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1.
The nanocrystallization behavior of 25K2O−25Nb2O5-(50−x)GeO2-xSiO2 glasses with x=0,25,and50 (i.e., KNb(Ge,Si)O5 glasses) and the chemical etching behavior of transparent nanocrystallized glass fibers have been examined. All glasses show nanocrystallization, and the degree of transparency of the glasses studied depends on the heat treatment temperature. Transparent nanocrystallized glasses can be obtained if the glasses are heat treated at the first crystallization peak temperature. Transparent nanocrystallized glass fibers with a diameter of about 100 μm in 25K2O-25Nb2O5-50GeO2 are fabricated, and fibers with sharpened tips (e.g., the taper length is about 450 μm and the tip angle is about 12°) are obtained using a meniscus chemical etching method, in which etching solutions of 10 wt%-HF/hexane and 10 M-NaOH/hexane are used. Although the tip (aperture size) has not a nanoscaled size, the present study suggests that KNb(Ge,Si)O5 nanocrystallized glass fibers have a potential for new near-field optical fiber probes with high refractive indices of around n=1.8 and high dielectric constants of around ε=58 (1 kHz, room temperature).  相似文献   

2.
Transparent glasses having molar composition (23−x)K2xNa2O·27Nb2O5·50SiO2 (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K2O by Na2O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO3 crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.  相似文献   

3.
Some K2O-TiO2-GeO2 glasses with a large amount of TiO2 contents (15-25 mol%) such as 25K2O-25TiO2-50GeO2 have been prepared, and their electronic polarizability, Raman scattering spectra, and crystallization behavior are examined to clarify thermal properties and structure of the glasses and to develop new nonlinear optical crystallized glasses. It is proposed that the glasses consist of the network of TiO6 and GeO4 polyhedra. The glasses show large optical basicities of Λ=0.88-0.92, indicating the high polarizabity of TiOn (n=4-6) polyhedra in the glasses. K2TiGe3O9 crystals are formed through crystallization in all glasses prepared in the present study. In particular, 20K2O-20TiO2-60GeO2 glass shows bulk crystallization and 18K2O-18TiO2-64GeO2 glass exhibits surface crystallization giving the c-axis orientation. The crystallized glasses show second harmonic generations (SHGs), and it is suggested that the distortion of TiO6 octahedra in K2TiGe3O9 crystals induces SHGs.  相似文献   

4.
Transparent crystallized glasses consisting of nonlinear optical Ba2TiSi2O8 nanocrystals (diameter: ∼100 nm) are prepared through the crystallization of 40BaO-20TiO2-40SiO2-0.5Dy2O3 glass (in the molar ratio), and photoluminescence quantum yields of Dy3+ ions in the visible region are evaluated directly by using a photoluminescence spectrometer with an integrating sphere. The incorporation of Dy3+ ions into Ba2TiSi2O8 nanocrystals is confirmed from the X-ray diffraction analyses. The total quantum yields of the emissions at the bands of 4F9/26H15/2 (blue: 484 nm), 4F9/26H13/2 (yellow: 575 nm), and 4F9/26H11/2 (red: 669 nm) in the crystallized glasses are ∼15%, being about four times larger compared with the precursor glass. It is found that the intensity of yellow (575 nm) emissions and the branching ratio of the yellow (575 nm)/blue (484 nm) intensity ratio increase largely due to the crystallization. It is suggested from Judd-Ofelt analyses that the site symmetry of Dy3+ ions in the crystallized glasses is largely distorted, giving a large increase in the yellow emissions. It is proposed that Dy3+ ions substitute Ba2+ sites in Ba2TiSi2O8 nanocrystals.  相似文献   

5.
The total conductivity of monoclinic La2Ti2SiO9 is mixed oxygen-ionic and n-type electronic, and increases on reduction of the oxygen partial pressure down to 10−21 atm at 973-1223 K. The substitution of Ti4+ with Nb5+ decreases both contributions to the conductivity, whilst Pr doping and reducing p(O2) have opposite effects. The oxygen ion transference numbers of La2Ti2SiO9−δ, LaPrTi2SiOδ and La2Ti1.8Nb0.2SiOδ ceramics, measured by the faradaic efficiency and e.m.f. methods, vary in the range 0.15-0.32, increasing when temperature decreases. In air, the activation energies for the ionic and electronic transport are 1.23-1.40 and 1.59-1.74 eV, respectively. Protonic contribution to the conductivity in wet atmospheres becomes significant at temperatures below 1000 K. The experimental data and the results of atomistic computer simulations suggest that the oxygen-ionic and electronic transport is primarily determined by processes involving TiO6 octahedra. The ionic conduction may occur via both the vacancy and interstitial migration mechanisms, but the former is more favorable energetically and should dominate, at least, in reducing atmospheres. The average thermal expansion coefficients of La2Ti2SiO9-based ceramics, calculated from dilatometric data in air, are (8.7−9.5)×10−6 K−1 at 300-1373 K. The lattice of lanthanum titanate-silicate is almost intolerant with respect to A-site deficiency and to doping with lower-valence cations, such as Sr and Fe.  相似文献   

6.
0.8[xB2O3-(1 − x)SiO2]-0.2K2O (with 0 ≤ x ≤ 1) glasses were synthesized by melt quenching techniques. DSC curves of the glasses exhibit only one glass transition. Calorimetric measurements of heats of dissolution in lead borate at 973 K indicated small negative enthalpies of mixing. Consequently phase separation was not observed over the whole composition range. The results are in good agreement with the structural data available in the literature.  相似文献   

7.
Phosphosilicate doped with a mixture of phosphotungstic acid and zirconium oxide (PWA/ZrO2–P2O2–SiO2) was investigated as potential glass composite membranes for use as H2/O2 fuel cell electrolytes. The glass membranes were studied with respect to their structural and thermal properties, proton conductivity, pore characteristics, hydrogen permeability, and performance in fuel cell tests. Thermal analysis including TG and DTA confirmed that the glass was thermally stable up to 400 °C. The dependence of the conductivity on the humidity was discussed based on the PWA content in the glass composite membranes. The proton transfer in the nanopores of the PWA/ZrO2–P2O5–SiO2 glasses was investigated and it was found that a glass with a pore size of ∼3 nm diameters was more appropriate for fast proton conduction. The hydrogen permeability rate was calculated at various temperatures, and was found to be comparatively higher than for membranes based on Nafion®. The performance of a membrane electrolyte assembly (MEA) was influenced by its PWA content; a power density of 43 mW/cm2 was obtained at 27 °C and 30% relative humidity for a PWA/ZrO2–P2O5–SiO2 glass membrane with a composition of 6–2–5–87 mol% and 0.2 mg/cm2 of Pt/C loaded on the electrode.  相似文献   

8.
This article aims to shed some light on the structure and thermo-physical properties of lithium disilicate glasses in the system Li2O–SiO2–Al2O3–K2O. A glass with nominal composition 23Li2O–77SiO2 (mol%) (labelled as L23S77) and glasses containing Al2O3 and K2O with SiO2/Li2O molar ratios (3.13–4.88) were produced by conventional melt-quenching technique in bulk and frit forms. The glass-ceramics (GCs) were obtained from nucleation and crystallisation of monolithic bulk glasses as well as via sintering and crystallisation of glass powder compacts. The structure of glasses as investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) depict the role of Al2O3 as glass network former with four-fold coordination, i.e., Al(IV) species while silicon exists predominantly as a mixture of Q 3 and Q 4 (Si) structural units. The qualitative as well as quantitative crystalline phase evolution in glasses was followed by differential thermal analysis (DTA), X-ray diffraction (XRD) adjoined with Rietveld-reference intensity ratio (R.I.R.) method, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The possible correlation amongst structural features of glasses, phase composition and thermo-physical properties of GCs has been discussed.  相似文献   

9.
Garnet-structure related metal oxides with the nominal chemical composition of Li5La3Nb2O12, In-substituted Li5.5La3Nb1.75In0.25O12 and K-substituted Li5.5La2.75K0.25Nb2O12 were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li5La3Nb2O12. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In3+(rCN6: 0.79 Å) for smaller Nb5+ (rCN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K+ for La3+ and In3+ for Nb5+ in Li5La3Nb2O12 exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li5.5La3Nb1.75In0.25O12 exhibits the highest bulk lithium ion conductivity of 1.8×10−4 S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10−10-10−7 cm2/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li5La3Nb2O12 was found to be unsuccessful under the investigated conditions.  相似文献   

10.
To obtain red luminants, MgO-GeO2 gel glasses and glass ceramics doped with manganese ions were prepared by a sol-gel method and their properties were investigated by measuring X-ray diffraction (XRD), electron spin resonance (ESR), and luminescence and excitation spectra. Under UV irradiation at 254 nm, the gel glasses and glass ceramics showed red luminescence at 620–665 nm, the intensity of which became strong with increasing the heat-treatment temperature. A glass ceramic with the composition 1.0MnO-25MgO-75GeO2 heat treated at 1000°C exhibited the strongest red luminescence at 661 nm. From the results of XRD and ESR, this luminescence is found to be due to the transition from the 4T1g to the 6A1g state of octahedrally coordinated Mn2+ ions located in MgGeO3 polycrystals. The luminescence wavelength of the glass ceramics (∼665 nm) is long compared with Eu3+-containing phosphors (612 nm), therefore the glass ceramics can be expected for red luminants.  相似文献   

11.
New glasses giving the crystallization of Sm3+-doped SrxBa1−xNb2O6 (SBN) ferroelectrics have been developed in the Sm2O3−SrO−BaO−Nb2O5−B2O3 system, and the formation of SBN crystal dots and lines by continuous wave Nd:YAG laser (wavelength:1064 nm, power: 1 W) irradiations, i.e., samarium atom heat processing, has been examined. The formation of Sm3+-doped SBN non-linear optical crystals is confirmed from X-ray diffraction analyses, micro-Raman scattering spectra, second harmonic generations, and photoluminescence spectra. Sm3+-doped SBN crystal dots with the diameters of 20-70 μm and lines with the widths of 20-40 μm are written at the surface of some glasses such as 10Sm2O3·10SrO·10BaO·20Nb2O5·50B2O3 (mol%) by Nd:YAG laser irradiations with the irradiation times of 20-70 s for the dots and with the scanning speeds of 1-5 μm/s for the lines. The present study suggests that the samarium atom heat processing has a potential for the patterning of optical waveguides consisting of ferroelectric SBN crystals in glass substrates.  相似文献   

12.
The devitrification behaviour of the glasses K2O·xGeO2 with x=4, 7 or 8 was examined by means of differential thermal analysis (DTA), the Fourier transformation infrared (FTIR) transmittance spectra and X-ray diffraction (XRD). The glass transition temperatures were related to the molar ratio GeO4/GeO6. For the glass with x=4, metastable K4Ge9O20> crystals are initially formed and then converted at higher temperatures into stable K2Ge4O9 crystals. The glasses with x=7 or 8 both devitrify into K2Ge7O15> crystals. The effects of the specific surface area of the samples on the devitrification mechanisms were established. Bulk nucleation predominates in the glass with x=4, while the glasses with x=7 or 8 crystallize from the surface. The activation energies for crystal growth were evaluated from the DTA curves. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

13.
Oxy-silicate and oxy-germanate, Ln2(TO4)O (Ln=La and Nd, T=Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd2(SiO4)O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd2(SiO4)O, La2(GeO4)O and Nd2(GeO4)O, respectively. The overall conductivities at 1173 K range from 1.2×10−4 S cm−1 for Nd2(SiO4)O to 1.3×10−6 S cm−1 for La2(GeO4)O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.  相似文献   

14.
The crystallization and microstructure of Li2O-Al2O3-SiO2 (LAS) glass ceramic with complex nucleating agents (TiO2 + ZrO2 + P2O5 +/or F) are investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the effects of P2O5 and F on the crystallization of LAS glass are also analyzed. The introduction of both P2O5 and F promotes the crystallization of LAS glass by decreasing the crystallization temperature and adjusting the crystallization kinetic parameters, allows a direct formation of β-spodumene without the transformation of LiAl(SiO3)2 into β-spodumene and as a result, increases the crystal size and crystallinity of LAS glass ceramic.  相似文献   

15.
Transparent glass ceramics in the system SiO2-B2O3-PbO-CdO-PbF2-CdF2-YbF3-ErF3 showing infrared to visible anti-Stokes (upconversion) luminescence are studied in the present work. The glass compositions have been optimized in order to reduce the melting temperature and, hence, to obtain laboratory scale samples with good optical quality. Erbium-doped nanoscale Pb4Yb3F17 crystals are precipitated in the precursor glasses during annealing at temperatures 30-40 K above Tg. A kinetically self-constrained growth explains the nano sizes of the crystals. Both the Stokes and anti-Stokes luminescence spectra of glasses could be explained with clustering of the Yb3+ and Er3+ ions in fluorine-rich regions. At the annealing temperature these regions act as nucleation precursors. The crystal growth further enhances the local concentration of these ions. Consequently, a series of energy transfer and energy cross relaxation processes occurs between adjacent rare earth ions leading to the observed luminescence spectra of the glass ceramics studied.  相似文献   

16.
综合ZnO-Al_2O_3-SiO_2系和锗酸盐玻璃陶瓷的优点,采用熔融-晶化法首次制备了Ho~(3+)/Yb~(3+)共掺以ZnAl_2O_4为主晶相的ZnO-Al_2O_3-GeO_2-SiO_2系玻璃陶瓷。因[GeO_4]四面体和[SiO_4]四面体都是玻璃网络形成体,讨论了GeO_2取代SiO_2对玻璃陶瓷样品硬度及发光性能的影响,最终确定GeO_2的取代量为10.55%(w/w)时,玻璃陶瓷综合性能最佳。在980 nm泵浦光的激发下,发现强的绿色(546 nm)和弱的红色(650 nm)上转换发光,并研究了不同Ho~(3+)/Yb~(3+)掺杂比对样品上转换发光的影响,最终结果表明当Ho~(3+)/Yb~(3+)掺杂比为1∶11(n/n)时样品荧光强度最强,在绿色上转换发光材料方面具有潜在的应用。  相似文献   

17.
Thermal and structural properties of model silicate-phosphate glasses containing the different amounts of the glass network modifiers, i.e. Mg2+ and Ca2+ were studied. To explain the changes of the parameters characterizing the glass transition effect (Tg, Δcp) and the crystallization process (Tc, ΔH) depending on the cations modifiers additions, analysis of the bonds and chemical interactions of atoms in the structure of glasses was used. 31P MAS-NMR spectra of SiO2–P2O5–MgO(CaO)–K2O glasses show that the phosphate complexes are mono- and diphosphate. It has been found that increasing amounts of Mg2+ or Ca2+ cations in the structure of glasses causes the reduction of the degree of polymerization of the phosphate framework (Q1→Q0). The influence of increasing of modifiers in the structure of silicate- phosphate glasses on the number of non-bridging oxygens per SiO4 tetrahedron and density of glasses was presented.  相似文献   

18.
Glass‐ceramics are multi‐phase materials that are comprised of one amorphous phase and at least one crystalline phase. Their versatile performance and properties can be engineered by alterations of the three fundamental steps – formulation and production of the amorphous base glass, nucleation, and crystallization. Efforts have been made on syntheses of glass‐ceramics with different components, yet little is known about the details of nucleation and crystallization processes that are essential for tailoring glass‐ceramic properties. Herein, we investigate the nucleation and crystallization mechanisms of a multi‐component, that is SiO2‐Al2O3‐CaO‐Li2O‐K2O‐P2O5‐F, glass‐ceramic system by a combined use of powder X‐ray diffraction (pXRD), solid‐state nuclear magnetic resonance (NMR), and electron microscopic (EM) techniques. The role of P2O5 in the nucleation and crystallization processes is particularly studied. We show that the formation of lithium silicate crystals being independent of the P2O5‐associated crystals, and the separation of P2O5 phases into individual growth domains of lithium orthophosphate and fluorapatite. We also observe the non‐uniform distribution of fluorapatite particles that explains the opalescence effect of this glass‐ceramic.  相似文献   

19.
The sol-gel processing method was used as an alternative route to obtain Nb2O5 phase homogenously dispersed in the SiO2 matrix, improving the thermal stability of the Brønsted acid sites, Nb-OH and Nb-OH-Si groups. The local niobium structure and the influence of the amount of niobia on the surface of the Nb2O5/SiO2 system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3d5/2 BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO2 matrix, probably associated to Si-O-Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of NbO species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 Å−1, which probably arises from Nb-O-Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb-Nb lengths and one Nb-Si.  相似文献   

20.
Na-A and/or Na-X zeolite/porous carbon composites were prepared under hydrothermal conditions by NaOH dissolution of silica first from carbonized rice husk followed by addition of NaAlO2 and in situ crystallization of zeolites i.e., using a two-step process. When a one-step process was used, both Na-A and Na-X zeolites crystallized on the surface of carbon. Na-A or Na-X zeolite crystals were prepared on the porous carbonized rice husk at 90 °C for 2-6 h by changing the SiO2/Al2O3, H2O/Na2O and Na2O/SiO2 molar ratios of precursors in the two-step process. The surface area and NH4+-cation exchange capacity (CEC) of Na-A zeolite/porous carbon were found to be 171 m2/g and 506 meq/100 g, respectively, while those of Na-X zeolite/porous carbon composites were 676 m2/g and 317 meq/100 g, respectively. Na-A and Na-X zeolites are well-known microporous and hydrophilic materials while carbonized rice husk was found to be mesoporous (pores of ∼3.9 nm) and hydrophobic. These hybrid microporous-mesoporous and hydrophilic-hydrophobic composites are expected to be useful for decontamination of metal cations as well as organic contaminants simultaneously.  相似文献   

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