Structure,crystallization, and performances of alkaline-earth boroaluminosilicate sealing glasses for SOFCs

We study the structure, crystallization, and performances of the sealing glasses with the composition (mol.%) of 12Al₂O₃·8B₂O₃·40SiO₂·40RO (R = Mg, Ca, Sr) for solid oxide fuel cells (SOFCs) before and after isothermal treatment at 700°C, which is within the operation temperature range (600-800°C) of SOFCs. The crystallization behavior has been investigated by differential scanning calorimetry and X-ray diffraction under both dynamic and isothermal conditions. The structural evolution is probed using the Raman and nuclear magnetic resonance spectroscopies. The performances of the sealing glasses are characterized in terms of the coefficient of thermal expansion, the crystallization-induced stress at glass–steel interface. We find that strong crystallization occurs at the operation temperature (700°C) far below the crystallization onset temperature measured by DSC. The structure origin of this anomalous crystallization is discussed in terms of structural heterogeneity of the three studied glasses. We determine the residual stress at the interface between the Ca-containing glass and the steel after isothermal treatment at 700°C for 48 h, but this stress does not lead to falling off the glass layer from the steel. This indicates that this glass is a good candidate to be applied in SOFCs.     

Preparation and sealing effects of Mg–Al–Si–Ba–O-based glass-ceramic coatings on NTC thermistors

Herein, Mg–Al–Si–Ba–O-based glass ceramics were studied as potential candidates to protect Mn–Co–Ni–O-based negative temperature coefficient (NTC) thermistors at high temperatures such as 900 °C. The ceramics were prepared in three glass formulations (1#: 15MgO–15Al₂O₃-44.7SiO₂–25BaO, 2#: 17MgO–17Al₂O₃–41SiO₂–25BaO and 3#: 17MgO–17Al₂O₃–41SiO₂–20BaO–5Y₂O₃ (in mol%)) and their glass-transition temperatures (T_g) were determined using the differential scanning calorimetry (DSC) method. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the parent glasses and glass-ceramic coatings. The sealing effects of the glass ceramics were examined by conducting an insulation test. The glass-ceramic sealing structures were subjected to 1000 thermal shock cycles at temperatures varying from room temperature to 900 °C. Notably, the sealing structure of glass-ceramic coating 1# was compact at a T_g of 760.9 °C. The glass-ceramic coatings effectively maintained the NTC properties of the sensitive ceramics in all three formulations. Interestingly, the glass-ceramic coating 3# containing Y₂O₃ demonstrated an increase in electrical resistance. Both the NTC thermistors coated with 1# and 2# glass formulations successfully passed 1000 thermal shock cycles without visible failures, and their resistance change ratios were well below the requisite 20%.     

Development of a gastight sealing material for ceramic components

Within the framework of a cooperation project, ceramic components of alumina, Al₂O₃, were sealed in a gastight manner for chemical applications using a sealing compound based on ceramic raw materials. The sealing is adapted to the Al₂O₃ ceramic with respect to expansion coefficient and wettability and it is inert, mechanically stable, chemically resistant and above all gastight at application temperatures of approx. 1000 °C. This was achieved with high reproducibility by a composition in the SiO₂–Al₂O₃–CaO–K₂O quaternary system. The ceramic sealing compound is selectively crystallized by adequate temperature control and shows then the required high temperature resistance. The strengths achieved are sufficient for the planned application as a sealing material for a ceramic microreactor. The temperature cycling resistance and the thermal shock resistance are also adapted to Al₂O₃. Additional tests of commercial glass sealants were carried out to evaluate the results obtained.     

Synthesis and long-term test of borosilicate-based sealing glass for solid oxide fuel cells

A series of borosilicate-based glasses, ternary BaO-SiO₂-B₂O₃ and quaternary BaO-SiO₂-B₂O₃-Al₂O₃ systems, are prepared as sealing materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion, crystalline phases, glass forming ability, and thermo-chemical stabilities of the glasses are characterized. Additionally, the effect of the B₂O₃/SiO₂ ratio and Al₂O₃ and BaO contents on the coefficient of thermal expansion (CTE) are discussed and compared. Test results show that one glass (G6) can fully wet various substrates at the sealing temperature of 1000 °C and match thermal expansion. Possible interfacial reactions between the glass and those cell components aging up to 5000 h are investigated by element mapping, XRD, SEM, and EPMA. Leakage testing was also performed at temperatures up to 650 °C. The results show that the glass (G6) remains amorphous after 5000 h test and is stable under these conditions and compatible with the other fuel cell components.     

Sealing Glass‐Ceramics with Near Linear Thermal Strain,Part I: Process Development and Phase Identification

A widely adopted approach to form matched seals in metals having high coefficient of thermal expansion (CTE), e.g. stainless steel, is the use of high CTE glass‐ceramics. With the nucleation and growth of Cristobalite as the main high‐expansion crystalline phase, the CTE of recrystallizable lithium silicate Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO glass‐ceramic can approach 18 ppm/°C, matching closely to the 18 ppm/°C–20 ppm/°C CTE of 304L stainless steel. However, a large volume change induced by the α‐β inversion between the low‐ and high‐ Cristobalite, a 1^st order displacive phase transition, results in a nonlinear step‐like change in the thermal strain of glass‐ceramics. The sudden change in the thermal strain causes a substantial transient mismatch between the glass‐ceramic and stainless steel. In this study, we developed new thermal profiles based on the SiO₂ phase diagram to crystallize both Quartz and Cristobalite as high expansion crystalline phases in the glass‐ceramics. A key step in the thermal profile is the rapid cooling of glass‐ceramic from the peak sealing temperature to suppress crystallization of Cristobalite. The rapid cooling of the glass‐ceramic to an initial lower hold temperature is conducive to Quartz crystallization. After Quartz formation, a subsequent crystallization of Cristobalite is performed at a higher hold temperature. Quantitative X‐ray diffraction analysis of a series of quenched glass‐ceramic samples clearly revealed the sequence of crystallization in the new thermal profile. The coexistence of two significantly reduced volume changes, one at ~220°C from Cristobalite inversion and the other at ~470°C from Quartz inversion, greatly improves the linearity of the thermal strains of the glass‐ceramics, and is expected to improve the thermal strain match between glass‐ceramics and stainless steel over the sealing cycle.     

Study on non-isothermal crystallization kinetics of the BaO-CaO-Al2O3-B2O3-SiO2 glass for IT-SOFCs sealing

The crystallization ability plays a key role in effecting thermal ability of sealing glass for intermediate temperature-solid oxide fuel cells (IT-SOFCs) to prevent fuel leakage during operation and insulate the cell stack from the external atmosphere. Herein, using differential thermal analysis (DTA) techniques, the growth mode of crystals precipitated in BaO-CaO-Al₂O₃-B₂O₃-SiO₂ (BCABS) sealing glass through the heat treatment was calculated in terms of non-isothermal crystallization kinetics for the first time. The calculated results showed that the average kinetic exponent n of the glass was approximatively 1, indicating that the crystal nucleuses became to form and further grew with one-dimensional mode from the surface inwards. Scanning electron microscope (SEM) observations clearly revealed that a large number of one-dimensional filamentous crystals have been formed on the interface between the sealing glass and the electrolyte after the heat treatment at 973?K for 100?h, which perfectly coincided with the theoretical calculations, and the glass was well combined with the electrolyte without any visible cracks or peeling at the interface. The one-dimensional growth of hexagonal BaAl₂Si₂O₈ crystals verified by X-ray diffraction (XRD) could effectively decelerate the decrease of thermal expansion coefficient of glass to ensure enhance the thermo-stability of the BCABS sealing glass for IT-SOFC.     

Phase transformation and microstructure of MgO–Al2O3–SiO2 system glass–ceramics under different heat treatment conditions

Abstract

Effects of heat treatment conditions on phase transformation, microstructure and thermal expansion coefficient (TEC) in MgO–Al₂O₃–SiO₂ system glass–ceramics were investigated by means of differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnesium aluminium titanate (MAT) precipitated firstly at 850°C and β-quartz solutions (β-QSS) formed at 950°C. Further increasing temperature to 1000°C, MAT disappeared and β-QSS became master phase, following little amount of α-cordierite, MgTi₂O₅, rutile and sapphirine. When glass was treated at 1050°C, β-QSS content decreased and α-cordierite became master phase. As temperature reached higher than 1100°C, β-QSS and sapphirine disappeared, and α-cordierite became master phase accompany with rutile and MgTi₂O₅ as secondary phase. The microstructure transformed gradually from particle shape crystallites to slat shape network with the increase in heat treatment temperature. By controlling heat treatment condition, an ideal glass–ceramics with proper TEC for matching sealing to 4J29 alloy has been obtained.     

Tailoring the efficient laser-absorption-melting behavior of Bi2O3-B2O3-ZnO-Nd2O3 glass for the OLED encapsulation

The efficient 810 nm laser energy conversion of glass frit had been proven to be the key to the long-term hermetic encapsulation of Organic Light Emitting Display (OLED). A direct laser energy conversion laser-assisted Bi₂O₃-B₂O₃-ZnO-Nd₂O₃ sealing glass material without extra laser absorbent such as carbon black, was designed and systematically investigated. The addition of Nd₂O₃, as glass modifiers with higher cationic field strength, could be conducive to enhancing the polymerization of glass network structure, manifesting that the glass-transition temperature Tg, onset-crystallization temperature Tc and thermal stability ΔT (ΔT = Tc-Tg) increased, while thermal expansion coefficient CTE dropped to 9.72×10⁻⁶/°C and advantageously matched with the glass substrate (8±1×10⁻⁶/°C). More importantly, the absorption rate of BBZ-Nd glass was more than 50 % between 800∼810 nm owing to the 4f-4f electron transition of Nd³⁺ ions, and yet the reflectivity and transmittance of the wavelength at 800–810 nm were lower. As optimal compositions, the addition of 3.0 wt% Nd₂O₃ in Bi₂O₃-B₂O₃-ZnO-Nd₂O₃ glass frit with higher absorption coefficients (80 %) led to instantaneous bonding encapsulation between glass substrates without interfacial cracks or pores with the 808 nm wavelength of the laser at 20 W and 2.4 mm/s.     

Hot corrosion behavior of YSZ/ZrW2O8 composites exposed to V2O5 at 700 °C and 850 °C in air

The hot corrosion behavior of YSZ/ZrW₂O₈ composites as a promising thermal barrier coating system exposed to V₂O₅ at 700 °C and 850 °C was investigated in order to better understand the influence of the incorporated ZrW₂O₈ with isotropic negative thermal expansion performance on the corrosion resistance. Results indicate that the ZrW₂O₈ incorporation could retard the degradation of YSZ from V₂O₅ attack and the corrosion process is significantly related to the inclusion content and the temperature. The corrosion resistance could be determined by the incorporation content, while the reaction products are only temperature dependent. At 700 °C, ZrV₂O₇, YVO₄ and m-ZrO₂ were the main corrosion products, while ZrW₂O₈ recrystallized under the acidic environment provided by V₂O₅. At 850 °C, ZrW₂O₈ decomposed and only WO₃, YVO₄ and m-ZrO₂ could be detected as final corrosion products. The corrosion mechanisms of YSZ/ZrW₂O₈ composites at 700 °C and 850 °C were discussed based on the phase diagrams and Lewis acid-base rule as well as the volume compensation of the positive and negative expansion ceramics.     

Interfacial evolution mechanism of MgAl2O4/MgAl2O4 joints bonded with lanthanum glass

The La₂O₃-SiO₂-B₂O₃ (LSB) glass filler with high softening temperature was first used to join MgAl₂O₄ ceramic. An interfacial layer composed of Al₂O₃ was formed due to the solubility difference of MgO and Al₂O₃ in the LSB glass filler. As a result, the addition of Al₂O₃ into the LSB glass filler caused the increase of interfacial layer thickness. On the contrary, the addition of MgO into the LSB glass filler led to the decrease of interfacial layer thickness. When the adding content of MgO was 6 wt%, the interfacial layer disappeared and completely amorphous brazing seam was obtained. The in-line transmittance of joints decreased with the increase of the thickness of interfacial layer. The optimal in-line transmittance of joint bonded with La₂O₃-SiO₂-B₂O₃-MgO (LSB6M) glass filler reached 82.9% at 1000 nm. Meanwhile, the average flexural strength of joints was about 196.2 MPa, which was equal to the strength of MgAl₂O₄ substrate.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Improved corrosion resistance of Al2O3-SiC-C castables through in situ carbon containing aluminate cement as binder

Slag corrosion is one of the main damage modes for refractory castables used in iron and steel metallurgy. The matrix plays vital roles for corrosion resistance of refractory castables. In the present paper, the properties of the Al₂O₃-SiC-C-based trough castables with in situ carbon containing calcium aluminate cement (CCAC) and ball pitch as carbon sources, respectively, were comparatively investigated. The microstructures of the trough castables after corrosion were observed by field-emission scanning electron microscopy. The results showed that after being corroded at 1450°C for 3 hours, the corrosion depth of Al₂O₃-SiC-C-castables with CCAC as binder (A2) was 1.2 mm, 65.71% lower than that of the trough castables with ball pitch as carbon source (A1), though the content of carbon materials in the former was much lower than that of the latter. The reasons for these observations were that the in situ carbon materials of CCAC exhibit improved distribution in the matrices of castables and excellent oxidation resistance, resulting in lower porosity within the castables, and hindered the penetration of the molten slag at high temperatures. In addition, the Al₂O₃-SiC-C-castables bonded by CCAC displayed good mechanical properties at room temperature and elevated temperature.     

钒酸盐玻璃陶瓷封接材料的制备及应用

采取熔融-晶化法制备了主晶相为Ba3(VO4)2的玻璃陶瓷,研究了热处理制度对玻璃陶瓷样品析晶情况、热学及力学性质的影响.结果表明:在最佳热处理条件晶化540 ℃,保温2 h下,玻璃陶瓷的转变温度Tg、软化温度Tf、热膨胀系数及硬度均优于基质玻璃,且热膨胀系数为17.81 ×10 -6℃-1与金属铜的热膨胀系数值( 17.7 × 10-6℃-1)临近,能够与之直接封接,将有毒的含铅封接材料取而代之.     

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al₂O₃ coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al₂O₃ coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al₂O₃ were detected. The formation of Al₂O₃ layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al₂O₃ coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al₂O₃ coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.     

Property evolutions of Si/mixed Yb2Si2O7 and Yb2SiO5 environmental barrier coatings completely wrapping up SiCf/SiC composites under 1300 °C water vapor corrosion

A bi-layer environmental barrier coating (EBC) consisting of silicon(Si) bond coat/mixed ytterbium disilicate (Yb₂Si₂O₇) and ytterbium monosilicate (Yb₂SiO₅) topcoats has been successfully prepared to completely wrap up the SiC_f/SiC composites and the protective effects of such EBC have been evaluated by soaking them in a mixed 50% O₂ and 50% H₂O corrosive gases at 1300 °C for various times. In topcoats, Yb₂Si₂O₇ is the major phase, providing good thermal expansion coefficient (CTE) matching with composite substrate and thus excellent thermal shock resistance, whereas Yb₂SiO₅ is the dispersing minor phase, providing improved water vapor corrosion resistance. The completely wrapping up of SiC_f/SiC composites by above EBC system is employed to avoid direct exposure to the corrosive conditions, making it possible to evaluate the genuine protection effects of current EBCs. Under 1300 °C water vapor corrosion, the mass change, the phase composition and the evolution of microstructure are investigated, which suggest that the bi-layer EBC has excellent performance on protecting SiC_f/SiC composites from water vapor corrosion at 1300 °C.     

P2O5-V2O5-B2O3-ZnO系无铅低熔电子封接玻璃的性能

研究了P2O5-V2O5-B2O3-ZnO系无铅封接材料,考察了V2O5和B2O3含量对低熔玻璃软化温度、热膨胀系数及热稳定性的影响. 结果表明,玻璃的软化点随V2O5含量增加而降低,提高B2O3含量使软化温度先升高后降低,出现硼反常现象. 当V2O5和B2O3摩尔含量为15%和8%时,低熔玻璃的软化温度、热膨胀系数及热稳定性能满足低温封接要求,但化学稳定性较差,而添加少量Al2O3和Fe2O3能明显提高低熔玻璃的化学稳定性. 优化组成为26.0P2O5-17.3V2O5-7.7B2O3-45.0ZnO-2.0Al2O3-2.0Fe2O3的玻璃转变温度为340℃,热膨胀系数为7.5×10-6 ℃-1 (25~300℃),在90℃的去离子水中恒温10 h,失重为0.63 mg/cm2,化学稳定性与传统的含铅封接玻璃相当,综合性能基本满足无铅低熔玻璃的要求.     

Glass forming region and bonding mechanism of low-melting V2O5–TeO2–Bi2O3 glass applied in vacuum glazing sealing

As a new kind of energy-saving glass, vacuum glazing has excellent thermal and sound insulation properties and is widely used in building, household appliances and solar photovoltaic. The edge sealing material, along with sealing method, is key to the fabrication of vacuum glazing. Low transition temperature (T_g) and good fluidity at sealing temperature (T_s) make low-melting glass of V₂O₅–TeO₂–Bi₂O₃ (VTB) system perfect to be the edge sealing material for vacuum glazing. The glass forming region of VTB ternary system was mapped for the first time in this work. Low-melting VTB glass of 40V₂O₅–50TeO₂–5Bi₂O₃–3ZnO–2Na₂O (wt%) was optimized to be the sealing material. Glass powder of this composition could be used to seal the edges of vacuum glazing at an extremely low temperature of 360°C. With the assistance of anodic-bonding method, the bonding strength of vacuum glazing was dramatically enhanced. Vacuum glazing fabricated under the optimized process parameters of 420°C, 600 V, and 60 min possesses a highest bonding strength of 4.31 MPa. Furthermore, anodic-bonding mechanism of low-melting VTB glass applied in vacuum glazing sealing has been thoroughly researched.     

Some thermomechanical properties of pure oxide ceramics

Conclusions The proposed vacuum furnace makes it possible to determine the deformation temperature under load and the coefficient of thermal expansion to high temperatures.Ceramics of pure oxides, Al₂O₃, ZrO₂, MgO, and BeO, have high temperatures of softening under load. The temperature of initial softening of Al₂O₃ ceramics containing additives lies in the range 1860–1930°C. The magnesia and beryllia specimens show high softening temperatures under load, but in vacuum at high temperatures they are very volatile. The initial softening temperature of ZrO₂ is about 2250°C.The linear expansion of pure-oxide ceramics reaches 2–3% at 1800–2000°C. Values obtained for the average coefficients of expansion for Al₂,O₃, ZrO₂, MgO, and BeO are little different from those in the literature.The compressive strength and bending strength of pure oxides at high temperatures are relatively low. The highest o_bend at high temperatures is possessed by specially pure zirconia, stabilized with MgO. Magnesia and beryllia in compressive strength at high temperatures exceed the other oxides.The highest spalling resistance is shown by beryllia ceramics. The combined addition to alumina of 1% TiO₂ and 5% ZrO₂ leads to a reduction in sintering temperature and an increase in thermal shock resistance. Ceramics based on specially pure zirconia stabilized with an optimum amount of CaO and MgO show a high thermal shock resistance.     

The effect of Fe2O3 addition on tialite formation

Aluminium titanate (Al₂TiO₅) exhibits interesting properties, such as low thermal expansion, high thermal shock resistance, high refractoriness, and good corrosion resistance. In this research, effect of hematite (Fe₂O₃) addition on the formation temperature of Al₂TiO₅ was studied starting from very fine (micronsized) rutile (TiO₂) and aluminum hydrate (Al(OH)₃) powders. Samples with (0, 2.5, 5, 7.5, and 10) wt.% hematite were prepared for comparison. Identification of phases was accomplished using X-ray diffraction. The samples were characterized by simultaneous thermal analysis and microstructural analysis using scanning electron microscopy. The addition of Fe₂O₃ changes the mechanism of tialite formation. The main results are reduction in tialite formation temperature down to 1150 °C, tialite grains growth, and decrease of porosity.