A SAXS study of silica aerogels

Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porod's limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.     

SAXS investigation of porous nanostructures

Fractal and aggregate structures of aerogels were investigated by small angle X-ray scattering in order to analyze the various evaluation methods of the SAXS data for porous nanostructures. Scattering data (SAXS, USAXS) for aerogels measured with laboratory equipment as well as synchrotron technique were interpreted in the terms of Guinier, Porod, Freltoft, Teixeira, and Emmerling theories. We modified the Freltoft fit in order to get information about the structure of elementary units. The performances of the evaluation programs were studied for different aerogels structures such as fractal of wide range, fractal of limited size, and aggregate systems. The evaluation of the scattering measurements resulted in fractal dimensions, sizes of the elementary units, sizes of the fractal domains or aggregates. Quality of the fits to SAXS data was characterized by a mathematical parameter and proved by TEM photography. TEM images confirmed the sizes of the elementary building units and clusters.     

USAXS studies of monosaccharide gels. II. The common features of structural changes

The results of SAXS studies are compared for two groups of samples: (i) obtained by in situ process of gelation of α-galactose-based gel with benzene as solvent, (ii) gels of various concentrations prepared from glucofuranose-based gelator with toluene. The experimental conditions of sample preparation and SAXS measurements were very similar for both groups. The results include fractal analysis of aggregates, and – for aggregate envelopes – the radii of gyration, pair functions, and simulated models. For both groups only two aggregate types were found to be formed: the first – of simple, loose fractal structure and disk-like shape, and the second – smaller aggregates, of well-developed fractal structure and cylinder-like shape. The conditions of the change from the first type to the second one are discussed. In addition, it was verified by the SAXS method on the basis of kind of fractal analysis for the loose structure that the fractal aggregate is built of gelator molecules joined by hydrogen interaction.     

Adsorption of alcohol vapors on alkoxide-derived silica gels

The adsorption behavior of alcohol vapors on acid-catalysed silica gels was investigated. The adsorption isotherms determined at two ambient temperatures were type IV or pseudo type I of BET classification, and were all interpreted by the duplicated processes of multilayer adsorption and capillary condensation. Some of the isotherms obtained gave both the monolayer capacity and the pore volume, which made it possible to calculate the mean pore radii and the fractal dimensions of the pore surfaces. The fractal dimensions obtained were close to 2, indicating the essentially smooth surface on the molecular scale. The gels prepared from the solutions containing smaller alkoxy groups were found to retain larger pore volume, which was explained in terms of the aging effect under acidic conditions.     

A SAXS study of the gelation process of silicon and titanium alkoxides

The gelation process of silicon ethoxide and titanium iso-propoxide solutions was studied as a function of water content and reaction time by small-angle X-ray scattering (SAXS). Approaching the gelation points, the SAXS intensities for titanium tetra-iso-propoxide solutions start to follow a power-law decay in the Porod region, except for a H₂O/Ti ratio greater than 4. For silicon ethoxide solutions, the fractal dimension, d_f, measured for aggregated clusters increases continually with the H₂O/Si ratio and can be related to the spinnability of the solutions. For solutions of both silicon and titanium alkoxides, a solution of fractal dimension d_f < 1.79 shows spinnability, whereas solutions having d_f > 1.79 and no fractal structures do not show spinnability.     

The effect of the H2O/TEOS ratio on the structure of gels derived by the acid catalysed hydrolysis of tetraethoxysilane

Silica gels were produced by the acid catalysed hydrolysis of tetraethoxysilane (TEOS) using H₂O/TEOS ratios from 2 to 50. After heat treatment the structure of the gels was studied using nitrogen adsorption, small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and bulk density measurements.All the gels possessed microporosity in the region of 30 Å or less. For H₂O/TEOS = 25 and 50 the matrix density was found to be uniform, but for gels from solutions with H₂O/TEOS = 2,4 and 10, density fluctuations in the matrix were detected from Porod analysis of the SAXS data. These results indicate that in higher water content solutions, rearrangement of the polymeric chains leads to small densified particles, but for lower water content solutions, gelation results from the entanglement of linear chains leaving free volume on a molecular scale between the chains.     

SAXS and DSC studies on the structural characteristics of siliconized s-triazine glassy hybrid materials

Three hybrid materials composed of planar s-triazine rings and polyhedral silica (SiO₂), phenylsilsesquioxane (PhSiO_1.5) and diphenylsiloxane (Ph₂SiO) building blocks were investigated by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) techniques. These measurements revealed that the geometrically dissimilar components were fully integrated into intact glassy hybrid structures. Their DSC thermograms showed that these hybrids are thermally stable below 350 °C with moderate glass transition temperatures (T_g) of 56–110 °C consistent with the increasing structural connectivity of the silicone component. The SAXS data was analyzed to obtain different structural information using Porod, Guinier and Kratky approximations. The general features of each of the SAXS profiles of these hybrids are very similar to those of polyphenylsilsesquioxane (PPhSQ). The SAXS profiles reveal that these hybrids can be described as nano-scale primary particles that are self-organized in macromolecular ensembles to form extended unfolded textures of varying scattering lengths (91–168 Å). The obtained hybrid particles adopt either 3-D bulk fractals with open structures or 2-D surface fractals with dense cores. The short interfacial thickness (< 3 Å) and the low thermal fluctuation parameters strongly suggest that these particles are held together by substantial cohesion forces.     

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

Poly(vinyl alcohol)/silica hybrid xerogels were prepared from sonohydrolysis of tetraethoxysilane (TEOS) and additions of water-solution of poly(vinyl alcohol) (PVA). The samples were studied by small-angle X-ray scattering (SAXS), nitrogen adsorption, and differential scanning calorimetry (DSC). On drying at room temperature the resulting xerogels exhibit a fairly bimodal porous structure composed by small mesopores and micropores. The pore size distribution of the mesopores was found to follow approximately a power-law with the pore size. The micropore structure was associated to an evolution at a high resolution level of the mass fractal structure of the original wet gels. The role of the PVA addition on the pore structure of the xerogels is to diminish the specific surface area and the pore volume without to change substantially the pore mean size.     

Shrinkage behavior of silica gels during drying

The shrinkage behavior during drying has been studied for silica gels prepared from a mixture of TEOS, water and ethanol. Significant differences in the shrinkage were noted between base- and acid-catalyzed gels after that the volumes shrank to about 40% of the initial value. The volume shrinkage was also affected by the alcohol/water ratio of the pore liquid. The gel had a tendency to shrink still more with increasing water concentration in the liquid. On the other hand, the higher the alcohol content of the pore liquid, the lower the bulk density of the dried gel. SEM pictures illustrate that both base- and acid-catalyzed gels have granular structures consisting of particles of similar size initially and, when dried, the packing state of the gel particles are different, depending on catalysis for the hydrolysis-polymerization and alcohol/water ratio of the pore liquid.     

The preparation of carbon aerogels based upon the gelation of resorcinol-furfural in isopropanol with organic base catalyst

Carbon aerogels with high BET surface area were developed by sol-gel polycondensation of resorcinol and furfural in isopropanol using hexamethylenetetramine (HMTA) as a catalyst, and then directly drying the organic gels under isopropanol supercritical conditions, followed by carbonization under a nitrogen atmosphere. The preparation conditions of carbon aerogels were explored by changing the mole ratio of resorcinol to basic catalyst HMTA (R/C), the ratio of resorcinol to isopropanol (R/I), and the mole ratio of resorcinol to furfural (R/F). The effect of these preparation conditions on the porous structure of the carbon aerogels obtained was studied by nitrogen adsorption isotherms. According to the characterizations of TEM, SEM and nitrogen adsorption, the carbon aerogels obtained have a three-dimensional network that consists of carbon nano-particles with size from 20 to 30 nm, which define numerous micropores, mesopores and macropores. HMTA reacts not only as a catalyst but also as a reagent in the gelation polymerization. XRD characterization indicates that carbon aerogels have disordered nanocrystalline structures similar to activated carbon.     

Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Structural model of gelation processes of a sodium silicate sol destabilized by calcium ions: combination of SAXS and rheological measurements

This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels.     

Structural investigation of alumina silica mixed oxide gels prepared from organically modified precursors

The development of the structure of silica/alumina gels was investigated by small angle X-ray scattering (SAXS) through all stages of the preparation process, that is gelation, aging, drying and calcination. Two series of gels were compared; the first was prepared from a single-source precursor, obtained by reaction of 3-oxoethyl-6-trimethoxysilyl-hexan-2-one (OTH-H) with Al(O^sBu)₃. The gels of the second series were prepared from the same precursors, but under conditions where hydrodeacylation of OTH-H occurs. It turned out that both series showed a similar structural development of the gels. Immediately after the start of the reaction small primary particles are formed, the size of which (r = 0.7 ± 0.1 nm) remains constant through the gelation and aging process. In some samples, Si(OEt)₄ was added as an additional source for Si/O which was incorporated between the primary particles. Condensation proceeds by a slower aggregation of the primary particles, which initially form a rather open network. This network densifies during aging and drying.     

Liquid crystals,fractals and art

We discuss the aesthetic appeal of liquid crystal textures and take a brief polarising microscopic tour through the wealth of the appearances of nematic, cholesteric, smectic and banana phases. Some examples are given on how these textures may be used in a creative process employing digital image processing to produce ‘computer art’, including images exhibiting self-similarity. A discussion of the ‘art’ of computer-generated mathematical fractals leads us to examples of fractal structures observed in the phase-ordering process of some liquid crystalline phases. This includes a first brief report of dilatation invariance observed for aggregates of a conventional SmC phase and percolation simulations to quantitatively explain the obtained textures. The circle between liquid crystals, fractals and arts is closed by a discussion of self-similarity in modern arts, especially works of the movement known as abstract expressionism. More than 100 paintings of the artists Wassily Kandinsky, Jackson Pollock, Mark Tobey and Franz Kline were analysed, placing an emphasis of the discussion on the ‘drip-paintings’ of Pollock in relation to recent reports.     

Positron annihilation study of pore size in ordered SBA-15

Ordered mesoporous materials such as SBA-15 possess a network of channels and pores with a well-defined size in the nanoscale range (2–50 nm). This particular pore architecture makes them suitable candidates for a variety of applications. Different techniques have been used to measure pore diameters. PALS (positron annihilation lifetime spectroscopy) nanoprobe has been used to investigate free volume in several materials, including mesoporous silica. PALS can be used to find out if the micropore and mesopore structures of samples prepared under different experimental conditions are different. Indeed, considering that the pores present a cylindrical shape, an equation was developed that uses specific pore volumes, theoretical density, and specific surface measurements to evaluate structural connectivity. Our goal is to determine the influence of aging temperature on the porous structure of SBA-15 samples. The structural evolution was studied by PALS, small-angle X-ray (SAXS), N₂ adsorption desorption isotherms and computational modeling to evaluate connectivity. The variation of aging temperature changes the pore structure, indicating the presence of micropores and connections between mesopores. Materials aged at high temperatures present the lowest microporosity.     

Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

A SAXS study of the nanostructural characteristics of TEOS-derived sonogels upon heat treatment up to 1100 °C

Xerogels obtained from the acid-catalyzed and ultrasound stimulated hydrolysis of TEOS were submitted to heat treatment at temperatures ranging from 60 to 1100 °C and studied by small-angle X-ray scattering (SAXS). The SAXS intensity as a function of the modulus of the scattering vector q was obtained in the range from q₀=0.19 to q_m=4.4 nm⁻¹. At 60 °C the xerogels exhibit an apparent surface fractal structure with a fractal dimension D_S∼2.5 in a length scale ranging from 1/q₁∼1 to 1/q_m∼0.22 nm. This structure becomes extremely rough at 120 °C (D_S∼3) and at 150 °C, it apparently converts to a mass fractal with a fractal dimension D∼2.4. This may mean an emptying of the pores with preservation of a share of the original mass fractal structure of the wet aged gel, for it had presented a mass fractal dimension D∼2.2. A well characterized porous structure formed by 2.0 nm mean size pores with smooth surface of about 380 m²/g is formed at 300 °C and remains stable until approximately 800 °C. At 900 °C the SAXS intensity vanishes indicating the disappearance of the pores in the probed length scale. The elimination of the nanopores occurs by a mechanism in which the number of pores diminishes keeping constant their mean size. The xerogels exhibit a foaming phenomenon above 900 °C and scatter following Porod's law as does a surface formed by a coarse structure.     

Nanoparticle Aggregation and Fractal Growth in Fluid Smectic Membranes

We observe directly the diffusion and aggregation of buckyball clusters dispersed in thin, freely-suspended films of smectic liquid crystal using reflected light microscopy. The buckyballs at early times are barely resolvable, nanoscale clusters, which diffuse stochastically in the film. Clusters eventually coalesce to form micron-scale, fractal aggregates whose effective radius increases approximately linearly with time, so that after several days, millimeter-size fractals extend across the entire film. The measured fractal dimension of these final networks suggests that the aggregation of the buckyball clusters in the film is a diffusion-limited process.     

Understanding and Controlling the Morphology of ZnO Crystallites under Hydrothermal Conditions

The morphological features of ZnO crystallites influenced by solution basicity under hydrothermal conditions have been studied from the standpoint of the incorporation of the growth units. A crystal chemistry approach is developed to understand and thus to control a desirable morphology of the crystallites. The effects of the additive OH⁻ on the crystal morphology of ZnO crystallites are qualitatively but satisfactorily explained by a mechanism considering two aspects: (1) solution structures and the structural forms of growth units under a certain growth condition, i.e. the interactions between the solvent molecules and the growth units; and (2) the influence of the solvent molecules or additives on growth interfaces, particularly on two polar faces of ZnO crystallites in terms of the inhibition or promotion of the incorporation of various growth units depending on the solution basicity. Since the incorporation rates of the growth units are different on positive and negative polar faces, the relative growth rates of these faces are different and thus lead to the habit modifications. The approach clearly demonstrates that the hindrance of crystal growth is a consequence of surface adsorption processes.     

Gel to glass conversion: Densification kinetics and controlling mechanisms

The densification kinetics and controlling mechanisms during the gel to glass conversion are revised. The goal has been to analyse the controlling factors that lead to species joining and porosity elimination. This review includes several sections. First of all, the current understanding of gel structures, the driving forces for densification and the mechanisms involved are examined. Further, an analysis is carried out on the shrinkage due to dehydration of gels. Finally, the general approaches that have been used to formulate the kinetics of viscous sintering analytically and the experimental results for viscous sintering of gels under conventional and applied load sintering are described.