We analyze charge/discharge currents in insulators in order to identify them (1) as charge injection/extraction currents, (2) as shift of internal charges, or (3) as relaxational polarization currents. General properties of the different kind of currents are highlighted. Experiments are carried out at the example of PVDF copolymers. For the experiments P(VDF-TrFE) films of 1100 nm thickness are prepared by a spin technique. Using evaporated Al electrodes a sequence of two positive pulses is applied with electrical field below and above coercive field, charge times of 25 ms to 25 s and discharge times of 25 s. Charging as well as discharging currents are measured. For fields smaller than the coercive field and times $\boldsymbol{t}<\mathbf{100 ms}$ charge and discharge currents follow a power law $\boldsymbol{j} \sim \boldsymbol{t}^{-\boldsymbol{\alpha}}$ having the same magnitude, but opposite directions, although they take place at different field strengths, i.e. $\boldsymbol{E}=\mathbf{3 6} \mathbf{V} / \boldsymbol{\mu} \mathbf{m}$ and $\boldsymbol{E}=\mathbf{0}$. For fields above the coercive field an additional current indicating ferroelectric polarization is observed in the first positive pulse, but not in the discharge processes and not in the second positive pulse, as expected for the development of a ferroelectric polarization. A variation of the length of the charging pulse has no influence on the discharge currents at short times. (1)In case of excess carriers injected from the electrodes the carriers cause an internal field. After the external field is switched off this internal field has a zero-field plane. The fields at both sides point in different directions and the carriers drift to both electrodes. The short circuit current is always smaller than the current with external field, in contrast to the experiments. (2)Two scenarios are possible for quasi free carriers drifting in the insulator. When they can move freely to the electrodes a constant current instead of a decreasing power law would be observed. In case of a blocking layer, e.g. due to a thin oxide layer on the electrodes, charges accumulate. The longer the field was applied, the more charges are collected, and the higher should be the discharge currents directly after removal of the field, which is again in contrast to the experimental results. (3)For a polarization process of permanent dipoles fluctuating in double well potentials the thermal energy is the driving force. The number of dipoles shifted from well 1 to well 2 depends on the applied field. When the field is switched off only the shifted dipoles can return. Their speed of turn and return is ruled by the temperature and not by the field. Therefore, charge and discharge currents have the same magnitude, as observed in the experiment. The charge/discharge currents in P(VDF-TrFE) are identified as relaxational polarization currents.

Polymer–semiconductor hybrid materials or composites have been investigated with respect to their microstructure, optical, photoconductive, and ferroelectric properties. For this purpose, either CdSe quantum dots or (Cd:Zn)S microparticles were dispersed in poly(vinylidenefluoride-trifluoroethylene) solution and hot pressed to films. In both material systems, the electrical conductivity and the polarization behavior could be controlled by the intensity of the optical excitation. The simultaneous high optical transparency of the CdSe quantum-dot-based hybrid materials makes them particularly interesting for applications in the field of flexible, high-resolution sensors.

The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures.

The potential suitability of P(VDF-TrFE-CFE) relaxor-ferroelectric terpolymer films for non-volatile memory devices is assessed from the $\varepsilon_{2}/(3\varepsilon_{0}^{2}\varepsilon_{1}^{3})$ ratio that is directly proportional to the remanent polarization. Non-linear dielectric spectroscopy was employed to obtain $\varepsilon_{2}/(3\varepsilon_{0}^{2}\varepsilon_{1}^{3})$ ratios vs. time or temperature. For comparison, dielectric hysteresis loops were measured in a Sawyer-Tower circuit. Optimum poling temperatures and polarization stabilities of non-annealed and annealed terpolymer samples were also determined. The results indicate that annealed samples show definite polarization values only when poled at low temperatures close to their respective glass transition. On the other hand, non-annealed samples allow poling at ambient temperature due to their higher content of the ferroelectric crystalline phase. Both, non-annealed and annealed terpolymer films show stable polarization values for several minutes after field removal – indicating their possible use at least in short-term memory devices.

Non-linear dielectric spectroscopy (NLDS) is employed as an effective tool to study relaxation processes and phase transitions of a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) relaxor-ferroelectric (R-F) terpolymer in detail. Measurements of the non-linear dielectric permittivity $${\varepsilon _{2}^{'}}$$ reveal peaks at 30 and 80 $$\,^\circ$$ C that cannot be identified in conventional dielectric spectroscopy. By combining the results from NLDS experiments with those from other techniques such as thermally stimulated depolarization and dielectric-hysteresis studies, it is possible to explain the processes behind the additional peaks. The former peak, which is associated with the mid-temperature transition, is found in all other vinylidene fluoride-based polymers and may help to understand the non-zero $$\varepsilon _\mathrm {2}^{'}$$ values that are detected on the paraelectric phase of the terpolymer. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 $$\,^\circ$$ C and is due to conduction and space-charge polarization as a result of the accumulation of real charges at the electrode–sample interface.

Most previous attempts on achieving electric-field manipulation of ferromagnetism in complex oxides, such as La0.66Sr0.33MnO3 (LSMO), are based on electrostatically induced charge carrier changes through high-k dielectrics or ferroelectrics. Here, the use of a ferroelectric copolymer, polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE)], as a gate dielectric to successfully modulate the ferromagnetism of the LSMO thin film in a field-effect device geometry is demonstrated. Specifically, through the application of low-voltage pulse chains inadequate to switch the electric dipoles of the copolymer, enhanced tunability of the oxide magnetic response is obtained, compared to that induced by ferroelectric polarization. Such observations have been attributed to electric field-induced oxygen vacancy accumulation/ depletion in the LSMO layer upon the application of pulse chains, which is supported by surface-sensitive-characterization techniques, including X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. These techniques not only unveil the electrochemical nature of the mechanism but also establish a direct correlation between the oxygen vacancies created and subsequent changes to the valence states of Mn ions in LSMO. These demonstrations based on the pulsing strategy can be a viable route equally applicable to other functional oxides for the construction of electric field-controlled magnetic devices.

Over the last half century, the existence of an additional thermal transition in between the glass transition and the Curie/melting transition has been frequently observed on vinylidenefluoride-based ferro-, pyro- and piezoelectric homo- and co-polymers. The transition has also been observed recently in some of the related relaxor-ferroelectric terpolymers. Despite its well-known existence and the rich history of its treatment in the literature, the origin(s) and a more or less complete picture of the mid-temperature transition have remained elusive until now. Over the years, several authors have put forth various explanations for the so-called mid-temperature transition - some complementary and some contradictory to each other. At the 17th IEEE International Symposium on Electrets (ISE-17) in Limerick, Ireland, in September 2019, the mysterious mid-temperature transition and its possible mechanism(s) became the subject of a panel discussion a) to mark the Golden Jubilee of the discovery of piezoelectricity in polyvinylidenefluoride (PVDF) by Heiji Kawai of Kobayashi Institute of Physical Research, Japan, as well as the Centennial of the first recognition of ferroelectricity in piezoelectric Seignette's or Rochelle salt. The panel put forward a new hypothesis that the mid-temperature transition is most likely a result of several interrelated processes that take place within the respective temperature range. The relevant processes include an upper glass transition or relaxation, a relaxation related to conformational disorder, possible imperfect/time-dependent structures formed as a result of thermal processing and secondary crystallization, as well as interface polarization effects at crystalline-amorphous boundaries. The article captures the essence of the panel discussion and the perspectives obtained therefrom to elucidate the complex mid-temperature transition in vinylidenefluoride-based ferro-, pyro- and piezoelectric homo-, co- and ter-polymers.

Electrical parameters of a symmetrical hot carrier solar cell double heterostructure with metallic absorber layer and high-pass energy filters are calculated within thermionic emission theory. An efficiency limit of 73% is predicted for infinite characteristic cooling time and full solar concentration. Carrier cooling is treated within linear thermal conductivity model. Heat loss through carrier cooling in the absorber layer depends not only on the characteristic cooling time, but also on the specific volumetric heat capacity of the carrier gas and on the thickness of the absorber layer. Optimal absorption of light together with small thermal loss to the lattice require high carrier mobility in the absorber layer.

The influence of optical excitation intensity on the electrical, ferroelectric and pyroelectric properties of ferroelectric-semiconductor-composites was investigated. For this purpose, composite thin films consisting of poly(vinylidene fluoride-co-trifluoroethylene) and 10 vol % (Cd:Zn)S particles with a thickness of 34 µm were fabricated. The samples were used to measure the absolute pyrocoefficient and to determine the relative pyroelectric depth profile using Laser Intensity Modulated Method. It was shown that a polarization of the samples without an optical excitation at the utilized relatively small peak-to-peak voltages could not be verified by the Sawyer–Tower circuit and the measurement setup of the pyroelectric coefficient, respectively. Both remanent polarization and pyroelectric coefficients increased with increasing optical excitation intensity during poling as well as increasing peak-to-peak voltage. The pyrocoefficient shows a temporal decay in the first hours after poling. The specific heat and thermal conductivity or the thermal diffusivity are required for the calibration of the pyroelectric depth profile. Rule of mixture and photo-acoustic investigations proved that the thermal properties of the utilized composites do not differ significantly from those of the pristine polymer. Based on the pyroelectric depth profile which is proportional to the polarization profile, the existing “three phase model” has been extended to generate a replacement circuit diagram, explaining the local polarization due to the optical excitation dependency for both local resistivity and local field strength.

The influence of semiconductor particle concentration and photoexcitation on the electrical and ferroelectric properties of ferroelectric-semiconductor-composites was investigated. For this purpose, 32 µm thin films of poly(vinylidene fluoride-co-trifluoroethylene) with (Cd:Zn)S particle concentrations of between 0 and 20 vol % were fabricated and characterized by scanning electron microscopy, Fourier transformed infrared spectroscopy, X-ray diffraction, and optical spectroscopy. It was shown that the particle concentration has only a negligible influence on the molecular structure of the polymer but strongly determines the optical properties of the composite. For (Cd:Zn)S particle concentrations below 20 vol %, the I-V characteristics of the composites is only marginally affected by the particle concentration and the optical excitation of the composite material. On the contrary, a strong influence of both parameters on the ferro- and pyroelectric properties of the composite films was observed. For particle fractions that exhibit ferroelectric hysteresis, an increased remanent polarization and pyroelectric coefficient due to optical excitation was obtained. A theoretical approach that is based on a "three phase model" of the internal structure was developed to explain the observed results.

We investigated the tunability of the transport and magnetic properties in 7.5 nm La0.7Sr0.3MnO3 (LSMO) epitaxial films in a field effect geometry with the ferroelectric copolymer P(VDF-TrFE) as the gate insulator. Two different switching behaviors were observed upon application of gate voltages with either high or low magnitudes. The application of single voltage pulses of alternating polarity with an amplitude high enough to switch the remanent polarization of the ferroelectric copolymer led to a 15% change of the resistance of the LSMO channel at temperature 300 K (but less than 1% change at 20 K). A minimal shift of the peak in the resistance-temperature plot was observed, implying that the Curie temperature T-C of the manganite layer is not changed. Alternatively, the application of a chain of low voltage pulses was found to shift T-C by more than 16 K, and a change of the channel resistance by a 45% was obtained. We attribute this effect to the field-assisted injection and removal of oxygen vacancies in the LSMO layer, which can occur across the thickness of the oxide film. By controlling the oxygen migration, the low-field switching route offers a simple method for modulating the electric and magnetic properties of manganite films. (C) 2017 Elsevier Ltd. All rights reserved.

Time dependent distributions of mobile ions in a sample material with blocking electrodes resulting from sinusoidal external electric fields have been simulated. Harmonic components appearing in the resulting time dependent electric displacement have been used to compute linear and nonlinear permittivity spectra.

A numerical procedure is introduced to calculate the profiles of ion densities and electric fields in polymer electrolytes with blocking electrodes. For low electric potentials the numerical results are in agreement with Jaffé's approximate analytical solutions. In contrast to the analytical solution the numerical procedure is also suitable for high electric potentials at which the relation between dielectric polarisation and electric potential becomes strongly nonlinear. Furthermore, the temporal development of ion distributions can be modelled after the application of a voltage step or under other transient voltages.

Rare-earth (RE) (Eu3+, Gd3+, Tb3+, and Dy3+)-doped BiFeO3 (BFO) ceramics were prepared by a modified solid-state reaction method, which adopted higher heating as well as cooling rates during sintering process. All the fabricated samples showed ferroelectric hysteresis loops with a remnant polarization of 21-35 mu C/cm(2). A piezoelectric coefficient (d(33)) of similar to 48 pC/N was obtained and this value was showed to be composition independent. The pyroelectric properties of our samples were studied as a function of temperature. Generally, the pyroelectric coefficient slightly decreased with temperature, and this is attributed to the increase of electrical conduction at higher temperatures. Among the different doped BFO ceramics, Gd-doped samples exhibited the largest pyroelectric coefficient of 146 mu C/m(2)K at room temperature. For the magnetic properties, slim hysteresis loop with remnant magnetizations of 0.016-0.044 emu/g were obtained in all the doped samples. Our results revealed that the RE-doped BFO ceramics posses an improvement in both the electrical and magnetic properties. On the basis of our studies, we demonstrate that RE-doped BFO is a potential candidate for magnetoelectric device applications.

Nanocomposites of a ferroelectric matrix polymer with dispersed ferroelectric ceramic nanoparticles can be polarized to be piezo-or pyroelectric alone at a given temperature, due to the different origin of piezoelectricity in polymers (dipole density) and ceramics (intrinsic). With a two-step poling procedure, which allows selective poling of the ceramic inclusion and the ferroelectric polymer, bifunctionality is achieved in the same material. The selective poling of filler and matrix phase is proved by ferroelectric hysteresis measurements at room temperature. Such bifunctional materials may be interesting for artificial skin, sensitive to changes in pressure and temperature.

The electric polarization in polyethylene oxide com-plexed with LiClO4 has been investigated over the frequency range from 10 mHz to 10 kHz. The exciting electric fields have been chosen so high that nonlinear components in the dielectric response are evident. Nonlinear dielectric and conductive spectra are determined and their temperature dependence is studied. It is found that the nonlinear relaxation shows a temperature dependence similar to the linear relaxation. With rising temperature the phenomena shift to higher frequencies and the relaxation strength increases. To evaluate relaxation frequencies and strengths theoretical functions are fitted to the experimental data. While the linear spectra are reproduced well over the whole frequency range, a reasonable agreement between experimental data and theoretical curves is found in a limited frequency range only for the nonlinear spectra.

BiFeO3 (BFO) ceramics were prepared by a modified solid-state-reaction method which adopts a higher heating/cooling rate during the sintering process than usually used. It was found that the calcination temperature T cal (from 400 to 750°C) does not influence the BFO phase formation, while the sintering temperature T sin (from 815 to 845°C) dominates the phase purity. The optimum sintering temperature was in the range from 825 to 835°C. The optimized samples exhibit saturated ferroelectric hysteresis loops with a remnant polarization of 13.2 μC/cm2. The measured piezoelectric coefficient d 33 was 45 pC/N. No remnant magnetization was observed in all of the samples. The pyroelectric properties were studied as a function of temperature and frequency. A pyroelectric coefficient as high as 90 μC/m2 K was obtained at room temperature in the optimized sample. An abrupt decrease of the pyroelectric coefficient was observed at temperatures between 70 and 80°C. On the basis of our results, BFO may have the potential for pyroelectric applications.

A monolithically integrated bifunctional frontplane is introduced to large area electronics. The bifunctional frontplane element is based on a composite foil of piezoelectric ceramic lead titanate nanoparticles embedded in a ferroelectric poly(vinylidene fluoride trifluoroethylene) polymer matrix. Bifunctionality to pressure and temperature changes is achieved by a sequential, area selective two-step poling process, where the polarization directions in the nanoparticles and the ferroelectric polymer are adjusted independently. Thereby, sensor elements that are only piezoelectric or only pyroelectric are achieved. The frontplane foil is overlaid on a thin-film transistor backplane. Our work constitutes a step toward multifunctional frontplanes for large area electronic surfaces.

A substantial improvement in the performance of pyroelectric 0-3 composites of ceramic particles in a polymer matrix has been achieved by doping the polymer matrix material. Readily prepared and polarized films with various volume fractions of lead zirconate-titanate (PZT) particles in polyurethane have been doped in a solution of lithium perchlorate in acetone to increase the conductivity. With an appropriate conductivity, the dielectric permittivities of the ceramic particles and the polymer matrix become matched, resulting in an improvement of the pyroelectric coefficient from about 6 mu C/(m(2)K) to about 50 mu C/(m(2)K). The experimental results are explained by theoretical predictions.

We have derived a general formula for the prediction of the effective pyroelectric coefficient of composites. In this relation, the pyroelectric coefficient of the composite is written as a function of the pyroelectric coefficients and dielectric constants of the constituents and the effective dielectric constant of the composite material. In contrast to other formulas in the field, shape and spatial arrangement of the components and the volume ratio are not directly involved in our relation. The influence of these is completely included in the effective dielectric constant of the composite. Therefore, the new relation gives a correct description for all kinds of connectivities and provides an easy tool for the optimization of pyroelectric composites.