JP5293209B2 - Method for thermoforming silica glass - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for heat molding silica glass, which gives uniform optical characteristics. <P>SOLUTION: The method for heat molding silica glass includes: storing silica glass 1 in a carbon container 11; and uniformizing the viscous flow in the silica glass 1 in the height direction of the silica glass 1 when the diameter of the silica glass 1 is expanded in a heating furnace 12 while a load is applied to the silica glass 1. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for thermoforming silica glass, and more particularly, to a method for thermoforming silica glass in which silica glass is accommodated in a carbon container and the silica glass is expanded in a heating furnace.

  Silica glass is known to have extremely high purity and high transparency. For this reason, it is widely used not only for communication optical fibers but also for electronic industry applications such as optical lenses for semiconductor exposure apparatuses and exposure photomasks.

  There are roughly the following two methods for producing silica glass. The first production method is called “direct method”, and is a method in which silicon tetrachloride is vapor-phase hydrolyzed in an oxyhydrogen flame to directly deposit glass. This method is easy to increase in size and is excellent in laser resistance, but has a characteristic that vacuum ultraviolet transmittance is inferior because it has about 1000 ppm of OH groups.

  Another manufacturing method is called “VAD method” or “soot method”, and after producing a porous intermediate called soot in which fine particles of silica glass are deposited, the soot is made transparent by heat treatment. It is a method of manufacturing. Although this method is difficult to increase in size as compared with the direct method, it has features such as low OH group content, easy addition of various dopants, and excellent infrared transmission, vacuum ultraviolet transmission, heat resistance, and the like.

  In order to make silica glass into optical parts such as optical fibers and optical fiber couplers, it is necessary to heat-process the glass, but when processing into optical fibers, a method of drawing rod-like glass called drawing or drawing. Is used.

  Such a deformation process in the steady portion of the uniaxial deformation has good consistency with a simple classic model. However, one-dimensional analysis is needed to model the processing process for imparting a three-dimensional shape, such as unsteady parts such as the starting and ending parts of stretching, and lenses as optical components, and to stabilize quality. It may be insufficient.

  On the other hand, as an example of the actual thermoforming method in the conventional silica glass, an upper surface heater is provided on the upper surface of the silica glass, a side heater is provided on the side surface of the silica glass, and the isothermal surface is parallel to the vertical direction. There is a heating method for improving the uniformity and for example (see, for example, Patent Document 1).

  In addition, as an example of another thermoforming method, there is a silica glass thermoforming method in which silica glass is housed in a carbon container and a load is applied to the silica glass with a weight (weight) via a felt material (for example, , See Patent Document 2).

JP2007-261194A JP 2002-53330 A

  However, in the conventional silica glass thermoforming methods disclosed in Patent Documents 1 and 2, the viscous flow of silica glass becomes non-uniform, and the uniformity of optical properties such as birefringence and refractive index distribution can be improved. was difficult.

  The reason is considered to be that frictional resistance works in the direction opposite to the viscous flow of silica glass from the top plate or bottom plate of the carbon container containing silica glass.

  That is, the viscous flow is suppressed on the upper and lower end surfaces of the silica glass, whereas the viscous flow is likely to occur near the center of the height of the silica glass and the flow amount is increased. Therefore, the glass base material flows into the upper and lower end surfaces of the silica glass, particularly near the lower end surface of the silica glass whose friction resistance is increased by its own weight, from the vicinity of the center of the height.

  This invention is made | formed in view of the situation mentioned above, The objective is to provide the thermoforming method of the silica glass which can acquire a uniform optical characteristic.

The silica glass thermoforming method according to the present invention capable of solving the above-mentioned problems is a thermoforming method in which silica glass is housed in a carbon container and the silica glass is expanded in a heating furnace, wherein the silica glass The temperature near the upper and lower end surfaces is set high so that the temperature difference between the center in the height direction and the upper and lower end surfaces in the height direction is 50 ° C. ± 30 ° C., and is predetermined in the height direction of the silica glass. By providing the above temperature distribution , the viscous flow from the center to the outer periphery in the silica glass is made substantially uniform in the height direction of the silica glass.

According to the silica glass thermoforming method thus configured, the temperature distribution applied to the silica glass is such that the temperature difference between the center in the height direction and the vicinity of the upper and lower end surfaces in the height direction is 50 ° C. ± 30 ° C. In addition, by setting the temperature near the upper and lower end surfaces high, the viscous flow from the center in the silica glass toward the outer periphery can be made substantially uniform in the height direction of the silica glass. Therefore, since the diameter is expanded without breaking the initial axial symmetry, the optical characteristics of the formed silica glass can be reliably stabilized. Thereby, the silica glass which has a uniform optical characteristic can be obtained.

  Further, in the method for thermoforming silica glass according to the present invention, it is desirable that the viscous flow be made uniform by contacting the silica glass with a top plate and / or a bottom plate having a frictional resistance smaller than that of carbon.

According to the silica glass thermoforming method thus configured, the friction resistance of the top plate and / or the bottom plate is reduced even if the friction resistance acts in the direction opposite to the viscous flow from the top plate or the bottom plate. Therefore, the viscous flow can be made uniform in the height direction of the silica glass. Therefore, the optical characteristics of the molded silica glass can be stabilized.
In addition, "carbon" here means general carbon (friction coefficient is about 0.1 to 0.2). Therefore, “the top plate or / and the bottom plate having a lower frictional resistance than carbon abuts” means that at least the material of the top plate itself or the bottom plate itself has a friction coefficient of 0.1 or less (for example, 0.02 to 0). Or a floor plate such as a carbon thin film (coefficient of friction of about 0.05) described later on the lower surface of the top plate or the upper surface of the bottom plate.

  According to the method for thermoforming silica glass according to the present invention, in the thermoforming method in which silica glass is housed in a carbon container and the silica glass is expanded in a heating furnace, the viscous flow in the silica glass is increased by the height of the silica glass. Uniform in direction. Thereby, the silica glass provided with the uniform optical characteristic can be obtained.

It is a partially broken external appearance perspective view of the silica glass thermoforming apparatus to which the silica glass thermoforming method according to the first embodiment of the present invention is applied. It is the longitudinal cross-sectional view which simplified FIG. It is a temperature distribution figure at the time of providing the temperature distribution of 50 degreeC of temperature differences to silica glass with the silica glass thermoforming method of FIG. It is the simplified longitudinal cross-sectional view of the silica glass thermoforming apparatus to which the silica glass thermoforming method which concerns on 2nd Embodiment of this invention is applied. It is a schematic diagram for obtaining an estimate of the amount of deformation of a silica glass sample. It is sectional drawing of the silica glass sample before heat forming. It is sectional drawing after heat forming of the silica glass sample without temperature distribution. It is a temperature distribution figure at the time of providing the temperature distribution of temperature difference 10 degreeC. It is sectional drawing which shows the heat molding of FIG. It is sectional drawing which shows after thermoforming at the time of providing the temperature distribution of 20 degreeC of temperature differences. It is sectional drawing which shows after thermoforming at the time of providing the temperature distribution of 50 degreeC of temperature differences. It is sectional drawing which shows after thermoforming when the temperature distribution of a temperature difference of 80 degreeC is provided. It is sectional drawing which shows after thermoforming at the time of providing the temperature distribution of 100 degreeC of temperature differences. It is a comparison graph of a temperature difference and a homogeneous part ratio.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of a method for thermoforming silica glass according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a partially broken external perspective view of a silica glass thermoforming apparatus to which a first embodiment of a silica glass thermoforming method according to the present invention is applied. FIG. 2 is a longitudinal sectional view of FIG. FIG. 2 is a temperature distribution diagram applied to silica glass by the silica glass thermoforming method of FIG. 1.

  As shown in FIGS. 1 and 2, a silica glass thermoforming apparatus 10 according to the first embodiment of the present invention includes a carbon container 11 that contains silica glass 1 and a carbon container 11 that contains silica glass 1. A heating furnace 12 for heating and a pressing mechanism 13 for applying a load to the silica glass 1 are provided.

  The carbon container 11 is formed in a bottomed cylindrical shape using a carbon material, for example, having an outer diameter of 350 mm and an inner diameter of 330 mm. The carbon container 11 includes a top plate 14, a peripheral plate 15, and a bottom plate 16.

  The heating furnace 12 has an outer shape in which the carbon container 11 is housed, and includes a heater (not shown). The heating furnace 12 houses a carbon container 11 in which the silica glass 1 is housed, drives a heater, and performs vacuum heating while imparting a predetermined temperature distribution described later to the silica glass 1.

The pressing mechanism 13 includes, for example, a 6.5 kg weight member 17 placed on the top plate 14 of the carbon container 11. The pressing mechanism 13 pressurizes the silica glass 1 accommodated in the carbon container 11 in the heating furnace 12. Further, the displacement amount of the silica glass 1 is monitored by a potentiometer 19.
In addition, although the form which provides a press mechanism and applies a load is described here, the form which is crushed with the dead weight of silica glass without applying a load may be sufficient.

  Next, the method for thermoforming silica glass of this embodiment will be described.

First, the silica glass 1 having a predetermined dimension is set at the center of the carbon container 11, and the carbon container 11 is placed in the heating furnace 12. Then, the silica glass 1 is pressurized by the weight member 17 of the pressing mechanism 13.
At this time, the heat-treated silica glass 1 is expanded in the direction of the peripheral plate 15 of the carbon container 11 with a predetermined temperature distribution in the height direction, and formed into a silica glass molded body.

Specifically, as shown in FIG. 3, it is desirable that the temperature of the upper surface 2 and the lower surface 3 of the silica glass 1 has a temperature difference of about 50 ° C. compared to the vicinity of the center. Here, the inside of the central portion 4 indicated by a broken line from the center of the silica glass 1 is + 10 ° C., + 20 ° C. at the broken line 7 portion in the height direction, + 30 ° C. at the broken line 8 portion, + 40 ° C. at the broken line 9 portion, The lower surfaces 2 and 3 have a temperature difference of + 50 ° C.
As a result, the viscous flow of the upper surface 2 and the lower surface 3 of the silica glass 1 in which frictional resistance is generated becomes large, and the viscosity of the silica glass molded body formed by making the viscous flow in the silica glass 1 substantially uniform in the height direction. Characteristics are stabilized.

  As described above, according to the silica glass thermoforming method of the present embodiment, the initial axial symmetry is destroyed by making the viscous flow from the center in the silica glass 1 toward the outer periphery substantially uniform in the height direction. Since the diameter of the silica glass is increased, the optical characteristics of the formed silica glass can be stabilized, and a silica glass having uniform optical characteristics can be obtained.

  Moreover, by providing a predetermined temperature distribution in the height direction of the silica glass 1, the viscous flow in the silica glass is made uniform in the height direction, and the optical characteristics of the formed silica glass are stabilized. . Thereby, a high-quality silica glass having uniform optical characteristics can be obtained.

  Next, a second embodiment of the method for thermoforming silica glass according to the present invention will be described with reference to FIG. FIG. 4 is a simplified longitudinal sectional view of a silica glass thermoforming apparatus to which the second embodiment of the silica glass thermoforming method according to the present invention is applied. In addition, the description regarding the same structure and effect | action as the said 1st Embodiment is abbreviate | omitted by attaching | subjecting the same code | symbol.

  As shown in FIG. 4, the silica glass thermoforming apparatus 20 according to the second embodiment of the present invention is made of a material having a lower frictional resistance than normal carbon on the lower surface of the carbon top plate 14 and the upper surface of the bottom plate 16. The floor boards 21 and 22 which consist of are arrange | positioned. The upper end surface and the lower end surface of the silica glass 1 are in contact with the floor plates 21 and 22. In addition, as the material of the floor plates 21, 22, it is desirable to use, for example, a material having a friction coefficient of about 0.02 or a carbon thin film having a friction resistance smaller than that of normal carbon and having a friction coefficient of about 0.05.

  By arranging the floor plates 21 and 22, the frictional resistance of the upper surface and the lower surface of the silica glass 1 becomes smaller than the frictional resistance when directly contacting the normal carbon top plate 14 and bottom plate 16. As a result, even if friction resistance acts in the direction opposite to the viscous flow from the top plate 14 or the bottom plate 16, the frictional resistance of the upper surface and the lower surface of the silica glass 1 is reduced. It can be made substantially uniform in the height direction. Therefore, the optical characteristics of the molded silica glass molded body are stabilized.

  As described above, according to the silica glass thermoforming method of the present embodiment, the friction resistance of the top plate or the bottom plate is smaller than that of normal carbon. Even when working from the plate or the bottom plate, the viscous flow can be made substantially uniform in the height direction. Thereby, the optical characteristic of the shape | molded silica glass can be stabilized.

  Next, examples (simulation results) performed for confirming the effects of the silica glass heat molding method according to the present invention will be described.

  As part of studying the viscous flow analysis method of silica glass, we analyzed uniaxial deformation using a classical model and viscous flow analysis using a three-dimensional thermal fluid program. As a model for the analysis, a process for expanding the diameter of silica glass for optical fibers produced by the VAD method to an outer diameter of φ300 mm (φ12 inches) or more by thermoforming in a heating furnace was examined.

[Analysis of thermoforming by classical model]
It is known that the uniaxial stretching deformation of a viscous body can be described as follows.

ここで、ｆは単位面積当たりの荷重または張力、Ｔは温度、η(Ｔ)はガラス粘度、Ｓは断面積、ｖは長手方向の変形速度、ｚは長手方向軸を表す。

Here, f is the load or tension per unit area, T is the temperature, η (T) is the glass viscosity, S is the cross-sectional area, v is the deformation rate in the longitudinal direction, and z is the longitudinal axis.

  As shown in FIG. 5, by performing a numerical calculation based on the above formula, an estimation of the deformation amount when an arbitrary heating temperature T and a load f are applied to a glass cylinder having an arbitrary cross-sectional area S and a thickness z. Is possible. In addition, about the viscosity (eta) (T) of silica glass, the result measured in the temperature range of 1100 degreeC-1500 degreeC by the separate penetration method was used.

[Three-dimensional analysis of viscous flow using FLOW-3D]
Furthermore, in addition to the classical model, in order to grasp the viscous flow phenomenon of glass in detail, the details of the deformation behavior were investigated using a three-dimensional analysis.

In order to simulate the viscous flow of silica glass during heat deformation, it is important to accurately calculate the large deformation behavior and heat transfer at the glass interface.
There are two types of glass interface modeling methods: a “direct representation method” represented by the Lagrangian method and the Euler method, and an “indirect representation method” represented by the VOF (Volume of fluid) method.

The former direct expression method has the advantage that an element conforming to the interface shape can be created, but large deformation analysis is known to have the disadvantage that the aspect ratio of the element increases and the calculation accuracy decreases. Yes.
In addition, the latter indirect expression method is effective for large deformation analysis. However, since glass is handled in a lattice shape, there is a concern that the flow near the boundary and the accuracy of heat transfer become insufficient.

  Then, in order to solve these pros and cons, the application of Flow Science's fluid analysis simulator “FLOW-3D” was examined. In this simulator, the VOF method is expanded using the FAVOR (Fractional Area Volume Obstacle Representation) method that expresses a smooth shape with a part of the calculation cell cut based on the orthogonal lattice, and the advantage of the indirect expression method Has the feature that the flow and heat transfer near the wall can be solved accurately.

  This simulator can handle a wide range of flows such as interface shape (free surface), phase change, compressibility, fluid / rigid coupled motion, etc. as one of the 3D thermal fluid programs. There are many reports on the validity of.

  However, there is no analysis example for a large and highly viscous material such as silica glass. Therefore, a new analysis example was examined.

[Preparation for analysis]
(1) Analysis conditions The analysis was modeled using a two-dimensional axisymmetric model of FLOW-3D. The viscosity of the silica glass was input in a format depending on the temperature, and the deformation due to the viscosity change was analyzed by giving the temperature condition from the outside.
The actual heating deformation takes 4 to 5 hours depending on the heater capacity of the heating furnace, but the analysis reflecting the real time is enormous in the amount of calculation, so the time is scaled by 100 times. The analysis time was shortened by scaling so that the viscosity of the glass was 1/100. The validity of 100 times scaling will be described later.

(2) Verification of time scaling Low viscosity setting that can be deformed with only a static load of 6.5 kg using a silica glass sample with an outer diameter of 150 mm and a height of 150 mm in order to grasp to what extent the time scaling used in the analysis is acceptable. We compared the displacement with a simple model.

As a result of the comparison of the displacement amount, when looking at the respective calculation results when the initial height is 150 mm and 51 mm is deformed with the same scale (1 time) scaling, the same magnification is obtained with the same 53 mm with the scaling of 10 times and 100 times. The displacement difference was 2 mm, and when the scaling was 1000 times, the displacement difference was 4 mm.
Therefore, it is judged that there is no big difference from the actual situation up to 100 times scaling, and thereafter, analysis was performed with 100 times scaling from both aspects of calculation load and accuracy.

[Analysis results of viscous flow mechanism]
In order to grasp the viscous flow mechanism of the whole glass during heat forming, a plurality of marking lines 6 are attached in advance to the silica glass sample 5 as shown in FIG. The state of change was investigated. The marking line visualizes deformation inside the glass, that is, the state of viscous flow, and corresponds to contour lines of physical properties such as refractive index, dopant concentration, and OH group concentration in terms of characteristics.
Hereinafter, the investigation results for both the case where the temperature distribution in the silica glass sample 5 is uniform and the case where the temperature difference occurs are described.

[No temperature distribution]
A silica glass sample 5 having an outer diameter of 170 mm and a height of 320 mm was used, the load was 6.5 kg, and the temperature in the sample was uniform.

  As shown in FIG. 7, when the temperature distribution is not given, the amount of flow toward the outer periphery (lateral direction in the figure) is relatively large near the center of the height of the silica glass sample 5, and it is in the vicinity of the outer periphery in the initial stage. The part flows into the bottom.

This is because friction force works between the upper surface / top plate of the sample and between the lower surface / bottom plate of the sample, and in particular, the sample's lower surface is loaded with its own weight (5.4 kg) in addition to the load due to the weight. The frictional force is larger than between the upper surface / top plate.
For this reason, the frictional force acts in the opposite direction of the viscous flow in the area near the upper and lower surfaces in the initial stage, so that the viscous flow is less likely to occur than in the vicinity of the center of the height, especially on the lower surface of the sample where the frictional force is large. become. Therefore, it is considered that the glass that was in the vicinity of the outer periphery in the initial stage flows into the lower surface side.

[With temperature distribution]
Next, the case where the above silica glass sample 5 is used for thermoforming by adding a temperature distribution will be examined.
Specifically, the silica glass sample 5 having an outer diameter of 170 mm and a height of 320 mm is set at the center of the carbon container 11, and the carbon container 11 is placed in the heating furnace 12 and pressed with a load of 11.0 kg (see FIG. 2).

  During the thermoforming, the silica glass sample 5 receives heat transfer from the top plate 14 and the bottom plate 16 in addition to the radiant heat from the heater of the heating furnace. Presumed to have occurred. This temperature difference can be an instability factor in process and quality in that the initial axial symmetry is lost without being molded in the vertical direction.

  Therefore, by giving a large amount of heat conduction to the upper and lower surfaces of the sample under the calculation conditions, analysis is performed by giving multiple temperature differences between the upper and lower surfaces and the vicinity of the center of the height. Compared.

[Temperature distribution available, temperature difference 10 ℃]
As shown in FIG. 8, 1800 ° C. was applied to the central portion 4 of the silica glass sample 5, and a temperature difference of + 10 ° C. of 1810 ° C. was applied to the upper surface 2 and the lower surface 3 thereof. Note that a broken line 18 has a temperature difference of + 5 ° C.

  As shown in FIG. 9, when a temperature difference of 10 ° C. is applied, the amount of flow around the center of the height is large and the marking line 6 is close to the load direction (the angle between the vertical axis is 0 to 20 degrees). It can be seen that the range () is as small as the length dimension L1.

[Temperature distribution present, temperature difference 20 ℃]
As shown in FIG. 10, 1800 degreeC was provided to the center part of the silica glass sample 5, and the temperature difference of +20 degreeC of 1820 degreeC was provided to the upper surface and the lower surface.

  When a temperature difference of 20 ° C. is applied, the amount of flow near the center of the height is large, and the substantially parallel part of the marking line 6 is slightly larger than the case of a temperature difference of 10 ° C. L2 (L1 <L2) It turns out that it is.

[With temperature distribution, temperature difference 50 ℃]
With reference to FIG. 3, 1800 degreeC was provided to the center part 4 of the silica glass 1, and the temperature difference of +50 degreeC of 1850 degreeC was provided to the upper surface 2 and the lower surface 3. As shown in FIG.

  As shown in FIG. 11, when a temperature difference of 50 ° C. is applied, the amount of flow around the center of the height is small, and the substantially parallel portion of the marking line 6 is longer than when the temperature difference is 10 ° C. and 20 ° C. It can be seen that the length is L3 (L1 <L2 <L3). That is, it can be estimated that the viscous flow in the silica glass sample 5 becomes substantially uniform, and the optical characteristics of the molded silica glass molded body are further stabilized.

[Temperature distribution available, temperature difference 80 ℃]
As shown in FIG. 12, 1800 degreeC was provided to the center part of the silica glass sample 5, and the temperature difference of +80 degreeC of 1880 degreeC was provided to the upper surface and the lower surface.

  When a temperature difference of 80 ° C. is applied, the amount of flow near the center of the height is small, and the substantially parallel portion of the marking line 6 has a length dimension L 4 that is substantially the same as that when the temperature difference is 50 ° C. I understand.

[Temperature distribution present, temperature difference 100 ℃]
As shown in FIG. 13, 1800 degreeC was provided to the center part of the silica glass sample 5, and the temperature difference of +900 degreeC of 1900 degreeC was provided to the upper surface and the lower surface.

  When a temperature difference of 100 ° C. is applied, the amount of flow around the center of the height is large, and the portion of the marking line 6 parallel to the load direction is as small as a length dimension L 5 that is substantially the same as when the temperature difference is 10 ° C. I understand. This is because when the temperature difference increases, the viscous flow near the top and bottom of the sample wins, so that the vicinity of the center of the height collapses.

As shown in FIG. 14, the range in which the angle between the marking line 6 and the vertical axis is 0 degree (parallel) to ± 20 degrees is defined as a homogeneous part, and the ratio of the homogeneous part to the whole varies depending on the temperature difference. As a result of comparison, it can be seen that the temperature difference at which the ratio of the homogeneous portion is 40% or more is in the range of 50 ° C. ± 30 ° C.
Accordingly, as the temperature distribution to be applied to the silica glass sample 5, the temperature near the upper and lower end surfaces is set high so that the temperature difference between the center in the height direction and the upper and lower end surfaces in the height direction is 50 ° C. ± 30 ° C. Thus, the viscous flow of the sample can be made substantially uniform in the height direction. Therefore, the optical characteristics of the molded silica glass can be reliably stabilized.

This is because, as described above, the viscosity in the vicinity of the upper and lower surfaces is lower because the temperature is higher than that in the vicinity of the center of the height, and the amount of flow to the outer peripheral portion is increased by overcoming the frictional force between the top plate and the bottom plate. This is probably because the amount of flow around the center of the height is relatively small.
Thus, it is suggested that controlling the viscous flow by positively imparting a temperature distribution in the heating furnace leads to stabilization of quality such as optical characteristics.

  In addition, this invention is not limited to embodiment mentioned above, A deformation | transformation, improvement, etc. are possible suitably. In addition, the material, shape, dimension, numerical value, form, number, arrangement location, and the like of each component in the above-described embodiment are arbitrary and are not limited as long as the present invention can be achieved.

DESCRIPTION OF SYMBOLS 1 Silica glass 11 Carbon container 12 Heating furnace 14 Top plate 16 Bottom plate

Claims (1)

Silica glass is housed in a carbon container and is a thermoforming method for expanding the silica glass in a heating furnace,
The temperature near the upper and lower end faces is set high so that the temperature difference between the vicinity of the center in the height direction of the silica glass and the upper and lower end faces in the height direction is 50 ° C. ± 30 ° C., and in the height direction of the silica glass A method for thermoforming silica glass, characterized in that a viscous flow from the center to the outer periphery of the silica glass is made substantially uniform in the height direction of the silica glass by applying a predetermined temperature distribution .

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