Materials electromechanical responses

Huang C, Zhang QM, deBotton G, Bhattacharya K (2004) All-organic dielectric-percolative three-component composite materials with high electromechanical response. Appl. Phys Lett 84 4391... 

Figure 18.1 Electromechanical responses for various materials, (a) Electromechanical ceramics [8] (b) Domain-engineered (001) single C7stal of Pb(ZnT/3Nb2/3)o,955Tio.o4503 [9].

It should be noted that only the A- or B-site donor dopants cause softening of ABO3 ferroelectric materials. As shown in Ref. [87], the fluorine doping of PZT leads to a hardening of the dielectric and electromechanical response. This effect can be explained by ordering of point defects or their associates involving the charged... 

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... 

In addition, this good amorphization resistance opens new prospects especially for the use of these oxides in nanostructured devices for nanoelectronics. Indeed, the nanopatterning step by focused ion beam can induce a loss or reduction in piezoelectric properties of some compounds such as Pb(Zr Tii J03 (PZT). Recently, in the case of crystallized La2Ti207 thin films with a Pt electrode on top, it has been shown that this step does not damage the electromechanical response of the material [91,92]. 

In order to interpret the eleetromechanical results, the performance of IPMCs is often reported alongside of a variety of characteristics such as tire capacitance of the actuator, current during the operation cycle, charge accumulated by the time of maximum displacement/blocking force, conductivity of the electrodes, viscoelasticity of the materials, etc. Finding out how all these parameters relate to the electromechanical response of IPMCs is a subject of ongoing research in the field of electroactive polymers. 

In any case, polyurethane dielectric elastomers have continued to be studied in the last decade, particularly with regard to the possibility of increasing their actuation performance. It is well known that both dielectric and mechanical properties are key parameters governing the electromechanical response of any dielectric elastomer, which can be in principle improved by an increase of the dielectric constant and by a decrease of the elastic modulus. In order to increase the dielectric permittivity of a polymer elastomeric matrix, various methods are available (Carpi et al. 2008), such as making composites or blends with highly polarizable phases. Table 1 constitutes a non-exhaustive list of works fi-om the literature, mostly relying on such methods for improving the performance of polyurethane dielectric elastomers. The studies are classified in terms of system complexity and component materials. 

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. 

Three-dimensional (3D) structuring of materials allows miniaturization of photonic devices, micro-(nano-)electromechanical systems (MEMS and NEMS), micro-total analysis systems (yu,-TAS), and other systems functioning on the micro- and nanoscale. Miniature photonic structures enable practical implementation of near-held manipulation, plasmonics, and photonic band-gap (PEG) materials, also known as photonic crystals (PhC) [1,2]. In micromechanics, fast response times are possible due to the small dimensions of moving parts. Femtoliter-level sensitivity of /x-TAS devices has been achieved due to minute volumes and cross-sections of channels and reaction chambers, in combination with high resolution and sensitivity of optical con-focal microscopy. Progress in all these areas relies on the 3D structuring of bulk and thin-fllm dielectrics, metals, and organic photosensitive materials. 

Scheme 2 Some possibilities for the pharmaceutical technologies and approaches to be used in personalized medicine, ranging from simple liquid oral dose forms where the dose can be varied by volume, through responsive systems, micro-electromechanical systems (MEMS), GPS-directed systems (see text) transdermal systems, thin film technologies with passive or active release mechanisms, combination tablet or capsule dose forms, and what we term dosed solid platforms, for example, aqueous dispersible polymer, solid gel or matrix material into which precise doses of drag can be absorbed.

Carbon nanotubes are responsible for spawning the age of nanotechnology, and creating a paradigm shift in our views on materials and their apphcations. In the field of MEMS and NEMS, CNTs have fonnd applications as electromechanical switches or as pliable nano-actuators, among others. The high elastic modnlns of... 

These materials have shown piezoelectric responses after appropriate poling [18]. Their piezoelectric actuation properties are typically worse than ceramic piezoelectric crystals however, they have the advantages of being lightweight, flexible, easily formed, and not brittle. Additionally, while ceramics are limited to strains on the order of 0.1%, ferroelectric polymers are capable of strains of 10% [91] and very high electromechanical coupling efficiencies [93]. 

Electrostriction, which is a change in sample dimensions in response to the application of an electric field to a dielectric, is a universal characteristic and provides another example of an electromechanical effect. Some materials get thinner while others get thicker in the direction of the electric field. This effect is not reversible and a deformation does not produce any polarisation. The effect is found in all materials, not just those that lack a centre of symmetry, including glasses and hquids. However, the electrostrictive effect is generally very small except for ferroelectric perovskites, especially relaxor ferroelectrics described in the following (Section 6.7). 

In any of the electromechanical devices, strong electric fields act on the permanent or induced dipoles present along the polymeric chains promoting coulombic interactions, forcing conformational movements on the polymeric chains and concomitant macroscopic changes of volume, which relax in the absence of the electric field. Similar coulombic interactions occur when a solvent and ions are present, giving electrokinetic (electroosmotic and electrophoretic) processes. So, electrostatic and mechanical models applied to polymeric materials are required to model the attained responses. No chemical reaction is required for the actuation of those devices. 

---