GB2100711A - Process for the production of silicon nitride type molded bodies - Google Patents

Abstract

A process for the production of silicon nitride type molded bodies, comprises the steps of: reacting together (1) a silicon halide or a polymerizable group- containing organohalogenosilane; (2) a monomer co-polymerizable with said polymerizable group, or a monomer containing a plurality of polymerizable groups, or glycerine or glycol; and (3) ammonia gas or an ammonia-producing compound to obtain a compound containing a silazane group copolymerized with the polymerizable group, molding the silazane group- containing compound together with or without a polyhydric alcohol or a base material and heating the resulting molded body in a N2 or NH3 atmosphere thereby to produce the silicon nitride type molded body.

Description

SPECIFICATION Process for the production of silicon nitride type molded bodies and the molded bodies produced This invention relates to novel silicon nitride type compounds in molded form and a process for the production thereof.

Silicon nitride is excellent in heat resistance and oxidation resistance and is, as its unique properties, advantageous in low thermal expansion, high heat impact resistance and high erosion resistance to molten metals. Since silicon nitride has such excellent and unique properties, they have heretofore been used as high-temperature electrical insulating material, material for electronic parts, refractory material for metal industrial use, material for high-temperature portions of structures, binders for sintering, coating material, wear resistant material, material for use in preparing alloys, and the like.

Conventional silicon nitride has hitherto been obtained by heating powdered silicon at 1200-1 3000C in the presence of nitrogen or ammonia or by heating a silicon-containing inorganic or organic monomer in the presence of nitrogen or ammonia. Thus, the conventional silicon nitride has been obtained usually in the powder form.

The reaction between silicon and nitrogen will involve heat generation. Thus, when powdered silicon is reacted with nitrogen, a silicon nitride film is formed on the surface of the powdered silicon thereby making it difficult to cause a nitriding reaction in the inner portion of the silicon powder with the result that it is difficult to obtain a nitride of high purity.

In a case where such silicon nitride powder is molded into a predetermined shape for the various uses mentioned above, the resulting moldings will be disadvantageous in their low strength. More particularly, it requires a binder such as magnesium oxide to mold conventional nitride into a desired shape since the conventional nitride is obtained usually in the powder form, and accordingly, the resulting moldings will be disadvantageous in that they are degraded in various properties such as heat resistance and mechanical strength.

Further, in the production of conventional silicon nitride molded bodies from silicon nitride powder or silicon powder, very careful attention must be paid to a temperature-raising operation and the like. It is thus extremely difficult to obtain molded bodies having excellent heat resistance and mechanical properties.

The present inventors made intensive studies in an attempt to provide silicon nitride type compounds in a molded state which are freed of such disadvantages as above and, as a result of their studies, found that a resin compound containing a silazane group and a vinyl or glyceride group is synthesized as an intermediate for silicon nitride, the thus synthesized compound is then treated, for example molded or melt spun, to obtain a desired molded body thereof and the molded body so obtained is finally heated in an ammonia or nitrogen atmosphere to obtain silicon nitride having a desired shape and that the silazane group-containing resin compound is incorporated or impregnated in a base material such as carbon fibers, carbon material, carbon fiber-reinforced carbon material, silicon carbide, silicon nitride, a metal oxide such as alumina, ceramic material or ceramic fiber-reinforced material and then heated in an ammonia or nitrogen atmosphere thereby to produce silicon nitride in the base material resulting in improving the latter in heat resistance and oxidation resistance. This invention is based on these findings.

The crux of this invention resides in the fact that a compound containing a silazane group copolymerized with a polymerizable group, a compound containing a polyhydric alcohol and silazane group, a mixture of said compounds or such a base material impregnated therewith, is heated to be molded into a predetermined form and then heated in a nitrogen or ammonia atmosphere to obtain a silicon nitride type compound having the predetermined molded body.

There may also the obtained silicon nitride type compound in the fibrous form by heating such a silazane group-containing polycondensate to be melted and then extruding the thus melted polycondensate through nozzles.

A compound containing a silazane group copolymerized with a polymerizable group may be converted to silicon nitride by heating in a nitrogen or ammonia atmsophere. More particularly, such silicon nitride may be obtained by reacting in an organic solvent an organohalogenosilane containing a polymerizable group such as a vinyl group with a monomer copolymerizable with said polymerizable group or with a monomer containing a plurality of polymerizable groups in the presence of ammonia gas oran ammonia-producing compound.

The polymerizable monomers include styrene, divinylbenzene, vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methyl acrylate, ethyl acrylate, other acryiic acid esters, methyl methacrylate, ethyl methacrylate and 2-ethylhexyi methacrylate.

The organic solvent which may be used herein includes methyl alcohol, ethyl alcohol; propyl alcohol, other aliphatic alcohols, polyhydric alcohols, xylene, n-hexane and other organic solvent.

The silazane group-containing compound mentioned above is obtained by reacting together in a solvent an organohalogenosilane containing a polymerizable group, a monomer copolymerizable with said polymerizable group or a monomer containing a plurality of polymerizable groups and ammonia gas or an ammonia-producing compound. The reaction temperature varies depending on the kind of starting material and it may be in the range of from room temperature to 900 C, preferably from 30 to 800C.

The organohalogenosilane to be reacted with a monomer containing at least one polymerizable group is represented by the following general formula R -Si-Xn wherein X is a halogen atom, n is an integer of 4 - 1 and R is a hydrogen atom, alkyl group, alkylene group, alkyl group, vinyl group or aryl group.

The compound containing a silazane group reacted with a polyhydric alcohol is obtained by reacting together the organohalosilene, the polyhydric alcohol and ammonia gas or ammonia-producing compound. The polyhydric alcohols include glycerine and glycols.

Molded bodies consisting of the silazane containing compound or a material impregnated therewith are baked at 1000-1 8000C, preferably 1 500-1 6000C in an ammonia or nitrogen atmosphere to be converted to those consisting of silicon nitride. If, in this case, the molded bodies of the silane group-containing compound, that is a polycondensate, were heated abruptly to a high temperature, then they would be subjected to heat impact and cause an abrupt decomposing reaction thereby to tend to damage their original shape. Thus, they may preferably be heated at a temperature raising velocity of 5-1 00C/hr until their temperature reaches 600-7000C and, thereafter, they may be heated at a higher temperature-raising velocity.

The silazane group-containing compound may be heated to a temperature higher than its melting point to be made liquid or half molten and then molded into fibers, films, sheets, plates and other desired shapes by the use of a usual method. Said compound may be mixed with carbon fibers, graphite powder, metal oxide powder such as alumina powder, or ceramic powder such as silicon nitride powder or silicon carbide powder and then molded to obtain a molded body of a silicon nitride type compound.

In addition, the silazane-containing compound may be impregnated in carbon fibers, carbon fiberreinforced carbon composite material, ceramic material or ceramic fiber-reinforced material and then molded to obtain a molded body of a silicon nitride type compound. These silicon nitride type compound molded bodies are obtained without the use of a binder unlike conventional silicon nitride molded bodies and, therefore, they are excellent in purity, mechanical strength, heat resistance and oxidation resistance.

Further, the fibrous silicon nitride may reinforce plastics and rubbers such as polyester, epoxy resins, phenolpolyimide and rubbers, also reinforce a metallic matrix such as aluminum, copper, titanium or magnesium matrix and further reinforce carbon, glass, silicon nitride or other various materials.

This invention will be better understood by the following non-limitative examples.

EXAMPLE 1 There were mixed together 1 OOg of vinylmethyldichlorosilane as the organochlorosilane, 1 00g of glycerine and 5g of methyl acrylate monomer containing 0.05 g of benzoyl peroxide as the polymerization initiator. The resulting mixture was incorporated at 300C in small portions with ammonium carbamate as the ammonia-producing compound to copolymerize the organochlorosilane with the methyl acrylate monomer to obtain a co-polymer which was then reacted with ammonia to obtain a polymer containing a silazane group. The reaction was carried out at 30-500C for one hour and then at 60-700C for 3 hours.The reaction product contained two layers, the upper layer consisting of a white-colored liquid material containing ammonium chloride as the side product and the lower layer consisting of a white-colored resinous solid. The reaction product was heated to 800C to dissolve the ammonium chloride as the side product in glycerine thereby to separate said white-colored resinous solid from the ammonium chloride. The resinous solid so obtained was heated and extruded through nozzles to obtain fibers thereof.

The fibers so obtained were raised in temperature at a rate of 5 C/hrto 1 0000C in an ammonia gas or stream in a tubular electric furnace and then further heat treated at 1 5000 C for 10 minutes in another electric furnace to obtain yellow-greenish black-coloured fibers.

The fibers so obtained were subjected to X-ray's diffraction whereby they were confirmed to consist mainly of silicon nitride. In addition, they had a tensile strength of 1 50 Kg/mm2.

EXAMPLE 2 The procedure of Example 1 was followed except that 11 5g of vinylmethyldichlorosilane were substituted for 1 OOg thereof, to obtain fibrous silicon nitride. The fibrous silicon nitride was subjected to X-ray's diffraction to confirm that it consisted mainly of silicon nitride.

EXAMPLE 3 One hundred (100) grams of vinylmethyldichlorosilane as the organolhalogenosilane containing a polymerizable group, 200g of glycerine and 1 5g of divinylbenzene containing 1 wt.% of benzoyl peroxide were mixed together to obtain a mixture which was incorporated at 300C with 1 Og of ammonium carbamate in small portions. The resulting mixture was heated to 650C for 4 hours to complete the reaction thereof. In this case, as the reaction proceeded, the ammonium carbamate was pyrolyzed to evolve ammonia with the consequent production of a white-colored resinous material.

The thus produced white-colored resinous material was separated from the glycerine as the solvent and the ammonium chloride as the by-product. The resinous material so separated was heated to 900C to obtain a viscous liquid which was then formed to a film. The film so formed was placed in a porcelain boat, further placed in a cylindrical electric furnace, thereafter heated at a temperature-raising rate of 50 C/hrto 1 0000C in an ammonia atmosphere and further heated to 1 5000C in a nitrogen gas atmosphere thereby to obtain a yellow-greenish black-colored film.

The thus obtained film was analyzed by the use of X-rays to confirm that it consisted mainly of - type silicon nitride with the balance being a minute amount of P type silicon carbide.

Further, the film was heat treated at a rate of 1 00C/hr to 1 3000C and maintained at this temperature for 2 hours in an air stream by the use of a differential calorimeter (produced by Rigaku Electric Co., Ltd.) with the result that the weight gain of the film was 2.1 mg/cm2.

EXAMPLE 4 One hundred (100) grams of vinylmethyldichlorosilane were mixed with 20g of methyl acrylate containing 1% of benzoyl peroxide to form a mixture which was reacted with 1 5g of ammonium carbamate in xylene. The reaction was effected at 650C for 4 hours, after which the xylene was removed from the reaction mixture to obtain a white-coloured rubbery elastic resin. The thus obtained resin was mixed with chopped silicon carbide fibers of 3mm length in an amount of 35 wt% of the resin and then molded into a sheet-like body, 30mm x 50mm x 5mm. The thus obtained sheet-like molded body was baked at a rate of30C/hrto 1 0000C in an ammonia (NH3) gas and further heat treated at 1 5000C in a N2 gas. The product obtained has a flexural strength of 460 Kg/cm2.In addition, the product was heat treated at 7000C for one hour in an air stream with the result that the change of the product in weight was quite negligible.

EXAMPLE 5 One hundred (100) g of silicon tetrachloride were mixed with 1009 of glycerine to form a mixture which was incorporated with 209 of ammonium carbamate. The whole was initially agitated at room temperature to proceed with its polycondensation, gradually raised in temperature to 650C and maintained at this temperature for 3 hours. The reaction mixture so obtained was freed of the glycerine and the NH4CI as the side product thereby to obtain a white-colored viscous resin.

A separately prepared circular carbon fiber-reinforced carbon matrix composite material having a size of 1 Omm in thickness x 100mm in diameter, was impregnated with said viscous resin under a reduced pressure, heat treated at 1 000 C in an ammonia (NH3) atmosphere and then further heat treated at 1 5000C in a N2 atmosphere. Such impregnation and heat treatment at 1 500C were repeated three times. Thus, the composite material so treated had a flexural strength of 650 Kg/cm2, this showing an about 20% increase of strength.

EXAMPLE 6 Fifty (50) grams of methyltrichlorosilane (CH3-Si-CI3) were incorporated in 1 00g of glycerine to form a mixture which was incorporated with 7.5g of ammonium carbamate in small portions and then reacted at 300C for 4 hours. The reaction was effected with the attendant vigorous foaming thereby to produce a white-colored resinous material. The reaction mixture so produced was freed of the excess glycerine to obtain a white-colored viscous resin which was suitable in impregnating in carbon material or ceramic material.

The above procedure was followed except that 1 50g of xylene were substituted for the glycerine with the result that the production of silazane was confirmed by an infrared analysis but there was produced a white-colored solid sediment which was not resinous.

It was thus necessary that glycerine was used for resinification in a case where such functional groups were present.

EXAMPLE 7 One hundred (100) grams of dimethyldichlorosilane ((CH3)2.SiC12) were mixed with 100g of glycerine to form a mixture which was incorporated at 300C with 1 5g of ammonium carbamate for reaction. Then, the reaction continued at 600C for 4.5 hours to obtain a white-colored jelly-like resin.

The thus obtained resin was incorporated with powdered alumina in an amount by weight of 2.5 times the amount of the resin, thoroughly mixed together, placed in a metallic mold, 1 Omm thick x 50mm wide x 80mm long, and then pressure molded under heat. The molding so obtained was baked at 1 3000C in a nitrogen gas to obtain a rigid rectangular molded body having a flexural strength of 380 Kg/cm2.

EXAMPLE 8 The compounds shown in Table 1 were reacted together under the reaction conditions indicated in Table 1 to obtain silazane group-containing compounds. The thus obtained silazane group-containing compounds were molded into the respective shapes shown in Table 1 and heat treated under the condition shown in Table 1 thereby to obtain silicon nitride type molded bodies having the properties shown in Table 1.

TABLE 1

Properties of silicon Compounds, Reaction Heat treating Nitride Type Molded Conditions Shape of Molded Bodies Conditions Bodies Bromotrimethylsilane, 10 x 20 x 20 mm Ammonia atmosphere, A weicht gain of 1.8 mg/cm divinylbenzene, glycerine obtained after treated at and acrylonitrile were block-like 1000~C. 1000~C for 2 hours in an reacted together at 60~C air atmosphere.

for 4 hours while blowing an excessive ammonia gas against the reaction mixture.

Dibromodiphenylsilane Incorporated with N2 atmosphere A weight gain of 1.2 mg/cm and glycol were reacted chopped carbon fibers obtained after treated at at 80~C for 2 hours while of 1 mm length in an 1600~C. 1000~C for 2 hours in an blowing an excessive amount by weight of air atmosphere.

ammonia gas against the 20% and then molded reaction mixture into 1 mm thick films.

lodotrimethylsilane, Incorporated with N2 atmosphere, Strength 1.7 times as high styrene, glycerine and powdered silicon as that of silicon nitride vinyl acetate were nitride in an amount 1500~C. sintered body obtained reacted together in the of 45% and then molded from conventional powdered presence of ammonium into blocks, silicon nitride.

carbamate at 80~C for 50 x 50 x 100 mm.

4 hours.

TABLE 1 (Continued)

Properties of silicon Compounds, Reaction Heat Treating Nitride Type Molded Conditions Shape of Molded Bodies Conditions Bodies Vinyltrichlorosilane Fibers of 15 in Ammonia atmosphere, tensile strength divinylbenzene were diameter.

reacted together at (Several % of poly- 1800~C. 120 Kg/mm 70~C for 2 hours while ethylene oxide added blowing an excessive during spinning.) ammonia gas against the reaction mixture.

Divinyldichlorosilane Fibers of in N2 atmosphere, Tensile strength and methyl acrylate were diameter.

reacted together at (Several % of poly/ 1500~C. 100 Kg/mm 80~C for 3 hours in ethylene oxide added the presence of during spinning.) ammonium carbamate.

Claims (9)

1. A process for the production of a silicon nitride type molded body, comprising the steps of: reacting together (1 ) a member selected from silicon halides and polymerizable group-containing organohalogenosilanes having the general formula R(4~n)SiXn wherein X is a halogen atom, n is an integer of 4 - 1 and R is a hydrogen atom, alkyl group, alkylene group, allyl group, vinyl group or aryl group, (2) a member selected from monomers copolymerizable with said polymerizable group, monomers containing a plurality of polymerizable groups and glycerine or glycols and (3) a member selected from ammonia gas and ammonia-producing compounds to obtain a compound having a silazane group copolymerized with the polymerizable group, molding thermally a member selected from (A) the thus obtained compounds containing the silazane group, (B) compounds containing a silazane group together with a polyhydric alcohol, (C) mixtures of the compounds (A) and (B), and (D) mixtures of each of the compounds (A) to (C) with a base material to obtain a molded body having a predetermined shape and then heating the thus obtained molded body in a nitrogen gas or ammonia atmsophere thereby to produce the silicon nitride type molded body.

2. A process according to claim 1, wherein the base material is carbon fibers, graphite powder, alumina powder, silicon nitride powder or silicon carbide powder.

3. A process according to claim 1, wherein the thermal molding is effected by heating the member to melt it and then extruding the melted member through nozzles to obtain fibers thereof as the molded body.

4. A process according to claim 1, wherein the thermal molding is effected by heating the member to melt it and then impregnating the melted member in the base material to obtain an impregnated base material as the molded body.

5. A process according to claim 4, wherein the base material is carbon fibres, carbon fiberreinforced carbon composite material, ceramic material or ceramic fiber-reinforced material.

6. A process according to claim 1, 3 or 4, wherein the monomer (2) is a vinyl compound, styrene, divinylbenzene, vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methyl acrylate, ethyle acrylate, methyl methacrylate, ethyl methacrylate or 2-ethylhexyl methacrylate.

7. A process according to claim 1, 3 or 4, wherein the silazane group-containing compound is converted to silicon nitride at 1 500-1 6000 C.

8. A molded body in the fibrous form produced by the process of claim 3.

9. A process according to claim 1 substantially as hereinbefore described in any one of the foregoing Examples.