Gary William Bohannan

Dielectric behavior in an oriented β-PVDF film and chain reorientation upon transverse mechanical deformation

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.

Interpretation of complex permittivity in pure and mixed crystals

The power law behaviors implied by the Cole-Cole plot are presented. The power law behavior in the frequency domain, indicated by the shape of the Cole-Cole plot, implies power law behavior in the time domain as well. The so-called “delayed response” in e(ω) should be interpreted as a long time transient response, which can impose significant experimental constraints. Dielectric materials in general, and ferroelectric crystals in particular, allow us to investigate power law decay in great detail and offer an experimental probe into a very fundamental question does power law behavior necessarily imply a fractal underlying structure?

Piezoelectric polymer actuators in a vibration isolation application

We present results from development and testing of lightweight actuators made of the piezoelectric polymer PVDF. The prototype being developed is intended for microgravity applications in space and has been tested aboard NASAs Reduced Gravity Platform. The design has been driven by the requirements for a full 3D environment. Incorporation of additional electrical leads into the actuators themselves may remove the need for a separate umbilical to the suspended experiment. Linear equations describing the displacement of piezoelectric bimorphs were developed and applied to the bellows actuator including the epoxy layer. Properties for the piezoelectric layers were obtained from the literature; properties for the epoxy layer were obtained through ultrasonic testing. To assess the validity of the assumed linearity of the actuator, we conducted nonlinear finite element analysis, which indicated a high degree of linearity on contraction and up to a maximum of 5% deviation on expansion to full deflection (about 6 mm). We have developed and tested a proportional-plus-derivative (PD) control system for use with the actuator in 1D using a novel folded pendulum to simulate a zero-g environment. Passive and active characteristics are both in agreement with theoretical predictions.

Piezoelectric polymer actuators for active vibration isolation in space applications

Abstract A lightweight actuator for active vibration isolation in space applications is being developed to replace the heavy electromagnetic systems now in use. The actuator has a low effective spring constant that provides for passive vibration damping down to sub-Hertz frequencies while allowing the isolated experiment to follow the near-dc bias motion of the spacecraft. The actuator is currently optimized for the vibration level of the Space Shuttle and assembled from a pair of bimorphs in a leaf-spring configuration. Changing the size and number of sheets used in construction can vary electromechanical properties. Passive damping has been demonstrated in one and two-dimensional tests. For large (greater than a few kilograms) suspended masses, the system is underdamped and relative velocity feedback must be used to remove the resonance. Real-time control of the resonance frequency is achieved by controlling the voltage applied to the actuator with feedback from a displacement sensor. A folded pendulum ...

Nonexponential relaxation in piezoelectric PVDF

Polymers are known for displaying nonexponential relaxation to both electrical and mechanical stresses. A numerical method for integrating fractional differential equations has made it possible to verify the connection between a power-law ac permittivity function and power-law decay currents resulting from application of dc step voltages. The responses show significant dependence on the duration of the applied pulse. The history dependence predicted by the simulations has been confirmed experimentally. The computational techniques should be applicable to a wide range of systems exhibiting nonexponential relaxation.