PNNL

Abstract

High temperature, solid-state electrochemical devices have received considerable research attention in recent years because of their technological importance in a wide array of applications, including chemical sensors, solid oxide fuel cells, gas separation devices, and electrocatalyzers. While significant advancement has been made in the fabrication and performance of the ceramic materials that form the heart of the working device, less effort has been placed on the materials that will be used in the balance of the device; in particular the seals that bond the functional ceramic component to the metallic structural component. Since solid-state devices of this nature operate due to an oxygen ion gradient across an electrolyte membrane, hermeticity across this membrane is paramount. Not only must the membrane, typically yttria-stablized zirconia (YSZ), be fully dense with no pinholes or interconnected porosity, but it must be connected to the body of the device with a gas-tight seal. Depending on the specific application, the seal will be exposed simultaneously to an oxidizing atmosphere on one side and a reducing environment on the other at an operating temperature between 500 - 800°C. It may also experience numerous thermal cycles over the lifetime of the device, typically on the order of several thousand hours, during which time the seal must remain hermetic, rugged, and stable. Commercially, high temperature glasses have been employed as the means of hermetically sealing electrochemical devices. Metal brazes, however, may offer better performance with respect to thermal and mechanical shock, thermal cyclability, and overall joint strength. Before a given braze alloy can be considered for this type of application, though, the oxidation behavior of the braze alloy and that of the as-brazed joint must be thoroughly characterized. With respect to this issue, the research effort at PNNL is currently focused on: (1) understanding what effect typical electrochemical device operating conditions have on the hermeticity and strength of ceramic-to-metal brazed joints and (2) how the joint properties can be improved either by making compositional changes in standard commercial brazes or by adopting an alternative brazing approach. As will be presented, systematic studies on commercially available ceramic-to-metal brazes indicate that braze oxidation is greatly accelerated through contact with the underlying metal substrate due to the diffusion of oxidizable species from the substrate through the braze to a high oxygen containing interface or surface. Two different methods of rectifying this problem have been developed, which appear to yield an oxidation resistant brazed joint.