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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.
Silica glass tubes were prepared by following a new sol-gel method. The pH of the sol containing hydrolyzed Si(OC 2H 5) 4 with HCl and colloidal silica was adjusted by adding ammonia solution. Sol was poured into a cylindrical tube and gelled while rotating cylinder at 1000 rpm to form a tube-shaped gel. Then the gel was dried carefully to be tube dry-gel and finally sintered to become a clear silica glass tube. The effects were examined of the speed of rotation before and after gelation, to form the tubular gel, upon the shape and appearance of the derived tubes. The best speed of rotation was about 1000 rpm, and an adequate long period of rotation after gelation was necessary to maintain the tube shape. One of the large silica tubes had dimensions of 26 mm outside diameter, 13 mm internal diameter and 1000 mm length, and the ovality of the silica glass tube was 0.02%. The derived tube can be applied in a silica glass tube for an optical fiber fabrication process or in a furnace tube for IC production. 相似文献
3.
V 2O 5 gels containing up to 18 mol% of TiO 2 were obtained through the simultaneous hydrolysis of alkoxides in ethanol solution. V 2O 5 gels containing Na 2O or Li 2O were obtained through the ion exchange method. The crystallization temperature, Tcr, of the gels increased and the H 2O content of the gels decreased by the addition of TiO 2 or Na 2O. These additives seem to stabilize the amorphous state of the gels. On the other hand, Tcr and the H 2O-content slightly varied with the addition of Li 2O. No ionic polarization was observed in coating films of the gels dried at temperatures below Tcr. The dc conductivity of the films was anisotropic, and increased with the addition of Li 2O or Na 2O. However, it decreased with increasing TiO 2 content. The fiber-like structure of gels was observed by TEM. The gels obtained from alkoxides were thin and short in comparison with the gels obtained through the ion exchange method. 相似文献
4.
This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC 4H 9) 4. The hydrolysis of Si(OC 2H 5) 4 can be carried out at about 75°C. The amount of added water (γ WI), which varied with TiO 2 content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC 4H 9) 4 is added. The total amount of added water (γ W) is related to the amount of solvent ( R). For example, if TiO 2 is 40mol%, γ W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when RSi(OC2H5)4 = 1, RTi(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO 2 content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO 2. 相似文献
5.
Small-angle X-ray scattering (SAXS) measurements on V 2O 5· nH 2O sols and gels, prepared by dissolving V 2O 5 glass into water at room temperature, show that there are V 2O 5 polymeric fibers entangled like random coils in the sol of n 5000, while the deviation from the random coil behavior occurs in the dilute sol of n 6000. A Bragg peak appears at the scattering vector h 0.02 Å−1 to be superimposed on an asymptotic h−2-course in the SAXS curve of the concentrated sol of n 680. This means that the spatial correlation between V2O5 polymeric fibers takes place even in the fluid state. V2O5·nH2O sols completely lose fluidity at n 250 to transfer to the gel state, where V2O5 polymeric fibers begin to pile up in the parallel with a substrate surface. Such a layer structure is preserved up to the gel of n 4. However, V2O5 polymeric fibers are randomly oriented within each layer. 相似文献
6.
The colorless and transparent glasses in the Al 2O 3---B 2O 3---SiO 3 system with high B 2O 3 and SiO 2 content were prepared from gels at low temperature. Their IR spectra not only revealed the evolution of the gel to glass conversion, but also showed that the formation of mixed bonds in the glasses obtained did not show any effect due to the B 2O 3 content. The accuracy of the glass composition is dependent upon the SiO 2/B 2O 3 molar ratio. The higher the ratio, the less the deviation of the analyzed compositions of the resulting glasses from their original calculated values. It is obvious that the higher the ratio, the lower the thermal expansion coefficient and the higher the transformation temperature of the glass, and the temperature at which the thermal contraction reaches an equilibrium is higher. 相似文献
7.
Alkoxide derived gels were prepared in the system Na 2O---B 2O 3---SiO 2. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram. Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na2O, B2O3 and SiO2 extracted from the attack gels were analyzed. The experimental results indicate that the amount of B2O3 has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B2O3 mol% the greater are the amounts of Na2O and B2O3 extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B2O3 and Na2O extracted slightly increases as the B2O3 mol% increases. Small amounts of extracted SiO2 were always observed. These results are complemented with other measurements so that an explanation of the controlling mechanism is given. 相似文献
8.
Using HCl as a transport agent, well-shaped crystals of rhombohedral Fe 2(SO 4) 3 with edge length up to 1 mm were formed in the less hot zone of a silica ampoule. The dependence of the deposition rate on the mean transport temperature for a constant concentration of the transport agent ( C = 4.5 × 10 −2 mmol HCl/cm 3) shows a distinct maximum at
= 560°C. Additionally, the influence of the concentration of the transport agent at a constant temperature gradient was investigated. On the basis of thermochemical calculations, an explanation of the observed transport behaviour is given in the present paper. 相似文献
9.
The gel formation of the (100- x)TiO 2· xSiO 2 ( x = 0–10 mol%) system has been studied. The progressive elimination of residues was followed by DTA and TGA curves. DTA curves showed that the formation of anatase during heat treatment could be sensibly slowed down with the increase of SiO 2. The relationship between the gel composition and crystallization temperature of anatase has been systematically investigated. The X-ray diffraction spectra demonstrated that the crystallization temperature of anatase is 400°C for TiO 2 gel and 430°C for 90TiO 2 - 10SiO 2 gel. The infrared absorption spectra were used to follow the structural transformation of gels heat-treated at different temperatures. With the help of EPR it is evident that titanium ions exist only in tetravalence. 相似文献
10.
The effects of drying temperature on the structural evolution of alkoxy-derived silica gel prepared using various catalysts have been investigated. The dependence of specific surface area, Sg, reflecting the structure, on the temperature of drying was remarkable for a non-catalyzed xerogel. The effect of drying temperature on the Sg of an ammonia-catalyzed xerogel was also found but was not very large. The Sg of xerogels obtained by drying at 60°C was always higher by 50% than the gels dried at 30°C without regard to the aging temperature. The Sg of xerogels from HCl-catalyzed solution was of the order of several m 2/g, however, the Sg of the aerogel obtained by hypercritical drying of the wet gel from a similar solution was about 800 m 2/g. These phenomena were understood on the basis of SAXS measurements on both wet gels and aerogels. 相似文献
11.
Commercially available fumed colloidal silica was gelled in an aqueous solution containing PO 4H 3. Instantaneous gelation was obtained by adding several drops of HF (48 wt%). The objective of this paper was to study the gel-to-glass conversion of these colloidal gels using constant heating rate (CHR) experiments. Gel densification was measured at temperatures ranging from 50 to 1550°C at different heating rates (1 to 10°C/min) using a dilatometer. Shrinkage and shrinkage rate as a function of temperature were measured and CHR equations were used to derive information on the densification mechanisms. The experimental results show that small additions of PO4H3 into pure silica dispersions give rise to gels which densify to high silica glass at much lower temperatures than pure colloidal silica gels. The bloating effects produced by pure colloidal silica gels at temperatures above 1280°C were also eliminated. For these P2O5-doped silica gels maximum shrinkage rates were found at temperatures between 1050 and 1150°C according to the heating rate used. The CHR analysis showed that several different mechanisms seem to operate in a complex interdependence in the whole range of temperature studied. This CHR analysis was compared with those results obtained from isothermal shrinkage experiments in the range where viscous sintering is the predominant shrinkage-controlling mechanism (between 1000 and 1100°C). 相似文献
12.
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
13.
Mössbauer absorption measurements have been made at room temperature on 57Fe in iron sodium silicate glasses containing 3–15 mol% Fe 2O 3 and various iron alkali silicate crystals in order to study the state of iron in these glasses. The spectra of all the glasses gave one doublet with a quadrupole splitting varying from 0.73–0.78 mm s −1, while those of Na 2O · Fe 2O 3 · 4 SiO 2 and 5 Na 2O · Fe 2O 3 · 8 SiO 2 crystals showed much smaller quadrupole splitting, 0.28 mm s −1 and 0.10 mm s −1, respectively, and an asymmetrical doublet of much narrower linewidth. When sodium was replaced by other alkali metals of larger size, such as K and Cs, in MFeSi 2O 6 and MFeSi 3O 8 crystals, the quadrupole splitting became wider and approached to 0.73 mm s −1. Such a variation was not observed for glasses. These results suggest that a larger number of non-identical sites exist in iron sodium silicate glasses than in the corresponding crystals. 相似文献
14.
The effects of vibration ball-milling of a mixture of two gels, prepared from mixing ZrO 2 and TiO 2 sols, on crystallization processes during heating are studied in order to determine enhanced mechanochemical reactions with the aid of hydroxyl groups. Data of differential thermal analysis (DTA) and X-ray diffractometry show that TiO 2 (anatase) and ZrO 2 (tetragonal) crystallized at 698 and 712 K, respectively, when a powdered gel mixture was heated to 1023 K to 10 K min −1 in air. No reactions between the two phases were detected. After vibro-milling a powdered gel mixture at room temperature for 3 to 30 h, a new exothermic DTA peak appeared around 950 K and ZrTiO 4 (orthorhombic) was detected on heating to 1023 K. After vibro-milling, the compositional micro-homogeneity of non-heated gels was estimated from the fluctuations of elemental compositions in local areas using energy dispersive spectrometer with a 25 nm electron beam. The fraction of ZrTiO 4 crystallized on heating the gels to 1023 K, determined by Raman spectroscopy, increased linearly with increasing the micro-homogeneity for non-heated gels. It is suggested that atomic movement and short-range ordering of ZrTiO 4 rather than crystallization of individual oxides occur when applying mechanical stress to a powdered mixture of ZrO 2 and TiO 2 gels. 相似文献
15.
Materials made of tin oxide nanocrystals homogeneously dispersed into a silica glass matrix have been elaborated through sol–gel processing. Addition of strong acids was used to control the hydrolysis–condensation in tin and silicon alkoxides mixed alcoholic solutions. Fourier transform infrared spectroscopy (FTIR) and Small angle X-ray scattering (SAXS) measurements show that the introduction of HCl allows to synthesize gels containing up to 50% of tin oxide precursor without significant modification of the pure silica gel network. Thermal treatments of slowly dried bulk gels induce the crystallization of tin oxide nanoparticles. After firing at 1000 °C, dense materials containing tin oxide nanocrystals (with a mean diameter close to 1–2 nm) are obtained. The crystal size distribution was estimated by X-ray diffraction line profile analysis. The narrowness of this distribution makes these materials interesting for optical applications. 相似文献
16.
Homogeneous and transparent V 2O 5–TiO 2 composite nanometer thin films were prepared on glass substrates by sol–gel processing and dip-coating technique. The films as well as the dried powder of bulk gel were characterized by different techniques like X-ray diffraction (XRD), high-resolution scanning electron microscopy (HRSEM), atomic force microscope (AFM) and thermogravimetry–differential thermal analysis (TG–DTA). The hydrophilicity of the films was determined by measuring the water contact angles on the films. The results showed that the dopant of V 2O 5 on TiO 2 thin films could produce a visible-light response to the films, and the introduction of V 2O 5 could suppress the structural phase transition and crystal growth of TiO 2 crystal. Finally, the relationship between crystalline size and hydrophilicity under sunlight was investigated in this article. 相似文献
17.
The sintering of three kinds of dried gels with different microstructures prepared from the tetramethoxysilane (TMOS) solutions containing N,N-dimethylformamide (DMF) with varying [H 2O]/[TMOS] mole ratios and different catalysts was investigated by measuring the pore size distribution, the Vickers hardness, the linear shrinkage and the apparent density of the gels heated to various temperatures. The marked shrinkage of the pores started at temperatures above 900 °C. The temperature required for the conversion of a gel to a pore-free silica glass depended on the average size of the pores contained in the dried gels; the average pore sizes of about 50 Å, 75 Å and 100 Å corresponded to 1040 °C, 1060°C and 1080°C, respectively. The Vickers hardness, the linear shrinkage and the density of the gel showed a corresponding change with a change in heating temperature. 相似文献
18.
The preparation of glass-lined coating mould from gels in the ternary system of SiO 2–ZrO 2–B 2O 3 has been investigated. The crystallization characterization and high temperature structure stability of this coating mould are demonstrated. We can find that the crystallization of t-ZrO 2 as well as the tetragonal to monoclinic phase transformation are, respectively, retarded and impeded owing to the encasement of SiO 2 matrix. While the inhibitive effect of B 2O 3 on crystallization of the SiO 2–ZrO 2–B 2O 3 coating mould is explained. Finally, DD3 single crystal superalloy melt can realize highly undercooled rapid solidification by adopting this coating mould, which further evinces that SiO 2–ZrO 2–B 2O 3 coating mould has an ideal nucleation inhibition for superalloy. 相似文献
19.
Homogeneous glass powders were prepared from multicomponent silicate gels in the BaO---Al 2O 3---SiO 2 system. Gels synthesized from TEOS, Al-sec-butoxide, and Ba-acetate were characterized by TGA, DSC, and single-point BET analysis. Amorphous oxide powders of controlled composition were obtained through an appropriate drying and heat treatment schedule. Inclusion-free clear oxynitride glasses were also prepared by co-melting gel synthesized oxide powders with silicon nitride powders. 相似文献
20.
Tellurite containing vanadate (50− x)V 2O 5– xBi 2O 3–50TeO 2 glasses with different bismuth ( x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi 2O 3 content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi 2O 3 content. The observed results confirm that the addition of Bi 2O 3 modifier changes the rigid formula character of TeO 2 to a matrix of regular TeO 3 and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO 4 trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi 2O 3 content are more stable than with high Bi 2O 3 content. 相似文献
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