Electrostrictive Response Research Articles

Preparation and properties of polyurethane/MMT nanocomposite actuators

This study dealt with the electrostrictive response of a polyurethane (PU)/clay nanocomposite film, which was a promising candidate for a material to be used in polymer actuators. The nanocomposites were produced by using three types of montmorillonites (MMTs) such as natural MMT (Cloisite®Na+), hydrophobic MMT (Cloisite® 20A), and hydrophilic MMT (Cloisite® 30B). The nanometer-scale silicate layers of organo-clay were completely exfoliated in PU for the cases of 1, 3 and 5 wt% PU/MMT nanocomposites as confirmed by wide X-ray diffraction (WAXD) profiles. Actuation tests indicated that the displacement of PU/MMT nanocomposite actuator was larger than pure PU actuator, caused by an increase in dielectric constant. Especially, PU/MMT nanocomposite actuator with Cloisite® 30B had the largest displacement and it became possible to operate at low voltage.

Modification of the structure and properties of poly(vinylidene fluoride–trifluoroethylene) 56/44 mol % copolymer by a proton‐irradiation treatment

AbstractThe effects of high‐energy proton irradiation on the structure and properties of 56/44 mol % poly(vinylidene fluoride–trifluoroethylene) copolymer were studied with differential scanning calorimetry (DSC), X‐ray diffraction (XRD), relative permittivity, and polarization hysteresis measurements. Copolymer films prepared by hot compression molding were irradiated with a broad range of proton dosages (10–107 Mrad) at room temperature. The DSC results showed that the ferroelectric transition was strongly affected by the proton dosages. The XRD data indicated the reduction of polar ordering in the copolymer by the proton‐irradiation treatment. From the relative permittivity and polarization behavior, the copolymer film was found to be converted from a normal ferroelectric material to a relaxor ferroelectric material as the proton dosage was increased to 50 Mrad. The electrostrictive coefficient of the 56/44 mol % copolymer was enhanced after irradiation, and the optimized proton dosage for attaining the highest electrostrictive strain response was determined. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2334–2339, 2005

Influence of composition on relaxor ferroelectric and electromechanical properties of poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene)

We investigate the influence of the composition, especially the chlorofluoroethylene content, on the ferroelectric and electromechanical properties of the poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene) (PVDF-TrFE-CFE). It was found that increasing the CFE from 0to9mol% gradually converts the normal ferroelectric of the copolymer (in the compositions range of VDF/TrFE mole ratio between 64∕36 to 75∕25) to a relaxor ferroelectric, resulting in a nearly hysteresis free polarization loop and high electrostrictive response. On the other hand, increasing CFE content causes reduction in crystallinity, which will affect the elastic modulus and the induced polarization level of the polymer. These competing effects determine the desired terpolymer compositions for given applications. The electromechanical strain as a function of induced polarization and macropolar phase fraction was modeled by a modified electrostrictive relation, which closely matches experimental data. It is found that the electrostrictive coefficient is relatively constant across the compositions investigated.

Finite Element Implementation of a Thermodynamic Description of Piezoelectric Microstructures

A model and numerical framework is developed for piezoelectric materials. The model treats the piezoelectric and electrostrictive effects by incorporating orientation‐dependent, single‐crystal properties. The method is implemented in Object Oriented Finite Element program, a public domain finite element code, so it can be applied to arbitrary two‐dimensional microstructures with crystallographic anisotropy. The model is validated against analytic solutions. Consistency of the method for known cases permits application of the technique to more complicated two‐dimensional systems. The piezoelectric and electrostrictive response is determined for a few simple device geometries and provides insight for design and convergence criteria.

Structural and property changes in poly (vinylidene fluoride–trifluoroethylene) 70/30 mol % copolymer induced by proton irradiation

Poly(vinylidene fluoride–trifluoroethylene) 70/30 mol % copolymer has been irradiated with 3 MeV protons at doses ranging from 43 to 200 Mrad. The effects of irradiation on the polarization hysteresis, dielectric properties, lattice spacing, phase transition behavior and electric-field-induced strain have been studied. The irradiated copolymer exhibits the characteristic behavior of a relaxor ferroelectric, including frequency dispersion of the dielectric constant, which follows the Vogel–Fulcher rule. These results indicate that the proton irradiation breaks up the coherent polarization domains in the copolymer into nano-sized regions, thereby converting the copolymer to a relaxor ferroelectric. X-ray diffraction measurements show that the nano-sized regions are in the non-polar phase. Since the lattice spacing of the non-polar phase is substantially different from that of the polar phase, the local phase transformation between these two phases induced by an external electric field gives rise to a large lattice strain and hence a giant electrostrictive response.

Scanning Probes of Nonlinear Properties in Complex Materials

Nonlinear materials have found wide applications in electronic devices. Of special importance and interest are those with varistor-type grain boundaries and those displaying large piezoelectric and electrostrictive responses to external fields. To access the local properties of these materials, we have designed two new contact mode scanning probe techniques. The first technique referred to as nanoimpedance microscopy/spectroscopy, which incorporates impedance spectroscopy with a conducting AFM tip as an electrode, has been demonstrated on measuring local grain boundary properties of a ZnO varistor, as well as the contact quality between AFM tip and a ferroelectric sample. The other technique referred to as second harmonic piezoresponse force microscopy developed to measure the electrosctriction has been demonstrated on ferroelectric polymer thin film. A theoretical model has been presented to describe the contrast formation of the second harmonic as well as the first harmonic (piezoresponse) images and to explain the observed hysteretic field dependence of the second harmonic amplitude signal.

Electro-optical response of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer

The electro-optical (EO) properties of the ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer were investigated in the wavelength region from 3 to 5 μm. A large Kerr effect was observed in which a refractive index change of −2.6% can be induced under an electric field of 80 V/μm. When combined with the electrostrictive strain, the terpolymer film exhibits a total −5.6% optical path-length change under a field of 80 V/μm. Calculations based on density functional theory suggest that such a large EO effect was caused mainly by the reorientation of the C–F dipoles under external field in the crystalline region. Furthermore, the results support the notion that in the ferroelectric relaxor terpolymer, there exist nanopolar regions that undergo reorientation under external field and generate the observed electrostrictive strain and EO responses.

Study of the structure and electrostrictive property of proton-irradiated P(VDF-TrFE) 56/44 mol% copolymer

High-energy proton (3 MeV) irradiation with dosages ranging from 43 to 200 Mrad have been carried out to investigate the potential for modifying both the structure and property of vinylidene fluoride-trifluoroethylene 56/44 mol% copolymer. The structural and transitional behavior of the irradiated copolymer was studied by X-ray diffraction and differential scanning calorimetry. The polarization hysteresis, relative permittivity properties and electrostrictive strain response of these copolymers were also measured. It was found that the ferroelectric copolymer could be successfully converted to a relaxor at a low proton dosage of about 75 Mrad at ambient temperature. A slim polarization hysteresis loop and a frequency dispersion of the relative permittivity observed in the irradiated copolymer imply that the high-energy protons break up the coherent polarization domains in the ferroelectric copolymer into nanosized regions. In addition, the irradiation leads to a significant change in the ferroelectric-to-paraelectric phase transition behavior. X-ray diffraction measurements show that the crystalline region in the copolymer is converted into a nonpolar phase upon irradiation, and the lattice spacing increases significantly. The electric field induced phase transformation of the nanosized regions between the nonpolar and polar phase leads to a high electrostrictive strain observed in the irradiated copolymer.

Large electrostrictive actuation of barium titanate single crystals

An experimental investigation of the electromechanical behavior of single crystals of the ferroelectric perovskite barium titanate is presented. An experimental setup has been designed to investigate large strain actuation in single crystal ferroelectrics subjected to combined electrical and mechanical loading. Experiments have been performed on initially single domain crystals of barium titanate with (1 0 0) and (0 0 1) orientation at compressive stresses between 0 and 5 MPa . Global strain and polarization histories have been recorded. The electrostrictive response is shown to be highly dependent on the level of applied stress with a maximum strain of 0.9% measured at a compressive stress of about 2 MPa and electric field of about 10 kV/ cm . This level of strain is about 5 times higher than in typical commercial piezoelectric PZT. Polarized light microscopy has been used to observe the evolution of the domain pattern simultaneously with the strain and polarization measurement. The observations reveal that the observed large strain behavior is the result of 90° domain switching.

Dielectric Properties of Relaxor-like Vinylidene Fluoride−Trifluoroethylene-Based Electroactive Polymers

The dynamic processes in poly(vinylidene fluoride−trifluoroethylene) (P(VDF−TrFE)) copolymer, lead lanthanum zirconate titanate (PLZT)−P(VDF−TrFE) composite, electron-irradiated P(VDF−TrFE) copolymer, and poly(vinylidene fluoride−trifluoroethylene−chlorofluoroethylene) (P(VDF−TrFE−CFE)) terpolymer have been studied by measurements of the temperature and frequency-dependent linear (ε1) and third-order nonlinear (ε3) dielectric constants. While a paraelectric-to-ferroelectric transition takes place in the crystalline region of the nonirradiated copolymer and composite, electron-irradiated P(VDF−TrFE) copolymer and P(VDF−TrFE−CFE) terpolymer show a significantly different dielectric response: a broad frequency dispersion in ε1 and ε3, asymmetric temperature evolution of the relaxation spectrum as the longest relaxation time diverges at a finite freezing temperature while the shortest relaxation times remain active down to the lowest temperatures, and a paraelectric-to-glass crossover in the temperature dependence of the dielectric nonlinearity a3 = ε3/ε03 ε14. All these properties, together with the temperature dependence of the static field cooled dielectric constant, are very similar to those observed in the classical relaxor systems and are reminiscent of the dynamic behavior observed in various spin glasses. Thus, additional confirmation that giant electrostrictive response of the electron-irradiated P(VDF−TrFE) copolymer and P(VDF−TrFE−CFE) terpolymer is the consequence of their relaxor-like structure is obtained.

Geometry and Electronic Structure of PVDF/TrFE Copolymer

Restricted Hartree-Fock self-consistent field method was employed to study all-trans molecular chain of P(VDF-TrFE) copolymer. Defect and gauche bonds in electron doped chain of P(VDF-TrFE) were found in our calculations, which could cause the relaxor behavior in electron-irradiated P(VDF-TrFE) copolymer. The exceptionally high electrostrictive response of electron-irradiated P(VDF-TrFE) copolymer is discussed by comparing the density of state and energy levels of all-trans chain and electron doped chains.

Dielectric and Field-Induced Piezoelectric Responses in Electron-Irradiated Poly(Vinylidene Fluoride- Trifluoroethylene) Copolymer

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers were transformed into relaxor ferroelectric phase by high-energy electron irradiation. Temperature and frequency dependence of dielectric properties were investigated under dc bias field. The relaxor ferroelectric copolymers show high electrostrictive strain response (Q12 = 10 m4/C2) with an effective d31 = 180 pm/V at a field of 50 MV/m. Direct piezoelectric response of the relaxor ferroelectric copolymer can be tuned by dc bias, d31 is measured to be 116 pC/N at 40 MV/m bias field. Electromechanical coupling (k31 = 0.28 at 60 MV/m bias field) was significantly enhanced in the relaxor ferroelectric copolymers as compared to that in the normal ferroelectric P(VDF-TrFE) copolymers.

Sensing Shear Strains with Electrostriction Effect in Solid Electrorheological Composites

Solid electrorheological (ER) composites are created by applying an electric field to a liquid suspension of isotropic, homogeneously distributed inclusions and cross-linking the obtained transverse isotropic structure. Electrostriction phenomenon in ER composites can be defined as change in dielectric properties with deformation. Solid dielectric material, which consists of inclusions arranged in a chain-like structure and embedded in a matrix phase, demonstrates strong electrostriction response to normal and shear deformations. Such high sensitivity of ER composites to shear electrostriction can be utilized for sensing shear stresses and strains in active damping of vibrations. In this article, we model an electrostriction response and measure electromechanical properties of solid ER composites.

Electrostrictive response of an ideal polar rubber

This article considers the electrostrictive response of an ideal polar rubber in which each random link carries a dipole which is constrained to remain perpendicular to the link but is free to rotate about it. It thus mimics typical polar polymers with dipoles essentially perpendicular to the chain backbone and which rotate preferentially about the local chain axis (Stockmayer type B). In such materials, there is a genuine interplay between mechanical deformation and the electrical permittivity since an applied strain can produce preferential segmental orientation, align dipole rotation axes, and thus cause a change in permittivity. Though this is a plausible mechanism of electrostriction in polar rubbers, the quantitative analysis presented in this article suggests that the effect makes only a small contribution to the observed deformation of actuators based on typical polar rubber films with compliant electrodes.

Inverse Piezoelectric and Electrostrictive Response in Freely Suspended FLC Elastomer Film as Detected by Interferometric Measurements

We report the electric field induced thickness variations of homeotropically oriented free standing films of a smectic (C* or A*) FLCE prepared from cross - linkable ferroelectric polysiloxanes. The changes in optical path length in free standing ferroelectric liquid crystal elastomer films have been detected by means of interferometric measurements at both the first and second harmonic of the exciting electric field (ω=33 Hz). The measured electrostrictive strain is above 2.7% (in the thickness direction) at a electric field around 1.5 MV /m. Our experiment reveal that the inverse piezoelectric and electrostrictive response increases sharply near the Sm-C* - Sm-A* phase transition temperature. Also X-ray reflection measurements on a spin cast FLCE film reveal the constriction of smectic layers.

Inverse Piezoelectric and Electrostrictive Response in Freely Suspended FLC Elastomer Film as Detected by Interferometric Measurements

We report the electric field induced thickness variations of homeotropically oriented free standing films of a smectic (C* or A*) FLCE prepared from cross - linkable ferroelectric polysiloxanes. The changes in optical path length in free standing ferroelectric liquid crystal elastomer films have been detected by means of interferometric measurements at both the first and second harmonic of the exciting electric field (ω=33 Hz). The measured electrostrictive strain is above 2.7% (in the thickness direction) at a electric field around 1.5 MV /m. Our experiment reveal that the inverse piezoelectric and electrostrictive response increases sharply near the Sm-C* - Sm-A* phase transition temperature. Also X-ray reflection measurements on a spin cast FLCE film reveal the constriction of smectic layers.

Effects of Proton Irradiation on the Structure and Properties of Poly(Vinylidene Fluoride-Trifluoroethylene) 80/20 mol.% Copolymer

Films of poly(vinylidene fluoride-trifluoroethylene) 80/20 mol.% copolymer have been irradiated with 200 Mrad of 3 MeV protons. The effects of irradiation on the electric field induced strain, polarization hysteresis, dielectric constant, lattice spacing and phase transition behavior have been investigated. The irradiated copolymer exhibits a typical relaxor ferroelectric behavior, indicating that the proton irradiation breaks up the coherent polarization domains in the normal ferroelectric copolymer into nano-sized regions, thereby converting the material into a relaxor ferroelectric. The transformation of the nano-sized regions between nonpolar and polar phase induced by an external electric field gives rise to a large lattice strain and hence a high electrostrictive response.

Effects of Proton Irradiation on the Structure and Properties of Poly(Vinylidene Fluoride-Trifluoroethylene) 80/20 mol.% Copolymer

Films of poly(vinylidene fluoride-trifluoroethylene) 80/20 mol.% copolymer have been irradiated with 200 Mrad of 3 MeV protons. The effects of irradiation on the electric field induced strain, polarization hysteresis, dielectric constant, lattice spacing and phase transition behavior have been investigated. The irradiated copolymer exhibits a typical relaxor ferroelectric behavior, indicating that the proton irradiation breaks up the coherent polarization domains in the normal ferroelectric copolymer into nano-sized regions, thereby converting the material into a relaxor ferroelectric. The transformation of the nano-sized regions between nonpolar and polar phase induced by an external electric field gives rise to a large lattice strain and hence a high electrostrictive response.

Mesoscale Thin Film Actuator for Promoting Fluid Motion in Microfluidic and Nanofluidic Channels

ABSTRACTMicrofluidic and nanofluidic devices often require actuators to induce fluid motion for applications such as pumping and mixing in small channels. Mixing, for instance, is important in systems where channel or chamber dimensions are on the order of 100 μm or larger as diffusive mixing can be prohibitively slow at these dimensions. In this work, a new mesoscale thin film polymer electromechanical actuator is introduced for use in the aforementioned applications. Unlike inorganic piezoelectric actuators, the devices based on these materials will be relatively easy to fabricate involving no high temperature processing, crystal growth, or microlithography. Fabrication of an array of actuators is simply achieved by spin casting the polymer over top of lithographically patterned gold electrodes at a thickness of less than 50 nm. This simple process enables a microfluidic device based on these actuators to be an integral part of a microfluidic channel rather than a separate unit operation. Depending on the application, the actuator array can be designed and controlled for random perturbations of the fluid flow field as required for mixing or for systematic actuation as required for pumping. These thin-film mesoscale actuators have been characterized and show extremely favorable properties such as a high electrostrictive response (compared to none in the bulk) and a frequency response of up to 50 kHz. In addition, finite element simulations show feasibility of these actuators for use in microfluidic mixing applications.

Highly electrostrictive poly(vinylidene fluoride–trifluoroethylene) networks

We report greatly improved electromechanical performance from a poly(vinylidene fluoride–trifluoroethylene) copolymer by chemical cross linking. Curing with an organic peroxide in combination with a free-radical trap, followed by crystallization, yields a ferroelectric network exhibiting a high electrostrictive response. The electric-field-induced strains were measured at low frequency using an air gap capacitance method and by a noninterferometric optical technique. At electric fields as low as 9 MV/m, longitudinal strains of 12% are obtained. This is more than two orders of magnitude better than conventional vinylidene fluoride materials, and more than an order of magnitude larger than the best results reported to date on any fluoropolymer. Since the mechanical modulus of the network is high (∼0.5 GPa), this work demonstrates the potential for extending the range of utility for polymer-based electromechanical materials.