Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Reexamination Certificate
2000-08-15
2003-10-14
Dunn, Tom (Department: 1725)
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C501S092000, C501S090000, C252S516000, C264S086000, C264S299000
Reexamination Certificate
active
06632761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon carbide powder which is a raw material of sintered silicon carbide, a method of producing a green body, and a method of producing a sintered silicon carbide. By green body is meant a compact of silicon carbide in which many pores are present.
2. Description of the Related Art
Conventionally, silicon carbide has attracted attention as a material used in high-temperature areas from the standpoint that silicon carbide has excellent strength even in high temperatures exceeding 1000° C., resistance to heat, resistance to thermal shock, resistance to abrasion, and the like. In recent years, as a metallurgical implement in the manufacture of semiconductors, it has been used as a substitute material for quartz.
As one method of producing a sintered body made of silicon carbide, there is a reaction sintering method. In the reaction sintering method, silicon carbide powder and a carbon powder or an organic substance consisting of a carbon source are dissolved in a solvent and dispersed to produce a mixed powder body in a slurry state. Then, the obtained mixed powder body is cast in a cast mold, an extrusion mold, a press mold or the like, and dried to obtain a green body. Then, the obtained green body is heated in a vacuum atmosphere or in an atmosphere of inert gas, and immersed in metallic silicon which has been melted. Then, the free carbon in the green body is made to react with the metallic silicon which is sucked into the green body, and a sintered silicon carbide is obtained.
Sintered silicon carbide obtained in accordance with such a reaction sintering method has extremely excellent strength, resistance to heat, resistance to thermal shock, and resistance to abrasion and the like. However, in recent years there has been a demand for sintered silicon carbide which has even greater high performance. Further improvements to increase density, which density greatly effects strength, resistance to heat, resistance to thermal shock, resistance to abrasion and the like, have been sought.
Increasing the density of the green body, which is an intermediate compact, is important for increasing the density of sintered silicon carbide, and attempts to achieve such are being researched and developed each day. If the density of the green body is not sufficient, not only does the density of the sintered silicon carbide to be obtained decline, but handling properties of the green body deteriorate, leading to cracks, particles and the like. Therefore, improvements have been sought after. Further, in cases where conductivity is imparted to the sintered silicon carbide (e.g., introducing a nitrogen source or the like), when the density of the green body is low, there are many instances in which the volume resistivity of the sintered silicon carbide to be obtained becomes higher, and electrical discharge machining and the like becomes difficult. Therefore, improvements have likewise been sought after.
SUMMARY OF THE INVENTION
The present invention has been created in light of the aforementioned prior art.
A first object of the present invention is to provide a silicon carbide powder which can increase the densities of a green body and a sintered silicon carbide.
A second object of the present invention is to provide a method of producing a green body having a high density and excellent handling properties, and a method of producing a sintered silicon carbide having a high density.
A means of overcoming the drawbacks described above is as follows.
A first aspect of the present invention is a silicon carbide powder formed of a silicon carbide powder (A) which has a modal diameter of 1.7 to 2.7 &mgr;m and a silicon carbide powder (B) which has a modal diameter of 10.5 to 21.5 &mgr;m, at a particulate volume ratio (the volume of silicon carbide powder (A)/the volume of silicon carbide powder (A) and the volume of silicon carbide powder (B)) of 20% to 80%.
Another aspect of the present invention is a method of producing a green body having the steps of dissolving and dispersing the aforementioned silicon carbide powder in a solvent to produce a silicon carbide mixed powder in a slurry state, and then casting into a cast mold the silicon carbide mixed powder in a slurry state and drying the silicon carbide mixed powder in a slurry state to produce a green body.
Another aspect of the present invention is a method of producing a sintered silicon carbide having a process wherein the green body obtained in accordance with the aforementioned method of producing a green body is, in one of a vacuum atmosphere and an atmosphere of inert gas, immersed in metallic silicon which has been melted, whereby the metallic silicon is made by suction to penetrate the pores in the green body and the pores in the green body are filled up.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Silicon Carbide Powder
A silicon carbide powder of the present invention is a silicon carbide powder which includes at a specific particulate volume ratio (the volume of silicon carbide powder (A)/the volume of silicon carbide powders (A) and (B)) silicon carbide powders (A) and (B) having different modal diameters. By using such a silicon carbide powder, the densities of a green body and a sintered silicon carbide can be raised.
In the silicon carbide powder of the present invention, modal diameter refers to a particle diameter most present within a particle size distribution of the powder.
An example of the silicon carbide powder of the present invention specifically includes a silicon carbide powder (AB) discussed below.
—Silicon Carbide Powder (AB)—
The silicon carbide powder (AB) is a silicon carbide powder which includes at a particulate volume ratio (the volume of silicon carbide powder (A)/the volume of silicon carbide powders (A) and (B)) of 20% to 80% a silicon carbide powder (A) whose modal diameter is 1.7 &mgr;m to 2.7 &mgr;m and a silicon carbide powder (B) whose modal diameter is 10.5 &mgr;m to 21.5 &mgr;m.
With regard to the volume of the silicon carbide powder, the weight of the silicon carbide powder is measured by a precision gravimeter, and the obtained weight is divided by a theoretical density of carbon silicon of 3.21 g/cm
3
. The value thus calculated is adopted for the volume of the silicon carbide powder.
The silicon carbide powder (AB) includes the silicon carbide powder (A) and the silicon carbide powder (B) at a particulate volume ratio of 20% to 80%. However, the particulate volume ratio is preferably 20% to 65% and more preferably 30% to 40%. When the particulate volume ratio falls below 20% or exceeds 80%, sometimes a sufficient density for the green body and the sintered silicon carbide cannot be obtained.
In the silicon carbide powder (AB), the modal diameter of the silicon carbide powder (A) is 1.7 &mgr;m to 2.7 &mgr;m. When a silicon carbide powder (A) having a modal diameter which falls below 1.7 &mgr;m or exceeds 2.7 &mgr;m is combined with the silicon carbide powder (B), sometimes a sufficient density for the green body and the sintered silicon carbide cannot be obtained.
In the silicon carbide powder (AB), the modal diameter of the silicon carbide powder (B) is 10.5 &mgr;m to 21.5 &mgr;m. When a silicon carbide powder (B) having a modal diameter which falls below 10.5 &mgr;m or exceeds 21.5 &mgr;m is combined with the silicon carbide powder (A), sometimes a sufficient density for the green body and the sintered silicon carbide cannot be obtained.
With regard to the silicon carbide powder (A) in the silicon carbide powder (AB), the ratio (D
90
/D
10
) of a cumulative diameter that accounts for 90% of all volume of powder (D
90
) to a cumulative diameter that accounts for 10% of all volume of powder (D
10
) calculated according to the particle size distribution is, from the standpoint of increasing density, preferably 10 or below, more preferably 7 or below, and particularly preferably 5 or below. When this ratio exceeds 10, sometimes extremely large particles become present and impede densification.
With regard to the
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