JP2561103B2 - Method for manufacturing glass article - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-purity glass article.

<Prior Art> Currently, there is an optical fiber in a field requiring a large amount of high-purity glass. Taking this optical fiber as an example, conventionally, precursors for forming glass such as SiCl ₄ , GeCl ₄ , POCl ₃ are subjected to flame hydrolysis or thermal oxidation reaction to form fine glass particles, which are formed at the tip or the outer periphery of the starting member. A method is adopted in which a transparent preform is formed by adhering and depositing it to form a porous preform, and then heating this to a high temperature to form a transparent glass preform.

However, according to this method, the porous glass preform is apt to be damaged, so that it needs to be handled with care, and therefore the transparent vitrification must be heat-treated one by one, which is extremely inefficient. Also, if the porous glass is quartz glass, a furnace core tube is usually placed inside the heating element to prevent contamination of impurities from the heating element due to high temperature processing, and a transparent glass preform is passed through this furnace tube. Although vitrification is performed, quartz glass or alumina is used as the material for the core tube because it can withstand high temperatures. Thus, in the case of a quartz furnace core tube, since the porous glass preform is also a silica-based glass, the furnace core tube itself is deformed and its life becomes a problem. On the other hand, the alumina core tube is superior to quartz glass in heat resistance, but has a drawback that it is susceptible to rapid cooling and rapid heat and is prone to cracking. When a carbon resistance heating element is used as the heating element for the heating furnace, the surface temperature of the heating element itself is 1750 because heat is supplied through the core tube even if the temperature at the center of the furnace is 1600 ° C. It is not uncommon to reach around ℃. When a heating element is actually heated to such a high temperature, reaction occurs between the quartz glass, which is the core tube, and carbon, and the SiO ₂ slightly evaporated from the surface of the core tube reacts with carbon to form silicon carbide (SiC). There is a problem that the heating element reacts with oxygen released from SiO ₂ to become CO or CO _{2 and is} consumed.

From the above, the present inventors
As No. 20, we stopped the porous glass preform from completely transparent vitrification, however, by low temperature heat treatment to the extent that it did not break during handling, and to the extent that bubbles were independently present inside, We have proposed an opaque glass preform that has a reduced volume of impurities, and has a smaller volume than the porous glass preform stage. In this way, it is possible to perform multiple vitrification in a batch and achieve high efficiency.Because the temperature of the opaque glass preform is lower than the vitrification temperature, the damage to the core tube can be reduced and the life is long. Can be realized. In addition, it is possible to prevent the consumption of the carbon heating element.

<Problems to be Solved by the Invention> However, when the diameter of the base porous glass preform is increased in order to increase the size of the preform, the opaque glass body inevitably becomes large in size. When I tried to do so, it was found that the bubbles inside did not disappear, and sometimes it eventually expanded and became a defective product.
The gas contained in the bubbles depends on the gas flowing in the furnace, but includes He, O ₂ , Ar, Cl ₂ , SO _2, ... The inventors have attempted to elucidate this phenomenon theoretically. The figure shows the measured diffusion time of He gas from cylindrical transparent quartz glass rods of various diameters with respect to temperature. The diffusion time is the He concentration at the center of the cylindrical glass is 1 / e (e: 2.718: natural logarithm). ) Is defined as the time. As is clear from the figure, the diffusion time of He does not have a very large temperature dependence like the viscosity of glass. For example, the viscosity of pure quartz glass is about 10 12 at 1100 ° C., while it decreases to 5 × 10 9 at 1400 ° C. That is, the viscosity decreases by two and a half digits when the temperature difference is 300 ° C. On the other hand, the diffusion time of He (helium) is at most tripled (about 0.5 digit increase) even if the temperature is lowered from 1400 ° C to 500 ° C as shown in the figure. This means that if a slightly lower temperature is selected so that the bubbles in the glass do not swell, the gas in the bubbles in even a fairly large base material is exposed to the high temperature of transparent vitrification before exposing the preform. It shows that it is possible to expel a lot. Fortunately at this time, it is not necessary to treat the preform until the concentration of the gas remaining in the bubbles is completely zero. That is, a gas with a large diffusion rate such as helium or hydrogen has a high solubility in glass, and, for example, at room temperature,
A standard volume of gas, approximately 2.0% of the glass volume, can be dissolved in the glass. Therefore, the bubble extinguishing step of the present invention may be carried out until the residual amount of gas in the glass falls below this dissolvable level. In addition, the figure shows that the diffusion time of the gas from the preform is 2 times the diameter of the preform.
As the size of the preform (base material) becomes larger in proportion to the power, diffusion of gas becomes very difficult. This is one of the objects of the present invention, when the transparent glass at a high temperature of the large-sized base material is performed without preliminary heating of the preform, diffusion and release of gas from the center of the preform diffuses in time. It has been shown before to thermally expand and ruin the base material.

By the way, the mechanism by which the closed cells remaining in the glass disappear during the transparent vitrification is considered as follows.

The gas in the bubbles tends to expand as the temperature rises. On the other hand, it is known that the force that tries to shrink the accumulation of bubbles is the surface tension of the glass, which does not depend much on the temperature. Also, the time required for expansion or contraction is determined by the viscosity of the glass, and if the viscosity increases by one digit, the time required for foam deformation increases by one digit.
The rate of diffusion / melting in glass differs depending on the type of gas in the bubbles, but this diffusion rate is also highly temperature dependent and is known to increase exponentially with increasing temperature. There is. If the gas diffusion rate is fast and the gas expansion with temperature rise, that is, the diffusion of the gas in the bubble into the glass progresses to such an extent that the increase in gas pressure in the bubble can be barred, the bubble is in the bubble. It will be crushed by the surface tension of the glass on the surface.

However, when the size of the opaque glass body is large and the diffusion and release of the gas in the glass is not performed at a sufficient rate, or the next step = the heating rate in the vitrification of the opaque preform is too fast, In the above-described bubble disappearance mechanism, the expansion of the gas in the bubble has a larger force, so that the bubble expands rather than collapses.

<Means for Solving Problems> From the above viewpoints, the present invention collects glass fine particles obtained from a glass-forming precursor to form a porous glass preform, and the porous glass preform is used in a heating furnace. An opaque glass preform containing independent bubbles inside is heat-treated to reduce its volume, and subsequently this opaque glass preform is heat-treated in the furnace while maintaining the state of opaque glass. The gas in the bubbles is released, and then the opaque glass preform from which the gas in the bubbles is released is taken out from the heating furnace and heated to a high temperature by another heating means so that it becomes transparent glass. .

 The present invention will be described below based on examples.

Example Using the so-called VAD method, four oxyhydrogen burners were used for S
A porous glass preform made of iO ₄ was created. At this time, one burner from below the growing end of the preform,
In other cases, the flame was directed to the preform from the side orthogonal to the growth axis of the preform. The oxyhydrogen burners had the same structure, and the operating conditions were as follows.

Amount of gas in the burner Flow rate Oxygen 9SLM (standard rectifier) Hydrogen 10SLM SiCl ₄ 4SLM Ar 2SLM The SiO ₂ porous glass preform thus obtained was 200 mm in diameter and 1500 mm in length.

This porous preform was inserted into a heating furnace operating at 1340 ° C. at a speed of 200 mm / h to obtain an opaque glass preform containing bubbles throughout. The size of this heating furnace is the inner diameter
The length of the high temperature region in the furnace is 250 mm, the maximum temperature and the temperature 50 ° C lower than that, 300 mm. The size of the obtained opaque glass preform is 90mm in diameter and 650m in length.
It was m (effective length). Next, this opaque glass preform was left in a heating furnace at 1100 ° C. for 6 hours. The size of this heating furnace is 120 mm in inner diameter, 800 mm in length (uniform heating zone length), and a heating element SiC. The heating furnace has a vacuum of about 10 TORR, but an atmosphere such as Ar may be used. Then, this preform was traversed at a rate of 50 mm / min while being heated with an oxyhydrogen burner at a temperature of 1950 ° C., whereby a completely transparent SiO ₂ glass preform was obtained. The size of this preform was 89.8 mm in diameter and 650 mm in length, which was almost the same as the size of the opaque glass preform.

Comparative Example Same as the example until obtaining an opaque glass preform, and without heat treatment, it was made transparent glass while traversing at 50 mm / min using an oxyhydrogen burner at 1950 ° C. The surface of the opaque glass preform From depth 20
The glass of about mm part was completely transparent, but
The bubbles existing inside did not disappear, but rather expanded, and it was impossible to obtain a completely transparent glass preform.

<Effects of the Invention> As described above, the present invention obtains an opaque glass preform containing bubbles therein, heat-treats it at a low temperature to release the gas in the bubbles, and the inside of the bubbles is vacuumed or in the glass of the gas. The opaque glass preform from which the gas in the bubbles is released is taken out of the heating furnace and heated to a high temperature by another heating means to become transparent glass. Therefore, it is possible to obtain a high quality product.

[Brief description of drawings]

The drawing is a graph showing the relationship between the diffusion time of gas in glass and the temperature.

Claims (1)

(57) [Claims] 1. A method for collecting glass fine particles obtained from a precursor for forming glass to form a porous glass preform.
This porous glass preform is heat-treated in a heating furnace to reduce its volume to form an opaque glass preform that encloses independent bubbles inside, and subsequently this opaque glass preform is heat-treated in the furnace. Then, the gas in the bubbles is released while maintaining the state of the opaque glass, and then the opaque glass preform in which the gas in the bubbles is released is taken out from the heating furnace and heated to a high temperature by the heating means to make the opaque glass. A method for producing a glass article, comprising: