Use of an External Electric Field to Convert the Paraelectric Phase to the Ferroelectric Phase in Ultra-thin Copolymer F
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Use of an External Electric Field to Convert the Paraelectric Phase to the Ferroelectric Phase in Ultra-thin Copolymer Films of P(VDF-TrFE) 1
Matt Poulsen, 1S. Adenwalla, 1Stephen Ducharme, 2V.M. Fridkin, 2S.P. Palto, 2N.N. Petukhova, and 2S.G. Yudin 1 Department of Physics and Astronomy and Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588 2 Institute of Crystallography, Russian Academy of Sciences, 117333 Moscow, Russia ABSTRACT X-ray diffraction was used to probe the structural changes associated with the conversion of the paraelectric phase to the ferroelectric phase that results from the application of a large external electric field. The samples under study are ultrathin (150 to 250 Å) Langmuir-Blodgett films of the copolymer vinylidene fluoride (70%) with trifluoroethylene (30%) deposited on aluminum-coated silicon. Theta-2theta X-ray diffraction was used to measure the change in inter-layer spacing perpendicular to the film surface. Upon heating at zero external electric field, the crystalline films undergo a structural phase transition, at 100± 5°C, from the all-trans ferroelectric phase to the trans-gauche paraelectric phase. [1,2] Above the phase transition temperature, the non-polar paraelectric phase can be converted back to the polar ferroelectric phase, in a smooth continuous process, using a large external electric field (~1 GV/m). For example, at 100° C the ferroelectric phase first appears above 0.2 GV/m and increases steadily in proportion while the paraelectric phase decreases until complete conversion to the ferroelectric phase is achieved at approximately 0.6 GV/m. INTRODUCTION Ferroelectricity was first realized and identified by John Valasek in 1920, with the observation that the spontaneous polarization of Rochelle salt was reversible in the presence of an applied electric field. [3] Ferroelectric materials are analogous to ferromagnetic materials in that they both possess a net macroscopic dipole, a hysteresis effect, and a phase transition at a Curie temperature. At the Curie temperature, ferroelectric materials undergo a structural phase transition from a structure with net electric polarization, the ferroelectric phase, to a structure with no net polarization, the paraelectric phase. One family of ferroelectric materials are the copolymers of vinylidene fluoride with trifluoroethylene or P(VDF-TrFE). In the present study all films are composed of copolymers of 70% vinylidene fluoride with 30% trifluoroethylene. Using Langmuir-Blodgett deposition it is possible to obtain ultrathin (5→5000 Å) films of P(VDF-TrFE). We have investigated the effect an applied electric field has on the ferroelectricparaelectric phase transition, using theta-2theta X-ray diffraction and dielectric measurements. We have shown that the application of a sufficiently large electric field causes the ferroelectricparaelectric phase transition to be shifted to higher temperature. [4] Sample Preparation Solvent-cast and spin-coated films of PVDF and P(VDF-TrFE) have long been studie
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