JP2022135123A - Sintering aid for silicon nitride and method for producing silicon nitride sintered body - Google Patents

Abstract

To provide a sintering aid for silicon nitride that enables the production of a silicon nitride sintered body having high thermal conductivity without using an oxide aid.SOLUTION: An inventive sintering aid for silicon nitride according to the invention is characterized by comprising a silicon carbonitride compound represented by a composition formula: M3Si8N12C2 (where M is Hf, Zr, Sc, Y or a lanthanoid element).SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a silicon nitride sintering aid and a method for producing a silicon nitride sintered body using the silicon nitride sintering aid.

A silicon nitride sintered body is known as a ceramic material having high mechanical strength, excellent oxidation resistance, corrosion resistance, and thermal conductivity, and is widely used industrially. For example, turbocharger rotors, automotive parts such as diesel engines, glow plugs and hot plugs, parts for machine tools such as grinding tips, heat engine parts such as turbine blades for gas turbines and combustion chamber walls, high frequency transistors and power devices It is used as an electrical insulating substrate, etc.

Silicon nitride sintered bodies are generally produced by firing a mixed raw material in which silicon nitride powder is mixed with a sintering aid. Physical properties such as strength and thermal conductivity of the obtained silicon nitride sintered bodies It is known to depend on the type of sintering aid used. Therefore, researches on sintering aids are actively conducted.
For example, Patent Document 1 describes an invention relating to a method for producing a silicon nitride sintered body that uses magnesium silicon nitride (MgSiN ₂ ) as a sintering aid and is sintered at a low temperature of 1900° C. or less. The example shows that a dense silicon nitride sintered body with high thermal conductivity can be obtained by using both MgSiN ₂ and Yb ₂ O ₃ which is a rare earth oxide as sintering aids.
Patent Document 2 describes an invention relating to a method for producing a silicon nitride sintered body by simultaneously adding a rare earth oxide and a magnesium compound to silicon powder or a mixed powder of silicon powder and silicon nitride powder. It is shown that a silicon nitride sintered body having high toughness and excellent thermal conductivity can be obtained.

As seen in these examples, in the production of silicon nitride sintered bodies, oxide aids (Sc ₂ O ₃ , Y ₂ O ₃ , _{Nd2O3 ,} Yb2O3 _{, HfO2} , _ZrO2 , etc.) _are commonly added. However, it is known that these oxide auxiliaries exist at the grain boundaries of silicon nitride crystals in the silicon nitride sintered body and cause a decrease in the strength and thermal conductivity of the silicon nitride sintered body. It is
From this point of view, Patent Document 3 describes an invention related to a silicon nitride sintered body that does not contain an oxide auxiliary agent, and LnSi ₃ N ₅ (Ln is Sc, Y or a lanthanide element), which is a silicon nitride compound ) or Ln ₂ Si ₄ N ₆ C (Ln is Sc, Y or a lanthanoid element) as a sintering aid, a silicon nitride sintered body can be produced without using an oxide aid. ing.

JP 2002-128569 A JP 2007-197226 A JP 2015-86125 A

As described above, according to the method of Patent Document 3, a silicon nitride sintered body can be produced without using an oxide auxiliary agent, and adverse effects such as a decrease in thermal conductivity due to the use of an oxide auxiliary agent are reduced to a certain extent. can be suppressed. However, in recent years, from the viewpoint of improving the performance of various industrial products, there has been a need for silicon nitride sintered bodies with higher thermal conductivity than ever before.
In addition, in the case of using an existing silicon nitride sintering aid, in order to increase the thermal conductivity of the silicon nitride sintered body obtained, it is necessary to sinter for a long time to promote grain growth and purify the silicon nitride particles. There was a problem that the production efficiency was poor.

The present invention has been made in view of the above-mentioned conventional problems, and is capable of producing a silicon nitride sintered body having a higher thermal conductivity than conventional ones in a relatively short time without using an oxide auxiliary agent. It is an object of the present invention to provide a silicon nitride sintering aid that enables this, and a method for producing a silicon nitride sintered body using the silicon nitride sintering aid.

The present inventors have made intensive studies in order to achieve the above object. As a result, the above problem was solved by a silicon nitride sintering aid comprising a silicon carbonitride compound represented by the composition formula M ₃ Si ₈ N ₁₂ C ₂ (where M is Hf, Zr, Sc, Y or a lanthanide element). We have found that the problem can be solved, and completed the present invention.

The gist of the present invention is the following [1] to [3].
[ ₁ ] A silicon nitride sintering aid comprising a silicon carbonitride compound represented by the composition formula M Si _{N C (} where M is Hf, Zr, Sc, Y or a lanthanide element) agent.
[2] A mixed raw material obtained by blending 3 to 50 parts by mass of the silicon nitride sintering aid described in [1] with respect to 100 parts by mass of silicon nitride powder is pressure-sintered in nitrogen gas at 5 to 300 MPa. A method for producing a silicon nitride sintered body, characterized by:
[3] The method for producing a silicon nitride sintered body according to [2] above, wherein the pressure sintering is performed by a hot press method or a spark plasma sintering method (SPS).

According to the present invention, a silicon nitride sintering aid that enables the production of a silicon nitride sintered body with high thermal conductivity in a relatively short time without using an oxide aid, and the silicon nitride A method for producing a silicon nitride sintered body using a sintering aid can be provided.

_Fig . ₃ is an _XRD pattern of _Ce3Si8N12C2 produced in Example. _Fig . ₃ is an _XRD pattern of _Zr3Si8N12C2 produced in Example.

[Silicon nitride sintering aid]
The silicon nitride sintering aid of the present invention comprises a silicon carbonitride compound represented by the composition formula M ₃ Si ₈ N ₁₂ C ₂ (where M is Hf, Zr, Sc, Y or a lanthanide element). By using the sintering aid, a silicon nitride sintered body having high thermal conductivity can be produced in a relatively short time without using an oxide aid.
The reason for this is not clear, but is presumed as follows. The silicon nitride sintering aid of the present invention is a C-rich compound. For this reason, the reducing action is stronger than that of the sintering aids disclosed in the prior art, and the oxygen content of the obtained silicon nitride sintered body can be further reduced, which is thought to improve the thermal conductivity. In addition, it is believed that the use of the silicon nitride sintering aid of the present invention promotes grain growth of silicon nitride, and a silicon nitride sintered body with high thermal conductivity can be obtained in a short period of time.

The silicon nitride sintering aid in the present invention comprises a silicon carbonitride compound represented by the composition formula M ₃ Si ₈ N ₁₂ C ₂ . M is Hf, Zr, Sc, Y or a lanthanide element. The lanthanoid element is a general term for 15 elements with atomic numbers from 57 to 71, that is, from lanthanum (La) to lutetium (Lu).
Among these, Hf, Ce, and Zr are preferable as M because the particle shape of the obtained silicon nitride sintered body can be easily adjusted and the strength characteristics can be easily improved.
In the present invention, the particle size of the silicon nitride sintering aid is not particularly limited, but the average particle size _D50 is preferably 1 to 10 μm, particularly 2 to 5 μm.

[Method for producing silicon nitride sintering aid]
As a method for producing a silicon carbonitride compound used as a silicon nitride sintering aid, metal oxide powder (MO ₂ : where M is Hf, Zr, Sc, Y or a lanthanide element), silicon nitride powder ( Si ₃ N ₄ ) and carbon powder (C) are mixed so that the ratio of the composition formula of the silicon carbonitride compound is obtained, and then heated in a nitrogen atmosphere at 1400 to 1800° C., preferably 1500 to 1700° C. for 1 hour or more. , preferably by reacting for 3 to 10 hours and pulverizing the reaction product with a known pulverizer.
Mixing of the raw materials can be performed using a known mixing device such as a planetary mill. In addition, mixing is preferably performed by wet-mixing each raw material in an organic solvent such as ethanol, and the slurry obtained by wet-mixing is, for example, using an evaporator, for example, under vacuum to remove the organic solvent. After drying, it is preferably subjected to the reaction. The treatment temperature is preferably about 90 to 120.degree. Furthermore, it is recommended that the reaction be carried out in a vacuum/pressurized atmosphere furnace.
Further, the silicon carbonitride compound, which is the reaction product, can be identified using X-ray diffraction and SEM-EDX, as shown in the Examples.

[Manufacturing method of silicon nitride sintered body]
A silicon nitride sintered body can be produced using the silicon nitride sintering aid of the present invention. A method for manufacturing a silicon nitride sintered body will be described below.

In the method for producing a silicon nitride sintered body in the present invention, the composition formula M Si ₈ N _{12 C 2 (} where M is Hf, Zr, Sc, Y or a lanthanoid element) is applied to 100 parts by mass of silicon nitride powder. A mixed raw material containing 3 to 50 parts by mass of a silicon nitride sintering aid composed of a silicon carbonitride compound represented by is pressure-sintered in nitrogen gas at 5 to 300 MPa.

(mixed raw material)
The mixed raw material is obtained by blending 3 to 50 parts by mass of a silicon nitride sintering aid with 100 parts by mass of silicon nitride powder.
Silicon nitride powder (Si ₃ N ₄ powder) that is generally available can be used, and silicon nitride powder produced by various methods such as reduction nitriding method, direct nitriding method, and imide decomposition method can be used without particular limitation. can be used.
The average particle size _D50 of the silicon nitride powder is not particularly limited, but is, for example, 0.5 to 10 μm, preferably 1 to 3 μm, considering ease of sintering.
In this specification, the average particle diameter _D50 is a value based on 50% volume measured by a laser diffraction scattering method.

The content of the silicon nitride sintering aid in the mixed raw material is 3 to 50 parts by mass with respect to 100 parts by mass of the silicon nitride powder. If the content of the silicon nitride sintering aid is less than 3 parts by mass, sintering will be difficult to proceed and a dense sintered body will not be obtained. If it exceeds 50 mass, it becomes difficult to obtain the effect corresponding to the amount added. The content of the silicon nitride sintering aid is preferably 3 to 30 parts by mass, more preferably 5 to 15 parts by mass with respect to 100 parts by mass of the silicon nitride powder.

The mixed raw material can be prepared by dry mixing or wet mixing, and wet mixing is preferred from the viewpoint of mixing efficiency.
Wet mixing may be performed by adding a solvent to the silicon nitride powder and the silicon nitride sintering aid and using a ball mill, planetary ball mill, rotation-revolution mixer or apex mill for 3 minutes to 3 hours. A slurry-like mixed raw material is obtained. Mixing for less than 3 minutes is insufficient, and mixing for more than 3 hours has little effect on mixing. If the silicon nitride sintering aid is unstable and easily oxidized in the air, it can be prevented from being oxidized during mixing by performing the mixing in an inert gas such as nitrogen gas.

There are two methods of sintering using mixed raw materials. One is to adjust the amount of solvent in the slurry mixed raw material, make a molded body using an injection molding machine or an extrusion molding machine, dry the molded body sufficiently, and then use an atmosphere sintering furnace. It is a method of sintering under pressure. The atmosphere for this sintering is nitrogen gas, and the pressure is in the range of 0.05 MPa to 300 MPa of atmospheric pressure. If the pressure is less than 0.05 MPa, nitrogen may evaporate from the nitride, and if it exceeds 300 MPa, the material of the metal container that supports this pressure will be limited. The sintering temperature ranges from 1500°C to 1900°C. At 1500° C. or less, sintering cannot proceed sufficiently. When the temperature reaches 1900° C. or higher, decomposition of the nitride begins. The sintering time is 5 minutes to 10 hours. If the time is less than 5 minutes, the sintering will not be sufficiently performed. Sintering proceeds sufficiently in 10 hours or less, and longer time is not required.

Another sintering method is a method of pressurizing and sintering the mixed raw material in nitrogen gas. Pressure sintering is preferably performed by a hot press method or a discharge plasma sintering method. Specifically, the slurry-like mixed raw material is dried, or the slurry-like mixed raw material is granulated to make pellets and dried, and these dry raw materials are packed in a graphite mold and hot-pressed or spark plasma sintered. This is a method of pressure sintering. The pressure sintering atmosphere is nitrogen gas, and the gas pressure ranges from 0.05 MPa to 0.2 MPa. The reason why the pressure is set to 0.2 MPa or less is that the pressure sintering machine is not a pressure-resistant container. The pressure for sintering is 5-300 MPa. When the pressure is less than 5 MPa, the effect of pressure is small and the product is the same as pressureless sintering. The reason why the upper limit is 300 MPa is that there is no graphite mold that can be pressurized beyond this.
The sintering temperature ranges from 1500°C to 1900°C. At 1500° C. or less, sintering cannot proceed sufficiently. When the temperature reaches 1900° C. or higher, decomposition of the nitride begins. The sintering time is 5 minutes to 10 hours. If the time is less than 5 minutes, the sintering will not be sufficiently performed. Sintering proceeds sufficiently in 10 hours or less, and longer time is not required.

By using the silicon nitride sintering aid of the present invention, a silicon nitride sintered body with high thermal conductivity can be produced in a short time without using an oxide aid.
In addition, the silicon nitride sintered body produced using the silicon nitride sintering aid of the present invention has a structure in which columnar crystals of different sizes are intertwined. Therefore, the strength and toughness are nearly doubled or more than those of other ceramic materials composed of spherical crystals. That is, the strength of the nitrogen-silicon-based sintered body of the present invention is 800 MPa or more, and the toughness is 8 MPa·m ^1/2 or more.

The silicon nitride sintered body produced in the present invention can be industrially utilized in many fields. Automotive parts such as turbocharger rotors, diesel engine glow and hot plugs, tappets and injector links, grinding tips, heat engine and heat exchanger components such as turbine blades and combustion chamber walls for gas turbines, and thermocouple protection. Pipes, nozzles, nozzle covers, rotors for plastic working, molten aluminum parts, abrasive cloth dressing plates, motor shafts, bearings, wear-resistant parts such as fishing gear thread paths, semiconductors such as IC inspection tables, clampers, chucks, and push-up tables It can be used as a manufacturing equipment part, an electrically insulating substrate for high-frequency transistors and power devices, and the like. Furthermore, by making it into a composite material with silicon carbide fiber, it can be used as a turbine blade for a jet engine, which requires high reliability.

EXAMPLES Hereinafter, examples will be shown to describe the present invention more specifically, but the present invention is not limited to these examples.

(Production of silicon carbonitride compound: Ce ₃ Si ₈ N ₁₂ C ₂ )
Cerium oxide powder (CeO ₂ : Kojundo Chemical Laboratory), silicon nitride powder (Si ₃ N ₄ : Ube Industries) and carbon powder (C: Mitsubishi Chemical) in a composition ratio of CeO ₂ :Si ₃ N ₄ :C=54 A planetary mill (manufactured by Fritsch Japan Co., Ltd.) was used to mix in ethanol at a rotational speed of 250 rpm for 2 hours. A silicon nitride pot and a silicon nitride ball (diameter 1.0 mm) were used for mixing.
The resulting slurry was vacuum-dried at 110° C. for 4 hours to volatilize the solvent with an evaporator to obtain a starting material. The obtained mixed powder was weighed in a BN crucible, set in a vacuum/pressurized atmosphere furnace (manufactured by Fuji Dempa Kogyo Co., Ltd., model: Multi500), and synthesized at 1650° C. in a nitrogen atmosphere. The synthesized powder was pulverized in a mortar to obtain a silicon nitride sintering aid having an average particle size D ₅₀ of 3.1 μm.
Compounds of the obtained powder were identified by X-ray diffraction (Rigaku Co., model: RINT2500) and SEM-EDX (JEOL Ltd., model: JSM-5600). XRD results are shown in FIG.

(Production of silicon carbonitride compound: Zr ₃ Si ₈ N ₁₂ C ₂ )
Zirconium oxide powder (ZrO ₂ : Kojundo Chemical Laboratory), silicon nitride powder (Si ₃ N ₄ : Ube Industries) and carbon powder (C: Mitsubishi Chemical) in a composition ratio of ZrO ₂ :Si ₃ N ₄ :C=46 A planetary mill (manufactured by Fritsch Japan Co., Ltd.) was used to mix in ethanol at a rotational speed of 250 rpm for 2 hours. A silicon nitride pot and a silicon nitride ball (diameter 1.0 mm) were used for mixing.
The resulting slurry was vacuum-dried at 110° C. for 4 hours to volatilize the solvent with an evaporator to obtain a starting material. The obtained mixed powder was weighed in a BN crucible, set in a vacuum/pressurized atmosphere furnace (manufactured by Fuji Dempa Kogyo Co., Ltd., model: Multi500), and synthesized at 1650° C. in a nitrogen atmosphere. The synthesized powder was pulverized in a mortar to obtain a silicon nitride sintering aid having an average particle diameter D ₅₀ of 3.3 μm.
Compounds of the obtained powder were identified by X-ray diffraction (Rigaku Co., model: RINT2500) and SEM-EDX (JEOL Ltd., model: JSM-5600). XRD results are shown in FIG.

(Production of comparative compound: LaSi ₃ N ₅ )
Weigh 1.914 g of silicon nitride powder and 2.086 g of LaN, mix them in nitrogen gas using a mortar made of agate, use a non-pressurized atmosphere furnace, and pressurize nitrogen gas at 0.2 MPa. LaSi ₃ N ₅ was produced by reacting at 1550° C. for 2 hours under low temperature.

(Example 1)
100 parts by mass of silicon nitride powder (95% or more α phase) and 10 parts by mass of Ce ₃ Si ₈ N ₁₂ C ₂ as a silicon nitride sintering aid were added to 15 mL of alcohol. A slurry mixed raw material was prepared by mixing for 5 minutes under the conditions of . After drying the mixed raw material, in a nitrogen atmosphere, using a discharge plasma sintering machine, the temperature was raised to 1700 ° C. in 20 minutes under the condition of a pressure of 40 MPa, and this temperature was maintained for 15 minutes to complete sintering. , to obtain a silicon nitride sintered body.
Thermal conductivity, bending strength and fracture toughness of the obtained silicon nitride sintered body were measured as follows, and the measurement results are shown in Table 1.
Thermal conductivity was measured by the laser flash method.
Bending strength was measured as three-point bending strength in accordance with JIS R1601.
The fracture toughness was measured by the SEPB (Single Edge Pre-Cracked Beam) method according to JIS R1607.

(Example 2, Comparative Example 1)
A silicon nitride sintered body was obtained in the same manner as in Example 1, except that the silicon nitride sintering aid was changed as shown in Table 1.
The thermal conductivity, bending strength and fracture toughness of the obtained silicon nitride sintered body were measured as described above, and the measurement results are shown in Table 1.

As is clear from the results of each example, silicon nitride sintered silicon nitride compounds made of silicon carbonitride compounds represented by the composition formula M ₃ Si ₈ N ₁₂ C ₂ (where M is Hf, Zr, Sc, Y or a lanthanide element) A silicon nitride sintered body was obtained in a relatively short firing time by using a cohesion aid without using an oxide aid. The obtained silicon nitride sintered body was excellent in thermal conductivity, bending strength and fracture toughness.

Claims (3)

A silicon nitride sintering aid comprising a silicon carbonitride compound represented by the composition formula M ₃ Si ₈ N ₁₂ C ₂ (M is Hf, Zr, Sc, Y or a lanthanide element). A mixed raw material obtained by blending 3 to 50 parts by mass of the silicon nitride sintering aid according to claim 1 with respect to 100 parts by mass of silicon nitride powder is sintered under pressure at 5 to 300 MPa in nitrogen gas. A method for producing a silicon nitride sintered body characterized by: 3. The method for producing a silicon nitride sintered body according to claim 2, wherein the pressure sintering is performed by a hot press method or a discharge plasma sintering method.

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