Bulletin of the American Physical Society
71st Annual Gaseous Electronics Conference
Volume 63, Number 10
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session KW3: Gas Phase Plasma Chemistry |
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Chair: Gilchiro Uchida, Osaka University Room: Oregon Convention Center A106 |
Wednesday, November 7, 2018 2:00PM - 2:15PM |
KW3.00001: First-principles reaction-path finding in gas discharge plasmas with QCEIMS Rei Sakuma, Satoshi Nakamura, Shogo Sakurai, Hiroyuki Kubotera, Kiyoshi Ishikawa, Sun-Taek Lim, Dae Sin Kim Current plasma simulations for semiconductor plasma processes all rely on accurate reaction databases in gas discharges, but prediction of the reactions between discharged electrons and gas molecules remains a major theoretical challenge as these reactions involve complex electronic processes far beyond the scope of standard electronic-structure approaches. Aiming at constructing reliable reaction databases for plasma process simulations from first-principles, in this contribution we report an application of the recently-proposed reaction-path-finding program QCEIMS\footnote{S. Grimme, Angew. Chem., Int. Ed., 52, 6306 (2013); V. \'{A}sgeirsson, C. A. Bauer, S. Grimme, Chem. Sci. 8, 4879 (2017).} for dissociative ionization of fluorocarbon gas molecules. QCEIMS is a molecular dynamics based approach which has been successful for reproducing experimental electron ionization mass spectra as well as describing dissociative electron attachment processes of several systems. As a first benchmark, we compute mass spectra of several fluorocarbon molecules and show that QCEIMS not only reproduces many of the major peaks of experimental mass spectra but also provides the corresponding trajectories, which makes it a promising tool for deducing dominant reaction pathways in plasma processes. [Preview Abstract] |
Wednesday, November 7, 2018 2:15PM - 2:30PM |
KW3.00002: A global model of microwave induced hexametyhldisiloxane/oxygen coaxial surface wave discharge Efe Kemaneci, Felix Mitschker, Jan Benedikt, Denis Eremin, Peter Awakowicz, Ralf Peter Brinkmann A volume-averaged global model is developed for a microwave induced coaxial hexamethyldisiloxane/oxygen surface wave discharge. Positive ion and neutral flux at the coaxial walls are analytically estimated [Efe Kemaneci et al 2017 J. Phys. D: Appl. Phys. 50 245203]. A total number of 1200 homogeneous and heterogeneous gas phase reactions are identified for a set of 100 different plasma species and implemented in the model. The simulation results are benchmarked against a variety of measurements for a variation of input power, pressure and oxygen to hexamethyldisiloxane flow ratios. An agreement is obtained between the simulation results and the measurements of electron temperature, electron density as well as of hexamethyldisiloxane, carbonmonoxide, carbondioxide and various hydrocarbon concentrations. Hexamethyldisiloxane, pentamethyldisiloxanyl, methane, carbonmonoxide, water, hydrogen and oxygen molecules are the most dominant species in the discharge. A significant amount of positive charge is formed by pentamethyldisiloxanyl ion (Si$_2$OC$_5$H$_{15}^+$) via electron impact dissociative ionization of the hexamethyldisiloxane molecule. Underlying reaction mechanisms in the plasma are identified and their relative contributions are quantified. [Preview Abstract] |
Wednesday, November 7, 2018 2:30PM - 2:45PM |
KW3.00003: Simulation of uranium plasma plume dynamics in atmospheric oxygen produced via femtosecond laser ablation Mikhail Finko, Davide Curreli, Magdi Azer, David Weisz, Jonathan Crowhurst, Timothy Rose, Batikan Koroglu, Harry Radousky, Joseph Zaug, Mike Armstrong The use of laser ablation for the study of uranium plasma chemistry in atmospheric ablation plumes is highly relevant for nuclear forensics and standoff detection, but the behavior of such systems is currently not well understood. One of the main difficulties with studying these systems is that the already considerable complexity of plume dynamics in vacuum conditions is further enhanced by shockwave formation and plasma-chemical behavior in reactive, atmospheric environments. In order to account for both the transport and kinetics of uranium in atmospheric oxygen, we have constructed a 2D compressible, reactive, multi-species fluid model of femtosecond laser ablation plumes. The model captures both the complex compressible dynamics of the ablation shockwave and the stratification of the ablation plume into regions of varying reactivity and molecular composition. The model allows for a detailed analysis of the spatial and temporal evolution of both the fluid moments and the major plasma-chemical species concentrations of the ablation plume. [Preview Abstract] |
Wednesday, November 7, 2018 2:45PM - 3:00PM |
KW3.00004: The role of O atoms in CO$_{2}$ plasma kinetics Olivier Guaitella, Ana-Sofia Morillo-Candas, Tiago Silva, Polina Ogloblina, Vasco Guerra Using CO$_{2}$ as a raw material instead of treating it as waste is one of the biggest challenges today. Raising it would both create a true green chemistry on Earth, and provide a source of O$_{2}$ for space missions on Mars. The interest of converting CO2 by plasma relies on the efficiency of the asymmetric stretch vibrational mode excitation, and on the control of the processes involving O atoms. Indeed O atoms can be responsible both for quenching of vibrationally excited CO$_{2}$, and for back reaction with CO. We have used time resolved in situ FTIR to obtain vibrational temperature of CO$_{2}$ and CO in a pulsed glow discharge with and without SiO$_{2}$ fibers on the wall. This porous material allows having a complete recombination of O atoms which has been confirmed by actinometry and TALIF measurements. By removing the O atoms, their influence on the vibrational temperatures and the conversion rate of CO$_{2}$ has been studied. In order to investigate deeper the O atoms exchange rate between CO and CO$_{2}$, isotopic measurements are also performed with IR absorption. [Preview Abstract] |
Wednesday, November 7, 2018 3:00PM - 3:15PM |
KW3.00005: Increasing electron density with increasing oxygen admixture? Competing reaction and recombination processes in an atmospheric N$_2$/O$_2$ dielectric barrier discharge Katharina Stapelmann, Friederike Kogelheide, Bjoern Offerhaus, Philip Krajinski, Nikita Bibinov, Peter Awakowicz, Julian Schulze A DBD is investigated for various N$_2$/O$_2$ mixtures in controlled atmosphere by OES in combination with numerical simulations and I/V-measurements. Surprisingly, an increasing electron density was found for increasing O$_2$ content. Due to the higher electron affinity of O$_2$, the opposite would be expected. Furthermore, the spatial electron distribution in the discharge volume differs comparing synthetic air with pure N$_2$ as process gases. While the synthetic air discharge shows a homogeneous electron distribution in the center of the discharge, the pure N$_2$ discharge appears to be more confined to the electrode. The rate constants, reaction rates, and life times of the positive ions of O$^{+}_{2}$ and N$^{+}_{2}$ are calculated and compared. The recombination rate of N$^{+}_{2}$ is more than an order of magnitude lower than the recombination rate of O$^{+}_{2}$. Calculating the N$^{+}_{4}$ production and recombination rates, we found that they are several orders of magnitude larger than both, O$^{+}_{2}$ and N$^{+}_{2}$ recombination rates. Since N$^{+}_{4}$ occurs in significant densities with higher N$_2$ fraction in the gas mixture, electrons are consumed in recombination with N$^{+}_{4}$ efficiently. Thus, the electron density decreases with higher N$_2$ content. [Preview Abstract] |
Wednesday, November 7, 2018 3:15PM - 3:30PM |
KW3.00006: Macroscopic visualization for statistics and microscopic identification of species roles in web-like plasma-enhanced chemical networks Osamu Sakai, Yasutaka Mizui, Tetsuya Kojima, Masataka Koshiba, Akinori Iwai, Shigeyuki Miyagi, Tomoyuki Murakami Weakly-ionized plasma has multiple complexities in various phase spaces of parameters. One of the phase spaces it possesses is a chemical reaction network. To investigate its structure and functions, we proposed a visualization method for complex networks of chemical reactions enhanced by high-energy electrons, based on theories of complex networks; a graph with nodes and edges is efficient for this purpose, where nodes are species like atoms, molecules, radicals, ions and electrons with edges bridging chemical agents and products [1]. To identify roles of species in the network, centrality indices like betweenness and closeness are good measures as microscopic viewers [1,2]. To survey macroscopic and statistical properties of the network, we made a histogram of statistical counting of species for a centrality-index distribution, and plasma-enhanced chemical networks exhibit a wide broadening of the distributions in contrast to cases of random graphs [2]. These facts suggest that web-like plasma enhanced chemical networks are well organized and well balanced to preserve reaction-network stabilities. [1] O. Sakai, K. Nobuto, S. Miyagi and K. Tachibana, AIP Advances 5, 107140 (2015). [2] Y. Mizui, T. Kojima, S. Miyagi and O. Sakai, Symmetry 9, 309 (2017). [Preview Abstract] |
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