JPH04331764A - Lanthanum chromite-type multiple oxide and its use - Google Patents

Abstract

PURPOSE:To easily increase the density of the subject oxide in a normal pressure atmosphere and to improve the electrical conductivity and mechanical property of the oxide by baking a powdery mixture of a raw material having the composition corresponding to formula I. CONSTITUTION:A powdery raw material composed of a lanthanum source, an alkaline-earth metal source, a chromium source and a cobalt source and having a composition corresponding to formula I (M is alkaline-earth metal excluding Mg; 0<x<=0.5; 0<y<=0.5; 0.95<=b/a<1 or 1<b/a<=1.05) is mixed in wet state and calcined at 1000-1600 deg.C for 1 to several tens hours in an oxygen- containing atmosphere. The calcined product is formed and sintered in an oxygen-containing atmosphere under normal or positive pressure at >=1300 deg.C for 1-10hr to obtain the objective lanthanum chromite multiple oxide composed mainly of a perovskite structure and having a density corresponding to >=90% of the theoretical density, an electrical conductivity of 20OMEGA<-1>cm<-1> in air and a flexural strength of >=10kg/mm<2>. The oxide is useful as a high-temperature electrical conductive material, especially as a separator of a solid-electrolyte fuel cell.

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 composite oxide and its use as a high temperature conductive material and a solid electrolyte fuel cell separator. This new lanthanum chromite-based composite oxide is highly conductive and dense, and can be used for solid electrolyte fuel cells, MHD power generation, and other high-temperature conductive materials.

0002

[Prior Art] Lanthanum chromite (LaCrO3) is an oxide 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 resistance and reduction resistance. It is a type of ceramic. Electrical conductivity can be improved by adding alkaline earth metals such as magnesium, calcium, strontium, and barium to lanthanum chromite as trace elements. 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

[Problem to be solved by the invention] Although the above-mentioned lanthanum chromite with added trace elements has sufficient performance in terms of electrical conductivity, 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.

Generally, sintering of particles occurs due to movement of the particle interfaces due to diffusion of atoms or ions within the solid. The reason why it is not easy to obtain a dense sintered body with lanthanum chromite is firstly because the vapor pressure of chromium oxide produced when lanthanum chromite decomposes at the firing temperature is high, which turns into a gas and obstructs the movement of the interface. Secondly, the diffusion of atoms within a solid is extremely slow, making it difficult for the interface of the raw material powder to move.

SUMMARY OF THE INVENTION The object of the present invention is 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 and strength compared to the conventional ones.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides the general formula (La1-xM x ) a (
Cr1-yCoy ) b O3 (where M is an alkaline earth metal excluding magnesium, 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 novel lanthanum chromite-based composite oxide, which is represented by the following formula and mainly consists of a perovskite structure, and a high-temperature conductive material and solid oxide fuel cell using the same.

[0007] La1-x M x Cr1-y Co disclosed by the present inventors in Japanese Patent Application No. 196785/1999
The lanthanum chromite-based composite oxide represented by yO3 has a perovskite structure, and most ideally, La,
It is thought that in the basic structure of lanthanum chromite in which Cr is placed at the B site, a portion of La is substituted with an alkaline earth metal, and a portion of Cr is further substituted with Co.

[0008] Then, the general formula (La1-xM x )
The lanthanum chromite-based composite oxide mainly consisting of a perovskite structure represented by a (Cr1-yCoy) b O3 has the above-mentioned perovskite structure (b/a=1
It is considered that a structure other than the perovskite structure is included by a slight deviation in the ratio b/a of the B site and A site from the case of . Conductivity is improved by replacing a portion of La with an alkaline earth metal. However, magnesium is not La at the A site but Cr at the B site.
is not used in the present invention. The amount of alkaline earth metal substitution is up to 0.5 in molar ratio, preferably 0.0.
5 to 0.4, more preferably 0.05 to 0.2. The more substitutions with these alkaline earth metals within this range, the higher the conductivity becomes, but if the amount increases beyond this range, La can no longer be substituted, and complex oxides other than perovskite structures (e.g. CaCrO4, SrC
rO4, etc.) and significantly deteriorate its properties.

The present invention basically maintains the properties of lanthanum chromite, which has excellent oxidation resistance and reduction resistance as well as high-temperature electrical conductivity, and aims to make the sintered body denser and to have high-temperature electrical conductivity. This is an attempt to further improve gender. Therefore, based on lanthanum chromite, its C
Part of r is replaced with Co. The reason for substituting a part of Cr with Co is that attention has been paid to the fact that lanthanum cobaltite has the following properties.

(1) La0.75Sr0.25CrO3
The structure of La0.9Sr0.1CoO3 is a very similar perovskite structure. JCPDS (Joint C
omitte on powder diffract
ion Standards), the lattice constants of La0.75Sr0.25CrO3 are ao = 5.403 Å, co = 13.301 Å, C
=2.462, and Sr0.9Sr0.1Co
The lattice constant of O3 is ao = 5.444 Å, co = 1
3.107 Å, C=2.4076, and their lattice constants match very well. (2) Lanthanum cobaltite is densely sintered at a lower temperature than lanthanum chromite. (3) Lanthanum cobaltite also has excellent high-temperature electrical conductivity, but is more susceptible to reduction than lanthanum chromite. That is, because of these properties, when Cr in lanthanum chromite is replaced with Co, it is possible to impart the characteristics of lanthanum cobaltite while taking advantage of the reduction resistance characteristics of lanthanum chromite without distorting the structure.

[0011] Cobalt replaces a part of chromium at the B site of the lanthanum chromite lattice to lower the vapor pressure of chromium oxide, suppress its evaporation, and facilitate the diffusion of atoms during sintering. For these reasons, it becomes possible to obtain a dense sintered body. Additionally, the addition of cobalt has the effect of improving the electrical conductivity of the sintered body, and the
More than double the conductivity can be obtained. Cr of lanthanum chromite
Although the electrical conductivity can be improved by replacing Co with Mn, Fe, etc. other than Co, sufficient densification of the sintered body cannot be achieved with these elements.

The amount of cobalt substituted is 0<y≦0 in terms of molar ratio.
5, more preferably 0<y≦0.3. When the amount of cobalt added increases, solid solution in the lanthanum chromite lattice becomes difficult, and lanthanum cobaltite (LaCoO3
) will now be generated. This lanthanum cobalt has oxygen ion conductivity in addition to electronic conductivity, and is unstable in a reducing atmosphere, which deteriorates the properties of lanthanum chromite. Therefore, it is desirable that the amount of cobalt added is such that no lanthanum cobaltite is produced, or that the amount that is produced is as small as possible. In order for the difference in electrical conductivity to be within three digits in a 1000° C. reducing atmosphere and in an air atmosphere, 0<y≦0.3 is required. The electrical conductivity difference is more than three orders of magnitude, but the stable composition in a reducing atmosphere is 0<y≦
It is 0.5. If more cobalt is added, it will decompose in a reducing atmosphere. Further, conductivity is improved by substituting a part of La with an alkaline earth metal. The amount of substitution is up to 0.5 in terms of molar ratio, and if it exceeds 0.5, a complex oxide other than a perovskite structure is produced and its properties are degraded. The preferred range is 0.1 to 0.4.

General formula (La1-X M X ) a (C
r1-y Coy ) b In O3, b/a does not necessarily have to be 1, and the same effect can be achieved even before or after that, and if b/a is slightly shifted from 1, the strength of the ceramic will be improved. I found that it works. The material of the present invention has excellent sinterability, especially by adding Co, and it is possible to obtain a dense sintered body, 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, b
By slightly shifting the ratio of /a from 1, a structure other than the perovskite structure can be introduced into the polycrystal, thereby controlling the grain size and thereby improving the strength of the ceramic. 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 within the range of 0.95 to 1.05. However, b
The case where /a=1 was disclosed separately and is therefore excluded here.

The novel lanthanum chromite complex oxide of the present invention can be produced as follows. That is, a powder mixture containing a lanthanum source, an alkaline earth metal source, a chromium source, and a cobalt source in a predetermined ratio is heated at a predetermined temperature, generally from 1000 to 1600°C, preferably from 1000 to 1000°C.
Calculate at 1200℃. 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.

Next, a calcined powder is formed and fired. The firing temperature is generally 1300°C or higher, preferably 1500°C.
The firing time is generally 1 to 10 hours, preferably 1 to 2 hours, depending on the shape of the fired body, and 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 thus obtained can achieve a density of 90% or more, especially 99% or more of the theoretical density even by firing in the atmosphere at normal pressure, and has an electrical conductivity of 20 s. /cm or more, in particular, it is possible to obtain a value that is more than twice that of the conventional composition (18 s/cm). In addition, this sintered body has excellent oxidation resistance, reduction resistance, and mechanical strength, so 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.

When used in a solid electrolyte fuel cell, it is desired that the linear thermal expansion coefficient be similar to that of stabilized zirconia, which is a typical solid electrolyte, and should be 9×10-6 to
It is desirable to satisfy 12×10−6. The thermal expansion coefficients of the lanthanum chromite-based composite oxide according to the present invention are all within these ranges. Figure 1 shows an example of the structure of a planar solid electrolyte fuel cell. In the figure, 1 is a solid electrolyte (
For example, a sheet of Y-stabilized zirconia) with a cathode (
For example, La0.9Sr0.1MnO3)2, an anode (eg, NiO/ZrO2 cermet) 3 is formed on the bottom 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 25 is a member that forms a flow path for air 6 and fuel 7 at both ends of the integrated unit cell, and is also a member that electrically connects with the anode 3 or cathode 2, and is also a heat-resistant component of the present invention. Configure. Further, although FIG. 1 shows a fuel cell in which two unit cells are integrated, it is also possible to integrate three or more unit cells, and in that case, a separator 4 is inserted between each unit cell.

[0018]

【Example】

Reference example (b/a=1) Lanthanum oxide 130.32g, strontium carbonate 2
9.52 g of chromic oxide, 68.40 g of dichromic oxide, and 8.03 g of tricobalt tetroxide were weighed and wet-mixed using an agate mortar. This composition is La0.8Sr0.2Cr0.9
Corresponds to Co0.1O3. This mixed powder was heated to 1200℃.
It was calcined for 1 hour. The temperature increase rate was 20°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 was solidly dissolved in the lanthanum chromite lattice having a perovskite structure.

[0019] This powder was float-molded under a load of 300 kgf/cm3, and then main fired at 1600°C for 2 hours (temperature increase: 5°C/min). The density and conductivity of the sintered body thus obtained were measured. As a result, the density was 6.5 g/cm3 (relative density 99% or more), and the electrical conductivity at 1000°C was 49 S/cm.
I got it.

[0020] Furthermore, the distribution of elements in this sintered body was observed using a scanning electron microscope and EDX spectroscopic analysis.
No segregation was observed, and it was found that the added cobalt was uniformly replaced by chromium. For comparison, a sintered body with a La0.8Sr0.2CrO3 composition (comparative example) manufactured using the same method as the one above had a density of 5.0 g/cm3.
(relative density 76%) and conductivity at 1000°C was 18 s/cm.

[0021] Thus, it can be seen that both the density and the conductivity are considerably improved by adding cobalt. Example 1 (b/a=0.96) Lanthanum oxide 133.58g, strontium carbonate 3
2.47 g of chromic oxide, 68.40 g of dichromic oxide, and 8.03 g of tricobalt tetroxide were weighed and wet-mixed using an agate mortar. This composition is La0.82Sr0.22Cr0
．． It corresponds to 9Co0.1O3. This mixed powder was fired in the same manner as in the reference example.

[0022] 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 the reference example, and the density, electrical conductivity, bending strength, and average particle size were measured. The results are shown in Table 1. Example 2 (b/a=1.04) Lanthanum oxide 130.32g, Strontium carbonate 2
9.52 g of dichromic oxide, 71.44 g of tricobalt tetroxide, and 8.03 g of tricobalt tetroxide were weighed and wet-mixed using an agate mortar. This composition is La0.8Sr0.2Cr0.
It corresponds to 94Co0.1O3. This mixed powder was fired in the same manner as in the reference example.

[0023] 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 the reference example, and the density, electrical conductivity, bending strength, and average particle size were measured. The results are shown in Table 1. Examples 3 to 4 The same oxide as in Example 1 was used as the raw material except that calcium oxide was used instead of strontium carbonate, and La0
．． 6Ca0.4Cr0.86Co0.09O3(b/a
=0.95), and La0.56Ca0.4Cr0.9
Two types of mixed powders corresponding to the composition of Co0.1O3 (b/a=1.04) were prepared and fired in the same manner as in Example 1 to obtain a sintered body.

The density, electrical conductivity, bending strength and average particle diameter of the sintered bodies are shown in Table 1 as Examples 3 and 4, respectively.

[0025]

[Table 1]

From the results in Table 1, it is clear that Sr and Co or Ca
The density (sinterability) and electrical conductivity of the sintered body are both improved by the addition of Co and Co, and the mechanical strength is improved by slightly shifting the composition ratio b/a of the A site and B site from 1. It can be seen that the density and conductivity are not impaired.

[0027]

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

[Brief explanation of drawings]

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 (6)

[Claims] [Claim 1] General formula (La1-xM x ) a
(Cr1-yCoy) b O3 (where M is an alkaline earth metal excluding magnesium, and 0<x≦0.5
,0<y≦0.5 and 0.95≦b/a<1
Or 1<b/a≦1.05. ) is a lanthanum chromite-based composite oxide characterized by mainly consisting of a perovskite structure. 2. The composite oxide according to claim 1, which has a density of 90% or more of the theoretical density. [Claim 3] Electrical conductivity in air at 1000°C is 20.
The composite oxide according to claim 1 or 2, which has a resistance of Ω-1 cm-1 or more. 4. The composite oxide according to claim 1 or 2, which has a bending strength of 10 kgf/mm2 or more. 5. A high temperature conductive material comprising the lanthanum chromite complex oxide according to claim 3. 6. A solid electrolyte fuel cell separator comprising the lanthanum chromite complex oxide according to claim 3.