Nano particles produced by a flame pyrolysis process coagulate and form aggregates whose size can reach up to several hundred nanometers in diameter. Since primary particles and aggregates have a large specific surface, sinter bridges and secondary structures form on the basis of mainly van-der-Waals forces. Therefore the preparation of suspensions of these particles proves to be very difficult [1]. In the present work experimental results on the production of stable aqueous nano-dispersions are presented. Electrostatically stabilised aqueous nano-dispersions with variable solid contents were produced by applying high pressure or ultrasound treatment. The pre-dispersed suspension was stressed up to a specific energy EV of 107 kJ/m3 by variation of the applied homogenization pressure or of ultrasound intensity or process time, respectively. The continuous dispersion and des-agglomeration process is mainly dominated by the specific energy [2] applied. In case of high pressure treatment, the dispersion result is in addition strongly affected by the nozzle layout, since micro turbulences and cavitation depend on nozzle geometry. Taking into account the influence of different mechanisms causing agglomerate disruption, specific variations of the nozzle geometry can lead to a target improvement of the dispersion efficiency. Therefore, different nozzle geometries were developed, their effect on the dispersion behaviour was determined, and compared to ultrasound and rotor stator systems by analysing the mechanisms for dispersion.

Figure1: Comparison of different nozzle configurations for high pressure dispersing of Aerosil 200 V Figure 1 shows some effects of downstream boundary conditions for the jet discharging from a nozzle. The diagram shows the particle size in suspension as a function of the specific dispersing energy. The particle size was analysed by dynamic light scattering. The pre-dispersed suspension enters the nozzle at high pressure and is forced to pass a hole of 0.08 mm in diameter. In the first configuration the dispersion discharges directly axial out of the nozzle into a fluid reservoir, resulting in a mean particle size of about 180 nm applying a pressure difference of 1,000 bar. An impingement of the jet directly after the outlet of the nozzle leads to further reduction of the mean particle size. The distance between the nozzle and the impingement plate is of prime importance as indicated in figure 1.

Figure2: Comparison of different systems for dispersing nano-particles (Aerosil 200 V) A comparison of different systems for dispersion is shown in figure 2. The usage of rotor stator systems results in no satisfying effect. An increase of the solid content leads to a slight improvement in dispersion efficiency, only. In contrast, ultrasound or high pressure systems enable to improve disruption efficiency and thus diminish clearly the mean agglomerate size. Compared to ultrasound high pressure systems are more efficient in terms of the specific energy EV. For achieving a mean particle size of 160 nm only 75 % of the specific energy is required when using optimised high pressure configurations compared to ultrasound. The dispersions were stable over several weeks without adding any surfactant. In contrast, dispersions produced by rotor stator systems are not stable due to their wide particle size distribution.

References [1] F. Hinze, 1. Dresdner Seminar Partikeltechnik 2000, 45. [2] H. Karbstein, Dissertation, Karlsruhe 1994. Acknowledgment Financial support was provided by the European Commission in the 6th framework program within the project PROFORM.