NEW DEVELOPMENTS IN VACUUM PLASMA SPRAYING OF INSULATING AND SEALING LAYERS FOR SOFC

The potential of metal supported high temperature solid oxide fuel cells (MS-SOFC) as power supply is undoubtedly high. Attributes like high efficient and environmental friendly conversion of different fuels from chemical to thermo-electrical energy and an excellent stability concerning red-ox and thermal cycling make them, in particular, an interesting candidate for mobile applications (e.g. as auxiliary power unit (APU) in vehicles or aircrafts). APU requires low weight and accordingly a high power to weight ratio. On this account plasma spraying provides the opportunity to be used in different areas of application in SOFC fabrication. Besides functional layers (electrodes, electrolyte) insulating and sealing coatings to assemble single MS- SOFC units to stacks can be made by thermal spraying. Current studies and new developments of the latter will be presented in this paper

Vacuum plasma sprayed insulating layers suitable for brazing in high temperature fuel cells

SOFCs for mobile applications require short starting times and capability of withstanding several and severe cycles. For such applications metallic cassette type cells with low weight and thermal capacity are beneficial where the active cell part is set in interconnects consisting of two sheets of ferritic steel. These cells are stacked serially to get higher voltage and power. This approach needs interconnect sheets that are electrically insulated from each other to prevent electrical short circuit. The technology discussed here is to use brazed metals, as sealants, and ceramic layers, as electrical insulators, which are vacuum plasma sprayed on the cassette rims. For reliable insulating layers, a variety of deposits were developed, starting from cermet-spinel multilayers with various compositions and constituents, where reactive metals (such as Ti, Zr) were part of the coatings, to pure ceramic layers. The qualities and characteristics of these coatings were investigated which included electric insulation at room temperature and at 800 °C (SOFC operating temperature), wettability of different brazes towards these deposits, phase stability and peeling strength. The single steps of development, characteristics of the insulating layers for SOFCs as well as some challenges that have to be taken into account in the process are described