Fluorine-containing silica glass powder, and method for producing fluorine-containing silica glass powder

Abstract

To provide a fluorine-containing synthetic silica glass powder that has an adequate fluorine content and that is capable of suppressing any decrease in fluorine concentration caused by fluorine desorption from silica.　The aforementioned problem is solved by a silica glass powder containing fluorine, the fluorine-containing silica glass powder being such that particles having a particle diameter greater than 150 μm and no greater than 300 μm constitute 25 wt% or more of the whole. Additionally, as a method for producing the glass powder, there is provided, inter alia, a method for producing a fluorine-containing silica glass powder in which silicon oxide is pre-fired in the presence of SiF4 at a temperature of less than 1000°C to produce fluorine-containing silica, and in which the fluorine-containing silica is subsequently fired at a temperature of at least 1000°C and less than 1400°C to form a silica glass powder.

Description

[Technical Field]

[0001] The present invention relates to a synthetic silica glass powder suitable for cladding or overcladding optical fibers, and a method for producing the same. [Background technology]

[0002] In fields such as optical communications and optical semiconductors, silica glass optical components such as optical fibers are widely used. Furthermore, low refractive index silica glass is required to suppress optical loss in optical fibers.

[0003] Specifically, in the manufacture of optical fibers, a layer of fluorine-containing silica glass is provided as a cladding and / or overcladding covering the core. By using low-refractive-index silica glass as the cladding, it is possible to use non-germanium-doped or low-germanium-doped silica glass for the core, thereby suppressing optical loss due to Rayleigh scattering of the core. Furthermore, by forming an appropriate refractive index distribution using low-refractive-index silica glass, optical loss when the optical fiber is bent can be reduced. Moreover, by forming an appropriate refractive index distribution using low-refractive-index silica glass, crosstalk in multi-core fibers can be reduced.

[0004] To produce silica glass with a low refractive index, it is known that boron or fluorine is doped into the silica glass, and methods for producing fluorine-doped silica glass are disclosed in Patent Documents 1 and 2. Patent Document 1 discloses a method for producing doped silica glass by preparing a dispersion in a liquid containing silica particles and a doping agent, treating the precipitate with a fluorine source-containing gas, and then calcining it. Patent Document 2 discloses a method for producing doped silica glass by irradiating silica glass containing a dopant element with electromagnetic waves and heating it. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2016-519641 [Patent Document 2] Japanese Patent Publication No. 2006-021952 [Overview of the project] [Problems that the invention aims to solve]

[0006] As mentioned above, although a method for manufacturing fluorine-doped silica glass has been proposed, it has not been fully understood what measures should be taken to ensure a sufficient amount of fluorine is doped into the fluorine-doped silica powder during the manufacturing process.

[0007] More specifically, it is important that the fluorine is stably retained within the silica in the manufactured fluorine-doped silica powder. Therefore, reducing the silica particle size, for example, is considered advantageous in that it increases the surface area when doping with fluorine, allowing the fluorine to penetrate from the silica surface and be retained inside. However, on the other hand, in the context of glass manufacturing, if fluorine is present at a shallow depth from the silica surface, it is possible that the fluorine will not remain in the silica glass during the processing of the silica powder or when melting to obtain silica glass with the desired refractive index, and the desired amount of fluorine may not be obtained. As a result, there was little knowledge about what conditions are preferable. [Means for solving the problem]

[0008] The inventors investigated how to dope the silica glass powder with a sufficient amount of fluorine to produce a practically necessary difference in refractive index under the circumstances described above. As a result, they discovered that if the particle size of the silica powder used as the raw material is within an appropriate range, it is possible to provide a fluorine-containing silica glass powder that contains sufficient fluorine and suppresses the decrease in fluorine concentration due to fluorine desorption from the fluorine-containing silica powder, thus leading to the invention.

[0009] A first embodiment of the present invention includes the following: [1] A synthetic silica glass powder containing fluorine, wherein particles with a particle size of more than 150 μm and 300 μm or less constitute 25 wt% or more of the total. [2] The fluorine-containing silica glass powder according to [1], wherein the fluorine content in the fluorine-containing silica glass powder is 0.1 wt% or more. [3] The fluorine-containing silica glass powder according to [1] or [2], wherein the content of metal impurities in the fluorine-containing silica glass powder is 1000 ppm or less. [4] Fluorine-containing silica glass powder according to any one of [1] to [3], wherein particles with a particle size of more than 150 μm and 300 μm or less constitute 50 wt% or more of the total. [5] Fluorine-containing silica glass powder according to any one of [1] to [4], wherein particles with a particle size exceeding 425 μm constitute 1 wt% or less of the total. [6] Fluorine-containing silica glass powder according to any one of [1] to [5], wherein particles with a particle size of 75 μm or less constitute 1 wt% or less of the total. A method for producing an optical fiber, comprising the step of forming a cladding or overcladding using fluorine-containing silica glass powder as described in any of [7][1] to [6]. A method for manufacturing a jacketed tube, comprising the step of forming a jacketed tube using fluorine-containing silica glass powder as described in any of [8][1] to [6]. A method for producing a substrate tube, comprising the step of forming a substrate tube using fluorine-containing silica glass powder as described in any of [9][1] to [6]. A method for manufacturing an overclad tube, comprising the step of forming an overclad tube using fluorine-containing silica glass powder as described in any of

[10] [1] to [6].

[0010] Under such circumstances, the present inventors also considered how to dope a sufficient amount of fluorine to cause a difference in refractive index. As a result, they found that by using SiF4 as a fluorine source, it is possible to provide fluorine-containing silica glass powder that contains sufficient fluorine and suppresses a decrease in fluorine concentration due to fluorine desorption from the fluorine-containing silica glass powder, thereby reaching the invention.

[0011] The second aspect of the present invention includes the following.

[11] A method for producing fluorine-containing silica glass powder, comprising pre-firing silicon oxide at a temperature below 1000°C in the presence of SiF4 to obtain fluorine-containing silica, and then firing at a temperature of 1000°C or higher and lower than 1400°C to obtain silica glass powder.

[12] The method for producing fluorine-containing silica glass powder according to

[11] , wherein the pre-firing in the presence of SiF4 is carried out at 100°C or higher and 800°C or lower.

[13] The method for producing fluorine-containing silica glass powder according to

[11] or

[12] , wherein annealing is carried out at 200°C or higher and lower than 1000°C after the pre-firing.

[14] The method for producing fluorine-containing silica glass powder according to any one of

[11] to

[13] , wherein the pre-firing has a step of heating silicon oxide to 200°C or higher and 800°C or lower before introducing SiF4, and then a step of introducing a gas containing SiF4 at 100°C or higher and 800°C or lower.

[15] The method for producing fluorine-containing silica glass powder according to any one of

[11] to

[14] , wherein the particle size is 1 mm or less.

[16] Fluorine-containing silica glass powder having a fluorine content of 2% by weight or more.

[17] A method for producing an optical fiber, comprising a step of melting and drawing the fluorine-containing silica glass powder obtained by any one of

[11] to

[15] .

[18] A method for producing a jacket tube, comprising a step of forming a jacket tube using the fluorine-containing silica glass powder according to any one of

[11] to

[15] . A method for manufacturing a substrate tube, comprising a step of forming a substrate tube using the fluorine-containing silica glass powder according to any one of

[19] ,

[11] to

[15] . A method for manufacturing an overclad tube, comprising a step of forming an overclad tube using the fluorine-containing silica glass powder according to any one of

[20] ,

[11] to

[15] .

[0012] Further, under the above circumstances, the present inventors studied how to dope an amount of fluorine that would result in a sufficient difference in refractive index. As a result, it was found that by allowing a specific substance to be present during pre-firing, fluorine-containing silica glass powder having a sufficient fluorine concentration can be obtained, and that by using this fluorine-containing silica glass powder, the energy efficiency during the production of fluorine-containing silica glass as a bulk can be improved. Also, it was found that the production method can provide fluorine-containing synthetic silica glass powder having a sufficient fluorine concentration and capable of suppressing a decrease in the fluorine concentration of the fluorine-containing silica glass due to fluorine desorption from the fluorine-containing silica glass powder, leading to the invention.

[0013] The third aspect of the present invention includes the following.

[21] A method for manufacturing fluorine-containing silica glass powder, comprising a first' step of pre-firing silicon oxide at a temperature of 750 °C or lower in the presence of ammonium fluoride to obtain fluorine-containing silica, and a second' step of firing the obtained fluorine-containing silica at a temperature of 1000 °C or higher and lower than 1400 °C to obtain silica glass powder.

[22] The method for manufacturing fluorine-containing silica glass powder according to

[21] , wherein the particle size of the obtained fluorine-containing silica glass powder is 1 mm or less.

[23] The method for manufacturing fluorine-containing silica glass powder according to

[21] or

[22] , wherein in the first' step, silicon oxide and ammonium fluoride powder are mixed and pre-fired.

[24] The method for manufacturing fluorine-containing silica glass powder according to any one of

[21] to

[23] , wherein the silicon oxide is produced by the sol-gel method.

[25] A method for producing fluorine-containing silica glass powder according to any one of

[21] to

[24] , wherein in the first step, the pre-firing temperature is 250°C or higher and 750°C or lower. A method for producing optical fibers, comprising the step of melting fluorine-containing silica glass powder obtained by any one of the manufacturing methods described in

[26] ,

[21] to

[25] , and drawing it into a wire. A method for producing a substrate tube, comprising the step of forming a substrate tube using fluorine-containing silica glass powder obtained by any of the manufacturing methods described in

[27] ,

[21] to

[25] . A method for manufacturing an overclad tube, comprising the step of forming an overclad tube using fluorine-containing silica glass powder manufactured by any of the manufacturing methods described in

[28] ,

[21] to

[25] .

[0014] Furthermore, the inventors investigated how to dope the silica glass powder with a sufficient amount of fluorine to produce a practically necessary difference in refractive index under the circumstances described above. As a result, they discovered that by using silicon oxide as a raw material, performing pre-calcination in the presence of fluorocarbons, followed by annealing, and then calcining at a specific temperature or higher, it is possible to provide a fluorine-containing silica glass powder that contains sufficient fluorine and suppresses the decrease in fluorine concentration due to fluorine desorption from the fluorine-containing silica glass powder, thus leading to the invention.

[0015] A fourth embodiment of the present invention includes the following:

[29] A method for producing fluorine-containing silica glass powder, comprising: a first'' step of pre-calcining silicon oxide at a temperature of 850°C or lower in the presence of fluorocarbons to obtain fluorine-containing silica; and a second'' step of annealing the obtained fluorine-containing silica at a temperature of 1000°C or higher, and then calcining it to obtain silica glass powder, wherein the temperature before pre-calcination in the presence of fluorocarbons does not exceed 850°C.

[30] The method for producing fluorine-containing silica glass powder according to

[29] , wherein the temperature during pre-calcination in the presence of fluorocarbons in the first'' step is 700°C or higher.

[31] The method for producing fluorine-containing silica glass powder according to

[29] or

[30] , wherein the particle size of the obtained fluorine-containing silica glass powder is 1 mm or less.

[32] A method for producing fluorine-containing silica glass powder according to any one of

[29] to

[31] , wherein fluorocarbons are present from the middle of the pre-firing process in the first'' step.

[33] A method for producing fluorine-containing silica glass powder according to any one of

[29] to

[32] , wherein the fluorocarbons are perfluorofluorocarbons. A method for manufacturing a jacketed tube, comprising the step of forming a jacketed tube using fluorine-containing silica glass powder manufactured by any one of the manufacturing methods described in

[34] ,

[29] , to

[33] . A method for manufacturing a substrate tube, comprising the step of forming a substrate tube using fluorine-containing silica glass powder manufactured by any of the manufacturing methods described in

[35] ,

[29] , to

[33] . A method for manufacturing an overclad tube, comprising the step of forming an overclad tube using fluorine-containing silica glass powder manufactured by any of the manufacturing methods described in

[36] ,

[29] , to

[33] . [Effects of the Invention]

[0016] The present invention provides a fluorine-containing silica glass powder that has a sufficient fluorine content and can suppress the decrease in fluorine concentration due to fluorine desorption. Furthermore, the present invention provides a manufacturing method that can improve energy efficiency during production. Moreover, this manufacturing method provides fluorine-containing silica glass powder that has a sufficient fluorine content, can be easily mass-produced industrially, and suppresses the decrease in fluorine concentration due to fluorine desorption. [Modes for carrying out the invention]

[0017] The present invention will be described in detail below, but the description of the constituent elements described below is just one example (representative example) of an embodiment of the present invention, and the present invention is not limited to these contents, and can be implemented in various ways within the scope of its gist.

[0018] <First form> One embodiment of the present invention (first form) is a synthetic silica glass powder containing fluorine, wherein particles with a particle size of more than 150 μm and less than or equal to 300 μm constitute 25 wt% or more of the total. "Over 150 μm" refers to particles that remain on a sieve with a nominal mesh size of 150 μm, conforming to the JIS metal mesh sieve (JIS Z8801-1:2006), after being shaken for 10 minutes. "300 μm or less" refers to particles that pass through a sieve with a nominal mesh size of 300 μm, conforming to the JIS metal mesh sieve, after being shaken for 10 minutes.

[0019] Although methods for doping synthetic silica glass with fluorine have been proposed in Patent Documents 1 and 2, these methods do not involve the production of powder, and the considerations for producing powder have been insufficient. Furthermore, sufficient consideration has not been given to suppressing the decrease in fluorine concentration due to fluorine desorption when producing powder. Under these circumstances, the present inventors have discovered that if the particle size of the silica powder is within an appropriate range, it is possible to provide a fluorine-containing synthetic silica glass powder that has a sufficient fluorine content and can suppress the decrease in fluorine concentration due to fluorine desorption.

[0020] Particles with a particle size exceeding 150 μm and less than or equal to 300 μm preferably make up 30 wt% or more of the total fluorine content in the fluorine-containing silica glass powder, more preferably 35 wt% or more, even more preferably 40 wt% or more, and most preferably 50% or more. Furthermore, there is no particular upper limit, and it may be 100 wt%, but it may also be 95 wt% or less.

[0021] Furthermore, it is preferable that particles with a particle size greater than 150 μm and less than or equal to 212 μm constitute 25 wt% or more of the total, and more preferably 30 wt% or more. The upper limit is not particularly limited and may be 100 wt%, but may also be 95 wt% or less.

[0022] Furthermore, particles with relatively small particle sizes, specifically those with a particle diameter of 75 μm or less, have a large surface area relative to their volume. This surface area is advantageous for reaction with the fluorine source, and at the time of fluorination, a fluorine-containing silica powder with a seemingly high fluorine content is obtained. However, when heated to high temperatures during the firing stage or for forming the glass, fluorine easily detaches from the surface, resulting in a reduced fluorine content after the glass product is made. It is preferable that particles with a particle diameter of 75 μm or less constitute 1 wt% or less of the total.

[0023] Furthermore, larger particles, specifically those exceeding 425 μm in diameter, require longer heating times during the firing process and when forming the glass, making them more susceptible to fluorine detachment. It is preferable that particles exceeding 425 μm in diameter constitute 1 wt% or less of the total.

[0024] The proportion of particles with a particle size greater than 150 μm and less than or equal to 300 μm, and particles with a particle size greater than 150 μm and less than or equal to 212 μm, can be calculated by sieving the silica powder. A particle size of over 150 μm and up to 212 μm refers to particles that pass through a stainless steel mesh with a nominal mesh opening of 212 μm but not through a stainless steel mesh with a nominal mesh opening of 150 μm. The mesh openings of the sieves used for these measurements are specified in JIS Z 8801-1:2006, and sieves with nominal mesh openings of 300 μm, 212 μm, and 150 μm should be used. The combing process should be performed for 10 minutes.

[0025] The fluorine-containing silica glass powder in this embodiment can contain fluorine at a high concentration, with the fluorine concentration usually being 0.05 wt% or more, preferably 0.1 wt% or more, more preferably 0.3 wt% or more, and even more preferably 0.5 wt% or more. There is no upper limit, but it is usually 40 wt% or less, and may be 10 wt% or less.

[0026] Fluorine-containing silica glass powder may contain metal impurities, preferably 1000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less. A low concentration of metal impurities suppresses light loss when fluorine-containing silica glass powder is used as cladding or overcladding for optical fibers. Typical metal impurities include transition metals, more specifically copper, iron, and chromium. Alkaline earth metals and alkali metals are preferable to reduce because they alter physical properties such as melt viscosity and catalyze the crystallization of silica glass, leading to devitrification. Methods known as conventional methods for producing undoped synthetic silica glass powder can be applied to reduce the concentration of metal impurities.

[0027] The fluorine concentration in fluorine-containing silica glass powder can be quantified by ion chromatography of an aqueous solution obtained by dissolving the fluorine-containing silica glass powder in an alkali and then dissolving it in water. Furthermore, the metal impurity concentration can be quantified by ICP-MS measurement after heating and dissolving the fluorine-containing silica glass powder in a mixed acid of hydrofluoric acid and sulfuric acid, then heating it further until sulfuric acid droplets remain and evaporating the solution. This solution is then dissolved in pure water.

[0028] The specific surface area of ​​fluorine-containing silica glass powder is not particularly limited, but is typically 0.001 m². 2 It is 1 / g or more, preferably 0.01m 2 It is 1 m or more. 2 It is less than / g and 0.3m 2 It is preferable that the value be less than or equal to / g.

[0029] The method for producing fluorine-containing silica glass powder is not particularly limited, but the following explanation will use the sol-gel method as an example.

[0030] An example of the sol-gel method involves charging an equivalent to 10 times the equivalent amount of alkoxysilane and pure water into a reactor, carrying out a sol-gel reaction, and then pulverizing and drying the resulting dry gel, which is a silica precursor. The obtained dry gel is then calcined at a temperature of approximately 400°C to 1400°C to obtain silica powder. During calcination, a fluorine source can be supplied to obtain fluorine-containing silica glass powder. A specific fluorine source is, for example, C2F6. A mixture of C2F6 and O2 gas is circulated, and the mixture is fluorinated at 700°C to 1200°C. If necessary, annealing is performed, and then the mixture is calcined to obtain fluorine-containing silica glass powder.

[0031] Furthermore, by using SiF4 as a fluorine source, it is possible to easily obtain fluorine-containing silica powder with a very high fluorine content, specifically a fluorine concentration of about 10 wt%. Furthermore, when NH4F is used as a fluorine source, fluorine-containing silica powder with a sufficiently high fluorine content can be obtained at relatively low temperatures.

[0032] The fluorine-containing silica glass powder obtained in this embodiment has a low refractive index and is suitable for use as a cladding or overcladding material for optical fibers. It can also be used as an optical component other than cladding or overcladding for optical fibers.

[0033] A method for manufacturing optical fibers, which includes the step of forming an overcladding using the fluorine-containing silica glass powder according to this embodiment, is described below. In one example, a primary preform (core rod) made of glass is placed in an overclad tube, and the space between the primary preform and the overclad tube is filled with fluorine-containing synthetic silica glass powder. Then, the overclad tube filled with silica glass powder is placed in a furnace to soften the glass (tube and silica glass powder), and optical fibers can be manufactured by drawing them. Alternatively, instead of directly drawing the overclad tube filled with silica glass powder, a secondary preform that has been molten and vitrified may be used as a secondary preform. The overclad tube can be made of silica glass, and the fluorine-containing silica glass powder of the present invention can be used as a raw material, but commercially available products may also be used. The temperature of the furnace for softening the glass can be set appropriately by those skilled in the art, and in one example, it may be 2000°C to 2500°C. It should be noted that the method of manufacturing optical fibers is not limited to this, and they may be manufactured by known methods.

[0034] Furthermore, the fluorine-containing silica glass powder according to this embodiment can also be used as a raw material for jacket tubes (sometimes called tubes or cylinders) when manufacturing the cladding portion by the jacketing method (Sleeving method), or for substrate tubes used to deposit soot by the MCVD method, etc.

[0035] <Second form> One embodiment of the present invention (second embodiment) is a method for producing fluorine-containing silica glass powder, comprising: a first step of pre-calcining silicon oxide at a temperature of less than 1000°C in the presence of SiF4 to obtain fluorine-containing silica; and a second step of subsequently calcining at a temperature of 1000°C or more and less than 1400°C to obtain silica glass powder. Preferably, the method includes an annealing step of annealing the fluorine-containing silica obtained in the first step at a temperature of 300°C or more and less than 1000°C.

[0036] The silicon oxide used in the first step is not particularly limited, but it is preferable to use a silicon oxide obtained by the sol-gel method, so-called dry gel. As an example of the sol-gel method, an equivalent to 10 times the equivalent amount of alkoxysilane and pure water is charged into a reactor, a sol-gel reaction is carried out, and then the mixture is pulverized and dried to obtain a dry gel, which is a silica precursor. Known methods can be used for the process up to obtaining this dry gel, for example, the method described in Japanese Patent Publication No. 2013-15380 can be used.

[0037] In the first step, silicon oxide is pre-calcined in the presence of SiF4 to obtain fluorine-containing silica. In a preferred embodiment, in the first step, the silicon oxide is heated to 200°C to 800°C before introducing SiF4 to remove a certain amount of water and hydroxyl groups, and then SiF4 is introduced to introduce fluorine into the silicon oxide. This prevents water from being generated from the silicon oxide in the presence of SiF4, which would cause the fluorine from the SiF4 introduced into the reaction system to escape out of the reaction system in the form of hydrogen fluoride (HF), thus preventing waste and allowing for effective use of the fluorine source.

[0038] The temperature for this pre-firing should be less than 1000°C. At this time, fluorine reacts with the surface of the silicon oxide, and fluorine is incorporated into the silicon oxide. The temperature at this time is preferably 100°C or higher, more preferably 200°C or higher, and most preferably 400°C or higher. On the other hand, even if the temperature is too high, the amount of fluorine introduced will not be large relative to the energy used, so 800°C or lower, more preferably 600°C or lower, is preferable.

[0039] The pre-firing time varies depending on the pre-firing temperature and the supply amount of silicon oxide and SiF4, but it is generally preferable to perform it for 30 minutes or more, preferably 1 hour or more, after the set temperature is reached, and also for 30 minutes or more, preferably 1 hour or more, after the introduction of SiF4. The upper limit is not particularly limited as it depends on the size of the reactor, etc.

[0040] Preferably, the fluorine-containing silica obtained in this way is annealed. Annealing allows the fluorine that has reacted on the surface of the silicon oxide to penetrate to the core of the silicon oxide. From a technical standpoint, annealing can be performed immediately after pre-firing, and its temperature is not particularly limited as long as it is below the firing temperature, and may be less than 1000°C, but preferably 200°C or higher, more preferably 300°C or higher, and more preferably 400°C or higher. On the other hand, the upper limit is 800°C or lower, more preferably 700°C or lower, and most preferably 600°C or lower, as raising the temperature more than necessary would be a waste of energy. The duration of annealing depends on the annealing temperature, but is not particularly limited and may be 20 minutes or more, 1 hour or more, or 24 hours or less.

[0041] After annealing, the second step is firing at a temperature of 1000°C to less than 1400°C. The upper limit of the firing temperature is below the melting point of silica (which varies depending on the desired fluorine concentration), and more preferably at least 50°C lower than the softening point of silica at the desired fluorine concentration. The firing time is not particularly limited, but is usually 1 hour or more.

[0042] The fluorine-containing silica glass powder obtained in this embodiment has a low refractive index and is suitable for use as a cladding or overcladding material for optical fibers. It can also be used as an optical component other than cladding or overcladding for optical fibers. Fluorine-containing silica glass powder can be made to contain any desired concentration of fluorine by adjusting the amount of SiF4 used. The fluorine concentration is usually 0.05% by weight or more, preferably 0.1% by weight or more, more preferably 0.15% by weight or more, and even more preferably 0.2% by weight or more. There is no upper limit, but it is usually 40% by weight or less, may be 10% by weight or less, or even 5% by weight or less. The method of using SiF4 as a fluorine raw material can easily increase the fluorine concentration and is particularly suitable for producing fluorine-containing silica glass powder with a fluorine concentration of 2% by weight or more.

[0043] Fluorine-containing silica glass powder may contain metal impurities, preferably 1000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less. A low concentration of metal impurities suppresses light loss when fluorine-containing silica glass powder is used as cladding or overcladding for optical fibers. Typical metal impurities include transition metals, more specifically copper, iron, and chromium. Alkaline earth metals and alkali metals are preferable to reduce because they alter physical properties such as melt viscosity and catalyze the crystallization of silica glass, leading to devitrification. Methods known as conventional methods for producing undoped synthetic silica glass powder can be applied to reduce the concentration of metal impurities.

[0044] The fluorine concentration in fluorine-containing silica glass powder can be quantified by ion chromatography of an aqueous solution obtained by dissolving the fluorine-containing silica glass powder in an alkali and then dissolving it in water. Furthermore, the metal impurity concentration can be quantified by ICP-MS measurement after heating and dissolving the fluorine-containing silica glass powder in a mixed acid of hydrofluoric acid and sulfuric acid, then heating it further until sulfuric acid droplets remain and evaporating the solution. This solution is then dissolved in pure water.

[0045] The specific surface area of ​​fluorine-containing silica glass powder is not particularly limited, but is typically 0.001 m². 2 It is 1 / g or more, preferably 0.01m 2 It is 1 m or more. 2 It is less than / g and 0.3m 2 It is preferable that the value be less than or equal to / g.

[0046] The particle size of the fluorine-containing silica glass powder is preferably 1 mm or less, more preferably 500 μm or less, preferably 1 μm or more, and more preferably 10 μm or more. A particle size of 1 mm or less for the entire fluorine-containing silica glass powder means that 50% by weight or more, preferably 75% by weight or more, passes through a sieve with a mesh size of 1 mm. If the lower limit is 20 μm or more, then 50% by weight or more, preferably 75% by weight or more, of the particles do not pass through a sieve with a mesh size of 20 μm as specified in JIS 8801-1. If the lower limit is set to less than 20 μm, then a laser particle size analyzer should be used to determine that particles with a particle size equal to or greater than the lower limit constitute 50% by weight or more, preferably 80% by weight or more of the total. Furthermore, when fluorine-containing silica glass powder is classified by sieving, it is preferable that particles with a particle size greater than 150 μm and 300 μm or less constitute 50% by weight or more of the total particles. Note that particles with a particle size greater than 150 μm and 300 μm or less refer to particles that pass through a stainless steel mesh with a mesh opening of 300 μm but do not pass through a stainless steel mesh with a mesh opening of 150 μm, and are sometimes referred to as particles with a particle size of 150-300 μm. The mesh openings of the sieves used for these measurements are specified in JIS Z 8801-1:2006, and sieves with nominal mesh openings of 300 μm and 150 μm should be used.

[0047] A method for manufacturing optical fibers, which includes the step of forming an overcladding using the fluorine-containing silica glass powder according to this embodiment, is described below. In one example, a primary preform (core rod) made of glass is placed in an overclad tube, and the space between the primary preform and the overclad tube is filled with fluorine-containing synthetic silica glass powder. Then, the overclad tube filled with silica glass powder is placed in a furnace to soften the glass (tube and silica glass powder), and optical fibers can be manufactured by drawing them. Alternatively, instead of directly drawing the overclad tube filled with silica glass powder, a secondary preform that has been molten and vitrified may be used. The overclad tube can be made of glass, and commercially available tubes may be used. The temperature of the furnace for softening the glass can be set appropriately by a person skilled in the art, and in one example, it may be between 2000°C and 2500°C.

[0048] The method for manufacturing optical fibers is not limited to this, and they may also be manufactured by known methods. Furthermore, the fluorine-containing silica glass powder of this embodiment can also be used as a raw material for jacket tubes (sometimes called tubes or cylinders) when manufacturing the cladding portion by the jacketing method (Sleeving method), or for substrate tubes used to deposit soot by the MCVD method, etc.

[0049] <Third form> One embodiment of the present invention (third embodiment) is a method for producing fluorine-containing silica glass powder, comprising: a first' step of pre-calcining silicon oxide at a temperature of 750°C or lower in the presence of ammonium fluoride to obtain fluorine-containing silica; and a second' step of calcining the obtained fluorine-containing silica at a temperature of 1000°C or higher and less than 1400°C to obtain silica glass powder.

[0050] The silicon oxide used in step 1' is not particularly limited, but it is preferable to use silicon oxide obtained by the sol-gel method, so-called dry gel. As an example of the sol-gel method, alkoxysilane and pure water are charged into a reactor in amounts from equimolar to 10 times equimolar, and after performing the sol-gel reaction, pulverization and drying are carried out to obtain a dry gel which is a silica precursor. A known method can be used for the steps until obtaining this dry gel, for example, the method described in JP-A-2013-15380 can be used. Also, the specific surface area of the silicon oxide is not particularly limited, but is usually 1 m 2 / g or more, preferably 10 m 2 / g or more, more preferably 300 m 2 / g or more, and usually 10000 m 2 / g or less, and preferably 1000 m 2 / g or less. The preferable value of the specific surface area at the time of charging the fluorine source is the same.

[0051] In the first' step, the silicon oxide is pre-fired in the presence of ammonium fluoride to obtain fluorine-containing silica. As the ammonium fluoride, not only NH4F but also acidic salts such as NF4HF2 and Me4NF in which the nitrogen of ammonium is substituted with an organic functional group can be used.

[0052] Regarding the temperature of this pre-firing, it is set to 750 °C or lower. At this time, fluorine reacts with the surface of the silicon oxide, and fluorine is incorporated into the silicon oxide. The temperature at this time is preferably 700 °C or lower, more preferably 650 °C or lower, and even more preferably 600 °C or lower. The lower limit is not limited, but in order to avoid that a part of it becomes HF without being incorporated into the silica after the decomposition of ammonium fluoride, it is preferably 250 °C or higher, and more preferably 400 °C or higher. Ammonium fluoride, such as NH4F, can be added in solid powder form by mixing it with silicon oxide, which is the raw material, thus simplifying the process. The ammonium fluoride used should preferably have few impurities, preferably a purity of 99.5% or higher, and more preferably 99.9% or higher. Furthermore, it is preferable for the ammonium fluoride to be in granular or powder form rather than in lump form, as this is advantageous when mixing with the pulverized silicon oxide; for example, a particle size of 1 mm or less is preferred.

[0053] Furthermore, since water is easily generated from silicon oxide in the initial stages of pre-calcination, at temperatures below 150°C, and especially below 100°C, it is preferable that, in the case of a batch system, the lid be made openable by internal pressure, a carrier gas be circulated to discharge moisture from the reactor, and / or that pre-calcination be performed at a temperature higher than the pre-calcination temperature before mixing with ammonium fluoride.

[0054] Although ammonium fluoride is supplied in powder form, it melts or decomposes at low temperatures after moisture is removed and reacts as a fluorinating agent. Furthermore, mixing silicon oxide with ammonium fluoride powder results in frequent contact between the silicon oxide and ammonium fluoride. Compared to other methods that supply fluorine raw materials as gas, fluorine can easily penetrate and react to the center of the dry gel. The low pre-calcination temperature contributes significantly to saving production energy, which is a major advantage of using ammonium fluoride as a raw material.

[0055] The pre-firing time varies depending on reaction conditions such as the pre-firing temperature and the amount of dry gel supplied, but it is generally preferable to perform the pre-firing for 30 minutes or more, preferably 1 hour or more, after the temperature reaches 250°C or higher. The upper limit is not particularly limited as it depends on the size of the reactor, etc. Furthermore, annealing may be performed at a temperature lower than the firing temperature after pre-firing. However, from an economic standpoint, it is preferable not to perform annealing.

[0056] In step 2', the obtained fluorine-containing silica is fired at a temperature of 1000°C or higher but less than 1400°C to obtain fluorine-containing silica glass powder. The firing temperature is more preferably 1100°C or higher. The upper limit of the firing temperature is below the temperature at which silica melts (this varies depending on the desired fluorine concentration), and more preferably 50°C or more below the softening point of silica with the target fluorine concentration. The firing time is not particularly limited, but is usually 1 hour or more. If the firing temperature exceeds 1400°C, the fluorine-containing silica glass powder will no longer be able to maintain its particle shape.

[0057] The fluorine-containing silica glass powder obtained in this embodiment has a low refractive index and is suitable for use as a cladding or overcladding material for optical fibers. It can also be used as an optical component other than cladding or overcladding for optical fibers. Fluorine-containing silica glass powder can contain any concentration of fluorine by adjusting the amount of ammonium fluoride used. The fluorine concentration is usually 0.05% by weight or more, preferably 0.1% by weight or more, more preferably 0.15% by weight or more, even more preferably 0.2% by weight or more, especially preferably 0.8% by weight or more, and particularly preferably 1.0% by weight or more. There is no upper limit, but it is usually 40% by weight or less, and may be 5% by weight or less.

[0058] Fluorine-containing silica glass powder may contain metal impurities, preferably 1000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less. A low concentration of metal impurities suppresses light loss when fluorine-containing silica glass powder is used as cladding or overcladding for optical fibers. Typical metal impurities include transition metals, more specifically copper, iron, and chromium. Alkaline earth metals and alkali metals are preferable to reduce because they alter physical properties such as melt viscosity and catalyze the crystallization of silica glass, leading to devitrification. Methods known as conventional methods for producing undoped synthetic silica glass powder can be applied to reduce the concentration of metal impurities.

[0059] (Method for measuring fluorine content) The fluorine concentration in fluorine-containing silica glass powder can be quantified by ion chromatography of an aqueous solution obtained by dissolving the fluorine-containing silica glass powder in an alkali and then dissolving it in water. Furthermore, the metal impurity concentration can be quantified by ICP-MS measurement after heating and dissolving the fluorine-containing silica glass powder in a mixed acid of hydrofluoric acid and sulfuric acid, then heating it further until sulfuric acid droplets remain and evaporating the solution. This solution is then dissolved in pure water.

[0060] The specific surface area of ​​fluorine-containing silica glass powder is not particularly limited, but is typically 0.001 m². 2 It is 1 / g or more, preferably 0.01m 2 It is 1 m or more. 2 It is less than / g and 0.3m 2 It is preferable that the value be less than or equal to / g.

[0061] The particle size of the fluorine-containing silica glass powder is preferably 1 mm or less, more preferably 500 μm or less, preferably 1 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. A particle size of 1 mm or less for the entire fluorine-containing silica glass powder means that 50% by weight or more, preferably 75% by weight or more, passes through a sieve with a nominal mesh size of 1 mm. The lower limit is set at 20 μm or more, meaning that 50% by weight or more, preferably 75% by weight or more, of the particles do not pass through a sieve with a nominal mesh size of 20 μm as specified in JIS 8801-1. If the lower limit is set to less than 20 μm, then a laser particle size analyzer should be used to determine that 50% by weight or more, preferably 80% by weight or more, of the particles exceeding the lower limit. Furthermore, when fluorine-containing silica glass powder is classified by sieving, it is preferable that particles with a particle size greater than 150 μm and 300 μm or less constitute 50% by weight or more of the total particles. Note that particles with a particle size greater than 150 μm and 300 μm or less refer to particles that pass through a stainless steel mesh with a mesh opening of 300 μm but do not pass through a stainless steel mesh with a mesh opening of 150 μm, and are sometimes referred to as particles with a particle size of 150-300 μm. The mesh openings of the sieves used for these measurements are specified in JIS Z 8801-1:2006, and sieves with nominal mesh openings of 300 μm and 150 μm should be used.

[0062] A method for manufacturing optical fibers, which includes the step of forming an overcladding using the fluorine-containing silica glass powder according to this embodiment, is described below. In one example, a primary preform (core rod) made of glass is placed in an overclad tube, and the space between the primary preform and the overclad tube is filled with fluorine-containing synthetic silica glass powder. Then, the overclad tube filled with silica glass powder is placed in a furnace to soften the glass (tube and silica glass powder), and optical fibers can be manufactured by drawing them. Alternatively, instead of directly drawing the overclad tube filled with silica glass powder, a secondary preform that has been molten and vitrified may be used as a raw material. The overclad tube can be a tube made of glass, and can be manufactured using the fluorine-containing silica glass powder of this embodiment as a raw material, but commercially available products may also be used. The temperature of the furnace for softening the glass can be set appropriately by a person skilled in the art, and in one example, it may be 2000°C to 2500°C.

[0063] The method for manufacturing optical fibers is not limited to this, and they may also be manufactured by known methods. Furthermore, the fluorine-containing silica glass powder of this embodiment can also be used as a raw material for jacket tubes (sometimes called tubes or cylinders) when manufacturing the cladding portion by the jacketing method (Sleeving method), or for substrate tubes used to deposit soot by the MCVD method, etc. The fluorine-containing silica glass powder of this embodiment, obtained in this way and having a high fluorine concentration, can be used for forming the cladding layer of optical fibers. In particular, it can be used to easily manufacture optical fibers containing multiple cores or optical fibers with a high allowable curvature. Alternatively, a viscosity modifier such as an alkali metal oxide like potassium oxide may be added to the fluorine-containing silica glass powder before use.

[0064] <Fourth form> One embodiment of the present invention (fourth embodiment) is a method for producing fluorine-containing silica glass powder, comprising: a first'' step of pre-calcining silicon oxide at a temperature of 850°C or lower in the presence of fluorocarbons to obtain fluorine-containing silica; and a second'' step of annealing the obtained fluorine-containing silica at a temperature below the calcination temperature, and then calcining it at a temperature of 1000°C or higher to obtain silica glass powder, wherein the temperature before pre-calcination in the presence of fluorocarbons does not exceed 850°C.

[0065] The silicon oxide used in the first step is not particularly limited, but it is preferable to use a silicon oxide obtained by the sol-gel method, a so-called dry gel. As an example of the sol-gel method, an equivalent to 10 times the equivalent amount of alkoxysilane and pure water is charged into a reactor, a sol-gel reaction is carried out, and then the mixture is pulverized and dried to obtain a dry gel, which is a silica precursor. Known methods can be used for the process up to obtaining this dry gel, for example, the method described in Japanese Patent Publication No. 2013-15380 can be used.

[0066] In the first step, silicon oxide is pre-calcined in the presence of chlorofluorocarbons to obtain fluorine-containing silica. The temperature for this pre-firing should be 850°C or lower. At this time, fluorine reacts with the surface of the silicon oxide, and the fluorine is incorporated into the silicon oxide. The temperature at this time is preferably 700°C or higher, and more preferably 750°C or higher. In the first step, supplying oxygen in the presence of the fluorocarbons is more preferable because it decomposes the fluorocarbons and prevents carbon derived from the fluorocarbons from remaining in the silica glass. The fluorocarbons used are not particularly limited as long as they decompose at 850°C or below and can supply fluorine, and solids such as polytetrafluoroethylene can also be used, but gases such as perfluorofluorocarbons, chlorofluorofluorocarbons, and hydrofluorofluorocarbons are preferred, and perfluorofluorocarbons that do not generate water during decomposition are particularly preferred, with C2F6 being the most preferred.

[0067] Furthermore, in the initial stages of pre-calcination, water may be generated from silicon oxides. To prevent the fluorine derived from the fluorocarbons introduced into the reaction system from escaping outside the reaction system in the form of hydrogen fluoride (HF) and being wasted, it is preferable to introduce the fluorocarbons from partway through the pre-calcination process. Specifically, it is preferable to start introducing the fluorocarbons into the reactor after the temperature reaches 100°C or higher during the heating process of pre-calcination, and more preferably 130°C or higher. As an upper limit, it is preferable to start introducing the fluorocarbons into the reactor at a temperature of 850°C or lower.

[0068] The pre-firing time varies depending on the pre-firing temperature and the amount of fluorocarbons supplied, but it is generally preferable to perform the pre-firing for 30 minutes or more, preferably 1 hour or more, after the temperature reaches 700°C or higher. The upper limit is not particularly limited as it depends on the size of the reactor, etc.

[0069] In the second step, the fluorine-containing silica obtained in this way is annealed. Annealing allows the fluorine that has reacted on the surface of the silicon oxide to penetrate to the core of the silicon oxide. From a technical standpoint, annealing can be performed immediately after pre-firing, and the temperature is not particularly limited as long as it is below the firing temperature, but is preferably 850°C or lower. The duration of annealing is also not particularly limited and can be 10 minutes or more, preferably 15 minutes or more, more preferably 1 hour or more, or 24 hours or less. Furthermore, there are no technical problems with continuously supplying fluorocarbons during annealing. However, from an economic standpoint, it is preferable to stop supplying fluorocarbons and use dry air or the like. Also, this annealing does not necessarily mean performing a heat treatment, and depending on the heat capacity of the equipment, it can be substituted by stopping heating after pre-firing and lowering the reactor temperature, or by raising the temperature to the firing temperature after pre-firing. When heating is performed immediately after pre-firing, it is preferable to raise the temperature to the firing temperature over 10 minutes or more, preferably 15 minutes or more, and more preferably 20 minutes or more.

[0070] After annealing, if energy efficiency is a priority, the glass is left as is. If the fluorine concentration in the silica glass powder is a priority, the glass is cooled down and then fired at a temperature above the pre-firing temperature, preferably above 1000°C. The upper limit of the firing temperature is below the melting temperature of silica (which varies depending on the desired fluorine concentration), and more preferably at a temperature at least 50°C lower than the softening point of silica with the desired fluorine concentration. The firing time is not particularly limited, but is usually at least one hour. Furthermore, the temperature should not exceed 850°C before pre-firing in the presence of CFCs. Exceeding 850°C will reduce the surface area and decrease the amount of fluorine doping required.

[0071] The fluorine-containing silica glass powder obtained in this embodiment has a low refractive index and is suitable for use as a cladding or overcladding material for optical fibers. It can also be used as an optical component other than cladding or overcladding for optical fibers. Fluorine-containing silica glass powder can contain any concentration of fluorine by adjusting the amount of fluorocarbons used. The fluorine concentration is usually 0.05% by weight or more, preferably 0.1% by weight or more, more preferably 0.15% by weight or more, even more preferably 0.2% by weight or more, especially preferably 0.8% by weight or more, and particularly preferably 1.0% by weight or more. There is no upper limit, but it is usually 40% by weight or less, and may be 10% by weight or less.

[0072] Fluorine-containing silica glass powder may contain metal impurities, preferably 1000 ppm or less, more preferably 500 ppm or less, and even more preferably 100 ppm or less. A low concentration of metal impurities suppresses light loss when fluorine-containing silica glass powder is used as cladding or overcladding for optical fibers. Typical metal impurities include transition metals, more specifically copper, iron, and chromium. Alkaline earth metals and alkali metals are preferable to reduce because they alter physical properties such as melt viscosity and catalyze the crystallization of silica glass, leading to devitrification. Methods known as conventional methods for producing undoped synthetic silica glass powder can be applied to reduce the concentration of metal impurities.

[0073] The fluorine concentration in fluorine-containing silica glass powder can be quantified by ion chromatography of an aqueous solution obtained by dissolving the fluorine-containing silica glass powder in an alkali and then dissolving it in water. Furthermore, the metal impurity concentration can be quantified by ICP-MS measurement after heating and dissolving the fluorine-containing silica glass powder in a mixed acid of hydrofluoric acid and sulfuric acid, then heating it further until sulfuric acid droplets remain and evaporating the solution. This solution is then dissolved in pure water.

[0074] The specific surface area of ​​fluorine-containing silica glass powder is not particularly limited, but is typically 0.001 m². 2 It is 1 / g or more, preferably 0.01m 2 It is 1 m or more. 2 It is less than / g and 0.3m 2 It is preferable that the value be less than or equal to / g.

[0075] The particle size of the fluorine-containing silica glass powder is preferably 1 mm or less, more preferably 500 μm or less, preferably 1 μm or more, preferably 10 μm or more, and even more preferably more than 20 μm. Having a particle size within the above range is preferable because it facilitates handling as a powder, easy vitrification, and easy introduction of fluorine. The particle size can be measured by general particle size distribution measurements such as sieving or laser diffraction.

[0076] For fluorine-containing silica glass powder to have a particle size of 1 mm or less as a whole, it means that 50% or more by weight, preferably 75% or more by weight, of the particles pass through a sieve with a nominal mesh opening of 1 mm. Furthermore, if the lower limit is set to exceed 20 μm, then 50% or more by weight, preferably 75% or more by weight, of the total particles shall not pass through a sieve with a nominal mesh size of 20 μm as specified in JIS Z8801-1. Furthermore, if the lower limit is set to 20 μm or less, then, using a laser particle size analyzer, 50% or more by weight, preferably 80% or more by weight, of the total particles shall be equal to or greater than the lower limit particle size.

[0077] More preferably, when fluorine-containing silica glass powder is classified by sieving, particles with a particle size greater than 150 μm and 300 μm or less constitute 3% by weight or more of the total particles, more preferably 50% by weight or more, with no upper limit, and may be 100% by weight, but is usually 95% by weight or less. By keeping the particle size within the above range, the amount of fluorine introduced can be increased, which is preferable.

[0078] A method for manufacturing optical fibers, which includes the step of forming an overcladding using the fluorine-containing silica glass powder according to this embodiment, is described below. In one example, a primary preform (core rod) made of glass is placed in an overclad tube, and the space between the primary preform and the overclad tube is filled with fluorine-containing synthetic silica glass powder. Then, the overclad tube filled with silica glass powder is placed in a furnace to soften the glass (tube and silica glass powder), and optical fibers can be manufactured by drawing them. Alternatively, instead of directly drawing the overclad tube filled with silica glass powder, a secondary preform that has been molten into glass may be used as a starting point. The overclad tube can be made of silica glass and can be manufactured using the fluorine-containing synthetic silica glass powder of the present invention as a raw material, but commercially available products may also be used. The temperature of the furnace for softening the glass can be set appropriately by those skilled in the art, and in one example, it may be 2000°C to 2500°C.

[0079] The method for manufacturing optical fibers is not limited to this, and they may also be manufactured by known methods. Furthermore, the fluorine-containing silica glass powder of this embodiment can also be used as a raw material for jacket tubes (sometimes called tubes or cylinders) when manufacturing the cladding portion by the jacketing method (Sleeving method), or for substrate tubes used to deposit soot by the MCVD method, etc. [Examples]

[0080] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to the following examples. <Example A: First Form> [Example A1] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was calcined at 800°C for 10 hours in a dry air atmosphere to prepare 4.3 kg of raw silica gel powder. 40 g of this raw silica gel powder was subjected to a fluorination treatment at 800°C for 2 hours in an atmosphere of 5 mL / min perfluoroethane and 5 mL / min oxygen mixed gas flow, and then allowed to cool naturally to prepare 40 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 1.0 wt%-F. Subsequently, 30 g of this fluorine-doped silica gel powder was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 30 g of fluorine-doped synthetic quartz glass powder. The fluorine concentration of this fluorine-doped synthetic quartz glass powder was 0.7 wt%-F.

[0081] [Examples and Comparative Examples] The 2.1 kg of silica gel used in Example A1 was sieved in 20 portions of approximately 110 g each. An example of the results is shown in Table 1. Each sample of the size exceeding 75 μm was subjected to a fluorination treatment at 800°C for 2 hours under a flowing atmosphere of a mixed gas of perfluoroethane at 5 mL / min and oxygen at 5 mL / min, to obtain fluorine-doped silica gel powder. Subsequently, each of the fluorine-doped silica gel powders was heat-treated at 1,000°C for 1 hour under a dry air atmosphere to obtain fluorine-doped synthetic quartz glass powder. The weight and fluorine concentration of each sample are shown in Table 2.

[0082] [Table 1]

[0083] [Table 2]

[0084] As is clear from the above results, it can be understood that particles between 150 μm and 300 μm are silica glass powders that maintain a high fluorine concentration. This makes it possible to easily manufacture optical fibers.

[0085] [Example A2] 100 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was calcined at 800°C for 11 hours and 40 minutes in a dry air atmosphere to prepare raw silica gel powder. 7 kg of this raw silica gel powder was subjected to a fluorination treatment at 800°C for 4 hours in a circulating atmosphere of a 1:1 mixed gas of perfluoroethane and oxygen, and then allowed to cool naturally to prepare fluorine-doped silica gel powder. Subsequently, the fluorine-doped silica gel powder was heat-treated at 1000°C for 36 hours in an oxygen atmosphere of 50 mL / min to obtain fluorine-doped synthetic quartz glass powder. The fluorine concentration of this fluorine-doped synthetic quartz glass powder was 1.6 wt%-F. 100g of the fluorine-doped synthetic quartz glass powder was sieved. The results are shown in Table 3.

[0086] [Table 3]

[0087] <Example B: Second form> [Example B1] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was pre-calcined at 500°C for 10 hours in a dry air atmosphere to prepare 4.2 kg of raw silica gel powder. 1 kg of this raw silica gel powder was reacted with 200 g of silicon tetrafluoride (SiF4) at 250°C for 9 hours (pre-calcination) to obtain 1.1 kg of fluorine-doped silica gel. The fluorine concentration of this fluorine-doped silica gel powder was 8.9 wt%-F. Subsequently, 40 g of this fluorine-doped silica gel powder was annealed at 400°C for 2 hours in a dry air atmosphere to obtain 39 g of annealed gel. The fluorine concentration of the annealed gel was 8.5 wt%-F. 30 g of this annealed gel was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 28 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 4.1 wt%-F.

[0088] [Example B2] 40 g of fluorine-doped silica gel powder (fluorine concentration 8.9 wt%) used in Example B1 was annealed at 600°C for 2 hours in a dry air atmosphere to obtain 37 g of annealed gel. The fluorine concentration of the annealed gel was 4.7 wt%-F. 30 g of this annealed gel was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 29 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 3.9 wt%-F.

[0089] [Example B3] 30 g of fluorine-doped silica gel powder (fluorine concentration 8.9% by weight) used in Example B1 was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 27 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 3.7% by weight-F.

[0090] [Example B4] 30 g of fluorine-doped silica gel powder (fluorine concentration 8.9 wt%) used in Example B1 was annealed at 250°C for 2 hours in a dry air atmosphere to obtain 29 g of annealed gel. The fluorine concentration of the annealed gel was 8.1 wt%-F. 27 g of this annealed gel was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 25 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 3.1 wt%-F. It can be seen that if annealing is not performed at the appropriate temperature, a gel with a seemingly very high fluorine concentration is obtained, but fluorine is removed during firing.

[0091] [Example B5] 40 g of fluorine-doped silica gel powder (fluorine concentration 8.9 wt%) used in Example B1 was annealed at 800°C for 2 hours in a dry air atmosphere to obtain 36 g of annealed gel. The fluorine concentration of the annealed gel was 3.6 wt%-F. 30 g of this annealed gel was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 30 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 2.6 wt%-F.

[0092] As described above, by contacting silicon oxide with SiF4 at an appropriate temperature, and more preferably annealing and firing at an appropriate temperature thereafter, a large amount of fluorine that could not be obtained conventionally can be added to silica glass powder. This not only makes it easier to manufacture optical fibers, but also, due to the large refractive index difference, they are resistant to bending and have good optical confinement efficiency, so it is possible to manufacture optical fibers with more complex structures for the same diameter.

[0093] <Example C: Third Form> [Example C1] Five kilograms of dried silica gel powder were obtained by hydrolysis condensation of tetramethoxysilane. This dried silica gel was subjected to a first pre-calcination at 600°C for 10 hours under a dry air atmosphere to prepare 4.1 kg of raw silica gel powder. The specific surface area of ​​this raw silica gel was 524.3 m². 2 The average pore size was 2.272 nm and the pore volume was 0.2978 cc / g. Measurements, including those in other examples, were performed by nitrogen adsorption. 5.1 g of the raw silica gel powder and 0.67 g of ammonium fluoride were mixed and placed in an alumina crucible, then calcined at 400°C for 2 hours (second pre-calcination) to obtain 5.0 g of fluorine-doped silica gel powder. The fluorine concentration of the fluorine-doped silica gel powder was 3.7 wt%-F. Subsequently, 2.8 g of the fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 2.6 g of fluorine-containing silica glass powder. The fluorine concentration of the fluorine-containing silica glass powder was 2.9 wt%-F.

[0094] [Example C2] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was subjected to a first pre-calcination at 500°C for 10 hours in a dry air atmosphere to prepare 4.2 kg of raw silica gel powder. The specific surface area of ​​this raw silica gel was 569.5 m². 2 The silica gel powder was found to have a density of 2.277 nm on the average pore size and a pore volume of 0.3241 cc / g. 5.0 g of the silica gel powder was mixed with 0.66 g of ammonium fluoride and placed in an alumina crucible, where it was fired at 400°C for 2 hours (second pre-firing) to obtain 4.9 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 4.5% by weight-F. Subsequently, 2.7 g of this fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 2.5 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 2.8% by weight-F.

[0095] [Example C3] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was subjected to a first pre-calcination at 400°C for 10 hours in a dry air atmosphere to prepare 4.5 kg of raw silica gel powder. The specific surface area of ​​this raw silica gel was 749.3 m². 2 The silica gel powder had a density of 2.263 nm on average and a pore volume of 0.424 cc / g. 4.9 g of the silica gel powder was mixed with 0.65 g of ammonium fluoride and placed in an alumina crucible, where it was fired at 400°C for 2 hours (second pre-firing) to obtain 4.8 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 4.4 wt%-F. Subsequently, 2.9 g of this fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 2.7 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 1.9 wt%-F.

[0096] [Example C4] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was subjected to a first pre-calcination at 700°C for 10 hours in a dry air atmosphere to prepare 4.1 kg of raw silica gel powder. The specific surface area of ​​this raw silica gel was 409.4 m². 2 The silica gel powder was 1.7% by weight, and the average pore size was 2.277 nm, with a pore volume of 0.2331 cc / g. 4.9 g of the silica gel powder was mixed with 0.67 g of ammonium fluoride and placed in an alumina crucible, where it was fired at 400°C for 2 hours (second pre-firing) to obtain 4.7 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 1.7% by weight-F. Subsequently, 3.4 g of this fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 3.3 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 1.5% by weight-F.

[0097] [Comparative Example C1] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was subjected to a first pre-calcination at 800°C for 10 hours in a dry air atmosphere to prepare 4.1 kg of raw silica gel powder. The specific surface area of ​​this raw silica gel was 406.7 m². 2 The average pore size was 2.303 nm and the pore volume was 0.2341 cc / g. 5 g of the raw silica gel powder and 0.66 g of ammonium fluoride were mixed and placed in an alumina crucible, then calcined at 400°C for 2 hours (second pre-calcination) to obtain 4.7 g of fluorine-doped silica gel. The fluorine concentration of the fluorine-doped silica gel powder was 0.4 wt%-F. Subsequently, 2.7 g of the fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 2.6 g of fluorine-containing silica glass powder. The fluorine concentration of the fluorine-containing silica glass powder was 0.1 wt%-F. The results are shown in Table 4. In Table 4, wt% means weight percent.

[0098] [Table 4]

[0099] As described above, by mixing and contacting silicon oxide with NH4F, and then performing firing after pre-sintering, it is possible to stably add a larger amount of fluorine than conventional methods, specifically more than 1% by weight of fluorine, to silica glass powder. Furthermore, because it is possible to manufacture glass while maintaining this fluorine concentration, not only can optical fibers be easily manufactured, but complex structures that were difficult to manufacture in conventional methods, such as optical fibers having multiple light-transmitting layers (paths of light) within a single optical fiber, can also be easily manufactured.

[0100] <Example D: Fourth form> [Example D1] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was pre-calcined at 800°C for 10 hours in a dry air atmosphere to prepare 4 kg of raw silica gel powder. 40 g of this raw silica gel powder was pre-calcined at 800°C for 2 hours in an atmosphere of 5 mL / min perfluoroethane and 5 mL / min oxygen mixed gas flow, and then annealed by natural cooling to prepare 40 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 1.6 wt%-F. Subsequently, 39 g of this fluorine-doped silica gel powder was heat-treated at 1,000°C for 1 hour in a dry air atmosphere to obtain 39 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 1.5 wt%-F. This demonstrates that an annealing effect can be obtained even through the process of natural cooling and reheating.

[0101] [Example D2] Using 40g of silica gel powder from Example D1, pre-calcination was performed in a perfluoroethane and oxygen flow atmosphere, and then, without removing the powder, the atmosphere was switched to dry air and annealed at 800°C for 2 hours. The temperature was then raised to 1,000°C and calcined for 1 hour to obtain 41g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 1.9% by weight-F. It can be seen that the amount of fluorine doping improved after annealing.

[0102] [Example D3] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was pre-calcined at 800°C for 10 hours in a dry air atmosphere to prepare 4.1 kg of raw silica gel powder. Using 30 g of this raw silica gel powder, pre-calcination was performed in a perfluoroethane and oxygen atmosphere in the same manner as in Example 1. Then, without cooling or removal (i.e., while continuously heating), the atmosphere was switched to dry air and the temperature was raised from 800°C to 1,000°C in 10 minutes, followed by calcination for 1 hour to obtain 30 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 0.9 wt%-F.

[0103] [Comparative Example D1] 5 kg of dried silica gel powder obtained by hydrolysis condensation of tetramethoxysilane was pre-calcined at 900°C for 10 hours in a dry air atmosphere to prepare 4.1 kg of raw silica gel powder. Using 31 g of this raw silica gel powder, 30 g of fluorine-doped silica gel powder was prepared by pre-calcining at 900°C for 2 hours in an atmosphere of flowing mixed gas of 5 mL / min of perfluoroethane and 5 mL / min of oxygen, followed by natural cooling. The fluorine concentration of this fluorine-doped silica gel powder was 0.4 wt%-F. Subsequently, 29 g of this fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 29 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 0.1 wt%-F.

[0104] [Comparative example D2] 31 g of the silica gel powder used in Comparative Example D1 was pre-fired at 700°C for 2 hours in an atmosphere of flowing mixed gas of perfluoroethane at 5 mL / min and oxygen at 5 mL / min, and then allowed to cool naturally to prepare 30 g of fluorine-doped silica gel powder. The fluorine concentration of this fluorine-doped silica gel powder was 0.1 wt%-F. Subsequently, 16 g of this fluorine-doped silica gel powder was heat-treated at 1,200°C for 1 hour in a dry air atmosphere to obtain 15 g of fluorine-containing silica glass powder. The fluorine concentration of this fluorine-containing silica glass powder was 0.1 wt%-F.

[0105] As described above, by contacting silicon oxide with fluorocarbons at an appropriate temperature, followed by annealing and firing, a large amount of fluorine can be added to silica glass powder, and the decrease in fluorine concentration due to fluorine desorption from the fluorine-containing silica glass powder can be suppressed, thereby enabling the easy manufacture of optical fibers.

Claims

1. Fluorine-containing silica glass powder, wherein particles with a particle size of more than 150 μm and 300 μm or less constitute 25 wt% or more of the total, particles with a particle size of 75 μm or less constitute 1 wt% or less of the total, and the fluorine content is 1.6 wt% or more.

2. The fluorine-containing silica glass powder according to claim 1, wherein the content of metal impurities in the fluorine-containing silica glass powder is 1,000 ppm or less.

3. The fluorine-containing silica glass powder according to claim 1 or 2, wherein particles with a particle size of more than 150 μm and less than or equal to 300 μm constitute 50 wt% or more of the total.

4. Fluorine-containing silica glass powder according to any one of claims 1 to 3, wherein particles with a particle size exceeding 425 μm constitute 1 wt% or less of the total.

5. A method for manufacturing an optical fiber, comprising the step of forming an overcladding using fluorine-containing silica glass powder according to any one of claims 1 to 4.

6. A method for manufacturing a jacketed tube, comprising the step of forming a jacketed tube using fluorine-containing silica glass powder according to any one of claims 1 to 4.

7. A method for manufacturing a substrate tube, comprising the step of forming a substrate tube using fluorine-containing silica glass powder according to any one of claims 1 to 4.

8. A method for manufacturing an overclad tube, comprising the step of forming an overclad tube using fluorine-containing silica glass powder according to claims 1 to 4.

9. A method for producing fluorine-containing silica glass powder according to claims 1 to 4, wherein SiF₄ is used as a fluorine source.

10. A method for producing fluorine-containing silica glass powder according to claims 1 to 4, wherein NH₄F is used as a fluorine source.

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