JPH05213673A - Production of silicon carbide-based sintered compact having high heat conductivity - Google Patents

Abstract

PURPOSE:To obtain a silicon carbide-based sintered compact having high mechanical characteristics and high heat conductivity by adding a carbon-contg. compd. and a boron-contg. compd. as sintering aids and carrying out sintering and heat treatment in a nitrogen atmosphere under a high pressure. CONSTITUTION:A carbon-contg. compd. such as carbon black and a boroncontg. compd. such as B4C as sintering aids are added to silicon carbide powder having 0.1-2mum average particle diameter as a base by 1-4wt.% (expressed in terms of carbon) and 0.2-0.4wt.% (expressed in terms of boron), respectivity. They are mixed with a mixing means such as a ball mill, a binder is added and the mixture is molded into a desired shape by press molding or other method and sintered at 1,950-2,000 deg.C in vacuum or in an inert atmosphere to obtain a silicon carbide-based sintered compact having >=88% relative density and low heat conductivity. This sintered compact is heat-treated at 1,800-2,100 deg.C in a nitrogen atmosphere under a high pressure of 1,500-2,000atm so that the weight is increased by 0.01-5% and a sintered compact having >=100 W/mXk heat conductivity is obtd.

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 carbide-based sintered body having high mechanical strength and excellent high thermal conductivity, which is applied to high thermal conductive substrates, heat sinks, heat exchangers and the like. ..

[0002]

2. Description of the Related Art Conventionally, a silicon carbide-based sintered body has been attracting attention as a material having excellent mechanical properties and having little strength deterioration particularly at high temperatures, and its application to various fields has been promoted.

In such a silicon carbide sintered body, carbon and boron are generally added to silicon carbide powder as a sintering aid, and after this is molded, it is pressureless in an inert atmosphere such as Ar. It is obtained by firing at 2000 to 2200 ° C.

The thermal conductivity of the silicon carbide based sintered body thus obtained is at most about 70 to 80 W / m · k.

On the other hand, as a technique for increasing the thermal conductivity of a silicon carbide sintered body, recently, BeO has been added to silicon carbide as a sintering aid and hot press firing has been carried out. For example, it has a high thermal conductivity of 270 W / m · k level and is used as a material for various substrates and heat sinks.

[0006]

However, this BeO-added silicon carbide-based sintered body must be handled with care during manufacture because BeO itself has toxicity, and the manufacturing method is hot pressing. Since the method is a method, the shape of the sintered body is limited and there is a problem in productivity.

[0007]

With respect to the above problems, the present inventors have studied a method for increasing the thermal conductivity of a silicon carbide based sintered material without using BeO, and as a result,
Heat treatment is performed in a high-pressure nitrogen atmosphere on a sintered body that has been sintered by a conventional method in which carbon and boron are added as sintering aids to maintain the high mechanical strength of the conventional silicon carbide-based sintered material. It was found that the thermal conductivity can be significantly improved as compared with the bonded body.

That is, according to the method for producing a silicon carbide-based sintered body of the present invention, a molded body containing silicon carbide as a main component and carbon and a boron-containing compound as a sintering aid is added in a vacuum or in an inert atmosphere. After firing in a high-pressure nitrogen atmosphere, a sintered body having a relative density of 88% or more is prepared.
The heat treatment is performed at 2100 ° C. so that the weight increase with respect to the sintered body before heat treatment is in the range of 0.01 to 5%.

The present invention will be described in detail below. According to the manufacturing method of the present invention, first, various compounds containing silicon carbide powder as a starting material and carbon and boron as a sintering aid are prepared. As the silicon carbide powder, either α type or β type,
Alternatively, these may be mixed and used, and the average particle diameter is suitably 0.1 to 2 μm. Further, as the sintering aid, in addition to carbon black, graphite and the like as the carbon component, phenol resin or coal tar pitch which can generate carbon by thermal decomposition can be used. In addition, examples of the boron component include B ₄ C and metallic boron. These sintering aids finally contain 1 to 4% by weight of carbon,
It is desirable to add boron in an amount of 0.2 to 0.4% by weight.

Next, these raw material powders are weighed at a predetermined ratio and thoroughly mixed by a mixing means such as a ball mill, and then a binder is added to this powder, and a known molding method,
For example, it is formed into a desired shape by press molding, extrusion molding, cast molding, cold isostatic molding, or the like. When a phenol resin or the like is added as a sintering aid, it is
Carbon can be generated by calcining the molded body at 800 ° C. in a non-oxidizing atmosphere and thermally decomposing it.

Next, the molded body obtained as described above is fired to form a sintered body having a relative density of 88% or more. Specifically, the molded body is fired at a temperature of 1950 to 2000 ° C. in vacuum or in an inert atmosphere such as Ar. Examples of the firing means at this time include a pressureless firing method and a hot pressing method.

The silicon carbide-based sintered body thus obtained has a thermal conductivity of about 70 to 80 W / m · k.

According to the present invention, this silicon carbide sintered body having a low thermal conductivity is heated to 1800 to 2100 ° C. in a high pressure nitrogen atmosphere.
A major feature is that the heat treatment is performed at the temperature of. By this heat treatment, nitrogen atoms are introduced into the silicon carbide crystal grains of at least the surface portion of the sintered body. If the nitrogen pressure at this time is too low, the effect of improving the thermal conductivity is not remarkable.
The pressure is controlled to 500 to 2000 atm. Further, if the temperature at this time is lower than 1800 ° C., densification does not proceed and 2100
If the temperature is higher than ° C, the silicon carbide crystals will grow abnormally and the strength will be lowered, or the nitriding will proceed too much and the effect of improving the thermal conductivity will be lowered.

According to the heat treatment under the above conditions, silicon carbide may react with nitrogen and change into silicon nitride and carbon.
However, silicon nitride itself has a thermal conductivity of 42 to 50.
Since it is as low as W / m · k, the formation of silicon nitride may reduce the thermal conductivity of the entire sintered body. Therefore,
According to the present invention, the weight increase after the heat treatment with respect to the weight of the sintered body before the heat treatment is 0.01 to 5%, particularly 0.03% to
It is important to heat treat at a level of 1.0%.
That is, if the weight increase is less than 0.01%, there is no effect of improving the thermal conductivity, and if the weight increase exceeds 5%, the formation of silicon nitride becomes excessive and the thermal conductivity decreases.

Further, in order to suppress the generation of excessive silicon nitride, it is necessary that the density of the sintered body before the heat treatment be high to some extent. That is, if the density of the sintered body before the heat treatment is low, and if it is a porous body, the contact area with nitrogen becomes large during the heat treatment, so that the nitriding reaction easily proceeds. Therefore, the relative density of the sintered body before heat treatment was set to 88% or more. However, if the density of the sintered body before the heat treatment is too high, the weight increase due to the heat treatment may be less than 0.01%. Therefore, the relative density before heat treatment is preferably set to 96% or less.

[0016]

According to the present invention, a silicon carbide-based sintered body obtained by solid-phase sintering using carbon and boron as a sintering aid is heat-treated in high-pressure nitrogen, so that excellent mechanical properties are not impaired. The thermal conductivity of the sintered body can be increased.

It is known that many stacking faults are usually present in the silicon carbide crystal grains of the silicon carbide based sintered body. In particular, when boron is used as the sintering aid, it is expected that the boron atoms diffuse into the silicon carbide crystal and enter the position of carbon, thereby causing an electric defect. According to the present invention, it is considered that when heat treatment is performed under the above-mentioned predetermined conditions, nitrogen is dissolved in the silicon carbide crystal as a solid solution to reduce crystal defects, thereby improving the thermal conductivity. Further, even if some silicon nitride is generated by nitriding, since it does not substantially include grain boundaries and is bonded to the silicon carbide crystal, there is no barrier for phonon transmission, so that high thermal conductivity is achieved. Guessed.

According to the present invention, the conventional 70-80 W / m.multidot.
From the thermal conductivity of about k, it is possible to obtain a silicon carbide based sintered body having a thermal conductivity of 100 W / m · k or more, particularly 190 W / m · k according to the examples described later.

Since it is not necessary to use the hot pressing method in the manufacturing method, it can be applied to any product shape.

The present invention will be described below with reference to the following examples.

[0021]

[Example] α-type silicon carbide powder 1 having an average particle size of 0.4 μm
After 0.4 part by weight of B ₄ C was added to 100 parts by weight and mixed well, a phenol resin having a carbonization rate of 40% was added so that the carbon equivalent amount was 2 parts by weight.

Using this mixed powder, a molding pressure of 1000
Molded into a shape having an outer diameter of 60 mm and a thickness of 10 mm at kg / cm ² to obtain a green molded body having a green density of 2.1 g / cc. Next, this molded body was calcined at 1850 to 2000 ° C. and then calcined under an argon atmosphere under the conditions shown in Table 1.
Then, this sintered body was heat-treated at the nitrogen pressure and the treatment temperature shown in Table 1 to obtain a sample.

In the above steps, the density of the sintered body before the heat treatment and the density of the sintered body after the heat treatment were measured by the Archimedes method, and the weight increase thereof was calculated. Further, the thermal conductivity of the sintered body after the heat treatment was measured by a laser flash method at room temperature for a sample having a thickness of 1.5 mm. Further, a bending test piece was cut out from the sintered body and kept at room temperature and 1400 ° C.
The flexural strength in Table 1 was measured by the 4-point bending method according to JIS R1601. The measurement results are shown in Table 1.

[0024]

[Table 1]

According to Table 1, the thermal conductivity of the conventional high-density silicon carbide-based sintered body (Sample No. 1) not subjected to heat treatment is 6
It is as low as 7W / mk. Further, when heat treatment was performed and the density of the sintered body was too high (Sample No. 2), the weight increase was less than 0.01%, and there was no significant improvement effect on the thermal conductivity. The density of the sintered body before heat treatment is 88% in relative density.
In Samples Nos. 8, 9 and 10 below which the heat treatment was performed, the nitridation due to the heat treatment progressed and the weight increase became large, and in this case as well, the thermal conductivity was not so different from the conventional one.

On the other hand, according to the present invention, a sintered body prepared by controlling the heat treatment conditions so that the sintered body before the heat treatment is in an appropriate range and the weight increase is in a predetermined range is It was possible to obtain a sintered body having a high thermal conductivity of 100 W / m · k or more without deterioration of mechanical properties as compared with a normal silicon carbide sintered body.

[0027]

As described in detail above, according to the present invention, the thermal conductivity of the silicon carbide based sintered material can be maintained while maintaining the excellent mechanical properties at high temperature without using a toxic substance such as BeO. The rate can be increased. Further, according to this manufacturing method, since the hot pressing method is not used, it can be applied to any product shape.

As a result, the application of the silicon carbide sintered body to a heat dissipation substrate for semiconductor parts, a material for a heat sink, a structural material for a heat exchanger, etc. can be expanded.

Claims (1)

[Claims] 1. A sintered body having a relative density of 88% or more is obtained by firing a molded body containing silicon carbide as a main component and containing a compound containing carbon and boron as a sintering aid in a vacuum or in an inert atmosphere. After making the sintered body,
A method for producing a highly heat-conductive silicon carbide sintered body, which comprises performing heat treatment at 0 to 2100 ° C. so that a weight increase with respect to a sintered body before heat treatment is in a range of 0.01 to 5%.