Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2000-02-25
2002-04-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C501S005000, C501S068000, C501S069000, C501S070000, C501S072000, C501S073000
Reexamination Certificate
active
06376084
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to glass-ceramics which has a high elastic modulus and is suitable for use as a substrate for information recording media, e.g., hard disks and optomagnetic disks, as a substrate for electrical/electronic parts, and as an optical part or a substrate for optical parts, and a process for producing the same. The present invention further relates to a substrate for an information recording medium having high performances with small deflection, utilizing advantages of high elastic modulus and high heat resistance of the glass-ceramics.
BACKGROUND OF THE INVENTION
Glass-ceramics is generally produced by, for example, maintaining an amorphous glass as a precursor therefor at a temperature of from the glass transition point to about the softening point for a certain period of time. Glass-ceramics combine high surface flatness inherent in glasses, and high mechanical strength and heat resistance improved by crystallization, and has no pores or voids that are difficult to eliminate in conventional ceramics. A further feature of glass-ceramics is that properties such as coefficient of thermal expansion can suitably be regulated according to applications of the glasses.
Due to those properties, glass-ceramics has conventionally been used as heat-resistant tableware and building members and is recently used as substrates for electrical/electronic parts, as optical parts, as substrates for optical parts, or as substrates for information recording media. The term “information recording media” used herein means something in which information is retained in any form. Specifically, an information recording medium is something having a recording layer which partly changes magnetically, physically, chemically, or mechanically by the action of magnetism, light, heat, etc., and retains the changed state permanently or temporarily. Furthermore, the term “substrates for information recording media” used herein means, for example, substrates for magnetic disks, optomagnetic disks, compact disks (CD), and the like to be integrated into hard disks for use in computers, digital cameras, digital video recorders, etc. However, the substrates for information recording media should not be construed as being limited to those.
Substrates for information recording media (hereinafter referred to as “substrates” for simplicity) are always required to attain higher information recording density and higher rate of recording or reading. In particular, with the recent trend toward the recording or processing of information as digital data, the above requirements are becoming highly important. In order to realize a substrate of higher information recording density, flatter surface of the substrate must be obtained. In hard disks, for example, higher recording density can be obtained by narrower distance between the magnetic head and the substrate (lower glide height) employed. However, if surface flatness of the substrate was low, lower glide height causes a serious problem that the magnetic head comes into contact with projections exist on the substrate surface and is thus damaged. On the other hand, in order to realize a substrate available on higher rate of information recording or readout, mechanical strength (elastic modulus) of the substrate must be improved. In hard disks, for example, a higher rate of recording or readout can be obtained by employing higher rotational speed of the substrate. However, higher substrate rotational speed results in larger substrate bending and a larger amplitude of substrate vibration, and this leads to a strong fear that the magnetic head hits on the substrate and the head, films on substrate, and information recorded are damaged.
Hitherto, substrates made of an aluminum alloy have generally been used for hard disks. Although such substrates made of an aluminum alloy (hereinafter referred to as “aluminum substrates”) have merits of inexpensive, good formability, etc., they have drawbacks, for example, that the elastic modulus thereof is insufficient and there is no way to improve the elastic modulus thereof, and the surface flatness thereof cannot be improved beyond a certain level. An elastic modulus (Young's modulus) of these aluminum substrates is 71 GPa, and it is thought that aluminum substrates are inapplicable to hard disks which are driven at a rotational speed of 10,000 rpm or higher and expected to become the mainstream in the future, due to the low elastic modulus thereof. This is because when rotated at such a high speed, the aluminum substrates bend more and vibrate at larger amplitude to cause a problem therefore the glide height cannot be lowered. In addition, since further size reduction will be required to various information recording devices including hard disks in the future, thinner substrates will also be required. However, aluminum substrates cannot meet the above requirement because a thickness reduction therein results in larger bending.
Substrates made of a glass-ceramics (hereinafter referred to as “crystallized substrates”) are superior to aluminum substrates in flatness and elastic modulus. Despite this, however, conventional crystallized substrates have become unable to meet the recent requirements concerning higher recording densities and higher rates of recording or reading sufficiently. Under these circumstances, crystallized substrates of higher elastic modulus have been proposed. For example, JP-A-8-91873 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a glass-ceramics of major crystalline phase containing nickel spinelloid (solid solution of NiAl
2
O
4
and Ni
2
SiO
4
). U.S. Pat. Nos. 5,476,821 and 5,491,116 and International Publication WO 98/22405 describe a devised technique in which the composition of a glass-ceramics is changed to improve the elastic modulus.
Further, JP-A-10-188260 discloses a technique of improving the flatness of a glass-ceramics containing lithium disilicate (Li
2
O.2SiO
2
) as a major crystalline phase and cristobalite (SiO
2
) crystals coexistent therewith by adding manganese (MnO) and chromium (Cr
2
O
3
) to the glass in an amount of from 1 to 3 wt %.
However, the prior art techniques have the following problems.
The glass-ceramics disclosed in JP-A-8-91873 contains nickel oxide (NiO) in a large amount. Since nickel is an expensive element, this glass-ceramics is expensive and unsuitable for mass production.
The glass-ceramics described in U.S. Pat. Nos. 5,476,821 and 5,491,116 and International Publication WO 98/22405 has a drawback that devitrification occurs during glass forming even if the glass is not cooled rapidly. Consequently, this prior art glass has high devitrification temperature and is hence difficult to mass-produce in a stable manner. Another drawback of this glass-ceramics is that it is difficult to examine the glass-ceramics for defects such as surface mars and adherent foreign substances because the glass-ceramics is milk-white or hazy.
Further, the glass-ceramics disclosed in JP-A-10-188260 has a drawback that its elastic modulus (Young's modulus) is low because the major crystalline phase thereof comprises lithium disilicate (Li
2
O.2SiO
2
).
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the above-described problems of the prior art techniques.
One object of the present invention is to provide a glass-ceramics which has a high elastic modulus, can be produced easily, and is inexpensive.
Another object of the present invention is to provide a process for producing the glass-ceramics.
Still another object of the present invention is to provide at low cost a crystallized substrate which can effectively suppress bending or vibrating.
Further object of the present invention is to provide an information recording medium.
Still further object of the present invention is to provide an information recording device.
The glass-ceramics according to the present invention has a major crystalline phase constituted of crystals containing manganese (Mn).
The proce
Kishimoto Shoichi
Koyama Akihiro
Tanaka Hiroyuki
Jones Deborah
McNeil Jennifer
Nippon Sheet Glass Co. Ltd.
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