Ferroelectric Phase Transition of VDF/TrFE Copolymer at High Pressures
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High pressure ultrasonic and D - E hystresis measurements were performed on a vinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer with 54 mol% VDF content to investigate the pressure effect on physical properties accompanying wiht a ferroelectric phase transition. Differing from the results at atmospheric pressure where the ferro-to-paraelectric phase transition proceeded simply in one-step, this copolymer exhibited at 350 MPa two-step temperature variations in ultrasonic velocity and absorption, and remanent polarization upon the ferroelectric phase transition. These results agree well with the previous results of DTA and X-ray diffraction experiments, suggesting that the changes in the nature of the ferroelectric phase transition is attributed to the pressure-induced structural transformation.Keywords:
Transition temperature
Ferroelectric Polymers
Ferroics
Abstract The first order ferroelectric-paraelectric transitions in ferroelectric polymers are discussed in the framework of the Diffuse Phase Transition models. In order to describe these transitions, we use the classical Landau-Devonshire treatment for the sample micro-regions undergoing normal phase transition and look for a critical temperature distribution function. Thus, this model is applied to study the phase transition of a vinylidene fluoride-trifluoroethylene copolymer. We show how to obtain the correct behaviour of the spontaneous polarization and of the dielectric constant besides the Landau expansion parameters.
Ferroelectric Polymers
Ferroics
Landau theory
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The authors of paper commented claim that trimethylammonium tetrachlorozincate crystal shows at 282 K the ferroelectric-paraelectric phase transition. But no ferroelectric hysteresis loop was observed below this temperature. Moreover, in the low-temperature phase the ferroelectric domain walls should exist giving dielectric relaxation in a low frequency electric field. The authors conclude that the phase transition is of the second order. This conclusion is contrary to the DSC data where the phase transition has a strong first order character. In the whole measured temperature range the dielectric loss is 100 times higher than the real part of dielectric constant.
Hysteresis
Atmospheric temperature range
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We present a theoretical study to explain the ferroelectric phase transition of polymers which has been experimentally observed in copolymers of vinylidene fluoride and trifluoroethylene. The phase transition of these copolymers changes its thermodynamic character from a first order phase transition to a diffuse transition with a decrease in a mole fraction of vinylidene fluoride. This fact has been confirmed experimentally by observing temperature hystereses seen as differences between transition temperatures in the heating process and cooling one. The ferroelectric spontaneous polarization of this polymer is formed by a cooperative orientation of dipoles belonging mainly to a vinylidene fluoride part in linear chains with a planar zigzag conformation. In order to represent these interesting ferroelectric phenomena, we examine a three-states model whose element can take three values of the dipole moment as +u, -u, 0. Monte Carlo simulation of this model reveals the physical essence of the ferroelectric characters described above.
Ferroelectric Polymers
Transition temperature
Zigzag
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In the last few years, considerable interest has centered on the ferroelectric phenomena in fluoropolymers. An extensive review is given on the recent development of structural studies connected with ferroelectric phase transitions in polyvinylidene fluoride (PVDF) and copolymers of vinylidene fluoride/trifluoroethylene (P·VDFx/TrFE100-x) .PVDF exhibits five polymorphic forms, although the form-V is still ambiguous in its structure and properties. Various types of transitions among these forms, induced mechanically, electrically and thermally, are introduced and their mechanisms are discussed.In PVDF, the ferroelectric to paraelectric transition is not yet clearly understood. On the other hand, the Curie temperature has been recently observed in P·VDFx/ TrFE100-x. There are two kinds of phase transition from the paraelectric phase, that is, to the low temperature phase or to the cooled phase, depending on the composition x. The structures of these three crystal phases and the unique properties of the transition are introduced. The mechanisms of the phase transitions are discussed in terms of statistical theory in relation to cooperative conformational changes in the molecular chains.Finally, molecular motions in the paraelectric phase, domain structure and future problems on ferroelectric polymers are described.
Polyvinylidene fluoride
Ferroelectric Polymers
Ferroics
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Vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymers are ferroelectric materials and exhibit ferroelectric to paraelectric phase transitions. In order to understand the ferroelectric phase transition of polymers, we performed Monte Carlo simulations for the three states model which an extra freedom was added to Ising model. The simulations reproduced the characters of the ferroelectric phase transition of VDF/TrFE copolymers. The temperature dependence of the specific heat obtained by MC simulations revealed that the transition changed from a first order transition to a diffuse one and that the critical phenomena existed between them. Keywords: Ferroelectric phase transitionMonte Carlo simulationspecific heatVDF-TrFE copolymer Acknowledgments Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005; received for publication January 26, 2006.
Ferroelectric Polymers
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The ferroelectric phase transition characteristics of the 0.32Pb(In1/2Nb1/2)O3-0.345Pb(Mg1/3Nb2/3)O3-0.335PbTiO3 (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FEM-FET) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FET-PC), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around TM-T and TC/Tm temperatures. The temperature dependent permittivity derivative ξ = dϵ/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FEM-FET phase transition and the first-order transition feature of the FET-PC phase transition. The FEM-FET phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the Pr and Ec values, and extremums of the pyroelectric performance.
Tetragonal crystal system
Monoclinic crystal system
Transition temperature
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Specific heat measurements performed on the ferroelectric (N(CH3)4)2ZnCl4 reveal that the phase transition occurring in this compound can be classified into two groups. The commensurate to incommensurate to ferroelectric to paraelectric sequence belongs to the first group, whereas in the second group, for which a new anomaly was detected, the authors have three phase transitions associated with the organic group movements. The critical behaviour for the commensurate-incommensurate phase transition is discussed.
Anomaly (physics)
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Abstract In the early 1970s it became clear that the polymer polyvinylidene fluoride is ferroelectric. There have been extensive studies of its properties and of the properties of copolymers of vinylidene fluoride with tri- or tetra-fluoroethylene. This work has led to a fairly complete understanding of the ferroelectric and related properties of these materials. The emphasis in this review is on the studies of these materials that are oriented toward showing that the polymers are indeed ferroelectric, determining the mechanism of polarization reversal, and understanding the origins of the observed piezoelectric and pyroelectric effects. Very recently some odd nylons (nylons with an odd number of carbon atoms in the monomer unit) were recognized to be ferroelectric, and ferroelectric liquid-crystalline polymers have been prepared by attaching ferroelectric liquid-crystal molecules as side chains to polymer backbones. These new findings are included in the review.
Ferroelectric Polymers
Polyvinylidene fluoride
Side chain
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High pressure ultrasonic and D - E hystresis measurements were performed on a vinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer with 54 mol% VDF content to investigate the pressure effect on physical properties accompanying wiht a ferroelectric phase transition. Differing from the results at atmospheric pressure where the ferro-to-paraelectric phase transition proceeded simply in one-step, this copolymer exhibited at 350 MPa two-step temperature variations in ultrasonic velocity and absorption, and remanent polarization upon the ferroelectric phase transition. These results agree well with the previous results of DTA and X-ray diffraction experiments, suggesting that the changes in the nature of the ferroelectric phase transition is attributed to the pressure-induced structural transformation.
Transition temperature
Ferroelectric Polymers
Ferroics
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Abstract Thermal bysteresis of the ferroelectric-ferroelastic transition has been observed in virgin and heat-treated samples by means of differential scanning calorimetry, X-ray diffraction of oriented samples and electron microscopy observation of replicas of cleavage surfaces. These replicas provided for the first time direct evidence of the ferroelectric domain structure. In the ferroelastic-paraelastic transition, transition temperature changes, under different atmospheric surroundings, have been observed.
Transition temperature
Cleavage (geology)
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