Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2001-03-29
2003-09-09
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
Reexamination Certificate
active
06617272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an Si
3
N
4
sintered body having a high thermal conductivity as well as an excellent mechanical strength and further to a method for producing the same.
2. Description of the Prior Art
Heretofore, since an Si
3
N
4
sintered body comprising Si
3
N
4
as a main component has an excellent high-temperature property, the body has been used as materials for structural parts and mechanical parts used under high temperature condition. However, although a silicon nitride sintered body obtained by a prior producing method is excellent in mechanical strength such as toughness, it has been difficult to apply it to parts or members in which a high thermal-shock resistance is required because of being inferior in thermal conductivity as compared with aluminum nitride (AIN), silicon carbide (SiC), or beryllium oxide (BeO).
The reason why the thermal conductivity of the Si
3
N
4
sintered body is low is that phonons as a carrier for thermal conduction are scattered by impurities such as oxygen ions which dissolve into the crystal of Si
3
N
4
and form a solid solution. In Si
3
N
4
powder which has heretofore been used as a raw material for an Si
3
N
4
sintered body, since oxygen dissolves in the state of solid solution during the producing process, usually, oxygen of approximately 1.5% is contained. Oxygen existing in Si
3
N
4
powder and oxygen in a naturally-formed oxide film composed mainly of SiO
2
being produced at the powder surface are diffused and dissolve into the crystals of Si
3
N
4
to form a solid solution in the sintering process, thereby causing phonon scattering.
In one of the methods for producing an Si
3
N
4
sintered body, there is a reactive sintering-method using silicon powder as a raw material powder and nitriding this silicon powder. However, also in the reactive sintering-method, oxygen in the naturally-formed oxide film existing on particle surfaces of the silicon powder dissolves into the silicon particles and Si
3
N
4
particles formed during nitriding process of silicon and forms a solid solution. Further, oxygen dissolves into the Si
3
N
4
particles also in a subsequent sintering process and forms a solid solution. Such a solid solution unavoidably reduces the thermal conductivity of the resultant Si
3
N
4
sintered body.
Therefore, a method for producing an Si
3
N
4
sintered body having a high thermal conductivity has been studied. For example, in Japanese Patent Laid-Open No. 9-268069, a method has been proposed in which Si
3
N
4
powder is used as a raw material and oxides of 3A group elements in the periodic table are added while the amount of oxygen and the amount of aluminum are controlled. In this method, the material is fired at a temperature of 1500 to 1800° C., and further fired at a temperature of 2000° C. or higher in nitrogen of 1.5 atm or higher for 5 to 10 hours. The resultant sintered body is further heat-treated in an non-oxidizing atmosphere at a temperature of 1000 to 1400° C. Although it is said that this method provides an Si
3
N
4
sintered body having a thermal conductivity at least 70 W/m.K, it can not be considered to be a method suitable for the mass production from the viewpoint of the energy cost because high temperature and high pressure are required in this method.
On the other hand, when an Si
3
N
4
sintered body is prepared using the silicon powder as a raw material, by the reactive sintering method, nitriding and sintering reactions do not progress sufficiently when using only Si powder. For this reason, in order to achieve a highly densified and strengthened product by nitriding and sintering, in general, rare earth oxide or alkaline earth oxide are added. For example, Japanese Patent Laid-Open No. 7-109176 describes a method for producing an Si
3
N
4
sintered body by reactive sintering at the temperature range of 1400 to 1850° C. applicable in mass production in which silicon powder and a sintering aid consisting of Y
2
O
3
powder and Al
2
O
3
powder are used as raw materials. However, the thermal conductivity of the Si
3
N
4
sintered body produced by this method is only approximately 40 W/m.K, which is not the value to be satisfied.
As described above, in order to obtain an Si
3
N
4
sintered body having a high thermal conductivity, there has been proposed a method for achieving a high thermal conductivity and a high strength by sintering Si
3
N
4
powder at a high temperature under a high pressure, but since the producing cost is increased due to an increase in the energy cost or the like, the method has not been practical. Moreover, even in the case where an Si
3
N
4
sintered body is produced at a low price by a reactive sintering method, there is a difficulty in obtaining a high thermal conductivity although the Si
3
N
4
sintered body obtained is excellent in mechanical strength.
SUMMARY OF THE INVENTION
In view of such previous circumstances, an object of the invention is to provide an Si
3
N
4
sintered body having a high thermal conductivity together with a high strength property inherent in an Si
3
N
4
sintered body, at a low cost, by a reactive sintering method.
In order to achieve the above-mentioned object, the invention provides a Si
3
n
4
sintered body. In accordance with the present invention, a Si
3
N
4
sintered body is produced by reactive sintering of silicon, the Si
3
N
4
sintered body comprising crystal grains of Si
3
N
4
and a grain boundary phase, wherein a compound of at least one element selected from the group consisting of Y, Yb and Sm is contained in an amount of 0.6 to 13% by weight in terms of oxide Ln
2
O
3
thereof, an oxygen content in the crystal grains of Si
3
N
4
is not more than 1% by weight, a ratio of oxygen and Ln in the Si
3
N
4
sintered body is within a range of 0.1 to 0.8 in a molar ratio SiO
2
/Ln
2
O
3
of the oxygen in terms of SiO
2
to the oxide Ln
2
O
3
, the Si
3
N
4
sintered body comprising a relative density in the range of 85 to 99.9%, a thermal conductivity of at least 70 W/m.K or more, and a three point bending strength of at least 600 Mpa.
Furthermore, the Si
3
N
4
crystal grains are &bgr;-type ones having an average grain size of not less than 2 &mgr;m in terms of major axis, and in the grain boundary phase, preferably a compound ln
a
Si
b
O
c
N
d
(wherein 2≦a≦4, 2≦b≦3, 0≦c≦7, 2≦d≦4), more preferably a compound Yb
4
Si
2
O
7
N
2
is contained.
A method for producing the Si
3
N
4
sintered body according to the invention comprises:
mixing 80 to 99% by weight of a silicon powder having an oxygen content not more than 1% by weight and 1 to 20% by weight of a powdered oxide Ln
2
O
3
(where Ln=Y, Yb or Sm) of at least one element selected from the group consisting of Y, Yb and Sm, thereby providing a powdered raw material;
molding the raw material into a molded body;
nitriding the molded body in an atmosphere containing nitrogen at a temperature of not higher than 1400° C.; and
sintering the nitride body obtained in an atmosphere containing nitrogen at a temperature of 1700 to 1950° C.
In this method for producing the Si
3
N
4
sintered body, a reducing coating agent in an amount of 1 to 10% by weight based on the weight of the silicon powder is further added to and mixed with the above-described powdered raw material, and a molded body of the resultant mixture is head-treated with in a vacuum of not more than 100 Torr or in an atmosphere containing nitrogen at a temperature of 200 to 800° C., and then the above-described nitriding and sintering can be performed. As the reducing coating agent of this case, a compound including C, H, O and metallic cations can be used, and specifically, a coupling agent including Si or Ti as the metallic cations is preferable.
Moreover, in the method for producing the Si
3
N
4
sintered body, the content of oxygen, Y, Yb and Sm in the powdered raw material is preferably within the range of 0.1 to 2 in terms of a molar ratio of oxides, SiO
2
/Ln
2
O
3
, and more preferably the range is within 0.1 to 0.8.
DETAILED
Itoh Ai
Miyanaga Michimasa
Group Karl
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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