氟锆酸盐光纤玻璃中的析晶和分相现象

氟化物光纤玻璃中的夹杂物,分相颗粒和微晶颗粒等缺陷,造成光纤的散射损失,影响了实际制备工作.本文利用大视场光学显微镜,X射线衍射,透射电子显微镜和差热分析,研究了ZBLA氟化物玻璃中的分相和析晶现象,观察到了以成核生长机理进行的分相颗粒,析晶颗粒的成分是β-BaF_2·ZrF_4和α-BaF_2·2ZrF_4,通过DTA分析确定了这些相的析晶活化能.     

Depth distribution of the fluorine concentration during radiative carbonization of PVDF

The XPS integral intensity of the F1s line and its satellite is measured during the long-term radiative carbonization of PVDF (polyvinylidene fluoride). A model is proposed that describes the effect of the fluorine depth distribution on the shape and intensity of the F1s spectra. A comparison of the experimental data with the model calculations provides estimates for the concentration inhomogeneity during the radiative carbonization of PVDF, for the photoelectron escape depth, and for the probability of a single energy loss by a photoelectron in its motion towards the surface. A technique determining the fluorine concentration is presented. It is based on the occurrence of chemical shifts of the C1s line towards larger bond energies for the carbon atoms chemically bonded to one or two fluorine atoms.     

19F spectroscopy and relaxation behavior of trifluorovinyldichloroborane

The 19F NMR spectra and spin-lattice relaxation rate, R1, of trifluorovinyldichloroborane as shown in were studied as a function of temperature, T, and magnetic field, B. All logR1 vs 1/T plots show a minimum at 299K indicating the presence if dipolar relaxation at lower T and spin-rotation relaxation at higher T. The R1 values increase with increasing B due to chemical shift anisotropy relaxation. Estimates of the fluorine chemical shift values for F3 (cf. Fig. 1) suggest that there is pi character in the F-C bond. The other two C-F bonds are largely single bonded. No evidence was found for intermolecular exchange of the trifluorovinyl group. Two of the three fluorine atoms show large increases in their NMR linewidth with increasing temperature while the third shows only a small increase but the activation energy for the process is the same for all. The increase in linewidths is due to scalar coupling to the boron atoms. The boron linewidths were measured between 253 and 363K and decreased with increasing temperature. A plot of logR2, where R2 is the linewidth of the boron as a function of 1/T shows some curvature indicating a second relaxation mechanism. This is ascribed to spin-rotation but not enough data are available to be conclusive. In all cases there is a second small set of fluorine peaks that are due to 10B interactions separated from the 11B peaks by amounts varying from 1 to 4 ppm depending on the field and fluorine atom.     

A 1H/19F minicoil NMR probe for solid-state NMR: application to 5-fluoroindoles

We show that it is feasible to use a minicoil for solid-state 19F 1H NMR experiments that has short pulse widths, good RF homogeneity, and excellent signal-to-noise for small samples while using low power amplifiers typical to liquid-state NMR. The closely spaced resonant frequencies of 1H and 19F and the ubiquitous use of fluorine in modern plastics and electronic components present two major challenges in the design of a high-sensitivity, high-field 1H/19F probe. Through the selection of specific components, circuit design, and pulse sequence, we were able to build a probe that has low 19F background and excellent separation of 1H and 19F signals. We determine the principle components of the chemical shift anisotropy tensor of 5-fluoroindole-3-acetic acid (5FIAA) and 5-fluorotryptophan. We also solve the crystal structure of 5FIAA, determine the orientation dependence of the chemical shift of a single crystal of 5FIAA, and predict the 19F chemical shift based on the orientation of the fluorine in the crystal. The results show that this 1H/19F probe is suitable for solid-state NMR experiments with low amounts of biological molecules that have been labeled with 19F.     

Structure of Na2O·MO·SiO2·CaF2 (M=Mg,Ca) oxyfluoride glasses

(9−x)CaO·xMgO·15Na₂O·60SiO₂·16CaF₂(x=0, 2, 4, 6, and 9) oxyfluoride glasses were prepared. Utilizing the Raman scattering technique together with ²⁹Si and ¹⁹F MAS NMR, the effect of alkaline metal oxides on the Q species of glass was characterized. Raman results show that as magnesia is added at the expense of calcium oxide, the disproportional reaction Q³→Q⁴+Q² (Qⁿ is a SiO₄ tetrahedron with n bridging oxygens) prompted due to the high ionic field strength of magnesia, magnesium oxide entered into the silicate network as tetrahedral MgO₄, and removed other modifying ions for charge compensation. This reaction was confirmed by ²⁹Si MAS NMR. ¹⁹F MAS NMR results show that fluorine exists in the form of mixed calcium sodium fluoride species in all glasses and no Si–F bonds were formed. As CaO is gradually replaced by MgO (x=6, 9), a proportion of the magnesium ions combines with fluorine to form the MgF⁺ species. Meanwhile, some part of Na⁺ ions complex F⁻ in the form of F–Na(6).     

Local atomic structures in Zr-Ni metallic glasses

Local atomic structures of Zr100−xNix (x=33.3, 36, 50 at%) binary metallic glasses were investigated by means of extended X-ray absorption fine structure (EXAFS) probe. Structural parameters show that the Zr-Ni bond length, RZr-Ni, keeps a constant value of 2.62 Å, regardless of alloy compositions. This result implies that there is a strongly chemical interaction between Zr and Ni atoms, leading to significant chemical short-range orders (CSROs) in the present metallic glasses. Further analysis indicates that the SRO structures in these metallic glasses are extremely similar with those in their crystalline counterparts. It is interesting to note that the CSROs in the eutectic Zr₆₄Ni₃₆ metallic glass consist of Zr₂Ni-like and ZrNi-like CSROs.     

A Refinement of the Hg2F2 Structure by the Use of NMR Data

Abstract

The crystal structure of Hg₂F₂ was studied in works.^1,2 The Hg₂F₂elementary cell of the tetragonal syngony with a=3.66 Å and c=10.89 Å contains two formula units, the space-group being 14/mmm. Hg₂F₂ belongs to those types of structures which possess one non-fixed coordinate (z_F and zHg); hence, the position of the fluorine nuclei may be determined from the value of the second moment of the NMR spectrum of ¹⁹F for powder sample.³ In this case it is ostensibly important, inasmuch as determining the positions of fluorine nuclei near the heavy mercury atoms presents a difficulty for X-ray analysis.     

First-principles study of the H and F centers in LiYF4

We have investigated the electronic structures of the LiYF₄ containing interstitial fluorine atoms and F center (a fluorine ion vacancy trapping an electron) using first-principles density functional theory. It is found that the interstitial fluorine atoms in two different interstitial positions would combine with its nearest neighbor two or three formal lattice fluorine ions forming fluorine molecular ions or by different ways, which would cause the 260 nm absorption band. Simultaneously, our study indicates that one electronic state appears in the forbidden band of the perfect LiYF₄ crystal resulting from the F center in the LiYF₄ crystal. And the energy difference of this electronic state and the bottom of the conduction band is 3.74 eV, corresponding to the 331 nm absorption band. It is predicted that the 330 nm absorption band could arise from the F center in LiYF₄ crystals.     

Medium-range order in cesium borate glasses probed by double-resonance NMR

Rotational-echo double-resonance NMR is used to probe the proximity of Cs+ network modifiers to network-forming boron in binary cesium borate glasses. Low- and high-alkali glasses show distinctly different dephasing curves, which indicate preferential association of Cs+ with four-co-ordinate boron ([4]degrees )B) at low-alkali contents only. Different [4]B sites within a given glass appear to be subject to the same 133Cs dipolar field, thus placing constraints on the possible assignments of multiple tetrahedral boron peaks to different types of medium-range order and guiding future structural modeling studies.     

Multinuclear NMR study of silica fiberglass modified with zirconia

Silica fiberglass textiles are emerging as uniquely suited supports in catalysis, which offer unprecedented flexibility in designing advanced catalytic systems for chemical and auto industries. During manufacturing fiberglass materials are often modified with additives of various nature to improve glass properties. Glass network formers, such as zirconia and alumina, are known to provide the glass fibers with higher strength and to slow down undesirable devitrification processes. In this work multinuclear (1)H, (23)Na, (29)Si, and (91)Zr NMR spectroscopy was used to characterize the effect of zirconia on the molecular-level fiberglass structure. (29)Si NMR results help in understanding why zirconia-modified fiberglass is more stable towards devitrification comparing with pure silica glass. Internal void spaces formed in zirconia-silica glass fibers after acidic leaching correlate with sodium and water distributions in the starting bulk glass as probed by (23)Na and (1)H NMR. These voids spaces are important for stabilization of catalytically active species in the supported catalysts. Potentials of high-field (91)Zr NMR spectroscopy to study zirconia-containing glasses and similarly disordered systems are illustrated.     

Glasslike two-level systems in minimally disordered mixed crystals

THz spectroscopy is used to identify a broad distribution of two-level systems, characteristic of glasses, in the substitutional monatomic mixed crystal systems, Ba(1-x)Ca(x)F(2) and Pb(1-x)Ca(x)F(2). In these minimally disordered systems, two-level behavior, which was not previously known to occur, begins at a specific CaF(2) concentration. The concentration dependence, successfully modeled using the statistics of the impurity distribution in the lattice, points to a collective dopant tunneling mechanism.     

Preparation and characterization of vinyltriethoxysilane based inorganic–organic gel glasses dispersed 4′-n-pentyl-4-cyanobiphenyl nematic liquid crystal

Vinyltriethoxysilane based inorganic–organic glasses dispersed with 4′-n-pentyl-4-cyanobiphenyl nematic liquid crystal are designed to fabricate a new type of gel glass dispersed with liquid crystal. Scanning electron microscopy shows that the nematic liquid crystal phase, occupying the interconnected cavities, is continuously embedded in the inorganic–organic matrixes. The effect of the nematic liquid crystal weight concentration on the morphology is investigated. The differential scanning calorimetry shows that the nematic-isotropic transition temperature depends on the nematic liquid crystal weight concentration. The Raman spectra show that there are no obvious Raman frequency shifts, compared with pure nematic liquid crystal. The in situ diffuse reflectance FTIR spectra show that the integral intensity of bands of hydroxyls has not varied.     

Dilithium zirconium hexafluoride Li2ZrF6 at high pressures: A new monoclinic phase

Dilithium zirconium hexafluoride, Li₂ZrF₆ (, Z=1), is studied at high pressures using synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell at room temperature. At atmospheric conditions, it has a structure with all the cations octahedrally coordinated to fluorine atoms. Above 10 GPa it transforms reversibly to a new polymorph (C2/c, Z=4), in which the coordination polyhedron of the Zr atoms is a distorted square antiprism, while the Li atoms are in the octahedral coordination. The LiF₆ octahedra form layers parallel to (100) that are connected by zig-zag chains of the edge-sharing Zr polyhedra running in the [001] direction. The relative change in volumes per one formula unit for both polymorphs is 6% at 11.8 GPa. The relations to other A₂BX₆-type structures are discussed.     

An advanced NMR protocol for the structural characterization of aluminophosphate glasses

In this work a combination of complementary advanced solid-state nuclear magnetic resonance (NMR) strategies is employed to analyse the network organization in aluminophosphate glasses to an unprecedented level of detailed insight. The combined results from MAS, MQMAS and (31)P-{(27)Al}-CP-heteronuclear correlation spectroscopy (HETCOR) NMR experiments allow for a detailed speciation of the different phosphate and aluminate species present in the glass. The interconnection of these local building units to an extended three-dimensional network is explored employing heteronuclear dipolar and scalar NMR approaches to quantify P-O-Al connectivity by (31)P{(27)Al}-heteronuclear multiple quantum coherence (HMQC), -rotational echo adiabatic passage double resonance (REAPDOR) and -HETCOR NMR as well as (27)Al{(31)P}-rotational echo double resonance (REDOR) NMR experiments, complemented by (31)P-2D-J-RESolved MAS NMR experiments to probe P-O-P connectivity utilizing the through bond scalar J-coupling. The combination of the results from the various NMR approaches enables us to not only quantify the phosphate units present in the glass but also to identify their respective structural environments within the three-dimensional network on a medium length scale employing a modified Q notation, Q(n)(m),(AlO)(x), where n denotes the number of connected tetrahedral phosphate, m gives the number of aluminate species connected to a central phosphate unit and x specifies the nature of the bonded aluminate species (i.e. 4, 5 or 6 coordinate aluminium).     

Distribution of magnetic dipole fields in amorphous ferromagnetic and speromagnetic alloys and in crystalline and amorphous spin glasses

For binary alloys with only one sort of magnetic atoms we have calculated, by means of computer simulations, the distribution of magnetic dipole fields at regular lattice sites and suitably defined interstitial sites for the magnetic ground state of amorphous ferromagnetic and speromagnetic alloys and crystalline and amorphous spin glasses. Thereby we have considered spatially random arrangements of atoms and have neglected all magnetic correlations in the case of speromagnets and spin glasses. We have derived symmetric and isotropic distribution functions, which are fairly well described by Gaussian distributions for large concentrations of magnetic atoms only. The distribution functions are characterized by their second moments, which are important for the discussion of NMR and μ⁺ SR relaxation rates in these materials.     

以AlF_3-YF_3为基础的氟化物玻璃的振动光谱及结构研究

本文报道了氟化物玻璃系统室温下的红外和喇曼光谱,并根据振动光谱结果讨论了玻璃中AIF_3YF_3的结构状态.     

NMR Insights into Wasteforms for the Vitrification of High-Level Nuclear Waste

Magic-angle spinning nuclear magnetic resonance of ¹¹B, ²⁹Si and ²⁷Al has been used to study the distribution of nonbridging oxygen atoms (NBO) in an alkali borosilicate glass to which surrogate oxides for high-level radioactive waste have been added. The properties of such glasses are shown to depend on the fraction N ₄ of four-coordinated boron atoms (B4) and on the fraction of silicate tetrahedra possessing one NBO, Q³. The aqueous corrosion rate increases with Q³ content, as does weight loss due to evaporation from the melt. The activation energy for direct current conduction scales with N ₄. Values of N ₄ obtained for these glasses deviate from those predicted by the currently accepted model and are strongly affected by the modifier or intermediate nature of the surrogate oxide and also by its effect on the distribution of NBO between the silicate and borate polyhedra. Authors' address: Diane Holland, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Photorefractive properties of Cu-doped KNSBN crystal with fluorine replacing oxygen

We report the photorefractive properties of a new kind of Cu-doped KNSBN crystal in which a small fraction of oxygen (O) atoms are replaced with fluorine (F) atoms. The replacement of oxygen with fluorine in Cu:KNSBN crystals leads to a much shorter response time of 30 ms at an incident intensity of 5 W/cm(2). We also discuss the effect of electron-hole competition on the coupling constant.