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1.
Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe2O3 and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe3+ to Fe2+, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na+ and Ca2+ on FeO4 and AlO4-tetrahedra, which strengthens the incorporation of Fe3+ into the glass structure.  相似文献   

2.
The structure of Li2O·SiO2 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 SiO4 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   

3.
Glasses of compositions 5ZrO2·5SiO2(ZS), 5ZrO2·Al2O3·4SiO2(ZAS) and 5 5ZrO2·0.5Al2O3·0.5Na2O·4SiO2(ZANS) were prepared by the sol-gel process from metal alkoxides and sintered to make glass-ceramics. Tetragonal ZrO2 was precipitated by heat treatment at 900 to 1300°C. The activation energy for tetragonal ZrO2 crystal growth was extremely high in Al2O3 containing glasses. ZAS and ZS were sintered to the near theoretical densities above 1200°C, at which the predominant phase was tetragonal ZrO2. On the other hand, for ZANS, high densification was not attained owing to the large pores enclosed by the glass phase. Strength and fracture toughness increased with the densification and the crystal growth of tetragonal ZrO2, reaching 450 MPa and 9 MN/m1.5, respectively.  相似文献   

4.
A.T. Aldred 《Journal of Non》1980,40(1-3):347-352
A solubility limit of ≈6 mol% PuO2 in sodium disilicate (Na2O · 2 SiO2) glass has been determined. Magnetic susceptibility measurements on these glasses yield approximate Curie—Weiss behavior, in contrast to the temperature-independent susceptibility of crystalline PuO2. This result is interpreted to indicate that the local site symmetry around the Pu ion in the sodium disilicate glass is very different than in crystalline PuO2. The effective paramagnetic moments determined from the temperature dependence of the susceptibility are found to be consistent with calculated free-ion values based on the most likely 5f electron configurations.  相似文献   

5.
J. Gtz 《Journal of Non》1976,20(3):413-425
The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO2 have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO2 and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO2 is of a polyanionic nature; in T---Pb2SiO4 dimetic groups [Si2O7]6− prevail; M1---Pb2SiO4 contains predominantly [Si4O12]8− rings and H---Pb2SiO4 is a typical polysilicate with chain anions [SiO32−]n. The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.  相似文献   

6.
K. Hirao  T. Komatsu  N. Soga 《Journal of Non》1980,40(1-3):315-323
Mössbauer absorption measurements have been made at room temperature on 57Fe in iron sodium silicate glasses containing 3–15 mol% Fe2O3 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 Na2O · Fe2O3 · 4 SiO2 and 5 Na2O · Fe2O3 · 8 SiO2 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 MFeSi2O6 and MFeSi3O8 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   

7.
The surface morphology of Na2O–B2O3–Al2O3–SiO2 vitrified bond with and without calcium oxide was studied by soaking vitrified bonded microcrystalline alumina composites in water. The content of water introduced to the vitrified bond was determined by thermal gravity analysis, and the effects of water and calcium on the phase separation and nucleation of the vitrified bond were investigated using scanning electron microscope and energy-dispersive X-ray spectrometer. Soaked in water for 72 h, the Na2O–B2O3–Al2O3–SiO2 vitrified bond presented a porous surface, and its bending strength declined with increasing sintering temperature. However, the Na2O–CaO–B2O3–Al2O3–SiO2 vitrified bond was more durable against aqueous coolant even needle-shape crystals were found clustered on the surface of the vitrified bond. The crystals were enriched with aluminosilicate tested by energy-dispersive X-ray spectrums. The appearance of crystals lessened the dissolution of the vitrified bond and made the bending strength increase in the sintering temperature region between 870 °C and 930 °C.  相似文献   

8.
The behavior of copper ions in the Cu2O·Al2O3·4SiO2 (in moles) glass on heating in air at temperatures up to 500°C was studied. When the glass, in which about 90% of Cu was present as Cu+ ions, was heated in air above 300°C, a CuO layer was formed on the surface. The amount of CuO was increased with heating temperature and time, corresponding to the decrease in weight of the glass. Furthermore, the fraction of Cu2+ ions in the glass increased. These observations suggest that oxygens do not diffuse into the glass, but Cu+ ions migrate to the surface from the interior to balance the surplus positive charge produced by the oxidation of Cu+ to Cu2+ ions inside the glass. The following reaction scheme for the formation of the CuO layer was proposed; 2Cu+(interior) + 21O2(surface) → Cu2+(interior) + CuO(surface).  相似文献   

9.
We determined the internal nucleation, crystal growth and overall crystallization kinetics of fresnoite crystal (2BaO · TiO2 · 2SiO2) in an almost stoichiometric fresnoite glass. Due to the extremely high nucleation rates (1017 m−3 s−1) that limit the maximum crystal size to 700 nm the nucleation densities and crystal sizes were estimated by scanning electron microscopy (SEM). The volume fraction crystallized was measured by X-ray diffraction. The nucleation rates obtained directly from SEM measurements reasonably agree with those calculated from the combination of overall crystallization with crystal growth kinetics. The activation enthalpies for viscous flow, transport of structural units across the nucleus/melt interface (nucleation) and crystal growth: ΔHη, ΔHτ and ΔHU respectively, follow a similar trend to that observed for other stoichiometric silicate glasses that nucleate internally: ΔHη=294>ΔHτ=87>ΔHU=61 kJ/mol. Fresnoite glass displays the highest internal nucleation rates so far measured in inorganic glasses. These rates are comparable to some metallic glasses and can lead to nanostructured glass-ceramics.  相似文献   

10.
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   

11.
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   

12.
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   

13.
The development of microstructure during crystallisation of a glass with composition Y15.2Si14.7Al8.7O54.1N7.4 has been studied by analytical and high resolution transmission electron microscopy. Crystal nucleation at temperatures in the range 965–1050°C occurs by the heterogeneous nucleation of lenticular-shaped yttrium, silicon and aluminium containing crystals on silicon-rich clusters that formed during glass preparation. The lenticular crystals have a wide range of composition after heat treatment at 1050°C; the yttrium cation percentage varies around that of the expected B-phase composition Y2SiAlO5N but the aluminium content is lower and the silicon content generally significantly higher than that. The crystals display the hexagonal crystal structure of B-phase, although the results from EDX analysis imply that the atomic arrangement of the lattice is not the previously proposed B-phase structure. Crystal growth during prolonged heat treatment at 1050°C occurs to a significant extent by coalescence.  相似文献   

14.
The La L1 and L3 XANES and L3 EXAFS have been investigated for the series of glasses 10K2O---50SiO2---x La2O3 (x = 1, 5, 10) and (10 − x)K2O---40SiO2−(x/3)La2O3 (x = 7.5, 5, 2.5) and model compounds La2O3, LaAlO3, LaPO4, La2NiO4, La2CuO4 and La(OH)3. An edge resonance at 25 eV above the L1 edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La2O3 concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.  相似文献   

15.
The ability to observe different ice nucleation processes in LiCl · RH2O electrolytes is demonstrated by light scattering experiments. The homogeneous ice nucleation mechanism is analyzed on the basis of Turnbull's theory. This nucleation operates in supercooled water, as deduced from the volumic dependence of the samples on R(5 < R < 9), and is compatible with the definition of the electrolyte as a heterogeneous submicroscopic LiCl·6H2O medium composed of H2O and LiCl·6H2O.  相似文献   

16.
Alkoxide derived gels were prepared in the system Na2O---B2O3---SiO2. 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   


17.
The colorless and transparent glasses in the Al2O3---B2O3---SiO3 system with high B2O3 and SiO2 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 B2O3 content. The accuracy of the glass composition is dependent upon the SiO2/B2O3 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   

18.
Fine-sized ZnO–B2O3–CaO–Na2O–P2O5 glass powders with spherical shape were directly prepared by high temperature spray pyrolysis. The ZnO–B2O3–CaO–Na2O–P2O5 powders prepared by spray pyrolysis at temperatures above 1200 °C had broad peaks at around 30° in the XRD patterns. The glass transition temperatures (Tg) of the glass powders obtained by spray pyrolysis at preparation temperatures between 900 °C and 1400 °C were near 480 °C regardless of the preparation temperatures. The dielectric layers formed from the glass powders prepared by spray pyrolysis at preparation temperatures above 1300 °C had clean surface and dense inner structure at the firing temperature of 580 °C. The transmittance of the dielectric layer formed from the glass powders obtained by spray pyrolysis at preparation temperature of 1400 °C was 90% at the firing temperature of 580 °C, in which the thickness of the dielectric layer was 13 μm. The UV cutoff edges gradually shift towards longer wavelength with increasing the preparation temperature of glass powders and the firing temperature of dielectric layers.  相似文献   

19.
We investigated the dependence of the Y2O3 film growth on Si surface at initial growth stage. The reflection high-energy electron diffraction, X-ray scattering, and atomic force microscopy showed that the film crystallinity and morphology strongly depended on whether Si surface contained O or not. In particular, the films grown on oxidized surfaces revealed significant improvement in crystallinity and surface smoothness. A well-ordered atomic structure of Y2O3 film was formed on 1.5 nm thick SiO2 layer with the surface and interfacial roughness markedly enhanced, compared with the film grown on the clean Si surfaces. The epitaxial film on the oxidized Si surface exhibited extremely small mosaic structures at interface, while the film on the clean Si surface displayed an island-like growth with large mosaic structures. The nucleation sites for Y2O3 were provided by the reaction between SiO2 and Y at the initial growth stage. The SiO2 layer known to hinder crystal growth is found to enhance the nucleation of Y2O3, and provides a stable buffer layer against the silicide formation. Thus, the formation of the initial SiO2 layer is the key to the high-quality epitaxial growth of Y2O3 on Si.  相似文献   

20.
An EMF cell using a Na-β″-alumina electrolyte has been designed for the quantification of the thermodynamic activity of Na2O (aNa2O) in a series of sodium-bearing silicate liquids at high temperature. Initial experiments have been performed using Na2O–0.663WO3 and Na2O–0.555MoO3 as reference liquids. Values of aNa2O derived for Na2O–2SiO2 binary melt are found to be in excellent agreement with data from the literature, confirming the validity of the method. To extend use of this experimental set-up to higher temperature, the aNa2O of industrial C-glass has been calibrated as a reference liquid at temperatures up to 1263 °C. The influence of additions of CaO, Al2O3 and B2O3 on the Na2O activity of binary sodium-silicates has been quantified. For each glass composition, measured values of aNa2O are a function of temperature, log(aNa2O) varying as a function of inverse absolute temperature. Activation energies derived from these data are all generally similar with the exception of industrial E-glass, which is rich in Al and poor in Na. At constant temperature, additions of network forming Al2O3 and B2O3 to a Na2O–SiO2 binary melt yield a decrease of the activity of Na2O, while addition of network modifying CaO results in an increase in (aNa2O). These changes are qualitatively consistent with predictions based upon expected modifications of melt structure. However, measured values of log(aNa2O) do not correlate perfectly with theoretical models of glass basicity, suggesting that either sodium activity is decoupled from melt basicity, or that current models are insufficient to calculate that parameter, in particular for the case of liquids poor in Na and rich in Al.  相似文献   

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