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Thermochemistry of brazing ceramics and metals in airDedicated to Prof. Bernhard Wielage on the occasion of his 65 th birthday Reactive air brazing offers economically and technologically advantageous joining of ceramics to metals. Solid oxide fuel cells and membranes for oxyfuel combustion are recent fields of application. However, it remains a problem that strong metallurgical reactions between brazes and base materials occur. These reactions were analysed by differential scanning calorimetry tests to get a better understanding. Therefore, three braze alloys (Ag8Cu, Ag8Cu0.5Ti and Ag4Cu4Ni) and five base materials (alumina, 3YSZ partially stabilised zirconia, BSCF perovskite ceramic, X1CrTiLa22 and X15CrNiSi25-20) were investigated. The reaction peaks correlate with the formation of reaction layers, which were observed in metallographic analysis of brazed specimens. The results help to explain the reaction mechanisms and allow optimised selection of filler metals and brazing temperature.

Nowadays with a rapid development of high temperature electrochemical devices, sealing technology is an issue of high practical relevance for the applicability of such equipment. Most commonly used joining technologies are glass joining, active metal brazing and reactive air brazing (RAB). [1] Glass joining is a relatively simple and cost-effective bonding method. However, for glass brazed joints, the maximum operation temperature is limited by the softening point of glass. [2,3] Active metal brazing requires a very stringent brazing atmosphere. Such joints have to be processed either under high vacuum or under very clean inert gasatmosphere, what causes comparatively higher processing costs than air-brazing processes. [1] Besides, recent studies also show that after long time operation in air, the active metal braze joints oxidize completely, maintaining little or no strength. [4,5] In an effort to find a cost-effective and stable brazing technique, RAB was developed. [2] Reactive air brazing uses reactive elements, which are at least partially dissolved in a noble metal solvent, to reactively modify the faying surface, such that the newly formed surface is readily wetted by the rest of the molten filler metal. [1] With the increased effort in the development of intermediate temperature SOFC systems, Ag and Ag-based materials for sealing and interconnection have gained increased interest. [6] As a precious metal, Ag possesses good chemical stability and lower costs than other precious metals like platinum and gold.

Reactive air brazing (RAB) is an emerging technology for the production of ceramic-to-ceramic and ceramic-to-metal joints. In this study, RAB was investigated with respect to the potential applications for solid oxide fuel cells (SOFCs) as one example of use. It was found that alumina could be well brazed by RAB with AgCu and AgCuTi brazes. Both braze composition and brazing temperature influenced significantly the wetting behavior and their mechanism of wetting. AgCu and AgCuTi braze alloys could also be used to produce brazed joints with the SOFC materials ceramic yttria stabilized zirconia and steel X1CrTiLa22. However, CuO reacts with the steel, forming a brittle oxide layer on the steel surface, which is undesirable for SOFC applications. The first trials with Ag0.5Al showed a promising solution.

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