Session XF4: Atmospheric Plasma Material Treatment and Synthesis I

Live

Our presentation deals with the graphene quantum dots production by an atmospheric pressure microplasma jet and its optimization. A metallic pipe on which a DC voltage was applied was facing a liquid solution containing Sodium Dodecyl Sulfate (SDS) connected to ground via a platinum electrode. We used photoluminescence and Raman spectroscopy to characterize the GQDs and UV-visible absorption to compare the GQD production yields. Optical emission spectroscopy and electrical passive probes were employed to characterize the discharges. We focus our study on the polarity applied to the jet. The positive and the negative polarities leads to two different discharge regimes leading themselves to several GQD production yields. The positive discharge, which produces more GQDs, shows unstable current waveforms, a constant emission intensity with time, and the emission of the $H_{\alpha}$line. The negative discharge shows a stable current, a decreasing emission intensity with time, as well as more intense molecular spectra compared to atomic lines. The reaction mechanisms were investigated by adding $H_2O_2$ which is a solvated electron scavenger. The results show that the reaction mechanism is not the same for both polarities.   

Live

Atmospheric pressure plasmas (APPs) sustained in air are used to functionalize commodity polymers for packaging and printing. Functionalizing polymers such as polystyrene (PS) for higher value applications in biotechnology often instead use rare-gas plasma jets seeded with oxygen. Plasma treatment creates alkyl radicals by abstraction of H from the polymer by O or OH (from impurities or air-mixing). O or O$_{\mathrm{2}}$ then fixes to the polymer to form alkoxy and peroxy sites, increasing hydrophilicity and wettability. Subsequent reactions produce a mix of hydroperoxy, alcohol and acid groups. Correlating plasma conditions with desired functionality would help in process design. We report on results from a computational investigation of surface functionalization of PS using APP jets sustained in Ar/O$_{\mathrm{2}}$ and He/O$_{\mathrm{2}}$ flowing into ambient air. GlobalKin, a global-plasma chemistry model, simulated RF excited plasmas exiting into room air onto PS a few mm from the jet nozzle, and then exposing the PS to ambient air. A surface site balance model addresses plasma-surface and surface-surface reactions leading to functionalization. Results for coverage of O-containing groups on the PS as a function of power, gas flow rate, distance of the PS from the nozzle and post-plasma air exposure will be discussed. Results from the model will be compared to experimental trends for water contact angle as a measure of oxygen content.   

Live

Recent developments in additive manufacturing have enabled printing of colloidal nanoparticles for diverse technologies, including energy conversion and storage, sensing, and electronics. However, the printed materials must be then processed at a high temperature or in a high pressure environment in order to be sintered and become conductive. Conventional plasma sintering methods require high temperature and pressure (e.g., spark plasma sintering) or low pressure (e.g., radio frequency plasmas). These extreme requirements are not conducive for high throughput processing or additive manufacturing on substrates that have relatively low melting temperatures. Here, a plasma jet sintering process is implemented at atmospheric pressure and room temperature, enabling printing on plastics and flexible materials and sintering aerosol jet-printed silver nanoparticles. The electrical conductivity of the sintered silver films is measured as a function of plasma exposure and correlated with plasma properties via electrical measurements and time-resolved optical emission spectroscopy (OES) and the thermal conditions via infrared (IR) imaging. Results show that sintering can be achieved using an argon plasma jet in a nitrogen environment with the substrate maintained below 30C.   

Live

We have developed a dielectric barrier discharge reactor with rotatable electrodes for particulate material treatment. It contains a cylindrical reactor made from polyoxymethylene and thin plate electrodes attached on a rotatable axial rod installed in the reactor. Two outer electrodes were attached to be surrounded on the cylindrical reactor, which were connected to an ac power supply. AC peak to peak voltage was set at 30 kV. Air gas was supplied to the reactor at a flow rate of 1.5 slm. We found uniform discharge generation conditions on the inner surface of the reactor by adjusting rotation speed of the axial rod although applied ac high voltage was operated at 60 Hz. Surface treatment test was made for particulate material of 300 mg TiO$_{\mathrm{2}}$ nano-powder Degussa P-25. Treated TiO$_{\mathrm{2}}$ nano-powder was analyzed using XRD, XPS, and FTIR. XRD measurements showed that the peak theta positions shifted, which could be attributed to the substitution of new functional groups in the TiO$_{\mathrm{2\thinspace }}$lattice. X-ray photoelectron spectra analysis indicated that the Ti 2p, O 1s peak shifted from a higher energy level to a lower energy level. The FTIR results indicated that hydroxyl groups significantly increased after 3 min DBD treatment.   

Live

Epidermal electronics is a growing field where electronic devices are placed directly onto human skin. These electronic devices have strict requirements of adhesion to the skin, biocompatibility, and high flexibility while still maintaining operability. This project seeks to develop a low temperature plasma-based deposition method for epidermal electronic applications. A helium floating-electrode dielectric barrier discharge jet is attached to the head of a three-dimensional printer, making a direct write system capable of depositing complex patterns onto substrates. A radio-frequency power supply is used to ignite the discharge. Copper patterns are deposited on various substrates including pig skin as a surrogate human skin model using sublimated copper (II) acetylacetonate as a precursor. A small admixture of hydrogen is necessary to achieve any copper deposition. Factors affecting the deposition (hydrogen and helium flow rates, copper precursor temperature, discharge power, etc.) are explored to maximize the deposition rate.   

Live

Extended solids are a class of materials reported to possess advanced optical, mechanical and energetic properties. They are solid phases of simple gas molecules such as carbon monoxide (CO) that are created at extremely high pressures in very small batches. [1] Lab-scale dielectric barrier discharge reactors were used in recent studies to generate CO-derived solids at atmospheric pressure. [2], [3] These solids were obtained as deposits on dielectric surfaces in pure CO plasma and were reported to be a polymerized form of CO with a molecular structure similar to CO-based extended solids. Our work studied the generation of CO-derived solids in a DC microplasma discharge jet because it enables separation of the solid product from the ``hot'' plasma zone, deposition on a variety of substrates, and collection of the product without reactor disassembly. Two types of CO-derived solids were obtained by changing the microplasma operating condition. The scalability of the microplasma technique for production of CO-derived solids was demonstrated. [1] Lipp, M. J. et al, Nature Mater. 4, 211-215 (2005). [2] Geiger, R. and Staack, D., J. Phys. D: Appl. Phys. 44, 274005 (2011). [3] Belov, I. et al, Plasma Process and Polymers, 14, 1600065 (2017).   

Live

In our previous experiments we synthesized boron nitride (BNNTs) and carbon nanotubes (CNTs) in volume by anodic arc discharges at near atmospheric pressure of nitrogen and helium, respectively. In order to understand NT formation, we determined the plasma and gas composition in the nucleation and growth regions using laser diagnostics, atomistic simulations, thermodynamic and fluid dynamics (CFD) modeling. Firstly, we performed validated arc modeling to predict how the arc can provide feedstock for nanomaterial synthesis. A complicated setup was implemented into ANSYS and included many complex effects: radiation, sheath boundary conditions near emitting electrodes, ablation/deposition of carbon on electrodes, and coupling of the thermal transport through electrodes. In addition, we developed several analytic models for key phenomena: 1) nonlinear dependence of the ablation rate as a function of arc current and interelectrode gap, 2) anode spot formation, in which the arc channel is constricted near anode, 3) radial narrow arc jet emanated from the arc. Thermodynamic modeling results show that at a temperature of 3000K, where CNT are thermally stable, carbon condenses into the long chains and then rolls into flakes and further converts into fullerenes. Therefore, the only carbon available for CNT formation is the carbon dissolved into metal catalyst particles. This also strongly supports the root growth mechanism model. For production of boron nitride nanotubes (BNNTs), boron is evaporated in the near-atmospheric-pressure arc in nitrogen atmosphere. We study precursors for the BNNTs' formation that can effectively convert molecular nitrogen (N$_{\mathrm{2}})$ into boron nitride. Using quantum chemistry methods, we discovered that formation of linear BNBN, and other more complex BN species from small boron clusters and N$_{\mathrm{2\thinspace }}$ proceeds through many sequential steps with activation barriers. Thus, based on our calculations we can conclude that N$_{\mathrm{2\thinspace }}$ is able to react with small boron clusters producing new BN clusters, and these clusters can be accumulated in the gas phase even at high temperature providing contribution to the BNNTs' growth.   

Live

Low temperature plasma catalysis is a potential technique to accelerate chemical processes. Many industrial dielectric catalysts or catalyst supports are porous for which penetration of plasma into the pores may be desirable. The shape of surface, including the pore opening, can restrict or enhance plasma penetration. For plasma to flow through the pore opening, the opening should be at least a few Debye lengths, which is about 10s of microns. Although plasma cannot directly flow through smaller openings, the plasma may generate a flux of photons that does penetrate inside the pore and which produces photoionization to seed a plasma. Even if a pore does not fill with plasma, the sub-surface pore can affect propagation of plasma across the surface due to the change in surface capacitance. In this paper we discuss results form a computational investigation of atmospheric pressure plasma propagation into the pores of a catalyst-like material having different shapes and small openings. The 2-D plasma-hydrodynamics model \textit{nonPDPSIM} was used in our study. Simulations were performed for surface ionization wave (SIW) treatment of the porous surface in humid air at atmospheric pressure. The likelihood of SIW propagation into the pore was found to depend on the pore diameter, spacing, topology (concave inwards or outwards) and pore opening. The influence of air pores under a flat surface on SIW propagation was also investigated.