Silica glass manufacturing method

Abstract

To provide a production method capable of producing silica glass whose OH group concentration is 10 ppm or less, by using a general-purpose electric furnace.SOLUTION: In a production method of silica glass, a material including a layering silica acid salt compound of 70 mass% or more is heated in the electric furnace and melted, and a molten material is cooled and vitrified to obtain the silica glass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing silica glass. [Background technology]

[0002] Silica glass with a low concentration of OH groups (hydroxyl groups) (low water or anhydrous silica glass) is used as an optically functional material for optical fibers, laser media, etc., mainly in the field of infrared communications. Because OH groups have a broad absorption peak in the infrared band, using silica glass with a low concentration of OH groups can reduce transmission loss in the infrared band.

[0003] Electrically fused glass is known as a silica glass with a low concentration of OH groups. Electrically fused glass is obtained by melting silica raw materials in an electric furnace and cooling the melt to vitrify it. Electrically fused glass can be produced by a relatively simple method using general-purpose glass manufacturing equipment, and the OH group concentration is usually around several tens of ppm.

[0004] Also known is synthetic glass in which the OH group concentration has been reduced to less than 1 ppm by dehydration treatment such as reduced pressure treatment or chlorine gas treatment. Such synthetic glass can be obtained by sintering synthetic silica raw materials while dehydrating them using a method such as the vapor axial deposition (VAD) method, then melting the sintered body and cooling the melt to vitrify it (Patent Documents 1 to 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-180725 [Patent Document 2] Special Publication No. 2005-523863 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-230814 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-230815 [Patent Document 5] Special Publication No. 2012-062240 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the OH group concentration (several tens of ppm) in conventional electrically fused glass is not sufficiently low for some applications. For example, in the field of infrared communications, it is desirable for the OH group concentration in silica glass to be less than 10 ppm. Furthermore, since the production of silica glass requires special equipment such as vacuum equipment and chlorine gas introduction equipment when performing dehydration treatment, it is desirable to be able to reduce the OH group concentration to less than 10 ppm without performing such dehydration treatment.

[0007] The present invention has been made in view of the above problems, and aims to provide a manufacturing method that enables the production of silica glass having an OH group concentration of less than 10 ppm using a general-purpose electric furnace. [Means for solving the problem]

[0008] The above-mentioned problem was solved by using a layered silicate compound (phyllosilicate compound) as a main raw material in a method for producing silica glass using a general-purpose electric furnace. Specifically, the above-mentioned problem was solved by the following means [1], preferably by the means [2] and subsequent means. [1] A method for producing silica glass, comprising heating and melting raw materials containing 70% by mass or more of a layered silicate compound in an electric furnace, and then cooling and vitrifying the melt to obtain silica glass. [2] The method according to [1], wherein the layered silicate compound comprises at least one of magadiite and Kenyaite. [3] The method according to [1] or [2], wherein the ratio of the layered silicate compound in the raw material is 80 mass % or more. [4] The manufacturing method according to [3], wherein the silica glass has an OH group concentration of less than 1 ppm. [5] The method according to any one of [1] to [4], wherein the heating temperature is 1600 to 1900°C and the heating time is 2 to 20 hours. [6] The manufacturing method according to any one of [1] to [5], wherein the steps from heating the raw materials to obtaining silica glass are carried out in air or in an inert gas atmosphere. [7] The manufacturing method according to any one of [1] to [6], wherein a dehydration treatment using a dehydrating agent and a dehydration treatment using reduced pressure are not carried out. [Effects of the Invention]

[0009] According to the method for producing silica glass of the present invention, it is possible to produce silica glass having an OH group concentration of less than 10 ppm even when using a general-purpose electric furnace. DETAILED DESCRIPTION OF THE INVENTION

[0010] <Method of manufacturing silica glass> The method for producing silica glass of the present invention comprises heating and melting raw materials containing 70% by mass or more of a layered silicate compound in an electric furnace, and cooling and vitrifying the melt to obtain silica glass.

[0011] The raw material may contain 70% by mass or more of a layered silicate compound based on the mass of the raw material, and may contain other materials. Examples of other materials include silicate compounds other than layered silicate compounds, such as crystalline silica and amorphous silica, as well as metal oxides, such as titania, that are commonly added to silica glass. From the viewpoint of reducing the OH group concentration, the proportion of the layered silicate compound in the raw material is preferably 80% by mass or more, or 85% by mass or more, and more preferably 90% by mass or more. The proportion of the layered silicate compound in the raw material may be 100% by mass or 95% by mass or less.

[0012] Layered silicate compounds are silicate compounds in which silicate forms a layered structure and has cleavage (easily peels off thin). Examples of layered silicate compounds include kanemite, makatite, illaite, magadiite, and kenyaite. There are no particular limitations on the type of layered silicate compound, and the layered silicate compound may be a natural mineral or a synthetic product. The layered silicate compound preferably contains at least one of magadiite and kenyaite, and more preferably consists of at least one of magadiite and kenyaite. A typical chemical formula for magadiite is Na2Si 14 O 29 nH2O, and the typical chemical formula of Kenyaite is Na2Si 22 O 45 nH2O. In the above chemical formula, n is an integer from 1 to 20, more commonly from 5 to 15. These compounds may have a structure in which a hydrogen atom is substituted for one Na atom (the "Na2" part is represented by "NaH") or a structure in which a hydrogen atom is added to two Na atoms (the "Na2" part is represented by "Na2H"). The synthesis method for magadiite and kenyaite is not particularly limited and is described, for example, in Kosuge Katsunori et al., "Hydrothermal Synthesis of Magadiite and Kenyaite," Journal of the Ceramic Society of Japan, Vol. 100(3), pp. 326-331 (1992).

[0013] The layered silicate compound is preferably in powder form from the viewpoint of reducing the OH group concentration. The layered silicate compound powder preferably has an average particle size of 10 to 500 μm, more preferably 50 to 200 μm. The average particle size of the layered silicate compound powder is a value measured by a laser diffraction method.

[0014] The manufacturing method of the present invention uses a highly versatile electric furnace. This allows existing facilities to be utilized, reducing manufacturing costs. The type and size of the electric furnace are not particularly limited and can be appropriately selected depending on the application of the silica glass. Examples of heating methods for electric furnaces include resistance heating, induction heating, and direct current. Examples of electric furnace structures include box type, crucible type, tube type, continuous type (tunnel type), hearth lift type (elevator type), and rotary type.

[0015] The heating temperature during heating of the raw materials is preferably 1600 to 1900°C and the heating time is preferably 2 to 20 hours, more preferably 1700 to 1850°C and the heating time is more preferably 3 to 10 hours.

[0016] The conditions for cooling and vitrifying the melt can be adjusted as appropriate to obtain the desired properties of silica glass. The cooling rate is, for example, preferably 50 to 150°C / hour, more preferably 75 to 125°C / hour. The cooling rate may be changed stepwise. By setting the cooling rate within the above range, silica glass with higher transparency can be obtained.

[0017] In the manufacturing method of the present invention, the steps from heating the raw materials to obtaining silica glass can be carried out in air or an inert gas atmosphere. From the viewpoint of stabilizing quality, an inert gas atmosphere is preferred. Examples of inert gases that can be used include nitrogen gas and argon gas. The pressure of the inert gas atmosphere is not particularly limited and can be approximately atmospheric pressure (101 kPa), for example, 50 to 150 kPa, and preferably 80 to 100 kPa. In particular, applying a reduced pressure below atmospheric pressure (less than 101 kPa) promotes degassing from the silica glass, making it easier to obtain silica glass with high transparency. Furthermore, while silica tends to sublimate under reduced pressure at high temperatures, the problem of silica sublimation is less likely to occur if the atmospheric pressure is 50 kPa or higher. The air or inert gas atmosphere is preferably maintained throughout the entire process from heating the raw materials to obtaining silica glass, but may also be applied to only some of the steps. The air atmosphere and the inert gas atmosphere may be switched between during the manufacturing process as needed.

[0018] In the manufacturing method of the present invention, it is not necessary to perform a dehydration treatment using a dehydrating agent or a dehydration treatment using reduced pressure. A dehydration treatment using a dehydrating agent is, for example, a treatment in which the raw materials are heated in an atmosphere of a dehydrating agent (e.g., a halogen-based gas, particularly a chlorine-based gas) at a temperature at which the raw materials do not melt (e.g., about 1100 to 1300°C). A dehydration treatment using reduced pressure is, for example, a treatment in which the raw materials are heated in a reduced pressure or vacuum atmosphere at a temperature at which the raw materials do not melt. The manufacturing method of the present invention has the advantage that silica glass having an OH group concentration of less than 10 ppm can be obtained without using special equipment for such dehydration treatment.

[0019] The manufacturing method of the present invention can produce silica glass having an OH group concentration of less than 10 ppm. Furthermore, the higher the proportion of the layered silicate compound in the raw material, the more the OH group concentration in the resulting silica glass can be reduced. According to the present invention, it is possible to achieve an OH group concentration of less than 5 ppm, less than 3 ppm, or less than 2 ppm, and even less than 1 ppm (substantially anhydrous silica glass).

[0020] Although there is no intention to be limited to a particular theory, the reason why silica glass having an OH group concentration of less than 10 ppm can be obtained by the production method of the present invention is thought to be as follows.

[0021] The OH group concentration in conventional electrically fused glass produced using a general-purpose electric furnace from raw materials that do not contain layer silicate compounds or raw materials with a low content of layer silicate compounds is generally around several tens of ppm, and it is difficult to reduce it to less than 10 ppm even by extending the heating time or increasing the heating temperature. The reason why the OH group concentration in such electrically fused glass cannot be reduced to less than 10 ppm is thought to be because the OH groups incorporated into the silica structure are difficult to remove by simple heat treatment, and the OH groups in the raw materials remain in the glass.

[0022] In contrast, the manufacturing method of the present invention produces silica glass from raw materials containing 70 mass% or more of a layered silicate compound. It is believed that OH groups in a layered silicate compound exist in the spaces between the layers of the layered structure (interlayer spaces) (see, for example, Takushi Ikeda et al., "Creating a New Zeolite, CDS-1, Using Layered Silicates Like Building Blocks," Shinku, Vol. 49, No. 4 (2006)). Therefore, it is believed that the OH groups in the layered silicate compound are likely to be released from the compound through the interlayer spaces during the period from the start of heating the raw materials to the melting of the raw materials.

[0023] In the manufacturing method of the present invention, by using a layered silicate compound that easily releases OH groups as a main raw material, it is possible to reduce the amount of OH groups remaining in the silica structure, and as a result, silica glass with an OH group concentration of less than 10 ppm can be obtained using a general-purpose electric furnace without performing a dehydration treatment. [Example]

[0024] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

[0025] Example 1 100g of Kenyaite powder (average particle size: 150μm) was placed in a 50mm diameter mold and heated for 10 hours at 1800°C in an argon gas atmosphere at atmospheric pressure to melt the raw material. The material was then cooled to 1000°C at a rate of 100°C / hour without changing the atmosphere, and then naturally cooled to room temperature. This procedure yielded transparent silica glass measuring approximately 45mm in diameter and 30mm in height.

[0026] The absorbance of the obtained silica glass at 2720 nm was measured using a Fourier transform infrared spectrophotometer FT / IR-6600 Plus (manufactured by JASCO Corporation), and the OH group concentration was calculated based on this absorbance. The OH group concentration in the silica glass was less than 1 ppm.

[0027] <Example 2> Instead of the raw material made of Kenyaite powder, a raw material made of 100 g of magadiite powder was used to obtain transparent silica glass in the same manner as in Example 1. The OH group concentration was measured in the same manner as in Example 1, and the OH group concentration in the silica glass was found to be less than 1 ppm.

[0028] Example 3 Instead of using the raw material consisting of Kenyaite powder, a raw material consisting of 70 g of Kenyaite powder and 30 g of quartz powder (natural crystalline silica powder) was used to obtain transparent silica glass in the same manner as in Example 1. The OH group concentration was measured in the same manner as in Example 1, and the OH group concentration in the silica glass was found to be less than 10 ppm.

[0029] <Comparative Example> Instead of the raw material made of Kenyaite powder, a raw material made of 100 g of quartz powder was used to obtain transparent silica glass in the same manner as in Example 1. The OH group concentration was measured in the same manner as in Example 1, and the OH group concentration in the silica glass was found to be 35 ppm.

Claims

1. A method for producing silica glass, comprising heating and melting a raw material containing 70% by mass or more of a layered silicate compound in an electric furnace, and cooling and vitrifying the melt to obtain silica glass, comprising: The method for producing a layered silicate compound includes at least one of kanemite, makatite, illaite, magadiite, and kenyaite.

2. The method according to claim 1 , wherein the layered silicate compound comprises at least one of magadiite and Kenyaite.

3. The method according to claim 1 or 2, wherein the ratio of the layered silicate compound in the raw material is 80 mass % or more.

4. The method according to claim 3 , wherein the silica glass has an OH group concentration of less than 1 ppm.

5. 3. The method according to claim 1, wherein the heating temperature is 1600 to 1900° C. and the heating time is 2 to 20 hours.

6. 3. The method according to claim 1, wherein the steps from heating the raw materials to obtaining the silica glass are carried out in air or in an inert gas atmosphere.

7. The method according to claim 1 or 2, wherein a dehydration treatment using a dehydrating agent and a dehydration treatment under reduced pressure are not carried out.