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Eine Hochtemperaturbrennstoffzelle (solid oxide fuel cell = SOFC) hat zwar ein sehr einfaches Funktionsprinzip, jedoch erfordern die sehr unterschiedlichen Materialien der Einzelkomponenten hochentwickelte thermische Fugeverfahren, um sie funktionsgerecht miteinander zu verbinden. Fur die SOFC existieren zwei Optionen einer moglichen Anwendung, erstens als dezentrale Hauskraftwerke (stationare SOFC), zweitens als Energiewandler fur Kraftfahrzeuge (mobile SOFC). Es haben sich daher im Forschungszentrum Julich zwei sehr verschiedene Konstruktionstypen etabliert, fur die auch die Entwicklung unterschiedlicher Verfahren zum Fugen der Einzelkomponenten erforderlich wurde. Beim stationaren Design besteht der Hauptfugeprozess in dem elektrisch isolierenden Verbinden einzelner Stahlplatten durch ein Glaslot zu einem SOFC-Stapel. Beim mobilen Design besteht der SOFC-Stapel aus einzelnen dunnen Stahlkassetten. Bei den Kassetten wird deren Fensterblech aus ferritischem Chromstahl mit der keramischen Schicht des Zirkonoxidfeststoffelektrolyten durch ein metallisches Lot verlotet. Es handelt sich bei diesem Lot um ein Silberbasislot mit nur geringen benetzungsbegunstigenden Zusatzen von Kupferoxid und Titanhydrid. Entscheidend fur beide Lotprozesse ist ihre Anwendbarkeit in normaler Atmosphare (Luft), also unter oxidativen Bedingungen. Um die SOFC zur Marktreife auf dem Energiesektor zu entwickeln, dauern die F&E-Arbeiten zur Wirkungsgradsteigerung und zur Langzeitbetriebsstabilitat an. Auch die beiden Lotverfahren sind noch nicht als normale Standardprozesse zu betrachten, sondern erfordern ebenso permanente Weiterentwicklungen, hier im Hinblick auf Reproduzierbarkeit, Bestandigkeit und Festigkeit der Lotverbunde. Auf der Suche nach geeigneten schnellen Ersatzmethoden, um die Fugeeigenschaften der Lotmaterialien genauer studieren und messtechnisch erfassen zu konnen, ohne den Bau aufwendiger komplexer SOFC-Stacks betreiben zu mussen, setzt die Arbeitsgruppe Sonderfugetechnik im Forschungszentrum Julich speziell abgewandte dilatometrische Messverfahren ein. Mit einem sog. Absenkdilatometer werden die Schwindungsprozesse der Glaslotfugen nachgestellt und im μm-Bereich gemessen. Dies ermoglicht, die fur Dichtheit und Kontaktierung entscheidende Glaslotmenge und die Prozessparameter vorherzubestimmen. Mit einem vertikalen Dilatometer erfolgen Untersuchungen des Schmelzverhaltens der reaktiven Silberlote im Hinblick auf Schmelzpunktverschiebung, Zahigkeit und Porenanteil und in Abhangigkeit von der Prozessatmosphare und von einem metallischen Additiv (Aluminium). The principle of operation of solid oxide fuel cells (SOFCs) is very simple. However, the fact that very different materials are used for the individual components requires advanced thermal joining techniques to join them in a functional manner. Two very distinct designs have established themselves for the two different intended applications: decentralised power generation (stationary SOFCs) on the one hand, and power converters for vehicles (mobile SOFCs) on the other hand. As a consequence, alternative techniques for joining the individual components are also required. The principal joining process for the stationary SOFC design consists of joining individual steel plates with a glass sealant in an electrically insulating way so that they form an SOFC stack. For the mobile fuel cell design, the SOFC stack consists of individual thin steel cassettes. The window frame of the cassettes, which is made of ferritic chromium steel, is brazed to the ceramic layer of the zirconium oxide solid electrolyte using a filler metal. The material used is a silver-based brazing filler metal which contains only small amounts of copper oxide (CuO) and titanium hydride (TiH2) as wetting agents. Both joining processes must be applicable in normal atmospheric air, i. e. under oxidative conditions. R&D activities continue for improving the efficiency and long-term operational stability of the technology to such an extent that SOFCs will become ready for the energy sector market. The two joining techniques described cannot yet be considered standard processes. They, too, will require continuous improvement with respect to reproducibility, endurance and strength of the joints. The Special Joining Techniques working group at Forschungszentrum Julich uses specially modified dilatometric techniques as suitable quick replacement methods for studying and measuring the joining characteristics of the materials without having to manufacture complex and expensive SOFC stacks. The shrinkage processes in the glass sealant joints are simulated and measured in the μm range using a special dilatometer. In this way, the amount of glass sealant – which is decisive for tightness and bonding – and the process parameters can be determined in advance. With a vertical dilatometer, the melting behaviour of the reactive silver filler metals is examined with respect to melting point shift, viscosity and void ratio, and as a function of the metal additives (Al) and the process atmosphere.

Carbon fiber composites (CFCs) are the first choice as plasma facing materials for the strike points of divertor targets for future nuclear fusion devices like WENDELSTEIN 7-X and ITER. For the application in these facilities several potential European 3D-CFCs were compared and qualified: (1) four material batches of NB31 produced by Snecma Propulsion Solide (SNECMA); (2) NB41, SNECMA, the upgraded version of NB31; (3) N31, SNECMA, which is densified by chemical vapor infiltration (CVI) instead of a final liquid pitch infiltration characteristic for NB31; (4) a new developmental 3D-CFC produced by DUNLOP. The characterization of the composites is comprised of thermo-physical measurements and tensile tests. The results are correlated to density and microstructure and summarized as follows: (1) NB41 provides the highest thermal conductivity in the ex-pitch direction of ∼375 W/(m K) at room temperature; (2) all material grades are, due to their heterogeneity, characterized by a relatively large scatter of mechanical properties; (3) the different densification process for N31 in comparison to NB31 has no influence on the material properties; (4) NB41 provides in all three directions a comparably high tensile strength with an average in the ex-pitch direction of ∼220 MPa; (5) the 3D-CFC from DUNLOP is comparable to NB41 but yet does not meet the specifications in the needling direction.

Copper–chromium–zirconium (CuCrZr) is a commercially available, precipitation-strengthened alloy. The combination of its good thermal conductivity and mechanical strength at low and moderate temperatures made it of interest for use as a heat sink material in high heat flux components in actual and future fusion facilities. Its only drawback is the microstructural modification and thereby, particularly, the loss of mechanical strength at temperatures above 500 °C. This limits the allowable operational temperatures and the temperature range in which cyclic and reproducible measurements of thermophysical properties can be performed. These difficulties and the international significance of the material makes it an excellent object of study for an interlaboratory test of several independent European thermophysical laboratories, in which the quality and the comparability of thermophysical measurements were determined. The main outcome (result, conclusion) of this study is that the different laboratory data are in good agreement providing maximum deviations of ~5 % for dilatometry, ~2 % for DSC measurements, and up to 10 % for thermal-diffusivity measurements. In addition to the determined high reproducibility within the particular thermophysical laboratories, it was found that the average thermal conductivity of CuCrZr produced in a certain compositional range is identical within a small-scatter band.