Diffusion of Oxygen in Vitreous Silica

The diffusivity of the oxygen ion in vitreous silica has been directly determined by exchange measurements employing the stable isotope ¹⁸O and mass spectrometer analysis. It was found that over the temperature range 925° to 1225°C the results can be represented by the equation D = 1.51 × 10⁻² exp (-71,200/ RT ) cm² sec⁻¹. These results are compared with other measurements of oxygen diffusion in silicate glasses. It is proposed that the controlling diffusion step in silicate glasses and nonstoichiometric silica is the rupture of a single oxygen bond to silicon and that the diffusion mechanism is interstitial motion through voids in the lattice. An analysis of theoretical expressions for the pre-exponential term D _O shows that present theories are unable to predict D _O for oxygen diffusion in glasses. It is also shown that the mechanism for electrical conduction in vitreous silica or in electrolytically purified quartz is not migration of oxygen ions.     

Effects of silica sol on ion exchange membranes: Electrochemical characterization of anion exchange membranes in electrodialysis of silica sol containing-solutions

The fouling potential of the negatively charged silica sol in electrodialysis (ED) by adsorption on the surface of an anion exchange membrane was investigated. Since the fouling potential is related to the physical and electrochemical properties of the silica sol and anion exchange membranes, it is important to characterize the properties of silica sol and membranes. The surface charge of silica sol was investigated by the electrophoretic mobility and its isoelectric point was determined as pH 3. The commercial anion exchange membranes were characterized in terms of exchange capacity, water content, the zeta potential and the electrochemical properties of the membranes using impedance spectroscopy to predict the effects on the electrodialysis performances. Among the characterized properties, exchange capacity and some electrochemical properties of the anion exchange membranes were rather improved after ED experiments. In the electrodialysis of solution containing silica sol, deposition of the silica sol did not decrease the desalting performance of the anion exchange membranes because of loosely packed cake layer on the membrane surface.     

Mechanism of methane hydrate formation in the presence of hollow silica

Methane hydrates are studied extensively as a prospective medium for storing and transporting natural gas due to their inherent advantages, including high volumetric energy storage density, being environmentally benign and extremely safe method compared to conventional compression and liquefaction methods. Enhanced formation kinetics of methane hydrates has been reported in hollow silica due to the increased gas/liquid contact surface area available for efficient conversion of water to hydrates. This work elucidates the mechanism of methane hydrate formation in light weight hollow silica. Hollow silica-to-water ratio was varied and its effect on the methane hydrate formation/dissociation morphology was observed. There exists a critical hollow silica-to-water ratio (1 : 6) beyond which the hydrates preferentially crystallize on the top of the bed by drawing water from the interstitial pores, whereas below this ratio the hydrate formation occurs within the bed between inter-particular spaces of hollow silica. Due to the very low bulk density, a small fraction of hollow silica was observed to be displaced from the bed during the hydrate formation above the critical hollow silica to water ratio.     

氟硅酸氨化制高补强性白炭黑干燥方式研究

以磷肥副产氟硅酸为原料,氨水为氨解剂制备了高补强性白炭黑。为获得团聚少．质量好的白炭黑产品,研究了烘箱干燥、真空干燥、喷雾干燥、溶剂置换干燥对白炭黑比表面积和粒度的影响。实验结果表明,通过溶剂置换干燥能有效防止白炭黑形成硬团聚,所得白炭黑粒度小,比表面积大。同时分析了4种干燥方式中白炭黑粉体硬团聚形成的机理。     

Synthesis of organic-inorganic composite membrane by sol-gel process

Silica was succesfully incorporated into cation exchange polymer membranes, CL-25T and Nafion 417, utilizing sol-gel process. As dipping time increased, increase in silica uptake in membrane was observed. In Nafion 417 membrane, no relationship was found between the silica uptake and the change in ion exchange capacity. But CL-25T which has larger pores than Nafion 417 shows proportional decrease in ion exchange capacity with increasing silica uptake. It suggests that the pore structure of membrane and the size control of silica sol are important to modify the structure of composite membranes. In CL-25T membranes modified by silica, the transport rate of IPA (isopropyl alcohol) increased with increasing OH^- concentration on the pore surface.     

Numerical Models for Predicting Tracer Profiles in Silicon Double-Oxidation Experiments

Standard numerical techniques are used to model the development of tracer profiles in "double-oxidation" experiments involving silicon or silicon-based ceramics. These models are more generally applicable than previously developed analytical models. In particular, the numerical model allows the relative contribution to oxidation by network and interstitial oxygen diffusion through the silica scale to be varied, and can accommodate a position-dependent diffusion coefficient. The results of calculations are compared to previously published profiles.     

Preparation,characterization, and properties of polyacrylonitrile–silica gel anion exchange composite fibers

This article reports the preparation of new anion exchange composite fibers (AECFs). These AECFs were prepared by simple stirring of polyacrylonitrile (PAN) and silica gel in different stoichiometric ratios. The physicochemical characterization of the prepared AECFs was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X‐ray diffraction analysis, thermogravimetric analysis (TGA, differential thermal analysis, and differential thermal gravimetry), and elemental analysis (CHNS/O). Ion‐exchange behavior was also observed. The results indicate that addition of silica gel affects the structure and properties of the composite fibers. The addition of silica gel also changes the thermal stability of PAN–silica gel AECFs. The ion‐exchange capacity of the resulting AECFs was found to be 1.98 mequiv/g. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers     

Prediction of crack propagation and fracture in residually stressed glass as a function of the stress profile and flaw size distribution

Engineered stress profile (ESP) glasses are noted for narrow strength distributions and the potential for stable growth of multiple surface cracks under applied tensile stress. This behavior depends on the interaction of the surface flaw size distribution with the residual stress profile in the material. In this work, several surface preparation methods were used to produce a range of flaw size distributions in soda lime silica glass specimens. Two ion exchange processes were then performed on these specimens to produce ESP glass. For each condition, crack growth behavior and fracture strength were experimentally observed. Residual stress profiles resulting from each ion exchange process were measured with an optical technique. These stress profiles were used to calculate stress intensity factors as a function of crack geometry, using a weight function method. Crack growth and fracture strength predictions based on these stress intensity factors were compared to experimental data, resulting in good agreement in most cases.     

Color Centers in Sulfur-Doped Silica Glass: Spectroscopic Manifestations of an Interstitial Molecule S2

The absorption and luminescence spectra of color centers in sulfur-doped silica glass are investigated. It is demonstrated that the most intense bands of absorption at 280 nm and luminescence at 385 nm have a resolved vibrational structure. These bands are assigned to interstitial molecules S₂, which are weakly bound to the glass network. The proposed model provides a simple interpretation of the shift observed for the luminescence excitation band with respect to the corresponding absorption band.     

Fabrication of inorganic/polymer nanocomposite membranes containing very high silica content via in situ surface grafting reaction and reactive dispersion of silica nanoparticles: Proton conduction,water uptake,and oxidative stability

In this study, we present a new fabrication process for proton exchange membranes based on inorganic/organic nanocomposite using in situ surface grafting reaction and reactive dispersion of silica nanoparticles in the presence of reactive dispersant, urethane acrylate nonionomer (UAN). Through in situ surface grafting reaction of silica nanoparticles, urethane acrylates were chemically introduced on the surface of silica nanoparticles, which were dispersed in DMSO solutions containing UAN and sodium styrene sulfonate (NaSS). After urethane linkage and copolymerization of NaSS, UAN and urethane acrylate moieties of silica nanoparticles, the solutions were converted to silica nanoparticle‐dispersed proton exchange membranes where silica particles were chemically connected with organic polymer chains. 5.89–29.45 wt % of silica nanoparticles could be dispersed and incorporated in polymer membranes, which were confirmed by transmittance electron microscopy (TEM) measurement. On varying weight % of silica nanoparticles dispersed within the membranes, water uptake and oxidative stability of nanocomposite membranes were largely changed, but membranes showed almost the same proton conductivity (greater than 10⁻² S cm⁻¹). At 5.89 wt % of silica nanoparticles, nanocomposite membranes showed the lowest water uptake and excellent oxidative stability compared to the sulfonated polyimide membranes fabricated by us. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Diffusion Mass Exchange in Dissolving Silica in Na2O – SiO2 Melts

Based on experimental materials on the kinetics of dissolution of silica grains accumulated in the literature, a generalized dependence is obtained, according to which kinetic dissolution curves in diffusion mass exchange are uniquely determined by the chemical composition (silica modulus), the melting temperature, and the initial granulometric composition of silica-bearing batch components.     

Characterization of surface charge and zeta potential of colloidal silica prepared by various methods

Colloidal silica is prepared by hydrolysis of TEOS, direct oxidation of Si powder, condensation of silicic acid, etc. There are differences in surface reactivity of silica particle due to the preparation routes. Therefore, it is useful to evaluate surface properties accurately in order to understand the physiochemical properties of the products. The surface charge density, site density and zeta potential with respect to three types of colloidal silica were estimated and discussed. The surface charge density was different depending on preparation method. It is decreasing in the order of direct oxidation, ion exchange, TEOS hydrolysis. The zeta potential is decreasing in the order of ion exchange, TEOS hydrolysis, direct oxidation. The order in surface charge density is different from that in zeta potential because of the difference in stability depending on the particle size and surface charge density.     

Gas Solubility in Glasses: Principles and Applications

There are numerous practical applications of the solubility of gases in glass, for example, gas separation and the outgassing and leak testing of vacuum systems. Analysis of the structure of glass provides an understanding of the mechanisms of gas solution, diffusion, and permeation. The gas atoms (or molecules) serve as structural probes. Noble gas solubility in vitreous silica indicates the distribution of interstitial size to be lognormal in nature, a result confirmed by computer-generated models. In related studies, computer models have indicated that the ring statistics in vitreous silica are normal (Gaussian) in nature over a wide range of pressures. The geometrical literature on convex polyhedra provides additional detail about the nature of the structure of glass and its relationship to gas transport. This paper focuses on gas solubility in rigid glasses below the glass transition temperature, but there are implications for the related problem of gas solubility in glass melts, another problem of substantial commercial interest. The understanding of gas solubility has also had a significant influence on models of geological importance, such as volcanic eruptions and formation of the earth's atmosphere. These studies in the earth sciences, in turn, enhance our understanding of engineered glasses.     

A facile method to fabricate monolithic alumina–silica aerogels with high surface areas and good mechanical properties

In this study, monolithic alumina–silica aerogels with high surface areas and good mechanical properties were synthesized via a facile sol–gel method without solvent exchange. Furthermore, surface areas, microstructures (up to 1300 °C), and mechanical properties of the prepared alumina–silica aerogels were investigated. The sintering and phase transformations of metastable alumina nanoparticles are suppressed owing to the uniformly distributed Si in the alumina–silica aerogels; therefore, the alumina–silica aerogels can maintain much higher specific surface areas after being calcined at 800 °C (575.5 m²/g), 1000 °C (443.2 m²/g), and 1200 °C (120.6 m²/g) compared to pristine alumina aerogels. In addition, the prepared high surface area alumina–silica aerogels show considerably higher strengths than those obtained in previous works. The compressive stress (3 % strain) and elastic modulus of the alumina–silica aerogels reached 1.78 and 65.6 MPa, respectively. The reported alumina–silica aerogels in this study can be good candidates as high-temperature thermal insulators and catalysts.     

As-synthesized MCM-41 silica: new adsorbent for perchlorate

Here we demonstrate that the as-prepared MCM-41 mesoporous silica material, which is synthesized using cetyltrimethylammonium bromide as a cationic surfactant exhibits very high capacity for perchlorate uptake from solutions. Thus we discover a new function for the as-synthesized mesoporous materials containing cationic surfactants. These materials are shown to have better capacity than the currently used activated carbon, which is preloaded with cationic surfactant. As-synthesized MCM-41 has a higher removal of perchlorate with 0.378 ± 0.038 meq/g than the surfactant modified activated carbon sample, which removed 0.304 ± 0.005 meq/g i.e., MCM-41 has a 24 % higher capacity than the surfactant modified activated carbon sample for perchlorate uptake because of higher surfactant content and higher positive charge in the former. The residual positive charge on the cationic surfactant micelles trapped in mesopores of silica is responsible for the high perchlorate uptake. The excess positive charge on the micelles in the as-synthesized MCM-41 silica is balanced by bromide ions and these ions are involved in exchange with perchlorate ions.     

Cation exchange of surface protons on silica gel with cupric ions

The exchange of surface protons on silica gel with cupric ions was investigated under two different conditions. The equilibrium constant was determined and the mechanism of the exchange was suggested. In ammonia water, two protons exchange with one tetrammine cupric ion and formed two ammonium ions. In acidic solution, where one proton exchanges with one cupric ion, the apparent equilibrium constant increases with an increase in the amount of copper loading. The structure of surface species on silica gel was suggested from the mechanism of the exchange and from ESR study. The surface species prepared in acidic solution is changed into the one prepared in ammonia water by exposure to ammonia. The change in the structure of surface species by the adsorption of foreign molecules was also studied.     

Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

Preparation of silica aerogel from oil shale ash by fluidized bed drying

The silica aerogel with high specific surface area and large pore volume was successfully synthesized using oil shale ash (OSA) via ambient pressure drying. The oil shale ash was burned and leached by sulfuric acid solution, and then was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel. After washing with water, the solvent exchange with n-hexane, and the surface modification with hexamethyldisilazane (HMDZ), the aged gel was dried by fluidization technique and also using a furnace to yield silica aerogels. The physical and textural properties of the resultant silica aerogels were investigated and discussed. The results have been compared with silica aerogel powders dried in a furnace. From the results, it is clear that the properties of silica powders obtained in fluidized bed are superior to that of powders dried in the furnace. Using fluidization technique, it could produce silica aerogel powders with low tapping density of 0.0775 g/cm³, high specific surface area (789 m²/g) and cumulative pore volume of 2.77 cm³/g.     

Environmentally Enhanced Crack Growth in Soda-Lime Glass

Crack growth data are presented for soda-lime glass in various chemical environments. It is shown that the same environments which govern crack growth rates in vitreous silica also do so in soda-lime glass. The slopes and positions of the crack growth curves in soda-lime glass are shown to differ from those in vitreous silica. It is hypothesized that the differences between the behavior of soda-lime glass and silica result from the effects of the modifier ions, Na⁺ and Ca²⁺, on the reactivity of the Si-O bond or through changes in the elastic properties of the bridging network. It is shown that sodium ion exchange and silica dissolution may also be important to crack growth, particularly at low crack velocities.     

Dissolution Kinetics of Crystalline and Amorphous Silica in Hydrofluoric-Hydrochloric Acid Mixtures

The dissolution kinetics of crystalline and amorphous silica were studied as a function of time, temperature, concentration of HF, concentration of HC1, and crystallographic orientation. The results indicate that dissolution is a surface reaction-controlled process. The dissolution rate depends strongly on HF concentration and weakly on HC1 concentration. The dissolution rates depend on crystallographic direction in α-quartz and are slower than for fused silica. The rate-controlling step is thought to be a ligand exchange process at the surface.