Abstract

The effect of quench conditions on the thickening and thixotropic ability of fumed silica was studied. The powder was generated by rapidly quenching a vapour stream containing SiO(g) with steam. Silicon monoxide vapour was produced by decomposing quartz in a transferred arc reactor using an Ar/NH,3 thermal plasma. The parameters investigated included pre‐quench temperature (1100‐2100 K), and supersaturation ratio (1 ‐ 107, quench rate (17000 ‐75000 K/s), and quench stoichiometry (27 ‐ 84). which was defined as the molar ratio of steam to SiO. The thickening and thixotropic ability of the powders increased with increasing quench rate and decreasing pre‐quench supersaturation. This was due to an increase in specific surface area. Use of large H2 O/SiO ratios caused a high degree of surface hydroxylation.

A novel process for silica decomposition consisting of transferring a thermal arc directly to a silica anode was investigated. The effects of current (150−250 A), plasma gas flow rate (10−20 L/min of Ar), and plasma gas composition (0−2.8% H2) on the rate of decomposition were examined. The decomposition rate ranged from 0.09 to 1.8 g/min and was determined to be heat-transfer-limited, with decomposition occurring below the arc root where the anode surface attained its boiling point. The decomposition rate was independent of plasma gas flow rate, suggesting that convective heat transfer was reduced significantly by the counterflow of decomposition products (SiO(g) and O2) from the surface. Increasing the current increased the decomposition rate because heat input to the anode because of electron flow and arc radiation increased. Adding H2 to the plasma gas increased the decomposition rate because of an increase in radiative heat transfer to the anode, a reduction in the theoretical energy requirement for decomposition, and a consumption of O2 which lowered the silica boiling point. The fumed silica produced had properties typical of commercial fumed silica products.

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