Ferroelectric Vinylidene Fluoride Research Articles

Temperature dependence of polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

AbstractIn this study, we report the temperature dependence of polarization fatigue in ferroelectric copolymer films. Studies on temperature effect show a non‐monotonic change of fatigue rate with increasing temperature. At temperature below ∼ 90°C, fatigue endurance is weakened at higher temperature; while, at temperature above ∼ 90°C, experimental data indicate that the higher the temperature, the lower the fatigue rate. X‐ray diffraction spectra reveal little changes of characteristic peak before and after polarization fatigue. On the basis of the hypothesis of charge‐injection‐induced fatigue, we propose a possible explanation of our experimental results. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy

In this paper, we reported the possibility to image non-conducting P(VDF-TrFE) copolymer films by STM. The films had the thickness of ∼25.0 nm and were spin-coated onto Au or graphite substrates. For films deposited on Au substrates, STM images showed grain structures of ∼100 nm, much larger than the grains of bare Au substrate. With increased scan rate, the film surface was damaged by STM tip and extreme protrusions and holes were observed. For films deposited on graphite substrates, we only obtained an image of very flat plane and could not observe the topography of the film surface. It seemed that the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. Asymmetrical current–voltage curves were observed from copolymer films deposited on HOPG. Experimental results were discussed.

Characterization of ferroelectric/metal interface under the repeated polarization switching

Polarization fatigue mechanism in organic ferroelectrics, structures at interface between ferroelectric vinylidene fluoride oligomer and Al electrode under the repeated polarization switching process were investigated by high-resolution X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Al 2O 3 layer at interface was formed with increasing the number of polarization reversal. The formation of oxide layer will be strongly related to polarization reversal, thus the repeated charge and discharge process by polarization reversal may promote the oxidation of Al electrode. Furthermore, the structural and orientation changes in ferroelectric molecular films by applying the electric field were observed. The formation of Al 2O 3 layer, as well as the structural changes in thin films, is affected to polarization fatigue process of ferroelectric organic devices.

Landau-Khalatnikov Switching Kinetics in the Ferroelectric Copolymer Nanostructures

In 1994 the first ultrathin Langmuir-Blodgett (LB) ferroelectric films were prepared from ferroelectric vinylidene fluoride and trifluoroethylene copolymer P(VDF-TrFE) [1]. It was shown that ferroelectricity exists even in two monolayers [2]. Later was shown, that these LB ferroelectric films do not reveal the critical size and ferroelectricity and polarization switching exist even in one monolayer with thickness 5 Å [3]. These films show typical phase transition of the first order with Curie temperature and phase transition temperature, which do not depend on the film thickness in the interval 1–30 ML [2, 3, 4, 5]. In contrast with usual well-known thick ferroelectric copolymer films, prepared by the spinning method, the Langmuir-Blodgett films do not contain amorphous phase and show perfect crystal structure of orthorhombic (or monoclinic) symmetry. Recently were obtained the experimental evidence of the existence of ferroelectricity in the perovskite films with thickness 1.2–5 nm [6, 7]. In this paper we show interesting peculiarities in switching kinetics of the ultrathin ferroelectric copolymer films.

Polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

In this letter polarization fatigue in ferroelectric polymers has been reported. Experimental results indicate that the driving voltages with lower frequency and higher amplitude should correspond to higher fatigue rates. A universal scaling behavior with N∕f, where N is the number of switching cycles and f is the frequency, is found in the frequency dependence of polarization fatigue. The profile of driving voltages is also found to affect the fatigue behavior. It is believed that the trapped charges, injected from electrodes into films, should make the major contribution to polarization fatigue.

A force modulation study on crystallization process in ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

This paper introduces the Force Modulation technique to the study of crystallization process in ferroelectric vinylidene fluoride and trifluoroethylene copolymer films. Using this technique we have successfully visualized ferroelectric crystalline domains and observed that these ferroelectric domains grow out from amorphous phase, unite into strip-like structures, and finally congregate into a union. Force Modulation can weaken the influence of topography on imaging of ferroelectric domains, and reveal more details, which are difficult to be observed in topographical image.

Surface structure of ultrathin copolymer films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) on graphite

The structure and local structural distortions, through the polarization manipulation, of crystalline films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) [P(VDF-TrFE)] copolymer on graphite were studied by scanning tunneling microscopy (STM). A quasispiral twist in $\mathrm{C}\mathrm{C}$ bonds with rotations about the polymer chain axis was observed by high-resolution STM, indicating a surface relaxation of the strained copolymer films. Such a relaxation behavior appears to be linked to the observed local dipole rotations accompanied by the reversal of the local polarization with biasing the STM tip. A structure model is proposed based upon the observations.

Determination of the optical dispersion in ferroelectric vinylidene fluoride (70%)/trifluoroethylene (30%) copolymer Langmuir–Blodgett films

We report measurements of the optical dispersion in ferroelectric Langmuir–Blodgett films of polyvinylidene fluoride (70%)-trifluoroethylene (30%) copolymer, using variable-angle spectroscopic ellipsometry over a wide spectral range from infrared to ultraviolet. Film thickness averaged 1.78±0.07 nm per deposition layer for films ranging from 5 to 125 deposition layers as determined from multi-sample analysis. This deposition rate was consistent with capacitance measurements, yielding a dielectric constant of 9.9±0.4 normal to the film, by quartz microbalance measurements, and by atomic force microscopy.

Structural characteristics of 0–3 ceramic/polymer composites

Calcium and lanthanum modified lead titanate (PCLT) powder sintered at 850°C has been prepared by using a sol-gel method and ferroelectric vinylidene fluoride trifluoroethylene [P(VDF-TRFE)] copolymer film has been prepared by the hot-press method. 0–3 PCLT/P(VDF-TrFE) composite films with fine PCLT powder dispersed uniformly in P(VDF-TrFE) matrix have been fabricated by using solvent-casting method. XRD and IR analysis have been used to determine and compare the structures of the single-phase materials with the composite. The XRD peaks and characteristic vibration bands of both single-phase ferroelectric PCLT and P(VDF-TrFE) can still be found which shows that the composite is composed of two ferroelectric phases.

Electrostrictive Phenomena Associated with Polarization Reversal in Ferroelectric Polymers

The electrostriction effect of ferroelectric vinylidene fluoride (VDF) and trifluoroethylene copolymers was studied by measuring their dimensional variation under an external electric field changing beyond a coercive electric field. The relationship between strain and polarization for the copolymer which includes a 54 mol% VDF reveals a parabolic electrostriction relationship over the entire measurement range of the strain-polarization relationship. The relationship for the copolymer which includes a 73 mol% VDF, however, deviates from the parabolic relationship. The result is discussed in terms of the thermodynamics of a ferroelectric phase transition. Furthermore, the origin of the ferroelectricity of these copolymers is discussed.

Structural and Electronic Characterization of Epitaxially-Grown Ferroelectric Vinylidene Fluoride Oligomer Thin Films

AbstractThe structural and electric properties of newly synthesized vinylidene fluoride (VDF) oligomer thin films evaporated on various substrate were investigated. The structural behavior of the VDF oligomer thin films strongly depended on the kinds of substrate and substrate temperature during film preparation. In particular, the VDF oligomers epitaxially grew on KBr(001), aligned their molecular chain along the <110> direction of substrate surface, and were similar to polar Form Itype crystals of poly(VDF). While, the thin films evaporated on NaCl(001), SiO2/Si were non-polar Form II-type or mixture of both Form I- and Form II-type crystals with their molecular chain normal to the surface. These facts indicated that crystal field of substrate, based on van der Waals and electrostatic interactions, strongly influenced to ferroelectric phase transition of the VDF oligomer. In addition, we demonstrate the formation of the local polarized domains in the epitaxial crystals by applying electric pulses from a conducting AFM probe used as a positionable topelectrode, and confirm their piezoelectricity of the VDF oligomers for the first time.

Optically sensitized ferroelectric polymers

The new photoferroelectric material is prepared by means of dye sensitization of ferroelectric vinylidene fluoride trifluoroethylene copolymers VDF/TrFE. The photoconductivity and bulk photovoltaic effects have been observed in the spectral bands connected with dye molecules absorption. At the ferroelectric phase transition the sharp changes of absorption and fluorescence spectra of sensitized films VDF/TrFE are revealed. These effects are denoted as structural optic effects. The influence of an external electric field on the fluorescence of VDF/TrFE doped by rhodamine 6G is revealed. This phenomena is explained by the influence of polarization on the radiationless electron transitions in the dye molecules.

Dielectric relaxations in ferroelectric copolymer vinylidene fluoride(65)/trifluoroethylene(35)

The molecular motion characteristics of ferroelectric copolymer vinylidene fluoride(65)/trifluoroethylene(35) have been investigated by means of dielectric relaxation. The frequency spectra of the copolymer consisting of low-frequency and high-frequency components are observed below room temperature. With the rise of crystallinity, the Curie point decreases while the dielectric constant increases in the region of ferroelectric-to-paraelectric phase transition. Based on the fitting results using one or two Cole-Cole functions, and calculating with the WLF equation and an Arrhenius formula, the glass transition temperature (= -28°C) and the activation energy of local relaxation (= 54.4 kJmol-1) of the copolymer are obtained. It is found that the distribution of relaxation time in the molten and the ferroelectric phases is located at a same line, which complies with the Arrhenius rule. This indicates that the elementary motions are of the same type for molecular motions provided by the high-temperature process and the low-temperature local mode process.

Effects of Electric Field on the Molecular Orientation in Evaporated Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

An electric field was applied during the evaporation process to obtain ferroelectric polymeric ultrathin films with uniaxial dipole orientation. The crystal structure and molecular orientation in the evaporated (VDF/TrFE) copolymer films were examined with a recently developed total reflection X-ray diffractometry, revealing that polar Form I crystals with the dipole moment oriented normal to the substrate can be formed by evaporating the samples under a high electric field (6 MV/m). Moreover, we evaluated the ferroelectric-paraelectric phase transition behavior of the evaporated films from a structural viewpoint.

Electron irradiation effects in ferroelectric vinylidene fluoride trifluoro-ethylene copolymers

Abstract We have studied the effect of 100 keV electron irradiation on the physical properties of the ferroelectric vinylidene fluoride trifluoro-ethylene (VF2-F3E) random copolymer. In situ dielectric measurements (in the range 100 Hz - 100 kHz) have been performed as a function of the irradiation dose for two different comonomer ratios, VF2/F3E, of respectively. 70/30 and 60/40 mol %. In both cases, a decrease in the Curie temperature, Tc, and in the maximum of the dielectric constant ϵ max, have been observed. Differential scanning calorimetry experiments have revealed that the melting temperature. Tm, was lowered by irradiation. In addition, the enthalpy changes associated to the para-ferroelectric transition and to the melting were continuously reduced for increasing irradiation doses. All these results are consistent with a reduction in the degree of crystallinity of the crystalline phase and a reduction of the crystallites size which are induced by the electron irradiation.

Modification of the ferroelectric properties of vinylidene fluoride trifluoro-ethylene copolymers induced by electron irradiation

The effect of electron irradiation on the physical properties of the ferroelectric vinylidene fluoride trifluoro-ethylene (VF 2-F 3E) random copolymer has been studied. In situ dielectric measurements (in the range 100 Hz to 100 kHz) have been performed as a function of the dose and of the irradiation temperature for two different comonomer ratios, VF 2 F 3 E , of, respectively, 70 30 and 60 40 mol% . In both cases, a decrease in the Curie temperature, T c, has been observed. In addition, differential scanning calorimetry experiments have revealed that the melting temperature, T m, was lowered by irradiation. We suggest that all these results are consistent with a reduction in the degree of crystallinity of the crystalline phase and a reduction of the crystallite size which are induced by the electron irradiation.