Electromechanical effects

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. 

The piezoelectric effect is an electromechanical effect in which mechanical evoke and reverse an electric reaction in a ferroelectric material and vice versa. The word piezo has been derived from the Greek piezein which means press . Compounds are composed of positive and negative ions and are electrically neutral as a whole. The fact that electrically charged particles are still present in the crystal can for example be demonstrated by means of the electric... 

Since the open-circuit voltage is developed, the electromechanical effects take place at constant D. Therefore, from... 

In Sect. 7.3, Eqs. (18) and (19) describe the Maxwell stress forces acting on a conductive tip when a combined d.c./a.c. voltage is applied. For the PFM set-up we have to complete the total interaction force by the additional effects of piezoelectricity, electrostriction and the spontaneous polarisation. Both electromechanical effects cause an electric field-induced thickness variation and modulate the tip position. The spontaneous polarisation causes surface charges and changes the Maxwell stress force. If the voltage U(t)=U[)c+UAc sin((Ot) is applied, the resulting total force Ftotai(z) consists of three components (see also Eq. 19) Fstatic, F(0 and F2m. Fstatic is the static cantilever deflection which is kept constant by the feedback loop. F2a contains additional information on electrostriction and Maxwell stress and will not be considered in detail here (for details see, e.g. [476]). The relevant component for PFM is F(0 [476, 477] ... 

Lehmaim W, Hartmann L, Kremer F, Stein P, Finkelmann H, Kruth H, Diele S (1999) Direct and inverse electromechanical effect in ferroelectric liquid crystalline elastomers. J Appl Phys 86 1647... 

The properties described in the previous section apply to networks exhibiting helical structures. This type of mesophase is obtained by introducing side mesogenic substituents with chiral end groups on the backbone [94, 138-141, 145-147], or by doping an achiral network with a chiral molecule [144,145]. As a result of the interaction between orientational (liquid-crystalline) and translational (network) degrees of freedom, these noncentrosymmetric systems exhibit rich electromechanical effects, such as piezoelectricity. 

While the first paper on liquid crystalline elastomers [30] already reports the detection of a cholesteric-isotropic transition using differential calorimetry and polarizing microscopy, comparatively little work has been done to characterize thy physical properties in the vicinity of this phase transition (compare, however, also the discussion of electromechanical effects in the next section) [9, 30, 31]. Combined liquid crystalline elastomers have been synthesized and various of these materials show a cholesteric-isotropic transition using X-ray scattering, polarizing microscopy and differential scanning calorimetry [31]. Dynamic mechanical investigations have been carried... 

Various electromechanical effects in cholesteric and chiral SmC phases were presented including a discussion of how to distinguish piezoelectricity from flexoelec-tricity and electrostriction [77, 78]. It was examined for cholesteric structures in general for which structures one can get longitudinal piezoelectricity [79]. 

Yusuf Y, Huh JH, Cladis PE, Brand HR, Finkelmann H, Kai S. 2005. Low voltage driven electromechanical effects of swollen liquid crystal elastomers. Phy Rev E 71 061702. 

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). 

N.V. Madhusudana, R. Pratibha and H.P. Padmini, Electromechanical effect in cholesteric liquid crystals with fixed boundary conditions. Mol. Cryst. Liq. Cryst. 202(1), 35-49, (1991). doi 10.1080/00268949108035658 H.P. Padmini and N.V. Madhusudana, Electromechanical effect in cholesteric mixtures with a compensation temperature, Liq. Cryst. 14(2), 497-511, (1993). doi 10.1080/02678299308027665... 

Induced-charge and second-kind electrokinetic phenomena arise due to electrohydrodynamic effects in the electric double layer, but the term nonlinear electrokinetic phenomena is also sometimes used more broadly to include any fluid or particle motion, which depends nonlinearly on an applied electric field, fit the classical effect of dielectrophoresis mentioned above, electrostatic stresses on a polarized dielectric particle in a dielectric liquid cause dielectro-phoretic motion of particles and cells along the gradient of the field intensity (oc VE ). In electrothermal effects, an electric field induces bulk temperature gradients by Joule heating, which in turn cause gradients in the permittivity and conductivity that couple to the field to drive nonlinear flows, e.g., via Maxwell stresses oc E Ve. In cases of flexible solids and emulsions, there can also be nonlinear electromechanical effects coupling the... 

Eber, N., Bata, L., Scherowsky, G., and Schliwa, A., Linear electromechanical effect in a polymeric ferroelectric liquid crystal, Ferroelectrics, 122, 139-147 (1991). Kozlovsky, M. V., Darius, M., and Haa.se, W., Frustrated pha.se behaviour of a chiral side chain polymer, European Polymer Journal, (in press). 

This pyroelectric effect can be utilized for sensors such as e.g. infrared cameras. However, in sensors that use the electromechanical effect p3To-electricity can be disturbing. The disturbing effects arise especially in low-frequency or quasi-static applications as the temperature drift is often a slow process. A one ohm resistance is applied in parallel to suppress this effect. That way, the pyroelectric induced charges are deflected and the cut-off frequency of the sensor is raised. 

A novel electromechanical effect has been observed in FLCs [130]. A periodic shear flow occurred parallel to the bounding plates and perpendicular to the helical axis (FLC layers were perpendicular to the substrates). The frequency of oscillation of the shear flow was equal to that of the applied field and the amplitude was proportional to the field strength. The electromechanical effect in FLCs seems to have many common features with a backflow effect in nematic liquid crystals, as it is caused by the coupling between the Goldstone mode and flow [131]. 

The coupling between the properties of conventional polymer networks and the properties of chiral liquid crystalline phases results in interesting, new opto- and electromechanical effects of the chiral liquid crystalline elastomers, as demonstrated by theoretical considerations and experiments. Knowledge about these new materials is still in its infancy. But the properties analyzed so far for these elastomers indicate promising aspects for application and are the basis for the new syntheses of optimized chiral liquid crystal networks. 

The ionic actuators with carbonaceous electrodes are considered, as a rule, as being non-Faradaic, i.e., the electromechanical effects are governed by the electrochemical double-layer buildup at the electrode-electrolyte boundary (Kosidlo et al. 2013). The absence of Faradaic charge-transfer reactions, which can deteriorate the actuator s performance in the long run, can be assured by not exceeding the electrochemical stability window (ESW) of the electrolyte. 

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