JPH0450129A - Heating furnace for production of glass preform for optical fiber - Google Patents

Abstract

PURPOSE:To stably obtain a heating furnace to heat-treat porous glass preform for optical fibers for a long period of time, by forming a gas impermeable coating layer having specific structure at least on the inside surface of a furnace core tube of carbon material. CONSTITUTION:In a heating furnace to heat-treat porous glass perform, equipped with a furnace core tube of carbon base, the furnace core tube is provided with a gas-impermeable coating layer at least on the inside surface and the coating layer consists of a surface coating layer 1 and a coating layer 3 impregnated into the carbon material 2. Isotropic carbon is preferable as the carbon material. The gas-impermeable coating layer is preferably made of carbon of graphite structure or amorphous structure or silicon carbide of beta- or amorphous structure.

Description

Detailed Description of the Invention [Industrial Application Field 1] The present invention relates to a heating furnace for heat-treating a porous glass base material such as a glass base material for optical fibers, and a porous glass base material for optical fibers using the heating furnace. The present invention relates to a method for heat treatment of a base material.

[Prior art] Typical methods for producing glass base materials for optical fibers include:
There are methods such as method for gas phase shaft attachment and method for external attachment. By these methods, first a cylindrical or cylindrical porous glass base material for optical fiber is made, and then this porous glass base material for fiber is heated with inert gas such as argon, helium, etc. and/or chlorine gas or Under an atmosphere of fluorine-based gas, etc.
It is heated in a heating furnace (heating, dehydration sintering, etc.) to turn it into a transparent, high-purity glass base material for optical fibers.

The operating conditions of the heating furnace used in such heat treatment vary depending on the type and content of the dopant added to the base material, but the temperature range is usually about 1200 to 1600°C. Further, since impurities are likely to get mixed into the base material at high temperatures, a furnace core tube made of high-purity quartz is used for boat fishing in order to reduce the mixing of impurities as much as possible.

However, such high-purity quartz furnace core tubes suffer from ``devitrification'', which is a transition from a glassy state to a crystalline state at high temperatures.
There is a problem in that a phenomenon called .

In order to solve this problem of the durability of quartz hearth tubes, there is a method of applying a gas-impermeable coating (for example, carbon, SiC, etc.) to the surface of a carbon material with an ash content of less than 20 ppm (Patent Application No. 1999). -167695 and patent application No. 1
-232378, etc.) cracks due to stress caused by the difference in thermal expansion coefficients (mismatch in thermal expansion coefficients) between the carbon base material and the coating layer, and the intended purpose of gas impermeability is not achieved. It may disappear.

Further, it is difficult to make the coating layer thicker than 100 μm due to the above-mentioned mismatch in thermal expansion coefficients, and cracks may occur in the coating layer due to mechanical impact or the like.

The problems of the conventional technology as described above are summarized as follows: (1) When a quartz furnace core tube is used, the furnace core tube softens and deforms at temperatures above 1400°C, and crystals (
Since cristobalite) is produced, the furnace temperature cannot be lowered below the crystal transition temperature (300° C. or below), and once heated, it must be used continuously.

(2) When a gas-impermeable coating is applied on a carbon substrate, cracks occur in the coating due to stress caused by a mismatch in the coefficient of thermal expansion between the substrate and the coating layer, and the gas-impermeable function is lost. I no longer have it.

[Problem to be Solved by the Invention 1 Therefore, the problem to be solved by the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a porous quartz glass base material for optical fibers for a long period of time. An object of the present invention is to provide a heating furnace and a heat treatment method that can stably perform heat treatment.

[Means for Solving the Problems] As a means for solving the above problems, the present invention has a furnace core tube made of a carbon base material, and a heating method for heat-treating a porous glass base material for Hikari 7 Eyeper. A furnace, wherein the furnace core tube has a gas-impermeable coating layer on at least an inner surface;
The present invention provides a heating furnace characterized in that the coating layer comprises a surface coating layer and an impregnated coating layer impregnated into a carbon substrate.

The furnace core tube needs to have a gas-impermeable coating layer at least on the inner surface of the carbon base material, which becomes hot due to heat treatment and is easily consumed, but if desired, a gas-impermeable coating layer can also be applied to the other surface area. It may have layers.

Furthermore, the present invention provides a method for heat treating a porous quartz glass preform for an optical fiber, which is characterized by using a heating furnace having a core tube as described above.

As used herein, the term "heat treatment" means a treatment involving heating of a glass base material, such as ordinary heating, dehydration,
Used to include operations including doping, drying, etc.

As used herein, the term "impregnated" is used to mean a state in which a substance to be coated has penetrated into the pores of a porous carbon substrate. Furthermore, the term "impregnated coating" as used below means coating such that at least a portion of the coating layer is in an impregnated state, that is, the coating layer consists of a surface coating layer and an impregnated coating layer.

In order to clarify the difference between the coating applied to the surface of the furnace core tube of a conventional heating furnace and the coating layer applied to the surface of the furnace core tube of the heating furnace of the present invention, FIGS. Figure 1(b) shows a schematic cross-sectional view of the furnace core tube portion.

Figure 1(a) shows the case of a conventional furnace core tube, which consists of a surface coating layer 1 and a furnace core tube base material body 2, and there is a portion between them where the base material and the coating layer do not match. exist. FIG. 1(b) shows the case of the furnace core tube of the present invention, in which the surface coating layer l with a thickness L2 and the furnace core tube base material body 2
There is an impregnated coating layer 3 of thickness Ll between.

In the furnace core tube of the heating furnace of the present invention, a carbon base material that does not undergo deformation at temperatures below 2000°C is used.

Furthermore, since a portion of the coating layer is impregnated into the carbon base material, the adhesion between the base material and the coating layer is significantly improved, and the above problem can be solved.

The thickness of the impregnated coating layer is not particularly limited, but considering adhesion with the base material and mechanical strength, it should be at least 5 times the average particle diameter of the carbon used as the carbon base material. is preferred.

Carbon materials that can be used as the base material of the furnace core tube of the present invention include:
Isotropic carbon having a density of at least 1.6 g/cm3 is preferred. Anisotropic carbon materials tend to cause coating cracks due to differences in thermal expansion in the axial direction. For example, in the case of a general carbon material having a density of about 1.75 g/c, the average particle size is 15 to 20 pm. Therefore, in the furnace core tube of the present invention, the thickness of the impregnated coating impregnated into the carbon substrate is preferably at least 100 μm.

If such an impregnated coating layer exists, even if the surface coating layer closer to the core tube surface corrodes or wears out, a high-density region will exist in the core tube base material, which will prevent gas from flowing. The advantage also arises that transparency is retained.

The materials used for the coating layer of the furnace core tube of the present invention are particularly limited as long as they have heat resistance, purity, and can withstand the halogen gases used to improve the purity of the optical fiber base material and control the refractive index. isn't it. From this perspective,
For example, carbon with a graphitic or amorphous structure, such as pyrolytic carbon, or β-5iC or amorphous SiC can be used as coating material.

Impregnation coating is performed by infiltrating the coating material into the pores of the porous substrate.

For impregnation coating, for example, a vapor phase deposition method (CVI method, chemical vapor phase impregnation method) using conditions in which the rate of gas phase reaction is rate-determining in a reduced pressure atmosphere and low temperature, and liquid phase coating on porous materials. material (for SiC, SiC1x (C
In the case of carbon, such as tHs)z, a method such as dipping PVC (polyvinyl chloride) and then thermally decomposing it at high temperature to obtain a coating having an impregnated region (liquid phase dipping method) can be applied. Considering purity, vapor phase deposition method (CVD method)
It is particularly preferred to form the surface coating layer and the impregnated coating layer by.

For example, a coating layer can be formed under the following conditions: ■ SiC impregnated coating total pressure 3 Q torr, S i (CH3) C (i
When the coating layer is formed for 20 hours at s/Hl-1/2 and a temperature of 1350°C, the thickness of the impregnated coating layer is 400 μm 1 The thickness of the surface coating layer is 50 μm
becomes.

■SiC normal coating total pressure 760 torr, St (CHs) CQs/Hx
-1/10 and a temperature of 1350°C for 3 hours, a SiC coating layer with a thickness of 60 μm is obtained.

■Pyrolytic carbon impregnation coating After impregnation coating for 40 hours at a total pressure of 1 torr, c H4/H*-1/3, and a temperature of 15000C,
Total pressure 50 torr SCH4/H! -1/3, temperature 180
When the surface is coated at 0° C. for 10 hours, the thickness of the impregnated coating layer is 200 to 300 μm, and the thickness of the surface coating layer is 30 μm.

■Pyrolytic carbon normal coating Total pressure 50 torr, CH4/Hl-1/3, temperature 180
When a coating layer is formed at 0° C. for 10 hours, a coating layer with a thickness of 30 μm is obtained.

In the case of the CVI method, the thickness (Ll) of the impregnated coating layer is determined by the total pressure P during coating formation, the pore diameter R of the substrate, and the deposition reaction rate K.

The thickness Ll of the impregnated coating layer is the pore diameter R to the 5th power of 0°,
It is expressed as a function proportional to the 0.5th power of the mean free path (CP) of molecules, which can be defined from the reaction rate to the 11th power and the total pressure P (proportional to 0'').In other words, the pore diameter of the carbon base material and The thickness L1 of the impregnated coating layer is determined based on the total pressure and temperature.The thickness L2 of the surface coating layer is determined based on the reaction rate and time.

Therefore, by appropriately changing the above-mentioned operating conditions during coating formation, surface coating layers and impregnated coating layers of various thicknesses can be formed.

In addition, since the surface coating layer and the impregnated coating layer are generally manufactured continuously, it is not possible to strictly distinguish between the two coating layers, but in this specification, for convenience, the pores of the base material are The coating portion above the substrate is defined as an impregnated coating layer, and the portion coated above the surface of the substrate is defined as a surface coating layer, and the two coating layers are divided and the thickness of each layer is measured. The thickness of these layers can be easily measured, for example, by cutting the surface portion of the base material on which the coating has been formed and observing it with an SEM (scanning electron microscope, etc.).

In addition, in recent years, carbon materials whose carbon matrix is carbon fiber with increased mechanical strength have been commercially available (for example, as C/C composite from Nippon Carbon Materials)8, which can be impregnated and coated with a coating material. This results in a mechanically stronger furnace core tube.

Next, an experimental example that led to the present invention will be specifically explained.

Experimental Example 1 Thermal Shock Resistance Experiment (SiC Impregnation Coating) An isotropic carbon base material with a specific gravity of 1.75 g/cm' was impregnated with β-5iC using S1(CHs)CI3 as a raw material by a reduced pressure CVD method. The impregnation coating layer thickness was 400 μm and the surface coating layer thickness was 50 μm.

This SiC-impregnated carbon material was heated for 100°C in a N2 atmosphere at a temperature range of 600°C to 1700°C.
The temperature was raised and lowered 30 times at a rate of °C/min. No cracks were observed on the surface of the carbon material, and the gas permeability was 1X10-" based on N2.
c was less than /see, and there was no practical problem.

Comparative Experiment Example 1 Thermal shock resistance experiment (SiC normal coating) β-
A surface coating layer with a thickness of 50μ was applied by coating 5iC.

When this SiC coated base material was repeatedly heat treated in the same manner as in Example 1, cracks were generated on the coated surface.

Experimental Example 2 Thermal Shock Resistance Experiment (Graphite Impregnated Coating) The same carbon substrate as in Example 1 was impregnated and coated with pyrolytic carbon graphite using CH as a raw material by low pressure CVD method, and the impregnated coating layer thickness was 500 μm. , the surface coating layer thickness was set to 35 μm.

This coated substrate was repeatedly heat treated in the same manner as in Example 1, but no cracks were observed.

In addition, the gas permeability after heat treatment is 2X 1 based on N2.
0-'-3, 5X l O-"crn"/sec, which was good (that is, the gas permeability was low).

Comparative Experimental Example 2 Thermal Shock Resistance Example (Normal Graphite Coating) A surface coating layer of pyrolytic carbon having a thickness of 35 μm was applied to the same base material as in Example 1 by the CVD method.

When the heat treatment was repeated in the same manner as in Example 1,
Cracks were generated in the coating layer, and the gas permeability was 5×IO-cm/sec based on N2, which was almost the same value as the carbon base material.

Experimental Example 3 Gas Resistance Experiment (Graphite Impregnated Coating) The graphite impregnated coating substrate used in Example 2 was treated at 800° C. under N2 containing 02800 ppm for 1 hour to remove 25 μm of the surface coating layer by oxidation.

When we measured the gas permeability of this coating base material, we found that
2 x 10-' to 8 x 10-” crn based on N2
It was good at 2/SeC.

Comparative Experimental Example 3 Gas Resistance Experiment (Normal Graphite Coating) The graphite coating substrate used in Comparative Example 2 was oxidized in the same manner as in Example 3.

The gas permeability of this coating base material is 3X based on N2.
l O−'cra”/sec, and the initial 2X1
It was much worse than 0-'cm''7sec (that is, the gas permeability was large).

Comparative Experiment Example 4 Gas resistance experiment (graphite impregnated coating) A carbon base material having a pyrolytic carbon impregnated coating layer (thickness 20 μm) and a pyrolytic carbon surface coating layer (thickness 30 μm) formed by low pressure CVD method was used as an example. It was treated in the same manner as in 3.

When the gas permeability of the coating substrate was measured, it was found that N2
Based on the standard, it was 6 x 10-''cm27 sec, and the gas permeability was deteriorated.

From the results of the above experimental examples and comparative experimental examples, it can be seen that thermal shock resistance and gas resistance are improved by impregnation coating.

Example 1 (SiC impregnated coating base furnace core tube) A schematic cross-sectional view is shown in FIG. On the inner surface of the furnace core tube base material 13, a 50 μm thick SiC surface coating layer and an 800 μm thick SiC impregnated coating layer (
A porous glass base material 21 is heated using a heating device 10 comprising a carbon-based furnace core tube 12 having a carbon-based furnace core tube 12 (both coatings are collectively designated by reference numeral 11) and a heater 14 through which the furnace core tube passes. was heat treated.

A porous glass base material 21 (SiO□, outer diameter 120
mm, bulk density 0.3 g/cm"), and the furnace temperature was 110
The temperature was 0°C. Dehydration - 5iCL300 as highly purified gas
cc/min and He1OL' min.
mm).

Subsequently, the furnace temperature was raised to 1340°C and He1OQ/
and 5iFa at a rate of 400 cc/min, and the porous glass preform was passed through a portion of the heater at a rate of 3 mm/min to add fluorine to the glass preform.

Then, the furnace temperature was raised to 1620°C and the feed gas was He1
O (1/min) and passed through the glass base material at a speed of 10 mm/min to obtain transparent vitrification.

The water content of the obtained glass base material was less than 0.005 ppm (
It was 5iOH (measurement of 3675c+i− absorption) by an infrared absorption photometer. The obtained glass base material has a diameter of 8 mm.
Hz/O! Diameter 25mm with glass lathe
It was heated and stretched. Thereafter, a glass rod made of anhydrous synthetic quartz glass with a diameter of 5 mm was inserted and melted and integrated to obtain a preform with a non-refractive index difference of 0.34% and a core/cladding ratio of 5.2.

After heating and stretching this preform to an outer diameter of 15 mm, an outer diameter of 3
It is inserted into a fluorine-doped glass pipe with a diameter of 5 mm and a hole diameter of 16.5 mm, heated and integrated, and the core/
A preform with a cladding ratio of 12.2 was manufactured. Incidentally, the above-mentioned fluorine-containing glass pipe was also produced in this SiC coating furnace.

The above preform was drawn into pure S with an outer diameter of 125 μm.
An iO2 core single mode fiber was fabricated. When we measured the transmission loss of this 7-eyeper, we found that the wavelength was 1.55μ.
It showed a loss of 0 and 178 dB/km at m.

When 40 base materials were processed using the above procedure, the average loss was 0.1748 dB/km. Further, during the treatment of 40 pieces, the temperature of the furnace was lowered to U at a rate of 80°C/min every 10 pieces, but no damage to the coating was observed.

Comparative Example 1 (SiC regular coating base core tube) A carbon base core tube (without impregnated coating layer) was provided with only a SiC surface coating layer (thickness 50 μm) by CVD method on the furnace core tube 12 of a heating furnace. Using the same procedure as in Example 4, ten fluorine-doped quartz glass base materials having a non-refractive index difference of 0.34% with respect to quartz glass were manufactured.

Using the obtained fluorine-doped silica glass base material, pure Sin,
Five core single mode fibers were manufactured. The average transmission loss is 0°1752 dB at a wavelength of 1.55 μm.
/km.

Thereafter, when the furnace temperature was lowered to room temperature at a rate of 80° C./min, cracks were observed in a portion of the furnace core tube heater.

When one fluorine-doped glass base material was made using this furnace core tube and made into a fiber, an increase in loss due to impurities (Fe'', Cu'') was confirmed, as shown by the line ■ in Figure 4. . Further, for comparison, the loss of the fiber obtained in Example 1 is shown by line I.

Example 2 (Graphite-impregnated coated base material furnace core tube) As the furnace core tube of the heating furnace shown in Fig. 2, a carbon-based furnace core tube impregnated and coated with graphite by a low-pressure CVD method (graphite coating layer thickness 50 μm 1 graphite impregnated layer) A fluorine-doped quartz glass base material was produced in the same manner as in Example 4 using a heating furnace having a thickness of 1200 μm.

During this time, the furnace temperature was increased to 100% every time 10 glass base materials were processed.
Although the temperature was lowered to room temperature at a rate of °C/min, no damage was observed to the furnace core tube.

Using the obtained fluorine-doped quartz glass base material, pure 5i02
When a core single mode fiber was manufactured, the average loss was 0.1735 dB/km at a wavelength of 1.55 μm.

In addition, we experimentally changed a part of the external coating to 0.21%.
After oxidative consumption was carried out by heat treatment at 800° C. with N299% or less, glass base material treatment was performed, but no leakage of the gas inside the furnace core tube to the outside was observed.

Example 3 (Graphite impregnated coating base furnace core tube) Heating furnace 1 having a cross-sectional structure as schematically shown in FIG.
0, a carbon-based furnace core tube 12 whose inner surface is impregnated and coated with graphite (formed by low-pressure CVD method, graphite surface coating layer thickness 35 μm)
m1 graphite impregnated coating layer thickness 1300μ
m) was used.

After inserting the porous base material 21 into the preliminary chamber 25,
mN! Gas replacement was performed for 10 minutes by supplying 20Ω. Thereafter, the inner lid 26 was opened and the base material was inserted into the furnace core tube. Furnace temperature 1080℃, dehydration - CL400cc as high purity gas
/min and He15Q/min into the furnace core tube,
The glass body was allowed to pass through a portion of the heater at a rate of descent of 10+nm/min.

Then, the gas was switched to He 15a/min and the furnace temperature was increased to 165
The temperature was set at 0° C., and the glass base material was made to pass through a portion of the heater at a descending speed of 5 mm/minute, thereby converting the base material into transparent glass.

The obtained glass base material had a water content of less than 005 ppm CCl. After stretching this 5iOz base material in a resistance heating furnace,
The fiber was inserted into a fluorine-doped glass tube (anhydrous) with a non-refractive index difference of 0.35% and heated and integrated to obtain a single mode fiber preform.

When the obtained preform was delineated, the wavelength was 1.55.
Transmission loss value at μIl is 0.173dB/kll
It was good.

Comparative Example 2 (Normal Graphite Coated Base Core Tube) A carbon base core tube whose inner surface was coated with graphite to a thickness of 70 μm by CVD was used as the core tube of the heating furnace having the structure shown in FIG.

When five fluorine-added glass base materials were produced using this heating furnace in the same manner as in Example 1, the water content of the third and subsequent base materials was 0.2 ppm. In addition, upon examining the condition of the coating on the furnace core tube, it was confirmed that cracks had formed in the coating at the top and bottom of a portion of the heater, and gas was leaking.

[Effects of the Invention] As can be understood from the Examples and Comparative Examples described above, when a heating furnace having a core tube coated with an impregnation coating according to the present invention is used, an optical fiber base material that does not contain impurities or moisture can be obtained. It can be manufactured stably, and furthermore, compared to conventional quartz furnaces and surface coating furnaces, there is no thermal damage due to temperature rises and falls, so the furnace core tube can be used for a long period of time, resulting in significant economic effects.

[Brief explanation of the drawing]

FIG. 1(a) is a schematic cross-sectional view of a conventional furnace core tube base material,
FIG. 1(b) is a schematic sectional view of the furnace core tube base material part used in the heating furnace of the present invention, FIGS. 2 and 3 are schematic sectional views of the heating furnace used in Examples or Comparative Examples, FIG. 4 is a graph showing the transmission loss of fibers obtained in Examples and Comparative Examples. DESCRIPTION OF SYMBOLS 1...Surface coating layer, 2...Furnace core tube base material body, 3...Impregnation coating layer, 10...Heating furnace, 1
1... Coating layer, 12... Furnace core tube, 13...
- Furnace core tube base material, 14... Heater 21... Glass base material, 25... Preliminary chamber, 26... Inner lid. Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Scope of Claims] 1. A heating furnace for heat-treating a porous glass base material for optical fiber, comprising a carbon-based furnace core tube, the furnace core tube having at least an inner surface impermeable to gas. 1. A heating furnace comprising a surface coating layer and an impregnated coating layer impregnated into a carbon substrate. 2. The thickness of the impregnated coating layer is at least 100 μm
The heating furnace according to claim 1. 3. The heating furnace according to claim 1 or 2, wherein the gas-impermeable coating layer is carbon with graphite structure or amorphous structure or silicon carbide with β- or amorphous structure. 4. The heating furnace according to any one of claims 1 to 3, wherein the carbon base material comprises isotropic carbon or a carbon fiber matrix. 5. A method of heat treating a porous glass preform for optical fiber using the heating furnace according to any one of claims 1 to 4.