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.     

Critical parameters in the processing of engineered stress profile glasses

A new approach to chemical strengthening is to move the maximum residual compression below the glass surface. With appropriate processing, this approach leads to strengthening and a decrease in the strength variability. This improvement in mechanical reliability is accomplished by introducing stable crack growth into the glass, which leads to multiple cracking of the glass surface prior to failure. In the current study, a fracture mechanics analysis is presented that uses a simple form for the stress profile. By determining the effect of this stress profile on the apparent fracture toughness of the material, it is possible to identify the conditions when spontaneous cracking occurs, the stress for the onset of multiple cracking and the ultimate strength of the glass. In particular, it is shown there is an important interplay between the depth of the maximum compression, the stress gradient and the magnitude of the maximum compressive stress. The current analysis is then used to define a strategy for the processing of glasses with engineered stress profiles.     

Crystallization of gels and glasses made from hot-pressed gels

Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na₂O the crystalline phase of silica which appears first is the cristobalite; with Li₂O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO₂B₂O₃ obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B₂O₃ it was impossible to crystallize the gels and the glasses of this system under two hours.     

Aging and drying of gels

This review describes the chemical and physical processes occurring during the aging and drying of an inorganic gel. The chemical reactions that lead to gelation are shown to continue long after the solution gels, causing changes in the composition, structure, and properties of the gel. The formation of new crosslinks produces shrinkage (syneresis) and increases the modulus and viscosity of the gel, so that aging reduces the subsequent shrinkage during drying. If the solubility of the solid phase is drying, the small pores of the gel develop high capillary pressures that results in shrinkage. If the exterior surface shrinks much faster than the interior, the differential strain can cause warping and cracking; very slow drying is generally required to prevent such catastrophes. Fracture is most likely at the moment that shrinkage stops and the liquid/vapor meniscus moves into the gel; after that point, the meniscus may remain smooth or may become unstable and ragged. The theory of drying is reviewed and shown to account for the observed behavior. Several strategies are described that minimize the tendency of the gel to crack.     

Stability of lithium silicate gels

Lithium silicate gels with 15 mol% Li₂O were prepared by combining a lithium nitrate solution with partially hydrolyzed tetraethyl orthosilicate (TEOS). Using low pH and high water levels, samples were cast, gelled, and dried to form cylinders 30 mm in length and 5 mm in diameter in about a week. The dried gels had a uniform lithium content throughout. Lithium disilicate crystals and quartz crystals appeared in samples heated above 600°C.     

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.     

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.     

The dynamics of wet granular matter

We present a numerical study of a shear-induced solid–fluid transition in wet granular matter in order to show the self-organized critical behavior close to the transition point. The continuous time simulation is based on a simple model considering both the cohesive forces induced by the adsorbed liquid amount and the repulsive forces due to the hard core interaction of the granules. Dissipation is assumed to be entirely due to the hysteretic character of the cohesive forces. The aim of our analysis concerns the crossover from a solid like behavior to a mobile ergodic state under the influence of an external force field F exceeding the critical force F_c. Diffusion coefficients, dissipation and kinetic order parameters can be expressed as characteristic scaling laws.     

Synthesis and thermal behavior of different aluminosilicate gels

In this study a series of different gels (inorganic polymers) was synthesized with a single procedure, although varying the pH of the synthesis medium across a wide range of values. The chemicals used were as follows: a 0.25 M solution of K₂O · SiO₂ · 9H₂O as the source of silica, a 0.25 M solution of Al(NO₃)₃ · 9H₂O as the source of aluminum; and a 0.25 M solution of KOH to regulate the pH. Gel thermal stability was also determined, up to 1200 °C. The results show that at acidic pH both Si-rich and Al-rich gels are formed, which may crystallize into cristoballite and mullite, respectively, under thermal treatment. The gels synthesized at alkaline pH, in turn, remain stable at temperatures over 1200 °C. Depending on their composition, such gels may crystallize into leucite.     

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.     

Crystal growth of potassium perchlorate in gelatin gels

The suitability of gelatin gel for the growth of good quality single crystals of potassium perchlorate has been investigated. Growth in a more rigid and simple apparatus has been accomplished and described here. The grown crystals were identified by X-ray analysis. A study of the relation of crystal morphology to concentration of feed solutions and growth temperature has also been made. Studies made on chemical etching of the cleavages have revealed that the crystals are relatively perfect. At times, large liquid inclusions which introduce stresses in the crystals are responsible for a large dislocation concentration.     

Growth of single crystals of potassium chloride from gels

The growth of single crystals of potassium chloride (KCl) in silica gels upto 3 × 3 × 3 mm³ in size is described. To crystallize KCl incorporated in the gel, hydrochloric acid has been used to crystallize the KCl in the gel media. The crystalline perfection has been studied by the chemical etch pit technique.     

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.     

Microwave assisted crystallization of zeolite A from dense gels

Pure zeolite NaA has been obtained, in a total processing time of 1 h, by exposing the reaction mixture to a microwave electromagnetic field under atmospheric pressure. The strong effect of microwave radiation has been used to progressively reduce the water content in the formulation, up to 86.9 mol%, with a 30% (in weight) yield in dried product. SEM images of microwave-produced zeolite have shown peculiar morphological differences from the zeolite obtained by conventional synthesis.     

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.     

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)₃ and aluminum halides AlX₃, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by ²⁷Al NMR spectroscopy.     

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.