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Fundamentals of Plasma
Published in Eiichi Kondoh, Micro- and Nanofabrication for Beginners, 2021
When a highly energetic electron collides with a molecule, two electrons appear, one is the primary electron and the other is from ionization. These electrons get accelerated and collide with other molecules, which multiplies the electrons, leading to an “electron avalanche” or the so-called Townsend ionization.
Drastic Improvement of Dielectric Performances by Nanocomposite Technology
Published in Toshikatsu Tanaka, Takahiro Imai, Advanced Nanodielectrics, 2017
Muneaki Kurimoto, Kazuyuki Tohyama, Yasuhiro Tanaka, Yoshinobu Mizutani, Toshikatsu Tanaka, Masayuki Nagao, Naoki Hayakawa, Takanori Kondo, Tsukasa Ohta
No clear evidence has yet been identified on whether or not tree initiation time is shortened by the incorporation of nanofillers in epoxy resins. However, it was found in polyethylene that tree initiation voltage increases with time as the content of nanofillers (MgO in this case) increases [4]. From this finding, it can be said that nanofillers play some role on tree initiation. It is proposed as a mechanism for this behavior that electrons injected from a needle electrode tip might be trapped on nanofillers to make it difficult for electron avalanche to happen. For that reason, nanofillers will retard tree initiation time and enhance breakdown voltage. Another mechanism is that electrons are scattered and decelerated by the electric field created by nanofillers, making it difficult for electron avalanche to take place. Figure 5.47 shows an interesting SEM photo in which a minute tree is generated from a bulky tree channel in epoxy nanocomposites. This gives some insight into the inner structures of tree channels in nanocomposites. This minute tree is approximately 1 μm in length extending from a big tree channel, and is considered to be similar to the initial tree generated from a needle electrode tip. The minute tree is hindered even in the region shorter than 1 μm and is forced to form zigzag paths and branches.
Ozone Production Influenced by Increasing Gas Pressure in Multichannel Dielectric Barrier Discharge for Positive and Negative Pulse Modes
Published in Ozone: Science & Engineering, 2018
Can Ding, Dingkun Yuan, Zhihua Wang, Yong He, Sunel Kumar, Yanqun Zhu, Kefa Cen
Generally, seed electrons increase by electron avalanche, absorbing energy in the electric field. Ozone production initiates via O2 dissociation due to high-energy electrons in the streamer, followed by a three-body collision to facilitate O3 formation as follows (Eliasson, Hirth, and Kogelschatz 1987; Yagi and Tanaka 1979):
Numerical simulation of air DBD under standard atmospheric pressure
Published in Radiation Effects and Defects in Solids, 2022
Xiaobing Wang, Chenyang Zhu, Lu Wang, Jinqiu Liu, An Jin
Non-equilibrium plasma is widely used in sewage treatment and air purification because it can produce a large number of strong, active particles in the reaction process. Studying the generation mechanism of non-equilibrium plasma can improve the practical application efficiency of non-equilibrium plasma. In order to reduce the calculation time of numerical simulation, the DBD equipment is simplified into a one-dimensional model, and a series of calculation parameters are determined by using the software BOLSIG+. The analysis of the results of the numerical simulation can provide a theoretical basis for the practical application of non-equilibrium plasma, and draw the following conclusions: When the power supply voltage is 18 kV and the frequency is 10 kHz, the electric field reaches the maximum near the discharge air gap sheath, the electric field intensity decreases in the middle of the cathode and anode, the potential decreases from the anode direction to the cathode direction, and the average value of the potential is less than the output voltage of the power supply.In the process of discharge, the maximum value of air gap voltage is about 12 kV, and the current density reaches the maximum value at the rising edge of air gap voltage, which is about 0.5 A m−2. The air gap voltage and current density increase with the increase of power supply voltage. However, the additional electric field is formed due to the accumulation of charge surface area, so that the air gap voltage remains basically unchanged until the power supply voltage reaches the peak and then decreases with the decrease of power supply voltage.In the process of electrons moving to the anode, electrons and air particles collide and ionize continuously, and air particles decompose into positive ions and electrons. The newly generated electrons collide and ionize with air particles to produce more electrons, and the free electrons in space will increase rapidly and accumulate to form an electron avalanche.At the initial stage of discharge, the electron temperature decreases due to the inelastic collision between electrons and air particles, which is about 0.5 eV. After a period of time, the secondary collision of electrons increases the electron temperature, and the maximum electron temperature is about 2.7 eV.The reduced electric field is at 3 × 10−4 reach a discharge steady state. With the increase of reduced electric field, the average electron energy of plasma increases from 0.95 to 1.45 eV. At the same time, at the end of the electron energy distribution function EEDF, the proportion of high-energy electrons in the total number of electrons becomes larger. At this time, electrons react actively with air particles and can produce more active substances.
Numerical and Experimental Investigation of the Channel Expansion of a Low-Energy Spark in the Air
Published in Combustion Science and Technology, 2019
K. V. Korytchenko, S. Essmann, D. Markus, U. Maas, E. V. Poklonskii
Optical access to the discharge was achieved via quartz glass windows (55 mm clear diameter) in each flange. The optical setup is shown in Figure 4. A schlieren setup was used to investigate the temporal evolution of the hot gas kernel that formed during the discharge, as well as the pressure wave that formed at the perimeter of this kernel. The setup consisted of two 500 mm focal-length field lenses and a vertically oriented razor blade as the schlieren stop. A spark flash lamp (Nanolite KL-L) with a very short flash duration of less than 25 ns was used as a light source in order to freeze the position of the pressure wave on the schlieren images, which were recorded on a CCD camera (LaVision Imager ProPlus 2M). The spatial resolution of the images was 50 pixels/mm. As will be seen later, the time scale on which the kernel expands necessitates that schlieren images be taken as early as a few hundred nanoseconds after the beginning of the discharge. However, intrinsic delays of the equipment used allowed images to be taken no earlier than 6.5 µs when triggering them by means of the discharge current. Therefore, the discharges were triggered by means of ultraviolet radiation from a laser (frequency-quadrupled Nd:YAG laser at 266 nm, 7 ns pulse duration, 50 mJ pulse energy, cf. Figure 4). The voltage chosen was close to but below the natural breakdown voltage; thus, the probability of discharges occurring spontaneously was low enough that no discharges were recorded in a time interval of several minutes. The natural breakdown voltage depends on the properties of the gas, the pressure, and the electrode distance. If the voltage applied is high enough, free electrons present in the inter-electrode region are sufficiently accelerated in the electric field that an electron avalanche is created, eventually leading to a breakdown due to the Townsend mechanism (Townsend, 1915). By decreasing the voltage, the limited number of start electrons may make the probability of a breakdown so remote that, ultimately, no discharges will occur under otherwise normal conditions. Using ultraviolet radiation, a large number of free electrons are generated at a specific point in space and time. The unfocussed laser beam partially hits the electrode tips, where electrons are emitted from the metal surface due to the photoelectric effect. Electrons may also become free from molecules in the gas phase due to multi-photon ionization, though this process is likely of minor importance. This temporarily reduces the electric field strength required for a breakdown, allowing a precisely timed generation of discharges to take place. The number of free electrons generated depends on the laser irradiance and the beam path. The laser energy chosen was as low as possible, so that pre-heating of the gas would not interfere with the experiment, but was high enough to reliably trigger discharges. This was confirmed by comparing experimentally obtained kernel and pressure wave radii with and without laser triggering at the same discharge energy for time intervals where this was possible (>6.5 µs). No significant differences were found between the two cases. Further details on this triggering method can be found in work (Essmann et al., 2017).