Patent Publication Number: US-2023163323-A1

Title: Metal member and manufacturing method thereof

Description:
CROSSREFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Application (Japanese Patent Application No. 2020-047270, filed on Mar. 18, 2020; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments of the present invention described herein relate generally to a metal member used in a solid-oxide type electrochemical stack, and a manufacturing method thereof. 
     BACKGROUND 
     An electrochemical device includes electrochemical cells each having an electrolyte membrane interposed between a fuel electrode and an air electrode. In general, the electrochemical device is formed of an electrochemical stack in which a plurality of electrochemical cells are stacked in order to increase the power output. 
     A solid-oxide type electrochemical stack includes electrochemical cells each having an electrolyte membrane composed of solid oxides, and the electrochemical cells can be used as solid oxide fuel cells (SOFC; Solid Oxide Fuel Cell) or solid oxide electrolysis cells (SOEC; Solid Oxide Electrolysis Cell). 
     Concretely, when used as the SOFC, electric energy is obtained by the reaction between hydrogen supplied to the fuel electrode and oxygen supplied to the air electrode through the electrolyte membrane, for example, under high temperature conditions. In contrast to this, when used as the SOEC, hydrogen is generated at the fuel electrode and oxygen is generated at the air electrode by water (steam) being electrolyzed, for example, under high temperature conditions. 
     The electrochemical cells are classified into a flat plate type, a cylindrical type, a cylindrical flat plate type, and so on according to their shape. For example, a flat-plate type electrochemical cell includes an air electrode, an electrolyte, and a fuel electrode each having a flat shape, and is composed of respective parts being stacked. Then, the electrochemical stack includes a separator interposed between a plurality of the electrochemical cells. The separator is a metal member and electrically connects a plurality of the electrochemical cells. Further, in the separator, a gas flow path is formed. 
     The metal member such as a separator used in the solid-oxide type electrochemical stack is required to have sufficient strength at high operating temperatures (600 to 1000° C.) and excellent oxidation resistance. In addition, the metal member is required to have a thermal expansion coefficient close to that of the electrochemical cell. For this reason, the metal member is often formed using, for example, ferritic stainless steel. 
     However, the ferritic stainless steel often contains chromium. Therefore, when chromium volatilizes, the ferritic stainless steel reacts with the materials composing the electrochemical cell and the performance of the electrochemical cell is sometimes degraded. In order to suppress the volatilization of chromium, the metal member generally includes a base of ferritic stainless steel coated with a coating layer. In addition to having the function of suppressing the volatilization of chromium, the coating layer of the metal member is required to have high electrical conductivity and a thermal expansion coefficient close to that of the electrochemical cell and the metal member. The coating layer of the metal member is formed using conductive oxides, for example, so as to meet these requirements. 
     Among the conductive oxides used in the coating layer of the metal member, spinel-based materials have attracted attention because they are high in conductivity and capable of effectively suppressing the volatilization of chromium. Conventionally, the deposition of the spinel-based material has been performed by attaching a spinel powder to the metal member. Recently, however, there has been proposed to electrochemically form a metal film on a base as a precursor by a plating method, and then sinter the metal film. For example, a Cu plating layer and a Mn plating layer are sequentially stacked on a thin Ni plating layer. Thereafter, Cu in the Cu plating layer and Mn in the Mn plating layer are mixed by heating to thereby form a composite oxide film containing Cu and Mn as the coating layer. This method is superior in terms of denseness, workability, and cost. 
     Conventionally, however, in the metal member used in the solid-oxide type electrochemical stack, the coating layer sometimes peels off from the base. 
     The problem to be solved by the present invention is to provide a metal member capable of effectively preventing a coating layer from peeling off from a base. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a view illustrating a cross section of a metal member 1 according to an embodiment. 
         FIG.  2    is a view illustrating a cross section of a metal member 1 according to a modified example of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A metal member in an embodiment is a metal member used in a solid-oxide type electrochemical stack, and includes a base formed of ferritic stainless steel and a metal film provided on the base, in which the metal film includes a first metal layer containing Co and a second metal layer made of Mn, and is a stack in which the first metal layer and the second metal layer are sequentially stacked from the side of the base. 
     According to this embodiment, it is possible to provide a metal member capable of effectively preventing a coating layer from peeling off from a base, and a manufacturing method thereof. 
     [A] Regarding a Metal Member  1   
       FIG.  1    is a view illustrating a cross section of a metal member  1  according to an embodiment. Here, a part of the metal member  1 , such as a separator used in a solid-oxide type electrochemical stack, is illustrated in an enlarged manner. 
     As illustrated in  FIG.  1   , the metal member  1  in this embodiment includes a base  10  and a metal film  20 . 
     In the metal member  1 , the base  10  is formed of ferritic stainless steel. The ferritic stainless steel of the base  10  is desirably made of, in mass%, C: 0.05% or less, Si: 0.15% or less, Mn— 0.5% or less, Cr: 25% or less, Al: 0.15% or less, Zr— 0.3% or less, La: 0.1% or less, W: 3.0% or less, Cu— 2% or less, and the balance composed of Fe and impurities. 
     In the metal member  1 , the metal film  20  is provided on the base  10 . Here, the metal film  20  is a stack composed of a first metal layer  21  and a second metal layer  22 , in which the first metal layer  21  and the second metal layer  22  are sequentially stacked from the side of the base  10 . 
     Of the metal film  20 , the first metal layer  21  is a Co layer made of Co. In contrast to this, the second metal layer  22  is a Mn layer made of Mn, for example. 
     The first metal layer  21  and the second metal layer  22  composing the metal film  20  can be formed by various deposition methods, such as a plating method, a sputtering method, a thermal spraying method, and a vapor deposition method. 
     Then, the formed metal film  20  is heated at around 600 to 1000° C., for example, during the operation of a solid-oxide type electrochemical stack. Thereby, in the metal film  20 , the metal element forming the first metal layer  21  and the metal element forming the second metal layer  22  are brought into a mixed state by thermal diffusion to be spinel (oxide), and thereby a coating layer (whose illustration is omitted) formed of a composite oxide is formed from the metal film  20 . 
     The thicknesses of the first metal layer  21  and the second metal layer  22  composing the metal film  20  are adjusted appropriately so as to make the element ratio of the composite oxide forming the coating layer desired. 
     [B] Summary 
     In this embodiment, the metal film  20  includes the first metal layer  21  made of Co and the second metal layer  22  made of Mn, and is a stack in which the first metal layer  21  and the second metal layer  22  are sequentially stacked from the side of the base  10 . Although details will be described later, this configuration can effectively prevent the coating layer of the composite oxide, which is formed by the metal film  20  turning into spinel, from peeling off from the base  10 . 
     [C] Modified Example 
     In the above-described embodiment, the case where the first metal layer  21  is a Co layer made of Co has been explained, but the above-described embodiment is not limited to this. The first metal layer  21  may be a Co-containing layer and may further contain at least one element of Fe, Cu, Ni, Zn, and Mo, in addition to Co. In this case, although details will be described later, the thermal expansion coefficient of a composite oxide is closer to that of the base  10  than the case where the first metal layer  21  is a Co layer made of Co. As a result, it is possible to more effectively prevent peeling of the coating layer of the composite oxide. 
     In the above-described embodiment, the case where the metal film  20  is a stack composed of two layers of the first metal layer  21  and the second metal layer  22  has been explained, but the above-described embodiment is not limited to this.  FIG.  2    is a view illustrating a cross section of a metal member  1  according to a modified example of the embodiment. As illustrated in  FIG.  2   , the metal film  20  may be a stack containing a third metal layer  23 , in addition to the first metal layer  21  and the second metal layer  22 . The third metal layer  23  is a metal layer made of one of Fe, Cu, Ni, Zn, and Mo. In this case, although details will be described later, the thermal expansion coefficient of a composite oxide is closer to that of the base  10  than the case where the third metal layer  23  is not included. As a result, it is possible to more effectively prevent peeling of the coating layer of the composite oxide. 
     Incidentally, in  FIG.  1    and  FIG.  2   , the metal film  20  is formed on one surface of the base  10 , but the metal film  20  may also be formed on the other surface of the base  10 . 
     Example 
     There will be explained examples and a comparative example while using Table 1. 
     Table 1 illustrates compositions of the metal film  20  and test results in terms of the examples and the comparative example.  
     
       
         
          TABLE  1 

           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 7 
                 Comparative example 
               
               
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
                 Element 
                 Element ratio 
               
             
            
               
                 Composition of metal film  20 
 
                 First metal layer  21 
 
                 Co 
                 7 
                 Co 
                 7 
                 Co 
                 7 
                 Co 
                 7 
                 Co 
                 7 
                 Co 
                 7 
                 CoNi 
                 8 
                 Mn 
                 3 
               
               
                 Second metal layer  22 
 
                 Mn 
                 3 
                 Mn 
                 2 
                 Mn 
                 2 
                 Mn 
                 2 
                 Mn 
                 2 
                 Mn 
                 2 
                 Mn 
                 2 
                 Co 
                 7 
               
               
                 Third metal layer  23 
 
                 - 
                 - 
                 Fe 
                 1 
                 Cu 
                 1 
                 Ni 
                 1 
                 Zn 
                 1 
                 Mo 
                 1 
                 - 
                 - 
                 - 
                 - 
               
               
                 Test results 
                 Thermal expansion coefficient of metal film  20  (x 10 -6 /°C) 
                 600° C. 
                 9.6 
                 11 
                 10 
                 10.1 
                 10 
                 10.3 
                 10.1 
                 Voids exist (measurement is impossible) 
               
               
                 1000° C. 
                 14.5 
                 14.6 
                 14 
                 14 
                 13.5 
                 13.7 
                 14 
                 Voids exist (measurement is impossible) 
               
               
                 Difference in thermal expansion coefficient between base  10  and metal film  20  ( X  10 -6 /°C) 
                 600° C. 
                 0.9 
                 -0.5 
                 0.5 
                 0.4 
                 0.5 
                 0.2 
                 0.4 
                 Voids exist (measurement is impossible) 
               
               
                 1000° C. 
                 -0.7 
                 -0.8 
                 -0.2 
                 -0.2 
                 0.3 
                 0.1 
                 -0.2 
                 Voids exist (measurement is impossible) 
               
            
           
         
       
     
     Fabrication of the Metal Member  1   
     Example 1 
     In Example 1, a metal film  20  was formed by sequentially stacking a first metal layer  21  and a second metal layer  22  on a base  10  as illustrated in Table 1 (see  FIG.  1   ). 
     Concretely, first, the base  10  formed of ferritic stainless steel was prepared. As the base  10 , a base formed of ferritic stainless steel having the following composition was prepared. Here, there was used ferritic stainless steel made of, in mass%, C: 0.02%, Si: 0.1%, Mn: 0.3%, Cr: 24%, Al: 0.1%, Zr: 0.25%, La: 0.071%, W: 2.0%, Cu: 1%, and the balance composed of Fe and impurities. 
     Then, the first metal layer  21  made of Co was formed on the base  10  by a plating method. Thereafter, the second metal layer  22  made of Mn was formed on the first metal layer  21  by a plating method. 
     Here, the thicknesses of the first metal layer  21  and the second metal layer  22  were adjusted so as to obtain an element ratio of Co: Mn = 7:3, to then form each part. Concretely, in consideration of the atomic weight and density, the first metal layer  21  was formed so as to obtain its thickness of 7 µm, and the second metal layer  22  was formed so as to obtain its thickness of 3 µm. In other words, the metal film  20  was formed so as to obtain its thickness of 10 µm. 
     Then, the metal film  20  was heated at around 700° C., which is an operating temperature of the solid-oxide type electrochemical stack. 
     Thereby, in the metal film  20 , the metal element forming the first metal layer  21  and the metal element forming the second metal layer  22  were brought into a mixed state by thermal diffusion to be spinel, and thereby a coating layer (whose illustration is omitted) formed of a composite oxide was formed. 
     Example 2 
     In Example 2, as illustrated in Table 1, unlike the case of Example 1, the first metal layer  21 , the second metal layer  22 , and a third metal layer  23  were stacked on the base  10  (see  FIG.  2   ). 
     Concretely, first, the first metal layer  21  made of Co was formed on the base  10  by a plating method. Thereafter, the second metal layer  22  made of Mn was formed on the first metal layer  21  by a plating method. Further, the third metal layer  23  made of Fe was formed on the second metal layer  22  by a plating method. 
     Here, the thicknesses of the first metal layer  21 , the second metal layer  22 , and the third metal layer  23  were adjusted so as to obtain an element ratio of Co: Mn: Fe = 7: 2: 1, to then form each part. Concretely, in consideration of the atomic weight and density, the first metal layer  21  was formed so as to obtain its thickness of 7 µm, and the second metal layer  22  was formed so as to obtain its thickness of 2 µm. Then, the third metal layer  23  was formed so as to obtain its thickness of 1 µm. In other words, the metal film  20  was formed so as to obtain its thickness of 10 µm. 
     Then, in the same manner as in the case of Example 1, the metal film  20  was heated. Thereby, in the metal film  20 , the metal elements forming the first metal layer  21 , the second metal layer  22 , and the third metal layer  23  were brought into a mixed state by thermal diffusion to be spinel, and thereby a coating layer (whose illustration is omitted) formed of a composite oxide was formed. 
     Example 3 
     In Example 3, as illustrated in Table 1, the fabrication was performed by the same step as in Example 2, except for the point that the third metal layer  23  was made of Cu. 
     Example 4 
     In Example 4, as illustrated in Table 1, the fabrication was performed by the same step as in Example 2, except for the point that the third metal layer  23  was made of Ni. 
     Example 5 
     In Example 5, as illustrated in Table 1, the fabrication was performed by the same step as in Example 2, except for the point that the third metal layer  23  was made of Zn. 
     Example 6 
     In Example 6, as illustrated in Table 1, the fabrication was performed by the same step as in Example 2, except for the point that the third metal layer  23  was made of Mo. 
     Example 7 
     In Example 7, as illustrated in Table 1, the fabrication was performed by the same step as in Example 1, except for the point that the first metal layer  21  was made of CoNi and the thickness of the first metal layer  21  was 8 µm. Incidentally, as for CoNi, the element ratio (molar ratio) of the amount of substance of the Co element [Co] and the amount of substance of the Ni element [Ni] has the following relationship. 
     
       
         
           
             
               
                 Co 
               
             
             : 
             
               
                 Ni 
               
             
             = 
             87.5 
             : 
             12.5 
           
         
       
     
     Comparative Example 
     In the comparative example, as illustrated in Table 1, a first metal layer  21  made of Mn was formed on a base  10  by a plating method so as to obtain its thickness of 3 µm. Thereafter, a second metal layer  22  made of Co was formed on the first metal layer  21  by a plating method so as to obtain its thickness of 7 µm. Except for this point, in the comparative example, the fabrication was performed by the same step as in Example 1 (see  FIG.  1   ). 
     Thermal Expansion Coefficient 
     Table 1 illustrates the thermal expansion coefficient of the metal film  20  and the difference in the thermal expansion coefficient between the base  10  and the metal film  20 . Here, the results obtained by performing measurements under the conditions of 600° C. and 1000° C. are illustrated. The measurement of the thermal expansion coefficient was performed in accordance with JIS Z 2285: 2003. 
     As illustrated in Table 1, in the examples, the difference in the thermal expansion coefficient between the base  10  and the metal film  20  was small, and the metal film  20  did not peel off from the base  10 . In contrast to this, in the comparative example, the metal film  20  peeled off from the base  10 , failing to measure the thermal expansion coefficient. 
     Incidentally, there has been explained, as an example, the case where the metal film  20  in each of the above-described examples has the ratio (element ratio) of the amount of substance of Co [Co], the amount of substance of Mn [Mn], and the amount of substance of Fe, Cu, Ni, Zn, or Mo [M] having the relationship described in the following equation (A). However, the metal film  20  can obtain suitable effects similarly even when the respective amounts of substances have the relationship described in the equation (B). 
     
       
         
           
             
               
                 Co 
               
             
             : 
             
               
                 Mn 
               
             
             : 
             
               M 
             
             = 
             7 
             : 
             2 
              to  
             3 
             : 
             0 
              to 1 
           
         
       
     
     
       
         
           
             
               
                 Co 
               
             
             : 
             
               
                 Mn 
               
             
             : 
             
               M 
             
             = 
             4 
              to 8 
             : 
             0.5 
              to  
             3 
             : 
             0.5 
              to  
             3 
           
         
       
     
     The case where the value of [Co] is other than the values described in (B) indicates that voids and a peeling problem sometimes occur. The case where the value of [Mn] is the value falling outside the range described in (B) indicates that voids and a peeling problem sometimes occur. The case where the value of [M] is other than the values described in (B) indicates that voids and a peeling problem sometimes occur. 
     Others 
     While certain embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 
     REFERENCE SIGNS LIST 
       1  ... metal member,  10 ...base,  20 ...metal film,  21 ...first metal layer,  22 ...second metal layer,  23 ...third metal layer