Broadband Dielectric Spectroscopy Studies Research Articles

Supercriticality, Glassy Dynamics, and the New Insight into Melting/Freezing Discontinuous Transition in Linseed Oil

The long-range supercritical changes of dielectric constant, resembling ones observed in the isotropic liquid phase of liquid crystalline compounds, are evidenced for linseed oil—although in the given case, the phenomenon is associated with the liquid–solid melting/freezing discontinuous phase transitions. This ‘supercriticality’ can be an additional factor supporting the unique pro-health properties of linseed oil. Broadband dielectric spectroscopy studies also revealed the ‘glassy’ changes of relaxation times, well portrayed by the ‘activated and critical’ equation recently introduced. In the solid phase, the premelting effect characteristic for the canonic melting/freezing discontinuous transition, i.e., without any pretransitional effect in the liquid phase, has been detected. It is interpreted within the grain model, and its parameterization is possible using the Lipovsky model and the ‘reversed’ Mossotti catastrophe concept. For the premelting effect in the solid state, the singular ‘critical’ temperature correlates with the bulk discontinuous melting and freezing temperatures. Consequently, the report shows that linseed oil, despite its ‘natural and complex’ origins, can be considered a unique model system for two fundamental problems: (i) pretransitional (supercritical) effects in the liquid state associated with a weakly discontinuous phase transition, and (ii) the premelting behavior in the solid side of the discontinuous melting/freezing discontinuous transition.

Microscopic mechanism of gas transport in mixed matrix membranes of coordination nanocages

The strong inter-phase attraction is required for the effective and stable integration of inorganics and polymers in mixed matrix membranes (MMMs), which would impact the structures and microscopic dynamics of the two phases and cause unexpected deviation of MMM performance from original design; however, the understanding of its structure-property relationship is still poor. Herein, 2 nm coordination nanocage (CNC) with well-defined, monodispersed structure is complexed with poly(4-vinyl pyridine-r-n-butyl acrylate) (P4VP-co-PnBA) via both coordination and hydrogen bonding to afford model systems. The surface interaction between the two phases is quantified by the thickness of absorbed layers of polymers on CNC surface as well as the segmental dynamics of polymers. In broadband dielectric spectroscopy (BDS) studies, accelerated chain dynamics can be observed in the nanocomposites with loadings of CNC, which is originated from the frustrated packing of polymers absorbed on CNC surface. The co-effect of interfacial layer thickness and chain dynamics leads to the non-monotonic dependence of the MMMs’ gas permeabilities on CNC concentrations. The temperature dependence of their gas permeability and chain dynamics are both explored and the two processes are fully decoupled at temperatures higher than 333 K due to the weakening of multiple supramolecular interaction. Our discoveries provide microscopic and quantitative understanding on structure-property relationship of MMMs, which benefits directly to the rational design of polymer structures for optimized gas separation performance.

The role of Zn and Ru substitution on the structural, magnetic and dielectric properties of ferrite cuprospinels

A comprehensive study on the structural, magnetic and dielectric behavior of ferrite cuprospinels with site specific substitution of non-magnetic Zn and weakly magnetic Ru ions has been presented. Such substitution leads to two classes of stable crystal symmetries, (i) tetragonal (I41/amd): CuFe1.95Ru0.05O4 and Cu0.95Zn0.05Fe1.95Ru0.05O4 and (ii) cubic (Fd-3m):Cu0.60Zn0.40Fe1.95Ru0.05O4 and Cu0.05Zn0.95Fe1.95Ru0.05O4 bulk polycrystals. The former set of compounds undergo significant Jahn-Teller (JT) distortion due to high Cu content and such distortion gets lifted with the substitution of non-magnetic Zn ions. Ferrimagnetic (FiM) behavior is predominant in all these compounds except the highest Zn (95 at.%) content cubic system with diluted substitution of Ru (2.5 at.%) which exhibits antiferromagnetic ordering below the Néel temperature (T N ∼ 10.9 K). We employed Néel’s expression for FiM systems applicable for the two sublattice model to extract the dominant exchange interactions (J AA , J AB and J BB ) between the A-site and B-site cations. Subsequently the high temperature (300 K T 900 K) inverse paramagnetic susceptibility data has been fitted to the above expression which yields dominant ∼ 391 K with very high asymptotic Curie temperature for the tetragonally distorted dilute Zn substituted cuprospinel, while the Zn-rich cubic system exhibits ∼ 300 K. On the other hand, the tetragonally distorted spinel devoid of Zn exhibits dominant ∼ 229 K with negligible and moderate (65 K) signifying that the JT active ions cause the onset of magnetic frustration and enhanced ordering temperatures. All the investigated systems exhibit significant magneto-crystalline anisotropy with a maximum anisotropy field H K ∼ 6 kOe and maximum anisotropy constant (K 1 ∼ 10.88×105 erg cm−3). The broadband dielectric spectroscopy studies reveal significant enhancement of mid-frequency dielectric constant ε R (f, T) with plateau-like feature over a wide temperature and frequency window, together with low-loss as a result of the incorporation of Ru ions into the tetragonally distorted spinel lattice. The ac resistivity ρ(f, T) studies reveal that the mode of conduction follows Mott’s variable-range hopping (VRH) model with a linear logarithmic variation of ρ ac versus T −0.25. The temperature dependence of the VRH activation energy of charge carriers follows a linear behavior which indicates the presence of localized polaron in all the investigated systems.

Critical-like behavior of low-frequency dielectric properties in compressed liquid crystalline octyloxycyanobiphenyl (8OCB) and its nanocolloid with paraelectric BaTiO3

The report presents pressure-related broadband dielectric spectroscopy (BDS) studies in liquid crystalline octyloxycyanobiphenyl (8OCB) and its nanocolloid with BaTiO3 paraelectric nanoparticles. It is focused on the low-frequency (LF) domain, and the impact of pretransitional fluctuations. Studies were carried out in isotropic liquid, nematic, Smectic A, and solid crystal phases. In each case, the critical-type behavior for the LF contribution to the real part of dielectric permittivity and DC electric conductivity is evidenced. Studies of dielectric constant, related to the static frequency domain, revealed in SmA mesophase the shift from the XY-type to the mean-field type criticality when adding nanoparticles. The ‘giant’ premelting effect, with the critical-like portrayal, in the solid crystal phase, is also evidenced. Studies are supplemented by the polarizing microscopy insight into all tested phases appearing on compressing in 8OCB, and the nanocolloid. Photos show prenematic fluctuations in the isotropic liquid phase, phase transitions, changes within nematic and smectic mesophases on compressing, and the impact of nanoparticles.

Dielectric relaxation of ice in a partially crystallized poly(N-isopropylacrylamide)microgel suspension compared to other partially crystalized polymer-water mixtures.

A broadband dielectric spectroscopy study was conducted on a partially crystallized 10 wt% poly(N-isopropylacrylamide) [PNIPAM] microgel aqueous suspension to investigate the dielectric relaxation of ice in microgel suspensions. The measurements covered a frequency range of 10 mHz to 10 MHz and at temperatures ranging from 123 K to 273 K. Two distinct relaxation processes were observed at specific frequencies below the melting temperature. One is associated with the combination of the local chain motion of PNIPAM and interfacial polarization in the uncrystallized phase, while another is associated with ice. To understand the temperature-dependent behaviour of the ice relaxation process, the relaxation time of ice was compared with those observed in other frozen polymer water mixtures, including gelatin, poly-vinylpyrrolidone (PVP), and bovine serum albumin (BSA). For concentrations ≥ 10 wt%, the temperature dependence of the relaxation time of ice was found to be independent. Therefore, the study primarily focused on the 10 wt% data for easier comprehension of the ice relaxation process. It was found that the microgel and globular protein BSA had no significant effect on ice crystallization, while gelatin slowed down the crystallization process, and PVP accelerated it. To discuss the mechanism of the dielectric relaxation of ice, the trap-controlled proton transport model developed by Khamzin et al. [Chem. Phys., 2021, 541, 111040.] was employed. The model was used to discuss the dynamic heterogeneity of ice observed in this investigation, distinguishing it from the spatial heterogeneity of ice commonly discussed.

Can Polymers be Irreversibly Adsorbed on Carbon Nanomembranes? A Combined XPS, AFM, and Broadband Dielectric Spectroscopy Study

Can Polymers be Irreversibly Adsorbed on Carbon Nanomembranes? A Combined XPS, AFM, and Broadband Dielectric Spectroscopy Study

Topological Interaction among Molecular Cluster Assemblies Affords Tunable Viscoelasticity.

In the assemblies of subnanoscale polyhedral oligomeric silsesquioxane, topological interaction makes the dominant contribution to their viscoelasticity with broad tunability. The assembly molecules are designed with dumbbell, triangular, and tetrahedral shapes, and they demonstrate an intrinsic glassy feature with neither long-range ordering nor supramolecular assembly formation in their bulk. Their viscoelasticity can be broadly tuned through the tailoring of molecular topologies, while the trimer and tetramer assemblies afford elastic moduli comparable to those of rubbers (∼0.5 MPa) even 80 K above their glass transition temperatures. Molecular dynamics studies reveal the topological constraints resulting from the topology-disrupted cooperative dynamics among the cluster assemblies, and this finally leads to the typical caging dynamics of the structural units and the elasticity of the bulk materials. Further broadband dielectric spectroscopy studies uncover the unique hierarchical relaxation dynamics, inspiring the strategy for the decoupling of mechanical strengths and toughness for the design of impact resistant materials.

Hard-to-soft transition-enhanced piezoelectricity in poly(vinylidene fluoride) via relaxor-like secondary crystals activated by high-power ultrasonication.

Although high piezoelectric coefficients have recently been observed in poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] random copolymers, they have low Curie temperatures, which makes their piezoelectricity thermally unstable. It has been challenging to achieve high piezoelectric performance from the more thermally stable PVDF homopolymer. In this report, we describe how high-power ultrasonic processing was used to induce a hard-to-soft piezoelectric transition and improve the piezoelectric coefficient d31 in neat PVDF. After high-power ultrasonication for 20 min, a uniaxially stretched and poled PVDF film exhibited a high d31 of 50.2 ± 1.7 pm V-1 at room temperature. Upon heating to 65 °C, the d31 increased to a maximum value of 76.2 ± 1.2 pm V-1, and the high piezoelectric performance persisted up to 110 °C. The enhanced piezoelectricity was attributed to the relaxor-like secondary crystals in the oriented amorphous fraction, broken off from the primary crystals by ultrasonication, as suggested by differential scanning calorimetry and broadband dielectric spectroscopy studies.

High ionic conductivity and ion conduction mechanism in ZIF-8 based quasi-solid-state electrolytes: a positron annihilation and broadband dielectric spectroscopy study.

The low ionic conductivity and electrode-electrolyte interface instability issues with solid polymer electrolytes jeopardize their electrochemical performances in lithium-ion batteries (LIBs). The use of quasi-solid-state electrolytes (QSSEs) with concentrated Li salt embedded inside the pore networks of metal organic frameworks (MOFs) can successfully address the aforementioned issues. Owing to the sieve effect of zeolitic imidazolate framework-8 (ZIF-8) towards selective cation permeability over anions through the interconnected pore network, a unique QSSE with LiTFSI salt concentrated in the ZIF-8 skeleton used as a filler in poly(ethylene oxide) has been synthesized. LiTFSI gets embedded inside the interconnected pore network of ZIF-8 that furnishes unhindered pathways for Li+ ion migration leading to a very high ionic conductivity of ∼6 × 10-4 S cm-1. The higher ionic conductivity is directly related to the Li+ ion conduction through the pore network of ZIF-8 which has been experimentally evident from complementary methods viz. Positron annihilation and broadband dielectric spectroscopy. The design route towards these types of QSSEs encompassing porous MOFs paves the way for realizing Li superionic conductors suitable for practical application in commercial LIBs with high safety and stability.

The fluctuation-driven dielectric properties of liquid crystalline 8OCB and its nanocolloids

Results of broadband dielectric spectroscopy studies in liquid crystalline octyloxycyanobiphenyl (8OCB) and its colloids with BaTiO3 nanoparticles (paraelectric, diameter d = 50 nm) are presented. Studies were carried out in isotropic liquid, nematic, smectic A and solid crystalline phases. They are supported by derivative-based and distortion-sensitive analyses, revealing a set of universal scaling patterns for temperature evolution of static and dynamic dielectric properties. All these yielded evidence for a pretransitional fluctuation impact on the dielectric constant, primary relaxation time, loss curve maximum and translational-orientational decoupling associated with anomalous values of the fractional Debye-Stokes-Einstein (DSE) exponent 0.2 < S < 2.2. The evidence for critical like changes in a distribution of relaxation times is shown. For mesophases, a tiny addition of nanoparticles causes permanent orientation of LC molecules, leading even to a 16% increase of the dielectric constant above a maximal value in pure 8OCB. A split of two regions, dominated by pre-isotropic and pre-smectic fluctuations, is evidenced. Model explanations for detected phenomena, particularly in the isotropic liquid phase, are presented. New evidence for a premelting effect in the solid phase is also shown.

Effects of partial crystallization on the glassy slowdown of aqueous ethylene glycol solutions

Combining differential scanning calorimetry, nuclear magnetic resonance, and broadband dielectric spectroscopy studies, we ascertain the glass transition of aqueous ethylene glycol (EG) solutions, in particular the effects of partial crystallization on their glassy slowdown. For the completely liquid solutions in the weakly supercooled regime, it is found that the dynamics of the components occur on very similar time scales, rotational and translational motions are coupled, and the structural (α) relaxation monotonously slows down with increasing EG concentration. Upon cooling, partial crystallization strongly alters the glassy dynamics of EG-poor solutions; in particular, it strongly retards the α relaxation of the remaining liquid fraction, causing a non-monotonous concentration dependence, and it results in a crossover from non-Arrhenius to Arrhenius temperature dependence. In the deeply supercooled regime, a recrossing of the respective α-relaxation times results from the Arrhenius behaviors of the partially frozen EG-poor solutions together with the non-Arrhenius behavior of the fully liquid EG-rich solutions. Exploiting the isotope selectivity of nuclear magnetic resonance, we observe different rotational dynamics of the components in this low-temperature range and determine the respective contributions to the ν relaxation decoupling from the α relaxation when the glass transition is approached. The results suggest that the ν process, which is usually regarded as a water process, actually also involves the EG molecules. In addition, we show that various kinds of partially crystalline aqueous systems share a common relaxation process, which is associated with the frozen fraction and differs from that of bulk hexagonal ice.

Role of free volumes and segmental dynamics on ion conductivity of PEO/LiTFSI solid polymer electrolytes filled with SiO2 nanoparticles: a positron annihilation and broadband dielectric spectroscopy study.

The limited ionic conductivity of polymer electrolytes is a major issue for their industrial application. Enhancement of ionic conductivity in the poly(ethylene oxide), PEO, based electrolyte has been achieved by loading passive nanofillers such as SiO2 nanoparticles (NPs). To investigate the role of modifications in free volume characteristics and the polymer chain dynamics induced by the loading of passive fillers on the ionic conductivity of the PEO based ternary electrolyte, a systematic investigation has been carried out using positron annihilation and broadband dielectric spectroscopy. As a result of interfacial interactions, the loading of SiO2 NPs alters the semi-crystalline morphology of PEO resulting in a higher crystallinity at lower loadings due to the surface confinement of PEO chains, and the formation of smaller PEO crystallites at higher loadings due to interparticle nanoconfinement. These modifications are accompanied by a decrease in free volume fraction at the lowest loading (0.5 wt%) followed by an increase at higher loadings (≥2.0 wt%). The Almond-West formalism considering two different universalities in different temperature and frequency ranges has been used to explain the ion-conduction process at different NP loadings. The Li ion conductivity is observed to be maximum for a 5.0 wt% loading of SiO2 NPs. The enhancement in ionic conductivity is observed to be directly correlated with the free volume characteristics and segmental dynamics of the PEO matrix, confirming their role in ion transport in polymer electrolytes.

Temperature and frequency dependence of dielectric relaxations in YFeO3

We report a dielectric study in wide frequency and temperature ranges of yttrium orthoferrite, YFeO3, in order to obtain the temperature-frequency dependence of the dielectric relaxation processes. A mixture of oxide powders (Y2O3 and Fe2O3) was activated by high-energy ball milling. The mechanically activated powders were pressed and sintered at 1073 K. The cylindrical test samples were characterized at room temperature by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Broadband Dielectric Spectroscopy (BDS) studies were carried out at several temperatures to the best understanding of dielectric properties. The X-ray diffraction patterns show an orthorhombic YFeO3 single phase. The magnetic hysteresis loop shows a weak ferromagnetic behavior, characteristic of the YFeO3. Dielectric spectroscopy analysis allows identifying two relaxation processes, the first at temperatures between 193 and 253 K and second above 333 K, defined as low-temperature dielectric relaxation (LTDR) and high-temperature dielectric relaxation (HTDR), respectively. It was determined the activation around 0.4 eV and 1.0 eV for the LTDR and HTDR, respectively. It indicates a single and doubly ionized oxygen vacancy. Moreover, it was proved that a hopping model is the dominating mechanism in the studied material, due to the conduction mechanism follows the Jonscher power law.

Polarization-Induced Quantum-Mechanical Charge Transfer in Perovskite–Graphene Nanocomposites with Superior Electro-optic Switching Modulation

Understanding the concept of light–matter interaction in organic–inorganic nanostructures such as graphene and metal-halide perovskites has been explored to realize photonic and optoelectronic devices. The strong light–matter interaction and impressive performance achieved by tuning the optical/electrical properties have enabled them as potential candidates for optoelectronic applications. In this perspective, we report the synthesis of metal-halide perovskite nanocomposites by incorporation of graphene sheets into the perovskite framework, which exhibits a significant enhancement in electro-optic (E-O) switching modulation. The carrier dynamics in the nanocomposite is investigated using time-resolved luminescence and broadband dielectric spectroscopy studies to get insights into the carrier relaxation and transfer mechanisms. The presence of graphene in the perovskite nanocomposites induced dielectric polarization with a strong electrical conduction and a nonlinear dielectric behavior with negative permittivity at the percolation threshold concentration because of the large resonance derived from the plasmonic oscillations of delocalized charges. A custom-designed optical fiber integrated with nanocomposites is scrutinized to explore the light modulation and E-O sensitivity under an applied electrical field. The E-O interference that leads to phase modulation of light by change in the refractive index of the graphene–CsPbBr3 nanocomposites resulted in a higher E-O sensitivity of 18 nm/V than those of CsPbBr3 nanostructures.

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions.

Combining broadband dielectric spectroscopy and nuclear magnetic resonance studies, we analyze the reorientation dynamics and the translational diffusion associated with the glassy slowdown of the eutectic aqueous dimethyl sulfoxide solution in nano-sized confinements, explicitly, in silica pores with different diameters and in ficoll and lysozyme matrices at different concentrations. We observe that both rotational and diffusive dynamics are slower and more heterogeneous in the confinements than in the bulk but the degree of these effects depends on the properties of the confinement and differs for the components of the solution. For the hard and the soft matrices, the slowdown and the heterogeneity become more prominent when the size of the confinement is reduced. In addition, the dynamics are more retarded for dimethyl sulfoxide than for water, implying specific guest-host interactions. Moreover, we find that the temperature dependence of the reorientation dynamics and of the translational diffusion differs in severe confinements, indicating a breakdown of the Stokes–Einstein–Debye relation. It is discussed to what extent these confinement effects can be rationalized in the framework of core-shell models, which assume bulk-like and slowed-down motions in central and interfacial confinement regions, respectively.

Dipolar glass state in BaCe0.3Ti0.7O3 perovskite solid solutions

An extensive broadband dielectric spectroscopy study of BaCe0.3Ti0.7O3 ceramics is presented. The distribution of relaxation times of the system derived from the frequency dependence of complex dielectric permittivity are similar to the ones observed in dipolar glasses. Therefore, it is demonstrated that the dipolar glass model, which is applied here for the first time for disordered perovskite oxides is able to properly describe the dielectric behavior of BaCe0.3Ti0.7O3 solid solution. The observed dipolar glass behavior in such BaTiO3-based solid solution is clearly different from the canonical Pb-based relaxors and the two types of dielectric behavior are comparatively discussed.

Dipole Relaxation in Semiconducting Zn2-xMgxInV3O11 Materials (Where x = 0.0, 0.4, 1.0, 1.6, and 2.0).

This paper reports on the electrical and broadband dielectric spectroscopy studies of Zn2−xMgxInV3O11 materials (where x = 0.0, 0.4, 1.0, 1.6, 2.0) synthesized using a solid-state reaction method. These studies showed n-type semiconducting properties with activation energies of 0.147–0.52 eV in the temperature range of 250–400 K, symmetric and linear I–V characteristics, both at 300 and 400 K, with a stronger carrier emission for the matrix and much less for the remaining samples, as well as the dipole relaxation, which was the slowest for the sample with x = 0.0 (matrix) and was faster for Mg-doped samples with x > 0.0. The faster the dipole relaxation, the greater the accumulation of electric charge. These effects were analyzed within a framework of the DC conductivity and the Cole–Cole fit function, including the solid-state density and porosity of the sample. The resistivity vs. temperature dependence was well fitted using the parallel resistor model. Our ab initio calculations also show that the bandgap increased with the Mg content.

Combustion synthesis, structural, magnetic and dielectric properties of Gd3+-doped lead molybdato-tungstates

Gd3+-doped lead molybdato-tungstates with the chemical formula of Pb1−3x□xGd2x(MoO4)1−3x(WO4)3x (where x = 0.0455, 0.0839, 0.1430, corresponding to 9.53, 18.32, 33.37 mol% of Gd3+, respectively, as well as □ denotes cationic vacancies) were successfully synthesized via combustion route. The XRD and SEM results confirmed the formation of single-phase, tetragonal scheelite-type materials (space group I41/a) with the uniform, spherical and oval grains ranging from 5 to 20 µm. Individual grains are strongly agglomerated into big clusters with the size even above 50 µm. The magnetic measurements as well as the Brillouin fitting procedure showed paramagnetic state with characteristic superparamagnetic-like behaviour and the short-range ferromagnetic interactions. The electrical and broadband dielectric spectroscopy studies revealed insulating properties with the residual electrical n-type conduction of 2×10−9 S/m and low energy loss (tanδ ≈ 0.01) below 300 K. Dielectric analysis showed that no dipole relaxation processes in the Gd3+-doped materials were observed. A fit of dielectric loss spectra of Gd3+-doped samples by sum of the conductivity and the Havriliak-Negami, Cole-Cole, and Cole-Davidson functions confirmed this effect.

Polyvinylidene difluoride-based composite: glassy dynamics and pretransitional behaviour

This paper presents results of broadband dielectric spectroscopy studies in the composite system for which particularly strong interactions between polyvinylidene difluoride (PVDF: ferroelectric polymer, TC = 453−473 K) matrix and barium strontium titanate (BST) ferroelectric micro-particles can be expected. For PVDF the super-Arrhenius (SA) dynamics, associated with segmental motions freezing at the glass temperature Tg = 235 K, is evidenced. The addition of BST particles qualitatively changes dynamics, converting the SA-type behaviour in PVDF to the clear Arrhenius one in BST/PVDF composite. The latter crossovers to the relaxor-type SA dynamics on cooling, exactly at the glass temperature of PVDF. The preliminary model explaining such unique behaviour is proposed. For the consistent portraying of the SA evolution of primary relaxation times in PVDF and BST/PVDF, the activation energy index analysis was carried out and the new equation, entropy and symmetry controlled, is introduced. Studies are accomplished by the analysis of the ferroelectric-paraelectric transition in PVDF and for the composite system. They led to the discovery of the strong pretransitional anomaly of dε∕dT, extending even to the vicinity of the room temperature, The semi-discontinuous nature of melting in PVDF and its composites, with the discontinuity metric △T ≈ 20 K is suggested.Graphical abstract

Investigating the Correlation of Segmental Dynamics, Free Volume Characteristics, and Ionic Conductivity in Poly(ethylene oxide)-Based Electrolyte: A Broadband Dielectric and Positron Annihilation Spectroscopy Study

Poly(ethylene oxide), PEO, based solid polymer electrolyte with different loadings of a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), has been investigated to study the role of...