@article{StubningDenesGerhard2021, author = {Stubning, Tobias and Denes, Istvan and Gerhard, Reimund}, title = {Tuning electro-mechanical properties of EAP-based haptic actuators by adjusting layer thickness and number of stacked layers}, series = {Engineering research express}, volume = {3}, journal = {Engineering research express}, number = {1}, publisher = {Institute of Physics}, address = {London}, issn = {2631-8695}, doi = {10.1088/2631-8695/abd286}, pages = {13}, year = {2021}, abstract = {In our fast-changing world, human-machine interfaces (HMIs) are of ever-increasing importance. Among the most ubiquitous examples are touchscreens that most people are familiar with from their smartphones. The quality of such an HMI can be improved by adding haptic feedback-an imitation of using mechanical buttons-to the touchscreen. Thin-film actuators on the basis of electro-mechanically active polymers (EAPs), with the electroactive material sandwiched between two compliant electrodes, offer a promising technology for haptic surfaces. In thin-film technology, the thickness and the number of stacked layers of the electroactive dielectric are key parameters for tuning a system. Therefore, we have experimentally investigated the influence of the thickness of a single EAP layer on the electrical and the electro-mechanical performance of the transducer. In order to achieve high electro-mechanical actuator outputs, we have employed relaxor-ferroelectric ter-fluoropolymers that can be screen-printed. By means of a model-based approach, we have also directly compared single- and multi-layer actuators, thus providing guidelines for optimized transducer configurations with respect to the system requirements of haptic applications for which the operation frequency is of particular importance.}, language = {en} } @article{GerhardKacprzyk2022, author = {Gerhard, Reimund and Kacprzyk, Ryszard}, title = {Paul B{\"o}ning - early electret researcher in Shanghai and Wroclaw (1922-1945)}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {29}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {3}, publisher = {Institute of Electrical and Electronics Engineers}, address = {New York, NY}, issn = {1070-9878}, doi = {10.1109/TDEI.2022.3168372}, pages = {853 -- 858}, year = {2022}, abstract = {The scientific career and the research activities of Paul Boening, especially during his tenures at Tongji University in Shanghai (Woosung Campus, 1922-1936) and the Technical University of Wroclaw (TH Breslau, 1936-1945), are briefly reviewed. In particular, Boening's pioneering investigations in the area of electrets and space charge in dielectrics are emphasized. We attempt to shed some light on the significant achievements of a virtually unknown contributor to the early history of electrets and of space-charge research and high-voltage engineering, during the 1920s and 1930s. It should be noted that dielectrics research was a truly international endeavor already at that time.}, language = {en} } @article{RamanVenkatesanSmykallaPlossetal.2022, author = {Raman Venkatesan, Thulasinath and Smykalla, David and Ploss, Bernd and W{\"u}bbenhorst, Michael and Gerhard, Reimund}, title = {Tuning the relaxor-ferroelectric properties of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Terpolymer films by means of thermally induced micro- and nanostructures}, series = {Macromolecules : a publication of the American Chemical Society}, volume = {55}, journal = {Macromolecules : a publication of the American Chemical Society}, number = {13}, publisher = {American Chemical Society}, address = {Washington}, issn = {0024-9297}, doi = {10.1021/acs.macromol.2c00302}, pages = {5621 -- 5635}, year = {2022}, abstract = {The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures.}, language = {en} } @article{RamanVenkatesanWuebbenhorstGerhard2022, author = {Raman Venkatesan, Thulasinath and W{\"u}bbenhorst, Michael and Gerhard, Reimund}, title = {Structure-property relationships in three-phase relaxor-ferroelectric terpolymers}, series = {Ferroelectrics}, volume = {586}, journal = {Ferroelectrics}, number = {1}, publisher = {Routledge, Taylor \& Francis Group}, address = {Abingdon}, issn = {0015-0193}, doi = {10.1080/00150193.2021.2014260}, pages = {60 -- 81}, year = {2022}, abstract = {Poly(vinylidenefluoride-trifluoroethylene)-based (P(VDF-TrFE)-based) terpolymers represent a new class of electroactive polymer materials that are relaxor-ferroelectric (RF) polymers and that offer unique and attractive property combinations in comparison with conventional ferroelectric polymers. The RF state is achieved by introducing a fluorine-containing termonomer as a "defect" into the ferroelectric P(VDF-TrFE) copolymer, which reduces the interaction between the VDF/TrFE dipoles. The resulting terpolymer exhibits a low Curie transition temperature and small remanent and coercive fields yielding a slim hysteresis loop that is typical for RF materials. Though the macroscopic behavior is similar to RF ceramics, the mechanisms of relaxor ferroelectricity in semi-crystalline polymers are different and not fully understood yet. Structure-property relationships play an important role in RF terpolymers, as they govern the final RF properties. Hence, a review of important characteristics, previous studies and relevant developments of P(VDF-TrFE)-based terfluoropolymers with either chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) as the termonomer is deemed useful. The role of the termonomer and of its composition, as well as the effects of the processing conditions on the semi-crystalline structure which in turn affects the final RF properties are discussed in detail. In addition, the presence of noteworthy transition(s) in the mid-temperature range and the influence of preparation conditions on those transitions are reviewed. A better understanding of the fundamental aspects affecting the semi-crystalline structures will help to elucidate the nature of RF activity in VDF-based terpolymers and also help to further improve their applications-relevant electroactive properties.}, language = {en} } @article{HeWangHeetal.2022, author = {He, Yushuang and Wang, Feipeng and He, Li and Wang, Qiang and Li, Jian and Qian, Yihua and Gerhard, Reimund and Plath, Ronald}, title = {An insight Into the role of Nano-Alumina on DC Flashover-Resistance and surface charge variation of Epoxy Nanocomposites}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {29}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {3}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2022.3173510}, pages = {1022 -- 1029}, year = {2022}, abstract = {The addition of nano-Al2O3 has been shown to enhance the breakdown voltage of epoxy resin, but its flashover results appeared with disputation. This work concentrates on the surface charge variation and dc flashover performance of epoxy resin with nano-Al2O3 doping. The dispersion of nano-Al2O3 in epoxy is characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The dc flashover voltages of samples under either positive or negative polarity are measured with a finger-electrode system, and the surface charge variations before and after flashovers were identified from the surface potential mapping. The results evidence that nano-Al2O3 would lead to a 16.9\% voltage drop for the negative flashovers and a 6.8\% drop for positive cases. It is found that one-time flashover clears most of the accumulated surface charges, regardless of positive or negative. As a result, the ground electrode is neighbored by an equipotential zone enclosed with low-density heterocharges. The equipotential zone tends to be broadened after 20 flashovers. The nano-Al2O3 is noticed as beneficial to downsize the equipotential zone due to its capability on charge migration, which is reasonable to maintain flashover voltage at a high level after multiple flashovers. Hence, nano-Al2O3 plays a significant role in improving epoxy with high resistance to multiple flashovers.}, language = {en} } @article{WangZhangYanetal.2022, author = {Wang, Feipeng and Zhang, Zheng and Yan, Yuyang and Shen, Zijia and Wang, Qiang and Gerhard, Reimund}, title = {Surface reconstruction on electro-spun PVA/PVP nanofibers by water evaporation}, series = {Nanomaterials}, volume = {12}, journal = {Nanomaterials}, number = {5}, publisher = {MDPI}, address = {Basel}, issn = {2079-4991}, doi = {10.3390/nano12050797}, pages = {7}, year = {2022}, abstract = {Tailoring the secondary surface morphology of electro-spun nanofibers has been highly desired, as such delicate structures equip nanofibers with distinct functions. Here, we report a simple strategy to directly reconstruct the surface of polyvinyl alcohol/polyvinylpyrrolidone (PVA/PVP) nanofibers by water evaporation. The roughness and diameter of the nanofibers depend on the temperature during vacuum drying. Surface changes of the nanofibers from smooth to rough were observed at 55 degrees C, with a significant drop in nanofiber diameter. We attribute the formation of the secondary surface morphology to the intermolecular forces in the water vapor, including capillary and the compression forces, on the basis of the results from the Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. The strategy is universally effective for various electro-spun polymer nanofibers, thus opening up avenues toward more detailed and sophisticated structure design and implementation for nanofibers.}, language = {en} }