This title appears in the Scientific Report :
2008
Please use the identifier:
http://hdl.handle.net/2128/3613 in citations.
Nickelreaktivlot / Oxidkeramik-Fügungen als elektrisch isolierende Dichtungskonzepte für Hochtemperatur-Brennstoffzellen-Stacks
Nickelreaktivlot / Oxidkeramik-Fügungen als elektrisch isolierende Dichtungskonzepte für Hochtemperatur-Brennstoffzellen-Stacks
The BMW Group and partners are developing a solid oxid fuell cell (SOFC) as an auxiliary power unit (APU) in order to make the electrical power supply of future car generations more efficient and environmentally friendly. Requirements like a high power density, a start-up-time of less than five minu...
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Personal Name(s): | Zügner, Stephan (Corresponding author) |
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Contributing Institute: |
Werkstoffsynthese und Herstellungsverfahren; IEF-1 |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
2008
|
Physical Description: |
136 S. |
Dissertation Note: |
Bochum, Univ., Diss., 2008 |
ISBN: |
978-3-89336-558-6 |
Document Type: |
Book Dissertation / PhD Thesis |
Research Program: |
Rationelle Energieumwandlung |
Series Title: |
Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment
30 |
Subject (ZB): | |
Link: |
OpenAccess |
Publikationsportal JuSER |
The BMW Group and partners are developing a solid oxid fuell cell (SOFC) as an auxiliary power unit (APU) in order to make the electrical power supply of future car generations more efficient and environmentally friendly. Requirements like a high power density, a start-up-time of less than five minutes and a high economical pressure pose a big challenge especially for the sealings. Being brittle, the established glass ceramics are less promising for aggregates put under high stress through fast temperature changes. Thus alternatives are sought for. It is a potential concept to use Reactive Air Brazing (RAB) of planar, anode supported cells with metallic interconnectors by Ag-CuO. This approach, however, fulfills the requirement to join the interconnectors in a hermetically and also electrically isolated way only partly. Hence, concepts based on it could not provide an acceptable likelihood to meet the demands of a SOFC-APU. To overcome these drawbacks, the aim of this work was to produce an alternative, highly resistant sealing concept by reactive metal brazing in combination with an oxid ceramic electrical isolating layer and to examine its behaviour under near service conditions. Fundamental research proved nickel brazes based on the standardized alloy Ni102 (Ni Si B Cr Fe) to be particularly suitable. Firstly, they insist the thermal and corrosive requirements, secondly, they can be brazed under 1100°C in inert as well as reductive atmospheres and thirdly, their coefficient of thermal expansion (cte) matches well the one of the cells and the chosen chromium ferritic interconnector. Magnesia (MgO) was chosen as element for electrical isolation because of its likewise good fitting cte and its negligible oxygen ion conduction. It was used in form of a self supported sintered foil (thickness 200 μm) or EBPVD-coating (Electron Beam Physical Vapour Deposition; thickness 6 to 24 $ \mu$m). The development and analysis of different ways to activate the nickel-brazing-alloy by addition of titanium or zirconium to join the Magnesia ceramics was the main focus of this study. In combination with a self-supported MgO-substrate, joints of real SOFC-interconnects (joining area 26 cm$^{2}$) which are homogenous, resilient and precise to contour could be fabricated under a high vacuum with Ni102+TiH$_{2}$-, Ni102+ZrH$_{2}$-pastes or by layering of Ni102 & CuTi and Ni102 & CuZr by a double screen printing. Through the joining process, they obtain an electrical area specific resistance of 2 x 10$^{7}$ $\omega$*cm$^{2}$ at 800°C and a gas leakage below 0.3 ml/min at a pressure of 200 mbar. With MgO-coatings by EBPVD, comparable joints with better mechanical stability and lower gas leakage could be produced. Due to corrugations resulting from the rolling of the steel sheets, a short circuit by infiltrated braze can not be ruled out if a simple 6 $\mu$m coating is applied. Improvements are provided by multi layer coatings with “braze infiltration stopping”, a mechanical polishing of the substrates and the application of preoxidized Aluchrom HfAl - substrates. With test specimens an area specific resistance of at least 2 x 10$^{4} \omega$*cm$^{2}$ at operating temperature was finally achieved. There is a well-defined formation of a Mg-Ti-O-reaction zone by using titanium-nickel reactive brazes whereas Ni-Si-Zr-phases in ceramic closeness without forming a clear reaction zone are characteristic with zirconium activated systems. This leads to a lower mechanical stability of joints brazed by zirkonium-nickel but also to a decreased sensitivity in atmospheres with higher oxygen content and with an overdose of the reactive element. Under SOFC-conditions the investigated brazing systems form a protecting chromia-layer up to a reactive element content of 5 wt.% and a copper content of 14 wt.% in the braze so that there are good prospects for a long time stable SOFC operation. Due to the promising results, work at an evaluation of the developed sealing in SOFC-APU-stacks is in progress. |