Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying

Porous carbon xerogels can be obtained by convective drying of resorcinol (R)–formaldehyde (F) hydrogels, followed by pyrolysis. Drying conditions have to be carefully controlled when crack-free monoliths with well-defined shape and size are required. The knowledge of the mechanical properties of the RF xerogels and their evolution with water content is essential to model their thermo-hygro-mechanical behavior during convective drying and avoid mechanical stresses leading to deformation and cracking of the sample. The shrinkage behavior and the mechanical properties of RF xerogels obtained with R/C ratio ranging from 300 to 1500 were investigated. R/C greatly influences the shrinkage and mechanical properties of the wet gel, on the one hand, and the mechanical and textural properties of the dried gel, on the other hand. The smaller the R/C, the higher the shrinkage, the stiffening, and the viscoelastic character of the xerogels. Water content has an influence on both the stiffness of the gels and the viscoelastic response. Generally, samples lose their mechanical viscous character and become more rigid when they are dried. Finally, mercury porosimetry measurements showed that the gels exhibit a marked lowering of their stiffness upon compression, interpreted as a result of the heterogeneity of the microstructure.     

Development of microporous carbon xerogels by controlling synthesis conditions

Textural properties of carbon gels can be controlled by varying the synthesis and drying process conditions. In this work, the influence of the initial pH and the drying method on the final properties of carbon gels, synthesized using methanol as a solvent, was evaluated. Furthermore, the use of microwaves as a drying method for the synthesis of carbon xerogels was assessed. In the light of the results obtained, in order to synthesize monolithic and microporous carbon gels in a short period of time, the use of a multimode microwave oven is proposed. The use of pH 7 also leads to shorter gelation times and more consistent monoliths. Furthermore, the multimode microwave drying can produce homogeneous microporosity and surface areas of up to 1341 m² g^?1, in a very short time (i.e., only 6 min is required for the drying step).     

Textural properties of low-density xerogels

The extent of shrinkage during drying is controlled by the balance between the capillary pressure developed in the pore liquid and the modulus of the solid network. One first method to obtain low-density xerogels consists in strengthening TEOS-based alcogels by providing new monomers to the alcogel after gelation. In the second method, low-density xerogels are produced by surface modification (silylation) of the wet gel with trimethylchlorosilane. The capillary pressure is reduced and the presence of non-reactive species on the surface makes the shrinkage reversible. A reduction of the capillary pressure can be achieved by introduction of a substituted alkoxide 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to a TEOS-based alcogel, synthesised in a single base-catalysed step. This additive acts as a nucleation agent leading to big silica particles (20 nm) with a low EDAS/TEOS ratio (0.03). The pores between those particles are also large and the drying stress is reduced. The textural properties of those three materials are compared: bulk densities of the samples modelled on the first and third method are varying in the same range (0.25–0.35 g/cm³) while xerogels obtained by the surface modification process are less dense (0.1–0.15 g/cm³). The biggest pores are observed in the third method.     

The structural evolution of alkoxy-derived gel during drying

The effects of drying temperature on the structural evolution of alkoxy-derived silica gel prepared using various catalysts have been investigated. The dependence of specific surface area, S_g, reflecting the structure, on the temperature of drying was remarkable for a non-catalyzed xerogel. The effect of drying temperature on the S_g of an ammonia-catalyzed xerogel was also found but was not very large. The S_g of xerogels obtained by drying at 60°C was always higher by 50% than the gels dried at 30°C without regard to the aging temperature. The S_g of xerogels from HCl-catalyzed solution was of the order of several m²/g, however, the S_g of the aerogel obtained by hypercritical drying of the wet gel from a similar solution was about 800 m²/g. These phenomena were understood on the basis of SAXS measurements on both wet gels and aerogels.     

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.     

Different morphologies of the dry physical gels as a function of the sample preparation method

Xerogels obtained by drying the gels formed by glucofuranose derivatives with organic solvents were studied. The xerogels were characterized using the SEM, XRD, DSC and OPM techniques. The morphology of a xerogel observed by SEM may change from ‘amorphous, fibrillar’ to ‘crystal-like’, which may be caused by time or temperature. The results suggest that a similar transition may take place when the xerogel is being prepared. Thus SEM pictures of xerogels should be treated with great caution as they may not reflect the gelator network morphology in the bulk gel.     

Effect of the counter-ion of the basification agent on the pore texture of organic and carbon xerogels

Organic and carbon xerogels were prepared by polycondensation of resorcinol with formaldehyde in water, followed by evaporative drying and, eventually, pyrolysis. The pH of the precursor’s solution was fixed at 6.0 in all cases by adding various hydroxides as basification agent. Three alkali metal hydroxides (LiOH, NaOH and KOH) and three alkaline earth metals hydroxides (Ca(OH)₂, Ba(OH)₂, Sr(OH)₂) were used. It was found that the pore texture of the organic and carbon xerogels is totally independent on the cation size, but depends on the charge and concentration of the counter-cation. Indeed, the pore size of the alkaline earth metal loaded samples is larger than that of the alkali metal-doped xerogels. As a matter of fact, to reach the same initial pH, the concentration in alkali metal hydroxide must be twice that of the alkaline earth metal base. The effect of ions on the pore texture was thus attributed to electrostatic effects on the microphase separation process that occurs prior to gelation.     

Application of mercury porosimetry to the study of xerogels used as stone consolidants

Alkoxysilanes, low-viscosity monomers that polymerize into the porous network of stone by a sol-gel process, are widely used in the restoration of stone buildings. We have used the mercury porosimetry technique to characterize changes in microstructure of three granites following their consolidation with two popular commercial products (Wacker OH and Tegovakon V). The suitability of this technique is questioned because a surprising increase of stone porosity is observed. In order to investigate the feasibility of porosimetry, we analyze the behavior of xerogels prepared from the two commercial products, under mercury pressure. Gels are basically compacted and not intruded by mercury. Thus, the increase of stone porosity after consolidation can actually be associated with gel shrinkage. Mercury porosimetry, therefore, has been found unsuitable for characterizing the microstructure of consolidated rocks. However, it can be employed usefully to evaluate shrinkage of gels under mercury pressure, which permits the behavior of a consolidant during the process of drying in stone to be predicted. It is a key factor because many problems of consolidants are related to their drying process within the stone. Gels under study exhibit a high rigidity and an elastic behavior, as consequence of their microporous structure. Finally, the reduction in the porous volume of gels after the porosimetry test demonstrates that the shrinkage mechanism is based on pore collapse.     

Preparation of cresol–formaldehyde carbon aerogels via drying aquagel at ambient pressure

Carbon aerogels were prepared from mixed cresol (Cm) and formaldehyde (F) via the sol–gel process followed by drying at ambient pressure and carbonization. The inexpensive feedstock of mixed cresol and the simple drying process could be as an alternative economical route to the classical resorcinol–formaldehyde synthesis process. In our process, organic precursor gels were synthesized via polycondensation of cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation the solvent was removed via drying at ambient pressure to obtain organic aerogels that exhibit a drying shrinkage below 5% (linear). Upon carbonization of the organic aerogels at 1173 K, monolithic carbon aerogels (denoted as CmF carbon aerogels) can be produced. Nitrogen adsorption results showed that the CmF carbon aerogels have abundant mesopores and micropores with a dominant pore diameter of 25–40 nm. An increase of the BET surface area and a modification of the pore size distribution of CmF can be realized by a CO₂ activation. The images of scanning electron microscopy (SEM) indicated that the microstructure of carbon aerogels can be effectively controlled and tailored by varying the synthetic conditions during the initial sol–gel process.     

Preparation of resorcinol formaldehyde (RF) carbon gels: Use of ultrasonic irradiation followed by microwave drying

In this work, ultrasonic irradiation (with suitable reactant ratios) during gelation of RF solution is a promising technique for maintaining porous properties of carbon gels prepared by microwave drying.     

Characterization of low-density silica xerogel formed into microspheres

Low-density silica xerogels were formed into spherical pellets using a process in which a xerogel suspension containing sodium alginate was solidified through free fall in a solution of divalent ions that induce the droplet solidification. Pellet preparation was also carried out at laboratory scale by using a syringe instead of the material-consuming unit used for scaling up. When beads are made from a suspension of dried-and-calcined xerogel material, the textural properties such as porosity and surface area are significantly altered. Both properties are reduced by 30-50% compared to the starting reference material. Properties were improved when a suspension of dried xerogel that is less sensitive to contact with water was used: specific surface area was maintained at its initial level and porosity decreased by 25%.     

Textural characterization of high temperature silica aerogels

Silica alcogels were synthetized by the sol–gel polymerization of tetraethylorthosilicate in acid media. Conventional and supercritical drying was performed in order to obtain xerogels and aerogels. Different process parameters of the supercritical drying were altered in order to control the texture of the resulting gel. The texture and the structural evolution of xero- and aerogels were studied by thermogravimetric-differential thermal analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy and N₂ physisorption at 77 K. ²⁹Si magic angle spinning nuclear magnetic resonance experiments on silica samples were used to resolve various silicon local environments. Hydrophilic microporous xerogels and hydrophobic micro- or mesoporous silica aerogels were obtained, whose microscopic structure is very similar. However, the samples obtained by different drying procedures exhibit a different structural evolution with temperature.     

Synthesis of transition metal-doped carbon xerogels by cogelation

The cogelation process, i.e. the co-polymerization of a metal complex with the gel precursors, was used for the synthesis of transition metal-doped resorcinol–formaldehyde gels. The aim of this process is to anchor the metal to the polymer so that the former does not sinter during the pyrolysis step leading to porous carbon. Cu-, Ni-, Pd- or Pt-loaded gels were prepared by this technique. After drying and pyrolysis, Pd and Pt were obtained as metal nanoparticles (2–5 nm in diameter) inserted in the carbon nodules, when the complexing agent and the synthesis conditions were well chosen. These small metal particles were inaccessible to reactive gases, probably due to carbon deposit at the metal surface during pyrolysis: CO almost did not chemisorb. Oxidation of the support or pyrolysis under reductive atmosphere was applied to the metal-doped gels and carbons in order to make the surface of the metal particles accessible, but these treatments develop the macropores only. The cogelation process is then suitable to prepare metal nanoparticles protected from the outside by encapsulation in the carbon matrix.     

Silica xerogels and aerogels synthesized with ionic liquids

The synthesis of silica gels with ionic liquids (IL) as either additives or co-solvents is described. The wet gels were either dried by supercritical CO₂ or by evaporation to obtain aerogels or xerogels, respectively. The effects of two ionic liquids: 1-butyl-3-methyl imidazolium tetrafluororate (IL1) and 1-butyl-4-methyl pyridinium tetrafluoroborate (IL2), on the structural and textural characteristics of gels were investigated. IL2 showed a more important influence than IL1, on the gelation time and gel structure, according to solid state NMR investigations. With both types of ionic liquids, the average pore radius of xerogels increased from ≈2 nm at an IL to Si molar ratio n_IL/n_Si = 0.07, to ≈4 nm at n_IL/n_Si ≈ 1.5 and the size distributions were rather well centered about each mean radius. Hence, ionic liquids appeared as interesting additives to target gels with a given pore size.     

Synthesis of ultrahigh-pore-volume carbon aerogels through a “reinforced-concrete” modified sol-gel process

Ultrahigh-pore-volume carbon aerogels were synthesized by adding rigid silica nanoparticles to resorcinol-formaldehyde sols, followed by supercritical drying, pyrolysis and HF leaching. The presence of silica nanoparticles in polymer gels dramatically inhibits volume shrinkage and framework collapse during the supercritical drying and pyrolysis processes, resulting in the obtained carbon aerogels exhibiting very low bulk density and high pore volume. By changing the mass ratio of silica nanoparticles/resorcinol-formaldehyde resin, pore volumes of carbon aerogels can be tuned in the range of 2.8-6.0 cm³/g.     

Arylene- and alkylene-bridged polysilsesquioxanes

Bis(triethoxysilyl)arylene monomers 1–5 and the 1,6-bis(trimethoxysilyl)hexylene monomer 6 were hydrolytically condensed under sol-gel conditions using both acid and base catalysts to produce their respective arylene- and alkylene-bridged polysilsesquioxane materials (X-1 through X-6). Polymerization reactions yielded gels within 24 h. One notable exception was the acid-catalyzed polymerization of 2,5-bis(triethoxysilyl)thiophene (X-5-S1) which required approximately 1 month to gel. The gels were processed by extracting with low dielectric solvents or by aqueous extraction. Solid state ¹³C and ²⁹Si CP MAS-NMR, infrared spectroscopy, and gas sorption porosimetry were measured on the xerogels. The materials were transparent, glass-like xerogels with surface areas as high as 1100 m²/g and porosity primarily confined to a micropore region (< 20 Å diameters). Xerogels prepared using the aqueous extraction had surface areas between 500 and 956 m²/g.     

Preparation and optical properties of samarium doped sol-gel materials

Transparent crack-free glassy monoliths containing up to 0.2 Sm/Si are prepared at room temperature by sol-gel technique. The sol-gel preparation conditions are: acid catalyzed hydrolysis followed by basic catalyzed gelation and controlled drying. The prepared xerogels are characterized by X-ray diffraction, thermogravimetry with Fourier transforming infrared spectroscopy, scanning electron microscopy and optical spectroscopy. The values of the phenomenological Ω-intensity parameters of Sm(III) ions in the gels are (in 10⁻²⁰ cm²) Ω₂ = 76 ± 16.71, Ω₄ = 3.01 ± 0.21, Ω₆ = 2.99 ± 0.41. The oscillator strengths of the Sm(III) ions in the gels are calculated. The results show a linear concentration dependence of Sm(III) in UV/Vis absorption spectra and formation of a Sm(NO₃)₃ · 6H₂O complex at high samarium concentrations.     

A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5–25 K), C_p, and low-frequency Raman scattering (<150 cm⁻¹) has been performed in amorphous silica samples synthesized by sol–gel method (xerogels) and thermally densified in a range of densities, from ρ=1250 kgm⁻³ to ρ=2100 kgm⁻³, close to the density of the melt quenched vitreous silica (v-SiO₂). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO₂, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870°C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm⁻¹ and of a peak in C_p(T)/T³, very close to those observed in v-SiO₂; (ii) in silica xerogels treated at temperatures less than about 800°C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.     

Attempts to avoid cracks during drying

A new technique for avoiding cracks during drying is proposed; increasing the hydroxy groups in the gels. By this technique a drying rate of 1.33 g/h was attained, which is approximately 5 times faster than non-treated gels (0.27 g/h). Further, this technique can easily be applied to change the microstructure of the gels.     

Structural evolution up to 1100 °C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

Silica xerogels were prepared from sonohydrolysis of tetraethoxysilane and exchange of the liquid phase of the wet gel by acetone. Monolithic xerogels were obtained by slow evaporation of acetone. The structural characteristics of the xerogels were studied as a function of temperature up to 1100 °C by means of bulk and skeletal density measurements, linear shrinkage measurements and thermal analyses (DTA, TG and DL). The results were correlated with the evolution in the UV-Vis absorption. Particularly, the initial pore structure of the dried acetone-exchanged xerogel was studied by small-angle X-ray scattering and nitrogen adsorption. The acetone-exchanged xerogels exhibit greater porosity in the mesopore region presenting greater mean pore size (∼4 nm) when compared to non-exchanged xerogels. The porosity of the xerogels is practically stable in the temperature range between 200 °C and 800 °C. Evolution in the structure of the solid particles (silica network) is the predominant process upon heating up to about 400 °C and pore elimination is the predominant process above 900 °C. At 1000 °C the xerogels are still monolithic and retain about 5 vol.% pores. The xerogels exhibited foaming phenomenon after hold for 10 h at 1100 °C. This temperature is even higher than that found for foaming of non-exchanged xerogels.