JP2009161376A - Manufacturing method of silicon nitride powder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently manufacturing a silicon nitride powder which has a low carbon content and is suppressed in generation of whisker. <P>SOLUTION: The manufacturing method of the silicon nitride powder comprises using a silica particle powder and a carbon powder of 5 μm or larger in volume-based average particle diameter (D<SB>50</SB>) as starting raw materials, and using a reduction nitriding method comprising heat-firing the mixture in a nitrogen atmosphere in the temperature range of 1,350 to 1,500°C to reduce and nitride it, and thereafter heating the nitrided material in an oxygen atmosphere at 600 to 1,000°C to decarbonize it. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for efficiently producing a silicon nitride powder having a low carbon content and suppressed whisker formation.

  Since silicon nitride sintered body has various excellent properties such as high heat resistance, high strength, high toughness, and corrosion resistance, various types including mechanical parts that require high strength at high temperatures. Application to various fields, such as structural materials, and the like, and silicon nitride particle powder as a raw material are also required to be highly pure and inexpensive.

  In general, (1) direct nitriding method, (2) imide decomposition method, (3) reductive nitriding method and the like are known as methods for producing silicon nitride powder. In particular, the reductive nitriding method (3) is relatively easy to operate, and by adding a silicon nitride seed crystal to the raw material, the particle diameter and crystal phase can be arbitrarily controlled, There is an advantage that silicon nitride powder can be obtained using inexpensive silica particle powder and carbon powder as raw materials.

  The reduction reaction of silica in the reductive nitriding method is a solid phase reaction on the surface of silica particles, and it is considered that the reaction proceeds more easily when the surface area of the silica particles is larger.

  Similarly, it is desirable that the carbon powder used in the reduction reaction has a large surface area in consideration of productivity, but generally, the powder tends to have a smaller particle size as the surface area increases, and the carbon powder used in the reduction reaction. When the particle size is reduced, the carbon powder enters between the silica particles during the reaction, and it becomes difficult to oxidize and remove excess or unreacted carbon powder after the formation of silicon nitride, and it tends to remain as impurities.

  On the other hand, when the surface area of the carbon powder used for the reduction reaction is not sufficiently large, whiskers are likely to be generated.Therefore, in the reduction nitriding method, reduction of the carbon content and suppression of whisker generation are in a trade-off relationship, Satisfying both characteristics has been difficult.

  Up to now, as a method for producing silicon nitride with a low content of impurities such as SiC by the reduction nitriding method, a method of limiting the particle size and composition ratio of silica particle powder and carbon powder as raw materials (Patent Document 1 and Patent Document 2) is disclosed.

  As a method for producing silica particle powder as a raw material for silicon nitride in the reduction nitriding method, a dry method and a wet method are mainly known. The dry method is a method in which silicon tetrachloride is used as a starting material and burned with oxygen and hydrogen, and high-purity silica particle powder can be obtained. However, it is expensive because it is expensive.

  On the other hand, the wet method is a method in which an inexpensive sodium silicate is reacted with sulfuric acid as a starting material, and the obtained silica particle powder has a large amount of silanol groups in the silica particles for reaction in an aqueous solution. In addition, the purity is inferior to the dry method using high-purity silicon tetrachloride as a starting material, but the price is lower than that of silica particle powder by the dry method.

  However, when silica powder by wet method is used in the production of silicon nitride powder by reductive nitriding, active hydrogen is generated from silanol groups in the silica particles, and the Si-O-Si bond is cut and recombined. To accelerate. For this reason, cristobalite of coarse crystals is generated and the reduction / nitridation reaction of silica by the carbon powder is hindered, so it has been difficult to obtain high-purity silicon nitride particle powder using inexpensive wet silica.

  As a method of using silica particle powder produced by a wet method as a starting material, a method (Non-patent Document 1) is disclosed in which wet silica particles are coated with carbon and then fired in a nitrogen stream.

  In the silicon nitride obtained by such a conventional manufacturing method, whisker is observed. When mixed with silicon nitride powder at the time of manufacturing, handling property is remarkably lowered, and when the molding is performed using this, a bridge is formed. It is known to cause sag and reduce the density of the compact.

  As a method for avoiding the formation of whiskers, silica having a total metal element content of 0.3% or less in silicon oxide as a raw material causing whisker generation is used (Patent Document 3), or silica powder. It has been suggested that the purity of the liquid is 99.8% or more (Patent Document 4).

JP-A-53-137899 JP 61-242905 A JP 61-91007 A Japanese Patent Laid-Open No. 1-226708 Yusuke Yoshida and 4 others, “Influence of raw material powder on the synthesis of Si3N4 powder by SiO 2 carbon reductive nitriding method”, Proceedings of 2005 Annual Meeting of the Ceramic Society of Japan, Ceramic Society of Japan, 2005, p. 161

  The reduction nitriding method is useful as a production method for obtaining silicon nitride powder at a low cost, but a production method for efficiently obtaining silicon nitride with low carbon content and suppressed whisker formation has not yet been obtained. .

That is, Patent Documents 1 and 2 describe a method of limiting the particle size and composition ratio of silica particle powder and carbon powder as raw materials, but the carbon powder described in the Examples of Patent Document 1 The specific surface area value is as small as 100 m ² / g, and it is difficult to suppress the formation of whiskers.

  Non-Patent Document 1 describes a method in which silica particle powder produced by a wet method is used as a starting material, and this is coated with carbon and then fired in a nitrogen stream, but the generation of cristobalite is suppressed. However, it is difficult to say that the production of whiskers is suppressed due to insufficient purification of the raw wet silica.

  Patent Document 3 describes using a silicon oxide powder having a total amount of metal elements that cause whisker generation of 0.3% or less, but does not describe the total amount of metal elements excluding Si. Moreover, since the metal element that causes the generation of whiskers is as high as 3000 ppm or less, it is insufficient in terms of suppressing the generation of whiskers.

  Patent Document 4 describes that the purity of the silica powder is 99.8% or more, but does not describe the content of metal elements that cause whisker generation, and the total amount of metal elements excluding Si. Even if it is 2000 ppm or less, since the whisker is likely to be generated when the content of the metal element causing the whisker exceeds 500 ppm, it is difficult to say that the generation of the whisker is suppressed.

  Accordingly, it is a technical object of the present invention to provide a method for producing silicon nitride in which the carbon content is low and whisker formation is suppressed.

  The technical problem can be achieved by the present invention as follows.

That is, the present invention provides a method for producing silicon nitride powder, wherein carbon powder having a volume-based average particle diameter (D ₅₀ ) of 5 μm or more is used as a starting material in a method for producing silicon nitride powder in a reduction nitriding method. (Invention 1).

The present invention is also a method for producing silicon nitride powder, wherein the carbon powder of the present invention 1 has a BET specific surface area value of 300 m ² / g or more (Invention 2).

  Further, the present invention, in the method for producing silicon nitride powder in the reduction nitriding method, the total content of V, Nb, Ta, Mo, Fe, Ni, Cr, Co, Cu as starting materials is 500 ppm or less, In addition, the present invention provides the method for producing a silicon nitride powder according to the first and second aspects of the invention (Invention 3), wherein a silica particle powder having a total amount of metal elements excluding Si of 1500 ppm or less is used.

  Further, the present invention is the method for producing a silicon nitride powder of the present invention 3 (invention 4), wherein the silica particles are produced by a wet method as a starting material.

  The method for producing silicon nitride powder according to the present invention can obtain silicon nitride powder having high purity and high nitrogen content at low cost, so that silicon nitride powder as a material for high-temperature structures such as automobile engine parts and gas turbines can be obtained. It is suitable as a production method.

  The configuration of the present invention will be described in more detail as follows.

  First, a method for producing silicon nitride powder according to the present invention will be described.

  The silicon nitride powder having a low carbon content and suppressed whisker formation in the present invention can be obtained by the reductive nitriding method shown in the following reaction formula 1.

<Reaction Formula 1>
3SiO ₂ + 6C + 2N ₂ → Si ₃ N ₄ + 6CO

  As the silica particle powder in the present invention, any of those synthesized by a dry method and those synthesized by a wet method can be used. In consideration of industrial properties such as cost, hydrous silicic acid synthesized by a wet method is advantageous, and among them, silica particles of gel type obtained by reaction in an acidic region in terms of impurities of silica particle powder A powder is more preferred.

  The metal impurities in the silica particle powder have a total content of V, Nb, Ta, Mo, Fe, Ni, Cr, Co, and Cu of 500 ppm or less, preferably 300 ppm or less, more preferably 200 ppm or less. The total amount of metal elements excluding Si is 1500 ppm or less, preferably 1250 ppm or less, more preferably 1000 ppm or less, and still more preferably 750 ppm or less. When the metal impurities in the silica particle powder exceed the above range, whiskers are mixed in the obtained silicon nitride powder, and the handling property is remarkably deteriorated. This is not preferable because the density of the raised molded body is lowered.

The BET specific surface area value of the silica particle powder is preferably 100 m ² / g or more. Considering the reaction efficiency and handling property in the reduction nitriding reaction, more preferably a BET specific surface area value is 150~1000m ² / g, and even more preferably 200~800m ² / g.

  As for the particle size of the silica particle powder, the average primary particle size is preferably 0.001 to 0.2 μm, more preferably 0.002 to 0.1 μm, still more preferably 0.003 for the same reason as above. ~ 0.05 μm.

As for the particle size of the carbon powder, the volume-based average particle diameter (D ₅₀ ) is 5 μm or more. Considering the carbon content of the obtained silicon nitride, the volume-based average particle diameter (D ₅₀ ) is preferably 6 to 100 μm, more preferably 7 to 80 μm.

  Moreover, it is preferable that the content rate of the fine particle of carbon powder is 5% or less in the ratio of the particle | grains whose volume reference particle diameter in carbon powder is 1 micrometer or less. Considering the carbon content of the obtained silicon nitride powder, it is more preferably 4% or less, still more preferably 3% or less.

The BET specific surface area value of the carbon powder is preferably 300 m ² / g or more. Considering suppression of whisker generation, it is more preferably 400 m ² / g or more, and even more preferably 500 m ² / g or more. The upper limit is preferably 2500 m ² / g, more preferably 2000 m ² / g.

  As the carbon powder in the present invention, any carbon black particle powder such as furnace black, channel black and acetylene black, activated carbon, porous carbon and graphite may be used as long as they satisfy the above-mentioned conditions. There are activated carbon and porous carbon.

The purity of the carbon powder is also preferably as high as possible in order to suppress the formation of whiskers. The metal impurities in the carbon powder are V, Nb, Ta, Mo, Fe, Ni, Cr, Co, Cu. The total content is 500 ppm or less, preferably 300 ppm or less, more preferably 200 ppm or less. Further, the total amount of metal elements excluding Si is 1500 ppm or less, preferably 1250 ppm or less, more preferably 1000 ppm or less, and even more preferably 750 ppm or less.

The mixing ratio of the silica powder and the carbon powder in the present invention is 30 to 200 parts by weight of the carbon powder with respect to 100 parts by weight of the silica particle powder. When the amount is less than 30 parts by weight, the carbon component relative to the silica particles is too small, so that the reduction reaction from SiO ₂ by carbon becomes insufficient, and unreacted SiO ₂ remains, so that the nitrogen content is reduced. . On the other hand, when the amount exceeds 200 parts by weight, the oxygen content increases due to excessive oxidation of the surface of the generated silicon nitride particles, and when the obtained silicon nitride powder is used as a sintered body, deterioration of properties such as strength is a problem. It becomes. Considering the nitrogen content and oxygen content of the resulting silicon nitride powder, the ratio of the carbon powder to 100 parts by weight of the silica particle powder is preferably 32-170 parts by weight, more preferably 34-140 parts by weight.

The silicon nitride powder in the present invention uses the above-mentioned silica particle powder and carbon powder as starting materials, and if necessary, adds silicon nitride powder as a seed crystal, and heat-fires it at a predetermined temperature in a nitrogen atmosphere for reduction and reduction. After nitriding, it can be obtained by heating at 600 to 1000 ° C. in an oxidizing atmosphere for decarbonization treatment. If necessary, it may be subjected to a dissolving and removing process of the remaining SiO ₂ by hydrofluoric acid.

  The silica particles powder and carbon powder (if necessary, silicon nitride powder as a seed crystal) may form a granulated body in advance if necessary. By forming a granulated body, handling property improves and a yield can be improved. Examples of the granulation method include compression granulation, extrusion granulation, rolling granulation, spray granulation and the like.

  As a binder used when forming a granulated body, a binder that does not remain as an impurity in the obtained silicon nitride is preferable. Specifically, starch, polyvinyl alcohol, acrylic resin, melamine resin, phenol resin, or the like can be used.

As a gas for forming a nitrogen atmosphere, N ₂ gas, NH ₃ gas, or a mixed system of these and an inert gas such as Ar gas can be used. N ₂ gas is preferred.

  As the seed crystal in the present invention, it is preferable to use silicon nitride powder having an alpha ratio of 85% or more. The higher the alpha conversion rate of the silicon nitride powder used for the seed crystal, the higher the alpha conversion rate of the resulting silicon nitride powder. Therefore, the alpha conversion rate of the silicon nitride powder of the seed crystal is more preferably 90% or more. .

  The addition amount of the seed crystal in the present invention is preferably 0.1 to 50 parts by weight, more preferably 0.5 to 40 parts by weight, and still more preferably 1 to 30 parts by weight with respect to 100 parts by weight of the silica particle powder. is there. If the amount is less than 0.1 part by weight, the alpha conversion rate of the obtained silicon nitride powder is lowered and it is difficult to control the particle size, and if it exceeds 50 parts by weight, the yield of the obtained silicon nitride powder Decreases and is industrially disadvantageous.

  The heating and baking temperature in a nitrogen atmosphere is preferably in the range of 1350 to 1550 ° C, more preferably 1400 to 1500 ° C. When the heating and baking temperature is less than 1350 ° C., the formation reaction of the silicon nitride powder hardly occurs, which is industrially disadvantageous. When the temperature exceeds 1550 ° C., silicon carbide is generated, and the purity of the resulting silicon nitride powder is unfavorable.

  End point determination of the reductive nitriding reaction by heating and baking in a nitrogen atmosphere is performed by monitoring the amount of CO generated in the reaction furnace, and the amount of CO generated is preferably 50 ppm or less, more preferably 30 ppm or less, and even more preferably 10 ppm or less. The end point was taken as the end point.

  The silicon nitride powder in the present invention is heated after the above-described reductive nitriding treatment, and further heated in an oxidizing atmosphere at a temperature range of 600 to 1000 ° C. for 1 hour or more, preferably 2 hours or more for decarbonization treatment. Process.

  The number of whiskers in the silicon nitride powder obtained by the production method of the present invention is 6 or less as the number of whiskers present in the field of view in 500 silicon nitride particles shown in the scanning electron micrograph, preferably 5 or less, more preferably 4 or less. In the case of 7 or more, when molding is performed using this, bridging occurs and the density of the molded body is lowered, which is not preferable.

  The average particle diameter of the silicon nitride powder obtained by the production method of the present invention is 0.05 to 5.00 μm, preferably 0.08 to 4.0 μm, more preferably 0.10 to 3.0 μm.

The BET specific surface area value of the silicon nitride powder obtained by the production method of the present invention is 1 to 30 m ² / g, preferably 1 to 20 m ² / g.

  The alpha ratio of the silicon nitride powder obtained by the production method of the present invention is 90% or more, preferably 93% or more, more preferably 95% or more.

  The nitrogen content of the silicon nitride powder obtained by the production method of the present invention is 37% by weight or more, preferably 37.5% by weight or more, more preferably 38.0% by weight or more.

  The oxygen content of the silicon nitride powder obtained by the production method of the present invention is 2.5% by weight or less, preferably 2.25% by weight or less, more preferably 2.0% by weight or less.

  The carbon content of the silicon nitride powder obtained by the production method of the present invention is 0.6% by weight or less, preferably 0.5% by weight or less, more preferably 0.4% by weight or less.

<Action>
The most important point in the present invention is to use a silicon nitride powder having a low carbon content and suppressed whisker formation by using a carbon powder having a specific BET specific surface area value and particle size as a starting material. The fact that you can get.

The present inventor considers that the silicon nitride production method according to the present invention can suppress the formation of whiskers and reduce the carbon content as follows.
In the production of silicon nitride by the reductive nitriding method, the carbon powder used as a starting material has a higher reactivity as the surface area is larger. Therefore, a carbon powder having a large surface area is also desirable from the viewpoint of suppressing the formation of whiskers. In general, however, the powder tends to have a smaller particle size when the surface area is larger. When the particle size of the carbon powder used for the reduction reaction is smaller, the carbon powder penetrates between the silica particles during the reaction, and the silicon nitride particles Oxidation and removal of excess or unreacted carbon powder after the production becomes difficult, and it tends to remain as impurities.
In the present invention, it is considered that the above-mentioned problems in a trade-off relationship have been solved by selectively using a carbon powder having a large surface area, a large particle size, and a small amount of fine powder.

Furthermore, the content of V, Nb, Ta, Mo, Fe, Ni, Cr, Co, and Cu known to promote whisker formation was reduced to 500 ppm or less, and metals other than the above metals except Si All elements that do not directly participate in whisker generation but contribute to whisker generation by V, Nb, Ta, Mo, Fe, Ni, Cr, Co, and Cu as a flux. It is considered that the production of whiskers could be suppressed even by setting the total amount of slag to 1500 ppm or less.

  Hereinafter, the present invention will be described in detail with reference to examples.

  The specific surface area value was indicated by a value measured by the BET method.

  The average primary particle diameters of the silica particle powder and the silicon nitride powder were expressed by measuring the particle diameters of 350 particles shown in the transmission electron microscope or scanning electron micrograph, respectively.

The particle size of the carbon powder is a volume-based average particle measured at a dispersion pressure of 0.3 MPa (3 bar) using a dry dispersion unit of “Laser diffraction particle size distribution analyzer model HELOS LA / KA” (manufactured by SYMPATEC). The diameter (D ₅₀ ) is shown.

  The fine particle content is measured by measuring the volume standard particle size at a dispersion pressure of 0.3 MPa (3 bar) using a dry dispersion unit of “Laser diffraction particle size distribution analyzer model HELOS LA / KA” (manufactured by SYMPATEC). The integrated value of particles having a volume reference particle diameter of 1 μm or less is shown.

  The metal element content in the silica particle powder was measured by ICP emission spectroscopic analysis after dissolving the silica particle powder with hydrofluoric acid.

  Regarding the metal element content in the carbon powder, the carbon powder was heated and burned in a muffle furnace, an aqueous solution of residual ash after combustion was prepared, and the amount of each element was measured by ICP emission spectroscopy.

  The number of whiskers in the silicon nitride powder was indicated by the number of whiskers present in the visual field per 500 silicon nitride particles shown in the scanning electron micrograph.

Α-conversion rate of the silicon nitride powder (%) is subjected to X-ray diffraction by CuKα-ray, the α phase (102) plane of the diffraction intensity _{I a102} (210) plane of the diffraction intensity _{I a210,} the β-phase (101) From the diffraction intensity I _b101 of the surface and the diffraction intensity I _b210 of the (210) surface, calculation was performed according to the following formula 1.

<Equation 1>
α conversion rate (%) = (I _a102 + I _a210 ) / (I _a102 + I _a210 + I _b101 + I _b210 ) × 100

  The nitrogen content (% by weight) and the oxygen content (% by weight) of the silicon nitride powder were measured using “Horiba Metal Oxygen / Nitrogen Analyzer EMGA620-W” (manufactured by Horiba, Ltd.). .

  The carbon content (% by weight) of the silicon nitride powder was determined by measuring the carbon content using a “Horiba Metal Carbon / Sulfur Analyzer EMIA-2200 Model” (manufactured by Horiba, Ltd.).

<Example 1: Production of silicon nitride powder>
Silica particle powder (silica particle 1) (particle shape: spherical, average primary particle size: 0.004 μm, BET specific surface area value: 798.5 m ² / g, V, Nb, Ta, Mo, Fe, Ni, Cr, Co , Cu content: 30 ppm, 100 parts by weight of metal elements excluding Si: 510 ppm) and carbon powder A (type: activated carbon, volume-based average particle diameter (D ₅₀ ): 17.9 μm, fine particles contained Rate: 0.18%, BET specific surface area value: 1473.3 m ² / g, V, Nb, Ta, Mo, Fe, Ni, Cr, Co, Cu content total: 41 ppm, total amount of metal elements excluding Si : 407 ppm) 120 parts by weight as a starting material, 2 parts by weight of silicon nitride powder (α conversion rate: 92%, average particle size: 0.66 μm) as a seed crystal are mixed, and 275 parts by weight of an 8% aqueous polyvinyl alcohol solution Additionally kneaded in a planetary mixer, to form a granule by an extrusion molding machine.

Subsequently, the obtained granulated body was put into a graphite container, and calcination was performed at 1450 ° C. for 4.5 hours while flowing N ₂ gas to perform reduction nitriding treatment. The CO concentration at the end of the reaction was 10 ppm. The obtained powder was heat-treated in air at 800 ° C. for 6 hours to burn off unreacted carbon to obtain a silicon nitride powder.

The obtained silicon nitride powder has an average primary particle diameter of 1.05 μm, a BET specific surface area value of 4.3 m ² / g, an α conversion of 96%, a nitrogen content of 39.3% by weight, and an oxygen content The amount was 1.73 wt%, the carbon content was 0.22 wt%, and the number of whiskers present in the field of view per 500 silicon nitride particles was one.

  According to Example 1, silicon nitride powder was produced. Various production conditions and various characteristics of the obtained silicon nitride powder are shown.

Silica particles 2 and 3:
A silica particle powder having the characteristics shown in Table 1 was prepared as a silica particle powder.

Carbon powders B to E
A carbon powder having the characteristics shown in Table 2 was prepared as the carbon powder.

<Method for producing silicon nitride>
Examples 2-4, Comparative Examples 1-3
The same procedure as in the manufacture of the silicon nitride powder of Example 1 except that the starting material type and blending ratio, the reaction temperature and reaction time in the reduction nitriding treatment, and the heating temperature and heating time in the decarbonization treatment were variously changed. A silicon nitride powder was obtained.

  The production conditions at this time are shown in Table 3, and various characteristics of the obtained silicon nitride powder are shown in Table 4.

The method for producing silicon nitride powder according to the present invention can obtain silicon nitride powder with low carbon content and reduced whisker generated during production at a low cost, so that it can be used at high temperatures such as automotive engine parts and gas turbines. It is suitable as a method for producing silicon nitride powder for structural materials.

Claims (4)

In the method for producing silicon nitride powder in the reductive nitriding method, a carbon powder having a volume-based average particle diameter (D ₅₀ ) of 5 μm or more is used as a starting material. A method for producing a silicon nitride powder, wherein the carbon powder according to claim 1 has a BET specific surface area value of 300 m ² / g or more. In the method for producing silicon nitride powder in the reduction nitriding method, the total content of V, Nb, Ta, Mo, Fe, Ni, Cr, Co, and Cu as starting materials is 500 ppm or less, and a metal excluding Si 3. The method for producing silicon nitride powder according to claim 1, wherein a silica particle powder having a total amount of elements of 1500 ppm or less is used. The method for producing a silicon nitride powder according to claim 3, wherein the silica particles are produced by a wet method as a starting material.