JPH04219364A - Lanthanum chromite-based oxide and its use - Google Patents

Abstract

PURPOSE:To improve electric conductivity by mixing raw material powders, then calcining the resultant powder mixture, subsequently burning the obtained double oxide and providing a sintered compact expressed by a specific formula. CONSTITUTION:Respective raw material powders are mixed at a prescribed ratio and the resultant powder mixture is then calcined at a prescribed temperature to provide a lanthanum chromite-based double oxide, expressed by the formula [M is alkaline earth metal except Mg; M' is two or more different elements in Fe, Ni, Co, Zn, Cu, Al, Mn, V, Ir, Mo, W, Pd, Y, Pt, Rh, Mg, Ti, Si and B, except combination of Co with Zn and Ni with Zn; 0<x<=0.50; 0<y<=0.50; 0.95<=(b/a)<1 or 1<(b/a)<=1.05] and having a perovskite structure. The resultant double oxide is subsequently burned at to afford a sintered compact, which is used as a separator 4 for an integrated cell of unit cells and external output terminals 5. Flow passages of air 6 and a fuel 7 are formed and separated to simultaneously electrically connect the anodes 3 to the cathodes 2.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a new lanthanum chromite complex oxide and its use as a high temperature conductive material and a high temperature fuel cell separator. This new lanthanum chromite-based composite oxide is highly conductive and dense, and can be used for high-temperature fuel cells, MHD power generation, and other high-temperature conductive materials.

0002

[Prior art] Lanthanum chromite (LaCrO3)
is an oxide-based ceramic that is highly promising as a conductive material for use in high-temperature corrosive atmospheres because it has electrical conductivity at high temperatures and has excellent oxidation and reduction resistance. By adding an alkaline earth metal such as magnesium, calcium, strontium, or barium to lanthanum chromite as a trace impurity element, it can act as a dopant and improve electrical conductivity. Lanthanum chromite has a perovskite structure (ABO3 [wherein A and B are metal elements and O is oxygen]). The added calcium, strontium, and barium are substituted in solid solution at the lanthanum position in the lanthanum chromite lattice, while magnesium is substituted in solid solution at the chromium position.

0003

[Problems to be Solved by the Invention] The above-mentioned lanthanum chromite added with trace elements has sufficient performance in terms of electrical conductivity, but it is difficult to obtain dense sintering in normal pressure atmosphere, and voids occur. The disadvantage is that the gas cannot be shut off sufficiently. Therefore, for example, when attempting to use lanthanum chromite as a separator material for a solid electrolyte fuel cell, it is impossible to completely separate fuel gas and air, and it cannot be used for this purpose.

[0004] The reason why it is not easy to obtain a dense sintered body in lanthanum chromite is that the vapor pressure of chromium oxide is high at the sintering temperature, and the chromium oxide vapor generated by the decomposition of lanthanum chromite flows into the grain boundaries of the sintered body. This is because they remain as fine voids in the sintered body to inhibit the movement of pores in the sintered body (Industrial Materials, November 1987 issue, special issue, 1).
(page 8), and secondly, the volumetric diffusion of ions is extremely slow and the interface of the raw material powder is difficult to move.

The object of the present invention is therefore to solve this problem and to make it possible to easily obtain a dense sintered body in normal pressure atmosphere, as well as to improve the electrical conductivity compared to the conventional one.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present inventors first replaced a part of lanthanum in lanthanum chromite with an alkaline earth metal, and replaced a part of chromium with cobalt. A novel lanthanum chromite-based composite oxide has been disclosed (Japanese Patent Application Laid-Open No. 196785/1999). Further, in the above, it has been found that the same effect can be achieved even when a part of chromium is replaced with two or more specific metal elements instead of cobalt, and the present invention has been achieved.

[0007] Thus, in order to achieve the above object, the present invention has the general formula La1-x Ax Cr1-y My O
3 (In the formula, A is an alkaline earth metal excluding magnesium, and M is two or more different elements selected from Fe, Ni, Zn, Cu, Co, and Al, but combinations of Co and Zn and Ni and Zn except 0<x≦0.5, 0<y≦0.5
It is. ) and is characterized by having a perovskite structure.

Similarly, the present invention provides the general formula (La1-
x Ax )a (Cr1-y My )b O3 [
In the formula, A is an alkaline earth metal excluding magnesium, and M is Fe, Ni, Co, Zn, Cu, Al, Mn, V
, Ir, Mo, W, Pd, Y, Pt, Rh, Mg, Ti
, Si, and B, excluding combinations of Co and Zn and Ni and Zn, 0<x≦0.5
, 0<y≦0.5, and 0.95≦b/a<1
Or 1<b/a≦1.05. The present invention provides a lantachromite-based composite oxide represented by the following formula, characterized in that it mainly consists of a perovskite structure.

Furthermore, the present invention provides a high temperature conductive material and a separator for a high temperature fuel cell using the above lanthanum chromite complex oxide. General formula La1-
The lanthanum chromite complex oxide having a perovskite structure represented by x Ax Cr1-y My O3 is most ideally a perovskite type (ABO3)
In the basic structure of lanthanum chromite, in which La is placed at the A site and Cr is placed at the B site, a portion of La is replaced with an alkaline earth metal, and a portion of Cr is also replaced with Fe.
It is thought that it has a structure substituted with .

[0010] Furthermore, the general formula (La1-x Ax) a
The lanthanum chromite-based composite oxide mainly consisting of a perovskite structure represented by (Cr1-y My )b O3 has a slight ratio b/a of the B site and A site from the above perovskite structure (when b/a = 1). It is thought that structures other than perovskite structures are included by the amount of deviation. Conductivity is improved by replacing a portion of La with an alkaline earth metal. However, since magnesium replaces La at the A site but Cr at the B site, it is not used in the present invention. The amount of alkaline earth metal substitution is up to 0.5 in molar ratio, preferably from 0.05 to 0.3.
It is. The more substitution by these alkaline earth metals within this range, the higher the conductivity becomes. However, if the amount increases beyond this range, it is no longer possible to replace La, and complex oxides other than perovskite structures (e.g. CaCrO4,
SrCrO4, etc.) and significantly deteriorate its properties.

Metal M is a dissimilar metal that replaces a part of chromium at the B site of the lanthanum chromite lattice to lower the vapor pressure of chromium oxide and suppress its evaporation, making it possible to obtain a dense sintered body. It is. Such metals are
The first is that the ionic valence of chromium is changed by adding a different metal, which lowers the vapor pressure.The second is that it promotes volumetric diffusion or grain boundary diffusion, which is important for sintering to achieve densification. preferable. Metals that satisfy this condition include Fe,
It can be selected from Ni, Co, Zn, Cu, and Al. However, combinations of Co and Zn and Ni and Zn are excluded from the present invention because they have been filed separately.

[0012] Addition of metal M also has the effect of improving the electrical conductivity of the sintered body, and the electrical conductivity can be more than twice that of the case where M is not added. The total amount of substitution of M is 0<y in molar ratio
≦0.5, preferably 0.05≦y≦0.3. M
As the amount of addition increases, solid solution in the lanthanum chromite lattice becomes difficult, and if Fe is excessive, Ni, Co, Zn, Co, Al, Mn, V, Ir, M
o, W, Pd, Y, Pt, Rh, Mg, Ti, Si, B
are LaNiO3, LaCoO3, ZnO, respectively.
CuO, Al2 O3, Mn2 O3, V2 O5
, Ir2 O3 , MoO3 , WO3 , PdO,
Y2 O3 , PtO, Rh2 O3 , MgO, Ti
O2, SiO2, B2 O3 come to be generated. LaNiO3, LaCoO3, CuO, etc. have oxygen ion conductivity in addition to electronic conductivity, and are unstable in a reducing atmosphere, which deteriorates the properties of lanthanum chromite. Further, ZnO, Al2 O3, etc. have low electronic conductivity in an oxidizing atmosphere and are unsuitable as electrically conductive materials. Therefore, it is desirable that the amount of M added be such that these oxides are not produced, or even if they are produced, the amount is as small as possible.

General formula (La1-x Ax )a (Cr
1-y My ) b O3, b/a does not necessarily have to be 1, and the same effect can be achieved even before or after that, but if b/a is slightly shifted from 1, the strength of the ceramic will be improved. It has the effect of The material of the present invention has excellent sinterability, especially due to the addition of Fe, and a dense sintered body can be obtained, but the sintered body is composed of polycrystals, and generally the improvement in sinterability is due to crystallization. It promotes grain size expansion, and as the crystal grain size increases, the ceramic strength decreases. This is a well-known phenomenon in alumina and stabilized zirconia. Therefore, by slightly shifting the b/a ratio from 1 and introducing a structure other than the perovskite structure into the polycrystal, the grain size can be suppressed, thereby improving the ceramic strength. However, if b/a deviates too much from 1, lanthanum oxides, chromium oxides, etc. other than the perovskite structure will increase, and these will be present at the particle interface and reduce electrical conductivity, which is not preferable. Therefore, b/a is 0.95~
It shall be within the range of 1.05.

The novel lanthanum chromite-based composite oxide of the present invention can be produced by any conventional method. Namely, lanthanum source, alkaline earth metal source, chromium source,
A powder mixture containing M sources in a predetermined ratio is heated to a predetermined temperature, generally 1000 to 1600°C, preferably 1000 to 1000°C.
It can be obtained by calcining at 1200°C. The calcination time is generally 1 to several tens of hours, preferably 1 to 10 hours. The calcination atmosphere is performed in an oxygen-containing atmosphere such as the air. The pressure during calcination may be atmospheric pressure.

[0015] Molding and firing of the calcined powder can also follow conventional methods, but the firing temperature is generally 1300°C or higher, preferably 1500°C to 1600°C, and the firing time is generally 1 to 100°C, although it depends on the shape of the fired product. 10 hours, preferably 1-2
The firing atmosphere is an oxygen-containing atmosphere. Although the lanthanum chromite-based composite oxide of the present invention is characterized in that a dense sintered body can be obtained even by normal pressure sintering, sintering under pressure is not excluded.

The trace element-added lanthanum chromite sintered body obtained in this way can obtain a relative density of 95% or more even by firing in the atmosphere at normal pressure, and has twice the electrical conductivity as compared to that of the conventional composition. It is possible to obtain the above values. Moreover, since this sintered body has excellent oxidation resistance and reduction resistance, it is useful as a high-temperature conductive material that requires both corrosion resistance and conductivity at high temperatures. In particular, it is useful as a separator material for solid oxide fuel cells because it has electrical conductivity, corrosion resistance, and denseness.

FIG. 1 shows an example of the structure of a planar solid electrolyte fuel cell. In the figure, 1 is a sheet of solid electrolyte (eg, Y-stabilized zirconia) with a cathode (eg, La0.
9 Sr0.1 MnO3 ) 2, and an anode (eg, NiO/ZrO2 cermet) 3 is formed on the lower surface. 4 is a separator made of the novel lanthanum chromite complex oxide of the present invention. 5 is made of lanthanum chromite complex oxide like 4, but is used as an external output terminal. As seen in FIG. 1, the separator 4 is a separator that forms a flow path for air 6 and fuel (e.g. hydrogen) 7 by grooves formed therein, and separates the air 6 and fuel 7 from adjacent unit cells. It also plays the role of electrically connecting the anode 3 and cathode 2 of. The external output terminal 5 forms a flow path for air 6 and fuel 7 at both ends of the integrated unit cell, and also connects to the anode 3 or cathode 2.
It is also a member for electrical connection with the lanthanum chromite complex oxide of the present invention. In addition, although Fig. 1 shows a fuel cell in which two unit cells are integrated, three
It is also possible to integrate more than one unit cell, in which case a separator 4 is inserted between each unit cell.

[0018]

[Example] Example 1 (b/a=1) Lanthanum oxide 26.
065g, strontium carbonate 5.905g, chromic oxide 12.919g, tricobalt tetroxide 0.803g
, nickel oxide 0.827g, zinc oxide 0.814
g was weighed and wet mixed using an agate mortar. This composition is La0.8 Sr0.2 Cr0.85CO0.0
It corresponds to 5Ni0.05Zn0.05O3. This mixed powder was calcined at 1200° C. for 1 hour. The heating rate is 2
The temperature is 0°C/min. As a result of analyzing the thus obtained lanthanum chromite powder by X-ray diffraction, the presence of a second phase could not be confirmed, and it was found that cobalt, nickel, and zinc were solidly dissolved in the lanthanum chromite lattice having a perovskite structure. Understood. 300Kgf/c of this powder
Floating molding with a load of m2, 2 at 1600℃
Main firing was carried out for an hour (temperature increase rate: 5° C./min). The density and conductivity of the sintered body thus obtained were measured. As a result, a density of 6.3 g/cm 3 and a conductivity in air at 1000° C. of 40 S/cm were obtained. In addition, the distribution of elements in this sintered body was observed using a scanning electron microscope and EDX spectroscopy, and no segregation was observed, indicating that the added cobalt, nickel, and zinc were uniformly replaced with chromium. .

For comparison, a sintered body (comparative example) with a composition of La0.8 Sr0.2 CrO3 produced by the same manufacturing method as the above one had a density of 5.0 g/cm3 (relative density 76%) and a sintered body of 1000 18S in terms of conductivity at °C
/cm. In this way, it can be seen that both density and electrical conductivity are improved by substituting a part of chromium in lanthanum chromite with other plurality of transition metal elements. Example 2 (b/a=0.97) Lanthanum oxide 26.06
5g, strontium carbonate 5.905g, chromic oxide 12.531g, tricobalt tetroxide 0.779g, nickel oxide 0.802g and zinc oxide 0.790
g was weighed and wet mixed using an agate mortar. This composition is La0.8 Sr0.2 Cr0.825 Co0
．． 048 Ni0.048 Zn0.048 O3. This mixed powder was fired in the same manner as in Example 1.

[0020] When the obtained fired product (powder) was analyzed by X-ray diffraction, it was found that most of it had a perovskite structure. A sintered body was prepared using this powder in the same manner as in Example 1, and the density, electrical conductivity, bending strength, and average particle size were measured. The results are shown in Table 1. Example 3 (b/a=1.02) Lanthanum oxide 26.06
5g, strontium carbonate 5.905g, chromic oxide 13.177g, tricobalt tetroxide 0.819g, nickel oxide 0.843g and zinc oxide 0.830
g was weighed and wet mixed using an agate mortar. This composition is La0.8 Sr0.2 Cr0.867 Co0
．． 051 Ni0.051 Zn0.051 O3. This mixed powder was fired in the same manner as in Example 1.

[0021] When the obtained fired product (powder) was analyzed by X-ray diffraction, it was found that most of it had a perovskite structure. A sintered body was prepared using this powder in the same manner as in Example 1, and the density, electrical conductivity, bending strength, and average particle size were measured. The results are shown in Table 1.

[0022]

[Table 1] From the results in Table 1, the addition of Sr, Co, Ni, and Zn improves both the density (sinterability) and electrical conductivity of the sintered body, and the composition ratio of the A site and B site b By slightly shifting /a from 1, mechanical strength is improved, and density,
It can be seen that the conductivity is intact.

[0023]

[Effects of the Invention] The novel lanthanum chromite-based composite oxide provided by the present invention is easily densified in normal pressure atmosphere and has excellent electrical conductivity, so it provides a stable conductive material that can be used at high temperatures. It is particularly useful as a separator for high-temperature fuel cells.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a flat plate solid electrolyte fuel cell.

[Explanation of symbols]

1...Solid electrolyte 2...Cathode 3...Anode 4...Zygote 5...External output terminal 6...Air 7...Fuel

Claims (4)

[Claims] Claim 1: General formula La1-x Ax Cr1-y
My O3 (wherein A is an alkaline earth metal excluding magnesium, M is Fe, Ni, Co, Zn, Cu
, Al, Mn, V, Ir, Mo, W, Pd, Y, Pt,
Two or more different elements selected from Rh, Mg, Ti, Si, and B, excluding combinations of Co and Zn and Ni and Zn, where 0<x≦0.5, 0<y≦0.5 . ) and is characterized by having a perovskite structure. Claim 2: General formula (La1-x Ax) a (
Cr1-y My )b O3 [In the formula, A is an alkaline earth metal other than magnesium, and M is Fe, Ni,
Co, Zn, Cu, Al, Mn, V, Ir, Mo, W,
Two or more different elements selected from Pd, Y, Pt, Rh, Mg, Ti, Si, and B, but Co and Zn, and Ni and Z
Except for the combination of n, 0<x≦0.5, 0<y≦0.5, and 0.95≦b/a<1 or 1<b/a≦1
．． It is 05. ] A lanthanum chromite-based composite oxide characterized by being mainly composed of a perovskite structure. 3. A high temperature conductive material comprising the lanthanum chromite complex oxide according to claim 1 or 2. 4. A high-temperature fuel cell separator comprising a separator made of the lanthanum chromite complex oxide according to claim 1 or 2.