Transition metal nitride, separator for fuel cells, fuel cell stack, fuel cell vehicle, method of manufacturing transition metal nitride, and method of manufacturing separator for fuel cells

Abstract

A transition metal nitride is obtained by a nitriding treatment of a surface of a base material including a transition metal or an alloy of the transition metal, and the transition metal nitride has a crystal structure of an M 4 N type and a crystal structure of an ε-M 2˜3 N type, and is formed over a whole area of the surface of the base material and continuously in a depth direction from the surface.

1 . A transition metal nitride obtained by a nitriding treatment of a surface of a base material including a transition metal or an alloy of the transition metal, wherein
 the transition metal nitride has a crystal structure of an M 4 N type and a crystal structure of an ε-M 2˜3 N type, and is formed over a whole area of the surface of the base material and continuously in a depth direction from the surface.   
     
     
         2 . The transition metal nitride as claimed in  claim 1 , wherein the transition metal nitride is a complex tissue including a crystal layer comprising a matrix of crystal structures of the M 4 N type and crystal structures of the ε-M 2˜3 N type formed in the matrix, and has a laminate structure in which crystal structures of the M 4 N type and crystal structures of the ε-M 2˜3 N type are repeatedly laminated. 
     
     
         3 . The transition metal nitride as claimed in  claim 1 , wherein the nitrided layer has crystal structures of the ε-M 2˜3 N type within a range of inter-layer distances of several tens to 100 nm. 
     
     
         4 . The transition metal nitride as claimed in  claim 1 , wherein the crystal structure of the M 4 N type comprises a structure having a nitrogen atom disposed in an octahedral void at a unit cell center of a face-centered cubic lattice formed by transition metal atoms selected from among Fe, Cr, Ni, and Mo. 
     
     
         5 . The transition metal nitride as claimed in  claim 1 , wherein the base material comprises a stainless steel containing transition metal atoms selected from among Fe, Cr, Ni, and Mo. 
     
     
         6 . The transition metal nitride as claimed in  claim 1 , wherein the base material contains 18 wt % or more of Cr and 10 wt % or more of Ni. 
     
     
         7 . The transition metal nitride as claimed in  claim 1 , wherein the base material contains 25 wt % or more of Cr. 
     
     
         8 . The transition metal nitride as claimed in  claim 1 , wherein the crystal structure of the M 4 N type comprises a structure having a nitrogen atom disposed in an octahedral void at a unit cell center of a face-centered cubic lattice formed by transition metal atoms selected from among Fe, Cr, Ni, and Mo. 
     
     
         9 . The transition metal nitride as claimed in  claim 1 , wherein the transition metal nitride has a nitrogen amount of 5 at % or more and an oxygen amount of 50 at % or less in a most superficial layer down to a 5 nm depth from a most superficial surface of the base material. 
     
     
         10 . The transition metal nitride as claimed in  claim 9 , wherein the transition metal nitride has an O/N ratio of 10.0 or less for the oxygen amount to the nitrogen amount in the most superficial layer down to the 5 nm depth from the most superficial surface. 
     
     
         11 . The transition metal nitride as claimed in  claim 1 , wherein the transition metal nitride has a nitrogen amount of 10 at % or more and an oxygen amount of 30 at % or less at a 10 nm depth from a most superficial surface of the base material. 
     
     
         12 . The transition metal nitride as claimed in  claim 1 , wherein
 the transition metal nitride contains transition metals selected from among Fe, Cr, Ni, and Mo, having Fe as a principal component, and   including a crystal layer comprising a matrix that has crystal structures of the M 4 N type having a nitrogen atom disposed in a position in an octahedral void of a face-centered cubic lattice, and crystal structures of the ˜M 2˜3 N type that have sizes of 10 nm to 30 nm and reside in the matrix, meeting formulas (1) to (4) below:
   (Fe 1-x-y-z Cr x Ni y Mo z ) 4 N 1.1˜1.7   (1) 
   0.19≦x≦0.28  (2) 
   0.11≦y≦0.20  (3) 
   0≦z≦0.01  (4) 
   
     
     
         13 . The transition metal nitride as claimed in  claim 12 , wherein crystal structures of the ε-M 2˜3 N type are finely dispersed in the matrix, and compounded. 
     
     
         14 . The transition metal nitride as claimed in  claim 12 , meeting formulas (5) to (8) below:
   (Fe 1-x-y-z Cr x Ni y Mo z ) 4 N 1.3˜1.7   (5)     0.26≦x≦0.28  (6)     0.13≦y≦0.19  (7)     0≦z≦0.01  (8)   
     
     
         15 . The transition metal nitride as claimed in  claim 12 , wherein a binding energy of Fe-2p electron by an X-ray photoelectron spectroscopy of a newly-formed surface on the transition metal nitride as acid cleaned has a highest relative intensity at a chemical shift position of Fe˜N. 
     
     
         16 . The transition metal nitride as claimed in  claim 12 , wherein crystal structures of the ε-M 2˜3 N type have a thickness within 5 nm to 30 nm, and an inter-layer distance within several tens to 100 nm. 
     
     
         17 . A separator for fuel cells, comprising:
 a base material comprising a transition metal or an alloy of the transition metal; and   a nitrided layer of a transition metal nitride according to  claim 1  formed in a depth direction from a surface of the base material.   
     
     
         18 . The separator for fuel cells as claimed in  17 , wherein the base material comprises a stainless steel containing transition metal elements selected from among Fe, Cr, Ni, and Mo, having Fe as a principal component, meeting formulas (8) to (10) below:
   18 wt %≦Cr≦26 wt %  (8)     11 wt %≦Ni≦21 wt %  (9)     0 wt %≦Mo≦2 wt %  (10)   
     
     
         19 . The separator for fuel cells as claimed in  17 , meeting formulas (11) to (13) below:
   24 wt %≦Cr≦26 wt %  (11)     14 wt %≦Ni≦20 wt %  (12)     0 wt %≦Mo≦1 wt %  (13)   
     
     
         20 . A method of manufacturing a transition metal nitride to be formed on a surface of a base material comprising a transition metal or an alloy of the transition metal, by a plasma nitriding, comprising
 forming, by the plasma nitriding, crystal structures of an M 4 N type and crystal structures of an ε-M 2˜3 N type, over a whole area of the surface of the base material and in a depth direction from the surface.   
     
     
         21 . The method of manufacturing a transition metal nitride as claimed in  claim 20 , wherein the plasma nitriding is performed by using a pulse plasma power supply adapted to repeat a discharge and an interruption of plasma by a period within 1 μsec to 1,000 μsec. 
     
     
         22 . The method of manufacturing a transition metal nitride as claimed in  claim 21 , wherein the base material comprises a stainless steel containing transition metal elements selected from among Fe, Cr, Ni, and Mo, having Fe as a principal component, meeting formulas (14) to (16) below,
 comprising forming, by the plasma nitriding, a nitrided layer including a crystal layer comprising a matrix that has crystal structures of the M 4 N type having a nitrogen atom disposed in a position in an octahedral void of a face-centered cubic lattice formed by transition metal atoms selected from among Fe, Cr, Ni, and Mo, and crystal structures of the ε-M 2˜3 N type of sizes of 10 nm to 30 nm in the matrix:
   18 wt %≦Cr≦26 wt %  (14) 
   11 wt %≦Ni≦21 wt %  (15) 
   0 wt %≦Mo≦5 wt %  (16) 
   
     
     
         23 . A method of manufacturing a separator for fuel cells, comprising plasma nitriding a surface of a base material comprising a transition metal or an alloy of the transition metal, and forming, by the plasma nitriding, a nitrided layer having crystal structures of an M 4 N type and crystal structures of an ε-M 2˜3 N type, over a whole area of the surface and in a depth direction from the surface. 
     
     
         24 . The method of manufacturing a separator for fuel cells as claimed in  claim 23 , wherein the plasma nitriding is performed by using a pulse plasma power supply adapted to repeat a discharge and an interruption of plasma by a period within 1 μsec to 1,000 μsec. 
     
     
         25 . A fuel cell stack having a separator for fuel cells according to  claim 18 . 
     
     
         26 . A fuel cell vehicle including a fuel cell stack according to  claim 25 , as a power source.