Growth and spectral properties of Nd:LaVO4 crystal

This paper reports the growth and spectral properties of 3.5 at% Nd³⁺:LaVO₄ crystal with diameter of 20×15 mm² which has been grown by the Czochralski method. The spectral parameters were calculated based on Judd–Ofelt theory. The intensity parameters Ω_λ are: Ω₂=2.102×10⁻²⁰ cm², Ω₄=3.871×10⁻²⁰ cm², Ω₆=3.235×10⁻²⁰ cm². The radiative lifetime τ_r is 209 μs and calculated fluorescence branch ratios are: β₁(0.88μm)=45.2, β₂(1.06μm)=46.7, β₃(1.34μm)=8.1. The measured fluorescence lifetime τ_f is 137 μm and the quantum efficiency η is 65.6%. The absorption band at 808 nm wavelength has an FWHM of 20 nm. The absorption and emission cross sections are 3×10⁻²⁰ and 6.13×10⁻²⁰ cm², respectively.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

Mössbauer studies on some glasses and crystals in the Na2O---Fe2O3---SiO2 system

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.     

Contributions on the thermal behaviour of sulfates XI. On the chemical vapour transport of Fe2(SO4)3 with HCl; thermochemical calculations and experimental results

Using HCl as a transport agent, well-shaped crystals of rhombohedral Fe₂(SO₄)₃ with edge length up to 1 mm were formed in the less hot zone of a silica ampoule. The dependence of the deposition rate on the mean transport temperature for a constant concentration of the transport agent (C = 4.5 × 10⁻² mmol HCl/cm³) shows a distinct maximum at = 560°C. Additionally, the influence of the concentration of the transport agent at a constant temperature gradient was investigated. On the basis of thermochemical calculations, an explanation of the observed transport behaviour is given in the present paper.     

Determination of diffusion coefficient of titanium ion in TiO2---SiO2 wet gel prepared from metal alkoxides during leaching

It was found that the radial gradient refractive index (r-GRIN) glass rod could be produced by the leaching of wet gel prepared by a sol-gel method from metal alkoxides and heat-treatment of the gel. The concentration gradient of the doped component resulting from its diffusion in the wet gel was studied. In order to estimate the exact diffusion coefficient of the titanium ion in the binary TiO₂---SiO₂ wet gel, its value was measured by using two methods. The observed diffusion coefficient by colorimetric analysis was about 1.1 × 10⁻⁶ cm²/s, while the value obtained from analysis of EPMA was about 1.2 × 10⁻⁶ cm²/s, and both values were nearly equal to each other. The activation energy of titanium ion diffusion in the wet gel was estimated to be about 11 kcal/mol.     

Growth and spectral properties of Nd-doped Sr3Y(BO3)3 crystal

A new crystal of Nd³⁺:Sr₃Y(BO₃)₃ with dimension up to 25×35 mm² was grown by Czochralski method. Absorption and emission spectra of Nd³⁺: Sr₃Y(BO₃)₃ were investigated . The absorption band at 807 nm has a FWHM of 18 nm. The absorption and emission cross sections are 2.17×10⁻²⁰ cm² at 807 nm and 1.88×10⁻¹⁹ cm² at 1060 nm, respectively. The luminescence lifetime τ_f is 73 μs at room temperature     

Structural transformation of the TiO2---SiO2 system gel during heat-treatment

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.     

Growth and properties of (Pb0.90La0.10)TiO3 thick films prepared by RF magnetron sputtering with a PbO buffer layer

The (Pb_0.90La_0.10)TiO₃ [PLT] thick films (3.0 μm) with a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering method. The PLT thick films comprise five periodicities, the layer thicknesses of (Pb_0.90La_0.10)TiO₃ and PbO in one periodicity are fixed. The PbO buffer layer improves the phase purity and electrical properties of the PLT thick films. The microstructure and electrical properties of the PLT thick films with a PbO buffer layer were studied. The PLT thick films with a PbO buffer layer possess good electrical properties with the remnant polarization (P_r=2.40 μC cm⁻²), coercive field (E_c=18.2 kV cm⁻¹), dielectric constant (ε_r=139) and dielectric loss (tan δ=0.0206) at 1 kHz, and pyroelectric coefficient (9.20×10⁻⁹ C cm⁻² K⁻¹). The result shows the PLT thick film with a PbO buffer layer is a good candidate for pyroelectric detector.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

Effect of the substrate temperature on the crystallization of TiO2 films prepared by DC reactive magnetron sputtering

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.     

Oscillator strength of the infrared absorption band near 1080 cm in SiO2 films

It has been well known that the absorption maximum of the peak near 1080 cm⁻¹ in amorphous SiO₂ films shifts continuously with variation of thickness and properties such as stress. This is a first report on the oscillator strength of the absorption against frequency at the absorption maximum. SiO₂ films on silicon wafers were prepared by thermal growth in either dry O₂ or an O₂/H₂ mixture or liquid-phase deposition in HF saturated with silica gel. The oscillator strength continuously decreased from 1×10⁻⁴ down to 1×10⁻⁵ with the frequency shift from 1099 to 1063 cm⁻¹.     

Study of electrical properties of MoO3–Fe2O3–P2O5 and SrO–Fe2O3–P2O5 glasses by impedance spectroscopy. II

The electrical and dielectric properties for three series of MoO₃–Fe₂O₃–P₂O₅ and one series of SrO–Fe₂O₃–P₂O₅ glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown in Part I that the MoO₃ is incorporated into phosphate network and the structure/properties are strongly influenced by the overall O/P ratio. The Fe₂O₃ content and Fe(II)/Fe_tot ratio in these glasses have significant effects on the electrical conductivity and dielectric permittivity. With decreasing Fe₂O₃ content in MoO₃–Fe₂O₃–P₂O₅ glasses with O/P at 3.5 the dc conductivity, σ_dc(ω) decreases for two orders of magnitude, which indicates that the conductivity for these glasses depends on Fe₂O₃ and is independent of the MoO₃ content. Also, the dielectric properties such as ^′(ω), ^″(ω) and σ_ac(ω) and their variation with frequency and temperature indicates a decrease in relaxation intensity with increase in the concentration of MoO₃. On the other hand, the dc conductivity for MoO₃–Fe₂O₃–P₂O₅ glasses with O/P > 3.5 increases with the substitution of MoO₃ which has been explained by an increase in the number of non-bridging oxygens and formation of Fe–O–P bonds that are responsible for formation of small polarons. The increase in the dielectric permittivity, ^′(ω) with increasing MoO₃ content is attributed to the increase in the deformation of glass network with increasing bonding defects. For SrO–Fe₂O₃–P₂O₅ glasses the conductivity and dielectric permittivity remained constant with increasing SrO.     

Vapor phase epitaxy of pure VO2 and V1−xCrxO2

The vapor phase epitaxy of thin epilayers of VO₂ and V_1−xCrxO₂ on TiO₂ transparent substrates is described. Chemical vapor deposition occurs by reacting a (VOCL₃/CrO₂Cl₂/H₂O/H₂) mixture at about 800°C using argon as a carrier gas. The preparation of pure VO₂ requires special care to make it homogeneously stoichiometric and to obtain steep concentration profiles at the TiO₂/VO₂ interface. Layers were obtained which had electrical and optical properties comparable to the best bulk crystals grown by other techniques. Homogeneous solid solutions of V_1−xCr_xO₂ epilayers were also grown for the first time in the range o < x < 0.17. Chromium concentration and homogeneity were determined by electron microprobe analysis. The separation coefficient k was also found to vary with x. It is close to unity below x = 0.001 and above this value Cr is incorporated more easily. High quality heteroepitaxial layers (1 cm² area, 1 to 30 μm thickness) of V_1−xCr_xO₂ have for the first time allowed the measurement of the optical absorption coefficient.     

Characteristics of ZnO–B2O3–CaO–Na2O–P2O5 glass powders prepared by spray pyrolysis

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 (T_g) 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.     

High porosity silica xerogels prepared by a particulate sol–gel route: pore structure and proton conductivity

The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2<d_poresize10 nm) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sol by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO₂ xerogels have been achieved in the calcination temperature range from 250 to 550 °C. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base=4.8) porosities up to 54.7±0.1% are achieved, at 250 °C of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58% and 81% RH. Xerogels with a low porosity (40.8±0.1%) and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84×10⁻³±5.11×10⁻⁵ S/cm to 4.0×10⁻³±7.2×10⁻⁵ S/cm, at 81% RH and 80 °C. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH.     

Measurement of the refractive index of glass fibers by the Christiansen-Shelyubskii method

The Christiansen principle was employed to measure the refractive index of borosilicate glass fibers (13–41 μm diameter) over the visible range. The refractive index for glass fibers at 589.3 nm was measured by temperature and wavelength scan and values obtained were in close agreement. The refractive index for glass fibers as a function of wavelength was measured to an accuracy of < 10⁻⁴. The uniformity of the refractive index for a bundle of fibers of slightly different diameter was calculated using the modified Shelyubskii method and compared to experimental values. Theoretical calculations of the transmission by the present work suggest that, for high optical clarity and transmission of Christiansen cell (or transparent composite consisting of glass fiber and polymer), the refractive index must be controlled to the fifth decimal place. For example, the maximum transmission of a fiber/liquid mixture cell at 25°C can increase from 89 to 97% when the standard deviation is reduced from 13 × 10⁻⁵ to 9 × 10⁻⁵.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

SIMS study of the concentration of dopants in LPCVD silicon thin films

Measurements of solid phase dopant concentration (S) of LPCVD Si thin films as a function of substrate temperature (T_s = 500−640 ° C) and gas phase doping ratio (R = 1 × 10⁻⁵ −4 × 10⁻²) by SIMS indicate different behaviors of P and B in the films. A linear relation S = b(T)R is observed for B-doped film with b(T) varying from 4 to 50 depending on T_s. Boron-doped microcrystalline film has a higher doping efficiency than that of P-doped ones.     

Optical third-harmonic generation from some high-index glasses

Optical third-harmonic generation from some high-index glasses was investigated. The highest χ⁽³⁾ was obtained from As₂S₃ glass 2.2 × 10⁻¹² esu/ This value was 100 times higher than that of pure SiO₂ glass and comparable with that of the polymer with monomer-doping, which are known as organic materials with quite high χ⁽³⁾. From the relationship between χ⁽³⁾ and composition, sulfide glasses were found to have higher χ⁽³⁾ than oxide or semiconductor-doped oxide glasses with similar refractive indices.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

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 FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.