Study on the gel casting of fused silica glass

Slip casting process is usually applied for the forming of fused silica products. Segregation always occurs and it will results in density deviation. By using gel casting process, green is fabricated by means of in situ polymerization with a three-dimensional network, holding the particles together and eliminating the tendency of migration. To prepare gel casting slurries, premix solutions were composed of acrylamide and N,N′-methylenebisacrylamide. Solid loading was kept 60% and the average particle size of silica powder 8–8.5 μm. Lactic acid was introduced as a dispersant to regulate the pH value 3–4. Mechanism of the dispersant was investigated by studying ζ-potential at different pH. Ammonium persulfate (NH₄)₂S₂O₄ was added as an initiator. Gelation took place with the help of initiator at 50–60 °C. Nanometer silica was introduced to boost sinterability so that the density and bending strength of fused silica ceramics have been increased to 2.1 g/cm³ and 40 Mpa, respectively.     

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.     

Elastic anomalous behavior of silica glass under high-pressure: In-situ Raman study

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.     

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.     

Raman,infrared, and optical spectra of some silver(I) sulfamides

Raman and infrared spectra of disilver sulfamide, Ag₂(HNSO₂NH), and tetrasilver sulfamide, Ag₄(NSO₂N), together with their¹⁵N and²H derivatives (at 300 and 80 K), are reported and interpreted. Resonance conditions for the Raman spectrum of the deep red tetrasilver sulfamide is assumed, but no overtone progression of any band is observed. Silver-nitrogen stretching bands appear in the frequency region 300–200 cm^–1. Although the X-ray crystal structures of the compounds reveal short AgAg distances, no frequency assignable to metalmetal stretching vibrations could be clearly located except for Ag₄(NSO₂N) in the case of the strong band at 288 cm^–1 appearing in the infrared only. This band is assigned to a lattice vibration having high frequency due to strong metalmetal interaction. Optical diffuse reflectance and fluorescence emission/excitation spectra are included and compared to the literature data for silver-exchanged zerolites.     

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.     

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.     

Raman spectra and thermal analysis of a new lead–tellurium–germanate glass system

Differential scanning calorimetry (DSC) and Raman scattering studies of a new glass system, lead–tellurium–germanate glasses in the form of (90−x)GeO₂·xTeO₂·27PbO·10CaO with x=0, 10, 20, 30, and 40, are reported. The glass samples were fabricated using a conventional melt-quenching method. The Raman spectra and possible glass structures are discussed for different TeO₂ contents. The results indicate that increasing TeO₂ content up to 40 mol% in the glass system decreases the glass transition temperature and melting temperature, and suppresses the crystallization tendency in the fiber pulling temperature range. The lead–tellurium–germanate glass, GTPC, possesses a larger refractive index and a smaller maximum phonon energy than that of a lead–germanate glass, 63GeO₂·27PbO·10CaO, and shows a better thermal stability compared to a tellurite glass, 75TeO₂·20ZnO·5Na₂O (TZN). These improved properties could be beneficial for fabricating rare-earth doped fiber devices.     

Mixed alkali glass spectra and structure

The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM₂O(1?x)M₂′O · P₂O₅. The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure. Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x. This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x. A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces.     

Incorporation of H2 in vitreous silica, qualitative and quantitative determination from Raman and infrared spectroscopy

Incorporation mechanisms of H₂ in silica glass were studied with Raman and infrared (IR) microspectroscopy. Hydrogenated samples were prepared at temperatures between 800°C and 955°C at 2 kbar total pressure. Hydrogen fugacities (f_H2) were controlled using the double capsule technique with the iron–wüstite (IW) buffer assemblage generating f_H2 of 1290–1370 bars corresponding to H₂ partial pressures (P_H2) of 960–975 bars. We found that silica glass hydrogenated under such conditions contains molecular hydrogen (H₂) in addition to SiH and SiOH groups. H₂ molecules dissolved in the quenched glasses introduce a band at 4136 cm⁻¹ in the Raman spectra which in comparison to that of gaseous H₂ is wider and is shifted to lower frequency. IR spectra of hydrogenated samples contain a band at 4138 cm⁻¹ which we assign to the stretching vibration of H₂ molecules located in non-centrosymmetric sites. The Raman and IR spectra indicate that the dissolved H₂ molecules interact with the silicate network. We suggest that the H₂ band is the envelope of at least three components due to the occupation of at least three different interstitial sites by H₂ molecules. Both, Raman and IR spectra of hydrogenated glasses contain bands at 2255 cm⁻¹ which may be due to the vibration of SiH groups. Under the assumption that the reaction Si–O–Si + H₂ → Si–H + Si–O–H describes adequately the ‘chemical dissolution' of H₂ molecules, the SiH concentrations in our samples were determined and the molar extinction coefficient for the SiH absorption band in the infrared (₂₂₅₅(SiH)) could then be estimated to be 45 ± 3 l/mol cm. The solubility of molecular H₂ in our hydrogenated samples was determined using the IR absorption band at 4138 cm⁻¹ and the extinction coefficient given by Shelby [J. Non-Cryst. Solids 179 (1994) 138]. Samples quenched with different cooling rates gave nearly identical Raman and IR spectra, suggesting that the chemical dissolution of hydrogen (SiH and SiOH) can be quenched to room temperature without changing relative concentrations and that no exsolution of hydrogen occurred during the quench.     

Distribution of hydroxyl groups on the silica gel surface

An experimental method for the investigation of the structure of surface hydroxyl coverage has been developed using the vapour of evaporable halides. It was found that the reaction of the surface hydroxyl groups with TiCl₄ proceeds to completion. A possibility for a quantitative characterization of the hydroxyl groups density was found on the basis of the proved applicability of the binomial distribution for the investigated.     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

Irradiation effects on the absorption edge of silica glass

Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica β-irradiated at different doses from ∼10³ to 5 × 10⁶ kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder degree induced by irradiation.     

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.     

Raman spectra and structure of sodium aluminogermanate glasses

Polarized Raman spectra of x NaAlO₂·(100 ? x) GeO₂ glasses (x = 0, 5, 10, 15, 20, 25, 33, 42, and 50) are presented. Analyses of the Raman data indicate that the aluminogermanate glasses have three-dimensional network structures consisting of interconnected AlO₄ and GeO₄ tetrahedra; Na⁺ ions are present in cavities and charge balance the Al³⁺ ions. Systematic changes are observed in the frequencies, intensities and polarization characteristics of spectral bands with variations in the NaAlO₂ content of these glasses. The antisymmetric stretching mode [ν_as (TOT), where T = Al, Ge] in the high-frequency region of the spectra (800–1000 cm^?1) appears as a doublet consisting of well-defined bands in the spectra of glasses along the entire join. Both components of the high-frequency doublet shift to a lower frequency with increasing NaAlO₂ content, indicating that the ν_as (GeO₄) and ν_as(AlO₄) stretching modes are coupled. The variations in the TO force constants and TOT bond angles with change in composition most likely cause the bands to shift. The frequencies of the Raman bands of sodium aluminogermanate glasses are compared with those of the corresponding bands in isostructural sodium gallogermanate glasses. On the basis of this comparison, the origin and delocalization of the vibrational modes producing characteristic Raman bands in the spectra of these glasses are discussed. The changes observed in the Raman spectra of aluminogermanate glasses with variation in NaAlO₂ content are analogous to those observed in the spectra of glasses along the NaAlO₂SiO₂ join.     

New observations on scratch deformations of soda lime silica glass

In spite of the wealth of literature, the role of the scratching speed in affecting the material removal mechanism in soda lime silica (SLS) glass is yet to be comprehensively understood. Here we report the surface and sub‐surface deformation mechanisms of SLS glass scratched under three different normal loads of 5, 10 and 15 N at various speeds in the range of 100–1000 μm/s with a diamond indenter of ~ 200 μm tip radius. The results show that at any given applied normal load, the width, depth, wear volume of the scratch grooves and wear rate of the SLS glass decreased with an inverse power law dependence on the applied scratching speed. The surface damage also reduced with the increase in scratching speed. A new, simple model was developed to explain these observations. The significant contributions of the time of contact, the tensile stress behind the indenter and the shear stress active just underneath the indenter in governing the material removal mechanisms of the SLS glass were discussed.     

Singular-value decomposition and boron NMR spectra in glass

We have applied the method of singular-value decomposition to the analysis of boron NMR spectra in glass. This procedure, which is useful for handling least-squares problems, clearly shows that there are a large number of solutions to any boron NMR spectral analysis problem. This limits the technique as a structure tool.