CA1207629A - Process for preparing porous body - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for preparing a porous body which comprises forming high silica wet gel containing an addition element through hydrolysis and then drying the wet gel to make a dry gel, whereby an aqueous solution for dilution obtained by mixing a solvent such as an alcohol and water for hydrolysis is employed for hydrolysis, instead of pure water, is disclosed. Ac-cording to such a process, a homogeneous and transparent gel can be obtained.

Description

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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 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.
~5 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 ~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 ~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:

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 - 8a -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

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 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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Claims (7)

WHAT IS CLAIMED IS:

1. A process for preparing a porous body which 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 said solvent and water for hydrolysis with said liquid of said raw material mixture to provide a solution mixture;
iii) subjecting said solution mixture to gelation in a container of a desired shape to make a wet gel; and iv) drying said wet gel to make a dry gel.

2. The process for preparing a porous body as described in claim 1, wherein said raw material liquid for forming silica gel is a silicon alkoxide represented by the general formula:
Si(OR)4.

3. The process as described in claim 2, wherein the concentration of water for hydrolysis in said aqueous solution for dilution in step ii) is between 10 and 80 vol%.

4. The process as described in claim 3, wherein said silicon alkoxide is Si(OCH3)4.

5. The process as described in claim 3, wherein said solvent is an alkanol.

6. The process as described in claim 1, wherein said addition element is selected from the group consisting of Ge, Ti, Zr, Ta, Nb and Sb.

7. The process as described in any one of claims 1 to 3, wherein said solvent is not added at step 1) described above.