Eidgenossische Technische Hochschule Zurich

Patrick Reichen

Philipp Rudolf Von Rohr

Axel Sonnenfeld

The present invention relates to a novel process for the remote plasma surface treatment of substrate particles at atmospheric pressure. The invention is motivated by the urge to overcome major drawbacks of particle treatment in low pressure plasmas and in-situ particle treatment at atmospheric pressure. The former requires complex and mostly expensive vacuum installations and vacuum locks usually prohibiting continuous processing. Independent of the system pressure, in-situ plasma treatment causes particle charging and therefore undesirable interaction with the electric field of the discharge, which is seen to contribute to the process of reactor clogging. Additionally, the filamentary discharges modes of atmospheric pressure plasmas are inflicted with inhomogeneous surface treatment. Furthermore, short radical lifetimes at elevated pressures complicate a remote plasma treatment approach as widely used in low pressure applications. The key-element of the invention is that by reducing the dimension of the atmospheric discharge arrangement to the micrometer range, transonic flow conditions can be achieved in the discharge zone while maintaining moderate flow rates. The resulting superimposition of high drift velocity in the gas flow and the inherent diffusion movement is to prolong the displacement distance of activated species, thus making a remote plasma treatment of substrate particles feasible and economically interesting. The circumferential arrangement of e.g. micro discharge channels around the treatment zone of variable length allows a remote plasma treatment independently of the discharge mode and benefits additionally from the aerodynamic focusing of a particle-gas stream to the center, reducing reactor clogging. Furthermore, taking advantage of non-thermal discharges, there is no restriction of the concept of the outlined invention in the material properties of the particulate solids especially not with regard to the treatment of temperature sensitive materials as often encountered in polymer or pharmaceutical industries. In conclusion, atmospheric pressure plasma treatment close to ambient gas temperature as well as continuous processing is a specialty of the invention disclosed here.

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Patrick Reichen