Method and apparatus for reproducibly producing a preform for glass fiber manufacture

Abstract

The invention relates to a method and a device for producing a preform for glass fiber production. The method comprises the steps of providing a carrier gas with a desired, precisely adjusted temperature, loading the carrier gas with halide vapor, mixing the loaded carrier gas with additional gases, and producing the preform in a reaction chamber with substrate.

Claims

1. Method for producing a preform for glass fiber production, comprising the steps a1) providing at least two oxygen gas-containing carrier gas streams with the same or different temperature, and a2) heating and/or cooling of at least one oxygen gas-containing carrier gas stream of said at least two oxygen gas-containing carrier gas streams so that said at least two oxygen gas-containing carrier gas streams have a different temperature, a3) Introducing said at least two oxygen gas-containing carrier gas streams into a mixing unit, the mixing unit being arranged in a temperature-controllable unit, wherein said at least two oxygen gas-containing carrier gas streams are each controlled by one mass flow controller, and producing a mixed carrier gas stream with a precisely set temperature T1, b) introducing the mixed carrier gas stream from step (a3) at temperature T1 into a vaporizer system comprising a vaporizer liquid at temperature T2 and at least one preform precursor, a vaporizer inlet and a vaporizer outlet, c) loading the mixed carrier gas stream with at least one preform precursor to form a loaded carrier gas stream in the vaporizer system by passing the mixed carrier gas stream through the vaporizer liquid, wherein the loaded carrier gas stream has the temperature T3 at the vaporizer outlet, d) mixing the loaded carrier gas stream from step (c) in a mixing chamber with one or more additional gases to obtain a mixed gas stream, and e) producing a preform for glass fiber production in a reaction chamber containing a substrate tube, using the mixed gas stream from step (d), and wherein the temperature T1 is below a transport temperature T4, wherein the transport temperature T4 corresponds to the lowest temperature value of the loaded carrier gas stream, on a transport path of the loaded carrier gas stream from the vaporizer outlet of step (b) across the mixing chamber of step (d), up to the reaction chamber, and wherein the transport temperature T4 is maintained above the dew point at the vaporizer outlet.

2. The method according to claim 1, wherein the at least two oxygen gas-containing carrier gas streams in step (a1) have the same temperature T0.

3. The method according to claim 1, wherein the temperature T1 is adjusted with an accuracy of +/−0.1° C. in view of a target temperature.

4. The method according to claim 1, wherein the temperature T1 is less than or equal to the temperature T2.

5. The method according to claim 2, wherein the temperature T0 is obtained by active or passive cooling of one oxygen gas-containing carrier gas stream as an outlet stream of a catalytic oxidation unit.

6. The method according to claim 1, wherein the loaded carrier gas stream is passed after the vaporizer outlet through a heated conduit system to the mixing chamber.

7. The method according to claim 1, wherein an additional gas is a carrier gas stream loaded with at least one further preform precursor.

8. The method according to claim 1, wherein at least one of the additional gases of step (d) comprises a carrier gas stream loaded with a further preform precursor to form at least one additional loaded carrier gas stream, wherein this additional loaded carrier gas stream is generated by passing an additional carrier gas stream through an additional vaporizer system containing an additional vaporizer liquid (precursor) having a temperature T2′.

9. The method according to claim 8, wherein the further preform precursor is selected from SF.sub.6, BCl.sub.3, GeCl.sub.4 and POCl.sub.3.

10. The method according to claim 1, wherein one oxygen gas-containing carrier gas stream in step (a1) originates from a catalytic oxidation unit and a pressure reduction is carried out before the temperature is increased or decreased in step (a2).

11. Device for producing a preform for glass fibre production comprising: i) at least one carrier gas source which supplies carrier gas to two temperature control units, wherein the temperature control units are suitable for heating and/or cooling the carrier gas; ii) a mixing unit connected to the temperature control units for obtaining two carrier gas streams generated in the temperature control units, wherein the mixing unit is capable of mixing the two carrier gas streams to generate a mixed carrier gas stream, iii) a vaporizer with temperature control and pressure control and vaporizer inlet and vaporizer outlet, wherein the vaporizer is capable of receiving said mixed carrier gas stream directly from the mixing unit, wherein the vaporizer comprises a container comprising an immersion tube, iv) a heatable piping system between the vaporizer outlet and a mixing chamber, wherein the mixing chamber is configured to mix a loaded carrier gas stream with one or more further gas streams from at least one further gas source to obtain a mixed gas stream; and v) a reaction chamber for the production of a preform for glass fiber production.

12. The device according to claim 11, wherein a mass flow controller is present to control each of the two carrier gas streams into the mixing unit to set a target pressure and temperature of the mixed carrier gas stream.

13. The device according to claim 11, wherein the device does not have a mass flow controller.

14. The device according to claim 11, wherein a heatable piping system is provided between the mixing chamber and the reaction chamber.

15. The method according to claim 1, wherein the mixed gas stream comprises oxygen, halide vapor and additional gas(es) and is passed after the mixing chamber through a heated conduit system to the reaction chamber with the substrate tube.

16. The method according to claim 1, wherein the temperature T1 is less than the temperature T3.

17. The device according to claim 11, wherein the device does not have a device to control the pressure of the mixed carrier gas stream downstream of the vaporizer.

18. The device according to claim 11, wherein the mixing unit is arranged in a temperature-controllable unit so that a temperature of the mixed carrier gas stream can be kept constant.

Description

(1) The present disclosure will be further explained by means of figures:

(2) FIG. 1: Temperature ratios of the carrier gas or carrier gas/halide vapor mixture, with the following steps shown: 1: Providing a carrier gas temperature with desired, exact temperature T1 and mass flow ms1; 2: Carrier gas flows into evaporator; 3: Vaporizer liquid temperature T2, loaded with halide vapor of temperature T2 and mass flow ms2; 4: Carrier gas is loaded with halide vapor in the vaporizer; 5: carrier gas+halide vapor, mixing temperature at the vaporizer outlet (T3), mass flow ms1+ms2; 6: carrier gas and halide vapor flows from vaporizer into the following tempered reactive gas piping system; 7: temperature of piping system T4 after vaporizer outlet, condensation if temperature falls below dew point

(3) FIG. 2: Vapor pressure of GeCl4 as a function of temperature.

(4) FIG. 3: Schematic structure of an arrangement for the temperature control of the carrier gas stream for a reproducible loading in the vaporizer of preform production systems, where the following steps are shown: 1: Carrier gas in the temperature range of e.g. 15 to 40° C. (T0); 2: Gas temperature control to 10±3° C.; 3: Gas temperature control to 45±3° C.; 4: MFC 1; 5: MFC 2; 6: Temperature measurement and MFC control to e.g. 23±0.05° C.; 7: Vaporizer inlet

(5) FIG. 4: Sketch of the carrier gas stream according to the invention from the COU to the reaction site of the reactants in the substrate tube.

EXAMPLES

Example 1—Estimation of the Carrier Gas/Halide Vapor Temperature

(6) In the following an estimation of the carrier gas/halide vapor temperature for the case without gas/liquid interaction is carried out. A heating/cooling of the carrier gas when passing through the vaporizer liquid by interaction is neglected and only a mixing temperature of the thermally unaffected carrier gas with the halide vapor at the vaporizer temperature level is considered. Both the carrier gas and the halide vapor are considered as ideal gases. This is common in the temperature range of interest from 10 to 50° C. as well as in the pressure range of interest around 1 bar. 1000 sccm O.sub.2 are passed through the vaporizer (GeCl.sub.4 channel, vaporizer temperature 26° C., vaporizer pressure at vaporizer outlet 1125 mbar), temperature of the piping system from vaporizer outlet 27.0° C. to the mixing chamber. The following calculations are based on a reference time of 1 min in order to be able to make the various estimates.

(7) The further explanations are limited to the GeCl.sub.4 channel in the gas cabinet of a MCVD system. However, the contents of the explanations can also be transferred to the other vaporizer channels SiCl.sub.4 and POCl.sub.3.

(8) For the calculation of the molar volume under the different temperature and pressure conditions the equation from literature reference 3 was used.

(9) The loading of a carrier gas volume flow (in the example: O.sub.2) in the vaporizer system under the assumption of a complete loading is calculated according to literature reference 4:

(10) $\stackrel{.}{V}\left(\mathrm{GeCl}4\right)=\frac{\mathrm{pDD}*\stackrel{.}{V}\left(O2\right)}{\left(P\left(\mathrm{vaporizer}\right)-\mathrm{pDD}\right)}$ {dot over (V)} (GeCl4): GeCl4 volume flow {dot over (V)}(O2): Carrier oxygen volume flow pDD: Vapor pressure GeCl4 P(vaporizer): Pressure at vaporizer outlet

(11) The calculation of the mixing temperature is done according to literature reference 5:

(12) $T_M=\frac{m_1{c}_1{T}_1+m_2{c}_2{T}_2}{m_1{c}_1+m_2{c}_2}$

(13) Table 1 shows the calculated mixture temperatures TM at the vaporizer outlet assuming no heat exchange between carrier gas and vaporizer liquid for the different carrier gas temperatures.

(14) The table also shows how the temperature of the gas/vapor mixture and the GeCl.sub.4 content x depends on the carrier gas and vaporizer temperature:

(15) TABLE-US-00001 TABLE 1 Mass of the carrier gas O.sub.2 at different gas temperatures, mass of the loaded GeCl.sub.4 vapor at two different vaporizer temperatures and a uniform vaporizer pressure of 1125 mbar, mixing temperature of the carrier gas/GeCl.sub.4 vapor mixture with neglected thermal interaction. Tem- perature Tem- Vaporizer gas/vapor x = perature Gas mass temperature vapor mass mixture m(GeCl4)/ O2 (° C.) O2 (g) GeCl4 (° C.) GeCl4 (g) (° C.) m(O2) 40 1.383 26 1.213 34.40 0.878 30 1.428 26 1.213 28.43 0.850 26 1.447 26 1.213 26.00 0.838 20 1.477 26 1.213 22.31 0.822 10 1.529 26 1.213 16.02 0.794 40 1.383 36 1.935 37.94 1.400 36 1.401 36 1.935 36.00 1.382 30 1.428 36 1.935 33.04 1.355 20 1.477 36 1.935 27.98 1.310 10 1.529 36 1.935 22.75 1.265

(16) The PFA piping system at the outlet of the vaporizer within the vaporizer chamber is, for example, tempered to only 27° C. Since the mixture temperature of the carrier oxygen/GeCl.sub.4 vapor mixture is sometimes significantly higher than the temperature of 27.0° C. of the line system connected after the vaporizer outlet, the carrier gas/vapor mixture will continue to cool down until the mixing chamber. To quantify the risk of condensation, the dew point at the vaporizer outlet is now calculated.

(17) The carrier gas can be loaded with different amounts of halide vapor depending on the carrier gas temperature. The dew point temperature is a measure of the temperature at which the halide begins to condense. At the dew point, the carrier gas is by definition saturated with vapor.

(18) The gas/vapor mixture as a function of the GeCl.sub.4 content x and the pressure at the vaporizer outlet P is described by the following equation (A, B, C are coefficients of the GeCl4 vapor pressure formula according to literature reference 1):

(19) $T\tau =\frac{B}{A-\log 10\left[\frac{x*P}{\left(6,702+x\right)}\right]}-C$ Tτ: Dew point temperature (in K) A: 3.68225 B: 1080.101 C: −63.588 P: Pressure at vaporizer outlet (in bar) x: GeCl.sub.4 vapor content (x=mGeCl.sub.4/mO.sub.2)

(20) The equation was derived using the system of formulas in literature reference 6 and the vapor pressure formula from literature reference 1.

(21) If the gas/vapor mixture is cooled below the dew point temperature, condensation of the GeCl.sub.4 vapor occurs, which leads to uncontrolled doping and thus refractive index fluctuations in the deposited core layers.

(22) The following table shows the dependence of the dew point temperature at the vaporizer outlet on the GeCl.sub.4 content x of the vapor-gas mixture and different pressures P at the vaporizer outlet.

(23) TABLE-US-00002 TABLE 2 expected condensation effects at the outlet of the vaporizer system at a gas/vapor transport pipe system temperature of 27° C. and at different GeCl4 contents x caused by different carrier gas temperatures and vaporizer temperatures at two different vaporizer pressures P. Condensation at Vaporizer temp. of piping x = pressure P system after m(GeCl4)/m(O2) at outlet (mbar) Tτ (° C.) vaporizer at 27.0° C. 0.8 1125 25.07 No 0.9 1125 27.41 Yes 1.0 1125 29.51 Yes 1.1 1125 31.42 Yes 1.2 1125 33.17 Yes 1.3 1125 34.78 Yes 0.8 1000 22.49 No 0.9 1000 24.78 No 1.0 1000 26.83 No 1.1 1000 28.70 Yes 1.2 1000 30.41 Yes 1.3 1000 31.98 Yes

(24) The method according to the invention is described below with reference to FIG. 4:

(25) After the carrier gas has left the catalytic oxidation unit (COU), it is fed to the gas cabinet without temperature control. There the pressure is reduced and regulated to 2 bar. After the pressure reducer and pressure regulator, the carrier gas usually has a temperature between 15 and 40° C. The carrier gas is then fed into one or more channels to produce a main gas and optionally one or more process gases. According to step (a), the carrier gas is then tempered before entering the vaporizer at a temperature lower than the temperature of the loaded carrier gas stream at the vaporizer outlet. The carrier gas is introduced into the vaporizer liquid through a tube in the vaporizer to be loaded with the precursor(s). Then the main gas and the additional gases are mixed without falling below the dew point. The mixed gas can be fed into the reaction chamber to produce the preform.

CITED REFERENCES

Reference 1

(26) http://webbook.nist.gov/cgi/cbook.cgi?ID=C10038989&Units=SI&Mask=4#Thermo-Phase

Reference 2

(27) http://www.dockweiler.com/fileadmin/user_upload/PDF/Broschueren/DW_Bubbler_EN.pdf)

Reference 3

(28) https://de.wikipedia.org/wiki/Molares_Volumen

Reference 4

(29) M. Minami, Vapor concentration control system for bubbling method, Horiba Technical Reports No. 41, 2013

Reference 5

(30) http://www.physik.uni-halle.de/Fachgruppen/bio/Lehre/exphysbiochem/Waerme.pdf, page 4

Reference 6

(31) http://www.wikiwand.com/de/Taupunkt U.S. Pat. Nos. 4,235,829, 4,220,460, 4,276,243, 4,582,480, 6,135,433, 7,011,299, DE 69922728, U.S. Pat. No. 4,412,853 R. Nagel et al. “An Overview of the Modified Chemical Vapor Deposition (MCVD) Process and Performance”, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 4, April 1982