JP2012180234A - Method for producing silicon nitride-based ceramic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method capable of easily giving a silicon nitride-based ceramic with high thermal conductivity.SOLUTION: The production method includes: a mixing step of mixing a raw material powder which is a silicon nitride powder and a sintering aid; a compacting step of compacting a mixed powder obtained by the mixing step to form a green compact; and a sintering step of sintering the green compact in a nitrogen atmosphere to form a sintered compact of a silicon nitride-based ceramic, wherein in the mixing step, (a) wt.% of YO, (b) wt.% of HfOand (c) wt.% (c is 0-1) of SiOare added as the sintering aid, with respect to the total weight of the raw material powder and the sintering aid, MgO is not added, and the sintering aid is added so as to satisfy b/a=1-2 and a+b+c=5.5-11 wt.%.

Description

The present invention relates to a method for producing silicon nitride (Si ₃ N ₄ ) ceramics.

Silicon nitride (Si ₃ N ₄ ) ceramics are excellent in heat resistance, wear resistance, toughness, etc., and have been developed as various structural materials such as engine component materials, bearing materials, tool materials, and molten metal components. Is underway. Recently, application as a material for semiconductor substrates has also been attempted.

  Since heat dissipation is required when applied as a semiconductor substrate, attempts have been made to increase the thermal conductivity of silicon nitride ceramics. For example, Patent Document 1 achieves a thermal conductivity of 65 W / m · K or more by containing an appropriate amount of Fe, Ca, Al, Mg, and Hf oxides in a silicon nitride sintered body. Patent Document 2 discloses that when 0.3 to 3.0% by weight of Mg is converted to oxide, the sinterability is improved, and a high thermal conductivity of 50 W / m · K or more is achieved. It is described to do.

International Publication No. 2005/113466 Pamphlet Japanese Patent No. 4221006

  As described in Patent Documents 1 and 2, oxides of various elements exist as sintering aids added to silicon nitride ceramics. The types of oxides added and the combinations of the amounts added are enormous, and the types of sintering aids required to achieve silicon nitride ceramics with high thermal conductivity, combinations of the amounts added, and sintering conditions Such optimization has not been fully elucidated.

  For example, in Patent Documents 1 and 2, MgO is added, but MgO exhibits a densification effect in low-temperature sintering, and is reduced to Mg and evaporated during high-temperature sintering in a nitrogen atmosphere. Disappears. Therefore, when MgO is added, it is premised that low temperature sintering is performed in order to exert the effect, and it is impossible to improve the thermal conductivity due to silicon nitride grain growth by high temperature sintering. From this point of view, even the techniques described in Patent Documents 1 and 2 do not sufficiently optimize the additives necessary for realizing a silicon nitride ceramic having high thermal conductivity.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a production method capable of easily obtaining a silicon nitride ceramic having high thermal conductivity.

As a result of intensive studies, the inventor has found that the ratio of the content of Y ₂ O ₃ and HfO _{2 in} the silicon nitride-based ceramics is related to the thermal conductivity of the silicon nitride-based ceramics. The knowledge that the conductivity can be increased was obtained. The present inventor has focused on such findings and found the present invention.

Specifically, the method for producing a silicon nitride ceramic according to the present invention comprises mixing a raw material powder that is silicon nitride powder and a sintering aid, and molding the mixed powder obtained by the mixing step. As a sintering aid in the mixing step, including a forming step for producing a green compact and a sintering step for producing a sintered body of silicon nitride ceramic by firing the green compact in a nitrogen atmosphere. The weight ratio of the raw material powder and the sintering aid to the total weight is a weight percent Y ₂ O ₃ , b weight percent HfO ₂ , and c weight percent (c is 0 or more and 1 or less) SiO _2. And adding MgO, and further adding a sintering aid so that b / a is 1 or more and 2 or less and a + b + c is 5.5% by weight or more and 11% by weight or less. Features.

The method for producing a silicon nitride-based ceramic of the present invention includes a mixing step of mixing a raw material powder that is either metal Si powder or a mixture of silicon nitride powder and metal Si powder, and a sintering aid, A forming step for forming a green compact by forming the mixed powder obtained by the mixing step, and a nitriding step for nitriding the metal Si in the green compact by firing the green compact in a nitrogen atmosphere. And a sintering step for producing a sintered body of silicon nitride ceramics by firing the green compact after the nitriding step in a nitrogen atmosphere, and in the mixing step, as a sintering aid, Y ₂ O ₃ having a weight ratio of a weight% to the total weight of the weight of the raw material powder and the weight of the sintering aid when it is assumed that the metal Si powder in the raw powder is nitrided, and b weight% HfO ₂ and c weight % (C is 0 or more and 1 or less) of SiO ₂ , MgO is not added, b / a is 1 or more and 2 or less, and a + b + c is 5.5% by weight or more and 11% by weight or less. Thus, a sintering aid is added.

In the above configuration, “without adding MgO as a sintering aid” means that MgO is not added intentionally, and is included as an impurity of other materials such as Y ₂ O ₃ and HfO _2. This does not exclude the addition of MgO. That is, in the case where only the inevitable MgO contained as an impurity of other materials is contained in the silicon nitride ceramic, it is included in the scope of the present invention as long as other configurations are satisfied.

It was confirmed that a silicon nitride ceramic with high thermal conductivity can be easily obtained with the above configuration. This is because the temperature at which the liquid phase of the Y ₂ O ₃ —HfO ₂ —SiO ₂ compound produced as a grain boundary phase starts to rise increases (1) the grain growth of the silicon nitride crystal to the maximum, 2) It is considered that the amount of the glass component of the grain boundary phase can be further reduced, and (3) the amount of unnecessary elements in the silicon nitride crystal can be reduced. Due to the effect of (1), the silicon nitride crystal occupying the ceramic can be enlarged, and heat can be more easily conducted. Moreover, the thermal conductivity of the grain boundary phase can be improved by the effect (2), and the thermal conductivity of the silicon nitride crystal itself can be improved by the effect (3).

  Furthermore, in the production method of the present invention, in the sintering step, the nitrogen pressure is preferably 0.5 MPa or more, and preferably fired at 1850 ° C. or more.

According to the above configuration, even when the temperature at which the liquid phase of the Y ₂ O ₃ —HfO ₂ —SiO ₂ compound produced as the grain boundary phase starts to rise, the nitrogen pressure is set to 0.5 MP or more. The decomposition temperature of silicon nitride is increased, and liquid phase sintering can be performed at a higher temperature. As a result, grain growth of the silicon nitride crystal can be promoted. In addition, grain growth can be further promoted by firing at 1925 ° C. or higher.

  Moreover, when said c is 0, it is preferable that a + b is 7.5 to 10 weight%. Thereby, it becomes easy to produce | generate the appropriate amount compound which comprises a grain boundary phase.

  According to the present invention, it is possible to easily obtain a silicon nitride ceramic having high thermal conductivity.

It is a flowchart which shows the flow of the manufacturing method of the silicon nitride ceramics concerning Embodiment 1 of this invention. It is a flowchart which shows the flow of the manufacturing method of the silicon nitride ceramics concerning Embodiment 2 of this invention. 4 is a SEM photograph of the sintered body of Example 2. FIG. It is a figure which shows the measurement result of the insulation resistance of Example 2. FIG.

  The present invention provides a manufacturing method capable of easily manufacturing silicon nitride ceramics having high thermal conductivity. The thermal conductivity of silicon nitride ceramics depends on both the thermal conductivity in the silicon nitride crystal and the thermal conductivity in the grain boundary phase. Therefore, in order to increase the thermal conductivity of the silicon nitride ceramics, it is necessary to increase both the thermal conductivity of the silicon nitride crystal and the grain boundary phase.

As a result of intensive studies, the present inventors have found that the content ratio of the sintering aids Y ₂ O ₃ and HfO _{2 in} the silicon nitride ceramics is related to the thermal conductivity of the silicon nitride ceramics. It was found that the thermal conductivity can be increased by controlling the ratio. The present invention has been made based on this finding.

Specifically, in the method for producing a silicon nitride ceramic, the total weight ratio of the raw material powder (silicon nitride) and the sintering aid is a weight percent of Y ₂ O ₃ , b weight percent of HfO ₂ , c % By weight (where c is 0 or more and 1 or less) of SiO ₂ , MgO is not added, b / a is 1 or more and 2 or less, and a + b + c is 5.5% by weight or more and 11% by weight % addition of sintering aids to be equal to or less than _to adjust the Y 2 _{O 3} and HfO ₂ and SiO _2.

Regarding HfO ₂ , for example, Patent Document 1 describes the function of crystallizing the grain boundary phase. Patent Document 2 describes 0.3 to 3% by mass, preferably 1.0 to 2.5% by mass, as the amount of HfO ₂ added. Thus, although the function of crystallization of the grain boundary phase was known as a function of HfO ₂ , the amount added was preferably lower than 3% by mass. In this regard, for example, Japanese Patent Publication No. 3115238 describes that when the amount of HfO ₂ added exceeds 3.0% by weight, the thermal conductivity and mechanical strength decrease. However, the inventors set the ratio b / a of Y ₂ O ₃ and HfO ₂ to 1 or more and 2 or less, and a + b + c to 5.5 wt% or more and 11 wt% or less, so that HfO ₂ Although the content is 3% by weight or more, the inventors have obtained a different knowledge from that that the thermal conductivity can be improved. Such an improvement in thermal conductivity is considered to be due to the following reason.

Y ₂ O _3, which is a sintering aid, contributes to densification and has a function of promoting grain growth of columnar crystals. This is because a compound of Y ₂ O ₃ and silicon oxide formed on the surface of silicon nitride (for example, Y ₂ Si ₂ O ₇ or Y ₂ SiO ₅ ) is generated, and the compound becomes a liquid phase. This is because it promotes ties. When _the further addition of HfO ₂ in Y 2 _{O 3,} the compound of Y ₂ O ₃ -HfO 2 -SiO ₂ system is present in the grain boundary, the sintering by the grain boundary in the liquid phase It will be promoted. Conventionally, when a Y ₂ O ₃ —HfO ₂ —SiO ₂ -based compound is present at the grain boundary, it has been usual to increase the amount of Y ₂ O ₃ added to that of HfO ₂ . _This, Y 2 amount of _{O 3} is present eutectic point of the compound of _{Y 2 O} 3 -HfO 2 -SiO 2 system in the region greater than the amount of HfO _2, form a liquid phase from the lower temperature This is because the grain growth of the silicon nitride crystal is promoted by adjusting the grain boundary component.

However, contrary to such conventional knowledge, the ratio b / a of the weight ratio (content) a of Y ₂ O _{3 and} the weight ratio (content) b of HfO ₂ is 1 or more and 2 or less. It was confirmed that the thermal conductivity of the silicon nitride ceramic was improved. This is because the ratio b / a of the weight ratio a of Y ₂ O _{3 and} the weight ratio b of HfO ₂ is 1 or more and 2 or less, so that SiO ₂ generated on the surface of the sintering aid and the silicon nitride or the firing is increased. The temperature at which the liquid phase of the Y ₂ O ₃ —HfO ₂ —SiO ₂ compound produced with SiO ₂ added as a co-agent is started is around 1900 ° C. (slightly lower than the decomposition temperature of silicon nitride). It is thought that it is to become. That is, the temperature at which the liquid phase of the Y ₂ O ₃ —HfO ₂ —SiO ₂ compound produced as a grain boundary phase starts to be close to the decomposition temperature of silicon nitride, The temperature is set slightly lower than the decomposition temperature. As a result, liquid phase sintering is performed at a higher temperature, and the grain growth of the silicon nitride crystal is promoted to the maximum extent.

In addition, since the temperature at which the liquid phase of the produced Y ₂ O ₃ —HfO ₂ —SiO ₂ compound starts to be close to the firing temperature, HfO ₂ is easily recrystallized when the temperature is lowered from the sintering temperature, and the grain boundary The amount of the glass component of the phase can be further reduced.

Further, when HfO ₂ is recrystallized, it is considered that impurities in the silicon nitride crystal are taken in and the silicon nitride crystal is purified. Therefore, as described above, since HfO ₂ is easily recrystallized, the amount of impurities taken in by HfO ₂ is increased, the amount of unnecessary elements in the silicon nitride crystal is reduced, and a higher-purity silicon nitride crystal is generated. It will be.

Thus, by setting the ratio b / a of the weight ratio (content) a of Y ₂ O _{3 and} the weight ratio (content) b of HfO ₂ to 1 or more and 2 or less, ((1) the silicon nitride crystal This is considered to be because the grain growth was promoted to the maximum, (2) the amount of glass components in the grain boundary phase was further reduced, and (3) the amount of unnecessary elements in the silicon nitride crystal was reduced. The effect of (1) can increase the size of silicon nitride crystals in the ceramic, making it easier to conduct heat, and the effect of (2) also improves the thermal conductivity of the grain boundary phase. In addition, the thermal conductivity of the silicon nitride crystal itself can be improved by the effect (3), and as a result, it is considered that the thermal conductivity of the silicon nitride ceramic is improved.

  In addition, when a + b + c is less than 5.5% by weight, the effects (1) to (3) are small, and when it is greater than 11% by weight, the grain boundary component is excessive and the thermal conductivity is lowered. Therefore, a + b + c is set to 5.5 wt% or more and 11 wt% or less.

  It is also possible to add other sintering aids, but the sintering temperature is higher than 1850 ° C. and is easily reduced and evaporated when sintered in a nitrogen atmosphere. An auxiliary agent (for example, MgO) is not added. This is because it functions as a sintering aid even when sintering at a temperature exceeding 1850 ° C., and the silicon nitride crystal is grown to increase the thermal conductivity of the silicon nitride crystal.

Hereinafter, a method for producing a silicon nitride ceramic according to an embodiment of the present invention will be described.
<Embodiment 1>
FIG. 1 is a diagram illustrating a flow of steps of the manufacturing method according to the first embodiment.

(S1. Preparation step)
First, silicon nitride powder and powders of Y ₂ O ₃ and HfO ₂ that are sintering aids are prepared as raw materials for silicon nitride ceramics. Incidentally, as a sintering aid, it may be added further to SiO _2. However, as described above, a sintering aid (for example, MgO) that easily disappears when sintered at a temperature exceeding 1850 ° C. is not added.

At this time, the weight ratio of Y ₂ O ₃ to the total weight of the silicon nitride powder and the sintering aid powder is a weight%, the weight ratio of HfO ₂ is b weight%, and the weight ratio of SiO ₂ is c weight%. In this case, b / a is set to 1 or more and 2 or less. Further, a + b + c is set to 5.5 wt% or more and 11 wt% or less. However, c is 0 or more and 1 or less. The reason for setting a, b, and c in this way is as described above.

In the case where the weight ratio of SiO ₂ is more than 1 wt%, SiO ₂ becomes excessive relative to the _{Y 2 O} 3 -HfO 2 -SiO 2 type compounds generated as a grain boundary phase.

When c = 0, a + b is preferably 7.5% by weight or more and 10% by weight or less. When c = 0, Y ₂ O ₃ and HfO ₂ react with SiO ₂ formed on the surface of the silicon nitride raw material to produce a Y ₂ O ₃ —HfO ₂ —SiO ₂ compound. The amount of SiO ₂ formed on the surface of the silicon nitride raw material also depends on the type and particle size of the silicon nitride powder. Therefore, even if the amount of SiO ₂ formed on the surface of the silicon nitride raw material is small, if a + b is 7.5% by weight or more, Y ₂ O ₃ and HfO ₂ present in the vicinity of SiO ₂ The amount increases, and an appropriate amount of Y ₂ O ₃ —HfO ₂ —SiO ₂ compound is more easily generated.

(S3. Mixing and granulating process)
Next, the powder obtained in the blending process is mixed (mixing process). As the mixing method, for example, wet mixing using a ball mill can be used, but the mixing method is not limited thereto. Specifically, it may be mixed for 24 hours using a Si ₃ N ₄ ball having a ball diameter of 10 mm. Thereafter, the mixed powder of each powder is dried and granulated (granulation step). At this time, a binder resin or the like may be appropriately added to form granules.

(S5. Molding process)
Subsequently, the granulated product is filled into a mold having a predetermined shape, and pressure-molded to form a green compact. As the pressure forming method, a uniaxial press forming method, a cold isostatic pressing method (CIP) method, or the like can be used. Moreover, after temporary molding by a uniaxial press molding method, the main molding may be performed using CIP.

(S7. Degreasing process)
The green compact obtained in the molding process contains organic substances such as a binder resin added in the granulation process. Therefore, degreasing treatment for removing these organic substances is performed by raising the temperature to 250 to 500 ° C. in air, for example.

(S9. Sintering process)
Subsequently, the green compact is densified by firing in a nitrogen atmosphere. The set maximum temperature of the firing temperature is preferably 1850 ° C. or higher. Further, it is preferable that the surrounding nitrogen pressure is 0.5 MPa or more in at least a temperature range from the set maximum temperature to a temperature lower by 100 ° C. than the set maximum temperature. Thereby, silicon nitride can be prevented from being decomposed and fired at a high temperature of 1850 ° C. or higher.

Furthermore, it is preferable that the set maximum temperature of the firing temperature is 1925 ° C. or higher. Furthermore, it is preferable that it is 1950 degreeC or more. Thereby, the grain growth of the silicon nitride crystal can be further promoted.
<Embodiment 2>
Next, a method for producing a silicon nitride ceramic according to another embodiment will be described. The manufacturing method of this embodiment is a method of manufacturing silicon nitride ceramics using metal Si powder as a starting material. FIG. 2 is a diagram illustrating a process flow of the manufacturing method according to the second embodiment.

(S1a. Raw material preparation step)
First, metal Si powder, which is a raw material powder of silicon nitride ceramics, and Y ₂ O ₃ and HfO ₂ powders, which are sintering aids, are prepared. Incidentally, as a sintering aid, it may be further added to the SiO ₂ and the like. However, as described above, a sintering aid (for example, MgO) that disappears when sintering at a temperature exceeding 1850 ° C. is not added. In addition, powders of various sintering aids are prepared. Moreover, you may add a silicon nitride powder with metal Si powder. In this case, a mixture of metal Si powder and silicon nitride powder becomes the raw material powder.

(S2a. Grinding step)
Next, the metal Si powder is pulverized by a wet pulverizer. There are various types of wet pulverizers, but it is preferable to use a bead mill capable of fine graining. A bead mill is a microscopic device in which a bead (pulverization media, bead diameter 0.015 to 2 mm) is filled in a container called a vessel and rotated, and a slurry in which raw material powder is mixed is fed into a liquid and collided with the beads. A crusher for crushing. Note that it is preferable to use Si ₃ N ₄ as the material of the beads. Thereby, mixing of impurities can be prevented. In addition, it is preferable to add various dispersants to the slurry to control the slurry viscosity.

(S3a. Mixing and granulating process)
Next, the pulverized raw material powder and the sintering aid are mixed so as to have a predetermined composition ratio (mixing step).

At this time, the weight ratio of Y ₂ O _{3 to} the weight of the raw material powder when the metal Si powder is nitrided and the total weight of the sintering aid is a weight%, and the weight ratio of HfO ₂ is b weight. %, When the weight ratio of SiO ₂ is c wt%, b / a is 1 or more and 2 or less. Further, a + b + c is set to 5.5 wt% or more and 11 wt% or less. However, c is 0 or more and 1 or less. The reason for setting a, b, and c in this way is as described above.

  When only the metal Si powder is used as the raw material powder, the metal Si powder is nitrided by multiplying the weight of the metal Si powder by (Si atomic weight × 3 + N atomic weight × 4) / (Si atomic weight × 3). It is possible to determine the weight of the raw material powder when it is assumed. Further, when a mixture of metal Si powder and silicon nitride powder is used as a raw material powder, a value obtained by multiplying the weight of the metal Si powder by (atom weight of Si × 3 + atom weight of N × 4) / (atom weight of Si × 3); The sum of the weight of the silicon nitride powder is the weight of the raw material powder when the metal Si powder is assumed to be nitrided.

As a mixing method, for example, wet mixing may be performed using Si ₃ N ₄ balls having a ball diameter of 10 mm. Thereafter, the mixed powder of the raw material powder and the sintering aid is dried and granulated (granulation step). At this time, a binder resin or the like may be appropriately added to form granules. Then, (S5. Molding process) and (S7. Degreasing process) are performed as in the first embodiment.

  In the granulation step, a pore forming agent that forms pores by melting or vaporizing, such as a phenol resin, may be added. By adding a pore-forming agent, S5. In the degreasing process, the pore-forming agent is melted or vaporized, and fine pores are formed in the green compact. As a result, the contact area between the metal Si and nitrogen can be increased, and the nitridation of the metal Si can be promoted in the nitriding step described later.

(S8. Nitriding process (reaction sintering process))
Next, the defatted green compact is fired in the range of 1200 to 1450 ° C. in a nitrogen atmosphere, thereby nitriding metal Si. Here, the firing temperature and the holding time thereof may be appropriately set depending on the size of the green compact and the like.

  In this embodiment, the metal Si powder is finely pulverized by a bead mill. Therefore, the surface area of the metal Si that comes into contact with nitrogen is increased, and the reaction with nitrogen is facilitated. As a result, the firing temperature in the nitriding step can be set lower than in the prior art.

  Then, (S9. Sintering process) is performed similarly to Embodiment 1 with respect to the reaction sintered compact which is the sintered compact obtained by the nitriding process.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  Hereinafter, the present invention will be specifically described based on examples. However, the present invention is not limited to these examples.

<Examples 1-17, Comparative Examples 1-7>
Samples of Examples 1 to 17 and Comparative Examples 1 to 7 were prepared according to the method described in Embodiment 1 above.

(S1. Preparation step)
The following were used as starting materials.
・ Silicon nitride powder: “SN-E10” manufactured by Ube Industries, Ltd.
Y ₂ O ₃ : “RU-P” manufactured by Shin-Etsu Chemical Co., Ltd.
・ HfO ₂ : manufactured by High Purity Chemical Laboratory Co., Ltd. ・ SiO ₂ : “So-C2” manufactured by Admatechs Co., Ltd.
The weight ratio is as described in Table 1 described later.

(S3. Mixing and granulating process)
100 g of the mixed powder obtained in the step S1 and 2.0 ml of a dispersant (“Celna E503” manufactured by Chukyo Yushi) were added to 120 ml of ethanol, and ball mill mixing was performed for 24 hours using Si ₃ N ₄ balls having a ball diameter of 5 mm. . Then, 4 wt% of paraffin and 1 wt% of DOP (dioctyl phthalate) were added to 100 g of the mixed powder and granulated.

(S5. Molding process)
Thereafter, using a mold having a cylindrical hollow portion having an outer diameter of 11 mm, a pressure of 50 MPa was applied for 30 seconds by a uniaxial press molding method. Furthermore, it was molded by applying a pressure of 200 MPa for 60 seconds by a cold isostatic press (CIP) method. Thereby, the compacted green compact was obtained.

(S7. Degreasing process) (S9. Sintering process)
The obtained green compact was held in air at 250 ° C. for 3 hours and then held at 500 ° C. for 3 hours for degreasing. Then, about each Example, the sintering process was performed on the baking conditions of Table 1 mentioned later. In the sintering step, firing was performed while flowing 0.9 MPa of nitrogen at a rate of 4 l / min.

<Examples 18 to 21>
Samples of Examples 18 to 21 were prepared according to the method described in Embodiment 2 above.

(S1a. Raw material preparation step)
As metal Si powder as a starting material, # 600 powder manufactured by Yamaishi Metal Co., Ltd. was used. The same raw material as Examples 1-17 was used for the sintering aid.

(S2a. Grinding step)
“Minizea” manufactured by Ashizawa Finetech was used as the bead mill, and 290 g of Si ₃ N ₄ beads having a bead diameter of 0.5 mm were used as the grinding media. Then, a slurry obtained by mixing 1200 g of metal Si powder and 4000 g of ethanol was passed through a bead mill for 10 passes and pulverized. The rotation speed of the bead mill is 3000 rpm.

(S3. Mixing and granulating process)
The composition ratio between the ground metal Si powder and the sintering aid was adjusted and mixed. The weight ratio of each Example is as having described in Table 1 mentioned later. Table 1 shows the composition ratio of silicon nitride and sintering aid when the metal Si powder is nitrided. Therefore, the blending amount of the metal Si powder is a value obtained by multiplying the weight of silicon nitride (Si ₃ N ₄ ) by (Si atomic weight × 3) / (Si atomic weight × 3 + N atomic weight × 4). In Examples 18 to 21, no silicon nitride powder was added.

Specifically, 57 g of a mixed powder of a bead mill pulverized product (metal Si powder) and a sintering aid, and a pore-forming agent (“Bellpearl R200” manufactured by Air Water Velpearl Co., Ltd.) are added to 120 ml of ethanol, Ball mill mixing was performed for 24 hours using Si ₃ N ₄ balls having a ball diameter of 5 mm. In addition, the addition amount of the pore forming agent was 30 vol% with respect to the total volume of the mixed powder and the pore forming agent. Then, 4 wt% of paraffin, 6 wt% of DOP (dioctyl phthalate), and 35 ml / 100 g of cyclohexane were added to 100 g of the mixed powder of each example and granulated.

(S5. Molding process)
Thereafter, using a mold having a cylindrical hollow portion having an outer diameter of 11 mm, a pressure of 6 MPa was applied for 30 seconds by a uniaxial press molding method. Furthermore, it was molded by applying a pressure of 200 MPa for 60 seconds by a cold isostatic press (CIP) method. Thereby, the compacted green compact was obtained.

(S5. Degreasing step) (S8. Nitriding step) (S9. Densification step)
The obtained green compact was held at 500 ° C. for 3 hours for degreasing. Further, in the nitriding step, nitriding was performed by holding at 1400 ° C. for 8 hours in a nitrogen atmosphere. Then, about each Example, the sintering process was performed on the baking conditions of Table 1 mentioned later. In the sintering step, firing was performed while flowing 0.9 MPa of nitrogen at a rate of 4 l / min.

<Evaluation>
The relative density was measured about the sintered compact after a sintering process. The relative density is obtained by calculating the ratio of the measured density by the Archimedes method to the theoretical density. As shown in Table 1, the relative density of each sample was 96% or more, and it was confirmed that a sufficient sintered body was obtained.

  FIG. 3 is a SEM (scanning electron microscope) photograph showing the sintered body of Example 2. As shown in FIG. 3, it can be seen that columnar silicon nitride crystals are generated and the sintered body is densified without voids.

  Next, thermal conductivity was measured by a laser flash method for all examples and comparative examples. As a measuring instrument, “LF / TCM-FA8510B” manufactured by Rigaku Corporation was used. Table 1 shows the measurement results of thermal conductivity.

As shown in Table 1, the ratio b / a of the content a wt% of Y ₂ O _{3 and} the content b wt% of HfO ₂ is 1 or more and 2 or less, and a + b + c is 5.5 wt% or more and 11 or less. In Examples 1-20 which satisfy | fill weight% or less, it was confirmed that heat conductivity becomes 80 W / mK or more.

On the other hand, in Comparative Examples 1, 2, 6, and 7 in which the ratio b / a was less than 1, the thermal conductivity was low. This is because the difference between the temperature at which the liquid phase of the Y ₂ O ₃ —HfO ₂ —SiO ₂ compound constituting the grain boundary phase starts and the sintering temperature is large, and the glass component tends to remain in the grain boundary phase, and It is considered that the effect of taking in the amount of unnecessary elements in the silicon nitride crystal by HfO ₂ was small. Further, even in Comparative Example 5 in which the ratio b / a exceeded 2, the thermal conductivity was low. This is because the grain boundary component is unlikely to become a liquid phase.

  Further, Comparative Examples 3 and 4 in which the value of a + b + c was 16 also had low thermal conductivity. This is because the grain boundary phase has become too much.

For the sample of Example 2, the three-point bending strength was measured by a three-point bending test with a span of 30 mm using a rectangular test piece of 3 × 4 × 35 mm based on JISR1601. As a result, it was confirmed that the strength was as high as 777 MPa. Moreover, the result of having measured the insulation resistance about the sample of Example 2 is shown in FIG. As shown in the figure, it was confirmed that a high insulation resistance was exhibited even when the electrolytic strength exceeded 10 ³ V / cm. Thus, it can be said that the silicon nitride ceramic of this example is a material that can be practically used as a semiconductor substrate because it has high strength and insulation resistance.

  The present invention can be used for silicon nitride ceramics used for semiconductor substrates and the like.

Claims (5)

A mixing step of mixing the raw material powder that is silicon nitride powder and the sintering aid;
A molding step for producing a green compact by molding the mixed powder obtained by the mixing step;
A sintering step of producing a sintered body of silicon nitride ceramics by firing the green compact in a nitrogen atmosphere,
In the mixing step, as a sintering aid, the weight ratio of the raw material powder and the sintering aid to the total weight is a weight percent of Y ₂ O ₃ , b weight percent of HfO ₂ , c weight percent ( c is 0 or more and 1 or less) SiO ₂ is added, MgO is not added, and b / a is 1 or more and 2 or less, and a + b + c is 5.5% or more and 11% or less by weight. A method for producing silicon nitride ceramics, comprising adding a sintering aid to A mixing step of mixing a raw material powder that is either metal Si powder or a mixture of silicon nitride powder and metal Si powder, and a sintering aid;
A molding step for producing a green compact by molding the mixed powder obtained by the mixing step;
A nitriding step of nitriding the metal Si in the green compact by firing the green compact in a nitrogen atmosphere;
And sintering the green compact after the nitriding step to produce a sintered body of silicon nitride ceramics by firing in a nitrogen atmosphere,
In the mixing step, as a sintering aid, the weight ratio of the weight of the raw material powder and the weight of the sintering aid when the metal Si powder in the raw material powder is nitrided is a weight. % Y ₂ O ₃ , b wt% HfO ₂ , c wt% (c is 0 or more and 1 or less) SiO ₂ , MgO is not added, and b / a is 1 or more and 2 A method for producing a silicon nitride-based ceramic, comprising adding a sintering aid so that a + b + c is 5.5 wt% or more and 11 wt% or less.   3. The method for producing a silicon nitride-based ceramic according to claim 1, wherein in the sintering step, the nitrogen pressure is 0.5 MPa or more and firing is performed at 1850 ° C. or more.   4. The method for producing a silicon nitride ceramic according to claim 3, wherein the sintering step includes firing at 1925 [deg.] C. or higher.   5. The method for producing a silicon nitride-based ceramic according to claim 1, wherein c is 0 and a + b is 7.5 wt% or more and 10 wt% or less.