JP3748910B2 - Heat treatment method for glass base material - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a heat treatment of a glass base material that performs a sintering (transparent vitrification) treatment step by heating a porous glass body in a step of producing an optical fiber preform by a VAD method or an external method. Regarding the method.
[0002]
[Prior art]
When an optical fiber preform is manufactured by the VAD method or the external method, an oxyhydrogen flame is produced from a burner, and glass source gas such as silicon tetrachloride and dopant material gas are fed into the oxyhydrogen flame, and hydrolysis reaction and Glass fine particles (silicon dioxide) are generated by a thermal oxidation reaction, and the glass fine particles are deposited on a target to form a cylindrical porous glass body. Then, the porous glass body is heated to be transparent vitrified (sintered). Alternatively, the step of removing (dehydrating) OH groups may be performed at the same time as this sintering process. In this way, a transparent glass body is made, and a thin optical fiber is made by melting and drawing and spinning.
[0003]
This heat treatment process for the glass base material is performed by suspending the porous glass body with a traverse device and moving (traverse) the furnace core tube heated by the heater of the heating furnace downward. That is, the heat treatment apparatus includes a heating furnace including a furnace core tube and a heater that heats the furnace tube, and a traverse apparatus disposed above the heating furnace.
[0004]
[Problems to be solved by the invention]
However, according to the conventional heat treatment apparatus for a glass base material, when a large (long) glass base material is heat-treated, there is a problem that a variation in the outer diameter of the resulting transparent glass body is large.
[0005]
This problem arises under the circumstances that the porous glass body, which is the glass base material to be processed, is becoming larger and larger in recent years in order to reduce the manufacturing cost of the optical fiber. In other words, since the heat treatment apparatus inevitably treats a long one that exceeds the size of the porous glass body planned, the tip of the porous glass body enters the area heated by the heater from the beginning. In such a positional relationship, the heater is heated in a state where the porous glass body is disposed, and when the sintering temperature is reached, the downward traverse is started. Therefore, the tip (lower end) portion put into the heating region from the beginning is excessively heated and tries to become a sphere due to the surface tension, so that the transparent glass body 6 after the heat treatment as shown in FIG. The lower end of the swells and the diameter becomes larger than other parts (upper part).
[0006]
And since a good optical fiber can only be made at a part with a uniform outer diameter, if there is such a change in the outer diameter, even if a large porous glass body is made, there are fewer parts that can be used effectively. Therefore, it cannot contribute to the reduction of the manufacturing cost.
[0007]
Of course, in order to avoid such a problem, it is only necessary to increase the size of the heat treatment apparatus in accordance with the long porous glass body, but this causes a problem of an increase in cost of the heat treatment apparatus. Moreover, even if the heat treatment apparatus is enlarged, when the porous glass body that exceeds the size that it is supposed to be processed has to be processed, the same problem will be encountered again. Therefore, it does not mean that the heat treatment apparatus should be easily enlarged.
[0008]
In view of the above, the present invention has improved so that a long porous glass body can be heat-treated without causing fluctuations in the outer diameter of the sintered transparent glass body even in a small heat treatment apparatus. It aims at providing the heat processing method of a glass base material.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, the porous glass body is suspended from above and traversed downward into the furnace core tube of the heating furnace, so that the region is heated by the heater of the heating furnace. In the method of sequentially passing the heat from the lower end to the upper end of the porous glass body, the length of the porous glass body is long in relation to the core tube, and the lower end of the porous glass body is heated at the starting point of the traverse. When the porous glass body can only be suspended so as to enter the region, the temperature of the heating region is increased by the heater while holding the porous glass body with the lower end entering the heating region. The traverse downward is started at a time before the temperature reaches the sintering temperature of the porous glass body.
[0010]
[Action]
In the case of a porous glass body longer than originally planned by the heat treatment apparatus for the glass base material, when this is suspended by the traverse means, the lower end portion of the porous glass body enters the heater from the beginning. End up. Therefore, if the heater temperature is raised in this state and the traverse downward is started when the sintering temperature is reached, the lower end portion that has been in the heater from the beginning is excessively heated, As a result, fluctuations in the outer diameter will occur, but since the traverse is started at a time before reaching the sintering temperature, the lower end is also the same as the other parts, Appropriate amount of heating is applied, and fluctuations in the outer diameter at the lower end can be suppressed. As a result, a transparent glass body having a large effective length can be obtained, which can contribute to reducing the manufacturing cost of the optical fiber.
[0011]
【Example】
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, the heat treatment apparatus for glass base material according to one embodiment of the present invention includes a heating furnace 1, a traverse apparatus 7, and a controller 8 for controlling them. The heating furnace 1 has a quartz glass core tube 2 at its center, and a heater 3 is provided around it. The traverse device 7 holds the porous glass body 5 formed at the lower end of the starting member 4 by hanging the starting member 4 from above. The porous glass body 5 is traversed downward while being rotated in the core tube 2 while being suspended by the traverse device 7. The core tube 2 is filled with a process gas of a dehydrating agent such as helium and chlorine gas fed from the lower part of the core tube 2 when the dehydration process is performed simultaneously with the sintering process.
[0012]
Thus, the porous glass body 5 is heated from the lower end to the upper end by passing through the region heated by the heater 3 from the upper side to the lower side, and from the lower end portion to the upper end. The glass is gradually made into transparent glass. That is, when the porous glass body 5 is traversed downward, a transparent glass body is completed from the lower end portion. The dotted line shows the porous glass body 5 in a state where this heat treatment step is nearing the end, and the transparent glass body 6 is formed at the lower end thereof. Since the porous glass body 5 having a low bulk density is sintered into the transparent glass body 6, the outer diameter of the transparent glass body 6 is smaller than the outer diameter of the porous glass body 5.
[0013]
When the heat treatment is performed in this manner, if the porous glass body 5 is long, the lower end of the porous glass body 5 is heated from the beginning by the heater 3 as shown in FIG. It has entered the state. Therefore, as in the conventional case, the temperature of the heater 3 is raised as shown in FIG. 2 in this state, and when the sintering temperature (for example, 1500 ° C.) is reached (for example, 50 minutes), the dotted line in FIG. If the downward traverse is started as in FIG. 3, the lower end portion is excessively heated, and a swollen portion is formed as shown in FIG.
[0014]
In order to avoid this, as described above, the position of the traverse device 7 is further upward, the furnace tube 2 is also long, and even if the porous glass body 5 is long, its lower end is sufficiently higher than the heater 3. In this state, the temperature is increased, and when the sintering temperature is reached, the downward traverse of the porous glass body 5 may be started. However, if such a configuration is adopted, the overall size of the heat treatment apparatus must be increased, resulting in an increase in size and an increase in cost.
[0015]
Therefore, in this embodiment, when the temperature reaches 1300 ° C. (for example, 44 minutes) before reaching the sintering temperature, the controller 8 controls to start the downward traverse as shown by the solid line in FIG. I have to. According to this, since the lower end portion of the porous glass body 5 that has entered the region heated from the beginning by the heater 3 is also heated to the same extent as the other portions above it, the swelling of the lower end portion is increased. Can be eliminated. Further, since the traverse is started before the heating furnace 1 reaches the sintering temperature, the sintering time can be shortened.
[0016]
Actually, a porous glass body 5 having an outer diameter of 140 mm and a length of 1000 mm manufactured by the VAD method is baked using a furnace core 2 having a furnace core tube 2 having an inner diameter of 200 mm and a heater 3 having a length of 390 mm. I tried to finish. The porous glass body 5 is suspended by the traverse device 7. Even when the porous glass body 5 is raised to the uppermost position, the lower end of the porous glass body 5 is 140 mm below the upper end of the heater 3. The temperature was increased by holding and heating the heater 3. The sintering temperature was set to 1500 ° C., and it took 50 minutes to rise to the sintering temperature. However, the temperature reached 1300 ° C. at 44 minutes before reaching the sintering temperature. Started. The traverse speed is 150 mm / hour.
[0017]
As a result of such heat treatment, the outer diameter of the transparent glass body 6 excluding the lower end portion was 70.1 mm, and the lower end portion was 70.7 mm when the maximum value of the outer diameter was shown. This indicates that the outer diameter fluctuation is suppressed to 0.08% of the steady portion. Incidentally, as a comparative example, when the traverse start time is 50 minutes when the temperature reaches 1500 ° C. as in the conventional case under exactly the same conditions as described above, the outer diameter of the transparent glass body 6 excluding the lower end is 71. 7 mm at the maximum outer diameter portion at the lower end, and the outer diameter fluctuation reached 3.3% of the steady portion.
[0018]
In the above, the time and temperature for starting the traverse are only examples, and are appropriately determined according to various conditions. That is, since the amount of heat applied to each part of the porous glass body 5 corresponds to the time integration of the temperature, it is basically determined by the residence time of each part in the heater 3 and the temperature during the stay. However, since it does not actually work as expected, it is desirable to determine it by trial and error according to actual conditions. However, even if the optimum value cannot always be found, it is easy to suppress the variation in the outer diameter as in the above example only by appropriately bringing the traverse start time earlier than the time when it reaches the sintering temperature.
[0019]
【The invention's effect】
As described above with respect to the examples, according to the heat treatment method for a glass base material of the present invention, a long porous glass body can be obtained after sintering even in a small heat treatment apparatus having no height. Heat treatment can be performed without causing a variation in the outer diameter of the transparent glass body. Therefore, even if the porous glass body is long, it is not necessary to make the glass base material heat treatment apparatus large in accordance with this, and it can contribute to the reduction of the manufacturing cost of the optical fiber.
[Brief description of the drawings]
FIG. 1 is a schematic view of a glass base material heat treatment apparatus according to an embodiment of the present invention.
FIG. 2 is a time chart showing a relationship between a temperature change and a traverse in the embodiment.
FIG. 3 is a perspective view showing an outer diameter variation in a conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Core tube 3 Heater 4 Starting member 5 Porous glass body 6 Transparent glass body 7 Traverse apparatus 8 Controller

Claims (1)

  By suspending the porous glass body from above and traversing downward into the furnace core tube of the heating furnace, the porous glass body sequentially passes from the lower end to the upper end in the region heated by the heater of the heating furnace. In the method of heat treatment, the porous glass body is long so that the length of the porous glass body is long in relation to the core tube and the lower end of the porous glass body enters the heating region at the starting point of the traverse. If the porous glass body is held in such a state that the lower end has entered the heating area when it cannot be suspended, the temperature of the heating area is raised by a heater, and the temperature is the sintering temperature of the porous glass body. A glass substrate heat treatment method, characterized in that a downward traverse is started at a time before reaching the point.

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