Transparent CaF2 surface crystallized CaO–2B2O3 glass possessing efficient photocatalytic and antibacterial properties

This study aims to fabricate surface crystallized transparent calcium borate glass (CaO–2B₂O₃) for photocatalytic and antibacterial applications. The CaO–2B₂O₃ glass was fabricated by using the conventional melt-quenching technique. Instead of using traditional high temperature and longer soaking time heat-treatment, the acid etching method was adopted to get surface crystallization. Surface etching of as-quenched glass was performed at room temperature by using three different concentrations of hydrogen fluoride in order to attain the different amount of calcium fluoride crystals at the surface. The characterization of surface crystallized glass was performed with an X-ray diffraction pattern and scanning electron microscope-EDS mapping system. Sufficient transparency (65%) was retained even at highest crystallization of glass surface. Surface crystallization with CaF₂ induces hydrophilicity and photocatalytic character in glasses which was evaluated with contact angle measurements and smart resazurin (Rz) ink test. Around 68% of methylene blue (MB) dye degradation was also displayed by the crystallized glass. Crystallized glasses portrayed good antibacterial property against Escherichia coli (gram-negative bacteria). Ninety-nine per cent of bacterial depletion was recorded in 3 hours exposure time with surface crystallized glass without any external intervention. Interesting morphological changes were observed in bacterial shape and physical appearance under field emission scanning electron microscope after exposure to crystallized glasses.     

Structure and viscosity of CaO–Al2O3–B2O3–BaO slags with varying mass ratio of BaO to CaO

The structure of CaO–Al₂O₃–B₂O₃–BaO glassy slags with varying mass ratio of BaO to CaO has been investigated by Raman spectroscopy, ¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and atomic pair distribution function (PDF). ¹¹B MAS-NMR spectra reveal the dominant coordination of boron as trigonal. Both simulations on ¹¹B MAS-NMR spectra and Raman spectroscopy indicate the presence of orthoborate as the primary borate group with a few borate groups with one bridging oxygen and minor four-coordinated boron sites. ²⁷Al MAS-NMR and PDF show the Al coordination as tetrahedral. Raman spectral study shows that the transverse vibration of Al^IV–O–Al^IV and Al^IV–O–B^III, stretching vibration of aluminate structural units and vibration of orthoborate and pyroborate structural groups. A broader distribution of Al–O bond lengths in PDF also supports the enhanced network connectivity. Viscosity measurements show the increase in viscosity of molten slags with increasing mass ratio of BaO to CaO, which further attributes to the enhanced degree of polymerization of the aluminate network.     

Application of BaO–CaO–Al2O3–B2O3–SiO2 glass–ceramic seals in large size planar IT-SOFC

BaO–CaO–Al₂O₃–B₂O₃–SiO₂ (BCAS) glass–ceramics can be used as sealant for large size planar anode-supported solid oxide fuel cells (SOFCs). BCAS glass–ceramics after heat treatment for different times were characterized by means of thermal dilatometer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the coefficients of thermal expansion (CTE) of BCAS glass–ceramics are 11.4×10⁻⁶ K⁻¹, 11.3×10⁻⁶ K⁻¹ and 11.2×10⁻⁶ K⁻¹ after heated at 750 °C for 0 h, 50 h, and 100 h, respectively. The CTE of BCAS matches that of YSZ, Ni–YSZ and the interconnection of SOFC. Needle-like barium silicate, barium calcium silicate and hexacelsian are crystallized in the BCAS glass after heat-treatment for above 50 h at 750 °C. The glass–ceramics green tape prepared by aqueous tape casting can be directly applied in sealing the cell of SOFCs with 10 cm×10 cm. The open circuit voltage (OCV) of the cell keeps 1.19 V after running for 280 h at 750 °C and thermal cycling 10 times from 750 °C to room temperature. The maximum power density is 0.42 W/cm² using pure H₂ as fuel and air as oxidation gas. SEM images show no cracks or pores exist in the interface of BCAS glass–ceramics and the cell.     

Effect of TiO2 addition amount on BaO–CaO–Al2O3 -SiO2 glass-bonded Al2O3 ceramics

A new type of glass-ceramic BaO–CaO–Al₂O₃–SiO₂ (BCAS) was developed to join Al₂O₃ ceramics, adding TiO₂ to the glass-ceramics can promote the crystallization behavior of the glass-ceramics. Through the observation of the joints, rutile TiO₂ whiskers can grow on the surface of Al₂O₃ ceramics, and the grown TiO₂ whiskers are one-dimensional needle-like whiskers growing in different directions in the joints, providing mechanical support for the joints. The aspect ratio of TiO₂ whiskers was changed by controlling the addition of TiO₂, and the crystallization behavior and microstructure of the joints were studied. The experimental results show that when the amount of TiO₂ added is 10% (wt%), the density of TiO₂ whiskers in the joint is the largest, the strengthening effect on the joint is the best, and the shear strength can reach 94.33 MPa.     

Effect of Al2O3 content on BaO–Al2O3–B2O3–SiO2 glass sealant for solid oxide fuel cell

The glass structure, wetting behavior and crystallization of BaO–Al₂O₃–B₂O₃–SiO₂ system glass containing 2–10 mol% Al₂O₃ were investigated. The introduction of Al₂O₃ caused the conversion of [BO₃] units and [BO₄] units to each other and it played as glass network former when the content was up to 10 mol%, accompanied by [BO₄] → [BO₃]. The stability of the glass improved first and then decreased as Al₂O₃ increased from 2 to 10 mol%, the glass with 5 mol% Al₂O₃ being the most stable one. The wetting behavior of the glasses indicates that excess Al₂O₃ leads to high sealing temperature. The glass containing 5 mol% Al₂O₃ characterized by a lower sealing temperature is suitable for SOFC sealing. Al₂O₃ improves the crystallization temperature of the glass. The crystal phases in the reheated glasses are mainly composed of Ba₂Si₃O₈, BaSiO₃, BaB₂O₄ and BaAl₂Si₂O₈. Al₂O₃ helps the crystallization of BaSiO₃ and BaAl₂Si₂O₈.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Effect of BaO substitution for CaO on the structural and thermal properties of SiO2–B2O3–Al2O3–CaO–Na2O–P2O5 bioactive glass system used for implant coating applications

The present study replaced 3.30 and 9.00 mol.% BaO for CaO in a SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O–P₂O₅ bioactive glass system used for implant coating applications. Variations of the glass structure, thermal properties, cytotoxicity, and radiopacity of glasses were studied. As demonstrated by the results, upon adding barium oxide to the glass structure, the weight density increased significantly, while a slight decrease in oxygen density was determined. Introducing barium oxide into glass composition did not cause any considerable change in the spectra of FTIR and Raman. It was demonstrated that the amount of bridging oxygen in the glass structure remained quite unaffected. The hot stage microscopy evaluations revealed further shrinkage of barium-containing frits due to lower viscosity and hence, higher viscous flow of these glasses. By substituting barium oxide for calcium oxide and increasing its concentration, the glass transition temperature (T_g) and the dilatometric softening temperature (T_d) decreased, while the thermal expansion coefficient increased. Moreover, upon substituting 9 mol.% barium oxide for calcium oxide, a 30 °C reduction in maximum sintering temperature (T_ms) of the glass was obtained, whereas the shrinkage rate was increased 1.7 times. It was indicated that the sintering process of barium-incorporated glasses would easily proceed without any phase crystallization. The barium-incorporated glasses exhibited more radiopacity. Additionally, no cytotoxic effect was caused by the substitution, and the Ba-containing glasses could be used for biomedical applications and implant coating as well.     

Optical and structural properties of ZnO NPs and ZnO–Bi2O3 nanocomposites

Pure ZnO and ZnO–Bi₂O₃ nanocomposites with 5 wt% and 10 wt% of Bi₂O₃ content were synthesized using the co-precipitation method. Optical properties such as refractive index (n), extinction coefficient (k), bandgap (E_g), and Urbach energies, as well as the band structure, were determined by modeling the experimental transmittance and reflectance UV–Vis spectra. The deduced bandgap and Urbach energies for pure ZnO (3.758 eV) increase with the increase of the doping degree of Bi₂O₃ in ZnO–Bi₂O₃ nanocomposite films. X-ray diffraction and scanning electron microscopy (SEM) was used to study the structural and morphological properties of these nanocomposite films. Pure ZnO and nanocomposites with Bi₂O₃ exhibit crystalline domains with wurtzite hexagonal structures, and as the doping degree of Bi₂O₃ increases, the crystallite size decreases. Based on SEM micrographs, the ZnO nanoparticles (NPs) structure shows the presence of aggregation. Moreover, Bi₂O₃ NPs in the nanocomposite film led to the further aggregation in the form of large rods. The elemental and chemical properties of the nanocomposites were investigated using infrared and energy-dispersive X-ray spectroscopy. The charge transfer process in the studied system is between ZnO and Bi₂O₃ conduction bands. Density-functional theory (DFT) calculations were performed for ZnO, Bi₂O₃, and ZnO-Bi₂O₃ compounds to investigate structural, optical, and electronic properties, being in agreement with the experimental results.     

Preparation of glass–ceramic glazes in the SiO2–Al2O3–CaO–MgO–K2O–Na2O–ZnO system by variable content of ZnO

This paper is focused on glass–ceramic glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O–Na₂O system with ZnO additions (2.5, 5, 10, 15, 15, 20 and 25 wt%). The compositions were designed based on constant molar ratio of SiO₂/Al₂O₃. In the resulting glazes diopside (CaMg[Si₂O₆]), willemite (Zn₂SiO₄) and vitreous phase were identified by X-ray diffraction. Morphological and structural date of these glazes were supplementary determined by EPMA, FTIR and Raman Spectroscopy. DSC analysis was carried out to characterize thermal properties of the materials.     

Tuning the interfacial reaction between CaO–SrO–Al2O3–B2O3–SiO2 sealing glass–ceramics and Cr-containing interconnect: Crystalline structure vs. glass structure

In this paper, Al₂O₃ was added to CaO–SrO–B₂O₃–SiO₂ sealing system to tailor the structure of sealing glass–ceramics and glass–ceramics/metal interfacial reaction. The addition of alumina in glasses contributes to increasing fraction of bridging oxygen in silica tetrahedral as well as the change in boron environment from three-fold to four-fold (BO₄ → BO₃). The devitrification tendency of glasses also decreases with increasing Al₂O₃ content. The condensed glass structure and increasing residual glass content play opposite roles on the interfacial reaction, consequently resulting in a maximum fraction of Cr⁶⁺ in reaction couples between Cr₂O₃ and glass containing 6 mole% Al₂O₃ at 700 °C. In addition, the good bonding can be observed at the interface between Cr-containing interconnect (Crofer 22APU) and glass containing 4 mole% Al₂O₃, held at 700 °C for 100 h. The reported results support the suitability of the prepared glass–ceramics as sealing materials for SOFC applications.     

Scintillation characteristics of transparent Eu2O3–BaO–TeO2 glass-ceramics

Tellurite glass (TeG) and its glass-ceramics (TeGC435 and TeGC455), with a composition of 10Eu₂O₃–10BaO–80TeO₂, were prepared and their luminescence properties were evaluated. TeG was prepared via the melt quenching technique, while TeGC435 and TeGC455 were fabricated by heat treating TeG at 435 and 455 °C, respectively, for 5 h each. The Eu₂Te₆O₁₅ crystal phase was formed in TeGC435 and TeGC455. Both the glass and glass-ceramics showed sharp photoluminescence and scintillation peaks, attributed to the 4f→4f transitions of Eu³⁺. The highest quantum yield was obtained for TeG, whereas the highest integrated scintillation intensity was obtained for TeGC455. The scintillation intensity of TeGC455 was approximately 10% of that of the Bi₄Ge₃O₁₂ single crystal. Furthermore, typical decay times derived from the 4f→4f transitions were obtained for TeG, TeGC435, and TeGC455 during photoluminescence and scintillation.     

The formation mechanism of pores and unbonding in the Al2O3/Al2O3 joints brazed by 50Bi2O3–35B2O3–15ZnO glass

50Bi₂O₃–35B₂O₃–15ZnO (mol. %) glass referred to as Bi50 glass, was used to braze Al₂O₃ ceramics. The phase transformations and wettability of the Bi50 glass on Al₂O₃ substrates at different temperatures were investigated. The results showed that the chemical compatibility of Bi50 glass and Al₂O₃ substrates at 650 °C was excellent. However, Al₂O₃/Al₂O₃ joints having a considerable volume fraction of pores and unbonding were obtained when the joining procedures were carried out by a one-step brazing method. Based on the experiments and simulation results, the prime determinants responsible for the presence of the pores and unbonding within the brazing joints can be divided into two aspects: (i) the intrinsic causes leading to the formation of closed pores (ii) the external factors causing the failure of pores and glass separation. Ultimately, an advanced joining procedure named two-step brazing was proposed, and joints nearly free of defects were acquired.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Effect of properties and crystallization behavior of BaO–ZnO–B2O3–SiO2 glass on the electrical conductivity of Cu paste

The extensive application of multilayer ceramic capacitors provides an attractive development for terminal electrode pastes. However, the growing requirement for advanced glass materials used in terminal electrode pastes is substantiated. Therefore, to advance the development of electrode pastes, better development and deeper exploration of glass powder are required. Here, a series of BaO–ZnO–B₂O₃–SiO₂ (BZBS) glasses were prepared by melt-quenching technique, which are used to investigate the effect of the introduction of BaO on structure and properties of the ZnO–B₂O₃–SiO₂ (ZBS) glass. With the introduction of BaO, the relative amount of [BO₄] was much less, which made the glass network structure loosen, decreased the glass transition temperature (T_g) and increased the coefficient of thermal expansion of the glass. The decreasing contact angle was observed on the surface of a barium titanate (BaTiO₃) substrate. When the BaO content was around 3–7 mol%, the stability of ZBS glass frit could be strengthened by inhibiting the precipitation of Zn₂SiO₄ crystal. In addition, to further characterize the effect of glass on terminal electrode paste, BZBS glass powder was adopted to prepare copper electrode paste, which was printed on the BaTiO₃ substrate and subsequently fired at 800°C for 10 min. With the related copper paste containing ZBS glass doped with 7 mol% BaO, the optimum value of acid resistance and sheet resistance (1.99 mΩ) were exhibited, at which the coated copper paste formed a dense conducting layer.     

Effect of BaO ratio on the structure of glass–ceramic composite materials from the SiO2–Al2O3–Na2O–K2O–CaO system

This article focused on effect of the content of barium oxide on microstructure of the glass–ceramic materials based on the system SiO₂–Al₂O₃–Na₂O–K₂O–CaO. The following characterisation techniques have been used: X-ray diffraction (XRD), scanning electron microscopy with micro-analyser (SEM–EDS), mid-infrared analysis (MIR), far-infrared analysis (FIR) and Raman Spectroscopy. Significant differences were observed in microstructure of silica–alumina network of glassy phase and phase composition related to changes in the amount of the barium oxide additive. Discussed results are part of a larger project implemented under the PBS Applied Research Programme, in order to determine the compositions of glass–ceramic materials with potential application as a chemically resistant hard coatings or/and resistant to thermal shock or as construction materials.     

Crystallization behavior of BaO–CaO–SiO2–B2O3 glass sealant and adjusting its thermal properties for oxygen transport membrane joining application

In this work, the thermal stability of a BaO–CaO–SiO₂–B₂O₃ glass sealant, named “H”, was investigated by differential scanning calorimetry (DSC). The crystallization behavior of glass H as the sealant matrix was investigated by a combination of experimental X-ray diffraction (XRD) analysis and thermodynamic simulation with the FactSage package. A good agreement was found between the Rietveld refinement of XRD experiments and the FactSage simulation. Particular attention was also given to the influence of the Sr₂SiO₄ filler added to the glass matrix “H” on the thermal expansion and microstructures of glass-Sr₂SiO₄ composites by means of dilatometry and scanning electron microscopy (SEM). The reinforced 20 wt% Sr₂SiO₄ composite (HS2S20) showed excellent properties and, thus, its joining performance was investigated using SrTi_0.75Fe_0.25O_3-δ (STF25) and Aluchrom as promising oxygen transport membrane (OTM) and counterpart, respectively. The joining behaviors were investigated by comparing different joining temperatures. 920 °C is the best joining temperature for HS2S20 sealant.     

Phase evolution and electrical properties of copper-electroded BaTi4O9 materials with BaO–ZnO–B2O3–SiO2 glass system in reducing atmosphere

BaTi₄O₉ (BT4) microwave dielectric ceramics using a copper electrode and containing 10 wt% BaO–ZnO–B₂O₃–SiO₂ (BZBS) glass frit were sintered under reducing atmosphere at 950 °C and were investigated on the phase evolutions, microstructures and dielectric properties of BT4 with various BaO/SiO₂ and ZnO/SiO₂ ratios of BZBS glasses. Experimental results show that the BaO/SiO₂ ratio contributes to wettability of glass with BaTi₄O₉ ceramics, and ZnO/SiO₂ ratio determines the densification of BaTi₄O₉ ceramics. The different Ba–Ti–O and Ba–Cu–O phases with various Ba/Ti and Ba/Cu ratios can be attributed to the contents of BaO in glass. Ba₄Ti₁₃O₃₀ and Ba₂Cu₃O_5+X may form when BaO contents are too high, and inducing copper diffusion due to the reactions of BaO and Cu, accompanying with degrading of the dielectric characteristics. If the ZnO contents of BZBS glasses were raised, a little bit of ZnSiO₃ and Ba₂Cu₃O_5+X phases appear without Cu diffusion due to non-reaction of ZnO and CuO. The high ZnO/SiO₂ ratio of glass reveals the lower softening point, indicating that the high ZnO glass could enhance the density and therefore increase the dielectric constant and quality factor.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.