JPH02188465A - Production of silicon carbide-based formed product - Google Patents

Abstract

PURPOSE:To improve the dynamic properties and heat resistance of the formed product by allowing a specified infusibilizing agent to be adsorbed and/or act on the formed product of an organosilicon polymer, and then calcining the formed product in an inert gas atmosphere. CONSTITUTION:Polysilane, polysilastyrene, etc., are heat-treated at 300-500 deg.C for 5 min to 10 hr and/or irradiated at 20-200 deg.C by a UV lamp with 5-500W/cm output to obtain an organosilicon polymer consisting of a polycarbosilane- polycarbosilylene-polycarbosilastyrene copolymer. The polymer is formed into the fiber, tape, film, sheet, tissue, etc., to obtain the formed product. The halide of a group IV element shown by the formula (M is a group IV element, R is H, a lower alkyl, and allyl, X is a halogen, and n is 0-3) as the infusibilizing agent is adsorbed and/or allowed to act on the formed product at >=50 deg.C in an inert gas or vacuum, then 1-400 equivalents of basic material (e.g. ammonia) based on the agent is allowed to act on the product, if necessary, and the product is precalcined at 200-800 deg.C for 1min to 3hr in an inert gas atmosphere, and then calcined at 800-1400 deg.C for 1min to 2hr.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is directed to the production of silicon carbide-based molded articles, fibers, tapes, films, sheets, etc., which have good mechanical properties and heat resistance, using organosilicon polymers as raw materials. This invention relates to a method for efficiently manufacturing thin film bodies, etc.

[Prior Art] Conventionally, polysilastyrene as described in U.S. Pat. No. 4,324,901 is molded into a hollow molded product, film, fiber, etc., and then crosslinked and made invincible by irradiation with ultraviolet rays, electron beams, etc. It is known to produce silicon carbide molded products by firing (Japanese Patent Laid-Open No. 58-21542)
No. 6) However, this method causes significant shrinkage and fusion of the molded product (eg, fibers) during crosslinking and infusibility, and is not suitable for industrial implementation.

To solve this problem, the present inventors first converted polysilastyrene into a polycarbosilastyrene copolymer by heat treatment and/or ultraviolet irradiation treatment, and then molded this by a melting method or a dry method. By heat-treating the obtained molded body in air to make it invulnerable and then firing it in an inert gas, we can produce high-quality silicon carbide molded bodies much more efficiently than conventionally known methods. how to(
(see U.S. Pat. No. 4,743,411). However, even with this method, a large amount of oxygen is incorporated into the polymer during the crosslinking and infusibility step, so the physical properties of the silicon carbide molded product finally obtained are not necessarily sufficient, and polycarbosilastyrene copolymer molding The problem remains that it takes a considerable amount of time to process the body to make it infusible.

There is also a known method for producing silicon carbide molded bodies using polycarbosilane as a precursor polymer (
Special Publication No. 57-53892, No. 5B-38534.

60-28927, etc.), as in the case described above, a large amount of oxygen is incorporated into the polymer in the crosslinking invulnerability step of thermal oxidation by heating to about 100 to 200°C in air, and the resulting silicon carbide The physical properties of the molded product are not sufficient,
Another problem is that it takes a long time to make it infusible.

Furthermore, there is also known a method for producing silicon carbide molded bodies using poly or carbosilanes containing small amounts of other metal elements such as poly or carbotitanosiloxane as precursor polymers (for example, Japanese Patent Publication No. 60-1405). ), this method also has the same problems as mentioned above.

[Object of the Invention] The first object of the present invention is to improve the physical properties such as strength, modulus, and heat resistance of the final molded product in a method for producing a silicon carbide molded product from an organosilicon polymer. The second purpose is to improve productivity by significantly shortening the crosslinking process for making organosilicon polymer molded articles invulnerable.

[Structure of the Invention] The object of the present invention as described above is to form an organosilicon polymer into a fiber, a tape, a film, a sheet, a thin film, etc., and then add an infusible agent to the organosilicon polymer molded product as shown in the following formula. A compound of group 1V of the periodic table; MRHXl-n (n=o~3) (wherein M is carbon, silicon, germanium, or tin.

Group Ⅰ element such as titanium, R is hydrogen, carbon number is 1 to 4
is a lower alkyl group or allyl group, and X is a halogen such as chlorine, bromine, iodine, etc.) is 0.01 based on the molded product.
After ~300 (weight i)% adsorption and/or action, if necessary, after reacting with a basic substance, and if necessary, after pre-calcining, firing in an inert gas atmosphere,
This is achieved by the process of the invention, which is characterized in that the organosilicon polymer is converted into silicon carbide.

Each component of the present invention will be explained in detail below.

In the method of the present invention, the organosilicon polymer used as a raw material is an organosilicon polymer that is a precursor of silicon carbide, such as polysilastyrenes, polycarbosilanes, polycarbosilastyrene copolymers, etc. used. Moreover, these 2N co-decomposed polymers may also be used, and among these, polycarbosilastyrene copolymers are particularly preferred.

As a method for synthesizing such polymers, for polysilastyrenes, for example, dichlorodimethylsilane and dichloromethylphenylsilane are reacted in an inert solvent such as toluene or xylene using a sodium metal catalyst at a temperature above their melting point. Can be adopted.

The composition of such polysilastyrenes is expressed by the following formula, (in the above formula,
R is CHs or C@Ha + n is an integer of 10 to 3000), and the value of x is 0.
A range of 2 to 0.9, preferably 0.3 to 0.7 is used.

In addition, a small amount of polysilanes may be used together with the above-mentioned polysilastyrenes.

Such a method for producing polysilastyrene is described in detail in, for example, US Pat. No. 4,324,901 and Japanese Patent Publication No. 62-96)2.

Further, the polycarbosilane used in the present invention can be synthesized, for example, by thermal decomposition transition reaction of polydimethylsilane. An example of such a method is the West German Patent Publication No. 2
, No. 236,078, U.S. Patent No. 4,052,430, Japanese Patent Publication No. 57-2F3527, etc.

On the other hand, the polycarbosilastyrene copolymer preferably used in the present invention is proposed by the present inventors in US Pat.
As described in No. 43,411, JP-A No. 62-275131, etc., it is a special organosilicon polymer, and the copolymer is produced by subjecting the polysilastyrenes to heat treatment and/or ultraviolet irradiation treatment. It is produced by converting it into a polycarbosilastyrene copolymer.

The heat treatment of polysilastyrenes at this time is 300 to 50
It is carried out in a temperature range of 0°C, preferably in a temperature range of 350-450°C. The heat treatment time is appropriately selected within the range of 5 minutes to 10 hours depending on the heat treatment temperature.

That is, the heat treatment temperature and time are approximately 3
~10 minutes, 10 to 100 minutes at 450°C, and 60 to 120 minutes at 400°C are sufficient.

In addition, in the treatment by ultraviolet irradiation, for example, the output 5
20~200 using ~500 W/■ ultraviolet lamp
Preferably, the irradiation is carried out at a temperature of .degree.

When polysilastyrenes are heat-treated or UV irradiated according to the above method, methane gas, methylsilane, benzene, etc. are generated as low-boiling substances, and at the same time, along with carbosilane due to rearrangement of methyl groups, some of them are increased in molecular weight by crosslinking and softened. point rises, and the molding temperature also rises.

The polycarbosilastyrene copolymer referred to in the present invention is a general term for organosilicon polymers having carbosilane bonds, silastyrene bonds, and partially crosslinked bonds. It is preferable that the copolymerization molar ratio of carbosilane bonds and silastyrene bonds is 3=7 to 7:3.

These various organosilicon polymers may contain a small amount of bonds such as -Ti-0-, -Zr-0-, -9t-〇-, etc. as a method for introducing such bonds. For example, as described in Japanese Patent Publication No. 60-1405, a mixture of polycarbosilane and polytitanosiloxane is heated in an organic solvent under an inert atmosphere to remove at least a portion of the silicon atoms of the polycarbosilane from the polytitanosiloxane. This is carried out by bonding at least a portion of the silicon atoms and/or titanium atoms of titanosiloxane via oxygen atoms.

In the method of the present invention, the organosilicon polymer used for molding may optionally contain a small amount (for example, 20% by weight or less of the organosilicon polymer in the case of fibers) of an organic lubricant, a modified JM compound,
It may contain crosslinkers, stabilizers, and other additives.

Organic lubricants are higher fatty acids and higher fatty acid esters.

Higher fatty acid amides, higher alcohols, etc. are used singly or in a mixture, and examples of these compounds include, but are not limited to, the following substances. Namely, as higher fatty acids, caprin, acid,
Lauric acid, palmitic acid, margaric acid, stearic acid, oleic acid, etc.; higher fatty acid esters include capric acid ester, nonyl acetate, lauric acid ester.

Esters of the above-mentioned higher fatty acids such as ethyl stearate, butyl stearate, etc.; Higher fatty acid amides include oleic acid amide, linoleic acid amide, linoleic acid amide, stearic acid amide, etc.; higher alcohols include caprylic alcohol, decyl alcohol, and lauryl. Examples include alcohol, oleyl alcohol, and stearyl alcohol. These are particularly useful when manufacturing sheet-like molded products.

Furthermore, small amounts of Si fine powder, Ti fine powder, SiC fine powder, Tic fine powder, etc. may be added within a range that does not impair moldability.

Organosilicon polymer fibers and tapes as described above.

Molding methods for films, sheets, thin sheets, etc. include melting method,
Either dry method (solution method) may be used; in the case of melt method,
The organic silicon polymer is discharged into a cooled atmosphere and cooled to solidify. In the dry method, a dope in which the organic silicon polymer is dissolved in an organic solvent is extruded through a nozzle or slit, and the solvent in the dope is removed by evaporation. A method of coagulating is adopted. Industrially, the melting method is preferable, and such 7
Regarding the 8-melting method, US Patent No. 4, proposed by the present inventors,
743.411 in detail.

After forming a molded article, such as a fiber, in this way, the molded article is rendered invulnerable and then fired to convert it into a silicon carbide-based molded article. In the conventional technology, infusibility is achieved through thermal oxidation by heating in air, but the oxygen contained in the molded product has a negative effect on the physical properties and visual characteristics of the resulting product.
And this oxygen uptake is mostly caused by infusibility carried out in air or an oxidizing atmosphere. Therefore, it is expected that the physical property 1 heat resistance of the silicon carbide molded product will be improved by making it infusible while suppressing the inclusion of oxygen as much as possible. However, there are no methods to complete infusibility without the action of oxygen, except for special cases using radiation (see "Kobunshi" Vol. 37, June issue, p. 466).
This has not been discovered, and it is still unknown to perform non-oxygen infusibility by an industrially practicable method.

In order to meet such demands, the present inventors previously proposed a low oxygen infusibility method using halogen. According to this method, the amount of oxygen taken in during infusibility can be dramatically reduced compared to the conventional air infusibility method. However, even in such a low oxygen infusibility method, it is necessary to incorporate a small amount of oxygen in order to suppress the variation in the degree of flame resistance caused by the type of polymer. As a result of further research in order to find a method for non-oxygen infusibilization, we found that a compound MRn x, -n (
n = 0 to 31 (in the formula, M is a Group 1 V element such as carbon, silicon, germanium, tin titanium, etc., R is hydrogen, carbon number 1
~4 lower alkyl group or allyl group, X is chlorine.

We have discovered that by treating with a compound represented by a halogen (such as bromine, iodine, etc.) and, if necessary, sequentially further treating with a basic substance, it is possible to effectively infusible in the substantially absence of oxygen, We have arrived at the present invention.

In the method of the present invention, a molded article such as a fiber, for example, is treated as an infusibility agent in an inert atmosphere, and a compound MRn ,M,R
, Infusibility can be completed without the action of Therefore, it is possible to suppress the oxygen content of the silicon carbide molded product obtained by firing to an extremely low level, and the mechanical properties and properties such as heat resistance of the molded product are dramatically improved.

Examples of Group 1V compounds of the periodic table that are suitably used as infusibility agents in the method of the present invention include carbon tetrachloride, chloroform, iodoform, and silicon tetrachloride.

Trichloromethylsilane, dichlorodimethylsilane, methyldichlorosilane, dichlorosilane, tita tetrachloride>・
, titanium trichloride, germanium tetrachloride.

Examples include tin tetrachloride.

The amount of the infusibility agent to be absorbed/acted is 0.01 to 300 (
f! jt)% is preferred.

As a method of adsorption/action, an organosilicon polymer molded product (for example, 1# immediately after spinning) is placed in an atmosphere of vaporized gas of the above-mentioned infusibility agent.
a method of immersing an organosilicon polymer molded body in a solution (for example, an aqueous solution) in which the above-mentioned infusibility agent is dissolved;
Any means such as the above may be selected and adopted as appropriate.

An inert gas atmosphere or a vacuum atmosphere is preferable as the atmosphere in which the infusible agent is adsorbed/acted on.This means that in such an atmosphere, substances (e.g. oxygen, benzene, etc.) that are generated along with the adsorption/action of the infusible agent are avoided. It can be understood that the aim is to reduce the negative effects of

In the method of the present invention, the temperature at which the infusibility agent is adsorbed/acted on the molded article is 50° C. or higher, preferably 100° C. or higher, and 10° C. below the melting point of the polymer.

In this way, the organosilicon polymer molded article on which the infusibility agent has been adsorbed/acted by the method of the present invention can be prepared by:
Furthermore, a basic substance is allowed to act in a non-oxidizing atmosphere.

Examples of the basic substance used in the present invention include ammonia, methylamine, ethylamine, ethylenediamine, sodium hydroxide, potassium hydroxide, and the like, with ammonia being the most preferred.

A non-oxidizing atmosphere is preferable as the atmosphere in which the basic substance is applied.The method for realizing such an atmosphere differs depending on whether the basic substance used is supplied in gaseous or liquid form. That is, when supplied in gaseous form,
Only the gas of the basic substance, or the gas of the basic substance and nitrogen.

A mixture of inert gases such as hydrogen, argon and helium is used. On the other hand, if the basic substance is supplied in liquid form, an aqueous solution of the basic substance is usually used, but in such a case, it is preferable to remove the excess basic substance by washing with water, etc. In order to prevent the incorporation of oxygen, it is preferable to operate under an inert gas atmosphere or at low temperatures.

The amount of the basic substance to be applied varies depending on the amount of the infusibility agent that has been adsorbed/acted on in advance, but it is preferable to act in sufficient excess to the amount of the infusibility agent that has been acted on. A range of equivalents to 400 equivalents is preferably adopted.

In the method of the present invention, there is no particular restriction on the temperature at which the basic substance is applied. In other words, the action of the basic substance and the organosilicon polymer adsorbed and/or acted upon by the infusibility agent is as follows:
This is because it starts very quickly. However, from the viewpoint of operability, boat fishing is carried out at temperatures ranging from room temperature to 200°C.

The melting point of the organosilicon polymer molded body treated in this way is significantly increased compared to the original (untreated) polymer, despite the absence of oxygen action, and the molded body shows that the original polymer is Even at a temperature at which high-order polymerization/crosslinking starts and the melting point begins to rise rapidly, for example at a temperature of 350°C to 400°C, it does not melt. That is, the shape of the molded body is maintained and the molded body is in a state where it can be converted into an inorganic substance without being fused to each other, thus achieving the purpose of infusibility. Furthermore, according to experiments conducted by the present inventors, organosilicon polymers treated according to the present invention are insoluble in good solvents such as toluene relative to the original polymer, even if the treatment with a basic substance is carried out at room temperature. Therefore, it is considered that cross-linking has already been formed in the method of the present invention.

For these reasons, the organosilicon polymer molded body treated by the method of the present invention can omit the crosslinking invulnerability process using oxygen and heat, which was previously essential, and can be subjected to high-temperature firing as it is to form a silicon carbide molded body. can do.

However, this must first be carried out using an inert gas (
After preliminary firing (heat treatment) for 1 minute to 3 hours in an inert gas (nitrogen, etc.) at 800 to 1400°C.

It is also possible to make a silicon carbide-based molded product by baking in argon, etc.) for 1 minute to 2 hours.

The silicon carbide fiber obtained in this way is
Since the yarn is mineralized without going through an extreme softening region, its cross section almost maintains the shape of the spinning nozzle, and extreme deformation does not occur. The cross-sectional shape of the obtained fiber is expressed as the ratio S/A of the actual circumference length (S) of the fiber cross section and the circumference length (A) of a circle having the corresponding cross-sectional area, from 1.1 to 1. It is in the range of 0.

In addition, the elemental composition of the silicon carbide fiber obtained by the method of the present invention is such that the oxygen content is the value obtained by dividing the oxygen contained in the polymer by the firing yield, since the invulnerability step is carried out in a non-oxidizing atmosphere. It falls within the range of 5% by weight.

This absolute value varies depending on the polymer, but is usually 10% or less, preferably 7% or less, and more preferably 5% or less.

In addition, if the infusibility agent is a compound such as germanium, tin, titanium, etc., 0.1 to 1.5% of the metal may be detected in the fired compact depending on the amount adsorbed/acted upon during infusibility. Introduction of different metals is extremely effective in improving heat resistance and corrosion resistance.

<Effects of the Invention> Since the silicon carbide molded product thus obtained contains only a small amount of oxygen, its heat resistance properties and mechanical properties in air are significantly improved.

Furthermore, according to the method of the present invention, the infusibility treatment for firing the organosilicon polymer molded article can be simplified, the preliminary firing time in an inert gas can be shortened, and the strength of the obtained molded article can be improved. Physical properties such as modulus are improved, and variations in physical properties within the same lot are also reduced.

Therefore, the method of the present invention is extremely useful as a method for industrially producing high quality silicon carbide fibers, tapes, films, etc.

<Examples> Next, Examples and Comparative Examples of the present invention will be described in more detail, but the present invention is not limited thereto.

Example 1 Polysilastyrene (softening point 86-94°C) obtained by polymerizing equimolar amounts of dichlorodimethylsilane and dichloromethylphenylsilane at 110°C using a sodium dispersed catalyst in a toluene solvent was polymerized at 400°C. C. for 60 minutes in an inert gas (nitrogen), and then treated under reduced pressure for 5 minutes at the same temperature to obtain a polycarbosilastyrene copolymer with a softening point of 220 to 230.degree. Its average molecular weight is 4
600, and the ratio of carbosilane bonds to silastyrene bonds was 45,155.

A polycarbosilastyrene copolymer fiber obtained by melt-spinning this copolymer at 280°C and 600 m/min is placed in a pressure vessel, the pressure is reduced to 0°2 Torr, and this is broken with high-purity nitrogen repeatedly. After the oxygen was completely replaced, a nitrogen atmosphere was created, and 10% titanium tetrachloride based on the weight of the spun yarn was adsorbed/acted on with steam at 180° C. for 1 hour. Thereafter, the inside of the container was again purged with nitrogen, and then replaced with ammonia gas, and left to work at room temperature for 1 hour. After that, take out the treated fibers,
It was fired in nitrogen at 1200° C. for 2 minutes under a load of 10 g/10000 de to convert it into silicon carbide fiber. The thread diameter of the silicon carbide fiber thus obtained was 6.
．． The silicon carbide fiber had a tensile strength of 330 kg/-, and after exposing the silicon carbide fiber to air at 1200° C. for 1 hour, the strength retention rate was measured and showed no deterioration at all.

Furthermore, when the content of titanium in this fired fiber was measured, 2.7% (M content) of titanium element was detected.

Table 1 Examples 2-9. Comparative Example A polycarbosilastyrene copolymer synthesized by a method similar to that shown in Example 1 was melt-spun at 280°C and 600 m/min, and the resulting polycarbosilastyrene copolymer fiber was Infusibility was carried out in the same manner as in Example 1 using various infusibility agents shown in Table 1 below under the conditions shown in Table 1. Then, 1 g/100d on the treated fibers
e Under load, slow temperature rise (10℃/min) and rapid temperature rise (500℃/min)
The results are shown in Table 1 below.

The fibers shown in Table 1 above show no fusion at all during firing, and each of these fibers weighs 300 kg.
It showed a fiber strength of /- or more. In addition, the oxygen content of the fired m rags was all 5% (by weight) or less. However, fibers that have become infusible in air are not sufficiently infusible;
The tm female had poor firing properties, and was so poor that it was difficult to prepare a sample for strength measurement due to fusion between the fibers during firing.

Example 10 Polycarbosilastyrene copolymer fibers synthesized by the same method as in Example 1 were melt-spun at 280°C and 600 m/min. , put it in a pressure-resistant container, evacuate it to 0.2 Torr, break it with high-purity nitrogen, and repeat the operation to completely replace the oxygen inside, and then create a nitrogen atmosphere.
Dichlorosilane (H2S i C1z
) was adsorbed/acted on a 1 m fiber of polycarbosilastyrene copolymer.

Subsequently, the inside of the container was replaced with nitrogen to create an inert atmosphere, and then the temperature was raised from room temperature to 350°C at a rate of 5°C/min, and the temperature was maintained for 3 hours to perform preliminary firing. The post-treated fibers were taken out and fired at 1200° C. in a nitrogen atmosphere under a load of 1 g/100 de to convert them into silicon carbide fibers. The silicon carbide fiber thus obtained has a thread diameter of 7°8 μm and a tensile strength of 285 kr/
− was shown. Further, when the strength of the fiber was measured after being exposed to air at 1200° C. for 1 hour, no deterioration in strength was observed.

Example 11 Dichlorodimethylsilane was subjected to a polymerization reaction at 110 to 120°C using a sodium dispersed catalyst in a xylene solvent to obtain a white powdery polysilane. This raw material was heat-treated at 450° C. for 24 hours while stirring under pressure, and then treated at 300° C. under reduced pressure to remove low-boiling substances to obtain polycarbosilane. After this raw material polymer was once dissolved in a toluene solvent, toluene-insoluble matter was removed by filtration, and toluene was further removed from the filtrate under reduced pressure to obtain a polymer free of foreign substances. This purified polymer was heated at 280℃ for 3
The fibers were melt-spun at 00 m/min to obtain polycarbosilane fibers. This fiber is placed in a pressure-resistant container, evacuated to 0.2 TOrr, and then broken with high-purity nitrogen, which is repeated to completely replace the oxygen inside, creating a nitrogen atmosphere.
Tin tetrachloride was applied at 1f/f for 1 hour at 80°C.

Thereafter, the atmosphere in the container was replaced with nitrogen and then with ammonia gas, and the atmosphere was left to work for 1 hour.

Subsequently, the inside of the container was replaced with nitrogen to create an inert atmosphere, and then the temperature was raised from room temperature to 350° C., and the temperature was maintained for 2 hours to perform preliminary firing.

After that, the treated fibers were taken out and weighed 1 g/100 de
It was fired at 1200° C. under load in nitrogen to convert it into silicon carbide fiber. The silicon carbide fiber thus obtained has a thread diameter of 8.7 μm and a tensile strength of 3.
25 kg/-, the result of elemental analysis of the fiber 3
．． It contained 5% tin and the oxygen content was below 5%. Further, when the silicon carbide fiber was exposed to air at 1200° C. for 1 hour and its strength was measured, no deterioration was observed.

Patent sponsor Teijin Ltd.

Claims (7)

[Claims] (1) After molding the organosilicon polymer into the shape of fiber, tape, film, sheet, thin film, etc., 0.0% per weight of the molded product as an infusibility agent is added to the organosilicon polymer molded product.
01 to 300% (by weight) of a group IV element halogen compound MR_nX_4_-_n (wherein, M is a group IV element, R is hydrogen, a lower alkyl group or an allyl group, X is a halogen atom, (n is an integer of 0 to 3)) is adsorbed and/or acted upon, a basic substance is further acted upon if necessary, and the silicon carbide molded body is then fired in an inert gas atmosphere. manufacturing method. (2) The organosilicon polymer according to claim (1), wherein the organosilicon polymer is a polycarbosilane, polycarbosilylene, or polycarbosilastyrene copolymer obtained by heat treating and/or ultraviolet irradiation treatment of polysilane or polysilastyrene. Production method. (3) As an infusibility agent, in the group IV element compound represented by the following formula, MR_nX_4_-_n (n = 0 to 3) where M is carbon, silicon, germanium, tin or titanium, and R is hydrogen and the number of carbon atoms 1 to 4 lower alkyl group or allyl group, and X is chlorine, bromine or iodine,
The manufacturing method according to claim (1) or (2), which uses a halogen compound. (4) Claims (1) to 10 in which the infusibility agent is adsorbed and/or acted on the organosilicon polymer molded body in an inert atmosphere.
The manufacturing method according to any one of (3). (5) Claims (1) to 1, wherein the basic substance is ammonia.
The manufacturing method according to any one of (4). (6) The manufacturing method according to any one of claims (1) to (5), wherein a basic substance is made to act on the organosilicon polymer molded article on which the infusibility agent has been adsorbed and/or acted upon in a non-oxidizing atmosphere. (7) The manufacturing method according to any one of claims (1) to (6), wherein the firing is performed in an inert atmosphere at a temperature of 800 to 1500°C.