Molecular dynamics simulation for the rapid solidification process of MgO–Al2O3–SiO2 glass–ceramics

A molecular dynamics simulation method was used to study the effects of the microstructure on the solidification process of different cooling rates in the MgO–Al₂O₃–SiO₂ glass–ceramics with cordierite as the main crystalline phase. The reasons for changes in the microstructure during the solidification process were analysed by the radial distribution function curve, the bond angular distribution, the coordination number and the volume changes. The results showed that the cooling rate greatly affected the crystallisation process and the glass transition process. When the cooling rate was too fast, the atoms could not undergo a massive displacement before they were “frozen”, and the ability of atoms to achieve an equilibrium position was limited. Some amorphous phases were formed as a result of the disorder of the atomic arrangement, then some crystalline phase precipitated from the vitreous, and a glass–ceramic material was eventually formed.     

Down conversion luminescence of Tb3+–Yb3+ codoped SrF2 precipitated glass ceramics

In the Tb³⁺–Yb³⁺ codoped glass ceramics with SrF₂ nanocrystals precipitated, the energy transfer mechanism from Tb³⁺ to Yb³⁺ was investigated. The excitation power dependence of emission intensity study showed that the quantum cutting occurs during the energy transfer from Tb³⁺ to Yb³⁺ with the excitation of Tb³⁺ high energy level. However, the one-photon process is the main reason that is responsible for the Yb³⁺ infrared emission. The external quantum yields of Tb³⁺ and Yb³⁺ were evaluated by using an integrating sphere measurement system with the excitation of 377 and 488 nm lasers, which are much lower than the expected quantum efficiencies calculated from Tb³⁺ lifetimes. The external quantum yields in the glass ceramics and as-made glasses were also compared.     

Crystallization of bismuth oxide nano-crystallites in a SiO2–PbO–Bi2O3 glass matrix

SiO₂–PbO–Bi₂O₃ glasses having the composition of 35SiO₂–xPbO–(65 ? x)Bi₂O₃ (where x = 5, 20 and 45; in mol%) have been prepared using the conventional melting and annealing method. Differential scanning calorimetry (DSC) was employed to characterize the thermal behavior of the prepared glasses in order to determine their crystallization temperatures (T_cr). It has been found that T_cr decreases with the decrease of Bi₂O₃ content. The amorphous nature of the prepared glasses as well as the crystallinity of the produced glass–ceramics were confirmed by X-ray powder diffraction (XRD) analysis. SiPbBi₂O₆ glass nano-composites, comprising bismuth oxides nano-crystallites, were obtained by controlled heat-treatment of the glasses at their (T_cr) for 10 h. Transmission electron microscopy (TEM) of the glass nano-crystal composites demonstrates the presence of cubic Bi₂O₃ nano-crystallites in the SiPbBi₂O₆ glass matrix. Nano-crystallites mean size has been determined from XRD line width analysis using Scherrer's equation as well as from TEM; and the sizes obtained from both analyses are in good agreement. These sizes varied from about 15 to 170 nm depending on the chemical compositions of parent glasses and, consequently, their structure. Interestingly, replacement of the Bi₂O₃ by PbO in the glass compositions has pronounced effect on the nature, morphology and size of the formed nano-crystallites. Decrease of the Bi₂O₃ content increases the size of the nano-crystallites, and at the lowest Bi₂O₃ extreme, namely 20 mol%, introduces minority of the monoclinic Bi₂O₄ in addition to the cubic Bi₂O₃. The crystallization mechanism is suggested to involve a diffusion controlled growth of the bismuth oxide nano-crystallites in the SiPbBi₂O₆ glass matrix with the zero nucleation rate.     

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.     

BaO–B2O3–SiO2–Al2O3 sealing glass for intermediate temperature solid oxide fuel cell

Glasses in the formulation close to BaSiO₃–BaB₂O₄ eutectic compound are developed for sealing of intermediate-temperature (500–650 °C) solid oxide fuel cell (IT-SOFC). Thermal and microstructural analyses of the glasses with 0–10 mol% Al₂O₃ are also conducted. Detailed crystallization kinetics and interfacial stability of the glass in contact with yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) are investigated and compared. The results show that the formulation, 47BaO–21B₂O₃–27SiO₂–5Al₂O₃ (G1A5), performs the best on glass forming ability (GFA) among all tested formulations, and shows matched thermal expansion and working temperature to CeO₂-based electrolytes of IT-SOFC. Two major crystalline phases that precipitate from G1A5 above 750 °C are platy hexacelsian and BaSiO₃ grains.     

Piezoelectric and elastic properties of ZnF2–PbO–TeO2: TiO2 glass ceramics

Piezoelectric coefficients d₃₃ as well as ultrasonic velocities and elastic coefficients of ZnF₂–PbO–TeO₂ glasses crystallized with different concentrations of TiO₂ (0.5 to 2.0 wt.%) were measured. The contribution to the piezoelectric coefficients is attributed to presence of Pb₅Ti₃F₁₉, PbTiO₃ and PbTeO₃ ferroelectric crystal phases. The piezoelectric coefficients show substantial sensitivity to presence of TiO₂. The ultrasonic velocities and the related elastic coefficients in these glass ceramics as a functions of concentration of nucleating agent TiO₂ exhibited minimal effect at 1.0 wt.%. This is ascribed to the larger presence of titanium ions in Ti³⁺ states which act as modifiers and finally de-polymerize glass ceramic network. The results have been further discussed quantitatively within a framework of different oxidation states of titanium ions and the nature of the crystal phases ingrained in the glass ceramic.     

The effect of composition and temperature on the amount and type of nanoferrite particles inserted in Fe2O3–ZnO–MgO–SiO2 glass–ceramics

Glass–ceramics with the composition 2Fe₂O₃.1ZnO.1MgO.96SiO₂ [4ZnMgFe] and 2Fe₂O₃.2ZnO.3MgO.93SiO₂ [7ZnMgFe] (mol%) were prepared using the sol–gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), electron diffraction (ED) and Mössbauer spectroscopy (MS) were used to investigate the glass–ceramics structure. The samples contain ferrite nanoparticles embedded in a glass matrix. However, zinc ferrite nanoparticles seems to be the preferential crystalline phase formed. The amount of ferrite particles depends on treatment temperature and sample composition. The Mössbauer spectroscopy measurements show that ferrite nanoparticles can exhibit a ferrimagnetic behaviour combined with superparamagnetism.     

Spectral investigations on VO2+ ion doped in CaO–SrO–Na2O–B2O3 glass systems

Glasses of the (20 ? x)CaO–xSrO–(20 ? y)Na₂O–60B₂O₃ ? y (CSNB) system with (5 ≤ x ≤ 15) mol% and y = 0.1 mol% of V₂O₅ were characterized by X-ray diffraction (XRD), EPR (Electron Paramagnetic Resonance), Optical absorption Spectra and FT-IR (Fourier transform Infrared Spectroscopy) studies. EPR spectra of all the glass samples exhibit resonance signals characterstic of VO²⁺ ions. The values of spin-Hamiltonian parameters indicate that the VO²⁺ ions in CSNB glasses were present in octahedral sites with tetragonal compression and belong to C_4v symmetry. Spin-Hamiltonian parameters ‘g’ and ‘A’ were evaluated. The Optical band energy (E_opt) and Urbach energy (ΔE) were calculated from their ultra violet edges. By correlating EPR and Optical data the molecular orbital coefficients have been evaluated. IR spectra of these glasses were analyzed in order to identify the contribution of each component to the local structure that determines the physical properties of these glasses.     

Influence of Al2O3 addition on the properties of Bi2O3–BaO–SiO2–RxOy (R = K,Zn, etc.) glass sealant

A series of Bi₂O₃–BaO–SiO₂–R_xO_y (designated BiBaSi glass) glass sealants doped with different contents of α-Al₂O₃ have been investigated. Al₂O₃ was added as a modifier to affect the structure and the behavior of the glass. The thermal properties such as glass transition temperature (Tg), softening temperature (Tf) and coefficient of thermal expansion (CTE) were measured by a dilatometer. The sealing performance was investigated by sealing a SOFC single cell stack and measuring its open circuit voltage (OCV). Tg, Tf and suitable usage temperature of the sealants increased with increasing Al₂O₃ content in the glass, while CTE decreased. When the Al₂O₃ content was lower than 10 wt.%, excellent sealing performance was observed.     

Photosensitivity of SiO2–Al and SiO2–Na glasses under ArF (193 nm) laser

Photosensitivity of SiO₂–Al and SiO₂–Na glass samples was probed by means of the induced optical absorption and luminescence as well as by electron spin-resonance (ESR) after irradiation with excimer-laser photons (ArF, 193 nm). Permanent visible darkening in the case of SiO₂–Al and transient, life time about one hour, visible darkening in the case of SiO₂–Na was found under irradiation at 290 K. No darkening was observed at 80 K for either kind of material. This investigation is dedicated to revealing the electronic processes responsible for photosensitivity at 290 and 80 K. The photosensitivity of both materials is related to impurity defects excited directly in the case of SiO₂–Na and/or by recapture of self-trapped holes, which become mobile at high temperature in the case of SiO₂–Al. Electrons remain trapped on the localized states formed by oxygen deficient defects.     

Synthesis and bioactive properties of macroporous nanoscale SiO2–CaO–P2O5 bioactive glass

A macroporous nanoscale bulk bioactive glass (SiO₂–CaO–P₂O₅ system) was prepared by sol–gel co-template method. Porosimeter analysis showed that the as-synthesized bioactive glasses (BGs) had a porosity of 85% and exhibited a multimodal pore size distribution, nanopores (10–40 nm) and macropores (100 nm–10 μm). Morphological and structural characterizations showed the pores were interconnected with pore walls of about 250 nm in width and 1 μm in length. In vitro bioactivity test indicated that the as-synthesized bulk BGs exhibited faster apatite layer formation capability than the conventional sol–gel BGs. Additionally, the deposited layer was identified as hydroxycarbonate apatite, which is similar to the inorganic part of human bone.     

Performance investigation of Li2O–Al2O3–4SiO2 based glass–ceramics with B2O3, Na3AlF6 and Na2O fluxes

Influence of single fluxes (10 wt.% B₂O₃), bi-component fluxes (4 wt.% B₂O₃ + 6 wt.% Na₃AlF₆), and complex fluxes (4 wt.% B₂O₃ + 4 wt.% Na₃AlF₆ + 2 wt.% Na₂O) on the thermal kinetic parameters, microstructure, flexural strength and coefficient of thermal expansion (CTE) of Li₂O–Al₂O₃–4SiO₂ (LAS) glass–ceramics was investigated through differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The results showed that complex fluxes could efficiently decrease transition temperature (T_g) and crystallization temperature (T_p), and accelerate the formation of needle-like β-spodumene crystals which benefit high flexural strength. The homogeneous LAS glass–ceramic (sample C₃) which has a high strength of 132.4 MPa and low CTE (100–650 °C) of 2.74 × 10^? 6/°C is obtained by doping of the initial LAS glass by complex fluxes of 4 wt.% B₂O₃, 4 wt.% Na₃AlF₆, and 2 wt.% Na₂O, nucleating at 630 °C/120 min and then crystallized at 780 °C/120 min. It is worthy of further investigation as a bonder of diamond composite material due to its outstanding prosperities.     

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.     

Surface modification and crystallization of the BaO–B2O3–SiO2 glassy system using CO2 laser irradiation

The surface modification and crystallization process of BaO–B₂O₃–SiO₂ glass compositions when exposed to CO₂ laser irradiation was evaluated as a function of the laser power, irradiation time and surface condition. The glass surface was modified by the application of laser power exceeding 0.40 W and an irradiation time of more than 300 s. Micro-Raman and X-ray diffraction measurements revealed at high laser power the formation of β-BaB₂O₄ (β-BBO) crystalline phase. The crystallization of the irradiated region was enhanced when β-BBO micrometer sized particles were dispersed on the surface of the glass sample. The intensity of the second harmonic generation observed in the crystallized region was found to depend mainly on the condition of the glassy surface prior to glass irradiation.     

Preparation and third-order optical nonlinearity of glass ceramics based on GeS2–Ga2S3–CsCl pseudo-ternary system

Chalcohalide glass-ceramics based on GeS₂–Ga₂S₃–CsCl pseudo-ternary system were prepared by heat treatment method. X-ray diffraction and scanning electron microscope studies confirmed the formations of Ga₂S₃ and GeS₂ phase grains with sizes of 2–5 and 80 nm, respectively. Z-scan technology was employed to investigate the third-order nonlinear optical characteristics of both precursor glass and its glass ceramics at 800 nm. The results show that nonlinear refractive index n₂ as well as nonlinear absorption coefficient β increase after heat treatment, which is due to quantum effects, and the largest n₂ of the glass ceramics is 4.3 × 10^? 11 esu which is 4 times larger than that of the host.     

Ultra-high thermal expansion glass–ceramics in the system MgO/Al2O3/TiO2/ZrO2/SiO2 by volume crystallization of cristobalite

Glasses in the system MgO/Al₂O₃/TiO₂/ZrO₂/SiO₂ with and without the addition of As₂O₃ and Sb₂O₃ were thermally treated. Up to a temperature of 950 °C, this resulted in the formation of ZrTiO₄, sapphirine and high quartz solid solution. Annealing at higher temperatures led to the formation of low quartz solid solutions, ZrTiO₄ and sapphirine. This resulted in a continuous increase of density, hardness, fracture toughness and thermal expansivity. In the glass doped with As₂O₅ and Sb₂O₅ annealing temperatures >1000 °C resulted in the formation of cristobalite instead of quartz. Then the density, hardness and strength decreased again, while the fracture toughness (up to 2.8 MPa m^1/2) and the thermal expansion coefficient increased strongly. In the dilatometric curves, a steep increase in expansion is observed in the temperature range from 100 to 200 °C, which is attributed to the transformation of low cristobalite to high cristobalite. The mean linear thermal expansion coefficient (25–200 °C) is 20 × 10^?6 K^?1 and the largest up to now reported in the literature for alkali-free silicate glass–ceramics.     

Microstructural characteristics and crystallization of CaO–P2O5–Na2O–ZnO glass ceramics prepared by sol–gel method

Porous phosphate-based glass ceramics prepared by the sol–gel method were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and differential thermal analysis (DSC). The 48CaO–45P₂O₅–2ZnO–5Na₂O glassy system can remain fully amorphous up to 550 °C. After heat treated at 650 °C, the obtained porous bodies consisted of dense struts and macropores where β-Ca₂P₂O₇ and Na₂CaP₂O₇ phases crystallized from the glass matrix. When treated at 750 °C, Ca₄P₆O₁₉ and NaZn(PO₃)₃ precipitated homogeneously as new phases among the residual glass matrix. The material was assessed by soaking samples in phosphate-based buffer solution (PBS) solution to determine the solubility and observe apatite formation.     

The role of coordination and valance states of tungsten ions on some physical properties of Li2O–Al2O3–ZrO2–SiO2 glass system

A series of Li₂O–Al₂O₃–ZrO₂–SiO₂ glasses doped with different concentrations of WO₃ (0 to 5.0 mol.%) have been synthesized. Differential thermal analysis of the samples indicated increasing glass forming ability with the increasing concentration of WO₃ in the glass matrix. A variety of spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties (over a range of frequency and temperature) of these glasses have been investigated. The optical absorption and ESR spectral studies have pointed out that a part of tungsten ions do exist in W⁵⁺ state in addition to W⁶⁺ state especially in the samples containing low concentration of WO₃. The IR and Raman spectral studies have suggested that there is a decreasing degree of disorder in the glass network with increase in the concentration of WO₃. The values of dielectric parameters viz., dielectric constant, loss and ac conductivity at any frequency and temperature are observed to decrease as the concentration of WO₃ is increased. Such changes have been attributed to decrease of redox ratio or decreasing proportions of W⁵⁺ ions that act as modifiers in the glass network. The quantitative analysis of the results of ac conductivity and dielectric properties have indicated an increase in the insulating character of the glasses with the concentration of WO₃; this is attributed to the presence of tungsten ions largely in W⁶⁺ ions that participate in the glass network forming with WO₄ structural units.