Dynamics of polarization growth and reversal in PVDF films

Measurements are presented of the time development of the dielectric displacement and the remanent polarization in polyvinylidene fluoride (PVDF) for poling times ranging from 1 μs to 1000 s and poling fields between 0.8 and 2.0 MV/cm. For longer times (0.1 to 1000 s), the time dependence of the polarization distribution across the film thickness is also determined. After application of a steep rectangular high-voltage pulse, the sample is shorted to zero voltage. The remanent polarization under the short-circuit conditions is compared to the maximum dielectric displacement under the external poling field. A significant time delay of the buildup of the remanent polarization was observed as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal, a flipping back' of the polarization was observed for shorter poling times of up to 200 μs. Under these conditions, a large part of the polarization is reversed under the field, but after the removal of the field, most of the polarization returns to the original direction. The results can be explained by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites

Dynamics of polarization growth and polarization reversal in PVDF films

The authors present measurements of the maximum displacement at the end of a rectangular high-voltage pulse together with the remanent polarization after the pulse under short-circuit condition as a function of the polarizing pulse end length. The pulse length varied from 10 μs to 100 s, and the field strength ranged from 0.8 MV/cm to 2 MV/cm. The original samples were reversely polarized or unpolarized 12-μm-thick films containing 37% β-material. It is concluded that the cooperative model combined with six-site potential alone cannot explain the observed switching back of the remanent polarization, its delayed development, and the long times necessary for polarization reversal. The results are qualitatively consistent with a model of charge injection and charge trapping at polarized crystallites

Synthesis of ordered mesoporous Fe3O4 and gamma-Fe2O3 with crystalline walls using post-template reduction/oxidation

Ordered mesoporous Fe3O4 with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous alpha-Fe2O3 (corundum structure) to Fe3O4 spinel then to gamma-Fe2O3 by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of alpha-Fe2O3 (corundum structure) to the cubic close packed subarray of Fe3O4 spinel and gamma-Fe2O3. Preliminary magnetic measurements reveal that the spins in both Fe3O4 and gamma-Fe2O3 are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (> 8 nm)

Mössbauer spectroscopy and magnetic measurements as complementary techniques for the phase analysis of FeP electrodes cycling in Li-ion batteries

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57Fe Mössbauer study of the electrochemical reaction of Li with FePy (y=1,2)

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Elucidation of capacity fading on CoFe2O4 conversion electrodes for lithium batteries based on 57Fe Môssbauer spectroscopy

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Thermal low spin-high spin equilibrium of Fe(II) in thiospinels CuFe0.5(Sn(1-x)Tix)1.5S4 (0 ≤ x ≤ 1)

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The study of electron transfer on advanced materials (electrodes for Li-ion batteries or catalysts)

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57Fe Mössbauer spectroscopy study of the electrochemical reaction with lithium of MFe204 (M=Co and Cu) electrodes

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X-ray absorption spectra of spinel LiCu0.5Mn1.5O4

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