JPH02175657A - Ceramic composition, production thereof and forming assistant - Google Patents

Abstract

PURPOSE:To obtain a high-density ceramic composition with hardly any shrinkage in calcining without requiring a degreasing step by kneading silicon nitride powder with calcining assistant therefor and polysilazane capable of producing silicon nitride by calcining, forming the kneaded mixture and subsequently calcining the kneaded mixture. CONSTITUTION:Silicon nitride powder and a calcining assistant therefor are blended with a polysilazane capable of producing silicon nitride in an atmosphere at 1650-2200 deg.C calcining temperature in a pressurized state of nitrogen and/or ammonia and the resultant blend is kneaded and then formed. The obtained compact is subsequently calcined. For example, the following are cited as the above-mentioned polysilazane. That is a polysilazane, having a skeletal structure consisting of recurring units expressed by formula I and plural precursor residues having units expressed by formula II partially converted into structural units expressed by formula III, partially and mutually connected with structural units expressed by formula IV. In formulas I to IV, R1 to R4 represent hydrogen, lower alkyl group, (substituted) vinyl group, (substituted) allyl group, (substituted) lower aryl group, etc.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for producing a sintered body containing silicon nitride (Si3N4), which is a fine ceramic with excellent heat resistance and corrosion resistance, and the sintered body. be.

[Prior art and problems to be solved by the invention] Generally,
Sintered silicon nitride crystals are manufactured through the following steps: powder treatment, molding, degreasing, firing, and processing. In the molding process, raw material powder prepared by adding molding aids and sintering aids is generally filled into a mold, and these molding methods include dry pressure molding, rocking molding, etc. Methods such as molding, potter's wheel molding, slurry casting, injection molding, and extrusion molding are being practiced.

On the other hand, molding aids are used to impart moldability to ceramic powder and give strength to molded bodies.The amount and composition used vary depending on the molding method and technology, but these agents are used after molding. It is necessary to degrease before firing to remove molding aids. As a degreasing method, a heating degreasing method is generally carried out, but since this decomposes and vaporizes the molding aid in the molded object, it requires a considerably long degreasing treatment, and the processing time is The larger the amount of auxiliary agent used, the longer the length, and the more severe the degreasing conditions. For example, the amount of molding aid used is 4 to 8% by weight in the mechanical press method, 8 to 14% by weight in the doctor blade method, 8 to 25% by weight in the extrusion method, and 10 to 25% by weight in the injection method. In any case, a degreasing process is essential for these, and the more complex the shape, the longer the degreasing time tends to be.

In addition, firing methods include reaction sintering, normal pressure sintering, and atmosphere sintering, but reactive sintering has a small sintering shrinkage rate and relatively high dimensional accuracy, but 1 during consolidation
Pores of 5% by volume or more remain, and a dense sintered body cannot be obtained. In addition, although the pressureless sintering method allows a relatively dense sintered body to be obtained, the volumetric shrinkage rate reaches 30 to 50% by volume, and cracks, cracks, distortions, etc. are likely to occur during sintering. Moreover, when the sintering temperature is increased, silicon nitride dissociates into silicon and nitrogen, so
It is necessary to sinter at relatively low temperatures of ~1700°C using sintering aids. Furthermore, atmospheric pressure sintering allows sintering at higher temperatures and with less sintering aid, and produces a dense sintered body with excellent high-temperature strength, but volume shrinkage cannot be avoided, and There is a problem in that a shrinkage of 50% by weight occurs.

In this way, in order to obtain a ceramic molded body, a molding aid is required, and in order to remove it, depending on the case, two or more molding aids are required.
A long degreasing process of 8 days is required, and the larger the amount of molding aid used and the more complex the shape, the longer the degreasing process takes, and the more likely it is that cracks, swelling, pores, deformation, etc. Defects occur. In order to prevent this, studies have been made on the types and combinations of molding aids, the amount of molding aids, etc., but this is not easy. Furthermore, after degreasing, a molded product with a filling rate of 100% will have pores equal to the volume occupied by the molding aid, and a molded product with a low filling rate will have a large amount of pores including the volume occupied by the molding aid. It will remain. When these degreased molded bodies are sintered to a high density in a firing process, the sintered bodies undergo large shrinkage, distortion, and deformation as well as deformation in the degreasing process.

Regarding the molding of a silicon nitride sintered body using polysilazane, a method of obtaining a sintered body from a compacted powder body is known as a prior art in Japanese Patent Laid-Open No. 63-25276. However, in this case, the polysilazane used is (CN3SiHNH). , 45・(CN3
SIHN CN3). ,. 3.(CN3SiN). ,
, 2, and when such polysilazane is used,
If the N2 pressure in the firing atmosphere is less than 10 atm, a good sintered body cannot be obtained, and there are further problems to be solved.

[Means for Solving the Problems] In view of the above-mentioned circumstances, the present invention provides a molding aid for a silicon nitride molded body, which causes less shrinkage during sintering and does not require a degreasing process, and a method for firing the molded body. As a result of extensive research into the possibility of obtaining a high-density ceramic composition obtained from these materials, we have finally completed this work.

Therefore, the present invention provides silicon nitride powder and a silicon nitride sintering aid with nitrogen or/and ammonia under pressure at a temperature of 1650 to
The present invention relates to a method for producing a ceramic composition, which comprises kneading polysilazane that produces silicon nitride in an atmosphere at a sintering temperature of 2200°C to form a molded body, and firing the molded body.

And as such polysilazane, formulas (a) and (b)
There is a polysilazane represented by (c).

The polysilazane of formula (A) has a skeleton structure consisting of repeating units, and a plurality of pre-waste residues having units of formula: R□ partially become structural units of formula: %formula%, Some of them are polysilazane that are connected to each other by structural units of -N-3i-R1.

[In the formula, R, R2, R3, and R4 are hydrogen (however,
Hydrogen can include R1, R2, and R3, but not R4. same as below. ), lower alkyl groups having 1 to 6 carbon atoms such as methyl group, ethyl group, n-propyl group, isopropyl group, substituted or unsubstituted vinyl group,
Substituted or unsubstituted allyl group, phenyl group, tolyl group,
Substituted or unsubstituted lower aryl groups having 6 to 10 carbon atoms such as xylyl, tri(lower)alkyl or di(lower)alkyl such as trimethyl, dimethyl, methylethyl, triethylsilyl; Silyl group, or di(lower) alkylamino group such as dimethyl-, diethyl-methylethyl-, diisopropyl-amino group (however, di(lower) alkylamino group is R1, R2,
R1 can be included and R4 is not included. same as below. )
And.

Furthermore, R□, R2, R3, and R4 may be the same or different. The polysilazane of the formula (B) has a skeleton structure consisting of repeating units, and residues that are repeating units of the formula: in the antecedent form are connected to each other by structural units of the formula: %formula% The precursor is a polysilazane characterized in that it is composed of units of the formula: % formula % and the formula: R5 i -NR,H.

[In the formula, R1 is as described in the first claim, and R5 and R6 are 1 to 6 carbon atoms such as methyl group, ethyl group, n-propyl group, isopropyl group, etc. Substituted or unsubstituted lower aryl having 6 to 10 carbon atoms, such as substituted or unsubstituted vinyl group, substituted or unsubstituted allyl group, phenyl group, tolyl group, xylyl group, etc. group, tri(lower)alkyl- or di(lower)alkylsilyl group such as trimethyl-, dimethyl-, methylethyl-, triethyl-silyl group, or dimethyl-, diethyl-,
A di(lower) alkylamino group such as methylethyl-, diisopropyl-amino group, and R1, R5, R
6 may be the same or different].

In addition, the polysilazane of formula (c) has a skeleton structure consisting of repeating units, and a plurality of precursor residues having a unit of the formula: R□, some of which become structural units of the formula: It is a polysilazane that is connected to each other by structural units of the formula: and is further composed of a portion of its precursor H and units of the formula: R5 GH.

[Note that R□, R2, R3, R4, RS, and R are as described in the second and third claims, and R□, R2, R3, R4, R, and R6 are They may be the same or different (the same applies below). Additionally, the present invention provides a method of kneading silicon nitride powder, a silicon nitride sintering aid, and a polysilazane that produces silicon nitride in an atmosphere with a sintering temperature of 1650 to 2200° C. under nitrogen or/and ammonia pressure. The present invention relates to a ceramic composition produced by forming a molded body and firing the molded body.

The polysilazane used in this case has the above formula (
Examples include those containing at least one of the polysilazane shown in (a), (b), and (c).

Furthermore, the present invention provides a molding aid that is kneaded with silicon nitride powder and a silicon nitride sintering aid and is used to enable molding without a degreasing step, the molding aid comprising nitrogen or/and a silicon nitride sintering aid. and 1650-2200℃ under ammonia pressure
The present invention relates to a molding aid for ceramic compositions, which is a polysilazane that produces silicon nitride under a sintering temperature atmosphere of .

In this case, polysilazane may also be represented by the formulas (a) and (b) above.
Examples include those containing at least one type of polysilazane shown in (c).

This polysilazane (Polysilazane) is
By kneading 3N4 powder, A1□03 powder, and Y2O3 powder as a molding aid, it gives good moldability and sufficient strength to the molded product, does not require a degreasing process, and has a low temperature of 10 atm or less in a nitrogen atmosphere. It is possible to obtain a high-density silicon nitride sintered body with small firing shrinkage under pressure.

[Left below] When the polysilazane is fired at a temperature of 1650 to 2200°C in an N2 or/and NH3 atmosphere, it can be converted to S in high yield.
Since it generates i3N4, there is no need to degrease it after molding, unlike commonly used organic molding aids, and it remains in the molded product as Si and N, so it can be used for the general organic molding mentioned above. This results in a reduced porosity compared to what would occur if the auxiliary agent were removed.

Therefore, the shrinkage and deformation rate of the final sintered body is small, and there is little gas to decompose and vaporize, so defects such as cracks and bulges are less likely to occur.

In carrying out the present invention, N is used as the reaction atmosphere.
It goes without saying that an inert gas such as argon or helium may be mixed with 2 and/or NH.

For example, when comparing the shrinkage rates of high-density sintered bodies when using 50vol% of polysilazane as a molding aid and when using 50vol% of an organic molding aid, it is found that when the polysilazane of the present invention is used, the density is 1. 0 polysilazane is heated in the molded body and changes to Si and N4 with a density of 3.2, resulting in a weight loss of approximately 70%, so the molded body shrinkage rate is 4.
The shrinkage rate is 0vo1%, but when an organic molding aid is used as in the conventional case, the shrinkage rate is 50vo1%. The polysilazane as a forming aid in the present invention and the method for producing a silicon nitride sintered body using the same can produce a good high-density silicon nitride sintered body even under N2 or NH3 pressure of 10 atmospheres or less, and the present invention The polysilazane in the invention is soluble in organic solvents like general organic polymers, and also has properties such as being softened by heating, and can be used in general ceramic forming methods without adding other organic substances, for example. Using general molding methods such as extrusion molding, injection molding, slurry casting, and doctor blade molding, it is possible to produce molded bodies without a degreasing process, and the molded bodies obtained have high dimensional accuracy and high quality. It is extremely important because it has high density, high strength, and is suitable for machining.

The Si, N, and powder used as the main reaction ingredients may be those conventionally used, but those with small particles and uniform particle sizes are preferable. Sintering aids include oxides and nitrides of Be, Mg, A1, Y, and rare earth elements. The amount of these sintering aids added is preferably 10% by weight or less, and it is preferably as small as possible within the range that allows sintering. Furthermore, the pressurization is Si3N4
It is necessary to select a pressure that does not cause dissociation. The firing temperature increase rate is preferably 15°C/min or less up to 700°C, more preferably 5°C/min or less. The firing temperature is 1
650℃~2200℃, 1800℃~1900℃
is preferred.

The polysilazane used in the present invention has a main chain skeleton of Si and N.
The molecular formula or molecular structure differs depending on the manufacturing method, and the ceramic yield also varies. In the present invention, the following ceramics having a high yield and heat softening properties can be suitably used. The amount of the polysilazane of the present invention added is preferably approximately 5 to 70 vol%, and varies depending on the molding method.

Regarding the method for producing polysilazane used in the present invention, for example, for the polysilazane (a), anhydrous ammonia is mixed with organodihalosilane R□SiHX
2 in a solution to form a cyclic or linear silazane precursor, and to the mixture of the former II compound, while coexisting in the precursor a silazane or silylamine compound represented by the formula: can be produced by reacting in the presence of a basic catalyst capable of deprotonating hydrogen from a nitrogen atom adjacent to a silicon atom, resulting in dehydrogenated cyclization crosslinking and polymerization.

In each formula, R□, R2, R3, and R4 are hydrogen (however,
Hydrogen can be included in the case of R1, R2, R3, and R4
is not included. same as below. ), methyl group, ethyl group, n-
Lower alkyl groups having 1 to 6 carbon atoms such as propyl group, isopropyl group, substituted or unsubstituted vinyl group, substituted or unsubstituted allyl group, phenyl group, tolyl group, xylyl group, etc. 6 to 10 Substituted or unsubstituted lower aryl group having up to 3 carbon atoms, trimethyl-
Tri(lower)alkyl- or di(lower)alkylsilyl groups such as dimethyl-, methylethyl-11-liethylsilyl groups, or di(lower)alkylamino groups such as dimethyl-, diethyl-, methylethyl-, diisopropyl-amino groups (However, a di(lower) alkylamino group may include R, R2, and R3, but does not include R4. The same applies hereinafter.), and R1, R2, R3, and R4 may be the same or different. is also good. Moreover, X is a halogen such as chlorine or bromine (the same applies hereinafter).

In addition, regarding the method for producing polysilazane represented by formula (b), anhydrous ammonia is mixed with R, 5iHX2 and R3RGS.
iX2 in solution with an organohalosilane mixture, thereby forming a cyclic or linear silazane precursor, which has the ability to deprotonate the hydrogen from the nitrogen atom adjacent to the silicon atom. The polymer can be produced by crosslinking through depro-cyclization in the presence of a basic catalyst.

Furthermore, for the polysilazane of formula (c), anhydrous ammonia is reacted in solution with an organohalosilane mixture of R1Si HX2 and R6R6SiX2 to form a cyclic or linear silazane precursor; , the reaction is carried out in the presence of a basic catalyst capable of deprotonating hydrogen from the nitrogen atom adjacent to the silicon atom while coexisting in the precursor a silazane or silylamine compound represented by the formula %(). The molecular weight can be increased by dehydrogenation and crosslinking. In this product, the content of (A) is 1 to 60 mol%, Rg R
The content of GSiX2 is 1 to 60 mol%, and (
The content of A) plus RsR, 5iX2 is preferably 2 to 60 mol%.

[Function and Effect] In summary, the present invention is constructed as described above, so when forming a ceramic composition using silicon nitride powder and its sintering aid, nitrogen or/and ammonia pressurization is not necessary. In this state, polysilazane, which produces silicon nitride, is kneaded as a forming aid in an atmosphere at a sintering temperature of 1,650 to 2,200°C. In this sintering process, Si3N4 is generated from the polysilazane at a high yield, so there is no need to degrease it after molding, unlike commonly used organic molding aids, and moreover, it is not necessary to remove fat from the molded body as Si3N4. As it remains as is, the porosity generated by removing the organic forming aid can be suppressed, resulting in less shrinkage and deformation of the final sintered body, and less gas that decomposes and vaporizes. Therefore, it is possible to create a ceramic composition that is less prone to defects such as cracks and swelling. Moreover, when firing a ceramic composition using this polysilazane, 1
It can provide sufficiently good formability even in a low-pressure atmosphere of 0 atmospheres or less, and can be applied to molding methods commonly used when molding ceramic compositions, such as thermoplastic molding, slurry casting, and press molding. The molding method can be performed at low pressure and completely eliminates the need for a degreasing process. Moreover, the obtained ceramic composition has a small firing shrinkage rate, a high-density fired body without cracks and defects, and is useful with low distortion and high dimensional accuracy.

Next, examples of the present invention will be described.

[Example 1] α-813N4 powder (5N-9S manufactured by Denki Kagaku Kogyo) was
7.0% by weight, 4.9% by weight of A1□03 powder (Alumina B type manufactured by Iwatani Chemical Co., Ltd.), 4.9% by weight of Y2O3 powder (Mitsubishi Chemical In1 Helia 0.8 μm), 3.
3% by weight of polysilazane, in which R□, R5, and R6 are all methyl groups and the number average molecular weight is 12oO, which is described in the above formula (b), is added in a toluene solution, kneaded, and vacuum-dried to obtain α-8.
Uniformly dispersed powders of i3N4 powder, A1□03 powder, Y2O3 powder and polysilazane were obtained. 5 of the powder
It was placed in a 0x60nwn iron mold and uniaxially pressed at about 30kg/ffl, followed by isostatic pressing at 3000kg/cm2 to obtain a molded body with a TD of 69° and 5%. The molded body was placed in a silicon nitride crucible and filled with Si3N4 powder. Firing was performed at 700°C at 3°C/min in a nitrogen atmosphere of 1 atm.
The temperature was raised to 'C, then the temperature was raised to 1100°C at 15°C/min, and further the temperature was raised to 1650°C at 10°C/min.

After that, while maintaining the pressure at 9.5 atm,
The temperature was raised to 1850° C., maintained at this temperature for 3 hours, and then naturally cooled to obtain a fired body. The density of this fired body is 98%T
D, linear shrinkage rate was 12%, and room temperature bending strength was 741 Mpa.

[Example 2] 79.7% by weight of α-8i3N4 powder, 4.4% by weight of Al2O powder, 4.4% by weight of Y2O3 powder, 11
．． 5% by weight of R□, R5, R6 described in the above formula (b)
A polysilazane having a methyl group and a number average molecular weight of 1200 was added in a toluene solution and molded in the same manner as in Example 1 to obtain a molded product with a TD of 67.8%. This was fired in the same manner as in Example 1. The density of the fired body is 97.4% TD, and the linear shrinkage rate is 1166%.

The room temperature bending strength was 682 MPa.

[Example 3] a-8i3N, 63.7% by weight of powder, A12o, 3.5% by weight of powder, 3.5% by weight of Y2O3 powder, Example 1
29.3% by weight of the polysilazane described in Example 1 was added in a toluene solution to obtain a uniformly dispersed powder in the same manner as in Example 1.

Put the powder into a cylinder heated to 90℃ and weigh 100 kg.
A cylindrical shaped body with a diameter of 1.0 mm was obtained at a pressure of /a++2. This molded body was fired in the same manner as in Example 1. The density of the fired body was 96% TD, and the linear shrinkage rate was 17%.

[Example 4] α-3i, N4 powder 75.0% by weight, A1□03 powder 4
．． 0% by weight and 4.0% by weight of Y2O3 powder, 17% by weight of the polysilazane described in Example 1 was added as a toluene solution and stirred to form a uniform slurry. This slurry is 70X
The mixture was poured into a 20×10 mm mold and dried under vacuum to obtain a molded product. This molded body was fired in the same manner as in Example 1. The density of the fired body was 91% TD, and the linear shrinkage rate was 12%.

[Example 5] α-5i3N, powder 83.8% by weight, Al2O3 powder 4
．． 6% by weight, 4.6% by weight of Y2O3 powder, 7% by weight of the polysilazane shown in (a) above, in which R1, R2, R3, and R2 are all methyl groups and the number average molecular weight is 1100, was added to a toluene solution. and calcined and baked in the same manner as in Example 1. The density of the obtained fired body was 98% TD, the linear shrinkage rate was 12.5%, and the room temperature bending strength was 614 MPa.

[Example 6] α-8i3N4 powder 79.7% by weight, A1□03 powder 4
．． 4% by weight, 4.4% by weight of Y2O3 powder, 11°5% by weight of R1, R2, R3, R4, shown in (c) above,
R5 and R6 are both methyl groups, and the number average molecular weight is 120
0 polysilazane was added as a toluene solution, and firing and firing were performed in the same manner as in Example 1. The density of the obtained fired body is 97
%TD, linear shrinkage rate is 11°6%, room temperature bending strength is 655
It was Mpa.

Claims (1)

[Scope of Claims] 1) Silicon nitride powder and silicon nitride sintering aid are kneaded with polysilazane that produces silicon nitride in a nitrogen or/and ammonia pressurized atmosphere at a sintering temperature of 1650 to 2200°C. 1. A method for producing a ceramic composition, which comprises: forming a molded body, and firing the molded body. 2) A method for producing a ceramic composition, wherein the polysilazane according to claim 1 is represented by formula (a). Formula (A) has a skeletal structure consisting of repeating units of the formula: ▲ There are mathematical formulas, chemical formulas, tables, etc. It is a polysilazane in which the groups are connected to each other by structural units, some of which are the formula: ▲Mathematical formulas, chemical formulas, tables, etc.▼, and some of which are connected to each other by structural units of the formula: . [In the formula, R_1, R_2, R_3, and R_4 are hydrogen (however, hydrogen can include R_1, R_2, and R_3, but not R_4. The same applies hereinafter), methyl group, ethyl group, n- Lower alkyl groups having 1 to 6 carbon atoms such as propyl group, isopropyl group, substituted or unsubstituted vinyl group, substituted or unsubstituted allyl group, phenyl group, tolyl group, xylyl group, etc. 6 to 10 Substituted or unsubstituted lower aryl radicals having up to 5 carbon atoms, tri(lower)alkyl- or di(lower)alkylsilyl radicals such as trimethyl-, dimethyl-, methylethyl-, triethyl-silyl radicals, or dimethyl- , diethyl-
, methylethyl-, diisopropyl-amino group, etc.
lower) alkylamino group (however, di(lower) alkylamino group can include R_1, R_2, R_3, R
_4 is not included. same as below. ) and R_1
, R_2, R_3, and R_4 may be the same or different. 3) A method for producing ceramics, wherein the polysilazane according to the first claim is represented by formula (b). Formula (b) has a skeletal structure consisting of repeating units of the formula: ▲There are mathematical formulas, chemical formulas, tables, etc.▼, and the formula in the precursor is: ▲There are mathematical formulas, chemical formulas, tables, etc.▼ It is a repeating unit of The precursor is a polysilazane in which the residues are connected to each other by structural units of the formula: ▲There are mathematical formulas, chemical formulas, tables, etc.▼ and the formula: ▲There are mathematical formulas, chemical formulas, tables, etc.▼ It is a polysilazane characterized by being composed of units. [In the formula, R_1 is as described in the first claim, and R_5 and R_6 are 1 to 6 carbon atoms such as methyl group, ethyl group, n-propyl group, isopropyl group, etc. a lower alkyl group having an atom, a substituted or unsubstituted vinyl group, a substituted or unsubstituted allyl group,
Substituted or unsubstituted lower aryl groups having 6 to 10 carbon atoms such as phenyl, tolyl, xylyl, tri(lower)alkyl such as trimethyl, dimethyl, methylethyl, triethylsilyl; - or a di(lower)alkylsilyl group, or a di(lower)alkylamino group such as dimethyl-, diethyl-, methylethyl-, diisopropyl-amino group, and R_1
, R_5, and R_6 may be the same or different]. 4) A method for producing ceramics, characterized in that the polysilazane according to claim 1 is represented by formula (c). Formula (c) has a skeletal structure consisting of repeating units of the formula: ▲There are mathematical formulas, chemical formulas, tables, etc.▼, and it has a skeleton structure consisting of repeating units of the formula: The group is a polysilazane that is connected to each other by structural units, some of which are the formula: ▲Mathematical formula, chemical formula, table, etc.▼, and some of which are the formula: ▲There are mathematical formulas, chemical formulas, tables, etc. Furthermore, some of its precursors are polysilazane characterized by being composed of units with the formula: ▲Mathematical formula, chemical formula, table, etc.▼ and formula: be. [In addition, R_1, R_2, R_3, R_4, R_5, R_
6 is in accordance with what is stated in the second and third claims, and R_1, R_2, R_3, R_4, R_5
, R_6 may be the same or different (the same applies hereinafter). ] 5) Silicon nitride powder, silicon nitride sintering aid, and nitrogen or/and ammonia in a pressurized state, 1650 to 2200
A ceramic composition characterized in that it is produced by kneading polysilazane that produces silicon nitride in an atmosphere at a sintering temperature of °C to form a molded body, and firing the molded body. 6) The polysilazane described in the fifth claim has the formulas (a), (b), and (b) as described in the second, third, and fourth claims.
A ceramic composition characterized by containing at least one type of polysilazane shown in c). 7) A molding aid that is kneaded with silicon nitride powder and a silicon nitride sintering aid and used to enable molding without a degreasing process, the molding aid being mixed with nitrogen or/and ammonia additive. A molding aid for a ceramic composition, characterized in that it is a polysilazane that produces silicon nitride under pressure in an atmosphere at a sintering temperature of 1650 to 2200°C. 8) The polysilazane of the seventh claim contains at least one of the polysilazane represented by formulas (a), (b), and (c) described in the second, third, and fourth claims. A forming aid for a ceramic composition, characterized in that: