Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-08-17
2002-06-11
Dunn, Tom (Department: 1725)
Compositions: ceramic
Ceramic compositions
Refractory
C501S098400
Reexamination Certificate
active
06403510
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to aluminum nitride sintered bodies and manufacturing methods thereof and, more specifically to an aluminum nitride sintered body with high heat conductivity and high strength and a method of inexpensively manufacturing the aluminum nitride sintered body at a low temperature.
2. Description of the Background Art
Recently, non-toxic aluminum nitride (AlN) with high heat conductivity and high insulative property is taking the place of alumina with low heat conductivity or toxic BeO, which has conventionally been used for a substrate of various kinds of electronic parts.
However, aluminum nitride is a material which cannot be readily sintered, so that a sintering temperature must be at least 1800° C. Accordingly, a sintering furnace or jig must be made of an expensive material to ensure sufficient heat resistance. In addition, such a sintering furnace or jig must be frequently replaced due to rapid wastage, resulting in a high running cost including a power consumption cost. This increases the prices of aluminum nitride sintered bodies, whereby the diffusion of the aluminum nitride sintered bodies is restricted.
Further, since it is difficult to sinter aluminum nitride independently, a sintering method has conventionally been employed which utilizes a liquid phase obtained by adding and heating a sintering agent of a rare earth element compound or an alkaline earth metal compound. Another contemplated approach is to add both of the rare earth element compound and alkaline earth metal compound to enable sintering at a low temperature.
For example, Japanese Patent Laying-Open No. 62-153173, Japanese Patent Publication No. 6-49613, Japanese Patent Laying-Open No. 9-175867 or the like discloses a method of manufacturing an aluminum nitride sintered body including an aluminum nitride, rare earth aluminum oxide and alkaline earth metal aluminum oxide by adding a rare earth element compound and alkaline earth metal compound to aluminum nitride. Further, Japanese Patent Laying-Open No. 63-190761, Japanese Patent Publication No. 7-17457 or the like discloses an aluminum nitride sintered body mainly including an aluminum oxide and also including as a sintering agent an yttrium compound of rare earth element and a calcium compound of alkaline rare earth metal element.
Such a combination of the rare earth compound and alkaline rare earth metal compound as the sintering agent enables an aluminum nitride to be sintered at a temperature lower than the conventional case. This enables the manufacture of the aluminum nitride sintered body with high density and conductivity. Recently, the aluminum nitride is finding its usage in a wider range.
However, there still remains a problem. For example, Y
2
O
3
+CaO is used as a typical sintering agent of the rare earth compound and alkaline earth metal compound. As described in Japanese Patent Laying-Open No. 63-190761, Japanese Patent Publication No. 7-17457 or the like, a sufficiently low melting point is not achieved even in the case of the typical sintering agent and, more specifically, only a relative density of 70% to 80% is achieved at a sintering temperature of 1600° C. Densification is achieved at 1650° C., but a condition is extremely unstable. In fact, a sintered body obtained at 1650° C. is not uniform in color. Thus, to increase the yield by stable sintering, a sintering temperature of at least 1700° C. is required. Moreover, the maximum heat conductivity of the aluminum nitride sintered body thus obtained is as low as 110 W/mk.
Japanese Patent No. 2742600 discloses a method of manufacturing an aluminum nitride sintered body with high heat conductivity by using an Yb metal compound and a Ca metal compound. However, the maximum heat conductivity of the sintered body obtained at the sintering temperature of 1600° C. is as low as 130 W/mK. Further, Japanese Patent Laying-Open No. 61-209959 discloses a method of decreasing a sintering temperature by adding a fluoride of rate earth elements. This method suffers from a problem that the method involves high material cost and the inner wall of an apparatus is subjected to corrosion due to a fluorine oxide produced by firing.
Further, Japanese Patent Publication No. 5-7349 discloses that an aluminum nitride sintered body which is highly densified and provided with high conductivity can be obtained by adding a nitride of 3A group elements according to the periodic low table, and at least one of oxide and fluoride of 3A group elements, and a nitride, oxide and fluoride of 2A group element as a sintering agent to an aluminum nitride. However, the nitride of 3A group elements is unstable, absorbing a vapor steam in the atmosphere and readily turning to an oxide, during which cracking or the like is caused by a change in volume. Thus, due care must be exercised in handling to eliminate moisture and the like.
On the other hand, Japanese Patent Publication No. 7-121830 discloses that an aluminum nitride sintered body with high conductivity of at least 200 W/mK is obtained by sintering for at least four hours at 1550° C. to 2050°&pgr;C. in a reducing ambient including a nitrogen gas and carbonic gas. When the component phases of the obtained sintered body is observed by using X-ray diffraction and an electron microscope, only AlN crystal grains are found, but no other phase is observed. As described in the embodiment of the publication, a grain boundary phase is actually reduced and removed by sintering for as long as 96 hours at a temperature as high as 1900° C. in a reduction ambient including a carbonic gas.
However, the AlN grain in the aluminum nitride sintered body which has been sintered for a long period of time at a high temperature tends to be large in size. Accordingly, although high heat conductivity is achieved by removing a grain boundary phase with low heat conductivity, cracking tends to be caused throughout the sintered body. Since the grain boundary phase is removed which connects the AlN grains, the strength of the AlN sintered body is significantly reduced. Moreover, sintering for a long period of time at a high temperature significantly increases cost.
SUMMARY OF THE INVENTION
In view of the above described conventional problems, an object of the present invention is to provide an aluminum nitride sintered body with high heat conductivity and high strength as well as a method of inexpensively manufacturing such an aluminum nitride sintered body at a low temperature.
To achieve the above mentioned object, an aluminum nitride sintered body provided by the present invention mainly includes an aluminum nitride. The aluminum nitride sintered body also includes a compound of at least one type of rare earth element (R) selected from a group of lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm) and europium (Eu), a calcium compound, and an aluminum compound other than an aluminum nitride. The total content of oxygen forming Al
2
O
3
existing in an aluminate including each of the above mentioned rare earth element (R), and yttrium, calcium, and aluminum compounds and oxygen forming independently existing Al
2
O
3
is calculated as 0.01 to 5.0% by weight, heat conductivity is 166 to 200 W/mK, and bending strength is at least 300 MPa.
The aluminum nitride sintered body of the above mentioned present invention preferably includes at least one type of rare earth element (R) selected from a group of lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm) and europium (Eu), where the content of the rare earth element (R) is calculated as 0.05 to 10% by weight in oxide R
2
O
3
form. The aluminum nitride sintered body preferably includes yttrium, where the content of yttrium is calculated as 0.05 to 10% by weight in oxide Y
2
O
3
form. The aluminum nitride sintered body preferably includes calcium, where the content of calcium is calculated as 0.05 to 5% by weight in oxide CaO form. Further, the strength of the aluminum nitride sintered body of the present invention can be increased without causin
Higaki Kenjiro
Ishii Takashi
Kuibira Akira
Nakata Hirohiko
Sasaki Kazutaka
Cooke Colleen P.
Dunn Tom
Fasse W. F.
Fasse W. G.
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