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고체산화물 연료전지 연결재용 페라이트계 스테인리스강의 개발과 단추형 전지를 이용한 평가

고체산화물 연료전지 연결재용 페라이트계 스테인리스강의 개발과 단추형 전지를 이용한 평가
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The planar type solid oxide fuel cell (SOFC) has been considered as a promising power generation system because of its high efficiency over a wide range of electricity generation capacity and high tolerance to impurities such as carbon monoxide and sulfur in fuel gas when compared to fuel cells operating at low temperature. In commercialization of SOFCs, one of the important factors is the development of a cost effective interconnect material. Among various metallic materials, ferritic stainless steels have advantages of their thermal expansion coefficient similarity with other cell components and cheap material cost. However, ferritic stainless steels are oxidized to form the oxide scale that gradually increases the contact resistance during operation. The Cr species, in the form of CrO3 and especially CrO2(OH)2 in a humid atmosphere, evaporate from the Cr based oxide scale and deposit on the cathode and deteriorate cathode performance. Crofer22 APU, which is FSS containing 22 wt% Cr and reactive element of La, developed by Thyssen Krupp shows promising properties of low oxidation rate, low area specific resistance of its oxide formed during operation and low Cr evaporation rate. However, the production of stainless steels containing the reactive elements requires a cupola type vacuum process due to high oxygen affinity of the reactive elements. This will increase the overall production cost and reduce productivity. Therefore, it is required to find an alloying element that can improve the required properties of ferritic stainless steels for SOFC interconnect, for while the commercial steelmaking process is applicable for production. To develop an interconnect material that can be produced with commercial mass production processes, the effect of tungsten and molybdenum on the oxidation resistance, area specific resistance and Cr evaporation rate of Fe-22Cr-0.5Mn steel is investigated. Addition of 4 - 6 wt% W reduces the oxidation rate, evaporation rate of Cr vapor species and area specific resistance of Fe-22Cr-0.5Mn steel. When more than 4wt% W is added, chi (χ) phase particles are formed, especially beneath the interface between the oxide layer and substrate steel. These second phase particles are responsible for suppressing the fast diffusion of Cr, so that the surface Cr content is reduced while Mn content is increased. This reduction in Cr content at the surface is responsible for the lower Cr evaporation rate and area specific resistance. Addition of 0.1 - 2 wt% Mo reduces the oxidation rate and especially area specific resistance of Fe-22Cr-0.5Mn steel. Mo addition of these contents increases the activation energy and suppresses the inward diffusion of oxygen, which alters the defect chemistry of oxide scale. This results in the increase of oxidation resistance and electric conductivity. When more than 4wt% Mo is added, the oxidation rate increases after 300 h of oxidation at 800 oC in ambient air. The evaporation of volatile Mo species reduces the stability of protective chromia, so that rapid growing Fe-rich spinel is formed after 300 h of oxidation. To evaluate the Mo alloyed steel and Nb alloyed steel as a SOFC interconnect in a simulated SOFC environment, the Fe-22Cr-0.5Mn ferritic stainless steels alloyed with Nb or Mo are evaluated in the button cell configuration at 750 oC in terms of degradation in ohmic resistance and cathodic polarization resistance. STS444 and Crofer22 APU are also evaluated for comparison. Each polarization element is separated by equivalent circuit analysis on the electrochemical impedance spectroscopy data. Cr deposition on the button cell cathode is also analyzed both qualitatively and quantitatively by transmission electron microscope and inductively coupled plasma. The Nb or Mo alloyed ferritic stainless steel shows comparable performance with Crofer22 APU in terms of the increase rate in ohmic resistance and Cr evaporation rate, even without the addition of reactive element such as La. When the same amount of Cr is deposited on the cathode, the cathode performance deteriorates more at the high Cr evaporation rate than at the low Cr evaporation rate.
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