Low-frequency Permittivity Research Articles

Communication—Tunable Lorentz-Type Negative Permittivity of PANI/Epoxy Resin Composites in the Frequency Range from 3 kHz to 1 MHz

Negative permittivity in percolation composites garnered significant interest due to its promising implications for practical applications. This study demonstrates that the percolation threshold of the polyaniline(PANI)/epoxy resin composite falls within the range of 40 wt% to 50 wt%. Beyond this percolation threshold, the composites exhibit a corresponding negative dielectric behavior. Notably, at a high PANI content level of 90 wt%, the permittivity exhibits characteristics akin to Lorentz resonance type behavior. This research presents an effective approach to exhibit tunable low-frequency negative permittivity through Lorentz resonance.

Effective Permittivity of Compacted Granular Materials: Effects of Interfacial Polarization and Pore-filling Fluids

Interfacial polarization dominates the permittivity spectra of heterogeneous granular materials for the intermediate frequency range (i.e., from kHz to MHz). In this study, we examine the corresponding dielectric responses of compacted glass sphere packings saturated with pore-filling fluids under various compressive stresses. The effective permittivity spectra are observed to exhibit consistently a plateau-to-plateau drop, described by low-frequency permittivity, characteristic frequency, and high-frequency permittivity. The permittivity spectra under different compressive levels are found to be influenced by the packing structure, compressive stress, and electrical property contrasts between solid and fluid (specifically permittivity and conductivity). For considered measurement conditions, the variation of packing structure and its associated porosity is found to be more significant than the stress evolution in controlling the interfacial polarization, thus the permittivity spectra, as supported by analytical and numerical results for unit cells. Furthermore, to gain a general rule for dielectric responses for saturated granular materials, we train multi-layer artificial neural network (ANN) models based on a series of simulations for unit cells with various structures, stresses, and electrical and dielectric properties. The predictions with two-layer ANN agree well with experimental measurements, presenting errors smaller than 5% for both low-frequency and high-frequency permittivity. This study offers an effective predicting approach for the dielectric behaviour of heterogeneous and multiphase materials.

Flexible Copper Nanowire/Polyvinylidene Fluoride Membranous Composites with a Frequency-Independent Negative Permittivity.

With the increasing popularity of wearable devices, flexible electronics with a negative permittivity property have been widely applied to wearable devices, sensors, and energy storage. In particular, a low-frequency dispersion negative permittivity in a wide frequency range can effectively contribute to the stable working performance of devices. In this work, polyvinylidene fluoride (PVDF) was selected as the flexible matrix, and copper nanowires (CuNWs) were used as the conductive functional filler to prepare a flexible CuNWs/PVDF composite film with a low-frequency dispersion negative permittivity. As the content of CuNWs increased, the conductivity of the resulting composites increased sharply and presented a metal-like behavior. Moreover, the negative permittivity consistent with the Drude model was observed when CuNWs formed a percolative network. Meanwhile, the negative permittivity exhibited a low-frequency dispersion in the whole test frequency range, and the fluctuation of the permittivity spectra was relatively small (-760 to -584) at 20 kHz-1 MHz. The results revealed that the high electron mobility of CuNWs is reasonable for the low-frequency dispersion of negative permittivity. CuNWs/PVDF composite films with a frequency-independent negative permittivity provide a new idea for the development of flexible wearable electronic devices.

Dielectric relaxations without colossal permittivity in Mg and Nb co-doped BaTiO3 ceramics

Magnesium (Mg) and Niobium (Nb) co-doped BaTiO3 ceramics are prepared using a solid-state reaction method under an air atmosphere, and their crystal structures, microstructures, valence states and dielectric properties are studied. The ceramics adopt a cubic perovskite structure, and the lattice parameter increases with doping content owing to the larger ionic radii of dopants. The complex permittivity and electric modulus as a function of frequency and temperature are analyzed. Dielectric relaxations without colossal permittivity are observed. The relaxation processes are thermally activated and their variations with temperature are analyzed. The activation energy values for the relaxation processes are determined by the Arrhenius relationship and increase with doping content. By analyzing the electric modulus, the activation values for high-frequency relaxation are found to be associated with the electrical properties of grain, and the contribution of grain boundary could be neglected based on internal barrier layer capacitor (IBLC) model. The electron hopping between the different Ti valences, which is confirmed by X-ray photoelectron spectroscopy, is supposed to play an important role in high-frequency relaxation. The permittivity plateaus are related to the inhomogeneous structures of grain and grain boundary, and the low-frequency permittivity plateau is attributed to the space-charge polarization at grain boundaries. By comparing with Cu/Nb co-doped BaTiO3, the absence of colossal permittivity is ascribed to high resistivity of the samples. Charge density and migration as well as the insulating grain boundaries are critical for the formation of space charge and a high permittivity.

The effect of a static magnetic field on the low-frequency dielectric permittivity of nominally pure and Co<sup>2+</sup> ions doped triglycine sulfate crystals

The effect of a weak static magnetic field on the low-frequency dielectric permittivity of the crystals of nominally pure triglycine sulfate (TGS): and TGS containing cobalt ions (TGS + Co2+) is investigated. In TGS, the magnetic effect is essentially anisotropic: it is completely absent when the vectors \(\vec {B}\) and \({{\vec {P}}_{{\text{s}}}}\) are collinear and is most pronounced when \(\vec {B} \bot {{\vec {P}}_{{\text{s}}}}\) and the magnetic field is oriented along the crystallophysical axis \(\vec {a}\). In TGS + Co2+ magnetic effect is noticeably stronger than in TGS, but practically does not depend on the mutual orientation of vectors \(\vec {B}\) and \({{\vec {P}}_{{\text{s}}}}\).

Influence of LiNbO3 layer thickness on structural and dielectric properties of CoFe2O4\\LiNbO3 multiferroic bilayers prepared by laser ablation

Multiferroic bilayer thin films composed by LiNbO3 and CoFe2O4 were prepared by laser ablation over (100) platinum-covered Si substrates, to understand the influence of the LiNbO3 layer thickness on their structural, electric and dielectric properties. The films were dense and polycrystalline, presenting rhombohedral LiNbO3 and cubic spinel CoFe2O4 layers. The CoFe2O4 unit cell experienced compression along the growth direction of the films, whereas the LiNbO3 layer exhibited a structural distortion in its unit cell. These strain effects were due to the mechanical interaction between both layers and to the presence of a higher amount of point defects in the thinner LiNbO3 samples, which gradually relaxed as the LiNbO3 layer thickness increased. The DC conductivity and activation energies of LiNbO3, determined from low-frequency dielectric permittivity behavior, were found to be dependent on the layer thickness. Thicker LiNbO3 layers displayed bulk-like characteristics, with a transition from polaronic to ionic conduction with increasing temperature. In contrast, thinner LiNbO3 layer films displayed polaronic behavior up to higher temperatures, due to the formation of bound polarons associated with lithium defects during deposition. AC conductivity analysis revealed that the CoFe2O4 layer conductivity mechanism primarily involved hopping between ions located at octahedral sites in the ferrite. These findings provide valuable insights into structure-function relationships and dynamical behavior within multiferroic bilayer thin films, promoting their applicability for emerging magneto-electric technologies.

Study of the Aid Effect of CuO-TiO2-Nb2O5 on the Dielectric and Structural Properties of Alumina Ceramics.

The aim of this work is to study the structural, dielectric, and mechanical properties of aluminum oxide ceramics with the triple sintering additive 4CuO-TiO2-2Nb2O5. With an increase in sintering temperature from 1050 to 1500 °C, the average grain size and the microhardness value at a load of 100 N (HV0.1) increased with increasing density. It has been shown that at a sintering temperature of 1300 °C, the addition of a 4CuO-TiO2-2Nb2O5 additive increases the low-frequency permittivity (2-500 Hz) in alumina ceramic by more than an order of magnitude due to the presence of a quadruple perovskite phase. At the same time, the density of such ceramics reached 89% of the theoretical density of α-Al2O3, and the microhardness value HV0.1 was 1344. It was observed that the introduction of 5 wt.% 4CuO-TiO2-2Nb2O5 in the raw mixture remarkably increases values of shrinkage and density of sintered ceramics. Overall, the results of this work confirmed that introducing the 4CuO-TiO2-2Nb2O5 sintering additive in the standard solid-phase ceramics route can significantly reduce the processing temperature of alumina ceramics, even when micron-sized powders are used as a starting material. The obtained samples demonstrated the potential of α-Al2O3 with the triple additive in such applications as electronics, microwave technology, and nuclear power engineering.

Optical and dielectric realisation of biomolecular detection using gold nanoparticles bio-conjugate with liquid crystal

Dielectric biosensor based on nematic liquid crystal platform has been fabricated using sandwich immunoassay technique for the detection of a biomolecule, human cardiac troponin I (cTnI). The optical response of Liquid crystal (LC) biosensor and its corresponding frequency-dependent dielectric properties have been investigated as a function of cTnI concentration. Gold nanoparticles conjugate with detection antibody (AB-GNP) is used for the enhancement of both the optical and dielectric signal of the biosensor. AB-GNP binding with cTnI alter the surface energy of the substrate causing disturbances in LC alignment resulting in a change in transmission. The transmission of the LC cell has been found to be varying from dark to bright texture under polarized microscope as a function of cTnI concentration. A change in low-frequency permittivity with increasing cTnI concentration that forms an increasing order of sandwich immunocomplex with AB-GNP revealed an interfacial surface phenomenon. A positive shift in the onset frequency of permittivity is observed due accumulation of immunocomplex with increasing concentration of cTnI at electrode/LC interface. The dielectric response ε′ of the biosensor is fitted with Hill-Langmuir equation over a concentration range of 0.01 to 10 ng mL−1 cTnI. The dissociation constant ‘Kd’ of 1.02 ± 0.24 ng/mL−1 indicates strong cTnI binding affinity for substrate antibody. A nearby value of 1, calculated for Hill coefficient ‘n’ revealed no co-operativity characteristic of cTnI binding to the primary capture antibody, wherein each binding site of the antibody functions independently at the electrode surface, causing accumulation of charged species revealing LC interfacial surface electrical property.

Dielectric properties of porous rocks with partially saturated fractures from finite-difference modeling

Fractures are common features of geologic rocks, and they have a profound influence on the fluid flow and transport characteristics as well as physical properties of the fracture-bearing rocks. Fractures are frequently partially saturated in hydrocarbon reservoirs, and the dielectric properties that are highly sensitive to the fluid types and concentrations are useful tools for the quantitative characterization of reservoir rocks with partially saturated fractures. However, the effects of partially saturated fractures on the dielectric properties of the porous rock are still poorly understood. Based on a 3D finite-difference model, we have bridged this knowledge gap by studying the frequency-dependent dielectric properties of a porous carbonate rock with partially saturated fractures obtained by the brine-displacing oil simulation. We find that the conductivity and high-frequency (higher than approximately 1 MHz) relative permittivity generally increase, whereas the low-frequency relative permittivity and peak dielectric loss decrease with the increasing brine saturation. We also find that the partially saturated fractured rock with lower brine saturation exhibits a greater low-frequency relative permittivity and maximum dielectric loss factor than the cases in which the fractures are completely filled with oil. A systematic investigation based on simplified intersecting fracture models suggests that the oil-brine distribution is the main reason for the abnormal dielectric responses of the low-frequency relative permittivity and the maximum dielectric loss factor at low brine saturations. The results provide new insights for the comprehensive understanding of the frequency-dependent dielectric responses of fractured sedimentary rocks and will pave the way for the quantification of the brine saturation in partially saturated fractures.

Interactions and dynamics of cations and interlayer quest molecules in nontronite and nontronite–formamide intercalation

Nontronite is a swelling clay mineral. Between the layers, it contains water and cations. Nontronite can be easily modified by intercalation. This work focuses on characterizing unmodified nontronite and nontronite intercalated with formamide. Thermal measurements, infrared spectroscopy, X–ray diffraction, and dielectric relaxation measurements were made to evaluate the interactions of the guest molecules with the host layers. A startling effect was observed in dielectric experiments: the low–frequency permittivity detected in nontronite was much larger than in the material intercalated with formamide, the molecule with a high dipole moment. To explain this effect, it was assumed that the low–frequency dielectric relaxation was associated with the movement of interlayer ions. Dehydration of nontronite, which is a stage of preparation for intercalation, can probably fix some ions in hexagonal cavities (privileged positions). Subsequent intercalation of nontronite by formamide does not bring back the mobility of ions, and the low–frequency electric permittivity remains relatively low. To achieve mobility of the cations, an expansion of the interlayer space is required, and ion–dipole interactions with guest molecules should be present.

Dielectric and ferroelectric properties of multilayer BaTiO3/NiFe2O4 thin films prepared by solution deposition technique

In this work different lead-free multilayered structures, composed of perovskite BaTiO3 and spinel NiFe2O4 thin layers, were obtained by solution deposition method. Structural characterization of the sintered thin films confirmed the well-defined layered structure with overall thickness from 160 to 600 nm, crystalline nature of perovskite BaTiO3 and spinel NiFe2O4 phases without secondary phases (after sintering below 900 °C) and grains on nanometer scale. Dielectric properties of the multiferroic multilayer BaTiO3/NiFe2O4 thin films were analyzed in temperature and frequency range from 30 °C to 200 °C and 100 Hz to 1 MHz, respectively. In comparison to the pure BaTiO3 films, the introduction of ferrite layer reduces dielectric response and increases low frequency permittivity dispersion of the multilayer thin films. The multilayer samples have shown relatively low dielectric loss with stronger contribution of conductivity at higher temperatures, and characteristic broad peak representing “relaxation” of the interface charge accumulation.

Experimental determination of frequency- and temperature-dependent electrical properties of water-saturated clays using spectral induced polarization and network analyzer technique

Impedance and vector network analyzer measurements are combined in order to measure frequency dependent conductivity and permittivity of two different saturated clays: montmorillonite and illite chlorite. In the low frequency range, the complex conductivity is determined over the 10 mHz to 45 kHz frequency range using a complex impedance analyzer with a 4-electrodes cell. In the high frequency range (50 MHz to 5 GHz), the complex permittivity is obtained from measured reflection factor of an open-ended probe. The ultrabroadband constitutive properties of the saturated clays were investigated in the temperature range 25°C down to 0°C. A mixture equation, based on a volume average approach, was applied to model the low and high frequency electrical properties based on the formation factor. The temperature dependency was considered in the following petrophysical quantities: low frequency conductivity, chargeability and high frequency permittivity. The modeled results are in reasonable agreement with measurements.

Communication—Tunable Negative Permittivity of Cobalt and Epoxy Composites at 3 kHz ∼ 1 MHz Frequency Regions

Negative permittivity in percolation composites has drawn much attention owing to their potential application. This study set out to explore whether tunable negative permittivity can be realized in metal composites at 3 kHz ∼ 1 MHz frequency regions. When the content of cobalt is 80 wt%, the permittivity turns to negative in the test band. These findings show that the Co conductive network has formed and provides large numbers of free electrons. Meanwhile, the percolation composites with negative permittivity show inductive characteristic. This research supports the idea that tunable low-frequency negative permittivity can be realized in metal composites.

Domain-wall freezing in Cd2Nb2O7 pyrochlore single crystal

Cadmium niobate Cd2Nb2O7 is a rare example of a bulk ferroelectric with a pyrochlore structure, which is characterized by an extremely complex phase diagram and contradictions in the structural phase transitions and polar states. Here, the dielectric spectroscopy studies of the Cd2Nb2O7 single crystals were performed. For the first time it was shown that dielectric behavior of these crystals demonstrates all typical features of the domain-wall "freezing" dynamics such as the characteristic "kink" on the temperature dependence of the real part of the dielectric permittivity as well as the fulfillment of the Vogel-Tammann-Fulcher relation of the experimentally determined temperature dependence of the relaxation time. It was found that the contribution of the domain-wall relaxation process to the total low-frequency dielectric permittivity is more than 85% at the temperature at which the dielectric dispersion is most pronounced. Thus, a limited family of the ferroelectrics exhibiting domain-wall freezing phenomena was expanded.

Synthesis and broadband dielectric-infrared spectroscopy of La1-xSrxMnO3@BaTiO3 nanocomposite

The study deals with design, structural and magnetic characterization, electric and magnetoelectric transport and broad-band dielectric spectroscopy of the all-perovskite core-shell composite nanoceramics of 0–3 connectivity, in which ∼50 nm cores of ferromagnetic La0.65Sr0.35MnO3 (LSMO) metal are embedded in ferroelectric BaTiO3 (BTO) shells of mean 13 nm thickness. The BTO spacers lead to significant barriers for itinerant carriers, at low temperatures being transmitted by tunnelling, but additional conduction paths are temperature activated by hopping across them. The dielectric, terahertz and infrared spectroscopy data of LSMO@BTO are modelled by effective medium approaches, which enables to explain the broad dielectric and AC conductivity dispersion including the giant low-frequency permittivity values. Temperature dependant terahertz transmission spectroscopy clearly indicates the ferroelectric transition in the nano-BTO shells in the same temperature range as in the bulk BTO.

Dielectric properties and electrical response of yttrium-doped Bi2/3Cu3Ti4O12 ceramics

([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ([Formula: see text] 0.00–0.30) ceramics were successfully prepared via the conventional solid-state method. X-ray powder diffraction confirmed the lattice constant gradually decreases with increasing Y[Formula: see text] content. SEM images displayed Y[Formula: see text] substitution for Bi[Formula: see text] gave rise to the large abnormal grains, and the size of abnormal grains became larger with the increase of Y[Formula: see text] substitution. ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics presented the relatively high dielectric constant of 7400 with the dielectric loss of 0.055 when [Formula: see text] 0.20. The analysis of complex impedance suggested the grains are semiconductive and the grain boundaries are insulating. For pure [Formula: see text]Cu3Ti4[Formula: see text] ceramics, the appearance of additional low-frequency peaks in electrical modulus indicated the grain boundaries are heterogeneous. The investigation of modulus peaks fitting with Arrhenius formula implied that the low-frequency permittivity for all ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics was ascribed to the Maxwell–Wagner relaxation at grain boundaries. In addition, a set of clear dielectric peaks above [Formula: see text]C associated with Maxwell–Wagner relaxation can be found for all ([Formula: see text]Bi[Formula: see text]Cu3Ti4[Formula: see text] ceramics in the temperature dependence of dielectric constant. This set of clear dielectric peaks showed a tendency to shift to higher temperatures with the increase of Y[Formula: see text] substitution. Meanwhile, a tiny dielectric anomaly at room temperature was found in Y-doped [Formula: see text]Cu3Ti4[Formula: see text] ceramics.

Seemingly anisotropic magnetodielectric effect in isotropic EuTiO3 ceramics

In the antiferromagnetic phase of $\mathrm{Eu}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics, a seemingly anisotropic magnetodielectric effect (up to 2.5%) was observed via the low-frequency dielectric permittivity measured in external magnetic field below 1.9 T. We explain the effect theoretically by taking into account the demagnetizing field which effectively reduces the internal magnetic flux density by 0.6 T when the external magnetic field is applied perpendicular to the plane of the disk-shaped sample. This finding is also confirmed experimentally by magnetization measurements. The refractive index between 0.2 and 0.5 THz exhibits a large anisotropy in an external magnetic field of up to 7 T. This anisotropy is due to a shift of the ferromagnetic resonance from the microwave to the sub-THz region with external magnetic field applied perpendicular to the magnetic component of the THz radiation. This magnon observation is performed in the strong external magnetic field (above 2 T) and in both paramagnetic and antiferromagnetic phases.

Broadband dielectric spectroscopy of La0.65Sr0.35MnO3@TiO2 core–shell nanocomposites

Core–shell composites of ferromagnetic conducting nanoparticles La0.65Sr0.35MnO3 (LSMO) embedded in an insulating matrix of TiO2 (LSMO@TiO2) have been processed, structurally and magnetically characterized, and their DC magnetoresistivity and complex dielectric response measured and fitted from Hz up to the infrared (IR) range (1014 Hz). XRD indicates that the TiO2 shells are amorphous. Modelling of the IR spectra using standard models based on the effective medium approximation has it confirmed and has characterized the effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite. Modelling of the lower-frequency spectra has shown that TiO2 shell thicknesses are rather non-uniform down to thin nm values, which leads to giant low-frequency permittivity values and non-negligible free-carrier tunnelling among the LSMO cores. Two main dielectric dispersion regions were observed and shown to be due to the inhomogeneous conductivity—the one occuring in the 1011–1012 Hz range relates to nonmagnetic less-conducting dead layers on the surface of LSMO nanocrystallites and the broad second one below the 1010 Hz range is due to the non-uniform thicknesses of the dielectric TiO2 shells. In the IR range, effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite were characterized from the reflectivity spectra.

Spontaneous Magnetodielectric Effect and Its Coupling to the Lattice Dynamics in Fluoroperovskites

Experimental results on temperature dependences of the low-frequency dielectric permittivity of the group of magnetic fluoroperovskites with different crystal and magnetic structures are presented. Orthorhombic NaCoF3 and NaNiF3, cubic RbFeF3, hexagonal RbNiF3 and tetragonal K2CoF4 and K2NiF4 were investigated. The analysis of experimental results in combination with those of our previous studies of other fluoroperovskites was carried out taking into account the influence of the spontaneous magnetodielectric effect on lattice dynamics. It revealed the role of the spin-phonon coupling and the anharmonic contribution, which leads an increase of dielectric permittivity at heating, and contribution of the hidden structural instability manifested as an increase of the dielectric permittivity at cooling. It was established that the relative contributions of these three main mechanisms to the temperature dependence of dielectric permittivity are significantly different in all fluoroperovskites under study but they are well correlated with the tolerance factor t, which characterizes a relation between ionic radii and is a measure of stability of AMF3 perovskite crystal structure. The obtained results and their analysis reliably demonstrate that the low-frequency dielectric spectroscopy is a highly sensitive method to study particular features of the lattice dynamics of fluoroperovskites at magnetic and structural phase transitions.

Fe3+/Nb5+ Co-doped rutile-TiO2 nanocrystalline powders prepared by a combustion process: preparation and characterization and their giant dielectric response.

Fe3+/Nb5+ co-doped TiO2 (FeNb-TO) nanocrystalline powders were prepared by a combustion process. A pure rutile–TiO2 phase of powders and sintered ceramics with a dense microstructure was achieved. Both co-dopants were homogeneously dispersed in the ceramic microstructure. The presence of oxygen vacancies was confirmed by Raman and X-ray photoelectron spectroscopy techniques. The low-frequency dielectric permittivity enhanced as co-doping concentration increased. The thermally activated giant-dielectric relaxation of FeNb-TO ceramics was observed. Removing the outer-surface layer had a slight effect on the dielectric properties of FeNb-TO ceramics. Density functional theory (DFT) calculation showed that, in the energy preferable configuration, the 2Fe atoms are located near the oxygen vacancy, forming a triangle-shaped FeVoTi defect complex. This defect cluster was far away from the diamond-shaped 2Nb2Ti defect complex. Thus, the electron-pinned defect-dipoles (EPDD) cannot be formed. The giant-dielectric relaxation process of the FeNb-TO ceramics might be attributed to the interfacial polarization associated with electron hopping between Ti3+/Ti4+ ions inside the grains, rather than due to the surface barrier layer capacitor (SBLC) or EPDD effect.