JP2003020224A - Method for manufacturing ceria powder in which individual particles are separated into nanosize - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing ceria powder in which individual particles are separated into a nanosize, and which is suitable as raw material powder for manufacturing a dense sintered compact, or a catalyst, or a grinding material. SOLUTION: In the method for synthesizing ceria powder in which the mean particle size of primary particles is 10 to 100 nm, and the hard flocculation of the primary particles is not observed, a cerium salt aqueous solution having a saturation concentration of 0.05 mol/l or higher and an ammonium carbonate aqueous solution or an ammonium hydrogen carbonate aqueous solution having a saturation concentration of 0.3 to 3 mol/l are mixed to precipitate ammonium cerium carbonate which can be described by the chemical formula of (NH4 )h CeOk (CO3 )m .nH2 O (wherein, (h) is 0.05 to 1.2, (k) is <=2, and (m) is <=4), thereafter, the ammonium cerium carbonate is aged in the temperature range of 50 to 90 deg.C for <=12 hr, is cleaned, and is calcined at 500 to <1,000 deg.C. In the above manufacturing method, instead of the cerium salt aqueous solution, an acidic aqueous solution containing cerium ions and one or plural kinds of metallic ions of 0.1 to 30 mol% in total to the cerium ions is used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a catalyst, an abrasive, or an electro-optical material, a fluorescent material, a magnetic refrigerating material, a laser material, a thermistor material, a piezoelectric material, a superconductor material, a dielectric material, and a sintering aid. The present invention relates to a method for producing a nano-sized dispersed ceria powder used as a raw material powder for the above.

[0002]

2. Description of the Related Art Conventionally, since ceria has a fluorite type crystal structure, it has been expected to be used as a solid electrolyte that has excellent ionic conductivity and controls the performance of fuel cells and oxygen pumps. Further, since the hardness of ceria is smaller than those of diamond and alumina, the powder thereof has been expected as a precision abrasive material that does not leave distortion on the polished surface. further,
Since the valence of cerium ions changes depending on the oxygen partial pressure of the atmosphere, fine ceria particles have been attracting attention as catalysts or catalyst supports. In order to enhance the function of these materials, it was essential to develop a raw material powder of ceria, in which individual particles were separated into nano-sized particles with a narrow particle size distribution.

As a conventional method of synthesizing a ceria precursor, a method of reacting a cerium salt with a basic agent such as aqueous ammonia, a hydrolysis method of a ceria salt, a uniform precipitation method utilizing thermal decomposition of urea or hexamethylenediamine, A cerium carbonate production method using a cerium salt and a carbonate, an electrochemical method, a hydrothermal synthesis method, and the like have been proposed.

[0004]

An aqueous solution such as a method of reacting a base agent such as aqueous ammonia with a cerium salt, a hydrolysis method of a cerium salt, a homogeneous precipitation method utilizing the thermal decomposition of urea or hexamethylenediamine is used. It is possible to synthesize ceria powder having primary particles of nanosize by the conventional chemical wet method.

However, the primary particles are strongly aggregated to form secondary particles having a diameter of 1 μm or more. Since the sinterability, catalytic activity, polishing characteristics, etc. of ceria powder are governed by the size and morphology of secondary particles rather than the size of primary particles,
There is a drawback in that the advantage of producing nano-sized primary particles is not utilized.

Cerium carbonate prepared by the conventional carbonate method has a plate shape with a width of 0.3 μm or more and a length of 1 μm or more,
The ceria powder obtained by calcination of the carbonate has a defect that the shape of the mother salt remains as a skeleton, so that a powder in which individual particles are separated into a nano size cannot be prepared.

The alkoxide method has the advantage of being able to produce a raw material powder monodispersed in the nanosize, but has the drawback that the expensive metal alkoxide has to be produced in advance and the cost is high. The hydrothermal synthesis method can produce raw material powder in which the primary particles are individually separated, but the primary particle size is 100n.
It has a drawback that it is relatively large, such as m or more, and that it requires an expensive high-pressure container and, at the same time, the workability is poor and the cost is high because of high-pressure processing.

[0008]

Means for Solving the Problems The present inventors have conducted various investigations and researches on ceria powder in which particles are separated into nano-sized particles. As a result, a precipitate produced by reacting a cerium salt and a carbonate in an aqueous solution can be aged and grown at a relatively high temperature to produce ceria powder in which individual particles are separated into nanosizes. It has been found that it is suitable as a raw material powder for producing a dense sintered body, or as a catalyst or an abrasive.

In the production method of the present invention, an aqueous solution of 0.05 mol / l to a saturated concentration of cerium salt and an aqueous solution of 0.3 to 3 mol / l of ammonium carbonate solution or ammonium hydrogen carbonate solution are mixed ((NH ₄ ) _h CeO _k (CO ₃ ) _m・ nH ₂ O (however, h is 0.
05-1.2, k is within 2 and m is within 4) Ammonium cerium carbonate (hereinafter referred to as ACC), which can be described by the chemical formula, is precipitated and then aged at a temperature in the range of 50 ° C to 90 ° C for up to 12 hours The average particle size of primary particles is 10 nm to 100 nm, which is characterized by performing, washing, and calcining at 500 ° C to 1000 ° C.
It is characterized by synthesizing ceria powder in which hard agglomeration of primary particles is not observed.

Further, in the above production method, it is preferable to use an acidic aqueous solution containing cerium ions and one or more kinds of metal ions in a total amount of 0.1 mol% to 30 mol% with respect to the cerium ions instead of the cerium salt aqueous solution. It is characterized in that a ceria powder containing metal oxide having an average primary particle size of 10 nm to 100 nm is produced.

It is necessary to limit h, k, and m of ACC that exhibits the features of the present invention to 0.05 to 1.2, 2 or less and 4 or less, respectively. The value of n varies depending on the dry state of ACC, and n of ACC dried by the usual method is limited to 4 or less. Among these, the value of h can be controlled by the pH of the aqueous solution and the ammonium ion concentration.
When carbonate is used, k has a value close to zero, and when hydrogen carbonate has a value close to 1. m can be controlled by the concentration of carbonate ion in the reaction solution.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Examples of the cerium salt used in the present invention include inorganic cerium acidic compounds such as nitrates, chlorides and sulfates, and organic cerium acidic compounds such as cerium oxalate. The type of cerium salt is not particularly limited as long as it has a high solubility in water.

The concentration of the cerium salt aqueous solution used in the present invention is preferably 0.05 mol / l or more. When the concentration is 0.05 mol / l or less, the amount of precipitation obtained is small for the aqueous solution used, which is not preferable. The concentration is preferably up to the saturation concentration of the cerium salt.

The ammonium carbonate or ammonium hydrogen carbonate used in the present invention is not particularly limited and may be a commercially available product or a self-made product. Since the purity of the ceria finally obtained is also affected by these chemicals, it is necessary to determine the purity of the chemical to be used depending on the intended use of the ceria.

The concentration of the carbonate or hydrogen carbonate used in the present invention is preferably 0.3 to 3 mol / l, particularly preferably 1 mol / l to 2 mol / l. When the concentration is 0.3 mol / l or less, the ammonium ion concentration in the reaction solution is low,
Cerium carbonate is formed with h substantially zero. The carbonate precipitate has a plate-like shape with a width of 0.3 μm or more and a length of 1 μm or more, and even if this precipitation is calcined, primary particles in which individual particles are separated into nano-sized particles cannot be produced, which is not preferable.

As the ammonium ion concentration in the aqueous solution increases, h increases and the solubility of ACC increases. h is 1.0
In the above cases, the growth of ACC precipitated particles and the bonding between the particles due to the dissolution-precipitation mechanism become remarkable. As a result, the precipitate has a diameter of 0.
Hard agglomerated particles in which particles of 4 μm or more are joined in a linear or plate shape. Even if the particles are calcined, the shape of carbonate remains,
It is not preferable because it is impossible to produce ceria powder in which individual particles are separated into nano-sized particles.

In the present invention, in order to grow the ACC precipitate to an appropriate size, the temperature range of 50 ° C. to 90 ° C., especially 60 ° C.
Aging is preferably carried out in the temperature range of 80 ° C. The aging temperature
If it is lower than 50 ° C., the carbonate precipitate is amorphous or fine particles close to amorphous, and if the precipitate is dried, it becomes a hard mass, which is not preferable. On the other hand, when the aging temperature is higher than 90 ° C., the precipitated particles grow during the aging, and at the same time, the precipitated particles are hard-bonded to form large aggregated particles, which is not preferable.

There are no particular restrictions on the temperature at which the cerium salt of the present invention is reacted with the basic carbonate, and the speed at which they are mixed.
It can be determined in consideration of workability and economy.

The aging time of the cerium carbonate of the present invention means the time for which all the cerium salt aqueous solution and the basic carbonate aqueous solution are mixed and then kept at the aging temperature (range of 50 ° C to 90 ° C). Even if the aging is performed for 12 hours or more, the aging effect is not improved so much, so that aging for 12 hours or more is not preferable.

The washing according to the present invention is carried out in order to remove unnecessary anions and ammonium ions generated during the formation of ACC precipitates. 3% by weight of anion and ammonium ion
If the ceria powder obtained by calcination remains in the precipitation product as described above, primary particles form hard agglomerated particles, which is not preferable.

The calcination of the present invention is intended to remove unnecessary components of the precipitation product in order to develop the characteristics of ceria and to obtain ceria powder separated into nanosizes to maximize the characteristics. To do. If the calcination temperature is 500 ° C. or lower, the ceria powder cannot achieve the purpose of adsorbing a large amount of gas, which is not preferable. On the other hand, the calcination temperature is 1000
When the temperature is higher than 0 ° C, the ceria particles grow to submicron during the calcination and the purpose cannot be achieved, which is not preferable.

When the calcination is carried out in the range of 500 ° C. to 600 ° C., the primary particles of the calcined ceria powder are fine and the friction between the particles is large, so that the powder filling becomes uneven. For this reason, cracking may occur during sintering if not properly molded.
On the other hand, when the calcination temperature is above 900 ° C., a temperature to achieve a sintered density of 98% or more of the theoretical density is higher than 1100 ^o ° C.. When the calcination temperature is 1000 ° C or higher, it is necessary to sinter at 1300 ° C or higher, and the sinterability is the same as that of the ceria powder produced by the conventional method, which is not preferable. That is, the preferable calcination temperature of the present invention is in the range of 500 ° C or higher and lower than 1000 ° C, and the range of 600 ° C to 900 ° C is particularly preferable.

Although the particle size of the ceria powder of the present invention strongly depends on the calcination temperature, it does not depend so much on the calcination time, and even if the calcination time is lengthened, the increase in the particle size can be ignored. The practical calcination time is 1 to 8 hours, but the characteristics of the present invention are exhibited even if the calcination temperature is out of this range as long as the calcination temperature is in the above range.

Various substances are used as additives to enhance the functionality of ceria. In order to exert the effect of addition, it is necessary to add the additive in an amount of 0.1 mol% or more. As the amount added increases, the desired function can be improved,
If the amount added exceeds 30 mol%, the function will be adversely affected.
The addition amount of the present invention is preferably 0.1 to 30 mol%. 30 added
Even if it is less than mol%, when the second phase appears, the original properties of ceria are impaired, which is not preferable, and the addition amount must be limited to the amount in which the additive is completely dissolved in ceria.

In the metal ion addition of the present invention, an aqueous solution containing cerium ions and one or more kinds of metal ions in a total amount of 0.1 mol% to 30 mol% with respect to the cerium ions is used instead of the cerium salt aqueous solution. It is possible to manufacture ACC in which additive ions are uniformly dispersed in atomic order. If this ACC is used, ceria powder in which additive ions are uniformly dispersed can be synthesized even if calcined at a low temperature as long as it is within the limit where ceria can form a solid solution.

Examples of the metal of the metal oxide added to ceria in the present invention include rare earth elements such as ytterbium and samarium, aluminum, alkaline earth metals and transition metals. Metals such as alkali metals which form a carbonate or hydrogen carbonate having a high solubility in water cannot be quantitatively added by the method of the present invention, which is not preferable.

The average particle size of the primary particles of the ceria powder produced by the method of the present invention is preferably 10 nm to 100 nm. If the average particle size is smaller than 10 nm, the packing property of the particles is poor and a dense sintered body cannot be produced, which is not preferable. On the other hand, calcination so that the average particle size becomes 100 nm or more is not preferable because bonding between the primary particles proceeds and hard agglomerated particles are formed like the ceria powder manufactured by the conventional method.

[0028]

Examples Example 1: 1.5 mol kept at 70 ° C with stirring
To 300 ml of ammonium carbonate solution / l, 300 ml of 0.15 mol / l cerium nitrate aqueous solution is dropped at a rate of 5 ml / min to form a precipitate. After holding for 1 hour, filtration and addition of distilled water are repeated 3 times to remove ions such as nitrate ions and ammonium ions remaining after the reaction. The finally filtered precipitate is dried at room temperature in a nitrogen stream. This dried sample is lightly loosened in an alumina mortar and calcined at 700 ° C for 2 hours while flowing oxygen gas in a tubular electric furnace.

SEM images of the obtained precursor and calcined powder are shown in FIGS. 1a and 2a, respectively. Although Fig. 1a shows that ACC aggregates in a flat plate shape, it can be seen from Fig. 2a that nano-sized ceria powder in which individual grains are separated into nano-sized particles is obtained by calcining and the aggregation is destroyed.

The powder obtained by calcination is 30M in a mold having an inner diameter of 6 mm.
10 tablets of isostatically pressed tablets at 200 MPa after molding with Pa
The shrinkage curve due to densification when the temperature was raised at a rate of ° C / min was investigated. The bulk density calculated from this shrinkage curve and the raw bulk density is shown by the curve a in FIG. It can be seen from this curve that densification is substantially complete at temperatures as low as 1000 ° C.

Example 2 Instead of the cerium nitrate solution of Example 1, an aqueous solution prepared by dissolving 0.12 mol of cerium nitrate and 0.03 mol of rare earth nitrate (Sm, Y, Yb as rare earth) in 1 liter of distilled water is used. Other experiments were performed by the method of Example 1. According to the chemical analysis and powder X-ray diffractometry, the chemical formula of the precursor could be divided into two types according to the kind of additive.

When samarium having a large ionic radius is added, (NH ₄ ) CeSm _0.25 (CO ₃ ) _2.375 · H ₂ O (1) When yttrium or ytterbium having a relatively small ionic radius is added, (NH ₄ ) It can be described by _.25 CeRE _0.25 (CO ₃ ) _2.0・ 2H ₂ O (2). Here, RE means yttrium or ytterbium.

SEM of ACC with 20 mol% samarium, ACC with 20 mol% yttrium, and ACC with 20 mol% ytterbium
The images are shown in b, c and d of FIG. 1, respectively, and the SEM images of the powders which are calcined at 700 ° C. for 2 hours are shown in b, c and d of FIG. 2, respectively.
When the addition amounts of samarium, yttrium, and ytterbium are calculated as oxides, they correspond to 10 mol% with respect to ceria, respectively, so in FIG. 2, 10 mol% Sm ₂ O ₃ added CeO ₂ ,
It was expressed as 10 mol% Y ₂ O ₃ added CeO ₂ and 10 mol% Yb ₂ O ₃ added CeO ₂ .

From FIGS. 1 and 2, it can be seen that the added ACC and the calcined powder are spherical. Curves b, c, and d in FIG. 3 are densification curves of ceria added with samarium, yttrium, and ytterbium under constant speed heating. In both cases, the densification is substantially 1100 ° C The following is completed, and it can be seen that the ceria powder produced by the method of the present invention has excellent sinterability.

Example 3: 400 ml of a 1.5 mol / l ammonium hydrogen carbonate solution was stirred while maintaining it at 70 ° C., and 50 ml of a 1 mol / l cerium nitrate aqueous solution was added dropwise at a rate of 3 ml / min to form a precipitate. . After stirring and holding for 30 minutes, the solution is filtered and distilled water is added to remove ions such as nitrate ions and ammonium ions remaining after the reaction. The finally filtered precipitate is dried at room temperature in a nitrogen stream. This dried sample is lightly loosened in an alumina mortar and calcined at 800 ° C for 2 hours while flowing oxygen gas in a tubular electric furnace. From the ratio of ammonia ions, nitrate ions, and carbon obtained by chemical analysis of the obtained precursor, and the ratio of cerium and water evaluated from the weight reduction of the precursor, the precursor was (NH ₄ ) _.1 It could be described by the chemical formula of Ce _2.0 O (CO ₃ ) _2.0 · H ₂ O.

SE of the precursor and powder obtained by calcining the precursor
M images are shown in Figures 4a and 5a, respectively. Figure 4a has a diameter of 30-50n
Fig. 5a shows elongated rice-shaped particles having a length of m and a length of 100 to 300 nm, and Fig. 5a shows dice-shaped particles having a side of 30 to 50 nm and rounded corners. The curve a in FIG. 6 shows the ceria particles shown in FIG. ℃ / min
The bulk density at each temperature when the temperature is raised at a speed is shown. From this curve, it can be seen that the densification to 97% was achieved by the constant rate heating to 1100 ° C.

Comparative Example 1: Example 1 except that aging is performed at room temperature
Ceria powder was synthesized by the above method, and a constant rate temperature rising sintering experiment was performed. Curve e in FIG. 3 shows the bulk density of the sintered body calculated from the measured values of the linear shrinkage. Shrinks sharply above 800 ° C, but 1000
Above ℃, the densification rate decreased, and the reached density was only 94% of the theoretical density even after heating up to 1300 ℃.

Comparative Example 2: Example 1 except aging at 90 ° C
Ceria powder was synthesized by the above method, and a constant rate temperature rising sintering experiment was performed. Curve f in FIG. 3 shows the bulk density of the sintered body calculated from the measured values of the linear shrinkage. In the temperature range from 1100 ℃ to 1400 ℃, the curve
Although f shows a higher density than curve e, the reached density was only 96% of the theoretical density even when the temperature was raised to 1300 ° C.

Comparative Example 3: The precursor prepared by the method of Example 3 was calcined at 1000 ° C. A curve b in FIG. 6 shows a change in bulk density due to sintering of the green compact of the powder. From the same curve, the compact density is as large as 67% to 68% of the theoretical density, but the densification does not proceed much even if the sintering temperature rises, and the sintering density is 95 or less even after firing at 1500 ° C for 2 hours. I know there was.

Comparative Example 4: SEM images of the precursor and calcined powder synthesized under the conditions of Example 3 except that the precipitate was aged at 30 ° C. are shown in FIGS. 4b and 5b, respectively. From the comparison of these figures, it can be seen that the shape of the precursor powder remains even after calcination. Figure 6 in curve c
5 shows a change in bulk density when the green compact of ceria powder shown in 5b is sintered. The molding and sintering were performed by the method of Example 3. It can be seen from curve c in Fig. 6 that the densification is very poor.

[Brief description of drawings]

FIG. 1 is a drawing-substitute SEM photograph of ACC of Example 1 (a: no addition, b: 20 mol% samarium added, c: 20 mol% yttrium added, d: 20 mol% ytterbium added).

FIG. 2 is a drawing-substitute SEM photograph of powder obtained by calcining the ACC shown in FIG. 1 at 700 ° C. for 2 hours.

FIG. 3 is a graph showing changes in bulk density of various types of ceria due to constant rate temperature rising sintering.

FIG. 4 Example 3 (a: aged at 75 ° C.) and Comparative Example 4 (b:
It is a drawing-substitute SEM photograph of ACC synthesized by aging at 30 ° C).

5 is a drawing substitute of the ceria powder obtained by calcining ACC synthesized in Example 3 (a: aged at 75 ° C.) and Comparative Example 4 (b: aged at 30 ° C.) shown in FIG. 4 at 700 ° C. It is a SEM photograph.

FIG. 6 is a graph showing a change in bulk density of ceria obtained in Examples, Comparative Examples 3 and 4 due to constant rate temperature rising sintering.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yusuke Moriyoshi             1-2-1 Sengen, Tsukuba-shi, Ibaraki Independent             National Institute for Materials Science (72) Inventor Toshiyuki Mori             1-2-1 Sengen, Tsukuba-shi, Ibaraki Independent             National Institute for Materials Science F-term (reference) 4G076 AA02 AA18 AB07 BA13 BA15                       BA38 BA45 BD02 CA04 CA05                       CA26 DA01 DA03 DA04 DA07                       DA11 DA30

Claims (2)

[Claims] 1. An aqueous solution of cerium salt having a concentration of 0.05 mol / l to a saturated concentration and an aqueous solution of ammonium carbonate or an aqueous solution of ammonium hydrogencarbonate having a concentration of 0.3 to 3 mol / l are mixed ((NH ₄ ) _h CeO _k (C
O ₃ ) _m・ nH ₂ O (however, h is 0.05 to 1.2, k is 2 or less, m is 4 or less) can be described by ammonium cerium carbonate, and then aging is performed at a temperature of 50 to 90 ° C. The average particle diameter of the primary particles is 10 nm, which is characterized by performing cleaning for up to 12 hours in the range, washing, and calcining at 500 ° C or higher and lower than 1000 ° C.
A method for synthesizing ceria powder in which hard agglomeration of primary particles of ~ 100 nm is not observed. 2. The synthetic method according to claim 1, wherein instead of the aqueous cerium salt solution, an acidic aqueous solution containing cerium ions and one or more kinds of metal ions in a total amount of 0.1 mol% to 30 mol% with respect to the cerium ions is used. A method for producing a metal oxide-added ceria powder having an average primary particle diameter of 10 nm to 100 nm, which is used.