Herstellung von Partikeln durch elektrostatische Zerstäubung von Flüssigmetallen in gasförmiger Umgebung
Fine solid particles, especially nano-particles, are of great interest for the synthesis of new materials. There is also a strong demand for newer and more precise contact technologies of electronic components due to the higher getting integration of electronic devices. These new technologies may be based on submicron conducting particles, e.g. for producing solder pastes. In the present thesis a process was developed for the production of micron and submicron conducting particles by the use of electrostatic atomisation of liquid metals. The atomization process is based on the instability of conducting liquids in a strong inhomogenous electric field. The free liquid surface is transformed into a cone shape, from where a thin liquid jet is emitted. This jet breaks up into a spray of fine unipolary charged droplets immediately after the emission. Under defined circumstances the droplets have a narrow size distribution and can be extracted for other applications without additional classification methods. The rapid solidification leads to defined spherical particles, depending on the used metal, and the unipolar charge to a low agglomeration. The high electrical field, which is necessary for the atomization, causes electrical discharges of the ambient gas and disturbes the atomization process. To reduce these discharges a special pressure chamber was constructed, which is filled with quenching gas as SF6 with over-pressures of up to 12 bar. The electrostatic atomisation was applicated for liquid metals only in an ultra high vacuum up to now. The production of aerosol particles may have advantages not only due to available cheaper online measurement techniques. By the use of aerosol measurement techniques a direct statement about the particle size, the particle number concentration and the mean charge on the particles is possible. The atomization was investigated for metals as well as for eutectic alloys with a low melting point and a low surface tension. The results show, that spherical particles with a diameter down to a few hundred nanometer could be produced. In addition the production of thin and homogenous granular films was possible. To compare and verify the experimental results a simplified numerical model of the spraying process with respect to the space charge was developed. By calculating the particle trajectories from the emission source to the collector plate an estimation of the spray cloud geometry and the emitted electrical current was possible. An interesting observation was the self induced size segregation of the particles, which occurred in the experiments and in the numerical simulations. This effect can be used for a defined expansion or focussing of the deposition area on the target. Here there may be found some interesting applications for the point spraying.
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