Laser cladding of glass-ceramic sealants for SOFC
Introduction
Solid oxide fuel cells (SOFCs) have the capability of directly convert chemical energy into electricity. SOFCs are the most efficient technology to make this conversion with efficiencies in the range of 45–65% that increase up to 85% in cogeneration or combined heat and power (CHP). The possibility of providing a wide range of power (100 W–1 MW) as a consequence of their modular structure and the different designs, the fuel flexibility from the use of conventional fuels (natural gas and gasoline) to future alternative fuels (H2 and biofuels) and the lower pollution of SOFCs, make these devices very attractive for the energy sector [1], [2], [3].
Despite of some reduction of operation temperature during the last decade [1], the production cost of the units are still too high to be cost-effective. Among the problems of the SOFCs are the reliability of the materials and long-term stability. SOFC planar designs (p-SOFC) are the best known because of its high specific power output, low electrical resistance and high volumetric power density [4].
Manufacturing of SOFC requires a joining process to obtain a gastight and electrically insulating joint between generally two metallic parts (window frame and interconnect) and to separate cathode and anode in an anode-supported p-SOFC in order to stack a serial repeat unit. The most common materials employed for this application are glass-ceramic (GC) sealants because of their good mechanical and electrical insulation properties and the possibility to use a wide range of chemical compositions to control some of its properties. However, these materials give rise to cracks in the seal during thermal cycles (especially important for light-stacks in portable applications) or thermal gradients own to their inherent fragility. This may cause leaks, with a drop of stack performance [5]. There are some innovative approximations to improve this behavior such as the use of seals composed of layers, glass seals with self-healing properties [6] and glass and glass-ceramic seals reinforced with particles [7], [8].
Generally, for the implementation of the seal a paste obtained from glass powder mixed with alcohol and a binder is prepared and applied to the parts to be sealed; after that the entire stack is heated up at temperatures between 800 and 1000 °C in an electrical furnace during some hours [4], [6]. An alternative to save production cost is a sealing time reduction. For this purpose, the laser joining technology could offer a huge reduction of time process, from hours to minutes. Other advantages of this approach are an accurate and localized energy (without the need of heating up all the stack) due to the high power density of the laser and an improvement of the glass wetting during joining due to the higher temperatures (which allows obtaining a better adhesion) but there are also some disadvantages like the appearance of bubbles and cracks due to the thermal stresses and the lack of a mechanism to reduce these stresses [9]. The repair of un-tight stack is another interesting application of the laser joining technology employing higher temperatures in this case to re-melt the glass-ceramic.
Laser technology has been widely used for the improvement of the joining of different materials [10], [11], [12], [13]. Two recent publications [9], [14] describe this new approach for sealing SOFCs. In the first publication the authors have employed three types of laser (Nd:YAG, CO2 and Diode laser), concluding that the best laser for the glass-melting is the CO2 laser but the best joining results were obtained using Nd:YAG [9]. In the second study the repair concept for un-tight stack was explored, noticing that this approximation is possible and the reparation process is influenced by the capillarity force of the residual glassy phase of the sealant inside the cracks [14].
In the present work, a CO2 laser was employed to obtain stable joints introducing some variables in the laser process to improve the adhesion. In order to achieve a high sealing quality several properties have been taken into account such as: laser radiation absorption, fitting of coefficient of thermal expansion (CTE) and an accurate wetting between glass and steel. After laser cladding, different properties of the seal have been studied to observe the influence of laser cladding in the composition, structure and crystallization of the seal. Due to the high thermal differences during laser process, another objective has been to evaluate residual stresses of glass-ceramics/metal joints in view of optimizing its overall mechanical performance. The (hkl)-dependence of the stress state has been determined by peak shift measurements of an unstressed piece of Crofer22APU steel compared with the steel covered with glass after laser cladding process with and without further thermal treatment.
Section snippets
Materials and characterization techniques
The glass composition was designed in the ternary system SiO2–BaO–MgO with additions of B2O3; this system has previously shown suitable properties for sealing applications [15]. The boron content was adjusted to the minimal amount to increase the wettability and ensure a good bonding with the interconnect material [16]. The selected composition was (in mol %): 47.5SiO2–27BaO–18MgO–7.5B2O3 (labeled 7.5B(Ba)). The raw materials used for the preparation of the glasses were: silica sand, BaCO3
Thermal characterization of the glass
The chemical composition was analyzed by X-ray Fluorescence Spectroscopy (XRF) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), the results are shown in Table 3. The experimental results after furnace melting show some impurities of SrO, K2O and Na2O which came from the raw materials and melting process. After laser cladding a new impurity appears (Fe2O3), Fe from the diffusion of Crofer22APU. It has to be noted also a small decrease in the boron content after laser
Conclusions
Laser joining technology was employed for the sealing of glass-ceramic/interconnect joints for their use in SOFC. The selected glass, a barium magnesium silicate composition with additions of boron oxide showed suitable properties to be employed as seal such as appropriate CTE and crystallization temperatures in the range of SOFCs working temperatures. Good adhesion of the GC/Crofer22APU joints was observed after laser joining as well as good stability against thermal aging and thermal cycling
Acknowledgments
The authors thank the project FP7-JTI-CP-FCH, Working towards Mass Manufactured, Low Cost and Robust SOFC stacks (MMLRC = SOFC), project reference: 278525, the research proposal XRD1-16163-Brazilian National Synchrotron Light Laboratory (LNLS), projects CNPq/PVE 400590/2013-1 and CNPq/PVE 313555/2013-3 for financial support and C-LABMU/UEPG for the use of research facilities. S. Rodríguez-López thanks the financial support given by the JECS Trust program of the European Ceramic Society through
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