Bulletin of the American Physical Society
69th Annual Gaseous Electronics Conference
Volume 61, Number 9
Monday–Friday, October 10–14, 2016; Bochum, Germany
Session UF3: Thermal Plasmas |
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Chair: Pierre Tardiveau, University of Paris Room: 2b |
Friday, October 14, 2016 8:30AM - 8:45AM |
UF3.00001: Unstable Behavior of Anodic Arc Discharge for Synthesis of Nanomaterials Sophia Gershman, Yevgeny Raitses Fast imaging and electrical current measurements reveal unstable behavior of the carbon arc discharge for synthesis of nanomaterials. The arc column and the arc attachment region to the anode move in a somewhat sporadic way with a characteristic time in a 10$^{\mathrm{-3}}$ sec range. The arc exhibits a negative differential resistance before the arc motion occurs. A physical mechanism is proposed based on the thermal processes in the arc plasma region interacting with the ablating anode which leads to the shift of the arc to a new anode region. According to the transient heat transfer analysis, the time needed to heat a new anode region is also in a 10$^{\mathrm{-3}}$ sec range. For a 0.6 cm diameter anode used in our experiments, this time yields a frequency of about 200-300 Hz, comparable to the measured frequency of the arc motion. The voltage and current measurements show oscillations with a similar characteristic frequency. The thermal model is indirectly supported by the measured negative differential resistance of the arc discharge during arc oscillations. The observed unstable behavior of the arc may be responsible for the mixing of the flow of nanoparticles during the synthesis of nanoparticles leading to poor selectivity typical for the arc synthesis. [Preview Abstract] |
Friday, October 14, 2016 8:45AM - 9:00AM |
UF3.00002: Coagulation of carbon nanoparticles in the acoustic field in the vicinity of the arc discharge Mikhail Shneider An arc discharge produced in a background inert gase between graphite electrodes is one of the popular methods of nanoparticle synthesis. Nanoparticles and microscopic soot particles are produced in the peripheral region of arc. Intensive soot generation significantly reduces the efficiency of the arc as the technological process for production of fullerenes and other nanoparticles. Experimental studies have shown that exposure of peripheral region of the arc to intense ultrasound leads to a noticeable increase in the efficiency of the nanoparticle synthesis and reduces the soot yield (see, e.g. [1]), because ultrasound causes coagulation of soot particles and decrease of their concentration without affecting the nanoparticles. The paper presents theoretical study of the threshold for the ultrasound intensity required for the coagulation as a function of particle sizes and charge, and background gas parameters. The charge acquired in a thermionic emission, as a result of particles heating by radiation from the arc, is calculated self-consistently [2,3]. I would like to thank Dr. Yevgeny Raitses, Dr. Igor Kaganovich, and Mr. James Mitrani for their interest in this work and fruitful discussions. 1. G.N. Churilov, Nanotubes and Carbon Nanostructures 16, 5-6, 395-403 (2008). 2. M.N. Shneider, Physics of Plasmas 22, 073303 (2015), 3. M.N. Shneider, AIAA 2016-1693, 54th AIAA Aerospace Sciences Meeting, San Diego, CA, 2016. [Preview Abstract] |
Friday, October 14, 2016 9:00AM - 9:15AM |
UF3.00003: Fluid simulation of carbon arc plasma Kentaro Hara, Yevgeny Raitses, Igor Kaganovich An arc discharge using graphite electrodes is known to produce carbon nanomaterials, e.g. nanotubes and fullerenes. In order to understand where and how such nanomaterials are synthesized, the plasma properties inside the arc discharge must be characterized. The mechanism of the carbon arc plasma is as follows. Carbon particles evaporate from the graphite anode, which is mainly heated by the electrons. Carbon atoms and ions condensate and form a deposit on the cathode, from which the electrons are thermionically emitted. A one-dimensional fluid model is developed to study the characteristics of the carbon arc plasma in atmospheric pressures. Sheath models for the anode and cathode are coupled to the fluid simulation to obtain the material temperature and sheath potential. In the model, thermal nonequilibrium is assumed and atomic carbon, dimer, and trimer are considered. A typical operating condition of a carbon arc plasma is discharge voltage of 20 V, discharge current of 60 A, the electron radius of 6 to 12 mm, and background pressure of 500 Torr. Transition from low to high ablation mode is obtained from the simulations with a smaller electrode radius and with a larger discharge current, which agrees with experimental observations. [Preview Abstract] |
Friday, October 14, 2016 9:15AM - 9:30AM |
UF3.00004: Two color interferometric electron density measurement in an axially blown arc Patrick Stoller, Jan Carstensen, Bernardo Galletti, Charles Doiron, Alexey Sokolov, René Salzmann, Sandor Simon, Philipp Jabs High voltage circuit breakers protect the power grid by interrupting the current in case of a short circuit. To do so an arc is ignited between two contacts as they separate; transonic gas flow is used to cool and ultimately extinguish the arc at a current-zero crossing of the alternating current. A detailed understanding of the arc interruption process is needed to improve circuit breaker design. The conductivity of the partially ionized gas remaining after the current-zero crossing, a key parameter in determining whether the arc will be interrupted or not, is a function of the electron density. The electron density, in turn, is a function of the detailed dynamics of the arc cooling process, which does not necessarily occur under local thermodynamic equilibrium (LTE) conditions. In this work, we measure the spatially resolved line-integrated index of refraction in a near-current-zero arc stabilized in an axial flow of synthetic air with two nanosecond pulsed lasers at wavelengths of 532 nm and 671 nm. Generating a stable, cylindrically symmetric arc enables us to determine the three-dimensional index of refraction distribution using Abel inversion. Due to the wavelength dependence of the component of the index of refraction related to the free electrons, the information at two different wavelengths can be used to determine the electron density. This information allows us to determine how important it is to take into account non-equilibrium effects for accurate modeling of the physics of decaying arcs. [Preview Abstract] |
Friday, October 14, 2016 9:30AM - 9:45AM |
UF3.00005: Electrical characteristics of TIG arcs in argon from non-equilibrium modelling and experiment Margarita Baeva, Dirk Uhrlandt, Erwan Siewert Electric arcs are widely used in industrial processes so that a thorough understanding of the arc characteristics is highly important to industrial research and development. TIG welding arcs operated with pointed electrodes made of tungsten, doped with cerium oxide, have been studied in order to analyze in detail the electric field and the arc voltage. Newly developed non-equilibrium model of the arc is based on a complete diffusion treatment of particle fluxes, a generalized form of Ohm’s law, and boundary conditions accounting for the space-charge sheaths within the magneto-hydrodynamic approach. Experiments have been carried out for electric currents in the range 5-200 A. The electric arc has been initiated between a WC20 cathode and a water-cooled copper plate placed 0.8 mm from each other. The arc length has been continuously increased by 0.1 mm up to 15 mm and the arc voltage has been simultaneously recorded. Modelling and experimental results will be presented and discussed. [Preview Abstract] |
Friday, October 14, 2016 9:45AM - 10:00AM |
UF3.00006: Course of organized structures in thermal plasma inside and outside argon plasma torch Jan Gruber, Jiri Sonsky, Jan Hlina Arc chamber of direct-current (dc) argon plasma torch and area just above the nozzle outside of this dc plasma torch were observed by hi-speed camera. System of reflecting mirrors and transparent silica arc chamber walls were used to obtain simultaneous records of both i) cathode area with electric arc inside the plasma torch and ii) nozzle exit with resulting plasma jet outside the plasma torch. Such experimental arrangement allowed us to track localized repeating patterns (organized structures) in the arc chamber and in the plasma flow. Identification of various organized structures - for different experimental conditions - according to their origin and typical development is presented in this paper. Impact of 300 Hz ripple in arc current was compared between different areas of the plasma. Additional simultaneous observation of plasma flow in the same system by series of photodiodes was used for verification of the results. [Preview Abstract] |
Friday, October 14, 2016 10:00AM - 10:15AM |
UF3.00007: Synthesis of boron-nitride nanocages and fullerenes in a BN plasma Predrag Krstic, Longtao Han Synthesis of boron-nitride fullerenes, nano-cocoons and nano-cages by self-organization of BN molecules in a high-temperature plasma is simulated using the DFT tight-binding method. No boron nano-cluster or catalysts nano-particle are needed to initiate this process. By varying the plasma temperature and the BN density, as well as the time of growth we can simulate growth of he sp$^{\mathrm{2}}$ cages of various shape, size and quality. Role of hydrogen in HBNH and H$_{\mathrm{2}}$BNH$_{\mathrm{2}}$ synthesis is also considered. [Preview Abstract] |
Friday, October 14, 2016 10:15AM - 10:30AM |
UF3.00008: Plasma-produced nanocrystals enable new insights in semiconductor physics Benjamin Greenberg, Zachary Robinson, Claudia Gorynski, Bryan Voigt, Lorraine Francis, Eray Aydil, Uwe Kortshagen The transition from semiconducting (insulating) to metallic behavior is a central problem of semiconductor physics. In bulk semiconductors, this insulator-to-metal transition is described by the well-known Mott criterion. However, in films of semiconductor nanocrystals the Mott criterion fails completely. Recent progress in the nonthermal plasma synthesis of films of highly doped silicon nanocrystals has contributed to the development of a new theory that presents a consistent analog to the Mott criterion for nanocrystal materials [Chen, T. {\em et al., Nat. Mater.} {\bf 2016}, {\em 15}, 299]. Here, we study films of nonthermal plasma produced zinc oxide (ZnO) nanocrystals to in detail investigate the insulator-to-metal transition. We produce high-purity monodisperse ZnO nanocrystals in a nonthermal plasma and form dense films via supersonic impact deposition. We then modulate the free carrier density, $n$, and nanocrystal contact facet radius, $\rho$, via xenon-flashlamp intense pulsed light annealing, which induces necking between the clean surfaces of adjacent nanocrystals. Preliminary electrical measurements indicate that the electron mobility can be finely tuned and that the films cross the insulator-to-metal transition for sufficiently high $n$ and $\rho$. [Preview Abstract] |
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