JPH0420853B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inexpensive method for producing synthetic quartz, particularly synthetic quartz that does not contain elemental or compound chlorine. The method for producing synthetic quartz involves converting silicon tetrachloride into a gaseous state and hydrolyzing it in an oxyhydrogen flame.
A method in which the silica (SiO ₂ ) generated here is blown onto a heat-resistant substrate along with flame, and is melted and grown by the sensible heat of the flame (see US Pat. No. 2,272,342). There is also a method in which this silica is grown as a porous mass without melting, and then heated in an electric furnace to 1400 to 1700°C to melt and vitrify it (US Patent No. 3806570).
However, the synthetic quartz obtained by the former method contains 10 to 200 ppm of elemental or compound chlorine and 700 to 1200 ppm of OH groups.
is also included, and what can be obtained by the latter method is also included.
Although the OH group content is about 100 ppm or less, the amount of chlorine is quite high at 10 to 100 ppm. In addition, regarding the manufacturing method of this synthetic quartz, a method is known in which the decomposition reaction of silicon tetrachloride is heated with high-frequency plasma, and according to this method, the OH group content can be reduced.
Although it can be lowered to below 10 ppm, the amount of chlorine is not reduced in this case and is still quite high at 10-500 ppm. However, synthetic quartz containing a large amount of chlorine is colored yellow, although the degree of yellowing varies depending on its content, which has the disadvantage of impairing its properties for optical communications and optical applications. This also has the disadvantage of inferior chemical resistance compared to those that do not contain chlorine. The present invention solves these disadvantages and relates to a method for producing synthetic quartz that does not substantially contain chlorine.This invention uses the general formula RnSi(OR') _4-o (where R is a hydrogen atom or a methyl group). , ethyl group, R' is a methyl group or ethyl group, n is a positive number from 1 to 4) or a mixture gas of this and a vaporized combustible substance that does not contain hydrogen gas or chlorine is combusted. The silica thus generated is deposited on a substrate, and the porous sintered silica thus obtained is heated and melted under vacuum or in an inert gas atmosphere containing no water. It is something to do. To explain this, the inventors of the present invention have investigated various methods for producing synthetic quartz that does not contain chlorine, and have found that it is sufficient to use silane gas, which does not contain chlorine atoms in its molecules, as the starting material. After researching this, we found that silane with the formula SiH ₄ could also be used, but it has an extremely low boiling point of -111.2°C and is a dangerous substance that can explode and burn in the air. It's expensive. In addition, tetraalkoxysilane is toxic, has a high melting point, and freezes in winter, making it difficult to handle.In addition, since it does not have an alkyl group, the heat of combustion is low, the hydrogen consumption rate is high, and the unit price is high, making it uneconomical. There is a problem that.
However, the ester silane represented by the general formula above does not contain any chlorine and has a boiling point of approximately 100°C.
As described above, it has a low burning rate, low toxicity, is easy to handle, and because it has an alkyl group, it has a low hydrogen consumption rate and is economical, and it can be supplied industrially at low cost. Therefore,
After repeated experiments on this, it was confirmed that by using this, synthetic quartz that does not contain chlorine can be easily and reliably produced, and the present invention was completed. The ester silane used as the starting material in the method of the present invention has the general formula RnSi(OR') _4-o as described above.
(n=1 to 4), and examples include methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, and methyltriethoxysilane, which are inexpensive and easy to handle. Therefore, industrially it is preferable to use methyltrimethoxysilane. This ester silane is produced by a direct reaction between methyl chloride and metal silicon, and methyl trichlorosilane, which is a by-product during the synthesis of dimethyldichlorosilane, which is the main raw material for silicone rubber, silicone varnish, and silicone oil, is mixed with alcohol such as methanol or ethanol. or by thermally decomposing polymethylpolychloropolysilane or polymethylpolychloropolysiloxane, which has the general formula (CH ₃ ) _o Si _n Cl _x O _y and is produced in the manufacturing process of dimethyldichlorosilane. Since it can be easily obtained by reacting a mixture of monomethyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, monomethyldichlorosilane, etc. with alcohol, it can be supplied industrially at low cost; In addition, it can be easily obtained as a purified product free of impurities by rectifying chlorosilane or ester silane, which is the raw material, so it has the advantage of being able to obtain highly pure synthetic quartz. give. In the method of the present invention, the ester silane is first combusted to generate silica, which is deposited on a substrate to form a porous sintered silica. It may be carried by argon gas or the like and burned in air. This combustion occurs when the ester silane contains methyl and ethyl groups that serve as fuel in its molecules.
Because it has an alkoxy group and because oxygen can be mixed with the ester silane in advance, it exhibits extremely high combustion efficiency, and complete combustion can be achieved. It is possible to obtain a sufficiently high temperature to obtain a porous silica sintered body made of appropriately sintered fine silica, and since this ester silane does not contain chlorine, it contains no chlorine at all. Silica sintered bodies can be easily obtained. The porous silica sintered body thus obtained has a concentration of ester silane for producing silica,
The bulk density differs depending on the reaction conditions such as supply rate and reaction temperature, but this is because it is extremely easy to collapse below 0.05 g/cm ³ and the shrinkage rate when melting and vitrifying it is large. However, it has the disadvantage that the shape after vitrification becomes irregular, and conversely, in order to achieve a shape exceeding 1.0 g/cm ³ , it is necessary to raise the forming temperature of this sintered body considerably. This is often in the range of 0.05 to 1.0 g/cm ³ and for this purpose it is necessary to use the starting material as a starting material. Hydrogen gas may be added to the ester silane as an auxiliary fuel, or vaporized combustible substances such as chlorine-free volatile hydrocarbons such as methane, ethane, propane, etc. may be added. Incidentally, the silica produced by combustion of this ester silane is grown on a heat-resistant substrate, and although this substrate may be of a heat-resistant material, it is possible to grow it on a heat-resistant substrate. A quartz rod offers the advantage of being able to produce a solid glass body by melting as described below. However, this quartz rod has Ge, P,
It may be doped with B, F, Ti, Al, etc., and this allows the base portion and the portion attached thereon to be glass bodies with different refractive indexes. Further, this substrate may be a rod-shaped body made of carbon or graphite, and by this, a hollow glass body can be obtained by extracting the rod after melting and cooling. Next, the method for producing a porous sintered silica body according to the method of the present invention will be explained based on the drawings. Figure 1 shows a system diagram thereof, and Figure 2 shows a longitudinal cross-section of the main part showing another aspect. Figures 3 and 3 illustrate a burner used in carrying out the method of the present invention, in which a is a top view and b is a partial vertical sectional view thereof. In carrying out the method of manufacturing a porous silica sintered body according to the method of the present invention, first, as shown in FIG. 1, ester silane stored in a raw material tank 1 is sent to an evaporator 3 by a metering pump 2. If the ester silane is vaporized at a high temperature in this evaporator, the ester silane will decompose and polymerize, so an inert gas such as argon or nitrogen is blown into the evaporator through the pipe 4 to vaporize the ester silane at a low temperature. Let them do it. The ester silane carried by this inert gas is discharged from the pipe 5 as necessary in order to completely burn it and to reduce or completely eliminate the use of hydrogen gas and combustible materials as auxiliary fuel. Oxygen gas is mixed in, but
This amount of oxygen does not necessarily have to be the stoichiometric amount required for complete combustion of the raw material, and the remaining amount may be supplied to the burner 6. This burner 6 is further supplied with oxygen gas or inert gas from a pipe 7, oxygen gas from a pipe 8, and hydrogen gas or combustible gas from a pipe 9 as required. The structure of this burner is shown in FIG. As described above, a concentric multi-ring structure may be used, and a raw material gas containing ester silane is supplied from the center, and oxygen gas, an inert gas, oxygen gas, hydrogen gas, or combustible gas is supplied around the center. The ester silane supplied from this burner 6 is completely combusted in coexistence with gas from the surrounding area, generating silica in the reactor. heat-resistant substrate 10
This causes a porous sintered body 11 to be formed. FIG. 2 shows another embodiment of this, in which the rotating heat-resistant base 10' repeats reciprocating motion in the horizontal direction relative to the burner 6'. For example, the porous silica sintered body can be formed into a cylindrical shape along the length direction of the base body 10'. In the method of the present invention, the porous silica sintered body thus obtained is then melted and vitrified to obtain synthetic quartz. All you have to do is heat it to about 1400℃, the vitrification temperature, and melt it.
In this process, it is necessary to dehydrate and condense the moisture and ≡SiOH groups adsorbed on this sintered body, so for this purpose, the vacuum level in the furnace must be kept at 1×10 ^-2 Torr or less. , the inside of the furnace is at its vitrification temperature
After carrying out dehydration treatment at around 1000°C, which is below 1400°C, the temperature inside the furnace may be heated to 1400 to 1600°C for vitrification. However, when this sintered body is melted and vitrified under a high vacuum, the silica evaporates, and the small amount of foreign matter present in the sintered body becomes bubbles and remains in the glass. This can be done by enclosing or circulating an inert gas that does not contain water, such as helium, nitrogen, or argon. According to this method, the OH group content can be reduced.
It can be reduced to 50ppm or less. In summary, the present invention generates silica using the ester silane represented by the above general formula, which does not contain any chlorine, as a starting material, obtains this as a porous sintered silica, and then melts it to produce synthetic quartz. Therefore, according to this method, synthetic quartz that does not contain any chlorine can be easily obtained, and if sufficient dehydration is performed in the vacuum treatment in the melting and vitrification process, the OH group content can be reduced to 50 ppm or less. Therefore, it is possible to easily obtain base materials for optical devices such as lenses and prisms, and even for optical communications, and the hollow quartz tube obtained by this method can be used to manufacture low-loss optical fibers. It is also useful as an internal CVD tube. Next, examples of the method of the present invention will be given. Example 1 Methyltrimethoxysilane [ _CH3Si ( _OCH3 ) ₃ ,
Argon gas is blown into the evaporator containing a boiling point of 102°C as a carrier gas.
A raw material gas containing 100g/h of methyltrimethoxysilane is prepared at 20N/h, and oxygen gas is added to this.
The mixture is sent to the burner at 50N/h, and 100N of hydrogen gas is added to this burner as auxiliary fuel.
/h, supplying 80N/h of oxygen gas for combustion,
A porous silica sintered body was prepared on a synthetic quartz substrate using the method shown in Figure 1. When this process was continued for 2 hours, a sintered body weighing 58 g was obtained. The skein density was 0.55 g/cm ³ . Next, this sintered body was placed in a vacuum furnace and held at 1100°C for 2 hours at a vacuum level of 1 × 10 ^-2 Torr, and then heated to 1550°C while maintaining the vacuum to vitrify it. Quartz was obtained, which does not contain any chlorine and its OH group content is also
It was 25ppm. Example 3 Methyltrimethoxysilane was used as a raw material, transported with argon gas as a carrier gas, mixed with oxygen gas, and then supplied to a burner. Hydrogen gas for auxiliary fuel was also supplied to this burner. The ester silane was combusted, and the resulting silica was deposited onto a rotating graphite rod, 20 mm in diameter, held horizontally as shown in FIG. On this occasion,
The burner was placed in the same horizontal plane as the substrate, and when it was reciprocated along the substrate at a distance of 400 mm, porous silica sintered with silica deposited with an approximately uniform thickness was formed on the substrate. Once the body was obtained, it was placed in a vacuum furnace under a heavy atmosphere of 1×10 ^-2 Torr.
After being maintained at 1150° C. for 2 hours, it was melted and vitrified in helium gas or under vacuum, and when the substrate was removed after cooling, a pipe-shaped synthetic quartz was obtained. The combustion conditions, melting conditions, and properties of the obtained sintered body and synthetic quartz were as shown in Table 1 below. 【table】

[Brief explanation of drawings]

Fig. 1 is a system diagram of the method of the present invention, Fig. 2 is a vertical sectional view of main parts showing another aspect of the present invention, and Fig. 3 is a diagram showing a burner used in the present invention, and a is a top view thereof. ,b is a partial longitudinal sectional view thereof. 1... Raw material tank, 2... Metering pump, 3...
Evaporator, 4, 5, 7, 8, 9... Gas supply pipe, 6, 6'... Burner, 10, 10'... Heat resistant substrate, 11, 11'... Porous silica sintered body.

Claims (1)

[Claims] 1 General formula R _o Si (OR') _4-o (where R is a hydrogen atom or a methyl group or an ethyl group, R' is a methyl group or an ethyl group, and n is a positive number from 1 to 4) ) or a mixed gas of this and a vaporized combustible substance that does not contain hydrogen gas or chlorine is burned, and the silica generated by this is deposited on a substrate, and then the silica obtained here is obtained. A method for producing synthetic quartz, which comprises heating and melting a porous sintered silica body under vacuum or in an inert gas atmosphere containing no water. 2. The method for producing synthetic quartz according to claim 1, wherein the ester silane is methyltrimethoxysilane. 3. The method for producing synthetic quartz according to claim 1 or 2, wherein the substrate is a carbon or graphite product.