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.     

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.     

Properties and microstructure of silica glass incorporated with tributyl phosphate by sol–gel method

An organic phosphate species tributyl phosphate (TBP) was incorporated into sol–gel-derived glass matrix. TBP could be directly added to the hydrolyzed silica source from tetraethylorthosilicate (TEOS) and immobilized in silica glass matrix. TBP was stably immobilized in silica glass matrix even in the case where the weight ratio of TBP to silica was unity, and where the volume fraction of the glass sample occupied by TBP moiety was as large as 69%. The glass sample showed an appearance of hard glassy solid even at such a large fraction of TBP which is an organic solvent in the neat state at room temperature. The FT-IR spectrum showed that TBP was immobilized in silica glass in an intact state without chemical bonding with the siloxane network. The Vickers hardness was large enough even at higher weight ratios of TBP to silica to be measured as data indicating that the immobilized TBP molecules could play a promotive role in forming the siloxane bonding. The wide-angle X-ray scattering experiments revealed that the siloxane bonding was expanded by TBP molecules entrapped in the siloxane network. Furthermore, TBP molecules are dispersed in the siloxane network in the molecular scale.     

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.     

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.     

Characteristics of composites based on PMMA modified gel silica glasses

Gel silica glass prepared by the sol-gel process can be modified by incorporating an organic phase into the intrinsically porous inorganic gel matrix, which results in a composite material with much improved mechanical and optical properties. Characterisation of PMMA modified gel silica glass prepared by the in situ polymerisation method using FT-Raman spectroscopy, gel permeation chromatography and the nitrogen adsorption technique are reported. Some essential problems encountered in the preparation are discussed.     

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.     

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.     

Study of the influence of some DCCAs on the structure of sol–gel silica membranes

We obtained silica gel membranes by sol–gel processing. In order to study the effect of some drying control chemical additives, we used an alkoxide/additive molar ratio of 1/1. The performance of the drying additives in obtaining crack-free gels was evaluated through monolithicity measurements. The structural evolution occurring in the interconnected network of the membranes during thermal treatment was monitored by Fourier transform infrared spectroscopy, structural density measurements and nitrogen gas sorption. The degree of cross-linking of the silica network was inferred from the frequency of the Si–O(Si) vibration in the range. We noted that in the presence of formamide and N,N-dimethylformamide, the Si–O–Si bonds are stronger and belong to a more cross-linked structure. Changes in the chemical properties of the membranes, evaluated by quantities of molecular water and silanol groups on their surface, were monitored by absorption bands in the range of 3800–3000 cm⁻¹. Membranes obtained with additives have surfaces covered by a large content of isolated silanol groups even when annealed at 800 °C. The membrane obtained in the presence of amide has larger pore volume and its pore structure is in the range of mesoporosity. The membranes without additive and with propylene carbonate are microporous. Formamide and N,N-dimethylformamide allowed the preparation of crack-free membranes stabilized at high temperatures.     

Study of the structure of alkali–silica reaction gel by high-resolution NMR spectroscopy

The alkali–silica reaction is a deleterious chemical process that can occur in concrete. The product of the reaction is an amorphous silicate material with gel characteristics, whose high expansion properties may cause cracking in the matrix and in the discrete aggregate of particles, leading to severe deterioration of the concrete structure. Structural information of this gel at the atomic scale can provide critical information on how to develop appropriate repair of the affected structure. Samples of this gel, produced under in-service conditions in a large concrete dam, were studied by ²⁹Si and ²³Na high-resolution nuclear magnetic resonance spectroscopy, triple quantum magic angle spinning ²³Na nuclear magnetic resonance, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The short-range atomic structure of the compound was determined as an amorphous potassium–hydroxide–silicate glass, with a Q³-like dominant silicate connectivity, having a silicate speciation highly disproportioned when compared with potassium–silicate glasses with the same K₂O content. Sodium ions are mostly segregated from the bulk amorphous silicate network, forming crystal domains attributed to the trona compound (sodium sesquicarbonate, Na₂CO₃ · NaHCO₃ · 2H₂O). The structural picture at atomic scale obtained in this study gives support for double-layer models of the expansive properties of the alkali–silica reaction gel.     

Evaluation of activation energy of viscous flow of sol–gel derived phenyl-modified silica glass

The temperature-induced softening behavior in sol–gel derived phenyl-modified low-melting glass (phenyl glass) was investigated in terms of the activation energy for the viscous flow. The temperature dependence of the relative viscous flow was measured from the falling rate of a needle loaded with a constant weight. The activation energy for the viscous flow of phenyl-modified silica glass was found to be irrespective of the time of drying the sample phenyl-modified silica glass, which directly affects the extent of polymerization. Furthermore, the obtained activation energy was in considerably good agreement with that for the viscous flow of potassium alkali glass, and approximately twice larger than that of linear amorphous polymer (polystyrene). This result suggests the common microstructural feature of glassy materials interspaced by additive substances like Na/K or covalently bonded chemical functions such as phenyl groups.     

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.     

Effect of the inclusion of a water soluble high molecular weight oligomer on the surface characteristics of silica gel

The surface characteristics of pure and polyglycol 4000 containing silica gels were investigated by nitrogen adsorption at ? 195°C and water adsorption at 35°C. The addition of polyglycol 4000 during the precipitation of silica gel led to a remarkable increase in the surface area as well as the pore volume and on the other hand a sharp drop in c-BET constant.Thermal treatment led to a variation in the surface characteristics of both pure and polyglycol containing silica gels, but in different ways. Inclusion of polyglycol 4000 led to a retardation of the dehydroxylation process that occurs at elevated temperature. The decrease in surface area at 400°C was much higher in polyglycol 4000 containing silica gel than in the pure simple, an effect which correlated with shrinkage of the polymer at high temperature. Slight increase in surface area at 500°C may be attributed to the phase change from β to α form. For the high content polyglycol sample the phase change effect was counteracted by the shrinkage of the polyglycol.Water vapor adsoprtion indicated that polyglycol 4000 containing silica gel was more lyophobic in character than the pure gel at all temperatures except at 200°C.     

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.     

Electrical and structural properties of y(MnxFe3−xO4)−(1 − y)B2O3 glasses between 400 and 1000 K

The structure of silica glass was investigated by high-resolution electron microscopy. Observed dark field images were compared with those calculated by a kinematical electron diffraction theory on the basis of random and crystallite models. Bright small spots with sharp contrast were observed in the dark field image, which were found to originate from crystallites. It was indicated that small crystallites of about 17 Å in size were present in silica glass.     

29Si CPMAS NMR and near-IR study of sol–gel microporous silica with tunable surface area

High-resolution solid-state NMR and IR techniques were used to investigate the surface structure of microporous silica with tunable surface areas, obtained by a sol gel method acting on a variation of gelation time. ²⁹Si MAS NMR and Med-IR evidenced a similar number of silanol groups in all the silicas. Near-IR results revealed the existence of a different ratio of hydrogen bonded silanols and free silanols as a function of surface area. ²⁹Si CP-MAS experiments, including CP spin dynamics studies, were employed to probe hydrogen bonding and local structural environments of various hydroxyl groups of silica surface. These studies showed that for high surface area silicas both free and hydrogen bonded silanols are present at the surface of the samples and the amount of free silanol groups increases with the increasing of the surface area of the samples.     

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.     

Synthesis and characterization of V2O5–SiO2 xerogel composites prepared by base catalysed sol–gel method

In this report, the base-catalyzed sol–gel synthesis and electrochemical properties of homogeneous mixed oxide xerogel composite composed of vanadium pentoxide xerogel dispersed in a silica matrix are described. It was verified that the immobilization of vanadium pentoxide xerogel in a silica matrix does not affect its lamellar structure or its electrochemical properties. The silica matrix provides an improvement of the electrochemical properties, mainly in relation to the lithium electroinsertion into the oxide matrix with little decrease in the total charge during successive redox cycles. Moreover, the voltammetric behavior is dependent on the support electrolyte solution with varying cationic species. The effects of different catalysts were investigated.     

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.     

Physical aging in PMMA/silica nanocomposites: Enthalpy and dielectric relaxation

We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided.