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1.
The gel formation of the (100-x)TiO2·xSiO2 (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 SiO2. 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 TiO2 gel and 430°C for 90TiO2 - 10SiO2 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   

2.
The influence of phase separation and crystallization on the internal friction of some oxide glasses is reviewed and discussed. In alkali-containing glasses, the internal friction peak caused by stress-induced diffusion of alkali ions decreases in magnitude and shifts to higher temperature slightly due to phase separation. And in alkali-free glasses phase separation only exerts a minor decrease upon the background of internal friction curves, whereas crystallization influences the internal friction of these glasses more strongly. Because of crystallization, in alkali-containing glasses alkali ions might diffuse in a residual glass phase and a crystal phase, respectively. This might cause corresponding internal friction peaks. And in alkali-free glasses, no evident internal friction peak is observed. However, the author found a high and wide internal friction peak at about 100°C in the crystallized MgO·Al2O3·SiO2·TiO2 and ZnO·Al2O3·SiO2·ZrO2 glasses. The peak occurring in the two glasses studied is probably connected with glass crystallization and crystallized crystals.  相似文献   

3.
Feng Liu  Gencang Yang 《Journal of Non》2001,290(2-3):105-114
The preparation of glass-lined coating mould from gels in the ternary system of SiO2–ZrO2–B2O3 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-ZrO2 as well as the tetragonal to monoclinic phase transformation are, respectively, retarded and impeded owing to the encasement of SiO2 matrix. While the inhibitive effect of B2O3 on crystallization of the SiO2–ZrO2–B2O3 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 SiO2–ZrO2–B2O3 coating mould has an ideal nucleation inhibition for superalloy.  相似文献   

4.
V2O5 gels containing up to 18 mol% of TiO2 were obtained through the simultaneous hydrolysis of alkoxides in ethanol solution. V2O5 gels containing Na2O or Li2O were obtained through the ion exchange method. The crystallization temperature, Tcr, of the gels increased and the H2O content of the gels decreased by the addition of TiO2 or Na2O. These additives seem to stabilize the amorphous state of the gels. On the other hand, Tcr and the H2O-content slightly varied with the addition of Li2O. 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 Li2O or Na2O. However, it decreased with increasing TiO2 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   

5.
The effects of the homogeneity of precursor non-crystalline Pb---Ti---O gels on their transformation to crystalline PbTiO3 were studied. Hydrous gels were prepared via two different routes, i.e., (i) mixing two unitary sols (MS), and (ii) co-precipitation (CP). Mechanical pretreatment was also carried out, primarily for the purpose of homogenization. The local homogeneity was evaluated by elementary microanalysis using a transmission electron microscope. The process of ball-milling the MS gel for 3 h improved its homogeneity only slightly. However, the homogeneity of the ball-milled MS gel suddenly increased to that of the CP gel on subsequent heating to a temperature as low as 483 K. This type of thermal homogenization was not observed for the unground MS gel. The evaporation of lead monoxide on further heating to 1023 K was also suppressed by preliminary ball-milling. All these results suggest that the ionic migration and the formation of Pb---O---Ti bonds in the gels take place during the mechanical activation with the aid of OH groups.  相似文献   

6.
The surface of a TiO2 film electrode, about 1 μm thick, prepared by the sol-gel method, was modified by being additionally coated with about 0.1 μm thick TiO2---SiO2, TiO2---ZrO2 or TiO2---Al2O3 films. The effect of the additional coating on the photoelectrochemical properties of a TiO2 film electrode was investigated in detail. On addition of the second additive to TiO2, the flat band potential was shifted toward negative potential for SiO2 and positive potential for ZrO2 and Al2O3, which is attributed to the change in the point of zero zeta potential (pzzp), not in the electron affinity (EA). However, enhancement in photocurrent was not observed for all the cases.  相似文献   

7.
Ceramic materials with a very low thermal expansion coefficient are synthesized by the sol-gel process. The binary gel is obtained by hydrolysis and polycondensation reactions of organometallic compounds of aluminium and titanium. The thermal evolution of the amorphous powder is followed by DTA and TGA measurements. Structural evolution is followed using X-ray diffraction. The crystallization of the TiO2 rutile and Al2O3 corindon starts at 700 and 900°C respectively. The transformation of Al2O3 and TiO2 into Al2TiO5 appears between 1200 and 1300°C. The densification of the powder is performed by the hot pressing process. The shrinkage of the powder was previously followed by dilatometric measurements. The physical properties of the final material are studied as a function of pressing parameters.  相似文献   

8.
Glasses in the system Na2O/B2O3/Al2O3/In2O3 were melted and subsequently tempered in the range from 500 to 700 °C. Depending on the chemical composition, various crystalline phases were observed. From samples without Al2O3, In2O3 could not be crystallized from homogeneous glasses, because either spontaneous In2O3 crystallization occurred during cooling, or other phases such as NaInO2 were formed during tempering. The addition of alumina, however, controlled the crystallization of In2O3. Depending on the crystallization temperature applied, the crystallite sizes were in the range from 13 to 53 nm. The glass matrix can be dissolved by soaking the powdered glass in water. This procedure can be used to prepare nano-crystalline In2O3-powders.  相似文献   

9.
A series of titania-silica glasses with 0–9% TiO2 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   


10.
Small-angle X-ray scattering was used to examine in situ formation of ZrO2---SiO2 structures in alcoholic solution of tetraethoxysilan (TEOS) as a function of the ratio of ZrO2 to SiO2. For the moment of the first measurement (15 min after the preparation) primary particles with Rg ≈ 1.5 nm exist in all investigated mixed gels. These particles aggregated to secondary clusters. The resulting clusters can be described by means of fractal theory, where the determining mechanism of formation is cluster-cluster aggregation (diffusion or chemical limited). The time of gelation is a function of the ZrO2 concentration. The higher the ZrO2 concentration in the solution, the faster is the aggregation to secondary clusters. Gelation times were between 170 and 970 h.  相似文献   

11.
Since 1980 much attention has been payed to hydrothermal processing. There are several papers related to hydrothermal fine Zirconia powders by authors 1), 2), 3) and 3).

The authors 3) prepared ZrO2 and Y2O3-ZrO2 powders under hydrothermal conditions. Several methods were used to prepare fine ZrO2 powders under hydrothermal conditions: hydrothermal precipatation, hydrothermal crystallization, hydrothermal oxidation, hydrothermal decomposition, hydrothermal synthesis, hydrothermal anodic oxidation, RESA (reactive electrode submerged arc) process, etc.

This paper describes hydrothermal precipatation processing of ZrO2 and Y-ZrO2 powder.  相似文献   


12.
In the present work, the influence of the nucleation time-lag on the non-isothermal glass crystallization is discussed. Differential thermal analysis (DTA) results of an iron-rich glass nucleated by Cr2O3 were obtained at different heating rates. The activation energy of crystallization, Ec, and the Avrami parameter, m, estimated by Kissinger's and Ozawa's equations were shown to be dependent on the heating rate. The value of Ec, obtained at 2.5, 5 and 7.5 K/min heating rates was calculated as 299 kJ/mol, while the value of Ec, obtained at 10, 15 and 20 K/min was as 499 kJ/mol. The value of m for ‘low' and ‘high' heating rates were 2.57 and 1.45, respectively. The results were interpreted on the basis of the non-steady state nature of the nucleation process. It was assumed that at high heating rates no nucleation takes place and the crystals grow on a existing fixed number of nuclei; the activation energy of crystal growth, Eg, can be estimated by applying the Kissinger equations. At low heating rates nucleation occurs and the number of nuclei formed is influenced by the heating rate; Eg can be estimated by the Matusita and Sakka equation.  相似文献   

13.
Zr---Al metallo-organic compounds (zircoaluminates), having (CH2)4COOH, (CH2)12CH3 and (CH2)2NH2 as the organofunctional groups, were treated preliminary by (1) spray-drying, (2) gelation of addition of 10% NH4OH aqueous solution followed by spray-drying and (3) rotary evaporation under a reduced pressure. After the treatment they were heated in air to prepare ZrO2---Al2O3 composite powders. The IR and DTA profiles for the treated compounds indicated that the procedures modified the structures for the zircoaluminates. The stability of tetragonal ZrO2 for the ZrO2---Al2O3 composite powder were dependent on the modification in the structure for the zircoaluminates. Balloon shaped particles, 0.5–2 μm in diameter, were obtained through procedure (1) and spherical particles, 1–4 μm in diameter, through (2). Tetragonal ZrO2 grains, 0.1–0.2 μm in diameter, were dispersed in the particles when heated at 1400°C.  相似文献   

14.
Surface crystallization in a rare-earth aluminosilicate glass (Nd2O3–Al2O3–SiO2–TiO2) was studied using an isothermal method and the crystal growth rate of the glasses was evaluated as a function of the composition. For measuring the surface crystal growth rate, two different methods: measurement of the crystal layer in the longitudinal and lateral growth direction. It was found that crystallization proceeded by surface crystallization only and TiO2 did not act as a nucleating agent. The growth rate was strongly dependent on the viscosity of glass and agreed with prediction from the Preston model using the known viscosity and melting temperature. As the Si/Nd and Si/Al ratios decreased, the crystal growth rate increased. TiO2 and Nd2O3 played the role of network modifier, which decreased the viscosity of the glass, facilitating crystallization of the rare-earth aluminosilicate glass.  相似文献   

15.
Monolithic glasses with compositions 2MO · 2Al2O3 · 5SiO2, being M=Ni and equimolar mixtures of Ni and Mg, were prepared at 1650 °C by melting mixtures of raw materials. The crystallization of monoliths was produced by heat-treatment at several temperatures up to 1200 °C. The crystallization sequence was followed by differential thermal analysis (DTA), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) and diffuse reflectance spectroscopies. Surprisingly, the only crystalline phase formed after heating up to 1100 °C was a nickel-containing aluminate spinel for both compositions. The microstructural characterization indicated the volume crystallization of well formed octahedral crystals of spinel with smaller size than 500 nm. Finally, it has been proved that nickel-containing aluminosilicate glasses could be converted into μ- or -cordierite- and spinel-based glass-ceramics by thermal treatment of powdered glasses and monolithic bodies, respectively.  相似文献   

16.
Aihu Chen  P.F. James 《Journal of Non》1988,100(1-3):353-358
A 10Li2O---90SiO2 (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li2O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.  相似文献   

17.
Various oxide films on SiO2 glass substrates were irradiated by a laser beam. A continuous CO2 laser source (wavelength 10.6 μm) was used for this purpose; the composition change at the surface layer was determined by Rutherford backscattering spectrometry (RBS). All the alkaline-earth oxides as well as those of lanthanum and yttrium, entered the glass after treatment. ZrO2 and CeO2, however, did not enter the SiO2 glass due to laser irradiation. It is interesting, however, that a film of ZrO2 + Al2O mixture easily entered into the SiO2 glass by laser processing. The conditions and mechanism of laser-enhanced interaction of ZrO2 or other oxide films with SiO2 glass surfaces are discussed especially in view of their structural behaviour in glass.  相似文献   

18.
Tellurium oxide glasses were prepared by the hammer and anvil technique. The glass systems are (0.85TeO2 + 0.15Z), where Z = K2O, TiO2, V2O5, MnO, Fe2O3, CoO, NiO or CuO. A second group is a ternary system 0.85TeO2+(0.15 − x)TiO2 + xFe2O3) with x=0.0, 0.05, 0.1, 0.15 mol. X-ray diffraction, infrared spectroscopy and differential thermal analysis measurements were carried out. The present study showed the different glass-forming groups, the glass transition and crystallization temperatures as well as the crystallization processes.  相似文献   

19.
Homogeneous and transparent V2O5–TiO2 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 V2O5 on TiO2 thin films could produce a visible-light response to the films, and the introduction of V2O5 could suppress the structural phase transition and crystal growth of TiO2 crystal. Finally, the relationship between crystalline size and hydrophilicity under sunlight was investigated in this article.  相似文献   

20.
Titanium oxide (TiO2) films were deposited on silicon substrates at the temperature in the range 50–600 °C by DC reactive magnetron sputtering. It was found that the anatase and rutile phases co-existed in the TiO2 films deposited at 450–500 °C, while only the anatase phase existed in those deposited at other temperatures. The mechanism of such a crystallization behavior of TiO2 films is preliminarily explained.  相似文献   

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