Electromechanical: behavior response

Figure 4. Effect of epitaxial (or deposition) strains on the average extrinsic ferroelectric behavior of a polycrystalline thin-film [83], Left inset summarizes the predicted out-of-plane hysteretic response, while the right inset embodies the predicted out-of-plane extrinsic electromechanical behavior. Note that while the electromechanical response for large fields corresponds to the equilibrium (intrinsic) behavior, a great potential for reaching electromechanical enhancements up to one order of magnitude greater than the ones currently available are possible by harnessing the time-dependent switching behavior.

The forth direction, analytical modeling for understanding the behaviors of these materials, has been popular approach. Testing and characterization have been conducted for developing the models. Such attempts have been done especially for ionic polymer metal composites (IPMCs)[58, 70, 72, 120]. Nemab Nasser and his co-workers carried out extensive experimental studies on both Nafion- and Flemion-based IPMCs consisting of a thin perfluorinated ionomer in various cation forms, seeking to imderstand the fundamental properties of these composites, to explore the mechanism of their actuation, and finally, to optimize their performance for various potential applications[121]. They also performed a systematic experimental evaluation of the mechanical response of both metal-plated and bare Nafion and Flemion in various cation forms and various water saturation levels. They attempted to identify potential micromechanisms responsible for the observed electromechanical behavior of these materials, model them, and compare the model results with experimental data[122]. A computational micromechanics model has been developed to model the initial fast electromechanical response in these ionomeric materials[123]. A number... 

Uncoupled solutions for current and electric field give simple and explicit descriptions of the response of piezoelectric solids to shock compression, but the neglect of the influence of the electric field on mechanical behavior (i.e., the electromechanical coupling effects) is a troublesome inconsistency. A first step toward an improved solution is a weak-coupling approximation in which it is recognized that the effects of coupling may be relatively small in certain materials and it is assumed that electromechanical effects can be treated as a perturbation on the uncoupled solution. 

From the mechanical viewpoint, ferroelectrics exhibit unsteady, evolving waves at low stresses. Waves typical of well defined mechanical yielding are not observed. Such behavior is sensitive to the electrical boundary conditions, indicating that electromechanical coupling has a strong influence. Without representative mechanical behavior, it is not possible to quantitatively define the stress and volume compression states exciting a particular electrical response. 

The discussion of this section follows the development presented by Frerking (1978). Because of the piezoelectric effect, the quartz resonator behaves physically as a vibrating object when driven by a periodic electric signal near a resonant frequency of the cavity. This resonant behavior may be characterized by equivalent electrical characteristics that may then be used to accurately determine the response and performance of the electromechanical system. 

The electrostrictive behavior of P(VDF-CTFE) copolymers was investigated (Li et al. 2004 Li et al. 2006). A high electromechanical response was obtained in these copolymer films. As other PVDF-based polymers, the processing condition plays a very critical role on its properties. For a well-stretched and annealed P (VDF-CTFE) 88/12 copolymer film, a longitudinal electrostrictive strain as much as 5.5 % was obtained. A linear relationship between the strain response and the was observed, which indicates the electrostrictive nature of the electromechanical response in P(VDF-CTFE) copolymers. The corresponding electric field-related electrostrictive coefficient for the copolymer film is obtained as Mss = —1.23 0.02 X 10 (m /V ) (Li et al. 2006). 

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