Takeo Furukawa

Electromechanical Properties in the Composites of Epoxy Resin and PZT Ceramics

The complex dielectric, elastic and piezoelectric constants were measured for the composites of epoxy resin and ferroelectric PZT ceramics at 10 Hz as a function of temperature, and the magnitudes and loss tangents of these constants were compared with the theoretical predictions. The piezoelectric constants for a two phase system with piezoelectric spherical inclusions were expressed in terms of the properties of the constituents by use of the known expressions of the dielectric and elastic constants. The observed values of the d-constant (electric displacement/stress) were about 3/4 of the predicted. In the local mode dispersion region of epoxy resin near -30°C, the g-constant (electric field/stress) showed a larger temperature variation than the theoretical prediction.

Factors governing ferroelectric switching characteristics of thin VDF/TrFE copolymer films

A series of ferroelectric switching measurements are performed on the 75/25mol% vinylidene fluoride/trifluoroethylene copolymer thin films with various structures to elucidate the factors governing the switching characteristics of such films. It is shown that the spin-coated and well-annealed samples exhibit a remanent polarization Pr of 85 mC/m2. Unlike that in previous reports, the switching time taus of these samples is independent of thickness down to 50 nm if Au electrodes are used instead of Al electrodes. Thin films with different metal electrodes exhibit anisotropic taus due to a potential difference between metal electrodes produced by their work function difference. A train of unipolar short on-off electric field pulses induce a slow but full polarization reversal whose time evolution supports the nucleation and growth mechanism. Continuous switching under bipolar on-off electric field pulses reveals an accelerated polarization reversal with decreasing pulse width. This means that taus is a function of not only applied electric field but also the preceding poling conditions. Fatigue is significantly improved using Au electrodes instead of Al electrodes. The results are discussed in relation to the microscopic features of the VDF/TrFE copolymer and the underlying nucleation-growth mechanism

Investigation of switching characteristics of vinylidene fluoride/trifluoroethylene copolymers in relation to their structures

Ferroelectric switching characteristics of VDF/TrFE copolymers have been investigated over the entire range of composition for the samples subjected to drawing and/or annealing. Switching phenomena were most remarkably observed in the copolymers containing 50–80 mol% VDF. The as-cast copolymer exhibits no switching behavior because of high a depolarization field associated with low crystallinity. Draw-induced orientation of the chain axis parallel to the film surface causes an increase in the amount of reversed polarization. Annealing at a temperature between the Curie point and the melting point results in a significant increase in the degree of crystallinity and the amount of reversed polarization. Extrapolating to 100% crystallinity allows an estimation of spontaneous polarization, which is 120–135 mC/m2 for the 73/27 mol% copolymer. This value suggests a small Lorentz factor in crystals of VDF/TrFE copolymers.

Structure and functional properties of ferroelectric polymers

Abstract Ferroelectric polymers represented by vinylidene fluoride copolymers with trifluoroethylene (VDF/TrFE) have been investigated with special interest on the relationship between their dynamical properties and structures. Copolymers containing 50–80 mol% VDF exhibit fast polarization reversal due to the rotation of chain molecules about their axes. Annealing of an extruded copolymer yields a highly crystalline film consisting of regularly stacked large lamellae. The switching transient of these extruded copolymers is consistent with the switching mechanism that progresses through a nucleation-growth process with a considerable waiting time (time between the generation of a nuclei and the onset of its growth) and an accelerated growth velocity. TrFE-rich copolymers exhibit a two-step switching transient, suggesting that polarization reversal occurs via a nonpolar state associated with the copolymers antiferroelectric-like nature. Piezoelectric and pyroelectric properties were examined for their high frequency and short time characteristics. The time evolution of the charge response induced by laser pulse irradiation consists of intrinsic pyroelectricity and the coupling of thermal expansion and piezoelectric resonance. The intrinsic pyroelectric response for well-annealed VDF-rich copolymers is in the sub-nanosecond range, which promises their use as fast thermal sensors.

Intrinsic Switching Characteristics of Ferroelectric Ultrathin Vinylidene Fluoride/Trifluoroethylene Copolymer Films Revealed Using Au Electrode

Time-domain measurements of ferroelectric polarization reversal were conducted for spin-coated ultrathin vinylidene fluoride/trifluoroethylene copolymer films deposited with an Au electrode. It was found that full polarization reversal occurs in 50–330-nm-thick films that have switching time independent of the film thickness. We conclude that the thinning-induced increase in switching time reported previously is not a general feature of this copolymer but is specific to an Al-electrode-deposited sample. The intrinsic switching characteristics revealed here include the broadening of the switching time distribution with thinning, which is attributed to the nonuniform electric field caused by surface roughness.

Effects of annealing on the structure and switching characteristics of VDF/TrFE copolymers

Abstract Structure-property relationship has been investigated in concern with the ferroelectric switching characteristics of a VDF(78)/TrFE(22) copolymer. Unoriented and oriented film samples prepared by melt-quenching and melt-extrusion, respectively, were subjected to annealing and resulting structural changes were characterized by SEM, X-ray, density and DSC measurements. It was found that annealing just above the Curie point (120–130°C) caused a marked increase in crystallinity, whereas annealing in the paraelectric state (130–145°C) induced a crystal growth into thick lamellae. Such structural changes resulted in the ferroelectric switching characteristics with larger remanent polarization and shorter switching time. The unoriented sample yielded a smaller remanent polarization than the oriented sample by a factor of π/4, which substantiated that polarization reversal occurred as a result of rotation of molecules rotation about their chain axis. The melt-extruded and well-annealed sample consisted o...

Enhanced electrical properties of highly oriented poly(vinylidene fluoride) films prepared by solid‐state coextrusion

Oriented poly(vinylidene fluoride) (PVDF) films with β-form crystals have been commonly prepared by cold drawing of a melt-quenched film consisting of α-form crystals. In this study, we have successfully produced highly oriented PVDF thin films (20 µm thick) with β-crystals and a high crystallinity (55–76%), by solid-state coextrusion of a gel film to eight times the original length at an established optimum extrusion temperature of 160°C, some 10°C below the melting temperature. The resultant drawn films had a highly oriented (orientation function fc = 0.993) fibrous structure, showing high mechanical properties of an extensional elastic modulus of 8.3 GPa and tensile strength of 0.84 GPa, along the draw direction. Such highly oriented and crystalline films exhibited excellent ferroelectric and piezoelectric properties. The square hysteresis loop was significantly sharper than that of a conventional sample. The sharp switching transient yielded the remnant polarization Pr of 90 mC/m2, and the electromechanical coupling factor kt was 0.24 at room temperature. These values are about 1.5 times greater than those of a conventional β-PVDF film. Thus, solid-state coextrusion near the melting point was found to be a useful technique for the preparation of highly oriented and highly crystalline β-PVDF films with superior mechanical and electrical properties. The morphology of the extrudate relevant to such properties is discussed.

Switching Phenomena in Vinylidene Fluoride / Trifluoroethylene Copolymers near the Curie Point

Ferroelectric switching and dielectric relaxation processes were simultaneously observed by a time-domain measurement of charge responses induced by a step voltage for a 65/35 mol% copolymer of vinylidene fluoride and trifluoroethylene. These processes were separated on the basis of the different field dependences of their characteristic time constants and resulting polarization changes. As the temperature approaches the Curie Point (Tc~68°C), both the activation field and the amount of reversed polarization associated with the switching process fall to zero. It is found that the switching times under various fields converge to the relaxation time at Tc. This result implies that the switching process is affected by dipole fluctuations, probably at an early stage when the nuclei of reversed domains are created.

Time evolution of laser-induced pyroelectric responses in a VDF/TrFE copolymer

Pyroelectric charge response induced by instantaneous heating using YAG laser pulse (7 ns) irradiation has been measured over a 1 ns to 10 s time range for a VDF/TrFE copolymer with composition 75/25 mol%. The observed spectra exhibited two damped oscillations associated with thermal expansion in the thickness and length directions coupled with piezoelectric effects. Removing these two components revealed pyroelectric responses consisting of an instantaneous component followed by a gradually increasing component. The instantaneous component appearing in parallel with the integrated laser intensity was attributed to primary pyroelectricity whose response time was <1 ns. The gradual component with time constant 200 ns was attributed to the relaxational nature of molecules and heat transfer from the electrode.

Ferroelectric VDF/TrFE copolymers studied by nonlinear dielectric spectroscopy

Abstract Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene have been investigated by means of nonlinear dielectric measurements conducted on both time and frequency domains. Two kinds of nonlinear responses were separately observed; one associated with a local change in orientational order of crystalline dipoles constituting remanent polarization P r and the other with ferroelectric polarization reversal from—P r to P r. The frequency-domain measurement using a relatively low sinusoidal field characterized the local dipole motion in terms of the frequency spectra of nonlinear complex permittivity e* ′′ = e′ ′′. It is shown that ϵ2 is negative and ϵ3 is positive being consistent with cooperative dipoles. The time-domain measurement using a high step-field provided dynamical information about polarization reversal in terms of the field-dependent switching time. The analysis of the switching profile revealed that a change in orientational order of crystalline dipoles precedes polarizat...