@article{AssagraAltafimdoCarmoetal.2020, author = {Assagra, Yuri A.O. and Altafim, Ruy Alberto Pisani and do Carmo, Joao P. and Altafim, Ruy A.C. and Rychkov, Dmitry and Wirges, Werner and Gerhard, Reimund}, title = {A new route to piezo-polymer transducers: 3D printing of polypropylene ferroelectrets}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {27}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {5}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2020.008461}, pages = {1668 -- 1674}, year = {2020}, abstract = {Here, a promising approach for producing piezo-polymer transducers in a one-step process is presented. Using 3D-printing technology and polypropylene (PP) filaments, we are able to print a two-layered film structure with regular cavities of precisely controlled size and shape. It is found that the 3D-printed samples exhibit piezoelectric coefficients up to 200 pC/N, similar to those of other PP ferroelectrets, and their temporal and thermal behavior is in good agreement with those known of PP ferroelectrets. The piezoelectric response strongly decreases for applied pressures above 20 kPa, as the pressure in the air-filled cavities strongly influences the overall elastic modulus of ferroelectrets.}, language = {en} } @article{RychkovYablokovRychkov2012, author = {Rychkov, Dmitry and Yablokov, M. and Rychkov, A.}, title = {Chemical and physical surface modification of PTFE films-an approach to produce stable electrets}, series = {Applied physics : A, Materials science \& processing}, volume = {107}, journal = {Applied physics : A, Materials science \& processing}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-012-6834-5}, pages = {589 -- 596}, year = {2012}, abstract = {The thermal stability of positive charge has been investigated in chemically and physically treated polytetrafluoroethylene (PTFE) films. It has been found that virgin films, oriented by the manufacturer, display an increase in thermal stability of positive charge with an increase of the initial value of surface potential. Such an anomalous behavior is explained by the influence of a negative tribocharge, trapped some small distance below the surface. In PTFE samples treated with orthophosphoric acid and with tetraethoxysilane, a considerable improvement of positive charge stability has been achieved, but no influence of the initial value of surface potential has been observed. However, this influence should be kept in mind when comparing charge stability in virgin and modified samples. In nonoriented PTFE films, no influence of the initial value of surface potential on charge stability has been observed. This could be due to the fact that these films did not possess a noticeable negative tribocharge. After the treatment in glow-discharge defluorination, oxidation and appearance of polar groups have been detected on the surface. These changes in chemical composition of a PTFE surface resulted in a noticeable improvement in thermal stability of positively charged electrets. This improvement is attributed to the formation of deeper traps on the modified surface.}, language = {en} } @article{WangRychkovGerhard2017, author = {Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund}, title = {Chemical modification with orthophosphoric acid enhances surface-charge stability on polypropylene electrets}, series = {Applied physics letters}, volume = {110}, journal = {Applied physics letters}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.4983348}, pages = {5}, year = {2017}, abstract = {The low surface-charge stability of polypropylene (PP) frequently limits its application as an electret material. In this paper, we demonstrate how the treatment of PP-film surfaces with orthophosphoric acid (H3PO4) enhances their charge stability. To discriminate between the effects of chemical modification and thermal treatment, as-received and annealed PP films are used as reference samples. The electret properties of treated and non-treated PP films are characterized with thermally stimulated discharge (TSD) and isothermal surface-potential decay (ISPD) experiments, from which considerable improvement in thermal and temporal charge stability is observed for samples modified with H3PO4. The half-value temperature (T-1/2) observed on TSD curves of chemically treated PP increases to 131 and 145 degrees C for positive and negative charges, respectively. The enhancement might be attributed to the phosphoric compounds detected on the H3PO4-modified surfaces via attenuated-total-reflection infrared spectroscopy. Deeper surface traps formed at the "foreign" phosphorus-containing structures are able to capture the charges over longer time periods and at higher temperatures, thus leading to significant improvements in the temporal and thermal surface-charge stabilities of PP electrets. Published by AIP Publishing.}, language = {en} } @misc{NguyenWangRychkovetal.2019, author = {Nguyen, Quyet Doan and Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund}, title = {Depth Profile and Transport of Positive and Negative Charge in Surface (2-D) and Bulk (3-D) Nanocomposite Films}, series = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)}, journal = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)}, publisher = {IEEE}, address = {New York}, isbn = {978-1-5386-8434-4}, doi = {10.1109/ICEMPE.2019.8727256}, pages = {298 -- 300}, year = {2019}, abstract = {In the present study, the charge distribution and the charge transport across the thickness of 2- and 3-dimensional polymer nanodielectrics was investigated. Chemically surface-treated polypropylene (PP) films and low-density polyethylene nanocomposite films with 3 wt \% of magnesium oxide (LDPE/MgO) served as examples of 2-D and 3-D nanodielectrics, respectively. Surface charges were deposited onto the non-metallized surfaces of the one-side metallized polymer films and found to broaden and to thus enter the bulk of the films upon thermal stimulation at suitable elevated temperatures. The resulting space-charge profiles in the thickness direction were probed by means of Piezoelectrically-generated Pressure Steps (PPSs). It was observed that the chemical surface treatment of PP which led to the formation of nano-structures or the use of bulk nanoparticles from LDPE/MgO nanocomposites enhance charge trapping on or in the respective polymer films and also reduce charge transport inside the respective samples.}, language = {en} } @article{RychkovKuznetsovRychkov2011, author = {Rychkov, Dmitry and Kuznetsov, Alexey and Rychkov, Andrey}, title = {Electret properties of polyethylene and polytetrafluoroethylene films with chemically modified surface}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {18}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {1}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2011.5704487}, pages = {8 -- 14}, year = {2011}, abstract = {This paper investigates the effect of chemical surface modification of polytetrafluoroethylene (PTFE) and low density polyethylene (LDPE) films on their electret properties. PTFE films were subjected to wet treatment with three different chemicals: orthophosphoric acid, tetrabutyl titanate and tetraethoxysilane. The technique based on the principles of molecular layer deposition (MLD) method was used to modify the surface of LDPE films with phosphorus trichloride vapors. The surfaces of the films were then corona charged, and the electret charge stability was studied by means of isothermal and thermally stimulated surface potential decay. Both PTFE and LDPE films, after the surface treatment, displayed a considerable enhancement in the charge stability compared to the virgin samples. It is important to note that the enhancement of the charge stability was achieved in the positively charged PTFE films, a result important to practical applications. We attribute this effect of charge stabilization to the formation of new energetically deep traps on the modified surface. Decrease in molecular mobility, due to attachment of new chemical structures to the surface macromolecules, may also contribute to the overall growth of the charge stability.}, language = {en} } @article{RychkovGerhardIvanovetal.2012, author = {Rychkov, Dmitry and Gerhard, Reimund and Ivanov, Vadim and Rychkov, Andrey}, title = {Enhanced electret charge stability on Polyethylene Films treated with Titanium-Tetrachloride vapor}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {19}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {4}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, pages = {1305 -- 1311}, year = {2012}, abstract = {Low-density polyethylene (LDPE) films have been treated with titanium-tetrachloride vapor by means of the molecular-layer-deposition method. It is shown that such a treatment leads to a considerable improvement of the electret properties for both positively and negatively charged films. The temperature stability of the electret homo-charge has been increased by approximately 60 degrees C. At the same time, the temporal stability of charge is also considerably improved. Modified low-density polyethylene films show no "cross-over phenomenon" when charged to higher voltages. Thus, it is now possible to produce electrets from polyethylene films with high initial charge densities, but without a strongly reduced charge stability. The influence of a chemical treatment with titanium-tetrachloride vapor on charge injection from aluminum electrodes into polyethylene films was also investigated. It is found that the interface between an aluminum electrode and a modified LDPE surface layer has different injection properties for positive and negative charges. Electrons can be injected across the modified interface, whereas injection of holes is either very limited or non-existent.}, language = {en} } @article{RychkovRychkovEfimovetal.2013, author = {Rychkov, Dmitry and Rychkov, A. and Efimov, N. and Malygin, A. and Gerhard, Reimund}, title = {Higher stabilities of positive and negative charge on tetrafluoroethylene-hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor}, series = {Applied physics : A, Materials science \& processing}, volume = {112}, journal = {Applied physics : A, Materials science \& processing}, number = {2}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-013-7821-1}, pages = {283 -- 287}, year = {2013}, abstract = {Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 A degrees C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 A degrees C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.}, language = {en} } @article{McCarthyStoyanovRychkovetal.2012, author = {McCarthy, Denis N. and Stoyanov, Hristiyan and Rychkov, Dmitry and Ragusch, Huelya and Melzer, Michael and Kofod, Guggi}, title = {Increased permittivity nanocomposite dielectrics by controlled interfacial interactions}, series = {Composites science and technology}, volume = {72}, journal = {Composites science and technology}, number = {6}, publisher = {Elsevier}, address = {Oxford}, issn = {0266-3538}, doi = {10.1016/j.compscitech.2012.01.026}, pages = {731 -- 736}, year = {2012}, abstract = {The use of nanoparticles in polymer composite dielectrics has promised great improvements, but useful results have been elusive. Here, the importance of the interfacial interactions between the nanoparticles and the polymer matrix are investigated in TiO2 nanocomposites for dielectric materials using surface functionalisation. The interface is observed to dominate the nanocomposite properties and leads to a threefold increase in permittivity at volume fractions as low as 10\%. Surface functionalisation of the filler nanoparticles with silanes allows control of this interface, avoiding significant degradation of the other important material properties, particularly electrical breakdown strength, and resulting in a material that is demonstrated successfully as an active material in a dielectric elastomer actuator application with increased work output compared to the pure polymer. Although further permittivity increases are observed when the interface regions have formed a percolation network, the other material properties deteriorate. The observation of percolation behaviour allows the interface thickness to be estimated.}, 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} } @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} }