Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
2000-09-26
2001-09-04
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S141000, C428S426000, C428S690000, C428S690000, C428S900000, C501S004000, C501S005000, C501S007000
Reexamination Certificate
active
06284340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a crystallized glass, a magnetic disc substrate using the crystallized glass, and a magnetic disc using the magnetic disc substrate.
2. Description of Related Art
Generally, predetermined thermal expansion coefficient, smooth surface, high strength and so on are required for a magnetic disc substrate. Particularly, in a use of the magnetic disc substrate, a glass having a thermal expansion coefficient of 60-90 ×10
−7
/k level at near room temperature is required. Moreover, in order to improve a record density of a hard disc, it is necessary to decrease a suspension amount of a magnetic head, and thus a highly smooth surface is required for the substrate. Further, a magnetic film decreases its characteristics if it reacts with alkali metal ion. Therefore, it is necessary to avoid strongly an intrusion of alkali metal ion from the substrate to the magnetic film.
In Japanese Patent Laid-Open Publication No. 11-16151 (JP-A-11-16151) and Japanese Patent Laid-Open Publication No. 11-16143 (JP-A-11-16143), it was disclosed a crystallized glass for the magnetic disc in which a main crystal phase was &agr;-quartz phase and lithium disilicate phase. In Japanese Patent Laid-Open Publication No. 10-226532 (JP-A-10-226532), it was disclosed a crystal glass for the magnetic disc in which a chemical durability was improved.
In the known crystallized glass having cristobalite phase and lithium disilicate phase as the main crystal phase, or, in the known crystallized glass having &agr;-quartz phase and lithium disilicate phase as the main crystal phase, crystal particles are liable to be large in both cases, and thus there arises a problem such that a surface roughness after a precise polishing treatment becomes not so smooth. Particularly, in the crystallized glass having &agr;-quartz phase and lithium disilicate phase as the main crystal phase, a thermal expansion coefficient is about 100×10
−7
/k, and thus it is too large to be used for the magnetic disc. Moreover, in the crystallized glass having cristobalite phase and lithium disilicate phase as the main crystal phase, or, in the crystallized glass having &agr;-quartz phase and lithium disilicate phase as the main crystal phase, an elution amount of lithium from the substrate is large, and thus an affection applying to a magnetic film is worried about.
For example, in the above JP-A-11-16151 and JP-A-1 1-16143, an elution of lithium is not mentioned at all. Moreover, since &agr;-quartz phase is precipitated as a condensed state, a surface roughness (Ra) after a precise polishing treatment is 3-9 angstrom at best. In the above JP-A-10-226532, a chemical durability of glass is described in relation to a specific composition and also it is explained with reference to a glass weight variation, but an elution of lithium is not described at all.
SUMMARY OF THE INVENTION
An object of the invention is to suppress an agglomeration of &agr;-quartz phase or a bulk generation of crystal particles, decrease a thermal expansion coefficient at a temperature range from −50° C. to +70° C. and prevent an elution of lithium from a crystallized glass, in a crystallized glass of SiO
2
-Al
2
O
3
-Li
2
O series having lithium disilicate phase and &agr;-quartz phase.
According to the invention, a crystallized glass, which has a main crystal phase of petalite (Li
2
O.Al
2
O
3
.8SiO
2
) phase and lithium disilicate (Li
2
O.2SiO
2
) phase and an auxiliary crystal phase of &agr;-quartz phase, comprises a crystal phase composition measured by Rietvelt method of:
20 wt %≦petalite phase≦40 wt %,
25 wt %≦lithium disilicate phase≦45 wt %, and
10 wt %≦&agr;-quartz phase≦25 wt %.
During various studies about the crystal glass of SiO
2
—Al
2
O
3
—Li
2
O series having lithium disilicate phase and &agr;-quartz phase, the inventors found such a phenomenon that, when a crystallized temperature was decreased at about 10-30° C., a precipitation amount of &agr;-quartz phase was decreased and instead a large amount of petalite was precipitated.
By precipitating the petalite phase whose amount is larger than &agr;-quartz phase, the petalite phase and the lithium disilicate phase were precipitated as a main crystal phase, and the &agr;-quartz phase was precipitated as an auxiliary crystal phase. In this case, it was possible to control a thermal expansion coefficient at a temperature range from −50° C. to 70° C. to a level of 60-90×10
−7
/k. In addition, it was found that, by precipitating the petalite phase as the main crystal phase, a surface roughness after precisely polishing the crystallized glass could be smaller. Moreover, it was found that, by increasing a crystal amount of petalite phase, an elution amount of lithium could be extraordinarily smaller. In this manner mentioned above, the present invention was achieved.
The petalite is less apt to be large particles. Therefore, if a precipitated amount of petalite phase is increased, it is supposed that a crystallized glass having fine crystal grain can be produced.
Moreover, during a step of precipitating &agr;-quartz phase, raw materials are deformed into &agr;-quartz via petalite. In this case, when &agr;-quartz (SiO
2
) phase is once precipitated, lithium component that is existent in petalite (Li
2
O.Al
2
O
3
.8SiO
2
) is moved into a non-crystallized remaining glass. Therefore, it is supposed that an amount of lithium in the remaining glass is increased and an elution amount of lithium is also increased. On the other hand, petalite includes lithium in its crystal. Therefore, it is also supposed that, if a precipitated amount of petalite is large, an amount of lithium existent in the petalite crystal is increased accordingly, so that an elution amount of lithium is decreased. However, the inventors find after an actual lithium elution test that, in the crystallized glass according to the invention, a lithium elution can be suppressed extraordinarily to such an extent that it is not explained by such a mechanism mentioned above.
That is to say, if the above mechanism is correct, it is supposed that an elution amount of lithium is in substantially proportion to an amount of lithium existent in the remaining glass. However, in the crystallized glass according to the invention, a decrease of lithium elution amount is remarkable beyond our prediction, and an amount of lithium existent in the remaining glass may be also decreased.
In the present invention, the lithium disilicate phase and the petalite phase are the main crystal phase and the &agr;-quartz phase is the auxiliary crystal phase. This indicates that an amount of lithium disilicate phase and an amount of petalite phase are respectively larger than that of &agr;-quartz phase. Differences between an amount of lithium disilicate phase or an amount of petalite phase and an amount of &agr;-quartz phase are not particularly limited, but such a difference is preferably not lower than 5 wt %, more preferably 10 wt %.
A crystal phase composition in the crystallized glass is preferable if it is as follows:
30 wt %≦petalite phase≦40 wt %,
25 wt %≦lithium disilicate phase≦40 wt %, and
10 wt %≦&agr;-quartz phase≦20 wt %.
A crystallinity of the crystallized glass according to the invention is preferable if it is not lower than 70 wt %, more preferably 75 wt %.
A method of producing the crystallized glass according to the invention is not limited, but one example is as follows.
If an amount of SiO
2
in a raw glass is limited to not lower than 70 wt %, a crystal grain can be made fine. If it is limited to not more than 80 wt %, the raw glass is easily melted. More preferably, an amount of SiO
2
is not lower than 72 wt % but not more than 77 wt %.
If an amount of Al
2
O
3
in the raw glass is less than 5 wt %, the &agr;-quartz phase is not precipitated and a thermal expansion coefficient of the crystallized glass is lowered. If an amount of Al
2
O
3
exceeds 10 wt %, a solubility of the glass is deteriorated.
Abe Masahiro
Takahashi Takahiro
Burr & Brown
NGK Insulators Ltd.
Resan Stevan A.
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