Room and elevated temperature shear strength of sealants for solid oxide fuel cells

The shear strength of glass-ceramic sealants was measured in torsion to aid the developments of robust sealants that can withstand the combined complex stress situation typical for solid oxide fuel cell stacks. The specimens consisted of hour-glass-shaped samples, with the sealant layer in typical application relevant thickness between two steel plates. Partially crystallized sealant materials with either silver particles or 8YSZ fibers as filler material were tested and compared with respect to their shear strength and failure behavior at room and elevated temperatures up to 800 °C. The results emphasize the importance of interfacial bonding as well the effect of creep properties of sealant and steel substrates onto the testing results. An outlook on improvements of testing procedure and specimens’ geometry is given.     

Simulation of a high temperature electrolyzer

Based on Solid Oxide Fuel Cell (SOFC) technology, Solid Oxide Electrolysis Cell (SOEC) offers an interesting solution for mass hydrogen production. This study proposes a multiphysics model to predict the SOEC behavior, based on similar charge, mass, and heat transport phenomena as for SOFC. However, the mechanism of water steam reduction on Nickel/Yttria-Stabilized Zirconia (Ni/YSZ) cermet is not yet clearly identified. Therefore, a global approach is used for modeling. The simulated results demonstrated that a Butler–Volmer’s equation including concentration overpotential provides an acceptable estimation of the experimental electric performance under some operating conditions. These simulations highlighted three thermal operating modes of SOEC and showed that temperature distribution depends on gas feeding configurations.     

Effect of Crystallites on Surface Damage and Fracture Behavior of a Glass-Ceramic

A study was conducted of the effect of crystallization on the fracture toughness, strength, and resistance to surface damage of glass-ceramic materials with a range of microstructures obtained by different heat treatments. The hardness indentation method was used as a quantitative tool to simulate mechanical surface damage. In the uncrystallized glass and in the glass-ceramic heat-treated to result in a uniform fine-grained structure, crack size increased monotonically with indentation load. In contrast, in the glass-ceramics heat-treated to result in a microstructure consisting of larger crystallites (a few micrometers) contained within a fine-grained matrix, a discontinuity in the crack size vs load curve presented evidence for crack-pinning at crack sizes which were a small multiple of the intercrystallite spacing. At the position of crack-pinning, the fracture toughness showed a discontinuous increase with increasing crack size that was attributed to crack deflection. The strength of the glass and fine-grained glass-ceramic measured in biaxial flexure decreased monotonically with indentation load. The strength at low values of indenter load of the glass-ceramic heat-treated to yield the coarser crystallites within the fine-grained matrix was independent of indentation load, indicating stable crack propagation prior to fast fracture. At the higher values of indenter load, the coarse-grained glass-ceramics exhibited a monotonic decrease in strength with increasing indentation load. The results of this study indicate that the strengthening observed on crystallization of a glass can be attributed to a combination of a decrease in flaw size achieved at a given mechanical surface treatment, an increase in fracture toughness, and a modification in the mode of crack propagation.     

Mechanical properties of solid oxide fuel cell glass-ceramic sealants in the system BaO/SrO-MgO-B2O3-SiO2

Planar solid oxide fuel cells (p-SOFCs) require materials that can satisfy the high mechanical demands related to their utilization in stationary and, especially, in mobile applications. Two suitable glass-ceramic sealants based on the system BaO/SrO-MgO-B₂O₃-SiO₂ have been characterized with respect to their mechanical properties such as hardness, Young’s modulus, flexural strength at room and elevated temperature, fracture toughness as well as creep behavior at relevant operation temperatures (800 °C). Fracture toughness was calculated from crack opening displacements (COD) and the results were compared with fracture toughness measured by bending tests of notched bar samples. The mechanical behavior has been discussed regarding different thermal aging times of the glass-ceramics and their microstructural evolution. The glass-ceramics containing SrO revealed a better mechanical behavior than glass-ceramics with BaO. In particular, several superior properties were found in comparison to previously reported materials for this application.     

阻燃型中空玻璃密封胶条的研制

用氢氧化镁作为阻燃剂制备中空玻璃用密封胶条。考查了氢氧化镁的粒径、用量对密封胶条力学性能及阻燃性能的影响。实验结果表明,密封胶条的剪切强度随氢氧化镁粒径的增大而减小;氢氧化镁粒径为6μm时可塑度达到最小值,氧指数(LOI)达到最大值;密封胶条的剪切强度、氧指数和平衡转矩均随氢氧化镁含量的增加而增大;氢氧化镁含量为250份时可塑度达到最小值。由此可见,当氢氧化镁粒径为6μm、添加量为250份时,密封胶条的综合性能最好。     

木粉填充中空玻璃密封胶条的研制

为降低中空玻璃密封胶条的生产成本,用不同粒径及表面处理的木粉填充再生丁基橡胶来制备密封胶条。力学性能测试结果表明,密封胶条的强度随木粉粒径的增大先增大后减小。红外光谱分析说明,经表面处理后的木粉中含有硬脂酸,木粉表面的羟基可能与硬脂酸发生了酯化反应;密封胶条的剪切强度随处理剂浓度的增大而减小,可塑度先减小后增大;剪切强度和平衡转矩随木粉含量的增加而增大。因此,采用粒径为180μm、3％的硬脂酸乙醇溶液处理的木粉填充制备的密封胶条性能较好,且木粉可以大量填充。     

Development and characterization of inks for screen printing of glass solders for SOFCs

Glass ceramics are supposed to produce stable sealants between ceramic electrolyte and ferritic steel in planar SOFC stacks. The screen printing method is a suitable industrial process in order to produce layers with tailored height and width reproducibly. An appropriate suspension was developed for this application and improved by adjusting the additive composition. This study investigates the rheological behavior of screen printing inks and the corresponding topography of the printed layers. Rotary measurements were carried out to analyse the impact of shear rates on viscosity showing shear thinning behavior. In addition, oscillatory rheometry is performed to gain information about the viscoelastic properties of the slurries and to understand the internal structure of the developed inks. The viscoelasticity and the recovery behavior after shear loading are crucial for the topography of printed films. The principal investigations are executed on solvent-based suspensions, whereby the knowledge gained is used for a transfer to water-based systems allowing a more environmentally friendly handling.     

喷涂型汽车油箱点焊密封胶的研制

从油箱喷塑固化工艺条件出发,以环氧树脂(EP)和小分子聚酰胺为主要原料,制备了两种不同配方的汽车油箱用点焊密封胶。测试了点焊胶在90#汽油和0#柴油中的耐油性和粘接强度,并对三种不同连接接头(胶接、点焊、胶焊)的粘接强度进行了对比和分析。结果表明:点焊胶的固化工艺与汽车镀锌板类油箱表面漆层的固化工艺相同,故不必增加点焊胶的固化设备和工序;两种点焊胶具有较好的粘接性能,其T型剥离强度分别为89.0kN/m和92.0kN/m,拉伸剪切强度分别为13.3MPa和12.9MPa;两种点焊胶在90#汽油和0#柴油中浸泡360d后,试样的粘接强度降幅不明显,说明其具有一定程度的耐油性;将两种点焊胶用于汽车油箱的密封中,既具有良好的粘接密封性能,又解决了镀锌板类油箱渗漏油等问题。     

不同冲击条件下微晶玻璃涂层冲蚀应力的有限元模拟

材料表面应力状态对材料的冲蚀磨损行为起着重要作用。本文利用有限元方法探讨了冲蚀角度、冲蚀速度、磨料粒径对微晶玻璃涂层冲蚀应力的影响,利用常温冲蚀试验机对微晶玻璃涂层在不同冲蚀角度下的体积磨损率进行了测定。结果表明,随冲蚀角度、冲蚀速度、磨料粒径增大,涂层的冲蚀应力均逐渐增大。在相同的冲蚀角度和冲蚀速度条件下,磨料粒径对涂层冲蚀应力的提升效果显著。微晶玻璃涂层冲蚀磨损率随冲蚀角度的增大而增加,其变化规律与冲蚀应力变化趋势基本一致,从而验证了有限元应力模拟的可靠性。     

Effects of solid loading on joining and thermal cycling performance of glass-ceramic sealing pastes for solid oxide fuel cells

The variation of the joining performance of glass-ceramic sealants in the form of a paste as a function of the solid powder content in the sealing paste after the formation and a number of thermal cycles are experimentally studied. Three different sealing pastes having 40, 50 and 60 wt % solid loadings are prepared and tested for this purpose. The pastes are applied between two metallic interconnector plates and subjected to a glass formation step for the joining. The fracture strengths of 24 samples prepared for each case are determined via tensile tests. Similarly, the mechanical performances of the sealants after 3, 6 and 9 thermal cycles are also obtained. The results reveal that the joining strength tends to increase with the amount of solid powder content in the paste. This can be attributed to increased number of crystalline phases in the sealants with increasing the solid loading. The thermal cycles, on the other hand, are shown to have an adverse effect on the joining performance regardless of the solid loading. However, the rate of decrease in the fracture strengths is found to decrease with the solid powder contents in the pastes. This can be elucidated by the amount of glassy phases in the sealants, which can be expected to increase with the solid loading and provide self-healing ability. The microstructures of the fracture surfaces of all samples are also investigated by a scanning electron microscopy. The obtained images confirm the tensile test results.     

有机硅密封剂高温力学性能研究

在温度25~250℃对3类硫化体系的5种有机硅密封剂的拉伸强度、180o剥离强度和剪切强度的研究表明:随着温度的升高,5种有机硅密封剂的拉伸强度、180o剥离强度和剪切强度均不断下降,其中拉伸强度和剪切强度降低的速率递减。有机硅密封剂的高温性能变化与硫化体系类型密切相关,脱氢硫化体系HM321密封剂在25~150℃拉伸强度、剪切强度和180o剥离强度均最高,但在25~250℃拉伸强度、剪切强度和180o剥离强度保持率均最低。脱氨硫化体系XY-602S有机硅胶粘剂在232~250℃的拉伸强度和剪切强度最高,25~250℃的拉伸强度保持率也最高。     

Effect of Crystallization on Thermal Shock Behavior of a Chain-Silicate Canasite Glass-Ceramic

The resistance of a canasite glass-ceramic to the initiation of thermal stress fracture due to a water quench was found to be higher than for the original glass, due to higher values of strength and thermal conductivity which offset increases in thermal expansion and Young's modulus. Relative strength retention behavior of the glass-ceramic was also higher than for the glass, attributed to its crack-size-dependent fracture toughness.     

有机硅密封胶的结构和性能（Ⅱ）

有机硅密封胶广泛应用于工业领域的粘接密封，为了系统地研究有机硅密封胶的结构-性能，采用甲基三甲氧基硅烷等4种不同结构的交联剂，设计合成了4种有机硅密封胶。通过测定4种不同结构的有机硅密封胶的拉伸强度、剪切强度和剥离强度，并采用线性回归方法研究了交联剂对有机硅密封胶的力学性能、粘接性能的影响。结果表明，交联密度与密封胶的硫化胶的拉伸强度有很好的线性相关性，交联密度是影响密封胶粘接强度和粘接功的重要因素。     

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.     

Effect of filler on the compression set,compression stress–strain behavior,and mechanical properties of polysulfide sealants

In this work, investigations were made on the mechanical properties, stress–strain behavior during compression, swelling and compression set properties of polysulfide sealants at different carbon black and silicon dioxide loadings, and dynamic mechanical thermal analysis was also presented. The results reveal that carbon black filler indeed has significant effects on reinforcing mechanical properties of polysulfide sealants. Increasing carbon black loading improves the tensile strength of sealants promptly, but compression performance increases slowly. The simultaneous use of carbon black and silicon dioxide filler in polysulfide sealants hardly changes the tensile strength of sealants, whereas the ultimate elongation and compression performance of sealants are enhanced remarkably. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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 Single-Point Diamond Scratching Behavior of a Cordierite Glass and Glass-Ceramic

A comparative study was conducted of the single-point diamond scratching response of a cordierite glass and glass-ceramic. For the glass, deformation and material removal occurred by viscous flow at the lowest values of load, viscous flow combined with crack formation adjacent to the contact area over the intermediate load range, and extensive spoiling at the highest values of load. The density of the spalls and spall size were found to be proportional and inversely proportional to perpendicular scratching load, respectively. The glass-ceramic exhibited plastic flow and abrasive wear at loads as low as could be reliably measured, the particle size of the debris being of the order of the grain size. At the highest ranges of load, the glass-ceramic exhibited material removal by spall formation with a spall size proportional to load. At the lower ranges of load, the widths of the scratch for the glass and glass-ceramic were comparable. However, at the highest values of load tne size of the spall of the glass-ceramic was significantly less than the size of the spall in the glass.     

Study of interfaces of high-performance glass fibers and DGEBA-based epoxy resins using single-fiber-composite test

The single-fiber-composite (SFC) technique was used to study the interfacial behavior between two flexible blends of diglycidylether of bisphenol A (DGEBA)-based epoxy and polyglycol epoxide and three glass fibers. Dog-bone-shaped SFC specimens were made and strained to obtain a distribution of fragment lengths. The fibers were tension-tested at two different gauge lengths. The fragment length distributions, the fiber strength data, and a Monte Carlo simulation of a Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results show that the interface does not fail. Instead, penny-shaped transverse cracks appear at every fiber break and grow as the specimen is strained. The interfacial shear strength values are many times higher than the yield shear strength values of bulk epoxy obtained from the tension test.     

Fracture Strength and Mirror Size in a Commercial Glass- Ceramic

Extensive observations on the strength-fracture mirror size relation for a commercial cordierite-based glass-ceramic indicate a systematic deviation from the expected behavior at high strength/small mirror sizes. Calculations of internal stresses from phase properties suggest that the observations result from direct effects of internal stresses on fracture-mirror formation or from indirect effects of internal stresses affecting the fracture toughness and hence fracture-mirror size.     

纳米CaSO_4晶须改性聚氨酯密封胶的制备

通过三种聚醚制成的密封胶进行对比,具体考察了CaSO4晶须对密封胶力学性能的影响。结果表明,在单组分湿固化聚氨酯密封胶中添加纳米CaSO4晶须后,密封胶的剪切强度得到提高,硬度和弹性变化不大。使用CaSO4晶须替代气相二氧化硅,在降低成本的同时,密封胶的触变性仍然很好。