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Relative density 1.00 0.95 0.90 0.85 0.80 0.75 0.70 1300°C 1150°C Film exp. Film model Bulk 0.65 0 2000 4000 6000 8000 10000 Isothermal time [s] Fig 1. Experiment and modelled densification curves for 2 sintering temperatures The effect of film thickness was also investigated. Thinner layers seem to be more constrained, as they densify more slowly than thicker ones. Particle rearrangement may be hindered close to the substrate, which leads to a lower local density than elsewhere in the film (Fig 2). Fig. 2 Polished cross-section of a sintered film Finally, an extension of the sintering model to anisotropic bodies has been proposed. In the case of transverse isotropy, 2 free sintering strain rates, 2 uniaxial viscosity as well as 3 viscous Poisson’s ratios are introduced. In specific cases, such as constrained sintering, only 2 Poisson’s ratios are required. The experimental determination of these parameters is still challenging, as anisotropic specimens have to be obtained before being loaded in different directions and their strains measured. - 66 -
Breakdown-induced light emission and poling dynamics of porous fluoropolymers S. Zhukov and H. von Seggern Recently, a new class of plastics with a very strong piezoelectricity has been discussed in literature: foams. It has been demonstrated that inner pores, when properly charged in an external electric field, are responsible for a high piezoelectric response of cellular polypropylene (PP) [1-3] and expanded polytetrafluoroethylene (ePTFE) [3-5] films. Surprisingly, a strong difference in the obtainable polarization and the related piezoelectric d33 coefficients has been recognized between samples with closed-pore (PP) and openpore (ePTFE) geometry. Most research groups report quasi-static or low-frequency dynamic coefficients up to 200 pC/N on standard grades of porous PP films, whereas the open-porous ePTFE films reveal rather small values of 10 to 20 pC/N. Up to date the charging mechanism of open-porous electrets materials is not completely <strong>und</strong>erstood. To clarify this issue, we investigated the emission of light during the corona poling of single ePTFE films and the same film sandwiched between two solid FEP films. Generally, the charging mechanism is supposed to arise from electrical breakdown within the voids during the poling process [4,5]. The separation of charges due to the electrical breakdown leads to the aspired polarization buildup. However, each breakdown event is accompanied by light emission. Thus the emission of light can be considered as a direct proof of the occurrence of breakdown events and indicates an ongoing poling process within the sample. Experiments were carried 12.5 µm FEP ePTFE 5µm 12.5 µm FEP out on porous ePTFE films from GoodFellow Cambridge Limited, England. The samples are 63 µm thick with a nominal porosity of 91% and a pore size of 1 µm. In order to handle the films they are mounted in a circular aluminum holder with an inner diameter of 42 mm. A gold electrode was deposited onto the bottom side of the film by thermal evaporation. The utilized sandwiches are fabricated from the same ePTFE film placed between an unmetallized FEP film (top) and a one-side-metallized FEP film (bottom) as displayed in Fig. 1. The employed FEP films have a thickness of 12.5 µm and are purchased from Sheldahl Company, USA. 63µm Figure 1. Artist view of the threelayer sandwich sample. Negative charging is conducted in air with a corona triode consisting of a point-to-plane corona discharge and a grid between needle and sample. The charging experiments were carried out with the unmetallized surface of the samples facing the corona needle. The grid can be vibrated sinusoidally to measure the surface potential VS of the sample by means of the Kelvin technique. The dc needle voltage (VC) was kept at VC = −8.0 kV, whereas dc grid voltage (VG) is varied between 0 and −2500 V to achieve different surface potentials. In order to measure the light emission during sample poling a photomultiplier (Hamamatsu R6094) was incorporated into the charging chamber. - 67 -
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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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Institute of Materials Science Phys
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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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Page 24 and 25: Publications Fleissner, A.; Schmid,
Page 26 and 27: • Surface analysis The UHV-surfac
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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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Hessische Industriemüll GmbH). Due
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Technical Personnel Josef Kolb, Dip
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Research Projects Environmental sca
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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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