Note: Descriptions are shown in the official language in which they were submitted.
~76Z~
BACKGROUND OF THE I~`IVENTION
The present inventlon relates to a process
for preparing a porous gel body capable of using as a
mother material of optical glass and having a variety
of applications thereof in case that it is not used
as optical glass.
Recently a method of preparing a non porous
block body (e.g., silica glass) by sintering a por~us
gel obtained by hydrolysis of a metal alkoxide
(e.g., Si(OCH3)4) (herea~ter referred to as a "sol-
gel method") has been proposed.
In the drawi~gs appended to this specification:
Fig. 1 is an outlined cross sectional view
describi~g outlines of respective steps in the
solgel method.
Fig. 1 shows an outlined process of the
sol-gel method in which, for example, Si(OCH3)4 is
employed as a ra~ material and mixed with CH30H
and H2O in stage 11; the resulting mixture is
stirred in stage 12 to prepare a raw material liquid
16 and transferred to a desired container 20 to gel
(this gel is hereafter referred to as wet gel 17~.
Next, in stage 13 the residual CH30H and
H2O are gradually evapoxated off and dried to make a
dry gel (this gel is referred to as dry silica
gel 18). Thereafter, in stage 14 the dry gel is si~tered
at temperatures of about 1000C or more to vitrify
the dry gel; thus, a glass body 19 is obtained.
In the sol-gel method, for example, Si(OC~3)~
~7~ 9
l is hydrolyzed to form a ~el, which is then dried to
obtain a dry gel.
The sol-gel method is advantageous from an
economical viewpoint since a great amount of glasses
can be synthesized at temp~ratures below the glass
melting point.
Furthermore, dry silica gel has, e.gO,
a specific surface of about 800 m2/g and a micropore
diameter of from 20 to 100 ~, though depending upon
conditions for preparation thereof~ in case that a
gelled state is considered to be an aggregate of
finely divided particles, such a dry silica gel cor-
responds to an aggregate of finely divided particles
having a diameter of about 100 A. Accordingly, it has
various applications such as catalysts for various
reactions or selective transmitting media of gases
utilizing a micropore interface, parts to conver~
directional rays into quasi-spherical rays due to
light scattering, etc.
Taking an example of Si(OCH3)4 as a raw
material liquid, proceAures of gelation are briefly
described below. A solution of Si(OCH3)4 in methanol
and water for hydrolysis (referred to as a sol liquid)
is maintained at, e.g., 70C. The sol liquid which
was originally transparent is changed to a suspended
state of a light blue color with the passage of time.
That is, it is assumed that the suspended
state would appear by finely divided SiO2 particles
_ ~ _
121i 7629
1 formed by hydrolysis sho~n by:
Si(OCH3)4 + 2H20 > Sio2 + 4CH30H
With the passage of time, the sol liquid proceeds with
suspension and is changed to a pudding state in which
fluidity is lost to become a wet gel. Here, methanol
is a solvent added for the purpose of allowing Si(OCH3)4
to be compatible or miscible with water. In the
case of metal alkoxides, an alcohol is generally employed.
As such, hydrolysis for forming pure silica gel proceeds
relatively slowly.
On the other hand, hydrolysis of metal
alkoxides other than Si as well as B or P which gives
a water-soluble hydrolysate is extremely rapid. For
lS example, Ge(OCH3)4, Ge(OC2~5)4, Zr(OC4Hg)4, Sb(OC2H5)3,
etc. undergo hydrolysis by moisture in the air to
form a white solid oxidation product. For this
rsason, for preparing a high silica gel containing an
addition element, a solvent is added to a liquid
mixture of Si~OCH3)4 and an~alkoxide of the addition
element and water for hydrolysis is then added to
the mixture; in this case, white precipitates are
immediately formed in many occasions, while it
depends upon the compositional amount of the addition
element. It is assumed to be because the alkoxide
of the addition element would be directly hydrolyzed.
Therefore, it is difficult to obtain a homogeneous dry
gel even though the technique of the prior art sol-gel
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76;;~
l method is applied to the preparation of high silica gel
containing an addition element. Further, even though,
e.g~$ an optical fiber is prepared using glass obtained
by heating and sintering such a heterogeneous high
silica gel~ a fiber having a serious trancmission
loss can only be obtained; such is not preferred.
Furthermore, a method of preparing a fibrous
SiO2-ZrO2 gel by hydrolyzing a metal alkoxide with
moisture in the air has been proposed (see, "Journal
of Materials Science", vol. 15, (1980), page 1765).
However, the above-mentioned method involves such
disadvantages that (l) a long period of time is required
because of hydrolysis with moisture in the air, (2)
the product is a fibrous gel but a gel having any
optional shape is not expected, etc.
The following references are cited to show
the state of the art: i) Japanese Patent KOKAI (Laid-
Open) No. 100231/80 (corresponding applications were
also filed in U.S.A., the United Kingdom, West Germany,
France, Netherlands and Canada, Glaiming the priority
therefrom), ii) M. Yamane et al.; "Journal of
Materials Science", vol. 13 (1978), pp. 865 to 870,
and iii) M. Yamane et al.; "Journal of Materials
Science", vol. 14 (1979), pp. 607 to 611.
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SUMMARY OF THE INVENTION
An object of the present invention is to
provide a proess for easily preparing a homogeneous
12~7629
1 porous gel body which has solved the problems in the
prior art mentioned above.
A further object of the present invention
is to provide a process for preparing a homogeneous
5 and transparent silica gel containing an addition which
can change its refractive index particularly when it
is converted into glass.
For achieving the objects described above,
the process for preparing a porous body in accordance
with the present invention comprises the following steps:
i) incorporating at least one alkoxide of
an addition element capable of changing the refractive
index of transparent quartz glass/ into a raw material
liquid for forming silica gel through hydrolysis and
further incorporating therein a solvent comprising only
a polar organic solvent containing no water to provide
a liquid of a raw material mixture,
ii) incorporating an aqueous solution for
dilution obtained by mixing the solven~ and water for
hydrolysis with the li~uid of the raw material mixture
to provide a solution mixture;
iii) subjecting the solution mixture to
gelation in a container of a desired shape to make a
wet gel; and
iv) drying the wet gel to make a dry gel.
DESCRIPTION OF THE lNv~NllON
In step i) which is a modification of step
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~2~7~i29
1 i) of the process of the present invention, a compound
of an addition element capable of changing a refractive
index of transparent quartz glass is incorporated in
a raw material liquid for forming silica gel throllgh
hydrolysis to provide a liquid of a raw material
mixture, in which the incorporation of the solvent in
step i) can be omitted.
As the aforesaid raw material liquid for
forming silica gel, silicon alkoxides represented by the
general formula:
Si(OR)4
which are liquid at ambient temperature, are generally
employ~d, and representative examples of silicon
alkoxides include Si(OCH3)4 and SitOC2H5)4, wherein
R represents an alkyl group.
The aqueous solution for dilution in step
ii) described above includes water for hydrolysis of
from 10 to 80 vol %. When the concentration of hydrolysis
water in the aqueous solution for dilution is higher
than the upper limit, precipitates are easily formed
upon hydrolysis and such becomes a cause for compo-
sitional segregation of a gel. When the concentration
of water for hydrolysis is lower than the lower limit,
~5 wet gelation becomes difficult. Anyway, it is not
preferred that the concentration be outside the range
described above.
As the aforesaid solvent, polar organic
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~B
solvents are emplo~ed~ Examples of polar oryanic
solvents include alkanols such as monovalent alcohols
that are liquid at room temperature, i.e., ethanol,
propanol, isopropanol, butanol, etc.~ other miscible
organic solvents such as ketones, amides, etc., i.e.,
dioxane, dimethylformamide, or a mixture thereof.
It is sufficient that the amount of the
aqueous solution for dilution which is added to the
liquid of a raw material mixture in step ii) be in a
range of that of water employed for hydroiysis in the
known sol-gel method in terms of the amount of water
for hydrolysis contained in the aqueous solution for
dilution.
The reason why the solvent is added in step
i) is that metal alkoxides are rendered miscible or
compatible with water for hydrolysis added in the
subsequent step, as is well known in th~ art.
Examples of the aforesaid addition
elements include Ge, Ti, Zr, ~a, Nb, Sb, etc. and
compounds of such addition elements are preferably
alkoxides of these addition elements. Furthermore,
B and/or P can also additionally be incorporated
therein.
Other conditions for preparation can be
determined with reference to the prior art or
techniques as disclosed in the following documents:
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1~ Japanese Patent Application No~ 150082/79 -
Laid-Open No. 73538/81 (Laid Open Date:
Jun~ 18, 1981),
2) Japanese Paten Application No. 169335/79 -
Laid-Open No. 92135/81 (Laid-Open Date-
July 25, 1981),
3) Japanese Patent Application No. 95755/80 -
Laid-Open No. 22127/82 [Laid-Open Date:
February 5, 1982),
:10 4) Japanese Patent Application No. 95758/80 -
Laid-Open No. 22130/82 (Laid-Open Date:
February 5, 1982),
Japanese Patent Application No. 4600/80 -
Laid-Open No. 104732/81 (Laid-Open Date:
August 20, 1981),
Japanese Patent Application No. 55485/80 -
Laid Open No. 155028/81 (Laid-Open Date:
Decembex 1, 1981~, and
Japanese Patent No. 95757/80 -
Laid-Open No. 22129/`82
(Laid-Open Date: February 5, 1982~.
The present inventi.on is based on the
present inventors~ dis~overy that, by the use of an
aqueous solution for dilution, in place of water in
the prior art, for the purpose of hydrolysis, bulky,
transparent homogeneous high silica gel containing an
addition element can easily be obtained.
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DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
Example 1
5 . 8 g of Sb (OC2H5 ) 3 was added to and mixed
with 15.2 g of Si (OCH3) 4. To the mixture, 26 cc of
ethanol was further added. The mixture was stirred
].5
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1 to obtain a solution. The solution was divided into
two and charged in containers A and B. Then, pure
water as water for hydrolysis was charged in container
A. White precipitates were immediately formed and
s even agitation over a long period of time could not
dissolve the precipitates. After about 4cc of pure
water - which was greater than an amount required
for hydrolysis of -the alkoxide - was further added to
the system and mixed, the mixture was settled as it
was and gelled to obtain a wet gel. Thereafter,
drying was further performed to obtain a dry gel.
The thus prepared high silica gel maintained white
turbidity formed at the stage of forming the wet gel
even in the dry gel after drying. It was clearly
observed even with the naked eye that it was a hetero-
geneous solid gel.
Next, an aqueous solution for dilution
obtained by adding 4 cc of pure water to 13 cc of
ethanol and mixing them was poured into container B
to prepare a mixture solution. A small quantity of
precipitates were formed at the initial stage oE the
mixing but the precipitates which were dissolved by
stirring were then dissolved to become a transparent
sol liquid. After the sol liquid was gelled to make
a wet gel, the wet gel was dried and made a homogeneous
dry gel having good transparency as it was. For
verification, the dry gels ~diameter of 4 mm and
length of ~0 mm) prepared in these containers A
7629
l and B were heated and sintered in 2~ N2 or inert gas,
respectively. The dry gel prepared in contalner A
still remained white-turbid; it became clear that the
dry gel was inappropriate for preparing glass therefrom.
5 To the contrary, the dry gel prepared in container B
was transparent and good results were obtained.
Here, ethanol was employed as the solvent;
with methanol, precipitates were formed and, propanol
as well as butanol gave similar results to ethanol.-
From this fact, it is assumed that an exchange reactionwould occur between Sb(OC2H5)3 and methanol and, a
solvent should be selected, depending upon compounds
of addition elements. However, hydrolysis described
above is widely applicable in general.
Example 2
To a mixture of 15.2 g of Si(OCH3)4 and 7.6 g
of Sb(OC4H9)3, 34 cc of propanol was further added and
the resulting mixture was stirred to obtain a solution.
The solution was divided into two portions, each of
which was charged in two containers A and B, respectivel~.
Pure water was added to container A. Precipitates
were formed as in the case of using Sb(OC2H5)3 in
Example l. In container B, an aqueous solution for
dilution obtained by mixing 17 cc of propanol and 5 cc
of H2O was charged and hydrolysis was performed to
obtain a wet gel. Thereafter, drying followed to
obtain a transparent dry gel. Furthermore, it was
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629
1 confirmed that high silica dry gel having good transparency
could be obtained also in the case of adding a solution
to which 34 cc of propanol, i.e., an aqueous solution
for dilution obtained by mixing 34 cc of propanol and
8 cc of pure water directly with a liquid of a ra~
material mixture of Si(OCH3)4 and Sb(OC4~9~3.
Example 3
To 15.2 g of Si(OCH3)4, 6.8 g of Zr(OC4Hg)4
was added and 34 cc of n-propanol was further added to
the mixture. Then, the mixture was divided into two
portions, each of which was charged in containers A and
B, respectively, While stirring a liquid of the raw
material mixture in container A with a magnetic stirrer,
5 cc of pure water was added thereto. As in the case
of Sb of the foregoing example, precipitates were
immediately formed. On the other hand, an aqueous
solution for dilution obtained by mixing 5 cc of pure
water and 17 cc of n-propanol was added to container B.
Precipitates were hardly formed. The mixture was gelled
to make a wet gel. Thereafter, dry-ing followed to
obtain a good dry gel.
Investigations were also made on alkoxides
of Ge as well as Ti, Ta and Nb belonging to transition
metals, as in the Zr alkoxide, and it was found that
the addition of an aqueous solution obtained by mixing
the alcohol described above and water for hydrolysis
was effective with the content ranging from 2 to 20 mol%.
~2~7~29
1 Obviously many modifications and variations of the
present invention are possible in the light of the
above teachings. It is therefore to be understood
that within the scope of the appended claims the
invention may be practiced otherwise than as specifically
described.
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B
