Low melting temperature silver braze alloy

Abstract

A brazing alloy comprising elemental silver alloyed with elemental silicon in a silver / silicon ratio between about 95 / 5 and 99 / 1, preferably about 97 / 3. Small amounts of silicon alloyed with silver depress the alloy liquidus curve significantly, the liquidus temperature of a silver and silicon eutectic alloy being about 837° C. Brazing alloys in accordance with the invention are useful in bonding ceramics to ceramics, ceramics to metals, and metals to metals. Copper, vanadium, or other oxygen-reactive surface bonding elements may also be included. Silver / silicon alloys are useful in applications such as assembly of components of solid oxide fuel cells. A variety of silver / silicon alloy brazes can be used within the same fuel cell so that subsequent brazing can be performed without reliquifying a previous braze. A brazing alloy comprising elemental silver and ruthenium in a silver / ruthenium ratio between 97 / 3 and 99 / 1 is also included.

Description

[0001] This invention was made with United States Government support under Government Contract / Purchase Order No. DE-FC26-02NT41246. The Government has certain rights in this inventionTECHNICAL FIELD [0002] The present invention relates to alloys for joining materials by brazing; more particularly, to silver-containing braze alloys for joining ceramics to metals; and most particularly, to silver braze alloys containing silicon to lower the liquidus temperature. BACKGROUND OF THE INVENTION [0003] Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide, such as yttrium-stabilized zirconium (YSZ). Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and abo...

AI Technical Summary

Benefits of technology

[0008] Briefly described, a brazing alloy in accordance with the invention comprises elemental silver alloyed with another element that serves to reduce the liquidus temperature of the alloy to a temperature below the melting point of silver. In a preferred embodiment, a brazing alloy comprises elemental silver alloyed with elemental silicon in a silver / silicon ratio between about 95 / 5 and 99 / 1, preferably about 97 / 3. Silver melts at 962° C., but small amounts of silicon alloyed with silver depress the alloy liquidus point significantly, the liquidus temperature of a silver and silicon eutectic alloy being about 837° C. Brazing alloys in accordance with the invention are useful in bonding ceramics to ceramics, ceramics to metals, and metals to metals. Such metals are preferably alumina-forming. Copper, vanadium, or other oxygen-reactive surface bonding elements may also be included, and use of such brazing alloys is preferably carried out in an oxidizing atmosphere.

Problems solved by technology

A problem in the use of such alloys is that the liquidus temperature is substantially the melting point of pure silver, 962° C. This temperature is high enough to preclude use of some fuel cell materials which can be damaged by such high temperatures.

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