Debye-like Relaxation Research Articles

Observation of a dynamical crossover in the shear relaxation processes in supercooled selenium near the glass transition.

Shear relaxation in supercooled selenium liquid near its glass transition over the viscosity range of 106 Pa s-1012 Pa s is studied using oscillatory parallel plate rheometry. The results demonstrate the presence of a slow, Debye-like relaxation process and a fast, cooperative relaxation process that are attributed, respectively, to the Se-Se bond scission/renewal dynamics and the segmental motion of selenium chains. The slow process displays a nearly-Arrhenius temperature dependence, while the fast process is strongly non-Arrhenius, and their combined contribution to viscosity is estimated using the Maxwell relation. The slow process is found to be coupled to viscous flow over the entire viscosity range. In contrast, the fast process becomes a major contributor to viscosity, and hence, to fragility only near Tg. This dynamical crossover is likely a fundamental characteristic of fragile liquids that represents a temperature dependent evolution of their free energy landscape. The fragility of supercooled selenium liquid appears to be remarkably closely linked to the temperature dependence of the shear modulus of the slow process, thus validating the prediction of the elastic shoving model.

Peculiar relaxation dynamics of propylene carbonate derivatives

The aim of this work is to analyze in detail the effect of the alkyl chain length on the dynamics of glass-forming propylene carbonate (PC) derivatives. Examined samples are low-molecular weight derivatives of the PC structure, i.e., the 4-alkyl-1,3-dioxolan-2-one series, modified by changing the alkyl substituent from methyl to hexyl. The molecular dynamics (MD) has been analyzed based on experimental data collected from differential scanning calorimetry, broadband dielectric spectroscopy (BDS), X-ray diffraction (XRD), and nuclear magnetic resonance relaxometry measurements as well as MD simulations. The dielectric results show in samples with the propyl- or longer carbon chain the presence of slow Debye-like relaxation with features similar to those found in associative materials. Both XRD and MD reveal differences in the intermolecular structure between PC and 4-butyl-1,3-dioxolan-2-one liquids. Moreover, MD shows that the probability of finding one terminal carbon atom of the side chain of BPC in the vicinity of another carbon atom of the same type is much higher than in the case of PC. It suggests that there is a preference for longer hydrocarbon chains to set themselves close to each other. Consequently, the observed slow-mode peak may be caused by movement of aggregates maintained by van der Waals interactions. Reported herein, findings provide a new insight into the molecular origin of Debye-like relaxation.

About dielectric relaxation in highly cross-linked poly(ethylene oxide)

The effect of lithium salt on dynamics in highly cross-linked poly(ethylene oxide) (PEO) has been investigated. The elaboration uses results of dielectric relaxation studies. It turns out that competition of electric and structural relaxation coins conductivity mechanism. Neat cross-linked PEO with low mesh size can be transferred in super-cooled liquid state. Then, cross-linked PEO behaves like a hydrogen-bonded liquid since crystallization is strongly suppressed. As a result, one observes slow Debye-like relaxation at low temperature. It disappears after addition of salt since interaction of salt with polymer chains is stronger than the hydrogen-bonded network in the neat polymer. Analysis of tangent-loss spectra shows: Particle density governing dc conductivity does not depend on temperature at low concentration of added salt. It increases with temperature for neat cross-linked PEO and PEO loaded with sufficiently high concentration of salt. Prevention of crystallization requires a tight network of cross-links. Scaled representations of relevant impedance data for neat cross-linked PEO over extended ranges of frequency and temperature reveal that electric and structural relaxations are independent of temperature to good approximation at low and high frequency. There is a range of damped network oscillations, sandwiched between these limits, where relaxation becomes dependent on temperature. This range lessens with temperature. It does not occur at all in salt-comprising cross-linked PEO.

Dynamics of two glass forming monohydroxy alcohols by field cycling 1H NMR relaxometry

The dynamics of 2,2-dimethylbutan-1-ol (2,2-DM-1-B) and 3,3-dimethylbutan-1-ol (3,3-DM-1-B), two glass forming monohydroxy alcohols, was investigated by field cycling 1H NMR relaxometry in their liquid phase, including the supercooled regime. Nuclear Magnetic Relaxation Dispersion (NMRD) curves (i.e. longitudinal relaxation rate R1vs1H Larmor frequency), acquired for the two alcohols at different temperatures in the 0.01-35 MHz frequency range, were analyzed in terms of suitable models for internal motions, overall molecular reorientations, and molecular self-diffusion, and the corresponding correlation times were quantitatively determined. In addition, trends of 1H R1 as a function of the frequency square root at low frequencies, where the contribution of translational motions dominates, were exploited to achieve an independent determination of the self-diffusion coefficients (D), which does not require the separation of different motional contributions to relaxation. Good agreement was found between D values determined by the two methods, thus corroborating the model used for the description of the NMRD curves. Self-diffusion was found to be slower and more strongly temperature dependent for 2,2-DM-1-B with respect to 3,3-DM-1-B, whereas molecular reorientations were quite similar for the two isomeric alcohols. Correlation times for molecular reorientations were found to be at least one order of magnitude shorter than those reported in the literature for the Debye-like relaxation observed by dielectric spectroscopy in the liquid phase.

Nature of the Debye-Process in Monohydroxy Alcohols: 5-Methyl-2-Hexanol Investigated by Depolarized Light Scattering and Dielectric Spectroscopy.

The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long-standing debate. In the present Letter, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized dynamic light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations that occur due to a slight anisotropy of the α relaxation caused by the chain formation.

About impedance spectra and dynamics of charge carriers in neat poly(ethylene oxide) structures

Impedance spectra of linear and cross-linked poly(ethylene oxide) (PEO) are analyzed in a wide range of temperature. Dielectric responses differ at low and high temperature due to tendency of PEO to crystallization below melting temperature. Extent of crystallization depends on cross-linking density. The network in PEO with high cross-linking density is rigid and morphology transition shifts to very low temperature. Debye-like relaxation appears at low temperature similar as in ionic liquids. Onset of polarization relaxation shifts to higher temperature with increasing mesh size that is coupling of electric and structural relaxation appears. This is also nicely reflected by scaled conductivity. It demonstrates that the structure in cross-linked systems is a superposition of chemical and physical networks. They relax separately at low temperature and frequency under condition of sufficient rigidity.

Improved dielectric, nonlinear and magnetic properties of cobalt-doped CaCu3Ti4O12 ceramics

The dielectric properties and voltage–current nonlinearity of the pure and various cobalt-doped CaCu3Ti4O12 (Co-doped CCTO) prepared by solid-state reactions were investigated. The improved dielectric properties in the Co-doped CCTO, with a dielectric constant ε' ≈7.4 × 104 and dielectric loss tanδ ≈ 0.034, were observed in the sample with a Co doping of 5% (CCTO05) at room temperature and 1 kHz. The related multi-relaxations, RII (−20 to 40 °C) and RIII (100–150 °C), were demonstrated to be a Debye-like relaxation and a Maxwell–Wagner relaxation related to oxygen vacancies. The low dielectric loss of CCTO05 was associated with the high grain boundary resistance and the increase in cation vacancies. The improved nonlinear electrical properties (CCTO05, with nonlinear coefficients α ≈ 5.22 and breakdown electric field Eb ≈ 300.46 V/cm) and the ferromagnetism in Co-doped CCTO were also discussed.

Structural, electrical, and multiferroic characteristics of lead-free multiferroic: Bi(Co0.5Ti0.5)O3-BiFeO3 solid solution.

A solid solution of bismuth cobalt titanate [Bi(Co0.5Ti0.5)O3] and bismuth ferrite (BiFeO3) with a composition Bi(Co0.40Ti0.40Fe0.20)O3 (abbreviated as BCTF80/20) was synthesized via a cost effective solid-state technique. Phase identification and basic structural symmetry of the samples were determined by analyzing powder X-ray diffraction data. Field emission scanning electron micrograph (FE-SEM) and energy dispersive X-ray (EDX) spectra were analyzed to evaluate the micro-structural aspects (shape and size, distribution of grains) as well as a quantitative evaluation of the sample. The average crystallite (particle) and grain size were found to be ∼30 nm and ∼1–2 micron, respectively. The electrical parameters (dielectric constant, tangent loss, impedance, modulus, and conductivity) of as-synthesized material were obtained in a temperature range of 300 to 773 K and frequency range of 1 kHz and 1000 kHz. The strong correlation of microstructure (i.e., grains, grain boundary, etc.) and electrical parameters of this material were observed. The frequency dependence of electrical impedance and modulus exhibited a deviation from an ideal Debye-like relaxation process. The dependence of dielectric relaxation mechanism on frequency and temperature is discussed in detail. The field dependent polarization (P–E hysteresis loop) of BCTF80/20 exhibited an enhanced value of remnant polarization as compared to that of BiFeO3 (referred as BFO). At room temperature (300 K), the magnetic hysteresis loop measurements also showed a significant improvement in the magnetization of BCTF80/20. Thus, based on these enhanced values of remnant polarization and magnetic parameters, we can assume that BCTF80/20 may be considered as a promising candidate for some new generations of electronic devices.

Influence of Cu2+ doping on the structure, dielectric and magnetic properties of NiFe2O4 prepared by the sol-gel method

CuxNi1−xFe2O4 powders were prepared by the sol-gel method and the corresponding temperature dependence of microstructure, magnetic and dielectric properties of different CuxNi1−xFe2O4 samples have been investigated. Results of XRD and SEM indicate that the Cu2+ concentration played an important role in both crystal phase and particle distribution of CuxNi1−xFe2O4. The measurement of magnetic properties shows that both Cu2+ doping and sintering temperature could change the saturation magnetization and remanent magnetization in a noticeable manner, and even the magnetic material type. The Cu0.2Ni0.8Fe2O4 sample sintered at 900 °C had the largest saturation magnetization (34.61 emu/g), remanent magnetization (17.85 emu/g) and the lowest coercivity (0.17 kOe). The dielectric measurements show strong frequency dependence for all the samples. The peak observed in frequency dependence of dielectric loss measurements shifts to higher frequency with the increasing Cu2+ concentration and sintering temperature, indicating a Debye-like dielectric relaxation. All the results indicate that the Cu2+ doping has a significant influence on NiFe2O4 ferrites.

Inertial effects in systems with magnetic charge

This short article sets out some of the basic considerations that go into detecting the mass of quasiparticles with effective magnetic charge in solids. Effective magnetic charges may be appear as defects in particular magnetic textures. A magnetic monopole is a defect in this texture and as such these are not monopoles in the actual magnetic field B, but instead in the auxiliary field H. They may have particular properties expected for such quasiparticles such as magnetic charge and mass. This effective mass may–in principle–be detected in the same fashion that the mass is detected of other particles classically e.g. through their inertial response to time-dependent electromagnetic fields. I discuss this physics in the context of the “simple” case of the quantum spin ices, but aspects are broadly applicable. Based on extensions to Ryzkhin's model for classical spin ice, a hydrodynamic formulation can be given that takes into account inertial and entropic forces. Ultimately, a form for the susceptibility is obtained that is equivalent to the Rocard equation, which is a classic form used to account for inertial effects in the context of Debye-like relaxation.

Debye-like relaxation behavior and electric field induced dipole re-orientation of the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic

Mechanisms of the dielectric relaxation and electric field induced dipole re-orientation process in the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic have been investigated by a combination of experiments and simulations. Using the modified Debye relaxation theory, the observed strong relaxation is related to the evolution of the distribution of relaxation times with increasing temperatures. The increased freezing temperature under dc bias electric fields indicates that the nonlinear dielectric behavior has positive contributions to the total polarization. In relaxor state, the dielectric response of the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic shows a maximum value with the increasing external fields, which is attributed to two main polarization mechanisms, lattice phonon polarization and re-orientation of polar clusters. The contribution of these two polarization mechanisms is separated by the multi-polarization model, indicating that the re-orientation of polar clusters is responsible for the increase of the dielectric constant. At last, Monte Carlo method is performed to simulate the dipole re-orientation process, and the simulated results are consistent with experimental findings. This study provides a different perspective to investigate the relaxor materials with isolated polar clusters.

Structural and Dielectric Relaxations in Vitreous and Liquid State of Monohydroxy Alcohol at High Pressure.

2-Ethyl-1-hexanol monoalcohol is a well-known molecular glassformer, which for a long time attracts attention of researchers. As in all other monohydroxy alcohols, its dielectric relaxation reveals two distinct relaxation processes attributed to the structural relaxation and another more intense process, which gives rise to a low-frequency Debye-like relaxation. In this monoalcohol, the frequency separation between these two processes reaches an extremely high value of 3 orders of magnitude, which makes this substance a rather convenient object for studies of mechanisms (supposedly common to all monoalcohols) leading to vitrification of this type of liquids. In this work, we apply two experimental techniques, dielectric spectroscopy and ultrasonic measurements (in both longitudinal and transverse polarizations) at high pressure, to study interference between different relaxation mechanisms occurring in this liquid, which could shed light on both structural and dielectric relaxation processes observed in a supercooled liquid and a glass state. Application of high pressure in this case leads to the simplification of the frequency spectrum of dielectric relaxation, where only one asymmetric feature is observed. Nonetheless, the maximum attenuation of the longitudinal wave in ultrasonic experiments at high pressure is observed at temperatures ≈50 K above the corresponding temperature for the transverse wave. This might indicate different mechanisms of structural relaxation in shear and bulk elasticities in this liquid.

Dielectric properties of unfilled tetragonal tungsten bronze Ba4PrFe0.5Nb9.5O30 ceramics

In order to found new dielectrics ceramics in tungsten bronze structure, unfilled tungsten bronze (TB) ceramics with nominal formula Ba4PrFe0.5Nb9.5O30 were prepared by the solid state reaction method. The microstructure and dielectric properties were studied using powder X-ray diffraction, field emission scanning electron microscope, and variable temperature dielectric test system. The results show that the ceramics have a single phase and belong to the space group of P4bm with the cell of a = b = 12.4839(3) A, c = 3.9409(5) A, V = 614.192(2) A3. The frequency dependent dielectrics properties show that the ceramics have a Debye-like relaxation at room temperature, while the temperature dependent dielectrics properties indicate that the ceramics are a relaxor and the relaxation is due to the nanopolars and oxygen vacancies. The ceramics have a potential application in electronic ceramics as temperature-stable multilayer ceramic capacitors.

Superior electromagnetic properties obtained by enhanced resistivity on multiferroic barium titanate and hexaferrite di-phase composite ceramics

Abstract Di-phase multiferroic composite ceramics of 0.9BaTiO3-0.1Ba(1-x)SrxFe12O19 have been successfully prepared using microwave hydrothermal and microwave sintering methods. The mobility is decreased resulting in increased resistivity due to the crystal structure becomes more compact (Sr2+ ionic radius is smaller than Ba2+) after doping Sr2+. The study of dielectric exhibits the tanδ decreasing and e′ increasing after doping Sr2+. To further study the two relaxation peaks, the activation energy is calculated which originate in the Debye-like relaxation and hopping of electrons between Fe2+/Fe3+, respectively. The P-E loops show slimmer loops as a result of reduced the dielectric loss and conductive leakage loss owing to higher resistivity obtained after doping Sr2+. All the magnetic hysteresis loops present a waisted shape which is attributed to the change of grain size and existence of non-magnetic phase BaTiO3. In summary, the nearly invariable ferromagnetism but enhanced dielectric and ferroelectric properties multiferroic composite ceramics are obtained by doping Sr2+. In addition, the sample of x = 0.6 has the best-integrated performance among all the contents.

Influence of composition on structure, morphology and dielectric properties of BixAlyOz composite films synthesized by atomic layer deposition

In this experiment we used atomic layer deposited Bi2O3 and Al2O3 films to fabricate the BixAlyOz nanocomposite films. Bismuth composition was modulated in a series of samples via altering the number of bismuth subsidiary cycles. We discovered that the bismuth composition in atomic percent did not monotonically increase with the rise of the number of bismuth cycles. To explain this unconventional variation trend we proposed a growth mechanism regarding the steric hindrance of Bi(thd)3 and the diffusion of TMA. A single Debye-like relaxation can be determined for the BixAlyOz films with a parallel RC element to reflect it. The change in the dielectric response, such as dielectric constant and ac conductivity, was found to be in agreement with the variation of bismuth concentration. This could be attributed to the effect of 6s2 lone pair electrons in Bi-O dipoles. To further confirm the conductive strength, we estimated the optical band gap (Eg) using plots of (αhν)2 versus hν. The value of Eg would decrease from 5.62 eV to 5.00 eV as the bismuth content rises in the BixAlyOz films.

On the non-exponentiality of the dielectric Debye-like relaxation of monoalcohols.

We have investigated the Debye-like relaxation in a series of monoalcohols (MAs) by broadband dielectric spectroscopy and thermally stimulated depolarization current techniques in order to get further insight on the time dispersion of this intriguing relaxation. Results indicate that the Debye-like relaxation of MAs is not always of exponential type and conforms well to a dispersion of Cole-Davidson type. Apart from the already reported non-exponentiality of the Debye-like relaxation in 2-hexyl-1-decanol and 2-butyl-1-octanol, a detailed analysis of the dielectric permittivity of 5-methyl-3-heptanol shows that this MA also presents some extent of dispersion on its Debye-like relaxation which strongly depends on the temperature. Results suggest that the non-exponential character of the Debye-like relaxation might be a general characteristic in the case of not so intense Debye-like relaxations relative to the α relaxation. Finally, we briefly discuss on the T-dependence and possible origin for the observed dispersion.

An effective optimization strategy and analysis for BaTiO3–Na0.5Bi0.5TiO3 system with colossal permittivity

Colossal permittivity (CP, ε＞104) behavior in BaTiO3–Na0.5Bi0.5TiO3 (BT-NBT) ceramics has been studied, which showed extremely high permittivity up to ~105. Dielectric properties of samples showed Debye-like relaxations in the frequency range 20Hz–30MHz. Two different polarizations located in grain boundaries and grains respectively are responsible for the CP behavior and the models of defect charge compensation achieved by niobium doping are proposed to explain the phenomenon of abnormal variation of dielectric constant.By using defect engineering, a Nb-doped BaTiO3 ceramics with stable colossal permittivity (εr =1.3×104 at 1kHz and room temperature)，high bulk resistivity (>1010Ω·cm) as well as relative low dielectric loss (tanδ~0.06) has been obtained over a wide temperature range of −55–150°C, satisfying IEA X8R specification, which has a potential application prospect in high capacity solid supercapacitor.

Weak ferromagnetism and short range polar order in NaMnF3 thin films

The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF3 thin films were grown on SrTiO3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.

Electrode polarization and unusual magnetodielectric effect in a transformer oil-based magnetic nanofluid thin layer.

In the present paper, we provide low-frequency dielectric spectra for a thin layer of a nanofluid based on transformer oil and iron oxide nanoparticles stabilized by oleic acid. The complex dielectric permittivity measured in the frequency range from 1 mHz to 200 kHz shows an obvious electrode polarization effect and a Debye-like dielectric relaxation process. Both effects stem from the presence of space charge in the oil due to impurity ions, and in the nanofluid represented predominantly by a residual surfactant and uncompensated particle surface charge. It is shown that the spectra, which were measured in the temperature range from 298 K to 358 K, can be well represented by a fitting function consisted of one Havriliak-Negami term and the Jonscher's power law. In the investigated magnetic nanofluid layer, we found that the onset of the electrode polarization is suppressed to lower frequencies by the application of an external magnetic field (300 mT). This phenomenon is explained by a slowed-down migration of the space charge due to the Lorentz force and by a hindering effect of the formed magnetic nanoparticle aggregates. Surprisingly, a moderate decrease in the whole permittivity spectrum was observed for both parallel and perpendicular orientations of the electric and magnetic fields. This is in contradiction with the usual magnetodielectric anisotropy effect. Based on our qualitative analysis, we discuss potential reasons accountable for the observed effect.

High Frequency Electrical Properties of Fe/SiO<sub>2</sub> Composites by Pressureless Sinter

In this paper, Fe/SiO2 composites with different Fe content were prepared through pressureless sinter in weakly reductive atmosphere after high energy ball milling. The electrical properties including ac conductivity, permittivity were investigated in detail. The results showed that the frequency dispersions of ac conductivity follow the power law, indicating the hoping conduction behavior. The real part of permittivity for composites increased with higher Fe content, which was attributed to the more interface between Fe and SiO2. Besides, Debye-like dielectric relaxations were observed, and the Debye-like loss peak appeared blue shift as increasing the Fe content.