The Effect of Discharge Characteristics on Dielectric Barrier Discharges According to the Relative Permittivity

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1 , pp The Effect of Discharge Characteristics on Dielectric Barrier Discharges According to the Relative Permittivity Don-Kyu Lee Electrical Engineering, Dong-Eui University, Busan , Korea Abstract. Different kinds of DBD (Dielectric barrier discharges) have been developing for a long time. The internal dielectric has a very important characteristic on DBD, thus we analyze the discharge characteristics according to the relative permittivity and the relations between frequency and real used dielectrics (Green sheet, Down dielectric, white dielectric) in this study. We investigated that increased relative permittivity occurs the decrease of the firing voltage and sustain voltage. Also, in a condition of drive within 1MHz, with regard to the change of real part according to frequency, it has quite stable dielectric constant in the condition of drive within 1MHz. Keywords: dielectric barrier discharges(dbd), relative permittivity(εr), dielectric, discharge voltage, sustain voltage, frequency. 1 Introduction Different kinds of DBD (Dielectric barrier discharges) have been developing for a long time. DBD is a device with a constant thickness of dielectric between two electrodes and produces a non-equilibrium discharge at about atmospheric pressure. This leads to a number of important DBD applications including industrial ozone generation [1], pollution control [2], plasma-chemical vapor deposition and surface activation [3], excitation of excimer lamps, more recently surface modification of diverse materials [4], and the most marketable product of all kinds was the plasma display panel. The most important characteristic of dielectric-barrier discharges is that non-equilibrium plasma conditions can be provided in a much simpler way than with other alternatives like low pressure discharges, fast pulsed high pressure discharges or electron beam injection. Its flexibility with respect to geometrical configuration, operating medium and operating parameters is unprecedented. Conditions optimized in laboratory experiments can easily be scaled up to large industrials installations. Efficient low cost power supplies are available up to very large powers. [5] As an AC PDP is a capacitive driven device, it has an inevitable energy loss during charging the capacitor as 0.5CV 2. Here, C is the capacitance and V is the applied voltage. The energy recovery ratio of a commercial PDP is high up to 87%, but it is still necessary to reduce the panel capacitance to improve the luminous efficacy. In ISSN: ASTL Copyright 2017 SERSC

2 the PDP structure, dielectric layers are formed on the front and rear panels in order to protect electrodes. Therefore, the easiest way to reduce the panel capacitance is to decrease the dielectric constant or to increase the thickness of the dielectric layers. However, the change of dielectric properties results in the change of discharge characteristics, especially for the driving voltage. If the dielectric constant decreases, the driving voltage should be increased, but the energy loss by the capacitor increases proportionally to the square of V. Thus, it may not be effective to decrease the dielectric constant for the improvement of efficacy, and it is necessary to find optimal conditions. [6] The internal dielectric has a very important characteristic on DBD, thus we analyze the discharge characteristics according to the relative permittivity in this study. First, we investigated the characteristics of the discharge through two-dimensional simulation because the permittivity of the actual used dielectric is 6 to 15, and analyzed these characteristics through actual experiment. 2 Simulation Model and Results Figure 1 shows the cell structure used in the simulation and table 1 is the specification of the test panel. Simulation used is a two-dimensional fluid simulation, which consists of a continuity equation, drift-diffusion approximation for momentum conservation equation, and Poisson's equation. In the simulation, Ne and Xe are static background gases, and considered species are electrons, Ne +, Xe +, Xe 2 +, Ne 2 +, NeXe + ions, and excited species like Xe ( 3 P 1), Xe ( 3 P 2), Xe 2 (O u + ), Xe2 ( 1 Σ u + ), Xe 2 ( 3 Σ u + ) and Xe, etc. Local field approximation (LFA) is used to calculate the electron impact rate coefficient. [7 10] Specifications for simulation model are described in Figure 1 which has the same cell size as the experimental one. In order to describe the address electrode, a half value of the firing and sustain voltage are applied at the boundary for the address electrode. In the simulation study, however, we cannot treat all of the cells in the panel but can simulate only one cell. For this reason, we measure the firing voltage by giving continuous pulses to check if the discharge reaches a steady state. The minimum voltage which can reach a steady state is treated as a firing voltage. In the case of sustain voltage measurement, we apply the firing voltage between two sustain electrodes at first, and measure the voltage of the subsequent pulses shown in Figure 2 if the next waveform can sustain the discharge. When the subsequent discharges reach a steady state, its minimum voltage is the sustain voltage. The frequency of the applied pulse is 50 khz for simulation and 10 khz for experiment. [11] Figure 3 shows the variation of the firing voltage V f and the sustain voltage V s at 7, 10, 13 and 16 relative permittivity (ε r). As the relative permittivity increases by 3, the V f decreases by 3 from 225V and the V s decreases by about 10V from 190V. Increased permittivity causes the increase in the capacitance value of the dielectric. That causes a greater voltage between the discharge electrode than the voltage that occurs between the electrodes and the dielectric layer. Therefore, the firing voltage decreases. Also, the lower firing voltage produces more wall charge inside the 22 Copyright 2017 SERSC

3 dielectric. As a result, the decrease in sustain voltage is about three times larger than the decrease in firing voltage. Fig. 1. Cell structure used in the simulation Table 1. Cell specification of the test panel. Front panel ITO electrode width Electrode gap Bus (Ag) electrode width Dielectric thickness 190 μm 70 μm 80 μm 30 μm Rear panel Address (Ag) electrode width Dielectric thickness Barrier rib height Barrier rib width Phosphor (green) thickness Working gas Gas pressure Cell size 100 μm 30 μm 130 μm 25 μm 30 μm Ne(92%)+Xe 8% 400 Torr (53.3 kpa) μm2 Copyright 2017 SERSC 23

4 Fig. 2. Voltage waveforms used in the simulation firing voltage sustain voltage Fig. 3. Firing voltage and sustain voltage according to the εr. 3 Experiment Method and Results We investigate the frequency characteristics of used dielectric based the simulation results in chapter 3. The actual used dielectric consists of transparent dielectric and white dielectric in a PDP. Transparent dielectric layer is formed on a front glass substrate to over the display electrodes. It is necessary for the dielectric layer to maintain discharge, to have a high dielectric strength, and to have good transparency. For the development of a reasonable dielectric layer for a PDP, several properties are required such as high transparency (above 80%), high break down voltage (above 9 kv at 20μm), a dielectric constant below 15, and a reasonable coefficient of thermal expansion (8~ /K) to match the glass substrate. The white dielectric layer, if 24 Copyright 2017 SERSC

5 whitened, can reflect light that is emitted from the fluorescent layers and travels toward the rear substrate, to the front substrate, thereby enhancing the luminance. A white dielectric layer has a larger reflectance with regard to visible light than a transparent one. To investigate electrical properties, Ag electrode was coated on the dielectric samples by printing method. The capacitance was measured by an impedance analyzer (HP4194A). The dielectric samples were measured over the frequency range of 100 Hz ~ 1 MHz. Figure 4 shows the measurement tool of temperature dependence of dielectric constant. Fig. 4. Schematic diagram of a measurement tool of dielectric properties Figure 5 (a), (b) and (c) illustrate the frequency dependence of the dielectric constant of dielectric layers (Green sheet, Down dielectric, white dielectric) measured at a frequency region of 100Hz to 10MHz. The dielectric constant of the Down layer dielectric, green sheet, and white dielectric are 12.7 ~ 12.3, 10.2 ~12.3, and 18.9 ~20.7 in frequency region under 1 MHz. The dielectric constant of dielectrics is rapidly increases by 4~9 times in high frequency region over 1MHz. But PDP drive frequency is below 300kHz, therefore, the frequency dependence of the dielectric constant of dielectric layers has a stability. The wall voltages play an important role in lowering the sustaining voltage through wall charges accumulated on the dielectric surface in AC-PDP. The wall charges and voltages, as well as capacitances, are strongly depend on the dielectric constant. The experimental results of the dielectric constant of the transparent dielectric layers show that the transparent dielectric helps increasing the discharge gap voltage, because of discharge gap voltage larger than dielectric gap voltage (above 97%). [11] Copyright 2017 SERSC 25

6 (a) Green sheet (b) Down dielectric (c) White dielectric Fig. 5. Variation of dielectric constant as a function of frequency 4 Conclusion In this study, we investigated the relations between discharge voltage and relative permittivity of dielectrics materials and the relations between frequency and condition of dielectrics materials. As the relative permittivity increases, the V f and the V s decreases. The V f decreased quantity is three times less than the decreased quantity by the V s. That causes a greater voltage between the discharge electrode and a more wall charge. To be higher voltage on discharge space than dielectric, the dielectric constant of front layer which is the closest with firing voltage keep from 10 to 13 such as lower layer dielectric (12.7~13.5), green sheet dielectric (10.2~12.3), and white dielectric (18.9~20.1). In a condition of drive within 1MHz, with regard to the change of real 26 Copyright 2017 SERSC

7 part according to frequency, it has quite stable dielectric constant in the condition of drive within 1MHz. References 1. U. Kogelschatz, IEEE Trans. Plasma Sci., 30 (2002) K. Takaki, K. Urashima, and J-S. Chang, IEEE Trans. Plasma Sci. 32 (2004) O. Goossens, E. Dekempeneer, D. Vangeneugden, R. Van de Leest and C. Leys, Surf. Coat. Technol (2001) G. Borcia, C. A. Anderson and N. M. D. Brown, Appl. Surf. Sci. 225 (2004) U. Konelschatz, B. Eliasson and W. Egli, J. PHYS IV FRANCE 7 (1 997) 6. Seung Bo Shim a, Sung-Yong Cho a, Don Kyu Lee b, In Cheol Song a, Chung Hoo Park a, Ho-Jun Lee a, Hae June Lee, Thin Solid Films 518 (2010) Shin Young Kyo, Shon Chae Hwa, Kim Woong, Lee Jae Koo, IEEE Trans. Plasma Sci. 27 (1999) Lee Hae June, Kim Hyun Chul, Yang Sung Soo, Lee Jae Koo, Phys. Plasmas 9 (2002) S.S. Yang, S.M. Lee, F. Iza, J.K. Lee, J. Phys. D: Appl Phys. 39 (2006) H.C. Kim, M.S. Hur, S.S. Yang, S.W. Shin, J.K. Lee, J. Appl. Phys. 91 (2002) Chung-Hoo Park, Plasma Display, PNU, 2001 Copyright 2017 SERSC 27