Boron oxide-silicon dioxide mixed oxide

Abstract

A boron oxide-silicon dioxide mixed oxide which has a BET surface of less than 100 m.sup.2 g, and optionally containing oxides of aluminium, titanium or zirconium, is prepared pyrogenically by flame hydrolysis. The mixed oxide is used in glass making.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a boron oxide-silicon dioxide mixed oxide, a process for its preparation, and its use.

2. Prior Art

Document DE-A 21 22 066 describes the use or pyrogenically produced mixed oxides of boron with silicon, aluminum, titanium and/or iron having a boron content of 2 to 20 wt. %, calculated as elementary boron, as a filler in organopolysiloxane compositions (for example bouncing putty). In particular these mixed oxides contain 5 to 10 wt. % of boron, calculated as elementary boron. These mixed oxides can be prepared by using metals and/or metal compounds according to Ullmanns Enzyclopadie der technischen Chemie, (Ullmanns Encyclopedia of Industrial Chemistry), Vol. 15 (1964), p.726. They are preferably prepared from volatile compounds of boron and silicon, aluminum, titanium and/or iron, especially the chlorides of the aforementioned elements, in the presence of water formed in situ at temperatures above 800.degree. C., i.e. by flame hydrolysis. The mixed oxides obtained have a BET surface of 100 to 400 m.sup.2 /g.

According to the example given in document DE-A 21 22 066, 4.8 kg/h of SiCl.sub.4 and 1.2 kg/h of boron trichloride are vaporised and burnt together with 1.5 m.sup.3 /h of hydrogen in a combustion chamber, under the addition of 4.2 m.sup.3 /h of air. The temperature in the combustion chamber is more than 800.degree. C. The resultant mixed oxide contains 82.5 wt. % of SiO2 and 17.5 wt. % of B.sub.2 03 (=5 wt. % of boron) calculated as boron and has a BET surface of 180 m.sup.2 /g.

The known boron oxide-silicon dioxide mixed oxide has the disadvantage that it occurs in very finely divided form having a BET surface greater than 100 m.sup.2 /g. It is unsuitable for use as a raw material in glass-making.

The known process has the disadvantage that boron trichloride is used as starting material for the boron oxide. Boron trichloride is a poisonous compound with a boiling point of 12.5.degree. C. When used at room temperature special safety measures are therefore necessary. The commercially available boron trichloride may contain noticeable amounts of poisonous phosgene as impurity.

The object of the invention is therefore to prepare a boron oxide-silicon dioxide mixed oxide that does not have these disadvantages.

SUMMARY OF THE INVENTION

The invention provides a boron oxide-silicon dioxide mixed oxide, which is characterised in that it has a BET surface of less than 100 m.sup.2 /g, preferably 10 to 80 m.sup.2 /g, in particular 25 to 50 m.sup.2 /g.

The proportion of boron oxide may be from 0.01 to 40 wt. %.

In a further development of the invention the boron oxide-silicon dioxide mixed oxide may contain a further oxide or further oxides of metals and/or metalloids as a constituent of the mixed oxide.

Such metals or metalloids may be aluminum, titanium or zirconium.

In a preferred embodiment the boron oxide-silicon dioxide mixed oxides according to the invention may be prepared pyrogenically.

The known process of flame hydrolysis may preferably be employed for this purpose. This process is described in Ullmanns Enzyclopadie der technischen Chemie, 4th Ed., Vol. 21 (1982), pp. 464 and 465.

The boron oxide-silicon dioxide mixed oxide according to the invention having a BET surface of less than 100 m.sup.2 /g, preferably 10 to 80 mg.sup.2 /g, in particular 25 to 50 m.sup.2 /g, can be prepared by vaporising silicon halides and/or organosilicon halides, for example methyl trichlorosilane, preferably the chloride SiCl.sub.4 with a vaporisable boron compound, for example trimethyl borate, separately or together, optionally with the addition of vaporisable compounds of the metalloids and/or metals aluminum, titanium, zirconium, for example AlCl.sub.3, ZrCl.sub.4, TiCl.sub.4 or the like, mixing the vapours together with a carrier gas, for example air and/or nitrogen, in a mixer unit, preferably in a burner of known construction, with hydrogen as well as air and optionally further gases such as oxygen and nitrogen, reacting the gases in a flame, then cooling the hot gases and the solid, separating the gases from the solid, and optionally removing halide or raw material residues adhering to the product by a heat treatment with moist air.

The process according to the invention has the advantage that the trimethyl borate that is used is miscible in all proportions with SiCl.sub.4, and it is therefore possible to adjust the ratio exactly. Since the boiling point of trimethyl borate of 68.degree. C. is sufficiently close to the boiling point of silicon tetrachloride, it can be vaporised jointly with silicon tetrachloride from a vaporisation unit.

The boron oxide-silicon dioxide mixed oxide according to the invention, which may optionally also contain TiO.sub.2, Al.sub.2 O.sub.3 and/or ZrO.sub.2, can be used to make high-purity glasses. The high purity and the adjusted particle fineness of the mixed oxide according to the invention are of particular advantage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention now will be described with respect to the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a burner arrangement used for Examples 1-4 of the invention;

FIG. 2 is a diagrammatic illustration of a burner arrangement used for Examples 5-9 of the invention;

FIG. 3 is an electron microscope photograph of the powder of Example 2;

FIG. 4 is an electron microscope photograph of the powder of Example 9;

FIG. 5 is a summation curve of the number distribution of Example 2;

FIG. 6 is a differential curve of the number distribution of Example 2;

FIG. 7 is a class frequency representation of the number distribution of Example 2;

FIG. 8 is a summation curve of the weight distribution of Example 2;

FIG. 9 is a differential curve of the weight distribution of Example 2;

FIG. 10 is a class frequency representation of the weight distribution of Example 2;

FIG. 11 is a summation curve of the number distribution of Example 9;

FIG. 12 is a differential curve of the number distribution of Example 9;

FIG. 13 is a class frequency representation of the number distribution of Example 9;

FIG. 14 is a summation curve of the weight distribution of Example 9;

FIG. 15 is a differential curve of the weight distribution of Example 9; and

FIG. 16 is a class frequency representation of the weight distribution of Example 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

EXAMPLES

Preparation of Boron Oxide-Silicon Dioxide Mixed Oxide

Examples 1 to 4

The burner arrangement for examples 1 to 4 is shown diagrammatically in FIG. 1.

The burner is housed in a heated furnace part, the temperature of the furnace being roughly the same as the outflow temperature of the gas mixture. The burner consists of a mixing chamber to which is connected a nozzle from which the gas mixture flows, as well as a second (jacket) nozzle annularly surrounding the first nozzle, from which in addition (jacket) hydrogen is blowN into the flame in order to avoid agglomerations (in the nozzle region).

Example 1

1.0796 kg/h of SiCl4 are vaporised at ca. 130.degree. C. and conveyed together with 0.1 Nm.sup.3 /h of nitrogen as carrier gas into the mixing chamber of the burner. 0.51 Nm.sup.3 /h of air as well as 0.107 Nm.sup.3 /h of oxygen and 0.3 Nm.sup.3 /h of (core) hydrogen are ) fed into the same mixing chamber.

0.0643 kg/h of trimethyl borate are vaporised at ca. 120.degree. C. in a separate vaporisation device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm.sup.3 /h). The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) from the nozzle opening of the burner and burns in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiCl.sub.4)-air-hydrogen, oxygen, trimethyl borate) is measured at the mouth of the burner, and is found to be 128.degree. C. 0.05 Nm.sup.3 /h of hydrogen (jacket hydrogen, room temperature) are fed into the annular nozzle surrounding the mouth of the burner.

After the flame hydrolysis the reaction gases and the boron oxide-silicon dioxide mixed oxide are sucked through a cooling system by applying a vacuum and are cooled to ca. 100.degree. to 160.degree. C. The solid is separated from the waste gas stream in a filter. The boron oxide-silicon dioxide mixed oxide occurs as a white, finely particulate powder.

The specific surface is 45 m.sup.2 /g, and the analytically determined B.sub.2 0.sub.3 content is 4.2 wt. % (=1.3 wt. % of boron, calculated as boron).

The adjustment parameters are summarised in Table 1.

Example 2

The procedure described in Example 1 is employed and the following amounts are used:

0.9387 kg/h of SiCl.sub.4 with 0.1 Nm.sup.3 /h of nitrogen as carrier gas, air 0.76 Nm.sup.3 /h, oxygen 0.159 Nm.sup.3 /h (core) hydrogen 0.268 Nm.sup.3 /h.

0.05 Nm.sup.3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 14 m/sec (referred to standard conditions) from the nozzle opening of the burner.

0.1777 kg/h of trimethyl borate are vaporized in a separate vaporization device and likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm.sup.3 /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 34 m.sup.2 /g, and the analytically determined B.sub.2 0.sub.3 content is 13.3 wt. % (=4.1 wt. % of boron, calculated as boron).

The adjustment parameters are summarised again in Table 1.

The boron oxide-silicon dioxide obtained has the following particle size distribution:

______________________________________Total number of particles (N) 3223.Particle diameter, arithmetic mean (DN) 45.72 (NM)Particle diameter, averaged over (DA) 60.81 (NM)surface______________________________________ Percentage Distribution ______________________________________ Sum SumDiameter Number Number Number % Wt. %D (NM) N % SN % SND %______________________________________7.450 40. 1.241 1.241 0.00310.210 123. 3.816 5.057 0.03012.970 155. 4.809 9.867 0.09915.730 105. 3.258 13.124 0.18218.490 56. 1.738 14.862 0.25421.250 64. 1.986 16.848 0.38024.010 64. 1.986 18.833 0.56026.770 78. 2.420 21.253 0.86529.530 98. 3.041 24.294 1.37932.290 86. 2.668 26.962 1.97035.050 109. 3.382 30.344 2.92637.810 127. 3.940 34.285 4.32540.570 173. 5.368 39.652 6.68043.330 198. 6.143 45.796 9.96346.090 193. 5.988 51.784 13.81448.850 191. 5.926 57.710 18.35251.610 178. 5.523 63.233 23.33954.370 160. 4.964 68.197 28.58057.130 153. 4.747 72.944 34.39559.890 135. 4.189 77.133 40.30662.650 140. 4.344 81.477 47.32265.410 113. 3.506 84.983 53.76768.170 109. 3.382 88.365 60.80570.930 86. 2.668 91.033 67.06073.690 64. 1.986 93.019 72.28076.450 52. 1.613 94.632 77.01579.210 43. 1.334 95.966 81.37181.970 26. 0.807 96.773 84.28984.730 28. 0.869 97.642 87.76187.490 26. 0.807 98.449 91.31090.250 22. 0.683 99.131 94.60693.010 10. 0.310 99.442 96.24695.770 3. 0.093 99.535 96.78398.530 5. 0.155 99.690 97.757101.290 3. 0.093 99.783 98.393104.050 7. 0.217 100.000 100.000______________________________________ DN = 45./2 A = 60.81 D50 = 45.27 K = 4 A(K) = 0.1247 FQS = 1.7246 DN/D50 = 1.0100 DA/D50 = 1.3434 DA/DN = 1.3301

These data are illustrated graphically in FIGS. 5 to 10.

Example 3

The procedure described in Example 1 is employed, the following amount being used:

0.5005 kg/h of SiCl.sub.4 with 0.1 Nm.sup.3 /h of nitrogen as carrier gas, air 0.726 Nm.sup.3 /h, oxygen 0.152 Nm.sup.3 /h, (core) hydrogen 0.14 Nm.sup.3 /h.

0.05 Nm.sup.3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 124.degree. C. from the nozzle opening of the burner.

0.225 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm.sup.3 /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 39 m.sup.2 /g, and the analytically determined B.sub.2 0.sub.3 content is 23.0 wt. % (=7.1 wt. % of boron, calculated as boron).

The adjustment parameters are again summarized in Table 1.

Example 4

The procedure described in Example 1 is adopted, the following amounts being used:

0.365 kg/h of SiCl.sub.4, with 0.1 Nm.sup.3 /h of nitrogen as carrier gas, air 0.759 Nm.sup.3 /h, oxygen 0.159 Nm.sup.3 /h, hydrogen 0.103 Nm.sup.3 /h.

0.05 Nm.sup.3 /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner.

0.2575 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm.sup.3 /h).

The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 125.degree. C. from the nozzle opening of the burner.

The specific surface of the boron oxide-silicon dioxide mixed content is 30 m.sup.2 /g, and the analytically determined B.sub.2 O.sub.3 content is 29.0 wt. % (=9.0 wt. % or boron calculated as boron).

The adjustment parameters are again summarized in Table

TABLE 1__________________________________________________________________________Experimental conditions and flame parameters calculated therefrom inthe preparation of pyrogenic boron oxide-silicon dioxide mixed oxides. SiCl.sub.4 TMB Air O.sub.2 H.sub.2 core N.sub.2 core gamma lambda BET B.sub.2 O.sub.3No. [kg/h] [kg/h] [Nm.sup.3 /h] [Nm.sup.3 /h] [Nm.sup.3 /h] [Nm.sup.3 /h] [-] [-] [m.sup.2 /g] [wt. %]__________________________________________________________________________1 1.0796 0.0643 0.510 0.107 0.300 0.12 1.27 1.01 45 4.22 0.9387 0.1777 0.760 0.159 0.268 0.12 1.78 1.03 34 13.33 0.5005 0.2250 0.726 0.152 0.140 0.12 2.72 1.04 39 23.04 0.3651 0.2575 0.759 0.159 0.103 0.12 3.67 1.04 30 29.0__________________________________________________________________________ TMB = Trimethyl borate B(OCH.sub.3).sub.3 Ratio H.sub.2 gamma = Ratio of fedin hydrogen in the core (taking into account the hydrogen contained in TMB) to stoichiometrically required hydrogen Ratio O.sub.2 lambda = Ratio of fedin oxygen (atmospheric oxygen + additionally added O.sub.2) in the burner to stoichiometrically required oxygen

Deacidification and Removal of Raw Material Residues

In order to remove raw material residues that have possibly incompletely reacted and are still adhering to the product, and to reduce the chloride content of the samples, the oxides prepared according to Examples 1 to 9 can undergo a further temperature treatment stage.

For this purpose the powders are treated with moist air in a countercurrent downpipe arrangement at temperatures between 400.degree. and 700.degree. C. (preferably 650.degree. C.). (Deacidification)

The analysis date of the powders prepared according to Examples 1 to 4 before and after deacidification are summarised in Tables 2 and 3.

TABLE 2__________________________________________________________________________Analysis data of the samples obtained according to Examples 1 to 4(B--Si-mixed oxide)Before deacidification After deacidification Cl Cl BET B.sub.2 O.sub.3 TV GV Content BET B.sub.2 O.sub.3 TV GV ContentNo m.sup.2 /g Wt. % Wt. % Wt. % Wt. % m.sup.2 /g Wt. % Wt. % Wt. % ppm__________________________________________________________________________1 45 4.2 1.5 0.4 200 46 3.6 0.6 0.1 1382 34 13.3 1.8 2.7 142 37 10.1 0.4 1.5 873 39 23.0 0.8 3.1 87 44 22.2 0.5 2.5 184 30 29.0 1.0 3.8 48 33 28.6 0.7 1.3 13__________________________________________________________________________ TV = Drying loss (2 h at 10.degree. C., according to DIN/ISO 787/II, ASTM D 280, JIS K 5101/21) GV = Annealing loss (2 h at 1000.degree. C., according to DIN 55921, ASTM D 1208, JIS K 5101/23, referred to the substance dried for 2 hours at 105.degree. C. TABLE 3______________________________________Further analysis data of the samples obtained according to Examples 1to 4 (B--Si-mixed oxide)Before deacidification After deacidification Bulk Tamped Bulk Tamped BET density density BET density densityNo. m.sup.2 g pH g/l g/l m.sup.2 /g pH g/l g/l______________________________________1 45 3.84 58 69 46 4.09 88 1132 34 3.89 46 52 37 3.93 91 1153 39 3.69 39 46 44 3.88 69 874 30 3.52 41 48 33 3.88 94 118______________________________________ pH = pH in 4 t aqueous suspension., Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened).

Examples of the Preparation of Boron Oxide-Aluminum Oxide-Silicon Dioxide Mixed Oxide

The burner arrangement for Examples 5 to 9 is illustrated diagrammatically in FIG. 2.

Example 5

The two liquids SiCl.sub.4 and trimethyl borate are mixed In the desired ratio before being vaporised and are then converted together into the gaseous phase in a vaporiser at 135.degree. C. 0.876 kg/h of SiCl.sub.4 is vaporised together with 0.231 kg/h of trimethyl borate. The gases are conveyed with a stream of nitrogen at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of the burner mentioned in Example 1.

The temperature of the furnace in which the burner is housed is ca. 240.degree. C. 0.056 kg/h of AlCl.sub.3 is vaporized in a separate vaporiser at temperatures of ca. 230.degree. C. and is similarly conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of this burner.

0.938 Nm3/h of air as well as 0.196 Nm.sup.3 /h of oxygen and 0.254 Nm.sup.3 /h of hydrogen are introduced into the burner mixing chamber.

The gas mixture flows at a rate of 17 m/sec (referred to standard conditions) from the nozzle opening of the burner and bums in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiC.sub.4, AlCl.sub.3, air, hydrogen, oxygen, and trimethyl borate) is measured at the mouth of the burner and found to be 235.degree. C. 0.05 Nm.sup.3 /h of hydrogen (jacket hydrogen, room temperature) is fed into the annular nozzle surrounding the mouth of the burner.

After flame hydrolysis the reaction gases and the boron oxide-silicon dioxide-aluminium oxide mixed oxide are sucked through a cooling system by applying a vacuum and cooled to ca. 100.degree. to 160.degree. C. The solid is separated from the waste gas stream in a filter. The solid (boron oxide aluminium oxide-silicon dioxide mixed oxide) is in the form of a white, finely divided powder.

The specific surface of the boron oxide-aluminum oxide silicon dioxide mixed oxide is 28 m.sup.2 /g, the analytically determined B.sub.2 0.sub.3 content is 12.3 wt. % (.congruent.3.8 wt. % of boron) and the analytically determined Al.sub.2 0.sub.3 content is 3.9 wt. %.

Example 6

The procedure described in Example 5 is adopted, the following amounts being used: 0.83 kg/h of SiCl.sub.4 is vaporised together with 0.219 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of the burner mentioned in Example 1. 0.108 kg/h of AlCl.sub.3 is vaporised in a separate vaporiser at a temperature of ca. 240.degree. C. and conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of this burner.

0.782 Nm.sup.3 /h of air as well as 0.164 Nm.sup.3 /h of oxygen and 0.255 Nm.sup.3 /h of hydrogen are introduced into the same mixing chamber.

The gas outflow temperature at the mouth or the burner is 235.degree. C.

After flame hydrolysis and oxidation, and separation of the gaseous reaction gases, a boron oxide-aluminium oxide silicon dioxide mixed oxide is obtained having a BET surface of 30 m.sup.2 /g, the analytically determined B.sub.2 0.sub.3 content being 13.9 wt. % (.congruent.4.3 wt. % of boron) and the analytically determined Al.sub.2 0.sub.3 content being 8.3 wt. %.

Example 7

The procedure described in Example 5 is adopted, the following amounts being used:

0.788 kg/h of SiCl4 is vaporised together with 0.208 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a i rate of 0.120 Nm.sup.3 /h into the mixing chamber of the burner mentioned in Example 1. 0.151 kg/h of AlCl.sub.3 is vaporised in a separate vaporiser at temperatures of ca. 240.degree. C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of this burner. 0.762 Nm3/h of air as well as 0.160 Nm.sup.3 /h of oxygen and 0.257 Nm.sup.3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235.degree. C.

After flame hydrolysis and separation ot the gaseous reaction gases, a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m.sup.2 /g, the analytically determined B.sub.2 0.sub.3 content being 14.3 wt. % (.congruent.4.4 wt. % of boron) and the analytically determined Al.sub.2 0.sub.3 content being 12.9 wt. %.

Example 8

The procedure described in Example 5 is adopted, the following amounts being used:

0.721 kg/h of SiCl4 is vaporised together with 0.190 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of the burner mentioned in Example 1. 0.232 kg/h of AlCl.sub.3 is vaporised in a separate vaporiser at a temperature of ca. 240.degree. C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of this burner.

0.842 Nm.sup.3 /h of air as well as 0.176 Nm.sup.3 /h of oxygen and 0.259 Nm.sup.3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235.degree. C.

After flame hydrolysis and separation of the gaseous reaction gases, a boron oxide-aluminium oxide-silicon 29 m.sup.2 /g, the analytically determined B.sub.2 0.sub.3 content being 14.6 wt. % (.congruent.4.5 wt. % of boron) and the analytically determined Al.sub.2 0.sub.3 content being 19.3 wt. %.

Example 9

The procedure described in Example 5 is adopted, the following amounts being used:

0.641 kg/h of SiCl.sub.4 is vaporised together with 0.169 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of the burner mentioned in Example 5. 0.321 kg/h of AlCl.sup.3 is vaporised in a separate vaporiser at a temperature of ca. 240.degree. C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm.sup.3 /h into the mixing chamber of this burner.

0.800 Nm.sup.3 /h of air as well as 0.167 Nm.sup.3 /h of oxygen and 0.261 Nm.sup.3 /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235.degree. C.

After flame hydrolysis and separation of the gaseous reaction gases a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m.sup.2 /g, the analytically determined B.sub.2 0.sub.3 content being 12.9 wt. % (.congruent.4.0 wt. % of boron) and the analytically determined Al.sub.2 0.sub.3 content being 29.0 wt. %.

The boron oxide-aluminium oxide-silicon dioxide mixed oxide obtained has the following particle size distribution:

______________________________________Total number of particles (N) 4349Particle diameter, arithmetic mean (DN) 77.66 (NM)Particle diameter, averaged over surface (DA) 101.81 (NM)Percentage distribution______________________________________ ______________________________________ Sum SumDiameter Number Number Number Wt. %D (NM) N % % %______________________________________14.900 367. 8.439 8.439 0.03820.420 90. 2.069 10.508 0.06225.940 53. 1.219 11.727 0.09231.460 47. 1.081 12.808 0.13836.980 54. 1.242 14.049 0.22442.500 95. 2.184 16.234 0.45448.020 143. 3.288 19.522 0.95353.540 198. 4.553 24.074 1.91259.060 214. 4.921 28.995 3.30264.580 255. 5.863 34.859 5.46870.100 293. 6.737 41.596 8.65075.620 315. 7.243 48.839 12.94681.140 297. 6.829 55.668 17.94986.660 306. 7.036 62.704 24.23092.180 300. 6.898 69.602 31.64097.700 278. 6.392 75.994 39.815103.220 248. 5.702 81.697 48.416108.740 176. 4.047 85.744 55.553114.260 123. 2.828 88.572 61.339119.780 129. 2.966 91.538 68.330125.300 84. 1.931 93.470 73.541130.820 86. 1.977 95.447 79.613136.340 64. 1.472 96.919 84.728141.860 42. 0.966 97.885 88.509147.380 29. 0.667 98.551 91.436152.900 25. 0.575 99.126 94.254158.420 15. 0.345 99.471 96.135163.940 8. 0.184 99.655 97.247169.460 4. 0.092 99.747 97.861174.980 4. 0.092 99.839 98.536180.500 3. 0.069 99.908 99.093186.020 1. 0.023 99.931 99.296191.540 1. 0.023 99.954 99.517197.060 2. 0.046 100.000 100.000______________________________________ DN = 77/66 DA = 101.81 D50 = 76.56 K = 6 A(K) = O.0991 FQS = 1.7952 DN/D50 = 1.U144 DA/D50 = 1.3298 DA/DN = 1.3110

These data are shown graphically in FIGS. 11 to 16.

TABLE 4__________________________________________________________________________Experimental conditions and flame parameters calculated therefrom inthe preparation of pyrogenic boron-silicon-aluminum mixed oxides SiCl4 TMB AlC.sub.3 Air O.sub.2 H.sub.2 gamma lambda BETNo [kg/h] [kg/h] [kg/h] [Nm.sup.3 /h] [Nm.sup.3 /h] [Nm.sup.3 /h] [-] [-] [m.sup.2 /g]__________________________________________________________________________5 0.876 0.231 0.056 0.938 0.196 0.254 1.95 1.10 286 0.830 0.219 0.108 0.782 0.164 0.255 1.90 0.97 307 6.788 0.208 0.151 0.762 0.160 0.257 1.87 0.97 278 0.721 0.190 0.232 0.842 0.176 0.259 1.78 1.10 299 0.641 0.169 0.321 0.800 0.167 0.261 1.70 1.11 29__________________________________________________________________________ TMB = Trimethyl borate B(OCH.sub.3).sub.3 Ratio H.sub.2 gamma = Ratio of fedin hydrogen in the core (taking into account the hydrogen contained in TMB) to stoichiometrically required hydrogen Ratio O.sub.2 lambda = Ratio of fedin oxygen (atmospheric oxygen + additionally added O.sub.2) in the burner to stoichiometrically required oxygen TABLE 5__________________________________________________________________________Analytical data of the samples (Al--B--Si mixed oxide) obtainedaccording to Examples 5 to 9Before deacidification After deacidification BET B.sub.2 O.sub.3 Al.sub.2 O.sub.3 TV GV BET B.sub.2 O.sub.3 Al.sub.2 O.sub.3 TV GV m.sup.2 /g Wt. % Wt. % Wt. % Wt. % m.sup.2 /g Wt. % Wt. % Wt. % Wt. %__________________________________________________________________________5 28 12.3 3.9 0.5 3.2 27 13.5 4.5 0.2 2.16 30 13.9 8.3 1.6 3.3 29 13.9 9.3 0.2 2.27 27 14.3 12.9 0.7 3.5 29 13.0 12.9 0.6 1.79 27 12.9 29.0 0.6 1.9 34 12.4 27.0 0.3 1.5__________________________________________________________________________ TV = Drying loss (2 h at 105.degree. C., according to DIN/ISO 787/II, AST D 280, JIS K 5101/21) GV = Annealing loss (2 h at 1000.degree. C., according to DIN 5bY21, ASTM D 1208, JIS K 5101/23, referred to the substance dried for 2 hours at 105.degree. C. TABLE 6__________________________________________________________________________Further analytical data of the samples (Al--B--Si-mixed oxide)obtained according to Examples 5 to 9Before deacidification After deacidification Cl Bulk Tamped Cl Bulk Tamped BET content density density content density densityNo m.sup.2 /g ppm pH g/l g/l m.sup.2 g ppm pH g/l g/l__________________________________________________________________________5 28 150 3.47 65 82 27 130 3.90 175 2366 30 148 3.79 76 91 29 85 4.06 190 2407 27 132 3.84 80 97 29 115 4.38 210 2708 29 223 4.21 74 97 32 310 4.45 202 2559 27 210 4.33 75 90 34 414 4.55 215 276__________________________________________________________________________ pH = pH in 4 percent aqueous suspension. Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened).

Claims

1. A boron oxide-silicon dioxide mixed oxide in powdered form, for making glass, having a BET surface of 30-100 m.sup.2 /g and a B.sub.2 O.sub.3 content of from 10.1 to 40 wt. %.

2. A boron oxide-silicon dioxide mixed oxide according to claim 1, containing at least one further oxide of a metal or a metalloid.

3. A boron oxide-silicon dioxide mixed oxide according to claim 1, wherein said BET surface is in the range of 30 to 80 m.sup.2 /g.

4. A boron oxide-silicon dioxide mixed oxide according to claim 3, containing at least one further oxide of a metal or a metalloid.

5. A boron oxide-silicon dioxide mixed oxide according to claim 1, wherein said BET surface is in the range of 30 to 50 m.sup.2 /g.

6. A boron oxide-silicon dioxide mixed oxide according to claim 5, containing at least one further oxide of a metal or a metalloid.