JPH04219367A - Lanthanum chromite-based double oxide and its use - Google Patents

Abstract

PURPOSE:To improve electric conductivity by mixing raw material powders containing an La component, an alkaline earth metallic component, a Cr component, an Ni component and a Zn component, then calcining the resultant mixed powder, subsequently burning the obtained double oxide and providing a sintered compact expressed by a specific formula. CONSTITUTION:An La source such as La2O3 is mixed with an alkaline earth metallic source, a Cr source such as Cr2O3, an Ni source such as NiO and a Zn source such as ZnO at a prescribed ratio and the resultant mixed powder 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; 0<x<=0.5; 0<y; 0<z, 0<(y+z)<=0.5; 0.95<=(b/a)<1 or 1<(b/a)<=1.05] and having a perovskite structure. The resultant double oxide is subsequently burned at >=1300 deg.C to afford a sintered compact, which is then used as a separator 4 for an integrated cell of unit cells. Flows of air 6 and a fuel 7 are separated and the anodes 13 and the cathodes 2 are simultaneously electrically connected. The sintered compact is also used as external output terminals 5 and the formation of flow passage of the air 6 and the fuel 7 and electrical connection to the anodes 3 or cathodes 2 are simultaneously carried out.

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

[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.

[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 18).
Page), 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). In the above, it has been found that the same effect can be achieved even when a part of chromium is replaced with nickel and zinc instead of cobalt, and the present invention has been achieved.

[0007] Thus, in order to achieve the above object, the present invention provides the general formula La1-x Mx Cr1-y-z N
iy Znz O3 (wherein M is an alkaline earth metal excluding magnesium, 0<x≦0.5, y>
0, z>0, 0<y+z≦0.5. ) and is characterized by having a perovskite structure.

Similarly, the present invention provides the general formula (La1-x
Mx )a (Cr1-y-z Niy Znz)b
O3 [wherein M is an alkaline earth metal excluding magnesium, 0<x≦0.5, 0<y, 0<z, 0<y
+z≦0.5, and 0.95≦b/a<1 or 1<b/a≦1.05. The present invention provides a lanthanum chromite-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-
Most ideally, a lanthanum chromite-based composite oxide with a perovskite structure represented by
In the basic structure of lanthanum chromite arranged by
a is partially substituted with an alkaline earth metal, and further C
It is considered that a part of r is substituted with Fe.

[0010] Furthermore, the general formula (La1-x Mx)a
A lanthanum chromite-based composite oxide mainly consisting of a perovskite structure represented by (Cr1-y-z Niy Znz)b O3 has a ratio of B site to A site b/
It is considered that a structure other than the perovskite structure is included due to the slight deviation of a.

[0011] The conductivity is improved by substituting a part 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. The more substitutions with these alkaline earth metals within this range, the higher the conductivity becomes; however, when the number increases beyond this range, it is no longer possible to replace La, and complex oxides other than perovskite structures (e.g. CaCrO4, S
rCrO4 etc.) and significantly deteriorate its properties.

Nickel and zinc replace part of the 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. Further, the addition of nickel and zinc 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 nickel and zinc are not added. The total amount of nickel and zinc substitution is 0<y+ in molar ratio
z≦0.5, preferably 0.05≦y+z≦0.3. When the amount of nickel and zinc added increases, solid solution in the lanthanum chromite lattice becomes difficult, and LaNiO3
and ZnO will be generated. LaNiO3 has oxide ion conductivity in addition to electronic conductivity, and is unstable in a reducing atmosphere, which deteriorates the properties of lanthanum chromite. Furthermore, ZnO has much lower electrical conductivity than lanthanum chromide in an oxidizing atmosphere, reducing its properties as an electrically conductive material. Therefore, it is desirable that the amounts of nickel and zinc added be such that LaNiO3 and ZnO are not produced, or even if they are produced, the amount is as small as possible.

General formula (La1-x Mx ) a (Cr
1-y-z Niy Znz ) b in O3
/a does not necessarily have to be 1, and the same effect can be achieved either before or after it, but when b/a is slightly shifted from 1, the strength of the ceramic can be improved. 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, b/a
If it deviates too much from 1, lanthanum oxides, chromium oxides, etc. other than those having a perovskite structure will increase, which is undesirable because they are present at the particle interface and reduce electrical conductivity. Therefore, b/a is 0.95 to 1.05
within the range of

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,
It can be obtained by calcining a powder mixture of a nickel source and a zinc source in a predetermined ratio at a predetermined temperature, generally from 1000 to 1600°C, preferably from 1000 to 1200°C. Calcination time is generally 1 to several tens of hours, preferably 1 to 10 hours.
It's time. 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, nickel oxide 1.654g
, 0.814 g of zinc oxide was weighed and wet-mixed using an agate mortar. This composition is La0.8 Sr0.2 C
Corresponds to r0.85Ni0.1 Zn0.05O3. This mixed powder was calcined at 1200° C. for 1 hour. The temperature increase rate is 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 nickel and zinc were dissolved in solid solution in the lanthanum chromite lattice having a perovskite structure. Ta.

[0019] This powder was float-molded under a load of 300 Kgf/cm2, and then main fired at 1600°C for 2 hours (heating rate was 5°C/min). The density and conductivity of the sintered body thus obtained were measured. As a result, the density was 6.1g/cm3, and the temperature was 1000℃ in air.
A conductivity of 36 S/cm was obtained. Furthermore, 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 nickel and zinc were uniformly replaced by chromium.

[0020] La0.
The sintered body (comparative example) having a composition of 8 Sr0.2 CrO3 had a density of 5.0 g/cm 3 and a conductivity of 18 S/cm at 1000° C. in air. As can be seen from the examples above, it can be seen that both density and electrical conductivity can be improved by substituting a part of chromium in lanthanum chromite with other plurality of transition elements. Example 2 (b/a=0.97) Lanthanum oxide 26.06
5 g of strontium carbonate, 5.905 g of strontium carbonate, 12.531 g of chromic oxide, 1.604 g of nickel oxide, and 0.790 g of zinc oxide were weighed and wet-mixed using an agate mortar. This composition is La0.8 Sr0.2 C
r0.825 Ni0.097 Zn0.049 O3
corresponds to 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. Example 3 (b/a=1.02) Lanthanum oxide 26.06
5 g of strontium carbonate, 5.905 g of strontium carbonate, 13.177 g of chromic oxide, 1.687 g of nickel oxide, and 0.830 g of zinc oxide were weighed and wet-mixed using an agate mortar. This composition is La0.8 Sr0.2
Cr0.867 Ni0.102 Zn0.051
Corresponds to O3. This mixed powder was fired in the same manner as in Example 1.

[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 Example 1, and the density, electrical conductivity, bending strength, and average particle size were measured. The results are shown in Table 1.

[0023]

[Table 1] From the results in Table 1, the addition of Sr, Ni and Zn improves both the density (sinterability) and electrical conductivity of the sintered body.
It is also seen that by slightly shifting the composition ratio b/a of the A site and B site from 1, the mechanical strength is improved, and the density and electrical conductivity are not impaired.

[0024]

[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 Mx Cr1-y
-zNiy Znz O3 (wherein, M is an alkaline earth metal excluding magnesium, 0<x≦0.5, y>
0, z>0, 0<y+z≦0.5. ) and is characterized by having a perovskite structure. Claim 2: General formula (La1-x Mx) a (
Cr1-y-z Niy Znz )b O3 [In the formula, M is an alkaline earth metal other than magnesium, and 0
<x≦0.5, 0<y, 0<z, 0<y+z≦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 comprising a separator made of the lanthanum chromite complex oxide according to claim 1 or 2.