JP6884516B2 - Manufacturing method of SiC sintered body - Google Patents

Description

An embodiment of the present invention relates to a method for producing a SiC sintered body.

The SiC / SiC composite material is a ceramic-based fiber composite material composed of a SiC fiber and a SiC matrix, and has high strength, and is excellent in toughness, heat resistance, and corrosion resistance. Therefore, SiC sintered bodies such as SiC / SiC composite materials are expected to be applied to structures constituting each system such as nuclear energy systems and fusion energy systems.

The methods for producing the SiC sintered body are mainly the CVD method (chemical vapor deposition method), the PIP method (polymer melt impregnation pyrolysis method), the RS method (reaction sintering method), and the LPS method (liquid phase sintering method). ). Of these, the LPS method is a method of forming a SiC sintered body by mechanically pressurizing the molded body at a high temperature, and it is possible to synthesize a dense SiC sintered body in a relatively short time. Is.

SiC is a compound having a high covalent bond property and is difficult to sinter. Therefore, when forming a SiC sintered body, a sintering aid (liquid phase sintering aid) is generally used in order to make the SiC sintered body denser. For example, an Al- _{BC based powder, a Y 2} O ₃ (yttria) -Al ₂ O ₃ (alumina) -based mixed powder, or the like is used as a sintering aid.

The Y ₂ O _3- Al ₂ O _{3 system mixed powder has a composition in which Y 2} O ₃ and Al ₂ O ₃ are eutectic at a eutectic point (1760 ° C.) in order to lower the sintering temperature. , 60% by weight Y ₂ O ₃ and 40% by weight Al ₂ O ₃ are mixed. The reaction products of the mixed powder of the Y ₂ O _3- Al ₂ O ₃ system are mainly YAG (yttrium aluminum garnet (Y ₃ Al ₅ O ₁₂ )) and Al ₂ O ₃ . In the reaction product, the ratio of both is 68.8 mol% for YAG and 31.2 mol% for Al ₂ O ₃ . In the reaction product, Al ₂ O ₃ has lower corrosion resistance than YAG, and therefore, we have found that it is a factor that lowers the corrosion resistance of the SiC sintered body.

Japanese Patent No. 5077937 JP-A-2002-356381

SiC sintered bodies generally have excellent corrosion resistance. However, the corrosion resistance of the SiC sintered body differs greatly depending on the manufacturing method. Specifically, the SiC sintered body manufactured by the CVD method has the best corrosion resistance, then the SiC sintered body manufactured by the LPS method has excellent corrosion resistance, and then the SiC fired body manufactured by the RS method. Excellent corrosion resistance of the body.

The RS method obtains a dense SiC sintered body by filling the matrix with metallic silicon (Si). However, when Si is oxidized to form SiO ₂ (silica), the SiO ₂ is easily eluted. As a result, it is not easy to improve the corrosion resistance of the SiC sintered body.

The SiC sintered body produced by the LPS method can be easily sintered with a sintering aid. In order to lower the sintering temperature as much as possible _{, a mixing ratio of two kinds of auxiliaries (Al 2} O ₃ and Y ₂ O ₃ ), which are eutectic points, is mainly used. _{When the mixing ratio of Y 2} O ₃ and Al ₂ O ₃ used as a sintering aid is changed from the mixing ratio of the eutectic temperature, it becomes necessary to raise the sintering temperature. As a result, the sintering temperature may be higher than the heat resistant temperature of the SiC fiber.

In the case of the CVD (CVI) method, since a sintering aid is not required, the corrosion resistance of the SiC sintered body is the best. However, in this case, since the construction time becomes long, there is a high possibility that the manufacturing cost will increase. Further, during manufacturing, the surface of the sprayed SiC is filled before filling the large pores between the fiber bundles and does not penetrate into the inside, so that the density of the SiC sintered body may not be sufficient.

By the way, the fuel cladding of the light water reactor needs to contain the nuclear reaction-producing gas generated by the nuclear reaction during the operation of the light water reactor. Conventionally, the fuel cladding of a light water reactor is formed using a zirconium alloy, but since a large amount of hydrogen is generated in the event of an accident, it is desirable to use a material that is stable against oxidation, such as SiC.

On the other hand, light water reactors are required to have excellent corrosion resistance. Specifically, there is a demand for corrosion resistance that enables operation for 5 years with a small amount of wall loss in a high temperature (about 300 ° C.) water environment.

Therefore, an object to be solved by the present invention is to provide a method for producing a SiC sintered body capable of improving corrosion resistance.

The method for producing a SiC sintered body according to the embodiment includes a slurry manufacturing step and a sintering step.
In the slurry preparation step, a slurry containing SiC powder and a sintering aid powder is prepared. In the sintering step, a SiC sintered body is obtained by sintering the slurry. In the slurry, the sintering aid powder contains Al ₂ O ₃ and Y ₂ O _3, and the sintering aid powder is based on the total weight of both the SiC powder and the sintering aid powder. The weight is 12% by weight or more and 25% by weight or less, and _{the weight X of Al 2} O _{3 and} the weight Y of Y ₂ O ₃ are in the relationship shown in the following formula (A).
37/63 ≤ X / Y ≤ 44/56 ... Equation (A)

According to the present invention, it is possible to provide a method for producing a SiC sintered body capable of improving corrosion resistance.

FIG. 1 is a diagram showing the results of performing a corrosiveness test on the SiC sintered body produced as the “Example” in the first embodiment. FIG. 2 is a diagram showing the results of X-ray diffraction of the SiC sintered body produced in the "Example" of the first embodiment. FIG. 3 is an enlarged cross-sectional view of a part of the SiC sintered body according to the second embodiment.

<First Embodiment>
The method for producing the SiC sintered body according to the first embodiment will be described. In the present embodiment, when the SiC sintered body is manufactured, the slurry preparation step and the sintering step are sequentially performed.

(Slurry preparation process)
In the slurry preparation step, a slurry containing SiC powder and a sintering aid powder is prepared. Specifically, the slurry is prepared by stirring and mixing both the SiC powder and the sintering aid powder in the liquid.

In this embodiment, the sintering aid powder contains Al ₂ O ₃ and Y ₂ O ₃ . Here, in the sintering aid powder, _{it is preferable that the weight X of Al 2} O _{3 and} the weight Y of Y ₂ O ₃ have a relationship shown in the following mathematical formula (A). The range specified by the formula (A) is _{the temperature in the range in which Al 2} O ₃ and Y ₂ O ₃ become "Liq (liquid phase) and YAG" in _{the Al 2} O ₃ −Y ₂ O ₃ binary system. Almost corresponds to the range in _{which Al 2} O ₃ does not become a surplus when the amount drops.

37/63 ≤ X / Y ≤ 44/56 ... Equation (A)

When the value (= X / Y) shown in the formula (A) is smaller than the lower limit value, the sintering temperature required for sintering becomes high, so that sintering may be insufficient. On the other hand, when the value (= X / Y) shown in the formula (A) is larger than the upper limit value, corrosion may increase in the SiC sintered body due to _{the excess Al 2} O _3. ..

As the sintering aid powder as a raw material, YAG powder may be used together with _{Al 2} O ₃ powder and Y ₂ O _{3 powder.} When you add a YAG powder as a raw material in the sintering aid powder, in formula (A), the weight X of _Al 2 _{O 3} is an _Al 2 _{O 3} powder having a weight X1, the weight of _Al 2 _{O 3} component in YAG powder It becomes the value obtained by adding X2 (X = X1 + X2). Similarly, in formula _(A), the weight Y of Y 2 _{O 3 is} a Y 2 _{O 3} powder weight Y1, a value obtained by adding the _Y 2 _{O 3} by weight of the components Y2 in YAG powder (Y = Y1 + Y2 ). If the amount of YAG powder in the sintering aid powder is small, Al ₂ O ₃ and Y ₂ O ₃ may be unevenly distributed after sintering, and the corrosion resistance of the produced SiC sintered body may be insufficient. .. On the other hand, when the amount of YAG powder in the sintering aid powder is large, the sintering temperature becomes high and it may be difficult to perform sufficient sintering.

The sintering aid powder is preferably in the range of 1 to 30% by weight with respect to the total weight of both the SiC powder and the sintering aid powder. If it is smaller than the lower limit, the density after sintering may not be sufficiently secured. On the other hand, if it is larger than the upper limit value, the characteristics of the sintering aid may become apparent and the characteristics as SiC ceramics may not be sufficiently secured. This range is particularly preferably 3% by weight or more and 25% by weight or less.

The average particle size (average diameter) of the SiC powder and the sintering aid powder is preferably in the following range, for example. In the sintering aid powder, YAG is used, for example, by sufficiently pulverizing flaky or powdery crystals so as to have the following average particle size fine particles. The average particle size of each particle is preferably small in order to facilitate sintering.
-Average particle size of SiC powder: 30 nm to 300 nm
-Average particle size of Al ₂ O ₃ : 0.05 μm to 0.5 μm
-Average particle size of Y ₂ O ₃ : 0.05 μm to 0.5 μm
-Average particle size of YAG: 0.05 μm to 0.5 μm

When preparing the slurry, the liquid containing both the SiC powder and the sintering aid powder is, for example, ethanol or xylene. The ratio (solid content) contained in the SiC powder and the sintering aid powder in the slurry is not particularly limited and may be a concentration that is easy to process, for example, 10 to 30% by weight.

(Sintering process)
In the sintering step, a SiC sintered body is obtained by sintering the slurry produced above. Specifically, the SiC sintered body is manufactured by sintering while applying pressure to the slurry placed in the mold. The SiC sintered body is formed, for example, in a rod-like or plate-like shape. Sintering is performed by a liquid phase sintering method using HIP, hot pressing, plasma sintering, or the like. Although details will be described later, in the SiC sintered body of the present embodiment, the main component of the reaction product of the sintering aid powder is YAG.

In this embodiment, the sintering is performed under the sintering conditions shown below, for example.
-Sintering temperature: 1850 to 1950 ° C
-Pressure: 10 MPa to 40 MPa

(Example)
FIG. 1 is a diagram showing the results of performing a corrosiveness test on the SiC sintered body produced as the “Example” in the first embodiment.

In FIG. 1, in "Example", a SiC sintered body (SiC monolithic) was produced using a sintering aid powder containing _{Al 2} O ₃ and Y ₂ O _3.

Specifically, in the "Example", as the sintering aid powder, 0.3 parts by weight of Al ₂ O ₃ powder, 0.2 parts by weight of Y ₂ O ₃ powder, and 11.5 parts by weight of YAG A mixture with the powder was used. Here, in the Y ₂ O ₃ powder, the ratio of the Al ₂ O ₃ component is 42% by weight, and the ratio of the Y ₂ O ₃ component is 58% by weight. Therefore, in "Example", _{the weight ratio (X / Y) of Al 2} O ₃ and Y ₂ O ₃ is 0.746, which is the relationship shown in the above formula (A) (X = (11). .5 x 0.42 + 0.3) x 100/12 = 42.75, X / Y = 42.75 / 57.25 = 0.746).

The average particle sizes of the SiC powder and the sintering aid powder used in "Examples" are as follows.
-Average particle size of SiC powder: 30 nm
-Average particle size of Al ₂ O _{3: 0.1 μm}
-Average particle size of Y ₂ O _{3: 0.1 μm}
-Average particle size of YAG: 0.1 μm

In "Example", 88% by weight of SiC powder and 12% by weight of sintering aid powder were put into ethanol and mixed to prepare a slurry. Here, the slurry was prepared so that the ratio (solid content) contained in the SiC powder and the sintering aid powder in the slurry was 20% by weight.

Then, in "Example", a SiC sintered body was obtained by sintering the slurry prepared as described above under the sintering conditions shown below.
-Sintering temperature: 1850 ° C
・ Pressure: 20MPa

FIG. 2 is a diagram showing the results of X-ray diffraction of the SiC sintered body produced in the "Example" of the first embodiment.

As shown in FIG. 2, the SiC sintered body of the "example" is composed of SiC and YAG. That is, it can be seen that the reaction product of the sintering aid powder _{does not contain Al 2} O _{3 and is almost YAG.}

In "Comparative Example 1", as the sintering aid powder, a mixture in which Al ₂ O ₃ and Y ₂ O ₃ are mixed in a eutectic composition (X: Y = 60: 40) without containing YAG. Was used. In "Comparative Example 1", except for this point, a SiC sintered body was produced under the same conditions as in the case of "Example".

"Comparative Example 2" is not SiC monolithic but YAG alone.

In the corrosiveness test, each sample of "Example", "Comparative Example 1", and "Comparative Example 2" was immersed in pure water (dissolved oxygen 8 ppm) at 320 ° C. for 168 hours. Then, the weight per unit area changed before and after immersion was determined as the "corrosion amount".

As can be seen from FIG. 1, the amount of corrosion in "Example" in which the sintering aid powder contains YAG is significantly reduced as compared with "Comparative Example 1" in which the eutectic composition does not contain YAG in the sintering aid powder. ing. In "Example", the amount of corrosion is closer to that of "Comparative Example 2" which is YAG alone.

As described above, in the present embodiment, the corrosion resistance of the SiC sintered body can be improved.

<Second Embodiment>
The method for producing the SiC sintered body according to the second embodiment will be described.

FIG. 3 is an enlarged cross-sectional view of a part of the SiC sintered body according to the second embodiment.

In the present embodiment, the SiC sintered body includes the SiC fiber 11, the SiC crystal layer 21, and the SiC monolithic layer 31, as shown in FIG. In the SiC sintered body, the surface of the SiC fiber 11 is coated with a low-density SiC crystal layer 21. Further, the SiC monolithic layer 31 is formed so as to cover the SiC crystal layer 21.

When forming the SiC sintered body of the present embodiment, first, the SiC fiber 11 is prepared. Here, the SiC fiber 11 having a heat resistant temperature higher than the sintering temperature is prepared so that the SiC fiber 11 is not deformed in the sintering in the subsequent process.

Next, a SiC amorphous layer (not shown) is formed on the SiC fiber 11. Here, the SiC amorphous layer is formed by the PIP method (polymer melt impregnation thermal decomposition method).

Specifically, the SiC fiber 11 is immersed in a high molecular weight polycarbosilane solution (not shown) and then dried. Then, the dried SiC fiber 11 is heat-treated. As a result, a SiC amorphous layer is formed on the surface of the SiC fiber 11. The heat treatment is performed in an Ar—H ₂ environment under the condition that the heat treatment temperature is, for example, 1200 ° C. Since it is difficult to make the SiC amorphous layer produced by the PIP method dense, it is not necessary to make it dense.

Next, the SiC monolithic layer 31 is formed by providing the slurry of the first embodiment on the surface of the SiC amorphous layer and then sintering the slurry. Here, by sintering, the slurry becomes the SiC monolithic layer 31, and the SiC amorphous layer crystallizes into the SiC crystal layer 21. If liquid crystal sintering with a YAG composition is applied to the SiC monolithic layer 31, it can be expected that the corrosion characteristics of the outer surface of the pipe to be wetted will be improved.

In the present embodiment, the gaps between the SiC fibers 11 are embedded by providing the SiC amorphous layer by the PIP method as described above. Therefore, in the present embodiment, it is possible to prevent the sintering aid in the slurry from being incorporated into the SiC fiber 11.

In addition, carbon is generally used as the SiC coating layer, but in this case, the oxidation characteristics at high temperatures are inferior. However, in the present embodiment, since the SiC coating layer is a SiC amorphous layer, it is excellent in oxidation characteristics at high temperatures.

The SiC sintered body of the present embodiment gives suitable results in a bending test and is excellent in corrosion resistance in high temperature water. Further, the SiC amorphous layer is likely to undergo a volume change due to crystallization under the influence of neutron irradiation, but in the present embodiment, as described above, the SiC amorphous layer is changed to the SiC crystal layer 21 by crystallization. Therefore, the effect of neutron irradiation is small. Therefore, the SiC sintered body of the present embodiment can be suitably used as a core material of a nuclear reactor. Specifically, the SiC sintered body of the present embodiment can be applied to a fuel cladding tube and a fuel channel box in a light water reactor.

In addition, the SiC sintered body of the present embodiment can be applied to an ultra-high temperature engine nozzle, an ultra-high temperature turbine blade, and the like in the aerospace industry.

For example, a SiC sintered body may be manufactured by separately forming the SiC monolithic layer 31 by the same method as in the first embodiment and then attaching the SiC monolithic layer 31.

<Others>
Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

11 ... SiC fiber, 21 ... SiC crystal layer, 31 ... SiC monolithic layer

Claims (3)

A slurry preparation process for producing a slurry containing SiC powder and a sintering aid powder, and
It has a sintering step of obtaining a SiC sintered body by sintering the slurry.
In the slurry, the sintering aid powder contains Al ₂ O ₃ and Y ₂ O ₃ .
The sintering aid powder is 12% by weight or more and 25% by weight or less with respect to the total weight of both the SiC powder and the sintering aid powder.
The weight X of Al ₂ O _{3 and} the weight Y of Y ₂ O ₃ are in the relationship shown in the following formula (A).
A method for producing a SiC sintered body.
37/63 ≤ X / Y ≤ 44/56 ... Equation (A) A slurry preparation process for producing a slurry containing SiC powder and a sintering aid powder, and
It has a sintering step of obtaining a SiC sintered body by sintering the slurry.
In the slurry, the sintering aid powder contains Al ₂ O ₃ and Y ₂ O ₃ .
The sintering aid powder is 12% by weight or more and 25% by weight or less with respect to the total weight of both the SiC powder and the sintering aid powder.
The weight X of Al ₂ O _{3 and} the weight Y of Y ₂ O ₃ are in the relationship shown in the following formula (A).
The SiC sintered body is used as a material for a nuclear reactor.
A method for producing a SiC sintered body.
37/63 ≤ X / Y ≤ 44/56 ... Equation (A) Including a SiC amorphous layer forming step of forming a SiC amorphous layer on the surface of a SiC fiber by a polymer melt impregnation thermal decomposition method.
In the sintering step, the SiC amorphous layer is formed on the surface of the SiC amorphous layer, and then the slurry is sintered to form a SiC monolithic layer, and the SiC amorphous layer is crystallized.
The method for producing a SiC sintered body according to claim 1 or 2.

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