JPH0686299B2 - Method for producing silica-based glass - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is applicable to optical fiber preforms and the like.
The present invention relates to a method for easily and inexpensively manufacturing a high-purity silica glass doped with sol-gel method.

[Conventional technology]

Conventionally, a silica-based optical fiber was mainly a combination of a pure silica-based clad material and a core material doped with Ge.
As a recent tendency, a type of a cladding material doped with F and a core material of pure silica glass is desired for higher performance. When synthesizing this type of optical fiber by the conventional vapor phase method (VAD method), the F-containing compound used as a raw material gas generally has a high vapor pressure, so that the yield of F is low due to the problem of scattering and its distribution. There is also a problem in terms of control.

In order to solve these problems, many attempts have been made to synthesize an optical fiber preform containing F by the sol-gel method.

For example, triethoxyfluorosilane [Si (O
C ₂ H ₅ ) ₃ F] with silicon ethoxide hydrolyzed with aqueous ammonia and mixed with colloidal silica sol to react,
A cylindrical silica gel containing F was prepared and fired to form a cylindrical glass, which was then reheated at a high temperature to scatter F from the surface layer of the hollow portion and crush the hollow portion to form a core portion of pure silica and silica. A method of synthesizing a preform having a clad portion having a -F composition is known.

Also, after forming a cylindrical gel from a colloidal solution in which powdered silica is dispersed in water and drying it, silicon tetrafluoride (Si
F ₄ ) Cylindrical silica glass containing F obtained by firing in an atmosphere was used as a clad portion of an optical fiber preform, and soot as a core material was deposited in the hollow portion by a conventional vapor phase method or a rod-shaped core material was inserted. After that, a method is known in which the preform is synthesized by heating this to vitrify the core and simultaneously crushing the hollow.

[Problems to be Solved by the Invention]

The above-described F-containing optical fiber preform synthesis by the sol-gel method has the following problems.

Triethoxyfluorosilane [Si (O
C ₂ H ₅ ) ₃ F] and silicon tetrafluoride (SiF ₄ ) must be used.

There is a limit to the F doping amount. That is, when triethoxyfluorosilane [Si (OC ₂ H ₅ ) ₃ F] is used, F is doped only in the surface portion of the colloidal silica, so that it is difficult to introduce a large amount and at the same time F is easily scattered during firing. . Furthermore, as the amount of Si (OC ₂ H ₅ ) ₃ F added increases, gelation is significantly promoted, which makes it difficult to control the reaction. On the other hand, when F is introduced as a gas such as silicon tetrafluoride (SiF ₄ ), diffusion of the gas into the gel skeleton cannot be expected so much, and F is mainly doped only by the exchange reaction between the OH group and F on the surface portion. It will be in the form of being.

In either case, since the surface of colloidal silica is doped with F, there is a problem in the uniform dispersibility of F.

Further, common problems in the production of silica-based glass by the sol-gel method include dry cracking of the gel and a large amount of residual OH groups.

[Means for Solving the Problems]

The present invention is a method of producing a gel by hydrolyzing a mixture containing a silicon alkoxide, and drying and firing the gel to prepare a silica-based glass, wherein hydrofluoric acid has a HF / base molar ratio of 2-5. After premixing with the base in a ratio and adding to the mixture containing the silicon alkoxide, it is hydrolyzed,
This is a method for producing a silica-based glass which is characterized by gelling, whereby the above problems can be solved.

That is, the present invention uses inexpensive hydrofluoric acid as an F source, which is added to a mixture containing a silicon alkoxide premixed with a base, preferably an amine-based strong base, more preferably piperidine, and then hydrolyzed. Since the gel obtained by decomposition has a relatively large pore size and a low specific surface area, it has been found that it can be easily dried and calcined, and a silica-based glass in which a large amount of F is uniformly doped can be produced. is there.

The hydrofluoric acid (hydrofluoric acid) used in the present invention means an aqueous solution of HF, and the concentration of HF is in the range of 20 to 70% by weight, preferably 40 to 50% by weight, and commercially available. It is also possible to use the one having the highest concentration of 46 to 50% by weight.

In the present invention, if hydrofluoric acid is used alone without being premixed with a base and mixed with silicon ethoxide, localized gelation and precipitation occur, and a homogeneous gel cannot be prepared. Premixing the base with hydrofluoric acid mitigates the acidity of hydrofluoric acid and results in a homogeneous wet gel.

In the present invention, HF in the mixing ratio of hydrofluoric acid and a base in a premixing / HF ratio means a HF molecule contained in hydrofluoric acid, and its value is 2 to 5, preferably 3
When the ratio is less than 2, that is, when the amount of base is large, gelation is too fast and control of gel synthesis is difficult. If the ratio is greater than 5 and the HF content is too high, a wet gel with high acidity is produced, and shrinkage and cracking of the gel during drying become a problem.

The base used in the present invention is not particularly limited as long as it does not violate the object of the present invention.

For example, if ammonia is used alone, ammonium fluoride salt, which is difficult to dissolve in alcohol, precipitates, and thus homogeneous gel synthesis is difficult. Therefore, use of such a base alone is not preferable in the present invention.

Preferred bases used in the present invention include strong amine bases such as piperidine, pyrrolidine, piperazine,
Examples thereof include pyridine, isopropylamine, N-ethyl-n-propylamine, n-propylamine, isopropylamine and diethylamine, with piperidine being particularly preferred.

These amine-based strong bases increase the pore size of the gel,
It also has the effect of making it difficult to crack when dried.

The method of mixing these bases and hydrofluoric acid is not particularly limited, and the bases can be used as they are or in the form of an aqueous solution or an organic solvent solution, preferably an alcohol solution. Similarly, hydrofluoric acid may be diluted with water or an organic solvent before use.

In the present invention, the mixture containing the silicon alkoxide to which the premixed hydrofluoric acid and the base are added is a silicon alkoxide alone, or a silicon alkoxide as a main component and a sub-addition containing a metal other than silicon. Composed.

In the present invention, the silicon alkoxide has the formula Si
Examples thereof include compounds represented by (OR) ₄ (R represents methyl, ethyl, propyl, butyl and the like), and partially hydrolyzed multimers thereof.

In the present invention, it is also possible to use only the silicon alkoxide monomer, but from the viewpoint of forming a gel having a large pore diameter, suppressing dry cracking, and high-purity vitrification,
30 mol% or more of the silicon alkoxide used,
More preferably, 50 mol% or more is composed of a multimer having an average degree of polymerization of 3 or more (the silicon alkoxide as used herein means the case of being converted into the above-mentioned silicon alkoxide monomer, Means the above-mentioned partially hydrolyzed polymer).

Examples of the auxiliary additive used in the present invention include alkoxides of other elements, for example, triethoxyborane [B (OC ₂ H
₅ ) ₃ ] etc., alkoxides, metal salts, simple metals such as N
Examples thereof include alkali metals such as a, Mg, Ba and Sr, alkaline earth metals and the like, and B-containing compounds such as boric acid (H ₃ BO ₃ ).

In the present invention, the amount of water used for hydrolysis for gel formation varies depending on the type of hydrolyzable compound such as silicon alkoxide, but is usually 0.2 to 5 times the hydrolyzable group of those compounds. Mol, preferably
It is in the range of 0.5 to 1.5 times mol. The amount of water in this case includes the amount of water such as hydrofluoric acid.

The method for drying the gel formed in the present invention is not particularly limited, and a conventionally known method can be applied.

Specific examples include room temperature and normal pressure drying, supercritical drying and the like.

The supercritical drying includes the following two types.

Gel solvent (eg alcohol) itself dried under supercritical drying conditions For ethanol; 243 ° C or higher, 64 atm or higher Methanol; 240 ° C or higher, 81 atm or higher CO ₂ -gel solvent mixed supercritical drying conditions Under dry CO ₂ -ethanol mixed system; 31 ° C or higher, 180 atm or higher [Action] The present invention uses inexpensive hydrofluoric acid as a F source, which is a base, preferably an amine-based strong base, More preferably, a gel obtained by premixing a predetermined amount with piperidine and adding it to a mixture containing silicon alkoxide, and hydrolyzing this mixture is due to the presence of F due to the combination of hydrofluoric acid and a strong amine base. A strong silica skeleton structure is formed,
And since the shrinkage and densification of the gel are suppressed by the presence of a strong base, it has a relatively large pore size and a low specific surface area.
Prevents dry cracking, can be easily dried and fired, and F
It is possible to produce a high-purity silica-based glass in which a large amount of is uniformly doped.

〔Example〕

Hereinafter, specific examples of the present invention will be described, but the present invention is not limited thereto.

Example 1 2.3 g of hydrofluoric acid containing 47% by weight of HF (0.053 at HF)
Mol) and piperidine 1 g (0.012 mol) ethanol 19 g,
It was added to a mixed solution of 6 g of water, stirred and mixed under ice cooling to obtain a uniform solution. 21g of an average pentamer oligomer of silicon ethoxide (SiO ₂ content SiO ₂ content 40%) was added to this, and after stirring and mixing at room temperature, it was cast into a polypropylene cylindrical container with an inner diameter of 36 mm and left to stand naturally. Gelled. Gradually dry this wet gel at room temperature and under heating to 25 mmφ
An integrated dry gel of x36 mmh was obtained. This dry gel has 1
It had a very low surface area of 08 m ² / g. 800 this
After heating to 0 ° C in an oxygen atmosphere to sufficiently remove residual organic matter by burning, and then heating to 1250 ° C in a helium atmosphere, a dense transparent glass body of 14 mmφ x 22 mmh was obtained. As a result of chemical analysis, this glass was a silica-F based glass containing 2% by weight of F.

(Example 2) 23 g of hydrofluoric acid containing 47% by weight of HF (0.53 mol of HF), 12 g of piperidine (0.14 mol), 190 g of ethanol,
It was added to a mixed solution of 60 g of water, and stirred and mixed under ice cooling to obtain a uniform solution. To this, 220 g of an average 3.5-mer oligomer of silicon ethoxide (SiO ₂ content SiO ₂ content 38%) was added, stirred and mixed at room temperature, and then a Teflon rod with a diameter of 10 mm was vertically fixed in the center with an inner diameter of 60 mm. It was cast into a polypropylene cylindrical container (1) and allowed to gel by leaving it to stand naturally, and this Teflon rod was pulled out to obtain a 60 mmφ × 200 mmh cylindrical gel having a hole with a diameter of 10 mm in the center. Immediately after that, an average 3.5-mer oligomer of silicon ethoxide (SiO ₂ equivalent S
18 g of iO ₂ content 38.5%) and 18 g of ethanol were mixed, and 5.4 g of piperidine aqueous solution adjusted to pH 11.7 was added to the sol. A large-scale composite gel composed of a plain silica gel and an F-containing silica gel on the outside was obtained. This gel at room temperature and
After the gel solvent was replaced with ethanol with aged for several days under 60 ° C. warming, CO ₂ CO ₂ by supercritical extraction apparatus - supercritical conditions with ethanol mixture system ^{(80 ℃, 160kg / cm 2} ) Gel The internal solvent was extracted and removed to obtain a dry gel body of 60 mmφ × 200 mmh without shrinkage. By heating this dry gel to 800 ° C in an oxygen atmosphere to 1300 ° C, the central part is made of pure silica and the outer part is made of F-containing silica glass, and the central part applicable to optical fiber preforms is a high refractive index glass ( A transparent glass rod of 25 mmφ × 85 mmh having a structure composed of silica only) and a low refractive index glass (F-containing silica glass) on the outside was obtained.

〔The invention's effect〕

In the present invention, since hydrofluoric acid is used as the F source, it is not necessary to use expensive triethoxyfluorosilane [Si (OC ₂ H ₅ ) ₃ F], silicon tetrafluoride (SiF ₄ ), etc. Highly pure silica-based glass that can be cost-effective, increases the dope amount of F and uniformly disperses it by using a mixture premixed with a base, prevents cracking during gel drying, and fires it. Can be manufactured.

Claims (3)

[Claims] 1. A method for producing a silica glass by hydrolyzing a mixture containing a silicon alkoxide to prepare a gel, and drying and firing the gel, wherein hydrofluoric acid is used in an HF / base molar ratio of 2 to 5. A method for producing a silica-based glass, which comprises premixing with a base at a ratio of 1 and adding to a mixture containing silicon alkoxide, and then hydrolyzing and gelling the mixture. 2. The base mixed with hydrofluoric acid is selected from the group consisting of piperidine, pyrrolidine, piperazine, pyridine, isopropylamine, N-ethyl-n-propylamine, n-propylamine, isopropylamine, diethylamine. The method for producing silica glass according to claim 1, wherein the silica glass is an amine-based strong base. 3. 30 of the silicon alkoxides used
3. The method for producing a silica-based glass according to claim 1, wherein mol% or more comprises a multimer having an average degree of polymerization of 3 or more.