Phase relations of the Li2O-Ta2O5-B2O3 and Li2O-WO3-B2O3 systems and promising nonlinear optical compounds in K2O-Ta2O5-B2O3 system

The subsolidus phase equilibria of the Li₂O-Ta₂O₅-B₂O₃, K₂O-Ta₂O₅-B₂O₃ and Li₂O-WO₃-B₂O₃ systems have been investigated mainly by means of the powder X-ray diffraction method. Two ternary compounds, KTaB₂O₆ and K₃Ta₃B₂O₁₂ were confirmed in the system K₂O-Ta₂O₅-B₂O₃. Crystal structure of compound KTaB₂O₆ has been refined from X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the orthorhombic, space group Pmn2₁ (No. 31), with lattice parameters a = 7.3253(4) Å, b = 3.8402(2) Å, c = 9.3040(5) Å, z = 2 and D_calc = 4.283 g/cm³. The powder second harmonic generation (SHG) coefficients of KTaB₂O₆ and K₃Ta₃B₂O₁₂ were five times and two times as large as that of KH₂PO₄ (KDP), respectively.     

Formation and third-order optical nonlinearities of silver nano-crystals embedded bismuthate glasses

We report the synthesis of silver nano-crystals (Ag-NCs) composited bismuthate glasses through a single-step thermal reduction approach. The Ag-NCs show broad surface plasmon resonance (SPR) absorption bands with spectral coverage from visible to near infrared region. Bi₂O₃ is found to be the key factor to the formation of Ag-NCs, e.g. their number density varies significantly and the corresponding SPR red-shifts as Bi₂O₃ content increased. Third-order optical nonlinearities (TON) of the silver-bismuthate nanocomposites were investigated by Z-scan and pump-probe techniques at the wavelength of 800 nm. The experimental results revealed SPR-assisted behavior and ultrafast temporal response (less than 100 fs) of TON properties of the nanocomposites.     

Role of bismuth and titanium in Na2O-Bi2O3-TiO2-P2O5 glasses and a model of structural units

0.60Na₂O-0.40P₂O₅ and (0.55−z)Na₂O-0.05Bi₂O₃-zTiO₂-0.40P₂O₅ glasses (0≤z≤0.15) were prepared by melting at 1000°C mixtures of Na₂CO₃, Bi₂O₃, TiO₂ and (NH₄)₂HPO₄. Differential Scanning Calorimetry (DSC) measurements give the variation of glass transition temperature (T_g) from 269°C (for 0.60Na₂O-0.40P₂O₅) to 440°C (for z=0.15). The density measurements increases from 2.25 to 3.01 g/cm³. FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 0.60Na₂O-0.40P₂O₅ to P₂O₇⁴⁻ groups in the glasses containing Bi₂O₃ or both Bi₂O₃ and TiO₂. When bismuth oxide and titania are added to sodium phosphate glass, phosphate chains are depolymerized by the incorporation of distorted Bi(6) and Ti(6) units through POBi and POTi bonds. Bi₂O₃ and TiO₂ are assumed to be present as six co-ordinated octahedral [BiO_6/2]³⁻and [TiO_6/2]²⁻ units again with shared corners. This is accompanied by the simultaneous conversion of [POO_3/2] into [PO_4/2]⁺ units which achieves charge neutrality in the glasses.     

Preparation of Y2O3-Al2O3-SiO2 glasses by combustion synthesis melt-casting under high gravity

Y₂O₃-Al₂O₃-SiO₂ glasses were prepared by combustion synthesis melt-casting under high gravity. The properties of the glasses strongly depended on the starting compositions and preparation conditions. With a higher SiO₂ content in the starting compositions, the glass-forming ability of the melt was improved, but the density and hardness of the prepared glasses decreased. Crystallization occurred more frequently for larger samples and by using quartz crucibles instead of graphite ones. By increasing the high-gravity factors, both the density and hardness of the samples were improved. It is proposed that enhancing the high-gravity field facilitates the removal of bubbles from the melt.     

High-purity TeO2–WO3–(La2O3, Bi2O3) glasses for fiber-optics

TeO₂–WO₃, TeO₂–WO₃–La₂O₃ and TeO₂–WO₃–La₂O₃–Bi₂O₃ glasses were produced by high-purity oxides mixtures melting in crucibles of gold or platinum in purified oxygen atmosphere. The total content of Cu, Mn, Fe, Co and Ni impurities was not more than 0.1–2 ppm wt in glasses and below 0.1 ppm wt in the initial oxides. The hydroxyl groups absorption coefficients of the tellurite glasses at the maximum of the absorption band (λ ∼ 3 μm) lay in the region of 0.01–0.001 cm⁻¹. Stability to crystallization characterized by T_x − T_g = 150–190 °C was demonstrated for high purity 75TeO₂–25WO₃ glasses by DSC-measurements. There were no thermal effects of crystallization and crystallized phases fusion in case of La₂O₃-containing glasses.The optical absorption losses, measured by the laser calorimetry method at λ = 1.06, 1.56 and 1.97 μm, did not exceed 40–120 dB/km for glass samples. The multimode optical fibers with optical losses of 50–250 dB/km at 1.2–2.2 μm were produced from high-purity TeO₂–WO₃–(La₂O₃, Bi₂O₃) glasses.     

Structural study of (50 − x)Na2O-xCuO-10Bi2O3-40P2O5 glasses

(50−x)Na₂O-xCuO-10Bi₂O₃-40P₂O₅ glasses (0≤x≤25) were prepared by melting at 900-1100°C mixtures of Na₂CO₃, Bi₂O₃, CuO and (NH₄)₂HPO₄. DSC measurements give the variation of glass transition temperature T_g from 318 (x=0) to 378°C (x=25). FTIR spectroscopy shows the evolution of the phosphate skeleton: (PO₃)_∞ chains for 60Na₂O-40P₂O₅ to P₂O₇ groups in the glass containing Bi₂O₃ or both Bi₂O₃ and CuO. When bismuth and copper oxides replace Na₂O, phosphate chains are depolymerized by the incorporation of Bi₂O₃ and CuO through POBi and POCu bonds. P₂O₇ groups are predominant structural units in the richest CuO glass. The variation of T_g also supports these results.     

Sol-gel preparation and properties of TiO2-P2O5 bulk glasses

Monolithic transparent and colorless, or Ti³⁺-free TiO₂-P₂O₅ glasses containing very large amounts of TiO₂ (up to 93 mol%) were successfully prepared by heat-treating the xerogels, which were made from titanium tetraisopropoxide and triethyl phosphate, through the sol-gel reaction. The density and refractive index n_632.8 nm of the sol-gel-derived glasses were higher than the melt-derived glasses of the corresponding compositions. The glasses of TiO₂ content of larger than 80 mol% seemed somewhat porous, but n_632.8 nm of these glasses was very high as 2.2-2.3. Higher density and higher n_632.8 nm than the melt-derived glasses were considered to be due to more abundance of six-fold coordinated Ti⁴⁺ ions.     

Electron microscopy and EDX-microanalysis of photochromic silver halide glasses of the composition systems Al2O3-B2O3-SiO2 and Na2O-CaO-SiO2

The microstructure and microanalysis study of two glasses containing AgCl precipitated particles have been carried out by transmission electron microscopy (TEM, replica method), scanning electron microscopy (SEM) and SEM/EDX (energy dispersive X-ray spectrometry). The composition of these glasses doped with silver halide and CuO was formulated from the Al₂O₃-B₂O₃-SiO₂ and Na₂O-CaO-SiO₂ systems respectively. In both glasses the seeds, nuclei, crystals and matrix were analysed, and the mean size and number of crystals were evaluated from the TEM and SEM observations. The microstructure in both glasses is different because of the different shape of the silver halide particles; the particles of the Al₂O₃-B₂O₃-SiO₂ glass are rounded while Na₂O-CaO-SiO₂ shows square precipitated particles. Likewise, the darkening behaviour is basically different; the Al₂O₃-B₂O₃-SiO₂ glass shows a higher darkening velocity than the Na₂O-CaO-SiO₂ glass, showing for this glass a very small slope value.     

Production and properties of high purity TeO2–ZnO–Na2O–Bi2O3 and TeO2–WO3–La2O3–MoO3 glasses

High-purity TeO₂–ZnO–Na₂O–Bi₂O₃ and TeO₂–WO₃–La₂O₃–MoO₃ glasses were produced by melting the high-purity oxides mixtures in platinum or gold crucible at 800 °C in hermetic chamber in the purified oxygen atmosphere. The content of limiting impurities in the produced initial oxides and glasses, the optical properties as well as the stability to crystallization were investigated. The optical fibers were produced from high-purity tellurite glasses with losses at the level of several hundreds of dB/km.The total content of 3d-transition metals in the glasses was not more than 1 ppm wt and content of hydroxyl groups corresponded to the absorption level of 0.001–0.002 cm⁻¹ at ∼3 μm. The absorption band, monotonically increasing with the increase in MoO₃ content, was observed in dry TeO₂–WO₃–La₂O₃–MoO₃ glasses with the maximum at about 3.7 μm.     

Multi-color long-lasting phosphorescence in Mn-doped ZnO-B2O3-SiO2 glass-ceramics

Multi-color LLP phenomenon was observed in Mn²⁺-doped ZnO-B₂O₃-SiO₂ glass-ceramics after the irradiation of a UV lamp at room temperature. Transparent ZnO-B₂O₃-SiO₂ glass emitted reddish LLP while opaque glass-ceramics prepared by the glass sample after heat treatment emitted yellowish or greenish LLP. The change of the phosphorescence is due to the alteration of co-ordination state of Mn²⁺. The phosphorescence of the samples was seen in the dark with naked eyes even 12 h after the irradiation with a UV lamp (λ_max=254 nm) for 30 min. Based on the approximative t⁻¹ decay law of the phosphorescence, we suggest that the LLP is attributed to the thermally assisted electron-hole recombination.     

Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis,characterization and application in recyclable photodegradation of organic dyes under visible light irradiation

Heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by a two-step method. First, Fe₃O₄ nanoparticles with the size of ca. 10 nm were synthesized by chemical method at room temperature and then heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by hydrothermal method at 180 °C for 24 h with the addition of 10 wt% Fe₃O₄ nanoparticles into the precursor suspension of Bi₂O₂CO₃. The pH value of synthesis suspension was adjusted to 4 and 6 with the addition of 2 M NaOH aqueous solution. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe₃O₄/Bi₂O₂CO₃ photocatalysts were obtained, respectively. Both photocatalysts demonstrate superparamagnetic behavior at room temperature. The UV–vis diffuse reflectance spectra of the photocatalysts confirm that all the heterostructured photocatalysts are responsive to visible light. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. The heterostructured photocatalysts prepared in this study exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of MB and MO, and they can be easily recovered by applying an external magnetic field.     

Preparation of grape-like Bi2O3/Ti photoanode and its visible light activity

Compact and grape-like bismuth oxide (Bi₂O₃) coated titania (Ti) anode was prepared by oxalic acid (H₂C₂O₄) etching, electrodeposition and calcination in order to explore its photoelectrocatalytic activities. The Bi₂O₃ coating was demonstrated to be full of pores, and a good combination between Bi₂O₃ layer and honeycomb-like Ti substrate was observed by scanning electron microscopy. The characteristic morphology of Bi₂O₃ coating indicated that the electrode is stable during degradation. The Bi₂O₃/Ti electrode was used in oxidative degradation of Acid Orange 7 by electrolysis, photocatalytic oxidation and photoelectrocatalytic oxidation processes. The pseudo-first order kinetics parameter (K_app) of photoelectrocatalytic process was 1.15 times of the sum of electrolysis and photocatalytic oxidation under visible light irradiation at 420 nm. The results indicated that the synergy of electrolysis and photocatalysis lead to an excellent photoelectrocatalytic property of the Bi₂O₃/Ti electrode.     

Influence of Sm2O3 on the crystallization and luminescence properties of boroaluminosilicate glasses

Sm₂O₃-doped SiO₂-B₂O₃-Al₂O₃-BaO glasses were prepared by melting method in order to study the influence of Sm₂O₃ on the crystallization behavior and luminescence properties. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy were used to characterize the rare earth glasses and the crystalline phases after heat-treatment. The course of phase separation and devitrification of the glasses were also investigated. The results show that the thermal stability of the glasses decreases with the increase of content of Sm₂O₃. The crystalline phase changed from SmAl_2.07(B₄O₁₀)O_0.6 to SmBO₃. Divalent Sm²⁺ ions were detected in the crystallization product after heat-treatment. The valence transformation from Sm³⁺ to Sm²⁺ in the crystal suggests the samarium atoms entering the barium sites. The charge carried in vacancy defect induced by the substitution led to the partial reduction process. The reduction of Sm³⁺ ions was promoted by the increasing of Sm₂O₃ content or the extending of heat-treated holding time in boroaluminosilicate glass.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

CO2 absorption and desorption of Bi2O3-La2O3 powders prepared by mechanical synthesis

In order to obtain CO₂-absorbents to eliminate CO₂ concentration locally, Bi₂O₃-La₂O₃ mixed powders were prepared by mechanical alloying (MA) method using a planetary ball-milling machine. CO₂-absorption and desorption properties were checked by TG-DTA for the obtained powder samples. As a result, the sample shown by (Bi₂O₃)_1−x(La₂O₃)_x [x≤0.50] was found to form α-Bi₂O₃-solid solution with repeated CO₂-adsorption and desorption around 400- 500 °C. Absorbed and desorbed CO₂ contents varied with MA time: the 72 h MA’ed sample had a larger CO₂ content than the 24 h MA’ed sample. The performance depended on the sample composition, and (Bi₂O₃)_0.70(La₂O₃)_0.30 was found to have the highest performance in the present system.     

Chemical stability and dielectric properties of RO-La2O3-B2O3 (R = Ca, Mg, Zn)-based ceramics

New lanthanum borate (La₂O₃-B₂O₃) glasses modified with divalent oxides, such as CaO, MgO and ZnO were investigated as potential low temperature dielectrics by understanding compositional dependence of dielectric properties and chemical leaching resistance. Firing behavior, such as densification and crystallization, depended strongly on the glass composition and is found to influence the resultant dielectric performance. Specifically, the dielectric composition of 20ZnO-20La₂O₃-60B₂O₃ glass with 40 wt% Al₂O₃ as a filler showed distinct enhancements of dielectric properties, i.e., k ∼ 8.3 and Q ∼ 1091 at the resonant frequency of 17.1 GHz, as a result of 850 °C firing. The result was believed related to earlier densification and unexpected evolvements of ZnAl₂O₄ and La(BO₂)₃ phases during firing. The Mg-containing glass sample was most stable in strong acid solutions and did not show any significant changes in microstructure even after 300 min exposure. The Ca-containing glass sample was not regarded as a promising candidate for low temperature dielectrics from the observed low quality factor and weak chemical durability.     

Mixed cation effect in xR2O-(40 − x)Na2O-50B2O3-10Bi2O3 (R = Li, K) glasses

Mass density, glass transition temperature and ionic conductivity are measured in xLi₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ and xK₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ glass systems with 0 ≤ x ≤ 40 mol%. The strength of the mixed alkali effect in T_g, dc electrical conductivity and activation energy has been determined in each glass system. The magnitudes of the mixed alkali effect in T_g for the mixed Li/Na glass system are much smaller than those in the mixed K/Na glasses. The impact of mixed alkali effect on dc electrical conductivity in mixed Li/Na glass system is more pronounced than in the K/Na glass system. The results are explained based on dynamic structure model.     

Structural studies of Fe2O3-Bi2O3-CdO glass system

xFe₂O₃·(100 − x)[Bi₂O₃·CdO] system with 0 ≤ x ≤ 50 mol% was prepared and investigated by X-ray diffraction, density, FT-IR and Raman spectroscopies. The XRD patterns confirm the formation of a vitreous structure for x < 35 mol% Fe₂O₃. The evolution of density and molar volume with the addition and increasing of iron content indicates structural changes in the structure of Bi₂O₃·CdO glass matrix. The FT-IR spectrum of the glass matrix reveals a structure realized from BiO₃ pyramidal and BiO₆ octahedral units. With the addition of iron the structure proposed by the glass matrix is changing by the appearance of FeO₄ units. Also the existence of FeO₆ units cannot be excluded. The Raman spectra suggest a structure build from BiO₆ octahedral units. By Raman scattering the presence of structural units characteristic to Fe₂O₃ was not directly observed but the evolution of the spectra is dependent of the iron content.     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.