Activation Energy Of Relaxation Research Articles

A Mechanical Model for Stress Relaxation of Polylactic Acid/Thermoplastic Polyurethane Blends

Polylactic acid (PLA) is considered a promising biodegradable polymer alternative. Due to its high brittleness, composite materials made by melt blending thermoplastic polyurethane (TPU) with PLA can enhance the toughness of PLA. To understand the forced aging caused by stress relaxation in polymer materials, this study explains the stress relaxation experiments of PLA/TPU blends with different mass ratios under applied strain through mechanical model simulations. The Kelvin representation of the standard linear solid model (SLSM) is used to analyze the stress relaxation data of TPU/PLA blends, successfully explaining that the Young’s moduli (E1 and E2) of springs decrease with increasing temperature and TPU content. The viscosity coefficient of the PLA/TPU blends decreases with increasing temperature, and its reciprocal follows the Arrhenius law. For TPU/PLA blends with increased concentration of TPU, the activation energy for stress relaxation shows a linear decrease, confirmed by the glass transition point measured by DMA, indicating that it does not involve chemical reactions.

Dielectric relaxation studies in Sm2O3 doped boro-zinc-vanadate glasses

In this work, the conventional melting procedure was adopted to synthesize glasses with a composition of (ZnO)0.3 – (V2O5)0.3 – (B2O3)0.4-x – (Sm2O3)x, x = 0.002, 0.005, 0.007, 0.01 and 0.02 mol%. The dielectric properties of these glasses were measured. The dielectric constant (εʹ) and dielectric loss (εʹʹ) of glasses are found to vary in the range from 1.9177x102 to 3.9456x103 and from 1.5751 to 6.9095x105 respectively for the temperature range 343 K – 573 K and frequency range 50 Hz to 10 MHz. With increase of frequency, both dielectric constant and loss decreased and increased with increase of temperature. The analysis of dielectric constant and dielectric loss confirmed that the phenomenon of dielectric relaxation is mainly due to the frequency-dependent polarization mechanism. Electric modulus and impedence spectroscopy revealed non-Debye type and a single phase relaxation process. Activation energy for dc conductivity and dielectric relaxation are found to be of same size indicating that the potential barrier encountered by the charge carriers is same in both the processes. The master curves suggested that temperature-independent relaxation is occurring in the present glasses. The high values of measured dielectric parameters suggest that the present glasses are suitable in semiconducting and energy storage applications.

Analyzing Temperature – Dependent Structural Changes and Frequency Response of a Ferroelectric Nanocomposite Based on Triglycine Sulfate with Oxidized Multiwalled Carbon Nanotubes

The present work is a continuing study on anomalous properties of the ferroelectric nanocomposite from triglycine sulfate (TGS) and oxidized multiwalled carbon nanotubes (OMWCNT). In this study, the nature of frequency response in a region of 102–107 Hz at low temperatures (63–273 K) will be clarified by analyzing structural changes when reducing temperature. It was found that the interaction between TGS and OMWCNT inhibited the switching of SO4 - groups, leading to a significant increase in relaxation time, activation energies and coercive fields. Consequently, the domain – wall freezing occurred at higher temperatures than those in pure TGS.

Gd3+ substitution effects, electrical and AC impedance characterizations of Gd/Cr co-doped SrBi4Ti4O15 Aurivillius-phase ceramics

Gd/Cr co-doped SrBi4Ti4O15 (Sr1-xGdxBi4Ti4O15 + 0.2 wt% Cr2O3, x = 0∼0.08) Aurivillius-phase ceramics were prepared by the traditional solid-state reaction process, the influences of Gd3+ doping concentration (x) on the structures and electrical properties of SrBi4Ti4O15 were investigated, as well as the AC impedance characterizations of the ceramics were analyzed. The incorporation of Gd2O3 and Cr2O3 into SrBi4Ti4O15 induced the lattice distortion of its orthorhombic structure. The donor substitution of Gd3+ for Sr2+ at A-site improved the Curie temperature (TC) and decrease the temperature coefficient of dielectric constant (TKε), as well as change the relaxation activation energy (Earel) of SrBi4Ti4O15 through the lattice distortion caused. Between 425 °C and 700 °C, the DC conduction behaviors of the ceramics was found with a transition from the mixed conduction mechanism involved in both the single-ionized oxygen vacancy and the double-ionized one to the single mode dominated by the latter one. Above TC, the imaginary part of modulus (M″) gradually shifts towards higher frequencies with increasing temperature, indicating the emergence of a new dielectric relaxation mode related to the phase transition process of the ceramics. Owing to the co-doping effects of Gd3+ and Cr3+, the optimized composition with x = 0.03 achieved excellent electrical properties (TC = 550 °C and d33 = 30 pC/N) and good thermal stability (TKε = 4.19 × 10−3/°C between RT and 500 °C, and the lose in d33 is only 3.3% after being annealed at 500 °C for 4 h), which make it promising for stable applications below 500 °C in the high-temperature piezoelectric devices.

Effect of compaction pressure on sinterability, microstructure and electrical behavior of Ba0.5Na0.48Sr0.02TiO3 system

The microstructure of electroceramics is a highly influencing factor to decide its functionality and applicability. Processing conditions used for the conversion of raw powder of electroceramics to the net product are playing a dominant role to modulate microstructure. In the present attempt, various compaction pressures (5 tons/cm2, 7 tons/cm2, and 9 tons/cm2) are used to densify the powder sample to the pellet. The effect of the pressure on sinterability, microstructure, and electrical behavior is systematically revealed. Ba0.5Na0.48Sr0.02TiO3 (BNST) with optimized composition has been prepared by the sol-gel combustion method. The structure is confirmed by X-ray diffraction and accompanied by Rietveld refinements. The compaction pressure effect on sinterability is systematically studied by revealing variation of relative density as a function of temperature at constant holding time as well as a function of holding time at a constant temperature. The final obtained relative density is found to be strongly affected by compaction pressure. Microstructural changes during the sintering process are closely monitored and found porosity, grain size, and grain size distribution are strongly affected by compaction pressure. That indirectly influenced electrical behavior. Relaxation frequency and activation energy of charge relaxation are also found to be varied with the compaction pressure of the powder samples.

Effects of cooling rate on <i>β</i> relaxation process and stress relaxation of La-based amorphous alloys

The dynamic relaxation process and stress relaxation process are two important processes which can reflect the microstructures of materials, for they are closely related to the diffusions, the glass transition phenomena and the microstructural heterogeneities. It is of great significance to explore the relationship among them. In the current research, the <i>β</i>-relaxation characteristics and stress relaxation behaviors of bulk and ribbon samples obtained by different cooling rates are systematically investigated by taking the typical La-based amorphous alloys as model systems. The experimental results demonstrate that the cooling rate is an important parameter for controlling the energy state of the glass system, which further affects its physical and mechanical properties. Based on the dynamical mechanical spectra, the larger the cooling rate, the greater the low-temperature internal friction is and the smaller the beta relaxation activation energy according to Arrhenius calculations, and the greater the broadening of the beta relaxation behavior in the temperature spectra, suggesting that the higher cooling rate leads to greater atomic mobility and a high degree of heterogeneity in the microstructure. Thermodynamic analysis is conducted to study the slow process of thermal activation and the fast process driven by stress. At low temperature, the activation volume of the strip sample is larger than that of the bulk sample, and the activation volume values of the two samples are almost the same, as the cooling rate only affect the <i>β</i> relaxation stage, but exert little effect on the <i>α</i> relaxation, which is consistent with the conclusion that the stress relaxation behavior and <i>β</i> relaxation behavior are related to the structural non-uniformity of the amorphous alloy. The stress relaxation tests show that the characteristic time of deformation decreases at higher cooling rate, the normalized stress decay is larger, it is easier to deform under an applied force field, and the deformation unit is more likely to activate to accommodate structural deformation. The correlation between stress relaxation and <i>β</i> relaxation of amorphous alloy is further confirmed, and the proportion of liquid-like region is proportional to the relaxation mode spectrum, which also shows that <i>β</i> relaxation and stress relaxation are consistent. Finally, by calculating relaxation enthalpy <inline-formula><tex-math id="M1">\begin{document}$ \Delta {H}_{{\mathrm{r}}{\mathrm{e}}{\mathrm{l}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231417_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231417_M1.png"/></alternatives></inline-formula>, the variation of microstructure heterogeneity with cooling rate is experimentally verified. The research sheds new light on further clarifying the relationship among <i>β</i> relaxation, deformation and microstructural heterogeneity of the amorphous alloy.

Investigation of dysprosium oxide substitution on physical, electrical and dielectric properties in sodium borosilicate glass system

Dysprosium oxide doped sodium borosilicate glasses were prepared in this study using a usual melt-quenching process. Further XRD, FTIR, density measurements, electrical conductivity, and dielectric properties were utilized to explore the prepared glassy samples. We found that all produced samples exhibit amorphous nature through X-ray diffraction. Increase in density and molar volume were found when dysprosium oxide concentration increased, which was ascribed to dysprosium oxide's greater molecular weight. Density data was used to calculate and explain several physical characteristics, including rare earth ion concentration (N), polaron radius (rp), internuclear distance (ri), and field strength (F).

Thermally reprocessable bio-based polyhydroxyurethane vitrimers

Vitrimers present one way to address growing demand for sustainably sourced, recyclable thermoset polyhydroxyurethanes (PHUs). We employ vinylogous urethane formation for dynamic bonding, using amine-terminated bio-sourced PHUs (prepared via terpolymerization of diglycerol dicarbonate (DGC) with Priamine 1074 and 1,10-diaminodecane (DAD)) as crosslinkers with ketone-functional copolymers. The latter are synthesized via atom transfer radical copolymerization (ATRP) of an alkyl methacrylate (C13MA, with average side-chain length of 13 carbons) with (2-acetoacetoxy) ethyl methacrylate (AAEMA) (initial AAEMA mole fraction = 0.1–0.4). By manipulating copolymer functionality, thermoset PHUs (Young's moduli spanning from 31 ± 1.5 kPa to 133 ± 5.7 kPa) are obtained. Furthermore, as ketone group concentration increases, networks became tighter, with slower relaxation rates and increased apparent activation energy of stress relaxation, in agreement with calculated molecular weight between crosslinks obtained rheologically. Finally, thermoset PHUs can be effectively recycled three times through remolding at 100 °C without compromising their mechanical properties.

Dielectric Spectroscopy Studies of Conformational Relaxation Dynamics in Molecular Glass-Forming Liquids.

We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules.

Investigation on the rheological behavior of PA6 film during biaxial stretching

In this work, the rheological behavior of polyamide 6 (PA6) film during biaxial stretching was investigated. Firstly, the cast film was subjected to different ratios of biaxial stretching, and the stress calculation expression of biaxial stretching was derived. Then the generalized Maxwell model was employed to fit the stress-time curves in the heat setting stage. It was found that the elastic-plastic deformation during biaxial stretching influenced the stress rebound or relaxation in the heat setting. Finally, the relaxation spectrum and activation energy spectrum were calculated according to the fitting curves. It was found that the stress relaxation process of PA6 film was divided into multiple stages, but the stress rebound process was relatively simple. According to the activation energy spectrum, we speculated that stress relaxation was a process of releasing energy, and stress rebound was a process of absorbing energy. With the increased stretching ratio, the peaks on the activation energy spectrum became sharper and the intensity increased. Indicating that more energy was released or absorbed under the larger stretching ratio. The obtained results would provide theoretical guidance for further understanding the biaxial stretching deformation mechanism of polymer films.

Synthesis of dynamic polymers by amino-yne click reaction using multifunctional amine

Covalent adaptable networks (CANs) have become sustainable alternatives to traditional thermosetting materials due to their powerful reprocessing and rearrangement capabilities. Therefore, developing and investigating reversible reactions that can be used in the construction of CANs is a popular research topic. Amino-yne click chemistry is known for its green characteristics—it is a catalyst-free and efficient reaction—and is widely applied in the synthesis of bio-organic molecules, but there have been few reports on CANs, especially multifunctional amine-based amino-yne click reactions. In this study, various CANs were synthesized via the amino-yne click reaction using multifunctional amines (diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA)), and they were compared with CANs that were prepared from a primary amine (tris(2-aminoethyl) amine (TREN) and a secondary amine (tris[2-(methylamino)ethyl]amine (TMEN)) under the same conditions. The results showed that H-DETA CANs exhibited the smallest glass transition temperature Tg (15 °C) and Young's modulus (105 MPa) because the crosslinker (DETA) contained a low content of primary amine and secondary amine, but its crosslinking density followed the trend of H-TMEN < H-DETA < H-TREN. In addition, the relaxation time, activation energy, and creep rate results for the CANs followed the same trend as the crosslinking density. With the increase in the secondary amine content of the multifunctional amines, the Tg of the CANs increased from 15 °C to 57 °C, the Young's modulus increased from 105 to 666 MPa, and the crosslinking density increased from 955 to 2690 mol/m3. Notably, the increase in the secondary amine content led to the weaker dynamics of the material, namely, prolonged relaxation time, increased activation energy, and decreased creep rate. This work also verified the dynamic exchange mechanism of the materials by the molecular model reaction and the amine solvent dissolution tests. Finally, hot-pressing experiments demonstrated the good plasticity and recovery ability of prepared CANs. This work provides an elegant path for preparing materials in energy and chemical fields such as coatings and rubber.

1H, 31P NMR, Raman and FTIR spectroscopies for investigating phosphoric acid dissociation to understand phosphate ion kinetics in body fluids

The study investigates the formation and transportation of ionic charge carriers in phosphoric acid-water system. This investigation encompasses an analysis of 1H and 31P NMR chemical shifts, self-diffusion coefficients, spin-lattice relaxation rates, spin-spin relaxation rates, activation energies, dissociation constants, electrical conductivity, and Raman shifts, along with FTIR spectra across various water concentrations. Significantly, the maxima observed in these curves at around 0.8 water molar fraction predominantly from the unique molecular arrangement between phosphoric acid and water molecules, influenced by a hydrogen bonding network. These findings yield valuable insights into phosphate ion kinetics within body fluids, covering essential aspects like hydrogen bonding networks, ionization processes, and the energy kinetics of phosphoric dissociation. A customized semiempirical model is applied to calculate dissociated species (water, phosphoric acid, and hydronium ion) at different water contents within a wide range of water mole fraction. Furthermore, this investigation extends to the dissociation of phosphoric acid in DMEM cell culture media, offering a more precise model for phosphate ionic kinetics within body fluids, especially at nominal phosphate concentrations of approximately 1:700μL.

Electrical and dielectric characteristics of molybdenum dioxide nanoparticles for high-performance electrocatalysis

As an attempt to improve the catalytic processes in different electrochemical systems, molybdenum dioxide nanoparticles were prepared using the hydrothermal method, and their electrical and dielectric properties were investigated. The nanoparticles were polycrystalline with an orthorhombic structure. AC electrical transport properties of the pressed disc were conducted over a temperature range of 303–423 K and a frequency range of 42–5 × 106 Hz. The AC conductivity follows Jonscher's universal dynamic law, and it has been determined that correlated barrier hopping (CBH) is the primary conduction mechanism. The maximum barrier height (WM) was found to be 0.92 eV. The low activation energy showed that hopping conduction is the dominant mechanism of transporting current. The dielectric parameters were analyzed using both complex permittivity and complex electric modulus, with a focus on how they vary with temperature and frequency. At relatively high temperatures and low frequencies, the dielectric parameters showed a high-frequency dependence. The dielectric modulus showed that relaxation peaks move towards lower frequency when temperature increases. The dielectric relaxation activation energy, Δ Eω was determined to be 0.31 eV.

Tunable optical, electro-optical and dielectric properties of eco-friendly graphene quantum dots-nematic liquid crystal composites

ABSTRACT The changes in physical properties obtained by dispersing hydrophilic carboxylated graphene quantum dots (GQDs, <10 nm), which are non-toxic, biocompatible, and eco-friendly, into a room-temperature nematic liquid crystal, 5CB (4-pentyl-4ˈ- biphenylcarbonitrile) are described. Nematic–isotropic transition temperatures, polarising microscopy, electro-optic, photoluminescence, and dielectric investigations are reported for two composites having concentrations of GQDs of 0.25 and 1.0 wt.% in 5CB and compared to pristine 5CB. At the lower concentration (0.25%) of GQDs there is an enhancement in the order parameter by 5.49%, the birefringence by 1.28%, and the activation energy for dielectric relaxation by 4.4% compared to pure 5CB. However, at the higher concentration (1%), aggregation of GQDs occurs. This is apparent in optical textures and resulted in the deterioration of optical and electrical properties compared to 5CB. The photoluminescence spectrum of both concentrations of GQDs are centred at the same wavelength; however, the 1% GQD-5CB mixture exhibits significant quenching and may be suitable for some optical devices. Overall, our results support that, in addition to promoting sustainable, heavy-metal-free, and environmentally safe technologies, a low concentration of GQDs in LCs produces composites that have superior electrical and optical properties; thus, making them suitable for a variety of photonics and display applications.

Features of low-frequency relaxation processes of sodium niobate ceramics in various structural phases

Comparative studies of low-frequency dispersion, analysis of the most probable relaxation time and activation energy of the low-frequency relaxation process of sodium niobate ceramics obtained under various technological conditions for the synthesis of sodium niobate material were carried out. It is shown that the temperature conditions of the synthesis of the sodium niobate material determine the nature of the low-frequency dispersion only at relatively low temperatures (structural Q or P phase) of sodium niobate samples.

The slow molecular mobility in the amorphous solid and supercooled liquid phases of brucine, an anti-cancer drug. A study by differential scanning calorimetry (DSC), thermostimulated currents (TSC) and dielectric relaxation spectroscopy (DRS)

In this work, we explore in detail the molecular mobility in amorphous brucine, i.e. in the glassy state and in the supercooled liquid immediately above the glass transition, where viscosity is high and where a spatially heterogeneous dynamic is detected. Brucine is an alkaloid used in ancient times by Chinese medicine in the treatment of cancer, it can be obtained in the amorphous form by cooling from the melt and the high stability exhibited makes it a good candidate for being use in novel formulations.Combining thermostimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) different types of motions have been probed. In the glassy state, two secondary relaxations γ and β were characterized, the latter with features compatible with a Johari-Goldstein mobility. The cooperative α relaxation was clearly identified, and by combining also with results taken from differential scanning calorimetry (DSC), it was possible to determine parameters such as the glass transition temperature, and fragility index which indicate a moderately fragile glass former. In addition, the determination of the βJG relaxation times and activation energy, both above and below Tg, were a new result of the current investigation.Additionally, TSDC measurements, although with some uncertainty, reveal the existence of a mobility above Tg. On the other hand, in the supercooled liquid region, the analysis of the conductivity obtained by DRS, shows a breakdown of the Debye-Stokes-Einstein relation pointing to a heterogeneous dynamic that could be in the origin of the observed mobility by TSDC.

Dipole behavior in thin film heterostructure composed of Er-doped SnO2 and GaAs: Influence of polarization bias, temperature and stray light

The heterostructure system GaAs/SnO2 is built by resistive evaporation of Er-doped SnO2 powder on the top of GaAs semi-insulating substrate. The SnO2 powder comes from the drying of SnO2 sol-gel solution. The possible formation of dipoles in this heterostructure is investigated by the thermally stimulated depolarization current (TSDC) technique, and the possibilities for dipole formation are explored, such as the EL2 defect in the GaAs side, oxygen vacancies and Er ions in the SnO2 layer. The dipole relaxation activation energies are found in the range 0.2 eV to 0.3 eV, in good agreement with the ionization energies of these defects. The main TSDC bands: 213 meV with peak of −298 pA, for positive bias, and 218 meV with peak of 128 pA for negative bias, are modified by stray (room) light, which may correspond to the second ionization level of oxygen vacancies in SnO2, which are excited by the room lights and do not return to the original orientation.

Giant permittivity in (Nb0.5La0.5)xTi1‐xO2 ceramics prepared by slip casting in a strong magnetic field

AbstractA series of textured (Nb0.5La0.5)xTi1‐xO2 (x = 0, 0.0025, 0.005, 0.01) ceramics were sintered in a nitrogen environment after magnetic slip casting (12 T). Component x ranges from 0.0025 to 0.01 while the degree of orientation increases from 0.49 to 0.88. (Nb0.5La0.5)0.01Ti0.99O2 ceramics in the parallel magnetic field's plane have a high permittivity ɛr ≈ 1.6 × 104 and the ultralow dielectric loss tanδ ≈ 0.0038 at 104 Hz. The temperature coefficient value of η ≤ ± 7.1% between 218–473 K, fulfilling the X9R requirements. The giant permittivity properties of textured ceramics are mainly derived from internal barrier layer capacitor impacts, electron hopping, and electron‐pinned defect‐dipoles polarization. The microstructure evolution of sintered ceramics was modified by texturing in a magnetic field, leading to higher activation energies of dielectric relaxations and resistance of grain boundaries and grains. This excellent performance is expected to show great potential in electronic devices' miniaturization and high‐density energy storage.

Ultrahigh thermal stability of dielectric permittivity in 0.6Bi(Mg1/2Ti1/2)O3–0.4Ba0.8Ca0.2(Ti0.875Nb0.125)O3

0.6Bi(Mg1/2Ti1/2)O3–0.4Ba0.8Ca0.2(Nb0.125Ti0.875)O3 ceramics with a pseudo-cubic structure and re-entrant dipole glass behavior have been investigated via x-ray diffraction and dielectric permittivity–temperature spectra. It shows excellent dielectric–temperature stability with small variations of dielectric permittivity (±5%, 420–802 K) and dielectric loss tangent (tanδ &amp;lt; 2.5%, 441–647 K) in a wide temperature range. Three dielectric anomalies are observed from 290 to 1050 K. The low-temperature, weakly coupled re-entrant relaxor behavior was described using the Vogel–Fulcher law and the new glass model. The mid- and high-temperature dielectric anomalies are characterized by isothermal impedance and electrical modulus. The activation energy of both dielectric relaxation and conductivity follows the Arrhenius law in the temperature ranges of 633–753 and 833–973 K, respectively. The ultrahigh thermal stability of dielectric permittivity is attributed to the weak coupling of polar clusters, the formation of diffuse phase transition, and the local phase transition of calcium-containing perovskite. The results provide new insights into the defects behavior and the modification way of re-entrant relaxor behavior.

Thermo-kinetic and structural characterization of Ce-doped glasses based on Bioglass 45S5

Thermo-kinetic behavior of Ce-doped 45S5 bioactive glass (addition of CeO2 up to 1.5 mol.%) was explored by means of calorimetry and thermomechanical analysis. The apparent activation energy of structural relaxation was estimated to vary in the 540–670 kJ mol−1 range. The relaxation process exhibited broad distribution of relaxation times and the relaxation motions were found to only weakly depend on the material's structure. The activation energy of viscous flow was found to vary in the 585–615 kJ mol−1 range; the viscosity itself showed Arrhenian behavior within 107–1011 Pa s. Thermal stability of the glasses was found to be non-monotonous and generally very low, with the most stable glass composition being 45S5 + 0.5 mol.% CeO2. The cold crystallization was found to behave autocatalytically, with the activation energy decreasing from 360 to 310 kJ mol−1 with the gradual addition of CeO2. While the bulk glasses were crystallization-resistant up to 620 °C, the powdered material with CeO2 content ≥1 mol.% was predicted to crystallize even at temperatures slightly above the glass transition temperature.