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1.
Non-crystalline solids within the liquid-liquid immiscibility region in the system SrO-SiO 2 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-SiO 2 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- SiO 2 glasses of high SrO content prepared by melting. The maximum amount Sr 2+ which could be introduced using an aqueous solution as the starting material corresponded to a composition of 10 SrO · 90 SiO 2 by weight. 相似文献
2.
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
3.
Acid dissolution of silicate glasses with different lead contents was rigorously investigated. Aqueous solutions containing 0.5, 1, and 2 N HNO 3, HCl and H 2SO 4 were used to measure the durability of the glass probes. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Inductively Coupled Plasma (ICP), X-ray Diffraction (XRD) and weight loss analyses were used to evaluate the morphological/compositional changes of the probes, the ash deposit, and the aqueous solutions produced due to the dissolution of the glass specimens. Empirical results showed that any increase in the lead content of the probes deteriorated the durability of the glasses by accelerating the hydrolysis of the silica network. ZrO 2 and TiO 2 additions had inverse effect and improved the chemical durability and the practical life-time of the lead glasses. 相似文献
4.
The resistance of Na 2OCaOSiO 2 glasses to acid solutions has been studied. The compositions studied were Na 2O · 4SiO 2, Na 2O · x CaO · (4? x) SiO 2 and a common electrode glass containing 22.63Na 2O and 5.58 CaO, mol%. The reaction was made at 40°C for about 3 h in 1N solutions of HCl, HNO 3 and H 2SO 4. Powdered glass samples were used and the reaction was followed by analyzing the solution for soda, lime and silica.The extraction rates of each constituent were measured. The effect of acid concentration was also studied for each glass using 10 ?3–10N solutions of the three acids for a fixed time. The quantity of calcium extracted increased slowly at first with increasing calcium content in the glass, but rapidly when the lime content exceeded ≈10 mol%. Above this concentration, both calcium and sodium appears to pass into solution in the same proportion in which they are present in the glass. The extraction rate was found to depend on the type and concentration of the acid used, being least in H 2SO 4 and much higher and almost equal in both HCl and HNO 3.An attempt was made to correlate the results of decomposition of the soda-lime-silica glasses to their membrane potentials in acidic solutions. 相似文献
5.
In this work, solid acid membrane consisting of poly(vinyl alcohol) (PVA), ammonium bromide (NH 4Br) and sulfuric acid (SA) has been prepared by a solution casting technique method. X-ray diffraction of the (PVA) 0.75(NH 4Br) 0.25(H 2SO 4) xM polymer matrix and pure (PVA) 0.75(NH 4Br) 0.25 revealed the difference in crystallinity between them. The effect of different amounts of SA on the conductivity of the polymer electrolytes was studied. The ionic conductivity of the prepared electrolytes can reach 3.1 × 10 −2 S cm −1 at room temperature. The conductivity measurements carried out at different temperatures indicate that all the films follow Arrhenius behavior and that the activation energy decreases with the increase in SA concentration. The a.c. conductivity seems to follow the universal power law. 相似文献
6.
In this work, the sintering process of different glasses in the system Na 2O-B 2O 3-SiO 2 has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data. 相似文献
7.
Solid state 1H, 29Si and 31P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P 2O 5 in P 2O 5-SiO 2 were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P 2O 5 gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix. 相似文献
8.
The elastic moduli of the entire vitreous range of the system CoPO that can be prepared by melting together Co 3O 4 and P 2O 5 oxides in open crucibles, have been measured by ultrasonic techniques at 15 MHz. The bulk, shear, longitudinal and Young's moduli and the Poisson ratio are found to be rather sensitive to the glass composition. It is found from this ultrasonic data, that the glass system can be divided into “three compositional regions”. This behaviour is qualitatively interpreted in terms of the cobalt co-ordination, crosslink densities, interatomic force constants and atomic ring sizes. Also presented is a full discussion of effects of annealing on elastic properties. 相似文献
9.
Optical characteristics of silica glass optical fibers containing Co 2+ doped ZnO-Al 2O 3-SiO 2 (ZAS) glass-ceramics prepared by slurry-doping method were investigated. The absorption and emission bands of the fibers were found to be originated from the tetrahedral Co 2+ in ZnAl 2O 4 crystals in ZAS glass-ceramics particles embedded in the core of the fibers. The crystal field strength of the Co 2+ ions in the optical fiber was found to be smaller than that of the Co 2+ ions in the bulk ZAS glass-ceramics. 相似文献
10.
A glass rod of the PbO---K 2O---B 2O 3---SiO 2 system having a radial gradient of refractive index has been prepared by the sol-gel process using aqueous solutions of lead acetate and potassium nitrate as the sources for index-modifying cations. A gel prepared by gelatinizing a sol from the mixture of tetramethoxysilane, tetraethoxysilane, and the aqueous solutions of boric acid and lead acetate was placed in an aqueous solution of potassium nitrate to form the concentration gradient of modifier cations by diffusion through the micropores of the gel. The glass rod of about 7 mm in diameter obtained by drying and sintering the gel had a refractive index of parabolic profile changing from the center toward the perimeter with a maximum difference of about 0.04. 相似文献
11.
Significantly different characteristics have been observed between Ge 42S 58 films prepared by thermal evaporation and bulk Ge 42S 58 glasses prepared by melt-quench. The film exhibits a higher electrical conductivity by about four orders of magnitude and contains a higher density of paramagnetic defect compared with the bulk. A large increase of the electrical conductivity is attained by doping with a small amount of Ag into the bulk, but the doping effect is hardly observed in the film. These differences are considered to result from larger topological disorder and larger defect density in the film network compared with the bulk network. 相似文献
12.
Orthorhombic Fe 5(PO 4) 4(OH) 3·2H 2O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H 2O 2 solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe 5(PO 4) 4(OH) 3·2H 2O dendritic nanostructures is time dependent: amorphous FePO 4· nH 2O nanoparticles are formed firstly, and then Fe 5(PO 4) 4(OH) 3·2H 2O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe 5(PO 4) 4(OH) 3·2H 2O products can be controlled by adjusting the amount of H 2O 2 solution, pH values, and types of iron precursors in the reaction system. 相似文献
13.
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
14.
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 10 3-10 4 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. 相似文献
16.
A new organic-inorganic hybrid γ-octamolybdate complex, (NH 4) 4[(C 3H 7NO) 2Mo 8O 26]·2CH 3CH 2OH·2H 2O was synthesized from the mixture of ethanol, DMF, 4-aminobenzoic acid, (NH 4) 6Mo 7O 24·4H 2O, HCl and H 2O. The structure was determined by X-ray crystallography. It crystallizes in monoclinic P2 1/ n space group with the crystal cell parameters of a = 8.8825(4), b = 21.1608(10), c = 11.1343(6) ?, β = 104.7720(10)°, V = 2023.64(17) ? 3 and Z = 2. The crystal X-ray analysis shows that two DMF molecules are bound to γ-octamolybdate anion. The molecular structure
is stabilized by the complex hydrogen bonding. 相似文献
17.
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
18.
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. 相似文献
19.
Samples of silica glass and soda-silica glasses (containing 1 wt% and 13 wt% Na 2O) were prepared by hydrolysis of alkoxides to form gels, followed by heating the gels at low temperatures, below the glass transformation. The water necessary for hydrolysis was either taken from the atmosphere or added to the starting solution to accelerate the process. In the latter case the gelation time exhibited a minimum for a certain quantity of water added. The heat treatment process to form a glass from a gel was studied by DTA and TGA. At low temperatures (below 100°C) an endothermic DTA peak was observed due to the loss of organic volatiles and water. A large exothermic peak was also found at 276°C for the silica and at 365–395°C for the soda-silica gels. At higher temperatures the 13Na 2O87SiO 2 (wt%) composition gave a DTA trace very similar to the same composition conventionally melted which had been rapidly quenched from well above the transformation range (i.e. with a high fictive temperature). The preparation of bulk samples of the glasses is discussed. 相似文献
20.
The immiscibility boundary and the critical point of SiO 2-Nd 2O 3 system glass were determined as a function of Al 2O 3 addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al 2O 3. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd 2O 3 in SiO 2. It is concluded that the Al 2O 3 addition to Nd 2O 3 containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al 2O 3 addition diminishes the concentration fluctuation of Nd 2O 3 in silica glass. 相似文献
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