Glass composition and substrate for information recording...

Details Glass composition and substrate for information recording... Glass composition and substrate for information recording...

1999-09-22

2002-06-04

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S070000, C428S690000

Reexamination Certificate

active

06399527

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a glass composition having a high elastic modulus, more particularly a glass composition which is suitable for use as an information recording medium substrate required to have excellent surface smoothness, a high elastic modulus, and a low specific gravity and is easy for mass production. The present invention further relates to an information recording medium substrate, an information recording medium, and an information recording device each comprising the glass composition.
BACKGROUND OF THE INVENTION
A larger and larger recording capacity and a shorter and shorter access time are required to information recording devices such as magnetic disks. One possible way to realize the latter requirement is to rev up the spinning speed of an information recording medium (hereinafter referred to simply as “recording medium”).
However, since substrates for information recording media (hereinafter referred to simply as “substrates”) are bent down by their own weight, and the distortion makes substrates resonate considerably when a higher spinning speed is employed. Such a vibrational resonance causes a fatal crash of recording media by a collision between the medium and magnetic head. This is the reason why the narrow gap between a magnetic head and a recording medium cannot be employed when using a current substrate (hereinafter, the magnetic head/recording medium gap is referred to as “flying height”), and this prevents information recording devices from increasing their recording capacity.
In order to reduce bending and resonance of substrates, we should use substrates whose elastic modulus represented by Young's modulus is higher than that of conventional substrates (hereinafter, this elastic modulus is referred to simply as “Young's modulus”). The aluminum alloy which has been most commonly used as the substrates of magnetic disks has a Young's modulus of 71 GPa and cannot cope with high spinning speeds of 10,000 rpm and above. Although there is a need to use thinner substrate for device miniaturization, the aluminum substrates need to have a larger thickness so as to cope with such high rotational speeds.
Substrates made of a tempered glass have a higher Young's modulus than that of such aluminum substrates. For example, a glass substrate made of a commercially available soda-lime glass tempered by immersing into a molten potassium salt for ion-exchange is on the market. This substrate has a Young's modulus of 72 GPa. Furthermore, a substrate comprising glass ceramics whose a Young's modulus is 90 GPa is also on the market. However, this glass ceramics substrate has residual crystal grains at the surface after surface polishing because the glass ceramics contains precipitated crystals inside. Namely, this glass ceramics substrate has a large drawback accompanied with the tempered glass substrates at a standpoint of surface smoothness after surface polishing.
JP-A-10-81542 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a glass which is based on SiO
2
—Al
2
O
3
—RO (wherein R is a bivalent metal) and contains at least 20 mol % Al
2
O
3
, at least 20 mol % MgO, and from 0.5 to 17 mol % Y
2
O
3
. Furthermore, JP-A-10-81540 discloses a glass having a glass transition point of 750° C. or higher, while JP-A-10-79122 discloses a glass based on SiO
2
—Al
2
O
3
—RO (wherein R is a bivalent metal) and containing TiO
2
, ZrO
2
, Y
2
O
3
, etc. However, since these glasses basically contain no alkali ingredients, they cannot be tempered by ion exchange,. and as a result, they do not have a sufficient breaking strength.
International Publication Wo 98/55993 discloses various glasses having a Young's modulus represented by Young's modulus of 100 GPa or higher and a liquidus temperature of 1,350° C. or lower and a substrate obtained by forming any of these glass compositions with a mold. However, the forming process using a mold where molten glass is formed to products basically one by one, seen in production of bottles or cups, is unsuitable for mass production. In addition, such glass compositions, whose a liquidus temperature is 1,350° C., are difficult to vitrify, and it is exceedingly difficult to mold these glasses by a continuous process, e.g., a float process.
In future improvements in information recording devices, improvements in substrates with higher spinning speed and thinner thickness become more important. Moreover, since the amount of stored information is growing larger and larger with the progress of an information-oriented society, there is a social demand for a substrate and a recording medium of inexpensive and high performance.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a glass composition which has a high Young's modulus and a low density and suitable for mass production by a continuous process and is easy to obtain high surface smoothness.
Other objects of the present invention are to provide a substrate comprising the glass composition and reduced in bending and resonance even upon high-speed rotation, to provide a recording medium usable at a lower flying height, and to provide an information recording device having a higher recording capacity and a shorter access time.
The present invention has been achieved in view of the problems and requirements of the prior art techniques described above. Specifically, the present inventors have carried out intensive investigations on relationships between the contents of components and properties including Young's modulus in aluminosilicate glasses. The glass system whose properties will be described below is found as the fruits of the investigations.
The present invention provides a glass composition comprising, in terms of mol %, 40 to 55% silicon dioxide (SiO
2
), 0.5 to 6% aluminum oxide (Al
2
O
3
), 2 to 20% lithium oxide (Li
2
O), 0 to 10% sodium monoxide (Na
2
O), 2 to 30% R
2
O (R
2
O=Li
2
O+Na
2
O), 5 to 25% magnesium oxide (MgO), 0 to 25% calcium oxide (CaO), 0 to 10% strontium oxide (SrO), 10 to 40% RO (RO=MgO+CaO+SrO), 0 to 10% titanium dioxide (TiO
2
), and 0 to 5% zirconium oxide (ZrO
2
).
Preferred modes of the glass composition of the present invention are as follows:
the glass composition comprising, in terms of mol %, 45 to 55% silicon dioxide (SiO
2
), 0.5 to 5% aluminum oxide (Al
2
O
3
), 10 to 20% lithium oxide (Li
2
O), 2 to 6% sodium monoxide (Na
2
O), 15 to 26% R
2
O (R
2
O=Li
2
O+Na
2
O), 10 to 17% magnesium oxide (MgO), 0 to 7% calcium oxide (CaO), 2 to 8% strontium oxide (SrO), 12 to 24% RO (RO=MgO+CaO+SrO), 0.5 to 5% titanium dioxide (TiO
2
), and 0 to 2% zirconium oxide (ZrO
2
);
the glass composition wherein the ratio of the content of Li
2
O to that of (Li
2
O+Na
2
O) in terms of mol % is from 0.75 to 0.95;
the glass composition wherein the ratio of the content of MgO to that of (MgO+CaO+SrO) in terms of mol % is from 0.35 to 0.85;
the glass composition wherein the ratio of the content of (Li
2
O+Na
2
O) to that of (MgO+CaO+SrO) in terms of mol % is 0.90 or larger;
the glass composition wherein the ratio of the content of Al
2
O
3
to that of (Li
2
O+Na
2
O) is 0.25 or smaller;
the glass composition which virtually contains neither yttrium oxide (Y
2
O
3
) nor any rare-earth oxide;
the glass composition which contains virtually no CaO;
the glass composition whose liquidus temperature is 1,200° C. or lower;
the glass composition whose elastic modulus as represented by Young's modulus is 95 GPa or higher and whose density of 3.0 g/cm
3
is lower;
the glass composition which can be easily polished to obtain optically polished surface with free abrasive grains, the rate of polishing, which is represented in terms of thickness reduction per unit time by polishing, is not lower than that of soda-lime glasses; and
the glass composition which is experienced an ion exchange treatment by immersing into a molte

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