DE102007012201A1 - Ceramic-metal sealing in solid-oxide automobile fuel cell stacks, using silver solder, employs atmosphere in which oxygen partial pressure is carefully selected - Google Patents

Abstract

Soldering is carried out in an atmosphere with an oxygen partial pressure up to 100 mbar (approximately). The oxygen partial pressure is 5-100 mbar, preferably 10-70 mbar. Cooling rate during solder setting, is kept to less than 5 K per minute. A solder foil made from reactive air brazing (RAB-) solder is used.

Description

The invention relates to a method for gas-tight joining of a ceramic structure with a metallic structure, in particular of corresponding elements of a solid oxide fuel cell, by means of a silver-based RAB solder in an oxidizing atmosphere. The known prior art is on the US 7,055,733 B2 directed.

It has been proposed for metallic, gas-tight joining of ceramic elements with metallic elements of solid oxide fuel cells or fuel cell stacking, d. H. So-called SOFC stacks (SOFC = solid oxide fuel cell) so-called Reactive-Air-Brazing-Lote (abbreviated "RAB-Lote") to use. These solders are typically around noble metal-based, in particular silver-based solders (such as, for example, AgCuO), which have the advantage that with them substantially in Ambient atmosphere can be worked. This is insofar required to be present as a sufficiently high oxygen partial pressure needs to be a - of course unwanted - chemical Destruction of the ceramic cathode layer of the individual fuel cells to prevent soldering. The ceramic layer of the fuel cell cathode would namely at too low oxygen partial pressures disintegrated.

It It can be observed that silver is just below its melting temperature (from about 955 ° C) has a tendency to oxygen uptake. By this uptake of oxygen from the soldering atmosphere into the molten solder and its subsequent delivery However, the melt during solidification can cause the formation of pores come in the joining area. This porosity can change negative for the gas tightness and mechanical strength of the Affect addition. Make pores as far as they are together are connected, namely leakage passages for the Operating gases of SOFC dar. Moreover, pores act as crack starters under mechanical stress. on the other hand It should be noted that a RAB lottery system itself also has a certain significant Oxygen partial pressure needed to wetting the ceramic surface ensured by the CuO contained in the Lotsystem.

Herewith now a measure will be shown, with the help of those the method according to the preamble of claim 1 performed can be without a disturbing porosity in the joint area is to be determined. The solution of this Task is characterized in that the soldering in one Atmosphere with an oxygen partial pressure of the order of magnitude is carried out of less than or equal to 100 mbar. advantageous Further developments are content of the subclaims.

It It should be noted that pure silver at an oxygen partial pressure in the order of magnitude ≤ 100 mbar (millibar) in the surrounding atmosphere no significant tendency for oxygen uptake, so that under this condition no To observe mass increase of the melt by oxygen uptake is. Will thus the oxygen partial pressure in the soldering atmosphere reduced compared to the usual ambient conditions, preferably by using synthetic "air" with less than the natural oxygen partial pressure, namely on the order of less than / equal 100 mbar, so pore formation can be successfully prevented. So hereby are reliable gas-tight as well as mechanical high-strength joints feasible. Particularly good results were achieved with soldering processes in an atmosphere with an oxygen partial pressure between 5 mbar and 100 mbar, in particular between 10 mbar and 70 mbar.

Furthermore, it has been found that the soldering result can be improved by adjusting the amount of solder used with the surface to be soldered and the load weight used. Particularly good results were noted, if in the solder composition no TiH _{2 is} contained. The latter "burns" in the air to form water vapor, which causes a blistering.

Further Improvements are noticeable when soldering foil instead of solder paste be used, since then eliminates the burn-out of organic binders. In particular, however, there is a reduction in the cooling rate to less than 5 Kelvin per minute in the solidification phase of the solder alloy, d. H. between 960 ° C and 930 ° C positive effects on the result of the soldering process.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 7055733 B2 [0001]

Claims (4)

Method for gas-tight joining of a ceramic structure having a metallic structure, in particular corresponding elements of a solid oxide fuel cell, by means of a silver-based RAB solder in an oxidizing atmosphere, characterized in that the soldering in an atmosphere having an oxygen partial pressure in the order of magnitude smaller / equal to 100 mbar is performed. Method according to claim 1, characterized in that that soldering in an atmosphere with an oxygen partial pressure between 5 mbar and 100 mbar, in particular between 10 mbar and 70 mbar is performed. Method according to claim 1 or 2, characterized that the cooling rate in the solidification phase of the solder is kept less than 5 Kelvin per minute. Method according to one of the preceding claims, characterized in that a solder foil of RAB solder used becomes.