Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
2000-11-06
2003-05-06
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S519500, C252S519540, C252S521200, C252S521300, C252S521500, C252S500000, C360S097010, C360S098040, C360S135000
Reexamination Certificate
active
06558582
ABSTRACT:
This application is based on patent application Nos. 9-360095 and 10-150445 filed in Japan, the contents of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention relates to semiconductive ceramics for use in ceramic heaters, ceramic sensors, resistor substrates and the like, and a supporting member for supporting a magnetic disk substrate.
In general, ceramics is an insulating material. Recently, however, there have been marketed semiconductive ceramics having the property of semiconductivity. Silicon carbide ceramics or Perovskite ceramics, such as lanthanum chromite, have been conventionally used as semiconductive ceramics for use in ceramic heaters, ceramic sensors or the like.
Also, there have been-proposed semiconductive structural ceramics in which conductive material, such as metal oxides, metal nitrides, or metal carbides, is added in a conventional structural ceramics. For example, Japanese Unexamined Patent Publications Nos. (HEI) 2-295009 and 1-243388 respectively disclose alumina semiconductive ceramics and zirconia semiconductive ceramics which are obtained by mixing TiO
2
, TiC, NiO, or CoO as conductive material with alumina material or zirconia material, and sintering the mixture in a reducing condition.
Further, there have been recently proposed use of a semiconductive ceramics as a supporting member for supporting a magnetic disk substrate in a magnetic disk unit.
As shown in
FIG. 1
, specifically, a magnetic disk unit
50
, which is used as an external storage means of a computer, comprises a hub
14
fixedly attached to a rotary shaft
13
, and magnetic disk substrates
15
placed on the periphery of the hub
14
, and ring-shaped spacers
11
placed on the periphery of the hub
14
. The spacers
11
are placed between the magnetic disk substrates
15
. Further, the magnetic disk substrates
15
and the spacers
11
are pressed with a ring-shaped shim
10
and a cramp
12
, and tightly fastened on the hub
14
by screws
16
. Indicated at
17
are magnetic heads.
The rotary shaft
13
is rotated by a driver. The magnetic disk substrates
15
rotates together with the rotation of the rotary shaft
13
without coming into contact with the magnetic heads
17
so that information is written and read at a specified position of a magnetic disk substrate
15
.
The magnetic disk unit
50
has been demanded to store a large amount of information at a higher density. Accordingly, it is required to make the distance between the magnetic head
17
and the magnetic disk substrate
15
shorter, and increase the flatness and the smoothness of the magnetic disk substrate
15
. For this reason, the magnetic disk substrate
15
has been proposed to be made of glass material because the glass material makes it possible to ensure a higher surface flatness and smoothness. In this case, the supporting member for supporting the magnetic disk substrate
15
, that is, the spacer
11
, the shim
10
, and the clamp
12
, are made of ceramics or glass in order to prevent the magnetic disk substrate
15
from deforming due to a difference in thermal expansion between the magnetic disk substrate
15
and the supporting member. Japanese Examined Patent Publication No. (HEI) 5-80745 and Japanese Unexamined Patent Publication No. (SHO) 61-148667 disclose such a technique.
However, the conventional supporting member is made of insulating glass or ceramics. Recently, it has been revealed that the magnetic disk substrate
15
held by the supporting member made of insulating ceramics or glass has an electric charge, consequently causing damages in the writing and reading of information on the magnetic disk substrate
15
. To solve this problem, a film of metal such as aluminum or zinc has been proposed to be coated in the surface area of the supporting member that comes into contact with the magnetic disk substrate
15
to ground the electric charge.
However, the provision of the metal film decrease the flatness of the contact surface of the supporting member, consequently causing a strain in the magnetic disk substrate
15
, and further a likelihood that the magnetic head
17
comes into contact with the magnetic disk substrate
15
to damage it. In addition, there is a likelihood that the metallic film peels off from the supporting member due to a thermal expansion difference relative to the glass or ceramics supporting member, then cutting off the grounding of electric charge in the magnetic disk substrate
15
.
In view of these problems, Japanese Unexamined Patent Publication No. (HEI) 2-226566 proposes a technique of making a supporting member of semiconductive ceramics to prevent electric charge from accumulating in the magnetic disk substrate
15
.
In the field of jigs and tools for assembly of electronic parts, further, the electric conductivity has been required to prevent accumulation of electric charge in addition to the property of high a abrasion resistance and corrosion resistance. In view thereof, the jig or tool is considered to be made of semiconductive ceramics.
However, it has been difficult to produce the above-mentioned semiconductive ceramics at low costs. For example, silicon carbide ceramics is a difficult-to-sinter material. Specifically, silicon carbide ceramics is required to be sintered at 2000° C. or more in a non-oxidizing condition. In addition, the hot pressing and the hot isostatic pressing are required. As a result, the cost is high but the productivity is low.
Also, perovskite ceramics may be sintered in an oxidizing condition. However, the unit price of raw material is high. Further, perovskite ceramics generally has a bending strength less than 100 MPa, which is too low to be used as a structural material.
In production of alumina or zirconia semiconductive ceramics containing NiO, CoO or the like as a conductive provider, the hot pressing and the hot isostatic pressing are required. Further, after sintering in an oxidizing condition, another sintering is required to be conducted in a reducing condition. Such complicated production manner lowers the productivity, and makes the production control difficult. The requirement of two sinterings considerably increases the production costs.
Further, in the case of a supporting member made of the above-mentioned semiconductive ceramics, a thermal expansion difference of 2×10
−6
/° C. to 5×10
−6
/° C. occurs between the supporting member and the magnetic disk substrate
15
made of glass. Such thermal expansion difference causes a strain in the magnetic disk substrates
15
, and impairs the parallelism between the magnetic disk substrates
15
.
Further, as material for jigs and tools capable of removing electric charges and preventing excessive current due to electric charge, an electric conductivity of 10
5
&OHgr;·cm to 10
11
&OHgr;·cm in terms of volume specific resistance is required. However, alumina semiconductive ceramics added with TiO
2
or TiC cannot have an electric conductivity greater than 10
−2
&OHgr;·cm in terms of volume specific resistance. Accordingly, alumina semiconductive ceramics are not suitable for electric charge preventive jig or tool.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide semiconductive ceramics, and a supporting member for supporting a magnetic disk substrate which have overcome the problems residing in the prior art.
According to an aspect of the present invention, a semiconductive ceramic comprises 80 to 93 weight percent of a compound of MgO and SiO
2
, and 7 to 20 weight percent of an iron oxide, and having an electric conductivity of 10
7
&OHgr;·cm or more but less than 10
12
&OHgr;·cm in terms of volume specific resistance.
According to another aspect of the present invention, a supporting member for supporting a magnetic disk substrate at a specified position is made of a semiconductive ceramic comprising 80 to 93 weight percent of a compound of MgO and SiO
2
, and 7 to 20 weight percent of an iron oxide, and having an electric conductivity of 10
7
&OHgr;·cm or more but less t
Hayasaki Tetsuji
Okumura Masahiro
Gupta Yogendra N.
Hamlin Derrick G.
Hogan & Hartson LLP
Kyocera Corporation
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