Activation Energy Of Relaxation Research Articles

Exploring the optical and dielectric properties of bifunctional and trifunctional epoxy polymers

Two polymers were prepared from diglycidyl ether of bisphenol A (DGEBA) and triglycidyl p-aminophenol (TGAP) epoxy resins, using the same hardener diethylenetriamine (DETA). The obtained polymers were compared in terms of structural, optical, mechanical, thermal and dielectric properties. Dielectric spectroscopy and dynamic mechanical analysis indicated the presence of dipolar dielectric relaxations in both polymers, and interfacial one only for TGAP/DETA, which is consistent with XRD findings. Different approaches were considered to fit electrical modulus and conductivity and to estimate the relaxation times and activation energies according to temperature. For optical measures, a novel result processing approach is proposed for an accurate determination of the polymers bandgap and for highlighting the existence of additional energy transition level for TGAP. Results indicate that this polymer could be a potential candidate for use as a protective film and as UV/blue-light filter for screens and windows.

A comparative study on dielectric properties of PVDF/GO nanosheets encapsulated with different organic insulating shell

ABSTRACT To suppress the high dielectric loss of graphene oxide (GO)/poly(vinylidene fluoride) (PVDF) while still maintaining a high dielectric constant (k) near the filler percolation threshold, herein, GO nanosheets coated with polyvinyl pyrrolidone (PVP), poly(dopamine) (PDA) and tannic and Fe coordination complexes (TA-Fe), respectively, were incorporated into PVDF to investigate the effects of the three kinds of insulating interlayers on the dielectric properties of the nanocomposites. The results indicate that compared with pristine GO, all surface modified GO exhibit homogeneous dispersion in the matrix owing to the enhanced interface between fillers and matrix, and that the dielectric loss of the nanocomposites are significantly suppressed to rather low levels thanks to the presence of various insulating interlayers between GO nanosheets, which prevent them from direct contact, thereby remarkably reducing the leakage current and conductivity. Moreover, Activation energies for the pristine and modified GO/PVDF nanocomposites are calculated, respectively, and the introduction of insulating interlayers significantly reduces the relaxation activation energy of the nanocomposites.

Role of Organic Phosphates and Phosphonates in Catalyzing Dynamic Exchange Reactions in Thiol‐Click Vitrimers

AbstractOwing to their strong Brønsted acidity, organic phosphates and phosphonates are able to catalyze dynamic transesterification reactions in hydroxyl ester networks. Compared to commonly used transesterification catalysts, they are highly soluble in a wide range of acrylate monomers and neither affect cure kinetics nor shelf‐life of photocurable acrylate and thiol‐acrylate resins. Additionally, they promote fast stress relaxation and, by using derivatives with functional groups, are covalently incorporated within the photopolymer network. These salient features make organic phosphates and phosphonates ideal catalysts for the design and processing of photoreactive vitrimers. Herein, the catalytic activity of selected methacrylate‐functional phosphates and vinyl‐functional phosphonates on dynamic transesterifications are studied comprehensively. They are incorporated in a thiol‐acrylate photopolymer providing functional –OH and ester moieties. Cure kinetics and thermomechanical properties are not significantly affected by the structure and functionality of the catalyst. In contrast, time‐dependent stress relaxation measurements show that relaxation time and activation energy correlate well with the logarithmic acid dissociation constants of the Brønsted acids. The better understanding of the role of organic phosphates and phosphonates expands the scope of transesterification catalysts and is particularly interesting for the design of photoreactive vitrimers, which can be additively manufactured by using vat polymerization techniques.

Emergence of cooperatively reorganizing cluster and super-Arrhenius dynamics of fragile supercooled liquids.

In this paper, we develop a theory to calculate the structural relaxation time τ_{α} of fragile supercooled liquids. Using the information of the configurational entropy and structure, we calculate the number of dynamically free, metastable, and stable neighbors around a central particle. In supercooled liquids, the cooperatively reorganizing clusters (CRCs) in which the stable neighbors form "stable" nonchemical bonds with the central particle emerge. For an event of relaxation to take place, these bonds have to reorganize irreversibly; the energy involved in the processes is the effective activation energy of relaxation. The theory brings forth a temperature T_{a} and a temperature-dependent parameter ψ(T) which characterize slowing down of dynamics on cooling. It is shown that the value of ψ(T) is equal to 1 for T>T_{a}, indicating that the underlying microscopic mechanism of relaxation is dominated by the entropy-driven processes, while for T<T_{a}, ψ(T) decreases on cooling, indicating the emergence of the energy-driven processes. This crossover of ψ(T) from high to low temperatures explains the crossover seen in τ_{α}. The dynamics of systems that may have similar static structure but very different dynamics can be understood in terms of ψ(T). We present results for the Kob-Anderson model for three densities and show that the calculated values of τ_{α} are in excellent agreement with simulation values for all densities. We also show that when ψ(T), τ_{α}, and other quantities are plotted as a function of T/T_{a} (orT_{a}/T), the data collapse on master curves.

Study of structural, morphological, Mössbauer and dielectric properties of NiFeCoO4 prepared by a sol gel method

A methodical study on structural, electrical and dielectric properties of NiFeCoO4 nanoparticles, synthesized via sol-gel technique has been reported in this article. X-ray diffractogram confirmed phase purity of the synthesized sample. Raman spectra show the five predicted Raman bands that appear around 600–720, 250–360, 500–590, 450–520 and 180–220 cm−1 corresponding to A1g, Eg, T2g (3), T2g (2) and T2g (1), respectively. From the dielectric measurements, we have determined the different dielectric parameters such as the conductivity σac, complex permittivity e*, complex impedance Z*, and the tangent loss (tanδ). The electrical properties strongly depend on temperature and frequency. The relaxation activation energy deduced from the Z” vs. frequency plots was similar to the conduction activation energy obtained from the conductivity. Hence, the same type of charge carriers are attributed to the relaxation process and the conduction mechanism. The complex impedance plots have revealed the presence of only one semicircular arc corresponding to grains and grain boundaries contributions at all the temperatures and an equivalent electric circuit was proposed as a model of the sample.

High-frequency K[formula omitted] internal friction peaks in polycrystalline aluminum and magnesium near the melting points at longitudinal and torsional resonance

By using a modified piezoelectric ultrasonic composite oscillator technique (M-PUCOT), the temperature dependence of Young’s modulus, shear modulus and related internal frictions of 4 N polycrystalline aluminum and magnesium were measured from room temperature to 900 K at longitudinal and torsional resonance under several tens of kilohertz. The grain boundary internal friction peaks (Keˆ peaks) accompanied by large moduli relaxations were observed near the melting point (~0.86 Tm and 0.78 Tm for Al and Mg respectively) in both vibration modes. The activation energy of grain boundary relaxation is almost the same for the two vibration modes, which is 31.2 kcal/mol for aluminum and 36 kcal/mol for magnesium. The peak values of grain boundary internal frictions are significantly dependent on the vibration mode, and for both materials, the torsional peak is about 1.5 times higher than that of longitudinal peak. Finally, the experimental results were evaluated by using existing grain boundary slipping models (including 3D and 2D Zener’s model, Budiansky and O’Connell’s model and Ghahremani’s model). Results show that the periodic hexagonal grains sliding model (Ghahremani’s model) can reproduce the experiments best.

Viscoelastic behavior of alkali and alkaline earth aluminophosphate glasses

• Viscoelasticity of alkali and alkaline-earth aluminphosphate glass was investigated. • Activation energy of stress relaxation changed with aluminum content. • Different change in two series would be due to existence of AlO 4 . The viscoelastic behavior of alkali and alkaline earth aluminophosphate glasses was studied. The energetic interpretation was based on the Arrhenius behavior of the relaxation modulus. With increasing aluminum content, the activation energy of the relaxation increased in the alkali series but decreased in the alkaline earth series. Dynamic viscoelasticity measurements revealed that the loss modulus, E ”(ω), showed a crossover of the relaxation process at the glass transition temperatures. The activation energy below the transition temperature was close to the energies of internal friction or ionic self-diffusion. The energy above the transition temperature corresponded to the dissociation energies of alkali-oxygen or alkaline earth-oxygen single bonds. The different behavior in the energy of the relaxation modulus between alkali and alkaline earth series was due to weak bonds caused by adding alkali ions and the formation of crosslinking by low-coordinate aluminum polyhedra because of the presence of alkaline earth ions.

Solid-solid phase-change materials with excellent mechanical property and solid state plasticity based on dynamic urethane bonds for Thermal Energy Storage

Solid-solid phase change materials (SSPCMs) with small volume change and leak-proof characteristic during the whole process of phase change play a vital role in development of PCM for thermal energy storage (TES). However, the non-recyclability of the materials due to their permanent cross-linking networks limited their practical application. Herein, a dynamic urethane bond cross linked SSPCM with PEG as phase change functional segments was successfully fabricated which was in solid state even at 130°C.Meantime, it exhibited excellent solid-state plasticity, resulting from dynamic urethane bond exchange. Moreover, the dynamic behavior of urethane bond in SSPCMs was firstly investigated. The topology freezing transition temperature (Tv) is about 113°C and the relaxation activation energy of SSPCM is calculated to be about 90 kJ/mol. The crystalline PEG endows SSPCM with high enthalpy value (reached up to 85 J/g), meantime, the cross-linked structure endows SSPCMs with remarkable mechanical flexibility (possess ultra-stretchability of 600%), thermal reliability and chemical stability. The combined thermal storage ability, appealing recyclability and flexibility endow the SSPCM promising application in the field of TES.

Effect of vanadium doping on the electrical charge transport and dielectric relaxation properties of Sodium Bismuth Titanate perovskite

In this work, we report the effect of vanadium doping on the conductivity and relaxation dynamics of the charge carriers in Sodium Bismuth Titanate perovskite material (Na0.5Bi0.5TiO3) prepared by a unique technique which is a combination of chemical and mechanical methods. The parent material was prepared using the citrate auto-ignition method and the mechanical alloying technique was used to incorporate V2O5 in the NBT structure. The X-ray diffraction analysis confirmed the formation of the rhombohedral phase of NBT perovskite with space group R3c in all the compositions. The microstructural analysis was done using Field Emission Scanning Electron Microscopy, Energy-dispersive X-ray spectra, and the elemental mapping of the samples. The conductivity mechanism was analyzed using the complex impedance method. The sample with 15% doping showed the highest conductivity with minimum activation energy. The activation energies for conduction, relaxation, and migration of charge carriers were found to be comparable with each other. The frequency-dependent dielectric data was analyzed using Havriliak–Negami (HN) formalism and suggested non-Debye type relaxation behaviour. The 15% dopped composition showed the highest dielectric strength. The scaling behaviour of impedance and modulus spectra confirmed the temperature-independent conduction and relaxation mechanisms.

Internal friction and dynamic shear modulus of a metallic glass in a seven-orders-of-magnitude frequency range

Abstract We report measurements of the internal friction and shear modulus of glassy Cu49Hf42Al9 at sub-hertz frequencies (0.03–1 Hz) and at a high frequency of 560 kHz at temperatures from the room one up to well above the glass transition temperature Tg. It is found that an increase of the frequency in this range results in a drastic decrease of the internal friction and shear modulus relaxation both below and above Tg. The relaxation kinetics is analyzed within the framework of a classical phenomenological approach in terms of the real and imaginary parts of the dynamic shear compliance both for the initial and relaxed states of the glass under investigation. A law describing the frequency dependence of the imaginary compliance component in the whole frequency range investigated is determined. A new method for the determination of the Gibbs activation energy of relaxation is derived. The underlying activation energy spectrum determined on this basis is found to smoothly increase with the activation energies accessible in the experiment. A change of the activation energy with temperature below and above Tg is determined. The phenomenological analysis is combined with a physical interpretation of the relaxation, which is assumed taking place due to the activation of interstitial-type defects (essentially elastic dipoles) frozen-in from the melt upon glass production. It is argued that there exist at least three mechanisms of energy losses, which are related to changes of the dipoles’ orientation in the same energy states and transitions between their low- and high-energy states.

Preparation of mechanically robust and autonomous self-healable elastomer based on multiple dynamic interactions

Self-healing materials have attracted considerable attention because of their sustainable property of prolonging service life and reducing resource consumption. However, as plenty of room-temperature self-healable materials have been studied, it still remains a great challenge to equip these materials with admirable mechanical strength since there is a compromise between intermolecular interactions and chain mobility. Herein, a novel strategy of integrating self-healing ability with robustness in polymeric elastomer at ambient condition was developed. The synthesized polyurethane was equipped with multiple reversible interactions including disulfide bonds, oxime urethane bonds, H-bonds and coordination interactions, and formed triple cross-linking network. What’s important is that Cu2+ could not only act as the coordination center, but also serve as the catalyst to accelerate disulfide exchange and oxime urethane exchange reaction. XPS and stress relaxation tests revealed that the introduction of CuCl2 reduced the relaxation activation energy of both the disulfide metathesis and oxime urethane exchange reaction. Thanks to the multiple interactions design and the effects of Cu2+, the elastomer was endowed with excellent mechanical strength of 19.5 MPa and self-healing efficiency of 83% at room temperature.

Structural, dielectric and electrical characteristics of manganese modified (Bi0.5Ba0.25Sr0.25) (Ti0.5Fe0.5)O3 relaxor

In this communication, detailed studies of the structural, microstructural, dielectric, and electrical properties of polycrystalline materials, (Bi0.5Ba0.25Sr0.25) (Ti0.5Fe0.5)O3 and (Bi0.5Ba0.25Sr0.25) (Ti0.25Mn0.25Fe0.5)O3, synthesized by using a high–temperature solid-state-reaction method, have been reported. X-ray structural and scanning electron micrograph studies exhibit phase pure tetragonal system and surface morphology (size and distribution of grains and grain boundaries) of the samples respectively. Analysis of the temperature and frequency dependence of dielectric and electrical (impedance, modulus, and conductivity) data reveals the ferroelectric relaxor behavior, relaxation mechanism, and semiconductor (negative temperature coefficient of resistance) properties of the bulk BFBST and Mn modified BFBST electro-ceramics. The relaxation time and activation energy (Ea) were calculated from the above data. The characteristics of Mn modified BFBST have been compared to that of it’s parent (BFBST) compound. The different inherent conduction mechanisms, such as Ohmic, hopping, space charge limited (SCLC) have been analyzed. The bulk- and interface-limited conduction processes were evidently found in the materials by the Poole–Frenkel (PF) and Schottky (SEmen modified BFBST have been compared to that of its parent (BFBST) compound) emission fitting of the J ∼ E characteristic data. The leakage data of BFBST-Mn (Mn modified Mn) quantified the average energy gap (Eg) in the range of 0.83–0. 87 eV for different applied voltages and in a wide range of temperature (25 °C–300 °C). With the increase in voltage, Eg decreases. This work suggests that Mn-substitution (Mn4+) at B (Ti4+) site keeping the stoichiometry undisturbed enhances structural, dielectric response (higher dielectric constant) and reduce the leakage behavior especially at low temperature and high-frequency range.

ICMMS-2: Dielectric Relaxation Behaviour and AC Electrical Conductivity of Cellulose Acetate-Molybdenum Trioxide Nanoparticle Blended Film

Cellulose acetate (CA) and CA blended with molybdenum trioxide MoO3 nanoparticle at concentration of (0.0, 0.25, 0.5& 1.0 wt. %) prepared by casting method. Ac conductivity and dielectric properties were studied. Our results indicated that the conduction mechanism of CA and CA blended with different concentration of MoO3 nanoparticle is controlled by the correlation barrier hopping (CBH) model. The ac conductivity was observed to increase by adding MoO3 nanoparticle which explained by the network formed as the nanoparticle gets closer to each other as the concentration increase. The maximum barrier height was calculated and found to decrease by increasing MoO3 nanoparticle from 0.15 t0 0.12 eV. The analysis of the dielectric constant and dielectric loss suggested that their behavior can be explained by Maxwell–Wagner–Sillar type polarization at high frequency. At low frequency the polarization is high as a result of interfacial polarization. Arrhenius equation was used to calculate the activation energy of relaxation which found to decrease by increasing MoO3 nanoparticle.

Enhanced Electro-optical Properties of Low Viscous Nematic Liquid Crystal Doped with Mixed Phase Anatase/Rutile TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Nanoparticles for Display Applications

Organic–inorganic composite based on liquid crystalline and TiO2 nanoparticles were obtained and investigated taking into account the crystallographic form of TiO2 i.e., anatase/rutile mixed phase. TiO2 is an important class of material having various dielectric and electro-optical properties. The existent research presents the electro-optical properties of nematic liquid crystal (NLC) E204 and TiO2 nanocomposites over an extensive range of frequencies. Various important display parameters such as response time, threshold voltage, pretilt angle and activation energy of pristine as well as TiO2 doped composites systems were measured and analyzed. In comparison with the pure, TiO2-doped composite systems has approximately 50% faster response time, owing to its remarkable decline in the relaxation time and activation energy of the LCs. The alteration in the optical intensity of the NLC composites as a function of the concentration of TiO2 nanoparticles (NPs) was also examined. It was established that the optical intensity in nano-nematic composites was decreasing with the concentration of TiO2 NPs. Also, it was observed that an escalation in the TiO2 NPs concentration in NLC composites indicates to an escalation in the birefringence. Probable mechanisms of the interactivity between NLC molecules and TiO2 NPs have been discussed. The current work shows that the TiO2 NPs doping has an encouraging application in the display devices, including other electro-optical as well as photonic applications.

Effects of ultralow concentration MXene (nano-Ti3C2Tx) on the electric and physical properties of ternary polyvinyl alcohol composites

High-dielectric-constant (high-κ) fillers are embedded in polymer materials to improve the dielectric constant of polymer and form high-κ composites for the latest generation of energy storage devices. However, the addition of high-κ fillers will introduce interface polarization and cause space charge accumulations that will increase dielectric loss and reduce breakdown strength. Herein introduces a conductive MXene(nano-Ti3C2Tx) with high surface area to remarkably suppress space charge in polyvinyl alcohol (PVA) matrix. A ternary PVA composite with high dielectric constant and low dielectric loss is formed by doping with ultralow concentration MXene and soluble ferroelectric [Hdabco]ClO4 (TEDA.C) via solution mixing. The space charge suppression capability of MXene sharply reduces the dielectric loss of 1M@1788PVA and 5M@1788PVA (PVA with 0.0001 wt.% and 0.0005 wt.% MXene) at low frequency (0.1−10 Hz). By comparing the activation energy of dielectric relaxation (Ea) and breakdown strength (BDS) under AC electric field between pure PVA and PVA composites, ultralow concentration MXene declines the Ea of 1M@1788PVA and 5M@1788PVA by 26.8 % and 60.9 %, and effectively improves the BDS of 1M@1788PVA and 1M5T@1788PVA (PVA with 0.0001 wt.% MXene and 4.7 wt.% TEDA.C) by 3%. The addition of ultralow concentration MXene and TEDA.C bring 5M5T@1788PVA(PVA with 0.0005 wt.% MXene and 4.7 wt.% TEDA.C) highest discharged energy density(17.32 J/cm3)which is 1.53 times of that of 1788PVA.

Thermally reprocessable bio-based polymethacrylate vitrimers and nanocomposites

Vitrimers are covalently cross-linked polymeric materials that can be processed upon heating wherein triggerable polymer networks shuffle chemical bonds through exchange reactions without losing their network integrity. In this work, we apply vitrimers to develop recyclable thermosets derived from materials with relatively high bio-source content. We explore the reaction between β-ketoesters derived from commercially available (2-acetoacetoxy) ethyl methacrylate (AAEMA) incorporated within isobornyl methacrylate (IBOMA, from pine sap), and a bi-functional amine (Priamine, derived from vegetable oils) to synthesize catalyst-free vitrimers. Controlled radical miniemulsion polymerization was also used to synthesize the water-borne copolymer analogs. Vitrimer nanocomposites were obtained by the incorporation of amine‐functionalized polyhedral oligomeric silsesquioxane (POSS–NH2) at different loadings (0, 5, 10 and 20 wt%). Incorporation of 20 wt% POSS-NH2 improved tensile modulus (from 96 to 176 MPa), tensile strength (from 2.5 to 5 MPa) and decomposition temperature (225–255 °C), and slowed down the relaxation rate and increased apparent activation energy of stress relaxation compared to the neat vitrimer. We further show that the vitrimers studied here and the nanocomposites containing vinylogous urethane cross-linking networks can be un-cross-linked by dissolving in excess mono-functional amine at 65 °C or recycled by grinding and remolding at 125 °C without compromising the mechanical properties.

The effect of short polystyrene brushes grafted from graphene oxide on the behavior of miscible PMMA/SAN blends

A new concept of utilization of particle-polymer hybrids as multifunctional additives for polymer blends was introduced in this study. Graphene oxide particles with short densely grafted polystyrene brushes (GO-g-PS) were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP). Melt rheology studies revealed that GO-g-PS suppressed the phase separation of miscible poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) blends. The studies suggested specific interactions of GO-g-PS with the PMMA phase and this was confirmed based on calculations of activation energies of segmental relaxations by broadband dielectric spectroscopy (BDS). These unusual interactions of GO-g-PS with PMMA phase were assigned to the specific and precise architecture of the GO-g-PS particles as well as chemical nature of PS polymer brushes. The short chains of PMMA and PS provide miscibility originating from UCST behavior of PMMA/PS blend of short polymer chains. Additionally, BDS also revealed improved charge transport in PMMA/SAN blend in presence of GO-g-PS hybrid.

Dielectric, impedance and resistivity studies on [Ba(Nd0.1Ti0.8Nb0.1)O3]0.40[(Na0.5Bi0.5)TiO3]0.40[CaTiO3]0.20 piezoelectric ceramic

[Ba(Nd0.1Ti0.8Nb0.1)O3]0.40[(Na0.5Bi0.5)TiO3]0.40[CaTiO3]0.20 ceramic was prepared through solid state double sintering route. Dielectric, Impedance and Resistivity studies were carried out on this piezoelectric ceramic experimentally and by simulating the data using the new equations proposed by us earlier and in the present case. Through simulation it was shown to estimate the activation energy for relaxation and conduction & relaxation times directly from the plots. These proposed equations can be directly applied for simulation of data instead of well known laws like Arrhenius law, modified Arrhenius law and Chen et al. law which are indirect ways to do so.

Scaling behavior and variable-range-hopping conduction of localized polarons in percolative BaTiO3-Ni0.5Zn0.5Fe2O4 ceramic composite with colossal apparent permittivity

The percolative BaTiO3-Ni0.5Zn0.5Fe2O4 (BTO-NZFO) ceramic composite represents a family of multifunctional materials exhibiting multiferroic properties and colossal apparent permittivity. It is of fundamental interest to investigate the conduction mechanism in such percolative composites from both macroscopic and microscopic perspectives. Herein, three representative systems with the NZFO content locating below the percolation threshold fc, near fc, and above fc, respectively, were investigated, using pure NZFO ceramic as a comparison. The conductivity of the composite as a function of NZFO content agrees well with the McLachlan model, which takes percolation into consideration and essentially equivalent to the Kirkpatrick model. The electrical conductivity of the composite conforms to Mott's variable-range-hopping (VRH) model in the temperature range of 303–573 K, suggesting that VRH conduction of localized polarons dominates the electrical behavior microscopically. Parameters including the most probable hopping range (R); the density of localized states at the Fermi level [N(EF)]; and the activation energies of VRH (W), dc conductivity (Edc), hopping (Eon), and relaxation (Er) were obtained and analyzed. Scaling behaviors of the conductivity and the imaginary part of complex impedance of the composite have been observed, implying that the distribution of relaxation times is temperature independent. The impedance data measured at different temperatures exhibit typical semiconducting behavior, which can be well fitted by an equivalent circuit model considering both grain and grain boundary responses. The correlation between conductivity and colossal apparent permittivity has also been revealed. The discoveries deepen the understanding of the conduction mechanism in such multifunctional composites composed of an insulating phase and a semiconducting phase.

A Cu-Y-Al glassy alloy with strong beta relaxation and low activation energies for structural relaxation and crystallization

Structural relaxation, beta relaxation and crystallization processes of the Cu53Y42Al5 metallic glass were studied in the present work. X-ray diffractometry was used to study the structure of the samples. Phase transformations were studied by differential scanning calorimetry by using a power compensated type differential calorimeter. Cu-mould casting resulted in the formation of crystalline samples while fully glassy Cu53Y42Al5 alloy samples were produced by melt spinning at a high cooling rate of about 105-106 K/s. This glassy alloy is characterized by strong beta relaxation and low activation energy values for crystallization and relaxation in comparison with other metallic glasses studied earlier.