JP5576146B2 - Conductive member of solid oxide fuel cell - Google Patents

Description

  The present invention relates to an interconnector of a solid oxide fuel cell made of ferritic stainless steel, or a conductive member such as a separator and a current collector plate.

In recent years, the practical use of new power generation systems such as wind power generation and solar power generation has been demanded due to problems such as depletion of fossil fuels represented by petroleum and global warming due to CO ₂ emissions. Fuel cells are attracting attention as a clean power generation system as a distributed power source or a power source for automobiles and general households.

  There are several types of fuel cells, mainly polymer electrolyte fuel cells (PEFC). Among them, solid oxide fuel cells (SOFC) have the highest operating temperature and energy efficiency among fuel cells. This power generation system is expected to be put to practical use.

  The operating temperature of solid oxide fuel cells (SOFCs) has conventionally been as high as about 1000 ° C. At that time, ceramics were mainly used, and metal materials with excellent high-temperature oxidation properties (high Cr, high Ni austenitic stainless steel) Steel) was very difficult to use. However, in recent years, the operating temperature can be lowered to about 600 to 800 ° C. by improving the solid electrolyte membrane. This is a temperature range in which application with a metal material is possible.

The required characteristics required for a low temperature operation type solid oxide fuel cell (SOFC) are good electrical conductivity (30 mΩ · cm ² or less) and oxidation resistance in a temperature range of 600 to 800 ° C., and a ceramic solid. A thermal expansion coefficient equivalent to that of an oxide (room temperature to 800 ° C., about 13 × 10 ⁻⁶ (1 / K)) can be sufficiently satisfied, and in addition, when starting and stopping are frequently repeated, heat fatigue resistance is also required.

  High Cr, high Ni austenitic stainless steel, which has excellent resistance to steam oxidation at high temperatures, has a very high coefficient of thermal expansion, causing thermal expansion and contraction in situations where it is frequently started and stopped. Unusable due to thermal deformation and scale peeling. On the other hand, since ferritic stainless steel has the same thermal expansion coefficient as that of the electrolyte, it is most suitable as a material for a high-temperature conductive part of a solid oxide fuel cell.

  For example, in Japanese Patent Application Laid-Open No. 2003-105503, in mass%, C: 0.2% or less, Si: less than 0.2% (not including 0%), Mn: 1.0% or less (including 0%) ), Cr: 15-30%, Ni: 2% or less (including 0%), Al: 1% or less (including 0%), Zr: 1% or less, and Y: 0.5% or less, rare earth elements : Steel for solid oxide fuel cell separators containing one or more of 0.2% or less, the balance being Fe and unavoidable impurities is disclosed. This forms an oxide film having good electrical conductivity at about 700 ° C. to 950 ° C., and these steel types have good oxidation resistance and scale peeling resistance over a long period of use, and are compatible with the electrolyte. It has the feature that the thermal expansion difference is small.

JP 2003-105503 A

  All of these means are expensive and contain rare earth metals and Y, which are rare metals, and thus are expensive and are elements that make manufacturability difficult, leading to an increase in manufacturing cost. That is, there has been a demand for a material for a high-temperature conductive part of a solid oxide fuel cell that has improved electrical conductivity and oxidation resistance without increasing costs more than necessary.

  The present invention is based on conventional ferritic stainless steel, and various studies are made on the steel composition in consideration of the environment of the high-temperature equipment of the fuel cell exposed to a high-temperature steam atmosphere and the electrical conductivity at high temperature. It has been completed. In addition, if necessary, the high temperature strength in the medium to high temperature range is further improved by adding Mo, Nb, etc., and if necessary, heat is added by adding rare earth elements (La, Ce, Nd, etc.), B, and Ca. An object of the present invention is to provide a high-temperature conductive member for a solid oxide fuel cell having improved inter-workability and oxidation resistance and having sufficient durability at 600 to 800 ° C.

In the invention according to claim 1, an oxide layer is formed on the surface layer, and a thickness 0.1 that Nb oxide, Ti oxide, and Al oxide are mixed between the Cr oxide layer and the base material. A conductive member for a high-temperature conductive part of a solid oxide fuel cell excellent in high-temperature electrical conductivity and oxidation resistance, characterized in that an oxide layer of 10 μm is present.
Here, “mixed” means that Nb, Ti, and Al all exist in a concentrated state in a lump shape or a layer shape having a count number more than five times that of the base material in cross-sectional observation by FE-EPMA. .

  The invention according to claim 2 is: C: 0.02 mass% or less, Si: 0.5 mass% or less, Mn: 1.5 mass% or less, S: 0.01 mass% or less, P: 0.05 mass% %: Cr: 18-35% by mass, Nb: 0.15-0.5% by mass, N: 0.02% by mass or less, Al: 0.01-0.3%, Ti: 0.08-0 0.5% by mass, and Nb / (Nb + Ti + Al), Ti / (Nb + Ti + Al), and Al / (Nb + Ti + Al) are all 0.1 or more, and the balance is Fe and inevitable impurities. The conductive member for a high-temperature conductive part of a solid oxide fuel cell excellent in high-temperature electrical conductivity and oxidation resistance according to claim 1.

  The invention according to claim 3 includes, in addition to the components of claim 1, Cu: 0.1 to 1.5 mass%, Mo: 0.1 to 4.0 mass%, W: 0.1 to 4.0 mass% %, Zr: 0.05 to 0.50% by mass, V: 0.02 to 0.2% by mass, Ta: 0.05 to 0.50% by mass, or two or more thereof. Or a conductive member for a high-temperature conductive part of a solid oxide fuel cell having excellent high-temperature electrical conductivity and oxidation resistance described in 2;

  The invention according to claim 4 is excellent in high-temperature electrical conductivity and oxidation resistance according to claims 1 to 3, further comprising 0.0005 to 0.005% of one or two of B and Ca in total. This is a conductive member for a high-temperature conductive part of a solid oxide fuel cell.

  As described above, the ferritic stainless steel of the present invention can provide a material for a high-temperature conductive part of a solid oxide fuel cell having good electrical conductivity and oxidation resistance at high temperatures. This is expected to improve the performance and durability of the solid oxide fuel cell, promote commercialization and popularization in general households, and are expected to lead to improvement of environmental problems.

It is a conceptual diagram of the cross section of the stainless steel surface layer vicinity containing an oxide layer. It is a schematic diagram of the measuring method of film thickness.

  When exposed to the high-temperature steam atmosphere (600 to 800 ° C.) of the fuel cell, the oxidation of the steel material proceeds easily, and the electrical resistance of the conductive portion increases to impair the function of the fuel cell. In addition, when starting and stopping are repeated frequently, damage due to thermal fatigue becomes a problem. These are major problems in applying ferritic stainless steel to materials for high-temperature conductive parts.

  As a means of having electric conductivity at high temperature, an oxide layer having good conductivity is formed on the surface layer, or a special second layer is formed, and electric conduction is performed via the second layer. Can be considered. On the other hand, in order to improve oxidation resistance, the second layer must be in a form having good oxidation resistance. As a means of making both of them compatible, the inventors focused on the combined addition of Nb, Ti, and Al.

  In the case where Al is added alone, an oxide having excellent oxidation resistance is formed. However, if this oxide single layer is used, the electrical conductivity is greatly reduced. Therefore, by forming a composite oxide of Nb and Ti in this single Al layer, it is possible to locally form a conductive portion and ensure oxidation resistance without impairing electrical conductivity. On the other hand, if it is biased toward the oxide of Nb or the oxide of Ti alone, the effect of improving the oxidation rate becomes poor.

  In order to form this Nb, Ti, Al composite oxide, the mass ratio of Nb / (Nb + Ti + Al)> 0.1, Ti / (Nb + Ti + Al)> with respect to the total amount of Nb, Ti, Al in the steel material> It is necessary to maintain the relationship of 0.1 and Al / (Nb + Ti + Al)> 0.1.

  Further, in order to form this composite oxide layer between the Cr oxide and the base material, the temperature is higher than the normal use temperature (600 to 800 ° C.), that is, at a temperature range of 900 to 1100 ° C. for 300 seconds or more. It is necessary to preheat. This makes it possible to obtain an effective composite oxide layer of 0.1 μm or more between the Cr oxide and the base material. When the composite oxide layer has a thickness of 10 μm or more, scale peeling due to thermal stress occurs.

The composition and composition of the ferritic stainless steel used for the high temperature conductive part material of the solid oxide fuel cell were determined as follows.
C, N: 0.02% by mass or less C, N is a component that improves high-temperature strength, particularly creep properties, but if added excessively to ferritic stainless steel, workability and low-temperature toughness are significantly reduced. In addition, carbonitrides are easily generated by reaction with Ti and Nb, and solid solution Ti and solid solution Nb effective in improving high temperature strength are reduced. Therefore, in this component system, the C and N contents are preferably as low as possible, and the upper limit is set to 0.02% by mass or less for both. Preferably it is less than 0.01 mass%.

Si: 0.5% by mass or less Si is an alloy component effective for stabilizing a Cr-based oxide, and is an effective component for improving steam oxidation resistance. However, if an excessive amount of Si exceeding 0.5% by mass or more is contained, a SiO ₂ oxide layer having a high electrical resistance is formed on the surface layer, and the electrical conductivity is lowered. Moreover, low temperature toughness is also reduced, so that flaws are easily generated on the steel surface, and manufacturability is also reduced. Preferably it is 0.2 mass% or less.
Mn: 1.5% by mass or less A component that improves the scale peelability of ferritic stainless steel. If an excess amount of Mn exceeding 1.5% by mass is contained, the steel material becomes hard, and workability and low temperature toughness are improved. descend. Preferably it is 0.3 mass% or less.

S: 0.01% by mass or less S is a component that adversely affects hot workability and weld hot cracking resistance, and also serves as a starting point for abnormal oxidation. Therefore, the S content is desirably as low as possible, and the upper limit is set to 0.01% by mass. Preferably it is 0.003 mass% or less.

Cr: 18 to 35% by mass,
It is a synthetic component necessary for imparting the necessary corrosion resistance, oxidation resistance and electrical conductivity to stainless steel. In order to ensure steam oxidation resistance at around 700 ° C. and good electrical conductivity, 18 mass% or more of Cr is necessary. When exposed to a steam atmosphere of 750 ° C. or higher, addition of 20% by mass or more is preferable. The addition of Cr exceeding 35% by mass reduces the workability, low temperature toughness and 475 ° C. embrittlement susceptibility of ferritic stainless steel, so the upper limit was made 35% by mass.

Cu: 0.1-1.5 mass%
Cu improves the high temperature strength and thermal fatigue characteristics by solid solution or precipitation strengthening, and remarkably improves the electrical conductivity at 600 to 800 ° C. This effect becomes remarkable by the addition of 0.1% by mass or more, but if the excessive amount of Cu is contained, the steel material becomes excessively hardened, so the upper limit was set to 1.5% by mass. Preferably it is more than 0.4 mass%-1.5 mass%, More preferably, it is more than 1.0%-1.5 mass%.

Mo: 0.1-4.0% by mass, W: 0.1-4.0% by mass
Since Mo improves the high temperature strength and heat fatigue resistance by solid solution strengthening, it is added as necessary, particularly in a portion where thermal fatigue characteristics are considered. If an excessive amount of Mo or W is contained, the steel material becomes excessively hardened, so the upper limit was set to 4.0% by mass.

Nb: 0.15 to 0.50 mass%, Ti: 0.08 to 0.50 mass%
It is an important element in the present invention, and exhibits conductivity by forming a composite oxide layer together with Al.
Moreover, it contributes to the high temperature strength of ferritic stainless steel by solid solution or precipitation strengthening. When Ti: 0.08% by mass or more and Nb: 0.15% by mass or more, the effect of addition becomes remarkable. However, since steel materials harden | cured excessively when contained excessively, each upper limit was set to 0.50 mass%.

Al: 0.01 to 0.3% by mass,
Al is not only used as a deoxidizing material, but also forms an Al ₂ O ₃ oxide film on the oxidation-resistant surface, and therefore is an element that remarkably improves high-temperature oxidation resistance, and is added as necessary. The effect becomes remarkable by addition of 0.02 mass% or more. However, excessive addition not only reduces workability and weldability, but also impairs the electrical conductivity of the high-temperature conductive part material, so the upper limit was made 0.3 mass%. Preferably it is 0.02-0.2 mass%.

Zr: 0.05 to 0.50 mass%, V: 0.02 to 0.20 mass%, Ta: 0.05 to 0.50 mass%
All are components added as necessary, and further improve the high temperature strength of ferritic stainless steel by solid solution or precipitation strengthening. V: 0.02% by mass or more, Zr, Ta: 0.05% by mass or more, the effect of addition becomes remarkable. However, when an excessive amount of Zr, V and Ta is contained, the steel material is excessively hardened, so the upper limits of the respective materials were set to V: 0.20% by mass and Ta, Zr: 0.50% by mass.

B, Ca: 0.0005 to 0.005 mass% in total
It is an element that improves the hot workability and oxidation resistance of stainless steel, and one or two are added as necessary. On the contrary, excessive addition lowers the hot workability and the surface properties of the steel surface, so the upper limit was made 0.005 mass% in total.

  Other components are not particularly defined in the present invention, but it is preferable to reduce general impurity elements such as O and Ni as much as possible. Usually, O: 0.02 mass% or less and Ni: 2.0 mass% or less are regulated, but O and Ni are more strictly regulated in order to ensure a high level of workability and weldability. Re, which is effective for improving strength, Sn, which is effective for improving free machinability, and elements such as Co, Hf, Sc, which are effective for improving hot workability, are also required if Re ≦ 2.0 mass%, Sn ≦ 1.0 mass%, Co: ≦ 2.0 mass%, Hf: ≦ 1.0 mass%, Sc: 0.1 mass% can be added to the upper limit.

  Various ferritic stainless steels having the components and compositions shown in Table 1 were melted in a 30 kg vacuum melting furnace and forged into ingots. After roughly rolling the ingot, a cold-rolled annealed material having a plate thickness of 1.5 mm or 0.2 mm was manufactured through hot rolling, annealing pickling, cold rolling, and finish annealing.

  A test piece was cut out from each ferritic stainless steel, and the material was polished and finished with # 400 by wet polishing as the final finishing condition, and the electrical resistivity at 800 ° C. and the steam oxidation resistance at 930 ° C. were measured. .

The electrical resistivity was measured using a test piece processed to a plate thickness of 1.5 mm and 20 mm × 20 mm, and this test piece was lanthanum strontium manganese (La _0.6 -Sr _0.4 -Mn ₁₎ having a radius of 9 mm from both sides. ) Pt paste is applied and sandwiched between solid oxide discs, platinum electrodes for current supply are arranged above and below the sandwich measurement sample, and the test piece and lanthanum strontium manganese (La _0.6 -Sr _0.4) -Mn ₁ ) A load was applied on the platinum electrode so that the contact pressure of the contact surface of the solid oxide disk made of 2 kg / cm ² was applied, and a constant current of 10 mA was passed between the platinum electrodes to sandwich the test piece. Resistance measurement was performed by measuring a potential difference between solid oxides made of lanthanum strontium manganese (La _0.6 -Sr _0.4 -Mn ₁ ). In this state, the temperature was raised to 1000 ° C. in the atmosphere, held for 6 hours, sintered, then lowered to 800 ° C., and the electrical resistance was measured every 200 hours up to 1000 hours while maintaining 800 ° C. Converted to (area resistivity). The electrical conductivity was evaluated by assuming that the value of electrical resistivity was 25 mΩ · cm ² or less as ◯, and the value having an electrical resistance value exceeding 25 mΩ · cm ² as x.

Oxidation resistance was obtained by using a test piece cut to a plate thickness of 0.2 mm and 25 mm × 35 mm and heat-treated at 1000 ° C. for 6 hours in an air atmosphere. The oxidation test was performed in an air atmosphere containing 930 ° C. and 20% water vapor as acceleration conditions. The oxidation increase after 400 h was 1.5 mg / cm ² or less, and the one exceeding 1.5 mg / cm ² was rated as x. All of these are necessary and sufficient specifications as a high-temperature conductive member of a solid oxide fuel cell used at 600 to 800 ° C. The formation state of the complex oxide was observed by cross-section using FE-EPMA, and it was confirmed that an oxide having a form as shown in FIG. 1 was formed. The film thickness was measured by performing FE-EPMA surface analysis and measuring layer A (oxide containing a mixture of Ti, Nb, and Al oxides) as shown in FIG. That is, in the layer A, it was confirmed that Ti, Al, and Nb were all present in a lump-like or layered state having a count number more than five times that of the base material. The thickness of the layer A is determined by measuring the average unevenness χ of the unevenness in the layer length of 50 μm, ○ in the range of 0.1 to 10 μm, and × in which no complex oxide was observed It was.

As can be seen from the test results in Table 2, the ferritic stainless steels of steel types A1 to A15 according to the present invention have an oxidation resistance within the range of 0.1 to 10 μm, and the electrical conductivity is within the range of 0.1 to 10 μm. Also, the electrical resistivity of each was 25 mΩ · cm ² or less. On the other hand, the ferritic stainless steels of comparative steel type numbers B1 to B6 were judged to be unsuitable as materials for high-temperature conductive parts because they could not achieve both oxidation resistance and electrical conductivity.

  According to the present invention, it is possible to provide a high-temperature conductive member for a solid oxide fuel cell with improved electrical conductivity and oxidation resistance without increasing costs more than necessary.

1 Stainless steel base material 2 Internal oxide 3 A layer (oxide mixed with oxides of Ti, Al, Nb)
4 Cr ₂ O ₃ layers

Claims (4)

An oxide layer is formed on the surface layer, and Nb oxide, Ti oxide, and Al oxide are mixed between the Cr oxide layer and the base material . Ti, Al, and Nb are all in the base material. What is confirmed to be present in a concentrated state in the form of a lump or a layer having a count number of 5 times or more is defined as a predetermined composite oxide layer, and the average distance χ thickness of unevenness in the layer length of 50 μm is 0 Conductive member for a high-temperature conductive part of a solid oxide fuel cell excellent in high-temperature electrical conductivity and oxidation resistance, characterized by being a ferritic stainless steel having an oxide layer of 1 to 10 μm . Ferritic stainless steel is C: 0.02 mass% or less, Si: 0.5 mass% or less, Mn: 1.5 mass% or less, S: 0.01 mass% or less, P: 0.05 mass% or less, Cr: 18 to 35% by mass, Nb: 0.15 to 0.5% by mass, N: 0.02% by mass or less, Al: 0.01 to 0.3%, Ti: 0.08 to 0.5% by mass %, And Nb / (Nb + Ti + Al), Ti / (Nb + Ti + Al), and Al / (Nb + Ti + Al) are each 0.1 or more, and the balance is made of Fe and inevitable impurities. The conductive member for high temperature conductive parts of the solid oxide fuel cell according to claim 1 .   In addition to the components of claim 2, Cu: 0.1-1.5% by mass, Mo: 0.1-4.0% by mass, W: 0.1-4.0% by mass, Zr: 0.05- 3. High-temperature electricity according to claim 1, comprising one or more of 0.50 mass%, V: 0.02 to 0.2 mass%, Ta: 0.05 to 0.50 mass%. A conductive member for a high temperature conductive part of a solid oxide fuel cell having excellent conductivity and oxidation resistance.   The solid oxide fuel cell excellent in high-temperature electrical conductivity and oxidation resistance according to claim 1, further comprising 0.0005 to 0.005% in total of one or more of B and Ca. Conductive member for high temperature conductive parts.

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