Simulation and experimental study of atmospheric pressure plasma jets depositing thin films for controlled release of chemical substances

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Πασσαράς, Δημήτριος

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The aim of this Thesis is the development of a computational framework for cold atmospheric pressure plasma (CAPP) jets and the application to the deposition of SiOx films on Si substrates. The study combines mathematical modeling, simulation, and experimental measurements. Additionally, it includes the incorporation of deposited SiOx films in devices for controlled release of chemical substances. The detailed simulation of the whole process, i.e., film deposition with CAPP jets, is an extremely difficult task with a huge computational cost as the plasma equations are coupled with the turbulence flow equations along with a large reaction set. The air species joining the plasma result in a large number of reactions (the feed gas is mixed with the air species). To deal with this complexity and cost, a hybrid computational framework is developed which allows fast calculations with detailed reactions sets (thousands of reactions) and a large number (hundreds) of chemical species. It consists of a detailed turbulence flow model, a global plasma model, and a model for the calculation of the electron energy probability function, coupling homemade, free, and commercial code, in order to predict the density of plasma generated species along the axial direction of plasma jets. The computational framework is modular as one can change the chemistry and apply it to different problems. The reduction of the computational cost is based on reasonable decoupling of flow from the plasma problem and the proper use of the global (volume averaged) model to describe the plasma. The results of the calculations are used for the evaluation and validation of the assumptions of the global model. The computational framework is validated through published experimental data in the kINPen jet reactor. The effect of different turbulence flow models [standard k-ε, realizable k-ε, and Large Eddy Simulation (LES)] on the density of plasma generated species along the axial direction of the jet is investigated. The choice of the turbulence model affects significantly the computed densities of only specific plasma generated species [O2(1Σg), O-, O2-, O(1D), O, H, H2(r), H-, N2O(v), H7O3+, H9O4+,H15O7+ and OH-]. The use of the simple k-ε turbulence models constitutes a good compromise between accuracy and computational cost. If the density of one of these species is crucial for an application then the LES model is recommended for higher accuracy. The framework is not self-consistent and as a result, experimental measurements of the gas temperature and the power absorbed by the plasma need to be conducted. The determination of the gas temperature is conducted by a combination of experimental measurements and 2D flow simulations. These experimental measurements coupled with the hybrid computational framework and additional turbulence flow simulations are utilized to calculate deposition rates of SiOx on a Si substrate by the lab Ar/Hexamethyldisiloxane (HMDSO) plasma jet reactor. A detailed reaction set including 845 reactions and 87 species is utilized by the computational framework. A new reaction set of this deposition process is proposed, with the addition of reactions of air species with HMDSO. The sticking coefficient of the depositing radical (fragment of the precursor) on the Si surface is not known and is determined by means of fitting the calculated deposition rates to the experimental ones for different distances of the substrate surface from the plasma jet reactor, with a recommended value of about 0.04 (0.03-0.06). Finally, SiOx coatings deposited through the lab Ar/HMDSO plasma jet device are tested as barrier coatings of drug eluting devices consisting of porous Si matrices. The use of SiOx coatings on the surface of porous matrices prevents the initial burst release of the loaded drug and the drug elution is retarded. Thus, Si porous matrices with SiOx coatings deposited by a CAPP jet are promising candidates in devices for controlled drug release.



Plasma, Plasma reactors, Plasma jets, SiOx deposition, Turbulent flow, Drug delivery