@article{RychkovAltafimQiuetal.2012, author = {Rychkov, Dmitry and Altafim, Ruy Alberto Pisani and Qiu, Xunlin and Gerhard, Reimund}, title = {Treatment with orthophosphoric acid enhances the thermal stability of the piezoelectricity in low-density polyethylene ferroelectrets}, series = {Journal of applied physics}, volume = {111}, journal = {Journal of applied physics}, number = {12}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/1.4729866}, pages = {5}, year = {2012}, abstract = {Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d(33) coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 degrees C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelectrets is governed by the relaxation of less stable positive charges. The treatment also leads to noticeable changes in the chemical composition of the LDPE surface. Infrared spectroscopy reveals absorption bands attributed to phosphorus-containing structures, while scanning electron microscopy shows new island-like structures, 50-200 nm in diameter, on the modified surface.}, language = {en} } @article{AltafimRychkovWirgesetal.2012, author = {Altafim, Ruy Alberto Pisani and Rychkov, Dmitry and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Melzer, Martin}, title = {Laminated tubular-channel ferroelectret systems from low-density Polyethylene Films and from Fluoroethylene-propylene Copolymer Films - A comparison}, 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 = {1116 -- 1123}, year = {2012}, abstract = {A template-based lamination technique for the manufacture of ferroelectrets from uniform electret films was recently reported. In the present work, this technique is used to prepare similar ferroelectret structures from low-density polyethylene (LDPE) films and from fluoro-ethylene-propylene (FEP) copolymer films. A comparative analysis of the pressure-, temperature-, and frequency-dependent piezoelectric properties has been performed on the two ferroelectret systems. It is observed that the FEP ferroelectrets exhibit better piezoelectric responses and are thermally more stable. The difference between the piezoelectric d(33) coefficients of the two ferroelectret systems is partially explained here by their different elastic moduli. The anti-resonance peaks of both structures have been investigated by means of dielectric resonance spectroscopy and electroacoustic sound-pressure measurements. A difference of more than 10 kHz is observed between the anti-resonance frequencies of the two ferroelectret systems.}, 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} } @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{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} }