JPH10273364A - Production of transparent yttrium oxide sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of producing a transparent yttrium oxide sintered body at a lower cost than a conventional method and free from the necessity for severely controlling production conditions. SOLUTION: An yttrium compound which becomes yttrium oxide powder by pyrolysis or yttrium oxide powder of <=0.5 μm in an average particle size of primary particles is mixed with a calcium compound which becomes calcium oxide by pyrolysis or calcium oxide powder in a range of 100 ppm or 4% based on the yttrium oxide, and the resultant powdery mixture is processed into a formed body. The obtained formed body is baked at a temperature in the range of 1400-2000 deg.C under an atmosphere whose nitrogen partial pressure is controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a transparent sintered body of yttrium oxide which is expected to be used for various arc tubes for discharge lamps, transparent furnace tubes for ultra-high temperatures, laser host materials, and the like.

[0002]

2. Description of the Related Art Since the crystal structure of yttrium oxide is cubic, which is optically isotropic and has no birefringence, scattering of light at grain boundaries can be ignored in principle. Therefore, even if the pores and inclusions can be completely removed from a polycrystalline material such as a sintered body, the optical material can have the same transparency as a single crystal. Further, since its melting point is as high as about 2400 ° C., it is expected as a high-temperature material. However, a high melting point generally means that it is necessary to sinter at a high temperature or to apply pressure simultaneously with firing. For this reason, in the conventional ordinary sintering method, firing was performed at a very high temperature of 2200 ° C. or more to produce a yttrium oxide transparent sintered body.
In the case of hot pressing or HIP, which pressurizes during firing,
Although it can be made transparent even at 1500 ° C., there is a drawback that the workability is poor and it is not suitable for mass production, and the production cost becomes extremely high.

[0003] Needless to say, a method of producing a transparent sintered body utilizing the effect of additives to promote densification has also been studied. In this case, it is necessary to satisfy the following two conditions as an additive. The first condition is that even after firing, it completely dissolves in yttrium oxide,
In addition, when the substance does not absorb light or forms a second phase with an additive, the substance must have optical properties close to those of yttrium oxide. The second condition is that the material promotes densification in the latter stage of sintering. Thorium oxide has been reported as a substance satisfying these conditions. However, even in this case, firing at a high temperature of 2000 ° C. or more was required to obtain a transparent sintered body. Further, thorium oxide is a radioactive substance, albeit slightly, and has a drawback that its handling and usable fields are limited. These disadvantages of the prior art stem from the use of commercially available yttrium oxide powder. That is, since the hygroscopicity of yttrium oxide powder cannot be ignored if it is fine, commercially available yttrium oxide powder is generally large in order to prevent moisture absorption that proceeds during distribution.
As a result, commercially available yttrium oxide powder has poor sinterability, and it was necessary to increase the sintering temperature in order to produce a transparent sintered body using the powder.

Recently, the present inventors have found that yttrium oxide powder obtained by calcining appropriately prepared yttrium carbonate has extremely good sinterability. Based on this discovery, we have developed a process for manufacturing yttrium oxide sintered bodies that can be made transparent even at a sintering temperature of 1600 ° C without using additives.
An application was filed as a patent (reference number 9652701). In this method, a sintered body having excellent transparency can be obtained in a small-scale production on a laboratory scale. However, when mass-producing on an industrial scale, it is necessary to strictly control the aging of yttrium carbonate in order to produce a sintered body having good transparency. Actually, such precise control is difficult, and there is a disadvantage that the yield of the production of the transparent sintered body is poor.

[0005]

SUMMARY OF THE INVENTION In view of the above problems of the prior art, the present inventor has made it possible to further improve the sinterability of an easily sinterable yttrium oxide powder which is finer than a commercially available ordinary powder. As a result of various studies to obtain a transparent sintered body with a good yield by promoting the present invention, the present invention has been completed, the object of the present invention is a lower cost than the conventional manufacturing method, and strict production conditions Provided is a method for producing a yttrium oxide transparent sintered body without requiring control.

[0006]

The gist of the invention of claim 1 of the present application is that an yttrium compound which becomes yttrium oxide powder by thermal decomposition or yttrium oxide powder having an average primary particle size of 0.5 μm or less, and yttrium oxide On the other hand, a calcium compound or a calcium oxide powder which is converted into calcium oxide by pyrolysis in the range of 100 ppm to 4% is mixed, and a molded body is formed from the obtained powdery mixture. A method for producing a transparent yttrium oxide sintered body characterized by firing in an atmosphere with a limited partial pressure of nitrogen gas. The gist of the invention of claim 2 is that the yttrium compound which becomes yttrium oxide powder by thermal decomposition Alternatively, the average primary particle size is 0.5
A powder obtained by mixing a yttrium oxide powder having a particle size of 1 μm or less with a yttrium oxide powder having a particle size of 1 μm or less or a zirconium compound which becomes zirconium oxide by thermal decomposition in a range of 200 ppm to 10% with respect to the yttrium oxide. A method for producing a transparent yttrium oxide sintered body, characterized in that a molded body is prepared from a mixture in the form of a mixture, and the molded body is fired in a range of 1400 ° C. to 2000 ° C. in an atmosphere with a limited partial pressure of nitrogen gas. . In these inventions, the transparency of the sintered body could be further improved by adding a compound which becomes a low-melting substance by combining with calcium oxide or zirconium oxide.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (A) Production of Yttrium Oxide Powder Examples of the yttrium compound that produces yttrium oxide used in the present invention include yttrium hydroxide, yttrium carbonate, yttrium oxalate, yttrium chloride, yttrium nitrate, and yttrium sulfate. There is no particular limitation on the type of yttrium compound as long as it is an yttrium compound that becomes yttrium oxide powder after calcination. Among these, agglomerated particles of yttrium oxide powder obtained by calcining yttrium hydroxide, yttrium carbonate, and yttrium oxalate are very fragile and are particularly preferable because they can be uniformly filled during molding. Other compounds are not particularly limited to the kind of salt, the calcination temperature, and the like, as long as the primary particles of the yttrium oxide powder have a particle size of 0.5 μm or less after calcination. Generally, strong agglomerated particles behave substantially as primary particles. For example, when yttrium sulfate or yttrium nitrate is calcined by a usual method, large and strong aggregated particles are generated. In this case, it is necessary to grind the aggregated particles of the powder obtained by calcining with a ball mill or the like. Yttrium oxide powder used in the present invention, if the particle size is 0.5μm or less,
A commercially available powder may be used. Moreover, you may obtain by calcining the various yttrium compounds illustrated above.

(B) Effect of Additives Examples of the calcium compound which becomes calcium oxide by thermal decomposition used in the present invention include calcium hydroxide and calcium carbonate. There is no particular limitation on the type of the compound as long as it is a compound that changes to calcium oxide after calcination. When a calcium compound which becomes 100 ppm or more of calcium oxide is added to yttrium oxide, yttrium oxide develops light-transmitting properties. It is particularly preferable that the addition in the range of 1000 ppm to 2% can provide a sintered body having extremely high translucency. When the amount exceeds 2%, the effect of addition decreases, and when 4% or more is added, the effect disappears. The calcium oxide powder used in the present invention may be obtained by burning metallic calcium. Further, a calcium compound may be pyrolyzed and calcined to obtain a calcium oxide powder. Calcium oxide readily reacts with atmospheric water vapor to form calcium hydroxide. Since the hydroxide is thermally decomposed during calcination or firing to change into fine calcium oxide, the calcium oxide when added may be relatively large. Examples of the zirconium compound which generates zirconium oxide powder by thermal decomposition used in the present invention include zirconium oxynitrate, zirconium oxychloride, zirconium hydroxide and the like. There is no particular limitation on the type of the compound as long as it is a compound that changes to zirconium oxide by calcination.

The zirconium oxide powder used in the present invention is obtained by calcining the above zirconium compound. Since the diffusion coefficient of zirconium ions in yttrium oxide is small, unreacted zirconium oxide particles remain even after sintering if the particle size of the zirconium oxide powder used is 1 μm or more. This is not preferable because it becomes a light scattering source. The amount of zirconium oxide dissolved in yttrium oxide is very large. Therefore, in order to promote sintering of yttrium oxide, it is necessary to add a larger amount of zirconium oxide than calcium oxide. In order to obtain a transparent yttrium oxide sintered body, it is necessary to add a zirconium oxide powder or a zirconium compound which becomes zirconium oxide in an amount of 200 ppm or more based on yttrium oxide. 500
When a zirconium compound corresponding to zirconium oxide in the range of ppm to 3% is added, a sintered body having extremely high translucency can be obtained. When the amount exceeds 3%, the effect gradually decreases, and when it exceeds 10%, the effect disappears. In the present invention, the amount of calcium oxide or zirconium oxide defined in claims 1 and 2 may be present in advance as an impurity, provided that the amount finally present satisfies the conditions of the claims. A favorable result is obtained. In addition, they may coexist as long as they are within the range defined in claims 1 and 2.

The compound of the present invention which becomes a low-melting substance by combining with calcium oxide or zirconium oxide is an oxide of an alkali metal such as silicon oxide, boron oxide or sodium oxide, or calcined silicon oxide or boron oxide; It refers to a compound represented by a compound that becomes an oxide of an alkali metal. When these substances are present, they react with calcium oxide and zirconium oxide to form a low-melting substance such as glass and promote sinterability of yttrium oxide. As a result, the transparency is improved. The effect of improving the transparency of the compound is exhibited when the compound and calcium oxide or zirconium oxide coexist. These compounds are one-to-one with yttrium oxide.
Even 0 ppm sufficiently promotes the sinterability of yttrium oxide. However, it is necessary to limit the content of the compound to 1% or less in terms of weight% because it causes a reduction in the high-temperature strength of the sintered body. These compounds can exert the effects of the present invention not only when they are newly added as additives but also when they are present as impurities in yttrium oxide, calcium compounds, zirconium compounds, or the like in advance. In the present invention, after mixing calcium oxide powder, zirconium oxide powder, silicon oxide powder, or the like, or after mixing and calcining an yttrium compound, a calcium compound, a zirconium compound, a silicon compound, and the like, a ball mill or the like is used. Mixing is preferable because the yttrium oxide powder and the added powder are uniformly mixed.

(C) Calcination In the present invention, the powdery mixture is preliminarily calcined, if necessary, before obtaining a compact from the powdery mixture. That is, when the raw materials are prepared by a method other than the combination of the oxide powder alone, it is necessary to calcine at an appropriate temperature to obtain the oxide powder in advance.
When the calcination temperature is low, the crystallite of the oxide powder generated by thermal decomposition is extremely fine, and the filling property of the oxide powder is poor. The density in a green compact obtained by molding such a powder differs for each fine region. Where the packing density is low, large holes are formed during sintering. Such voids cannot be substantially removed by sintering. As a result of the study, when calcined at 700 ° C. or higher, fine powders that cause problems in filling substantially disappear.
When the thermal decomposition temperature of the compound to an oxide is higher than 700 ° C., if the compound is calcined at a temperature equal to or higher than the thermal decomposition temperature, extremely fine oxide nuclei generated immediately after the thermal decomposition grow during the calcining. Thus, the fine particles that hinder filling disappear. On the other hand, if the calcination temperature is 1300 ° C. or higher, the grain growth of the oxide powder proceeds in the calcination stage, and the sinterability decreases, which is not preferable.
When only oxide powder is used, calcining after mixing the oxide powder is not always necessary. However, depending on the production history of the oxide powder, calcining may be preferable. The calcination is simply performed to obtain an oxide powder having good sinterability. For this reason, since the atmosphere gas does not remain in the sintered body as in the case of firing, the calcining atmosphere is not particularly limited.

(D) Molding Molding methods include dry molding, cast molding, and extrusion molding. As long as the density distribution of the molded body is not wide, the effect of the present invention is not limited by the molding method. (E) Sintering If calcium oxide or zirconium oxide is added to yttrium oxide powder with very good sinterability to further improve the sinterability, the sintered body is densified to almost the theoretical density even at a sintering temperature of 1400 ° C or less. Can be manufactured. However, when the sintering temperature is so low, the grain growth is insufficient, and the light transmittance of the actual sintered body is low. Therefore, in order to solve this problem, 1
It is necessary to sinter at 400 ° C. or higher. Of course, as the firing temperature increases, the grain growth progresses, and the translucency increases. However, when the temperature exceeds 1600 ° C, the firing temperature rises,
The rate at which the translucency is improved decreases. For the above reasons,
In the present invention, it is preferable to fire at 1600 ° C. or higher. On the other hand, when the firing temperature is 2000 ° C. or higher, there is no significant difference in the sintering temperature from the conventional method.

(F) Firing atmosphere When firing is performed in an atmosphere containing a large amount of an inert gas having a large atomic radius, such as nitrogen atom or argon atom, which does not substantially diffuse in yttrium oxide, those gases become bubbles in the sintered body. It is not preferable because it remains as. Among these gases,
Since gases other than nitrogen gas are hardly contained in the atmosphere, there is no industrial problem. Since nitrogen gas occupies 80% of the atmosphere, it is necessary to take measures to reduce the amount of nitrogen gas in the atmosphere. Needless to say, the smaller the amount of nitrogen gas in the atmosphere, the better the density. Therefore, it is preferable to replace the atmosphere with hydrogen gas or oxygen gas, or to drive out the atmosphere to form a vacuum atmosphere. However, in the actual process of manufacturing the transparent sintered body, it is not necessary to completely remove the gas that inhibits densification from the atmospheric gas, and a preferable result can be obtained by limiting the gas to about 1/3 atmosphere or less.

[0014]

Examples and Comparative Examples Examples of the present invention are shown below.
The effects of the present invention are not limited to these. Example 1 Commercially available fine yttrium oxide powder (having a specific surface area of 14 M ²
/ G) Disperse 10 g in ethyl alcohol. To this dispersion stirred with a magnetic stirrer, 0.3% of a calcium oxide powder dispersed in ethyl alcohol and having an average particle size of 0.5 μm is added to yttrium oxide.
Heat with stirring to evaporate the ethyl alcohol.
The dried product is lightly loosened in an alumina mortar, and 3
After primary molding at 0 MPa, isostatic pressing is performed at 200 MPa to form a molded body. The molded body was placed in a vacuum electric furnace and a high vacuum of 10 ⁻⁵ Torr or more was obtained.
The temperature was raised to 0 ° C at a rate of 10 ° C / min.
Hold for hours. After that, the sample is cooled at a rate of 15 ° C./min. When the density of the sintered body was measured by the Archimedes method using water, a value very close to the theoretical density was obtained. When the wavelength is 50
The linear transmittance of the sintered body having a thickness of 1 mm with respect to light of 0 nm was about 40%. Comparative Example 1 A green compact of an additive-free yttrium oxide powder was prepared and sintered according to the procedure of Example 1 without adding the calcium oxide powder. The relative density of the obtained sample was 9 of theoretical density.
At 8%, the sintered body was milky white.

Example 2 After dissolving 20 g of yttrium nitrate in 300 ml of distilled water, the mixture is heated to about 90 ° C. while stirring with a magnetic stirrer equipped with a heater. 1N aqueous ammonia is dropped into the solution until the pH becomes 8, and the solution is maintained for 3 hours. After filtration and washing, it is dried at room temperature. Disperse lightly in an alumina mortar and disperse 5 g of the powder in ethyl alcohol. To this dispersion stirred with a magnetic stirrer, the average particle size dispersed in ethyl alcohol was 0.5 μm.
m of calcium oxide powder is added to yttrium oxide at 0.3%. Heat with stirring to evaporate the ethyl alcohol. The dried product is lightly loosened in an alumina mortar, placed in an alumina boat, and placed in a tubular electric furnace in an oxygen stream.
Calcinate at 100 ° C for 4 hours. Thereafter, the calcined powder is lightly loosened in an alumina mortar, and then primary molded at 30 MPa using a mold, and then subjected to isostatic pressing at 200 MPa to form a molded body. The molded body is placed in a vacuum electric furnace, and 10 ⁻⁵
After a high vacuum of more than Torr, 10 to 1700 ° C
The temperature is raised at a rate of ° C./min and maintained at that temperature for one hour. After that, the sample is cooled at a rate of 15 ° C./min. When the density of the sintered body was measured by the Archimedes method using water, a value very close to the theoretical density was obtained. The linear transmittance of the sintered body having a thickness of 1 mm with respect to light having a wavelength of 500 nm was about 40%.

Example 3 15 g of yttrium nitrate and 0.1 g of calcium nitrate are dissolved in 100 ml of distilled water, and the pH is adjusted to 5 by adding a 2 mol / l aqueous solution of ammonium hydrogen carbonate to form a precipitate. The resulting precipitate is cured for 1 day with good stirring and then filtered. The precipitate obtained by dispersing the filtered precipitate in distilled water and then filtering is repeatedly washed four times, and then washed.
Dry at room temperature. After the dried body is lightly loosened with an alumina mortar, it is put into an alumina boat and 1100 with a tubular electric furnace.
Calcination is performed in an oxygen stream at 4 ° C. for 4 hours. The calcined powder is molded by the method of Example 1 and sintered. The bulk density of the obtained sintered body was almost close to the theoretical density, and the linear transmittance of the sintered body having a thickness of 1 mm with respect to light having a wavelength of 500 nm was about 60%. Comparative Example 2 A yttrium oxide sintered body was produced by the method of Example 2 without adding calcium nitrate. The linear transmittance of this sintered body having a thickness of 1 mm for light having a wavelength of 500 nm is 20.
%Met.

Example 4 Instead of calcium oxide powder, a commercially available zirconium oxide powder having a particle size of 27 nm is treated in the same manner as in Examples 1 and 2 by adding 1% to yttrium oxide powder.
When the density of the sintered body was measured by the Archimedes method using water, a value very close to the theoretical density was obtained. Wavelength is 500n
The linear transmittance of the sintered body having a thickness of 1 mm with respect to light of m was 40% and 50%, respectively.

Example 5 In the same manner as in Example 3, except that 0.2 g of zirconium oxychloride was used instead of 0.1 g of calcium nitrate, a carbonate was produced from a mixed aqueous solution with yttrium nitrate, which was then aged.
Filter, calcine and mold. The molded body is placed in an oxygen stream for 1 hour.
Bake at 650 ° C. for 4 hours. The bulk density of the obtained sintered body was close to the theoretical density, and the linear transmittance of the sintered body having a thickness of 1 mm with respect to light having a wavelength of 500 nm was about 45%.

Example 6 Not only 0.3% of calcium oxide but also 100 ppm of silicon oxide are simultaneously added to produce a transparent sintered body according to the method of Example 1. The wavelength of a 1mm thick sintered body is 500nm
Was about 50%, and the light transmittance was better than that of the transparent sintered body manufactured by the method of Example 1.

[0020]

As described above, according to the method of the present invention, a transparent yttrium oxide sintered body can be obtained without requiring strict conditions as in the conventional method. As shown in the above, it has excellent light transmittance as compared with the case where calcium oxide or zirconium oxide is not added.

Claims (3)

[Claims] An average particle diameter of an yttrium compound or a primary particle which becomes yttrium oxide powder by thermal decomposition is 0.5.
μm or less yttrium oxide powder, and a calcium compound or calcium oxide powder that becomes calcium oxide by pyrolysis in the range of 100 ppm to 4% of yttrium oxide are mixed, and a molded body is formed from the obtained powdery mixture; The molded body is in the range of 1400 ° C to 2000 ° C,
A method for producing a transparent yttrium oxide sintered body, characterized by firing in an atmosphere with a limited partial pressure of nitrogen gas. 2. The average particle diameter of an yttrium compound or primary particles which becomes yttrium oxide powder by thermal decomposition is 0.5.
A powder obtained by mixing a yttrium oxide powder having a particle size of 1 μm or less with a yttrium oxide powder having a particle size of 1 μm or less or a zirconium compound which becomes zirconium oxide by thermal decomposition in a range of 200 ppm to 10% with respect to the yttrium oxide. A method for producing a transparent yttrium oxide sintered body, characterized in that a molded body is prepared from a mixture in the form of a mixture, and the molded body is fired in a temperature range of 1400 ° C. to 2000 ° C. in an atmosphere with a limited partial pressure of nitrogen gas. 3. A compound which is combined with calcium oxide or zirconium oxide to form a substance having a low melting point.
A method for producing a transparent yttrium oxide sintered body according to claim 2.