Porous Nickel Interlayer for Ceramic Brazing in Solid Oxide Fuel Cells
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Solution Overview
Problem
Conventional brazing methods for solid oxide fuel cells face challenges in wetting ceramic substrates with molten braze materials, leading to the formation of pores and mechanical weaknesses due to oxidation and reduction reactions, which reduce the reliability and lifetime of the braze joints.
Innovation Solution
A brazing method involving the formation of a porous metal layer on a substrate to assist wetting with a lower melting point braze metal, allowing for effective joining in an inert atmosphere that prevents oxidation and reduces pore formation, using a pre-sintering process to create a porous nickel layer for silver brazing in solid oxide fuel cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If reactive air brazing is used to improve wetting behavior on ceramic substrates, then wetting is improved, but Type I pores are formed during manufacturing and Type II pores are formed during operation
Solution Approach 1:
A porous nickel interlayer is introduced as an intermediary between the silver braze and the ceramic substrate. This interlayer facilitates wetting during brazing while its porous structure allows controlled oxidation that prevents pore formation. The nickel layer acts as a mediator that enables the braze to wet the ceramic surface without creating the harmful pores associated with direct reactive air brazing.
Solution Approach 2:
The invention utilizes a porous nickel layer with controlled porosity to achieve both wetting enhancement and pore prevention. The porous structure provides capillary action to draw the molten braze onto the ceramic surface, improving wetting. Simultaneously, the controlled porosity allows gradual oxidation that prevents the formation of Type I and Type II pores, resolving the contradiction between wetting improvement and pore formation.
2Object-affected harmful factors
If vacuum brazing is used to prevent oxidation, then oxidation is avoided, but many Solid Oxide Fuel Cell materials are decomposed due to low oxygen partial pressure
Solution Approach 1:
The invention creates different oxygen partial pressure conditions in different locations: the bulk brazing atmosphere is maintained at a higher oxygen partial pressure to prevent decomposition of SOFC materials, while the immediate interface between the porous nickel layer and ceramic substrate experiences controlled local oxidation. This spatial differentiation of oxygen partial pressure allows both oxidation prevention and material stability.
Solution Approach 2:
The invention uses an inert or controlled atmosphere during brazing that maintains sufficient oxygen partial pressure to prevent decomposition of SOFC materials, unlike vacuum brazing. The porous nickel interlayer enables controlled oxidation at the interface without requiring the low oxygen partial pressure conditions that would cause material decomposition, thus resolving the contradiction between oxidation prevention and material stability.
3Reliability
If reducing agents are added to improve braze wetting by locally reducing oxygen partial pressure, then wetting is improved, but deleterious reactions occur with Solid Oxide Fuel Cell materials
Solution Approach 1:
The porous nickel layer serves as an intermediary that enables controlled local reduction of oxygen partial pressure at the braze-ceramic interface to improve wetting, without requiring the addition of reducing agents. The nickel itself undergoes controlled oxidation, providing the necessary local atmosphere modification without introducing harmful reducing agents that would react deleteriously with SOFC materials.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enhances the reliability and operational robustness of the braze joints by reducing Type I and Type II pores, increasing the lifetime of the solid oxide fuel cell components and maintaining mechanical strength during thermal cycling.
Implementation Method 1
the capillary pressure which wicks the silver braze into the porous nickel network
Implementation Method 2
using a pre-sintering process to create a porous nickel layer for silver brazing
Implementation Method 3
brazing the pre-braze joint at a temperature and pressure sufficient to melt the braze second metal material
Data Source
AI summary
The disclosure relates to a brazing method for joining substrates, in particular where one of the substrates is difficult to wet with molten braze material. The method includes formation of a porous metal layer on a first substrate to assist wetting of the first substrate with a molten braze metal, which in turn permits joining of the first substrate with a second substrate via a braze metal later in an assembled brazed joint. Ceramic substrates can be particularly difficult to wet with molten braze metals, and the disclosed method can be used to join a ceramic substrate to another substrate. The brazed joint can be incorporated into a solid-oxide fuel cell, for example as a stack component thereof, in particular when the first substrate is a ceramic substrate and the joined substrate is a metallic substrate.