GLASS FIBRES AND PRE-FORMS MADE OF HOMOGENEOUS QUARTZ GLASS

Abstract

One aspect relates to a light guide comprising a jacket and one or more cores, wherein the jacket surrounds the cores. Each core has a refractive index profile perpendicular to the maximum extension of the core, wherein at least one refractive index n.sub.K of each refractive index profile is greater than the refractive index n.sub.M1 of the jacket. The jacket is made of silicon dioxide and has an OH content of less than 10 ppm, a chlorine content of less than 60 ppm, and an aluminium content of less than 200 ppb. One aspect also relates to a silicon dioxide granulate I, characterized by a chlorine content of less than 200 ppm and an aluminium content of less than 200 ppb, in each case based on the total weight of the silicon dioxide granulate I.

Claims

1-24. (canceled)

25. A light guide comprising: a jacket; and one or more cores; wherein the jacket surrounds the cores; wherein each core has a refractive index profile perpendicular to the maximum core extension, wherein at least one refractive index n.sub.K of each refractive index profile is greater than the refractive index n.sub.M1 of the jacket; wherein the jacket is made of silicon dioxide and comprises: an OH content of less than 10 ppm; a chlorine content of less than 60 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the jacket M1.

26. The light guide according to claim 25, comprising two or more cores, wherein the jacket surrounds the cores as a matrix.

27. The light guide according to claim 25, wherein the jacket comprises at least one of: an ODC content of less than 510.sup.15/cm.sup.3; a metal content of metals different to aluminium of less than 1 ppm; a viscosity (p=1013 hPa) in a range from log.sub.10 ((1200 C.)/dPas)=13.4 to log.sub.10 ((1200 C.)/dPas)=13.9 or log.sub.10 ((1300 C.)/dPas)=11.5 to log.sub.10 ((1300 C.)/dPas)=12.1 or log.sub.10 ((1350 C.)/dPas)=1.2 to log.sub.10 ((1350 C.)/dPas)=10.8; a curl parameter of more than 6 m; a standard deviation of the OH content of not more than 10%, based on the OH content of the jacket; a standard deviation of the Cl content of not more than 10%, based on the Cl content of the jacket; a standard deviation of the Al content of not more than 10%, based on the Al content of the jacket; a refractive index homogeneity of less than 110.sup.4; and a transformation point T.sub.g in a range from 1150 to 1250 C., wherein the ppb and ppm are each based on the total weight of the jacket.

28. The light guide according to claim 25, wherein the content by weight of the jacket is at least 60 wt.-%, based on the total weight of the cores and the jacket.

29. A silicon dioxide granulate I comprising: a chlorine content of less than 200 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the silicon dioxide granulate I.

30. The silicon dioxide granulate I according to claim 29 further comprising at least one of the following: a metal content of metals which are different to aluminium of less than 1000 ppb; a BET surface area in a range from 20 to 50 m.sup.2/g; a pore volume in a range from 0.1 to 1.5 mL/g; a residual moisture content of less than 10 wt.-%; a bulk density in a range from 0.5 to 1.2 g/cm.sup.3; a tamped density in a range from 0.5 to 1.2 g/cm.sup.3; a carbon content of less than 50 ppm; a particle size distribution D.sub.10 in a range from 50 to 150 m; a particle size distribution D.sub.50 in a range from 150 to 300 m; and a particle size distribution D.sub.90 in a range from 250 to 620 m, wherein the wt.-%, ppb and ppm are each based on the total weight of the silicon dioxide granulate I.

31. A process for the preparation of a silicon dioxide granulate I, at least comprising: providing a silicon dioxide powder, wherein the silicon dioxide powder comprises: a chlorine content of less than 200 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the silicon dioxide powder; providing a liquid; mixing the components of steps I. and II. to obtain a slurry; and spray drying the slurry to obtain a silicon dioxide granulate I.

32. The process according to claim 31, wherein the silicon dioxide powder comprises at least one of the following: a BET surface area in a range from 20 to 60 m.sup.2/g; a bulk density in a range from 0.01 to 0.3 g/cm.sup.3; a carbon content of less than 50 ppm; a total content of metals which are different to aluminium of less than 5 ppm; at least 70 wt.-% of the powder particles have a primary particles size in a range from 10 to 100 nm; a tamped density in a range from 0.001 to 0.3 g/cm.sup.3; a residual moisture content of less than 5 wt.-%; a particle size distribution Dio in the range from 1 to 7 m; a particle size distribution Dso in the range from 6 to 15 m; and a particle size distribution D90 in the range from 10 to 40 m; wherein the wt.-%, ppb and ppm are each based on the total weight of the silicon dioxide powder.

33. A silicon dioxide granulate I obtainable by the process according to claim 31.

34. A silicon dioxide granulate II comprising: a chlorine content of less than 500 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the silicon dioxide granulate II.

35. The silicon dioxide granulate II according to claim 34 comprising at least one of the following: a metal content of metals which are different to aluminium of less than 1000 ppb; a BET surface area in a range from 10 to 35 m.sup.2/g; a pore volume in a range from 0.1 to 2.5 m.sup.2/g; a residual moisture content of less than 3 wt.-%; a bulk density in a range from 0.7 to 1.2 g/cm.sup.3; a tamped density in a range from 0.7 to 1.2 g/cm.sup.3; a particle size D.sub.50 in a range from 150 to 250 m; and a carbon content of less than 5 ppm, wherein the wt.-%, ppb and ppm are each based on the total weight of the silicon dioxide granulate II.

36. A process for the preparation of silicon dioxide granulate II, comprising: providing silicon dioxide granulate I; and treating the silicon dioxide granulate Ito obtain silicon dioxide granulate II.

37. A silicon dioxide granulate II obtainable by the process according to claim 36.

38. A process for the preparation of a quartz glass body, comprising: providing silicon dioxide granulate II; making a glass melt from the silicon dioxide granulate II; and making a quartz glass body from at least a part of the glass melt.

39. The process according to claim 38, further comprising: making a hollowing body with at least one opening from the quartz glass body.

40. A quartz glass body obtainable by the process according to claim 38.

41. A quartz glass body, wherein the quartz glass body is made of silicon dioxide, wherein the silicon dioxide comprises: an OH content of less than 10 ppm; a chlorine content of less than 60 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the quartz glass body.

42. The quartz glass body according to claim 41, further comprising at least one of the following: an ODC content of less than 510.sup.15/cm.sup.3; a metal content of metals which are different to aluminium of less than 300 ppb; a viscosity (p=1013 hPa) in a range from log.sub.10 ((1200 C.)/dPas)=13.4 to log.sub.10 ((1200 C.)/dPas)=13.9 or log.sub.10 ((1300 C.)/dPas)=11.5 to log.sub.10 ((1300 C.)/dPas)=12.1 or log.sub.10 ((1350 C.)/dPas)=1.2 to log.sub.10 ((1350 C.)/dPas)=10.8; a standard deviation of the OH content of not more than 10%, based on the OH content of the quartz glass body; a standard deviation of the Cl content of not more than 10%, based on the Cl content of the quartz glass body; a standard deviation of the Al content of not more than 10%, based on the Al content of the quartz glass body; a refractive index homogeneity of less than 110.sup.4; a cylindrical form; a transformation temperature T.sub.g in a range from 1150 to 1250 C.; and a fictive temperature in a range from 1055 to 1200 C.; wherein the ppb and ppm are each based on the total weight of the quartz glass body.

43. A process for the preparation of a light guide, comprising: providing one of: a hollow body with at least one opening obtainable by the process according to claim 39; and a quartz glass body according to claim 41, wherein the quartz glass body is first processed obtaining a hollow body with at least one opening; introducing one or multiple core rods into the quartz glass body through the at least one opening to obtain a precursor; and drawing the precursor in the warm to obtain the light guide with one or more cores and a jacket.

44. A light guide obtainable by the process of claim 43.

45. The light guide according to claim 44, wherein the light guide has a jacket and one or multiple cores, wherein the jacket surrounds the cores, wherein each core has a refractive index profile perpendicular to the maximum extension of the core, wherein at least one refractive index n.sub.K of each refractive index profile is greater than the refractive index n.sub.M1 of the jacket; wherein the jacket is made of silicon dioxide and comprises an OH content of less than 10 ppm; and a chlorine content of less than 60 ppm; and an aluminium content of less than 200 ppb; wherein the ppb and ppm are each based on the total weight of the jacket.

46. A light guide cable comprising at least two light guides according to one of claim 25.

47. A process for the preparation of a light guide cable comprising: providing at least two light guides according to claim 25; and processing the at least two light guides from to obtain a light guide cable.

48. A use of the silicon dioxide granulate according to claim 29 for the preparation of products selected from the group consisting of jacket materials for optical fibres, light guides and light guide cables.

Description

FIGURES

[0649] FIG. 1 flow diagram (process for the preparation of a quartz glass body)

[0650] FIG. 2 flow diagram (process for the preparation of a silicon dioxide granulate I)

[0651] FIG. 3 flow diagram (process for the preparation of a silicon dioxide granulate II)

[0652] FIG. 4 schematic representation of a spray tower

[0653] FIG. 5 schematic representation of a cross section of a light guide

[0654] FIG. 6 schematic representation of a top view of a light guide

[0655] FIG. 7 schematic representation of a gas pressure sinter oven (GDS oven)

DESCRIPTION OF THE FIGURES

[0656] FIG. 1 shows a flow diagram containing the steps 101 to 104 of a process 100 for the preparation of a quartz glass body according to the present invention. In a first step 101, a silicon dioxide granulate is provided. In a second step 102, a glass melt is made from the silicon dioxide granulate.

[0657] Preferably, for the melting, moulds are employed which can be introduced into and removed from an oven. Such moulds are often made from graphite. They provide a negative form of the casting. The silicon dioxide granulate is filled into the mould in the third step 103 and first brought to melting in the mould. Subsequently, the quartz glass body forms in the mould by cooling the melt. This is then freed from the mould and processed further, for example in an optional step 104. This procedure is discontinuous. The formation of the melt is preferably performed at reduced pressure, in particular in a vacuum. It is also possible, during step 103, to intermittently charge the oven with a reducing, hydrogen containing atmosphere.

[0658] In another procedure, the crucible is preferably a hanging or standing crucible. In this case, the silicon dioxide granulate is introduced into the melting crucible and warmed therein until a glass melt is formed. In this case, the melting is preferably performed in a reducing hydrogen containing atmosphere. In a third step 103, a quartz glass body is formed. The forming of the quartz glass body is performed here by removing at least part of the glass melt from the crucible and cooling, for example through a nozzle on the underside of the of the crucible. In this case, the form of the quartz glass body can be determined by the layout of the nozzle. In this way, solid bodies can be obtained for example. Hollow bodies are obtained for example, if a mandrel is additionally present in the nozzle. This exemplary process for the preparation of quartz glass bodies, and in particular step 103, is preferably performed continuously. In an optional step 104, a hollow body can be formed from a solid quartz glass body.

[0659] FIG. 2 shows a flow diagram containing the steps 201, 202 and 203 of a process 200 for the preparation of a silicon dioxide granulate I. In a first step 201, a silicon dioxide powder is provided. A silicon dioxide powder is preferably obtained from a synthetic process in which a silicon containing material, for example a siloxane, a silicon alkoxide or a silicon halogenide is converted into silicon dioxide in a pyrogenic process. In a second step 202, the silicon dioxide powder is mixed with a liquid, preferably with water, to obtain a slurry. In a third step 203, the silicon dioxide contained in the slurry is transformed into a silicon dioxide granulate. The granulation is performed by spray granulation. For this, the slurry is sprayed through a nozzle into a spray tower and dried to obtain granules, wherein the contact surface between the nozzle and the slurry comprises a glass or a plastic.

[0660] FIG. 3 shows a flow diagram containing the steps 301, 302, 303 and 304 of a process 300 for the preparation of a silicon dioxide granulate II. The steps 301, 302 and 303 proceed corresponding to the steps 201, 202 and 203 according to FIG. 2. In step 304, the silicon dioxide granulate I obtained in step 303 is processed to obtain a silicon dioxide granulate II. This is preferably performed by warming the silicon dioxide granulate I in a chlorine containing atmosphere.

[0661] In FIG. 4 is shown a preferred embodiment of a spray tower 1100 for spray granulating silicon dioxide. The spray tower 1100 comprises a feed 1101 through which a pressurised slurry containing silicon dioxide powder and a liquid are fed into the spray tower. At the end of the pipeline is a nozzle 1102 through which the slurry is introduced into the spray tower as a finely spread distribution. Preferably, the nozzle slopes upward, so that the slurry is sprayed into the spray tower as fine droplets in the nozzle direction and then falls down in an arc under the influence of gravity. At the upper end of the spray tower there is a gas inlet 1103. By introduction of a gas through the gas inlet 1103, a gas flow is created in the opposite direction to the exit direction of the slurry out of the nozzle 1102. The spray tower 1100 also comprises a screening device 1104 and a sieving device 1105. Particles which are smaller than a defined particle size are extracted by the screening device 1104 and removed through the discharge 1106. The extraction strength of the screening device 1104 can be configured to correspond to the particle size of the particles to be extracted. Particles above a defined particle size are sieved off by the sieving device 1105 and removed through the discharge 1107. The sieve permeability of the sieving device 1105 can be selected to correspond to the particle size to be sieved off. The remaining particles, a silicon dioxide granulate having the desired particle size, is removed through the outlet 1108.

[0662] In FIG. 5 is shown a schematic cross section through a light guide 1200 according to the invention which has a core 1201 and a jacket M1 1202 which surrounds the core 1201.

[0663] FIG. 6 shows schematically a top view of a guide 1300 which has cable structure. In order to represent the arrangement of the core 1301 and the jacket M1 1302 around the core 1301, a part of the core 1301 is shown without the jacket M1 1302. Typically however, the core 1301 is sheathed over its entire length by the jacket M1 1302.

[0664] FIG. 7 shows a preferred embodiment of an oven 1500 which is suitable for a vacuum sinter process, a gas pressure sinter process and in particular a combination thereof. The oven has, starting from the outside and going inward, a pressure resistant jacket 1501 and a thermally isolating layer 1502. The space which is surrounded thereby, also referred to as oven interior, can be charged via a gas feeder 1504 with a gas or with a gas mixture. Further, the oven interior is provided with a gas outlet 1505, through which the gas can be removed. According to the relationship in the gas transport between gas feeding 1504 and gas removal at 1505, an over pressure, a vacuum or also a gas flow can be produced in the interior of the oven 1500. Further, heating elements 1506 are provided in the interior of the oven 1500. These are often placed on the isolating layer 1502 (not shown here). For protecting the melt material from contamination, the interior of the oven is provided with a so called liner 1507 which separates the oven chamber 1503 from the heating elements 1506. Melt moulds 1508 with melt material 1509 can be introduced into the oven chamber 1503. The melt mould 1508 can be open on one side (shown here) or can entirely surround the melt material 1509 (not shown).

[0665] Test Methods

[0666] a. Fictive Temperature

[0667] The fictive temperature is measured by Raman spectroscopy using the Raman scattering intensity at about 606 cm.sup.1. The procedure and analysis described in the contribution of Pfleiderer et. al.; The UV-induced 210 nm absorption band in fused Silica with different thermal history and stoichiometry; Journal of Non-Crystalline Solids, volume 159 (1993), pages 145-153.

[0668] b. OH Content

[0669] The OH content of the glass is measured by infrared spectroscopy. The method of D. M. Dodd & D. M. Fraser Optical Determinations of OH in Fused Silica (J.A.P. 37, 3991 (1966)) is employed. Instead of the device named therein, an FTIR-spectrometer (Fourier transform infrared spectrometer, current System 2000 of Perkin Elmer) is employed. The analysis of the spectra can in principle be performed on either the absorption band at ca. 3670 cm.sup.1 or on the absorption band at ca. 7200 cm.sup.1. The selection of the band is made on the basis that the transmission loss through OH absorption is between 10 and 90%.

[0670] c. Oxygen Deficiency Centres (ODCs)

[0671] For the quantitative detection, the ODC(I) absorption is measured at 165 nm by means of a transmission measurement at samples with thickness between 1-2 mm using a vacuum UV spectrometer, model VUVAS 2000, of McPherson, Inc. (USA).

[0672] Then:

N=/

with [0673] N=defect concentration [1/cm.sup.3] [0674] =optical absorption [1/cm, base e] of the ODC(I) band [0675] =effective cross section [cm.sup.2]

[0676] wherein the effective cross section is set to =7.5.Math.10.sup.17cm.sup.2 (from L. Skuja, Color Centres and Their Transformations in Glassy SiO.sub.2, Lectures of the summer school Photosensitivity in optical Waveguides and glasses, Jul. 13-18 1998, Vitznau, Switzerland).

[0677] d. Elemental Analysis

[0678] d-1) Solid samples are crushed. Then, ca. 20 g of the sample is cleaned by introducing it into a HF-resistant vessel fully, covering it with HF and thermally treating at 100 C. for an hour. After cooling, the acid is discarded and the sample cleaned several times with high purity water. Then, the vessel and the sample are dried in the drying cabinet.

[0679] Next, ca. 2 g of the solid sample (crushed material cleaned as above; dusts etc. directly without pre-treatment) is weighed into an HF resistant extraction vessel and dissolved in 15 ml HF (50 wt.-%). The extraction vessel is closed and thermally treated at 100 C. until the sample is completely dissolved. Then, the extraction vessel is opened and further thermally treated at 100 C., until the solution is completely evaporated. Meanwhile, the extraction vessel is filled 3 with 15 ml of high purity water. 1 ml HNO.sub.3 is introduced into the extraction vessel, in order to dissolve separated impurities and filled up to 15 ml with high purity water. The sample solution is then ready. d-2) ICP-MS/ICP-OES Measurement

[0680] Whether OES or MS is employed depends on the expected elemental concentrations. Typically, measurements of MS are 1 ppb, and for OES they are 10 ppb (in each case based on the weighed sample). The measurement of the elemental concentration with the measuring device is performed according to the stipulations of the device manufacturer (ICP-MS: Agilent 7500ce; ICP-OES: Perkin Elmer 7300 DV) and using certified reference liquids for calibration. The elemental concentrations in the solution (15 ml) measured by the device are then converted based on the original weight of the sample (2 g).

[0681] Note: It is to be kept in mind that the acid, the vessels, the water and the devices must be sufficiently pure in order to measure the elemental concentrations in question. This is checked by extracting a blank sample without quartz glass.

[0682] The following elements are measured in this way: Li, Na, Mg, K, Ca, Fe, Ni, Cr, Hf, Zr, Ti, (Ta), V, Nb, W, Mo, Al.

[0683] d-3) The measurement of samples present as a liquid is carried out as described above, wherein the sample preparation according to step d-1) is skipped. 15 ml of the liquid sample are introduced into the extraction flask. No conversion based on the original sample weight is made.

[0684] e. Determination of Density of a Liquid

[0685] For measuring the density of a liquid, a precisely defined volume of the liquid is weighed into a measuring device which is inert to the liquid and its constituents, wherein the empty weight and the filled weight of the vessel are measured. The density is given as the difference between the two weight measurements divided by the volume of the liquid introduced.

[0686] f. Fluoride Detection

[0687] 15 g of a quartz glass sample is crushed and cleaned by treating in nitric acid at 70 C. The sample is then washed several times with high purity water and then dried. 2 g of the sample is weighed into a nickel crucible and covered with 10 g Na.sub.2CO.sub.3 and 0.5 g ZnO. The crucible is closed with a Ni-lid and roasted at 1000 C. for an hour. The nickel crucible is then filled with water and boiled up until the melt cake has dissolved entirely.

[0688] The solution is transferred to a 200 ml measuring flask and filled up to 200 ml with high purity water. After sedimentation of undissolved constituents, 30 ml are taken and transferred to a 100 ml measuring flask, 0.75 ml of glacial acetic acid and 60 ml TISAB are added and filled up with high purity water. The sample solution is transferred to a 150 ml glass beaker.

[0689] The measurement of the fluoride content in the sample solution is performed by means of an ion sensitive (fluoride) electrode, suitable for the expected concentration range, and display device as stipulated by the manufacturer, here a fluoride ion selective electrode and reference electrode F-500 with R503/D connected to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH. With the fluoride concentration in the solution, the dilution factor and the sample weight, the fluoride concentration in the quartz glass is calculated.

[0690] g. Determination of Chlorine (>50 ppm)

[0691] 15 g of a quartz glass sample is crushed and cleaned by treating with nitric acid at ca. 70 C. Subsequently, the sample is rinsed several times with high purity water and then dried. 2 g of the sample are then filled into a PTFE-insert for a pressure container, 15 ml NaOH (c=10 mol/l) are added, closed with a PTFE-lid and placed in the pressure container This is closed and thermally treated at ca. 155 C. for 24 hours. After cooling, the PTFE-insert is removed and the solution is transferred entirely to a 100 ml measuring flask. There, 10 ml HNO.sub.3 (65 wt.-%) and 15 ml acetate buffer are added, allowed to cool and filled to 100 ml with high purity water. The sample solution is transferred to a 150 ml glass beaker. The sample solution has a pH value in the range between 5 and 7.

[0692] The measurement of the chloride content in the sample solution is performed by means of an ion sensitive (Chloride) electrode which is suitable for the expected concentration range, and a display device as stipulated by the manufacturer, here an electrode of type C1-500 and a reference electrode of type R-503/D attached to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH.

[0693] h. Chlorine Content (<50 ppm)

[0694] Chlorine contents <50 ppm up to 0.1 ppm in quartz glass are measured by neutron activation analysis (NAA). For this, 3 bores, each of 3 mm diameter and 1 cm length are taken from the quartz glass body under investigation. These are given to a research institute for analysis, in this case to the institute for nuclear chemistry of the Johannes-Gutenberg University in Mainz, Germany. In order to exclude contamination of the sample with chlorine, a thorough cleaning of the sample in an HF bath on location directly before the measurement was arranged. Each bore is measured several times. The results and the bores are then sent back by the research institute.

[0695] i. Optical Properties

[0696] The transmission of quartz glass samples is measured with the commercial grating- or FTIR-spectrometer from Perkin Elmer (Lambda 900 [190-3000 nm] or System 2000 [1000-5000 nm]). The selection is determined by the required measuring range.

[0697] For measuring the absolute transmission, the sample bodies are polished on parallel planes (surface roughness RMS<0.5 nm) and the surface is cleared off all residues by ultrasound treatment. The sample thickness is 1 cm. In the case of an expected strong transmission loss due to impurities, dopants etc., a thicker or thinner sample can be selected in order to stay within the measuring range of the device. A sample thickness (measuring length) is selected at which only slight artefacts are produced on account of the passage of the radiation through the sample and at the same time a sufficiently detectable effect is measured.

[0698] The measurement of the opacity, the sample is placed in front of an integrating sphere. The opacity is calculated using the measured transmission value T according to the formula: O=1/T=I.sub.0/I.

[0699] j. Refractive Index and Refractive Index Profile in a Tube or Rod

[0700] The refractive index profile of tubes/rods can be characterised by means of a York Technology Ltd. Preform Profiler P102 or P104. For this, the rod is placed lying in the measuring chamber the chamber is closed tight. The measuring chamber is then filled with an immersion oil which has a refractive index at the test wavelength of 633 nm which is very similar to that of the outermost glass layer at 633 nm. The laser beam then goes through the measuring chamber. Behind the measuring chamber (in the direction of the radiation) is mounted a detector which measures the angle of deviation (of the radiation entering the measuring chamber compared to the radiation exiting the measuring chamber). Under the assumption of radial symmetry of the refractive index profile of the rod, the diametric refractive index profile can be reconstructed by means of an inverse Abel transformation. These calculations are performed by the software of the device manufacturer York.

[0701] The refractive index of a sample is measured with the York Technology Ltd. Preform Profiler P104 analogue to the above description. In the case of isotropic samples, measurement of therefractive index profile gives only one value, the refractive index.

[0702] k. Carbon Content

[0703] The quantitative measurement of the surface carbon content of silicon dioxide granulate and silicon dioxide powder is performed with a carbon analyser RC612 from Leco Corporation, USA, by the complete oxidation of all surface carbon contamination (apart from SiC) with oxygen to obtain carbon dioxide. For this, 4.0 g of a sample are weighed and introduced into the carbon analyser in a quartz glass dish. The sample is bathed in pure oxygen and heated for 180 seconds to 900 C. The CO.sub.2 which forms is measured by the infrared detector of the carbon analyser. Under these measuring conditions, the detection limit lies at 1 ppm (weight-ppm) carbon.

[0704] A quartz glass boat which is suitable for this analysis using the above named carbon analyser is obtainable as a consumable for the LECO analyser with LECO number 781-335 on the laboratory supplies market, in the present case from Deslis Laborhandel, Flurstrae 21, D-40235 Dusseldorf (Germany), Deslis-No. LQ-130XL. Such a boat has width/length/height dimensions of ca. 25 mm/60 mm/15 mm. The quartz glass boat is filled up to half its height with sample material. For silicon dioxide powder, a sample weight of 1.0 g sample material can be reached. The lower detection limit is then <1 weight ppm carbon. In the same boat, a sample weight of 4 g of a silicon dioxide granulate is reached for the same filling height (mean particle size in the range from 100 to 500 m). The lower detection limit is then about 0.1 weight ppm carbon. The lower detection limit is reached when the measurement surface integral of the sample is not greater than three times the measurement surface integral of an empty sample (empty sample=the above process but with an empty quartz glass boat).

[0705] 1. Curl Parameter

[0706] The curl parameter (also called: Fibre Curl) is measured according to DIN EN 60793-1-34:2007-01 (German version of the standard IEC 60793-1-34:2006). The measurement is made according to the method described in Annex A in the sections A.2.1, A.3.2 and A.4.1 (extrema technique).

[0707] m. Attenuation

[0708] The attenuation is measured according to DIN EN 60793-1-40:2001 (German version of the standard IEC 60793-1-40:2001). The measurement is made according to the method described in the annex (cut-backmethod) at a wavelength of ==1550 nm.

[0709] n. Viscosity of the Slurry

[0710] The slurry is set to a concentration of 30 weight-% solids content with demineralised water (Direct-Q 3UV, Millipore, Water quality: 18.2 Mcm). The viscosity is then measured with a MCR102 from Anton-Paar. For this, the viscosity is measured at 5 rpm. The measurement is made at a temperature of 23 C. and an air pressure of 1013 hPa.

[0711] o. Thixotropy

[0712] The concentration of the slurry is set to a concentration of 30 weight-% of solids with demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm). The thixotropy is then measured with an MCR102 from Anton-Paar with a cone and plate arrangement. The viscosity is measured at 5 rpm and at 50 rpm. The quotient of the first and the second value gives the thixotropic index. The measurement is made at a temperature of 23 C.

[0713] p. Zeta Potential of the Slurry

[0714] For zeta potential measurements, a zeta potential cell (Flow Cell, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is set to 7 through addition of HNO.sub.3 solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The measurement is made at a temperature of 23 C.

[0715] q. Isoelectric Point of the Slurry

[0716] The isoelectric point, a zeta potential measurement cell (Flow Cell, Beckman Coulter) and an auto titrator (DelsaNano AT, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is varied by adding HNO.sub.3 solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The isoelectric point is the pH value at which the zeta potential is equal to 0. The measurement is made at a temperature of 23 C.

[0717] r. pH Value of the Slurry

[0718] The pH value of the slurry is measured using a WTW 3210 from Wissenschaftlich-Technische-Werksttten GmbH. The pH 3210 Set 3 from WTW is employed as electrode. The measurement is made at a temperature of 23 C.

[0719] s. Solids Content

[0720] A weighed portion m.sub.1 of a sample is heated for 4 hours to 500 C. reweighed after cooling (m.sub.2). The solids content w is given as m.sub.2/m.sub.1*100

[0721] t. Bulk Density

[0722] The bulk density is measured according to the standard DIN ISO 697:1984-01 with an SMG 697 from Powtec. The bulk material (silicon dioxide powder or granulate) does not clump.

[0723] u. Tamped Density (Granulates)

[0724] The tamped density is measured according to the standard DIN ISO 787:1995-10.

[0725] v. Measurement of the Pore Size Distribution

[0726] The pore size distribution is measured according to DIN 66133 (with a surface tension of 480 mN/m and a contact angle of 140). For the measurement of pore sizes smaller than 3.7 nm, the Pascal 400 from Porotec is used. For the measurement of pore sizes from 3.7 nm to 100 m, the Pascal 140 from Porotec is used. The sample is subjected to a pressure treatment prior to the measurement. For this a manual hydraulic press is used (Order-Nr. 15011 from Specac Ltd., River House, 97 Cray Avenue, Orpington, Kent BR5 4HE, U.K.). 250 mg of sample material is weighed into a pellet die with 13 mm inner diameter from Specac Ltd. and loaded with 1 t, as per the display. This load is maintained for 5 s and readjusted if necessary. The load on the sample is then released and the sample is dried for 4 hat 1052 C. in a recirculating air drying cabinet.

[0727] The sample is weighed into the penetrometer of type 10 with an accuracy of 0.001 g and in order to give a good reproducibility of the measurement it is selected such that the stem volume used, i.e. the percentage of potentially used Hg volume for filling the penetrometer is in the range between 20% to 40% of the total Hg volume. The penetrometer is then slowly evacuated to 50 m Hg and left at this pressure for 5 min. The following parameters are provided directly by the software of the measuring device: total pore volume, total pore surface area (assuming cylindrical pores), average pore radius, modal pore radius (most frequently occurring pore radius), peak n. 2 pore radius (m).

[0728] w. Primary Particle Size

[0729] The primary particle size is measured using a scanning electron microscope (SEM) model Zeiss Ultra 55. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm), to obtain an extremely dilute suspension. The suspension is treated for 1 min with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) and then applied to a carbon adhesive pad.

[0730] x. Mean Particle Size in Suspension

[0731] The mean particle size in suspension is measured using a Mastersizer 2000, available from Malvern Instruments Ltd., UK, according to the user manual, using the laser deflection method. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL suspension with a concentration of 1 g/L. The suspension is treated with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) for 1 min.

[0732] y. Particle Size and Grain Size of the Solid

[0733] The particle size and grain size of the solid are measured using a Camsizer XT, available from Retsch Technology GmbH, Deutschland according to the user manual The software gives the D.sub.10, D.sub.50 and D.sub.90 values for a sample.

[0734] z. BET Measurement

[0735] For the measurement of the specific surface area, the static volumetric BET method according to DIN ISO 9277:2010 is used. For the BET measurement, a NOVA 3000 or a Quadrasorb (available from Quantachrome), which operate according to the SMART method (Sorption Method with Adaptive Dosing Rate), is used. The micro pore analysis is performed using the t-plot process (p/p.sub.0=0.1-0.3) and the meso pore analysis is performed using the MBET process (p/p.sub.0=0.0-0.3). As reference material, the standards alumina SARM-13 and SARM-214, available from Quantachrome are used. The tare weight of the measuring cell (clean and dry) is weighed. The type of measuring cell is selected such that the sample material which is introduced and the filler rod fill the measuring cell as much as possible and the dead space is reduced to a minimum. The sample material is introduced into the measuring cell. The amount of sample material is selected so that the expected value of the measurement value corresponds to 10-20 m.sup.2/g. The measuring cells are fixed in the roasting positions of the BET measuring device (without filler rod) and evacuated to <200 mbar. The speed of the evacuation is set so that no material leaks from the measuring cell Baking is performed in this state at 200 C. for lh. After cooling, the measuring cell filled with the sample is weighed (raw value). The tare weight is then subtracted from the raw value of the weight=nett weight=weight of the sample. The filling rod is then introduced into the measuring cell, this is again fixed at the measuring location of the BET measuring device. Prior to the start of the measurement, the sample identifications and the sample weights are entered into the software. The measurement is started. The saturation pressure of nitrogen gas (N2 4.0) is measured. The measuring cell is evacuated and cooled down to 77 K using a nitrogen bath. The dead space is measured using helium gas (He 4.6). The measuring cell is evacuated again. A multi point analysis with at least 5 measuring points is performed. N2 4.0 is used as absorptive. The specific surface area is given in m.sup.2/g.

[0736] za. Viscosity of Glass Bodies

[0737] The viscosity of the glass is measured using the beam bending viscosimeter of type 401from TA Instruments with the manufacturer's software WinTA (current version 9.0) in Windows 10 according to the DIN ISO 7884-4:1998-02 standard. The support width between the supports is 45 mm. Sample rods with rectangular cross section are cut from regions of homogeneous material (top and bottom sides of the sample have a finish of at least 1000 grain). The sample surfaces after processing have a grain size=9 m & RA=0.15 m. The sample rods have the following dimensions: length=50 mm, width=5 mm & height=3 mm (assignment: length, width, height as in the standards document). Three samples are measured and the mean is calculated. The sample temperature is measured using a thermocouple tight against the sample surface. The following parameters are used: heating rate=25 K up to a maximum of 1500 C., loading weight=100 g, maximum bending=3000 m (deviation from the standards document).

[0738] zc. Residual Moisture (Water Content)

[0739] The measurement of the residual moisture of a sample of silicon dioxide granulate is performed using a Moisture Analyzer HX204 from Mettler Toledo. The device functions using the principle of thermogravimetry. The HX204 is equipped with a halogen light source as heating element. The drying temperature is 220 C. The starting weight of the sample is 10 g10%. The Standard measuring method is selected. The drying is carried out until the weight change reaches not more than 1 mg/140 s. The residual moisture is given as the difference between the initial weight of the sample and the final weight of the sample, divided by the initial weight of the sample.

[0740] The measurement of residual moisture of silicon dioxide powder is performed according to DIN EN ISO 787-2:1995 (2 h, 105 C.).

EXAMPLES

[0741] The invention further illustrated in the following with examples. The invention is not limited to the examples.

[0742] A. 1. Preparation of Silicon Dioxide Powder (OMCTS Route)

[0743] The aerosol formed by atomising a siloxane with air (A) is introduced into a flame using pressure which is formed by igniting a mixture of oxygen enriched air (B) and hydrogen. Further, a gas flow (C) surrounding the flame is introduced and the process mixture subsequently cooled with process gas. The product is separated off at a filter. The process parameters are given in table 1 and the specification of the resulting product in table 2. Experimental values for this example are indicated with A1-x.

[0744] 2. Modification 1: Raised Carbon Content

[0745] The process described in A.1. was performed, except that the burning of the siloxane was performed in such a way that an amount of carbon was also formed. Experimental values for this example are marked with A2-x.

TABLE-US-00002 TABLE 1 Example A1-1 A2-1 A2-2 Aerosol formation Siloxane OMCTS* OMCTS* OMCTS* Feed kg/h 10 10 10 (kmol/h) (0.0337) (0.0337) (0.0337) Air feed (A) Nm.sup.3/h 14 10 12 Pressure barO 1.2 1.2 1.2 Burner feed Oxygen enriched air Nm.sup.3/h 69 65 68 (B) O.sub.2 content Vol. % 32 30 32 Total O.sub.2 feed Nm.sup.3/h 25.3 21.6 24.3 kmol/h 1.130 0.964 1.083 Hydrogen feed Nm.sup.3/h 27 27 12 kmol/h 1.205 1.205 0.536 Feed of carbon compounds Material Methane5.5 Amount Nm.sup.3/h Air flow (C) Nm.sup.3/h 60 60 60 Stoichiometric ratio V 2.099 1.789 2.011 X 0.938 0.80 2.023 Y 0.991 0.845 0.835 V = molar ratio of O.sub.2 employed/O.sub.2 required for complete oxidation of the siloxane; X = O.sub.2/H.sub.2 as molar ratio; Y = (mol. ratio of O.sub.2 employed/O.sub.2 required for stoichiometric conversion of OMCTS + fuel gas); *OMCTS = Octamethylcyclotetrasiloxane.

TABLE-US-00003 TABLE 2 Example A1-1 A2-1 A2-2 BET m.sup.2/g 30 33 34 Bulk density g/ml 0.114 0.011 0.105 0.011 0.103 0.011 Tamped density g/ml 0.192 0.015 0.178 0.015 0.175 0.015 Primary particle size nm 94 82 78 Particle size distribution D.sub.10 m 3.978 0.380 5.137 0.520 4.973 0.455 Particle size distribution D.sub.50 m 9.383 0.686 9.561 0.690 9.423 0.662 Particle size distribution D.sub.90 m 25.622 1.387 17.362 0.921 18.722 1.218 C content ppm 34 4 73 6 80 6 Cl content ppm <60 <60 <60 Al content ppb 20 20 20 Total content of metals other than Al ppb <700 <700 <700 Residual moisture wt.-% 0.02-1.0 0.02-1.0 0.02-1.0 pH value in water 4% (IEP) 4.8 4.6 4.5 Viscosity at 5 rpm, aqueous suspension mPas 753 1262 1380 30 wt-%, 23 C. Alkali earth metal content ppb 538 487 472

[0746] B. 1. Preparation of Silicon Dioxide Powder (Silicon Source: SiCl.sub.4)

[0747] An amount of silicon tetrachloride (SiCl.sub.4) is vaporised at a temperature T and introduced with a pressure P into a flame of a burner which is formed by igniting a mixture of oxygen enriched air and hydrogen. The mean normalised gas flow at the mouth of the burner is held constant. The process mixture is subsequently cooled with process gas. The product was separated off at a filter. The process parameters are given in table 3 and the specifications for the resulting products in Table 4. They are marked with B1-x.

[0748] 2. Modification: Raised Carbon Content

[0749] The process was performed as described under B.1., except that the burning of the silicon tetrachloride was performed in such a way that an amount of carbon was also formed. Experimental values for this example are marked with B2-1.

TABLE-US-00004 TABLE 3 Example B1-1 B2-1 Aerosol formation Feed of silicon tetrachloride kg/h 50 50 (kmol/h) (0.294) (0.294) Temperature T C. 90 90 Pressure p barO 1.2 1.2 Burner feed Oxygen enriched air, Nm.sup.3/h 145 115 O.sub.2 content therein Vol. % 45 30 Feed of carbon compounds Material Methane Amount Nm.sup.3/h 2.0 Hydrogen feed Nm.sup.3/h 115 60 kmol/h 5.13 2.678 Stoichiometric ratio X 0.567 0.575 Y 0.946 0.85 X = O.sub.2/H.sub.2 as molar ratio; Y = mol. ratio of O.sub.2 employed/O.sub.2 required for stoichiometric reaction with SiCl.sub.4 + H.sub.2 + CH.sub.4)

TABLE-US-00005 TABLE 4 Example B1-1 B2-1 BET m.sup.2/g 49 47 Bulk density g/ml 0.07 0.01 0.06 0.01 Tamped density g/ml 0.11 0.01 0.10 0.01 Primary particle size nm 48 43 Particle size distribution D.sub.10 m 5.0 0.5 4.5 0.5 Particle size distribution D.sub.50 m 9.3 0.6 8.7 0.6 Particle size distribution D.sub.90 m 16.4 0.5 15.8 0.7 C content ppm <4 76 Cl content ppm 280 330 Al content ppb 20 20 Total of the concentrations of ppb <1300 <1300 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Residual moisture wt.-% 0.02-1.0 0.02-1.0 pH value in water 4% (IEP) pH 3.8 3.8 Viscosity at 5 rpm, aqueous mPas 5653 6012 suspension 30 wt-%, 23 C. Alkali earth metal content ppb 550 342

[0750] C. Steam Treatment

[0751] A particle flow of silicon dioxide powder is introduced through the top of a standing column. Steam with a temperature (A) and air are introduced through the base of the column. The column is held by an internal heater at a temperature (B) at the top of the column and a second temperature (C) at the column base. After leaving the column (holding time (D)) the silicon dioxide powder has in particular the properties shown in Table 6. The process parameters are given in Table 5.

TABLE-US-00006 TABLE 5 Example C-1 C-2 Educt: Product from B1-1 B2-1 Educt feed kg/h 100 100 Steam feed kg/h 5 5 Steam temperature (A) C. 120 120 Air feed Nm.sup.3/h 4.5 4.5 Column height m 2 2 Timer diameter mm 600 600 T (B) C. 260 260 T (C) C. 425 425 Holding time (D) of silicon dioxide s 10 10 powder

TABLE-US-00007 TABLE 6 Example C-1 C-2 pH value in water 4% (IEP) 4.6 4.6 Cl content ppm <60 <60 C content ppm <4 36 Viscosity at 5 rpm, aqueous suspension mPas 1523 1478 30 wt-%, 23 C.

[0752] The silicon dioxide powder obtained in examples C-1 and C-2 each have a lower chlorine content, as well as a moderate pH value in suspension. The carbon content of example C-2 higher than for C-1.

[0753] D. Treatment with Neutralising Agent

[0754] A particle flow of silicon dioxide powder is introduced through the top of a standing column. Through the base of the column, a neutralising agent and air are added. The column is held by an internal heater at a temperature (B) at the top of the column and a second temperature (C) at the column base. After leaving the column (Holding time (D)), the silicon dioxide powder has in particular the properties shown in Table 8. The process parameters are given in Table 7.

TABLE-US-00008 TABLE 7 Example D-1 Educt: Product from B1-1 Educt feed kg/h 100 Neutralising agent Ammonia Feed of neutralising agent kg/h 1.5 Specification of neutralising Obtainable from Air Liquide: agent Ammonia N38, purity 99.98 Vol. % Air feed Nm.sup.3/h 4.5 Column height m 2 inner diameter mm 600 T (B) C. 200 T (C) C. 250 Holding time (D) for silicon s 10 dioxide powder

TABLE-US-00009 TABLE 8 Example D-1 pH value in water 4% (IEP) 4.8 Cl content ppm 210 C content ppm <4 Viscosity at 5 rpm, aqueous suspension mPas 821 30 wt-%, 23 C.

[0755] E. 1. Preparation of Silicon Dioxide Granulate from Silicon Dioxide Powder

[0756] A silicon dioxide powder is dispersed in fully desalinated water. For this, an intensive mixer of type R from Maschinenfabrik Gustav Eirich is employed. The suspension so produced is pumped through a membrane pump, charged with pressure, and converted into droplets by a nozzle. These are dried in a spray tower and collect on the floor thereof. The process parameters are given in Table 9, the properties of the obtained granulates in Table 10. Experimental values for this example are marked with E1-x.

[0757] 2. Modification: Raised Carbon Content

[0758] A process is performed analogous to the description E.1. Additionally, carbon powder is dispersed into the suspension as additive. Experimental values for this example are marked with E2-1. In E2-21 to E2-23, aluminium oxide is added as additive.

TABLE-US-00010 TABLE 9 Example E1-1 E1-2 E1-3 E1-4 E1-5 E2-1 E2-21 E2-22 E2-23 Educt = Product A1-1 A2-1 B1-1 C-1 C-2 A1-1 A1-1 A1-1 A1-1 from Amount of educt kg 10 10 10 10 10 10 1000 1000 1000 Additive Material C** Al.sub.2O.sub.3.sup.+ Al.sub.2O.sub.3.sup.+ Al.sub.2O.sub.3.sup.+ Max. Particle size 75 m 65 m 65 m 65 m Amount 1 g 0.32 g 0.47 g 0.94 g Water Grade* FD 5.4 FD 5.4 FD 5.4 FD 5.4 FD 5.4 FD 5.4 FD 5.4 FD 5.4 FD 5.4 Litre Dispersion Solids content wt.-% 65 65 65 65 65 65 65 65 65 Nozzle Diameter mm 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 Temperature C. 25 25 25 25 25 25 25 25 25 Pressure bar 16 16 16 16 16 16 16 16 16 Mounting height m 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 Spray tower Height m 18.20 18.20 18.20 18.20 18.20 18.20 18.20 18.20 18.20 Inner diameter m 6.30 6.30 6.30 6.30 6.30 6.30 6.30 6.30 6.30 T (Feed gas) C. 380 380 380 380 380 380 380 380 380 T (exhaust) C. 110 110 110 110 110 110 110 110 110 Air flow m.sup.3/h 6500 6500 6500 6500 6500 6500 6500 6500 6500 Mounting height = distance between the nozzle and the lowest point of the spray tower interior in the direction of the gravitational force vector. *FD = fully desalinated, conductivity 0.1 S; **C 006011: graphite powder, max. particle size: 75 m, high purity (obtainable from Goodfellow GmbH, Bad Nauheim (Germany). .sup.+Aeroxide Alu 65: highly dispersed pyrogenic aluminium oxide, particle size 65 m (Evonik Industries AG, Essen (Germany).

TABLE-US-00011 TABLE 10 Example E1-1 E1-2 E1-3 E1-4 E1-5 E2-1 E2-21 E2-22 E2-23 BET m.sup.2/g 30 33 49 49 47 28 32 30 32 Bulk density g/ml 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 0.8 0.1 Tamped density g/ml 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 0.9 0.1 Mean particle size m 255 255 255 255 255 255 255 255 255 Particle size distribution m 110 110 110 110 110 110 110 110 110 D.sub.10 Particle size distribution m 222 222 222 222 222 222 222 222 222 D.sub.50 Particle size distribution m 390 390 390 390 390 390 390 390 390 D.sub.90 SPHT3 dim.-less 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 0.64-0.98 Aspect ratio W/L (width dim.-less 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 0.64-0.94 to length) C content ppm <4 39 <4 <4 32 100 <4 <4 <4 Cl content ppm <60 <60 280 <60 <60 <60 <60 <60 <60 Al content ppb 20 20 20 20 20 20 190 270 520 Total of the concentrations ppb <700 <700 <1300 <1300 <1300 <700 <700 <700 <700 of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Residual moisture wt.-% <3 <3 <3 <3 <3 <3 <3 <3 <3 Alkali earth metal content ppb 538 487 550 550 342 538 517 490 541 Pore volume ml/g 0.33 0.33 0.45 0.45 0.45 0.33 0.33 0.33 0.33 Angle of repose 26 26 26 26 26 26 26 26 26

[0759] The granulates are all open pored have a uniform and spherical shape (all by inspection with microscope). They do not tend to cement or stick together.

[0760] F. Cleaning of Silicon Dioxide Granulate

[0761] Silicon dioxide granulate is first optionally treated in a rotary kiln at a temperature T1 with oxygen. Then, the silicon dioxide granulate is treated in co-flow with chlorine containing components, wherein the temperature is raised to a temperature T2. The process parameters are given in Table 11, the properties of the obtained treated granulate in Table 12.

TABLE-US-00012 TABLE 11 Example F1-1 F1-2 F2-1 F2-21 F2-22 F2-23 Educt = Product from E1-1 E1-2 E2-1 E2-21 E2-22 E2-23 Rotary kiln.sup.1) Length cm 200 200 200 200 Inner diameter cm 10 10 10 10 Throughput kg/h 2 2 2 2 Rotation speed rpm 2 2 2 2 T1 C. 1100 absent absent 1100 1100 1100 Atmosphere pure O.sub.2 absent absent O.sub.2 pure O.sub.2 pure O.sub.2 pure Reactant O.sub.2 absent absent O.sub.2 O.sub.2 O.sub.2 Feed l/h 300 absent absent 300 l/h 300 l/h 300 l/h Residual moisture wt.-% <1 <3 <3 <1 <1 <1 T2 C. 1100 1100 1100 1100 1100 1100 Co-flow Component 1: HCl l/h 50 50 50 50 50 50 Component 2: Cl.sub.2 l/h 0 15 15 0 0 0 Component 3: N.sub.2 l/h 50 35 35 50 50 50 Co-flow total l/h 100 100 100 100 100 100 .sup.1)For the rotary kilns, the throughput is selected as the control variable. That means that during operation the mass flow exiting from the rotary kiln is weighed and then the rotational speed and/or the inclination of the rotary kiln is adapted accordingly. For example, an increase in the throughput can be achieved by a) increasing the rotational speed, or b) increasing the inclination of the rotary kiln away from horizontal, or a combination of a) and b).

TABLE-US-00013 TABLE 12 Example F1-1 F1-2 F2-1 F2-21 F2-22 F2-23 BET m.sup.2/g 25 27 23 26 26 23 C content ppm <4 <4 <4 <4 <4 <4 Cl content ppm 100-200 100-200 100-200 100-200 100-200 100-200 Al content ppb 20 20 20 190 270 520 Pore volume mm.sup.3/g 650 650 650 650 650 650 Total of the concentrations of ppb <200 <200 <200 <200 <200 <200 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Alkali earth metal content ppb 115 55 35 124 110 116 Tamped density g/cm.sup.3 0.95 0.05 0.95 0.05 0.95 0.05 0.95 0.05 0.95 0.05 0.95 0.05

[0762] The granulates following the cleaning step have in the case of F1-2 and F2-1 a significantly reduced carbon content (like low carbon granulates, e.g. F1-1) and a significantly lower content of alkali earth metals. SiC formation was not observed.

[0763] G. Formation of a Glass Body

[0764] Silicon dioxide granulate according to row 2 of Table 13 was used as raw material. A graphite mould with a ring shaped hollow space and an outer diameter of the mould body of d.sub.a, an inner diameter of the mould body of d.sub.i and a lengthl was prepared. A high purity graphite foil having a thickness of 1 mm, was applied to the inner wall of the outer mould body and a graphite foil of the same high purity graphite with a thickness of 1 mm was applied to the outer wall of the inner mould body. A high purity graphite line made out of high purity graphite with a bulk density of 1.2 g/cm.sup.3 and a density of 0.4 mm was applied to the base of the ring shaped hollow space of the mould (for G-2: cylinder shaped hollow space). The high-purity graphite mould with the graphite foil was filled with the silicon dioxide granulate. The filled graphite mould is introduced into an oven and the oven is evacuated. The silicon dioxide granulate which was introduced was brought from the temperature T1 up to the temperature T2 at a heating rate R1 and held there for a duration t2. Then, it was warmed at a heating rate R2 up to T3, then, without further tempering, up to the temperature T4 at a heating rate R3, further up to the temperature T5 at a heating rate R4, and held at there for a duration t5. During the last 240 minutes, the oven is charged with nitrogen at a pressure of 1.6*10.sup.6 Pa. Subsequently, the mould was gradually cooled down. On reaching a temperature of 1050 C., the mould was held at this temperature for a duration of 240 min. Subsequently, it was gradually further cooled to T6. The process parameters are collected in Table 13, the properties of the quartz glass body which is formed in Table 14.Gradual cooling means that the mould is left in a switched off oven without further cooling measures, in other words cooled down only by giving off of heat to the surroundings.

TABLE-US-00014 TABLE 13 Example G1-1 G1-2 G2-1 G2-21 G2-22 G2-23 Educt = F1-1 F1-2 F2-1 F2-21 F2-22 F2-23 Product from T1 C. 25 25 25 25 25 25 R1 C./ +2 +2 +2 +2 +2 +2 min T2 C. 400 400 400 400 400 400 t2 min 60 60 60 60 60 60 R2 C./ +3 +3 +3 +3 +3 +3 min T3 C. 1000 1000 1000 1000 1000 1000 R3 C./ +0.2 +0.2 +0.2 +0.2 +0.2 +0.2 min T4 C. 1350 1350 1350 1350 1350 1350 R4 C./ +2 +2 +2 +2 +2 +2 min T5 C. 1750 1750 1750 1750 1750 1750 t5 min 720 720 720 720 720 720 T6 C. 25 C. 25 C. 25 C. 25 C. 25 C. 25 C.

TABLE-US-00015 TABLE 14 Example G1-1 G1-2 G2-1 G2-21 G2-22 G2-23 Length (quartz glass body) mm 2000 1000 2000 2000 2000 2000 Outer diameter (quartz glass body) mm 260 560 260 260 260 260 Inner diameter (quartz glass body) mm 45 45 45 45 45 (solid) OH content* ppm 0.3 0.2 0.4 0.2 0.4 0.2 0.3 0.2 0.3 0.2 0.3 0.2 C content ppm <4 <4 <4 <4 <4 <4 Cl content* ppm <60 <60 <60 <60 <60 <60 Al content* ppb 14 5 13 5 12 5 185 5 280 5 510 5 ODC content /cm.sup.3 0.8*10.sup.15 1.7*10.sup.15 1.1*10.sup.15 0.8*10.sup.15 0.8*10.sup.15 0.8*10.sup.15 Total of the concentrations of Ca, Co, Cr, ppb 153 62 171 160 166 172 Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Refractive index homogeneity ppm 30 30 30 30 30 30 Fictive temperature C. 1109 1137 1148 1120 1113 1244 Viscosity Lg(/dPas) @1250 C. 12.6 12.4 12.7 12.6 12.6 12.7 @1300 C. 11.8 11.8 11.8 11.8 11.8 11.8 @1350 C. 11.1 11.1 11.1 11.1 11.1 11.2 -indicates the standard deviation. All glass bodies have very good values for OH content, carbon content and aluminium content.

[0765] H. Preparation of a Glass Fibre

[0766] The quartz glass body prepared previously in example G is first mechanically worked analogously to Example 1 of EP 0598349 B1. The outer diameter of the worked quartz glass body was 250 mm, the inner diameter 50 mm. Subsequently, the worked quartz glass body is processed further analogously to example 6 of EP 0598349 B1, wherein this is used in place of the natural quartz glass body of example 6. A core rod with 45 mm outer diameter was used. The process parameters are given in Table 15, the properties of the glass fibre which is formed in Table 16.

TABLE-US-00016 TABLE 15 Example H1-1 H2-1 H2-21 H2-22 H2-23 Material = G1-1 G2-1 G2-21 G2-22 G2-23 product from Oven temperature of C. 2100 2100 2100 2100 2100 fibre drawing oven Drawing speed m/min 1200 1200 1200 1200 1200

TABLE-US-00017 TABLE 16 Example H1-1 H2-1 H2-21 H2-22 H2-23 Length (glass fibre) km 120 120 120 120 120 Total diameter m 125 125 125 125 125 Jacket specifications OH content* ppm <1 <1 <1 <1 <1 C content ppm <4 <4 <4 <4 <4 Cl content* ppm <60 <60 <60 <60 <60 Al content* ppb 14 5 12 5 185 5 280 5 510 5 ODC content /cm3 0.3*10.sup.15 0.5*10.sup.15 0.3*10.sup.15 0.3*10.sup.15 0.3*10.sup.15 Total of the concentrations ppb 153 171 153 153 153 of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Curl m 17.3 22.1 9.1 5.5 4.3 *-indicates the standard deviation. Large curl values are better than small values.