JPH02243567A - Production of silicon carbide-based compact - Google Patents

Abstract

PURPOSE:To produce a silicon carbide-based compact having excellent mechanical physical properties and heat resistance by adsorbing a specific infusibilization treating agent on a compact of an organosilicon polymer and then calcining the resultant compact in an inert gas atmosphere. CONSTITUTION:An organosilicon polymer, such as polycarbosilastyrene copolymer, as a precursor of SiC is formed into a fiber, tape, sheet, thin substance, etc., and a compound of group III element of the periodic table expressed by formula I as an infusibilizing agent in an amount of 0.01-300wt.% based on the compact is then adsorbed thereon. A basic substance, such as ammonia, as necessary, is further reacted therewith. The resultant compact, as necessary, is previously calcined at 800-1,500 deg.C in an inert gas atmosphere to convert the organosilicon polymer into the SiC. A halogen compound expressed by formula II including the case in which n is 0 or in formula I may be used as the infusibilizing agent. Thereby, the objective high-quality fiber, tape, film, etc., can be readily produced from the SiC.

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, thin sheets, etc.) that have good mechanical properties and heat resistance using organosilicon polymers as raw materials. The present invention relates to a method for efficiently manufacturing a body (such as a body).

[Prior Art] Conventionally, polysilastyrene as described in U.S. Pat. It is known that silicon carbide molded products can be manufactured by
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.

For this reason, 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 stage, 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. 58-38534 60
-28927, etc.), as in the case described above, a large amount of oxygen is incorporated into the polymer during the crosslinking and infusibility step of thermal oxidation by heating to approximately 100 to 200°C in air, resulting in a silicon carbide molded product. There are problems in that its physical properties are not satisfactory and it takes a long time to make it infusible.

Furthermore, there is also known a method of producing silicon carbide molded bodies using polycarbosilanes containing a small amount of flexible metal elements such as polycarbotitanosiloxane as a precursor polymer (for example, Japanese Patent Publication No. 14N5 of 1983). However, this method also has the same problems as described above.

[Object of the Invention] The first object of the present invention is to reduce the amount of oxygen incorporated into the polymer in the process of making the organosilicon polymer crosslinkable after molding, in a method for producing a silicon carbide molded article from an organosilicon polymer. The second purpose is to reduce the crosslinking and infusibility process of organosilicon polymer moldings and improve the physical properties such as strength, modulus, and heat resistance of the final molded product. The purpose is to improve productivity. A third purpose is to improve the surface properties of the molded article.

[Structure of the Invention] The object of the present invention as described above is to form an organosilicon polymer into fibers, tapes, films, sheets, thin sheets, etc., and then add an infusible agent to the organosilicon polymer molded product as shown in the following formula. Compounds of group Ⅰ elements of the periodic table; MRn is a lower alkyl group or an allyl group, and X is chlorine.

(halogens such as bromine, iodine, etc.) are adsorbed and/or acted upon in an amount of 0.01 to 300% (by weight) based on the molded body, and if necessary, a basic substance is applied, and further, if necessary, pre-calcination is performed. This is achieved by the method of the present invention, which is characterized in that the organosilicon polymer is converted into silicon carbide by calcination in an inert gas atmosphere.

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. Alternatively, a polymer of these two types of iron decomposition may be used. 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,
R1 is CH3 or hydrogen, R2 is CH3 or C6H5, n
is a polymer compound represented by an integer of 10 to 3QOO, and the value of X is in the range of 0.2 to 0.9, preferably 0.
A value in the range of 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. 62-9612.

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. Pat. No. 4,052,430, and Japanese Patent Publication No. 57-26527.

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, Japanese Patent Publication No. 63-39617, etc., it is a special organosilicon polymer, and the copolymer can be obtained by heat-treating the above-mentioned polysilastyrenes and/or by subjecting them to 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
The heat treatment is carried out in a temperature range of 0''C, preferably in a temperature range of 350 to 4SO'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, it is sufficient for the temperature and time of the heat treatment to be approximately 3 to 10 minutes at 500°C, 10 to 100 minutes at 450°C, and 60 to 120 minutes at 40°C.

In addition, in the treatment by ultraviolet irradiation, for example, the output 5
~20~200 using a ~500W/cy 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. The temperature rises from the molding temperature.

The polycarbosilastyrene copolymer referred to in the present invention is a general term for organosilicon polymers having carbosilane bonds, silastyrene bonds, and partially cross-linked or cross-bridged bonds. The copolymerization molar ratio of carbosilane bonds and silastyrene bonds is preferably 2:8 to 8:2.

These various organosilicon polymers may contain small amounts of bonds such as -Ti-0-, -Zr-0-, -3t-〇-, etc. as methods for introducing such bonds. For example, as described in Japanese Patent Publication No. 60-14n5, 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. 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 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 modifier 1, a crosslinking agent, a stabilizer, etc. may be included before the addition of.

Organic lubricants include higher fatty acids, higher fatty acid esters, higher fatty acid amides, higher alcohols, etc. used singly or in mixtures, and examples of these compounds include, but are not limited to, the following substances: That is, higher fatty acids include capric acid, lauric acid, balmitic acid, margaric acid, stearic acid, oleic acid, etc.; higher fatty acid esters include capric acid ester, nonyl acetate, and 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 bodies.

Furthermore, 5iFR powder, T
I fine powder, SiC fine powder, Tic fine powder, etc. can also be added.

Organosilicon polymer fibers, tape films, as described above;
The method for forming sheets, thin sheets, etc. may be either a melting method or a dry method (solution method). In the case of the melting method, a method is adopted in which the organosilicon polymer is discharged into a cooling atmosphere and cooled and solidified. In the case of the dry method, a method is employed in which a dope in which an organosilicon polymer is dissolved in an organic solvent is extruded or cast through a nozzle, slit, etc., and the solvent in the dope is evaporated and solidified. Industrially, a melting method is preferable, and this melting method is described in US Pat. No. 4.4 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 made infusible and fired to convert it into a silicon carbide molded article. In the conventional technology, infusibility is achieved by thermal oxidation by heating in air, but the oxygen contained in the molded product has a negative effect on the physical properties and heat resistance characteristics of the obtained product.
And this oxygen uptake occurs mostly through infusibility, which is carried out in air or an oxidizing atmosphere. Therefore, it is expected that the physical properties and heat resistance of the silicon carbide molded article will be improved by suppressing oxygen uptake as much as possible to make it infusible. However, no method has been found to complete infusibility without the action of oxygen, except for a special example of using radiation (see "Kobunshi" Vol. 37, June issue, p. 466), and it is industrially feasible. It is not yet known that non-oxygen infusibility can be achieved by such a 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 the compound M RnX3 0 (n
= O ~ 3) (wherein, M is a Group Ⅰ element such as boron, aluminum, etc., R is hydrogen, hydroxyl group, 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 is used as an infusibility agent in an inert atmosphere, and a compound MRn M
, R, Infusibility can be completed without the action of oxygen. For this reason, it becomes 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 I compounds of the periodic table that are suitably used as infusibility agents in the method of the present invention include boron chlorides such as boron trichloride and boron tetrachloride, boron tribromide, and boron hydroxide (
boron compounds such as boric acid), methyl, dichloroboron,
Also, aluminum compounds such as aluminum trichloride, aluminum tribromide, aluminum triiodide, ethyldichloroaluminum, diethylchloroaluminum, and polyaluminum chloride are listed.

The amount of the above-mentioned infusibility agent to be adsorbed/acted on is preferably 0.01 to 300 (t, t)% based on the weight of the organosilicon polymer molded article (for example, fibers immediately after spinning).

Adsorption/action methods include placing an organosilicon polymer molded body (for example, a fiber immediately after spinning) in an atmosphere of vaporized gas of the above-mentioned infusibility agent, and placing an organosilicon polymer molded body in a solution (for example, an aqueous solution) in which the above-mentioned infusibility agent is dissolved. Any method, such as a method of dipping the molded body, can be selected and adopted as appropriate.

The atmosphere in which the infusibility agent adsorbs/acts is preferably a non-oxidizing atmosphere, such as an inert gas atmosphere or a vacuum atmosphere. This means that in such an atmosphere, the infusibility agent adsorbs/acts.
This can be effective in reducing the harmful effects of substances (e.g., oxygen, benzene, etc.) generated along with the reaction. If necessary, an infusible agent such as iodine as previously proposed may be present in the atmosphere.

In the method of the present invention, the temperature at which the infusibility agent (halide of group Ⅰ element) is adsorbed/acted on the molded article is 50°C.
℃ or higher, preferably 100℃ or higher, polymer melting point -1
A temperature of 0°C is adopted.

The organosilicon polymer molded article on which the above-mentioned infusibility agent has been adsorbed and acted upon in this way is further treated with a basic substance in a non-oxidizing atmosphere, if necessary.

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 infusible agent that has been adsorbed/acted on in advance, but it is preferable to apply the basic substance in sufficient excess to the infusible agent type. A range of equivalents to 4n0 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, the temperature is generally 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 begins and the melting point begins to rise rapidly, for example at a temperature of 350°C to 40°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. Moreover, according to the experiments of the present inventors, organosilicon polymers treated with the above-mentioned infusibility agent according to the present invention have a good resistance to the original polymer, such as toluene, even if the treatment with a basic substance is carried out at room temperature. It is insolubilized in the solvent, and it is considered that crosslinking 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 is first pre-calcined (heat treated) in an inert gas (nitrogen, etc.) at 200-800°C for 1 minute to 3 hours, and then heated in an inert gas (nitrogen, etc.) at 800-14n0°C.

A silicon carbide-based molded product can be obtained by baking in argon, etc.) for 1 minute to 2 hours.

When silicon carbide fibers are produced in this way, they are inorganicized without going through an extreme softening region, so the cross section almost retains the shape of the spinning nozzle, and extreme deformation does not occur.1For example, a circular nozzle The cross-sectional shape of the fiber obtained by infusible and firing the stranded yarn according to the method of the present invention is as follows:
When expressed as the ratio S/A of the actual circumference (S) of the fiber cross section and the circumference (A) of a circle having the corresponding cross-sectional area, it is 1.1 to 1.0.
within the range of

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, compounds such as boron (boron), aluminum, etc., which are infusible agents, can be
Although 0.01 to 5% by weight of the above metals are detected in the fired compact, the introduction of these different metals is extremely effective in improving heat resistance and corrosion resistance.

For example, when manufacturing silicon carbide fibers, if the method of the present invention is applied, the above-mentioned metal will be present near the fiber surface after firing, so when manufacturing a composite material from the fibers and metal, 71 g of the fibers will be It can prevent the deterioration of the fiber surface that occurs when gold parent metal is used.

<Effects of the Invention> The silicon carbide molded product obtained in this way contains only a small amount of oxygen and has been treated with the above metal, so it has excellent heat resistance and R
Not only are the mechanical properties significantly improved, but the problems when compounding with metals are also reduced.

Furthermore, according to the method of the present invention, the infusibility treatment for firing the organosilicon polymer molded body can be simplified, the preliminary firing time in an inert gas can be shortened, and the strength of the molded product obtained 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 4n0 A polycarbosilastyrene copolymer with a softening point of 220-230°C was obtained by heat treatment in an inert gas (nitrogen) for 60 minutes at "C" and then at the same temperature under reduced pressure for 5 minutes. Its average molecular weight was 4600, and the ratio of carbosilane bonds to silastyrene bonds was 45/55.

This copolymer was spun at a spinning temperature of 280°C and a spinning (winding) speed of 6
Polycarbosilastyrene copolymer #1m fibers melt-spun at 00 m/min were placed in a pressure vessel, the pressure was reduced to 0.2 Torr, and this was broken with high-purity nitrogen. This was repeated to completely replace the oxygen inside. After that, a nitrogen atmosphere is applied,
10% based on yarn weight spun at 180°C for 1 hour
of boron tribromide was adsorbed/acted on with steam. 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, the treated fibers were taken out and 10 g/IQOOO
The fibers were fired in nitrogen at 1200° C. for 2 minutes under a de load to convert them into silicon carbide fibers. The silicon carbide fiber thus obtained had a thread diameter of 6.2 μm and a tensile strength of 330 kg/-. Further, after exposing the silicon carbide fiber to air at 1200° C. for 1 hour, the strength retention rate was measured, and as a result, no deterioration was observed.

Furthermore, when the content of boron in this fired fiber was measured, 2.7% (by weight) of boron element was detected.

A composite material with aluminum was produced using this fiber, but no deterioration of the fiber was observed when it came into contact with molten aluminum.

Examples 2-5. Comparative Example A polycarbosilastyrene copolymer (e) synthesized in the same manner as in Example 1 was prepared at a spinning temperature of 280°C and a spinning speed of 60°C.
Melt spinning was performed at 0 m/min, and the obtained polycarbosilastyrene copolymer fiber was subjected to the same operation method as in Example 1 using various infusibility agents shown in Table 1 below, and subjected to the conditions shown in Table 1. The infusibility treatment was performed below.

The treated fibers were then subjected to a load of 1 g/100 de.
The mixture was heated to 1200'C by slow heating (10°C/min) and rapid heating (500'C/min) to obtain silicon carbide fibers. The conditions at that time are shown in Table 12 below.

Table 1 MTI subjected to the treatments shown in Table 1 above showed no fusion at all during firing, and none of the silicon carbide fibers obtained showed a fiber strength of 300 kg/- or more. The oxygen content in all cases was 5% (by weight) or less.

However, fibers made invulnerable in air have insufficient infusibility, resulting in poor sinterability, and during sintering, the fibers fuse together, making it difficult to prepare samples for strength measurement. Met.

Example 6 A polycarbosilastyrene copolymer synthesized in the same manner as in Example 1 was spun at a temperature of 280° C. and a spinning speed of 600 m/min.
The polycarbosilastyrene copolymer fiber obtained by melt-spinning for 60 minutes was immersed in a 10% aluminum chloride aqueous solution for 60 minutes, then dried and opened, and held in a nitrogen atmosphere at 180''C for 1 hour. , then 250°C for 1 hour at 220°C.
After holding for 1 hour, the temperature was raised to 350℃ at 1℃/min.
It was maintained at this temperature for 2 hours to perform preliminary firing.

After that, the treated fibers are taken out and 1. c/100
The fibers were converted into sintered silicon carbide fibers at 1200° C. under a nitrogen atmosphere under a deload. The silicon carbide fiber thus obtained had a thread diameter of 7.5 μm and a tensile strength of 285 hg/-. In addition, the fiber was heated to 1200°C.
, After being exposed to air for 1 hour, the strength was measured.
No strength deterioration was shown.

Example 7 Dichlorodimethylsilane was subjected to a polymerization reaction at 110 to 120°C using a thorium dispersed catalyst in a xylene solvent to obtain a white powdery polysilane. This raw material and the same polysilastyrene as in Example 1 were mixed at a ratio of 3:1 (weight ratio), heated at 450°C for 24 hours while stirring under pressure, and then heated to 300°C under reduced pressure. The organic silicon polymer was obtained by removing low boiling point substances. After this raw material polymer was once dissolved in a toluene solvent, toluene-insoluble matter was removed by r-filtration, and toluene was further removed from the filtrate under reduced pressure to obtain a purified polymer free of foreign substances. This purified polymer was melt-spun at 280°C and 300 m/min.
I maintained it. The fibers were immersed in an aqueous solution containing 1% anti-fusing agent and 3% boric acid, left to stand for 60 minutes, taken out, dried, opened, heated in a nitrogen atmosphere at 180°C for 1 hour, and then heated at 220°C for 1 hour.
f&1 held at ℃ for 1 hour and then at 250℃ for 1 hour.
The temperature was raised to 350° C. at a rate of ° C./min and maintained at that temperature for 3 hours to perform preliminary firing. The post-treated fibers were taken out and subjected to a load of 1 g/100 de, under a nitrogen atmosphere for 12 hours.
It was fired at 00°C and converted into silicon carbide fiber. The thread diameter of the silicon carbide fiber thus obtained is 7.
, 2 μm, and the tensile strength showed a value of 302 kir/−.

Claims (7)

[Claims] (1) Organosilicon polymer can be used as fiber, tape, film, etc.
After molding into the shape of a sheet, thin film, etc., a halogen compound of a group III element MRnX_3n (where M is a group III element and R is a Hydrogen, hydroxyl group, lower alkyl group or allyl group, X is a halogen atom, n is 0 to 3
After adsorbing and/or acting on the compound (an integer of A method for producing a silicon carbide-based molded body, which is characterized by firing inside a molded body. (2) The organosilicon polymer is a polycarbosilane, polycarbosilylene, or polycarbosilastyrene copolymer obtained by heat treating and/or ultraviolet irradiation treatment of polysilane or polysilastyrene. manufacturing method. (3) As an infusibility agent, in the group III element compound represented by the following formula, MRnX_4n (n = 0 to 3) where M is boron or aluminum and R is hydrogen, a hydroxyl group, or a lower group having 1 to 4 carbon atoms. The manufacturing method according to claim (1) or (2), wherein a halogen compound is used, which is an alkyl group or an allyl group, and X is chlorine, bromine, or iodine. (4) Claims (1)-(
The manufacturing method according to any one of 3). (5) Claims (1) to (1) wherein the basic substance is ammonia.
4) The manufacturing method according to any one of 4). (6) The production method according to any one of claims (1) to (5), wherein the organosilicon polymer molded body on which the infusibility agent is adsorbed and/or acted upon is acted upon with a basic substance 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.