JPH0365517A - Lanthanum chromite-based compound oxide and use thereof - Google Patents

Abstract

PURPOSE:To obtain a lanthanum chromite-based compound oxide, capable of readily providing a dense calcined compact in the air under ordinary pressure, having a high electric conductivity at high temperatures and make the above- mentioned compound oxide useful as solid electrolyte type fuel cell separators, etc., by providing a compound oxide of an La-alkaline earth metal-Cu-O-based perovskite structure. CONSTITUTION:A compound oxide, expressed by the general formula La1-xMnCr1-yCoyO3 (M is alkaline earth metal other than Mg; 0<=x<=0.5; 0<y<=0.5) of a perovskite type structure and exhibiting a density of >=90% based on the theoretical density and >=20OMEGA<-1>cm<-1> electric conductivity (at 1000 deg.C in air) is converted into a lanthanum chromite-based compound oxide used as high-temperature electrically conductive materials and solid electrolyte type fuel cell separators.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel lanthanum chromite-based composite oxide and its use as a high-temperature conductive material and a solid oxide fuel cell separator. This new lanthanum chromite-based composite oxide is highly conductive and dense, and is useful for solid electrolyte fuel cells, MHD power generation [prior technology] Lanthanum chromite (LaCrOz) has electrical conductivity at high temperatures, oxidation resistance, In order to have excellent reduction resistance,
This is an oxide-based ceramic material that is considered extremely promising as a conductive material for use in high-temperature corrosive atmospheres.

By adding alkaline earth metals such as magnesium, calcium, strontium, and barium to lanthanum chromite as trace impurity elements, the electrical conductivity can be improved. Lanthanum chromite has a perovskite structure (ABO, in the formula C, A and B are metal elements, and ○ 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.

[Problem to be solved by the invention]

Although the trace element-added lanthanum chromite described above has sufficient performance in terms of electrical conductivity, it has the disadvantage that it is difficult to obtain dense sintering in normal pressure atmosphere, and voids are formed, making it impossible to sufficiently block gas. be.

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.

The reason why it is not easy to obtain a dense sintered body with lanthanum chromite is that the vapor pressure of chromium oxide is high at the firing temperature, and the chromium oxide produced by the decomposition of lanthanum chromite evaporates and forms bubbles that are sintered. This is because they remain in the body, and secondly, the diffusion of ions is extremely slow and the interface of the raw material powder is difficult to move.

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

[Means to solve the problem]

In order to achieve the above object, the present invention provides general formula Lal-
xMJr+-YCOYO3 (where M is an alkaline earth metal excluding magnesium, Q<x≦0.5,
0<y<1. The present invention provides a novel lanthanum chromite-based composite oxide represented by

This new lanthanum chromite-based composite oxide has a perovskite structure represented by the above general formula, and most ideally, a lanthanum chromite structure in which La is placed at the A site and Cr is placed at the B site of the perovskite type (ABO3) structure. It is thought that in the basic structure of chromite, a part of La is substituted with an alkaline earth metal, and a part of Cr is further substituted with CO.

Conductivity is improved by replacing a portion of La with an alkaline earth metal. However, magnesium is L at the A site.
Since it is substituted with Cr at the B site instead of a, it is not used in the present invention. The amount of alkaline earth metal substitution is 0.
5, preferably from 0.2 to 0.5. The more substitutions with these alkaline earth metals within this range, the higher the conductivity becomes, but when it increases beyond this range, it is no longer L.
It becomes impossible to replace a with a, and a complex oxide other than the perovskite structure (for example, CaCr01.5rCr04, etc.) is generated, and its properties are significantly deteriorated.

Cobalt replaces a part of chromium at the B site of the lanthanum chromite lattice, lowers the vapor pressure of chromium oxide, and suppresses its evaporation, making it possible to obtain a dense sintered body. Further, the addition of cobalt 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 cobalt is not added. The amount of cobalt substitution is O<y< in molar ratio
More preferably, O<y≦0.5. When the amount of cobalt added increases, it becomes difficult to form a solid solution in the lanthanum chromite lattice, resulting in lanthanum cobaltite (LaCoOs).
will now be generated. This lanthanum cobaltite 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.

The method for producing the novel lanthanum chromite complex oxide of the present invention itself can follow a conventional method.

That is, a powder mixture in which a lanthanum source, an alkaline earth metal source, a chromium source, and a cobalt source are mixed in a predetermined ratio is heated at a predetermined temperature - 1000 to 1600°C, preferably 1000 to 1600°C for boat fishing.
It can be obtained by calcining at 000 to 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.

Shaping and firing of the calcined powder can also be done according to the conventional method, but
The firing temperature is generally 1300°C or higher, preferably 150°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 obtained in this way can obtain a relative density of 99% or more even when fired in the atmosphere at normal pressure, and has an electrical conductivity that is more than twice that of that of the conventional composition. can be obtained.

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 (e.g., Ca-stabilized zirconia) with a cathode (e.g., Lao, 5sro) on the top surface.
, +Mn0s) 2. Anode (e.g., NiO/
ZrO□cermet) 3 is formed. 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 units. It also plays the role of electrically connecting the anode 3 and cathode 2 of the cell. 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. Furthermore, 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, in which case a separator 4 is placed between each unit cell.
Insert.

〔Example〕

Example 1 Lanthanum oxide 11.057 g, strontium carbonate 2.505 g, dichromium oxide 5.803 g,
Copal Oxide) 0.636 g was weighed and wet-mixed using an agate mortar. This composition is LaolsrO,z
Corresponds to Cro-9COO-103. This mixed powder was calcined at 1200° C. for 1 hour. The temperature increase rate is 20℃/m
It is in. 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.

This powder was float-molded under a load of 300 kgf/cut, and main firing was performed at 1600°C for 2 hours (heating temperature was 5°C/min).

The density and conductivity of the sintered body thus obtained were measured.

As a result, the density was 6.5 g/crl (relative density 99% or more), and the electrical conductivity at 1000°C was 4.
9s/cm was obtained.

In addition, the element distribution of this sintered body was observed using a scanning electron microscope and EDX spectroscopy, and no segregation was observed, indicating that the added cobalt was uniformly replaced by chromium.

For comparison, Lao produced using the same method as above
, asro, 2Cry3 composition sintered body has density 5, Og/c++I (relative density 76%), 1000°C
The conductivity was 18 s/am.

Thus, it can be seen that both density and conductivity are significantly improved by adding cobalt.

Examples 2 to 6 Using almost the same method as in Example 1, however, after weighing the raw material powder, wet mixing in a ball mill for 48 hours or more, drying, and calcination at 1200°C for 2 hours (heating rate 20°C/m1n). )
A sintered body having the following composition was prepared. The results of measuring the conductivity and density are shown in the table below.

3 Lao, , Sro, *Cra, 9Co
,,+0*49 S/Cm4 L
ao, 5sro, 4cro, 5co (1, +03
69 s/cm5 Lao, 7sr0.
3Cro, acoo10* 68S/Cm
6 1, ao, tsro, 3cro, ffco
Q, ffo3 81 S / Cm6.3g/cm (0,97) 6,1 g /cffl (0,94) 6,3 g /cnf (0,97) 6.3g/cffl (0,97) In the table The relative density of lanthanum chromite is 6
．． It is set at 5g/cd.

From this table, it can be seen that the conductivity improves and the density decreases as the Sr substituent increases, and that the conductivity improves as the amount of CO substitution increases.

X-ray charts of these sintered bodies are shown in FIGS. 2-6.

〔Effect 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 can provide a stable conductive material that can be used at high temperatures. It is useful as a separator for solid oxide fuel cells.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a planar solid oxide fuel cell, and FIGS. 2 to 6 are X-ray charts of the sintered bodies of Examples 2 to 6, respectively. 1... solid electrolyte, 2... cathode, 3...
・Anode, 4... Joined body, 5... External output terminal, 6... Air, 7... Fuel.

Claims (3)

[Claims] 1. General formula La_1_-_xM_xCr_1_-_yC
o_yO_3 (wherein M is an alkaline earth metal excluding magnesium, 0<x≦0.5, and 0<y<1) and is characterized by having a perovskite structure. Lanthanum chromite complex oxide. 2. A high temperature conductive material comprising the lanthanum chromite complex oxide according to claim 1. 3. A solid electrolyte fuel cell separator comprising the lanthanum chromite complex oxide according to claim 1.