Hiroji Ohigashi

Piezoelectricity and related properties of vinylidene fluoride and trifluoroethylene copolymers

The piezoelectric properties and ferroelectric transition behaviors of vinylidene fluoride and trifluoroethylene copolymers, P(VDFx−TrFE1−x), were studied as a function of x in the range of x=0.37–0.94. The electromechanical coupling factor kt and piezoelectric constant d31 are strongly dependent on both x and annealing temperature. In the copolymers of 0.65≲x≲0.82, a large kt value of 0.27–0.3 has been found when annealed above ferroelectric‐to‐paraelectric transition temperature Tc. The large kt is attributable to their high remanent polarization (∼110 mC/m2) which arises from high degrees of crystallinity and preferred orientation of well‐grown crystallites. Structural and morphological changes induced by annealing and poling were characterized by SEM, DSC, and x‐ray diffraction studies. Phase diagram of the copolymers were obtained by DSC measurements. Tc increases with increase of x and coincides with melting temperture Tm at x≂0.82. In the range of x≳0.82, Tc=Tm.

Electromechanical properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method

The piezoelectric resonance was observed in free vibrators of uniaxially oriented and biaxially oriented polyvinylidene fluoride films polarized at high electric fields. Using the piezoelectric resonance method, piezoelectric and elastic constants, and electromechanical coupling factors of such films were determined at high frequencies (20 kHz–30 MHz) in the temperature range −170–100 °C. For the uniaxially oriented films, the coupling factors k33 and k32 are independent of temperature, while k31 increases noticeably above the primary dispersion temperature. The value of k33 is about 0.2, one of the largest values ever reported for piezoelectric polymers. Similar results were obtained for the biaxially oriented films. The temperature dependence of elastic and piezoelectric constants of the uniaxially oriented films is interpreted by a model in which the crystalline and amorphous phases are combined in series along the stretching direction and in parallel along the perpendicular directions. The origin of t...

Piezoelectric and ferroelectric properties of P (VDF-TrFE) copolymers and their application to ultrasonic transducers

Abstract The copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) have been found to be effective ultrasonic transducer materials because of their large electromechanical coupling factor kt . On the basis of the results obtained by SEM, X-ray diffraction, and ferroelectric polarization reversal studies of copolymer films, the large kt is ascribed to their high remanent polarization, which may originate from high crystallinity and preferential orientation of well-grown crystallites. Various single element transducers for medical ultrasound imaging have been developed utilizing PVDF and P(VDF-TrFE) films. The performance of these transducers is in good agreement with a theoretical prediction in which their dielectric and mechanical losses are taken into account. The P(VDF-TrFE) transducers have a two-pathway sensitivity 10 dB higher than that of PVDF transducers. They give high resolution medical images much superior in quality to those obtained with conventional ceramic transducers. Linear ...

Characteristics of Schottky barrier solar cells using polyacetylene, (CH)x

Abstract We have constructed Schottky barrier solar cells using semiconductive polyacetylene, and investigated their characteristics which are influenced significantly by the surface conditions of the polyacetylene films. The surface contacting the glass wall during polymerization has greater trap density, and the device having a Schottky barrier on the surface shows inferior characteristics to that having a barrier on the other surface. The energy conversion efficiency was improved up to 1%, when the incident light intensity was 7 mW/cm 2 and the energy of 0.2 mW/cm 2 was input into the barrier region through the Al electrode. From the strong dependence of the photovoltaic current on the backward biasing voltage, it is inferred that excitons contribute to the photocarrier generation.

Ferroelectric properties of poly(vinylidenefluoride‐trifluoroethylene) copolymer thin films

The ferroelectric, piezoelectric, and pyroelectric characteristics in thin films (0.5–2.5 μm thick) of a copolymer composed of 74 mol % vinylidenefluoride and 26 mol % trifluoroethylene have been studied. They maintain remarkable ferroelectricity similar to that found in the thicker films of this copolymer. Both the temperature dependence of the reciprocal dielectric constant 1/e and the biasing field dependence of the phase transition temperature Tc indicate the phase transition to be of a first order. A large pyroelectric coefficient (35–50 μC/m2 K) and the significant piezoelectricity related to e33 have been found in thin films of this copolymer.

Generation of very high‐frequency ultrasonic waves using thin films of vinylidene fluoride‐trifluoroethylene copolymer

Electromechanical properties of thin films of vinylidene fluoride‐trifluoroethylene copolymer [P(VDF‐TrFE)] and their application to ultrasonic transducers were studied in the frequency range of 50–250 MHz at room temperature. A 5‐μm‐thick P(VDE‐TrFE) film resonating at 250 MHz in thickness extensional mode exhibited a large electromechanical coupling factor of 0.26. The dielectric and elastic constants were also studied in this frequency region. A transducer using a 3‐μm‐thick film exhibited a large conversion efficiency (6.5 dB at 150 MHz), and good impulse response (30 ns ring‐down time width) in water. A concave transducer transmitted a well‐focused ultrasonic beam (35‐μm beamwidth).