JP5838108B2 - Manufacturing method of glass base material - Google Patents

Description

  The present invention is a glass mother that can be introduced by controlling the concentration of an additive of an alkali metal oxide or alkaline earth metal oxide with a purity that is applicable to optical applications such as optical fibers and optical components. The present invention relates to a method for manufacturing a material.

Although many optical fiber optical fibers made using silica glass doped with alkali metal oxides or alkaline earth metal oxides have been shown to reduce transmission loss (e.g. The technology for industrially mass-producing this is not yet complete. Since the existing optical fiber manufacturing method uses a hydrolysis reaction in the gas phase or a thermal oxidation reaction with oxygen, a gaseous raw material such as silicon tetrachloride (SiCl ₄ ) or germanium tetrachloride (GeCl ₄ ) is used. Need to use.

  However, since alkali metal ions or alkaline earth metal ions, which are so-called hard cations, form very strong ionic bonds, these compounds (salts) are almost solid at room temperature and near normal pressure. . Therefore, since it hardly forms a compound that becomes a gas, it has been difficult to apply to the production of optical fibers. Therefore, in order to commercially produce optical fibers doped with alkali metal oxides or alkaline earth metal oxides, it is necessary to develop a manufacturing method different from the methods established in this field.

  Various efforts have been made to deal with such issues. For example, utilizing the fact that alkali metal compounds are readily water-soluble, an aqueous solution of an alkali metal compound is made into a mist, mixed into a raw material gas, introduced into an oxyhydrogen flame, and simultaneously hydrolyzed with other raw materials to produce glass. The method of forming was tried. In addition, it is known that a composite salt obtained by reacting a certain kind of alkali metal compound with another metal compound has a higher vapor pressure than the original alkali metal compound, and this composite salt is used as a raw material. Attempts to do so were also made.

  More recently, a method has been tried in which an alkali metal halide is heated to alkali metal vapor, and an optical fiber precursor glass is exposed to this for doping (for example, see Patent Documents 1 and 2). Furthermore, a method of supplying raw materials as an aerosol by heating the alkali metal halide to vapor and then immediately cooling it to form fine particles and transporting this in an appropriate gas flow was also tried.

JP 2005-537210 A JP-T-2007-513862

M.E. Lines, “A possible non-halide route to ultralow loss glasses”, Journal of Non-Crystalline Solids, 1988, Vol. 103, p. 279-288

The above-described method using an aqueous solution of an alkali metal compound is a manufacturing method that is contrary to the viewpoint that in the manufacture of an optical fiber, mixing of moisture that would cause an increase in loss should be avoided. In addition, the above-described method of forming a complex salt having a high vapor pressure and introducing it as a vapor has a small degree of increase in vapor pressure and has a very limited effect. Therefore, there is a concern that transmission loss will increase.
Furthermore, the method of obtaining an alkali metal vapor by strongly heating an alkali metal compound is unrealistic because the reaction mechanism of the reduction reaction is unclear. In the method using an aerosol, the controllability of the raw material supply amount is somewhat difficult as compared with the conventional method using a gas as a raw material.

  The present invention has been made in view of the above circumstances, and has a purity so high that it can be applied to optical applications such as optical fibers and optical components, and has an additive concentration of an alkali metal oxide or an alkaline earth metal oxide. It aims at providing the manufacturing method of the glass base material which can be controlled and introduce | transduced.

  In order to solve the above problems, the present invention provides a method for producing a glass base material doped with an alkali metal oxide or an alkaline earth metal oxide, wherein a porous silica glass is converted into an alkali metal compound or an alkaline earth metal. The alkali metal compound or alkaline earth metal compound is held together with the metal compound in a closed heating furnace so that the alkali metal compound or alkaline earth metal compound is in a gas-liquid equilibrium state on the surface of the silica glass. The silica glass is doped with an alkali metal oxide or an alkaline earth metal oxide by maintaining the temperature above the melting point and lower than the temperature at which the silica glass crystallizes within the dope treatment time, and then the heating Temperature rise of 10 ° C./min or more until the temperature reaches a sintering temperature at which the silica glass is sintered into a transparent glass body in a furnace To provide a method of manufacturing a glass base material, which comprises heating in degrees.

  According to the present invention, it is easy to dope silica glass by controlling the additive concentration of alkali metal oxide or alkaline earth metal oxide, and crystallization of silica glass can be suppressed.

It is a figure which shows an example of the manufacturing apparatus used for this invention. It is a diagram showing a phase diagram of the silica (SiO _2).

Hereinafter, the present invention will be described based on preferred embodiments.
The method for producing a glass base material according to this embodiment is a method for producing a glass base material doped with an alkali metal oxide or an alkaline earth metal oxide, wherein silica glass is alkali metal oxide or alkaline earth metal oxide. A dope process for doping an object and a sintering process for sintering silica glass to form a transparent glass body.

Alkali metals, ie, lithium, sodium, potassium, rubidium, cesium compounds (salts) and alkaline earth metals, ie, beryllium, magnesium, calcium, strontium, barium compounds (salts) generally have the following properties: Indicates.
For example, many of halides such as chloride, bromide, fluoride and iodide, oxides and hydroxides are chemically stable and exhibit melting points without decomposition. However, an alkali metal oxide having a large ionic radius, such as potassium oxide (K ₂ O), is decomposed into a peroxide (K ₂ O ₂ ) and potassium (K). Hydrogen carbonate decomposes at a considerably low temperature to become carbonate, but carbonate has a melting point and decomposes into oxide when heated further. Sulfides react with oxygen and carbon dioxide in the atmosphere to turn into oxides and carbonates, respectively, but exhibit melting points in inert gases. Nitrate has a relatively low melting point, but decomposes into nitrite above the melting point. Some examples are shown in Table 1.

  The melting point of these alkali metal compounds or alkaline earth metal compounds is almost always lower than the melting point of silica glass, and even in the temperature region below the melting point of silica glass, it can be vaporized with a low vapor pressure. Even when it is decomposed into an oxide or carbonate, it can be vaporized as an oxide or carbonate. Therefore, the vapor or droplet of the alkali metal compound or alkaline earth metal compound is brought into contact with the silica glass, and the alkali metal compound or alkaline earth metal compound is maintained for a certain period of time in a state where gas-liquid equilibrium is maintained on the surface of the silica glass. By doing so, a certain amount of alkali metal ions or alkaline earth metal ions diffuses into the silica glass, and as a result, the silica glass can be doped with an alkali metal oxide or an alkaline earth metal oxide.

  As an alkali metal compound or an alkaline earth metal compound, two or more compounds of the same alkali metal element can be used in combination, or two or more compounds of different alkali metal elements can be used in combination. Two or more kinds of metal element compounds can be used together, two or more kinds of different alkaline earth metal element compounds can be used together, and two or more kinds of alkali metal compounds and alkaline earth metal compounds can be used in combination. You can also. It is also possible to dope two or more kinds of alkali metal oxides or alkaline earth metal oxides into silica glass.

The silica glass to be doped may be pure silica (SiO ₂ ) glass, or may be silica glass doped with additives used in ordinary optical fibers such as fluorine (F) and germanium oxide (GeO ₂ ). Silica glass doped with less alkali metal oxide or alkaline earth metal oxide may be used. Silica glass containing two or more kinds of additives can be used as doped silica glass.

  In FIG. 1, an example of the manufacturing apparatus used for the manufacturing method of the glass base material of this embodiment is shown. The manufacturing apparatus 10 includes a heating furnace 11 and a shaft 13 that supports the silica glass 12 to be processed. The heating furnace 11 shown in FIG. 1 includes a plurality of parts that can be opened and closed when accommodating the silica glass 12 to be treated therein, that is, a main body (furnace core tube) 11a and a lid 11b. The shaft 13 may have a structure that can be rotated around the center line or moved along the longitudinal direction. Here, the “silica glass to be treated” is a silica glass as a raw material of a glass base material, that is, a silica glass to be doped before doping, and a silica glass in the middle of doping and after doping derived from this silica glass to be doped. It is a general term and does not include other members that can be made of silica glass in the manufacturing apparatus.

  The heating furnace 11 includes a glass heating part (soaking / sintering part) 14 for heating the silica glass 12 to be treated, and a dope material heating part (vaporization part) for heating the dope material 16 of the alkali metal compound or alkaline earth metal compound. 15 Thereby, the temperature of the silica glass 12 to be processed and the temperature of the dope material 16 can be controlled separately.

  One or two or more glass heating heaters H <b> 1 are provided on the outer periphery of the glass heating unit 14. In the case of FIG. 1, the glass heating heater H1 is integrally provided with a cylindrical heater longer than the entire silica glass 12 to be treated, and can uniformly heat the entire silica glass 12 to be treated. It constitutes a soaking furnace. It is also possible to install a plurality of heaters in the longitudinal direction of the silica glass 12 to be treated. In this case, all heaters are heated to heat the entire silica glass 12 to be used as a soaking furnace, or some heaters are heated to heat a part of the silica glass 12 to be processed. Or can be used as a tilt furnace. In the dope process, it is preferable to uniformly heat the entire silica glass 12 to be treated. In the sintering step, when a portion of the silica glass 12 to be treated is heated, the silica glass 12 to be treated is moved in the longitudinal direction, and the silica glass 12 to be treated is gradually sintered from one end side. When the gas contained in the inside of the glass becomes solid, it is easily released to the outside of the silica glass 12 to be treated, and it is possible to suppress the bubbles from remaining in the transparent glass body. Moreover, when providing a some heater in the glass heating part 14, the temperature of each heater can be changed and a temperature gradient can also be provided.

On the outer periphery of the dope material heating unit 15, one or more dope material heaters H <b> 2 are provided. When the dope material 16 (alkali metal compound or alkaline earth metal compound) is heated above its melting point by the heater H2 for heating the dope material, vapor 18 evaporated from the dope material 16 is generated in the heating furnace 11, and the silica glass to be treated It is possible to contact 12 surfaces (outer surface or porous inner surface).
The dope material 16 is held in a dope material holding unit 17 installed inside the heating furnace 11. The dope material holding part 17 can support a container such as a dish by a support such as a support, an arm, and a lattice (not shown). Thus, by installing the dope material 16 at a position away from the inner wall of the heating furnace 11, the carrier gas for transporting impurities such as moisture around the dope material can be efficiently used during the drying process of the dope material. It can be distributed well.

The heating furnace 11 has an inlet (gas inlet) 19 and an outlet (gas outlet) 20 through which gas flows. When the drying process of the dope material 16 and the dehydration process of the porous silica glass 12 to be processed are performed in the same heating furnace 11 as a pre-process of the doping process, the dope material 16 and the silica glass 12 to be processed are discharged. In order to promote the discharge of moisture, a carrier gas composed of an inert gas such as helium (He), argon (Ar), or nitrogen (N ₂ ) can be circulated from the gas inlet 19 to the gas outlet 20. . In the dehydration step, it is preferable that a chlorine-based gas such as chlorine (Cl ₂ ) or thionyl chloride (SOCl ₂ ) is mixed with a carrier gas and used as a dehydrating agent for porous silica glass. The gas inlet 19 is connected to a gas supply device (not shown) that supplies a carrier gas, a dehydrating agent, and the like. Further, the gas discharge port 20 is connected to an appropriate removal device (scrubber, not shown) in order to remove glass fine particles, chlorine-based gas and the like mixed in the discharged gas. In the production of an optical fiber, it is desirable that the gas introduced into the heating furnace 11 is a dry gas with a sufficiently reduced amount of water in order to avoid the mixing of moisture that causes an increase in loss.

  In the dope process, the porous silica glass 12 to be treated is held in the heating furnace 11 with the inlet 19 and the outlet 20 closed together with the alkali metal compound or alkaline earth metal compound 16. Since the internal temperature of the heating furnace 11 in the doping process is not necessarily equal to or higher than the boiling point of the doping material 16, the vapor 18 of the doping material may contain droplets or fine particles of the doping material. By closing the inlet 19 and the outlet 20 of the heating furnace 11 during the doping process, it becomes easy to keep the concentration of the vapor 18 of the dope material in a vapor-liquid equilibrium state constant throughout the heating furnace 11. . The dope material 16 is in a vapor-liquid equilibrium state on the surface of the silica glass 12 to be treated, so that the vapor 18 of the dope material of the dope silica glass 12 is higher than the case where the dope material 16 is a high-temperature steam having a boiling point or higher. It becomes easy to adhere to the surface. Further, when the dope material 16 is heated to a temperature higher than the boiling point, depending on the temperature, the silica glass 12 to be treated may be crystallized or softened (and liquefied) to maintain the glass state, shape, and composition distribution. It can be difficult. Therefore, the temperature within the heating furnace 11 is higher than the melting point of the dope material 16 (alkali metal compound or alkaline earth metal compound) by the glass heating heater H1, and the silica glass to be treated crystallizes within the dope treatment time. By keeping the temperature low, the silica glass 12 to be treated can be efficiently doped with an alkali metal oxide or an alkaline earth metal oxide. By adjusting the treatment temperature, treatment time, etc., it is possible to control and introduce the additive concentration of alkali metal oxide or alkaline earth metal oxide.

The dope amount by the dope material in the vapor-liquid equilibrium state is proportional to the contact time and the contact area with respect to the silica glass. For this reason, the silica glass to be treated (doped) is preferably a porous material such as an aggregate of fine glass soot rather than a solid glass lump. The porous silica glass can be formed by a method such as a VAD (Vapor phase axial deposition) method or an OVD (Outside vapor deposition) method. The soot aggregate is grown to a desired size by depositing soot on the outer periphery or tip of a support member made of silica glass, for example, a rod (rod) or tube (tube) made of silica glass. be able to. In the case of the VAD method, since a soot is grown in the length direction from the front end portion of the starting member, a large aggregate formed by the soot can be formed to the center portion in the radial direction, which is preferable. As shown in FIG. 1, when the porous silica glass 12 to be processed is produced by the VAD method, a part or all of the starting base material on which the glass soot is deposited is used as the shaft 13 that supports the silica glass 12 to be processed. be able to. For example, porous silica glass produced by the VAD method has a specific surface area of 5 to ²⁰ m ² / g, and can be suitably used.

  The temperature of the dope treatment needs to be at least the melting point of the alkali metal compound or alkaline earth metal compound because it vaporizes. The higher the temperature, the faster the alkali metal ions or alkaline earth metal ions diffuse into the silica glass, so that it can be doped in a shorter time. However, when an alkali metal salt or the like is present, the silica glass is easily crystallized, and crystalline silica such as cristobalite or tridymite is easily generated. Pure bulk silica glass is most easily crystallized in the temperature range of about 1000 to 1700 ° C., but the temperature at which silica glass is easily crystallized as the concentration of alkali metal ions or alkaline earth metal ions in silica glass increases. Range goes down. In order to suppress the crystallization of the silica glass to be treated, the alkali metal oxide or alkaline earth metal oxide must be doped at a temperature lower than the temperature range in which the silica glass to be treated crystallizes during the dope treatment time. . For this reason, the dope treatment temperature is preferably 700 to 950 ° C., although it depends on the required dope amount and the dope treatment time.

The sintering process can be performed following the dope process while the silica glass 12 to be treated is kept in the heating furnace 11. Before carrying out the sintering step, a step of discharging unreacted dope material present in the atmosphere of the heating furnace and on the surface of the silica glass to be treated can be provided. Thereby, it can suppress that the dope amount of an alkali metal oxide or an alkaline-earth metal oxide changes (increases) during a sintering process. Even in this case, if the dope material heating unit 15 is sufficiently cooled, the sintering process may be performed while the dope material 16 in the dope material holding unit 17 is kept in the heating furnace 11.
A transparent glass body can be obtained by sintering the porous silica glass 12 doped with alkali metal oxide or alkaline earth metal oxide. At this time, crystallization of the silica glass to be treated can be suppressed and the glass state can be maintained by heating at a high temperature rising rate that passes through the temperature range where the silica glass is easily crystallized as soon as possible.
The temperature range in which silica glass is easily crystallized is approximately 1000 to 1300 ° C., although it depends on the doping amount of the alkali metal oxide or alkaline earth metal oxide, and the temperature is increased at a high speed during this temperature range. As a temperature increase rate, it is 10-20 degrees C / min or more, for example, 10-20 degrees C / min is preferable.

FIG. 2 shows a phase diagram of silica (SiO ₂ ). Silica is known to have different crystal structures such as low-temperature quartz (low Q in FIG. 2), high-temperature quartz (high Q in FIG. 2), tridymite (T in FIG. 2), cristobalite (C in FIG. 2), and the like. ing. Since tridymite and cristobalite have a lower density than quartz, they exist stably under relatively low pressure conditions. At normal pressure (1 atm), the upper limit temperature (melting point of silica) at which cristobalite is stably present is about 1730 ° C. However, when alkali metal oxide or alkaline earth metal oxide is doped, cristobalite is reduced by viscosity reduction. The stable temperature range decreases by about 200 to 300 ° C., and the cristobalite becomes stable at about 1200 ° C. or more.

  Considering the fact that the production rate of quartz is very slow and that the glass state is maintained in a region where quartz is stable, and that tridymite is not produced directly from glass but is produced by phase transition from cristobalite, In order to prevent crystallization of glass, it is important to suppress the formation of cristobalite. Although the phase transition without a large change in atomic arrangement proceeds rapidly as between high-temperature type quartz and low-temperature type quartz, the phase transition between quartz, tridymite, cristobalite, etc. is accompanied by a large change in atomic arrangement. Less likely to occur under low conditions. Therefore, by increasing the rate of temperature rise, generation of cristobalite can be suppressed and sintering can be performed.

The temperature at which such high temperature increase starts (high temperature increase start temperature) is slightly lower than the temperature range in which the silica glass to be treated is easily crystallized, and is preferably 1000 ° C. or lower. If the atmosphere in the heating furnace is lower before the sintering step, it is necessary to heat to the high temperature rise start temperature at the start of sintering. In the temperature range below the high temperature temperature rise start temperature, the temperature rise rate is not particularly limited, and the temperature can be raised at a lower speed in order to suppress distortion of the silica glass to be treated. The temperature increase rate below the high temperature temperature increase start temperature is, for example, 1 to 10 ° C./min.
The temperature at which the high temperature increase is completed (the high temperature increase end temperature) is a sintering temperature at which the porous silica glass 12 is sintered to form a transparent glass body. This sintering temperature is preferably lower than the melting point of the silica glass to be treated. If the silica glass to be treated is heated to a high temperature at which it becomes liquid, crystallization can be prevented, but the glass body cannot maintain its shape and additive concentration distribution. Therefore, the high temperature temperature rise end temperature is, for example, 1300 to 1400 ° C. in the temperature range in which the silica glass 12 to be treated can maintain the glass state. After reaching the sintering temperature, it is kept within the range of the sintering temperature until a transparent glass body is obtained, and then allowed to cool. While the temperature is maintained within the range of the sintering temperature, the sintering temperature may be constant, or the temperature may be increased or decreased arbitrarily within the range of the sintering temperature.

  The resulting transparent glass body is doped with an alkali metal oxide or alkaline earth metal oxide in a dry atmosphere, so that the content of hydroxyl group (—OH) is low, the purity is high, and the loss of transmitted light is reduced. Therefore, it can be used as a glass base material for manufacturing optical components such as optical fibers, lenses, and prisms.

  Hereinafter, the present invention will be specifically described with reference to examples.

A soot made only of silica glass was prepared by a normal VAD method, and this was put in a heating furnace. As an alkali metal ion source, potassium chloride (KCl, manufactured by Junsei Chemical Co., Ltd., purity 99.9% or more) was placed as a powder in a silica glass dish and placed in a furnace. Silica glass soot and KCl are installed in the same furnace, but are spatially separated from each other and can be heated separately. The pressure of the atmosphere in the furnace is 1 atmosphere.
First, the temperature of KCl contained in the dish was kept at 200 ° C. sufficiently lower than its melting point, and dried helium (He) was circulated in the furnace to dry KCl. The silica glass soot was dehydrated and dried by exposing it to He containing chlorine (Cl ₂ ) while being kept at 950 ° C.
Next, after replacing the entire inside of the furnace with only He and sufficiently discharging moisture and chlorine, the heating temperature of silica glass soot and KCl is set to 800 ° C. (slightly higher than the melting point of KCl), respectively, and the flow of He is further increased. And the atmosphere in the furnace was kept constant. This state was left for 1 hour. Thereafter, while the temperature of the silica glass soot is kept at 800 ° C., the temperature of KCl in the dish is cooled to 200 ° C. which is sufficiently lower than the melting point, and then He is circulated again in the furnace so that the atmosphere and the silica glass Unreacted KCl present on the surface of the soot was discharged from the furnace.
After replacing the inside of the furnace with He containing oxygen (O ₂ ), the silica glass soot doped with KCl was heated to reach 1300 ° C. at a temperature rising rate of 10 ° C./min or more with the mixed gas flowing. Further, the silica glass soot was left to stand for 2 hours while maintaining the temperature of the silica glass soot at 1300 ° C. to sinter the silica glass. Finally, it was allowed to cool, and when the temperature of the silica glass became 500 ° C. or lower, the silica glass was taken out from the furnace.

DESCRIPTION OF SYMBOLS 10 ... Manufacturing apparatus, 11 ... Heating furnace, 11a ... Main body (core tube), 11b ... Lid, 12 ... Silica glass, 13 ... Shaft, 14 ... Glass heating part (soaking sintering part), 15 ... Dope material heating Part (vaporization part), 16 ... dope material (alkali metal compound or alkaline earth metal compound), 17 ... dope material holding part, 18 ... vapor or droplet of dope material, 19 ... inlet (gas inlet), 20 ... Outlet (gas discharge port), H1... Heater for glass heating, H2... Heater for heating dope material.

Claims (1)

A method for producing a glass base material doped with an alkali metal oxide or an alkaline earth metal oxide,
Hold the porous silica glass together with the alkali metal compound or alkaline earth metal compound in a closed heating furnace,
The silica glass is above the melting point of the alkali metal compound or alkaline earth metal compound and within the dope treatment time so that the alkali metal compound or alkaline earth metal compound is in a vapor-liquid equilibrium state on the surface of the silica glass. By maintaining the silica glass at a temperature lower than the temperature at which it crystallizes, the silica glass is doped with an alkali metal oxide or an alkaline earth metal oxide,
Next, in the heating furnace, the glass base material is heated at a heating rate of 10 ° C./min or more until reaching a sintering temperature at which the silica glass is sintered to become a transparent glass body. Production method.

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