JPH07267614A - Production of silicon nitride powder, sintered compact of silicon nitride and its production - Google Patents

Abstract

PURPOSE:To provide a method for producing silicon nitride powder having uniform properties independently of a lot of raw material powder, obtain a silicon nitride sintered compact, having a high density and strength and prepared by using the powder and provide a method for producing the sintered compact. CONSTITUTION:This method for producing silicon nitride powder is to add water to silicon nitride raw material powder, provide a mixture, then mix and pulverize the mixture while keeping the temperature thereof at 30-80 deg.C and the viscosity thereof at 20-1000cP, slurry the mixture and regulate the oxygen content in the resultant silicon nitride powder to 0.5-3.0wt.% and further the ratio (0s/Ss) of the amount of oxygen (Os) on the surface of the powder to the specific surface area (Ss) to 30-100. This silicon nitride sintered compact is obtained by using the prepared silicon nitride powder and has >=20mm wall thickness. This method for producing the silicon nitride sintered compact is to form the resultant silicon nitride powder and sinter the formed compact.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride powder, and more specifically, a method for producing a silicon nitride powder having specific properties and a thick and high strength obtained by using the same. The present invention relates to a high-density silicon nitride sintered body and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been attracting attention because of its high strength at high temperature, chemical stability, etc., and its application to structural materials for heat engines such as diesel and gas turbines. Are being studied. In general, silicon nitride is difficult to burn, so that in order to improve the sinterability, a sintering aid such as MgO, Y ₂ O ₃ or the like is added to silicon nitride powder, and the mixture is molded and fired to obtain a suitable property. A silicon nitride sintered body having strength is manufactured.

In such a method for producing silicon nitride,
When the temperature of the liquid phase generated during firing is high, the liquid phase is generated, the phase transformation from α-Si ₃ N ₄ to β-Si ₃ N ₄ , β
To be controlled crystal growth and densification of -Si ₃ N _4, it is known that not be obtained sintered bodies of high density and high strength. In such a situation, for a thin-walled product, a high-density and high-strength sintered body is obtained by highly controlling the firing temperature, the temperature raising rate, the temperature lowering rate, the firing atmosphere and the like.

On the other hand, for thick-walled products, since the liquid phase generation rate and the phase transformation temperature differ depending on the atmosphere, temperature, etc. on the surface and inside of the molded body, the material preparation technique and firing applied to conventional thin-walled products. Even with control technology, the wall thickness is 2
There is a problem that it is not possible to obtain a fired body of 0 mm or more at a high density. On the other hand, the applicant of the present application filed Japanese Patent Application No.
No. 64203, in order to control the behavior of SiO ₂ involved in the liquid phase formation and the phase transformation of silicon nitride when firing a silicon nitride sintered body, the amount of oxygen in the vicinity of the surface of the silicon nitride powder (the amount of surface effective oxygen It has been proposed that a thick silicon nitride sintered body with high density and high strength can be obtained by adjusting (1).

[0005]

However, as a result of further study by the present inventors, when adjusting the amount of surface effective oxygen of the silicon nitride powder, water was particularly added as a solvent to form a slurry, which was crushed. However, when adjusting the amount of oxygen, the viscosity of the slurry may vary depending on the lot of the raw material powder of silicon nitride, or the viscosity may change during pulverization, so that the desired amount of effective surface oxygen cannot be obtained. It turned out to be. The present invention has been made in view of such findings, and an object thereof is to provide a method for producing a silicon nitride powder having a uniform property that does not depend on a lot of raw material powder, and to obtain using the same. High density,
It is to provide a high-strength silicon nitride sintered body and a method for manufacturing the same.

[0006]

Means for Solving the Problems As a result of intensive research to achieve the above object, the present inventor has found that the above object can be achieved by appropriately controlling the temperature and viscosity of the slurry in the slurry pulverizing step. The present invention has been completed. Therefore, in the method for producing a silicon nitride powder of the present invention, water is added to the silicon nitride raw material powder, the temperature of the obtained mixture is 30 to 80 ° C., and the viscosity is 20 to 1000 cP.
The oxygen content of the obtained silicon nitride powder is 0.5 to 3.
This silicon nitride powder was adjusted to 0% by weight, and the silicon nitride powder had the following formula Os / Ss = 30 to 100 (where Os is the oxygen amount (% by weight) on the powder surface, and Ss is the specific surface area of the powder (m ² / mg))) is adjusted so as to satisfy the amount of surface effective oxygen represented by

The method for manufacturing a silicon nitride sintered body of the present invention is characterized in that the silicon nitride powder obtained by the above manufacturing method is molded and fired. Further, the silicon nitride sintered body of the present invention is the silicon nitride sintered body obtained by the above manufacturing method, and is characterized by having a wall thickness of 20 mm or more.

[0008]

In the present invention, the temperature and viscosity of the slurry are appropriately controlled in the step of pulverizing the silicon nitride raw material powder slurry. Thus, by controlling the temperature and viscosity of the slurry, the surface effective oxygen amount of the obtained silicon nitride powder, that is, the reason that the ratio between the oxygen amount on the powder surface and the specific surface area of the powder can be appropriately controlled is It is considered to be as follows.

It is known that silicon nitride chemically reacts with water to form SiO ₂ as shown by the following formula. Si ₃ N ₄ + 6H ₂ O → 3SiO ₂ + 4NH _{3 The} above reaction proceeds in the step of wet pulverizing the silicon nitride raw material powder, but at the same time, the silicon nitride raw material powder collides with grinding media such as boulders and It is crushed by being sandwiched between or separated. As described above, the wet pulverization step has two functions for the silicon nitride raw material powder to cause the oxidation reaction of the surface of the silicon nitride raw material powder and the pulverization of the silicon nitride raw material powder. In such a wet pulverization process, it is also an important matter to consider that the chemical reaction on the surface of the silicon nitride raw material powder is promoted by the so-called mechanochemical action of pulverization.

Therefore, in order to control the specific surface area and the amount of surface oxygen of the obtained silicon nitride powder to desired values, the reaction rate of the chemical reaction and the crushing rate as described above should also be considered in consideration of the mechanochemical action. It is necessary to appropriately control the temperature, and for that purpose, it can be considered that it is extremely effective to keep the temperature and viscosity of the slurry constant during the pulverization as in the present invention.

As described above, according to the present invention, not only the chemical reaction speed but also the grinding speed can be controlled by keeping the temperature and viscosity of the slurry during grinding constant.
Therefore, it is possible to provide a silicon nitride powder in which the variation in the amount of surface effective oxygen defined by the surface oxygen amount and the specific surface area is extremely small. As a result, it is possible to provide a silicon nitride powder having a certain characteristic regardless of the lot of the silicon nitride raw material powder. It is also possible to obtain a thick and high-strength silicon nitride sintered body. The viscosity of the slurry containing the silicon nitride raw material powder can also be controlled by adjusting the pH of this slurry.
According to the adjustment, there is also an advantage that it can be controlled in real time in the manufacturing process of the silicon nitride powder.

In the present invention, the grain boundary phase of the sintered body is sintered by controlling the oxygen content of the silicon nitride powder, particularly the oxygen content in the vicinity of the surface (for example, the area from the surface to several nm). The following reasons can be considered as a reason for obtaining a thick, high-density, high-strength silicon nitride sintered body having a uniform structure from the surface to the inside of the body. That is, in the sintering process of the silicon nitride sintered body, first, the sintering aid (metal oxide) acts to generate a liquid phase (glass phase). Next, α-Si ₃ N ₄ is dissolved in this liquid phase to precipitate as β-Si ₃ N ₄ . In order for this dissolution-precipitation reaction to occur uniformly inside the sintered body, uniform liquid phase generation in a wide area is essential.

By the way, not only the sintering aid but also SiO ₂ existing on the surface of the Si ₃ N ₄ powder influences the formation of the liquid phase. For example, in the sintering of Si ₃ N ₄ containing a sintering aid having a considerably low melting point such as MgO or CaO, the sintering aid (metal oxide) and SiO ₂ react at a considerably low temperature to form a liquid phase. To generate. On the other hand, in the auxiliary agent containing a rare earth element oxide as a main component, the rare earth element oxide and SiO
Eutectic temperature of the composite oxide of ₂ higher than the melting point of SiO _2, SiO ₂ content present in Si ₃ N ₄ powder surface becomes large influence on the liquid phase product of該助agent. Therefore,
It is considered that the presence of Si ₃ N ₄ particles having an appropriate SiO ₂ coating is important for achieving uniform liquid phase generation in a wide area.

The method for producing the silicon nitride powder of the present invention will be described in detail below. In this manufacturing method, the acid content of the obtained silicon nitride powder is adjusted to 0.5 to 3.0% by weight, and the silicon nitride powder has the following formula Os / Ss = 30 to 100 (Os in the formula: Is the surface oxygen content (% by weight) of the powder, and Ss is the specific surface area (m ² / mg) of the powder). Then, in this manufacturing method, in order to obtain the silicon nitride powder having the above characteristics, water is added to the silicon nitride raw material to obtain a mixture, the temperature of the obtained mixture is 30 to 80 ° C., and the viscosity of the mixture is Is maintained at 20 to 1000 cP and mixed and pulverized to make a slurry of this mixture.

When the above Os / Ss is less than 30, the obtained Si ₃ N ₄ powder has a small amount of Os or a large amount of Ss, which is a state in which the amount of Os per surface area of the Si ₃ N ₄ powder is small. Indicates. When the silicon nitride sintered body is fired using the Si ₃ N ₄ powder in such a state, it is not possible to generate a uniform liquid phase in a wide range, and Si ₃ N ₄ containing a sintering aid is present.
Dissolution of Si ₃ N ₄ particles only in the vicinity of powder (particles)
This is not preferable because the precipitation reaction proceeds, and it becomes easy to obtain a silicon nitride sintered body that does not have uniform properties such as strength.

On the other hand, when Os / Ss exceeds 100, O
If s is large or Ss is small, the amount of oxygen per surface area of the obtained Si ₃ N ₄ powder is too large. For this reason, in the firing of the silicon nitride sintered body, liquid phase generation proceeds more than necessary, and the dissolution / precipitation reaction locally causes the shrinkage of the silicon nitride compact to be fired.
If such a phenomenon occurs during the firing process of the thick-walled molded product, the shrinkage proceeds near the surface having a high temperature, which hinders the internal shrinkage. Furthermore, since β-Si ₃ N ₄ does not precipitate uniformly from the liquid phase, a small amount of β-Si ₃ N ₄ becomes nuclei and abnormal grain growth occurs. And, this is not preferable because it causes the generation of pores in the vicinity of the Si ₃ N ₄ particles with abnormal grain growth. In addition,
Oxygen content and surface effective oxygen content Os of the above silicon nitride powder
/ Ss is 1.0 to 2.5% by weight and 50, respectively.
It is more preferable to control to be 70.

The slurry of the silicon nitride raw material powder will be described. First, it is preferable to adjust the water content of the mixture of the silicon nitride raw material powder and water to 30 to 80% by weight. If the water content is less than 30% by weight, it is difficult to form a slurry, and if it exceeds 80% by weight, the amount of water evaporated during drying becomes too large, which is not preferable. The temperature is controlled to 30 to 80 ° C. when making the mixture into a slurry. When the temperature is lower than 30 ° C, the reaction rate is low and the production efficiency is low, and when the temperature is higher than 80 ° C, the water content changes due to water evaporation, which is not preferable. Further, the viscosity of the mixture (slurry) is controlled to 20 to 1000 cP. If it is less than 20 cP, the crushing action is too large to control, and 1000
If it exceeds cP, the crushing action is not performed, which is not preferable.

The viscosity of the slurry can be adjusted by adjusting the pH of the slurry.
This is because the change in pH is accompanied by electron transfer.
This is an example of an adjusting method based on a change in the zeta (ζ) potential that affects the dispersibility (viscosity) of the slurry due to the change. In the above-mentioned reaction between silicon nitride and water, the pH of the slurry also changes due to the generation of NH ₃ , so the viscosity of the slurry is also affected by this reaction. Here, the pH value of the slurry to be adjusted is
Although it is affected by the origin and lot of each silicon nitride raw material powder and is not uniquely determined, a pilot test was conducted using the silicon nitride raw material powder to be used and A pH value that matches the raw powder can be determined.

The method of adjusting the pH is not particularly limited, but can be carried out, for example, by appropriately adding an acid or alkaline solution to the slurry. The pH can be adjusted by adding a pH buffer, and according to the addition of the pH buffer,
Compared to the case of adding an acid or alkaline solution, the influence of pH fluctuation during pulverization can be suppressed, and the pH can be adjusted easily. The pH buffer is not particularly limited, and a conventionally known buffer can be used,
Acetic acid-acetate and hydrochloric acid-sodium chloride have a large buffering effect and can be preferably used. It is more preferable that the buffer agent does not contain metal ions that may adversely affect the properties of the obtained silicon nitride sintered body.

In adjusting the pH of the slurry, the pH value of the slurry can be detected in real time during mixing and pulverization of the slurry by using a pH meter or the like. Therefore, the pH thus detected in real time
Depending on the value, acids, alkalis or pH buffers or any mixture thereof can be added and the slurry p
It becomes possible to precisely adjust H in real time.

The mixing and crushing machine used for making the above-mentioned raw material powder of silicon nitride into a slurry may be a mixing and crushing machine of a type using a crushing medium such as ball stone, for example, a ball mill,
A pot mill etc. can be illustrated. Further, a mixing and crushing machine having a temperature adjusting mechanism capable of adjusting the slurry temperature, such as an attritor mill, can be preferably used, and a mixing and crushing machine having a pH detecting mechanism can be particularly preferably used. The material of the boulder to be used is not particularly limited, and ceramic balls such as alumina, zirconia, and silicon nitride can be used in addition to iron balls, but in the present invention, mixing of impurities is avoided. Therefore, it is preferable to use a ball stone made of silicon nitride.

In addition to the silicon nitride raw material powder and water, a sintering aid can be mixed in the above slurry formation. By mixing the sintering aid in this manner to form a slurry, the nitriding process is performed. Since one step of manufacturing the silicon sintered body can be omitted, the manufacturing efficiency can be improved. At this time, since the sintering aid is usually an oxide, it is not oxidized in wet mixing and pulverization, and does not adversely affect the property control of the slurry.

Next, the silicon nitride sintered body of the present invention and the method for manufacturing the same will be described. The silicon nitride sintered body of the present invention is
It is obtained by molding and firing the above-mentioned silicon nitride powder of the present invention. This silicon nitride sintered body is a homogeneous sintered body having high density and high mechanical strength. In particular, even if the thickness is 20 mm or more, a feature is that a silicon nitride sintered body having high density, high strength and uniform characteristics can be obtained. The method for producing a silicon nitride sintered body of the present invention uses the above-mentioned silicon nitride powder of the present invention, and the powder is subjected to molding and firing according to a conventional method.

[0024]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. (Examples 1 to 5) As shown in Table 1, 50 wt% of water was added to silicon nitride raw material powders of different lots and heated to 50 ° C. Then, the obtained mixture was mixed with 5 mmφ of Si ₃ N ₄
A ball-making stone was added, nitric acid or aqueous ammonia was added so that the slurry viscosity became about 500 cP (see Table 1), and the mixture was ground by an attritor mill for 5 hours. A part of the obtained slurry was sampled, and the specific surface area (BET specific surface area), the oxygen content and the surface available oxygen content were measured. The results obtained are shown in Table 1. The slurry characteristics are also shown in Table 1.

Here, the amount of oxygen and the amount of surface effective oxygen were analyzed and measured by the following procedure. First, a part of the above slurry was dried at 110 ° C. for 2 hours, then gradually cooled to room temperature in a desiccator, 1 g of the obtained analytical sample was evaluated, and a sealable polytetrafluoroethylene container (for example, Sanai Science HU-
It was placed in a Teflon inner cylinder of a 100-type pressure decomposition container. Next, hydrofluoric acid solution (hydrofluoric acid: water = 1: 9 (weight ratio)) 5
After adding 0 ml and sealing, the mixture was stirred at 70 ° C. for 1 hour.
It was cooled to room temperature with water and immediately filtered with a 0.65 μm membrane filter. The obtained filtration residue is washed with 5 mm of water,
This washing was repeated several times. The washed residue was put in a drier and dried at 110 ° C. for 2 hours.

Next, with respect to the analysis sample before being treated with the hydrofluoric acid solution and a part of the dried residue, an oxygen analyzer,
For example, the oxygen content was quantified using an ECM-2800 nitrogen / oxygen analyzer manufactured by Horiba Ltd. The quantitative amount of oxygen from the analytical sample before the hydrofluoric acid treatment is the oxygen amount (total oxygen amount), and the quantitative value from the dry residue is the internal oxygen amount. The surface oxygen content (Os) was determined as the difference between the total oxygen content and the internal oxygen content.

Next, 3% by weight of yttrium oxide and 7% by weight of aluminum oxide were added to the slurry obtained above as sintering aids, and the mixture was further mixed for 3 hours. The obtained slurry was granulated by a spray dry method to obtain a molding powder.
Then, the obtained molding powder is subjected to a die press and CIP.
Molding was performed to prepare a disk-shaped molded body having a diameter of 80 mm and a thickness of 25 mm. This compact was fired at 1900 ° C. for 3 hours in a N ₂ atmosphere of 9 kg / cm ² to obtain a silicon nitride sintered body. Test pieces with a thickness of 4 mm x 3 mm x 40 mm were cut out from the inside (center of thickness) and the vicinity of the surface of the obtained sintered body, and these test pieces were used at room temperature and 1400 ° C in JIS.
A 4-point bending strength test was conducted in accordance with R 1601, and the obtained results are shown in Table 1. The relative density of each test piece is also shown.

Comparative Examples 1 to 7 The same operations as in Examples 1 to 5 were repeated except that the viscosity of the slurry was not adjusted, and the results obtained are shown in Table 1.

[0029]

[Table 1]

As is clear from Table 1, the surface oxygen content and the specific surface area of the silicon nitride powders obtained in Examples 1 to 5 are small, and the variations in the bending strength of the silicon nitride sintered bodies obtained from them are also small. It has excellent characteristics.

(Examples 6 to 10) The slurry viscosity was adjusted by adjusting 0.2 M potassium dihydrogen phosphate (27.2 g KH ₂
Done by the addition of PO ₄ / l) and consisting of 0.2N sodium hydroxide buffer solution, the same procedure as in Example 1-5, to give a slurry and molding powder. The properties of the obtained slurry and molding powder are shown in Table 2. For reference, the slurry characteristics and the characteristics of the molding powder in Comparative Examples 1 to 5 are also shown.

[0032]

[Table 2]

From Table 2, it can be seen that according to Examples 6 to 10, the slurry viscosity can be suitably controlled and the variation in the characteristics of the obtained molding powder can be reduced.

[0034]

As described above, according to the present invention,
Since it was decided to appropriately control the temperature and viscosity of the slurry in the slurry crushing step, a method for producing a silicon nitride powder having uniform properties that does not depend on the lot of the raw material powder, and a high density obtained by using the method. A high-strength silicon nitride sintered body and a method for manufacturing the same can be provided.

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[Procedure amendment]

[Submission date] April 4, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction content]

On the other hand, For the thick product, the atmosphere in the forming surface and internal, for liquid phase generating speed and phase transformation temperature by a difference in temperature and the like are different, material preparation technology that is applied to a conventional thin-walled products Ya Even if the firing control technique is used, there is a problem that a fired body having a wall thickness of 20 mm or more cannot be obtained at a high density. On the other hand, the applicant of the present application has disclosed in Japanese Patent Application No. 5-64203 that nitriding is performed in order to control the behavior of SiO ₂ involved in the liquid phase formation and the phase transformation of silicon nitride when firing a silicon nitride sintered body. It was proposed that a high-density, high-strength, thick-walled silicon nitride sintered body can be obtained by adjusting the amount of oxygen near the surface of the silicon powder (the amount of surface effective oxygen).

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0025

[Correction method] Change

[Correction content]

Here, the amount of oxygen and the amount of surface effective oxygen were analyzed and measured by the following procedure. First, a part of the slurry is dried at 110 ° C. for 2 hours and then gradually cooled to room temperature in a desiccator, 1 g of the obtained analytical sample is evaluated, and a sealable polytetrafluoroethylene container (for example, Sanai Science HU-
It was placed in a Teflon inner cylinder of a 100-type pressure decomposition container. Next, hydrofluoric acid solution (hydrofluoric acid: water = 1: 9 (weight ratio)) 5
After adding 0 ml and sealing, the mixture was stirred at 70 ° C. for 1 hour.
It was cooled to room temperature with water and immediately filtered with a 0.65 μm membrane filter. The residue obtained by filtration is washed with 5 ml of water,
This washing was repeated several times. The washed residue was put in a drier and dried at 110 ° C. for 2 hours.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0029

[Correction method] Change

[Correction content]

[Table 1]

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

[Table 2]

Claims (8)

[Claims] 1. Water is added to a silicon nitride raw material powder, and the temperature of the resulting mixture is 30 to 80 ° C. and the viscosity is 20 to 10.
The content of oxygen in the obtained silicon nitride powder is adjusted to 0.
5 to 3.0% by weight, and this silicon nitride powder has the following formula Os / Ss = 30 to 100 (where Os is the oxygen amount (% by weight) on the powder surface, Ss is the specific surface area of the powder). (M ² / mg)), which is adjusted so as to satisfy the amount of surface effective oxygen represented by (). 2. By controlling the pH of the slurry,
The manufacturing method according to claim 1, wherein the viscosity of the slurry is controlled to 20 to 1000 cP. 3. The method according to claim 2, wherein the pH control is performed by adding an acid or an alkali. 4. The method according to claim 2, wherein the pH of the slurry is monitored during the mixing and grinding, and the viscosity of the slurry is controlled by adding an acid or an alkali accordingly. Method. 5. The method according to claim 2, wherein the pH control is performed by adding a pH buffer. 6. The production method according to claim 1, wherein the mixing and pulverizing are performed in the presence of a sintering aid. 7. A method for producing a silicon nitride sintered body, which comprises molding the silicon nitride powder obtained by the method according to claim 1 and firing it. 8. A silicon nitride sintered body obtained by the manufacturing method according to claim 7, wherein the silicon nitride sintered body has a wall thickness of 20 mm or more.