In addition to the principal peak at around 8 A, fresh peaks appear in the polydisperse Horvath-Kawazoe (HK) pore size distribution owing to the burnoff of intervening walls. The average width of the slitlike pores calculated from the Dubinin-Radushkevich (DR) plot increases from 8.4 to 11 A. The minimum slit width where the applied probe molecules, that is, nitrogen and hexane, can enter increases from about 5 to about 5.4 A. The separation distance of the basic structural units is practically unchanged. When, however, this carbon is in contact with hexane, this distance expands from about 19 to 27 A. The swelling is consistent with the deformable nature of this sample also illustrated by the low-pressure hysteresis and the reduced helium density. Particular attention was paid to the surface areas derived from low-temperature nitrogen adsorption and X-ray measurements. Owing to the wide spatial range of the structures in these samples, estimates of the specific surface area of activated carbons can be substantially in error unless both upper and lower q ranges of the SAXS spectra are taken into account. Surface areas derived from the adsorption data either by the BET or the DR approaches were always below the values obtained by standard SAXS. As an example, the carbon sample functionalized at room temperature gave surface area values of 1114, 1293, and 1970 m2/g, respectively. The possibility that this difference is caused by inaccessible pores was excluded by contrast variation measurements with hexane.

Previously published results have shown that washing and aging treatments (respectively in water/ethanol and water/polyethoxydisiloxane solutions) of silica gels (synthesized from polyethoxydisiloxane precursors through HF catalysis) significantly enhance both the permeability and mechanical properties of the wet gels. Unfortunately, scaling-up the process induces severe cracks during supercritical drying. This phenomenon has been related to a pore-size distribution gradient between the surface and the bulk of the gel that appears after aging of thick gels. We report efforts to optimize the aging step. Aging in less-concentrated polyethoxydisiloxane solutions yields gels with a more homogeneous pore-size distribution and has enabled us to obtain large monolithic and crack-free aerogels that remain light and transparent.

Elastomers are soft materials that can be reinforced by dispersing into them nanosized solid particles. Common examples of the latter are silica or carbon black aggregates. However, the mechanism of reinforcement is still not yet fully understood. Our work consists in investigating by small-angle X-ray scattering (SAXS) the structure of the aggregate network spreading throughout the matrix in the initial sample and its modification during and after straining (elongation). The goal is to relate the macroscopic mechanical behaviour with the structure of the aggregate network. The present paper is a qualitative overview of recent results obtained on well defined composites.

Ordered nanoporous carbons can be prepared by a replica technique starting from an organized silica template. The silica template used, SBA-15, displays hexagonal arrangements of mesopores interconnected by micropores. Two routes are possible for introducing carbon into the pores of the silica: liquid impregnation by a solution of sucrose followed by carbonization or chemical vapor infiltration (CVI). After dissolution of the silica template by hydrofluoric acid treatment, a carbon material is obtained. Small-angle X-ray scattering (SAYS) measurements were performed over a broad range of wave vector q in order to investigate the multiscale structure of the carbon replica as a function of the method of infiltration (liquid or gas route) and the amount of infiltrated carbon. Because of the close match in electron density between silica and carbon, it is possible to investigate the silica mesopore filling. It appears that at least 50% of the pore volume must be filled in order to obtain an organized carbon replica after dissolution of the silica template. It is also shown that the gas route (CVI) prevents the spatially proportional (i.e., affine) shrinkage observed for replicas prepared by liquid impregnation. TEM confirms that the organized carbon is a nearly perfect negative replica of the silica porous structure.

When dispersed inside a gel matrix a ferrofluid loses its translational degree of freedom, but the particles remain free to rotate and locally deform the surrounding polymer matrix. Small angle X-ray scattering methods were used to investigate the structure of dilute suspensions of two different ferrofluids dispersed in soft polyacrylamide hydrogels. The particles both in the free fluid and in the gel are shown to be fractal aggregates composed of smaller elementary units of radius ca. 5 nm. The volume fractal dimension D of the aggregates is strongly sample dependent, taking the value 1.7 in one case and 2.9 in the second. In a magnetic field the aggregates tend to orient along the direction of the field, but owing to the translational constraint are unable to form linear arrays as in the free fluid. At high magnetic field strengths (ca. 1 T), the magnetization of the elementary particles aligns along the field and the aggregates tend to stretch in that direction. The effect of the gel elasticity is treated as a potential barrier to the orientation process.

Anomalous small angle X-ray scattering is used to determine the distribution of divalent ions in a neutralized polyelectrolyte gel of sodium polyacrylate in the vicinity of the volume transition. At the five different energies of the incident beam used to vary the contrast, the scattering curves have similar shapes, and are separated only by constant multiplying factors. This result, in conjunction with SANS results from the same sample, indicates that the divalent ion (strontium) is confined on the polymer backbone.

To carry out small-angle scattering experiments with X-rays in the scattering vector range q < 10 −3 Å −1 with area detectors, background must be carefuly reduced. The principal source of background is slit scattering. Careful polishing of slits allows slit diffraction to be observed, which can be calculated from wave equations. In this case, the background takes the shape of thin streaks perpendicular to the slit edges. This introduces the design of new beamstops, which are cross-shaped. Typical results obtained on the D2AM beamline of the ESRF are shown, especially for the case of coherent scattering.

Small-angle x-ray scattering, nitrogen adsorption, and scanning tunneling microscopy show that a series of activated carbons host an extended fractal network of channels with dimension D(p) = 2.8-3.0 (pore fractal), channel width 15-20 A (lower end of scaling), network diameter 3000-3400 A (upper end of scaling), and porosity of 0.3-0.6. We interpret the network as a stack of quasiplanar invasion percolation clusters, formed by oxidative removal of walls between closed voids of diameter of approximately 10 A and held in registry by fibrils of the biological precursor, and point out unique applications.

During the last decade, several new precursors have been proposed for the synthesis of organic aerogels. In a large majority of cases, the sol–gel reaction is base catalyzed and water is used as the solvent. Because of the poor solubility of liquid CO2 in water, a time consuming two-step exchange is necessary (exchange of water by acetone followed by exchange of acetone by liquid CO2). To eliminate this step, a new process with the solvent acetone was developed. Also in order to reduce gelation time, acid catalysis is used. The aim of the present work is to compare the solid and the porous texture of resorcinol–formaldehyde (RF) aerogels prepared by the conventional method (in water and base catalysis) to that of samples prepared by the new method (in acetone and acid catalysis), all other parameters (mass ratio, catalyst concentration R/C, supercritical extraction, pyrolysis conditions) remaining the same. Determination of pore size distribution (PSD) in different series of organic and pyrolyzed aerogels by thermoporometry and characterization of the solid structure by small-angle X-ray scattering (SAXS) measurements over a wide range of length are reported. The main difference between the two series of aerogels lies in the aggregation of the primary particles: the new series prepared in acetone and acid catalysis displays fractal scaling over more than a decade in length, while the conventional one does not. It is also shown that thermoporometry yields reliable information for both series of aerogels.

The electrical resistivity of carbon black filled high density polyethylene increases significantly when the composite is heated to the melting temperature of the matrix. The paper aims to provide new insights on this feature, known as the Positive Temperature Coefficient (PTC) effect for resistivity. The influence of the volume fraction φ of carbon black and of the temperature on the DC conductivity is analyzed and compared to their influence on matrix expansion. Measurements of the dependence on frequency of the AC conductivity of carbon black–polyethylene composites, at different loadings and at different temperatures, are performed. They yield experimental evidence of a change of connectivity of the conducting network during heating near the melting temperature of polyethylene. It is shown that the initial volume fraction of carbon black strongly affects the PTC mechanism.

An investigation is described into polyethoxydisiloxane (PEDS-Px) silica precursors of aerogels prepared in ethylacetoacetate (etac) under HF conditions and dried under CO2 supercritical conditions. The influence upon the internal nanostructure of aerogels of the number of water molecules (n*) used to synthesize the precursors is studied, as well as that of their volume fraction (xPrec) in solution during the second catalytic step. Correlation between structure and optical transmission (%TR) is necessary to improve optical behavior of monolithic silica aerogels for double-window applications. For this reason, small angle X-ray scattering (SAXS) was performed to characterize the nanostructural silica skeleton, and especially to investigate particle and cluster properties. Optical transmission was also measured to estimate visible optical quality of the samples (%TR). Among other results, it is shown in this study that increasing n* and xPrec contributes to the decrease of the particle and cluster sizes, which indirectly improves %TR in the visible range. These observations confirm and help to explain previously published results.

Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a significant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stiffen the gel. As expected, a significant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiffness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS).

Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol–gel reaction. It is therefore highly desirable to investigate the sol–gel mechanism. For this purpose, dynamic light scattering (DLS) measurements have been performed at different stages of the reaction for base- or acid-catalyzed gelation of resorcinol–formaldehyde (RF) using water or acetone as solvents. The structure of aged gels was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO2 and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this series of aerogels consists of mass fractal aggregates (Dm=2.5). The partial collapse of this polymeric gel yielding colloidal particles in the aerogel can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS.