Fundamental Studies of the Durability of Materials for Interconnects in Solid Oxide Fuel Cells

Abstract

Table of Contents 4 List of Graphical Materials 5 Introduction 8 Executive Summary 9 Project Description 11 Experimental 12 Results and Discussion 20 References 44 EXECUTIVE SUMMARY This project had four objectives: • To develop mechanism-based evaluation procedures for the stability of SOFC interconnect materials and to use these procedures to study and modify a group of alloys, which have already been identified as candidate interconnect materials, i.e. ferritic stainless steels. • To study fundamental aspects underlying the thermomechanical behavior of interconnect materials and develop accelerated testing protocols. (CMU Subcontract) • To investigate the potential for the use of "new" metals as interconnect materials. • Development of a durable, conductive ceramic/metal (cermet) material, suitable for long-term use as a contacting material in the cathode chamber of SOFC. (WVU Subcontract) The program consisted of four major tasks aligned with its four objectives. Task 1: Mechanism-based Evaluation Procedures A variety of chromia-forming interconnect alloys were subjected to thermal cycling in air, in simulated anode gas (Ar-H 2-H 2 O) and with simultaneous exposure to air on one side and simulated anode gas on the other. Exposure temperatures range from 700°C to 900°C. Oxidation kinetics were tracked by mass change measurements and corresponding changes in oxide scale resistances were measured. Exposed specimens were examined in cross-section by scanning electron microscopy (SEM) to document changes in structure with exposure. Methods were studied to slow the growth of chromia scales on Cr and Ferritic alloys upon exposure to oxidizing gases. The effect of alloying additions (e.g. Mn, Ti) to ferritic steels to reduce harmful CrO 3 and CrO 2 (OH) 2 evaporation by forming a sealing outer layer over the chromia scale was evaluated. The ability of chromite coatings to reduce evaporation from chromia-forming interconnect alloys was investigated. Task 2: Fundamental Aspects of Thermomechanical Behavior (CMU) Understanding the resistance of growing chromia scales to spallation requires a fundamental understanding of the mechanics of chromia adhesion. From a fracture mechanics standpoint, the adherence of protective oxide scales to alloy substrates is governed by 1) the stored elastic energy in the scale, which drives delamination and 2) the fracture toughness of the alloy/oxide interface, which quantifies the resistance to fracture. The stored elastic energy in the scale is increased by increases in the scale thickness and increases in the residual stress in the scale. In this task, x-ray diffraction (XRD) was used to measure stresses in chromia films formed on pure chromium and chromia-forming alloys after the exposures described for Task 1. An indentation test was also used to measure the fracture toughness of chromia/alloy interfaces for the same exposures. In the test, the chromia scale is penetrated by the indenter and the plastic deformation of the underlying substrate induces compressive radial strains in the...