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Fig. 2 shows the time dependencies for the charging currents IC (a), surface potentials VS (b) and the light intensities ∆Ilight (c) for a single ePTFE film at grid voltages ranging from −500 to −1500 V. As soon as the applied electric field exceeds a threshold value of -700 V the film starts to emit light. The light emission is accompanied by a charging current IC and a buildup of surface potential VS. After 5 s the current IC, the surface potential VS and the light emission ∆Ilight saturate to non-zero values. The constant current can be explained by an equivalent circuit model consisting of the sample capacitor and a leakage resistor in parallel. The leakage current (saturated current IC) is caused by charge transport initiated by ongoing local breakdowns. Hence, the charge carriers generated in the material voids must permanently vanish during the poling process, most likely due to ejection at the metal electrode and neutralization of corona induced charges at the ePTFE top surface. As a consequence, the surface charge potential does not change for grid voltages VG < −700 V [4]. I C , [µA] V S , [ V ] ∆I light , [a.u.] 0,00 -0,05 -0,10 -0,15 -0,20 -800 -600 -400 -200 0 2,0 1,5 1,0 0,5 0,0 a) 0 5 10 15 20 b) -700... -1500V 0 5 10 15 20 c) 0 5 10 15 20 t 1 -500V -700V -800V -900V -1000V -1250V -1500V -500V -1500V -1250V -1000V -900V -800V -700V -500V time, [sec] Figure 2. Time dependencies of charging currents (a), surface potentials (b) and light intensities (c) during negative poling of an individual ePTFE film at different grid voltages as indicated. I C , [µΑ] V S , [ V ] ∆I light , [a.u.] 0,00 -0,05 -0,10 -0,15 -2000 -1000 0 3 2 1 0 a) 0 25 50 75 b) 0 25 50 75 t 1 c) 0 25 50 75 time, [sec] Figure 3. Time dependencies of charging currents (a), surface potentials (b) and light intensities (c) during negative poling of three-layer sandwich at grid voltage of -2500 V. In the contrary to the results obtained from single ePFTE films, the light intensity ∆Ilight and the charging current IC detected during the poling of the sandwich structure exhibit initially maxima and decay with time to zero. This is depicted in Fig. 3. Time t1 indicates the onset of light emission. Concomitantly, the saturated surface potential coincides for all applied grid voltage with VG. Due to the blocking character of the FEP films, on can anticipate that the positive and negative charges generated during breakdown are trapped at opposite - 68 -
ePFTE/FEP film interfaces and cannot leave the ePFTE film. The buildup of oppositely charged surfaces results in a polarization field compensating for the electric field induced by the corona charging. Once the internal electric field falls below the threshold field for electrical breakdown, further electrical breakdown is inhibited and the light intensity as well as the charging current diminished to zero. The macroscopic dipole formation accounts also for the drastic difference in piezoactivity between the single porous films and sandwiched structures. Opposite to the low piezoactivity of the single ePTFE film (20 pC/N), the three-layer sandwich reveals a large quasi-static piezoelectric d33 coefficient up to 800 pC/N [5]. In summary, it has been shown that the monitoring of light emission during poling of porous electrets give an important additional information on the charging mechanism. The emitted light itself is a direct proof of breakdown in the porous material, which was indeed observed for the single as well as the sandwiched porous structures. The good agreement of the poling current and the light intensity progression in time disclose basic features of the charge separation process in ePTFE layers. The authors acknowledge the financial support of Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF) “Otto von Guericke” e.V. (ref. Nr.: 09518/03) References [1] G. M. Sessler and J. Hillenbrand, Appl. Phys. Lett. 75 (1999) 3405. [2] J. Peltonen, M. Paajanen, and J. Lekkala, J. Appl. Phys. 88 (2000) 4787. [3] R. Gerhard-Multhaupt, IEEE Trans. Dielectr. Electr. Insul. 9 (2002) 850. [4] Z. Hu and H. von Seggern, J. Appl. Phys. 98 (2005) 014108. [5] Z. Hu and H. von Seggern, J. Appl. Phys. 99 (2006) 024102. - 69 -
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Staff Members Head Prof. Dr. Jürge
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Engineering Geology Engineering Geo
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Investigation of a Doline in NE-Hes
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Applied Sedimentology Sedimentary r
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econstruction.(Diploma thesis in co
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Darmstadt University of Technology
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Geomaterials Science Geomaterial Sc
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Gregori, G.; Kleebe H.-J.; Sorarù,
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Fig. 1: Aerial photo of the Lake Li
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Lithological Map of the Japanese Ar
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tower stripping, and therefore much
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Diffusion and reaction in micro- an
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Rift dynamics, uplift and climate c
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Geology, land use change and erosio
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nearly half a century has been perf
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The Effect of LiF Addition on the S
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In case of the undoped sintered B6O
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Microstructures in Omphacite and Ot
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Modelling phase-assemblage diagrams
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As an example, Fig. 2 shows the che
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MATERIALS SCIENCE Physical Metallur
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