The cyclic oxidation of a variety of chromia-forming ferritic stainless steels has been studied in the temperature range 700-900°C in atmospheres relevant to solid-oxide fuel-cell operation. The most detrimental environment at 800°C and 900°C was found to be air with 10% water vapor. This resulted in excessive oxide spallation or rapid scale growth. Impurities in the alloys, particularly Al and Si, were found to have a significant effect on the oxidation behavior. Oxide growth was slow at 700°C but the higher-Cr-content alloys were observed to form sigma-phase at this temperature. The sigma phase formation was accelerated by higher silicon contents, and remarkably, by the presence of water vapor in the exposure environment. Alloys containing Mn were observed to form an outer layer of MnCr 2 O 4 over the chromia scale. The potential for this overlayer to suppress reactive evaporation of the chromia scales has been analyzed.

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...

Two basalts with compositions relevant to the crusts of Mars and Earth were synthesized at igneous temperatures and held at 650°C for 21 to 257 days under quartz-fayalite-magnetite fO 2 buffer conditions. The run products are germane to slowly cooled igneous intrusions, which might be a significant volumetric fraction of the Martian crust and carriers of magnetic anomalies in the Southern Highlands. Both basalts acquired intense thermoremanent magnetizations and intense but easily demagnetized anhysteretic remanent magnetizations carried by homogeneous multidomain titanomagnetite. Hypothetical intrusions on Mars composed of these materials would be capable of acquiring intense remanences sufficient to generate the observed anomalies. However, the remanence would be easily demagnetized by impact events after the cessation of the Mars geodynamo. Coercivity enhancement by pressure or formation of single domain regions via exsolution within the multidomain grains is necessary for long-term retention of a remanence carried exclusively by multidomain titanomagnetite grains.