JPH06191850A - Lanthanum chromite based powder - Google Patents

Abstract

PURPOSE:To obtain a lanthanum chromite based powder exhibiting excellent sintering property and thereby capable of obtaining a sintered compact dense and excellent in characteristics by making the constitution mutually different in composition in the vicinity of the center of a particle and in the vicinity of the surface. CONSTITUTION:This lanthanum chromite based powder is constituted so that the larger quantity of chromium component exists in the vicinity of the center of the particle and the larger quantity of lanthanum component exists in the vicinity of the suface. The powder mainly contains La and Cr usually and contains alkaline earth metal element such as Sr, Ca and another addonal components such as Zn, Cu, Li, Fe, Ni, Co, Al, Mn or does not contain the addtional component and has oxide form, hydroxide form, carbonate form or organic acid salt form. The powder having (40/60) to (60/40) (mol ratio) composition ratio of La to Cr has excellent characteristics and practically used. The powder exhibits excellent sintering property, and thus the lanthanum chromite based sintered compact dense and excellent in characteristics is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lanthanum chromite powder having excellent sinterability, and in particular, a lanthanum chromite powder having conductivity and being chemically stable in an oxidizing and reducing atmosphere even at a high temperature. It relates to powder.

[0002]

2. Description of the Related Art Lanthanum chromite (LaCrO ₃ ) is a perovskite-type composite oxide, and a lanthanum chromite-based sintered body doped with an additive such as Sr has conductivity and is oxidized and oxidized even at high temperature. It is chemically stable in a reducing atmosphere, and is attracting attention as a material for electrodes for MHD power generation, heating elements, separators (plate type) of solid electrolyte fuel cell electrodes, or interconnectors (cylindrical type).

Generally, a lanthanum chromite powder, for example, a composition powder to which Sr is added has a problem that it is difficult to sinter and a dense sintered body cannot be obtained. This is because CrO ₃ is evaporated in the sintering of lanthanum chromite in the air, and the sintering is promoted by the surface diffusion or evaporation-condensation mechanism, so that the lanthanum chromite is not densified. Therefore, firing under a low oxygen partial pressure has been proposed to suppress the evaporation of CrO ₃ and densify it (L. G.
roupp and HU Anderson, "Densification of La1-xS
rxCrO3 ", J. Am. Ceram. Soc., Vol. 59, p449 (197
6).). However, firing under low oxygen partial pressure is not practical, and firing in air is desired. Also, Ca component, Zn,
Cu component, Li component, or Fe, Ni, Co, Zn, Cu, Al, Mn,
Components such as V, Ir, Mo, W, Pd, Y, Pt, Rh, Mg, Ti, Si, B (JP-A-4-219364, JP-A-4-219365, JP-A-4-219
366, Japanese Patent Laid-Open No. 4-219367, Japanese Patent Laid-Open No. 4-219369) and the like have been attempted for densification.

However, these additional components are electrically conductive.
Deterioration of characteristics such as deterioration of electrical properties is a problem.
For high-temperature operating temperature (about 1000 ℃)
Reactions of these additives with other constituent components of the component occur.
However, this may cause deterioration of long-term characteristic stability.
is there. Also, the composition of La site is excessive (Cr site is insufficient).
Therefore, a method of achieving densification by this is proposed (M.
 Mori, N. Sakai, T. Kawada, H. Yokokawa and M. Doki
ya, "Low-Temperature Air-Sinterable Lanthanum Calc
ium Chromite with Chromium Deficit for SOFC Separa
tor ", DENKI KAGAKU, Vol. 59, p314 (1990).)
With an excessive La composition of₂O ₃Exists and
La₂O₃ Reacts with the water in the air to form hydroxide and collapse.
There is a problem that it is broken or the characteristics are deteriorated.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above problems, and as a result of extensive studies on the structure of particles for improving the sinterability of lanthanum chromite powder, the composition components were The present invention has been completed by finding that the sinterability can be improved by making different configurations in the vicinity of the center of the particle and the vicinity of the surface of the particle.

[0006]

That is, the present invention is a lanthanum chromite powder characterized in that a large amount of chromium component is present near the center of the particle and a large amount of lanthanum component is present near the surface.

The present invention will be described in more detail below. In the present invention, the particles mean single primary particles or secondary particles formed by aggregating primary particles. In addition, the powder here means a state in which these primary particles are simply aggregated or a state in which the primary particles are aggregated.

The lanthanum chromite-based powder of the present invention generally contains La and Cr as main components, alkaline earth metal elements such as Sr and Ca, and Zn, Cu, Li, Fe, Ni, and
Co, Zn, Cu, Al, Mn, V, Ir, Mo, W, Pd, Y, Pt, Rh, M
The particles are composed of particles containing additive components such as g, Ti, Si and B, but the additive components are not particularly limited to these. Further, the additive component may be omitted.

The state of the substance constituting the particles is not particularly limited, and it is sufficient that a large amount of chromium component exists near the center of the particle and a large amount of lanthanum component exists near the surface of the particles, such as oxides, hydroxides, Examples include forms such as carbonates and organic acid salts. The fact that a large amount of chromium component is present in the vicinity of the center means that a large amount of chromium component is present in the vicinity of the center rather than the chromium component of the average composition of the particles. The same applies to the lanthanum component. The region in the vicinity of the surface varies depending on the size of the particles and therefore cannot be generally stated, but is usually about several millimeters μm to several μm, but is not limited to this. The composition ratio of La and Cr varies depending on the amount of the added components, etc., but the La / Cr molar ratio of 40/60 to 60/40 is excellent and the properties are practical.

As a method for analyzing particles having different compositions in the vicinity of the surface and in the vicinity of the center, various surface analysis methods capable of composition analysis in the depth direction can be applied. For example, X-ray photoelectron spectroscopy and electron probe. Micro analysis, Auger electron spectroscopy, Particle beam excitation X-ray spectroscopy, Ion scattering spectroscopy, Angle-resolved X-ray spectroscopy, Rutherford backscattering spectroscopy, Atomic probe field ion microscopy, Secondary ion mass spectrometry, Ion microprobe mass spectrometry Law etc. are mentioned. Further, the powder X-ray diffraction method is suitable for the analysis of the crystal phase constituting the particles, and instrumental analysis methods such as the chemical analysis method and the fluorescence X-ray analysis method can be applied to the composition analysis of the entire powder.

As the method for producing the lanthanum chromite powder of the present invention, a solid phase method, a liquid phase method, a gas phase method, or a combination thereof can be applied. In the solid phase method, a powder containing a chromium component as a main component, for example, a powder containing Cr ₂ O ₃ as a main component, a powder slurry containing a lanthanum component as a main component, for example,
The surface of the particles mainly composed of the chromium component is coated with the powder slurry mainly composed of La ₂ O ₃ to coat the particles mainly composed of the lanthanum component, and a calcination treatment is carried out if necessary to obtain the particles of the present invention. An example is a method of forming the structure of the lanthanum chromite powder. At this time, La ₂ O ₃ coating the surface
The particle diameter of the particles containing as a main component should be considerably smaller than the particle diameter of the Cr ₂ O ₃ particles forming the central portion.

As the raw material powder, a substance containing constituents,
Examples include carbonates, oxides, hydroxides, chlorides, nitrates, sulfates, and organic acid salts such as oxalates, formates, and acetates, but those that do not leave impurities in the sintered body preferable. Further, as a method applying the vapor phase method, there is a method of forming a coating layer mainly containing a lanthanum component on the surface of particles mainly containing a chromium component by a known vapor phase method such as a chemical vapor deposition method.

The liquid phase method is suitable as a method for synthesizing the powder of the present invention in view of controllability of composition, controllability of particle diameter and particle diameter distribution of powder, productivity and the like. In the liquid phase method, after a powder composed of particles mainly composed of a chromium component is dispersed in a solution mainly composed of a lanthanum component, a precipitate is formed from a solution mainly composed of the lanthanum component to form a particle mainly composed of the chromium component. Examples thereof include a method of forming a precipitation layer mainly containing a lanthanum component on the surface. Examples of the solution mainly containing a lanthanum component include various acidic aqueous solutions prepared from various inorganic salts and organic solvent solutions of organic metal compounds such as alkoxides.

Further, as the powder mainly composed of the chromium component dispersed in the solution mainly composed of the lanthanum component, those composed of particles which are not dissolved in the solution mainly composed of the lanthanum component are preferable, and a powder having a narrow particle size distribution is formed. It is preferable because it improves the powder properties such as properties. The method of forming the precipitate depends on the nature of the solution mainly composed of the lanthanum component, and as is generally done, for example, various alkalis are used as the precipitating agent in the acidic aqueous solution, and water or water and alcohol or alkali is used in the alkoxide solution. The containing solution can be used as a precipitating agent. Also, the hydrothermal synthesis method can be applied.

The calcination temperature when the powder prepared by the solid phase method, the gas phase method, the liquid phase method or a combination thereof, or any method, requires calcination, depends on the additive composition, but is 300 ° C. ~ 1300 ℃ is preferable, and 400 ℃ ~ 1000
C is preferred. If the calcination temperature is too high, strong agglomeration of powder progresses and the powder characteristics deteriorate. A specific example of the calcination method is calcination in an ordinary electric furnace or the like.

As in general sintering, the lanthanum chromite powder of the present invention prepared by any method has a small particle size, a narrow particle size distribution, and a powder capable of preparing a uniform compact. Is preferred. Thus, in order to improve the powder characteristics, it is also preferable to pulverize with a crusher such as a ball mill, a vibration mill, a jet mill, a stirring mill, a planetary mill or a dyno mill to prepare the powder characteristics. Also,
The method of adding the additive component is not particularly limited, and it may be added in an intermediate step of the above-mentioned powder manufacturing method, or may be added to the finally obtained powder.

[0017]

The sinterability of the lanthanum chromite-based powder of the present invention is improved because the powder surface layer mainly composed of the lanthanum component suppresses the evaporation of the internal chromium component which is a factor inhibiting the sintering during the sintering process. It is considered that this is because the surface layer having an excessive lanthanum component has a composition that facilitates sintering and promotes sintering. And, the non-uniformity of the composition near the center and near the surface of the particles present in the powder becomes more uniform as the sintering progresses, and finally the composition is highly uniform and dense and has excellent characteristics such as electrical characteristics. Further, it is possible to obtain a lanthanum chromite sintered body having a desired lanthanum chromite composition.

[0018]

EXAMPLES Examples of the present invention will be specifically described below. [Example] Chromium oxide powder (Cr ₂ O ₃ ) was molded and sintered at 1400 ° C., and the obtained sintered body was pulverized to prepare a chromium oxide powder having an average particle diameter of 1.2 μm. A solution of lanthanum nitrate in water (La (NO ₃ ) ₃ solution: concentration 1.1 mol / kg)
And strontium nitrate aqueous solution (Sr (NO ₃ ) ₂ aqueous solution: concentration
A dispersion was prepared by dispersing in a mixed aqueous solution (1.8 mol / kg). At this time, the molar composition ratio of each component in the dispersion is La
_It was blended so as to have a composition of _0.92 Sr _0.08 CrO ₃ . The thus-prepared dispersion was introduced into a precipitation-producing reaction vessel maintained at pH 9 to 10 to form a precipitate. A solution prepared by mixing ammonia water (NH ₄ OH aqueous solution) and ammonium carbonate aqueous solution ((NH ₄ ) ₂ CO ₃ aqueous solution) at a ratio of 1: 1 (molar ratio) was used to maintain the pH.

The slurry thus obtained was thoroughly washed with a dilute aqueous ammonia solution, dried with acetone and alcohol to remove water, and then calcined at 500 ° C. The obtained lanthanum chromite powder was used as a scanning electron microscope (JEOL: JSM-820) with an energy dispersive X-ray analyzer (Phillips EDAX PV-9100) and Auger electron spectroscopy (JEOL Ltd.). : As a result of analysis by JAMP-30), some particles did not contain chromium component, but many powders have much chromium component near the center of the particle and many lanthanum components near the surface. It was confirmed to be the powder of the present invention having characteristics.

To evaluate the sinterability of the obtained lanthanum chromite powder, the powder was molded and sintered at 1700 ° C.
The density of the obtained sintered body was 6.53 g / cm ³ (relative density of about 97
%) And the sintered body was high and dense. Further, when identified by powder X-ray diffraction, only the peak corresponding to the LaCrO ₃ phase was measured and the presence of a by-product phase was not recognized. In addition, the composition of the sintered body was measured by the fluorescent X-ray analysis method (System 3370E manufactured by Rigaku Denki KK) by the glass bead method, and as a result, the powder had the target La _0.92 Sr _0.08 CrO ₃ composition ratio within the error range. There was found.

[0021] [Comparative Example] aqueous solution of lanthanum nitrate and strontium nitrate aqueous solution and a chromium nitrate aqueous solution used in Example (Cr (NO _{3) 3} solution: Concentration 0.98 mol / kg) identical La _0.92 Sr _{0.0 8} CrO and examples _It was mixed so as to have _three compositions. This mixed solution was introduced into a precipitation-generating reaction vessel maintained at pH 9 to 10 in the same manner as in Example to generate a precipitate. For maintaining the pH, a solution obtained by mixing ammonia water (NH ₄ OH aqueous solution) and ammonium carbonate aqueous solution ((NH ₄ ) ₂ CO ₃ aqueous solution) at a ratio of 1: 1 (molar ratio) was used. The obtained precipitate was thoroughly washed with dilute ammonia water, dried to remove water using acetone and alcohol, and then calcined at 500 ° C. As a result of analyzing the obtained calcined powder by the same method as in the example, no particles having a large amount of chromium component near the center and a large amount of lanthanum component near the surface were not seen.

Sintered bodies were prepared under the same conditions as in the following examples. The density of the obtained sintered body was 5.33 g / cm ³ (relative density of about 7
9%), which was considerably lower than that of the example and was not densified. In addition, when identified by powder X-ray diffraction, the crystal phase was LaCr as in the example.
Only O ₃ phase was present and no by-product phase was found. Moreover, when the composition of the sintered body was measured by a fluorescent X-ray analysis method (glass bead method), the target La was within the error range.
It was found that the powder had a composition ratio of _0.92 Sr _0.08 CrO ₃ .

[0023]

The lanthanum chromite powder of the present invention is
A lanthanum chromite-based sintered body having a special particle structure and excellent sinterability, which is dense and excellent in characteristics, is obtained, and is used as a separator for MHD power generation electrodes, heating elements, and solid electrolyte fuel cell electrodes ( It can be applied to materials for flat plate type) or interconnector (cylindrical type), and its industrial value is great.

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[Procedure amendment]

[Submission date] June 4, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

The present invention will be described in more detail below. In the present invention, the particles mean single primary particles or secondary particles formed by aggregating primary particles. In addition, the powder referred to here, the state was a mere collection of these particles child addition,
Refers to the state in which the grain child are aggregated.

Claims (1)

[Claims] 1. A lanthanum chromite-based powder characterized in that a large amount of chromium component is present near the center of the particle and a large amount of lanthanum component is present near the surface.