Materials investigation of multi-walled carbon nanotubes doped silica gel glass composites

Multi-walled carbon nanotubes (MWNTs) doped silica gel glass matrix nano-composites were successfully prepared by sol–gel technique. Morphology of the composites was characterized by scanning electrical microscope and transmittance electrical microscope images. Pore structure of net MWNTs, silica gel glass matrix and resulted MWNTs doped composites were studied and compared. The results show that MWNTs are well dispersed in the gel glass matrix and sol–gel processing does not appear to affect the morphology of MWNTs. Pore structure of the silica matrix is changed by the introduction of MWNTs.     

Phase separation in the solution of water glass and poly(vinyl alcohol)

Silica gel samples with macropores were prepared from solutions of silicate and poly(vinyl alcohol) (PVA), where macropores were formed by fixing a transitional structure of phase separation. Among the silica sources tested, tetraethoxysilane (TEOS), colloidal silica and water glass, only the system with water glass shows phase separation and forms macroporous silica gel. In the system with TEOS, ethanol formed during hydrolysis of TEOS becomes good solution and stabilizes the system not to induce phase separation. In the system with colloidal silica, dense structure of silica is probably not suitable for controlling phase separation and gelation. In the system with water glass, driving force of phase separation is considered to be a repulsive interaction between solvent molecules and PVA interacting with silica surface and the solution separates into a phase rich in solvent and that rich in silica and PVA. One of the features in the water glass-PVA system is insensitivity of macropore size against compositional change in the solution, i.e. macroporous morphology in the resultant silica gel hardly changes by changing the composition ratio in the solution. This would be an advantage in the preparation of well-defined macroporous silica from water glass, whose composition varies among the product lot number, because reproducibility in macroporous morphology is ensured regardless of the lot number of the water glass.     

Silica glass tubes by new sol-gel method

Silica glass tubes were prepared by following a new sol-gel method. The pH of the sol containing hydrolyzed Si(OC₂H₅)₄ with HCl and colloidal silica was adjusted by adding ammonia solution. Sol was poured into a cylindrical tube and gelled while rotating cylinder at 1000 rpm to form a tube-shaped gel. Then the gel was dried carefully to be tube dry-gel and finally sintered to become a clear silica glass tube.

The effects were examined of the speed of rotation before and after gelation, to form the tubular gel, upon the shape and appearance of the derived tubes. The best speed of rotation was about 1000 rpm, and an adequate long period of rotation after gelation was necessary to maintain the tube shape. One of the large silica tubes had dimensions of 26 mm outside diameter, 13 mm internal diameter and 1000 mm length, and the ovality of the silica glass tube was 0.02%.

The derived tube can be applied in a silica glass tube for an optical fiber fabrication process or in a furnace tube for IC production.     

Glass formation through hydrolysis of Si(OC2H5)4 with NH4OH and HCl solution

Monolithic silica glass was obtained by heating the gels prepared by hydrolyzing Si(OC₂H₅)₄ with NH₄OH and HCl solution. The effect of the condition of hydrolysis of Si(OC₂H₅)₄ on glass formation was examined by measuring the bulk density, the infrared spectra and the thermal shrinkage of the gel on heating. The gel prepared by hydrolysis with NH₄OH solution consisted of numerous spherical particles, the bulk density being about 0.8. This gel abruptly shrank at about 1050°C, being converted to the pore free material similar to fused silica. The conversion of the gel to glass followed the sintering model in which the viscous flow controlled the sintering process. The viscosity and the activation energy for viscous flow were calculated on the basis of the Frenkel equation. On the other hand, the spherical particles were not observed in the gel prepared by hydrolysis with HCl solution. The bulk density of the gel was about 1.8. This gel was converted to glass at about 700°C, which was lower than the temperature of glass formation for the gel obtained by hydrolysis with NH₄OH solution.     

Silica gel with continuous macropores prepared from water glass in the presence of poly(acrylic acid)

Monolithic silica gel with macropores was prepared in the solution of water glass and polyacrylic acid (HPAA) by freezing transitional structures of phase separation. In the system, phase separation proceeds between silica polymers and HPAA, so that the porous morphology varies from closed macropores to particle aggregates through bicontinuous morphology, where both macropores and silica skeleton are three-dimensionally interconnected, with increasing HPAA/silica ratio. In addition, we can control the macropore size in bicontinuous morphology by varying the concentration of both silica and HPAA, or by changing the molecular weight of HPAA. The pore size distribution is quite sharp indicating the presence of pores with the same size all through the monolithic samples.     

Investigation on sol–gel silica coatings for the protection of ancient glass: Interaction with glass surface and protection efficiency

To hinder the phenomenon of weathering of ancient stained glass, the present work proposes the application of sol–gel coatings to the glass surface. Previous investigations [1], [2], [3], [4], [5] and [6], in fact, show that sol–gel silica coatings do not change the appearance of artistic glasses when deposited on their surface. Moreover, the film thickness is so small (around 200 nm) and its composition and structure so compatible with that of the original glass, that the characteristics of the coating and original glass are not distinguishable. In this work, several recipes used to produce sol–gel coatings have been tested in order to understand their behavior when adopted for covering ancient weathered glass. The coatings are made of sol–gel silica prepared with two different catalysts: H⁺, Pb²⁺ and without catalyst. All the investigated samples show a good adhesion of the coating to the glasses used to simulate the behavior of ancient artefacts. The sol–gel silica coatings have been studied before and after accelerated ageing to test the resistance of the protective coatings to weathering. Another important index to test of the efficiency of the sol–gel coatings for the protection of an ancient glass is the lead ion mobility. In ancient stained glass, in fact, this element is present in the metallic lead network, in the grisaille paintings and constitutes a main component of many glass tesserae. The action of water on this highly mobile ion involves the degradation of the glass itself and the release of the ion in the rain solution. Ageing tests show the efficiency of H⁺ and Pb²⁺ catalyzed coatings and the inefficiency of the non-catalyzed sol–gel layers.     

The gel to glass transition: Chemical and microstructural evolution

The chemical and microstructural changes occurring in the conversion of a died gel to fully dense glass are reviewed. The main emphasis is on gels prepared from alkoxide precursors, including silica and more complex silicate compositions. The gel to glass conversion in these systems is contrasted with that in colloidal systems. The processes occurring in the conversion are crucially dependent on the composition of the starting solution and the chemistry of the sol to gel transformation. Shrinkage is governed by four processes operating at different stages during the gel to glass transition: capillary contraction, condensation-polymerization, structural relaxation and viscous sintering. A variety of techniques have recently been applied to study the changes in the porous gel as a result of heat treatment, including dilatometry, gas adsorption, DTA, TGA, TEM, infra-red spectroscopy (to monitor OH content, in particular), Raman spectroscopy, resonance techniques and SAXS. The conversion of dried gels into monolithic glass samples using the slow drying and firing method is discussed, including removal of hydroxyl content and prevention of bloating. Other processing routes are also briefly reviewed including hypercritical drying and sintering, the use of drying control additives prior to sintering, and colloidal techniques.     

Energetics of high surface area silicas

The energetics and structure of high surface area, amorphous silicas prepared by low pressure chemical vapor deposition (LPCVD), flame hydrolysis and sol-gel were studied by high temperature transposed temperature drop calorimetry and solution calorimetry. Utilizing appropriate thermodynamic cycles, the total stored energy (measured as ‘fast’ energy release during drop experiments and as ‘slow’ energy release during solution experiments) of impurity free amorphous silicas relative to fused silica glass was determined. The ‘fast’ energy release involves the healing of point defects, reduction of surface area, release of strain, rearrangement of 2- and 3-fold rings by pore collapse or annealing of 2-fold rings (in conjunction with an appropriate concentration of 3- and 4-fold rings). The ‘slow’ energy release differences in the distribution of 3-fold and higher rings in annealed silica relative to fused silica glass.

LPCVD film silicas had been deposited at 0.4 Torr pressure by the reaction of SiH₄ and excess O₂ and 523, 643 and 703 K. The total stored energy of 22 to 44 kJ/mol is mainly due to the presence of 2- and 3-fold rings, consistent with Raman and infrared spectra of films and diffraction studies on related ‘snows’. The metastability of the LPCVD films decreases with increasing temperature of deposition due to the increased capacity to anneal metastable siloxane bonds. This trend continues to higher temperatures. An amorphous silica prepared by flame hydrolysis at 1073 K by the combustion of SiCl₄ in O₂ shows little or no stored energy and is energetically almost identical to fused silica glass.

Acid- (pH 1) and base- (pH 11) catalyzed dry silica gels were prepared by mixing TEOS : ethanol: water in molar proportion 1 : 4 : 4, then aged at 333 K for 24 h and dried at 423 K for 2–3 days. ‘Fast’ energy release accounts for most of the total stored energy of 7.3 kJ/mol for acid-catalyzed and 66.2 kJ/mol for base-catalyzed dry silica gel. It is unlikely that high concentrations of 2- and 3-fold rings percontact with the aqueous medium during the sol-gel process. Therefore, the total stored energy arises predominantly from structural relaxation and rearrangement in the base-catalyzed gel and rearrangement of surface siloxane by pore collapse during volatile loss in the acid-catalyzed gel. The creation of metastable siloxanes from the rapid condensation of monomers (present due to the high solubility of silica in the basic solution) during the drying of the base-catalyzed gel may be the source of its extremely large metastability.     

Rare-earth photoluminescence in sol–gel derived confined glass structures

The characteristics of rare-earth luminescence in selected sol–gel derived confined structures have been examined. Erbium and erbium/ytterbium doped photonic materials and structures have been prepared by sol–gel processing, in the form of silicate optical planar waveguides, modified with titania and hafnia, and 1-D photonic bandgap structures consisting of multilayer stacks of silica and titania. The Er³⁺ ions were found to be sensitive probes of the waveguide glass matrix structure, especially when hafnia-containing nanocrystallites were present, which narrowed and resolved different Stark components of the photoluminescence peaks. In 1-D Fabry–Perot coupled microcavities, efficient energy transfer was observed from Yb³⁺ to Er³⁺ ions when these were present simultaneously in the same defect layer, but not when the two types of ions were isolated in separate defect layers.     

Low temperature synthesis of non-crystalline solids of the system SrO-SiO2

Non-crystalline solids within the liquid-liquid immiscibility region in the system SrO-SiO₂ have been prepared from a gel obtained by the hydrolysis of silicon tetramethoxide with an aqueous solution of strontium nitrate. The gel which was porous and translucent at room temperature increased in transparency with heating due to the collapse of micropores until it became completely clear. The gel became opaque again due to the precipitation of α-quartz at higher temperatures. The critical temperatures below which clear solids were obtained fell on a line connecting the glass transition temperatures of vitreous silica and those of SrO-SiO₂ glasses prepared by melting. The density and refractive index of the pore-free, clear glassy solid, changed continuously with the SrO content along lines connecting those of vitreous silica and SrO- SiO₂ glasses of high SrO content prepared by melting. The maximum amount Sr²⁺ which could be introduced using an aqueous solution as the starting material corresponded to a composition of 10 SrO · 90 SiO₂ by weight.     

Characterization of organic adsorbates on model glass surfaces

Model silicate glasses in planar thin film structures are prepared using sol/gel techniques. A typical structure consists of 3.0–12.0 nm of glass on vapor deposited Ag. In this study, a pure silica and a 50/50 binary alumina-silica films are examined. Poly(methyl methacrylate) (PMMA) is adsorbed from chlorobenzene solution onto the surfaces. Infrared reflection (IR) spectroscopy, ellipsometry and quartz oscillator microgravimetry (QOM) are applied to characterize the PMMA/silicate glass interfaces formed. The QOM and IR data show that PMMA adsorption on the pure silica surface is irreversible with respect to removal by pure solvent rinse while on the binary glass surface and on a pure alumina surface, solvent rinses remove the initially adsorbed polymer. These results are interpreted in terms of Bronsted acid-base interactions involving the basic properties of the PMMA C=O group, the acidic nature of the silica and the more basic nature of the alumina containing surfaces. Further evidence for this interpretation is given by the IR spectral data which show broadening of the C=O stretching mode to lower frequencies for the irreversible adsorbed polymer on silica as compared with simulated spectra of non-surface bonding PMMA thin films. This spectral perturbation is interpreted as evidence for a hydrogen bonding interaction between OH groups on the silica surface and the C=O groups. The overall conclusion is that the surface of a 50/50 binary alumina-silica composition is dominated by the basic nature of alumina.     

Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica

In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO₃ for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping the glass samples into a sol suspension prepared by hydrolysis of Si(OEt)₄ in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass.     

Influence of thermal history on the structure and properties of silicate glasses

We studied a set of float glass samples prepared with different fictive temperatures by previous annealing around the glass transition temperature. We compared the results to previous measurements on a series of amorphous silica samples, also prepared with different fictive temperatures. We showed that the modifications on the structure at a local scale are very small, the changes of physical properties are moderate but the changes on density fluctuations at a nanometer scale are rather large: 12% and 20% in float glass and silica, for relative changes of fictive temperature equal to 13% and 25% respectively. Local order and mechanical properties of silica vary in the opposite way compared to float glass (anomalous behavior) but the density fluctuations in both glasses increase with temperature and fictive temperature.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Sol-gel preparation and optical properties of SiO2-Ta2O5 glass

Tantalum-doped silica glass was fabricated by the sol-gel process in order to obtain a glass with a high refractive index for optical use. A crack-free, clear glass rod was successfully prepared from a low-density gel and used as the core material for fabricating optical fibers. Transmission loss in the fabricated fibers was high, in the range of 10³-10⁴ dB/km, which may be caused by coloration due to the multivalency of tantalum; however, the loss was reduced by nearly one order of magnitude by heat treatment at 800 °C, that is, to 75 dB/km at a wavelength of 0.8 μm.     

Crystallization of Na2OSiO2 gel and glass

The crystallization behavior of a 19 wt% soda silica gel and gel-derived glass was compared to that of the ordinary glass of the same composition. Both bulk and ground glass samples were utilized. X-ray diffraction measurements were made to identify the crystalline phases and gauge the extent of crystallization. It was found that the gel crystallized in a distinctive manner, while the gel glass behavior was not qualitatively different from that of the ordinary glass.     

Preparation, characterization and catalytic properties of singly and doubly titanium-modified mesoporous silica gel

A series of titanium-modified mesoporous silica gel have been synthesized using tetrabutyl titanate. The samples were characterized by nitrogen adsorption-desorption, FT-IR and Raman spectroscopy, as well as by solid state diffuse reflectance UV-VIS spectroscopy. Physicochemical characterization of the materials showed that Ti atoms were part of the framework of silica gel, and it was probably in a tetrahedral coordination for low Ti contents. The resulting titania modified the inner walls of the mesoporous silica gel after hydrolysis and calcination. Actually, the titanium precursor reacted and condensed with the active silanol groups on silica gel via Si-O-Ti bonds. In addition, the titanium-modified mesoporous silica gel showed distinct activity behavior in the catalytic oxidation of cyclohexene and styrene with hydrogen peroxide as oxidant.     

Sol → gel → glass: I. Gelation and gel structure

The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.     

Porous characteristics of carbon-coated silica gel beads prepared by liquid phase deposition of organic colloidal nanoparticles

Carbon-coated silica gel beads could be prepared by immersing porous silica gel beads in a resorcinol–formaldehyde (RF) aqueous solution and then following by drying and pyrolysis in nitrogen atmosphere. With the proposed method, deposition of colloidal nanoparticles formed during the sol–gel polycondensation of the RF solution was successfully prepared on the mesopores of the silica gel beads. Both size and concentration of the colloidal particles could be controlled by the gelation time and the mixing ratio (V_EtOH/V_RF) of the RF solution to ethanol. The porous characteristics of the prepared carbon-coated silica gel beads and the amounts of deposited carbon could be controlled by changing V_EtOH/V_RF. Without any significant decrease in mesoporosity, the maximal carbon contents of carbon-coated silica gel beads could be increased up to 18 wt% in the inner part and 53 wt% on the outer surface, respectively.     

Synthesis of silica aerogel blanket by ambient drying method using water glass based precursor and glass wool modified by alumina sol

Silica aerogel blankets have been synthesized by ambient drying technique using cheap water glass as the silica source and glass wool modified by alumina sol. One step solvent exchange and surface modification were simultaneously conducted by immersing the wet hydrogel blanket in EtOH/TMCS/hexane solution. The synthesized silica aerogel blanket was light with the density of 0.143-0.104 g/cm³ and 89.4-95% porosity. The microstructure of silica aerogel blanket exhibits a porous structure consisting of glass fibers of diameter ∼2.5 μm interconnected with solid silica clusters (5-20 μm).