Method for producing porous glass base material

Abstract

In a device for producing a large-sized porous base material by a VAD process, the cracking and variation of the outer diameter of the base material are suppressed by forming a smooth tapered part, without changing the length of a non-effective part. In producing the porous base material by a VAD process, the time for a gas to reach a flow amount of the gas in a steady state from starting of the deposition is extended more in a burner that deposits glass microparticles on a layer closer to the outside of the base material.

Claims

1. A method for producing a porous glass base material, comprising producing a porous glass base material by disposing glass microparticles generated by burners, starting from a starting element that is rotating around its central axis as a rotation axis, along the central axis of the starting element, while the starting element is relatively drawn up with respect to the burners, the burners generate the glass microparticles by hydrolyzing a gas containing glass raw materials in oxyhydrogen flame, wherein a first burner of the burners, which is configured to form a core part of the porous glass base material, a second burner of the burners, which is configured to form a first clad part of the porous glass base material on the outer periphery of the core part, and a third burner of the burners, which is configured to form a second clad part of the porous glass base material on the outer periphery of the first clad part, are used, and wherein Ta<Tb<Tc, wherein Ta (min) is a time for a flow amount of the gas that is fed to the first burner to reach a flow amount at a steady state by increasing the flow amount from a flow amount at starting of the deposition, Tb (min) is a time for a flow amount of the gas that is fed to the second burner to reach a flow amount at a steady state by increasing the flow amount from a flow amount at the starting of the deposition, and Tc (min) is a time for a flow amount of the gas that is fed to the third burner to reach a flow amount at a steady state by increasing the flow amount from a flow amount at the starting of the deposition.

2. The method for producing a porous glass base material according to claim 1, wherein the relationships of (Tb−Ta)<(H1/v) and (Tc−Ta)<(H2/v) are satisfied, wherein v (mm/min) is a draw-up velocity of the starting element, H1 (mm) is a distance between a cross point of an extended line of the central axis of the first burner and the central axis of the starting element and a cross point of an extended line of the central axis of the second burner and the central axis of the starting element, and H2 (mm) is a distance between a cross point of an extended line of the central axis of the first burner and the central axis of the starting element and a cross point of an extended line of the central axis of the third burner and the central axis of the starting element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a drawing showing an example of a production device for carrying out the method for producing a porous glass base material of the present invention;

(2) FIG. 2 is a drawing showing the schematic shape of the porous glass base material;

(3) FIG. 3 is a drawing showing the schematic shape of a non-product tapered part;

(4) FIG. 4 is a drawing showing the change in a gas flow amount over the time, which was applied to the verification of a conventional production method;

(5) FIG. 5 is a drawing showing an example of the schematic shape of the non-product tapered part in the case when the size of the part was increased by a conventional production method;

(6) FIG. 6 is a drawing showing the change in the gas flow amount over the time, which was applied to the verification of the production method of the present invention;

(7) FIG. 7 is a drawing showing an example of the schematic shape of the non-product tapered part in the case when the size of the part was increased by the production method of the present invention;

(8) FIG. 8 is another drawing showing the change in the gas flow amount over the time, which was applied to the verification of the production method of the present invention; and

(9) FIG. 9 is another drawing showing an example of the schematic shape of the non-product tapered part in the case when the size of the part was increased by the production method of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(10) FIG. 1 shows an example of a production device for carrying out the method for producing a porous glass base material of the present invention. The method for producing a porous glass base material of the present invention is that, in a conventional production method including producing a porous glass base material by disposing glass microparticles generated by a plurality of burners, starting from a starting element 2 that is rotating around its central axis as a rotation axis, along the central axis of the starting element 2, wherein the burners generate the glass microparticles by hydrolyzing a gas containing glass raw materials in oxyhydrogen flame, while the starting element 2 is relatively drawn up with respect to the burners, different times for flow amounts of the gases at the starting of the deposition of the glass microparticles to reach flow amounts of the gases at a steady state by increasing the flow amount are used for the respective burners.

(11) Specifically, for example, a first burner 3, which is configured to form a core part of the porous glass base material, a second burner 4, which is configured to form a first clad part on the outer periphery of the core part, and a third burner 5, which is configured to form a second clad part on the outer periphery of the first clad part, are used, and Ta, Tb and Tc are adjusted so that Ta<Tb<Tc, wherein Ta (min) is a time for a flow amount of the gas that is fed to the first burner to reach a flow amount at a steady state by increasing the flow amount from the flow amount at a starting of the deposition, Tb (min) is a time for a flow amount of the gas that is fed to the second burner to reach the flow amount at the steady state by increasing a flow amount from the flow amount at the starting of the deposition, and Tc (min) is a time for a flow amount of the gas that is fed to the third burner to reach a flow amount at a steady state by increasing the flow amount from a flow amount at the starting of the deposition.

(12) Since the effect of the increasing of the diameter of the base material with respect to the degree of the change from the base material at the time of starting of the deposition to the gas flow amount at a steady state is greater in the layer closer to the outside of the base material, the amount of the change in the gas flow amount of the outer burner per unit time can be decreased by elongating the time for reaching a flow amount of a gas at a steady state in the burner that deposits the glass microparticles in the layer closer to the outside of the base material. By this way, generation of concave and convex on the surface of a non-product tapered part can be suppressed, and the deposition can be carried out with a smooth tapered shape.

(13) Furthermore, in order to prevent deficiency in the thickness of the clad at the part where the non-product tapered part 8 transfers to a product cylindrical part 9, which may occur by using different times for reaching the flow amounts of the gases at a steady state in respective burners, it is preferable to constitute as follows.

(14) FIG. 2 shows the schematic shape drawing of the porous glass base material, and FIG. 3 shows the schematic shape of the tapered part deposited by the three burners, respectively. The product cylindrical part 9 of the porous glass base material is positioned below the position where the gas flow amount of the first burner 3 for forming a core part has reached a flow amount at a steady state, and the shape of the deposited core part 11 has become a shape of a steady state (line a in FIG. 3). Therefore, it is substantially sufficient that the gas flow amounts of the second burner 4 for forming a first clad part and the third burner 5 for forming a second clad part have reached constant until the line a, and that the deposited first clad part 12 and second clad part 13 has become shapes of a steady state.

(15) Accordingly, it is preferable to constitute Ta, Tb and Tc so that 0<(Tb−Ta)<(H1/v) and 0<(Tc−Ta)<(H2/v) are satisfied, wherein v (mm/min) is a draw-up velocity of the starting element, H1 (mm) is a distance between a cross point of an extended line of the central axis of the first burner and the central axis of the starting element and a cross point of an extended line of the central axis of the second burner and the central axis of the starting element, and H2 (mm) is a distance between a cross point of an extended line of the central axis of the first burner and the central axis of the starting element and a cross point of an extended line of the central axis of the third burner and the central axis of the starting element.

(16) By this way, the gas flow amounts of the second and third burners have reached flow amounts at a steady state and the deposition shapes of the first and second clad parts are stabilized at above from the position where the gas flow amount of first burner has reached a flow amount of a steady state and the deposition shape of the core part is stabilized, whereby deficiency in the thickness of the clad at the part where the non-product tapered part transfers to the product cylindrical part can be prevented, and thus a better base material can be provided.

(17) The method for producing a porous glass base material of the present invention can be suitably modified within the scope of the technical concept that is expressed in the present invention, and embodiments obtained by adding such modification or improvement are also encompassed in the technical scope of the present invention.

(18) <Confirmation of Effect>

(19) A glass raw material gas (SiCl.sub.4), a combustible gas (H2), a combustion-assisting gas (O.sub.2) and the like were fed to a first burner 3, a second burner 4 and a third burner 5, glass microparticles were generated by a flame hydrolysis reaction, and the glass microparticles were deposited on a starting element 2 disposed in a reaction container 1, and the starting element 2 is drawn up above while the starting element 2 was rotated, thereby a porous glass base material was produced.

Conventional Example 1

(20) The draw-up velocity of the starting element 2 was set to 1.0 (mm/min), the first burner 3, the second burner 4 and the third burner 5 were disposed at a positional relationship wherein H1=70 (mm) and H2=140 (mm), and SiCl.sub.4 was fed to the respective burners at the flow amounts shown in Table 1, and the gas flow amount were continuously changed over 5 hours from the starting of the deposition to a steady state. The gas flow amounts of the combustible gas and the combustion-assisting gas were suitably adjusted depending on the flow amount of SiCl.sub.4 to be fed, and after reaching flow amounts at a steady state, the deposition was carried out at those flow amounts, and the deposition was completed at after 24 hours. Ten pieces of the porous glass base material were deposited under the same conditions, and porous glass base materials having an average outer diameter of 180 mm in which the surface of the non-product tapered part 8 was formed to be smooth were consequently produced, and failures such as variation in the outer diameter of the product cylindrical part 9 (concave and convex) and cracking during the deposition did not occur.

(21) TABLE-US-00001 TABLE 1 Core deposition First clad Second clad Start of deposition 0.1 L/min 0.2 L/min 1.0 L/min Steady state 0.3 L/min 0.8 L/min 3.0 L/min

Conventional Example 2

Comparative Example

(22) The gas flow amount were continuously changed over 5 hours from the starting of the deposition to a steady state as shown in FIG. 4 in a similar manner to Conventional Example 1, except that the flow amounts of SiCl.sub.4 to be fed to the respective burners were significantly increased as shown in Table 2 so as to increase the size of the base material. Ten pieces of the porous glass base material were deposited under the same conditions, and porous glass base materials having an average outer diameter of 200 mm were produced. However, as shown in FIG. 5, the variation (concave and convex) of the outer diameter of the non-product tapered part 8 was significant, and the variation also affected the product cylindrical part 9, and 5% of the product cylindrical parts 9 was judged to be defective due to the variation in the outer diameter. Furthermore, two pieces of the produced ten pieces were cracked beginning at the convex part of the concave and convex of the non-product tapered part 8.

(23) TABLE-US-00002 TABLE 2 Core deposition First clad Second clad Start of deposition 0.1 L/min 0.2 L/min 1.0 L/min Steady state 0.4 L/min 1.2 L/min 4.0 L/min

Example 1

(24) As shown in FIG. 6, deposition was carried out in a similar method to that of Conventional Example 2, except that the gas flow amounts were changed from the starting of the deposition to a steady state over 5 hours for the first burner 3, over 6 hours for the second burner 4 and over 7 hours for the third burner 5. The gas flow amounts of the combustible gas and the combustion-assisting gas were suitably adjusted depending on the flow amounts of SiCl.sub.4 to be fed. Ten pieces of the porous glass base material were deposited under the same conditions, and porous glass base materials having an average outer diameter of 200 mm in which the surface of the non-product tapered part 8 was formed to be smooth as shown in FIG. 7 were consequently produced, and failures such as variation in the outer diameter of the product cylindrical parts 9 (concave and convex) and cracking during the deposition did not occur.

Example 2

Comparative Example

(25) As shown in FIG. 8, deposition was carried out in a similar method to that of Example 1, except that the gas flow amounts were changed from the starting of the deposition to a steady state over 5 hours for the first burner 3, over 7 hours for the second burner 4 and over 9 hours for the third burner 5, that is, the time presetting was changed so as not to satisfy the relationships of (Tb−Ta)<(H1/v) and (Tc−Ta)<(H2/v). The gas flow amounts of the combustible gas and the combustion-assisting gas were suitably adjusted depending on the flow amounts of SiCl.sub.4 to be fed. Subsequently, porous glass base materials having an average outer diameter of 200 mm in which the surface of the non-product tapered part 8 was formed to be smooth as shown in FIG. 9 were produced, and failures such as variation in the outer diameter of the product cylindrical part 9 (concave and convex) and cracking during the deposition did not occur, but the shapes of the deposited first clad part 12 and second clad part 13 did not become shapes at a steady state and the deposited amounts were insufficient at the line a where the shape of the deposited core part 11 became a shape of a steady state, and thus 3% of the product cylindrical parts 9 had defective characteristic.

(26) It is understood from the comparison of Conventional Examples and Examples that, according to the production method of the present invention, the surface of a non-product tapered part is formed to be smooth even the diameter of a base material is increased, and thus a porous glass base material in which variation in the outer diameter and cracking during deposition are difficult to occur can be produced. Furthermore, it is understood from the comparison of Example 1 and Example 2 that deficiency in the thickness of the clad at the part where the non-product tapered part transfers to the product cylindrical part can be prevented by optimizing the relationship of the gas flow amounts of the respective burners, the positions of the respective burners, and the draw-up velocity of the starting element, and thus a better base material can be provided.

REFERENCE SIGNS LIST

(27) 1 Reaction container 2 Starting element 3 First burner 4 Second burner 5 Third burner 6 Hang shaft 8 Non-product tapered part 9 Product cylindrical part 11 Core part 12 First clad part 13 Second clad part