VDF TrFE Research Articles

Oriented polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) films by Langmuir-Blodgett deposition: a synchrotron X-ray diffraction study.

Langmuir-Blodgett films of polyvinylidene fluoride trifluoroethylene - P(VDF-TrFE)-copolymers possess substantially improved electrocaloric and pyroelectric properties, when compared with conventionally spin-cast films. In order to rationalize this, we prepared single-layered films of P(VDF-TrFE) (70 : 30) using both deposition techniques. Grazing incidence wide-angle X-ray scattering (GIWAXS), reveals that Langmuir-Blodgett deposited films have a higher concentration of the ferroelectric β-phase crystals, and that these films are highly oriented with respect to the substrate. Based on these observations, we suggest alternative means of deposition, which may substantially enhance the electrocaloric effect in P(VDF-TrFE) films. This development has significant implications for the potential use of P(VDF-TrFE) in solid-state refrigeration.

Asymmetric polarization and hysteresis behaviour in ferroelectric P(VDF-TrFE) (76 : 24) copolymer thin films spatially resolved via LIMM.

The concept of charge-polarization coupling in the ferroelectric copolymer PVDF-TrFE (poly[vinylidene fluoride-co-trifluoroethylene]) has been revisited by employing high resolution (<50 nm) pyroelectric depth profiling (LIMM). By virtue of probing the pyroelectric activity over the film thickness in the presence of a variable, external electrical field, the local ferroelectric response and spatially resolved hysteresis could be obtained. A large asymmetry was found between the positive and negative electrode-polymer interface. Differences in amplitude of polarization, contribution of interface charges and length scale of the polarization-free zone near the electrodes suggest a higher availability of compensation charges at the negative electrode that finally stabilize the local polarization. The resulting asymmetries in the polarization distribution are hence attributed to excess charges as a result of electron injection as a specific charge generation mechanism at the negative electrode.

Impact of structural changes on dielectric and thermal properties of vinylidene fluoride–trifluoroethylene-based terpolymer/copolymer blends

We report dielectric and thermal properties of the poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) terpolymer [P(VDF–TrFE–CFE), a member of the relaxor polymer family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect] blended with the ferroelectric poly(vinylidene fluoride–trifluoroethylene) copolymer, P(VDF–TrFE). Although the differential scanning calorimetry (DSC) clearly reveals that both components form separate crystalline phases, at low copolymer content blends entirely exhibit a relaxorlike linear dielectric response, since the interfacial couplings to the bulky defects in the terpolymer convert the normal ferroelectric copolymer into a relaxor. On the other hand, dielectric experiments evidence that in blends with 20–50wt% of P(VDF–TrFE) the ferroelectric and relaxor states coexist. This coexistence is confirmed by DSC results, which further reveal the influence of blending on the terpolymer crystallinity and melting point. At last, the crystallinity data appropriately explain the variation of the dielectric constant in P(VDF–TrFE–CFE)/P(VDF–TrFE) blends.

A study toward the development of an electromechanical poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) buckling membrane actuator

Buckling membrane micro-actuators based on relaxor ferroelectric polymer poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) (P(VDF–TrFE–CFE)) were fabricated and actuated near their resonant frequency. The devices produced displacements of several microns under applied biases of ⩽60 V μm−1. In this buckling actuator configuration the electrodes are located only at the membrane’s circumference and do not coat the entire membrane. Besides for preserving the optical transparency of the polymer, the critical advantage of this configuration is that the unique electro-mechanical properties of the polymer can be exploited without constriction of displacements by the metal electrodes which have a comparatively higher Young’s modulus.Electrical characterization of P(VDF–TrFE–CFE) thin films was performed through the use of micro-capacitor test structures. Dielectric properties were examined as a function of temperature (25–115 °C), frequency (6 kHz–2 MHz) and processing conditions, most notably thermoplastic nano-imprint lithography (T-NIL).

Nanostructured piezoelectric polymers

ABSTRACTAmong the wide variety of piezoelectric materials available, polymers offer an interesting solution because of their high mechanical flexibility, easy processing, and conformable features; they maintain good ferroelectric and piezoelectric properties. The most prominent examples of these are poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene difluoride–trifluoroethylene) [P(VDF–TrFE)]. An attractive prospective consists of the preparation of nanostructured polymers. It has been shown that the dimensional confinement of such macromolecules down to the nanoscale can improve their piezoelectric properties because the tailoring of the chemical structure is performed at the molecular level. In this review, we show how nanostructured polymers can be obtained and discuss reports on the ferroelectric and piezoelectric properties of nanostructured PVDF and P(VDF–TrFE) materials. In particular, we show how dimensional confinement leads to piezoelectric nanostructures with relevant performances, with a focus on the macromolecular structural arrangement that enhances their behavior. Experimental results and applications are also reported to compare the performances of different nanostructuration processes and the polymer efficiencies as piezoelectric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41667.

Interface morphology driven control of electrical properties of P(VDF–TrFE) and PMMA blend M–I–M capacitors

In this manuscript, we report on the role of interfacial morphology between poly(methyl methacrylate) or PMMA and poly(vinylidenedifluoride–trifluoroethylene) or P(VDF–TrFE) thin films upon improvement in the ferroelectric and leakage characteristics of the P(VDF–TrFE)–PMMA devices. Whilst keeping the overall thickness of 200nm, PMMA is incorporated in different proportions along with P(VDF–TrFE) in three different configurations: in the form of blend, and two bilayer structures, one with PMMA films below P(VDF–TrFE) films and another with PMMA films on top of P(VDF–TrFE) films. In the blend form, addition of PMMA decreases the crystallinity of P(VDF–TrFE) resulting in deterioration of the ferroelectric properties of the blend before complete disappearance at 30% PMMA when the blend becomes amorphous. In the bilayer structures, whilst overall polarization of the bilayer structure decreased in comparison to a 200nm P(VDF–TrFE) only device, ferroelectric polarization of P(VDF–TrFE) in the bilayer was larger than P(VDF–TrFE) only device for the similar voltage drop. This is attributed to the development of morphology in the bilayer structure. Importantly, PMMA incorporation leads to nearly two orders of magnitude reduction in the electrical leakage rendering bilayer devices more suitable for device applications than P(VDF–TrFE) only device.

Alternating current-driven, white field-induced polymer electroluminescent devices with high power efficiency

A solution-processed, all-phosphor, three-color (i.e., blue, green, and red), alternating current-driven white field-induced polymer electroluminescent device (WFIPEL), with low operational voltage, high luminance, high efficiency, high color-rendering index (CRI), and excellent color-stability, was demonstrated. The devices employed poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) [P(VDF–TrFE–CFE)] dielectric modified by single-walled carbon nanotubes (SWNTs) to further improve the dielectric characteristics, as the insulating layer. This significantly lowers the driving voltage of the device. Moreover, hole-generation layer and electron-transporting layer with high conductivity were used to more efficiently form and confine excitons in the emissive layer. The resulting WFIPEL devices show significant improvements in performance as compared to previous reports. Specifically, the devices exhibit a low turn-on voltage of 10V, a maximum luminance of 7210cdm−2, a maximum current efficiency and power efficiency of 33.8cdA−1 and 10.5lmW−1, and a CRI of 82. The power efficiency is even 10 times higher than the highest previous report (1lmW−1).

The field and temperature dependence of hysteresis loops in P(VDF–TrFE) copolymer films

The ferroelectric hysteresis loops of poly(vinylidene fluoride/trifluoethylene) [P(VDF–TrFE)] copolymer films are investigated as a function of external field and temperature. Starting from a plateau at the low frequency side the coercive field increases with increasing frequency. It exhibits a maximum in the kHz-range and decreases then. The remanent polarization is almost constant at low frequencies and decreases above the kHz-range. For a constant frequency, the coercive field increases with increasing the amplitude of the external field. Furthermore, the hysteresis loops at different temperatures at a given frequency and amplitude exhibit a linear decrease of coercive field with increasing temperature. A double hysteresis loop is observed close to the Curie point and a sharp jump of the remanent polarization is obtained in samples as thick as 600nm indicating a first order phase transition. For samples as thin as 90nm, the double hysteresis loop of the polarization is absent and the change of remanent polarization dependent on temperature is smoother. The above experimental results can be explained and simulated in the Weiss mean field model. The simulation results are compared with the experiments and show a good consistency.

Inkjet printed micropump actuator based on piezoelectric polymers: Device performance and morphology studies

All inkjet printed piezoelectric actuators based on poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF–TrFE)) for applications as pump actuators in microfluidic lab-on-a-chip systems (LOC) are manufactured and investigated in terms of their morphology and actuator performance. Furthermore, a pump demonstrator with an all-printed P(VDF–TrFE) actuator is characterized here for the first time. The actuators are manufactured in a fully additive and flexible way by successive inkjet printing of a P(VDF–TrFE) film sandwiched between two silver electrodes on a polyethylene terephthalate (PET) substrate. Different from most current micropumps where actuator elements are fabricated separately, no additional joining step is required in the manufacturing approach employed here. Actuator performance is investigated by measurements of piezoelectric d31 coefficients as well as remanent polarization Prem for different thermal treatments of the as-printed P(VDF–TrFE) films. A strong dependence of the device performance on the annealing temperature is found with maximum values for d31 and Prem of approximately 10pmV−1 and 5.8μCcm−2, respectively. Morphology investigations of the printed films by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) indicate an increased degree of crystallinity of the piezoelectric β-phase for samples annealed at temperatures above 110°C, which coincides with improved device performance. A basic pumping function with pump rates of up to 130μLmin−1 is demonstrated, which is promising for future applications in LOC. Furthermore, the process chain and characterization presented here can be employed to design and manufacture also other P(VDF–TrFE)-based devices and allows the combination with additional printed on-chip functionalities in future LOC.

Ferroelectric properties of vinylidene fluoride–trifluoroethylene copolymer thin films fabricated on amorphous alloy electrode

We have investigated the structure and ferroelectric properties of vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymer thin films fabricated on amorphous alloy electrodes. Thin films of amorphous Pd–Cu–Si alloys with various compositions were successfully prepared by sputtering deposition. An atomically flat surface resulting from a fully amorphous structure of Pd–Cu–Si was obtained upon achieving a uniform surface of a spin-coated VDF–TrFE copolymer thin film. X-ray diffraction (XRD) measurements revealed that the crystalline structure of the VDF–TrFE copolymer thin films was of ferroelectric β-phase, being independent of the composition and crystalline state of the Pd–Cu–Si alloy. The clearly observed D–E hysteresis loops showed a remanent polarization of 0.075 C/m2 and a coercive field of 90 MV/m at a measurement frequency of 10 Hz for a 50-nm-thick film, which is almost consistent with the results obtained with Pt electrode samples. We also observed the thinning-induced reduction of remanent polarization, which was explained by the depolarization field induced by the surface dead layer in VDF–TrFE copolymer rather than the oxidized layer in Pd–Cu–Si alloy electrode.

Ferroelectric characteristics of MFIS structure with P(VDF–TrFE)/BaTiO3 nanocomposite as ferroelectric layer

A metal–ferroelectric–insulator–semiconductor (MFIS) structure has been made using poly(vinylidene difluoride–trifluoroethylene)/barium titanate [P(VDF–TrFE)/BaTiO3] nanocomposite as ferroelectric layer, on silicon/silicon dioxide (Si/SiO2) substrate. Different concentrations of BaTiO3 were added to P(VDF–TrFE) polymer using bath sonication method, and the films were prepared using spin coating method. The structure was annealed to 120 °C for 2 h and then the top aluminium electrode was deposited by thermal evaporation method. Capacitance–voltage shows an increase in accumulation capacitance as the BaTiO3 nanoparticle concentrations increases. Dielectric constant was estimated from the capacitance voltage (C–V) characteristics and found to be changing as the concentration of BaTiO3 is varied. Polarization–electric field analyses show hysteresis behaviour of the nanocomposite. A comparison of MFIS and metal–ferroelectric–semiconductor structures was done with varying ferroelectric film thicknesses. All these results suggest that this polymer nanocomposite can be a promising material which can be used in non-volatile memory devices.

Combination of electrostrictive polymers composites and electrets for energy harvesting capability

More recently, the development of electrostrictive polymers has generated novel opportunities for high‐strain actuators. At present, the investigation of using electrostrictive polymers for energy harvesting is beginning to show potential for this application. Basically, the relative energy gain depends on the current induced by the mechanical strain and frequency. The aim of the present experimental work is to study the composites on the basis of terpolymer P(VDF–TrFE–CFE) filled with low concentrations of copper (Cu) powders. The scanning electron microscopy was performed essentially to verify the dispersion of the fillers within the polymeric matrix. The obtained results showed a relatively homogeneous dispersion for the microsized fillers and the existence of agglomerates for their nanosized counterparts. On the other hand, our experimental data show that the harvested power as well as the current is significantly important for nano‐Cu fillers with respect to micro‐Cu fillers by a factor of 45.9% in the case of the hybridization of electrostrictive polymers and electrets. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effect of polymer electrode thickness on the acoustical properties of all-screen printed piezoelectric PVDF copolymer transducers

High frequency poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymer ultrasonic transducers were prepared by screen printing as an all-printed device with conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) used the electrode material. A large number of prototypes with different electrode areas and electrode thickness were obtained by printing single or dual layers of the conductive polymer. The printed transducers were characterized using impedance and acoustic backscattering measurements. Effects imposed by the PEDOT:PSS conductivity were also evaluated both through experiments and finite element method (FEM) modeling, pointing out important limitations of conductive polymers when applied in high frequency ultrasonic transducers.

Influencing dielectric properties of relaxor polymer system by blending vinylidene fluoride–trifluoroethylene-based terpolymer with a ferroelectric copolymer

We report the influence of blending the poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) terpolymer [P(VDF–TrFE–CFE), a member of the relaxor polymer family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect] with the ferroelectric poly(vinylidene fluoride–trifluoroethylene) copolymer [P(VDF–TrFE)] on its dielectric response. Although both components form separate crystalline phases, at low copolymer content, the P(VDF–TrFE–CFE)/P(VDF–TrFE) blends entirely exhibit a relaxorlike linear dielectric response, since the interfacial couplings to the bulky defects in the terpolymer convert the normal ferroelectric copolymer into a relaxor. On the other hand, the linear and particularly nonlinear dielectric experiments, i.e., temperature dependences of the second and the third harmonic dielectric response, clearly evidence that in blends with 20–50 wt. % of P(VDF–TrFE), the ferroelectric and relaxor states coexist. The nonlinear dielectric response further reveals the onset of ferroelectric behavior also in blends with low copolymer amount, due to a high VDF content in the terpolymer, which increases the ferroelectric interactions: While in relaxor polymers with lower VDF content, the third order nonlinear dielectric constant, in accordance with the theoretical predictions, exhibits solely positive values, here it changes sign even in the pure P(VDF–TrFE–CFE).

Design and modeling of a fluid-based micro-scale electrocaloric refrigeration system

A refrigeration system composed of silicon MEMS cooling elements is designed based on the electrocaloric (EC) effect in a P(VDF–TrFE–CFE) terpolymer, poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) 59.2/33.6/7.2mol%. Each cooling element includes two diaphragm actuators fabricated in the plane of a silicon wafer, which drive a heat transfer fluid back and forth across terpolymer layers that are placed between them. In the EC effect, reversible temperature and entropy changes related to polarization changes appear in a material under the application and removal of an electric field. Finite element simulations are performed to explore the system performance. The effect of the applied electric field is studied, and the time lag between the electric field and the diaphragm motion is found to significantly affect the cooling power. A parametric study of the operating frequency, externally-applied temperature span, and the electric field amplitude are conducted. The results indicate that when the system is operated at a temperature span of 15K, a cooling power density of 3W/cm2 and a percent of Carnot COP of 31% are achieved for one element.

Analysis of carrier behavior in α-NPD/P(VDF–TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation

The effect of ferroelectric polymer on carrier behaviors in indium–tin oxide (ITO)/poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)]/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/Au structure was investigated. The internal electric field in α-NPD introduced by the dipole moments from ferroelectric P(VDF–TrFE) together with the α-NPD/P(VDF–TrFE) interface accumulated charges plays an important role in the carrier motion in α-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to α-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in α-NPD layer was directly probed and gave a support that only part of the dipoles in P(VDF–TrFE) was polarized. We proposed that the formation of an amorphous insulating layer between amorphous α-NPD layer and partially crystallized P(VDF–TrFE) layer markedly decreased the voltage directly applied on the P(VDF–TrFE) film and caused a reduced polarization. Our findings here will be helpful in analyzing the carrier behavior in organic electronic devices using ferroelectric layers.

Changing molecular band offsets in polymer blends of (P3HT/P(VDF–TrFE)) poly(3-hexylthiophene) and poly(vinylidene fluoride with trifluoroethylene) due to ferroelectric poling

Photoelectron emission and inverse photoemission spectroscopy studies of polymer blends of poly(vinylidene fluoride (70%) – trifluoroethylene (30%)) P(VDF–TrFE 70 : 30) and regio-regular poly(3-hexylthiophene) (P3HT) provide evidence of changes in the molecular band offsets as a result of changes in the ferroelectric polarization in P(VDF–TrFE). Investigation of the blends with higher concentrations of the semiconducting P3HT component revealed that the organic semiconductor component of the blend dominates the electronic structure in the vicinity of the chemical potential. Specifically, the states of P3HT at the conduction band minimum and valence band maximum fall within the HOMO–LUMO gap of the dielectric ferroelectric P(VDF–TrFE) but the P3HT does not exhibit a change in the molecular band offset with respect to the Fermi level or band bending with polarization reversal, unlike P(VDF–TrFE).

Cooling rate controlled microstructure evolution and reduced coercivity in P(VDF–TrFE) devices for memory applications

We report on the tunability of ferroelectric properties of Poly(vinylidenedifluoride–trifluoroethylene) P(VDF–TrFE) thin films by controlling the cooling rate during transformation from high temperature paraelectric α-phase to low temperature ferroelectric β-phase. A faster cooling rate of P(VDF–TrFE) thin films leads to an increased polarization by 30% and much decreased coercivity by 60%. The origin of these improvements in the ferroelectric characteristics is attributed to evolution of a favorable microstructure and crystallographic alignment leading to (110) oriented films that are cooled faster. The microstructure of the films changes from a fine fibrous structure at fast cooling rate to a flatter ripple containing structure in the slow cooled samples. This dramatic change in the microstructure is attributed to the combination of incorporation of large stresses arising from almost 50% change in the molar volume of P(VDF–TrFE) upon α→β transformation and the cooling rate assisted stress relaxation, nucleation and growth. Infrared spectroscopy further showed that the substantial improvement in the device performance of the fast cooled samples arises from a favorable alignment of C–F dipoles due to short and ordered fibers lying on the substrate plane whose orientation becomes more random as the cooling rate is decreased.

A nanocomposite approach to tailor electrocaloric effect in ferroelectric polymer

The electrocaloric effect (ECE) is enhanced in ferroelectric relaxor terpolymer poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) (P(VDF–TrFE–CFE))/ZrO2 nanocomposites. It was observed that the interface effects between the polymer matrix and nano-fillers enhance the polarization response and provide additional electrocaloric entropy changes. As a consequence, the nanocomposites exhibit a larger ECE than that of the neat terpolymer, i.e., the adiabatic temperature change of the nanocomposite with 3 volume percent of nano-fillers is 120% of that of the neat terpolymer. The results, for the first time, demonstrate that ECE can be tailored and enhanced through nanocomposite approach in the ferroelectric polymers.

Electrical and thermal properties of vinylidene fluoride–trifluoroethylene-based polymer system with coexisting ferroelectric and relaxor states

Dielectric, thermal, and electrocaloric investigations of poly(vinylidene fluoride–trifluoroethylene) P(VDF–TrFE) copolymer, irradiated with high-energy electrons, are reported. While the ferroelectric copolymer is transformed into a relaxor system at high irradiation doses, dielectric investigations, particularly nonlinear dielectric experiments, i.e., temperature dependences of the second and the third harmonic dielectric response, clearly evidence that at lower doses ferroelectric and relaxor states coexist in the P(VDF–TrFE). This is confirmed by the differential scanning calorimetry, which further reveals the influence of irradiation on the copolymer crystallinity and melting point. Finally, it is shown that large electrocaloric response of VDF–TrFE-based polymers is further enhanced in systems with coexisting relaxor and normal ferroelectric states.