JPH0688792B2 - Manufacturing method of perovskite type fine powder - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing perovskite type fine powder, and more specifically, it has a single composition having a very large BET surface area and extremely few heterogeneous phases. The present invention relates to a method for producing perovskite type fine powder.

(Problems to be solved with conventional technology) Conventionally, it is composed of an oxide of a rare earth element or an element obtained by substituting a part thereof with an alkaline earth element, and a transition metal composed of cobalt, manganese, nickel, iron, or the like. Perovskite type compounds are known, have excellent catalytic activity and high conductivity, and have been widely studied for various catalysts, electrode materials, sensors and the like.

The present inventor has previously proposed a perovskite type fine powder having a very large BET specific surface area and a method for producing the same by a wet precipitation method characterized by oxidation in a liquid phase (JP-A-61-22926). ). According to this, a uniform perovskite phase having a large BET specific surface area of 10 m ² / g or more can be obtained at a relatively low temperature of about 800 ° C.

However, the fine powder obtained in this way is 800
At a temperature of about ℃, it is a single phase of perovskite, but when heated to a temperature higher than that, in the case of LaCoO _3, a cobalt oxide (tricobalt tetroxide) phase and a small amount of lanthanum oxide phase are mainly observed, and it is completely uniform Not a phase. It is unlikely that these different phases were generated as a result of the decomposition of the perovskite, and there is a high possibility that they are potentially mixed in the respective powders. Further, even when the content of lanthanum or cobalt is increased or decreased within the range of 0 to 7% of the amount derived from the composition and the same synthesis is performed, the different phases do not disappear, and the phase does not become a complete single phase.

Another problem is that in a system in which a part of the rare earth element is replaced with an alkaline earth metal, the solubility of the alkaline earth metal is high, so that it may flow out during washing with water after the formation of precipitates, or the particles may be uneven. Therefore, there is a drawback that the advantage of wet coprecipitation is lost.

It is an object of the present invention to solve the above-mentioned drawbacks of the prior art and to provide a method for obtaining a perovskite-type fine powder having a high specific surface area and a single perovskite phase having no different phases in a wide firing temperature range. It is what

(Means for Solving the Problems) The present invention has been completed as a result of intensive research on a method capable of solving the above-mentioned drawbacks of the conventional technology.

That is, the present invention is a perovskite represented by the general formula ABO ₃ (however, A: an element obtained by substituting a rare earth element or a part thereof with an alkaline earth element, B: a transition metal element consisting of cobalt, manganese, nickel, and iron). In the production of fine powder, after neutralizing and precipitating hydroxide in the presence of an oxidizing agent in an aqueous medium of water-soluble salts of constituent elements, carbon dioxide gas is blown in to change some of the constituent elements to carbonate. Then, the gist is a method for producing a perovskite type fine powder, which is characterized by firing the obtained precursor.

The present invention will be described in more detail below.

(Function) The rare earth element of the component A used in the present invention is a rare earth element such as yttrium, lanthanum, praseodymium, neodymium, samarium, and europium, which can be used alone or as a mixture. Further, alkaline earth metals such as calcium, strontium, and barium can be used instead of a part of these rare earth elements.

In the present invention, the rare earth elements are alkaline earth elements used as trivalent and divalent water-soluble salts, respectively. These water-soluble salts may be any water-soluble salts such as chlorides, nitrates, sulfates and acetates, and these compounds may be used alone or as a mixture.

The transition metal such as cobalt, manganese, nickel and iron used as the component B in the present invention is used as a divalent water-soluble salt, and any water-soluble salt such as chloride, nitrate, sulfate or acetate can be used. However, these compounds may be used alone or as a mixture.

In the present invention, a water-soluble salt of a rare earth element or a part thereof substituted with an alkaline earth element as a component and a water-soluble salt of a transition metal as a component B are used at a metal molar ratio of about 1: 1. Preferably.

Then, a mixed aqueous solution in which the water-soluble salt of the component A and the water-soluble salt of the component B are dissolved in water is prepared and neutralized. As the method, there can be used a method of adding an alkaline agent or an aqueous solution thereof for neutralization, a method of adding the solution in reverse, or a method of adding both solutions at the same time, and the method is not limited thereto. In short, any method may be used as long as it can be obtained in a state where the hydroxide of the component A and the hydroxide of the component B are uniformly mixed. In such a mixed precipitation method, the alkaline agent used is sodium hydroxide, potassium hydroxide,
Any alkali agent such as ammonium hydroxide, sodium carbonate, sodium hydrogen carbonate, etc. may be used.

The amount of the alkali used may be an amount that can sufficiently neutralize the aqueous solution of the component A and the component B, but it is preferable to use an excess amount of the alkaline agent. Further, the slurry concentration of the hydroxides of the component A and the component B generated by the neutralization of these is 2 to 6
About wt% is preferable.

In the present invention, the divalent metal ions present in the slurry thus obtained are trivalently oxidized in the slurry simultaneously with or after the precipitation.

The oxidizing agent to be used may be any oxidizing agent such as hydrogen peroxide, oxygen, sodium chlorate, etc., but an oxidizing agent which does not generate impurities by oxidation, for example, hydrogen peroxide or oxygen gas is preferable.

The oxidizing agent is preferably used in a certain excess ratio because the divalent metal is oxidized to the trivalent metal ion.

The slurry thus obtained is preferably heated to 80 ° C. or higher and aged with stirring for 1 hour or more to complete the precipitation reaction.

Next, after cooling, it is necessary to blow carbon dioxide gas to convert part or most of the hydroxides of the A component and the B component into carbonates. As a result, a precursor having excellent reactivity can be obtained, and as a result, a fine powder of a perovskite single phase having no heterogeneous phase can be obtained at a lower firing temperature. Further, in the process of blowing carbon dioxide gas, the unevenness of the size of the precipitated particles during the alkali neutralization precipitation becomes small, and fine powder having a sharp particle size distribution can be obtained. At the same time, in the presence of an alkaline earth metal, all of them are converted into carbonates and taken into the coprecipitation system to obtain a uniform precursor as a whole.

The carbon dioxide gas may be blown through a fine tube such as a capillary or through a device that generates a large amount of fine bubbles, and the point is to blow the carbon dioxide gas uniformly into the slurry. As a matter of course, the blowing amount of carbon dioxide gas is not effective when the amount is small, and adversely affects when the amount is too large.

In this regard, the present inventor has studied in detail the relationship between the amount of carbon dioxide gas blown and the properties of the obtained perovskite fine powder, and found the optimum amount of gas blown therein.

First, the properties of the LaCoO ₃ perovskite powder were judged by X-ray diffraction and measuring the density and conductivity of the sintered body. The sintered body was formed by preforming the obtained perovskite powder into pellets of 16 mmφ and then molding it under a hydrostatic pressure of 17 MPa.
It was obtained by atmospheric pressure sintering at 5 ° C. for 5 hours. The density was judged from the shape, and the conductivity was judged by measuring the specific resistance by the two-terminal DC method.

As a result, the slurry pH after precipitation aging is about 13 because of excess alkali, but when carbon dioxide gas is blown in, the excess alkali is neutralized and at the same time the hydroxides of the A component and B component become carbonate. The pH starts to change and the pH gradually decreases. for that reason,
The amount of carbon dioxide gas blown in can be expressed by the pH of the slurry. Fig. 1 shows the relationship between the green compact density of the calcined powder, the density calcined at 1300 ° C and the pH at the end of the precipitation reaction of the specific resistance.
The following is clear from the X-ray diffraction diagram. At pH 13 where no carbon dioxide gas was blown in, a heterogeneous phase consisting mainly of tricobalt tetroxide was observed, and as a result, the resistivity was high and the sintered density did not rise. As carbon dioxide is blown, that is, pH
As a result, the number of different phases decreases, the specific resistance decreases, and the density of the sintered body increases. However, at pH 6.5, the lanthanum oxide phase begins to be visible, the resistivity slightly increases, and the sintered body density also decreases.

Therefore, the blowing amount of carbon dioxide gas is preferably in the range of pH 9.5 to 7.0. Also, from thermal analysis (TG) of the precursor in this range, the change from hydroxide to carbonate is about 10-20%.
It became clear that it corresponds to.

Next, it is preferable to remove unnecessary ions by filtering the slurry in which the optimum carbon dioxide gas is blown and washing with water, and then drying to obtain a perovskite precursor.

Further, in the present invention, the above precursor is fired. It is desirable to bake in a normal atmosphere, preferably a non-reducing atmosphere, at 700 to 800 ° C. for 30 minutes to 1 hour. As a result, it is possible to obtain a fine powder of a single perovskite phase having no heterogeneous phase.

(Example) Next, the Example of this invention is shown. In the text,% means% by weight unless otherwise specified.

Example 1 32.6 g of lanthanum oxide was completely dissolved in 43 cc of a 60% nitric acid aqueous solution, 58.2 g of cobalt nitrate was dissolved in this aqueous solution, and water was added to make a total amount of 200 cc. On the other hand, a solution of 43 g of sodium hydroxide dissolved in 200 cc of water and 100 cc of a 15% hydrogen peroxide aqueous solution.
Was prepared, and the above-mentioned 3 solutions were simultaneously injected into a vessel equipped with a stirrer containing 500 cc of water in advance. During this time, the pH of the slurry liquid
Was maintained around 9, and the temperature was maintained around 25 ° C.

After the precipitation reaction and the oxidation reaction were completed, an excessive sodium hydroxide solution was added, and the pH of the slurry was 12.8. Then, it heated at 80 degreeC and aged for 1 hour.

After cooling, remove 0.11 / m of fine carbon dioxide gas through the capillary.
It was blown into the slurry at a flow rate of in and the pH change in the solution was observed. After 15 minutes, the pH of the slurry dropped to 7 and the reaction was completed.

The resulting blackish brown product was filtered and washed with water to remove unnecessary impurities such as cations and anions, and then dried sufficiently at 100 ° C.

The dried material was calcined at 800 ° C. for 1 hour to obtain fine powder of perovskite type oxide. This fine powder showed a single perovskite phase from the X-ray diffraction diagram. 900 this fine powder
Even when reheated to ℃, no heterogeneous phase such as cobalt oxide was observed. The density and resistivity of the sintered body obtained by sintering this fine powder for 5 hours at 1300 ° C is 7.1 g / cm2 corresponding to pH7 in Fig. 1.
³ and 0.3 Ω · cm. From the X-ray diffraction diagram, this sintered body showed a single perovskite like fine powder. The BET specific surface area (800 ° C calcination) of this fine powder was 10 m ² / g,
According to the transmission electron micrograph, the particle diameter was around 0.1 μm.

Example 2 Except that the pH at the end of the reaction of blowing carbon dioxide was 9.5,
In the same manner as in Example 1, fine powder of perovskite type compound was obtained.

This fine powder consisted of a single perovskite phase according to the X-ray diffraction pattern, and even if it was reheated to 900 ° C., no heterogeneous phase was observed.
The density and resistivity of the sintered body obtained by sintering this fine powder for 5 hours at 1300 ° C are 7.04 g / cm ³ and 0.
It was 15 Ω · cm. From the X-ray diffraction diagram, this sintered body showed a single phase of perovskite like fine powder. The BET (800 ° C calcination) specific surface area of this fine powder was 14 m ² / g.

Example 3 A perovskite type compound fine powder was obtained in the same manner as in Example 1 except that 32.8 g of a mixed rare earth element was used instead of lanthanum oxide.

The characteristics were almost the same as in Example 1. The mixed rare earth elements used here consisted mainly of lanthanum oxide 50 to 70%, neodymium oxide 20 to 30%, and praseodymium oxide 5 to 5.
13%, samarium oxide 0.5-2%.

Example 4 26.1 g of lanthanum oxide was completely dissolved in 37 cc of a 60% nitric acid aqueous solution, and 9.5 g of calcium nitrate and 5 parts of cobalt nitrate were added to this aqueous solution.
8.2 g was dissolved and water was added to make the total amount 200 cc. Thereafter, in the same manner as in Example 2, fine powder of perovskite type compound was obtained.

This fine powder is a complete perovskite single phase from the X-ray diffraction diagram, and the BET (800 ° C calcination) specific surface area of the fine powder is 12 m ² / g.
Met. The density of the sintered body at 1250 ° C was 6.8 g / cm ³ , and the specific resistance was significantly reduced to 0.05 Ω · cm.

Comparative Example 1 The same operation as in Example 1 was performed except for the step of blowing carbon dioxide gas.

The obtained product showed a single phase of perovskite from the X-ray diffraction diagram, but when reheated to 900 ° C., a different phase mainly containing cobalt oxide was observed. The density and resistivity of the sintered body obtained by sintering this fine powder for 5 hours at 1300 ° C. were 6.95 g / cm ³ and 4.9 Ω · cm, which correspond to the position of pH 13 in FIG. The density of the sintered body was low and the specific resistance was high. The X-ray diffraction diagram of this sintered body showed that cobalt oxide was precipitated as a different phase.

Comparative Example 2 The same operation as in Example 1 was performed except that the amount of carbon dioxide gas blown was adjusted to pH 6.5.

In the calcined powder of the obtained product, a lanthanum oxide phase is seen in addition to the single perovskite. In addition, this calcined powder was
The density and specific resistance of the sintered body that was sintered for 7.0 hours were 7.06 g / cm ³ and
It was 0.4 Ω · cm.

(Effects of the Invention) As described in detail above, according to the present invention, a perovskite-type fine powder composed of a single perovskite phase which is in a very soft state and has a high specific surface area and has no heterogeneous phase in a wide firing temperature range. The body is obtained. Therefore, if this fine powder is used, an excellent sintered body having high sintering density and high conductivity can be produced. Further, since it has a high surface area, it is also useful as a catalyst.

[Brief description of drawings]

Figure 1 shows the amount of carbon dioxide blown in (that is, the amount of slurry
It is a figure which shows the relationship between sintering density and specific resistance with respect to (pH).

Claims (2)

[Claims] 1. ABO ₃ (wherein A is a rare earth element or an element obtained by substituting a part thereof with an alkaline earth element, B: cobalt,
Manganese, nickel, transition metal element consisting of iron) in the production of perovskite type fine powder represented by, after the hydroxide is precipitated by alkali neutralization in the presence of an oxidizing agent in an aqueous medium of a water-soluble salt of the constituent elements, A method for producing perovskite type fine powder, which comprises blowing carbon dioxide gas to change a part of constituent elements into carbonate, and then calcining the obtained precursor. 2. The amount of carbon dioxide gas blown depends on the pH value of the slurry.
The method according to claim 1, wherein the method is adjusted within the range of 9.5 to 7.0.