Browsing by Author "Bohannan, Gary W."

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Damping and electromechanical energy losses in the piezoelectric polymer PVDF
(2004) Vinogradov, A. M.; Schmidt, V. Hugo; Tuthill, G. F.; Bohannan, Gary W.
Polyvinylidene fluoride (PVDF) is a piezoelectric polymer that has been used in many applications including microphones, transducers, sensors and actuators. The electromechanical properties of PVDF are commonly defined by the constitutive equations of piezoelectricity. This paper presents experimental evidence that the assumptions underlying the theory of piezoelectricity have certain limitations in terms of representing adequately the electromechanical properties of PVDF. It is shown that PVDF tends to demonstrate time-dependent behavior in the form of viscoelastic creep and dielectric relaxation, and measurable energy losses under cyclic loading conditions. Moreover, the response of PVDF strongly depends on temperature and cyclic frequencies.
Dielectric behavior in an oriented β-PVDF film and chain reorientation upon transverse mechanical deformation
(2002) Lanceros-Mendez, S.; Moreira, S. C.; Mano, J. F.; Schmidt, V. Hugo; Bohannan, Gary W.
Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the g -phase have been studied by means of dielectric measurements and Fourier Transform Infrared Spectroscopy (FTIR). The main goal of the study was to compare the dielectric relaxations of f - and g -PVDF and to improve the understanding of the structural changes that occur in g -PVDF during a mechanical deformation process and their impact in the electromechanical properties of the polymer. A reorientation of the chains and a decrease in the degree of crystallinity with increasing deformation was observed.
Domain wall freezing in KDP-type ferroelectrics
(2000-02) Schmidt, V. Hugo; Bohannan, Gary W.; Arbogast, Darin; Tuthill, G. F.
Hysteresis loops in KH2PO4 (KDP) and its ferroelectric (FE) isomorphs disappear some 60 K below Tc. This disappearance may result from an order–disorder transition of the domain wall. The lowest energy wall consists of a single layer of nonpolar H2PO4 groups of Slater energy ε0. Including only the Slater/Takagi interactions predicts that a domain wall can become wider by having small protrusions that then diffuse along the wall. Reducing temperature would decrease domain wall mobility without causing a freezing transition. However, if one includes the Ishibashi dipolar interaction, this dipolar energy is minimized for a zero-entropy smooth domain wall with a particular ordered H-bond arrangement. Accordingly, there could be an order–disorder transition within the wall, if the bias “field” favoring this H-bond ordering is not great enough to smear out the transition. We are applying this model to predict domain wall mobility temperature dependence, and simultaneously measuring FE hysteresis in KDP-ferroelectrics to determine the nature and sharpness of this proposed domain wall transition.
Piezoelectric polymer actuators for vibration suppression
(1999) Schmidt, V. Hugo; Conant, J.; Bohannan, Gary W.; Eckberg, J.; Halko, S.; Hallenberg, J.; Nelson, C.; Peterson, N.; Smith, C.; Thrasher, C.; Tikalsky, B.
We have designed and built piezoelectric polymer actuators in a 'bellows' configuration and have used them in a near-zero-g environment vibrations suppression apparatus. The actuator is based on poly(vinylidene fluoride) (PVDF) sheets produced by AMP and electroded to our specifications. The actuator consists of two bimorphs, each with a double-bend precurvature, glued together at their ends so that the actuator has its thickest air gap at the middle. Each bimorph consists of two sheets glued together. Each sheet is electroded completely on the outside (ground) side, and has three electrode areas on the other side. If the electrode on the middle half is positive, and on the outer two quarters are negative, the bimorph curvature and the actuator length increase; with opposite polarities they decrease. In the vibration isolation application, the box to be isolated has actuators mounted between it and its surrounding enclosure on the vibrating vehicle. Feedback control is provided to change actuator length to compensate for vehicle motions and vibrations. This feedback is provided by accelerometers and by laser diode position sensors. The inherent softness of the actuator provides good passive damping of higher frequencies. So far, a one-dimensional test of the system has been made using a mass on a 'folded pendulum' as a 'weightless' (no restoring force for small displacements) load. Also, a two- dimensional version was flown on NASA's KC-135, which provided 25-second near-zero-g intervals during parabolic flight segments. Our goal is three-dimensional isolation for space vehicle applications.