Nitridation of SiO2–B2O3 aerogels

SiO₂–B₂O₃ aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO₂–B₂O₃ aerogels has been between 312 and 359 m²/g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO₂–B₂O₃ aerogels a typical bands for SiO₂ are observed. After nitridation a shift and broadening of 1100 cm^?1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels.     

Crystal structure of NH3(CH2)2NH3BiCl5·2H2O

The ethylenediammonium pentachlorobismuthate(III) dihydrate salt is monoclinic with the following unit cell dimensions: a = 10.902(8)Å, b = 7.926(6)Å, c = 15.199(6)Å, = 96.40(1)°, space group P2₁/n with Z = 4. The structure shows a layer arrangement parallel to the axis: planes of the [Bi₂Cl₁₀]^4– bioctahedra alternate with planes of [NH₃(CH₂)₂NH₃]²⁺ dications. The [Bi₂Cl₁₀]^4– bioctahedra are connected through O(W)–H··· Cl hydrogen bonds, so that infinite unidimensional chains of composition [Bi₂Cl₁₀(H₂O)₂] _n ^4n– are formed in the structure parallel to the axis. These chains are themselves interconnected by means of the N–H···Cl bonds originating from the [NH₃(CH₂)₂NH₃]²⁺ entities, forming a three-dimensional network.     

Influence of Bi2O3 content on the crystallization behavior of TeO2–Bi2O3–ZnO glass system

Glass ceramic materials with composition 75TeO₂–xBi₂O₃–(25-x)ZnO (x = 13, 12, 11) possessing transparency in the near- and mid-infrared (MIR) regions were studied in this paper. It was found that as the Bi₂O₃ content increased in the glass composition, the observed crystallization tendency is enhanced, and high crystal concentrations were obtained for the glasses with high Bi₂O₃ content while maintaining transparency in the MIR region. Crystal size in the glass ceramic was reduced by adjusting the heat treatment conditions; the smallest average size obtained in this study is 700 nm. Bi_0.864Te_0.136O_1.568 was identified using X-ray Diffraction (XRD) and found to be the only crystal phase developed in the glass ceramics when the treatment temperature was fixed at 335 °C. The morphology of the crystals was studied using Scanning Electron Microscopy (SEM), and crystals were found to be polyhedral structures with uniform sizes and a narrow size distribution for a fixed heat treatment regime. Infrared absorption spectra of the resulting glass ceramics were studied. The glass ceramic retained transparency in the infrared region when the crystals inside were smaller than 1 μm, with an absorption coefficient less than 0.5/cm in the infrared region from 1.25 to 2.5 μm. The mechanical properties were also improved after crystallization; the Vickers Hardness value of the glass ceramic increased by 10% relative to the base glass.     

Effect of TiO2 addition on the chemical durability of Bi2O3–SiO2–ZnO–B2O3 glass system

The effect of the substitution of ZnO for TiO₂ on the chemical durability of Bi₂O₃–SiO₂–ZnO–B₂O₃ glass coatings in hot acidic medium (0.1 N H₂SO₄ at 80 °C) for different times was studied. The thick films produced by a screen-printing method and heat treated at 700 °C/5 min were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The glass from the Bi₂O₃–SiO₂–ZnO–B₂O₃ system developed Zn₂SiO₄ and a glassy phase that were readily attacked by hot 0.1 N sulfuric acid, whereas the heat treated coating from the Bi₂O₃–SiO₂–TiO₂–ZnO–B₂O₃ system presented a finer microstructure with thin interconnected Bi₄Ti₃O₁₂ crystals and a glassy phase more resistant to hot 0.1 N sulfuric acid attack etching.     

Formation of Co2+-doped nanocrystals in La2O3–MgO–Al2O3–SiO2 glass-ceramics

Transparent glass-ceramics were synthesized by heat-treatment of glass with a composition of 5La₂O₃–13.2MgO–28.8Al₂O₃–46SiO₂–4.5TiO₂–2.5ZrO₂–0.15CoO (LMAS) (wt.%). The activation energy of crystallization and the Avrami parameter for the LMAS glass were determined from the DTA curves at different heating rates. The most two intense bands of Raman spectrum of initial glass at ~ 810 cm^?1 and ~ 900 cm^?1 were connected with the presence of [SiO₄] and [TiO₄] tetrahedral, respectively. After heat-treated at 700 °C/10 h+820 °C/8 h, the intensity of the band for [TiO₄] tetrahedral weakened, while an intensive band at ~ 800 cm^?1 for the Ti–O bond appeared. Other bands were characteristics of high-silicate network and x(MgTi₂O₅)·y(Al₂TiO₅) polycrystals. The changes reflected phase separation after heat-treatment of the initial glass. The strong absorption band of glass-ceramics centered at 580 nm can be assigned to ⁴A₂(⁴F)→⁴T₁(⁴P) and the broad absorption band at 1100–1700 nm to ⁴A₂(⁴F)→⁴T₁(⁴F) transitions of tetrahedral coordinated Co²⁺ ion. Two broad emission bands, one was around 660 nm, the other was from 800 nm to 1050 nm, of glass-ceramics correspond to the ⁴T₁(⁴P)→⁴A₂(⁴F) and ⁴T₁(⁴P)→⁴T₂(⁴F) transitions of tetrahedral coordinated Co²⁺ ions. The absorption and emission features clearly demonstrated that Co²⁺ ions were incorporated into nanocrystals and located in tetrahedral sites.     

Spectroscopic properties of Co2+ ions in La2O3–MgO–Al2O3–SiO2 glass-ceramics

Co²⁺-doped La₂O₃–MgO–Al₂O₃–SiO₂ (LMAS) glass-ceramics was synthesized by conventional method. The microstructure of LMAS GCs heat-treated at 760 °C/12 h + 930 °C/4 h was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The spectroscopic properties of Co²⁺-doped LMAS GCs were studied by absorption spectrum, excitation spectrum, and temperature dependent luminescence spectra. XRD results revealed the sizes of MgAl₂O₄ crystalline phases are about 9.1 ± 1.5 nm. The three peaks in the visible absorption band of LMAS GCs at 549 nm, 585 nm and 626 nm are connected with the transitions from ⁴A₂ level to ²A₁/²T₂(²G), ⁴T₁(⁴P) and ²E/²T₁(²G) levels, respectively, and excitations into them emit the radiation at around 666 nm. The luminescence intensity increased with temperature increasing from 10 K to 150 K, while it weakened with temperature increasing from 150 K to 350 K. These features were explained by the effects of two competing mechanisms.     

Effect of the ytterbium concentration on the upconversion luminescence of Yb3+/Er3+ co-doped PbO–GeO2–Ga2O3 glasses

The effect of Yb³⁺ concentration on the frequency upconversion (UPC) of Er³⁺ in PbO–GeO₂–Ga₂O₃ glasses is reported for the first time. Samples were prepared with 0.5 wt% of Er₂O₃ and different concentrations of Yb₂O₃ (1.0–5.0 wt%). The green (523 and 545 nm) and red (657 nm) emissions are observed under 980 nm diode laser excitation. The dependence of the frequency UPC emission intensity upon the excitation power was examined and the UPC mechanisms are discussed. An interesting characteristic of these glasses is the increase of the ratio of red to green emission, through an increase of the Yb³⁺ concentration due to an efficient energy transfer from Yb³⁺ to Er³⁺.     

Excited state absorption and up-conversion luminescence of Ho3+ centres in 3CaO–Ga2O3–3GeO2 glass

3CaO–Ga₂O₃–3GeO₂ glass excited state absorption spectra-activated with Ho³⁺ (Ho₂O₃ – 0.7 wt% content) have been measured and analysed. Up-converted emission channels have been identified and the predicted up-converted emission bands have been registered under Ar ion laser (λ = 488 nm) excitation according to the excited state absorption data. A mechanism of up-converted transitions for Ho³⁺ centres in this prototype glass network is proposed on the basis of the obtained results.     

Synthesis and photoluminescence of Eu3+-doped ZnO–Ga2O3–SiO2 nano-glass-ceramics

Transparent rare-earth Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics were obtained by a sol–gel method. X-ray diffraction and transmission electron microscopy were used to characterize the as-synthesized materials. Results showed that ZnGa₂O₄ nanocrystals with the size of 5 nm were precipitated from ZnO–Ga₂O₃–SiO₂ system and dispersed in the SiO₂-based glass when the heat-treatment temperature was up to 800 °C. Photoluminescence characterization of Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics was carried out and the results show that the as-synthesized material display intense emission at 615 nm belonging to ⁵D₀ → ⁷F₂ transition.     

A study of two geometric isomers of 1,1′-bi-3-cyanocyclohex-2-enylidene

The preparation, separation, crystal structure and ¹H NMR spectra are reported for the two geometrical isomers of 1,1-bi-3-cyanocyclohex-2-enylidene. The E-isomer crystallized as thin plates in the monoclinic space group P2₁/n with a = 5.3980(5), b = 7.0757(7), c = 15.300(2) Å, = 94.571(2)°, and Z = 2. The structure has symmetry C_2h. The Z-isomer crystallized as needles in the triclinic space group P1¯ with a = 7.0790(6), b = 11.3155(9),c = 15.386(1) Å, = 104.943° = 90.164(2)°, = 99.494(2)°, and Z = 4. The compound crystallized with two molecules per asymmetric unit. In C₆D₆, ¹H NMR signals of the 2-vinyl protons appear almost identical. However, in a mixture of C₆D₆ and pyridine-d ₅, the 2-vinyl protons can be distinguished. The structures compare favorably with MM2 calculations.     

Thermal expansion of glasses in the As2Se3–AsI3 system

Thermal expansion coefficients (α) of glasses in the As₂Se₃–AsI₃ system are measured in the glass transition region and temperature dependence of the fictive temperature is calculated on the basis of relaxation model. It is found that the increase of AsI₃ content results in: an increase of α, decrease of the glass transition temperature (T_g), increase of the α change at T_g, an effect of quenching rate on α, and also changes in the structural relaxation times spectrum. The data are discussed within the framework of the assumption that the addition of AsI₃ to As₂Se₃ results in: (1) destruction of the As₂Se₃ glass network, (2) structural inhomogeneity of the glasses increase, (3) the temperature dependence of chemical–structural equilibria occurring in the liquid state increases.     

Magnetite nanoparticles prepared by the crystallization of Na2O–Fe2O3–B2O3–SiO2 glasses

A glass with the composition of 35Na₂O–24Fe₂O₃–20B₂O₃–20SiO₂–1ZnO (mol%) was melted, quenched, using a twin roller technique, and subsequently heat treated in the range 485–750 °C for 1–2 h. This led to the crystallization of magnetite as the sole or the major crystalline phase.Heat treatment at lower temperatures resulted in the crystallization of magnetite crystals 7–20 nm in diameter, whereas heat treatment at higher temperatures produced higher quantities of magnetite and much larger crystals. The room temperature magnetization and coercive force values were in the range of 6–57 emu g^? 1 and 0–120 Oe, respectively for the heat treated glasses.     

Electrical and dielectric properties of MnF2–ZnF2–NaPO3 glasses

Direct electrical conductivity and dependencies of complex electrical modulus vs. temperature and frequency have been measured on glasses from the MnF₂–ZnF₂–NaPO₃ system. These glasses are sensitive to atmospheric humidity and as a consequence, the electrical conductivity increases up to temperature of 50 °C. A hydrated layer is created by the effect of water and leads to the significant increase of the electrical conductivity in the case of 0MnF₂–20ZnF₂–80NaPO₃ glass. This behavior is governed by Arrhenius relation where the values of activation energy are increasing and values of the electrical conductivity are decreasing with the amount of MnF₂. Dielectric measurements show that a heterogeneous phase is formed in the bulk of glasses. This may be seen when plotting complex electrical modulus in the complex plane. The records made by the light microscope confirmed the occurrence of the other phase in the bulk of glasses.     

Silver metal enhanced photoluminescence of Tm3+ doped GeS2–Ga2S3–CsCl glasses

We report silver metal enhanced near-IR and infrared-to-visible upconversion luminescence in Tm³⁺ doped 70GeS₂–10Ga₂S₃–20CsCl (in mol. %) glasses. The metal embedded glasses are prepared under controlled crystallization. Upon 808 nm excitation three fold enhancement of emissions is observed in the visible (446 nm, 496 nm, and 532 nm) and near infrared (1230 nm, 1450 nm and 1480 nm) regions. The possible mechanism responsible for the enhanced luminescence is discussed.     

Effect of lead oxide on optical properties of Dy3+ ions in PbO–H3BO3–TiO2–AlF3 glasses

The effect of lead oxide (PbO) on optical properties of Dy³⁺-doped PbO–H₃BO₃–TiO₂–AlF₃ (LBTAFDy) glasses is investigated. The LBTAFDy glasses were prepared with different PbO contents ranging from 30 to 60 mol%. The Judd–Ofelt intensity parameters (Ω_{λ = 2, 4, 6}) are obtained by the least square fit analysis. It is found that the Ω₂ parameter and yellow-to-blue intensity ratio (Y/B) of the Dy³⁺ emission depend on the PbO content in LBTAFDy glass. The structural asymmetry around the Dy³⁺ ion and the DyO covalency are responsible for the changes in Ω₂ parameter and Y/B ratio. The variation of decay time of ⁴F_9/2 emission level with the PbO content also supports the changes in structural asymmetry and DyO covalency in LBTAFDy glass.     

Structure and properties of NaPO3–ZnO–Nb2O5–Al2O3 glasses

In an effort to design low-melting, durable, transparent glasses, two series of glasses have been prepared in the NaPO₃–ZnO–Nb₂O₅–Al₂O₃ system with ZnO/Nb₂O₅ ratio of 2 and 1. The addition of ZnO and Nb₂O₅ to the sodium aluminophosphate matrix yields a linear increase of properties such as glass transition temperature, density, refractive index and elastic moduli. The chemical durability is also significantly, but nonlinearly, improved. The glass with the highest niobium concentration, 55NaPO₃–20ZnO–20Nb₂O₅–5Al₂O₃ was found to have a dissolution rate of 4.5 × 10^? 8 g cm^? 2 min^? 1, comparable to window glass. Structural models of the glasses were developed using Raman spectroscopy and nuclear magnetic resonance spectroscopy, and the models were correlated with the compositional dependence of the properties.