DECAIR

DECAIR aims to demonstrate the scientific and technological feasibility of a roll-to-roll plasma process at atmospheric pressure in air applied to the deposition of a thin and homogeneous layer. To keep the substrate intact and to realize a homogeneous coating on a large-scale surface, we will use a Dielectric Barrier Discharge (DBD). DBDs have been studied since the beginning of the century to realize thin layer coatings in nitrogen or rare gas atmosphere. In both cases, the quality of the coating is influenced by the regime of the discharge. When the discharge is filamentary, the coating is porous and fails to become dense. It consists of a large number of nanoparticles stacked together. By contrast, when the discharge is homogenous, the energy is uniformly transferred on the surface, then the coating is thick and consists on a homogeneous layer. Working in the homogenous mode will therefore be essential in the context of this project. To obtain a homogeneous discharge in a DBD, different pre-ionization mechanisms are required in order to create seed electrons prior to the ionization. These seed electrons result from a memory effect of the previous discharge. In rare gases or in nitrogen, the memory effect mainly depends on gas mechanisms. For a long time, it was considered impossible to obtain a diffuse discharge in air. However, recent studies as well as our preliminary results prove the opposite. Unlike in the other gases, the memory effect in air seems restricted to mechanisms occurring on the dielectric surface. Up to date, this process has not been precisely characterized. To fill this gap in knowledge, it is urgent to measure the electric charge dynamics on the dielectric during the discharge process. An interesting technique to quantify these electric charges in-situ is based on the electro-optic Pockels effects. This technic coupled to a systematic analysis of the dielectric material and the working conditions of homogeneous discharges will help understand the memory effect in air at atmospheric pressure. Simultaneously, the optimal working conditions will be established. Moreover, an optimization of the power transferred to the discharge will be carried out through a careful design of the power supply. After this optimization, it will then be possible to work at higher frequencies and at higher applied voltages, while remaining in diffuse discharge. With the increase of the excitation frequency, gas phase mechanisms are susceptible to play a non-negligible role. These mechanisms will be studied with a 1D Kinetic model and advanced optical diagnostic (OES, LIF). We will then realize SiOx coatings from the organosilicon precursor HMDSO. The choice of this precursor is due to its well-known dissociation mechanisms and our expertise working with it. We will first study the influence of the coating on the memory effect, before analyzing the coating properties and the deposition speed. The analysis of all the realized coatings will bring out the optimal conditions to obtain a dense coatings in air. These coatings will be compared to reference deposits made in atmospheric N2 homogeneous discharge. We will then demonstrate the feasibility to performing a dense and homogeneous coating of SIOx using a DBD at atmospheric pressure in air. From a scientific point of view, this project is designed to address fundamental questions, which critically limit our current understanding of the mechanisms behind homogenous DBD in air. Moreover, this process will be transferable to an industrial process through a slight modification of an existing Corona reactor treatment. The current atmospheric pressure processes use nitrogen or rare gas. By using only dry and filtered air, the financial and energetic costs of the process will be drastically reduced, which will make the coating of thin films more respectful of the environment.