Method for producing synthetic quartz glass of SiO2 granulate and SiO2 granulate suited therefor

Abstract

A method for producing synthetic quartz glass by fusion of SiO.sub.2 granulate involves synthesizing amorphous SiO.sub.2 primary particles, granulating the amorphous SiO.sub.2 primary particles to form an open-pore SiO.sub.2 granulate, sintering the open-pore SiO.sub.2 granulate by heating in a sintering atmosphere at a sintering temperature and for a sintering period to form a densified SiO.sub.2 granulate, and melting the densified SiO.sub.2 granulate at a melting temperature to form the synthetic quartz glass. To provide an inexpensive production of low-bubble transparent components of quartz glass despite the use of still open-pore SiO.sub.2 granulate, the sintering atmosphere, sintering temperature and sintering duration are adjusted such that the densified SiO.sub.2 granulate still comprises open pores but manifests a material-specific infrared transmission T.sub.1700 at a wavelength of 1700 nm. This transmission is in the range of 50-95% of the infrared transmission T.sub.1700 of quartz glass granules of the same material.

Claims

1. A method for producing synthetic quartz glass, comprising the following method steps: (a) synthesizing amorphous SiO.sub.2 primary particles, (b) granulating the amorphous SiO.sub.2 primary particles to form an open-pore SiO.sub.2 granulate, (c) sintering the open-pore SiO.sub.2 granulate by heating in a sintering atmosphere at a sintering temperature and for a sintering period to form a densified SiO.sub.2 granulate, and (d) melting the densified SiO.sub.2 granulate at a melting temperature to form the synthetic quartz glass; wherein for the production of transparent quartz glass, the sintering atmosphere, the sintering temperature, and the sintering duration are adjusted during sintering in step (c) such that the densified SiO.sub.2 granulate still comprises open pores and manifests a material-specific infrared transmission T.sub.1700 at a wavelength of 1700 nm, wherein the transmission is in a range of 50-95% of the infrared transmission T.sub.1700 of reference quartz glass granules formed by complete vitrification of same SiO.sub.2 granulate.

2. The method according to claim 1, wherein the infrared transmission T.sub.1700 is in a range of 60-90% of the infrared transmission T.sub.1700 of reference quartz glass granules formed by complete vitrification of same SiO.sub.2 granulate.

3. The method according to claim 1, wherein the sintering atmosphere, the sintering temperature, and the sintering duration are adjusted during sintering in step (c) such that the densified SiO.sub.2 granulate manifests a material-specific infrared transmission T.sub.500 at a wavelength of 500 nm, and wherein a ratio T.sub.500/T.sub.1700 is 0.8 or less.

4. The method according to claim 1, wherein the melting in step (d) comprises heating a bulk material of SiO.sub.2 granulate or a compact comprising the SiO.sub.2 granulate under negative pressure at the melting temperature.

5. The method according to claim 1, wherein the densified SiO.sub.2 granulate has a bulk density in a range of 0.9 kg/l to 1.3 kg/l.

6. The method according to claim 1, wherein the densified SiO.sub.2 granulate has a specific BET surface area in a range of 1 m.sup.2/g to 25 m.sup.2/g.

7. The method according to claim 1, wherein the sintering atmosphere contains chlorine and the sintering temperature is at least 1000 C.

8. The method according to claim 1, wherein the sintering in step (c) is performed in a rotary furnace.

9. The method according to claim 1, wherein the granulating in step (b) is performed by spray granulation, and wherein granulate particles with a spherical morphology are obtained.

10. The method according to claim 1, wherein the granulating in step (b) is performed by freeze granulation, and wherein granulate particles with a platelet-shaped morphology are obtained.

11. The method according to claim 1, wherein during granulating in step (b), open-pore SiO.sub.2 granulate is produced from granulate particles having a mean particle size of at least 150 m, and wherein the open-pore SiO.sub.2 granulate has a BET surface area in a range of 20 m.sup.2/g to 100 m.sup.2/g.

12. The method according to claim 1, wherein the granulate particles of the densified granulate have a narrow particle size distribution, wherein the particle size assigned to a D.sub.90 value is at the most three times as large as the particle size assigned to a D.sub.10 value.

13. The method according to claim 1, wherein during step (b), pyrogenically obtained SiO.sub.2 primary particles are used, and wherein the SiO.sub.2 primary particles are obtained by oxidation or flame hydrolysis of a chlorine-free silicon-containing feedstock.

14. An open-pore granulate consisting of synthetically produced amorphous SiO.sub.2 primary particles, wherein at a wavelength of 1700 nm the granulate manifests a material-specific infrared transmission T.sub.1700 in a range of 50-95% of the infrared transmission T.sub.1700 of reference quartz glass granules formed by complete vitrification of same SiO.sub.2 granulate.

15. The granulate according to claim 14, wherein the infrared transmission T.sub.1700 is in a range of 60-90% of the infrared transmission T.sub.1700 of reference quartz glass granules formed by complete vitrification of same SiO.sub.2 granulate, and wherein at a wavelength of 500 nm the SiO.sub.2 granulate manifests a material-specific infrared transmission T.sub.500, and wherein a ratio T.sub.500/T.sub.1700 is 0.8 or less.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

(2) In the drawings:

(3) FIG. 1 is a table which summarizes parameters of the production process and properties of different granulates;

(4) FIG. 2 shows infrared transmission spectra of the granulates; and

(5) FIG. 3 shows graphs of bubble-frequency distributions in different samples.

DETAILED DESCRIPTION OF THE INVENTION

(6) In the processing of initially porous SiO.sub.2 granulate into transparent quartz glass, a distinction can be made between hot treatment steps which are less complicated in terms of time, energy, and material, and those in which complicated measures have to be taken for achieving the desired absence of bubbles, e.g., evacuation measures at a high temperature. The method according to the invention aims at reducing the share of the complicated hot treatment steps in favor of less complicated ones. To achieve this aim, the following measures are taken.

(7) The pre-production of fully vitrified quartz glass granules which is standard for producing high-quality transparent quartz glass is avoided. As a consequence, the efforts entailed by this intermediate step in terms of time, energy, and material are also not required. Instead, a still porous SiO.sub.2 granulate which can be produced with less effort is provided for the melting process. Normally, however, porous granulate cannot be processed into transparent quartz glass in a melting process with an economically reasonable melting period and melting temperature. Although the use of porous granulates for fusion into transparent quartz glass is technologically possible in this respect, it is limited. Surprisingly, it has however been found that this limitation can be eliminated if the porous granulate to be used shows a specific transparency in the infrared wavelength range. The infrared (IR) transparency should not be too low or too high. Such a granulate shall also be called IR transparency-optimized SiO.sub.2 granulate in the following. It may be melted by taking comparatively small efforts in terms of time, energy, and material into a high-quality quartz glass. This aspect of the method according to the invention shall be explained in more detail hereinafter.

(8) To solve the problem regarding the formation of bubbles in the melting process, degassing measures have to be taken, independently of whether vitrified, dense quartz-glass granules or a still open-pore SiO.sub.2 granulate is used. Known degassing measures involve applying a negative pressure and in replacing gases prevailing within bulk particles with helium or hydrogen. The use of still open-pore granulate is advantageous in this respect because it can still be exposed to such degassing measures in the melting process, in the sense that gases present in the granulate can be eliminated and the inclusion of gases can be counteracted.

(9) The problems encountered by using the still porous granulate with respect to the thermal insulation and the supply of the heat energy required for fusion are mitigated according to the invention by way of a SiO.sub.2 granulate which is optimized with respect to IR transparency. This granulate has a specific transparency for IR radiation in the wavelength range of about 500 nm to about 2400 nm. Within this wavelength range, absorption bands are found that are normally due to hydroxyl groups or water and that need not be considered in this context. The transmission at the wavelength of 1700 nm that is substantially unaffected by absorptions by hydroxyl groups and water is representative of the IR transparency of the granulate.

(10) Due to their transparency for IR radiation, the scattering effect of the granulate particles for this radiation is reduced and the supply of the thermal energy for fusing the SiO.sub.2 granulate particles is thereby facilitated. It is thereby possible to significantly reduce the fusion period and/or the fusion temperature in the use of a SiO.sub.2 granulate optimized to IR transparency, as compared with a non-optimized granulate.

(11) A suitable measuring method for determining the IR transmission shall be explained hereinafter. The IR transmission of the granulate particles in bulk form or of a molded body made from the granulate particles depends on the degree of the thermal densification of the SiO.sub.2 granulate. The transmission curves of FIG. 2 show that the IR transmission at the wavelength of 1700 nm is increasing with the thermal densification of the SiO.sub.2 granulate, reaching a maximum shortly before complete densification. By comparison, the fully densified SiO.sub.2 granulate, i.e., quartz glass granules, may again show a slightly lower IR transmission, which can be attributed to increased reflection on the reflective vitrified surface. The IR transmission of the completely vitrified and transparent quartz glass granules without bubbles represents a suitable reference value for the IR transmission of the SiO.sub.2 granulate particles. The quartz glass granules which serve as a reference material and the SiO.sub.2 granulate particles consist of chemically identical SiO.sub.2 material. In the simplest case, the reference quartz-glass granules are formed by complete vitrification of the granulate whose IR transmission has to be determined.

(12) The IR transmission is measured, for example, by using an integrating sphere on the SiO.sub.2 granulate in bulk form with a thickness of 4.2 mm in a measuring cuvette. Over the measurement length of 4.2 mm, a high-porosity granulate yields a smaller SiO.sub.2 mass occupation than low-porosity granulate. To make the measurement values comparable despite these different granulate densities, they are normalized to the bulk density of the respective granulate. The normalization consists of multiplying the transmission measurement value determined by the integrating sphere (in %) by the specific bulk density of the SiO.sub.2 granulate (in g/cm.sup.3). The transmission value T determined thereby at the measurement wavelength of 1700 nm is here called T.sub.1700.

(13) As has been described above (and is apparent from FIG. 2), the IR transmission (particularly the T.sub.1700 value) of SiO.sub.2 granulate particles in bulk form may be even higher than the reference values measured on the fully vitrified quartz glass granules. However, granulate particles with such a high IR transmission (more than 95% of the reference value) surprisingly do not show the advantageous fusion behavior of the granulate optimized to IR transmission. Possibly, the very high IR transparency is a sign that the granulate particles are almost completely dense and no longer manifest any open porosity. The closed residual pores of such granulate particles can no longer be subjected to degassing measures in the fusion process.

(14) Therefore, according to the invention a SiO.sub.2 granulate which is as dense as possible but still shows open pores and the porosity of which is preferably at least 10% is used for melting according to method step (d). Under the boundary condition regarding the maintenance of the open porosity, the SiO.sub.2 granulate is densified mechanically and/or thermally to such an extent that it shows an IR transparency that is as high as possible. It has been found that the appropriate degree of densification is accompanied by an IR transmission that is in the range of 50-95% of the reference value at the wavelength of 1700 nm, preferably in the range of 60-90%.

(15) The porosity of the SiO.sub.2 granulate remaining after mechanical and/or thermal densification depends on the densification-relevant properties of the SiO.sub.2 primary particles, on the respective granulation process and the resulting morphology, porosity and size distribution of the granulate particles, and on the type and intensity of the densification process itself. This shall be explained in more detail hereinafter.

(16) A typical transmission curve of a still open-pore SiO.sub.2 granulate shows a rise in transmission in the wavelength range between 500 and 1300 nm. This transmission curve, which strongly depends on the wavelength, can be ascribed to scattering on structures in the sub-micrometer range. It evidently depends on the porosity of the SiO.sub.2 granulate and in the case of a highly densified granulate, the rise is small or is even missing altogether. A measure of this rise is the difference in the infrared transmission values T.sub.500 and T.sub.1700, respectively measured at wavelengths of 500 nm and 1700 nm. A small difference indicates a high densification of the granulate. Empirically, it has been found that in cases where the ratio T.sub.500/T.sub.1700 is more than 0.8, such a high densification of the granulate is apparently reached that closed pores are formed which impede fusibility.

(17) Therefore, it has turned out to be advantageous when sintering atmosphere, sintering temperature and sintering duration are adjusted during sintering in step (c) that the densified SiO.sub.2 granulate manifests a material-specific infrared transmission T.sub.500 at a wavelength of 500 nm, and that the ratio T.sub.500/T.sub.1700 is 0.8 or less. The ratio T.sub.500/T.sub.1700 of approximately 0.8 marks the transition between still open-pore SiO.sub.2 granulate pores and closed-pore SiO.sub.2 granulate that is too densified.

(18) A procedure is preferred in which melting in step (d) comprises heating a bulk material of SiO.sub.2 granulate or a compact consisting of the SiO.sub.2 granulate under negative pressure at the melting temperature.

(19) An advantageous property of the SiO.sub.2 granulate optimized to IR transparency is that it generates a comparatively small scattering for infrared ration and is therefore more easily fusible than SiO.sub.2 granulate that is not optimized for IR transparency. A further advantage is that it has a minimum density that is given by the lower limit value of T.sub.1700. This property also promotes easier fusibility of the granulate. Therefore, when this SiO.sub.2 granulate is used, the energy- and time-consuming hot treatment step with additional degassing measure, namely application of negative pressure or vacuum (<2 mbar), can be similarly short in the method according to the invention as in the fusion of completely vitrified quartz glass granules, and at any rate significantly shorter than in the use of a conventional SiO.sub.2 granulate. This is also true when the granulate is exposed to a mechanical pressure prior to the fusion process, for instance by way of cold-isostatic uniaxial pressing or during the fusion process, in addition to the use of negative pressure (vacuum), e.g., by hot isostatic pressing.

(20) In this connection it has also turned out to be useful when the densified SiO.sub.2 granulate has a bulk density in the range of 0.9 kg/l to 1.3 kg/l and preferably of at least 1.1 kg/l.

(21) Due to the comparatively high bulk density, the SiO.sub.2 granulate particles are easily fusible.

(22) In the invention, it is the open porosity of the individual SiO.sub.2 granulate particles that is of importance and not the porosity of the bulk granulate material. Open porosity of a material manifests itself basically in that it is permeable to gases and thus absorbtive for liquids, which can be demonstrated by a dye penetration test. A granulate particle which is not permeable to gases, like nitrogen, is defined as being dense and having no open porosity. As a measure of open porosity of the individual SiO.sub.2 granulate particles, the invention uses the ratio T.sub.500/T.sub.1700 that is 0.8 or less in the case of open porosity.

(23) The specific BET surface area determined by the BET method is in the range of 1 m.sup.2/g to 25 m.sup.2/g, preferably in the range of 3 m.sup.2/g to 20 m.sup.2/g. A BET surface area of more than 20 m.sup.2/g impedes fast fusion of the granulate.

(24) Sintering is carried out in the simplest case in an atmosphere consisting of air or inert gas. However, it has turned out to be particularly useful when the sintering atmosphere contains chlorine and when the sintering temperature is at least 1000 C., preferably at least 1100 C.

(25) The chlorine-containing atmosphere contains, for instance, HCl or Cl.sub.2. This treatment particularly effects a reduction of alkali and iron impurities from the SiO.sub.2 granulate and an elimination of hydroxyl groups. At temperatures below 100 C., the treatment periods are long and temperatures above 1000 C. pose the risk of dense sintering of the porous granulate with inclusion of chlorine or gaseous chlorine compounds.

(26) A small hydroxyl group content is obtained in the densified SiO.sub.2 granulate after the dehydration treatment. The chlorine content may be lowered by an after-treatment in an oxygen-containing atmosphere. Low concentrations of hydroxyl groups and chlorine facilitate the bubble-free fusion of the granulate particles.

(27) The sintering treatment of the SiO.sub.2 granulate is preferably carried out in a bulk state, i.e., in a loose, flowable state. For the prevention of agglomerate formations between the granulate particles it has turned out to be advantageous when sintering in step c) is carried out in a rotary furnace.

(28) Furthermore, a method variant is preferred in which granulation in step (b) is performed by freeze granulation, and granulate particles with a platelet-shaped morphology are obtained.

(29) In freeze granulation, a dispersion of the SiO.sub.2 primary particles is deep-frozen and subsequently freeze-dried, with the frozen liquid evaporating by sublimation in high vacuum. A low pore SiO.sub.2 granulate is thereby obtained.

(30) In another procedure that is also suited, granulation in step (b) is performed by spray granulation, and granulate particles with a spherical morphology are obtained.

(31) In the granulation of SiO.sub.2 particles, an undesired fine fraction is obtained with a granulate that is not agglomerated or is insufficiently agglomerated; this fraction poses problems in the subsequent sintering or fusion of the granulate. In spray granulation, the given size of the granulate particles may be adjusted relatively exactly and the fine fraction is relatively small. Due to the manufacturing method spray granulate particles have a morphology that promotes the flowability of the granulate. This facilitates the sintering treatment of the SiO.sub.2 granulate in its bulk state.

(32) It has been found that the IR transmission of SiO.sub.2 granulate depends on the particle size of the granulate particles. At mean particle sizes of more than 100 m (D.sub.50 value), the greater the IR transmission, the greater is the mean particle size.

(33) Therefore, during granulation in step (b), preferably open-pore SiO.sub.2 granulate is produced from granulate particles having a mean particle size of at least 150 m, preferably at least 200 m. Preferably, the open-pore SiO.sub.2 granulate obtained by granulation in step (b) has a BET surface area in the range of 20 m.sup.2/g to 100 m.sup.2/g.

(34) Particle size and particle size distribution of the granulate particles are characterized by the D.sub.50 value of a particle size distribution curve (cumulative volume of the SiO.sub.2 granulate particles relative to the particle size). The D.sub.50 value characterizes a particle size which is not reached by 50% of the cumulative volume of the SiO.sub.2 granulate particles. The particle size distribution is determined by scattered-light and laser-diffraction spectroscopy according to ISO 13320. The mean particle size is determined in the simplest case by sieve analysis according to DIN 66165-2. Here, mean particle size stands for the D.sub.50 value of the granulate.

(35) Approximately identical particle sizes are advantage to fusion of granulate particles that are as homogeneous as possible. In this respect, it has also turned out to be useful when the granulate particles have a narrow particle size distribution, in which the particle size assigned to the D.sub.90 value is at most three times as large as the particle size assigned to the D.sub.10 value.

(36) Preferably, pyrogenically obtained SiO.sub.2 particles are used during granulation in step (b). These SiO.sub.2 primary particles are obtained by oxidation or by flame hydrolysis of a silicon-containing feedstock, particularly preferably a chlorine-free, silicon-containing feedstock. Such SiO.sub.2 primary particles are distinguished by a particularly high purity and sintering activity, so that their granulation may be carried out by known granulation methods without the addition of binders.

(37) A reliable feedstock for the production of synthetic quartz glass is silicon tetrachloride (SiCl.sub.4). This substance may be converted by flame hydrolysis in the presence of hydrogen and oxygen and by pyrolysis, respectively, in the presence of oxygen into SiO.sub.2. This entails, however, the drawback that chlorine or chlorine compounds that impede the use of unprotected metallic components because of their corrosiveness are formed during conversion. To avoid this drawback, the use of another silicon-organic compound is therefore suggested, from which SiO.sub.2 can be formed by hydrolysis or oxidation. Polyalkylsiloxanes should here be mentioned by way of example.

(38) Synthesis of SiO.sub.2 Primary Particles

(39) SiO.sub.2 primary particles are obtained as so-called soot in the production of synthetic quartz glass. Soot is deposited using flame hydrolysis burners that are reversingly reciprocated in a joint burner row along a rotating carrier tube. The flame hydrolysis burners are respectively fed with oxygen and hydrogen as burner gases and with an SiO.sub.2 feedstock vapor as feedstock for the formation of the SiO.sub.2 particles. The vapor contains a mixture of evaporated polyalkylsiloxanes, including hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). D4 is the main component of the polyalkylsiloxane feedstock with an amount of 98% by weight.

(40) The SiO.sub.2 soot particles which are produced are present in the form of agglomerates or aggregates of the SiO.sub.2 primary particles with particle sizes in the nanometer range. The SiO.sub.2 soot particles are deposited on the cylindrical outer surface of the carrier tube rotating about its longitudinal axis, so that a soot body is built up layer by layer.

(41) Part of the SiO.sub.2 soot particles passes via suction into a filter system and is obtained as so-called soot dust or filter dust.

(42) This pyrogenically obtained SiO.sub.2 soot material of high purity is present in a particle size of less than 100 nm and typically has a specific (BET) surface area of about 30 m.sup.2/g and a bulk density of about 0.1 kg/l. It is used for producing SiO.sub.2 granulate.

(43) Production of SiO.sub.2 Spray Granulate

(44) A SiO.sub.2 slip is produced by dispersing the pyrogenically obtained SiO.sub.2 soot material of high purity in deionized water. The dispersion is adjusted to a liter weight of 1380 g/l. The slip viscosity is 450 mPas. The slip is then sprayed at a hot air temperature of 400 C. and at a slip pressure of 10.5 bar using a commercially available spray dryer.

(45) A spray granulate with a mean grain diameter of 214 m is obtained. The fine fraction with a diameter below 100 m is separated due to the manufacturing method during spray granulation by a cyclone. Each granulate particle is present as an agglomerate of individual spherical SiO.sub.2 primary particles. The mean particle size of the SiO.sub.2 primary particles is about 50 nm. The agglomerate of the SiO.sub.2 primary particles is loose, so that it can be ground and crushed by applying a slight mechanical pressure. Open pore channels are formed between the SiO.sub.2 primary particles. The specific (BET) surface area of the spray granulate is 30 m.sup.2/g and the bulk density is 0.8 kg/l.

(46) Production of Freeze Granulate

(47) In a frost-proof, closable PTFE bottle, a SiO.sub.2 slip is prepared by dispersing the pyrogenically obtained SiO.sub.2 soot material of high purity in deionized water. The solids content of the slip is 12% by weight. For homogenization, the SiO.sub.2 slip is stirred for several hours. The PTFE bottle is subsequently deep-frozen overnight in a freezer at 18 C.

(48) During defrosting, the agglomerated SiO.sub.2 particles separate as sediment from the water, so that the sediment is present in the lower half of the container and a more or less clear liquid is present above the sediment. The liquid is poured off. The remaining residual water is removed with a centrifuge from the moist sediment of agglomerated SiO.sub.2 particles. At a speed of 5000 rpm, a SiO.sub.2 granulate is obtained after about five minutes; this granulate is dried thoroughly in the drying cabinet at a temperature of 200 C.

(49) The fine fraction with a diameter below 100 m is sieved off. A freeze granulate with a mean grain diameter of 216 m is obtained. The specific (BET) surface area is 30 m.sup.2/g and the bulk density is 0.54 kg/l.

(50) Cleaning and Sintering of the Spray Granulate

(51) The SiO.sub.2 spray granulate is subsequently cleaned in a continuous furnace in an HCl/Cl.sub.2 gas mixture and thereby thermally pre-densified. In a test series, the maximum temperature and the throughput (and thus the mean residence time in the rotary furnace) were varied, as indicated in the second column of Table 1. Mean throughput means about 15 kg/h. The accompanying mean residence time is about 30 minutes. Low throughput and high throughput are about 5 kg/h lower or higher. In the samples treated in the rotary furnace, the last hot treatment step (second column of the table) is marked as dynamic.

(52) In some samples of the test series, the granulates were not cleaned and sintered in the rotary furnace, but in a graphite crucible with a corresponding HCl/Cl.sub.2 mixture. The residence time was 4 hours each time. In the samples densified in this way, the last hot treatment step is called static in Table 1.

(53) After this cleaning and sintering treatment, the granulate particles are thermally densified. The individual SiO.sub.2 primary particles are grown together relatively firmly by so-called neck formation. The pore channels existing before sintering have narrowed, but may still be permeable and at least partly open, depending on the intensity of the thermal action (sintering temperature and sintering duration). The corresponding degree of porosity is called open or partly open in column 5 of Table 1. In the transition region, in which parts of the previously open pore channels are closed and other parts are still open, the SiO.sub.2 granulate no longer shows optimum properties in the vitrification to quartz glass. The pore channels are closed at least superficially and are noted as closed in column 5. The corresponding example of Sample G is a comparative example. The porosity of the granulate particles is low in these cases, and there is the risk that gases of the rotary-furnace atmosphere are entrapped, so that a bubble-rich quartz glass is produced during fusion. As a measure of this, the ratio T.sub.500/T.sub.1700 is used, which is greater than 0.8 in this case.

(54) Each densified granulate is characterized by its particle size distribution. A narrow particle size distribution is desired on account of facilitated fusibility. A particle size distribution, in which the particle size assigned to the D.sub.90 value is not more than three times as great as the particle size assigned to the D.sub.10 value, is desired. As far as necessary, a possible fine fraction of the granulate or a fraction of coarse granulate particles that may, e.g., have formed during cleaning and sintering by agglomeration is removed for this purpose prior to melting. The particle size distribution of Sample E (Table 1) is, e.g., characterized by the following figures:

(55) D10: 130 m

(56) D50: 230 m

(57) D90: 380 m

(58) Moreover, the specific BET surface area, the porosity, the bulk density and the purity have been determined for each granulate. The total content of the impurities of Li, Na, K, Mg, Ca, Fe, Cu, Cr, Mn, Ti, and Zr is each time less than 200 wt. ppb. The other measurement results are also shown in Table 1.

(59) Dense Sintering of Spray Granulate for the Preparation of a Reference Sample

(60) A batch of the SiO.sub.2 spray granulate is introduced into an evacuable crucible for producing dense transparent quartz glass granules and is heated in vacuum (residual pressure <2 mbar) for a holding period of four hours to a temperature of 1450 C.

(61) The completely vitrified SiO.sub.2 particles are transparent and have no or only a few small bubbles. Their BET surface area is zero. The mean particle size is 166 m and the bulk density is 1.20 kg/l. These quartz glass granules are used as a reference sample for IR transmission measurements.

(62) Fusion of the Granulates into Quartz Glass

(63) The granulates and the quartz glass granules were fused into quartz glass. Fusion was each time carried out under the same conditions by introducing a batch of 1 kg of the granulate into an evacuable melting crucible of graphite, heating up the melting crucible to an end temperature of 1700 C. in vacuum (residual pressure <2 mbar) and holding at that temperature for a holding period of 200 minutes. After expiration of half the holding period, the softened quartz glass mass was subjected to a mechanical uniaxial pressure of 15 bar. The cooled quartz glass bodies were visually checked for bubbles and evaluated.

(64) In principle, each of the SiO.sub.2 granulates could possibly be fused into a bubble-free transparent quartz-glass body if only the heating intensity (temperature and holding period) is chosen to be sufficiently high. The present fusion process, however, is conceived such that the heating intensity for fully vitrified granulate (quartz glass granules) is just sufficient to obtain a bubble-free transparent quartz glass body as the end product.

(65) The question whether and to what extent this fusion process, which is very tight with respect to the applied heating energy, also yields high-quality quartz glass for less densified SiO.sub.2 granulates (namely Samples A to G and the not additionally thermally densified spray granulate and freeze granulate, respectively) is indicated in the last column of Table 1. The so-called TBCS value as a measure of the bubble content of the respectively obtained quartz glass body is listed and is representative of the quality of the quartz glass. The TBCS value designates the sum of the cross-sectional areas (in mm.sup.2) of all bubbles within a sample volume of 100 cm.sup.3. This value is determined by visually detecting the bubbles and adding the bubble cross-sections, bubbles with diameters of less than 0.08 mm being not recorded.

(66) FIG. 2 shows the infrared transmission spectra of the granulates and the SiO.sub.2 soot, respectively, and of the completely vitrified quartz glass granules as a reference sample over the wavelength range of 500-2500 nm. The IR transmission was measured using an integrating sphere (company Perkin-Elmer, lambda 900) on a bulk material of SiO.sub.2 granulate with a thickness of 4.2 mm in a measuring cuvette of quartz glass, normalized to an empty sample (cuvette without filling=empty intensity I.sub.0). The distance of the measuring cuvette from the entrance of the integrating sphere is 4 mm. The opening of the integrating sphere has a diameter of 23 mm and its diameter is 60 mm. The measurement beam has a cross section of about 1010 mm. The measured transmission (T(measured)=I/I.sub.0) follows from the measured intensity I by measurement of the diffuse and directed transmission of the sample in the integrating sphere.

(67) The SiO.sub.2 granulates differ in their density, i.e., their mass occupation per volume unit. A measure of the different mass occupation is the bulk density. To make the transmission measurement values comparable despite different mass occupation over the measurement length, the transmission values are therefore normalized to the bulk density of the respective granulate. This means that they are multiplied by the specific bulk density of the granulate as indicated in column 5 of Table 1. The transmission values T plotted on the ordinate are therefore determined as T (measured)bulk density, as: T=measuring intensity I/empty intensity I.sub.0bulk density. They have the unit [% g/cm.sup.3] and are valid for a measurement length of 4.2 mm.

(68) It can be seen that the wavelengths 500 nm and 170 nm, which are particularly used for characterizing the invention, are hardly influenced by absorption bands that are caused by water and OH groups. As a rule, the transmission is increasing with an increasing densification (in the order of the samples soot, freeze granulate, spray granulate, thermally densified Samples A to G). The maximum transmission of Sample G is even slightly higher than the maximum transmission of the reference sample consisting of quartz glass granules. The material-specific transmission T.sub.1700 at the wavelength of 1700 nm can serve as a measure of the degree of densification of the respective granulate.

(69) Furthermore, it can be seen that the still completely or at least substantially open-pore SiO.sub.2 granulates of Samples A to E show a significant rise in transmission in the wavelength range between 500 nm and 1300 nm. In the particularly highly densified Samples F and G (and in the reference sample) this rise is small or does not exist at all. The transition between Sample E (partly closed porosity) and Sample F (also still partly closed porosity) is particularly significant. The porosity of both samples is characterized as partly closed. Evidently, already small differences in porosity are distinctly noticeable in the rise of the transmission curve in the wavelength range of 500 nm to 1300 nm. This rise is thus also a measure of the porosity or optical scattering effect of the respective granulate. It is characterized by the difference of the material-specific infrared transmission values T.sub.500 and T.sub.1700, respectively, measured at the wavelengths 500 nm and 1700 nm. Column 8 of Table 1 shows that in Sample E this material-specific transmission ratio is still 0.24, whereas it has risen suddenly to 0.8 in Sample F, which is evaluated as a limit value of the open porosity of the densified granulate particles. The ratio is already 0.97 in Comparative Sample G.

(70) As shown in the last column of Table 1, the quartz glass with the lowest bubble content (measured as TBCS value) is achieved by using the completely vitrified quartz glass granules. This result had to be expected per se. A comparatively good quartz glass quality with respect to bubble content is, however, also achieved by using the two granulates of Sample D and particularly of Sample E. The granulates of Samples D and E, however, show a low pre-densification, as shown both in the transmission value T.sub.1700 of 85% and 93.7%, respectively, and in the low values for the ratio T.sub.500/T.sub.1700, namely 0.2 and 0.24, respectively. Hence, these granulates yield a quartz glass quality with similar TBCS values as the completely vitrified quartz glass granulates under the same fusion conditions. The preparation of the quartz glass from the only pre-densified granulates, however, requires comparatively smaller efforts in terms of energy and time. The reason is that the dense sintering of the spray granulate is more complicated than the preparation of the only partly densified granulates.

(71) However, sample F, which is only slightly more densified in comparison with Sample E, shows a slightly higher bubble content and Sample G even a significantly higher bubble content (measured as TBCS value). Although in Sample F the mean densification, represented by T.sub.1700, is also 93.8% (and thus similarly high as in Sample E), the ratio T.sub.500/T.sub.1700 is 0.8 and thus much higher than in Sample E. The quartz glass body produced from the granulate of Sample E turns out to be better in relation to the bubble content than that of Sample F and much better than that of Sample G.

(72) The quartz glass obtained from Sample F is regarded as just acceptable, but nevertheless as suitable. The granulate of the very strongly densified Sample G yields, however, a quartz glass body with a rather disadvantageous transmission and emission behavior in the infrared wavelength range, represented by a high value of the material-specific transmission ratio T.sub.500/T.sub.1700 of 0.97, which surprisingly has many and/or large bubbles in addition, represented by the TBCS value of 2. The latter can be ascribed to the fact that, in the fusion process, gases which are entrapped in the granulate particles could expand due to heating, but could no longer escape.

(73) Also, the slightly densified granulate of Sample A yields a bubble-containing quartz glass. The existing bubbles are, however, small, so that the quartz glass is still acceptable for applications where small demands are made on the absence of bubbles.

(74) In the histograms of FIG. 3, the bubble number N is respectively plotted on the ordinate (per 100 cm.sup.3 sample volume) against the bubble diameter d (in mm) in the range of 0.08 mm to 1 mm (and more).

(75) Hence, the spray granulate (histogram at the top at the left side) exclusively shows bubbles with diameters of 1 mm and more; the bubble number is about 50.

(76) This bubble number is approximately halved in Sample B, which is only moderately densified thermally (see Table 1; histogram at the top at the right side), but particularly the bubble size is much reduced. Most bubbles only have bubble diameters in the range of the detection limit (0.08 mm).

(77) This densification effect is also noticeable in the bubble number and diameter of the more strongly densified Samples D and E (lower histograms).

Definitions

(78) For better understanding of the individual method steps and terms of the above description, the following definitions are provided.

(79) 1. Synthesis of SiO.sub.2

(80) Synthetic quartz glass consists of undoped or doped silicon dioxide which has preferably been produced by hydrolysis or oxidation of silicon-containing start material. Halogenated silicon compounds, such as SiCl.sub.4, SiHCl.sub.3 and alkoxides (tetramethoxysilane, TMOS, methyltrimethoxysilane, MTMS) or siloxanes (such as polyalkylsiloxanes, for instance octamethylcyclotetrasiloxane OMCTS) or mixtures of these starting materials are suitable as starting materials.

(81) SiO.sub.2 primary particles in the nanoscale range with a SiO.sub.2 content of more than 99.99% and hardly any impurities are obtained by hydrolysis or oxidation. In their entirety, these form a finely divided SiO.sub.2 powder which is also called soot dust or soot.

(82) SiO.sub.2 powder has a high purity in the sense that the main component is SiO.sub.2 and only small amounts of other elements in the ppm or ppb range are present. Impurities in the SiO.sub.2 powder or in the quartz glass produced therefrom are determined by ICP-OES or ICP-MS methods, the concentrations being indicated in weight percentages.

(83) The specific (BET) surface area is determined according to the method of Brunauer, Emmet and Teller (BET) on the basis of DIN 66132 and is based on gas absorption on the surface to be measured.

(84) A particularly interesting group of chlorine-free feedstock is formed by the polyalkylsiloxanes (also briefly called siloxanes). Siloxanes can be subdivided into open-chain and closed-chain polyalkylsiloxanes. The polyalkylsiloxanes have the general sum formula Si.sub.pO.sub.p(R).sub.2P, where P is an integer 2. The residue R is an alkyl group, in the simplest case a methyl group. Polyalkylsiloxanes are distinguished by a particularly high amount of silicon per weight percentage, which makes their use economic in the production of synthetic quartz glass. The polyalkylsiloxane feedstock, however, preferably contains three different polyalkylsiloxanes selected from hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6). The abbreviations D3, D4, and D5 are based on a notation introduced by General Electric Inc., wherein D is also designated as D4 and D represents the group [(CH.sub.3).sub.2Si]-0-. In a preferred variant, D4 is the main component of the polyalkylsiloxane feedstock. Thus, the amount of D4 is at least 70% by wt., particularly at least 80% by wt., preferably at least 90% by wt. and particularly preferably at least 94% by weight.

(85) 2. Preparation of an SiO.sub.2 Slip

(86) The term slip is used for a suspension consisting of a liquid and SiO.sub.2 powder. Water may be used as the liquid, which is cleaned by distillation or deionization to minimize the content of impurities. The SiO.sub.2 powder is almost insoluble in water at room temperature, but may be introduced into water in high weight percentages.

(87) The particle size in the suspension designates the particle size distribution of the solid particles contained in a suspension. It is measured by dynamic light scattering (DLS). The scattering behavior of laser light depends on the particle size and varies in time due to the Brownian motion of the particles. The diffusion coefficients of the particles follows from the analysis of intensity fluctuations. The particle size, in turn, may be calculated from the diffusion coefficients by the Stokes-Einstein equation.

(88) 3. Granulation

(89) A distinction can be made between build-up granulation and press granulation and in terms of the technical process between wet and dry granulating methods. Known methods are roll granulation in a pan granulator, spray granulation, centrifugal atomization, fluidized-bed granulation, granulating methods using a granulating mill, compaction, roller presses, briquetting, flake production, or extrusion.

(90) In the granulating process, discrete, rather large agglomerates, which are here called SiO.sub.2 granulate particles or briefly granulate particles are formed by agglomerations of the SiO.sub.2 primary particles. In their entirety, the granulate particles form an SiO.sub.2 granulate.

(91) Spray granulation belongs to the group of wet granulating methods. During spray granulation, fine drops of the slip are formed, which are subsequently and simultaneously dried by a hot gas stream into a spray granulate. The spray granulate particles may be obtained in different sizes in the range of a few micrometers up to a few millimeters in diameter.

(92) The particle size of the spray granulates designates the macroscopic dimension of each solid granulate particle and is determined as the particle size distribution, normally by sieve methods or laser scattering methods. In sieve analysis, the powder is separated by sieve bottoms with different sieve sizes. This method is particularly simple and is therefore preferred. The weight percentage of the sieve material within a size range defined by two sieve sizes is plotted against the particle size, so that the particle size distribution is obtained. Typical size specifications are D.sub.10, D.sub.50 and D.sub.90, designating the weight percentage of the sieve material in percent that is smaller than the corresponding value specification.

(93) The term bulk density (also called bulk weight) of a granulate or powder is indicated in mass per volume unit. The bulk density is defined as the mass of many material particles based on the total volume occupied by them. It is measured using a container of a known volume that is filled and weighed. The bulk density of substances present in powder or granulate form is determined according to the international standard ISO 697 (formerly DIN 53912). The term tapped density designates the density which is determined after mechanical densification of the powder or of the granulate, e.g., by vibration of the container. It is determined according to DIN/ISO 787 Part 11.

(94) The pore volume of a porous material designates the free volume occupied by voids within the material. The pore volume is, e.g., measured by a porosimeter. A non-wetting liquid (such as mercury) is pressed under the action of an external pressure into the pores of a porous material against the counteracting surface tension forces. The force needed for this is inversely proportional to the pore size, and the pore size distribution of the sample can therefore also be determined apart from the total pore volume. Mercury porosimetry only detects pore sizes above 2 nm (mesopores and macropores).

(95) Micropores are pores with pore sizes of less than 2 nm. Their contribution to the porosity and the specific surface area is determined by the V-t method by nitrogen absorption, with a sample being held at differently small pressures and 77 K. The method is similar to the BET method, wherein the pressure range is expanded to higher pressures, so that surfaces of the non-microporous part of the material are also recorded.

(96) 4. Cleaning of the Granulate

(97) During cleaning the content of impurities in the granulate is reduced. The main impurities are residual water (OH groups), carbonaceous compounds, transition metals, alkali metals, and alkaline earth metals that derive from the starting material or are introduced by the processing. A small impurity content may be achieved by using pure starting materials and by corresponding equipment and processing under cleanroom conditions. To fulfill even higher demands made on purity, the granulate may be treated in a rotary furnace at a high temperature (<900 C.) in a chlorine-containing and oxygen-containing atmosphere. Residual water evaporates, organic materials react to form CO and CO.sub.2, and many metals (such as iron and copper) may be converted into volatile chlorine-containing compounds.

(98) After this treatment, the spray granulate shows an impurity content of less than 1 ppm for each of the metallic impurities.

(99) 5. Sintering/Densifying and Vitrifying

(100) Here, sintering or densifying designates a method step in which an SiO.sub.2 granulate is treated at an elevated temperature of more than 1100 C. either in a dynamic furnace (for instance in a rotary furnace) or in a static furnace. The specific (BET) surface area is reduced, whereas the bulk density and the mean particles size can increase due to agglomerations of granulate particles.

(101) During vitrification, the pre-densified sintered SiO.sub.2 granulate is vitrified to form a quartz glass body. The granulate is filled into a melt mold which consists, for instance, of graphite or is lined with graphite foil. The melt mold is then heated in an electric furnace to a temperature of about 1700 C. or more. Heating is performed under vacuum, which stands for an absolute gas pressure of less than 2 mbar. The softened quartz glass may be subjected to a static pressure (5 to 20 bar). After cooling to room temperature, a quartz glass body which is without bubbles, if possible, is obtained; this body is, e.g., suited for optical or semiconductor applications.

(102) Absence of bubbles means that the total cross-sectional area of all bubbles within a sample based on a specific volume of 100 cm.sup.3 is not more than 0.5 (TBCS value, total bubble cross section). The total cross section of all bubbles is indicated in square millimeters, with bubbles having a diameter of less than 0.08 mm being not counted.

(103) It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.