Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-10-18
2002-09-17
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S069000, C501S070000, C501S073000, C428S064200, C428S065100, C428S410000
Reexamination Certificate
active
06451720
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a glass composition having a high modulus of elasticity, more particularly a glass composition which is suitable for use as an information recording medium substrate required to have high surface smoothness and a high modulus of elasticity. The 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 recording capacity and a shorter access time are required to information recording devices such as hard disks. One possible way to realize the latter requirement is to rev up the spinning speed of an information recording medium (hereinafter referred to as “recording medium” for simplicity)
However, since substrates for information recording media (hereinafter referred to simply as “substrates”) have a slight distortion by their own weight, 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 a magnetic head. This is the reason why the narrow gap between a magnetic head and a recording medium cannot be employed, and this prevents information recording devices from decreasing their recording capacity.
In order to reduce the bending and resonance of substrates, we should use substrates whose modulus of elasticity (Young's modulus) is higher than that of conventional substrates. The commercial substrates made of an aluminum alloy, whose modulus of elasticity is 71 GPa, cannot cope with high rotational speeds of 10,000 rpm and above. Although there is a need to use thinner substrate for device miniaturization, aluminum substrates need to have a larger thickness so as to compensate for the deficiency in modulus of elasticity. Glass substrates made of chemically tempered soda-lime glass are also commercially available. However, Young's modulus of this substrate is only 72 GPa and there is the same drawback as the aluminum substrates.
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
or MgO and from 0.5 to 17 mol % Y
2
O
3
, as a glass free from the drawback described above. JP-A-10-81540 discloses a glass having a glass transition point of 750° C. or higher and a glass containing a rare-earth oxide such as Y
2
O
3
, Nd
2
O
3
, Gd
2
O
3
, Ho
2
O
3
, or La
2
O
3
in an amount or 18 mol % or larger, 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
, or the like. International Publication WO 98/55993 discloses various glasses having a modulus of elasticity as represented by Young's modulus of 100 GPa or higher and a density of 3.5 g·cm
−3
or lower. Furthermore, International Publication WO 98/09922 discloses a glass in which the specific modulus as determined by dividing the modulus of elasticity represented by Young's modulus by the density is 36 GPa·g
−1
·cm
3
or higher.
However, since most of the glasses shown in the Examples given in the references cited above contain no alkali ingredients, they cannot be tempered by ion exchange and, hence, they do not have a sufficient breaking strength. Although glasses containing an alkali ingredient are described therein, most of these have a Young's modulus as low as below 110 GPa and the remaining glasses, having a Young's modulus of 110 GPa or higher, have a density as high as 3.1 g·cm
−3
or above. These glasses have a specific modulus lower than 39.0 GPa·g
−1
·cm
3
. When such glasses are used as substrates, they bend considerably at a high rotational speed, or impose an excessive load on the drive. Specific modulus is the value obtained by dividing modulus of elasticity represented by Young's modulus by density. Another drawback of those glasses is that since they contain any of Al
2
O
3
, MgO, CaO, TiO
2
, and ZrO
2
in an excessively large amount, they easily devitrificates during glass production and, hence, it is hard to obtain a homogeneous glass.
It is inevitable that information recording devices will come to be more strongly required to attain a larger recording capacity, a shorter access time, etc. However, any of the conventional substrates cannot sufficiently meet these requirements.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the above-described problems of conventional techniques.
One object of the invention is to provide a glass composition which has a high modulus of elasticity and a low density, i.e., has a high specific modulus.
Another object of the invention is to provide at low cost a glass composition which is suitable for use as an information recording medium substrate, for example, because it can be formed easily, is less apt to devitrificate, is suitable for mass production, and can be easily made to have high surface smoothness by polishing.
Still another object of the invention is to use this, glass composition to provide a substrate and a recording medium each contributing to an increase in recording density and a reduction in access time in information recording devices to thereby contribute to further progress in an information-oriented society.
These objects are accomplished with a glass composition comprising, in terms of mol %, 35 to 45% silicon dioxide (SiO
2
), 15 to 20% aluminum oxide (Al
2
O
3
), 3 to 10% lithium oxide (Li
2
O), 0.1 to 5% sodium oxide (Na
2
O), 15 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 2 to 10% titanium dioxide (TiO
2
), 0.5 to 4% zirconium oxide (ZrO
2
), 4 to 12% TiO
2
+ZrO
2
, and 0 to 4% yttrium oxide (Y
2
O
3
).
The invention further provides a glass composition comprising, in terms of mol %, 35 to 42% silicon dioxide (SiO
2
), 15 to 20% aluminum oxide (Al
2
O
3
), 5 to 10% lithium oxide (Li
2
O), 0.1 to 2% sodium oxide (Na
2
O), 20 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 5 to 10% titanium dioxide (TiO
2
), 1 to 4% zirconium oxide (ZrO
2
), 6 to 12% TiO
2
+ZrO
2
, and 0 to 4% yttrium oxide (Y
2
O
3
).
The invention furthermore provides a glass composition comprising, in terms of mol %, 35 to 40% silicon dioxide (SiO
2
), 15 to 20% aluminum oxide (Al
2
O
3
), 3 to 10% lithium oxide (Li
2
O), 0.1 to 5% sodium oxide (Na
2
O), 15 to 30% magnesium oxide (MgO), 0 to 10% calcium oxide (CaO), 0 to 4% strontium oxide (SrO), 25 to 35% RO (MgO+CaO+SrO), 2 to 10% titanium dioxide (TiO
2
), 1 to 4% zirconium oxide (ZrO
2
), 4 to 12% TiO
2
+ZrO
2
, and 0 to 4% yttrium oxide (Y
2
O
3
).
In a preferred embodiment of the glass compositions of the invention described above, the ratio of the content of Li
2
O to that of (Li
2
O+Na
2
O) is from 0.76 to 0.99.
In another preferred embodiment of the glass compositions of the invention described above, the ratio of the content of MgO to that of (MgO+CaO+SrO) is from 0.76 to 1.00.
In still another preferred embodiment of the glass compositions of the invention described above, the ratio of the content of Al
2
O
3
to that of (Li
2
O+Na
2
O) is from 1.5 to 4.0.
The invention still further provides a glass composition which has a modulus of elasticity as represented by Young's modulus of 110 GPa or higher and a density lower than 3.1 g/cm
3
and in which the specific modulus G as determined by dividing the modulus of elasticity by the density thereof satisfies the following relationship (1).
G
≥
9.36
{
∑
i
u
i
x
i
/
d
i
}
·
{
∑
i
v
i
x
i
}
/
{
∑
i
w
i
x
i
}
(
1
)
A further preferred embodiment of the glass compositions of the invention described above i
Kishimoto Shoichi
Koyama Akihiro
Tanaka Hiroyuki
Nippon Sheet Glass Co. Ltd.
Sample David
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