Multiple Dielectric Relaxations Research Articles

Rheological and dielectric behavior of sodium carboxymethyl cellulose (NaCMC)/Ca2+ and esterified NaCMC/Ca2+ hydrogels: Correlating microstructure and dynamics with properties

Polyelectrolyte-based conductive hydrogels are being extensively explored for applications in energy storage and as electrode materials for batteries. We synthesized ionically crosslinked sodium carboxymethyl cellulose (NaCMC), esterified NaCMC, and Ca2+ doped esterified NaCMC hydrogels. This work aims to understand the effect of Ca2+ ions on the NaCMC and esterified NaCMC. FTIR, SEM, Rheology and EIS studies were performed to understand the structure and dynamics of hydrogels. Results confirmed that Ca2+ ions have an important role in determining the rheological and dielectric response of hydrogels. Power law behavior was observed in their rheological response with exponent (n) of 0.81 for G′ and 0.76 for G″ of ionically crosslinked NaCMC, 5.38 for G′ and 4.70 for G″ of esterified NaCMC, whereas, negative exponents −1.44 for G′ and −1.10 for G″ of Ca2+ ion doped esterified NaCMC. Ionically crosslinked NaCMC hydrogels have relaxation times (τ) in the range of 8.9 × 10−5 s–2.8 × 10−5 s may be due to the formation of temporary dipoles by electrostatic bridge formations with dc conductivity of (0.1 S/cm–5 S/cm), whereas, esterified NaCMC showed relaxation times (10−3 s–8.9 × 10−5 s) with increasing ester crosslinks and dc conductivity of (0.05 S/cm–0.8 S/cm). Interestingly, Ca2+ ion doped esterified hydrogels showed multiple dielectric relaxations on Ca2+ ion addition with different relaxation times may be due to change in ionic environment. The understanding obtained from this work may be useful for designing tuneable hydrogels with optimum electrical and mechanical properties.

The SHI irradiation induced transition to negative dielectric constant phase in K2Bi4Ti4WO18

In present communication, a new Aurivillius family compound K2Bi4Ti4WO18 was synthesized, and the impact of swift heavy ion (SHI), Ni+11 irradiation on its surface and dielectric properties has been studied in detail. The phase formation in this complex oxide, and crystallization to B2cb symmetry was confirmed by the X-ray diffraction. However, post irradiation the XRD, SEM and AFM studies shows the surface amorphization, in agreement with the theoretical calculations. Furthermore, the effect of irradiation was also observed in the bulk dielectric properties as the system transform to a phase with negative dielectric constant above 350 K in the radio frequencies. This transition is in correlation with significant change in other dielectric parameters such enhancement in AC conductivity, a helical Nyquist plot and multiple dielectric relaxations. This conspicuous changes in the dielectric response post irradiation is attributed to the SHI induced defect formation, modification of energy barriers and their consequences on the electronic structure. Thus, current study suggests that the dielectric properties of Aurivillius K2Bi4Ti4WO18 could be tailored by ion irradiation and opens a new possibility of tuning functional properties.

Multiple Dielectric Relaxations in CaMn2O4 Polycrystalline Compound

The dielectric response of the CaMn2O4 polycrystalline compound has been reported in this paper. The orthorhombic room-temperature structure (space group Pbcm) is confirmed by powder X-ray diffraction (XRD). The temperature dependent magnetization M(T) data reveal that antiferromagnetic ordering below T N = 220 K. Further decreasing the temperature magnetization shows increasing trend of canted spin structure. Dielectric anomalies are clearly observed at the onset of the magnetic ordering (∼220 K) and also at the onset of the canted structure (50 K) in CaMn2O4. Such type of dielectric anomaly around long range order and canted spin transition will correlate the influence of underlying magnetic structure with electric ordering.

Cu, Mg Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, XPS Spectra, Thermal Expansion, and Electrical Properties.

The pyrochlore-type solid-solution formation in a Bi1.6Mg0.8-xCuxTa1.6O7.2-Δ system, synthesized for the first time, is observed at x ≤ 0.56. High-temperature X-ray diffraction showed that the pyrochlore phase exists in air up to 1080 °C, where its thermal decomposition leads to the segregation of (Mg,Cu)Ta2O6. The thermal expansion coefficients of the end member, Bi1.6Mg0.24Cu0.56Ta1.6O7.2-Δ, increase from 3.3 × 10-6 °C-1 at room temperature up to 8.7 × 10-6 °C-1 at 930 °C. Rietveld refinement confirmed that the pyrochlore crystal structure is disordered with space group Fd3̅m:2 (Z = 8, no. 227). Doping with copper results in a modest expansion of the cubic unit cell, promotes sintering of the ceramic materials, and induces their red-brown color. X-ray photoelectron spectroscopy demonstrated that the states of Bi(III) and Mg(II) are not affected by doping, and the effective charge of tantalum cations is lower than +5, while the Cu(II) states coexist with Cu(I). The electron spin resonance spectra display a single line with g = 2.2, ascribed to the dipole-broadened Cu2+ signal. The dielectric permittivity of Bi1.6Mg0.8-xCuxTa1.6O7.2-Δ ceramics may achieve up to ∼105, with the dielectric loss tangent varying in the range from 0.2 up to 12. Multiple dielectric relaxations are found at room temperature and above for all samples.

Shielding performance of MnxNi0.8–xZn0.2Fe2O4 (0.1≤x≤0.7) for electromagnetic interference (EMI) in X-band frequency

A series of MnxNi0.8–xZn0.2Fe2O4 (0.1≤x≤0.7) nanoferrites were prepared by sol-gel auto combustion method. The TGA-DSC studies revealed the weight loss up to 700 °C and after no weight loss was found confirming the complete ferrite phase. Therefore, the as-burnt samples subjected to conventional sintering at 900 °C for 2h. The traces of XRD patterns revealed that formation of ferrite phase co-existed with secondary phases. The complex permittivity and permeability of present ferrite systems are found to be dependent of Mn2+ ion concentration. The real part of dielectric constant (ε′) for the all compositions are in the range of 5.184–6.951 at 8.2 GHz while it is 5.269–7.663 at 12.4 GHz. The values of ε′ at 8.2 GHz are slightly lower than the values at 12.4 GHz. The imaginary part of dielectric constant (ε″) was found to be in the range of 0.046–0.315 for all compositions over the entire range of X-band frequency (8.2–12.4 GHz). Compared to the variation of ε′, the variation of ε″ is more disorder. The number of semicircles in Cole-Cole plots is a witness of multiple di-electric relaxations of EM energy dissipation. The real part of magnetic permeability (μ′) is increasing, while imaginary part of magnetic permeability (μ″ ) is decreasing for all compositions as frequency is increasing from 8.2 to 12.4 GHz. Compared to the other ferrite compositions, the ferrite with low content of Mn2+ i.e. Mn0.1Ni0.7Zn0.2Fe2O4 ferrite system has higher value of μ″ and seems to be invariant over the entire X-band frequency range. The total effective shielding (SET) for all compositions is found to be in the range 37–12 dB. The highest values of SET 37 dB at 8.2 GHz and 28 dB at 12.4 GHz were reported for the ferrite composition Mn0.1Ni0.7Zn0.2Fe2O4. The shielding studies revealed that Mn–Ni–Zn ferrite systems are effectively suppressing electromagnetic interference (EMI) in X-band frequency. Comparatively, the ferrite system Mn0.1Ni0.7Zn0.2Fe2O4 is useful for the fabrication of EMI suppressor.

Flammulina velutipes-like Co@NCNTs enhancing the electromagnetic wave absorption performance

Magnetic nitrogen-doped carbon nanotubes have been recognized as promising microstructures for electromagnetic wave absorption, benefiting from the internal hollow structure, excellent electrical conductivity and easy functionalization. Herein, a new composite composed of the thin-wall carbon nanotubes coated with the cobalt nanoparticles (hereafter labelled as: Co@NCNTs) on the tip was designed and synthesized through high temperature pyrolysis process. Revealed by scanning electron microscopy and X-ray photoelectron spectroscopy, the geometric and electronic structures of product were susceptible to the carbonization temperature. With the multiple dielectric polarization relaxations, higher magnetism and good impedance matching, the flammulina velutipes-like Co@NCNTs-700 exhibited the strongest dielectric loss, magnetic loss and electromagnetic wave absorption performance. The minimum reflection loss value and effective absorption bandwidth was determined to be −21.0 dB at 4.96 GHz and 5.28 GHz, respectively, superior to the previous reports. Furthermore, the thickness is at 2 mm and the filler mass loading is only 10%. Therefore, our present Co@NCNTs materials will be an ultralight material as potential high-efficiency EMW absorber.

Multiple dielectric relaxations and superior sonocatalysis of bismuth iron niobate pyrochlores via high-level Co-doping

Manipulating the structure of Bi-based pyrochlores can produce excellent physicochemical properties and novel phenomena due to lattice disorders and distortions. Herein, bismuth iron niobate pyrochlores via high-level Co-doping were successfully fabricated by a sol-gel auto-combustion method and the doping effects on their structural, magnetic, dielectric and sonocatalytic properties were systematically investigated. Rietveld refinements revealed substituting Fe sites with Co ions (at a ratio of 30%–80%) can maintain the pyrochlore phase and induce larger lattice distortion. New dielectric relaxation and outstanding magneto-dielectric effect with a maximal 4% coefficient were clearly observed in the heavily Co-doped sample at low temperatures, which correlate with structural disorder and magnetic state transitions. New types of dielectric anomalies that gradually change the relative permittivity but maintain the dielectric loss almost constant with increasing temperature or frequency and can be tuneable at different doping levels were first observed above room temperature. The activation of ultrasonic vibration-driven degradation in the specimens was evaluated by the catalytic oxidation of dye, and the Fe/Co-coexisting catalysts exhibited superior sonocatalytic activity with magnetic recyclability, opening up a new pathway of developing feature-rich pyrochlores in the catalytic field.

Multifunctional BiFeO3 composites: Absorption attenuation dominated effective electromagnetic interference shielding and electromagnetic absorption induced by multiple dielectric and magnetic relaxations

BiFeO3 (BFO) grains prepared by molten salt method were fabricated in paraffin wax based composite structure to evaluate electromagnetic interference (EMI) shielding and electromagnetic (EM) absorption in 2–18 GHz. In the composites, BFO750 paraffin wax composite exhibits excellent EMI shielding and EM absorption performance. The EMI shielding effectiveness of BFO750 paraffin wax composite surpasses 10 dB, and its reflection loss shows three absorption peaks which reach −17 dB. The absorption efficiency and attenuation constant are close to 80% and 70, respectively. The results demonstrate that absorption attenuation is the dominant mechanism of EMI shielding and EM absorption. This is attributed to multiple dielectric and magnetic relaxations, where electric and magnetic dipoles coexisting in the composite can respond intensively to electromagnetic wave in the frequency range and thus cause high electromagnetic loss. The work highlights BFO composites as promising multifunctional materials in electromagnetic wave protection fields.

Observation of multiple dielectric relaxations in BaTiO3-Bi(Li1/3Ti2/3)O3 ceramics

Dense (1 − x)BaTiO3-xBi(Li1/3Ti2/3)O3 ceramics were fabricated by the solid state reaction route. Powder X-ray diffraction analyses revealed an increase in the unit cell volume with increasing x and a change on the average crystal structure from tetragonal (space group P4mm) to cubic ( $$ Pm\bar{3}m $$ ) at x > 0.10. Raman spectroscopy analyses corroborated a change of symmetry, but also showed the local structure for x > 0.10 to be inconsistent with the centrosymmetric ( $$ Pm\bar{3}m $$ ) space group. The dielectric measurements revealed for the first time, to our knowledge, a double relaxor behaviour in a BaTiO3-based solid solution. Basically, with increasing x, the sharp ferroelectric anomaly at the Curie temperature (T c) shifts towards lower temperatures until a relaxor-type response is observed, but simultaneously, another relaxation emerges above T c. The first arises from poor coupling between polar nanoregions, whereas the later obeys the Arrhenius Law and may be associated either with a defect-dipole reorientation or a Skanavi-type mechanism.

Dielectric data analysis: recovering hidden relaxations by fourth-order derivative spectroscopy

Complex materials exhibit dielectric behavior that can shed light on their chemophysical structure through comprehension of the physical mechanisms originating that behavior. For the interpretation of complex dielectric shape, where multiple relaxations are present (and usually described by semi-empirical functions), correct interpretation of each simple relaxation is necessary. The separation of often closely overlapped relaxation curves requires the verification of the meaningfulness of such multiple relaxations and to determine their number and relative magnitudes, in other words to resort to fitting procedure. However, often the physical interpretation of the fitting parameters is not straightforward, and their physical meaning is arguable. The application of even-order derivative spectroscopy is here demonstrated to be able to detect the presence and location of multiple dielectric relaxations, acting on the experimental data without any a priori hypothesis.

Effect of Preparation Conditions on Magnetic and Dielectric Properties of Y2MMnO6 (M = Co, Ni)

Herein we report the structure, magnetic and dielectric properties of Y2CoMnO6 (YCMO) and Y2NiMnO6 (YNMO) prepared under various conditions. On annealing in inert atmosphere, YCMO sample shows a partial decomposition while YNMO remains unaffected. Both YCMO and YNMO samples show paramagnetic to ferromagnetic transition around 80 K, but only the magnetic properties of YCMO samples depend on the preparation conditions. YCMO samples show antiferromagnetic ground state in zero field and a metamagnetic transition at relatively low field (~2 T). Differences in the annealing environment do not destroy the initial antiparallel spin order in YCMO, but affect the critical fields of metamagnetic transition. The field‐dependent magnetic properties indicated soft ferromagnetic nature of YNMO samples. The dielectric measurements show distinct frequency and temperature‐dependent features in YCMO samples compared to YNMO samples. A large relative permittivity with ε׳ of about 104 is observed in both O2 and air‐annealed YCMO samples. Multiple dielectric relaxations are observed in all the annealed YCMO and YNMO samples.

Dynamics of multiple phases in a colossal-magnetoresistive manganite as revealed by dielectric spectroscopy

Electronic phase separation is one of the key features in correlated electron oxides. The coexistence and competition of multiple phases give rise to gigantic responses to tiny stimuli producing dramatic changes in magnetic, transport and other properties of these compounds. To probe the physical properties of each phase separately is crucial for a comprehensive understanding of phase separation phenomena and for designing their device functions. Here we unravel, using a unique p-n junction configuration, dynamic properties of multiple phases in manganite thin films. The multiple dielectric relaxations have been detected and their corresponding multiple phases have been identified, while the activation energies of dielectric responses from different phases were extracted separately. Their phase evolutions with changing both temperature and applied magnetic field have been demonstrated by dielectric response. These results provide a guideline for exploring the electronic phase separation phenomena in correlated electron oxides.

Phase coexistence and multiple dielectric relaxations in the single layered manganite Nd0.22Sr1.78MnO4

We report the magnetization, dc and ac conductivities, and dielectric response of the single layered manganite with formulation Nd0.22Sr1.78MnO4. The sample undergoes charge/orbital ordering approximately below 260 K, and a region of thermal hysteresis is observed in the magnetization data between 175 and 50 K. The dc electrical resistivity indicates Arrhenius type thermally activated behavior at high temperature and a complicated conduction behavior at low temperature. In the ac conductivity measurement, the universal dielectric response is observed over the selected frequency range at low temperature (below 155 K). The sample shows dielectric response very similar to polycrystalline CaCu3Ti4O12 and other manganites, with a value of the room temperature dielectric permittivity as large as 5500 measured at 5 kHz of frequency. The disorder induced charge localization and the polaron hopping mechanism are possibly responsible for the overall electronic conduction and dielectric response of the sample. The sample shows signature of multiple relaxations, which is intrinsic to the sample and presumably related to the phase separation around the first order structural transition in the charge/orbital ordered phase.

Chain Dynamics in Solid Polymers and Polymerizing Systems as Revealed by Broadband Dielectric Spectroscopy

AbstractBroadband Dielectric Spectroscopy (BDS) provides a direct, powerful means of studying the chain dynamics of solid polymers. The present work discusses experimental data, phenomenological theories, molecular theories and recent molecular dynamics simulations of the relaxation functions and average relaxation times for the multiple dielectric relaxations observed for amorphous polymers. Examples of current applications of BDS are given, including studies of the dynamics of ultra‐thin polymer films and real‐time studies of (i) the crystallization behaviour of bulk polymers and (ii) thermo‐polymerizing systems.

Low-Frequency Dielectric Responses of Barium Strontium Titanate Thin Films with Conducting Perovskite LaNiO3 Electrode

Highly (h00)-oriented (Ba,Sr)TiO3 [BST] thin films were deposited by pulsed laser depositi on on the perovskite LaNiO3 metallic oxide layer as a bottom electrode. The LaNiO3 films were deposited on SiO2/Si substrates by the rf-magnetron sputtering method. The crystal line phases of the BST film were characterized by X-ray θ–2θ, ω-rocking curve and Φ-scan diffraction measurements. The surface microstructure observed by scanning electron mi croscopy was very dense and smooth. The low-frequency dielectric responses of the BST films grown at various substrate temperatures were measured as a function of frequency in the frequency range from 0.1 Hz to 10 MHz. The BST films have the dielectric constant of 265 at 1 kHz and showed multiple dielectric relaxations in the measured frequency region. The origins of these low-frequency dielectric relaxations are attributed to ionized space charge carriers such as the oxygen vacancies and defects in the BST film, the interfacial polarization in the grain boundary region and the electrode polarization. We also studied the capacitance-voltage characteristics of BST films.

Multiple dielectric relaxations in liquid crystals, their analysis and interpretation

Abstract Dielectric measurements on laterally substituted molecules were carried out in the frequency range 10 Hz to 10 MHz. In these systems, the main part of the molecules exhibits only a very small dipole moment in the direction of the para-axis, whereas the lateral substituent is strongly polar. Depending on the position of the dipole (o,m,p), a more or less strong dipolar correlation in the parallel direction was detected. The absorption data at higher frequencies were fitted to two Cole-Cole mechanisms. The low frequency relaxation was interpreted as angular vibration of the dipole moment of the lateral group, and the high frequency one as the reorientation about the para-axis of the main part of the molecule and of the lateral group. The appearance of two high frequency mechanisms is unexpected and demonstrates that in complicated molecules the dynamics also become differentiated.

Time Domain Analyses for Various Types of Dielectric Relaxations by a Lumped Capacitor Method

Several formulas are proposed for analyzing by a lumped capacitor method time domain responses to an incident wave of finite transient time for different types of dielectric relaxations with dc electric conductivities. The experimental problems associated with the transient behavior of the responses and the measurements of the multiple relaxation type dielectric relaxations are discussed.

A multi-state barrier model for the multiple dielectric relaxations of bulk amorphous polymers and related glass-forming systems

Abstract The dynamics of a fluctuation model, broadly based on an earlier model of Bueche, is considered for the motion of a molecule in a glass-forming material. The molecule is assumed to occupy a number of local states in four equivalent manifolds and relaxes by motion between the local states and between manifolds. The dipole-moment correlation function, and hence the dielectric relaxation behaviour, is calculated for a range of relaxation parameters. Two general processes are found being (i) a family of ‘fast’ relaxation modes due to motion between the local states and (ii) a single ‘slow’ relaxation mode due to motion out of one manifold into adjacent manifolds. This pattern is qualitatively in accord with observations of and α processes in glass-forming systems but the observed α-processes are generally rather broader than a ‘slow’ single relaxation-time process which results from the present model.