U.S. patent application number 09/933016 was filed with the patent office on 2002-04-25 for glass composition and process for producing the same.
This patent application is currently assigned to NIPPON SHEET GLASS CO., LTD.. Invention is credited to Kishimoto, Shoichi, Koyama, Akihiro, Kurachi, Junji, Yamamoto, Nobuyuki.
Application Number | 20020049128 09/933016 |
Document ID | / |
Family ID | 26461000 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020049128 |
Kind Code |
A1 |
Koyama, Akihiro ; et
al. |
April 25, 2002 |
Glass composition and process for producing the same
Abstract
A glass composition is disclosed which has a high modulus of
elasticity (Young's modulus) and a high rigidity (modulus of
elasticity/specific gravity) and is capable of being effectively
inhibited from bending or vibrating. Also disclosed is a process
for producing the glass composition. The glass composition
comprises, in terms of mol %, 40 to 65% SiO.sub.2, 5 to 25%
Al.sub.2O.sub.3, 2 to 20% Li.sub.2O, 0 to 9% Na.sub.2O, 0 to 10%
TiO.sub.2, 0 to 10% ZrO.sub.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.sub.2O)/2 and Na.sub.2O is from 1 to 10
mol %, said glass composition further containing, as clarifiers,
from 0.01 to 5 mol % SnO.sub.2 and up to 0.1 mol % sulfur (S) in
terms of the amount of SO.sub.3.
Inventors: |
Koyama, Akihiro; (Osaka,
JP) ; Kishimoto, Shoichi; (Osaka, JP) ;
Kurachi, Junji; (Osaka, JP) ; Yamamoto, Nobuyuki;
(Osaka, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN,
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, NW
Washington
DC
20037-3213
US
|
Assignee: |
NIPPON SHEET GLASS CO.,
LTD.
|
Family ID: |
26461000 |
Appl. No.: |
09/933016 |
Filed: |
August 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09933016 |
Aug 21, 2001 |
|
|
|
09293991 |
Apr 19, 1999 |
|
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6306786 |
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Current U.S.
Class: |
501/69 ;
501/70 |
Current CPC
Class: |
C03C 3/062 20130101;
Y10T 428/315 20150115; C03C 3/085 20130101; C03C 3/087
20130101 |
Class at
Publication: |
501/69 ;
501/70 |
International
Class: |
C03C 003/085; C03C
003/087 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 1998 |
JP |
HEI. 10-124308 |
Claims
What is claimed is:
1. A glass composition comprising the following basic components of
mol %: 40 to 65% SiO.sub.2, 5 to 25% Al.sub.2O.sub.3, 2 to 20%
Li.sub.2O, 0 to 9% Na.sub.2O, 0 to 10% TiO.sub.2, 0 to 10%
ZrO.sub.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.sub.2O)/2 and Na.sub.2O is from 1 to 10 mol %, said
glass composition further containing from 0.01 to 5 mol % SnO.sub.2
and up to 0.1 mol % sulfur (S) in terms of the amount of
SO.sub.3.
2. The glass composition as claimed in claim 1, which has a
rigidity as defined by (Young's modulus)/(specific gravity) of 30
GPa.multidot.g.sup.-1.multidot.cm.sup.3 or higher and a modulus of
elasticity as represented by young's modulus of 90 GPa or
higher.
3. The glass composition as claimed in claim 2, which has undergone
an ion exchange treatment in at least one molten salt containing
ions of potassium, sodium, or both.
4. A process for producing a glass composition comprising the
following basic components in terms of mol %: 40 to 65% SiO.sub.2,
5 to 25% Al.sub.2O.sub.3, 2 to 20% Li.sub.2O, 0 to 9% Na.sub.2O, 0
to 10% TiO.sub.2, 0 to 10% ZrO.sub.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.sub.3 and sulfur are further added as
clarifier to the composition, so that said glass composition
further contains, as clarifiers, from 0.01 to 5 mol % SnO.sub.2 and
up to 0.1 mol % sulfur (S) in terms of the amount of SO.sub.3 when
the sum of (Li.sub.2O)/2 and Na.sub.2O is from 1 to 10 mol %.
5. The process for producing a glass composition as claimed in
claim 4, wherein a sulfuric acid salt compound is used as part of
batch materials for the composition.
6. The process for producing a glass composition as claimed in
claim 4, where the glass composition is molded by a float
process.
7. A substrate for information recording media which comprises a
glass composition comprising the following basic components in
terms of mol %: 40 to 65% SiO.sub.2, 5 to 25% Al.sub.2O.sub.3, 2 to
20% Li.sub.2O, 0 to 9% Na.sub.2O, 0 to 10% TiO.sub.2, 0 to 10%
ZrO.sub.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.sub.2O)/2 and Na.sub.2O is from 1 to 10 mol %, said
glass composition further containing from 0.01 to 5 mol % SnO.sub.2
and up to 0.1 mol % sulfur (S) in terms of the amount of SO.sub.3.
having a rigidity as defined by (Young's modulus)/(specific
gravity) of 30 GPa.multidot.g.sup.-1.multidot- .cm.sup.3 or higher
and a modulus of elasticity as represented by young's modulus of 90
GPa or higher.
8. The substrate for information recording media as claimed in
claim 7, which has undergone an ion exchange treatment in at least
one molten salt containing ions of potassium, sodium, or both.
9. An information recording medium using the substrate as claimed
in claim 7.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.sup.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.
[0005] In contrast, substrates made of a chemically strengthened
glass are superior to the aluminum substrate in both modulus of
elasticity and specific gravity.
[0006] 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.sup.3/g.
[0007] 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.sup.3/g, these properties are still
insufficient.
[0008] 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 nd a rigidity of 38
GPa-cm.sup.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.
[0009] 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
[0010] 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.
[0011] Another object of the present invention is to provide a
glass substrate for information recording media which comprises the
glass composition.
[0012] Still another object of the present invention is to improve
glass quality by diminishing bubbling during glass article
production.
[0013] The present invention has been achieved in view of the
above-described problems of prior art techniques and the
above-described requirements.
[0014] The present invention provides a glass composition
comprising the following basic components in terms of mol %: 40 to
65% SiO.sub.2, 5 to 25% Al.sub.2O.sub.3, 2 to 20% Li.sub.2O, 0 to
9% Na.sub.2O, 0 to 10% TiO.sub.2, 0 to 10% ZrO.sub.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.sub.2O)/2 and
Na.sub.2O is from 1 to 10 mol %, said glass composition further
containing from 0.01 to 5 mol % SnO.sub.2 and up to 0.1 mol %
sulfur (S) in terms of the amount of SO.sub.3.
[0015] The glass composition preferably has a rigidity as defined
by (Young's modulus)/(specific gravity) of 30 GPa-g.sup.-1-cm.sup.3
or higher and a modulus of elasticity as represented by Young's
modulus of 90 GPa or higher.
[0016] 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.
[0017] 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.sub.2, 5 to 25%
Al.sub.2O.sub.3, 2 to 20% Li.sub.2O, 0 to 9% Na.sub.2O, 0 to 10%
TiO.sub.2, 0 to 10% ZrO.sub.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.sub.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.sub.2 and up to 0.1 mol %
sulfur (S) in terms of the amount of SO.sub.3 when the sum of
(Li.sub.2O)/2 and Na.sub.2O is from 1 to 10 mol %.
[0018] In the process for producing a glass composition, a sulfuric
acid salt compound is preferably used as part of batch materials
for the composition.
[0019] Furthermore, in the process for producing a glass
composition, the glass composition is preferably molded by a float
process.
[0020] 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.
[0021] 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.sub.2O and
K.sub.2O, which are alkali metal oxides, to or below certain
levels. In particular, since K.sub.2O 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.sub.2O which can come into a glass as an impurity. More
preferably, the content of K.sub.2O is substantially zero.
[0022] Na.sub.2O is the most effective after K.sub.2O in lowering
Young's modulus. The content thereof is hence preferably 9 mol % or
lower, more preferably 5 mol % or lower.
[0023] In producing soda-lime glass compositions to be generally
molded by a float process, a sulfuric acid salt compound,
especially Na.sub.2SO.sub.4, is used as a clarifier. The
Na.sub.2SO.sub.4 decomposes in the course of glass melting, and
part of the sulfur dissolves as SO.sub.2, SO.sub.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.
[0024] It has however been found that glasses having a low content
of alkali metal oxides, especially Na.sub.2O and Li.sub.2O, have a
reduced content of residual sulfur.
[0025] The inventors have further found that when the residual
sulfur content in a glass is expressed in terms of the amount of
SO.sub.3, this amount correlates with the content of Li.sub.2O and
Na.sub.2O in the glass, i.e., with (Li.sub.2O)/2+Na.sub.2O (mol %).
For example, soda-lime glasses in which the amount of (Li.sub.2O)
/2+Na.sub.2O has been reduced to 10 mol % or 5 mol % have an amount
of residual SO.sub.3 about one-half or about one-fifth,
respectively, the amount of residual SO.sub.3 in ordinary soda-lime
glasses.
[0026] There has hence been a problem that in glasses having a low
residual SO.sub.3 content, a sufficient clarifying effect cannot be
obtained with Na.sub.2SO.sub.4, which is an ordinary clarifier.
[0027] The present inventors furthermore made investigations on a
method for glass composition clarification applicable also to a
float process, which is a molding technique capable of producing
especially highly flat glass plates.
[0028] As a result of intensive studies made by the present
inventors in order to eliminate the above-described problem, they
have found that a sufficient clarifying effect is obtained in a
glass in which the amount of (Li.sub.2O)/2+Na.sub.2O is 10 mol % or
smaller and which contains SnO.sub.2 in an amount of from 0.01 to 5
mol %. The presence of SnO.sub.2 is effective especially in a glass
in which the amount of (Li.sub.2O)/2+Na.sub.2O is 5 mol % or
smaller.
[0029] Usable materials for the SnO.sub.2 include SnO.sub.2, SnO,
and other tin compounds.
[0030] It has also been found that the above clarifying effect is
further enhanced by using a sulfuric acid salt compound as one of
the batch materials for the above glass. The present invention has
been thus achieved.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Examples of the sulfuric acid salt used as a batch material
include Li.sub.2SO.sub.4, Na.sub.2SO.sub.4, MgSO.sub.4, CaSO.sub.4,
SrSO.sub.4, and BaSO.sub.4. Especially preferred of these are
Li.sub.2SO.sub.4 and Na.sub.2SO.sub.4. If the sulfur content in the
batch materials is too low, the clarifying effect thereof is
insufficient. Even if the content thereof is too high, the
clarifying effect thereof is not heightened anymore and the result
is increased emission of SO.sub.x gases causative of air pollution.
Consequently, the sulfuric acid salt as a batch material is used in
an amount of generally from 0.1 to 10%, preferably from 0.2 to 2%,
in terms of sulfur amount based on the SiO.sub.2 (mol %) contained
in the glass to be produced. When the sulfuric acid salt is used in
a proportion within the above range, the glass obtained has a
residual sulfur content of from 0 to 0.1 mol %.
[0032] Although As.sub.2O.sub.3, Sb.sub.2O.sub.3, and compounds
thereof have hitherto been generally employed as clarifiers, these
compounds are undesirable because of their toxicity. In the case of
using a float process, use of As.sub.2O.sub.3 or Sb.sub.2O.sub.3 is
more undesirable in that it is reduced by tin metal to generate
defects on the glass surface.
[0033] The reasons for limitations of the makeup of the glass
composition are as follows. Hereinafter, all percents are by
mole.
[0034] SiO.sub.2 is the main component constituting the glass. If
the proportion of SiO.sub.2 is lower than 40%, the glass has
impaired chemical durability. On the other hand, if the proportion
thereof exceeds 64%, the desired modulus of elasticity and rigidity
are not obtained. Consequently, the proportion of SiO.sub.2 should
be from 40 to 65%.
[0035] Al.sub.2O.sub.3 is an ingredient which improves the rigidity
of the glass and increases the depth of a compression stress layer
formed by ion exchange. Al.sub.2O.sub.3 further serves to improve
the water resistance of the glass. If the proportion of
Al.sub.2O.sub.3 is lower than 5%, these effects are insufficient.
On the other hand, if the proportion thereof exceeds 25%, the
results are an increased viscosity, an increase in liquidus
temperature which is severer than the viscosity increase, and
impaired meltability. Consequently, the proportion of
Al.sub.2O.sub.3 should be from 5 to 25%, and is preferably from 10
to 20%.
[0036] Li.sub.2O, which is an ingredient to be replaced in ion
exchange, serves to lower the melting temperature of the glass to
thereby enhance its meltability. If the proportion of Li.sub.2O is
lower than 2%, ion exchange cannot be conducted and the glass has
too high a melting temperature. On the other hand, if the
proportion thereof exceeds 20%, the substrate has impaired
weatherability and impaired acid resistance. Consequently, the
proportion of Li.sub.2O should be from 2 to 20%, and is preferably
from 2 to 15%.
[0037] Na.sub.2O, which is an ingredient to be replaced in ion
exchange, serves to lower the melting temperature and the liquidus
temperature to thereby enhance meltability. If the proportion of
Na.sub.2O exceeds 9%, the glass has a reduced modulus of elasticity
and impaired weatherability and acid resistance. Consequently, the
proportion of Na.sub.2O should be 9% or lower, and is preferably 5%
or lower.
[0038] TiO.sub.2 is an ingredient which improves the modulus of
elasticity and weatherability of the glass. However, if the
proportion thereof exceeds 10%, the glass has an elevated liquidus
temperature and impaired devitrification resistance. Consequently,
the proportion of TiO.sub.2 should be 10% or lower.
[0039] ZrO.sub.2 is an ingredient which improves the modulus of
elasticity and weatherability of the glass. However, if the
proportion of ZrO.sub.2 exceeds 10%, the glass has an elevated
liquidus temperature and impaired devitrification resistance.
Consequently, the proportion of ZrO.sub.2 should be 10% or
lower.
[0040] MgO is an ingredient which heightens the modulus of
elasticity and meltability of the glass. However, if the proportion
of MgO exceeds 25%, the glass has an elevated liquidus temperature
and impaired devitrification resistance. Consequently, the
proportion of MgO should be 25% or lower.
[0041] CaO is an ingredient which heightens the modulus of
elasticity and meltability of the glass. However, if the proportion
of CaO exceeds 25%, the glass has an elevated liquidus temperature
and impaired devitrification resistance. Consequently, the
proportion of CaO is preferably 25% or lower.
[0042] SrO is an ingredient which enhances the meltability of the
glass. However, if the glass contains SrO in a large amount, it
disadvantageously has an increased specific gravity and a reduced
rigidity. Consequently, the proportion of SrO is preferably 10% or
lower.
[0043] If the total amount of MgO, CaO, and SrO (i.e., the amount
of RO) is below 2%, the glass has a low modulus of elasticity and
insufficient meltability If the total amount thereof exceeds 40%,
the glass has an elevated liquidus temperature and impaired
devitrification resistance. Consequently, the total amount of RO is
preferably from 2 to 40%.
[0044] SnO.sub.2 is an ingredient which not only improves the
modulus of elasticity and weatherability of the glass, but also
serves as an effective clarifier. If the proportion of SnO.sub.2 is
lower than 0.01%, sufficient effects cannot be obtained. On the
other hand, proportions thereof exceeding 5% are undesirable in
that devitrification occurs to generate SnO.sub.2 crystals.
Consequently, the proportion of SnO.sub.2 is generally from 0.01 to
5%, preferably from 0.1 to 2%.
[0045] Besides the ingredients described above, other ingredients
may be added in a total amount of up to 3%. Examples of such
optional ingredients include Fe.sub.2O.sub.3, CoO, NiO, and MnO for
coloring, and further include ZnO, Y.sub.2O.sub.3, La.sub.2O.sub.3,
and CeO.sub.2.
[0046] This glass, which contains Li.sub.2O or contains Li.sub.2O
and Na.sub.2O, can be made to have an increased fracture strength
by immersing the glass in at least one molten salt containing ions
of potassium and sodium or of either at a temperature not higher
than the distortion point of the glass to thereby interchange these
ions and thus generate a compression stress on the surface of the
glass.
[0047] When this glass composition is used as a substrate for
information recording media, this substrate is less apt to bend or
suffer resonant vibration because it has a high rigidity.
Therefore, the recording medium employing this glass composition is
especially suitable for use in recording apparatuses of the high
rotational speed type.
[0048] Since the glass composition described above contains not
As.sub.2O.sub.3 nor Sb.sub.2O.sub.3, which are toxic, but SnO.sub.2
as a clarifier, it is excellent in quality with respect to freedom
from bubbles. The glass composition is preferred also in that due
to the nonuse of As.sub.2O.sub.3 or Sb.sub.2O.sub.3, it develops no
surface defects even when molded by a float process.
[0049] The present invention will be explained below in more detail
by reference to Examples. However, the invention should not be
construed as being limited to these Examples.
EXAMPLES 1 TO 10
[0050] Ten compositions as examples of the glass composition of the
present invention are shown in Table 1 together with properties
thereof.
1 TABLE 1 Example 1 2 3 4 5 6 7 Component (mol %) SiO.sub.2 41.9
47.0 62.6 63.0 54.9 45.4 45.0 Al.sub.2O.sub.3 9.5 12.0 10.3 10.0
10.0 15.0 20.0 Li.sub.2O 16.7 3.0 19.0 19.0 5.0 10.0 2.0 Na.sub.2O
0.0 0.0 0.0 0.0 7.0 4.0 8.0 K.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 0.0
MgO 12.5 12.0 2.5 2.5 0.0 20.0 19.8 CaO 18.3 22.0 4.5 4.5 5.0 0.0
4.0 SrO 0.0 0.0 0.0 0.0 10.0 0.0 1.0 ZrO.sub.2 0.0 3.0 0.0 0.0 5.0
0.0 0.0 TiO.sub.2 0.0 0.0 0.0 0.0 0.0 5.0 0.0 SnO.sub.2 1.0 1.0 1.0
1.0 3.0 0.5 0.1 SO.sub.3 0.058 0.004 0.081 0.000 0.085 0.063 0.069
As.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Total 100.0 100.0 100.0
100.0 100.0 100.0 100.0 Sulfate Material (%/SiO.sub.2)
Li.sub.2SO.sub.4 1.0 1.0 1.0 Na.sub.2SO.sub.4 2.0 0.5 0.2 Number of
bubbles (per 100 g) 0.0 0.0 0.6 9.0 0.0 0.0 0.3 Specific gravity
(g/cm.sup.3) 2.77 2.86 2.45 2.45 2.76 2.95 2.73 Modulus of
elasticity (GPa) 108 104 90 90 92 107 91 Rigidity (GPa .multidot.
cm.sup.3/g) 39 36 37 37 33 36 33 Example Comparative Example 8 9 10
1 2 3 4 Component (mol %) SiO.sub.2 50.1 50.1 54.9 63.0 58.2 71.4
67.0 Al.sub.2O.sub.3 15.0 15.0 15.6 11.0 15.6 0.9 10.4 Li.sub.2O
5.0 5.0 10.8 19.0 10.8 0.0 0.0 Na.sub.2O 0.0 0.0 4.2 0.0 4.2 12.7
13.0 K.sub.2O 0.0 0.0 0.0 0.0 0.1 0.5 2.3 MgO 17.9 17.9 3.0 2.5 3.0
6.0 5.2 CaO 5.0 5.0 4.5 4.5 4.5 8.4 0.5 SrO 0.0 0.0 3.0 0.0 3.0 0.0
0.0 ZrO.sub.2 5.0 5.0 0.0 0.0 0.0 0.0 0.0 TiO.sub.2 0.0 0.0 0.9 0.0
0.6 0.0 0.6 SnO.sub.2 2.0 2.0 3.0 0.0 0.0 0.0 0.0 SO.sub.3 0.005
0.000 0.082 0.000 0.000 0.170 0.000 As.sub.2O.sub.3 0.0 0.0 0.0 0.0
0.0 0.0 1.2 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Sulfate
Material (%/SiO.sub.2) Li.sub.2SO.sub.4 4.0 Na.sub.2SO.sub.4 1.0
1.0 Number of bubbles (per 100 g) 0.9 7.9 0.0 68.0 235.0 0.0 12.0
Specific gravity (g/cm.sup.3) 2.93 2.93 2.58 2.45 2.58 2.50 2.46
Modulus of elasticity (GPa) 106 106 91 90 91 72 72 Rigidity (GPa
.multidot. cm.sup.3/g) 36 36 35 37 35 29 29
[0051] An explanation is first given on the Examples. Common raw
materials for glass including silica, alumina, lithium carbonate,
lithium sulfate, sodium carbonate, sodium sulfate, basic magnesium
carbonate, calcium carbonate, potassium carbonate, strontium
carbonate, barium carbonate, titania, zirconia, and tin oxide were
mixed together to prepare batches as base materials for substrate
glasses so that these batches gave compositions respectively having
the components shown in Table 1 and each weighing 400 g.
[0052] The batch material used as a sulfur source in each batch is
shown in Table 1 together with the amount of sulfur contained in
the batch in terms of the amount of SO.sub.3 (%) based on
SiO.sub.2.
[0053] Each batch prepared was held at 1,500.degree. C. for 4 hours
with a platinum crucible and then poured on an iron plate, provided
that in Comparative Example 4, the batch was held at 1,600.degree.
C. for 16 hours. This glass was held at 650.degree. C. for 30
minutes in an electric furnace. Thereafter, the furnace was
switched off to allow the glass to cool to room temperature. Thus,
sample glasses were obtained.
[0054] The number of bubbles, specific gravity, modulus of
elasticity (Young's modulus), rigidity (Young's modulus/specific
gravity), and compression stress layer thickness of each of the
sample glasses were counted, measured, or calculated in the
following manners.
[0055] The number of bubbles in each sample glass was visually
counted with edge-lighting. The results obtained are shown in Table
1.
[0056] In each of the glasses of the Examples, the number of
bubbles was not larger than 10 per 100 g of the glass. In
particular, in each of the glasses for which a sulfuric acid salt
had been used as a batch material, the number of bubbles was not
larger than 1 per 100 g of the glass.
[0057] A bubble-free part of each sample glass was cut into a piece
and each side thereof was subjected to mirror polishing to obtain a
platy sample having dimensions of 5.times.30.times.30 mm. Each
sample was examined for density by the Archimedes method.
Furthermore, the Young's modulus of each sample was calculated by
the ultrasonic method using a sing-around oscillator.
[0058] The glasses of Examples 1 to 10 according to the present
invention each had a rigidity as defined by (Young's
modulus)/(specific gravity) exceeding 30
GPa.multidot.g.sup.-1.multidot.cm.sup.3 and a modulus of elasticity
exceeding 90 GPa in terms of Young's modulus. These results are
shown in Table 1.
[0059] Subsequently, glass samples were prepared from the sample
glasses of Examples 1 to 10 in the same manner as in the
determination of Young's modulus. These samples were immersed for 1
hour in a melt of an 80:20 by weight salt mixture of KNO.sub.3 and
NaNO.sub.3 heated at 380.degree. C. to conduct chemical
strengthening. Thereafter, each strengthened glass was examined
with a polarizing microscope to determine the thickness of the
resultant compression stress layer.
[0060] In each of the sample glasses of Examples 1 to 10, a
compression stress layer having a thickness of 50 .mu.m or larger
had been formed. The components of these glasses thus proved to be
suitable for chemical strengthening.
APPLICATION EXAMPLE
[0061] The sample glass of Example 1 described above was cut into a
ring form having an outer diameter of 95 mm and an inner diameter
of 20 mm. This disk was ground and polished and subsequently
subjected to chemical strengthening under the same conditions as
the above and then to mirror polishing (surface roughness R.sub.a:
.ltoreq.1 nm; JIS B 0601-1994) to regulate the thickness thereof to
1.0 mm. Thus, a substrate for magnetic recording media was
obtained.
[0062] Using the thus-produced substrate for magnetic recording
media, a magnetic disk medium was produced in the following
manner.
[0063] Chromium, Co--Cr--Ta, and carbon were deposited on the
substrate as an undercoat layer, recording layer, and protective
layer, respectively, by sputtering. A lubricating layer was further
formed thereon to obtain a magnetic disk medium. The medium thus
obtained was set in a closed type magnetic-disk drive and
continuously rotated at each of 10,000 rpm and 12,000 rpm. In
either case, the medium was found to be free from troubles such as
head crushing caused by substrate vibration, because the glass
substrate had a high Young's modulus and a high rigidity.
COMPARATIVE EXAMPLES 1 TO 4
[0064] An explanation is given below on Comparative Examples 1 to
4. The compositions of Comparative Examples 1 to 4 are outside the
scope of the present invention. Sample glasses were prepared in the
same manner as in the Examples, except that in Comparative Example
4, the batch was held at 1,600.degree. C. for 16 hours.
[0065] The glass compositions of Comparative Examples 1 to 4 each
contained no SnO.sub.2 as a clarifier. A sulfuric acid salt was
used only in Comparative Example 3. As.sub.2O.sub.3 as a clarifier
was used only in Comparative Example 4.
[0066] The number of bubbles, specific gravity, modulus of
elasticity (Young's modulus), rigidity (Young's modulus/specific
gravity) , and compression stress layer thickness of each of these
sample glasses were counted, measured, or calculated in the same
manners as in the Examples. The results obtained are shown in Table
1.
[0067] In each of the sample glasses of Comparative Examples 1 and
2, which contained no SnO.sub.2 as a clarifier, the number of
bubbles was larger than 50 per 100 g of the glass, which was far
larger than the numbers of bubbles of 10 or smaller in the
Examples.
[0068] With respect to modulus of elasticity (Young's modulus) and
rigidity (Young's modulus/specific gravity), the glasses of
Comparative Examples 3 and 4 each had a Young's modulus lower than
90 GPa and a rigidity lower than 30 GPa.multidot.g.sup.-1 cm.sup.3.
These found values were as small as about 80% of the corresponding
values in the Examples.
[0069] The sample glass of Comparative Example 3 only was subjected
to the same chemical strengthening as in the Examples, and the
thickness of the resulting compression stress layer was measured.
However, the compression stress layer formed on the sample glass of
Comparative Example 3 had a thickness as small as 10 .mu.m.
[0070] As described above in detail, the following effects are
brought about by the present invention.
[0071] According to one embodiment of the invention, a glass
composition having a higher rigidity and a higher modulus of
elasticity than conventional glasses can be obtained.
[0072] According to the preferred embodiment of the invention, a
limited high-rigidity high-modulus glass composition is provided.
This glass composition can have a higher rigidity and a higher
modulus of elasticity than the conventional glasses and
conventional aluminum alloy.
[0073] According to another preferred embodiment of the invention,
chemical strengthening is easy as in the case of general soda-lime
glasses, and a surface compression stress layer can be formed on
the glass composition to a larger depth than on the general
soda-lime glasses.
[0074] According to another embodiment of the invention, it is
possible to produce a glass composition which has both a high
rigidity and a high modulus of elasticity and is excellent in
quality with respect to freedom from bubbles even without using a
harmful clarifier such as As.sub.2O.sub.3 or Sb.sub.2O.sub.3.
Because of the high quality with respect to freedom from bubbles, a
higher yield is attained due to diminished bubble defects, whereby
the production cost can be reduced.
[0075] According to the preferred embodiment of the invention, it
is possible to produce a glass composition of even higher quality
with respect to freedom from bubbles which has both a high rigidity
and a high modulus of elasticity.
[0076] According to another preferred embodiment of the invention,
it is possible to produce a high-rigidity high-modulus glass
composition which has excellent surface smoothness and is
advantageous also in cost.
[0077] According to still another embodiment of the invention, a
glass substrate which has both a high rigidity and a high modulus
of elasticity and is suitable for use as the substrate of an
information recording medium can be obtained because of the use of
the above glass composition.
[0078] According to preferred embodiment of the invention, a glass
substrate which has a high fracture strength and is suitable for
use as the substrate of an information recording medium can be
obtained because of the use of the above glass composition.
[0079] According to another preferred embodiment of the invention,
a recording medium employing the above glass substrate having both
a high rigidity and a high modulus of elasticity and/or having a
high fracture strength can be obtained. This recording medium can
be rotated at a higher speed, bends less, and is free from resonant
vibration. Hence, the gap between the magnetic disk head and the
recording medium can be narrowed, making it possible to attain an
increase in storage capacity and a reduction in access time.
Therefore, the recording medium employing the glass composition is
especially suitable for use in recording apparatuses of the high
rotational speed type.
[0080] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof.
* * * * *