JP6896009B2 - Heat treatment method and heat treatment equipment for porous glass base material - Google Patents

Description

The present invention relates to a heat treatment method and a heat treatment apparatus for a porous glass base material.

In the manufacturing process of the optical fiber, the vitrification step is performed through the dehydration step of the porous glass base material which is the material of the optical fiber.
In this vitrification step, the dehydrated porous base material is suspended in a furnace and heated at a higher temperature than during dehydration.
If the porous glass base material is continuously heated at a high temperature, it may stretch vertically downward more than necessary due to its own weight, and the porous glass base material itself may be deformed. It may reach and damage the core tube.
For this reason, in the conventional manufacturing equipment, a radiation thermometer is provided with the detection range below the porous material in the furnace, and when it is detected that the detection range is extended from the change in the detection temperature, an emergency stop is performed. (See, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2010-90017

However, the manufacturing apparatus of Patent Document 1 detects when the porous base material extends to the lower detection range of the porous material in the furnace, so that the porous base material has sufficiently extended. It can be detected only later, and there is a problem from the viewpoint of stable production.

An object of the present invention is to more appropriately vitrify a porous glass base material for an optical fiber.

In the present invention, a porous glass base material for an optical fiber arranged in a heating furnace is heated by a heating unit in a predetermined atmosphere, and the exhaust gas in the heating furnace is exhausted during the heating. It is a heat treatment method for materials.
A temperature raising step of raising the temperature of the porous glass base material to a target temperature, and
A vitrification process for maintaining the porous glass base material at a target temperature, and
Including a temperature lowering step of lowering the temperature of the porous glass base material,
In the vitrification step, a specific component in the exhaust gas exhausted from the heating furnace is measured, and when the partial pressure of the component drops below a predetermined threshold value, the process shifts to the temperature lowering step. ..

Further, in the present invention, a heating furnace for storing the porous glass base material, a heating unit for heating the porous glass base material, an exhaust unit for exhausting the exhaust gas in the heating furnace, and the heating unit are provided. A heat treatment device for a porous glass base material provided with a control unit for controlling.
A measuring unit that measures the partial pressure of a specific component contained in the exhaust gas from the heating furnace, and
The control unit
A temperature raising step of raising the temperature of the porous glass base material in the heating furnace to a target temperature, a vitrification step of maintaining the porous glass base material in the heating furnace at a target temperature, and the porous glass base material. In addition to performing heat treatment step by step in multiple steps including the temperature lowering step of lowering the temperature of the material,
In the vitrification step, when the partial pressure of the specific component measured by the measuring unit falls below a predetermined threshold value, control is performed to shift to the temperature lowering step.

Further, in the above heat treatment apparatus,
The exhaust unit has an exhaust passage through which the exhaust of the exhaust gas passes.
The measuring unit may have a bypass path branched from the exhaust passage, and may be configured to measure the partial pressure of the specific component contained in the exhaust gas passing through the bypass path.

Further, in the above heat treatment apparatus,
The measuring unit may be configured to be connected with a suction means for drawing the exhaust gas from the exhaust passage into the bypass path.

According to the present invention, it becomes possible to more appropriately vitrify the porous glass base material for optical fibers.

It is sectional drawing which shows the schematic structure of the heat treatment apparatus which is an embodiment of an invention. It is a flowchart of the control of the heat treatment executed by the control unit. The porous glass base material was heat-treated by a heat treatment device, and the change in the partial pressure of the detected component based on the output of the gas analyzer obtained during the vitrification step at that time was continuously performed until it became undetectable. It is a diagram of the case. It is a diagram which showed the change of the partial pressure of the detection component of the measuring part in the vitrification step of the porous glass base material of Example 1. FIG. It is a diagram which showed the change of the partial pressure of the detection component of the measuring part in the vitrification process of the porous glass base material of Comparative Example 1. FIG. It is a diagram which showed the change of the partial pressure of the detection component of the measuring part in the vitrification process of the porous glass base material of Comparative Example 2.

[Outline of Embodiment of the Invention]
Hereinafter, the heat treatment apparatus 10 for the porous glass base material for the optical fiber according to the embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a schematic configuration of the heat treatment apparatus 10.

[About porous glass base material]
The porous glass base material 100 for an optical fiber to be heat-treated includes a core portion 101 and a soot body 102 formed on the outer periphery thereof.
The porous glass base material 100 is heated with a burner while spraying quartz glass fine particles, which are the raw material of the soot body 102, on the core portion 101 together with mist, and the quartz glass fine particles are deposited around the core portion 101. , Soot body 102 is formed.
Then, dehydration is performed by heating the porous glass base material 100 at a temperature lower than the sintering temperature.
The heat treatment apparatus 10 is for performing a heat treatment (vitrification step) for converting the soot body 102 of the dehydrated porous glass base material 100 into transparent vitrification by heat sintering.

[Outline configuration of heat treatment equipment]
The heat treatment apparatus 10 includes a furnace body 20 as a heating furnace having a core tube 21 for storing the porous glass base material 100, a support portion 30 for suspending and supporting the porous glass base material 100 in the furnace body 20, and a furnace. A heating unit 40 that heats the porous glass base material 100 in the body 20, a gas supply unit 50 that supplies a predetermined gas into the core tube 21, and an exhaust unit 60 that exhausts the exhaust gas in the core tube 21. A measuring unit 70 for measuring the partial pressure of a specific component contained in the exhaust gas, and a control unit 90 for controlling each unit of the device are provided.

[Furnace body]
The reactor body 20 is a housing for insulating the inside, and the core tube 21 is housed inside.
The core tube 21 is a hollow cylindrical body made of quartz glass. The upper part of the core tube 21 can be opened, and the porous glass base material 100 suspended and supported in the vertical vertical direction by the support portion 30 can be stored inside.
Further, the lower end of the core tube 21 is provided with an intake port 22 connected to the gas supply unit 50, and the upper end is provided with an exhaust port 23 connected to the exhaust unit 60.
The heat treatment apparatus 10 is a so-called decompression reactor, in which gas is supplied from the gas supply unit 50 to the inside of the core tube 21, and the inside of the core tube 21 is negatively pressured (compared to the atmosphere) by the exhaust unit 60. For example, it can be 100 [Pa] or less).

[Heating part]
The heating unit 40 includes a heater 41 that heats the porous glass base material 100 inside the core 20 via the core tube 21.
The heater 41 of the heating unit 40 is arranged so as to surround the periphery of the core tube 21, and has a length capable of heating over substantially the entire length of the core tube 21.
The heating unit 40 provided with such a heater 41 can simultaneously heat the entire length of the porous glass base material 100.
The heater 41 is connected to the adjusting unit 42, and the amount of heat generated can be adjusted according to the amount of power supplied by the adjusting unit 42. The adjusting unit 42 adjusts the calorific value of the heater 41 so that the porous glass base material 100 reaches the target temperature under the control of the control unit 90.

[Support]
The support portion 30 is provided on the upper side of the reactor body 20 and grips and supports the upper end portion of the core portion 101 of the porous glass base material 100 extending outward from the small holes provided at the upper end portion of the core tube 21. To do.
The support portion 30 includes a motor (not shown) as a drive source, a speed reducer for decelerating the output rotation of the motor, and the like, and imparts deceleration rotation to the porous glass base material 100 centering on the gripped core portion 101. ..
The support unit 30 is connected to the control unit 90, and is controlled so as to rotate the porous glass base material 100 at a predetermined rotation speed during the heat treatment of the porous glass base material.

[Gas supply unit]
The gas supply unit 50 includes a gas supply source 51 for vitrified gas, a connecting pipe 52 connecting the gas supply source 51 to the intake port 22 of the core pipe 21, and a gas flow provided in the middle of the connecting pipe 52. It is equipped with a meter 53.
The gas supply source 51 stores vitrified gas to be supplied into the core tube 21, for example, an inert gas such as _{He gas, Ar gas, N 2 gas, Cl 2} gas, etc., and these gases are appropriately stored in the core. Supply to tube 21.
The gas flow meter 53 is connected to the control unit 90, and the amount of gas supplied from the gas supply source 51 to the core pipe 21 via the connecting pipe 52 can be adjusted to a target amount.

[Exhaust section]
The exhaust unit 60 is connected to the dry pump 61 that takes in air from the core pipe 21, the exhaust pipe 62 as an exhaust passage that connects the exhaust port 23 of the core pipe 21 and the dry pump 61, and the exhaust side of the dry pump 61. It is equipped with an exhaust gas treatment device 63.
The dry pump 61 takes in air from the core tube 21 and depressurizes the inside thereof. Since the dry pump 61 is a vacuum pump that does not use oil or liquid in the vacuum chamber, it is possible to reduce the mixing of oil or liquid with respect to the exhaust sent to the exhaust gas treatment device 63.
The exhaust gas treatment device 63 collects components that cannot be exhausted from the exhaust gas.

[Measurement unit]
The measuring unit 70 includes a bypass pipe 71 as a bypass path that branches from the exhaust pipe 62 and returns to the exhaust pipe 62, and a gas analyzer 72 as a detection unit that detects a predetermined component provided in the middle of the bypass pipe 71. A turbo pump 73 provided on the downstream side of the gas analyzer 72 and a dry pump 74 provided on the downstream side of the turbo pump 73 are provided.

The dry pump 74 sucks the exhaust pipe 62 from the downstream side of the bypass pipe 71. On the other hand, since the exhaust pipe 62 takes in the exhaust gas by the dry pump 61 on the downstream side thereof, it is necessary to make the pressure inside the bypass pipe 71 more negative than that inside the exhaust pipe 62. Therefore, the bypass pipe 71 is also provided with a turbo pump 73, and a part of the exhaust gas is drawn into the bypass pipe 71 by using these two pumps 73 and 74 as suction means.
Further, since the dry pump 74 is also a vacuum pump that does not use oil or liquid in the vacuum chamber, it is possible to reduce the mixing of oil or liquid with respect to the exhaust sent to the exhaust gas treatment device 63.

The gas analyzer 72 is a gas analyzer such as a quadrupole mass spectrometer, and detects the concentration of a specific component contained in the exhaust gas. The concentration detected by the gas analyzer 72 is input to the control unit 90, and the control unit 90 calculates the partial pressure of a specific component.
Therefore, strictly speaking, the function of the control unit 90 to calculate the partial pressure from the detected concentration of a specific component is also a part of the configuration of the measurement unit 70.
As a specific component, hydrogen chloride is exemplified in Example 1 described later.

[Control unit]
The control unit 90 includes a CPU (Central Processing Unit), a storage device that stores a control program and control data, and a memory in which the CPU expands the data.
The control unit 90 is connected to the adjustment unit 42 of the heating unit 40 and the gas flow meter 53 of the gas supply unit 50 via an interface (not shown), controls the calorific value of the heater 41, and connects to the core tube 21. It is also possible to control the amount of gas supplied.
Further, the control unit 90 is connected to the gas analyzer 72 of the measurement unit 70 via an interface, and calculates the partial pressure of the detected component from the output of the gas analyzer 72.
Further, the control unit 90 is also connected to the radiation thermometer 24 as a temperature detection unit for detecting the temperature of the porous glass base material 100 stored in the core tube 21 via an interface, and the detection temperature is measured. Entered.
Further, the control unit 90 is connected to the dry pump 61 of the exhaust unit 60 and the turbo pump 73 and the dry pump 74 of the measurement unit 70 via an interface, and drives and stops these pumps 61, 73, 74. Control.

[Heat treatment control by control unit]
In the control unit 90, the CPU executes a control program of the storage device to realize heat treatment control based on a characteristic heat treatment method for the porous glass base material 100.
That is, the control unit 90 raises the temperature of the porous glass base material 100 in the core tube 21 to a target temperature suitable for vitrification, and sets the porous glass base material 100 in the core tube 21 to the target temperature. The heat treatment is carried out stepwise in a plurality of steps including a vitrification step for maintaining and a temperature lowering step for lowering the temperature of the porous glass base material 100 in the core tube 21.

The control of the heat treatment executed by the control unit 90 is shown in the flowchart of FIG.
First, the control unit 90 controls the gas flow meter 53 in a state where the porous glass base material 100 is stored in the core tube 21 before the temperature raising step, and the gas supply unit 50 enters the core tube 21. While supplying nitrogen gas, the dry pump 61 of the exhaust unit 60 and the turbo pump 73 and the dry pump 74 of the measuring unit 70 are operated to reduce the pressure in the core tube 21 to a predetermined negative pressure state (preparation step: step). S1).

Then, the control unit 90 supplies electric power to the heater 41 and heats and raises the temperature while monitoring the temperature of the porous glass base material 100 with the radiation thermometer 24 (heating step: step S2).
The control unit 90 determines whether the detection temperature of the porous glass base material 100 by the radiation thermometer 24 reaches a target temperature suitable for vitrification, and if not, continues to raise the temperature, and if it reaches, raises the temperature. The process shifts from the temperature step to the vitrification step (step S3).

Next, the control unit 90 controls the adjustment unit 42 to maintain a target temperature suitable for vitrification while monitoring the temperature of the porous glass base material 100 with the radiation thermometer 24, and supplies power to the heater 41. (Vitrification step: step S4).
Further, in the vitrification step, the control unit 90 calculates the partial pressure of the detected component from the output of the gas analyzer 72 of the measurement unit 70, and sequentially records the value.
Then, the control unit 90 compares the partial pressure of the detection component sequentially obtained with a predetermined threshold value, and if it is higher than the threshold value, continues to maintain the target temperature. Further, when the partial pressure of the detected component drops below the threshold value, it is assumed that the end point of the vitrification step has been reached, and the power supply to the heater 41 is stopped (step S5).
Then, the process shifts from the vitrification process to the temperature lowering process.

Next, the control unit 90 stops the turbo pump 73 and the dry pump 74 of the measurement unit 70, and ends the recording of the output of the gas analyzer 72.
Further, the control unit 90 monitors the temperature of the porous glass base material 100 with the radiation thermometer 24, and controls the gas flow meter 53 when the temperature of the porous glass base material 100 drops to a predetermined cooling temperature. Then, the supply of nitrogen gas is stopped, and the exhaust is stopped by stopping the dry pump 61 of the exhaust unit 60 (temperature lowering step: step S6).
Then, the heat treatment is completed.

[Example 1]
Here, Example 1 of the present invention will be described.
FIG. 3 shows that the porous glass base material 100 is heat-treated by the heat treatment apparatus 10, and the change in the partial pressure of the detected component based on the output of the gas analyzer 72 obtained during the vitrification step at that time is detected. It is a diagram when it is continued until it becomes impossible. Further, FIG. 4 is a diagram showing a change in the partial pressure when the vitrification step is performed until the partial pressure reaches a predetermined threshold value.

In Example 1, the porous glass base material 100 was put into the core tube 21 heated to 1100 [° C.], and the inside of the core tube 21 was depressurized to about 20 [Pa].
Then, the temperature was raised until the target temperature suitable for vitrification of about 1400 [° C.] was reached (heating step).
Further, when the detection temperature of the porous glass base material 100 reached about 1400 [° C.], a constant target temperature was maintained until the vitrification was completed (vitrification step). At this time, a part of the exhaust gas generated in the core tube 21 was drawn into the measuring unit 70, and the partial pressure of hydrogen chloride was sequentially recorded as a specific component.
As shown in FIG. 3, the partial pressure of hydrogen chloride in the vitrification step temporarily increased at the beginning of detection and then decreased. After that, when the detection temperature of the porous glass base material 100 continued to be about 1400 [° C], the partial pressure value ^{fell below 3 × 10 -6} [Pa] due to the influence of noise from other substances. It became undetectable.

On the other hand, the vitrification step is carried out in the same manner as above, the threshold value of the ^{partial pressure is set to 5 × 10 -6} [Pa] as shown in FIG. 4, and the vitrification step is terminated when the partial pressure drops to the threshold value. When the heating of the heater 41 was stopped and the process proceeded to the temperature lowering step, the result was as follows.
That is, when the porous glass base material 100 sufficiently cooled by the temperature lowering step is taken out from the core tube 21, the porous glass base material 100 is sintered substantially uniformly in the longitudinal direction and the whole is vitrified. It was confirmed.

Regarding this result, the inventor of the present invention presumed that the following phenomenon occurred in the vitrification step of the heat treatment. That is, the porous optical fiber base material contains chlorine inside, and is gradually discharged into the heating furnace in the dehydration step.
When the vitrification step of the porous glass base material is performed, a small amount of water and chlorine remaining in the porous glass base material 100 are discharged into the core tube 21 and hydrogen chloride is discharged as in the case of the dehydration step. Is generated. As the sintering of the base metal progresses, the amount of chlorine released decreases, so the amount of hydrogen chloride produced also decreases. Therefore, the end point of vitrification can be determined by determining the timing at which the detected amount of hydrogen chloride becomes sufficiently small.

[Comparative Example 1]
In the same manner as in Example 1, the porous glass base material 100 is charged into the core tube 21 and the temperature is raised to about 1400 [° C.]. Then, while maintaining 1400 [° C.], the concentration of hydrogen chloride in the exhaust gas generated in the core tube 21 was detected by the gas analyzer 72, and the change in partial pressure based on this was monitored. The change in partial pressure at this time is shown in FIG.
In this Comparative Example 1, when the numerical value of the decreasing partial pressure ^{became almost constant at 4 × 10 -6} [Pa], the process proceeded to the temperature lowering process.
Then, when it was taken out after being sufficiently cooled, it was confirmed that the porous glass base material 100 was sufficiently sintered, but a thin portion was formed in a part thereof. This is because the vitrification became excessive and the vitrified base material was stretched.

[Comparative Example 2]
In the same manner as in Example 1, the porous glass base material 100 is charged into the core tube 21 and the temperature is raised to about 1400 [° C.]. Then, while maintaining 1400 [° C.], the concentration of hydrogen chloride in the exhaust gas generated in the core tube 21 was detected by the gas analyzer 72, and the change in partial pressure based on this was monitored. The change in partial pressure at this time is shown in FIG.
In this Comparative Example 2, the process proceeded to the temperature lowering step when the numerical value of the decreasing partial pressure ^{showed 6 × 10 -6 [Pa].}
Then, when it was taken out after being sufficiently cooled, the vitrified base material was insufficiently sintered. That is, there were some spots in the soot body 102 of the porous glass base material 100 that remained cloudy without being made transparent.

From the results of Example 1 and Comparative Examples 1 and 2 described above, as shown in FIG. 3, the partial pressure of hydrogen chloride was 4 × 10 ^-6 [Pa] at the end point of the vitrification step of the porous glass base material. When set to the point E2, the vitrification of the porous glass base material 100 progresses excessively to cause elongation and deformation, and when set to the point E3 where the ^{partial pressure of hydrogen chloride becomes 6 × 10 -6 [Pa],} It turned out that the vitrification was insufficient. Then, it was found that when the end point of the vitrification step was set at the point E1 where the partial pressure of hydrogen chloride between them was 5 × 10 ^{-6 [Pa], good vitrification was performed without excess or deficiency.}
Therefore, it was shown that the end point of vitrification can be determined from the change in the partial pressure of hydrogen chloride.

[Comparative Example 3]
In the same manner as in Example 1, the porous glass base material 100 is charged into the core tube 21 and the temperature is raised to about 1400 [° C.]. Then, the porous glass base material 100 was heated while maintaining 1400 [° C.]. Based on the weight of the base metal of about 100 [kg], the process proceeded to the temperature lowering process after the preset time had elapsed. Then, the same heat treatment was performed on a plurality of porous glass base materials 100 under the same conditions.
When each of the porous glass base materials 100 is taken out after being sufficiently cooled, many of them are sintered almost uniformly in the longitudinal direction, but some of the base materials are insufficiently sintered. There was a base metal that was stretched. It is considered that this is influenced by the density of the base metal before sintering and whether the dehydration process was sufficiently performed.
Further, it was concluded that it is difficult to control the vitrification process by the processing time because the results of the plurality of porous glass base materials 100 vary even if the vitrification process is executed at the same time. Obtained.

[Technical Effects of Embodiments of the Invention]
As described above, the heat treatment apparatus 10 controls the control unit 90 to shift to the temperature lowering step when the partial pressure of a specific component based on the measurement of the measuring unit 70 drops below a predetermined threshold value in the vitrification step. Is going.
Therefore, it is possible to properly vitrify the entire porous glass base material 100 while suppressing excessive elongation.
In addition, since the end point of the vitrification process is determined based on the change in the partial pressure of a specific component, the porous glass matrix is compared with the case where the porous glass matrix is actually stretched and then detected. It is possible to suppress the occurrence of elongation of the material, and it is possible to obtain a good vitrified base material.

Further, in the heat treatment apparatus 10, the measuring unit 70 measures the partial pressure of a specific component contained in the exhaust gas passing through the bypass pipe 71 branched from the exhaust pipe 62, so that the vacuum inside the core pipe 21 is on the exhaust pipe 62 side. Even when suction is performed for pulling or depressurizing, it becomes easy to pull in the exhaust gas with a constant amount on the measuring unit 70 side, and it becomes possible to measure the partial pressure of a specific component with high accuracy. It is possible to realize good vitrification of the porous glass base material.

Further, since the turbo pump 73 and the dry pump 74 that draw the exhaust gas into the bypass pipe 71 are connected to the heat treatment apparatus 10, it becomes easy to adjust the suction force that draws the exhaust gas to the measuring unit 70 side, and the gas analyzer It is possible to adjust the pressure difference before and after 72, and it is possible to protect the gas analyzer 72.

[Other]
In the above-described embodiment, various technically preferable restrictions are given for carrying out the invention, but the scope of the present invention is not limited to the above-described embodiment and the illustrated examples.
For example, although hydrogen chloride is exemplified as the specific component to be measured by the measuring unit 70, other components that may be generated in the vitrification step may be targeted.
Further, the numerical value of the partial pressure that determines the end of the vitrification step is also an example, and for example, a more appropriate numerical value obtained by a comparative test in which the numerical value is changed may be selected.

10 Heat treatment equipment 20 Furnace body (heating furnace)
21 Core tube 24 Radiation thermometer 40 Heating section 41 Heater 42 Control section 50 Gas supply section 53 Gas flow meter 60 Exhaust section 61 Dry pump 62 Exhaust pipe (exhaust passage)
70 Measuring unit 71 Bypass pipe (bypass path)
72 Gas analyzer 73 Turbo pump (suction means)
74 Dry pump (suction means)
90 Control unit 100 Porous glass base material 101 Core part 102 Suit body

Claims (4)

A method for heat-treating a porous glass base material for an optical fiber arranged in a heating furnace by heating the porous glass base material for optical fibers in a predetermined atmosphere by a heating unit and exhausting the exhaust gas in the heating furnace during the heating. And
A temperature raising step of raising the temperature of the porous glass base material to a target temperature, and
A vitrification process for maintaining the porous glass base material at a target temperature, and
Including a temperature lowering step of lowering the temperature of the porous glass base material,
In the vitrification step, a specific component in the exhaust gas exhausted from the heating furnace is measured, and when the partial pressure of the component drops below a predetermined threshold value, the process shifts to the temperature lowering step. A method for heat-treating a porous glass base material. A heating furnace for storing the porous glass base material and
A heating unit that heats the porous glass base material and
An exhaust unit that exhausts the exhaust gas in the heating furnace and
A heat treatment apparatus for a porous glass base material including a control unit for controlling the heating unit.
A measuring unit for measuring the partial pressure of a specific component contained in the exhaust gas from the heating furnace is provided.
The control unit
A temperature raising step of raising the temperature of the porous glass base material in the heating furnace to a target temperature, a vitrification step of maintaining the porous glass base material in the heating furnace at a target temperature, and the porous glass base material. In addition to performing heat treatment step by step in multiple steps including the temperature lowering step of lowering the temperature of the material,
In the vitrification step, when the partial pressure of the specific component measured by the measuring unit falls below a predetermined threshold value, the heat treatment of the porous glass base material is characterized in that the transition to the temperature lowering step is controlled. apparatus. The exhaust unit has an exhaust passage through which the exhaust of the exhaust gas passes.
The porosity according to claim 2, wherein the measuring unit has a bypass path branched from the exhaust passage and measures the partial pressure of the specific component contained in the exhaust gas passing through the bypass path. Heat treatment equipment for quality glass base material. The heat treatment apparatus for a porous glass base material according to claim 3, wherein the measuring unit is connected to a suction means for drawing the exhaust gas from the exhaust passage into the bypass path.

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