Activation Energy Of Relaxation Research Articles

Reinforcing self-healing and Re-processable ionomers with carbon black: An investigation on the network structure and molecular mobility

While most self-healing elastomers are mechanically weak, using the most commonly used carbon black (CB) to reinforce self-healing elastomers has not been reported. The possible reason is that the introduction of carbon black will cause complex changes in molecular dynamics, which leads to unpredictable self-healing effects. Herein, carbon black is introduced into a self-healing ionomer based on brominated butyl rubber (BIIR) grafted with tert-butyl pyridine (BP). Interestingly, the hierarchical microstructure and multilevel molecular dynamics of ionomers are well regulated by the strong interfacial π-cation interactions between CB and ionic aggregates. The strong interfacial interaction leads to high content of bound rubber, uniform dispersion of CB and compact physical network. Although such structural change has negligible influence on the segmental motion, it enhances the relaxation time and activation energy of ionic clusters, and thus endows the ionomer with high mechanical properties. Meanwhile, the structural change leads to higher relaxation temperature, thereby increasing the self-healing temperature. Despite this fact, the healing efficiency can still be regulated between 50% and 100%, depending on the filler content and temperature. Moreover, the CB reinforced ionomer is re-processable and recyclable. This study demonstrates that CB reinforcement is feasible to improve the mechanical properties of self-healing elastomers.

Substituted Thiols in Dynamic Thiol–Thioester Reactions

The thiol–thioester reaction has emerged as a promising method for developing covalent adaptable networks (CANs) due to its ability to exchange rapidly under low temperature conditions in a number of solvents, orthogonality among other functional groups, and tunability. Here, the effects of thiol substitution (i.e., primary vs secondary) were assessed with respect to their reactivity in two dynamic thioester reactions: the thiol–thioester exchange and the reversible thiol–anhydride addition. Model NMR experiments were conducted using small-molecule compounds to observe how polymers of similar components would behave in thiol–thioester exchange. It was determined that the Keq was near unity for mixtures of primary thiols and secondary thioesters, and vice versa, in both a polar solvent, DMSO-d6, and at most slightly favors primary thioesters in a relatively nonpolar solvent, CDCl3. Dielectric spectroscopy and stress relaxation experiments were used to determine the relaxation times and activation energies of the two thioester-containing networks: Thiol-ene networks, which undergo thioester exchange, displayed activation energies of 73 and 71 kJ/mol from dielectric measurements and 36 and 53 kJ/mol from stress relaxation for the primary and secondary thiols, respectively. Thiol–anhydride-ene networks, which undergo both thioester exchange and reversible thiol–anhydride addition, displayed activation energies of 94 and 114 kJ/mol from dielectric and 111 and 139 kJ/mol from stress relaxation for primary and secondary thiols, respectively. In both types of networks, the secondary thioester-based networks demonstrated slower dynamics as compared to the same primary network by at least one order of magnitude. In the anhydride network, the secondary thiol also biased the dynamics toward reversible addition.

Nickel Manganite-Sodium Alginate Nano-Biocomposite for Temperature Sensing

Nanocrystalline nickel manganite (NiMn2O4) powder with a pure cubic spinel phase structure was synthesized via sol-gel combustion and characterized with XRD, FT-IR, XPS and SEM. The powder was mixed with sodium alginate gel to form a nano-biocomposite gel, dried at room temperature to form a thick film and characterized with FT-IR and SEM. DC resistance and AC impedance of sensor test structures obtained by drop casting the nano-biocomposite gel onto test interdigitated PdAg electrodes on an alumina substrate were measured in the temperature range of 20–50 °C at a constant relative humidity (RH) of 50% and at room temperature (25 °C) in the RH range of 40–90%. The material constant obtained from the measured decrease in resistance with temperature was determined to be 4523 K, while the temperature sensitivity at room temperature (25 °C) was −5.09%/K. Analysis of the complex impedance plots showed a dominant influence of grains. The decrease in complex impedance with increase in temperature confirmed the negative temperature coefficient effect. The grain resistance and grain relaxation frequency were determined using an equivalent circuit. The activation energy for conduction was determined as 0.45 eV from the temperature dependence of the grain resistance according to the small polaron hopping model, while the activation energy for relaxation was 0.43 eV determined from the Arrhenius dependence of the grain relaxation frequency on temperature.

Sintering Temperature, Frequency, and Temperature Dependent Dielectric Properties of Na0.5Sm0.5Cu3Ti4O12 Ceramics.

NSCTO (Na0.5Sm0.5Cu3Ti4O12) ceramics have been prepared by reactive sintering solid-state reaction where the powder was prepared from the elemental oxides by mechanochemical milling followed by conventional sintering in the temperature range 1000–1100 °C. The influence of sintering temperature on the structural and dielectric properties was thoroughly studied. X-ray diffraction analysis (XRD) revealed the formation of the cubic NSCTO phase. By using the Williamson–Hall approach, the crystallite size and lattice strain were calculated. Scanning electron microscope (SEM) observations revealed that the grain size of NSCTO ceramics is slightly dependent on the sintering temperature where the average grain size increased from 1.91 ± 0.36 μm to 2.58 ± 0.89 μm with increasing sintering temperature from 1000 °C to 1100 °C. The ceramic sample sintered at 1025 °C showed the best compromise between colossal relative permittivity (ε′ = 1.34 × 103) and low dielectric loss (tanδ = 0.043) values at 1.1 kHz and 300 K. The calculated activation energy for relaxation and conduction of NSCTO highlighted the important role of single and double ionized oxygen vacancies in these processes.

Modified Li7P3S11 Glass-Ceramic Electrolyte and Its Characterization.

Li7P3S11 glass ceramics have high conductivities competitive with liquid electrolytes, making them good candidates as solid-state electrolytes for all-solid-state lithium-ion batteries. However, the metastable nature and performance of Li7P3S11 glass ceramics remain mysterious. Herein, modified Li7P3S11 glass ceramics with compositions of 70Li2S-30P2S5 were prepared via two-step mechanical milling and thermal annealing. Li7P3S11 glass ceramics synthesized using the conventional method (mechanical milling and thermal annealing) were again ball-milled to obtain amorphous 70Li2S-30P2S5 with a peculiar glass structure. Further thermal annealing was carried out to crystallize the glass. The obtained crystalline phase was analogous to the original Li7P3S11 phase, but the conductivity was enhanced by a factor of 1.7. Based on 31P solid-state nuclear magnetic resonance (NMR) spectroscopy, the Li7P3S11 phase contained an additional PS43- unit. A rational deconvolution procedure for the 31P solid-state NMR spectra based on crystalline Li7P3S11 was developed and applied to the samples. The analysis can resolve the additional crystalline PS43- unit in the Li7P3S11 structure. Based on two-dimensional double-quantum 31P NMR spectroscopy, the additional PS43- unit is located adjacent to the P2S74- unit, suggesting that P2S74- is divided into two PS43- units in the Li7P3S11 phase. The flip motion of Li+ was also investigated based on the 7Li spin-lattice relaxation time. The independent activation energy of spin-lattice relaxation with respect to temperature in the Li7P3S11 phase was attributed to a conduction path between the two PS43- units. The findings provide a synthetic route that can be used to develop metastable solid-state electrolytes.

Rheological and NMR Studies of Cellulose Dissolution in the Ionic Liquid BmimAc.

Solutions of two types of cellulose in the ionic liquid 1-butyl-3-methyl-imidazolium acetate (BmimAc) have been analyzed using rheology and fast-field cycling nuclear magnetic resonance (NMR) spectroscopy, in order to analyze the macroscopic (bulk) and microscopic environments, respectively. The degree of polymerization (DP) was observed to have a significant effect on both the overlap (c*) and entanglement (ce) concentrations and the intrinsic viscosity ([η]). For microcrystalline cellulose (MCC)/BmimAc solutions, [η] = 116 mL g-1, which is comparable to that of MCC/1-ethyl-3-methyl-imidazolium acetate (EmimAc) solutions, while [η] = 350 mL g-1 for the commercial cellulose (higher DP). Self-diffusion coefficients (D) obtained via the model-independent approach were found to decrease with cellulose concentration and increase with temperature, which can in part be explained by the changes in viscosity; however, ion interactions on a local level are also important. Both Stokes-Einstein and Stokes-Einstein-Debye analyses were carried out to directly compare rheological and relaxometry analyses. It was found that polymer entanglements affect the microscopic environment to a much lesser extent than for the macroscopic environment. Finally, the temperature dependencies of η, D, and relaxation time (T1) could be well described by Arrhenius relationships, and thus, activation energies (Ea) for flow, diffusion, and relaxation were determined. We demonstrate that temperature and cellulose concentration have different effects on short- and long-range interactions.

Grain boundary relaxation behavior and phase stability of AlCrTiVx (x = 0, 0.5 and 1) high-entropy alloys

The defect relaxation behavior of AlCrTiVx (x = 0, 0.5, and 1) high entropy alloys was reported in this paper. A pronounced internal friction (IF) peak superimposing on a monotonously increasing high-temperature background (HTBG) was observed in all samples, which was ascribed to grain boundary peak. The activation energy of grain boundary relaxation increases with the increasing V content, implying an increasing complexity and difficulty of atomic diffusion with the increasing mixing entropy. After annealing at 1100 °C for 5 h, the HTBG of AlCrTiV alloy abnormally increases, which was attributed to the appearance of misfit dislocations induced by precipitation of nano-sized Ti-rich phases as confirmed by microstructure analysis. These findings may provide a new degree of freedom for evaluating and designing high entropy alloys with high thermal stability.

Structure evolution, dielectric, and conductivity behavior of (K0.5Na0.5)NbO3-Bi(Zn2/3Nb1/3)O3 ceramics

(1−x)K0.5Na0.5NbO3−xBi(Zn2/3Nb1/3)O3 ((1−x)KNN−xBZN, x = 0.010, 0.015, 0.020, 0.025, and 0.030) lead-free ceramics were fabricated via a traditional solid-state method. The crystal structure, microstructure, dielectric, and conductivity behavior of this system were studied. Combined with X-ray diffraction (XRD) patterns, Rietveld refinement, and dielectric spectroscopy, an orthorhombic phase was determined for x = 0.010, an orthorhombic-tetragonal mixed phase was identified for x = 0.015, and a rhombohedral symmetry appears in 0.020 ⩽ x ⩽ 0.030. Both 0.98KNN−0.02BZN and 0.975KNN−0.025BZN ceramics exhibit stable permittivity and low dielectric loss tangent (tanδ) in wide temperature ranges owing to the combination of rhombohedral-tetragonal step-like feature and the diffuse phase transition from tetragonal to cubic. The activation energies of dielectric relaxation and conductivity behavior at high temperatures initially decrease slightly, then drop sharply, and finally decline slowly, which could be attributed to microstructure morphologies and the concentration of oxygen vacancies.

The relaxation processes of dielectric, modulus and impedance in Dy0.55Sr0.45Mn1- xFexO3

The ac electrical properties of orthorhombic ceramics, Dy0.55Sr0.45Mn1-xFexO3 (x = 0, 0.4 and 1.0) in the range of frequency 0.5–107 Hz and temperature 126–299 K have been studied. Across the series, dielectric permittivity and loss increase with increase in Fe concentration. Non-Debye type dielectric relaxation peak was found in x = 0.4 compound among Dy0.55Sr0.45Mn1-xFexO3 materials and consequently activation energy of the dielectric relaxation was calculated. The results of impedance spectroscopy revealed that for higher concentration of Fe in the series, impedance (Z′) is low. The relaxation peaks in impedance, Z″ are observed at different frequency ranges for different Fe concentration in the compounds. From the temperature dependence of these peaks, activation energies are determined. The results of modulus spectra indicated a similar behavior as impedance, but x = 0.4 compound showed a large activation energy among the compounds in the series. Cole-Cole plots displayed negative temperature coefficient of resistance, a normal behavior of semiconductors. These plots also suggested that the dielectric relaxation is of non-Debye type. The scaling behavior in the modulus and impedance suggested that temperature dependence of relaxation processes exists in these materials. These studies and results are significant for materials which exhibit high resistance in low temperature.

The influences of lattice distortion on the antiferroelectric transition and relaxation of oxygen vacancies in high-entropy perovskites (Bi0.2Na0.2Ba0.2K0.2X0.2)TiO3 with X=Ca, Sr or La

High entropy ceramics (HECs) (Bi0.2Na0.2Ba0.2K0.2X0.2)TiO3, X=Ca, Sr or La, with a perovskite structure have been successfully synthesized using a modified citrate acid method. Lattice distortion is a main feature of the HECs which significantly influences their antiferroelectric phase transition behaviors. It is shown that the activation energy of oxygen-vacancy relaxation could quantitatively reflect the lattice distortion of perovskite HECs, as characterized by X-ray diffraction, dynamical mechanical analysis and Raman spectroscopy. Based on the activation energy, the lattice-distortion driven antiferroelectric transition in HECs can be also characterized, providing a viable strategy to design the structural, dielectric and ferroelectric properties as finely tuned by chemical disorder at A or B site in perovskite HECs.

Dielectric and ferroelectric properties of multicomponent equiatomic calcium lead strontium titanate (Ca0.33Pb0.33Sr0.33)TiO3

Perovskite oxides are well-known for their dielectric and ferroelectric properties. In this work, the dielectric properties of calcium lead strontium titanate synthesized by reverse co-precipitation process with two different solvent concentrations were studied. Formation of single phase was confirmed by X-ray diffraction and tetragonality present in the lattice was analysed through Rietveld refinement. The grain size, morphology and composition were studied using scanning electron microscope and energy dispersive spectroscopy. The influence of frequency in the range of 1 ​Hz–107 ​Hz and temperature range -100 ​°C to 300 ​°C on dielectric response were studied. Curie-Weiss fits showed both the systems to be relaxor ferroelectrics. The dielectric relaxation mechanism and activation energy were determined from the electric modulus. The relaxation time and the dissipation factors were obtained from Cole-Cole analysis. The ferroelectric properties were assessed using a polarization-electric field loop tracer and efficiencies of memory storage were analysed.

Investigation of Ga doping for non-stoichiometric sodium bismuth titanate ceramics

The electrical properties of Ga3+ doping Na0.5Bi0.5TiO3-based oxygen ionic conductors were studied. The Na0.52Bi0.47Ti1−xGaxO3−δ (x = 0, 0.01, 0.015, 0.02) samples were fabricated by the means of traditional solid-state reaction. The results of AC impedance measurements showed that the bulk conductivity of Na0.52Bi0.47Ti1−xGaxO3−δ samples decreased monotonously with the increase of Ga3+ doping concentration. At 673 K, the bulk conductivity of the Na0.52Bi0.47Ti0.98Ga0.02O3−δ sample is 7.19 × 10–4 S/cm, which is lower than that of the Na0.52Bi0.47TiO3−δ sample under the identical test temperature. The highest total conductivity emerged in the Na0.52Bi0.47Ti0.99Ga0.01O3−δ (x = 0.01) sample with 1.387 × 10–4 S/cm at 623 K, which demonstrated that the slight Ga3+ doping supported the enhancement of the total conductivity. A relaxation peak was observed in Na0.52Bi0.47Ti1−x GaxO3−δ compounds. As the Ga3+ ions were introduced into the Na0.52Bi0.47TiO3−δ compound, there was an increasing trend of the relaxation activation energy educed by the internal friction test. In addition, the oxygen relaxation height decreased with Ga3+ doping, which suggested that the introduction of Ga3+ ions resulted in the decrease of mobile oxygen vacancy.

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.