High Dielectric Response Research Articles

Normal ferroelectric to ferroelectric relaxor conversion in fluorinated polymers and the relaxor dynamics

To elucidate the molecular origin of the polarization dynamics in the ferroelectric relaxor poly(vinylidene fluoride—trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer, a broadband dielectric study was carried out in the frequency range from 0.01 Hz to 10 MHz and temperatures from −150?C to 120?C for the terpolymer and a normal ferroelectric P(VDF-TrFE) copolymer. The relaxation processes were also studied using dynamic mechanical analysis. It was shown that in the terpolymer, which was completely converted to a ferroelectric relaxor, there is no sign of the relaxation process associated with the ferroelectric-paraelectric transition which occurs in the P(VDF-TrFE) copolymer. In the copolymer, three additional relaxation processes have been observed. It was found that the relaxation process β a, which was commonly believed to be associated with the glass transition in the amorphous phase, in fact, contains significant contribution from chain segment motions such as domain boundary motions in the crystalline region. In the temperature range studied, the terpolymer exhibits the latter three relaxation processes with the one (termed β r) near the temperature range of β a significantly enhanced. This is consistent with the observation that in conversion from the normal ferroelectric to a ferroelectric relaxor, the macro-polar domains are replaced by nano-polar-clusters and the boundary motions as well as the reorientation of these nano-clusters generate the high dielectric response. The experimental data also reveal a broad relaxation time distribution related for the β r process whose distribution width increases with reduced temperature, reflecting the molecular level heterogeneity in the crystalline phase due to the random introduction of the CFE monomer in the otherwise ordered macro-polar domains. The random interaction among the nano-clusters as well as the presence of the random fields produces ferroelectric relaxor behavior in the terpolymer.

Polarization of High‐Permittivity Dielectric NiO‐Based Ceramics

Polarization and dielectric behavior of Li and Ti co‐doped NiO (LTNO) ceramics was investigated. Analysis of the ceramic microstructure and composition indicate that an obvious grain boundary was formed and surrounded the grains, and Ti ions distributed in grains and grain boundaries. The concentration of Ti had a remarkable effect on the dielectric properties of the LTNO ceramics. Positron annihilation spectra measurements implied that a large amount of defects exist in the LTNO ceramic samples. Anomaly polarization–electric field hysteresis loops were observed in the high‐permittivity dielectric LTNO ceramics, which are totally different from those for ferroelectric materials and are due to defect‐dipoles induced, and electrical conduction effects of samples could also have an influence on the hysteresis loops. The high dielectric constant response of the LTNO ceramics is enhanced partially by the boundary layer capacitor mechanism, and partially by the huge polarization of a large amount of defect dipoles.

Enhanced tunability and phase transition studies in compositionally varying lead magnesium niobate–lead titanate multilayered thin films

Multilayers of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 thin films were prepared with a compositional variation of PbTiO3 on each layer across the film by pulsed laser ablation technique. The fabricated films were found to possess an enhanced diffusiveness in the dielectric phase transition, which actually seemed to be an envelope of the phase transition of the individual layer relaxor ferroelectrics of a particular composition. They also showed a very high nonlinear dielectric response and hence a high tunability about 70%, an increase of about 40% than that observed for single layer composition. This increase in tunability has been attributed to the lattice strain and the artificially enhanced chemical heterogeneity of the system. These multilayer films also showed a slim hysteresis loop characteristic with a saturation and remnant polarization of 20 and 6μC∕cm2, respectively.

Microstructure and dielectric properties of pulsed-laser-deposited CaCu3Ti4O12 thin films on LaNiO3 buffered Pt/Ti/SiO2/Si substrates

CaCu3Ti4O12 (CCTO) thin films have been prepared by a pulsed-laser-deposition method on LaNiO3 buffered Pt/Ti/SiO2/Si substrates, and their microstructure and dielectric properties have been compared with those of the films deposited directly on Pt/Ti/SiO2/Si substrates. The crystalline structure and the surface morphology of the CCTO thin films were markedly affected by the bottom electrodes. Both the films show temperature-independent dielectric properties in a wide temperature range, which is similar to those properties obtained in single-crystal or epitaxial thin films, while the room-temperature dielectric constant of the 350-nm-thick CCTO films on LaNiO3/Pt/Ti/SiO2/Si substrates at 100 kHz was found to be 2300, which was increased significantly compared with that obtained in the films on Pt/Ti/SiO2/Si substrates. Using the impedance spectroscopy technique, it has been suggested that the high dielectric constant response of the CCTO thin films originates from the grain boundary layer mechanism as found in internal barrier layer capacitors.

Enhanced Dielectric Response of Liquid Crystal Ferroelectric Suspension

It was found that doping of tiny ferroelectric particles to a nematic liquid crystal(LC)strongly affects dielectric properties of the system. The doping increases dielectric response of the LC due to interaction between LC molecules and particles, which possess large dipole moment and high polarisability. The suspension reveals high dielectric response on either side of the Curie temperature, TCurie, and peculiarity at the vicinity of TCurie. This result points at retention of dipole ordering above Curie temperature that may be a result of a memory effect.

Radiation-Induced Changes in the Dielectric Response of Poly(Vinylidene Fluoride) Type Polymers

High and dispersive dielectric response, characteristic of the relaxor state, was observed in ferroelectric P(VDF/TrFE)(50/50) copolymers irradiated with 1.0 MeV and 1.5 MeV electrons. Relaxor-like behaviour of the copolymer is a result of overlapping of the dielectric anomaly characteristic of the glass transition and that related to the Curie point, which is shifted downwards by electron irradiation. The results of ESR, IR and NIR Raman spectroscopy studies of the radiation damage to P(VDF/TrFE)(50/50) show that radiation-induced irreversible transformation of the ferroelectric copolymer to the relaxor state is related to the existence of polar clusters, consisting of a variety of short range coherence of trans-conformation, stabilized by random fields of C=C and conjugated C=C bonds.

Repulsive Casimir forces.

We discuss repulsive Casimir forces between dielectric materials with nontrivial magnetic susceptibility. It is shown that considerations based on the naive pairwise summation of van der Waals and Casimir-Polder forces may not only give an incorrect estimate of the magnitude of the total Casimir force but even the wrong sign of the force when materials with high dielectric and magnetic responses are involved. Indeed repulsive Casimir forces may be found in a large range of parameters, and we suggest that the effect may be realized in known materials. The phenomenon of repulsive Casimir forces may be of importance both for experimental study and for nanomachinery applications.

Phase diagrams and dielectric response of epitaxial barium strontium titanate films: A theoretical analysis

We develop phase diagrams for single-domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau–Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. The biaxial epitaxy-induced internal stresses enable phase transformations to unusual ferroelectric phases that are not possible in single crystals and bulk ceramics. The dielectric response of the films is calculated as a function of the misfit strain by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that by adjusting the misfit strain via substrate selection and film thickness, a high dielectric response can be obtained, especially in the vicinity of structural instabilities. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films on (001) Si, MgO, LaAlO3, and SrTiO3 substrates as a function of misfit strain and film thickness is provided. An order-of-magnitude increase in the dielectric constant with increasing film thickness is expected for films on LaAlO3 and SrTiO3 substrates. A structural instability around 40 nm is predicted in films on MgO substrates accompanied by a substantial increase in the dielectric constant. For films on MgO substrates thicker than 40 nm, the analysis shows that the dielectric constant decreases significantly. We show that the theoretical approach not only predicts general trends but is also in good quantitative agreement with the experimental data reported in literature.

Atomistic Mechanism of Relaxor Ferroelectricity

Relaxor ferroelectrics are widely used in various applications because of their high dielectric and piezoelectric response over a wide range of temperature. The microscopic mechanism of the relaxor behavior, however, is highly complex and still inadequately understood. We discuss the atomistic mechanisms that create the relaxor behavior and suggest that adjusting the balance between ordering and disordering forces may be the key to achieving high dielectric and piezoelectric responses.

Low dielectric response in enzyme active site.

The kinetics of charge transfer depend crucially on the dielectric reorganization of the medium. In enzymatic reactions that involve charge transfer, atomic dielectric response of the active site and of its surroundings determines the efficiency of the protein as a catalyst. We report direct spectroscopic measurements of the reorganization energy associated with the dielectric response in the active site of alpha-chymotrypsin. A chromophoric inhibitor of the enzyme is used as a spectroscopic probe. We find that water strongly affects the dielectric reorganization in the active site of the enzyme in solution. The reorganization energy of the protein matrix in the vicinity of the active site is similar to that of low-polarity solvents. Surprisingly, water exhibits an anomalously high dielectric response that cannot be described in terms of the dielectric continuum theory. As a result, sequestering the active site from the aqueous environment inside low-dielectric enzyme body dramatically reduces the dielectric reorganization. This reduction is particularly important for controlling the rate of enzymatic reactions.

Chemical, mechanical and dielectric properties after sintering of hydrothermal nickel–zinc ferrites

NiZn ferrites microstructurally controlled were produced after sintering of hydrothermal nanopowders. Chemical microanalyses showed that the zinc-loss phenomenon did not occur significantly, and that the elements iron and nickel probably control the diffusion process in the initial stage of sintering. The hardness and the dielectric response of the sintered NiZn ferrites exhibited a greater dependence on the grain size than on the density of the samples. The variations of the static dielectric permittivity and the relaxation frequency with the sintering conditions were correlated with the microstructural parameters. Complex impedance spectroscopy showed materials with high dielectric response, attributed to electron hopping conduction mechanism, and revealed the presence of a single relaxation, occurring at the grain boundaries.

Study of high frequency dielectric response of AgNbxTa1−xO3mixed system for x = 0.65, 0.82 and 1

Abstract The quasi-elastic diffusion measured near the specific M2-M3 phase transition in the AgTa1−xNbxO3 system is described by a modified Debye relaxational function from which dielectric dispersion can be calculated in the 1 to 100 cm−1 frequency range. An alternative description of the low-frequency Raman spectra, using Barker's formula is proposed in order to explain the structural phase transition.

The high frequency dielectric response of a ferroelectric liquid crystal

Abstract The temperature and frequency dependences of the complex dielectric susceptibility of a ferroelectric liquid crystal near the smectic C*-smectic A phase transition have been calculated using the classical and generalized Landau models. It is shown that although the dielectric response of the S*C phase consists generally of four modes (soft, Goldstone, and two high frequency polarization modes) only three bands appear in the dielectric loss spectrum of ferroelectric liquid crystals at the SA–S*C phase transition. These results are in agreement with recent experimental data.

High Frequency Dielectric Response of Calcium Aluminate Cements: Potential Packaging Substrate Materials

ABSTRACTDielectric permittivity and loss tangent were measured on: 1) pressed disks of four different calcium aluminate cements shear mixed with water and poly(vinyl alcohol) and cured at room temperature and 2) laminates of SECAR 71 cement sintered at 1450°C. Post resonance and perturbation methods were used for microwave frequency measurements. Far infrared dielectric response of the cements was determined by FTIR in the diffuse reflectance mode. The real and imaginary parts of the relative dielectric permittivity were obtained via a conventional Kramers-Kronig analysis.

Dielectric and pyroelectric properties of nylon 5,7 as a function of molecular orientation

Abstract Nylon 5,7 has been made in film form with two different molecular orientations. X-ray studies show that the H-bonds in rolled and annealed Nylon 5,7 are oriented approximately perpendicular to the film surface. Because of this, the film has a low dielectric response and reasonably high pyroelectric response. In gold nucleated, directionally crystallized Nylon 5,7 the H-bonds are oriented parallel to the film surface. Such films have a high dielectric response and low pyroelectric response. This can be understood in the light of the proposed model of H-bond twinning. The disappearance of the alpha relaxation in film A was ascribed to pinning of the amorphous phase since the oriented film was annealed under tension. High temperature dielectric loss of film A follows Jonscher's power law with n close to 1 showing that it is probably due to ionic conduction. Dielectric loss in film B gives an n value of 0.30 in the temperature range of 130–160°C, indicating that much of the loss is not due to ionic c...

Some Comments on the Plasmon Spectrum of Tetrathiafulvalene Tetracyano-p-quinodimethane (TTF-TCNQ)

The random-phase-approximation model calculation of the high frequency dielectric response of a quasi--one-dimensional metal is generalized to the case of n conducting strands per unit cell. For a model of the two-band system tetrathiafulvalene tetracyano-p-quinodimethane one obtains good agreement with recent experiments, and also predicts an acoustic-plasmon branch. Some implications for the physics of the material are discussed.

High frequency dielectric response of dipolar liquids

The dynamical dielectric response of a condensed system of molecules with permanent electric dipole moments is studied. We start from the dynamical extension of the Onsager theory developed by Nee and Zwanzig, but generalized so as to be applicable at high frequencies. Specifically, we retain inertial terms and introduce relaxation effects in a way valid in a high frequency limit. The dielectric function obtained incorporates a diversity of both collective and single-particle behavior, consistently, over the entire range of frequencies below those of intramolecular excitation. It is noteworthy that not only is a collective mode, or dipolar plasmon, exhibited, but that its frequency is also a resonance of the single-dipole motion. The properties of the dielectric function are analyzed and illustrated in detail, and specific applications are made to water, to hydrogen chloride, and to chloroform.