The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform

J Schulze - One of the best experts on this subject based on the ideXlab platform.

  • experimental and computational investigations of the effect of the Electrode Gap on capacitively coupled radio frequency oxygen discharges
    Physics of Plasmas, 2019
    Co-Authors: J Schulze, A Derzsi, Z Donko, H J Yeom, D J Seong
    Abstract:

    Geometrically symmetric capacitively coupled oxygen plasmas are studied experimentally by optical emission spectroscopy and probe measurements as well as via numerical simulations using the kinetic Particle-in-Cell/Monte Carlo collision (PIC/MCC) approach. The experiments reveal that at a fixed pressure of 20 mTorr and a driving frequency of 13.56 MHz, the central electron density increases with an increased Electrode Gap, while the time averaged optical emission of atomic oxygen lines decreases. These results are reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations show that the electron density increases due to a mode transition from the Drift-Ambipolar-mode to the α-mode induced by increasing the Electrode Gap. This mode transition is due to a drastic change of the electronegativity and the mean electron energy, which leads to the observed reduction of the emission intensity of an atomic oxygen line. The observed mode transition is also found to cause a complex non-monotonic dependence of the O 2 + ion flux to the Electrodes as a function of the Electrode Gap. These fundamental results are correlated with measurements of the etch rate of amorphous carbon layers at different Gap distances.Geometrically symmetric capacitively coupled oxygen plasmas are studied experimentally by optical emission spectroscopy and probe measurements as well as via numerical simulations using the kinetic Particle-in-Cell/Monte Carlo collision (PIC/MCC) approach. The experiments reveal that at a fixed pressure of 20 mTorr and a driving frequency of 13.56 MHz, the central electron density increases with an increased Electrode Gap, while the time averaged optical emission of atomic oxygen lines decreases. These results are reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations show that the electron density increases due to a mode transition from the Drift-Ambipolar-mode to the α-mode induced by increasing the Electrode Gap. This mode transition is due to a drastic change of the electronegativity and the mean electron energy, which leads to the observed reduction of the emission intensity of an atomic oxygen line. The observed mode transition is also found to cau...

  • striations in electronegative capacitively coupled radio frequency plasmas effects of the pressure voltage and Electrode Gap
    Physics of Plasmas, 2017
    Co-Authors: I Korolov, J Schulze, Z Donko, Edmund Schungel, Younian Wang
    Abstract:

    Capacitively coupled radio-frequency (CCRF) CF4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF3+ and F– ions) resonates with the excitation frequency. In this work, we explore the effects of the gas pressure, the RF voltage, and the Electrode Gap on this striated structure by phase resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency, the minima of the CF3+ or F– ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enh...

  • striations in electronegative capacitively coupled radio frequency plasmas effects of the pressure voltage and Electrode Gap
    arXiv: Plasma Physics, 2017
    Co-Authors: I Korolov, Z Donko, Edmund Schungel, Younian Wang, J Schulze
    Abstract:

    Capacitively coupled radio-frequency (CCRF) CF_4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF_3^+ and F^- ions) resonates with the excitation frequency. In this work we explore the effects of the gas pressure, the RF voltage, and the Electrode Gap on this striated structure by Phase Resolved Optical Emission Spectroscopy and Particle-In-Cell/Monte Carlo Collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency the minima of the CF_3^+ and F^- ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enhanced spatial modulation of plasma parameters. The striation Gap (defined as the distance between two ion density peaks) is approximately inversely proportional to the pressure, while it exhibits a weak dependence on the RF voltage and the Electrode Gap. A transition between the striated and non-striated modes can be observed by changing either the pressure or the RF voltage; for 13.56 MHz and 18 MHz driving frequencies we present a phase diagram as a function of the pressure and voltage amplitude parameters.

Nicholas A. Melosh - One of the best experts on this subject based on the ideXlab platform.

  • Back-gated graphene anode for more efficient thermionic energy converters
    Nano Energy, 2017
    Co-Authors: Hongyuan Yuan, Piero A. Pianetta, Daniel C. Riley, Nicholas A. Melosh, Zhi-xun Shen, Roger T Howe
    Abstract:

    Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-Electrode Gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-Electrode Gap as small as 17 µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000 °C, the highest reported by far. We first demonstrate that electrostatic gating of graphene by a 20 nm HfO2dielectric layer changes the graphene anode work function by 0.63 eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the Gap from 1 mm down to 17 µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Finally, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69 eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-Electrode Gap.

  • Microbead-separated thermionic energy converter with enhanced emission current
    Physical Chemistry Chemical Physics, 2013
    Co-Authors: Karl A. Littau, Kunal Sahasrabuddhe, Dustin Barfield, Hongyuan Yuan, Zhi-xun Shen, Roger T Howe, Nicholas A. Melosh
    Abstract:

    The efficiency of thermionic energy converters is a strong function of the inter-Electrode separation due to space-charge limitations. Here we demonstrate vacuum thermionic energy converters constructed using barium dispenser cathodes and thin film tungsten anodes, separated by size specific alumina microbeads for simple device fabrication and inter-Electrode Gap control. The current and device efficiency at the maximum power point are strongly dependent on the inter-Electrode Gap, with a maximum device efficiency of 0.61% observed for a Gap on the order of 5 μm. Paths to further reductions in space charge and improved anode work function are outlined with potential for over an order of magnitude improvement in output power and efficiency.

Roger T Howe - One of the best experts on this subject based on the ideXlab platform.

  • Back-gated graphene anode for more efficient thermionic energy converters
    Nano Energy, 2017
    Co-Authors: Hongyuan Yuan, Piero A. Pianetta, Daniel C. Riley, Nicholas A. Melosh, Zhi-xun Shen, Roger T Howe
    Abstract:

    Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-Electrode Gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-Electrode Gap as small as 17 µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000 °C, the highest reported by far. We first demonstrate that electrostatic gating of graphene by a 20 nm HfO2dielectric layer changes the graphene anode work function by 0.63 eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the Gap from 1 mm down to 17 µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Finally, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69 eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-Electrode Gap.

  • Microbead-separated thermionic energy converter with enhanced emission current
    Physical Chemistry Chemical Physics, 2013
    Co-Authors: Karl A. Littau, Kunal Sahasrabuddhe, Dustin Barfield, Hongyuan Yuan, Zhi-xun Shen, Roger T Howe, Nicholas A. Melosh
    Abstract:

    The efficiency of thermionic energy converters is a strong function of the inter-Electrode separation due to space-charge limitations. Here we demonstrate vacuum thermionic energy converters constructed using barium dispenser cathodes and thin film tungsten anodes, separated by size specific alumina microbeads for simple device fabrication and inter-Electrode Gap control. The current and device efficiency at the maximum power point are strongly dependent on the inter-Electrode Gap, with a maximum device efficiency of 0.61% observed for a Gap on the order of 5 μm. Paths to further reductions in space charge and improved anode work function are outlined with potential for over an order of magnitude improvement in output power and efficiency.

Z Donko - One of the best experts on this subject based on the ideXlab platform.

  • experimental and computational investigations of the effect of the Electrode Gap on capacitively coupled radio frequency oxygen discharges
    Physics of Plasmas, 2019
    Co-Authors: J Schulze, A Derzsi, Z Donko, H J Yeom, D J Seong
    Abstract:

    Geometrically symmetric capacitively coupled oxygen plasmas are studied experimentally by optical emission spectroscopy and probe measurements as well as via numerical simulations using the kinetic Particle-in-Cell/Monte Carlo collision (PIC/MCC) approach. The experiments reveal that at a fixed pressure of 20 mTorr and a driving frequency of 13.56 MHz, the central electron density increases with an increased Electrode Gap, while the time averaged optical emission of atomic oxygen lines decreases. These results are reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations show that the electron density increases due to a mode transition from the Drift-Ambipolar-mode to the α-mode induced by increasing the Electrode Gap. This mode transition is due to a drastic change of the electronegativity and the mean electron energy, which leads to the observed reduction of the emission intensity of an atomic oxygen line. The observed mode transition is also found to cause a complex non-monotonic dependence of the O 2 + ion flux to the Electrodes as a function of the Electrode Gap. These fundamental results are correlated with measurements of the etch rate of amorphous carbon layers at different Gap distances.Geometrically symmetric capacitively coupled oxygen plasmas are studied experimentally by optical emission spectroscopy and probe measurements as well as via numerical simulations using the kinetic Particle-in-Cell/Monte Carlo collision (PIC/MCC) approach. The experiments reveal that at a fixed pressure of 20 mTorr and a driving frequency of 13.56 MHz, the central electron density increases with an increased Electrode Gap, while the time averaged optical emission of atomic oxygen lines decreases. These results are reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations show that the electron density increases due to a mode transition from the Drift-Ambipolar-mode to the α-mode induced by increasing the Electrode Gap. This mode transition is due to a drastic change of the electronegativity and the mean electron energy, which leads to the observed reduction of the emission intensity of an atomic oxygen line. The observed mode transition is also found to cau...

  • striations in electronegative capacitively coupled radio frequency plasmas effects of the pressure voltage and Electrode Gap
    Physics of Plasmas, 2017
    Co-Authors: I Korolov, J Schulze, Z Donko, Edmund Schungel, Younian Wang
    Abstract:

    Capacitively coupled radio-frequency (CCRF) CF4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF3+ and F– ions) resonates with the excitation frequency. In this work, we explore the effects of the gas pressure, the RF voltage, and the Electrode Gap on this striated structure by phase resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency, the minima of the CF3+ or F– ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enh...

  • striations in electronegative capacitively coupled radio frequency plasmas effects of the pressure voltage and Electrode Gap
    arXiv: Plasma Physics, 2017
    Co-Authors: I Korolov, Z Donko, Edmund Schungel, Younian Wang, J Schulze
    Abstract:

    Capacitively coupled radio-frequency (CCRF) CF_4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF_3^+ and F^- ions) resonates with the excitation frequency. In this work we explore the effects of the gas pressure, the RF voltage, and the Electrode Gap on this striated structure by Phase Resolved Optical Emission Spectroscopy and Particle-In-Cell/Monte Carlo Collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency the minima of the CF_3^+ and F^- ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enhanced spatial modulation of plasma parameters. The striation Gap (defined as the distance between two ion density peaks) is approximately inversely proportional to the pressure, while it exhibits a weak dependence on the RF voltage and the Electrode Gap. A transition between the striated and non-striated modes can be observed by changing either the pressure or the RF voltage; for 13.56 MHz and 18 MHz driving frequencies we present a phase diagram as a function of the pressure and voltage amplitude parameters.

Hongyuan Yuan - One of the best experts on this subject based on the ideXlab platform.

  • Back-gated graphene anode for more efficient thermionic energy converters
    Nano Energy, 2017
    Co-Authors: Hongyuan Yuan, Piero A. Pianetta, Daniel C. Riley, Nicholas A. Melosh, Zhi-xun Shen, Roger T Howe
    Abstract:

    Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-Electrode Gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-Electrode Gap as small as 17 µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000 °C, the highest reported by far. We first demonstrate that electrostatic gating of graphene by a 20 nm HfO2dielectric layer changes the graphene anode work function by 0.63 eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the Gap from 1 mm down to 17 µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Finally, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69 eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-Electrode Gap.

  • Microbead-separated thermionic energy converter with enhanced emission current
    Physical Chemistry Chemical Physics, 2013
    Co-Authors: Karl A. Littau, Kunal Sahasrabuddhe, Dustin Barfield, Hongyuan Yuan, Zhi-xun Shen, Roger T Howe, Nicholas A. Melosh
    Abstract:

    The efficiency of thermionic energy converters is a strong function of the inter-Electrode separation due to space-charge limitations. Here we demonstrate vacuum thermionic energy converters constructed using barium dispenser cathodes and thin film tungsten anodes, separated by size specific alumina microbeads for simple device fabrication and inter-Electrode Gap control. The current and device efficiency at the maximum power point are strongly dependent on the inter-Electrode Gap, with a maximum device efficiency of 0.61% observed for a Gap on the order of 5 μm. Paths to further reductions in space charge and improved anode work function are outlined with potential for over an order of magnitude improvement in output power and efficiency.