Electrostriction, ferroelectrics

Most of the inorganic and polymeric actuators present in current technological world are based on volume variations produced by electric fields. Engineers and physicists are familiar wifli fliose physical and mafliematical models developed during the past century to describe piezoelectric, electrostrictive, ferroelectric, coulombic, or electroosmotic actuators. Some of those models have been translated to try to describe actuators based on conducting polymers, driven by charge, not by fields. 

Zhang QM, Pan WY, Jang SJ, Cross LE (1988) The pressure dependence of the dielectric response and its relation to the electrostriction. Ferroelectrics 88 147-154... 

Piezoelectric and Electrostrictive Device Applications. Devices made from ferroelectric materials utilizing their piezoelectric or electrostrictive properties range from gas igniters to ultrasonic cleaners (or welders) (72). 

Due to their high piezoelectric response, electrostriction in ferroelectrics, induced by an applied electric field, can be used as strain-inducing components (just as ferromagnetic materials can be exploited for their magnetostriction). Thus barium... 

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

Ferroelectrics Poly(vynidilene fluoride) undergoes electrostriction when subjected to high ac fields, thus can be made into actuators applied pressure produces a piezoelectric response useful in sensors. 

Ferroelectric materials above their Curie point behave electrostrictively and comparison of the electrostriction coefficient with d j2eP shows them to be of similar magnitude. This suggests that the large -coefficients shown by some ferroelectric materials are due to a combination of large electrostriction coefficients and large spontaneous polarization and permittivity values. 

Electrostrictive materials offer important advantages over piezoelectric ceramics in actuator applications. They do not contain domains (of the usual ferroelectric type), and so return to their original dimensions immediately a field is reduced to zero, and they do not age. Figure 6.24(a) shows the strain-electric field characteristic for a PLZT (7/62/38) piezoelectric and Fig. 6.24(b) the absence of significant hysteresis in a PMN (0.9Pb(Mg1/3Nb2/303-0.1 PbTi03) electrostrictive ceramic. 

Shrout, T.R. et al. (1987) Grain size dependence of dielectric and electrostriction (properties) of Pb(Mgi/3Nb2/3)03 -based ceramics, Ferroelectrics, 76, 479-87. 

In Eq. (20) the three terms are related to the Maxwell stress (first), piezoelectric effect (second) and electrostriction (third). In order to obtain information about ferroelectricity via piezoresponse measurements, we need a link between the spontaneous polarisation and the piezoelectric constant. According to Furukawa and Damjanovic, piezoelectricity in ferroelectrics can be explained as electrostriction biased by the spontaneous polarisation if their paraelectric phase is nonpolar and centrosymmetric [461, 495, 496]. Therefore the d33 constant depends on the spontaneous polarisation P5 ... 

The ferroelectric Pb(Mgy3Nb2/3)03 (PMN) ceramic has been the snbject of extensive investigations due to its high dielectric coefficient and high electrostrictive coefficient, which renders it suitable for use in capacitors and electrostrictive actuators. However, the successful exploitation of this material is limited by the difficulty of producing a single-phase material with the perovskite structnre. Conventional solid state synthesis techniques invariably resnlt in the formation of one or more pyrochlore phases, which exhibit poor dielectric properties. 

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

EAPs can be broadly divided into two categories based on their method of actuation ionic and field-activated. Further subdivision based on their actuation mechanism and the type of material involved is also possible. Ionic polymer-metal composites, ionic gels, carbon nanotubes, and conductive polymers fall under the ionic classification. Ferroelectric polymers, polymer electrets, electrostrictive polymers, and dielectric elastomers fall under the electronic classification. 

Recent advances in PVDF-based materials have led to the elimination of the hysteretic behavior characteristic of ferroelectrics. For this reason, these PVDF-based materials are classified as relaxor ferroelectric polymers they will be discussed under the Electrostrictive Polymers heading. 

Zhang QM, Bharti V, Zhao X (1998) Giant electrostriction and relaxor ferroelectric bahavior in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Science 280 2101... 

Guo S, Zhao XZ, Zhuo Q, Chan HLW, Choy CL (2004) High electrostriction and relaxor ferroelectric behavior in proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Appl Phys Lett 84 3349... 

Lehmann W, Skupin H, ToUtsdorf C, Gebhard E, Zentel R, Kruger P, Losche M, Kremer F (2001) Giant lateral electrostriction in ferroelectric liquid-crystalhne elastomers. Nature 410 447... 

Liu Y, Ren KL, Hofmann HF, Zhang Q (2005) Investigation of electrostrictive polymers for energy harvesting. IEEE Trans Ultrason Ferroelectr Freq Control 52(12) 2411-2417... 

Interface polarization Dipole stretching Ferroelectric hysteresis Electric domain wall resonance Electrostriction Kezoelectricity Nuclear magnetic resonance Ferromagnetic resonance Ferrimagnetic resonance... 

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). 

Usually, artificial muscle based on electrostrictive, piezoelectric, electrostatic, or ferroelectric materials have been manufactured as a film of the dry polymer, both sides coated with a thin metallic film required to apply the electric field. Electrokinetic artificial muscles [5,6] are constituted by films of polymeric gel (polymer, solvent, and salt) and two electrodes, located as close as possible to the material or coating both on sides, which are required to apply the electric field that drives the electroosmotic process. Any of the actuators described in this paragraph has a triple layer structure metal-electroactive polymer-metal (Figure 16.2). 

The presence of water and ions in electrostrictive, piezoelectric, or ferroelectric polymers produces an overlapping of both actuation processes electromechanical and electrokinetics. The water also generates a problem the high applied-potentials (from several volts to thousand of volts) in an electrolytic media... 

Considering the existence of volume changes, Baughman et al. suggested the possibility for using conducting polymers as basic materials for the construction of actuators, similar to those developed from piezoelectric, electrostrictive, or ferroelectric inorganic actuators [53,54]. 

It follows from Eq. (3.20), that the second term in the brackets represents the contribution of stress Um and is independent on film thickness, while the thickness dependence of transition temperature is described by the third term originating from surface contribution, polarization gradient and depolarization field. Keeping in mind that for perovskite structure, which is characteristic for SrTiOs and KTaOs, the electrostriction coefficient Qu < 0, one can see, that 7/(/) increases for compressive misfit strain < 0 as well as for Xs < 0. The dependence of 7 (/) calculated on the basis ofEq. (3.20) for KTaOs is reported in Fig. 3.2. It follows from the Fig. 3.2, that Tf depends essentially on Xs value. The second term in the brackets of Eq. (3.20) is independent of the film thickness and determines the shift of phase transition temperature (approximately 50 K) at / -> 00. Note that only this contribution has been taken into account in Ref. [17]. With a film thickness decrease the third term contribution prevails so that in sufficiently thin films (/ < 50 nm) the appearance of ferroelectric phase at T < 100 K can be expected. The appearance of ferroelectricity in the films of several nm thicknesses even at room temperature (see Fig. 3.2) cannot also be excluded. 

A piezoelectric solid (e.g., quartz) acquires an electrical dipole moment upon mechanical deformation and, conversely, if it is subjected to an electric field E it becomes distorted by an amount proportional to the field strength E. The dipole moment disappears without the mechanical force. Piezoelectricity is only possible in lattices that do not have an inversion center. Electrostriction is also mechanical distortion in an electric field (strain proportional to E ) but ionic lattices that have a center of symmetry also show this effect. Figure 4.25 is a schematic representation of the source of these effects using the interatomic potential curve. A ferroelectric material is not only piezoelectric but its lattice has a permanent electric dipole moment (below its Curie temperature), which most other piezoelectric materials (such as quartz) do not have. 

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