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1.
A series of titania-silica glasses with 0–9% TiO 2 were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens. In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering. The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10−7 and −0.2 × 10−7°C−1 in the range 0 to 9% TiO2. The glass with 7.2% TiO2 exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm. 相似文献
2.
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. 相似文献
3.
Basic glasses are prepared by chemical polymerization in a sol-gel process. Nucleation and crystallization of these glasses are analyzed in dependence of the composition of the basic glasses and “additions” of TiO 2 and LiO 2. A comparison of gel glasses with conventionally molten glasses is made. Gel glass-ceramics are prepared as bulk materials and thin coatings. 相似文献
4.
The EPR spectra of Fe 3+ and V 4+ ions, and radiation centres have been studied in α-tridimite. Consideration of the EPR data in all crystalline and vitreous forms of the SiO 2 system is made. It is shown that the crystal field strength and symmetry did not change considerably in different polymorphs of silica, the changes in hyperfine structure constants being significant. The relation between electronic structure of vitreous silica and different crystalline forms is established. 相似文献
5.
Cathodoluminescence on wet and dry thermal SiO 2 films shows significant time, temperature, and current dependence in the intensities of the observed emission bands at 1.9, 2.7 and 4.3 eV. The low temperature behavior of the 2.7 and 4.3 eV lines suggests a relation between the corresponding luminescence centers. The 1.9 eV line is not found to behave similarly. 相似文献
6.
Anomalous SiO 2 films have been prepared by sputtering Si in a mixture of Ar-10% O 2 at 77 K. The same sputtering conditions at room temperature yield normal SiO 2 which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO 2 is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO 2 is only slightly different from bulk SiO 2 but esr experiments reveal about 3 × 10 18 spins cm which do not exist in bulk SiO 2. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10 5) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO 2 films consist of essentially bulk like SiO 2 clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties. 相似文献
7.
Four glasses of the SiO 2-GeO 2 binary system have been synthesized via a sol-gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO 4 and SiO 4 tetrahedra, with GeO 4 tetrahedra similar to the GeO 4 units in GeO 2 glass and continuous compositional variations from GeO 2-rich regions to SiO 2-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge-O-Si angles are close to the Ge-O-Ge mean angle in GeO 2 glass, 134° and 130°, respectively. This misfit with the Si-O-Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO 2 glasses. 相似文献
8.
The synthesis of mixed ZnO/SiO 2 oxides has been carried out using sol gel technique. Gelation time of the produced oxides gel has been measured experimentally by using turbidity change with time using Turbidimeter. In addition, gelation time was estimated visually. It is found that the gelation time is decreased by increasing the concentration of ZnO and SiO 2. Correlations between gelation times and concentrations of products are also discussed. Surface energy between the formed gel and solution is calculated as 12.1 mJ/m 2. The gelation rate is increased and the Gibbs free energy for the formation of critical nucleus is decreased by increasing the concentration product of ZnO · SiO 2. The critical radius of the nucleus is decreased from 5.75 to 5.02 Å when the concentration product is increased from 6.42% to 15.72%. On the other hand, the number of molecules in the critical nucleus is decreased from 11 to 8 when the concentration product changed under the same conditions. This approach can be used as a model to discuss the effect of any additives on the enhancing or inhibition the gelation rate for any gel. 相似文献
9.
In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO 2GeO 2 glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO 2 and SiO 2 melts results in a continuous random network structure of TO 4 tetrahedra (T Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch ( vas) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO 2GeO 2 join. The presence of three distinct vas (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO 2GeO 2 glasses indicates that the symmetric stretch modes ( vs) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the vs (TOT) band in the spectra of SiO 2GeO 2 glasses with increasing concentration of GeO 2 may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm ?1 region of the alumino-silicate glasses, spectra to the vas(AlOSi) and vas(SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO 2GeO 2 glasses have a much higher degree of disorder of the network-forming cations. 相似文献
10.
SiO 2 samples were implanted with 45 keV Zn ions at doses ranging from 5×10 15 to 1.0×10 17 ions/cm 2, and were then subjected to furnace annealing at different temperatures. Several techniques, such as ultra-violet–visible spectroscopy (UV–vis), grazing incidence X-ray diffraction spectroscopy (GXRD) and atomic force microscopy (AFM), have been used to investigate formation of nanoparticles and their thermal evolution. Our results clearly show that Zn nanoparticles could be effectively formed in SiO 2 at doses higher than 5×10 16 ions/cm 2. The subsequent thermal annealing at oxygen ambient could induce the growth of Zn nanoparticles at intermediate annealing temperature range. While at temperature above 600 °C, Zn nanoparticles could be transformed into ZnO, or even Zn 2SiO 4 nanoparticles. The results have been tentatively discussed in combination with Zn diffusion and migration obtained by Rutherford backscattering spectroscopy (RBS) measurements. 相似文献
11.
The effect of the interaction between colloidal particles in SiO 2 and TiO 2 sols on the viscosity of the sols has been examined. It was found that the interaction was significantly influenced by the zeta potential of electrical double layers and terminal bonds associated with particle surfaces. Water/alkoxide mole ratio and pH affect the zeta potential and the terminal bonds and, consequently, change the interaction between the particles and the viscosity of the sols. In addition, the relationship of viscosity with the volume fraction of the colloidal particles in SiO 2 sol has been described by a Dougherty-Kreiger equation when the viscosity reached a stable value with time. 相似文献
13.
A room temperature method for the encapsulation of pyrene in SiO 2 nanoparticles is described. The relation between alkoxysilane surfactant chain length, reactant molar ratios and the uptake of dye, sample morphology, photophysical properties, and the ability of the silicate matrix to protect the encapsulated dye was examined. The synthesis can easily be adapted for the encapsulation of other hydrophobic and thermolabile substances, and used in the development of nanostructured optically active coatings, films and monoliths. 相似文献
14.
The structure of Li 2O·SiO 2 glass has been determined by the pair-function analysis of the radial distribution function (RDF) obtained by X-ray and neutron diffraction measurements. The structure models were constructed by the molecular dynamics method (MD). The calculated RDFs, the summation of the pair-functions of the model, were compared with the observed RDFs while varying the MD parameters. Taking advantage of the negative scattering length of Li for neutron diffraction, the glass structure was investigated in detail, and it was found that the chains of SiO 4 tetrahedra bend at 20 °, which is a little less than for the crystal (23.48 °). It is known that the larger the size of the alkali ions (Na + → K + → Cs +), the smaller the bending angle of the chains. It was found that this rule also applies to Li +. 相似文献
15.
Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO 2Fe 2O 3 system (A: 5 wt% Fe 2O 3, B: 10 wt% Fe 2O 3, C: 20 wt% Fe 2O 3, D: 40 wt% Fe 2O 3).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe 3+ and Fe 2+ oxidation states was also determined, together with the attribution of the probable coordination states for Fe 3+ ions.Complex magnetic structure appeared in samples treated above 800°C. 相似文献
16.
Glasses with the compositions xNa 2O · 10MgO · (90 − x)SiO 2, 10Na 2O · xMgO · (90 − x)SiO 2, 5Na 2O · 15MgO · xAl 2O 3 · (80 − x)SiO 2, xNa 2O · 10MgO · 10Al 2O 3 · (80 − x)SiO 2, 10Na 2O · 10MgO · xAl 2O 3 · (80 − x)SiO 2, 10Na 2O · 5MgO · 10Al 2O 3 · (80 − x)SiO 2 were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm −1. At [Al 2O 3] > [Na 2O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm −1 attributed to Fe 2+ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm −1 increases. This is explained by the incorporation of iron in the respective glass structures. 相似文献
17.
Different solutions containing alkoxides of silicon, titanium, and zirconium have been prepared. Some of their properties like the time dependent viscosity and the gelling time have been measured and are reported here for different H 2O, HCl, ethanol and/or formamide contents. Microscope slides have been dip coated in these solutions. After baking, film thickness and chemical durability have been determined. In order to get good SiO2---TiO2---ZrO2 glass coatings, the withdrawal speed should not exceed 5 cm/min unless the viscosities of the solutions are reduced by the addition of ethanol. By such a dilution, the film thickness could also be reduced, while the addition of formamide caused a delayed increase of the viscosity and increased gelling times. For this reason, solutions containing formamide can be used for longer periods. The chemical durability of the substrates against boiling NaOH solution is enhanced by the SiO2---TiO2---ZrO2 glass coatings. 相似文献
18.
Glass films of pure SiO 2 and TiO 2 have been prepared on sodalime silica flat slide glasses by the sol-gel process using the dip-coating technique from TEOS and Ti(OC 3H 7) 4 solutions. The various parameters such as chemicals concentrations, viscosity, type of catalyst, withdrawal speed and temperature of densification leading to the obtention of good and adherent coatings with definite film thicknesses are reported. The same technique has been used for the depositon of layers of colored films SiO 2---M xO y (M = Co, Mn, Nd and Cr). Brilliant yellow coatings have been obtained with TiO 2---CeO 2. 相似文献
19.
Pure chemosynthetic Al 2O 3-2SiO 2 geoploymers displaying positive alkali-activated polymerization properties and high compressive strength at room temperature were effectively fabricated utilizing a sol-gel method. The molecular structure of the precursor powder and resulting geopolymers were investigated by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analysis. In addition, the mechanical and alkali-activated polymerization properties of these materials were also studied. NMR data revealed that the chemosynthetic powders began to contain 5-coordinated Al atoms when the calcination temperatures exceeded 200 °C. These calcined powders were capable of reacting with sodium silicate solutions at calcination temperatures exceeding 300 °C, which is, however, much lower than the temperature required to convert kaolin to Metakaolin. 相似文献
20.
A brief review of theoretical models which have been proposed to describe electronic processes in pure SiO 2 is presented. The models include the valence bond model which allows bond breaking and changes in local environment after an electronic transition, as well as more traditional molecular orbital models. The models are critically compared with experimental data for the pure and glassy materials in order to determine their degree of applicability to the properties involved. Data for the ultraviolet spectrum, the oxygen X-ray emission spectrum as well as that for the intrinsic E′ 1 defect center are included in the discussion. Further experiments are suggested which may determine in greater detail the range of applicability of the various models. 相似文献
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