ICMCTF2016 Session TS5: Atmospheric Plasma Applications

The electrical discharge process in a liquid environment, so called solution plasma process (SPP), is an effective method for the synthesis of metallic nanoparticles and nanofluids, sterilization and decomposition of organic compounds since solution plasma provides strong reaction fields, where active species with highly energetic ions, radicals and electrons react. Especially, SPP for the synthesis of metallic nanoparticles shows the most important advantages such as a short processing time, minimization of chemical uses and the preparation in room temperature and atmospheric pressure In this work, solution plasma process as a design tool for the authentic nanoparticles will be introduced and their applications for different industrial sectors will be illuminated, especially heat transfer application, renewable energy applications, etc.

Special attention will be paid to the synthesis of bimetallic nanoparticles such as Pt/Ag, Pt/Pd, Pt/Cu, and their electrochemical activities by means of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) for fuel cell applications will be illuminated in detail. Various processing variable in SPP and their effects on electrochemical activities will be presented in detail and future direction for the development of nanoparticles will be suggested.

Acknowledgments

This study was supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (NRF-2013R1A1A2060918).

The plasma source with a coaxial geometry based on the Fused Hollow Cathode (FHC) geometry was used for generation of plasma inside water and the ethanol-water mixtures. The effect of various generation regimes on the performance of plasma, on the hydrogen production efficiency and on the reaction rise-time was examined. A role of the solution temperature, composition of the mixture and current delivered to the discharge are investigated.

Complex interactions between a downstream atmospheric plasma discharge and metal alloy surfaces can potentially produce beneficial effects related to surface preparation and cleaning, and subsequent properties such as surface adhesion, wear, and corrosion. The plasma treatment results in visual changes (e.g. scattering and interference of light) to surfaces after treatment. This study used advanced materials characterization tools to better understand the morphology, structure, and chemistry of resulting surface structures after an air plasma treatment of a metal surface. An Inconel 718 alloy was chosen as the substrate alloy and resulting surface features from air plasma treatment were compared with surface features formed by standard furnace annealing.

Inconel 718 was subjected to an air atmospheric plasma operating at a power level of 2.5kW for seven seconds at a height of 15mm above the surface which resulted in a multi-colored region on the samples surface that measured ~3mm in diameter. The atmospheric plasma exposure created a thin film on the Inconel 718 surface and FESEM revealed densely packed nodular features measuring 15-40nm in diameter. STEM bright field images showed the nanometer scale nodules to be crystalline and revealed that the atmospheric plasma produced chemically distinct layers on the Inconel 718 surface. Both XPS and EELS showed the top most layer, measuring 45-55nm thick, was rich in nickel and oxygen. Below this first layer was an iron and oxygen rich layer measuring 35-40nm and below this a 15-20nm thick chromium and oxygen rich layer directly above the Inconel 718 substrate. In comparison, the 800 degree centigrade, 1 hour duration furnace annealed sample produced faceted platelet features measuring 300-500nm across on the Inconel 718 surface. XPS and STEM/EDS/EELS showed a surface layer 20-30nm thick rich in iron, chromium and oxygen and a thicker 80-90nm chromium rich oxide at the Inconel 718 substrate.

The chemically distinct oxide layers formed by the air atmospheric plasma were created in seconds and used a fraction of the energy to form a comparably thick oxide than the 1hr long furnace anneal. The atmospheric plasma treated surface exhibited a distinct morphology and chemical composition when compared to the furnace annealed Inconel 718 surface. The use of an air atmospheric plasma may provide a rapid and energy efficient method to grow controllably thin oxide layers on metal substrates.

Atmospheric plasma deposition is a versatile, low cost and flexible process that enables deposition on large and/or complex shapes in laboratory air and at low temperature. The low temperature plasma and solvent free process allows deposition on, and simultaneous functionalization of plastic substrates in a single step.

Building on our previous studies, we demonstrate a highly efficient deposition method using single and dual precursors to deposit multilayer organosilicate transparent coatings on selected polymer, silicon and glass substrates. The films exhibited ~100% transmittance in the visible wavelength range. FTIR and Raman spectroscopy of the films showed that we are able to control the incorporation of organic components (-C-C-)_n in the molecular network despite the oxidizing environment. This incorporation increased the film deposition rate and also resulted in mechanical plasticity in the films. We show how significant increases in the film adhesion with plastics can be achieved from ~2 J/m² to ~10 J/m² along with exceptional Young’s modulus ranging from 22GPa to 34GPa. Film structures including composition and thickness to achieve optimized hardness and adhesive properties are reported.

Plasma Electrolytic Oxidation (PEO) is a promising plasma-assisted surface treatment of light metallic alloys (e.g. Al, Mg, Ti). Although the PEO process makes it possible to grow oxide coatings with interesting corrosion and wear resistant properties, the physical mechanisms of coating growth are not completely understood yet.

Typically, the process consists in immersing the metal work piece in an electrolyte bath and applying a high voltage difference between the work electrode and a counter-electrode. Compared with anodising, the main differences concern the electrolyte composition and the current and voltage ranges which are at least two orders of magnitude higher in PEO. These significant differences in current and voltage imply the dielectric breakdown and consequently the appearance of micro-discharges (MD) on the surface of the sample under processing. Those MD are recognized as being the main contributors to the formation of a dielectric porous crystalline oxide coating.

By using a bipolar pulse current generator, it was shown that a judicious setting of the cathodic half-period parameters (amplitude, duration) with respect to the anodic ones improves strongly the coating compactness and growth kinetics. This regime called “soft regime” [1] is associated with a quasi-disappearance of MD and a significant voltage drop. The surface of aluminium samples treated in the conditions of “soft regime” exhibits a thin sponge-like glass phase that covers a thick alumina (α and γ-Al₂O₃) layer.

Transmission electron microscopy (TEM) observations of cross sections showed that the glass phase is composed of amorphous SiO₂ with inclusions of sodium. The presence of those elements comes from the electrolyte which is composed of KOH and Na₂SiO₃.

More surprisingly, we also lighted on that what was previously considered as a dense alumina layer is in fact composed of a stack made alternatively of 80 nm-thick alumina and 10-15 nm-thicksilicasublayers . The formation of such a composite is responsible for the increase in layer compactness and therefore the improvement of mechanical and anticorrosive properties. Two scenarii of growth mechanisms will be discussed, one dealing with single step spinodal transition and one consisting in the formation of porous alumina subsequently filled with silica.

[1] F. Jaspard-Mécuson, T. Czerwiec, G. Henrion, T. Belmonte, L. Dujardin, A. Viola, J. Beauvir, Surf. Coat. Technol. 201 (2007) 8677