Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1999-05-11
2001-06-05
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S097300, C264S683000
Reexamination Certificate
active
06242374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high thermal conductive silicon nitride sintered body and a method of producing the same, more particularly, to a high thermal conductive silicon nitride sintered body which achieves high strength characteristics, high thermal conductivity, and good heat-radiating characteristics, and has an excellent surface characteristics even if the sintered body is not ground or polished, and is preferably used as various semiconductor substrate or a heat-radiating plate, and a method of producing the high thermal conductive silicon nitride sintered body.
2. Description of the Related Art
Ceramic sintered bodies containing silicon nitride as a main component have strong heat resistance. They resist temperatures as high as 1,000° C. or higher. Silicon nitride ceramic sintered bodies also have strong thermal shock resistance due to their low thermal expansivity. Because of these characteristics, silicon nitride ceramic sintered bodies are expected to be widely used as high-temperature structural materials, most of which are currently made of heat-resistant super alloys. In fact, silicon nitride ceramic sintered bodies are already used for high-strength heat-resistant components and parts of, for example, gas turbines, engines or steel making machines. Further, because of their high corrosion resistance to metal, some silicon nitride ceramic sintered bodies are applied to melt-resistant material for molten metal. Still further, because of their high abrasion resistance, some silicon nitride ceramic sintered bodies are applied to or tested for cutting tools or sliding parts such as bearings.
Various sintering compositions for silicon nitride w ceramic sintered bodies are known: silicon nitride-yttrium oxide-aluminum oxide system; silicon nitride-yttrium oxide-aluminum oxide-aluminum nitride system; and silicon nitride-yttrium oxide-aluminum oxide-oxide of titanium, magnesium or zirconium system.
The oxides of rare earth elements, such as yttrium oxide (Y
2
O
3
) in the sintering compositions listed above, have been widely used as sintering assistant agents. Such rare earth element oxides enhance the sintering characteristics of sintering materials and, therefore, achieve high density and high strength of the sintered bodies.
According to the conventional art, silicon nitride sintered bodies are generally mass-produced as follows. After a sintering assistant agent as mentioned above is added to the powder of silicon nitride, the mixture is molded to form a compact. Then, the compact is sintered in a sintering furnace at about 1,600-1,900° C. for a predetermined period of time followed by cooling in the furnace.
However, though the silicon nitride sintered body produced by the conventional method achieves high mechanical strengths such as toughness, the thermal conductivities thereof are significantly lower than those of aluminum nitride (AlN) sintered bodies, beryllium oxide (BeO) sintered bodies or silicon carbide (SiC) sintered bodies. Therefore, conventional silicon nitride sintered bodies are unsuitable for electronic materials, such as semiconductor substrates, that need good heat-radiating characteristics. Accordingly, the use of silicon nitride sintered body is thus limited.
On the other hand, aluminum nitride sintered bodies have high thermal conductivity and low thermal expansivity, compared with other ceramic sintered bodies. Therefore, aluminum nitride sintered bodies are widely used as packaging materials or materials of circuit base boards for semiconductor chips, which have been progressively improved in operational speed, output power, variety of functions and size. However, no conventional aluminum nitride sintered bodies achieve sufficiently high mechanical strengths. Therefore, there is a growing need for a ceramic sintered body having both high thermal conductivity and high strength.
To cope with the growing need described above, the inventor of this invention had developed a silicon nitride sintered body which is excellent in both mechanical strength and thermal conductivity. However, in the conventional silicon nitride sintered body containing no magnesium oxide (MgO) there is caused a drawback that surface roughness of a surface of the sintered body after sintering operation (hereinafter referred to simply as “sintered surface”) is disadvantageously increased and a size of pore (void) to be formed at a surface portion of the sintered body becomes large. Accordingly, in the conventional sintered body, it was necessary to post-work the sintered surface of the sintered body thereby to expose a worked surface having a desired strength, followed by producing a final product using the sintered body. As a result, there is posed problems that the manufacturing process of the sintered body will become complicated and production cost will be disadvantageously increased.
SUMMARY OF THE INVENTION
The present invention has been made to cope with the problems and demands mentioned above, and an object of the present invention is to provide improvement of a silicon nitride sintered body having a high thermal conductivity, good heat-radiating characteristics, and excellent surface characteristics even if the sintered body is not ground or polished, as well as the high strength characteristics generally inherent in silicon nitride sintered body, and a method of producing such the silicon nitride sintered body.
To achieve the above object, the present inventor studied the effects of the types of silicon nitride powder, sintering assistant agent and additives, the amounts thereof used, and the sintering conditions on the characteristics of the final products, that is, the sintered bodies, by performing experiments.
As the results, the experiments provided the following findings. That is, a silicon nitride sintered body having both high strength and high thermal conductivity can be obtained by: adding certain amounts of a rare earth element, and if necessary, at least one compound selected from the group consisting of oxides, carbides, nitrides, silicides and borides of Ti, Zr, V, Nb, Ta, Cr, Mo and W, to a highly-pure fine powder of silicon nitride to prepare a material mixture; molding the material mixture to form a compact and degreasing the compact: maintaining the compact at a predetermined high temperature for a certain period of time to sinter the compact so as to enhance the density thereof; and then moderately cooling the sintered body at a certain cooling rate.
Further, the following acknowledgement could be obtained. That is, formation of a glass phase (amorphous phase) in the grain boundary phase is effectively suppressed by using a highly pure powder of silicon nitride containing reduced amounts of oxygen and impurity cationic elements, and preparing a silicon nitride molded compact having a reduced thickness before sintering. Thereby, a silicon nitride sintered body having a high thermal conductivity of 70 W/m·K or higher, more preferably, 90 W/m·K or higher can be obtained even if only an oxide of rare earth element is added to a silicon nitride material powder.
If a sintered body in a sintering furnace is cooled by switching off a heating power source of the furnace as performed according to the conventional producing method, the cooling rate was rather high and rapid, that is, about 400-800° C. per hour. However, an experiment performed by the present inventor provided the following findings. That is, the grain boundary phase in the structure of a silicon nitride sintered body is changed from an amorphous phase to a phase including crystal phases by moderately cooling the sintered body at a rate of 100° C. per hour or lower while controlling the cooling rate, thereby achieving both high strength and high thermal conductivity.
The above-mentioned high thermal conductive silicon nitride sintered body itself is partially applied for a patent by the present inventor, and is disclosed in Japanese Unexamined Patent Publication No.
6-135771
, Japanese Unexamined Patent Publication No.
7-4817
Group Karl
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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