Glass composition and process for producing the same

Details Glass composition and process for producing the same Glass composition and process for producing the same

2001-08-21

2002-04-23

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S070000, C501S072000, C501S073000, C428S410000

Reexamination Certificate

active

06376403

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a glass composition having a high rigidity and a high modulus and a process for producing the same. More particularly, this invention relates to a glass composition suitable for use as a substrate for information recording media which is required to be of high quality and have excellent surface smoothness and a high modulus of elasticity, and to a process for producing the glass composition. The present invention further relates to a substrate for information recording media and an information recording medium both comprising the glass composition.
BACKGROUND OF THE INVENTION
Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, the substrates or media currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For reducing the bending of a substrate or medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young's modulus) of the substrate and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity. However, the aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPa·cm
3
/g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a chemically strengthened glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity.
For example, a glass substrate obtained by subjecting a commercial soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa·cm
3
/g.
Also known besides the above one is a glass substrate obtained by chemically strengthening commercial Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa·cm
3
/g, these properties are still insufficient.
High-rigidity substrates for information recording media which are made of a material other than chemically strengthened glasses are on the market. These substrates comprise a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPa·cm
3
/g. However, these substrates, after polishing, inevitably have residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, these crystallized-glass substrates have a drawback that they are inferior in surface smoothness to the substrates made of a chemically strengthened glass.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young's modulus and a high rigidity, can be easily strengthened chemically, and gives a substrate having high surface smoothness through polishing.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a glass composition which has a high value of modulus of elasticity represented by Young's modulus and a high value of rigidity defined by (Young's modulus)/(specific gravity) and is capable of being effectively inhibited from resonantly vibrating or reduced in the amplitude of resonant vibration.
Another object of the present invention is to provide a glass substrate for information recording media which comprises the glass composition.
Still another object of the present invention is to improve glass quality by diminishing bubbling during glass article production.
The present invention has been achieved in view of the above-described problems of prior art techniques and the above-described requirements.
The present invention provides a glass composition comprising the following basic components in terms of mol %: 40 to 65% SiO
2
, 5 to 25% Al
2
O
3
, 2 to 20% Li
2
O, 0 to 9% Na
2
O, 0 to 10% TiO
2
, 0 to 10% ZrO
2
, 0 to 25% MgO, 0 to 25% CaO, and 0 to 10% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 2 to 40% and the sum of (Li
2
O)/2 and Na
2
O is from 1 to 10 mol %, said glass composition further containing from 0.01 to 5 mol % SnO
2
and up to 0.1 mol % sulfur (S) in terms of the amount of SO
3
.
The glass composition preferably has a rigidity as defined by (Young's modulus)/(specific gravity) of 30 GPa·g
−1
·cm
3
or higher and a modulus of elasticity as represented by Young's modulus of 90 GPa·or higher.
Furthermore, the glass composition is preferably one which has undergone an ion exchange treatment in at least one molten salt containing ions of potassium, sodium, or both.
The present invention further provides a process for producing a glass composition comprising the following basic components in terms of mol %: 40 to 65% SiO
2
, 5 to 25% Al
2
O
3
, 2 to 20% Li
2
O, 0 to 9% Na
2
O, 0 to 10% TiO
2
, 0 to 10% ZrO
2
, 0 to 25% MgO, 0 to 25% CaO, and 0 to 10% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 2 to 40%, wherein SnO
2
and sulfur are further added to the composition, so that the glass composition further contains, as clarifiers, from 0.01 to 5 mol % SnO
2
and up to 0.1 mol % sulfur (S) in terms of the amount of SO
3
when the sum of (Li
2
O)/2 and Na
2
O is from 1 to 10 mol %.
In the process for producing a glass composition, a sulfuric acid salt compound is preferably used as part of batch materials for the composition.
Furthermore, in the process for producing a glass composition, the glass composition is preferably molded by a float process.
The present invention still further provides a substrate for information recording media which comprises the above-described glass composition which has undergone the ion exchange treatment. An information recording medium containing the substrate is also provided by the invention.
The present inventors made intensive investigations on glass compositions having a high Young's modulus. As a result, they have found that it is necessary to limit the contents of Na
2
O and K
2
O, which are alkali metal oxides, to or below certain levels. In particular, since K
2
O is highly effective in lowering Young's modulus, the content thereof is preferably not higher than 0.1 mol %, which is an allowable limit of the amount of K
2
O which can come into a glass as an impurity. More preferably, the content of K
2
O is substantially zero.
Na
2
O is the most effective after K
2
O in lowering Young's modulus. The content thereof is hence preferably 9 mol % or lower, more preferably 5 mol % or lower.
In producing soda-lime glass compositions to be generally molded by a float process, a sulfuric acid salt compound, especially Na
2
SO
4
, is used as a clarifier. The Na
2
SO
4
decomposes in the course of glass melting, and part of the sulfur dissolves as SO
2
, SO
3
, and other forms in the glass. The dissolved sulfur ingredients absorb bubbles in the course of glass cooling, whereby a bubble-free glass of high quality can be produced.
It has however been found that glasses having a low content of alkali metal oxides, especially Na
2
O a

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