Silicon nitride powder, method for producing the same, and method for producing a silicon nitride sintered body

Abstract

Provided is a silicon nitride powder having an internal oxygen content of 0.6 mass% or less. Also provided is a production method that is for a silicon nitride powder and that has a step for firing a silicon powder having an oxygen concentration of 0.4 mass% or less in a mixed atmosphere of nitrogen and hydrogen to obtain a fired product, and a step for processing the fired product using hydrofluoric acid having a hydrogen fluoride concentration of 10-40 mass%.

Description

Technical Field

[0001] The present disclosure relates to silicon nitride powder, a method for producing the same, and a method for producing a silicon nitride sintered body.

Background Art

[0002] Silicon nitride is a material excellent in strength, hardness, toughness, heat resistance, corrosion resistance, thermal shock resistance, etc., and is used in various industrial parts such as die-casting machines and melting furnaces, and automotive parts. Further, since silicon nitride also has excellent mechanical properties at high temperatures, it has been studied for application to gas turbine parts that require high-temperature strength and high-temperature creep properties. For example, in Patent Document 1, as a method for improving the high-temperature properties of a silicon nitride sintered body, the total oxygen content of the silicon nitride powder is set to 1.5% by mass or less to reduce the grain boundary phase to be purified during sintering and maintain a high melting point to improve the high-temperature properties. has been studied.

[0003] The use of a silicon nitride substrate as an insulating substrate for power modules such as automobiles and machine tools has also been studied. For example, Patent Document 2 proposes using a silicon nitride substrate for an aluminum-ceramics bonding substrate. In such applications, high insulation and heat dissipation are required.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] As a silicon nitride substrate, in order to achieve excellent heat dissipation performance, it is required to have a high thermal conductivity. In the silicon nitride sintered body used for the substrate, the amount of defects present in the silicon nitride sintered body is cited as a factor affecting the thermal conductivity. It is considered that the amount of defects in the silicon nitride sintered body is affected not only by the sintering conditions but also by the physical properties of the silicon nitride powder used for the silicon nitride sintered body. Therefore, in the present disclosure, a silicon nitride powder and a method for producing the same capable of obtaining a silicon nitride sintered body having a high thermal conductivity are provided. Further, in the present disclosure, a method for producing a silicon nitride sintered body having a high thermal conductivity is provided.

Means for Solving the Problems

[0006] The silicon nitride powder according to one aspect of the present disclosure has an internal oxygen content of 0.6 mass% or less. Since such a silicon nitride powder has a sufficiently low internal oxygen content, when used as a sintering raw material, a silicon nitride sintered body having a high thermal conductivity can be obtained. The reason is considered to be that the defects inside the silicon nitride sintered body can be reduced by using a silicon nitride powder with a low internal oxygen content.

[0007] The surface oxygen content of the above silicon nitride powder may be equal to or less than the internal oxygen content. Thereby, a silicon nitride sintered body having an even higher thermal conductivity can be obtained.

[0008] The surface oxygen content of the above silicon nitride powder may be greater than the internal oxygen content. When the surface oxygen content increases, a liquid phase is likely to be formed during sintering, and the strength of the silicon nitride sintered body can be improved.

[0009] The method for producing silicon nitride powder according to one aspect of the present disclosure includes a step of firing silicon powder having an oxygen concentration of 0.4 mass% or less in a mixed atmosphere containing at least one selected from the group consisting of nitrogen, hydrogen, and ammonia to obtain a fired product, and a step of treating the fired product with hydrofluoric acid having a hydrofluoric acid concentration of 10 to 40 mass%. According to this production method, since silicon powder with a sufficiently low oxygen concentration is used, silicon nitride powder with a sufficiently low internal oxygen content can be obtained. Further, since the fired product is treated with hydrofluoric acid having a hydrofluoric acid concentration of 10 to 40 mass%, the surface oxygen content can also be adjusted to a range that does not significantly differ from the internal oxygen. Therefore, it can be suitably used as silicon nitride powder for producing a silicon nitride sintered body having high thermal conductivity while having sufficient strength.

[0010] The method for producing a silicon nitride sintered body according to one aspect of the present disclosure includes a step of molding and firing a sintering raw material containing silicon nitride powder produced by the above-described method for producing silicon nitride powder. According to this production method, a silicon nitride sintered body having high thermal conductivity can be produced.

Advantages of the Invention

[0011] According to the present disclosure, it is possible to provide silicon nitride powder and a method for producing the same capable of obtaining a silicon nitride sintered body having high thermal conductivity. Further, it is possible to provide a method for producing a silicon nitride sintered body having high thermal conductivity.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings as appropriate. However, the following embodiments are examples for explaining the present disclosure and are not intended to limit the present disclosure to the following contents.

[0014] The internal oxygen content of silicon nitride powder (Si3N4 powder) according to one embodiment is 0.6 mass% or less. The internal oxygen is oxygen that exists inside the powder without being exposed on the surface of the silicon nitride powder. From the viewpoint of further increasing the thermal conductivity of the silicon nitride sintered body when used as a raw material for the silicon nitride sintered body, the internal oxygen content may be 0.5 mass% or less, or may be 0.4 mass% or less. The lower limit of this internal oxygen content is not particularly limited, but from the viewpoint of ease of production, it may be 0.1 mass% or more, or may be 0.2 mass% or more. The internal oxygen content can be adjusted by changing the oxygen concentration in the raw material of the silicon nitride powder.

[0015] From the viewpoint of sufficiently lowering the thermal conductivity of the silicon nitride sintered body, the surface oxygen content of the silicon nitride powder may be equal to or less than the internal oxygen content. The surface oxygen is oxygen that is bonded or attached to the surface of the silicon nitride powder. The surface oxygen content may be, for example, 0.6 mass% or less, may be 0.5 mass% or less, or may be 0.4 mass% or less. The lower limit of this surface oxygen content is not particularly limited, but from the viewpoint of ease of production, it may be 0.05 mass% or more, or may be 0.1 mass% or more, or 0.2 mass% or more. The surface oxygen content can be adjusted by performing surface treatment on the silicon nitride powder.

[0016] In another embodiment, from the viewpoint of increasing the strength of the silicon nitride sintered body when used as a raw material for the silicon nitride sintered body, the surface oxygen content of the silicon nitride powder may be greater than the internal oxygen content, may be 0.7 mass% or more, or may be 0.8 mass% or more. The upper limit of this surface oxygen content is not particularly limited, but from the viewpoint of ease of production, it may be 1.5 mass% or less, or may be 1.0 mass% or less.

[0017] The total oxygen content of the silicon nitride powder may be 0.5% by mass or more, may be 0.7% by mass or more, and may be 1.0% by mass or more. The total oxygen content of the silicon nitride powder may be 2.0% by mass or less, and may be 1.5% by mass or less. As an example, it may be 0.5 to 2.0% by mass, and may be 0.7 to 1.5% by mass. In the present disclosure, the total oxygen content is the ratio of the mass of oxygen to the total mass of the silicon nitride powder. On the other hand, the internal oxygen content is the ratio of the mass of internal oxygen to the total mass of the silicon nitride powder. Also, the surface oxygen content is the ratio of the mass of surface oxygen to the total mass of the silicon nitride powder. Therefore, the following equation holds. Total oxygen content (% by mass) = Internal oxygen content (% by mass) + Surface oxygen content (% by mass)

[0018] The internal oxygen content, surface oxygen content, and total oxygen content in the present disclosure are determined by the following procedure. The oxygen content and nitrogen content of the silicon nitride powder are analyzed using an oxygen-nitrogen analyzer. The sample for measurement is heated from 20°C to 2000°C at a heating rate of 8°C / second in an atmosphere of helium gas. As the temperature rises, the desorbing oxygen is detected. At the beginning of the heating, the oxygen bonded to the surface of the silicon nitride powder desorbs. The surface oxygen content is determined by quantifying the amount of desorbing oxygen.

[0019] After that, when the temperature reaches near 1400°C, the silicon nitride starts to decompose. The start of the decomposition of the silicon nitride can be grasped by the start of the detection of nitrogen. When the silicon nitride starts to decompose, the oxygen inside the silicon nitride powder desorbs. The internal oxygen content is determined by quantifying the oxygen desorbing at this stage.

[0020] Figure 1 is an example of a chart obtained by oxygen-nitrogen analysis of silicon nitride. Peak 1 is the peak of surface oxygen, and Peak 2 is the peak of internal oxygen. Peak 3 is the peak of nitrogen. Straight line 4 indicates the temperature rising line. Peak 1 and Peak 2 are demarcated by the temperature T1 at which nitrogen begins to be generated. The temperature T1 is the temperature at which the detection of Peak 3 starts and is usually between 1350 and 1500 °C. The temperature at which the detection of Peak 1 starts (the temperature at the left end of Peak 1) is, for example, between 750 and 1200 °C. The temperature at which the detection of Peak 2 ends (the temperature at the right end of Peak 2) is, for example, between 1600 and 1800 °C. The amounts of internal oxygen and surface oxygen are determined based on the calibration curve from the integrated values (areas) of Peaks 1 and 2. Also, the sum of the amounts of internal oxygen and surface oxygen is the total oxygen amount.

[0021] In Figure 1, the left end of Peak 3 (temperature T1) coincides with the deepest part of the valley between Peaks 1 and 2, but they do not have to coincide exactly. However, normally, the temperature T1 (the left end of Peak 3) will be located between the temperatures at which the respective peaks of Peak 1 and Peak 2 are detected.

[0022] Since the amount of internal oxygen in silicon nitride powder like that in Figure 1 is sufficiently low, when used as a sintering raw material, a silicon nitride sintered body with excellent thermal conductivity can be obtained. The reason is considered to be that the defects inside the silicon nitride sintered body can be reduced by using silicon nitride powder with a small amount of internal oxygen. As shown in Figure 1, the amount of surface oxygen in the silicon nitride powder may be less than or equal to the amount of internal oxygen. Thereby, a silicon nitride sintered body having a higher thermal conductivity can be obtained.

[0023] In FIG. 1, the integrated value of peak 2 is larger than that of peak 1, but it is not limited to this magnitude relationship. For example, the integrated value of peak 1 may be larger than that of peak 2. In this case, the amount of surface oxygen is larger than the amount of internal oxygen, and a silicon nitride sintered body excellent in strength can be produced. The ratio of the integrated value of peak 1 to the integrated value of peak 2, that is, the ratio of the amount of internal oxygen to the amount of surface oxygen, may be 1 or more, may be 1.2 or more, or may be 1.3 or more from the viewpoint of sufficiently increasing the thermal conductivity of the silicon nitride sintered body.

[0024] The ratio of the amount of surface oxygen to the amount of internal oxygen may be 0.8 or more, may be 1.0 or more, or may be 1.5 or more. In particular, by setting the total oxygen content to 1.0% by mass or less and the ratio of the amount of surface oxygen to the amount of internal oxygen to 1.5 or more, the thermal conductivity can be further improved. The ratio is preferably 1.8 or more, more preferably 2.0 or more. The upper limit of the ratio of the amount of surface oxygen to the amount of internal oxygen may be 5.0, or may be 4.0.

[0025] A method for producing silicon nitride powder according to an embodiment includes a pretreatment step of pretreating silicon powder using a pretreatment liquid containing hydrofluoric acid to obtain silicon powder having an oxygen concentration of 0.4% by mass or less, a firing step of firing the silicon powder in a mixed atmosphere containing nitrogen and hydrogen to obtain a fired product, a pulverization step of pulverizing the fired product, and a post-treatment step of treating the pulverized fired product with hydrofluoric acid having a hydrofluoric acid concentration of 10 to 40% by mass.

[0026] In the pretreatment step, the oxygen bonded to the silicon powder is reduced using a pretreatment liquid containing hydrofluoric acid. The pretreatment liquid may be a mixed acid that is a mixture of hydrofluoric acid and hydrochloric acid, or only hydrofluoric acid may be used. The temperature of the pretreatment liquid in the pretreatment step is, for example, 40 to 80°C. Also, the immersion time in the pretreatment liquid is, for example, 1 to 10 hours.

[0027] The oxygen concentration of the silicon powder obtained in the pretreatment step is 0.4% by mass or less, preferably 0.3% by mass or less, more preferably 0.2% by mass or less. There is no particular limitation on the lower limit of the oxygen concentration, and it may be 0.1% by mass or more from the viewpoint of ease of production.

[0028] In the firing step, the silicon powder is fired in a mixed atmosphere containing at least one selected from the group consisting of nitrogen, hydrogen, and ammonia to obtain a nitride. The total content ratio of hydrogen and ammonia in the mixed atmosphere may be 10 to 40% by volume. The firing temperature may be, for example, 1100 to 1450 °C, or may be 1200 to 1400 °C. The firing time may be, for example, 30 to 100 hours.

[0029] When the silicon nitride obtained in the firing step is in the form of an ingot, a pulverization step of pulverizing the fired product is performed. The pulverization may be performed in multiple stages including coarse pulverization and fine pulverization. The pulverization may be performed wet using, for example, a ball mill. The silicon nitride may be pulverized until the specific surface area reaches 8.0 to 15.0 m 2 / g.

[0030] In the post-treatment step, the pulverized fired product is blended and treated with hydrofluoric acid having a hydrogen fluoride concentration of 10 to 40% by mass. For example, the fired product may be dispersed in hydrofluoric acid for treatment. The hydrogen fluoride concentration in the hydrofluoric acid may be 12 to 30% by mass. The temperature of the hydrofluoric acid in the post-treatment step is, for example, 40 to 80 °C. Also, the time for immersing the silicon nitride powder in the hydrofluoric acid is, for example, 1 to 10 hours.

[0031] By such a production method, the total oxygen amount, internal oxygen amount, and surface oxygen amount of the silicon nitride powder can be adjusted within the above-mentioned ranges. The silicon nitride sintered body formed using the silicon nitride powder thus obtained has high thermal conductivity.

[0032] The manufacturing method of the silicon nitride sintered body according to one embodiment has a step of molding and firing a sintering raw material containing the above-described silicon nitride powder as a main component. The sintering raw material may contain an oxide-based sintering aid in addition to the silicon nitride powder. Examples of the oxide-based sintering aid include Y2O 3、 MgO, Al2O3, and the like. The content of the oxide-based sintering aid in the sintering raw material may be, for example, 3 to 10% by mass.

[0033] In the above step, the above-described sintering raw material is pressurized at a molding pressure of, for example, 3.0 to 30 MPa to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP. Further, it may be fired while being molded by hot pressing. The firing of the molded body may be performed in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure during firing may be 0.7 to 1 MPa. The firing temperature may be 1860 to 2100 °C, or may be 1880 to 2000 °C. The firing time at the firing temperature may be 6 to 20 hours, or may be 8 to 16 hours. The heating rate to the firing temperature may be, for example, 1.0 to 10.0 °C / hour.

[0034] The silicon nitride sintered body produced in this way has high thermal conductivity and thus excellent heat dissipation performance. Further, by increasing the surface oxygen amount of the silicon nitride powder used as the raw material, a silicon nitride sintered body excellent in strength can be obtained. The thermal conductivity of the silicon nitride sintered body may be, for example, 100 W / mK or more, or may be 110 W / mK or more in an environment of 25 °C. The three-point bending strength of the silicon nitride sintered body may be, for example, 500 MPa or more at room temperature, or may be 600 MPa or more.

[0035] As described above, several embodiments have been described, but the present disclosure is not limited to the above embodiments at all.

Examples

[0036] The content of the present disclosure will be described in more detail with reference to Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

[0037] (Example 1) (Preparation of silicon nitride powder) Commercially available silicon powder (specific surface area: 3.0 m 2 / g) was immersed in mixed acid for pretreatment. For the pretreatment, the silicon powder was placed in the mixed acid whose temperature was adjusted to 60°C and immersed for 2 hours. As the mixed acid used for the pretreatment, a mixture of commercially available hydrochloric acid (concentration: 35% by mass) and hydrofluoric acid (concentration: 55% by mass) blended at a mass ratio of 10:1 was used. Thereafter, the silicon powder was taken out from the mixed acid, washed with water, and dried in a nitrogen atmosphere. The oxygen concentration of the dried silicon powder was 0.4% by mass. This oxygen concentration was measured by the infrared absorption method.

[0038] Using the dried silicon powder, a molded body (bulk density: 1.4 g / cm 3 ) was produced, and a silicon nitride ingot was produced by firing at 1400°C for 60 hours using an electric furnace. The atmosphere during firing was a mixed atmosphere of nitrogen and hydrogen (N2:H2 = 80:20, volume basis). After the obtained ingot was roughly pulverized, it was wet pulverized with a ball mill. Water was used as the solvent during wet pulverization. Intermittent firing After the silicon nitride powder obtained by wet pulverization was immersed in hydrofluoric acid (hydrogen fluoride concentration: 15% by mass) at a temperature of 60°C for 2 hours, a post-treatment was performed. Thereafter, the silicon nitride powder was taken out from the hydrofluoric acid, washed with water, and dried in a nitrogen atmosphere. Thus, the silicon nitride powder of Example 1 was obtained.

[0039]

[0040] (Evaluation of silicon nitride powder) The internal oxygen amount and surface oxygen amount of the silicon nitride powder were measured by the following procedure. 0.01 g of a sample for measurement was set in an oxygen-nitrogen analyzer (manufactured by Horiba, Ltd., device name: EMGA-920). In an atmosphere of helium gas, the temperature was raised from 20°C to 2000°C at a heating rate of 8°C / second. During the temperature rise, oxygen and nitrogen were detected. The measurement results are shown in Figure 2. As shown in Figure 2, peak 1 derived from surface oxygen, peak 2 derived from internal oxygen, and peak 3 derived from nitrogen were detected. Line 4 indicates the temperature.

[0041] The temperature at which peak 3 rises, that is, the temperature T1 that demarcates peak 1 and peak 2, was 1392°C. The surface oxygen content and the internal oxygen content were determined from the calibration curves of the integrated values of peaks 1 and 2, the separately determined peak integrated value, and the oxygen amount. The results were as shown in Table 1.

[0042] <Fabrication of silicon nitride sintered body> 90 parts by mass of the prepared silicon nitride powder, 5 parts by mass of Y2O3 powder with an average particle size of 1.5 μm, and 5 parts by mass of Yb2O3 powder with an average particle size of 1.2 μm were blended and wet-mixed in methanol for 4 hours. Then, the mixed powder obtained after drying was die-molded at a pressure of 10 MPa, and then further CIP-molded at a pressure of 25 MPa. The obtained green body was set in a carbon crucible together with a packing powder composed of a mixed powder of silicon nitride powder and BN powder, and fired at 1900°C for 12 hours in a nitrogen pressurized atmosphere of 1 MPa to produce a silicon nitride sintered body.

[0043] <Evaluation of silicon nitride sintered body> The silicon nitride sintered body was ground to produce a disk-shaped body of 10 mmφ × 3 mm for thermal conductivity measurement. The thermal diffusivity and specific heat capacity were measured by the laser flash method (in accordance with JIS R1611), and the product of the density, thermal diffusivity, and specific heat capacity of the sintered body was calculated as the thermal conductivity at room temperature. Also, a test piece for strength measurement was fabricated in accordance with JIS R1601:2008, and the three-point bending strength at room temperature was measured. The measurement results are shown in Table 1 as relative values based on the measured values of Example 1.

[0044] (Examples 2 to 8, Comparative Examples 1 to 3) The oxygen concentration of the silicon powder was changed as shown in Table 1 by changing the immersion time of the silicon metal powder in the mixed acid during pretreatment between 1 and 5 hours, and silicon nitride powder was prepared in the same manner as in Example 1 except that the concentration of hydrofluoric acid (concentration of hydrogen fluoride) used in the post-treatment was changed as shown in Table 1. In Examples 4 to 8, the immersion time of the silicon metal powder in the mixed acid during pretreatment was set to 2 hours, the same as in Example 1. In Examples 2 and 3, this immersion time was set to 3 hours and 5 hours, respectively. In Comparative Examples 1 to 3, this immersion time was set to 1 hour. In the same manner as in Example 1, the surface oxygen amount and the internal oxygen amount of each example and each comparative example were determined. Further, the ratio of the surface oxygen amount to the internal oxygen amount (indicated as "surface / internal" in Table 1) was determined. The results were as shown in Table 1.

[0045] In the same manner as in Example 1, a silicon nitride sintered body was produced using the silicon nitride powder and evaluated. The measurement results are shown in Table 1 as relative values based on the measured values of Example 1.

[0046]

Table 1

Industrial Applicability

[0047] According to the present disclosure, it is possible to provide a silicon nitride powder and a method for producing the same that can obtain a silicon nitride sintered body having high thermal conductivity. Further, a method for producing a silicon nitride sintered body having high thermal conductivity can be provided.

Claims

1. The internal oxygen content is 0.2% by mass or less, and the surface oxygen content is 0.6% by mass or more, wherein the internal oxygen content and the surface oxygen content are obtained by setting a 0.01 g sample in an oxygen-nitrogen analyzer, heating from 20°C to 2000°C at a heating rate of 8°C / second in an atmosphere of helium gas, and during the heating, detecting peak 1 derived from surface oxygen, peak 2 derived from internal oxygen, and peak 3 derived from nitrogen, and are determined from the integrated values of peak 1 and peak 2 partitioned at the temperature at which the detection of peak 3 starts, the total oxygen content, which is the sum of the internal oxygen content and the surface oxygen content, is 0.8% by mass or less, and the ratio of the surface oxygen content to the internal oxygen content is 3.0 or more, silicon nitride powder.

2. A step of firing silicon powder having an oxygen concentration of 0.4% by mass or less in a mixed atmosphere containing at least one selected from the group consisting of nitrogen, hydrogen, and ammonia to obtain a fired product, and a step of treating the fired product with hydrofluoric acid having a hydrofluoric acid concentration of 10 to 40% by mass to obtain silicon nitride powder having an internal oxygen content of 0.4% by mass or less, a surface oxygen content of 0.6% by mass or more, and a total oxygen content, which is the sum of the internal oxygen content and the surface oxygen content, of 1.0% by mass or less, and a ratio of the surface oxygen content to the internal oxygen content of 1.5 or more, wherein the internal oxygen content and the surface oxygen content are obtained by setting a 0.01 g sample in an oxygen-nitrogen analyzer, heating from 20°C to 2000°C at a heating rate of 8°C / second in an atmosphere of helium gas, and during the heating, detecting peak 1 derived from surface oxygen, peak 2 derived from internal oxygen, and peak 3 derived from nitrogen, and are determined from the integrated values of peak 1 and peak 2 partitioned at the temperature at which the detection of peak 3 starts, a method for producing silicon nitride powder.

3. A method for producing silicon nitride powder according to claim 2, comprising a step of reducing oxygen in silicon powder using a pretreatment liquid containing hydrofluoric acid and hydrochloric acid to obtain the silicon powder having an oxygen concentration of 0.4% by mass or less.

4. A method for producing a silicon nitride sintered body, comprising a step of molding and firing a sintering raw material containing the silicon nitride powder produced in claim 2 or 3.

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