Viscous behaviour of Bi2O3–B2O3–ZnO glass composites with ceramic fillers

Abstract

Variation in the viscous flow behaviour, nature and extent of glass fluidity in glass/filler composites are addressed with respect to various factors such as filler type, content, size, density and migration distance. The characterisation of a glass (Bi₂O₃–B₂O₃–ZnO) composite consisting of two different fillers (cordierite and willemite) was determined using hot stage microscopy, a differential scanning calorimeter and a flow button test. The microstructure was analysed using a scanning electron microscope. The apparent viscosity of the glass composites increased on increasing concentration and density of the filler. The variation in the viscosity is due to the diffusion of the glass matrix through channels in the cordierite filler of the composite. Based on the calculated migration distance of the filler in the glass matrix, the present work suggests that the interfacial behaviour and the density of the filler play a significant role in determining the viscous flow of the glass composites.     

Microstructure and characterization of interfacial phases of sapphire/sapphire joint bonded using Bi2O3–B2O3–ZnO glass

Bismuth borate zinc glass, 50Bi₂O₃–30B₂O₃–20ZnO (mol.%), was used as a braze intended to join sapphire. Thermal properties of the glass were all experimentally determined. The wettability of the glass on sapphire was investigated. The contact angle not greater than 11.3° was obtained in air at temperatures of ≥700 °C. Subsequently, sapphire was successfully joined to themselves at 700 °C for 20 min. The microstructure of the joint was studied and the interfacial phases were characterized. The results showed that the ZnAl₂O₄ phase was formed as a result of the reaction between the sapphire substrate and ZnO from the glass. The mechanical integrity of the joints was investigated. The joints with an average room shear strength of 70 MPa were achieved. The fracture analysis indicated that the fracture occurred within the glass interlayer rather than at the sapphire/glass interface, hence indicating a good bonding of the glass with sapphire.     

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.     

Growth mechanism of house-of-cards aggregates of alumina platelets containing Na2O–B2O3–SiO2 glass flux

Porous alumina bodies, intended for use as heat-insulating refractory materials, were fabricated by a high-temperature evaporation method and characterized. A series of flux systems was used by adding a third component to Na₂O–B₂O₃ glass in addition to boric acid and sodium carbonate. When SiO₂ was added as the third component, the primary alumina particles grew anisotropically, forming a plate-like shape, and the house-of-cards structure was self-organized. The anisotropic growth of alumina platelets was promoted by the solid solution of Si⁴⁺ ions in the flux on the α-Al₂O₃ surface. Furthermore, the bonding between the alumina platelets was strengthened by the high-SiO₂-concentration flux. Our typical alumina body had a porosity of 71.5%, a compressive strength of 3.7 MPa, a shrinkage rate of 2.6% when reheated at 1700 °C, and a thermal conductivity of 0.24 W m⁻¹•K⁻¹ at 1000 °C. Thus, the present alumina bodies are expected to find application as high-performance heat-insulating refractory materials.     

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.     

Effects of La2O3–B2O3 on the flexural strength and microwave dielectric properties of low temperature co-fired CaO–B2O3–SiO2 glass–ceramic

The La₂O₃–B₂O₃ (LB) addition, synthesized using the traditional solid-state reaction process, was chosen as a novel sintering aid of the low temperature co-fired CaO–B₂O₃–SiO₂ (CBS) glass–ceramic. The effects of LB on the flexural strength and microwave dielectric properties have been investigated. The LB addition promotes the crystallization of the CaSiO₃ but high amount of the LB addition leads to the formation of more pores. The CBS sample with 4 wt% LB addition sintered at 850 °C for 15 min shows good properties: flexural strength = 193 MPa, ?_r = 6.26 and loss = 9.96 × 10^?4 (10 GHz).     

Joining MgAl2O4 ceramics using ZnO–Al2O3–SiO2 glass ceramic with precipitated ZnAl2O4

In this work, crystallization, thermal expansion and wetting behavior of ZnO–Al₂O₃–SiO₂ (ZAS) glass were first investigated. The results showed that ZnAl₂O₄ was precipitated from ZAS glass after crystallization treatment. Crystallization increased the coefficient of thermal expansion (CTE) of ZAS glass ceramic due to the high CTE of ZnAl₂O₄. In addition, ZAS glass exhibited good wettability on the surface of MgAl₂O₄ substrate. On this basis, ZAS glass was used to join MgAl₂O₄ ceramic, and the microstructure and mechanical properties of joints obtained with different cooling methods were investigated. The flexural strength of joints was related to the content of ZnAl₂O₄ crystals in the brazing seams. Additional nucleation and crystallization treatment during cooling process improved the crystallinity of brazing seam, resulting in better matching of the CTE of brazing seam with that of MgAl₂O₄ ceramic. The maximum flexural strength of joints reached 201 MPa, which was equivalent to the strength of MgAl₂O₄ ceramic.     

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.     

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.     

Growth and optical properties of ZnO–In2O3 heterostructure nanowires

ZnO–In₂O₃ heterostructure nanowires were grown on a Si (111) substrate using the thermal evaporation method. Scanning electron microscopy results showed that the ZnO nanowires had spherical caps. The X-ray diffraction (XRD) pattern and energy-dispersive X-ray (EDX) spectrum indicated that these caps were In₂O₃. An analysis of the early growth process revealed that indium oxide might have played a self-catalytic role. Therefore, it was plausible that the vapor–liquid–solid mechanism (VLS) was responsible for the growth of the ZnO–In₂O₃ heterostructure nanowires. The optical properties of the products were characterized using a photoluminescence (PL) technique. The PL results for the ZnO–In₂O₃ heterostructure nanowires showed a strong peak in the ultraviolet region as a result of the near band emission and a negligible peak for the visible emissions that occurred as a result of the defects. Based on these PL results, it was found that the In₂O₃ nanostructures not only introduced the caps at the tips of the ZnO nanowires but also partially passivated the nanowire surfaces, leading to an improved near band edge emission and the suppression of the defect luminescence.     

Effects of CaO–B2O3–SiO2 glass additive on the microstructure and electrical properties of BCZT lead-free ceramic

Addition of CaO–B₂O₃–SiO₂ (CBS) glass was performed to lower the sintering temperature of lead-free Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) ceramics. Orthorhombic and tetragonal phases coexisted in CBS-free BCZT ceramics. The BCZT ceramics transformed into a pseudo-cubic phase when sintered at 1300 °C with increasing CBS glass content. Additionally, the secondary phase, Ba₂TiSi₂O₈, was observed when CBS glass was added. The density initially increased, reached a maximum value with 2 wt% CBS glass, and then decreased rapidly with further increase in CBS glass content, which was consistent with the microstructure. The ɛ, T_c, P_r, and d₃₃ depend on microstructure, and the results agree with the density. Evident relaxation behavior was observed. Observed results were inferred to be dependent on the microstructure, phase structure, lattice distortion, and secondary phase. The sample with 2 wt% CBS glass showed the excellent performance, which could be a promising substitute to lead-free piezoelectric ceramics for lead-based materials.     

Synthesis,physical, optical,mechanical, and radiation attenuation properties of TiO2–Na2O–Bi2O3–B2O3 glasses

A series of bismo-borate (50-x)B₂O₃-xTiO₂-15Na₂O–30Bi₂O₃ glass samples (where x = 0, 2.5, 5, 7.5, and 10 wt%) doped with TiO₂ were fabricated via the melt-quenching technique. The gamma and neutron shielding, physical, optical, and mechanical properties of the prepared samples were investigated. The experimental results were measured using an HPGe detector. ¹⁵²Eu, ¹³³Ba, ¹³⁷Cs, and ⁶⁰Co radioactive sources were used with energies in the range of 81–1408 keV. The experimental results were compared with both the FLUKA code and the XCOM database. The addition of TiO₂ increased the density of the glass samples and decreased their molar volume. The mass attenuation coefficient (MAC) decreased as photon energy decreased, while it increased as TiO₂ concentration increased. The half value layer (HVL) and mean free path (MFP) of the glass samples increased when the photon energy increased and decreased as the TiO2 concentration increased. The absorbance of the present samples is enhanced by using TiO₂, meaning they can be used to protect humans from UV light. Both direct and indirect band gaps decreased as TiO₂ content increased from 0 to 10 wt %. Moreover, the electronic transition between localized states is valid in the present samples. The radiation shielding, optical, physical, and mechanical properties of the fabricated glass samples demonstrate their utility for diagnostic gamma shielding.     

Dielectric Relaxation in CaO–Bi2O3–B2O3 Glasses

Glasses in the system CaO–Bi₂O₃–B₂O₃ (in molar ratio) have been prepared using melt-quenching route. Ion transport characteristics were investigated for this glass using electric modulus, ac conductivity and impedance measurements. The ac conductivity was rationalized using Almond–West power law. Dielectric relaxation has been analyzed based on the behavior of electric modulus behavior. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.76 eV, close to that the activation energy for dc conductivity (1.71 eV) indicating that the same species took part in both the processes. The stretched exponent β (0.5–0.6) is invariant with temperature for the present glasses.