Complex oxide ceramic sintered body and solid-electrolyte...

Details Complex oxide ceramic sintered body and solid-electrolyte... Complex oxide ceramic sintered body and solid-electrolyte...

1999-05-04

2001-06-19

Dunn, Tom (Department: 1725)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S047000, C429S047000

Reexamination Certificate

active

06248465

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a complex oxide ceramic sintered body, which is particularly used as a material for separators in solid-electrolyte fuel cells, and to a solid-electrolyte fuel cell using the same.
2. Description of the Related Art
Separators in solid-electrolyte fuel cells must be highly conductive, dense and stable in both oxidizing and reducing atmospheres. The separators must also have a thermal expansion coefficient close to that of yttria-stabilized-zirconia (hereinafter referred to as “YSZ”) which is generally used as a solid electrolyte material.
Although lanthanum chromite (hereinafter referred to as “LaCrO
3
”) is known to be stable in a high-temperature oxidizing atmosphere and in a low oxygen partial pressure atmosphere, LaCrO
3
has poor sintering properties and insufficient denseness. Therefore, when LaCrO
3
is used for separators of solid-electrolyte fuel cells, the fuel gas and air are not completely separated from each other, which is not satisfactory. Additionally, since LaCrO
3
has a significantly low thermal expansion coefficient in comparison with the thermal expansion coefficient (10.0 to 11.0×10
−6
K
−1
) of YSZ used as a solid electrolyte, bonded sections are easily separated because of the difference in volumetric change during heating and cooling. Moreover, LaCrO
3
does not have satisfactory conductivity.
Ceramics in which the site of La in LaCrO
3
is partially substituted by an alkali metal such as Ca or Sr, and in which the site of Cr is partially substituted by a transition metal such as Co, Ni, Cu, Zn, Fe or Mn, are disclosed as perovskite-type oxides which are highly conductive and are easily sintered (refer to Japanese Unexamined Patent Publications Nos. 3-65517, 4-214069, 4-331764, and 6-16471). However, ceramics sintered bodies having these compositions expand in a low oxygen partial pressure atmosphere and are unstable because of a decrease in strength, and thus warping may occur in cells or failure may be caused by small stresses if used under operating conditions in solid-electrolyte fuel cells.
The stability of the ceramic sintered bodies in a low oxygen partial pressure atmosphere is improved by controlling the amount of an alkali metal (for example, Ca or Sr) which partially substitutes the La site and the amount of metallic elements (at least one of Co and Ni, and at least one of Al and Mg) which partially substitute the Cr site, as disclosed in Japanese Unexamined Patent Publication No. 8-59341. However, this ceramic requires a sintering temperature of 1,750° C. or more, and the phase transition temperature thereof has not been examined. Therefore, when the ceramic sintered body is used as a separator in a solid-electrolyte fuel cell, contact failure with the solid-electrolyte material or other components, deformation or the like may occur because of volumetric change due to the phase transitions during repeated heating and cooling.
The phase transition temperature of the ceramic sintered body is decreased to 25° C. or less by partially substituting the La site by an alkali metal (for example, Ca or Sr) and by partially substituting the Cr site by Al only, as disclosed in Japanese Unexamined Patent Publication No. 9-196098. With respect to this ceramic, however, a phase transition temperature of 25° C. is not sufficient in consideration of use and transportation in winter and in cold areas, and a much lower phase transition temperature is desirable. Additionally, it is believed that the thermal expansion coefficient of the ceramic sintered body is not brought close to the thermal expansion coefficient of YSZ used as a solid electrolyte, and thus bonded sections may separate due to differences in volumetric changes during heating and cooling. Moreover, stability in an atmosphere of low oxygen partial pressure has not been fully examined.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to solve the problems described above and to provide a LaCrO
3
-based complex oxide ceramic sintered body which has high sintering density, high strength stability in a low oxygen partial pressure atmosphere, a phase transition temperature of 0° C. or below and a thermal expansion coefficient which is about the same as that of YSZ, and also to provide a solid-electrolyte fuel cell having excellent operating characteristics.
The complex oxide ceramic sintered body has a perovskite-type crystal structure and is represented by a general formula: (La
1-x
Sr
x
)
a
(Cr
1-y-z
Al
y
Co
z
)
b
O
3
, where x+z≧0.06; z≧0.02; x+y+3z≦0.25; 2x+3y+5z≧0.40; x+y≧0.12; and 1.00≦b/a≦1.04. The sintered body has a relative density of about 94% or more and a bending strength of about 15 kgf/mm
2
or more both at room temperature and at room temperature after the sintered compact is treated for 24 hours in an atmosphere of humidified hydrogen having an oxygen partial pressure of 10
−18
atm at 1,000° C. The sintered body further has a lattice volumetric expansion coefficient of about 0.3% or less at room temperature after treated for 24 hours in an atmosphere of humidified hydrogen having an oxygen partial pressure of 10
−18
atm at 1,000° C., an average thermal expansion coefficient in a range of about 9.8 to 11.0×10
−6
K
−1
at 30 to 1,000° C., and a phase transition temperature from a rhombic system to a rhombohedral system of about −20° C. or less.
According to the present invention, it is possible to obtain a LaCrO
3
-based complex oxide ceramic sintered body which has high sintering density, high stability in a low oxygen partial pressure atmosphere at high temperatures, a low phase transition temperature, and a thermal expansion coefficient substantially the same as that of the YSZ solid electrolyte.
Accordingly, by using a complex oxide ceramic sintered body in accordance with the present invention as a practical material for a separator in a solid-electrolyte fuel cell or the like, a solid-electrolyte fuel cell having a long operating life in a general environment as well as in winter and in cold areas and having excellent operating characteristics can be obtained.
For the purpose of illustrating the invention, there is described below several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The inventors of the present invention have found that a LaCrO
3
-based oxide obtained by partially substituting the La site of LaCrO
3
by Sr, by partially substituting the Cr site by Al and Co, and by controlling substitution amounts (x, y, and z) of the individual metallic elements and the amount ratio b/a between atoms located in the B site and atoms being located in the A site in the perovskite crystal structure (ABO
3
) can shape a dense sintered body at relatively low temperatures, and that the obtained sintered body is stable in high-temperature air and in a low oxygen partial pressure atmosphere. It has been further found that the stability of the sintered body in high-temperature air and in a reducing atmosphere is greatly improved by controlling the types and substitution amounts of the individual constituents substituted and dissolved in the sintered compact within specific ranges, respectively, and by setting the relative density of the sintered compact, bending strength before and after reduction treatment, coefficient of lattice volumetric expansion due to reduction treatment, thermal expansion coefficient and phase transition temperature within given ranges, respectively.
The complex oxide ceramic sintered body in accordance with the present invention has a perovskite-type crystal structure represented by a general formula: (La
1-x
Sr
x
)
a
(Cr
1-y-z
Al
y
Co
z
)
b
O
3
, where x+z≧0.06, z≧0.02, x+y+3z≦0.25, 2x+3y+5z≧0.40, x+y≧0.12 and 1.00≦b/a≦1.04. The compl

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