Hiroji Ohigashi

Bio: Hiroji Ohigashi is an academic researcher from Yamagata University. The author has contributed to research in topics: Ferroelectricity & Ultrasonic sensor. The author has an hindex of 21, co-authored 81 publications receiving 2333 citations. Previous affiliations of Hiroji Ohigashi include Daikin.

Piezoelectricity and related properties of vinylidene fluoride and trifluoroethylene copolymers

Abstract: 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

Abstract: 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...

Temperature dependence of elastic, dielectric, and piezoelectric properties of “single crystalline’’ films of vinylidene fluoride trifluoroethylene copolymer

Abstract: Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

Ferroelectric Copolymers of Vinylidenefluoride and Trifluoroethylene with a Large Electromechanical Coupling Factor

Abstract: A large electromechanical coupling factor kt of about 0.3 has been found in vinylidenefluoride and trifluoroethylene copolymers containing 70–80 mol% VDF. These copolymers exhibit clear ferroelectric behavior. The strong piezoelectricity and distinctive ferroelectricity are attributable to high crystallinity, strong preferred orientation, and reduced conformational defects in crystallites, which are induced by annealing and successive or simultaneous poling. The polarization is inferred to grow through rotation of the polar axis by nπ/3 about the molecular axis (c-axis) under an external electric field above the coercive field.

Polarization Behavior in Vinylidene Fluoride-Trifluoroethylene Copolymer Thin Films

Abstract: The polarization behavior in films of vinylidene fluoride trifluoroethylene copolymer (P(VDF-TrFE), VDF/TrFE molar content ratio of 75/25) of thickness less than 1 µm is reported. Ferroelectricity is maintained even in 0.06 µm-thick film, whose saturated polarization is as large as that for thick films (100 mC/m2). However, the crystallites are much smaller than in thick film, and the coercive field in the thin film is higher (125 MV/m) for the 0.06 µm-thick film) than that of the thick one (40 MV/m). The switching time does not follow the exponential law, but is inversely proportional to the applied field. To analyze these data, we introduce a model in which the thin film contains many defects in its crystalline phase, and the local field in the film is modulated by the defects. The observed switching behavior is consistent with the model.

Electroactive phases of poly(vinylidene fluoride) : determination, processing and applications

Abstract: Poly(vinylidene fluoride), PVDF, and its copolymers are the family of polymers with the highest dielectric constant and electroactive response, including piezoelectric, pyroelectric and ferroelectric effects. The electroactive properties are increasingly important in a wide range of applications such as in biomedicine, energy generation and storage, monitoring and control, and include the development of sensors and actuators, separator and filtration membranes and smart scaffolds, among others. For many of these applications the polymer should be in one of its electroactive phases. This review presents the developments and summarizes the main characteristics of the electroactive phases of PVDF and copolymers, indicates the different processing strategies as well as the way in which the phase content is identified and quantified. Additionally, recent advances in the development of electroactive composites allowing novel effects, such as magnetoelectric responses, and opening new applications areas are presented. Finally, some of the more interesting potential applications and processing challenges are discussed.

Recent Progress on Ferroelectric Polymer-Based Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects.

Abstract: Dielectric polymer nanocomposites are rapidly emerging as novel materials for a number of advanced engineering applications. In this Review, we present a comprehensive review of the use of ferroelectric polymers, especially PVDF and PVDF-based copolymers/blends as potential components in dielectric nanocomposite materials for high energy density capacitor applications. Various parameters like dielectric constant, dielectric loss, breakdown strength, energy density, and flexibility of the polymer nanocomposites have been thoroughly investigated. Fillers with different shapes have been found to cause significant variation in the physical and electrical properties. Generally, one-dimensional and two-dimensional nanofillers with large aspect ratios provide enhanced flexibility versus zero-dimensional fillers. Surface modification of nanomaterials as well as polymers adds flavor to the dielectric properties of the resulting nanocomposites. Nowadays, three-phase nanocomposites with either combination of fillers...

High performance piezoelectric devices based on aligned arrays of nanofibers of poly(vinylidenefluoride-co-trifluoroethylene)

Abstract: Multifunctional capability, flexible design, rugged lightweight construction and self-powered operation are desired attributes for electronics that directly interface with the human body or with advanced robotic systems. For these applications, piezoelectric materials, in forms that offer the ability to bend and stretch, are attractive for pressure/force sensors and mechanical energy harvesters. Here, we introduce a large area, flexible piezoelectric material that consists of sheets of electrospun fibres of the polymer poly[(vinylidenefluoride-co-trifluoroethylene]. The flow and mechanical conditions associated with the spinning process yield free-standing, three-dimensional architectures of aligned arrangements of such fibres, in which the polymer chains adopt strongly preferential orientations. The resulting material offers exceptional piezoelectric characteristics, to enable ultra-high sensitivity for measuring pressure, even at exceptionally small values (0.1 Pa). Quantitative analysis provides detailed insights into the pressure sensing mechanisms, and establishes engineering design rules. Potential applications range from self-powered micro-mechanical elements, to self-balancing robots and sensitive impact detectors.

Ferroelectric properties of vinylidene fluoride copolymers

Abstract: Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Polymer electronic memories: Materials, devices and mechanisms

Abstract: As an emerging area in organic electronics, polymer memories have become an active research topic in recent years, because they are likely to be an alternative or supplementary technology to the conventional memory technology facing the problems and challenges in miniaturizing from microscale to nanoscale. This review provides a summary of the widely reported electrical switching phenomena in polymers and the corresponding polymer electronic memories. A general introduction to the current state of memory technology and some basic concepts of electronic memories is first presented, followed by a brief historical development and some key advances in polymer electronic memories. The subsequent sections give a comprehensive review of three categories of polymer electronic memories, classified by drawing the mechanistic analogy between the polymer switching element and one of the three primary circuit elements, viz. , capacitor, transistor and resistor. Emphasis is placed on the relationships among material structures and properties, memory devices and operating mechanisms. Finally, the challenges facing the research and development in the field of polymer electronic memories are summarized.