Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2002-04-26
2004-08-17
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S070000, C148S325000
Reexamination Certificate
active
06776956
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a steel used in the separators of solid-oxide type fuel cells.
Because of their excellent features, such as high power generation efficiency, small emissions of SOx, NOx and CO
2
, good response to load variations and compact size, fuel cells are expected to be used in a wide range of applications in power generation systems such as a large-scale centralized generation type, a decentralized generation type provided near cities, and an independent power plant type, as a substitute for thermal power generation.
Types of fuel cells are sorted according to the used electrolyte into the phosphoric acid type, the fused carbonate type, the solid-oxide type and the solid-polymer type. Among others, the solid-oxide type fuel cells use as the electrolyte thereof ceramics such as stabilized zirconia and have been operated at high temperatures near 1000° C.
The above solid-oxide type fuel cell is regarded to be very promising as the next-generation power supply source because it has excellent features as explained below. That is, because the solid-oxide type fuel cell is operated at high temperatures, it is unnecessary to use a catalyst for electrode reactions, the internal modification of fossil fuels by high temperatures being possible and various kinds of fuels such as coal gas being able to be used, the high-efficiency power generation being possible by a so-called combined-cycle power generation in which a combination with a gas turbine, steam turbine, etc. is adopted by utilizing high-temperature waste heat, and the solid-oxide fuel cell is compact because all components are solids.
However, there are still many problems to be examined in the practical application of the solid-oxide type fuel cell, and particularly in the case of a planar type fuel cell that permits a high output density, a separator is present as an important component.
This separator supports the three layers of electrolyte, anode and cathode, define gas passages, and at the same time causes electric currents to flow. Therefore, the separator is required to provide properties such as electrical conductivity at high temperatures, oxidation resistance, and a small difference in thermal expansion from the electrolyte and hence in consideration of such required properties, electrically conductive ceramics have been frequently used. However, because ceramics have inferior machinability and are expensive, there are problems in terms of large size design and practical application of fuel cells.
For this reason, the development of a separator made of a metallic material that is inexpensive and reliable is demanded. When a usual metallic material is used at 1000° C., the surface of the metallic material is oxidized and an oxide film is formed. However, when a metallic material is used as the material for the separator, it is necessary that this oxide film be stable so that oxidization does not proceed, and at the same time, it is necessary that the oxide film have an electrical conductivity.
In order to meet such required properties, in JP-A-6-264193 is proposed as the metallic material for solid-oxide type fuel cells an austenitic stainless steel that consists of not more than 0.1% C, 0.5 to 3.0% Si, not more than 3.0% Mn, 15 to 30% Cr, 20 to 60% Ni, 2.5 to 5.5% Al, and the balance of Fe.
Also, in JP-A-7-166301 is proposed as a separator of solid-electrolyte type fuel cells an alloy containing 60 to 82% Fe, 18 to 40% Cr, and additive elements that reduce the contact resistance between the above-described single cell and the cathode (La, Y, Ce or Al being singly added). Furthermore, in JP-A-7-145454 is proposed as a metallic material for solid-electrolyte type fuel cells a material that comprises 5 to 30% Cr, 3 to 45% Co, not more than 1% La, and the balance of Fe.
Recently, however, dramatic progress has been made in the improvement of solid-electrolyte type fuel cells and it has become possible to lower operating temperature from conventional levels from about 1000° C. to the range of 700 to 950° C. or so. Therefore, it is expected that practical application will be accelerated.
Because the material disclosed in JP-A-6-264193 contains considerable amounts of Al and Cr, the oxide film on the surface comprises Al-base oxides as the main composition and further contains Cr-base oxides. However, as is described below, because Al-base oxides have a low electrical conductivity, this material is not always sufficient for use in separators of solid-oxide type fuel cell, and besides because austenitic stainless steels have a larger coefficient of thermal expansion than the stabilized zirconia of electrolyte, they are apt to cause a deterioration of the performance of cells caused by the crack formation etc. of the electrolyte due to the heat cycles associated with the start and stop of cells, thus posing a problem in stability in the case of long time of use. In addition, because expensive Ni is contained in large amounts, the price is high and this is not sufficient for the practical application of fuel cells.
In contrast to this, the materials disclosed in JP-A-7-166301 and JP-A-7-145454 have lower values of coefficient of thermal expansion than the austenitic stainless steels and these values of coefficient of thermal expansion are close to that of the stabilized zirconia of electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use and besides they have good electrical conductivity. However, the oxidation resistance after long time of use is insufficient, and particularly these materials promote the phenomenon of exfoliation associated with an increase in oxide films with the result that the grooves provided in the separator as gas passages in the cell are narrowed, thus posing the problem of a deterioration of the cell function.
Also, the materials disclosed in JP-A-8-35042 and JP-A-8-277441 have lower values of coefficient of thermal expansion than austenitic stainless steels, and these values are close to that of the stabilized zirconia of the electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use. However, electrical conductivity, which is important as a property of a separator material, is not taken into consideration at all.
SUMMARY OF THE INVENTION
Also, the materials disclosed in JP-A-9-157801 and JP-A-10-280103 have low values of coefficient of thermal expansion up to 1000° C. and these values are close to that of the stabilized zirconia of electrolyte. Therefore, these materials are favorable in terms of stability in the case of long time of use and besides they have also good oxidation resistance at 1000° C. and have a good electrical conductivity.
However, the known materials described above were all developed to obtain good properties as separators of solid-oxide type fuel cells that operate at 1000° C. and the properties at the operating temperatures of recent solid-oxide type fuel cells of 700 to 950° C. or so are not taken into consideration at all.
Because the solid-oxide type fuel cells repeat operations and stops, stresses due to heat cycles act on the separator, which is a cell component. Because there is a fear of a failure of the separator due to thermal stresses especially in the case of low impact properties at room temperature, it is necessary for a steel for separators to have impact properties high enough to withstand thermal stresses. In the known materials, however, impact properties at room temperature are not taken into consideration at all.
The object of the invention is to provide a steel for separators of solid-oxide type fuel cells, which steel makes oxide films having good electrical conductivity at 700 to 950° C. or so, which steel has good oxidation resistance and, in particular, the resistance to exfoliation even in the case of long time of use, which steel is excellent in impact properties at room temperature, which steel is small in the difference of thermal expansion between the electrolyte and the steel, and which steel is
Ohno Takehiro
Toji Akihiro
Uehara Toshihiro
Hitachi Metals Ltd.
Sughrue & Mion, PLLC
Yee Deborah
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