Infuence of Gas Temperature on Electrical Breakdown in Cylindrical Electrodes

Han S. Uhm,Hyoung S. Kim,Seh J Jung 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.III

The in uence of the gas temperature on electrical breakdown properties is investigated for the cylindrical electrode system. A theoretical model of the electrical breakdown in a cylindrical electrode system is brie y summarized, by making use of Paschen's law. The breakdown voltage increases, reaches its peak and decreases, as the aspect ratio a/b increases from 0.01 to unity, where a and b are radii of the inner and outer electrodes, respectively. The experimental data agree remarkably well with the theoretical predictions. The breakdown voltage V in a high gas temperature Tg is given by V = (Tr/Tg)V0 where V0 is the breakdown voltage at the room temperature Tr. Obviously, the breakdown voltage decreases as the gas temperature Tg increases. The experimental data agree well with the theoretical values. It is, therefore, concluded that the breakdown voltage is inversely proportional to the gas temperature Tg as predicted from the theoretical analysis.

Electrical Breakdown In Flames

Han S. Uhm 한국진공학회(ASCT) 2000 Journal of Korean Vacuum Science & Technology Vol.4 No.1

Properties of electrical discharge in flames and influence of plasma electrons on gas neutrals are investigated by making use of the ionization cross section of air. Frames have three distinctive features. They are hot, emit light and are weakly ionized. We investigate influence of these three characteristics of flames on the electrical breakdown. It is found that the breakdown electric field in flames is inversely proportional to the flame temperature T_g, thereby easily generating plasmas in flames. A swarm of low-energy electrons in flames would allow a significant population of electronically excited states of flame molecules to be formed. Therefore, the analysis shows that the electronic excitation of flame molecules may also considerably reduce the breakdown field. Plasma electrons generate atomic oxygen by the electron attachment of oxygen molecules in high-pressure flames. These oxygen atoms are the most reactive radicals in flames for material oxidation.

A decontamination study of simulated chemical and biological agents

Uhm, Han S.,Lee, Han Y.,Hong, Yong C.,Shin, Dong H.,Park, Yun H.,Hong, Yi F.,Lee, Chong K. American Institute of Physics 2007 Journal of Applied Physics Vol.102 No.1

Characterization of electronic structure in dielectric materials by making use of the secondary electron emission

Han S. Uhm,Joon H. Choi,조광섭,박병주,정난주,최은하 한국물리학회 2013 Current Applied Physics Vol.13 No.2

The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ε0 corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ε0 =7.4 eV, whereas the (110) orientation has a deep characteristic energy ε0 = 9.6 eV, consistent with the observed coefficient g of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient r of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena.

Hydrophobic Coating of Y2O3:Eu Phosphors by Using HMDSO/Toluene Plasma at Low Pressure and Their Wettability

Han S. Uhm,Kyung H. Lee,Soon C. Cho,Sung Y. Jo,Chan U. Bang,Dong K. Lee 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.2

Surface treatments of Y2O3:Eu phosphors for improving the hydrophobic property were carried out using plasmas generated from a O-(Si(CH3)3)2 (HMDSO) in a low-pressure environment. The super-hydrophobic properties of the treated Y2O3:Eu phosphors were investigated. The samples treated by using the HMDSO/toluene glow plasma were analyzed by FTIR and OES and analyzed in terms of their contact angles (CAs). The effects of the HMDSO/toluene plasma treatments on the Y2O3:Eu phosphors were investigated in terms of the total surface free energy by using probe liquids to measure their contact angles (CAs). The total surface free energy of the Y2O3:Eu phosphors before and after a surface treatment with the HMDSO/toluene plasma, as estimated via the Owens-Wendt equation, was found to decrease from 82.2 mJ/m2 to 0.021 mJ/m2. This represents a significant improvement of the hydrophobicity of the Y2O3:Eu phosphors after the plasma treatment.

Underwater Discharge and Cell Destruction by Shockwaves

Han Y. Lee,Han S. Uhm,Bang K. Kang,Han N. Choi,Hee C. Yoo,Yoon J. Jung 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.III

An arc discharge in water can produce a shockwave, ultraviolet radiation and radical species. In this paper, we have investigated arc discharge characteristics and sludge cell destruction by a shockwave. An arc discharge was generated by applying a pulsed voltage of V = 20 30 kV to the tip or coaxial electrodes. The capacitance of C = 30 nF was used. We used the Closed Re ux Titrimetric Method (K2Cr2O7 Oxidation method) to measure the COD (Chemical Oxygen Demand) increase caused by cell destruction. The shockwave is produced only by the arc discharge. Shockwave intensity is mostly related to the discharge energy. It is also a function of the discharge circuit, pulse duration, water conductivity, electrode size, gap distance and reactor conguration. However, discharge energy is the most eective parameter of the sludge cell destruction. We introduce a discharge model. The !-R graph is obtained from the experiments. Data on sludge treatment by underwater arc discharge is also presented.

Sterilization of Medical Equipment and Contaminated Articles by Making Use of a Resistive Barrier Discharge

Han S. Uhm,Jung G. Kang,Eun H. Choi,Guang S. Cho 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.4

Presented here is an apparatus consisting of an atmospheric resistive-barrier discharge for the sterilization of medical tools wrapped in typical hospital cloths, for the sterilization of manufactured drugs in typical packaging materials, and for the sterilization of biologically contaminated articles. The sterilization apparatus consists of layers of the resistive-barrier discharge device operating at room temperature, a sterilization chamber, and an ozone destruction device. An electrical discharge in the resistive-barrier discharge system generates atmospheric plasma in oxygen gas, efficiently generating ozone, which in turn sterilizes medical tools and biologically contaminated articles at room temperature. A sterilization experiment was carried out at an apparatus volume of 100 liters, with a sterilization chamber volume of 60 liters, and a discharge device volume of 40 liters. The sterilization in this experiment required 60 W of power for 5 hours of residence time. For a given sterilization time, the required electrical power is proportional to the apparatus volume. Ozone in the sterilization chamber was destroyed safely after sterilization.

Atmospheric Pressure Plasma Research Activity in Korea

Han S. Uhm 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.42 No.III

Plasma is generated by electrical discharge. Most plasma generation has been carried out at low-pressure gas typically less than one millionth of atmospheric pressure. Plasmas are in general generated from impact ionizations of neutral gas molecules by accelerated electrons. The energy gain of electrons accelerated in an electrical field is proportional to the mean free path. Electrons gain more energy at low-pressure gas and generate plasma easily by the ionization of neutrals, because the mean free path is longer. For this reason conventional plasma generation is carried out at low pressures. However, many practical applications require plasmas at high-pressure. In order to avoid the requirement for vacuum pump, researchers in Korea start to develop plasmas in high-pressure chambers where the pressure is 1 atmosphere or greater. Material processing, environmental protection/restoration and improved energy production efficiency using plasmas are only possible for inexpensive bulk plasmas. We thus generate plasmas by new methods and plan to set foundations for new plasma technologies for 21st century industries. This technological research will play a central role in material processing, environmental and energy production industries.

Electrical Breakdown In flames

Uhm, Han S. 한국진공학회 2000 Journal of Korean Vacuum Science & Technology Vol.4 No.1

Properties of electrical discharge in flames and influence of plasma electrons on gas neutrals are investigated by making use of the ionization cross section of air. Frames have three distinctive features. They are hot, emit light and are weakly ionized. We investigate influence of these three characteristics of flames on the electrical breakdown. It is found that the breakdown electric field in flames is inversely proportional to the flame temperature T$\_$g/, thereby easily generating plasmas in flames. A swarm of low-energy electrons in flames would allow a significant population of electronically excited states of flame molecules to be formed. Therefore, the analysis shows that the electronic excitation of flame molecules may also considerably reduce the breakdown field. Plasma electrons generate atomic oxygen by the electron attachment of oxygen molecules in high-pressure flames. These oxygen atoms are the most reactive radicals in flames for material oxidation.

Production of Hydrogen-Rich Syngas from Dimethyl Ether by Using a Microwave Steam Plasma

Han S. Uhm,Chang H. Choi,Joo Y. Lee,Dong J. Kim,Sang G. Woo,Minwoo Woo,Dong C. Lee 한국물리학회 2019 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.75 No.6

Dimethyl ether (DME) is disintegrated by a microwave steam-plasma raising the gas temperature in a reaction chamber of 100 liters in a swirl-type gasifier. With additional heating of DME by partial oxidation, the gasifier temperature reaches 1250 °C. The initial breakdown species of DME molecules are hydrogen, carbon monoxide and methane, which eventually disintegrate to CO and H2 by steam modification at temperatures higher than 1200 °C. The relative concentrations of hydrogen, carbon monoxide and carbon dioxide produced solely from DME modification by steam were 67.5 percent, 30.3 percent and 2.2 percent, respectively, achieving a conversion rate of 88 percent at a temperature of 1058 °C. The total energy efficiency of the present experiment was 81 percent.