Low Frequency Dielectric Relaxation Research Articles

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.

Impact of strontium titanate nanoparticles on the dielectric, electro-optical and electrical response of a nematic liquid crystal

The impact of ferroelectric nanoparticles (FNPs), namely strontium titanate (SrTiO3) on the dielectric, electro-optical and electrical response of a room temperature nematic liquid crystal (NLC), 5CB (4-pentyl-4′-cyanobiphenyl) is investigated through dielectric, electro-optical and electrical techniques. The dielectric studies clearly show that low frequency dielectric relaxation shifts towards higher frequency as concentration of FNPs increases but decreases to the value even lower than pure sample at 2 wt% concentration. Afterwards, the short axis molecular relaxation observed in homeotropic cell obtained from homogeneous aligned cell by applying bias voltage higher than threshold shifts towards higher frequency by ∼ 28 % in composites but remains independent of concentration. Interestingly, after the removal of bias voltage, dielectric spectra does not return to its original state even after ∼ 22 min in the composites unlike pure sample and could be considered as apparent dielectric memory (i.e. memory effect). However, the voltage dependent optical textures confirm the short term memory in the composites. The electrical response recorded as a function of frequency in the composite is also significantly different than pure sample which further confirms the impact of FNPs on NLC. We believe that the observed effects of FNPs on dielectric, electro-optical and electrical behavior could be due to the plausible favorable interaction among FNPs and NLC molecules. Such investigations would certainly be pivotal in tuning the dielectric and electro-optical properties of NLCs using FNPs and helpful in designing the next-generation tunable electro-optical and soft memory based devices.

Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films.

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol–gel-derived Sr1−1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Dielectric Response of KTaO3 Single Crystals Weakly Co-Doped with Li and Mn

Alkali tantalates are encouraging functional materials with versatile properties that make them potential players in microelectronics, photocatalytic processes or medicine. Here, KTaO3 single crystals with 0.27% Li and 0.15% or 0.31% Mn contents determined by inductively coupled plasma spectroscopy analysis are studied at low temperatures using dielectric spectroscopy in a frequency range of 102–106 Hz. Both Li and Mn doping are found to induce separate low-frequency dielectric relaxations of comparable strength in KTaO3. The relaxation dynamics follow the Arrhenius law with activation energy values of ~77 and 107 meV, attributing the relaxation origin to the dipoles formed by off-centre Li+K and Mn2+K ions, respectively.

Dielectric Relaxation, Local Structure and Lattice Dynamics in Mn-Doped Potassium Tantalate Ceramics.

Alkaline niobate and tantalate perovskites have attracted attention as polar dielectrics for electronics and telecommunications. Here, we studied the polar behaviour, lattice dynamics, and local structure in conventionally processed K0.985Mn0.015TaO3±δ ceramics using a combination of variable-temperature dielectric and Raman spectroscopies, and X-ray absorption fine structure (XAFS) measurements, respectively. Mn doping induces a low-frequency dielectric relaxation in KTaO3 (KT), which follows the Arrhenius law with an activation energy U ≈ 105 meV and the characteristic relaxation time τ0 ≈ 4.6 × 10−14 s. Our XAFS results support preferential Mn occupancy of the cuboctahedral sites as Mn2+, with these cations strongly off-centred in the oversized oxygen cages. Such disordered Mn displacements generate electric dipoles, which are proposed as the source of the observed dielectric relaxation. We show that in Mn-doped ceramics, the low-frequency polar TO1 mode softens on cooling and, at low temperatures, exhibits a higher frequency than in undoped KT. This mode displays no detectable splitting, which contrasts with Li-doped KT that also contains off-centred Li+ species on the cuboctahedral sites. Therefore, we conclude that the coupling between the Mn displacements and the lattice is weaker than in the Li case, and Mn-doped KT therefore exhibits a dielectric relaxation but no ferroelectric transition.

Influence of magnetic nanoparticles on dielectric properties of Shell oil transformer oil

The influence of two types of nano-impurities MF1 and MF2 on the dielectric properties of Shell oil transformer oil at the temperature 293 K has been studied. It has been shown that these magnetic impurities have no significant effect on the dielectric permittivity value of Shell oil, but more significantly increase its conductivity, in so doing, the impurity MF1 increases the conductivity of transformer oil 4 times larger than the impurity MF2. It has been ascertained that the low-frequency dielectric relaxation appearing in the studied samples can be described by the Cole–Cole equation. The parameters of this relaxation process and the influence of different types of magnetic impurities on them have been estimated.

Low-Frequency Dielectric Relaxation in Structures Based on Macroporous Silicon with Meso-Macroporous Skin-Layer.

The spectra of dielectric relaxation of macroporous silicon with a mesoporous skin layer in the frequency range 1–106 Hz during cooling (up to 293–173 K) and heating (293–333 K) are presented. Macroporous silicon (pore diameter ≈ 2.2–2.7 μm) with a meso-macroporous skin layer was obtained by the method of electrochemical anodic dissolution of monocrystalline silicon in a Unno-Imai cell. A mesoporous skin layer with a thickness of about 100–200 nm in the form of cone-shaped nanostructures with pore diameters near 13–25 nm and sizes of skeletal part about 35–40 nm by ion-electron microscopy was observed. The temperature dependence of the relaxation of the most probable relaxation time is characterized by two linear sections with different slope values; the change in the slope character is observed at T ≈ 250 K. The features of the distribution of relaxation times in meso-macroporous silicon at temperatures of 223, 273, and 293 K are revealed. The Havriliak-Negami approach was used for approximation of the relaxation curves ε″ = f(ν). The existence of a symmetric distribution of relaxers for all temperatures was found (Cole-Cole model). A discussion of results is provided, taking into account the structure of the studied object.

Colossal dielectric response in erbium iron garnet ceramics

Polycrystalline Er3Fe5O12 ceramic sample was synthesized by a solid-state reaction technique. The dielectric properties of the sample were experimentally studied. A colossal dielectric constant more than 103 was observed over a wide temperature and frequency range. The dielectric relaxation in the sample was identified by the measurement of permittivity modulus, impedance spectroscopy, and conductivity in detail. At relatively low temperatures (T < 600 K), the dielectric relaxation under high frequencies (f ≥ 50 kHz) is triggered by the dipolar effect accompanied by the Fe2+ and Fe3+ charge transitions, while that under low frequencies (f ≤ 50 kHz) is mainly be associated with the Maxwell–Wagner effect. Moreover, a low-frequency dielectric relaxation at high temperatures (T ≥ 600 K) is probably evolving from the contribution of oxygen vacancies and grain boundaries, which is also suggested to be the origin of the colossal dielectric constant.

General Fabrication of 3D Hierarchically Structured Bamboo-like Nitrogen-Doped Carbon Nanotube Arrays on 1D Nitrogen-Doped Carbon Skeletons for Highly Efficient Electromagnetic Wave Energy Attenuation.

Hierarchically three-dimensional (3D) micro-nanostructures have promising applications in multifarious fields. Herein, we report a general strategy, that is, in situ catalysis process, for fabrication of nitrogen-doped carbon nanotube (NCNT) arrays on one-dimensional (1D) nitrogen-doped carbon (NC) skeletons. The NCNT arrays branch out from the 1D NC surfaces, resulting in the formation of hierarchically 3D micro-nanostructures. The strategy is involved in the pyrolysis of M-precursor (M = Fe, Co, and Ni) nanowires with the assistance of dicyandiamide. During the synthesis process, the metal components in the precursors serve as catalysts for growing NCNTs, while dicyandiamide provides carbon and nitrogen sources. With the ongoing reaction, the NCNTs were catalytically grown and branched out from 1D NC skeletons. Through the strategy, three kinds of hierarchically 3D structures with encapsulated Fe/Fe3C, Co, and Ni nanoparticles, respectively, were fabricated successfully. As functional materials for attenuating electromagnetic wave energy, these hierarchically 3D structures exhibit satisfactory performances even at a low matching thickness, exceeding most of the carbon-based materials. Typically, the minimal reflection losses of the 3D structures can reach -10.0 dB even as the matching thickness is in the range of 1.4-2.0 mm. Experimental results demonstrate that the excellent attenuation properties toward electromagnetic wave energy are relative to high conduction loss at a low frequency and high dielectric relaxations at a high frequency as well as better impedance matching with the input impedance of the free space. Our method presented here opens a general way for the development of hierarchically 3D carbon-based micro-nanostructures for their practical applications.

Low Frequency Dielectric Relaxation and Conductance of Solid Polymer Electrolytes with PEO and Blends of PEO and PMMA.

Solid polymer electrolytes are mixtures of polymer and inorganic salt. There are quite a number of studies dealing with the relationship between electric conductivity and structural relaxation in solid polymer electrolytes. We present a phenomenological approach based on fluctuation-dissipation processes. Phase heterogeneity appears in poly(ethylene oxide) (PEO) of high molecular mass and its blends due to crystallization and accompanying phase segregation. Addition of salt hampers crystallization, causing dynamic heterogeneity of the salt mixtures. Conductivity is bound to amorphous phase; the conductivity mechanism does not depend on content of added salt. One observes dispersion of conductivity relaxation only at low frequency. This is also true for blends with poly(methyl methacrylate) (PMMA). In blends, the dynamics of relaxation depend on glass transition of the system. Glassy PMMA hampers relaxation at room temperature. Relaxation can only be observed when salt content is sufficiently high. As long as blends are in rubbery state at room temperature, they behave PEO-like. Blends turn into glassy state when PMMA is in excess. Decoupling of long-ranging and dielectric short-ranging relaxation can be observed. Conductivity mechanism in PEO, as well as in blends with PMMA were analyzed in terms of complex impedance Z*, complex permittivity, tangent loss spectra and complex conductivity.

Low-frequency dielectric processes in deformed helix ferroelectric liquid crystals

A low-frequency dielectric relaxation mode in deformed helix ferroelectric liquid crystal (DHFLC) has been observed at the interface of strongly rubbed substrates and DHFLC material which may find applications in low power consumption FLC devices. The surface-induced dielectric relaxation process at the interface of DHFLC and substrate is called the partially unwound helical mode (p-UHM) due to the unwinding of the helical structure at this interface. After investigation of the material under various parameters such as temperature, variation of the amplitude of probing ac voltage and dc bias voltage, the relaxation frequency of p-UHM is found to be shifted towards Goldstone mode and merged with it. The relaxation frequency of Goldstone mode is found to decrease, whereas the relaxation frequency of p-UHM process increases with the increase in temperature of DHFLC. Finally, both the modes merge and the resultant relaxation frequency is found to be lower than Goldstone mode in SmC* phase. It seems that phason mode and partial helical unwinding mode are coupled together due to dipole moment that is resulting in a new relaxation frequency. p-UHM process is significant for low-power displays and non-displays applications like a part of sensor where weak electric signal is required to be realized without pre-amplification.

High conductivity, percolation behavior and dielectric relaxation of hybrid ZIF-8/CNT composites

The electrical behavior of hybrid ZIF-8/conducting composites remains poorly understood. Here, the hybrid ZIF-8/CNT composites are synthesized by an in situ growth of ZIF-8 on carbon nanotubes (CNT). The crystal structure, morphology and dielectric properties of the composites were investigated. Dielectric permittivity of as-prepared composites is improved more than 10 times by introduced CNT. The composites have a low percolation threshold (3.2%), where the dc conductivity increases by 4–5 orders of magnitude near the percolation threshold. Furthermore, we find three dielectric relaxation processes in the as-prepared ZIF-8 and ZIF-8/1%CNT composite by electrical modulus. One low-frequency dielectric relaxation associates with Maxwell-Wagner effect between composites and electrodes, while the two high-frequency dielectric relaxation attributes to the short-range hopping process of carries or dipoles orientation. The work provides deep insights on the intrinsic and extrinsic dielectric responses of ZIF-8 based composites and on the percolation behavior of hybrid ZIF-8/conducting materials composites.

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm2, 240 mPa.s, 150 µs and 28ᴼ, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.

Probing the impact of carbon quantum dots on partially unwound helical mode in ferroelectric liquid crystals

We report the modulation in dielectric properties of a ferroelectric liquid crystal (FLC) by using carbon quantum dots (CQDs) of size ∼4–5 nm. The appearance of a low frequency dielectric relaxation mode in the FLC, called the partially unwound helical mode (p-UHM), is controlled by CQDs with a FLC material in two different ways. Firstly, CQDs are dispersed into the bulk of the FLC before filling into a sample cell, and secondly, they are deposited onto the surface of a substrate. In both cases, the p-UHM has been found suppressed due to the modulation of a helicoidal structure at the interface of the FLC and the surface of the substrate of the sample cell. It has also been confirmed that CQDs at the interface of the FLC and the surface of the substrate have not affected the intrinsic properties of the FLC material. On the other hand, CQDs in the bulk of the FLC have shown remarkable variations in the fundamental properties of the FLC material. The suppression of the dielectric mode is confirmed by high-resolution dielectric spectroscopy, optical textures, and contact angle measurements. The concept of appearance and disappearance of the p-UHM process leads to the understanding of FLC systems in confined geometries for various display and non-display applications.

Understanding the validity of impedance and modulus spectroscopy on exploring electrical heterogeneity in dielectric ceramics

Combined modulus and impedance spectra are widely employed to explore electrical inhomogeneity and carriers' behaviors in dielectric ceramics based on equivalent circuit. However, discrepancies are found between practical dielectric responses and widely proposed equivalent circuits. Taking ZnO varistor ceramics as an example, a low-frequency dielectric relaxation, which can be detected in practical dielectric spectroscopy, is overlooked in simulated dielectric spectroscopy based on the proposed equivalent circuit according to modulus and impedance spectra. Therefore, equivalent circuits are frequently incomplete because the real low-frequency dielectric response is unable to be characterized from them. The problem originates from debatable understanding of frequency responses in modulus and impedance spectra. The low-frequency peak in modulus spectroscopy is proved originating from DC conductance instead of a real dielectric relaxation and the involvement of DC conductance component makes a low-frequency dielectric relaxation unable to be characterized in modulus spectroscopy. Therefore, improved dielectric spectroscopy eliminating the component of DC conductance is proposed and a clear peak corresponding to the low-frequency dielectric relaxation appears. In addition, a modified equivalent circuit which is in accordance with practical dielectric responses in not only modulus and impedance spectra but also dielectric spectroscopy is presented.

Oxygen-vacancy-related giant permittivity and ethanol sensing response in SrTiO3-δ ceramics

The ethanol sensing properties of SrTiO3-δ (δ = 0.075 and 0.125) ceramics was analyzed by dielectric measurements. The ceramics were prepared by solid state reaction method followed by the creation of oxygen vacancies-δ, through a thermal activated process. The crystal symmetry, space group and unit cell dimensions were derived from the X-ray diffraction (XRD) data using FullProf software whereas grain's size distribution was assessed by scanning electron microscopy (SEM). The prepared samples have been analyzed by impedance spectroscopy over the frequency range from 100 Hz to 1 MHz and temperature range from 240 to 340 K. The dielectric properties of SrTiO3-δ ceramics showed a quite remarkable stability of giant permittivity (>104) as well as a low dielectric loss, which open ways for several applications such as over voltage protections of electronic devices. A low-frequency dielectric relaxation behavior was found, and the carriers for electrical conduction result from the first-ionization of oxygen vacancies. The conductivity and gas sensitivity of SrTiO3-δ-based sensors were investigated. Results demonstrated that the conductivity decreases after the introduction of the ethanol gas, and p-type semiconductor gas-sensing materials were obtained. Both characteristics present higher responses at lower optimal operating temperatures.

Electrical conduction mechanisms and dielectric relaxation in Al2O3 thin films deposited by thermal atomic layer deposition

In the present work, aluminum oxide (Al2O3) thin films were deposited on p-type silicon substrates by thermal atomic layer deposition technique. The structural properties of as-deposited Al2O3 thin films were characterized by grazing-incidence x-ray diffraction and it was determined that the films had an amorphous structure. Electrical transport mechanisms and dielectrical properties of the amorphous Al2O3 thin films were then investigated by fabricating and characterizing Al/Al2O3/p-Si metal–oxide-semiconductor (MOS) capacitors by performing the dc current-voltage and frequency-dependent dielectric measurements. As a function of the applied gate electric fields, the current conduction in low electric fields consists of Schottky emission, but in relatively high electric fields current conduction is dominated by space-charge limited conduction. From the frequency-dependent dielectric measurements, the dielectric loss showed two peaks at the frequencies of 10 kHz and 0.7 MHz which evidence of the dielectric relaxation. The low and high frequency dielectric relaxation behaviors were attributed to space charge polarization and orientation-polarization of dipoles, respectively. It was concluded that defects related localized states in the Al2O3 films may contribute to dielectric properties at high frequency.

Effects of dc bias on dielectric relaxations in CaCu3Ti4O12 ceramics

Effects of dc bias on dielectric relaxations in CaCu3Ti4O12 ceramics were investigated via an improved dielectric spectroscopy. A new low-frequency dielectric relaxation, which was assigned to space charge polarization, was found shifting towards higher frequency with increasing bias voltage in the improved spectra. It was suggested that the Schottky barrier at grain boundary was lowered under dc bias resulting in higher possibility for carriers to migrate. Therefore, the relaxation time was decreased, which was in accordance with rightward shift of this relaxation under increased dc bias. In addition, dependence of the widely reported high-frequency relaxation (> 105 Hz) and middle-frequency relaxation (103–105 Hz) on bias voltage was also discussed. Permittivity contributed by either high-frequency or middle-frequency relaxation presented inverse dependence on dc bias. Discrepancy on barrier parameters was obtained assuming both of them physically correlated with the barrier at grain boundary.

Space charge polarization modulated instability of low frequency permittivity in CaCu3Ti4O12 ceramics

A low frequency dielectric relaxation process of CaCu3Ti4O12 ceramics, which is generally overlapped by dc conduction in traditional dielectric spectra, is detected and clarified based on an improved analytical methodology in this letter. This relaxation process remarkably contributes to instability of low frequency permittivity and it clearly demonstrates the correlation between charge migration and space charge polarization. It is revealed that conduction will be achieved if carriers migrate and combine in the electrodes, while space charge polarization will be achieved if carriers migrate to the electrodes without combination.

Polaron-mediated low-frequency dielectric anomaly in reduced LiNbO3:Ti

Measurements of the real and imaginary parts of the dielectric permittivity of reduced Ti-doped lithium niobate reveal a low-frequency dielectric relaxation as well as a strong increase in the conductivity compared to undoped samples. The low-frequency permittivity is increased by about an order of magnitude due to the relaxation. The analysis of the temperature-dependence of the relaxation frequency reveals a thermally activated behavior with an activation energy of Ea = 0.65 eV, which is in agreement with a transition between NbLi4+ small bound polarons and NbNb4+ free polarons. Both low-field ac conductivity and high-field dc conductivity indicate a Poole-Frenkel-type electron hopping mechanism between NbLi4+ sites at low temperature, turning into similar hopping between NbNb4+ at high temperatures.