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.     

Surface structure and chemistry of high surface area silica gels

Combined Raman and ²⁹Si NMR investigations of high surface area silica gels indicate that dehydroxylation of the a-SiO₂ surface results preferentially in the formation of cyclic trisiloxanes (3-membered rings). Estimates of the maximum experimentally observed concentration of 3-membered rings correspond to a surface coverage of about 28–58%. Two consequences of 3-membered rings on the a-SiO₂ surface are enhanced hydrolysis rates and increased skeletal densities.     

Vanadyl ion stability in silica gels

Silica gels doped with vanadium were prepared from tetra ethyl orthosilicate and an aqueous solution of NH₄VO₃, at a final sol pH of 1.5–2.5. Absorption and electron paramagnetic resonance (EPR) spectral studies establish that under ambient conditions the incorporated pentavalent vanadium is stabilized as tetravalent vanadyl ion in the gel monoliths. Dried gels were very stable under ambient conditions, however, transformation of V⁴⁺ to V⁵⁺ was noted during gel densification.     

Drying evolution of wet silica gels

Two kinds of porosity have been found in wet gels prepared by an esterification-controlled hydrolysis: macropores and mesopores. The textural evolution during the during shows that the most dilute gel shrinks the most rapidly. The dynamical part of the Scherer model gives a good account of this observed behavior.     

Structural study of fractal silica humid gels

The structures of gels of silica obtained from solutions of TMOS, methanol and water have been studied by small angle X-ray scattering (SAXS) using synchrotron radiation. The SAXS results indicate that all the gels studied exhibit fractal structures. It is inferred that gels obtained under basic conditions are volume fractals built up by surface and volume fractal units for high and low water content, respectively. The gels obtained under acidic conditions are all volume fractals having structural units much smaller than those of basic gels. The influence of aging on the structure of basic gels has also been established.     

Etching of CaF2 single crystals in silica gels

Etching of {111} cleavage faces of CaF₂ crystals in aqueous solution of 50% HCl is carried out in citric acid set silica gel, and the kinetics of growth of etch pits at the sites of dislocations is investigated as a function of temperature, time of etching and height of gel column above the crystal surface. It is observed that the transient period required to initiate etch pit at the sites of a dislocation decreases (1) at particular temperature, with a decrease in gel height, and (2) for a particular gel height, with an increase in the temperature of etching. It is also observed that the morphology of dislocation etch pits remains triangular irrespective of the gel height and the temperature of etching. The results are compared with those of solution etching and discussed.     

Diffusion of gases in porous silica gel

Gas diffusion in porous silica gels prepared by the sol-gel process is studied at room temperature. It is shown from the measurement of helium or oxygen gas diffusion in the gels that the gas diffusion is limited by the average pore diameter of the gel, R_a; that is, the mean free path of gas in a porous gel can be regarded as equal to R_a. The results also indicate that the gas diffusing length is about three times larger than the geometrical thickness of the sample gel. Some adsorption of oxygen gas appears to take place on the silica surface of the gel at room temperature.     

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.     

Infiltration under isostatic pressure of porous silica glasses with silica sols

In this work a SiO₂ matrix with more than 50% porosity was developed and infiltrated with a pure silica sol under isostatic pressure, as a prior step to the immobilization of radioactive waste using this technique. The silica glass was prepared through the acidic leaching of phase-separated and partially-sintered sodium–borosilicate glass powder compacts. Phase separation was promoted at different stages of the sintering process to obtain different total porosity or pore size distributions, which in all cases showed macro, meso and micropores. Infiltration leads to a significant increase in weight, reflecting the initial porosity of the substrates. Porosimetry techniques (Hg porosimetry and N₂ adsorption isotherms) show that the silica sol fills practically all the pores with diameters over 3 nm. Preliminary sintering tests show that the infiltration technique lowers the sintering temperature by more than 150 °C.     

Densification kinetics of porous colloidal P2O5-doped silica gels using constant heating rate measurements

Commercially available fumed colloidal silica was gelled in an aqueous solution containing PO₄H₃. Instantaneous gelation was obtained by adding several drops of HF (48 wt%). The objective of this paper was to study the gel-to-glass conversion of these colloidal gels using constant heating rate (CHR) experiments.

Gel densification was measured at temperatures ranging from 50 to 1550°C at different heating rates (1 to 10°C/min) using a dilatometer.

Shrinkage and shrinkage rate as a function of temperature were measured and CHR equations were used to derive information on the densification mechanisms.

The experimental results show that small additions of PO₄H₃ into pure silica dispersions give rise to gels which densify to high silica glass at much lower temperatures than pure colloidal silica gels. The bloating effects produced by pure colloidal silica gels at temperatures above 1280°C were also eliminated. For these P₂O₅-doped silica gels maximum shrinkage rates were found at temperatures between 1050 and 1150°C according to the heating rate used.

The CHR analysis showed that several different mechanisms seem to operate in a complex interdependence in the whole range of temperature studied. This CHR analysis was compared with those results obtained from isothermal shrinkage experiments in the range where viscous sintering is the predominant shrinkage-controlling mechanism (between 1000 and 1100°C).     

Growth of octacalcium phosphate and dicalcium phosphate dihydrate crystals in silica gels

The crystallization of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) have been studied at ambient temperature in silica gel using double diffusion system. The first stage of evolution of the OCP spherulites as well as their growth have been observed by optical microscopy. The growth rate of the OCP spherulites under stable external conditions was constant. The formation of DCPD crystals has been observed in the last stages of experiments.     

Sol-gel synthesis of porous and dense silica microspheres

An acidic silica sol (35 ± 2 wt% equivalent SiO₂) having a gelling time of 9-10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl₄ as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the -Si-OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m²/g, 167 m²/g and 81 m²/g indicating the gradual densification. The decreased surface area and unchanged -Si-O-Si- asymmetric stretching vibration at 1084 cm⁻¹ up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the -Si-O-Si- asymmetric stretching vibration at 1104 cm⁻¹ and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.     

A 3-D numerical heat transfer model for silica aerogels based on the porous secondary nanoparticle aggregate structure

A 3-D finite volume numerical model based on the porous secondary nanoparticle random aggregate structure was developed to predict the total thermal conductivity of silica aerogels. An improved 3-D diffusion-limited cluster–cluster aggregation (DLCA) method was used to generate an approximately real silica aerogel structure. The model includes the effects of the random and irregular nanoparticle aggregate structure for silica aerogels, solid–gas coupling, combined conduction and radiation, nanoparticle and pore sizes, secondary nanoparticle porosity and contact length between adjacent nanoparticles. The results show that the contact length and porosity of the secondary aerogel nanoparticle significantly affect the aerogel microstructure for a give density and, thus, greatly affect the total thermal conductivity of silica aerogels. The present model is fully validated by experimental results and is much better than the model based on a periodic cubic array of full density primary nanoparticles, especially for higher densities. The minimum total thermal conductivity for various silica aerogel microstructures can be well predicted by the present model for various temperatures, pressures and densities.     

Growth of single crystals of barium iodate and strontium iodate in silica gels

Single crystals of iodates of barium and strontium grown by gel method are reported. Optimum conditions for good quality single crystals are worked out. Different habits of these crystals are reported. A brief report for characterization of these crystals by different methods is given.     

The preparation and characterization of silica gels doped with copper complexes

Silica gels doped with [Cu(C₅H₅N)₄]²⁺ (C₅H₅N = Py), CuCl₂-HOCH₂CH₂OH and Cu(CH₃COO)₂-NH₂(CH₂)₃Si-(OC₂H₅)₃ (CH₃COO = OAc, C₂H₅ = Et) have been prepared and examined by X-ray diffraction, electron spin resonance, transmission electron microscopy (TEM), thermal gravimetric analysis and Fourier transform infrared spectroscopy. Different dopants have different interactions with the silica alcogels depending on the chemistry of the dopants: [Cu(Py)₄]²⁺ forms chemical bonds with the hydroxyl oxygens on the surface of the silica matrix; in Cu(OAc)₂-NH₂(CH₂)₃Si(OEt)₃-doped alcogels, the siloamine enters into the sol-gel reaction and Cu(OAc)₂ coordinates to the -NH₂(CH₂)₃ ligand of the precursor. However, there is no evidence for bond formation between copper and the hydroxyl groups on the silica surface in the CuCl₂-HOCH₂CH₂OH-doped alcogels. The TEM images show a homogeneous and uniform distribution of copper particles on the reduced CuCl₂-HOCH₂CH₂OH- and Cu(OAc)₂-NH₂(CH₂)₃Si(OEt)₃-doped xerogel surface. The type of nitrogen adsorption isotherms of doped silica xerogels are unaffected by the dopant concentration. However, their surface areas do depend on the level of dopant. The microstructure of the silica matrix is primarily determined by the pH of the initial reactants, at least in the [Cu(Py)₄]²⁺-doped case.     

Dynamic properties of solvent confined in silica gels studied by broadband dielectric spectroscopy

We report the results of a broadband (10⁻²–10⁷ Hz) dielectric spectroscopy study on a solvent system (glycerol–water solution) confined in a porous silica matrix. The dielectric relaxation of the system is studied as a function of both temperature (120–280 K) and solvent composition (0–36 glycerol molar percentage), at constant matrix composition. Our data show that glycerol–water systems confined inside silica gel are characterized by a very complex dynamics quite different from that observed in solution, thus indicating that confinement may deeply modify solvent dynamics. Indeed in addition to the relaxation processes similar to those occurring in bulk samples, new dielectric relaxations are detected: two non-collective relaxations, attributed to water molecules strongly interacting with pore surfaces and to the glycerol trapped within the matrix structure, respectively; a relaxation in the glycerol free sample (and in samples at very low glycerol content) almost coincident with that observed in other different confinement conditions and governed by geometrical confinement per se. Moreover, at high glycerol content we observe two non-Arrhenius processes at least 4 order of magnitude slower than solution-like main relaxation; at low glycerol content the two above relaxations merge and show a fragile to strong transition at about 200 K.     

Characterization of HF-catalyzed silica gels doped with Degussa P25 titanium dioxide

SiO₂/TiO₂ composites were synthesized by adding Degussa P25 TiO₂ to a liquid sol that was catalyzed by HNO₃ and HF acids. Various composites were synthesized by altering the mass loading of TiO₂ and concentration of HF added to the liquid sol before gelation. The resulting materials were characterized by SEM, nitrogen adsorption-desorption, streaming potential, XRD, diffuse reflectance and TiO₂ surface area analyses. Approximate characteristics include an isoelectric point of 3, TiO₂ particle size of 30 nm, and a band gap energy of 3.2 eV. Small variations in these properties were noted for the different composites. Physical characteristics were largely affected by HF concentration and TiO₂ loading. Nitrogen adsorption-desorption isotherms were type IV for all materials and exhibited trends of decreased pore volume with an increase in TiO₂ loading and an increase in pore diameter with increased HF concentration. Surface areas of the composites ranged from 167 to 630 m²/g. Available TiO₂ surface area of the composite was also dependent upon TiO₂ loading and increased as the mass composition of TiO₂ increased but was not largely affected by HF concentration.