Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-05-29
2004-03-30
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S069000
Reexamination Certificate
active
06713418
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a glass article which is suitable for use as a substrate for information recording media such as, e.g., hard disks and optomagnetic disks, a substrate for electrical/electronic parts, an optical part, and a substrate for optical parts, and which has a high coefficient of thermal expansion even at low temperatures and can be easily produced by the float process. The invention further relates to a glass article capable of readily giving a clean glass surface through cleaning with an acidic cleaning fluid.
DESCRIPTION OF THE RELATED ART
With recent progress in the handling of digital information, the desire for a small storage medium having a large capacity is becoming extremely strong. For satisfying the desire, it is essential to heighten the recording density. At present, the storage medium which has a satisfactory balance among storage capacity, device size, cost, etc. is the hard disk device (HDD) employing a glass substrate. Besides being used in computers heretofore, hard disks are expected to be used in large quantities in domestic electrical appliances such as video recorders, cameras, and television receivers in the future. For realizing a hard disk having a heightened recording density, it is necessary to employ a substrate which not only has enhanced surface flatness but can be easily mass-produced.
A glass which is for use as a glass substrate for information recording media and is suitable for mass production is disclosed in Japanese Patent Laid-Open No. 357318/2000 so as to meet the desire. There is a description therein to the effect that this glass for substrate use has a Young's modulus of 85 GPa or higher and excellent weatherability and is suitable for mass production by a continuous forming method, e.g., the float process.
On the other hand, in hard disks employing a glass substrate, the other mechanical parts include ones made of a metal, e.g., stainless steel, although the substrate is made of a glass. Glasses differ from metals in the coefficient of thermal expansion. The glass substrates which have hitherto been used as substrates for hard disks generally have a lower coefficient of thermal expansion than metals, and there has been a considerable difference between these two kinds of materials in the coefficient of thermal expansion especially in the low-temperature range of from −50° C. to 70° C. (a temperature range centering around room temperatures at which appliances are used). Such a difference in thermal expansion coefficient results in a difference in expansion/contraction behavior with changing temperature between the substrate and the metallic mechanical parts. As a result, it becomes difficult to maintain the accuracy of the relative positions of a magnetic head for recorded-signal readout and recording bits.
Substrates made of a crystallized glass as a material relatively close to metals in thermal expansion coefficient have been disclosed. For example, the crystallized glasses disclosed in Japanese Patent Laid-Open Nos. 16142/1999, 16143/1999, and 16151/1999 are ones formed by precipitating lithium disilicate, &agr;-quartz, a solid solution thereof, &agr;-cristobalite, and a solid solution thereof so as to have a coefficient of thermal expansion close to that of metals. Furthermore, Japanese Patent Laid-Open No. 301732/1997 discloses a glass for substrate use which has a high coefficient of thermal expansion.
However, the related-art techniques have had the following problems. The glass for substrate use disclosed in Japanese Patent Laid-Open No. 357318/2000, which includes no description concerning thermal expansion coefficient, can arouse troubles attributable to a difference in thermal expansion coefficient between the glass substrate and other components of a hard disk. Another drawback is that since the content of alkali oxides (R
2
O=Li
2
O+Na
2
O+K
2
O) in the glass is low, the glass has poor suitability for chemical strengthening (a compression layer reaching a sufficient depth from the surface cannot be formed; a high surface compression stress cannot be attained) and hence has low mechanical strength even after an ion-exchange treatment. Furthermore, since the glass has a high total content of SiO
2
and Al
2
O
3
besides the low alkali oxide content, the glass tends to have a high melt viscosity and a high melting temperature (temperature T
2
at which the viscosity reaches 10
2
P). Namely, a high temperature is necessary for melting, and this not only results in an increased fuel cost and considerable deterioration of glass-melting apparatus but also poses a problem that the glass melt has poor homogeneity and hence gives a substrate glass having poor homogeneity. Because of this, the glass plate produced has unallowable defects such as warpage and undulation.
The crystallized glasses disclosed in Japanese Patent Laid-Open Nos. 16142/1999, 16143/1999, and 16151/1999 necessitate a heat treatment for crystal precipitation after the production of a raw glass. For example, in the technique disclosed in Japanese Patent Laid-Open No. 16143/1999, a raw glass is heat-treated at 450 to 550° C. for 1 to 12 hours, subsequently further heat-treated at 600 to 800° C. for 1 to 12 hours, and then polished. Namely, this related-art technique necessitates heat treatments in the period of from raw glass production to polishing. This is an important factor inhibiting the reduction of production cost. Furthermore, the glass in which crystals can be precipitated by a heat treatment has a serious drawback that since this glass is more susceptible to crystal precipitation, i.e., devitrification, than other glasses (glasses to be finally used as amorphous glasses), it undesirably devitrifies in glass production by a continuous process such as, e.g., the float process.
The glass for use as substrates disclosed in Japanese Patent Laid-Open No. 301732/1997 has a coefficient of thermal expansion as high as 75×10
−7
/° C. or above in the temperature range of from 50 to 350° C. However, there is no description therein concerning thermal expansion coefficient at low temperatures of from −50° C. to 70° C. Furthermore, this glass for substrate use has a drawback that since it does not contain Li
2
O, it has poor suitability for chemical strengthening and hence has low strength even after an ion-exchange treatment.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the above-described problems of the related-art techniques. An aim of the invention is to provide a glass which has a high coefficient of thermal expansion at low temperatures (around room temperature), is capable of giving a clean surface free from defects such as mars through cleaning with, e.g., an acidic cleaning fluid, and can be easily mass-produced at low cost. Another aim of the invention is to provide at low cost a substrate which satisfactorily matches in thermal expansion coefficient with metallic components of an information recording device.
The invention has been achieved in order to eliminate the problems of the related-art techniques described above and to accomplish a technical subject for obtaining a substrate suitable for high-density recording. As a result of intensive investigations on the compositions of aluminosilicate glasses, it has become possible to provide a glass having a high coefficient of thermal expansion at low temperatures and having a moderate rate of etching with hydrofluoric acid by ingeniously balancing the contents of components. It has further becomes possible to provide a glass article which can be produced by the float process, which can yield glass plates on a large scale, without raising technical difficulties in melting and plate formation. The present invention has the following constitution.
(1) A glass article having:
a rate of dissolution in an acidic liquid of from 10 to 100 nm/min in terms of an etching rate in immersing in a 0.1% by weight 50° C. aqueous solution of hydrofluoric acid; and
an average linear thermal expansion coeffici
Kishimoto Shoichi
Koyama Akihiro
Tanaka Hiroyuki
Yamamoto Nobuyuki
Bolden Elizabeth A.
Nippon Sheet Glass Co. Ltd.
Sample David
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