SOFC Chromium Interconnector Oxidation for Nitride Control
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Solution Overview
Problem
The formation of chromium nitrides in solid oxide fuel cell (SOFC) interconnectors leads to dimensional changes, impairing contact uniformity and accelerating degradation of electrical efficiency over time.
Innovation Solution
A controlled oxidation process for porous chromium components, involving exposure to an oxidation temperature range in a furnace with a controlled atmosphere comprising at least 30 volume % nitrogen, at least 10 volume % oxygen, and at least 20 volume % water vapor, to reduce nitride formation and enhance impermeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chromium components are exposed to nitrogen-containing atmospheres during oxidation, then nitrogen is absorbed into the material, but this causes excessive nitride formation leading to dimensional changes and warping beyond allowable tolerances
Solution Approach 1:
The patent applies an inert atmosphere principle by using a controlled oxidation environment with limited nitrogen exposure. The oxidation process is conducted in a atmosphere that prioritizes oxygen for oxide layer formation while minimizing nitrogen absorption, thereby preventing excessive nitride formation that would cause dimensional changes and warping beyond allowable tolerances.
Solution Approach 2:
The patent employs parameter changes by carefully controlling oxidation temperature, time, and atmospheric composition. By optimizing these parameters, the process achieves sufficient oxide layer growth for impermeability while limiting nitrogen absorption and nitride formation to levels that maintain dimensional tolerance within allowable ranges.
2Reliability
If oxidation temperature and time are increased to improve impermeability, then oxide layer growth is enhanced, but dimensional changes due to nitride formation are also exacerbated
Solution Approach 1:
The patent applies parameter changes by optimizing oxidation temperature and time within specific ranges. The process uses controlled temperature profiles and duration that are sufficient to grow an impermeable oxide layer on the chromium component while limiting the extent of nitride formation and associated dimensional changes to acceptable levels.
Solution Approach 2:
The patent employs composite material principles by creating a controlled atmosphere composition during oxidation that combines oxygen for oxide formation with limited nitrogen. This composite atmospheric environment enables selective oxidation while suppressing excessive nitride formation, achieving both impermeability and dimensional stability.
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
The process effectively reduces the nitrogen content in the interconnectors by less than 0.1 weight %, minimizing dimensional changes and improving the long-term dimensional accuracy and electrical efficiency of the SOFC stack.
Implementation Method 1
An elevated temperature oxidation process step is often used in the PM manufacturing process whereby growth of an oxide layer is encouraged on the walls of the internal porosity such that internal pore channels become blocked by the formed oxide films
Implementation Method 2
The present invention provides a method of oxidising a porous component... The oxidizing increases a nitrogen content of the porous component by less than 0.1 weight %
Data Source
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AI summary
An interconnector for a solid oxide fuel cell is manufactured by single-press compacting a powder blend to form a green interconnector with a desired shape of a final interconnector. The powder blend includes chromium and iron, and may include an organic lubricant. At least 50 wt% or more of an iron portion of the powder blend comprises iron particles smaller than 45 um. The green interconnector is then sintered and oxidized to form the final interconnector. The oxidation step occurs in a continuous flow furnace in which a controlled atmosphere (e.g., humidified air) is fed into the furnace in the travel direction of the interconnector. The final interconnector comprises at least 90 wt% chromium, at least 3 wt% iron, and less than 0.2 wt% nitrogen. An average density within a flow field of the final interconnector may be less than 6.75 g/cc.