Metallic materials in Solid Oxide Fuel Cells: Oxidation and chromium evaporation properties

Solid oxide fuel cells (SOFCs) are high-temperature energy conversion devices which have great potential due to their high efficiency, low operating costs and flexibility in using conventional hydrocarbon based fuels. However, system cost, durability and performance stability in long-term operation are barriers to the widespread commercialization of SOFC technology.Due to the high-temperature operation and aggressive environment in SOFCs, metallic materials - used for interconnects and balance of plant (BOP) - are subject to corrosion. Interconnects are typically made of ferritic stainless steel, which forms a protective chromia scale at high temperatures. This results in two main degradation mechanisms: 1) chromium evaporation, which leads to cathode poisoning and 2) chromia scale growth, which leads to increased electrical resistance. To date, research into metallic materials in SOFC has focused mostly on interconnects. However, metallic materials used inBOP components can be a significant source of volatile chromium species and are often overlooked. The aim of this thesis is to find high performance, cost-effective metallic materials for SOFC systems. Five metallic materials AISI 441, AISI 444, A197/Kanthal\uae EF101, alloy 800H and alloy 600, were studied for potential use in BOP components. Lowcost steels, AISI 441 and AISI 444, and tailor-made Crofer 22 APU in combination with different coatings were evaluated for the interconnect application. Chromium evaporation and oxide-scale growth of the materials are investigated, and the oxide scale is studied further, using XRD, SEM, EDX and ASR.The alumina former, A197, showed the lowest chromium evaporation and oxidation in all exposure conditions. Alloy 800H showed poor oxidation behaviour at lower temperatures but its performance improved significantly after pre-oxidation. Alloy 800H has higher chromium evaporation than A197 but significantly lower than 441 and 444. This low chromium evaporation is due to the formation of an Fe, Ni-rich oxide cap layer. Alloy 600 showed intermediate performance. 441 and 444 showed the highest oxidation and chromium evaporation of the selected alloys, making them a poor choice for BOP components.The uncoated low-cost steels, 441 and 444, showed higher chromium evaporation and/or oxide scale growth than the tailor-made Crofer 22 APU. The oxide scale structure was similar for all the steels, with (Cr, Mn)3O4 spinel on top and Cr2O3 scale underneath after 500 hours. The Ce/Co coated steels showed lower oxide scale growth and a chromium evaporation at least 60 times lower than the uncoated steels. In addition, all the coated steels showed similar chromium evaporation, oxide scale structure ((Co, Mn)3O4 spinel on top and Cr2O3 scale underneath), oxide scale thickness and area specific resistance after 1,000 hours

Design for manufacturability (computer aided optimum gear design)

Design for manufacturing has design as the first manufacturing step. It is important to recognize the importance of design clubing with manufacturing to reduce the production, set up time. In this thesis concepts of design, manufacturing and four different ways of bringing design and manufacturing together are discussed in detail. As there are large number of variables in the gear rating equations, it is very difficult to achieve optimum design by manual methods unless an extensive trial and error process is carried out. An algorithm has been developed which allows rapid convergence to the optimum gear design. The algorithm has the capability to solve for the optimum solutions over a wide range of input parameters. This algorithm has been computerized in the form of an interactive gear design package. This thesis presents the flow chart, the whole program in FORTRAN, and explanation of the algorithm. The optimization described in this thesis has a solution criterion whose objective is to eliminate unwarranted capacity of a gear pair. The technique developed significantly reduces the computing time required to obtain the solution. This involves a trimming of possible trials based on restrictions as well as knowledge of trends associated with the empirical formulae governing variables in the power calculations

Metallic materials for solid oxide fuel cells and electrolysers - Mitigating high temperature corrosion

Solid oxide cells (SOCs) are high-temperature energy conversion devices that have great potential due to their high efficiency, low operating costs, and flexibility. SOCs can produce electricity from a variety of fuels as solid oxide fuel cells (SOFCs), and they can convert electricity to fuels as solid oxide electrolyser cells (SOECs). However, the wide-spread commercialisation of this technology is hindered by high system cost, lack of durability, and poor performance stability during long-term operation. Owing to the high-temperature operation and aggressive environment of SOCs, metallic materials used for interconnects and balance of plant (BOP) components are subject to corrosion. Interconnects are typically made of ferritic stainless steel (FSS), which forms a protective chromia scale at high temperatures. The degradation mechanisms, such as Cr (VI) evaporation and chromia scale growth, lead to electrode poisoning and increased electrical resistance, which degrade cell performance.The primary objective of this thesis is to develop alternative materials and understand the degradative mechanisms so as to effectively reduce the costs and improve the performances of metallic materials in SOC systems. The Cr evaporation, oxide scale growth, the microstructural evolution of the oxide scale, and the area-specific resistances are investigated for the selected materials. The majority of the thesis is focused on Ce/Co coatings. Ce/Co-coated, low-cost, commercial FSS (AISI 441, AISI 430, and AISI 444) are compared to tailor-made Crofer 22 APU in air-side atmospheres. Ce/Co-coated steels are further investigated under dual-atmosphere conditions. The Ce/Co coating is compared to various coatings from research laboratories and universities world-wide. Furthermore, the underlying causes for the improvement in the oxidation resistances of FSS that occur in the presence of the reactive element Ce (in the Ce/Co coating) are investigated, and a new mechanism is proposed. Finally, a model to predict the lifetimes of the coated steels is proposed. Moreover, a new coating system, the Ce/FeNi coating, is proposed as an alternative to the Ce/Co coating. The Ce/FeNi coating is found to be more effective than Ce/Co coating in reducing chromia scale growth.While research on metallic materials for SOC has centred on the interconnects, the metallic materials used in BOP components, which can be a significant source of volatile chromium species, have been largely neglected. Five metallic materials (AISI 441, AISI 444, A197/Kanthal\uae EF101, alloy 800H, and alloy 600) are examined for potential usage in BOP components. The oxidation and Cr evaporation behaviours of these materials are discussed and correlated to the observed microstructures

Evaluating candidate materials for balance of plant components in SOFC: Oxidation and Cr evaporation properties

Balance of plant (BOP) components made of metallic materials in solid oxide fuel cells are subject to high-temperature corrosion and are a significant source of volatile chromium species. Prospective Fe and Ni-base alloys, AISI 441, AISI 444, a FeCrAl alloy A197/Kanthal\uae EF101, alloy 600, and alloy 800H are investigated for their suitability to BOP components. Oxidation kinetics and chromium evaporation were employed to study the selected alloys at 650 \ub0C and 850 \ub0C for 500 h. A197 performed the best while AISI 441 and AISI 444, performed the worst. Pre-oxidation significantly improved the performance of the alloys at 650 ⁰C

Reevaluating the Cr Evaporation Characteristics of Ce/Co Coatings for Interconnect Applications

Cathode poisoning by chromium evaporation from the interconnects is one of the major degradation mechanisms in SOFC. Coatings have proved to be very effective in suppressing chromium evaporation on interconnects. The quantification of chromium evaporation is important for determining the chromium consumption in the interconnect and predicting the lifetime of the interconnect. Chromium evaporation of uncoated and Ce/Co coated Crofer 22 APU is reevaluated at 800 C. The chromium evaporation of Ce/Co coatings on steel sheets and precut steels is studied. Coupons cut from Ce/Co coated sheets have uncoated edges, which influence the chromium evaporation measurements. The true chromium evaporation of the coated interconnects is evaluated. The PVD Ce/Co coatings on Crofer 22 APU reduce the chromium evaporation by at least 60 times compared to the uncoated at 800 C

An analytical model to study scalling capability of deep sub-micron double gate gallium nitride (gan) mesfets

A two dimensional (2-D) analytical model of a Gallium Nitride (GaN) Metal- semiconductor field effect transistor (MESFET) has been developed to present the submicron double gate, and it is expected to reduce the short channel effects for deep submicron GaN-MESFET based low power applications. This model also includes the two dimensional Poisson's equation to accurately determine the electric field in the drain side and the effect of drain induced barrier lowering effect. This analytical model expects that threshold voltage is developed greatly when compared to single-gate GaN-MESFET. By simulating this analytical model using the device parameters and DC biasing conditions, this Double Gate GaN MESFET model can solve the critical problems like immunity of short channel effects of GaN based circuits for the low power applications.California State University, Northridge. Department of Electrical and Computer Engineering.Includes bibliographical references (pages 62-68

Investigation of coated FeCr steels for application as solid oxide fuel cell interconnects under dual-atmosphere conditions

Dual-atmosphere conditions are detrimental for the ferritic stainless steel interconnects used in solid oxide fuel cells, resulting in non-protective oxide scale growth on the air side. In this paper, low-cost steels AISI 441 and AISI 444 and the tailor-made Crofer 22 APU, were investigated at 800 \ub0C and 600 \ub0C under dual-atmosphere conditions: air-3%H2O on one side and Ar-5%H2-3%H2O on the other side. At 800 \ub0C, the uncoated and Ce/Co-coated steels formed protective layers of (Cr,Mn)3O4/Cr2O3 and (Co,Mn)3O4/Cr2O3 respectively on the air side after 336 h. However, at 600 \ub0C, the Ce/Co-coated AISI 441 and AISI 444 showed ∼20–25 μm thick Fe2O3/(Fe,Cr)3O4 oxide scale on the air side after 336 h. Ce/Co coated Crofer 22 APU remained protective after 772 h at 600 \ub0C, indicating better resistance to the dual-atmosphere. The effect of Ce/Co coatings on the air side and the need for coatings on the fuel side are discussed and compared with experimental data

11–23% Cr steels for solid oxide fuel cell interconnect applications at 800 \ub0C – How the coating determines oxidation kinetics

The present work investigates the low-cost steels AISI 441, AISI 430, and AISI 444 against the tailor-made high Cr steel Crofer 22 APU (22.9 wt% Cr) at 800 \ub0C in simulated solid oxide fuel cell (SOFC) cathode conditions. Furthermore, a low Cr steel, AISI 409 (11.4 wt% Cr) is included in the study. The oxidation, chromium evaporation, and area-specific resistance (ASR) of the uncoated and Ce/Co-coated steels are studied for up to 3000 h. Ce/Co-coated steels showed significant improvement in behaviour compared to their uncoated counterparts. The oxidation and chromium evaporation behaviour between the uncoated steels varied substantially while the Ce/Co coated steels exhibited highly similar behaviour. The area-specific resistance of the coated low-cost steels was on par with Crofer 22 APU. However, 430 formed a continuous silica layer, resulting in a higher ASR after 3000 h. Cross-sections of the uncoated and Ce/Co-coated steels were analysed using a scanning electron microscope and energy dispersive X-ray spectroscopy