@article{RamanVenkatesanGulyakovaFruebingetal.2018, author = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Fr{\"u}bing, Peter and Gerhard, Reimund}, title = {Relaxation processes and structural transitions in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor-ferroelectric terpolymers as seen in dielectric spectroscopy}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {25}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {6}, publisher = {Institut of Electrical and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2018.007440}, pages = {2229 -- 2235}, year = {2018}, abstract = {Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation.}, language = {en} } @article{QiuGrothWirgesetal.2018, author = {Qiu, Xunlin and Groth, Frederick and Wirges, Werner and Gerhard, Reimund}, title = {Cellular polypropylene foam films as DC voltage insulation and as piezoelectrets}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {25}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {3}, publisher = {Institut of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2018.007192}, pages = {829 -- 834}, year = {2018}, abstract = {Polymer foams are in industrial use for several decades. More recently, non-polar polymer foams were found to be piezoelectric (so-called piezoelectrets) after internal electrical charging of the cavities. So far, few studies have been carried out on the electrical-insulation properties of polymer foams. Here, we compare the piezoelectric and the DC-voltage electrical-insulation properties of cellular polypropylene (PP) foams. Their cavity microstructure can be adjusted via inflation in high-pressure nitrogen gas in combination with a subsequent thermal treatment. While inflation is effective for improving the piezoelectricity, it is detrimental for the electrical-insulation properties. The original cellular PP foam shows a breakdown strength of approximately 230 MV/m, within the same range as that of solid PP. The breakdown strength decreases with increasing degree of inflation, and the dependence on the foam thickness follows an inverse power law with an exponent of 1.2. Nevertheless, up to a thickness of 140 mu m (3.5 times the original thickness), the breakdown strength of cellular-foam PP films is at least 7 times that of an air gap with the same thickness. In addition, the influence of high temperatures and high humidities on the piezoelectricity and the breakdown strength of cellular PP was studied. It was found that the piezoelectric d(33) coefficient decays rapidly already at 70 degrees C, while the breakdown strength slightly increases during storage at 70 or 90 degrees C. Under a relative humidity of 95\%, the breakdown strength increases with storage time, while the piezoelectric d(33) coefficient slightly decreases.}, language = {en} } @article{RychkovGerhardKuznetsovetal.2018, author = {Rychkov, Dmitry and Gerhard, Reimund and Kuznetsov, Alexey and Rychkov, Andrey}, title = {Influence of charge density on the trap energy spectrum in fluoroethylenepropylene copolymer films with chemically modified surfaces}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {25}, 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.2018.007437}, pages = {840 -- 844}, year = {2018}, abstract = {Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor by means of molecular-layer deposition. The treatment leads to considerable improvements of the electret-charge stability on positively charged films. A slight improvement is also observed for negatively charged films. In line with our previous findings, we attribute the improvement in electret properties to the formation of deeper traps on the chemically modified polymer surfaces. Here, we investigate the influence of the charge density on electret stability of FEP films with modified surfaces. Trap-energy spectra obtained from thermally-stimulated-discharge measurements indicate that the charge stability on modified FEP films depends on how the surface traps are populated and on the availability of additional deeper traps.}, language = {en} } @misc{CheilakouTsopelasAnastasopoulosetal.2018, author = {Cheilakou, E. and Tsopelas, N. and Anastasopoulos, A. and Kourousis, D. and Rychkov, Dmitry and Gerhard, Reimund and Frankenstein, B. and Amditis, A. and Damigos, Y. and Bouklas, C.}, title = {Strain monitoring system for steel and concrete structures}, series = {Procedia Structural Integrity}, volume = {10}, journal = {Procedia Structural Integrity}, publisher = {Elsevier}, address = {Amsterdam}, issn = {2452-3216}, doi = {10.1016/j.prostr.2018.09.005}, pages = {25 -- 32}, year = {2018}, abstract = {The present work is part of a collaborative H2020 European funded research project called SENSKIN, that aims to improve Structural Health Monitoring (SHM) for transport infrastructure through the development of an innovative monitoring and management system for bridges based on a novel, inexpensive, skin-like sensor. The integrated SENSKIN technology will be implemented in the case of steel and concrete bridges, and tested, field-evaluated and benchmarked on actual bridge environment against a conventional health monitoring solution developed by Mistras Group Hellas. The main objective of the present work is to implement the autonomous, fully functional strain monitoring system based on commercially available off-the-shelf components, that will be used to accomplish direct comparison between the performance of the innovative SENSKIN sensors and the conventional strain sensors commonly used for structural monitoring of bridges. For this purpose, the mini Structural Monitoring System (mini SMS) of Physical Acoustics Corporation, a comprehensive data acquisition unit designed specifically for long-term unattended operation in outdoor environments, was selected. For the completion of the conventional system, appropriate foil-type strain sensors were selected, driven by special conditioners manufactured by Mistras Group. A comprehensive description of the strain monitoring system and its peripheral components is provided in this paper. For the evaluation of the integrated system's performance and the effect of various parameters on the long-term behavior of sensors, several test steel pieces instrumented with different strain sensors configurations were prepared and tested in both laboratory and field ambient conditions. Furthermore, loading tests were performed aiming to validate the response of the system in monitoring the strains developed in steel beam elements subject to bending regimes. Representative results obtained from the above experimental tests have been included in this paper as well.}, language = {en} } @misc{RamanVenkatesanFruebingGerhard2018, author = {Raman Venkatesan, Thulasinath and Fr{\"u}bing, Peter and Gerhard, Reimund}, title = {Influence of Composition and Preparation on Crystalline Phases and Morphology in Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Relaxor-Ferroelectric Terpolymer}, series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, journal = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, publisher = {IEEE}, address = {New York}, isbn = {978-1-5386-6389-9}, doi = {10.1109/ICD.2018.8514758}, pages = {4}, year = {2018}, abstract = {The influence of chemical composition and crystallisation conditions on the ferroelectric and paraelectric phases and the resulting morphology in Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with 55.4/37.2/7.3 mol\% or with 62.2/29.4/8.4 mol\% of VDF/TrFE/CFE was studied. Poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) with 75/25 mol\% VDF/TrFE was employed as reference material. Fourier-Transform Infrared Spectroscopy (FTIR) was used to determine the fractions of the relevant terpolymer phases, and X-Ray Diffraction (XRD) was employed to assess the crystalline morphology. The FTIR results show an increase of the fraction of paraelectric phases after annealing. On the other hand, XRD results indicate a more stable paraelectric phase in the terpolymer with higher CFE content.}, language = {en} } @misc{WangRychkovGerhard2018, author = {Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund}, title = {Influence of Charge Density on Charge Decay in Chemically Modified Polypropylene Films}, series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, journal = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, publisher = {IEEE}, address = {New York}, isbn = {978-1-5386-6389-9}, doi = {10.1109/ICD.2018.8514718}, pages = {4}, year = {2018}, abstract = {Previous work has shown that surface modification with orthophosphoric acid can significantly enhance the charge stability on polypropylene (PP) surface by generating deeper traps. In the present study, thermally stimulated potential-decay measurements revealed that the chemical treatment may also significantly increase the number of available trapping sites on the surface. Thus, as a consequence, the so-called "cross-over" phenomenon, which is observed on as-received and thermally treated PP electrets, may be overcome in a certain range of initial charge densities. Furthermore, the discharge behavior of chemically modified samples indicates that charges can be injected from the treated surface into the bulk, and/or charges of opposite polarity can be pulled from the rear electrode into the bulk at elevated temperatures and at the high electric fields that are caused by the deposited charges. In the bulk, a lack of deep traps causes rapid charge decay already in the temperature range around 95 degrees C.}, language = {en} } @misc{NguyenGerhard2018, author = {Nguyen, Quyet Doan and Gerhard, Reimund}, title = {LDPE/MgO Nanocomposite Dielectrics for Electrical-Insulation and Ferroelectret-Transducer Applications}, series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, journal = {2018 IEEE 2nd International Conference on Dielectrics (ICD)}, publisher = {IEEE}, address = {New York}, isbn = {978-1-5386-6389-9}, pages = {4}, year = {2018}, abstract = {Published results on LDPE/MgO nanocomposites (3wt\%) show that they promise to be good electrical-insulation materials. In this work, the nanocomposites are examined as a potential (ferro-)electret material as well. Isothermal surface-potential decay measurements show that charged LDPE/MgO films still exhibit significant surface potentials after heating for 4 hours at 80 degrees C, which suggests good capabilities of LDPE/MgO nanocomposites to hold electric charges of both polarities. Open-tubular-channel ferroelectrets prepared from LDPE/MgO nanocomposite films show significant piezoelectricity with d(33) coefficients of about 20 pC/N after charging and are stable up to temperatures of at least 80 degrees C. Thus LDPE/MgO nanocomposites may become available as a new ferroelectret material. To increase their d(33) coefficients, it is desirable to optimize the charging conditions and the ferroelectret structure.}, language = {en} } @article{GidionGerhard2018, author = {Gidion, Gunnar and Gerhard, Reimund}, title = {The Bow on a String}, series = {Acta Acustica united with Acustica}, volume = {104}, journal = {Acta Acustica united with Acustica}, number = {2}, publisher = {Hirzel Verlag}, address = {Stuttgart}, issn = {1610-1928}, doi = {10.3813/AAA.919174}, pages = {315 -- 322}, year = {2018}, abstract = {The interaction between a bowed string instrument and its player is conveyed by the bow, the vibrational properties of which can be measured either separately on the bow or during the bowing procedure. Here, two piezoelectric film sensors, made of a ferroelectret material, are installed on a violin bow, one sensor at the tip and one at the frog. With these sensors, a violin is played under normal conditions, and the signals are analysed. The features in the resulting spectrograms are identified as string harmonics and longitudinal bow-hair resonances. The bow-hair sections on both sides of the bow-string contact exhibit separate resonances which are observed as absorption dips in the spectra. Owing to the sensor positions at the bow-hair terminations, it can be inferred that the two bow-hair sections act as mutual vibration absorbers. From a regression of the observed resonances, the longitudinal bow-hair velocity can be obtained. With additional film sensors under the violin bridge, body vibrations were also detected providing further details of the coupling mechanisms.}, language = {en} }