Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2000-04-10
2002-05-14
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S064200, C428S065100, C428S195100, C428S702000, C501S055000, C501S064000, C501S068000, C501S069000, C501S070000, C501S072000
Reexamination Certificate
active
06387510
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass for a data storage medium substrate, particularly a glass for a magnetic recording medium substrate and a glass substrate for data storage media, particularly a glass substrate for magnetic recording media, having a high Young's modulus.
2. Discussion of Background
Glass substrates have been used as substrates for data storage media, particularly as substrates for magnetic disks (hard disks), and a glass (hereinafter referred to as conventional glass) consisting of 65.3 mol % of SiO
2
, 8.6 mol % of Al
2
O
3
, 3.5 mol % of ZrO
2
, 12.5 mol % of Li
2
O and 10.1 mol % of Na
2
O is mentioned as an example. The conventional glass is usually subjected to chemical tempering treatment before use.
In recent years, in order to increase the storage capacity, it has been desired to increase the number of magnetic disks mounted by making the substrates thin. On the other hand, the space between a magnetic disk and a magnetic reading head has become small in order to increase the storage density, and accordingly, increase in bending or warp of the substrates caused by making the substrates thin, may break the magnetic disks.
To solve such a problem of bending and warp of the substrates, a glass having a high Young's modulus has been desired. Here, the Young's modulus of the above conventional glass is 82 GPa.
Further, it is required for a substrate for data storage media that its surface state does not significantly change during stock, and that films formed on said substrate, such as a base film, a magnetic film and a protective film, are not likely to separate off. Namely, corrosion resistance is required.
The corrosion resistance of the conventional glass is by no means high, but it will improve to an acceptable level by chemical tempering treatment. This is considered to be because an alkali component which is considered to be the main cause to decrease the corrosion resistance of the conventional glass is extracted and removed from the surface of the glass by the chemical tempering treatment. However, there are problems in the chemical tempering treatment such that the steps will increase, the surface of the substrate after the chemical tempering treatment is likely to be soiled, etc.
Further, in order to increase the storage density, it is effective to increase the coercive force of a magnetic layer as a magnetic recording layer, and accordingly, it is necessary to carry out heat treatment to form the magnetic layer at a higher temperature. From this point of view, a glass to be used for a substrate is required to have a high glass transition temperature. Here, the glass transition temperature of the above conventional glass is 500° C.
Conventionally, the glass substrates for magnetic disks have been mainly used as 2.5 inch substrates (glass substrate outer diameter: 65 mm) to be used for e.g. notebook size personal computers. However, there is a high possibility that the glass substrates are increasingly used as larger substrates to be used for e.g. servers, i.e. 3.0 inch substrates (glass substrate outer diameter: 84 mm), 3.5 inch substrates (glass substrate outer diameter: 95 mm), etc. Accordingly, the glass to be used for such glass substrates, is required to be suitable for mass production.
The mass production of the glass is carried out by means of a glass melting furnace. An AZS (Al
2
O
3
—ZrO
2
—SiO
2
) type electrocast brick is usually used for the glass melting furnace at a portion which will be directly in contact with molten glass. Accordingly, the corrosiveness of molten glass against the AZS type electrocast brick is required to be small.
Further, the mass production of a sheet glass is widely carried out by a continuous forming method represented by float process. As the continuous forming method, in addition to the float process, fusion down draw process and slot down draw process may, for example, be mentioned. Accordingly, the glass is required to be produced by a continuous forming such as float process.
WO99/06333 discloses a method for producing a glass substrate for data storage media, which comprises direct press forming of a glass having a Young's modulus of at least 90 GPa and consisting of from 0.1 to 30 mol % of TiO
2
, from 1 to 45 mol % of CaO, from 5 to 40 mol % of MgO+CaO, from 3 to 30 mol % of Na
2
O+Li
2
O, from 0 to 15 mol % of Al
2
O
3
and from 35 to 65 mol % of SiO
2
. Here, the direct press forming is not a continuous forming method.
The present inventors have conducted a follow-up examination on some of glass as disclosed in WO99/06333 (Examples 41 and 42 in Table 5 as shown hereinafter). As a result, they have considered that it is difficult to produce a sheet glass by applying a continuous forming method to the glass as disclosed in WO99/06333.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problems, and to provide a glass for a data storage medium substrate, particularly a glass for a magnetic recording medium substrate and a glass substrate for data storage media, particularly a glass substrate for magnetic recording media.
The present invention provides a glass for a data storage medium substrate, which consists essentially of the following components as represented by mol %:
SiO
2
60 to 72,
Al
2
O
3
2 to 9
MgO
3 to 9,
CaO
2 to 10,
SrO
0 to 15,
ZnO
0 to 4,
TiO
2
0 to 8,
ZrO
2
0 to 4,
Li
2
O
1 to 12,
Na
2
O
0 to 8,
K
2
O
0 to 5,
Y
2
O
3
0 to 5,
La
2
O
3
0 to 5, and
Li
2
O + Na
2
O + K
2
O
4 to 15,
and which has a Young's modulus of at least 85 GPa, and a glass substrate for data storage media comprising the glass for a data storage medium substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail with reference to the preferred embodiments.
The Young's modulus of the glass for a data storage medium substrate of the present invention (hereinafter referred to simply as glass of the present invention) is at least 85 GPa. If it is less than 85 GPa, the problem of bending and warp may arise. It is preferably at least 88 GPa, more preferably at least 90 GPa.
The glass transition temperature of the glass of the present invention is preferably at least 550° C. If it is less than 550° C., the temperature for heat treatment for forming a magnetic layer can not be made adequately high, hereby it tends to be difficult to increase the coercive force of the magnetic layer. It is more preferably at least 560° C., furthermore preferably at least 570° C., still furthermore preferably at least 580° C., particularly preferably at least 600° C., and most preferably at least 610° C.
The glass of the present invention preferably satisfies T
L
−T
4
<50, where T
L
is the liquidus temperature (unit: ° C.), and T
4
is the temperature (unit: ° C.) at which the viscosity becomes 10
4
P (poise). If T
L
−T
4
≧50, it may be difficult to form the sheet glass by a continuous forming such as float process. More preferably T
L
−T
4
<40, particularly preferably T
L
−T
4
<30.
Now, the composition of the glass of the present invention as represented by mol % (hereinafter “mol %” will be referred to simply as “%”) will be explained.
The glass of the present invention preferably consists essentially of:
SiO
2
60 to 72,
Al
2
O
3
2 to 8,
MgO
3 to 8,
CaO
3 to 10,
SrO
0 to 8,
ZnO
0 to 4,
TiO
2
1 to 8,
ZrO
2
0.2 to 4,
Li
2
O
1 to 12,
Na
2
O
0 to 3,
K
2
O
0 to 2,
Y
2
O
3
0 to 5,
La
2
O
3
0 to 5, and
Li
2
O + Na
2
O + K
2
O
4 to 14.
More preferably, the glass of the present invention consists essentially of:
SiO
2
62 to 70,
Al
2
O
3
3 to 7,
MgO
3.5 to 7.5,
CaO
3.5 to 8,
SrO
0.5 to 7,
ZnO
0 to 3.5,
TiO
2
1.5 to 7,
ZrO
2
Koike Akio
Nakao Yasumasa
Nakashima Tetsuya
Asahi Glass Company Limited
Jones Deborah
McNeil Jennifer
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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