JP3681550B2 - Rare earth oxide and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a rare earth oxide having a small Fischer diameter, a large specific surface area, and a small crystallite size, which are useful as a raw material for ceramics containing rare earth elements, a ceramic sintering aid, and the like, and a method for producing the same.
[0002]
[Prior art]
In general, rare earth oxides are obtained by firing their oxalates, carbonates, hydroxides, their double salts, and other salts. In addition, rare earth oxides produced by these methods are generally used for ceramics sintered with rare earth oxides or ceramics containing rare earth oxides. There is a difference in the ease of increasing the sintered density. It is known that the ease of increasing the sintering density is influenced by the particle size, specific surface area, crystallite size, etc. of the rare earth oxide used. Moreover, when mixing with other ceramic powder etc., the cohesive force of particle | grains is weak, and it is an important element whether it can mix, without applying a big effort. As rare earth oxide powders used for ceramics, it is generally preferred that the particle size is fine, the specific surface area is large, and the crystallite size is small.
[0003]
[Problems to be solved by the invention]
Therefore, the present inventors have conducted research for the purpose of obtaining rare earth oxides that tend to increase the sintering density, have a small particle size, a large specific surface area, and a small crystallite size when used in a sintered body. It was.
[0004]
In general, the characteristics required of rare earth oxides as raw materials for ceramics containing rare earth elements include an increase in sintering density at a lower temperature and a good dispersion state between powders. In order to impart such characteristics to the rare earth oxide, it is generally effective that the particle diameter is small, the specific surface area is large, and the crystallite size is small.
By the way, when making rare earth oxides, there are usually forms as salts such as oxalates and carbonates as precursors thereof, and in order to synthesize oxides by firing these, The characteristics of the salt will affect the properties of the oxide produced. Therefore, in order to obtain rare earth oxide particles suitable for ceramics containing rare earth elements, it is necessary to produce a salt as a precursor suitable for the purpose.
For example, when oxalate or a double salt thereof is used as a precursor for the production of rare earth oxides, a powder having good dispersibility is generally obtained, but on the other hand, the particle diameter is not so small and is only about 1 μm at most. There are drawbacks. In addition, when hydroxide is used as an intermediate raw material, the particle size can be small, but because the precipitate is in a gel form, solid-liquid separation and other handling are difficult, and the particle size is increased by aggregation during firing. There is a drawback of becoming. Thus, there are advantages and disadvantages in using any of the precursors in making rare earth oxide powder as a ceramic raw material.
An object of the present invention is to efficiently provide a rare earth oxide useful as a ceramic raw material, having a small particle size, a large specific surface area, and a small crystallite size.
[0005]
[Means for Solving the Problems]
The present inventors focused on the advantages of each salt as a rare earth oxide precursor and considered to make a precursor having both advantages.
First, attention was paid to the fineness of the particles of the hydroxide precipitate, and it was considered to make use of this. However, if the hydroxide precipitate is left as it is, the particle size becomes large during firing as described above. Therefore, once the hydroxide precipitate is generated, and the hydroxyl group in the hydroxide precipitate is replaced with another group, for example, oxalate ion or carbonate ion, the poor filterability, which is a drawback of the hydroxide precipitate, is reduced. We thought that it would be possible to improve and suppress the growth of the particle size during firing, and studied.
As a result, a part or all of the hydroxide can be changed to carbonate or basic carbonate by adding a carbonate ion generating substance in the process of depositing rare earth hydroxide using alkali. Thus, a precipitate having good filterability is obtained, and this is subjected to solid-liquid separation, and the obtained cake is baked, so that the required particle size is small, the specific surface area is large, and the crystallite size is required as a ceramic raw material. It has been found that a rare earth oxide having a small particle size can be obtained.
In addition, the precipitate obtained by this method is different from ordinary hydroxides in that it is easy to handle solid-liquid separation and other operations, and has the advantage that the particle size does not increase even during firing.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the production conditions of the rare earth oxide in the present invention will be described in more detail.
The rare earth oxide referred to in the present invention includes one kind of rare earth oxide of lanthanoids having atomic numbers 37 to 71 in the periodic table containing yttrium oxide to those composed of a plurality of rare earth oxides.
First, an aqueous solution containing a rare earth element salt is prepared.
Examples of the salt include chloride, nitrate, acetate and the like, and nitrate is preferred.
The solution concentration of the rare earth element is preferably 1 mol / liter or less. This is because a higher concentration than this is inconvenient to handle after crystal precipitation.
[0007]
Next, an alkali is added to the aqueous solution containing the salt of the rare earth element to form a rare earth hydroxide precipitate.
The amount of alkali added needs to be 2 to 10 times the number of moles of rare earth elements, preferably 3 to 3.5 times the number of moles, and precipitation is less than 2 times the number of moles. Becomes insufficient, and if it exceeds 10 times the number of moles, it becomes uneconomical. However, in the case where an acid is previously contained in the rare earth element salt solution, an alkali in an amount for neutralizing the acid can be added.
As the type of alkali, ammonia is preferable, but when an alkali metal element cannot be mixed in the rare earth oxide to be used, an alkali metal hydroxide such as caustic soda can also be used.
[0008]
It is possible to obtain hydroxides with small crystallite size even from rare earth hydroxide precipitates that are usually generated by adding alkali, but solid-liquid separation is not easy because the resulting precipitates are gel-like. In addition, when the oxide is synthesized through a firing step or the like, the particle size becomes large due to dehydration or aggregation from the hydroxyl group, and particles having a small particle size cannot be obtained.
Therefore, in the present invention, a step of changing a part or all of the hydroxide precipitate into a carbonate or a basic carbonate by adding a carbonate ion generating substance in the middle of the rare earth hydroxide precipitate generating step with an alkali is provided. .
As the carbonate ion generating substance, ammonium bicarbonate is generally used, but ammonium carbonate can also be used.
The rare earth hydroxide precipitate once produced is converted into carbonate or basic carbonate by adding a carbonate ion generating substance.
Rare earth carbonates or basic carbonates synthesized in this way are more granular than carbonates and basic carbonates made by adding carbonate ion-generating substances to solutions containing rare earth element salts from the beginning. It was found that the diameter was small and the crystallite size was small. This is because the particle size, crystallite size, etc. inherit the characteristics of hydroxide precipitation. In addition, rare earth carbonates usually have a feature that the particle diameter is difficult to increase upon firing, and therefore an oxide having a small particle diameter can be obtained.
[0009]
The addition amount of the carbonate ion generating material must be 0.5 to 2 times the number of moles of the rare earth element, and if the number of moles is less than 0.5 times and more than 2 times, the filterability is low. This produces a bad precipitate.
The timing of adding the carbonate ion generator may be any time as long as it is added in a state where the rare earth hydroxide precipitate is generated. However, since there is no effect in a state where hydroxide precipitates are not generated so much, it is preferable to add the alkali at the time of adding about twice the number of moles of the rare earth element contained in the solution.
The temperature of the solution does not need to be specified, but a temperature at which the operation is easy at 10 ° C. or higher and 90 ° C. or lower is preferable. After the addition of the carbonate ion generating substance, it is preferable to take stirring and aging time of 20 minutes or longer in order to wait for the hydroxyl group of the rare earth hydroxide and the carbonate radical of the carbonate ion generating substance to be replaced. The precipitate thus obtained is separated into solid and liquid, and dried if necessary.
Precipitated rare earth element is in a state like a coprecipitation product of rare earth element hydroxide and carbonate, and it is not gel like hydroxide precipitation and cake with good filterability Therefore, the workability of solid-liquid separation is greatly improved compared to hydroxide precipitation.
[0010]
The baking temperature of the cake separated into solid and liquid needs to be 300 ° C. or higher and 900 ° C. or lower, preferably 600 to 900 ° C. If it is lower than 300 ° C., the change to oxide is insufficient, If it exceeds 900 ° C., sintering will begin.
Since the oxide immediately after firing is in the form of a flock, it can be used as a ceramic raw material if it is loosened by means of a hand, a sieve or a crusher.
[0011]
The rare earth oxide thus obtained has a Fischer diameter of 0.5 μm or less, a specific surface area measured by the BET method of 30 m ² / g or more, and a crystallite size of 250 μm measured by the X-ray diffraction method. It is as follows.
When the sintered density of the sintered body using this rare earth oxide was measured, it was found that the density of the sintered body was improved at a lower temperature and the sintering characteristics were better than the oxide synthesized by the conventional method. all right.
[0012]
【Example】
Embodiments of the present invention will be specifically described below with reference to examples and comparative examples, but these do not limit the present invention.
(Physical property measurement method)
(1) Particle size: measured by the Fisher method.
(2) Specific surface area: measured by the BET method.
(3) Crystallite size: Measured by the Scherrer method with an X-ray diffractometer.
(Example 1)
To 10 liters of an aqueous solution of yttrium nitrate having a concentration of 0.1 mol / liter, 2 mol of 25% aqueous ammonia was added while stirring the solution. After the addition of the aqueous ammonia, a 10% solution of ammonium bicarbonate was added as 1. Add 5 moles and stir for about 60 minutes. Further, 1 mol of 25% aqueous ammonia was added and stirring was continued for 10 minutes to form a precipitate. The pH of the solution at this time was about 9. The precipitate solution was filtered through a Buchner funnel to separate the precipitate, which was easily performed. The precipitate was then calcined at 680 ° C. for 6 hours to obtain about 112 g of yttrium oxide powder. The Fischer diameter of this yttrium oxide powder was 0.2 μm, the specific surface area was 38 m ² / g, and the crystallite size was about 210 mm. Also, when this powder was used as a raw material, a sintered body of yttrium oxide was prototyped. The molding density was about 1.5 g / cm ³ (about 30% of the theoretical density), and this was sintered at 1500 ° C. for 4 hours under normal pressure. The sintered density was about 4.5 g / cm ³ (about 90% of the theoretical density).
[0013]
(Comparative Example 1)
In Example 1, oxalic acid was added instead of aqueous ammonia to precipitate yttrium oxalate, which was treated in the same manner as in Example 1 to obtain yttrium oxide powder. The Fischer diameter of this yttrium oxide powder was 0.7 μm, the specific surface area was 10 m ² / g, and the crystallite size was about 1000 mm. Moreover, when this powder was used as a raw material, a sintered body of yttrium oxide was prototyped. The molding density was about 2.5 g / cm ³ (about 50% of the theoretical density), and this was sintered at 1500 ° C. for 4 hours under normal pressure. The sintered density was about 3.8 g / cm ³ (about 75% of the theoretical density).
[0014]
【The invention's effect】
According to the present invention, a rare earth oxide having a small Fischer diameter, a large specific surface area, and a small crystallite size, which is useful as a raw material for rare earth element ceramics or a sintering aid for ceramics, is economically obtained by a simple method. It can be manufactured and its industrial utility value is extremely high.

Claims (5)

A rare earth oxide having a Fischer diameter of 0.5 μm or less, a specific surface area measured by the BET method of 30 m ² / g or more, and a crystallite size of 250 μm or less measured by an X-ray diffraction method. Claims obtained by firing at a temperature of 300 ° C. or more and 900 ° C. or less using a rare earth hydroxide precursor partially or entirely substituted with carbonate or basic carbonate. Item 2. The rare earth oxide according to Item 1. In the course of the process of adding an alkali having a mole number of 2 to 10 times the mole number of the rare earth element to the solution containing the rare earth element ion, and depositing a hydroxide precipitate, the carbonate ion generating material is added to the rare earth element. A method for producing a rare earth oxide, comprising adding 0.5 to 2 times the number of moles of the catalyst, solid-liquid separation of the resulting precipitate, and firing at a temperature of 300 ° C to 900 ° C. The method for producing a rare earth oxide according to claim 3, wherein the alkali is ammonia. The method for producing a rare earth oxide according to claim 3 or 4, wherein the carbonate ion generating material is ammonium bicarbonate.