Development of a novel compressive h-BN based seal for planar intermediate temperature SOFC

A novel compressive boron nitride (h-BN) based seal has been developed for planar intermediate temperature solid oxide fuel cell (SOFC). It exhibited extremely low leakage rates and thermal cycling stability under simulated stack conditions. The h-BN based seal showed leakage rates of 0.01 sccm/cm under gas pressure of 6.8 kPa in the temperature range of 650–800 °C, and maintained similar leakage rates during 10 thermal cycles. The excellent sealing performance can be explained by oxidation of h-BN surface to form liquid B₂O₃ layers beyond the critical temperature of 700 °C. Two of compliant B₂O₃ glass layers were observed on the surface of the h-BN to adjoin well cell and metallic interconnect. The sandwich structure with liquid B₂O₃ layer on two sides of the h-BN seal could not only enhance interfacial adherence but also eliminate the gas leakage paths at SOFC operation temperature. The applicability of the h-BN based seal has also been verified by single cell test.     

Development of ceramic fiber reinforced glass ceramic sealants for microtubular solid oxide fuel cells

Ceramic fibers in various forms with different fiber sizes are tested to improve the sealing performance of glass ceramic seals for microtubular solid oxide fuel cell applications. In this regard, several sealing pastes are prepared by mixing each ceramic fibers type with glass ceramics at 1.25 wt %. Five layered microtubular anode supported cells are also fabricated by extrusion and dip coating methods to evaluate the sealing performance of the composite sealants. The pastes are applied between the cells and gas manifolds made of Crofer22 APU. The electrochemical and sealing performances at an operating temperature of 800 °C under hydrogen are investigated after the glass forming process. Microstructures of the sealants are also examined by a scanning electron microscope. Experimental investigations reveal that the cells sealed by the pastes with ceramic bulk fiber and ceramic fiber rope gasket show acceptable open circuit potentials close to the theoretical one. These cells can be also pressurized up to around 150 kPa back pressure in the sealing performance tests. On the other hand, the pastes without any filler, with ceramic rope and with ceramic blanket exhibit poor sealing performance due to gas leakage originated from flowing of the main glass ceramic matrix from the joints. Therefore, ceramic bulk fiber and ceramic fiber rope gasket are found to behave as a stopper and can be used to prevent glass ceramics from flowing for microtubular solid oxide fuel cells or similar applications.     

Synthesis of ceramic nanotubes using AAO templates

In this paper, the metal aluminum oxide ceramic nanotubes (M–Al–O, M = Zn, Mg and Ba) were synthesized by the use of porous anodic aluminum oxide (AAO) template. Metal oxide nanotubes were firstly formed from the thermal decomposition of respective nitrates within the pores of AAO templates. Then M–Al–O nanotubes were synthesized through the solid-state chemical reaction between the metal oxide nanotubes and AAO. TEM observation shows that the as-prepared samples are nanotubes and XRD measurement reveals that M–Al–O nanotubes (M = Zn, Mg and Ba) are crystalline in nature with spinel structure.     

Optimum processing parameters to improve sealing performance in solid oxide fuel cells

Glass–ceramic composites are among the favorable candidates as a sealing material for solid oxide fuel cells (SOFC). In order to obtain a reliable, robust and hermetic sealing, the glass–ceramics must chemically bond to both the metallic interconnector and the ceramic electrolyte. A high-bonding strength and good wetting, which strongly depend on the thermal treatment, are always preferred to ensure gas-tight sealing. The thermal treatment involves three stages: binder burnout (stage-I), sintering (stage-II), and cooling (stage-III). This study investigates effects of various parameters on the sealing quality at the sintering stage. The effects of sintering temperature, clamping pressure and sealant thickness are considered. The glass–ceramic laminates are produced employing a tape casting method. The sealing quality is evaluated by measuring leakage and final macro-structure of the sealing region. It is suggested that a 900–930 °C sintering temperature and 1.5–7.6 N cm⁻² clamping pressure ranges are better for successful sealing. The initial thickness of glass–ceramic laminates is also desired to be between 0.25–0.5 mm thickness range for both a cost-effective and reliable sealing.     

Sealing Evaluation of an Alkaline Earth Silicate Glass for Solid Oxide Fuel/Electrolyser Cells

Glass is the most recognised material to seal solid oxide fuel/electrolyser cell components. We have developed a SrO‐La₂O₃‐Al₂O₃‐SiO₂ (SABS‐0) based seal glass with all the desired thermophysical properties and thermochemical stability. In this study, the SABS‐0 seal glass is sandwiched between AISI 441 interconnect alloy and fully stabilised ZrO₂ electrolyte for sealing ability evaluation. The sealing performance and the thermal cycling resistance of the tri‐layer assembly are tested by a pressure leakage test with 3–20 K min^–1 heating rates for 2,500 h and 100 thermal cycles. The sealing strength is evaluated by a rupture strength test. The results show that the SABS‐0 glass is hermetic for at least 2,500 h and has high rupture strength for solid oxide fuel/electrolyser cell component sealing. The stress state for the tri‐layer assembly has been analysed using three different approaches.     

Fabrication of glass ceramic sealants with ceramic fiber filler for solid oxide fuel cells

In this study, ceramic fibers are used as a filler material for glass ceramic sealant in solid oxide fuel cells to improve the thermal cycle behavior. Beside the bare glass ceramic sealant for comparison, multilayered sealants with different ceramic fiber contents are fabricated to investigate the effect of ceramic fiber quantity also. The mechanical performances of the samples are measured via tensile tests by placing them between two metallic interconnector plates after the glass formation process as well as after 1, 5 and 10 thermal cycles. The results show that the mechanical strength in general tends to decrease with increasing the ceramic filler content, which can be attributed to poor adhesion due to reduced glass ceramic composition. On the other hand, thermal cycle behavior of the samples with ceramic fibers is found to be improved at some extend. This may be due to the behavior of ceramic filler network and relatively slow crystallization with increasing the amount of the filler as proven by microstructural observations. Especially for the sample including 4 ceramic fiber interlayers each having 0.030 g ceramic fibers, the mechanical strength shows an increasing trend with the number of thermal cycles.     

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.     

Sealing Technology for Solid Oxide Fuel Cells (SOFC)

A variety of seals such as metal–metal, metal–ceramic, and ceramic–ceramic are required for a functioning solid oxide fuel cells (SOFC). These seals must function at high temperatures between 600 and 900°C and in oxidizing and reducing environments of the fuels and air. Among the different type of seals, the metal–ceramic and ceramic–ceramic seals require significant attention, research, and development because the brittle nature of ceramics and glasses can lead to fracture and loss of seal integrity and functionality. This paper addresses the needs and possible approaches for high-temperature ceramic–metal seals for SOFC and seals fabricated using some of these approaches. A new concept of self-healing glass seals is proposed, developed, and used for making metal—glass–ceramic seals for potential application in SOFC in order to enhance the reliability and life of a cell. In this study, glasses displaying self-healing behavior are investigated and used to fabricate seals. The performance of these seals under long-term exposure at higher temperatures coupled with thermal cycling is characterized by leak tests. The self-healing ability of these glass seals is also demonstrated by leak tests along with the long-term performance.     

Thermal expansion coefficient tailoring of LAS glass-ceramic for anodic bondable low temperature co-fired ceramic application

The Li–Al–Si glass-ceramics were prepared by conventional glass-ceramic fabrication method. The influences of Na₂O content on the sintering property, microstructure, and coefficient of thermal expansion were investigated. The results show that the coefficient of thermal expansion of LAS glass-ceramics can be tailored to match that of silicon by the addition of Na₂O content. Na₂O has a remarkable influence on the crystallinity of Li–Al–Si glass-ceramic. The coefficient of thermal expansion of Li–Al–Si glass-ceramic is thus tunable between that of glass phase and crystal phase. The Si–O bond length change in stretch vibration modes introduced by Na₂O also contributes to the variation of coefficient of thermal expansion of the Li–Al–Si glass-ceramics. The coefficient of thermal expansion of the Li–Al–Si glass-ceramic with 1.5 wt% Na₂O addition is about +3.34 ppm/°C at 350 °C and shows a good compatibility to that of silicon in a wide temperature range, which makes it a promising candidate for anodic bondable low temperature co-fired ceramic substrate applications.     

Low leakage rate of silicate glass modified with Al2O3 for solid oxide fuel cell

The gas tightness of glass sealing materials is a big challenge for the solid oxide fuel cell (SOFC) stacks operating at high temperature. Thermal, sintering, crystallization behavior and gas tightness properties of the glass-based with two different Al₂O₃ contents sealants are evaluated and discussed. The study showed that the sealants avoid cracks at the interface on NiO-YSZ (NiO-yttria stabilized zirconia) and SUS430 stainless steel interconnect substrates. The Al₂O₃ embedded in the glass matrix as a second phase, and promoted crystallization of K[AlSi₃O₈] at the early stage. This may because some ultrafine Al₂O₃ particles whose structure is destroyed by prolonged high temperature treatment according XRD and TEM analysis. Especially, the sealant containing 5 wt% Al₂O₃ undergoes a thermal cycle and maintains a stable leakage rate below 10^?4 sccm?cm^?1 for about 1000 h at 750 °C. The above results prove the possibility of using the Al₂O₃-doped sealing glass for SOFC stacks.     

Novel Compressive Mica Seals with Metallic Interlayers for Solid Oxide Fuel Cell Applications

A novel hybrid compressive mica seal was developed that showed an exceptionally low leak rate of ∼8.9 × 10⁻⁴ sccm/cm (standard cubic centimeters per minute per unit leak length of seal) at 800°C and a compressive stress of 100 psi, about 740 times lower than that of the conventional compressive mica seals (6.6 × 10⁻¹ sccm/cm), at a pressure gradient of 2 psi. The hybrid compressive mica seals were composed of two compliant metallic layers and a mica layer. Three commercially available micas were tested in this study. All showed substantial improvements in reducing the leak rates by using the hybrid design. The best results were obtained using muscovite single-crystal mica and 125 μm silver layers. Using the paper form of muscovite and phlogopite mica, the leak rates were still far superior (∼1 × 10⁻¹ sccm/cm) compared with mica without the compliant silver layer (about 6–9 sccm/cm). The microstructure of the mica was examined before and after the 800°C leak tests. These results are compared with results for hybrid seals using glass interlayers. In addition, an explanation for the substantial reduction of the leak rates is proposed, and the application of the hybrid compressive mica seals in planar SOFC stacks is briefly addressed.     

Sintering and thermal expansion behaviors of glass and glass–YSZ composites as self-repairable seals for SOFC

Glass and glass–ceramics are used as sealants in solid oxide fuel cell (SOFC) because their thermophysical properties can be tailored to meet the stringent requirements of the SOFC stack. The processing, sintering, and thermal expansion behaviors of self-healing and non-crystallizing glass and glass containing 10%–30 wt.% non-reacting yttria-stabilized zirconia (YSZ) are studied. The addition of inert YSZ to glass significantly retarded the sintering behavior. Thermal expansion behaviors of glass and glass–YSZ are also measured to study the role of YSZ addition on the glass transition, softening point, and coefficient of thermal expansion (CTE). It is shown that the densification is controlled by the viscous sintering mechanism, in which the addition of YSZ increased the effective viscosity of the glass–YSZ as evident from higher glass transition and softening temperatures and decreased CTE. These results demonstrated that the addition of YSZ to glass is promising for achieving optimum thermophysical properties useful as seals for SOFC.     

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 the crystalline layer on the electrical behaviour of 17.7Li2O·5.2ZrO2·68.1SiO2·9.0Al2O3 glass ceramic monoliths

Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass ceramic systems are usually obtained from powder technology to obtain materials with a low thermal expansion coefficient (CTE). However, in these cases, there is a high residual porosity. An alternative to reduce the porosity involves the production of monoliths. Nevertheless, there is still a lack of crystallisation kinetics and the final properties of glass ceramic monoliths are affected such as electrical properties. This study aims to evaluate the electrical behaviour as function of the crystalline layer thickness formed on the monolith surface of a 17.7Li₂O·5.2ZrO₂·68.1SiO₂·9.0Al₂O₃ (molar basis) glass ceramic LZSA composition. Monoliths thermally treated at 750, 800, and 850 °C were chosen to evaluate based on the range of the crystalline layer growth. Electrochemical impedance spectroscopy was used for the electrical characterisation of LZSA glass and the glass ceramics. The resistivity increased with increasing thermal treatment temperature due to the formation of lithium-based crystalline phases. The electrical conductivity at 25 °C of the glass ceramic thermally treated at 850 °C decreased to 1.4 × 10^?13 S cm^?1 from 8.7 × 10^?11 S cm^?1 for LZSA glass. Based on the electrical behaviour, monoliths thermally treated at 850 °C can be considered potential for dielectric industrial applications.     

Measurement of Mechanical Strength of Glass‐to‐Metal Joints

Three different test geometries were used to apply shear loading to fracture glass‐to‐metal joints typical of seals intended for use in planar solid oxide fuel cells (SOFCs): asymmetric compression; symmetric compression; and four‐point asymmetric bending. The measured apparent shear strengths were found to differ by an order of magnitude depending on the test configuration employed. In particular, the apparent shear strength measured in the asymmetric compression test was very low. Conversely, the highest apparent shear strengths were measured using the symmetric compression test and the four‐point asymmetric bend test gave an intermediate result. It is shown, by finite element modelling, that these differences are caused by differences in the normal stresses transverse to the joint. The locus of failure was always along the glass/metal interface in all test geometries. It is concluded that mechanical test procedures used to characterize glass‐ceramic seals in SOFC stacks need to be selected and interpreted with great care.     

Comparative Study of the Leak Characteristics of Two Ceramic/Glass Composite Seals for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have the potential to play a significant role in a future clean energy economy. However, SOFCs still face major obstacles before they can be commercialized, with efficient sealing being among the most prominent. The present research focuses on the comparative study of microstructure, crystal phase evolution, and leak rates, for two ceramic/glass seals used in an SOFC. The leak test apparatus is a controlled facility designed to incorporate different mechanical loading, stack configurations, and thermal cycles. Simultaneous leak testing with an acoustic emission (AE) sensor was also used to identify any micro‐damage in seals. A two‐level factorial design was applied to the first sealing composition to identify the main and the interactive factors for leak rates. MINITAB^® was also used to determine a linear regression‐based leak rate model. The second seal formulation employed a more stable glass which led to reduced leak rates. Additional factors in a two‐level factorial design were investigated for the second seal formulation. Based on multiple experiments with different stack components, it was determined that the number of interfaces is most critical for leak rate, showing that even in the presence of thermal cycling, leakage is an interfacial dominated phenomenon.     

New glass and glass–ceramic sealants for planar solid oxide fuel cells

This work describes the design of three new glass and glass ceramic compositions to join the ceramic electrolyte (YSZ wafer) to the metallic interconnect (Crofer22APU) in planar SOFC stacks. The designed sealants are low-sodium, barium free and boron-oxide free silica-based glasses.The sealing process was optimized for the most promising composition and joined Crofer22APU/glass–ceramic sealant/YSZ samples were morphologically characterized and tested for 300 h in humidified hydrogen atmosphere at the fuel cell operating temperature of 800 °C. The study showed that the use of the glass–ceramic was successful in joining the YSZ ceramic electrolyte to the Crofer22APU metallic interconnect and in preventing severe corrosion effects at the Crofer22APU/glass–ceramic interface after static treatments in humidified hydrogen at 800 °C for 300 h.     

Effect of SnO–P2O5–MgO glass addition on the ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte

NASICON-type structured compounds Li_1+xM_xTi_2-x(PO₄)₃ (M = Al, Fe, Y, etc.) have captured much attention due to their air stability, wide electrochemical window and high lithium ion conductivity. Especially, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) is a potential solid electrolyte due to its high ionic conductivity. However, its actual density usually has a certain gap with the theoretical density, leading the poor ionic conductivity of LATP. Herein, LATP solid electrolyte with series of SnO–P₂O₅–MgO (SPM, 0.4 wt%, 0.7 wt%, 1.0 wt%, 1.3 wt%) glass addition was successfully synthesized to improve the density and ionic conductivity. The SPM addition change Al/Ti–O bond and P–O bond distances, leading to gradual shrinkage of octahedral AlO₆ and tetrahedral PO₄. The bulk conductivity of the samples increases gradually with SPM glass addition from 0.4 wt% to 1.3 wt%. Both SPM and the second-phase LiTiPO₅, caused by glass addition, are conducive to the improvement of compactness. The relative density of LATP samples increases first from 0 wt% to 0.7 wt%, and then decreases from 0.7 wt% to 1.3 wt% with SPM glass addition. The grain boundary conductivity also changes accordingly. Especially, the highest ionic conductivity of 2.45 × 10^?4 S cm^?1, and a relative density of 96.72% with a low activation energy of 0.34 eV is obtained in LATP with 0.7 wt% SPM. Increasing the density of LATP solid electrolyte is crucial to improve the ionic conductivity of electrolytes and SPM glass addition can promote the development of dense oxide ceramic electrolytes.     

Influence of crystallization inside glass frit on seal stress in ceramic metal halide lamps

Ceramic metal halide lamps use polycrystalline aluminum oxide as an arc tube material; cracks inside the glass frit—used as the seal material—have been known to occur occasionally. This study measured the stress on the lamp seals caused by changes in the cooling rate during the sealing process by a 2D stress measurement method. Seal stress decreased with reducing cooling rate. Therefore, we discuss the influences of the glass frit's microstructure and the coefficients of thermal expansion (glass frit, capillary, and Nb wire) on the seal stress. The coefficient of thermal expansion of the annealed glass frit was essentially closer to those of the capillary and Nb wire, while that of the rapidly cooled glass frit differed greatly. Moreover, the glass frit of the rapidly cooled lamp seal contained only an amorphous phase (Dy, Si, Al, and O), while the glass frit of the annealed lamp seal contained both an amorphous phase (Dy, Si, Al, and O) and a crystalline one (Dy₂SiO₅ and Al₂O₃). Fracture toughness was found to be larger in the crystals than in the amorphous phase area. Moreover, it was larger in the area where crystalline Dy₂SiO₅ and Al₂O₃ were present compared to the area where only crystalline Dy₂SiO₅ was present.     

Modification of glass network and crystallization of CaO–Al2O3–MgO–SiO2 based glass ceramics with addition of iron oxide

Modification of glass network and crystallization process of a CaO–Al₂O₃–MgO–SiO₂ (CAMS) based glass ceramic to form diopside through addition of iron oxide were investigated using differential thermal analysis (DTA), Raman spectrum, X-ray diffraction, SEM and EBSD techniques. The experimental results showed that addition of Fe₂O₃ led to remarkable reductions in both the glass transition temperature (Tg) and crystallization temperature (Tp) of the CAMS glass ceramic. At addition level below 5 wt%, the Tg and Tp temperatures were 651°C and 903°C, respectively, and the crystallization only occurred on the surface of the glass ceramic samples. Increasing the addition level to 10 wt% and 15 wt%, not only led to reduction in the Tg and Tp temperatures to 643-641°C and 892-876°C, respectively, but also promoted the formation of crystalline diopside throughout the CAMS samples. Based on the results of Raman spectrums, it was confirmed that Fe₂O₃ addition reduced the strength of glass connection as a result of chemical reactions between the isolated Si–O tetrahedron and Fe³⁺ ion, forming Fe³⁺O₄–SiO₄, which can be regarded as Q₂ unit. And this is the first experimental evidence that proving the approach of Fe³⁺ mending glass network. Microstructural examination also identified the formation of large numbers of spherical Fe-enriched regions within the CAMS glass matrix as a result of the amorphous phase separation due to the Fe₂O₃ addition. The interfaces between the Fe-enriched regions and the glass matrix acted as preferred nucleation sites for the diopside, facilitating the crystallization. Crystallographic analysis using EBSD technique determined the <001> as the most favorite growth direction for the diopside crystals in the CAMS based glass ceramic.