Dean M. Paxton

Selection and Evaluation of Heat-Resistant Alloys for SOFC Interconnect Applications

Over the past several years, the steady reduction in SOFC operating temperatures to the intermediate range of 700~850oC [1] has made it feasible for lanthanum chromite to be supplanted by metals or alloys as the interconnect materials. Compared to doped lanthanum chromite, metals or alloys offer significantly lower raw material and fabrication costs. However, to be a durable and reliable, a metal or alloy has to satisfy several functional requirements specific to the interconnect under SOFC operating conditions. Specifically, the interconnect metal or alloy should possess the following properties: (i) Good surface stability (resistance to oxidation, hot corrosion, and carburization) in both cathodic (air) and anodic (fuel) atmospheres; (ii) Thermal expansion matching to the ceramic PEN (positive cathode-electrolyte-negative anode) and seal materials (as least for a rigid seal design); (iii) High electrical conductivity through both the bulk material and in-situ formed oxide scales; (iv) Bulk and interfacial thermal mechanical reliability and durability at the operating temperature; (v) Compatibility with other materials in contact with interconnects such as seals and electrical contact materials.

Materials Properties Database for Selection of High-Temperature Alloys and Concepts of Alloy Design for SOFC Applications

To serve as an interconnect / gas separator in an SOFC stack, an alloy should demonstrate the ability to provide (i) bulk and surface stability against oxidation and corrosion during prolonged exposure to the fuel cell environment, (ii) thermal expansion compatibility with the other stack components, (iii) chemical compatibility with adjacent stack components, (iv) high electrical conductivity of the surface reaction products, (v) mechanical reliability and durability at cell exposure conditions, (vii) good manufacturability, processability and fabricability, and (viii) cost effectiveness. As the first step of this approach, a composition and property database was compiled for high temperature alloys in order to assist in determining which alloys offer the most promise for SOFC interconnect applications in terms of oxidation and corrosion resistance. The high temperature alloys of interest included Ni-, Fe-, Co-base superalloys, Cr-base alloys, and stainless steels. In the US alone, there are hundreds of commercial compositions produced, over 250 of which are listed in Appendix A. Two initial criteria (oxidation resistance and oxide scale electrical conductivity) were used to reduce the list of alloys to manageable proportions. Thermal expansion and fabrication characteristics were then considered to further reduce the list of stainless steels. Due to their outstanding oxidation resistance and their potential to be used in SOFC components that can exclude alumina scales from the stack electrical path, alloys with a sufficient amount of aluminum were classified into a separate alumina-forming alloy category. The down-selected compositions (approx. 130 in number) and their characteristics and/or applications are listed in the Selected Alloy Compositions tables (Appendix B). Following the down-selection of alloy compositions, materials properties of interest corresponding to the their functional requirements in SOFC stacks were compiled in a tabular form (Appendix C). For comparison, the properties of selected noble metals and intermetallics were also collected and compiled and are listed in a separate table in Appendix C. Analysis of the pertinent literature indicated that, for a wide variety of alloys, there remains a lack of information on specific materials properties. Also, we have observed a large scatter in the reported database. For those cases, we employed general alloying principles as a tool of choice to approximate the unavailable data and to evaluate the reliability and consistency of collected data. Though numerous high temperature alloys look promising, it is anticipated that there will be few, if any, “off the shelf” alloy compositions which could completely satisfy the materials requirements as an interconnect, especially for a long term in a specific SOFC design. Therefore, some concepts of alloy design, including composition, constitution, and structure, as well as their effects on properties relevant to SOFC applications, are elaborated in an attempt to provide guidance for modification of current compositions and development of new alloys. Acknowledgement: This work was funded by the U.S. Department of Energy’s National Energy Technology Laboratory (NETL) under the Core Technology Program (CTP) of the Solid-State Energy Conversion Alliance (SECA).

Rolling Process Modeling Report: Finite-Element Prediction of Roll- Separating Force and Rolling Defects

Pacific Northwest National Laboratory (PNNL) has been investigating manufacturing processes for the uranium-10% molybdenum (U-10Mo) alloy plate-type fuel for the U.S. high-performance research reactors. This work supports the Convert Program of the U.S. Department of Energy’s National Nuclear Security Administration (DOE/NNSA) Global Threat Reduction Initiative. This report documents modeling results of PNNL’s efforts to perform finite-element simulations to predict roll separating forces and rolling defects. Simulations were performed using a finite-element model developed using the commercial code LS-Dyna. Simulations of the hot rolling of U-10Mo coupons encapsulated in low-carbon steel have been conducted following two different schedules. Model predictions of the roll-separation force and roll-pack thicknesses at different stages of the rolling process were compared with experimental measurements. This report discusses various attributes of the rolled coupons revealed by the model (e.g., dog-boning and thickness non-uniformity).

Rolling Process Modeling Report. Finite-Element Model Validation and Parametric Study on various Rolling Process parameters

Pacific Northwest National Laboratory (PNNL) has been investigating manufacturing processes for the uranium-10% molybdenum alloy plate-type fuel for high-performance research reactors in the United States. This work supports the U.S. Department of Energy National Nuclear Security Administration’s Office of Material Management and Minimization Reactor Conversion Program. This report documents modeling results of PNNL’s efforts to perform finite-element simulations to predict roll-separating forces for various rolling mill geometries for PNNL, Babcock & Wilcox Co., Y-12 National Security Complex, Los Alamos National Laboratory, and Idaho National Laboratory. The model developed and presented in a previous report has been subjected to further validation study using new sets of experimental data generated from a rolling mill at PNNL. Simulation results of both hot rolling and cold rolling of uranium-10% molybdenum coupons have been compared with experimental results. The model was used to predict roll-separating forces at different temperatures and reductions for five rolling mills within the National Nuclear Security Administration Fuel Fabrication Capability project. This report also presents initial results of a finite-element model microstructure-based approach to study the surface roughness at the interface between zirconium and uranium-10% molybdenum.

Influence of Homogenization on the Mechanical Properties and Microstructure of the U-10Mo Alloy

............................................................................................................................................ iii Acronyms and Abbreviations ...........................................................................................................vii 1.0 Introduction ................................................................................................................................ 1 2.0 Experimental ............................................................................................................................... 2 2.1 Materials ............................................................................................................................. 2 2.2 Homogenization Heat Treatment ....................................................................................... 2 2.3 Compression Testing .......................................................................................................... 3 2.4 Characterization of Microstructure .................................................................................... 4 3.0 Results ........................................................................................................................................ 5 3.1 Homogenization: Microstructure of the As-Cast U-10Mo ................................................. 5 3.2 Mechanical Properties (Compression Testing) .................................................................. 7 3.3 Microstructure of Compression Tested Samples ................................................................ 9 3.3.1 Sample Compression Tested at 500°C .................................................................... 9 3.3.2 Sample Compression Tested at 650°C .................................................................... 9 3.3.3 Sample Compression Tested at 800°C .................................................................. 13 4.0 Discussion ................................................................................................................................. 15 5.0 Summary and Conclusions ....................................................................................................... 16 6.0 References ................................................................................................................................ 17

Concept Feasibility Report for Using Co-Extrusion to Bond Metals to Complex Shapes of U-10Mo

In support of the Convert Program of the U.S. Department of Energy’s National Nuclear Security Administration (DOE/NNSA) Global Threat Reduction Initiative (GTRI), Pacific Northwest National Laboratory (PNNL) has been investigating manufacturing processes for the uranium-10% molybdenum (U-10Mo) alloy plate fuel for the U.S. high-performance research reactors (USHPRR). This report documents the results of PNNL’s efforts to develop the extrusion process for this concept. The approach to the development of a co-extruded complex-shaped fuel has been described and an extrusion of DU-10Mo was made. The initial findings suggest that given the extrusion forces required for processing U-10Mo, the co-extrusion process can meet the production demands of the USHPRR fuel and may be a viable production method. The development activity is in the early stages and has just begun to identify technical challenges to address details such as dimensional tolerances and shape control. New extrusion dies and roll groove profiles have been developed and will be assessed by extrusion and rolling of U-10Mo during the next fiscal year. Progress on the development and demonstration of the co-extrusion process for flat and shaped fuel is reported in this document

Overview of Lightweighting Materials Research & Development in the United States Freedomcar and Fuel Partnership

The United States of America’s (USA’s) transportation system is strongly dependent on petroleum as an energy source. Petroleum is used to satisfy 95 percent of the USA’s transportation energy needs, consuming two-thirds of all the petroleum used in the USA. Since roughly 60 percent of the petroleum is imported, the implications of this dependency on energy security are readily apparent. Since 2002, the United States Department of Energy (USDOE) and the United States Council for Automotive Research (USCAR) have worked cooperatively through the FreedomCAR and Fuel Partnership (FC&FP) to fund high-risk, high-payoff research and development (R&D) into advanced automotive technologies with the potential for lowering this dependence. The FC&FP succeeded and built upon the Partnership for a New Generation of Vehicles (PNGV) initiative that ran from 1993 to 2001. The long-term transition of vehicles from gasoline to non-petroleum energy sources is viewed as critical in lowering the dependence of the USA economy on foreign oil, and in reducing the environmental impact of the personal transportation sector. The FC&FP supports research on technologies with the potential for energy-efficiency and renewable energy benefits, such as new engine concepts, lightweight materials, alternate non-petroleum based fuels, and hybrid propulsion components. This paper will highlight the research in the lightweight metals portion of the FC&FP. Cooperative R&D projects will be discussed which focus on processing and manufacturing technologies such as casting of magnesium (Mg) and aluminium (Al) alloy components, advanced forming techniques for Al sheet, and warm-forming of Mg sheet. The overall objective of these efforts is not only to demonstrate new technologies, but to reduce the cost of manufacturing lightweight materials and enable implementation of the technologies in high-volume automotive applications.