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Abstract

Pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles having a BET surface area of 300.+-.25 m.sup.2/g, wherein the aggregates display an average surface area of 4800 to 6000 nm.sup.2, an average equivalent circle diameter (ECD) of 60 to 80 nm and an average circumference of 580 to 750 nm. It is produced by a pyrogenic process in which silicon tetrachloride and a maximum of up to 40 wt. % of a second silicon component comprising H.sub.3SiCl, H.sub.2SiCl.sub.2, HSiCl.sub.3, CH.sub.3SiCl.sub.3, (CH.sub.3).sub.2SiCl.sub.2, (CH.sub.3).sub.3SiCl and/or (n-C.sub.3H.sub.7)SiCl.sub.3 are mixed with primary air and a combustion gas and burnt into a reaction chamber, secondary air also being introduced into the reaction chamber, and the feed materials being chosen such that an adiabatic flame temperature of 1390 to 1450.degree. C. is obtained. It can be used as a filler.

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First Claim