ICMCTF2014 Session TS6: Atmospheric Plasma Applications

Superhydrophobicity has recently drawn a great deal of attention for both fundamental understandings and practical applications due to its potential applications in various technologies and consumer products such as weather-resistant or self-cleaning fabrics, windshields, display panels, microfluidic devices, etc. Superhydrophobicity requires both right surface chemistry (mostly hydrophobic) and proper surface roughness. In previous plasma-based superhydrophobic coatings, fluoro-hydrocarbons were often used as a process gas to deposit fluorinated coatings on textured surfaces. Recently, the direct superhydrophobic coating with atmospheric rf plasma on flat surfaces without pre-texturing was demonstrated. But, it still used fluoro-hydrocarbon which generates environmental hazard issues. This talk will discuss the direct deposition of hydrocarbon coatings with a static water contact angle higher than 150o using non-fluorinated precursor gases in helium plasma generated in ambient air without any pre-roughening of the silicon (100) substrate. Two types of precursor gases were investigated – pure hydrocarbons and organic-inorganic hybrids. Since the plasma is generated in air, all films show some degree of oxygen incorporation. These results imply that the incorporation of a small amount of oxygenated species in hydrocarbon films due to excitation of ambient air is not detrimental for superhydrophobicity, which allows the atmospheric rf plasma with the non-fluorinated precursor to produce rough surface topography needed for superhydrophobicity.

Polymeric materials are usually less expensive and more convenient to manufacture than alternative materials. In addition, the lightweight nature of these materials makes them desirable for aerospace applications, where weight is a crucial factor. However, polymeric surfaces are susceptible to abrasion and erosion damage, resulting in increased haze and decreased clarity for transparent parts. For example, when materials such as polycarbonate or stretched acrylic are used in windows, windshields, and canopies, one of the drawbacks is the tendency to scratch and craze. Polymeric windows have been historically coated with polysiloxane or polyurethane based coatings to overcome this limitation by improving the surface resistance to scratches. Improvements to the coating processes can decrease manufacturing times, improve durability, and can offer long-term solutions in which additional functionalities can be incorporated.

Advanced thin films created using Atmospheric Pressure Plasma-Enhanced Chemical Vapor Deposition (AP-PECVD) can improve the durability or functionality of many components on aircraft. These technologies can be exploited to generate materials with high performance which are also environmentally friendly and produced with solvent-free processes. At Boeing, some areas of AP-PECVD research include transparent durable films for protection of polymeric substrates such as passenger windows and military canopies, adherent films for carbon fiber reinforced polymers (CFRPs), multilayer films with tailored conductivity and durability, and oxygen barrier films for CFRP substrates to prevent degradation of the polymer at high temperatures. In addition, multiple materials are under investigation in order to create films with specific electrical, thermal, or optical properties in order to meet certain requirements such as static dissipation or hydrophobicity.

Surface modification of polymers for biomedical applications is a thoroughly studied area. The goal of this paper is to show the use of atmospheric pressure plasma technology as a useful addition as a pre-treatment for polyethylene (PE) shoulder implants. Atmospheric pressure plasma polymerization of methyl methacrylate (MMA) will be performed on PE samples to increase the adhesion between the polymer and a PMMA bone cement. For the plasma polymerization, a dielectric barrier discharge (DBD) is used, operating in a helium atmosphere at ambient pressure. Parameters such as treatment time, monomer gas flow and discharge power are varied one at a time. Chemical and physical changes at the sample surface are studied making use of X-ray photon spectroscopy (XPS) and atomic force microscopy (AFM) measurements. Coating thicknesses are determined by making use of optical reflectance spectroscopy. After characterisation, the coated samples are incubated into a phosphate buffer solution (PBS) for a minimum of one week at 37°C, allowing to test the coating stability, when exposed to implant conditions. These simulations are done both for healthy and infected tissue. The results show that PMMA coatings can be deposited with a high degree of control concerning chemical composition and layer thickness. In a final stage, adhesion of the plasma coated samples to bone cement is tested through a pull-out test. All samples are cut to standard dimensions and immersed in bone cement in a reproducible way with a sample holder specially designed for this purpose. The results prove indirectly that the plasma modifications lead to a more reliable implant material, with a longer implant life-time.

Pulsed submerged arc (SA) treatment generates plasma within a vapor bubble submerged within a liquid. Most previous investigations of SA treatment used high voltage (~kV’s) and very high currents, e.g. kA’s. In contrast, we report on a low voltage technique, which is easier and less expensive to use. It was obtained that low energy repetitively pulsed submerged arcs are effective and efficient in inactivating E. coli bacteria, breaking down Sulfadimathoxine (SDM) antibiotic and Methylene Blue (MB) dye, and producing various nanoparticles (NPs).

Water was treated with a SA using C, Fe, Ti, and Cu electrodes, and their combinations, both without and with the addition of (0.01-0.5%) H₂O₂. It was found that MB was decomposed both during and after arc treatment. The treated solutions were examined by Raman and absorption spectroscopy. Particles produced during the arc treatment were studied by SEM, XPS and XRD.

It was found that the particles eroded from electrodes defined the character of removal and the level of the removal ratio after SA treatment. With C/C electrodes, the MB concentration exponentially decreased for the duration of the experiment, while with the other electrodes the MB concentration saturated. The saturation is explained by a decrease of the oxidative species concentration with SA treatment time for these electrodes. The aging of the SA treated solutions in the presence of H₂O₂ with all combinations of electrodes removed ~99 % of the MB contaminant. The effect of aging may be associated with accumulation of oxidative species, particularly peroxides, on the surface of eroded particles which gradually oxidized MB. A high MB removal yield of G_99.6=90 g/kWhr was obtained using SA water treatment with Ti electrodes and 0.5% H₂O₂ addition.

Two SA modes were used to produced micro- and nano-particles: contact mode – in which the electrodes periodically contacted and separated; and breakdown mode – in which high voltage pulses broke down the inter-electrode gap maintained at a constant separation. Different metal, metal oxides, and metal carbides were produced as nano-particles. W-C alloy wastes were crushed and recycling as WC powder. Super-paramagnetic carbon nano-particles with critical temperature > 300 K were synthesized. Ni-C particles coated with a layer of carbon were also produced. These particles had a C-concentration in the Ni alloy ~3× greater than the maximum solid equilibrium solubility. These particles were almost insoluble in acids and had super-paramagnetic properties in a wide temperature interval T>T_B=8 K. The material had a very narrow hysteresis loop, i.e. H_c<8 Oe (i.e. it is a soft magnet) in the super-paramagnetic state.

The plasma source with a coaxial geometry was used for generation of plasma inside water and the ethanol-water mixtures. Atomic hydrogen and OH groups were observed by OES, indicating the hydrogen forming reactions. The hydrogen detector placed at the outlet from the hermetic reactor measures the hydrogen content up to 60% in the outlet gas flow. Various regimes of plasma generation were examined. The effects of the mixture composition and of the power delivered to the discharge on the hydrogen production and the reaction kinetics are investigated. An important role of the plasma source design is discussed.

The removal of paint and other organic coatings from metal and composite surfaces represents a substantial cost burden and source of environmental concern across many industries. Most coating removal methods produce large quantities of potentially hazardous waste that must then be disposed of properly. An alternative, media free, coating removal process is needed to reduce the generation of these waste materials. A 2kW plasma torch operating at atmospheric pressure using compressed air as a working gas has been developed to safely remove such organic coatings. The atmospheric plasma coating removal (APCR) process uses the oxygen and nitrogen in air to produce activated chemical species which oxidize the organics in the coatings. The activated chemical species in the plasma plume convert much of the organic coating matrix into water vapor and carbon dioxide leaving behind only a fraction of the original mass of coating. No additional media, other than air, is used to perform the coating removal process. After APCR there is typically no need for any secondary cleaning before coating reapplication can begin.

Alternatively, the plasma torch can operate with other gas mixtures in order to change the chemistry of the plasma plume. When operated using hydrogen rich gas mixtures, a chemically-reducing plasma can be produced. The reducing plasma has been used to remove inorganic oxides from certain metal surfaces.

It will be shown that an APCR process can be used to effectively remove both organic and selected inorganic coatings from a wide range of metal and composite substrates.

Plasma discharging in water, so-called solution plasma process (SPP), has been recently paid much attention for nanoparticles synthesis. SPP is faster (microseconds), simpler (one-step method) and more cost-effective (only requiring an ion source) than traditional colloid methods and SPP has been employed for the synthesis of various nanoparticles for many applications including biomedical, heat exchanger for improved efficiency, catalysts, etc.

In this approach, various electrocatalysts such as Pt and heterogeneous Pt/M (M=Ag, Pd, Ni, Cu) were successfully synthesized using SPP, in an attempt to enhance fuel cell efficiency with reduction of Pt amount. Nano-porous (or mesoporous) and dendritic Pt electrocatalysts showed better electrocatalytic performace than ultra-fine Pt because interconnected structures potentially could not only have high surface area, but also supply enough absorption sites for all absorbed molecules involved over a close range. In methanol environment, the Pt/Ag bimetallic electrocatalyst exhibited enhanced activity and much improved stability toward CO with respect to pure Pt. This could be attributed to the electronic interaction between individual components of bimetal catalysts and the geometric change (particle shape and dispersion state). In addition, various processing variable in SPP and their effects on electrochemical activities of Pt/M (M=Ag, Pd, Ni, Cu) bimetallic electrocatalysts will be presented in detail.

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013M2A8A1042684).