Journal of the Korean Physical Society, Vol. 61, No. 8, October 2012, pp. 1246∼1251
Effects of Pressure and Electrode Resistance on AC-plasma Display Panels (AC-PDPs) for Various Panel Temperatures Hong Tak Kim and Sang Ho Sohn∗ Department of Physics, Kyungpook National University, Daegu 702-701, Korea (Received 2 May 2012, in final form 29 August 2012) In this study, the effects of panel temperature on AC-plasma display panels (AC-PDPs) were investigated. Macroscopic properties, such as the luminance, discharge delay time lag, and discharge current decreased while the breakdown voltage increased with increasing panel temperature. These data implied that changes in the materials properties with the panel temperature strongly affected the macroscopic properties, including the gas-discharge characteristics of AC-PDPs. The reason can be explained by the change in the collision kinetics due to the increase in the pressure in the AC-PDPs, the voltage drop due to the increase in the resistivity of the metal electrodes, and the decrease in the effective voltage due to the decrease in the surface resistivity of a MgO protective layer. The properties of other components such as the indium-tin-oxide (ITO) transparent electrode and the dielectric layer changed little in the considered temperature range (0 ∼ 80 ◦ C), and, thus, are not thought to affect the macroscopic properties of AC-PDPs with changing panel temperature. PACS numbers: 51.30.+i, 52.75.-d, 52.80.-s, 85.60.Pg Keywords: AC-PDP, Delay time lag, Heating, Plasma display, Temperature DOI: 10.3938/jkps.61.1246
I. INTRODUCTION Alternating current plasma display panels (AC-PDPs) have been considered as promising display devices because of their large scale, slim profile, wide-view angle, and excellent image quality. In spite of progressive developments of AC-PDP devices, some problems are still unsolved. Especially, the change in the panel temperature, which always occurs in AC-PDPs due to plasma discharges on the cell and atmospheric temperature outside the panel, causes variations in the macroscopic properties, such as the luminance, discharge current, and discharge time lag, and results in an unstable discharge. This implies that the discharge patterns are not the same under different temperature conditions. Even though, many researchers have reported results related to the temperature effects of AC-PDPs [1–4], definite explanations of those results have only been given for plasma panels that were partially heated or for small sized panels that were heated. However, there is a big difference between the temperature effects for partially heated and completely heated panels. In this study, the effects of the panel temperature in AC-PDPs have been investigated from a material point of view, when the AC-PDPs were completely-heated. First, the difference between partial and complete heating is explained in terms of an ideal gas equation. Also, ∗ E-mail:
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we measured not only the macroscopic parameters of the AC-PDP but also the material properties as functions of the panel temperature. Consequently, the relationship between the measured parameters and the material properties were evaluated under different temperature conditions.
II. EXPERIMENTS AND DISCUSSION AC-PDP panels, 50 , were used to investigate the temperature effects. The cell structure of the plasma panel was a closed-well-type rib with a height of 125 µm, the electrodes were indium tin oxide (ITO) with a gap of 60 µm, and the pixel-resolution was 1366 × 768. The pressure of the plasma panel was 480 Torr for a gaseous mixture of Xe (8%) and Ne (92%). A panel with the pressure of 400 Torr was also fabricated to study the pressure effect. In addition, test panels, 42 , were used to study the effects of resistivity. The detailed specifications of the AC-PDPs are shown in Table 1. Panel temperatures were controlled by using a temperature controller system in an environment chamber and were measured by using an infrared thermometer (Raytek, ST60XBUS) at the panel surface. The discharge current and the breakdown voltage of the AC-PDPs were measured using a multimeter (Keithley, 2000). The luminance of the AC-PDP was measured using a photometer
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Effects of Pressure and Electrode Resistance on AC-plasma Display · · · – Hong Tak Kim and Sang Ho Sohn
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Table 1. Specifications of the AC-PDPs tested in this research. Specification Pressure (Torr) Xe concentration (%) Ne concentration (%) ITO gap (µm) Dielectric thickness (µm) Barrier rib height (µm) Ag line resistance (Ω) Panel size (inch) Panel resolution (pixel)
Reference 480 8 92 60 38 125 70 50 1336 × 768
Sample 1 440 8 92 60 38 125 70 50 1336 × 768
Sample 2 500 6 94 60 40 125 75 42 1024 × 768
Sample 3 500 6 94 60 40 125 130 42 1024 × 768
Table 2. Optoelectronic properties of 50-inch panels with different panel pressures (Vs = 195 V, Va = 60 V). Property Luminance (cd/m2 ) Breakdown voltage (V) Discharge current (A) Discharge time lag (ns)
Reference 210 181 1.92 597 ± 27
Sample 1 207 177 1.90 605 ± 31
Fig. 1. Schematic diagram of an experimental setup for investigating the effects of temperature in AC-PDPs.
(Minolta, CA-100+) and the discharge time lag was measured using an avalanche photodiode detector (Hamamatsu, C5460) and an oscilloscope (Lecroy, LC534AL). Figure 1 shows a schematic diagram of the experimental setup for measuring the temperature effects in AC-PDPs. In addition, the properties of materials were investigated to compare the characteristics of AC-PDPs. Ag and ITO were grown on glass substrates with thicknesses of 5 µm and 1200 ˚ A, respectively, and their electrical properties were measured by using a 4-point probe system (Mitsubishi Ltd, MCP-T600). To study the temperature dependence of the dielectric constant of the insulator, we made a pellet with a diameter of 15 mm and a thickness of 3 mm. The both sides of pellet were coated with Al, and the dielectric constant was measured by using a LCR meter (Hioki, 3532-50).
III. RESULTS AND DISCUSSION Figure 2 illustrates the difference between partial and complete heating. A partial and complete heating in ACPDPs should be distinguished because different physical phenomena occurs and this difference between partial and complete heating of AC-PDPs can be explained in
Fig. 2. (Color online) Schematic illustrations of the difference between partial and complete heating: (a) un-heated, (b) partially-heated, and (c) completely-heated states. (For convenience, 11 particles per one cell were placed in the normal state).
terms of an ideal gas equation. In complete heating of AC-PDPs, the momenta of the particles are entirely increased by heating which causes an increase in the total pressure in the panel. On the other hand, the momenta of the particles are increased inside heated cells in partial heating, which causes an increase in the pressure inside the heated cells, the pressure difference between the heated and the unheated cells causes diffusion of particles from the heated cells to the unheated cells. This diffusion process continues until the pressure reaches equilibrium. If the ratio of the heated region to the entire area is small,
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Journal of the Korean Physical Society, Vol. 61, No. 8, October 2012
the change in total pressure can be ignored because the unheated region acts as a pressure reservoir and the particle density in the heated region only decreases. However, the ratio of the heated region gradually increases, so the total pressure in the AC-PDP is affected. These relationships can be evaluated by using an ideal equation, and the equations for the heated and the unheated region are given by ∆P V1 = n1 R(T1 − T0 ) − ∆nRT1 , ∆P V2 = n2 R(T2 − T0 ) − ∆nRT2 ,
(1) (2)
where ∆P is the change in the panel pressure, V is the volume inside the panel, ∆n is the change in the gas density, R is the gas constant, and T is the temperature. Equations (1) and (2) are the ideal gas equations in the heated cells and the unheated cells after the heating process, respectively. In these equations, the subscripts 0, 1, and 2 describe variables for the initially unheated cells, for the partially-heated cells, and for the unheated cells during heating, respectively. For complete heating and small-area heating, Eq. (1) can be expressed by ∆P V = nR∆T, P V1 ≈ ∆nR∆T.
(3) (4)
From Eqs. (3) and (4), a variation in the temperature leads to different phenomena in complete and partial heating. The effects of the temperature variation were studied using AC-PDPs with different panel pressures. Table 2 shows the macroscopic properties at different panel pressures. The results are similar for the reference panel and the panel with complete heating (below explained). However, the increase in pressure due to heating should be distinguished from a higher pressure AC-PDPs because there is a difference in the number of moles. Figures 3(a) – (c) show the variations of the discharge current, the breakdown voltage, and the luminance the functions of the panel temperature. The breakdown voltage linearly increased while the discharge current and the luminance decreased with increasing temperature. This can be explained in terms of the ideal gas equation, the kinetic mechanism in the gas discharge, and the variation in the resistivity in metal electrode. From the Eq. (3), the panel pressure in complete heating only changed as a function of the panel temperature; the number of particles and the volume inside the AC-PDP were kept constant. In the plasma discharge, the average energy gained from collisions between particles is given by [5,6] W = eλe E,
(5)
where e is the electric charge, λe is the mean free path of electrons and E is the electric field. The parameter λe is inversely proportional to the pressure and the average energy is proportional to E/P [5,6]. This relationship implies that the increases in the pressure and in
Fig. 3. Macroscopic properties of AC-PDPs as functions of the panel temperature: (a) discharge current, (b) breakdown voltage, and (c) luminance (Vs = 195 V, Va = 60 V).
the electrical resistivity of metal electrodes, originating from the increased panel temperatures, reduce the energy acquired from the plasma discharge. Furthermore, secondary electron emission (SEE) plays an important role in controlling the discharge properties, and the rate of SEE decreases with increasing temperature [7]. This means that the breakdown voltage decreases with increasing SEE rate and that the variation in breakdown voltage affects the discharge properties, including the discharge margin [8]. Generally, the breakdown voltage can be simply expressed using Paschen’s law as follows [9] Vf =
BP d , ln(AP d/ ln(1 + 1/γ))
(6)
where γ is the secondary ionization coefficient, d is the electrode gap, and A and B are constants. Paschen’s law, Vf = f(Pd), can be stated in terms of temperature by using an ideal gas equation, and the breakdown voltage can simply be expressed as a function of the temperature, Vf = f(T). As a result, the breakdown voltage gradually increases when the panel is completely heated. This variation in the breakdown voltage with to panel temperatures is caused by a reduction in the discharge margin and sometimes leads to an unexpected or unstable discharge. Furthermore, the breakdown voltage plays an important role in determining the driving voltage and the pattern of the driving waveform. Commercial AC-PDPs have been used to vary the driving waveform with a temperature dependency to prevent the unexpected discharge. The discharge current and the luminance measured at different panel temperatures (see Figs. 3(b) and 3(c)) were closely related to the temperature dependency of the electrical resistivities for both metal electrodes and for the MgO layer. Figure 4(a) shows the sheet resistivity (ρs ) of the Ag
Effects of Pressure and Electrode Resistance on AC-plasma Display · · · – Hong Tak Kim and Sang Ho Sohn Table 3. Optoelectronic properties of 42-inch test panels with different line-resistances at room temperature (Vs = 193 V, Va = 60 V). Property Luminance (cd/m2 ) Breakdown voltage (V) Discharge current (A) Discharge time lag (ns)
Sample 2 179 183 1.32 815 ± 25
Sample 3 161 187 1.18 823 ± 31
Fig. 4. Sheet resistivity (ρs ) of (a) Ag and (b) ITO, and (c) dielectric constant (εr ) of the dielectric layer as functions of the panel temperature.
layer as a function of the panel temperature, and the sheet resistivity of the Ag layer increases significantly with increasing panel temperatures. Generally, the resistivity ρ of the metal linearly increases with increasing temperature T and is theoretically given by [10] ρ = ρ0 (1 + α(T − T0 )),
(7)
where ρ0 is the resistivity at 293 K, T0 is the temperature at 293 K, and α is the temperature coefficient, respectively. From Eq. (7), the resistivity linearly increases with increasing temperature, and a current loss due to the increased resistivity is induced by the voltage drop in the discharge gap. To verify the temperature effects for the resistivity, we tested 42 test panels with line resistances of 75 Ω and 130 Ω for Ag bus electrode at room temperature. For the panel with the high lineresistance, the luminance and the discharge current were decreased by 10% and 11%, respectively. This tendency was in good agreement with the results of Fig. 3. Thus, the change in the resistivity of the Ag electrodes plays an important factor in affecting the discharge current and the luminance. The detailed results for the 42 test panels with different line-resistances are shown in Table 3. Figures 4(b) and 4(c) show the sheet resistance of the
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ITO and the dielectric constant of the dielectric material as functions of the panel temperature, respectively. The ITO film, as a transparent conductive layer, is a well-known n-type semiconductor and can be described as indium oxide heavily doped with Tin [11]. Thus, the resistivity of the ITO films is expected to decreases, as with other semiconductor materials, when the temperature is increased [10]. In this case, the resistivity of the ITO films slightly decreased, and the value was about 12 Ω/. In addition, the dielectric constant (εr ) of an insulator is nearly constant within the temperature region under 100 ◦ C, and the measured dielectric constant for the dielectric pellets was about 12. Consequently, the two factors are not thought to be sufficiently important to affect the characteristics of the AC-PDP for changing panel temperature. Besides the electrodes, the MgO protecting layer was also considered to be one of crucial factors for determining the temperature dependency for the panels. Recently, Ha et al. and Chou et al. reported the temperature dependency for the surface resistivity of MgO films, and the surface resistivity of the MgO films exponentially decreased with increasing temperature [12,13]. The relationship between the surface resistivity of the MgO films and the temperature can be roughly expressed by a , (8) ρ ∝ exp − T where ρ is the resistivity of the MgO layer, a is the coefficient and T is the temperature of the MgO layer. The variation in the panel temperature caused the change in the resistivity of the MgO surface. In AC-PDPs, the electric field is very strong due to the small dimension of the discharge gap, and the electric field between the two electrodes can be roughly estimated as ∼106 V/m (gap: ∼10−4 m, voltage: ∼102 V). If the surface resistivity of the MgO films is reduced with increasing panel temperature, there is another current path because of the high electric field in the gap. This leakage current is neutralized by wall charges, and the loss of wall charges reduces the effective voltage, composed of the input voltage and the induced voltage due to wall charges. From the above argument, we can draw a conclusion: the temperature effects of the Ag electrodes and the MgO protective layer reduce the input voltage and the voltage induced by wall charges, respectively. The reduced effective voltage directly also affects the discharge to decrease the luminescence and the discharge current. Figure 5 shows the distributions of discharge delay time lag according to the panel temperature. The distributions of discharge delay time lag became sharp and the mean discharge time lag (tm ) decreased with increasing panel temperature. Generally, the discharge time lag (td ) consists of two parts: the formative (tf ) and the statistical delay time lags (ts ). The td is usually given by [14–16] td = tf + ts .
(9)
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Journal of the Korean Physical Society, Vol. 61, No. 8, October 2012
Fig. 5. Distributions of a discharge delay time lag at different panel temperatures: (a) 80 ◦ C, (b) 60 ◦ C, (c) 40 ◦ C, (d) 20 ◦ C, and (e) 0 ◦ C (Vs = 195 V, Va = 60 V).
Fig. 6. Discharge delay time lag as a function of panel temperatures: (a) statistical, (b) formative, and (c) mean discharge delay time (Vs = 195 V, Va = 60 V).
Figures 6(a) – (c) show the statistical, formative, and mean discharge time lags, respectively. As the panel temperature is increased, the tm and ts , including σ, become exponentially shorter while the tf is slightly decreased with increasing panel temperature. The tf is well to be mainly related to the electric field, and the ts to be closely related to the number of priming particles and the pressure [2,3,16]. These are simply expressed as d 1 1 tf = , (10) + E µi µe 1 , (11) ts = n0 Ps
Fig. 7. Discharge delay time lag at room temperature as a function of address voltage: (a) statistical, (b) formative, and (c) mean discharge delay time (Vs = 195 V).
where n0 is the number of seed electrons, and Ps is the probability that an electron will ignite a discharge, and µi and µe are the mobility of ions and electrons, respectively. As mentioned above, an increase in the pressure due to an increase in the temperature rising could enhance the collision rate between electrons and particles, which means that the probability of a discharge firing increases with increasing of pressures. Thus, an increase in the panel pressure due to panel heating is caused by a reduction in the ts . For the formative delay time, the variation with panel temperatures is very small and the relationship between the weakness of the electric field and the increased mobility for electrons and ions are thought to cancel out temperature effects during panel heating. To remove the effects of the mobility variations, we measured the discharge delay time lag at room temperature as a function of the address voltage. And adjustment of the voltage is equivalent to a variation in the resistivity for the Ag electrodes due to an increase in the temperature. The variation in the discharge delay time with the address voltages is shown in Fig. 7. As the address voltage decreases, the formative, the statistical, and the mean discharge time lags increased, which means that a rise in the resistivity due to a change in the temperature leads to a weakness in the electric field between the gap, and the effect of which is important for increasing the discharge time lag. Another reason for the shortening of the tm and the ts is the priming particles (or seed particles). Priming particles include electrons due to cosmic rays, thermal electrons, and photo-electrons due to artificial vacuum ultra-violet radiation. These initial particles also play an important role in controlling the discharge delay time lag.
Effects of Pressure and Electrode Resistance on AC-plasma Display · · · – Hong Tak Kim and Sang Ho Sohn
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IV. CONCLUSION
REFERENCES
In this study, the effects of the panel temperature in AC-PDPs have been investigated from materials point of view during heating of the entire panel. In addition, the difference between partial and complete heating was studied in terms of the ideal gas equation. A pressure decrease is thought to exist in the partial heated region because of particles diffusion. Otherwise, a pressure increase is thought to exist for complete heating due to an increase in the momenta of the particles. This difference caused different phenomena in the two situations. For complete heating, as the panel temperature was increased, the luminance, the discharge time lag, and the discharge current of the AC-PDPs decreased while the breakdown voltage increased. These on the panel temperature were closely related to property changes in the materials. Especially, the properties of the Ag electrode, the MgO layer and the gases depended sensitively on the panel temperature, but those of the ITO and the dielectric layer rarely did. These changes in materials strongly affected the electric field between the discharge gap, the collision probabilities between particles, and the effective applied voltage. From these results, we can conclude that materials with sensitive temperature dependencies play an important role in affecting the panel properties. However, the changes in the macroscopic properties in the AC-PDPs were complexly intertwined with the change in the materials properties, so it was difficult to distinguish the individual effect of each material in the AC-PDP.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0006092).
