METHOD OF MANUFACTURING B TYPE SILICON NITRIDE WHISKERS

Abstract: Silicon nitride whiskers having a long fiber length in which β type silicon nitride is well developed are produced by reacting a mixture of silica, carbon and cryolite in the specific molar ratio in a mixed gas atmosphere of N₂ and NH₃ by heating the mixture at a temperature of 1,250- 1,450°C. Kira of a ceramic industry waste may be used in place of silica and in this case N₂ alone is used as a nitriding atmos­ phere.

Description: The present invention relates to a method of manufacturing β type silicon nitride whiskers, and particularly to a method of manufacturing β type silicon nitride whiskers by which well developed β type silicon nitride whiskers having a long fiber length can be easily produced at a relatively low temperature of 1,250-1,450°C. In general, the application of β type silicon nitride whiskers has been eagerly expected as a fiber reinforcement for a composite material because of excellent high temperature strength and thermal shock resistance of silicon nitride. As the prior techniques relating to the present invention, there are the following methods, but they are quite different from the present invention for the reasons also stated below. Therefore, the invention cannot easily be inferred therefrom. The invention is to provide a method of commercially advantageously producing β type silicon nitride whiskers. (A) Japanese Patent Application Publication No. 12,320/1976 "A Method of Manufacturing Silicon Nitride" This reference is similar to the present invention in view of addition of a fluoride to a mixture of silica and carbon and in the composition thereof, but is different from the present invention in a way of adding the fluoride. Further, this reference is completely silent about the molar ratio of silica, carbon and cryolite, and it also fails to refer to the crystal type of the whiskers. (B) Japanese Patent Application Publication No. 21,160/1975 "A Method of Manufacturing Fibrous Silicon Nitride Crystals" This is quite different from the present invention in that silicon is used as the source of Si of silicon nitride. (C) Japanese Patent Application Publication No. 4,480/1975 "A Method of Manufacturing Silicon Nitride Fibers" This reference uses a metal additive as a catalyst of the nitriding reaction, and therefore it quite differs from the present invention. This reference is utterly silent about the crystal form of the obtained whiskers. (D) Japanese Patent Application Publication No. 27,755/1974 "A Method of Manufacturing Silicon Nitride Whiskers" This reference adds Cl component in N₂ atmosphere, and therefore it differs from the present invention. Further, it does not to the crystal form of the produced whiskers at all. Briefly speaking, there have been conventionally known the following methods as the method of manufacturing the silicon nitride whisker. (a) Direct nitriding of metallic silicon powder. (b) Catalytic reaction of silicon halide with ammonia. (c) In a silica reduction method, a fluoride or chlorine component is added, and silica is reacted with N₂ gas. According to the methods (a) and (b), merely powdery whiskers having a short fiber length can be obtained. According to the method of (c), although the whiskers of a slightly longer fiber length can be obtained, the structure is mainly of α type when the treating temperature is a relatively low temperature of 1,300 to 1,400°C. In the conventional techniques, a problem is the phase transition of Si₃N₄ in the case that the Si₃N₄ whiskers are mixed into Si₃N₄ powder, and are used as a reinforcement of the Si₃N₄ sintered body. Si₃N₄ has two crystal phase, α type and β type, which belong to the hexagonal system. The dimension of crystal lattices of both the crystal types is that a o =7.76 Å and c o =5.62 Å in the case of the α type, while a o =7.61 Å and c o =2.91 Å in the case of the β type. The dimension of the crystal lattice in the c-axis direction is largely different between the α type and the β type. β is a high temperature phase which is changed from the α type at about 1,400-1,600°C. The α type Si₃N₄ powder having a high purity and a high sinterability is suitable for the raw material of the Si₃N₄ sintered body. This material is considered to impart a high strength to the sintered body through phase transition from the α type to the β type in the course of sintering. It is reported that when Si₃N₄ whiskers abundant in the α type are added to such an α type Si₃N₄ powder and the mixture is sintered, the whiskers themselves undergo the phase transition in the course of the sintering, so that the strain is caused between matrices to lower the strength of the sintered body. It is considered that the influence of the phase transition of the Si₃N₄ whiskers similarly occurs in the case of the reinforcing of a sintered body other than Si₃N₄. Thus, it has been long demanded to develop a method of manufacturing Si₃N₄ whiskers composed mainly of the β type. Chemical Abstracts, Vol. 96, No. 20, 17th May 1982, page 668, column 1, abstract No. 172276t, discloses the influence of fluorides on vapor phase growth of silicon nitride whiskers. In view of the above circumstances, upon strenuously having made studies, the present inventors have accomplished the invention. That is, it is therefore an object of the present invention to provide a method of manufacturing a well developed β type Si₃N₄ whiskers of a long fiber length at a relatively low temperature, for instance, 1,250-1,450°C. According to the method of manufacturing the β type silicon nitride whiskers in the present invention, a mixture of silica, carbon and cryolite in a molar ratio of 1:(2∼10):(1/12∼1/4) is heated, at a relatively low temperature of 1,250-1,450°C in a mixture atmosphere of N₂+NH₃, a molar ratio of NH₃ to N₂ being from about 1/48 to about 1/5. These and other objects, features and advantages of the present invention will be well appreciated upon reading of the following description of the invention when taken in connection with the appended claims and the attached drawings. The invention will now be described in detail with reference to the accompanying drawing, wherein: Fig. 1 is a schematically sectional view of a device used in carrying out a process according to the present invention. The production reaction of silicon nitride (Si₃N₄) whiskers according to a silica reduction process using N₂ gas is shown by the following two formulae: SiO₂ (solid) + C (solid) → SiO (gas) + CO (gas) (1) 3SiO (gas) + 2N₂ (gas) + 3CO (gas) → Si₃N₄ (solid) + 3CO₂ (gas) (2) The inventors have found that β type Si₃N₄ whiskers can be manufactured at a relatively low temperature by reacting silica with carbon in the presence of cryolite in N₂ atmosphere to which a small amount of NH₃ is added. Since the reaction in the formula (1) is a solid reaction between SiO₂ and C, the reaction rate is slow. When cryolite is present in the reaction between SiO₂ and C, however, SiO₂ is changed to a silicate fused body by the action of this flux, which results in the reaction between the liquid and the solid C and is represented by the following formula (3): SiO₂ (silicate, liquid) + C (solid) → SiO (gas) + CO (gas) (3) The reaction rate of this reaction is far promoted as compared with the conventional solid reaction and the production of the SiO is extremely activated. Meanwhile, when NH₃ is added to the reaction atmosphere, gaseous SiO produced in the formula (3) reacts according to the following formula (4) due to a powerful reducing action of H₂ gas generated through thermal decomposition of NH₃. SiO (gas) + H₂ (gas) → Si (gas) + H₂O (gas) (4) The thus produced Si (gas) reacts with N₂ to produce crystal nuclei of β type Si₃N₄ according to the following formula (5): 3Si (gas) + 2N₂ (gas) → Si₃N₄ (solid) (5) It is in conformity with the ordinary crystal growth theory that the crystal nuclei of β type Si₃N₄ thus formed grow as the β type crystals even under conditions where the α type crystals are produced according to the reaction (2). Therefore, the production of the β type Si₃N₄ whiskers proceeds according to the reactions (4), (5) and (2). It is a question why the β type Si₃N₄ is produced according to the reaction (5) although when a single crystal of Si is heated and converted to Si vapor, which is reacted with N₂ gas, α type Si₃N₄ is produced. This is because Si vapor obtained through reduction of SiO under heating is different in physical properties from that obtained by heating the Si single crystal. This may be illustrated by the similar phenomenon in the case where a single crystal is produced from a fused body as another embodiment in which a single crystal containing Fe II cannot be obtained from a fused body in which the starting material is Fe II , while a single crystal containing Fe II can be obtained from a fused body obtained by reducing a starting material of Fe III at a high temperature into Fe II and this is based on the difference of the physical properties of both Fe II . This is due to the difference in the activity and the ion radius and in this case, in order to reduce the strain of the crystals, the β type product having a smaller c o value is produced. Further, in this case, the reaction (2) may independently take place depending upon the reaction conditions to produce the α type product simultaneously. According to the invention, the mixing ratio of silica to carbon is 1:2 to 1:10, preferably about 1:4 in a molar ratio. If the molar ratio of carbon is lower than the above range, the reaction (1) is difficult to proceed, while on the contrary, when it exceeds the above range, the amount of unreacted carbon residue increases, and the reaction yield unfavorably lowers. The mixing ratio of silica to cryolite cannot be definitively determined because the synergistic effects of the cryolite and NH₃ added in the nitrogen atmosphere is remarkable, but is preferably about 1:1/12-1:1/4 in a molar ratio of SiO₂:Na₃AlF₆. When the molar ratio of N₂ to NH₃ in the mixed atmosphere (N₂+NH₃) is 24:1, the ratio of SiO₂ to Na₃AlF₆ is more preferred to be 1:1/10-1:1/6. If the amount of cryolite is more than the above range, the amount of fused silicate formed in the reaction is too much, so that the flowing movement of N₂ gas and SiO gas is interrupted. Thereby, the reactions (4) and (5) are difficult to proceed and SiC is unfavorably formed in the residue. Reversely, when it is less than the above range, the reaction (3) is retarded and the unreacted SiO₂ is unfavorably formed in the residue. The mixing ratio of NH₃ to N₂ is preferably not higher than 1/5, more preferably not higher than 1/12 in a molar ratio. If the mixing ratio exceeds 1/5, the partial pressure of N₂ in the mixing gas atmosphere is too lower and a large amount of SiC is likely to be unfavorably produced in the residue. In the case of N₂ gas alone, α type Si₃N₄ whiskers are likely to be produced and no β type Si₃N₄ whiskers are obtained. As mentioned above, an appropriate amount of NH₃ and cryolite are indispensable for the present invention. The reaction temperature is 1,250-1,450°C. If the temperature is lower than 1,250°C, the reaction is difficult to proceed and the yield is extremely low. On the other hand, if it exceeds 1,450°C, no silicon nitride is produced and silicon carbide is produced instead. The reaction temperature is preferably 1,300-1,500°C. The present invention will be explained more in detail with reference to Examples in comparison with Comparative Examples. Examples using silica Silicate anhydride (silica), activated carbon and cryolite each being powder passing through 100 meshes were dry-mixed with one another in molar ratios as shown in the following Table to obtain a sample material 5. 1 g of each of the mixed raw material samples 5 was charged into a vessel 4 made of carbon as shown in Fig. 1, which was then placed into a protection tube 3 made of mullite. This mullite tube was placed into an electric furnace 1 using a furnace core tube 2. While a mixed gas 6 of N₂ and NH₃ having a molar ratio as shown in the following Table was introduced into the furnace core tube 2 in a direction shown by the arrow in Fig. 1, the reaction was effected in the atmosphere of N₂+NH₃ at a temperature of 1,350°C for 24 hours under heating. On the other hand, while an exhaust gas 7 was exhausted in a direction shown by the arrow in Fig. 1. In this case, the flow rate of the introduced mixed atmosphere gas 6 was 50 cc/min, and the reaction was effected while the pressure of the interior of the furnace core tube 2 was kept at about atmospheric pressure. The silicon nitride whiskers produced on the inner surface of the protection tube 3 was observed by X-ray diffraction. The contents (weight %) of α phase and β phase are also shown in the following Table 1. As apparent from the above Table, the amount of cryolite in the raw material 5 in Comparative Examples 1 and 5 is outside the range in the method according to the present invention, and these Comparative Examples are unfavorable in that the content of the α type silicon nitride whiskers is high or β' cyalone (Si 6-z Al z O z N 8-z :1≦z≦4), and SiC are produced. In Comparative Examples 2 to 4, the molar ratio of N₂+NH₃ in the mixed atmosphere gas 6 was outside the range of the present invention. They are no good in that the content of the α type silicon nitride whiskers is high, or β' cyalone and SiC are produced. On the other hand, in any of Examples 1-5 of the method according to the present invention, well developed β type silicon nitride whiskers having a high purity, a content of the β type silicon nitride of not lower than 80%, a diameter of 0.2-1.5 µ and a length of about 5 mm in the average were obtained. Namely, the conspicuous effects of the present invention can be observed in these Examples. As mentioned above, according to the present invention, there is provided a method of easily manufacturing the flexible and well developed β type silicon nitride whiskers having a long fiber length at a relatively low temperature.

Claims: Method of manufacturing β-type silicon nitride whiskers comprising the reaction of a silica source in an N₂-containing atmosphere characterized in that a mixture of silica, carbon, cryolite in a molar ratio of 1:(2-10): (1/12-1/4) is reacted in a mixed gas atmosphere of N₂ and NH₃ in a molar ratio from about 1/48 to about 1/5 based on N₂ by heating the mixture at a temperature of 1250-1450°C. The method according to claim 1, wherein the mixing ratio of silica to carbon (SiO₂:C) is about 1:4 in the molar ratio. The method according to claim 1, wherein the mixing ratio of silica to cryolite (SiO₂:Na₃AlF₆) is 1:1/10-1/6 in the molar ratio. The method according to claim 1, wherein the mixing ratio of NH₃ to N₂ is not higher than 1/12 in the molar ratio. The method according to claim 1, wherein the reaction temperature is 1300-1400°C.

Inventors: SAITO, HAJIME, URAKAWA, TETSURO
Applicants: SAITO, HAJIME, TOSHIBA CERAMICS CO., LTD.
Application year: 1984
Main classification: C04B35
IPC codes: C30B29, C01B21, C30B25