JPH0585823A - Sintered silicon carbide-silicon nitride-mixed oxide and its production - Google Patents

Abstract

PURPOSE:To obtain a highly reliable sintered compact having high strength and toughness by baking a mixture of powder of rareearth metal oxide-alumina- silica, SiC and Si3N4). CONSTITUTION:The objective silicon carbide-silicon nitride-mixed oxide sintered compact free from void, pore, etc., can be produced by mixing 5-95wt.% of a mixed oxide consisting of 95-5wt.% of rare-earth metal oxide, 4.9-94.9wt.% of alumina and 0.1-10wt.% of silica and 95-5wt.% of a non-oxide mixture consisting of 90-5wt.% of SiC powder, whisker or their mixture and 10-95wt.% of Si3N4 powder, whisker or their mixture, forming the obtained mixture, heating the formed product in a non-oxidizing atmosphere at a heating rate of 5-2000 deg.C/min and maintaining at 1500-1900 deg.C for 0.1-600 min. The above mixed oxide is e.g. composed of 99.9-90wt.% of Ln4AlO9 or LnAlO3 (Ln is rare-earth elements or their mixture) and 0.1-10wt.% of silica.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention eliminates pores and microcracks in ceramics, which cause deterioration of strength and reliability, and thereby has high strength and reliability. Silicon carbide (SiC) -silicon nitride (Si ₃ The present invention relates to a sintered body of N ₄ ) -based oxide ceramics, in particular, SiC-Si ₃ N ₄
We propose a (Ln-Al-Si) oxide-based sintered body (where Ln is a rare earth element and compounds thereof) and a method for producing the same.

[0002]

2. Description of the Related Art Generally, ceramic materials are promising as structural materials because they have high strength at high temperature and excellent heat resistance, oxidation resistance and corrosion resistance. In particular, there is great interest in application of such ceramics to structural materials used in a temperature range exceeding the use limit of metals. Among these ceramics, SiC and S
Since i ₃ N ₄ has excellent heat resistance and oxidation resistance, it is one of the most promising structural materials that can be used at high temperatures.

However, this SiC and Si ₃ N ₄ are
Since it is difficult to sinter, it is extremely difficult to obtain a dense sintered body without adding an auxiliary agent. For this reason, conventionally, SiC powder and Si ₃ N ₄ powder have been used as sintering aids in Y ₂ O ₃ and Al ₂ O.
It was baked by adding ₃ . However, SiC sintered bodies and Si ₃ N ₄ sintered bodies obtained by using these oxides alone or in combination as a sintering aid are dense and have high strength and excellent oxidation resistance. It has lower toughness than ceramics. As a result, it can be said that the SiC sintered body and the Si ₃ N ₄ sintered body are poorly reliable materials for use as structural materials.

On the other hand, a technique for improving the strength and toughness of a SiC sintered body and improving the reliability by compounding SiC with other ceramics was previously proposed by one of the present inventors. Proposed. For example, a technique for producing a dense SiC-rare earth oxide-alumina-based composite sintered body by pressureless sintering is described in "J, Am. Ceram. Soc., 65 ( 1982) C-92 ”. In addition, a technique for improving the toughness value and the reliability of a Si ₃ N ₄ sintered body by anisotropically growing a Si ₃ N ₄ crystal has been proposed by Kawashima et al. (Ken Kawashima, Hiromi Okamoto, Shuji Yamamoto, Akira Kitamura, Silicon Nitride Ceramics 2, Uchida Old Crane, P135-14
6) proposed by.

[0005]

However, since the SiC sintered body and the Si ₃ N ₄ sintered body synthesized by the above method always have a defect inside the sintered body, the defect occurs at the starting point. In addition, the reliability as a material was still lacking, and there was a big problem in practical use.

The generation mechanism of the sintered body of the SiC powder and the Si ₃ N ₄ powder and the rare earth oxide-alumina powder is considered as follows. First, in the first stage of pressureless sintering, an oxide solid solution or an oxide compound is produced from a rare earth oxide and alumina. Then, in the second stage, the SiC and
Si ₃ N ₄ forms a solid solution in the oxide and diffuses to form SiC grains and Si.
Grain growth of ₃ N ₄ grains is achieved and shrinks to form a dense sintered body.

However, in the second step, the SiC powder and the Si ₃ N ₄ powder are solid-solved in the oxide, and at the same time, a chemical reaction occurs, and the SiC and Si ₃ N ₄ react with alumina to decompose, and CO, CO Generates gases such as ₂ and NO. as a result,
Defects such as voids and pores occur in the sintered body, and SiC and
This causes a drawback that the strength of the interface between Si ₃ N ₄ and the oxide is reduced.

The object of the present invention is to obtain SiC-Si ₃ obtained by firing SiC and Si ₃ N ₄ and a rare earth oxide-alumina-based oxide.
Regarding the N ₄ -rare earth oxide-alumina-based sintered body, there is further provided a highly reliable sintered body having high strength and high toughness without defects such as voids and pores as described above. It is to propose a new technique for an advantageous manufacturing method.

[0009]

Means for Solving the Problems As a result of earnest research for realizing the above-mentioned object, the present inventors have found that the rare earth oxide-
The alumina-based oxide further by mixing silica, rare earth oxides - alumina - it was decided to perform a mixture firing the silica powder and SiC and Si ₃ N _4. In such a mixture firing, the reaction between SiC and Si ₃ N ₄ and alumina is suppressed, so that the generation of pores is reduced and the sintering between SiC and Si ₃ N ₄ and the oxide also proceeds gently. For,
It was found that a dense sintered body was easily obtained.

That is, the present invention relates to rare earth oxides 95 to 5
wt%, alumina 4.9-94.9wt% and silica 0.1-10wt
% Of mixed oxide consisting of 5% to 95% by weight; powder or whisker, or SiC 90 to 5% by weight of powder or whisker, or mixture thereof
Si ₃ N ₄ Non-oxide mixture consisting of 10-95wt% 95-5wt%
The mixture of and is molded, and then this molded body is heated in a non-oxidizing atmosphere at a temperature rising rate of 5 to 2000 ° C./min and held in the temperature range of 1500 to 1900 ° C. for 0.1 to 600 minutes. and performing firing Te _{SiC- Si 3 N 4 - (Ln} -Al-Si)
Method for producing oxide sintered body and Si synthesized by the method
_{C- Si 3 N 4 - (Ln} -Al-Si) oxide sintered body, and, Ln
₄ Al ₂ O ₉ or LnAlO ₃ (where Ln is a rare earth metal and compounds thereof) 99.9 to 90 wt% of a mixture of silica 0.1 to 10 wt% of 5 to 95 wt%; 〜5wt% and powder or whiskers, or mixture of these, Si ₃ N ₄ 10〜95
a mixture of 95 to 5 wt% of a non-oxide mixture consisting of wt%; 1900
℃ temperature range at 0.1 to 600 minutes holding and performing firing _{SiC- Si 3 N 4 - (Ln} -Al-Si) and method for manufacturing the oxide sintered body, whereby SiC is synthesized − Si ₃ N ₄ −
(Ln-Al-Si) oxide sintered body.

[0011]

[Function] Generally, it is considered that silica added to other oxide ceramics has a low melting point of 1550 ° C., becomes a glass state, and precipitates at grain boundaries to reduce high temperature strength.
For that reason, this silica was rather one of the compounds that we did not want to add.

On the other hand, in the present invention, attention is paid to this silica, and SiC and Si ₃ N ₄ and (rare earth oxide-alumina-
When it is mixed with silica powder, it becomes possible to sinter,
Moreover, this silica does not become a glass by precipitating at the grain boundaries, and further forms a solid solution in the rare earth oxide-alumina system, or, if present in a solid solution amount or more, reacts with the rare earth oxide or alumina. Since a silicate compound is produced by the above method, it has been found that there is no problem that the high temperature strength is lowered.

According to the findings of the present inventors, the addition of this silica to other compounds effectively acts to obtain a dense sintered body, and the oxide portion of the sintered body is effective. It is effective in improving the strength and toughness of.

It should be noted that unoxidized SiC and Si ₃
The very small amount of silica present on the surface of N ₄ is
It is insufficient to increase the strength and toughness of the oxide.

[0015] Next, the present invention _{SiC- Si 3 N 4 - (Ln} -Al-S
i) An oxide-based sintered body is a mixed oxide of 5 to 95 wt% of rare earth oxide powder, 94.9 to 4.9 wt% of alumina powder and 0.1 to 10 wt% of silica; powder or whiskers,
Alternatively, it is obtained by mixing SiC and 95 to 5 wt% of a non-oxide mixture of Si ₃ N _{4 in} the mixture and firing the mixture.

If the rare earth oxide powder is less than 5 wt%, the characteristics of the mixed oxide deviate from the characteristics of the alumina alone, and the sinterability with SiC and Si ₃ N ₄ deteriorates.
On the other hand, when the content of alumina is less than 4.9 wt%, the characteristics of the mixed oxide are almost biased to the characteristics of the rare earth oxide alone, and the sinterability with SiC and Si ₃ N ₄ deteriorates.

On the other hand, when the rare earth oxide exceeds 95 wt%, the properties of the oxide are the same as those of the rare earth oxide, and alumina is
If it exceeds 94.9 wt%, the properties of the oxide will be the same as those of alumina. Therefore, the rare earth oxide powder and alumina are limited to the above range.

If the amount of silica added is less than 0.1 wt%, abnormal growth of the crystal grains of the sintered body cannot be suppressed, so the strength and toughness of the oxide portion of the sintered body are small, and a dense sintered body is obtained. Can't get On the other hand, the amount of silica added is
If it exceeds 10 wt%, the effect of addition does not change, but silica, which exceeds the solid solution amount, reacts with the rare earth oxide or alumina to form a silicate compound, which is not preferable.
Therefore, the amount of silica added is limited to the above range of 0.1 to 10 wt%.

Further, if the amount of the mixed oxide is less than 5 wt%, the sinterability with SiC and Si ₃ N ₄ is deteriorated and a dense sintered body cannot be obtained. On the other hand, if the amount of the non-oxide mixture of SiC and Si ₃ N ₄ is less than 5 wt%, that is, if the amount of the mixed oxide exceeds 95 wt%, the hardness and high temperature strength which are the addition effects of SiC and Si ₃ N ₄ No excellent sintered body can be obtained.
Therefore, the mixing ratio of the mixed oxide and the non-oxide mixture is limited in this way.

As the rare earth oxide (Ln ₂ O ₃ ),
Sc ₂ O ₃ ,, Y ₂ O ₃ ,, La ₂ O ₃ ,, CeO ₂ ,, Pr ₂ O ₃ ,, Nd ₂ O ₃ ,, Sm ₂ O ₃ ,, Eu ₂ O ₃ ,,
Gd ₂ O ₃ ,, Tb ₂ O ₃ ,, Dy ₂ O ₃ ,, Ho ₂ O ₃ , Er ₂ O ₃ , Tm ₂ O ₃ , Yb ₂ O ₃ ,
Lu ₂ O ₃ is preferably used.

For mixing the mixed oxide and the non-oxide mixture, an ordinary machine used for mixing or kneading powders can be used. This mixing may be either dry type or wet type, and particularly in the case of wet type, it can be effectively mixed by using a surface active agent such as ethylamine or fish oil.

Next, the mixed raw material obtained as described above is once dried, and subsequently, a green molded body having a predetermined shape is molded. In this molding step, an organic polymer (polyethylene glycol, polyvinyl alcohol, etc.) as a molding aid can be added to the above mixed raw material, and molding can be carried out by applying a known molding technique of a usual method.

Next, the manufacturing method of the present invention will be described.
In the method of the present invention, first, a mixture of SiC and Si ₃ N ₄ and (Ln-Al-Si) mixed oxide is formed in the mixing ratio as described above. Then, the green body is heated at a temperature rising rate of 5 to 2000 ° C./min and is subjected to a firing treatment of holding it at a temperature of 1500 to 1900 ° C. for 0.1 to 600 minutes to obtain a sintered body.

If the rate of temperature rise is slower than 5 ° C./min, a dense sintered body cannot be obtained. On the other hand, it is impossible to raise the temperature faster than 2000 ° C./min. It is limited to the range of / minute.

If the firing temperature is lower than 1500 ° C., a dense sintered body cannot be obtained due to insufficient sintering, while if it is higher than 1900 ° C., SiC and Si ₃ N ₄ are produced. Alumina reacts with each other to form pores, which prevents densification. From this, the range of suitable firing temperature is limited to the range of 1500 to 1900 ° C.

Next, the firing time is related to the above firing temperature, and it is preferable that the firing time be long when the firing temperature is low and short when the firing temperature is high. No solids can be obtained, while the sintering effect hardly changes after 600 minutes, or
Since SiC and Si ₃ N ₄ react with alumina to form pores, the range is limited to 0.1 to 600 minutes.

The atmosphere during firing is preferably a non-oxidizing atmosphere (for example, an inert gas such as nitrogen gas, argon gas or helium gas) because SiC and Si ₃ N ₄ are oxidized in the oxidizing atmosphere. .. Among them, a nitrogen gas atmosphere is good, and in a nitrogen high-pressure gas atmosphere, decomposition of Si ₃ N ₄ can be suppressed, which is effective for synthesizing a dense sintered body at high temperature.

The thus obtained SiC-S of the present invention
The i ₃ N ₄ -mixed oxide system sintered body has good oxide and SiC and Si ₃ N ₄ , and the toughness and strength of the oxide are remarkably improved as compared with those without conventional silica. It was

Next, according to the present invention, the oxide containing silica and SiC and Si ₃ N ₄ are fired to produce an oxide sintered body which is excellent in strength and toughness and can be used at high temperature. The technology is Ln ₄ Al ₂ O ₉ which causes martensitic transformation during firing.
Alternatively, it is particularly effective in the case of a rare earth oxide-alumina-silica based sintered body containing a compound of LnAlO ₃ as a main component.
That is, Ln ₄ Al ₂ O ₉ or LnAlO ₃ 99.9 to 90 wt% and mixed oxide 5 to 95 wt% consisting of 0.1 to 10 wt% of silica powder;
Sprinkle on powder or whiskers, or mixtures of these
SiC-Si synthesized by the above method by mixing 10 to 95 wt% of SiC and 95 to ₅ wt% of non-oxide mixture consisting of 90 to ₅ wt% of Si ₃ N _{4 on} powder or whiskers or a mixture thereof. _Since the crystal grain size of the ₃ N ₄ -mixed oxide-based sintered body was controlled to be 30 μm or less, twin crystals were not formed by the martensitic transformation, and the strength and toughness were excellent.

The reason is that twin crystals do not form in the constituent particles in the sintered body, and a small number of twin crystals form only with the progress of cracks. And the movement of the twin planes can also absorb the energy of this strain, which is considered to increase the strength and toughness of the sintered body. this is,
This is a novel discovery of the inventors by a toughening mechanism that has not been known so far.

[0031]

【Example】

(Example 1) In a mixed solution of 150 ml of distilled water and 4 ml of diethylamine, 30 g of Si ₃ N ₄ powder, 30 g of SiC whiskers, and Y ₂ O were added.
₃ powder 71.2 g, was charged with a mixed powder of Al ₂ O ₃ powder 16.1g and SiO ₂ powder 2.7 g, further added diethylamine 2 ml, were wet-mixed for 24 hours using a ball mill. Next, polyethylene glycol was added so that the mixed solution became a 10% solution, and the mixture was stirred and mixed for 3 hours. After the completion of mixing, this suspension was molded into a green molded body by the slip casting method. Next, this green body was heated to 500 ° C. at a heating rate of 2 ° C./min in air, held at 500 ° C. for 2 hours, and calcined. Then, the calcined sample is heated to 50 ° C. under a pressure of 10 kgf / cm ² of nitrogen gas.
The temperature was raised to 1775 ° C. at a heating rate of 1 / min, and the temperature was maintained at 1775 ° C. for 30 minutes to obtain a sintered body.

The obtained sintered body was densely sintered, and almost no pores were desired to be observed. Further, the bending strength of the sintered body is 1500 MPa, and the fracture toughness value K _IC = 20MP ・
It was m ^1/2 .

Example 2 10 g of SiC powder and Si ₃ N were added to a mixed solution of 90 ml of ethyl alcohol and 2 ml of diethylamine.
₄ Whiskers 10g, Yb ₂ O ₃ powder 69.42g, Al ₂ O ₃ powder 6.98g
And 1.6 g of SiO ₂ powder were mixed and wet-mixed for 72 hours using a ball mill. After completion of the mixing, the mixture was heated to 60 ° C. to evaporate the alcohol, and then the mixture was put into a 5% polyethylene glycol aqueous solution and mixed by stirring, and this was dried. Then, it was formed into a green compact having a size of 45 × 20 × 5 mm ³ . Next, this green body was heated in air to 500 ° C. at a temperature rising rate of 2 ° C./min, held at 500 ° C. for 2 hours, and calcined. Next, the calcined sample was heated in nitrogen gas to a temperature of 1700 ° C. at a heating rate of 80 ° C./min, and then heated to 1700 ° C. for 5 minutes.
It was held for a minute to sinter, and further subjected to HIP treatment at 1650 ° C. under a pressure of 100 kgf / cm ² of nitrogen gas to obtain a sintered body.

The obtained sintered body was densely sintered, and it was almost desired that pores were observed. Further, the bending strength of the sintered body is 1300 MPa, and the fracture toughness value K _{IC is} 18 MPa.
It was m ^1/2 .

Example 3 In a mixed solution of 100 ml of distilled water and 1.5 ml of diethylamine, 60 g of Si ₃ N ₄ powder, 25 g of SiC whiskers, 12.76 g of Gd ₂ O ₃ powder, 1.79 g of Al ₂ O ₃ powder and
Put a mixed powder of 0.45g of SiO ₂ powder, and use a ball mill for 24
Wet mixed for hours. Then, 12 g of polyethylene glycol was added and mixed with stirring for 3 hours. After the completion of mixing, this suspension was molded into a green molded body by the slip casting method. Next, this green body was heated in air at a temperature rising rate of 2 ° C./min to 500 ° C., held at 500 ° C. for 2 hours, and calcined. This calcined sample was heated to 1770 ° C at a heating rate of 400 ° C / min under a pressure of nitrogen gas of 30 kgf / cm ² ,
It was held for 20 minutes to obtain a sintered body.

The obtained sintered body was densely sintered, and pores were almost observed. The bending strength of the sintered body is 1400MPa, and the fracture toughness value K _{IC is} 18MP ・
It was m ^1/2 .

Example 4 In a mixed solution of 100 ml of distilled water and 2 ml of diethylamine, Si ₃ N ₄ powder 40 g, SiC whiskers 20 g, Ho ₂ O ₃ powder 52.34 g, Al ₂ O ₃ powder 3.75 g and Si were added.
A mixed powder of O ₂ powder 0.6 g was put and wet mixed for 24 hours using a ball mill. Next, 12 g of polyethylene glycol was added and mixed with stirring for 3 hours. After the completion of mixing, this suspension was molded into a green molded body by the slip casting method. Next, this green body was heated in air at a temperature rising rate of 2 ° C./min to 500 ° C., held at 500 ° C. for 2 hours, and calcined. This calcined sample was heated to 1700 ° C at a heating rate of 50 ° C / min under a pressure of 10 kgf / cm ² of nitrogen gas, and at 1700 ° C.
It was held for 30 minutes to obtain a sintered body.

The obtained sintered body was densely sintered, and it was almost desired that pores were observed. Further, the bending strength of the sintered body is 1500 MPa, and the fracture toughness value K _{IC is} 19 MPa.
It was m ^1/2 .

Example 5 In a mixed solution of 100 ml of distilled water and 2 ml of diethylamine, 50 g of Si ₃ N ₄ powder, 20 g of SiC whiskers, 25.65 g of Sm ₂ O ₃ powder, 3.75 g of Al ₂ O ₃ powder and Si.
A mixed powder of O ₂ powder 0.6 g was put and wet mixed for 24 hours using a ball mill. Next, 12 g of polyethylene glycol was added and mixed with stirring for 3 hours. After the completion of mixing, this suspension was molded into a green molded body by the slip casting method. Next, this green body was heated in air at a temperature rising rate of 2 ° C./min to 500 ° C., held at 500 ° C. for 2 hours, and calcined. Then, the calcined sample was heated to 1750 ° C. at a heating rate of 80 ° C./min under a pressure of nitrogen gas of 10 kgf / cm ² ,
It was kept at 1750 ° C for 30 minutes to obtain a sintered body.

The obtained sintered body was densely sintered, and almost no pores were desired to be observed. The bending strength of the sintered body is 1100MPa, and the fracture toughness value K _IC = 15MP ・
It was m ^1/2 .

Example 6 In a mixed solution of 100 ml of distilled water and 2 ml of diethylamine, 25 g of Si ₃ N ₄ powder, 20 g of SiC whiskers, 42.36 g of Y ₂ O ₃ powder, 7.7 g of Al ₂ O ₃ powder and SiO _{2 were} added. ₂
Powder 1.1 g of mixed powder was put and wet mixed for 24 hours using a ball mill. Next, 12 g of polyethylene glycol was added and mixed with stirring for 3 hours. After the completion of mixing, this suspension was molded into a green molded body by the slip casting method. next,
This green body was heated in air at a temperature rising rate of 2 ° C./min.
The temperature was raised to 500 ° C., and the temperature was kept at 500 ° C. for 2 hours for calcination.
Next, this calcined sample was put under a pressure of 100 kgf / cm ² of nitrogen gas. The temperature is raised to 1800 ° C at a heating rate of
It was kept at 00 ° C for 10 minutes to obtain a sintered body.

The obtained sintered body was densely sintered, and it was almost desired that pores were observed. Moreover, the bending strength of the sintered body is 1200 MPa, and the fracture toughness value K _IC = 15 MPa.
It was m ^1/2 .

Thus, the SiC-obtained by the method of the present invention
The Si ₃ N ₄ -mixed oxide-based sintered body is dense, has no pores, and is composed of particles having an average crystal grain size of 30 μm or less. Therefore, the sintered body of the present invention has a bending strength of 1000 MPa.
As described above, the fracture toughness value is 15 MP · m ^1/2 or more, and the strength and the fracture toughness value are sufficient for practical use. Moreover, since the strength can be maintained even at high temperatures, it is possible to provide an excellent ceramic material that can be used in air at both room temperature and high temperature.

[0044]

As described above, according to the present invention, the rare earth oxide-alumina-silica mixed oxide and SiC and Si ₃ N ₄ are fired so that the crystal grain size of the sintered body is 30 μm or less. Controlled to prevent the formation of pores, it has both excellent toughness of the oxide sintered body and excellent characteristics of SiC and Si ₃ N ₄ at high temperature strength. it can be easily obtained mixed oxide sintered body - uniform SiC-Si ₃ N ₄ in.

Therefore, according to the present invention, engine parts,
Gas turbine blades, parts for gas turbines, parts for corrosion resistance equipment, crucibles, parts for ball mills, heat exchangers for high-temperature furnaces, heat-resistant materials, heat-resistant materials for high-aircraft, combustion tubes, parts for die casting,
Insulation materials, fusion reactor materials, nuclear reactor materials, solar reactor materials,
Tools, heat shielding materials, electronic circuit substrates, sealing materials, joint and valve parts, biomaterials such as artificial bones and artificial tooth roots, dielectric materials, blades and cutter blades, sports equipment, pumps, nozzles, magnetic heads, rollers, We can provide guides, bearings, ferrules, and other products that are effectively used in a wide range of fields.

Claims (4)

[Claims] 1. Silicon carbide 90 to 5 wt% and silicon nitride 10 to 10
It consists of a sintered body of 95 to 5 wt% of non-oxide mixture consisting of 95 wt% and 5 to 95 wt% of mixed oxide consisting of 95 to 5 wt% of rare earth oxide, 4.9 to 94.9 wt% of alumina and 0.1 to 10 wt% of silica. Silicon carbide-silicon nitride-mixed oxide sintered body. 2. The mixed oxide is Ln ₄ Al ₂ O ₉ or Ln
AlO ₃ (however, Ln is a rare earth element and their compounds)
The sintered body according to claim 1, comprising 99.9 to 90 wt% and silica 0.1 to 10 wt%. 3. Silicon carbide 90 to 5 wt% and silicon nitride 10 to 10
A mixture of 95 to 5 wt% of a non-oxide mixture of 95 wt% and 5 to 95 wt% of a mixed oxide of 95 to 5 wt% of a rare earth oxide, 4.9 to 94.9 wt% of alumina and 0.1 to 10 wt% of silica is molded, Then, the obtained molded body is heated in a non-oxidizing atmosphere at a temperature rising rate of 5 to 2000 ° C./min for 15 minutes.
A method for producing a silicon carbide-silicon nitride-mixed oxide-based sintered body, which comprises firing in a temperature range of 00 to 1900 ° C for 0.1 to 600 minutes. 4. The mixed oxide is Ln ₄ Al ₂ O ₉ or Ln
AlO ₃ (however, Ln is a rare earth element and their compounds)
The manufacturing method according to claim 3, comprising 99.9 to 90 wt% and 0.1 to 10 wt% silica.