Relaxor Ferroelectric Polymer Research Articles

Comparison of directly and indirectly measured electrocaloric effect in relaxor ferroelectric polymers

We report the directly measured electrocaloric effect (ECE) (the adiabatic temperature change ΔT) of relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and its blend with poly(vinylidene fluoride-chlorotrifluoroethylene). The results show that the directly measured ΔT in the relaxor terpolymer is much larger than that deduced from Maxwell relation and that the relaxor terpolymer possesses a giant ECE at room temperature. The large difference between the directly measured ΔT and that deduced indicates that the Maxwell relation, which is derived for ergodic systems, is not suitable for deducing ECE in the relaxor ferroelectric polymers, which are nonergodic (polar-glass) material systems.

Electrocaloric effect in the relaxor ferroelectric polymer composition P(VDF–TrFE–CFE)0.90–P(VDF–CTFE)0.10

The electrocaloric effect (ECE), i.e., the change of the temperature of the electrocaloric material due to the applied electrical field under adiabatic conditions, is a very attractive phenomenon used in a broad range of applications, for example in an electrical refrigerator. The first observations of a giant ECE were made by indirect measurements of the electrical polarization and not by direct measurements of the ECE itself. In our case, this effect was measured directly. Here a method and analysis of these direct measurements are presented and the obtained results are shown for P(VDF–TrFE–CFE) relaxor terpolymers blend with 10% composition of P(VDF–CTFE) copolymers.

Relaxor fluorinated polymers: novel applications and recent developments

It has been found that by introducing defects into the P(VDF-TrFE) copolymers, it is possible to convert the polymer from a normal ferroelectric to a relaxor ferroelectric. A new class of ferroelectric polymers, i.e., the terpolymers of P(VDF-TrFE-CFE) or of P(VDF-TrFE- CTFE), was developed from the normal ferroelectric PVDF-TrFE polymer by employing proper defect modifications which eliminate detrimental effects associated with a normal first order F-P transition while maintaining high material responses. Relevant studies show that this class of electroactive polymers offers unique properties in comparison with other ferroelectric polymers. The syntheses of these relaxor ferroelectric polymers have been done by a combination of the suspension polymerization process and an oxygen-activated initiator at a temperature of 40 °C. Films from cast solution can be made in different length and thicknesses. Stretching of these films increases the performance as well as the mechanical properties. These relaxor-ferroelectric terpolymers P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) are multifunctional i.e. electrostrictive material, dielectric for electric energy storage. The terpolymer exhibits high electrostrictive strain (>7%) with relatively high modulus (>0.4GPa) and high electrocalorific effect. Dielectric polymers with high dipole density have the potential to achieve very high energy density. Examples of devices applications using unimorphe systems are presented. Micropump and Optical device concerning a liquid-filled varifocal lens on a chip are described.

Thermally mediated multiferroic composites for the magnetoelectric materials

A magnetoelectric (ME) composite through thermal mediation is presented, which is different from the traditional strain/stress mediated ME composites. The ME laminate uses the large magnetocaloric effect, that is, a temperature change induced in the ferromagnetic Gd crystal by a magnetic field, and a large pyroelectric response in the relaxor ferroelectric polymer. Consequently, a simple laminate composite can produce a ME response ∼0.5 V/(cm Oe). The ME coefficient was further enhanced to ∼0.9 V/(cm Oe) by exploiting the magnetic flux concentration effect. The approach opens up an avenue in developing ME materials for broad range of applications.

Dielectric relaxation of relaxor ferroelectric P(VDF-TrFE-CFE) terpolymer over broad frequency range

Dielectric properties of a relaxor ferroelectric polymer, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer, were investigated over a broad range of frequency (from 0.1 kHz to 1 GHz) and a broad range of temperature (-20 degrees C to 76 degrees C). Time-temperature superposition was used to extrapolate the dielectric constant to high frequencies (approximately 1 GHz) from low frequency data (1 MHz). The consistency between the directly measured and the extrapolated data indicate that the time-temperature superposition can be applied at temperature ranging from the glass transition to the broad ferroelectric-paraelectric transition peak of relaxor, indicating that the glass transition is still the dominating relaxation process at room temperature for the ferroelectric relaxor. Compared with the dielectric properties of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE] copolymer, the terpolymer shows a higher dielectric constant even at 1 GHz, which is considered to originate from the random defects modification converting the long-chain polar-molecular conformation to short-range molecular microstructures and enhancing the molecular motions in both polar and nonpolar nanodomains.

Large Electrocaloric Effect in Ferroelectric Polymers Near Room Temperature

Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70 degrees C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12 degrees C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

Relaxor Ferroelectric Polymers–Fundamentals and Applications

We present recent studies in the fundamentals and applications of the relaxor ferroelectric polymers, i.e., Poly(vinylidene fluoride-trifluroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE) terpolymers and high energy electron irradiated P(VDF-TrFE) copolymers. We show that the dynamic processes in these relaxor ferroelectric polymers are very similar to these observed in various polar-glasses. We further show that the large and reversible polarization change in these polymers leads to giant electrostriction and large electro-optic effect. By employing active electric boundary conditions, we demonstrate that the large electromechanical responses in these electroactive polymers can be made use of effectively for energy harvesting with high harvested electric energy density and efficiency. Terpolymer composites with ZnS nanoparticles provide an advanced E-O polymer which refractive index can be varied widely by small amount of ZnS in the composites while maintaining the large E-O effect of the matrix.

Relaxor Ferroelectric Polymers, Thin Film Devices, and Ink-Jet Microprinting for Thin Film Device Fabrication

The relaxor ferroelectric polymer, based on poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer, exhibits high electromechanical response and dielectric response, suitable for a wide range of army applications such as micro-actuators, high efficiency pulse power capacitors, and solid state cooling devices. Recent experimental results show that with a simple blends approach, the terpolymer glass transition temperature can be raised quite markedly, resulting in a significant increase in the elastic modulus and consequently the elastic energy density, while the polarization response and electrostrictive strain remaining nearly the same. Utilizing the high electromechanical response, thin film microactuators and micropumps were fabricated, which showed very high microactuator displacement and force capability in comparison with the traditional piezoceramic based microdevices. Ink-jet microprinting technique was investigated for the fabrication of thin film devices and with a proper control of the solution conditions and surface chemistry, thin films with feature size down to less than 30 μ m can be fabricated.

Relaxor Ferroelectric Polymers

The relaxor ferroelectric polymers, i.e., the poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] based terpolymers and high energy electron irradiated copolymers, provide unique opportunity to investigate the ferroelectric relaxor behavior in polymeric materials. Experimental evidence shows that there is fundamental difference in the microstructures between the relaxor terpolymer and relaxor irradiated copolymer even though both exhibit similar macroscopic ferroelectric relaxor response.

Finite difference simulations of permittivity and electric field statistics in ceramic-polymer composites for capacitor applications

Finite difference quasielectrostatic modeling is used to predict the complex dielectric permittivity of barium titanium oxide (BTO)—polymer composites of interest for capacitor applications. The simulations explore the effects of the microstructural arrangement of spherical ceramic particles, the volume filling fraction of ceramic, and the type of polymer on the composite permittivity. For composites with randomly positioned ceramic particles, a soft percolation regime is found between volume filling fractions of 0.35 and 0.5 that leads to a more gradual growth in permittivity compared to ordered arrangements of particles. For BTO dispersed in a representative relaxor ferroelectric polymer, dielectric constants as high as 300 are predicted at a filling fraction of 0.45. Electric field statistics inside the composites are also computed, and localized intensification factors in the range of three to eight times the applied field are predicted, with an incrementally linear growth in high-field probability with increasing filling fraction in the soft percolation regime. The deleterious effect of air voids on composite permittivity is also explored.

Microstructure and Dielectric Properties of P(VDF−TrFE−CFE) with Partially Grafted Copper Phthalocyanine Oligomer

A new class of nanocomposites in which the high dielectric constant copper phthalocyanine oligomer (o-CuPc) was partially grafted to the poly(vinylidene fluoride−trifluoroethylene−chlorofluoroethylene) (P(VDF−TrFE−CFE)), a relaxor ferroelectric polymer, was developed. The new approach developed allowed for higher degree of grafting of o-CuPc to the polymer backbone and resulted in a nanocomposite [P(VDF−TrFE−CFE)-g-CuPc/CuPc] with o-CuPc inclusion size at about 60−100 nm. Compared with simple blend of the terpolymer and o-CuPc, a lower dielectric loss and higher breakdown field were achieved in the grafted nanocomposite which can be attributed to a more uniform distribution of o-CuPc particles in the polymer matrix as well as much reduced inclusion size. Moreover, the dielectric constant of the grafted nanocomposite at frequencies above 1 kHz is much higher than that of the simple blend, indicating an increased interface effect such as the exchange coupling which can enhance the dielectric response, as su...

Tunable electric-field-induced piezoelectricity in high strain relaxor ferroelectricP(VDF-TrFE) copolymer

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer (50% ofVDF) was transformed into the relaxor ferroelectric state by high-energy electron irradiation.The field-induced piezoelectric response in the relaxor P(VDF-TrFE) copolymer wasobtained by measuring the electrostrain induced by a small ac voltage superimposed on adc bias voltage. It was shown that the effective piezoelectric coefficient in the relaxorferroelectric copolymer, which was enhanced as compared to the unirradiated ferroelectriccopolymer, could be tuned by dc biasing field. The effective piezoelectric coefficientd31 initially increased with the dc biasing field and reached a value of84 pm V−1 at30 MV m−1;afterwards, d31 began to decrease with the biasing field and had a value of52 pm V−1 at50 MV m−1. The mechanism for the tunable field-induced piezoelectricity is discussed based on thehigh-field electromechanical coupling behaviour in the relaxor ferroelectric polymer.

Piezoelectric polymers

ABSTRACTThe science and technology of piezoelectric polymers has long been dominated by ferroelectric polymers from the polyvinylidene fluoride (PVDF) family. The piezoelectricity in this polymer class arises from the strong molecular dipoles within the polymer chain and from the resulting change of the dipole density upon application of a mechanical stimulus. Ferroelectric polymers show moderate piezoelectric coefficients (d33 and d31,32 around 20-30 pC/N) in comparison to ceramic piezoelectrics, with an acoustic impedance comparable to that of water. The thermal stability of the piezoelectric effect is limited to below 100°C, though stability up to 125°C has recently been announced. Applications of ferroelectric polymers emerged in many niches. A good example of a success story for PVDF applications are clamp-on transducers used as pressure sensor for Diesel injection lines, with selling numbers over 50 million pieces per year. A relatively new development are relaxor ferroelectric polymers, based on electron-irradiated poly(vinylidene fluoride) trifluoroethylene copolymers or on terpolymers of vinylidene fluoride, trifluoroethylene and chlorofluoroethylene. Relaxor ferroelectric copolymers exhibit strong electrostriction and thus large piezoelectric coefficients, when used under electric dc-bias fields. Internally charged cellular polymer foam electrets (ferroelectrets) resemble close similarities to ferroelectrics. They display large intrinsic piezoelectric d33-coefficients well above 100 pC/N and very small d31 and d32 coefficients, coupled with a limited thermal stability up to 50°C in the polypropylene workhorse material. The materials are pioneered in Finland and already entered the market in niches, for example in musical pick-ups. They promise large area applications, for example in surveillance and intruder systems. Finally, organic semiconductors have shown a rather unusual electromechanical response, governed by a power law S=V3/2 of strain S versus voltage V, located in between traditional piezoelectricity and electrostriction. The field of piezoelectric polymers therefore received new stimulus, and the material class of piezoelectric polymers has been significantly broadened recently.

Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers: High-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) copolymers

The dielectric behavior of the high-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) 68/32 mol % copolymer was characterized in the dose range from 0 to 175 Mrad. It was found that as the dose increases, the copolymer evolves from a normal ferroelectric to a relaxor ferroelectric and then to a simple relaxor. Correspondingly, the crystalline morphology changes from a two-phase coexistence between a polar and nonpolar phases at low doses (<50 Mrad) to a macroscopically uniform nonpolar phase for doses beyond 50 Mrad, as revealed by x-ray data. Interestingly, it was observed that in the whole dose range, the Vogel–Fulcher (V–F) relationship could describe the measured relationship between the frequency and the peak temperature of dielectric constant well. For the copolymers irradiated with dose near 75 Mrad, which show a typical relaxor ferroelectric response, the V–F behavior is a result of freezing of the relaxation time (into polar glass state). For the copolymers irradiated with doses lower that 50 Mrad where a polar phase and nonpolar phase coexist, the dielectric data at near the ferroelectric-paraelectric temperature show a dielectric peak with V–F type relaxation followed by a transition from a relaxor ferroelectric and normal ferroelectric as the temperature is lowered, suggesting a possible freezing process even in this dose range.

High-dielectric-constant ceramic-powder polymer composites

A ceramic-powder polymer composite, making use of a relaxor ferroelectric polymer that has a high room-temperature dielectric constant as the matrix, is developed. The experimental data show that the dielectric constant of the composites with Pb(Mg1/3Nb2/3)O3–PbTiO3 powders can reach more than 250 with weak temperature dependence. In addition, the composites under a proper preparation procedure exhibit a high breakdown field strength (>120 MV/m), leading to a maximum energy storage density of more than 15 J/cm3. Experimental results also indicate that the high electron irradiation does not have much effect on the dielectric behavior of Pb(Mg1/3Nb2/3)O3–PbTiO3 powders, possibly due to the relaxor nature of the ceramic.

High Dielectric Constant Polymer Ceramic Composites

AbstractA new polymer-ceramic composite, using the newly developed relaxor ferroelectric polymer that has a high room temperature dielectric constant as the matrix, is reported. Different kinds of ceramic powders were studied and homogeneous composite thin films (20μm) were fabricated. It was observed that the increase of the dielectric constant of the composites with the ceramic content could be described quite well by the expression developed by Yamada et al., when the ceramic content is below 60% by volume. The experimental data shows that the relative dielectric constant of composites using PMN-PT powders can reach more than 250 with weak temperature dependence (i.e., the dielectric constant changes little in a broad temperature range). In addition to high permittivity, the composite prepared in clean environment also has high breakdown field strength (120MV/m), which yields an energy storage density more than 14J/cm3. The dielectric behavior of the composite at various frequencies was also studied and the results show that the material is promising for high frequency applications.