Raman spectroscopic investigations of the effect of the doping metal on the structure of binary tellurium-oxide glasses

Raman studies over the range 10 to 1000 cm⁻¹ have been performed on binary tellurium-oxide glasses (1-x)TeO₂-xMO (M = Pb,Zn or Mg) prepared using a conventional melt technique. The intensities and positions of Raman bands observed in the range above 250 cm⁻¹ were found to depend both on the compound oxide and on the amount of doping. Indeed, the ratio of the intensity of the band around 680 cm⁻¹ (assigned to vibrations of TeO₄ trigonal bipyramids) with respect to that of the component around 750 cm⁻¹ (related to stretching-vibrations in TeO₃₊₁, TeO₃ and MO groups) are affected in different ways for the glass-modifier MgO and for the intermediate glass-formers PbO and ZnO. Concurrently, an explicit dependence on the compound oxide and amount of doping was also observed on the maximum of the boson peak (BP) in the low-frequency region around 40 cm⁻¹. The structural correlation lengths in the glasses, calculated using the model described by Shuker and Gammon, were found to be about 0.50 nm (1-x)TeO₂-xMgO glasses and around 0.65 nm for (1-x)TeO₂-xMO (M = Pb or Zn) glasses. All these results are interpreted in terms of the effect of the metal oxide on the changes induced in the structural arrangements of ¹_χ[TeO₄-TeO₃] chains.     

Raman spectroscopic study of hydrogen-containing vitreous silica

It is shown by Raman spectroscopy that high-temperature hydrogen-impregnated vitreous silica contains, in addition to physically dissolved hydrogen, ≡SiOH and ≡SiH groups. A Raman-scattering peak with a wave-number shift of 3685 cm^?1 is assigned to the OH stretching vibration, a peak with a wave-number shift of 2254 cm^?1 to a SiH stretching vibration and a peak with a wave-number shift of 4135 cm^?1 to the presence of physically dissolved hydrogen. When hydrogen was replaced by deuterium, appropriate shifts of the peaks mentioned were found.     

Raman spectroscopic study of bioactive silica based glasses

A study of the structure and bonding configuration of the bioactive glasses in the system Na₂O–CaO–P₂O₅–SiO₂ by Fourier transform Raman spectroscopy is presented. The assignment of the Raman lines, the changes in the Si–O–Si bond environment and the identification of the non-bridging silicon–oxygen groups (Si–O–NBO) for a wide range of silicate glasses are discussed. The frequency shifting and intensity variations of the Raman lines as a function of the bioactive glass composition are attributed to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the vitreous silica network. Correlation plots for the quantification of the Si–O–NBO groups as a function of the glass composition are also presented. These Raman analyses contribute to a better knowledge of the structural role of the network modifiers in the bioactive glasses and, as a consequence, improve the understanding of the bioactive process and the chemical routes of the CaP layer formation when exposed body fluids.     

EPR investigations of solid solutions of TSCC and TSCB doped with Mn2+

EPR investigations of Mn²⁺-doped solid solutions of TSCC and TSCB were performed between 4.2 K and 300 K. The Curie temperature vs. molar fraction of bromide x is nonlinear, for x ≧ 0,75 no ferroelectric EPR-line splitting occurs. The relation between the line widths and the molar fraction of bromide is discussed by means of a simple statistical model.     

Effects of poly(vinyl alcohol) additions on the structure of silica xerogels

Poly(vinyl alcohol)/silica hybrid xerogels were prepared from sonohydrolysis of tetraethoxysilane (TEOS) and additions of water-solution of poly(vinyl alcohol) (PVA). The samples were studied by small-angle X-ray scattering (SAXS), nitrogen adsorption, and differential scanning calorimetry (DSC). On drying at room temperature the resulting xerogels exhibit a fairly bimodal porous structure composed by small mesopores and micropores. The pore size distribution of the mesopores was found to follow approximately a power-law with the pore size. The micropore structure was associated to an evolution at a high resolution level of the mass fractal structure of the original wet gels. The role of the PVA addition on the pore structure of the xerogels is to diminish the specific surface area and the pore volume without to change substantially the pore mean size.     

Effects of permanent densification on the vibrational density of states of vitreous silica

We present a detailed Raman scattering study of the boson peak evolution in vitreous silica as a function of density, from the normal one up to ~ 22% of densification. We show that the distribution of low frequency modes in the boson peak range does not change as a function of density, at least until the densification process starts to modify the glassy network structure from a fourfold coordinated structure into a sixfold coordinated one.     

The role of the main silica precursor and the additive in the preparation of low-density xerogels

The incorporation of an additive during sol-gel synthesis reduces shinkage during ambient drying. The following additives have been studied: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES) and 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) and the main silica precursors were tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS). When the additive contains methoxy groups (EDAS), it acts as a nucleation agent of the silica particles and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained whether the main reagent is TEOS or TPOS. The nucleation mechanism is based on the difference in reactivity between additive and main reagent. In case of nucleation by the additive, the nucleation agent fixes the properties whatever the main silica precursor is. When both the additive and the main reagent contain ethoxy groups (series AES-TEOS and EDAES-TEOS), there is no nucleation mechanism by the additive, and the silica particle size remains nearly constant. With less reactive main reagent (series AES-TPOS and EDAES-TPOS), pore volumes up to 17 cm³/g have been obtained with pore sizes up to nearly 10 μm and very big particles (∼100 nm). The absence of nucleation by the additive for the couples AES-TPOS and EDAES-TPOS could be due to the fact that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to initiate the nucleation mechanism by the additive. In the absence of nucleation by the additive, the main reagent plays a role: highly porous materials with very large pores are prepared with TPOS.     

Structural and optical investigations on Mn doped Li2B4O7 crystals

Lithium tetraborate crystals have great demand due to its non‐critical phase matching at 90°. Transparent and good optical quality single crystals of undoped and Mn doped Li₂B₄O₇ were grown by Czochralski technique in air atmosphere. The crystalline phase was determined by X‐ray diffraction. Doping of Mn in the grown crystals was confirmed by energy dispersive analysis of X‐rays (EDAX) technique. Transmission spectra show that the crystal is transparent in the visible region. Birefringence interferograms were recorded to qualitatively analyse the optical homogeneity of the grown crystals and to quantify the birefringence of the samples in desired crystal direction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on the vertical alignment of nematic liquid crystals by fumed silica doping

The alignment of liquid crystals is important in the manufacturing of devices. Usually, the alignment layer is prepared using a thin-film coating, thermal processing and processes such as rubbing or photo-alignment. Several trials without processes related to the alignment layer were performed for realizing vertical alignment. The main approaches were to utilize the self-assembly of the alignment agent on the substrate. Here, tiny amount of fumed silica, a kind of nano-particle, was added to nematic liquid crystals. With a low frequency electric field and thermal treatment, the region of vertical alignment was obtained. In addition, the efficiency of alignment was related to the polarity of the electric field. The vertical alignment by the fumed silica was active for negative polarity. This results in an easy and convenient way of obtaining vertical alignment.     

Electron Spin Resonance of Mn2+ in Hemimorphite

The Electron Spin Resonance of Mn²⁺ in natural crystals of hemimorphite was studied at RT, X-band. No fine structure other than the central M = | + 1/2 〉 ⇆ | −1/2 > transition with allowed (Δm = 0) and forbidden (Δm = ±1) hyperfine transitions were observed. The spectrum was fitted with the spin Hamiltonian parameters g = 2.001, A = −84.6 × 10⁻⁴ cm⁻¹. The crystal field parameter D was estimated equal to (8 ± 2) × 10⁻³ cm⁻¹. The most striking result is the size of the hyperfine splitting constant favoring occupancy of sixfold coordinated sites.     

Microstructural Effects of Mn2+ ions in ZnO Nanoparticles

Semiconducting ZnO nanoparticles with manganese content 0 at. %, 5 at. % and 10 at. % were prepared by co‐precipitation technique and XRD reveals that the average crystallite size D is 10.2 nm, 7.62 nm and 7.06 nm respectively. The individual contributions of the size and strain to the XRD peak broadening were investigated by Williamson‐Hall (W‐H) analysis. The strain, stress and energy density were calculated more precisely by uniform deformation model (UDM), uniform stress deformation model (USDM), uniform deformation energy density model (UDEDM) and size‐strain plot (SSP) method. The average particle size from HRTEM analysis is 13 ± 2 nm. The evaluated average crystallite size from the W‐H analysis and SSP method is commensurate with the particle size from HRTEM. Diamagnetic to ferromagnetic phase transition observed is interpreted interms of the anisotropic strain due to the dopant.     

Glow Curves of Thermally Pretreated KCl:Mn2+ Crystals

Thermoluminescence studies of KCl crystals doped with 0.5 mole% of Manganese both in “as grown” state and after quenching them from various elevated temperatures indicate that the position of the glow peaks is found to depend upon the state of dispersion of impurity. In quenched crystals two additional peaks one around 130 and other around 215°C are observed. Dipoles near dislocations are suggested to be responsible for 130°C peak and 215°C peak to a complex formed by the associated of one or more Mn²⁺ ions with a single vacancy or an aggregate of vacancies, the complex being situation in the dislocation region.     

EPR studies on Mn2+ centres in solid solutions of TSCC and TSCB

EPR investigations of Mn²⁺-doped solid solutions of TSCC and TSCB were performed between 130 K and 300 K. The axial fine structure parameter D vs bromine concentration x is nonlinear. The relation between the axial fine structure parameter and bromine concentration is discussed by means of a simple statistical model.     

Observation of Raman scattering from N2 in a silica optical fiber

We have observed Raman scattering from N₂ in a silica optical fiber which had been pressurized in an N₂ atmosphere. The Raman shift is 2190 cm^?1 and the half-width of the line is 90 cm^?1 (fwhm).     

Raman investigation of the drawing effects on Ge-doped fibers

We have investigated the Raman activity of various germanosilicate fibers and their associated preforms. Our data indicate an enhancement in small rings' (3-member rings) concentration in the silica-based matrix of the fibers during the drawing process. The generation of such rings appears compatible with an increase of the sample density and fictive temperature. The data regarding the drawing effects on the fiber stress appear less clear, and it is possible to suggest that in some cases the drawing could lower the tensile stress. Finally we have also provided evidence that changing the drawing conditions within the usual range of application leads to no significant changes of the matrix structure as can be observed using Raman spectroscopy.     

新型W-LEDs用黄色荧光粉YAG:Mn2+的光谱性能研究

通过还原气氛下的高温固相法合成了荧光粉YAG: Mn,并对其光谱性质进行了表征.结果表明:荧光粉YAG:Mn中Mn的价态是+2价;其发射光谱为在590 nm有最强发射的宽带状光谱,激发光谱在411～424 nm的范围内最强,所以此荧光粉是理想的可与紫光LED (411 ～424 nm)相匹配的黄色荧光粉