Dynamics Of Relaxation Processes Research Articles

Characteristic of dynamic mechanical relaxation processes in Cu46Zr46Al8 and La43.4Ce18.6Ni24Al14 metallic glasses

Dynamic mechanical relaxation processes are very important to understand physical and mechanical properties of metallic glasses. In the current work, dynamic mechanical behaviors of La43.6Ce18.4Ni24Al and Cu46Zr46Al8 metallic glasses were investigated by mechanical spectroscopy. Unlike Cu46Zr46Al8 metallic glass, La43.6Ce18.4Ni24Al14 glassy system shows an evident slow β relaxation process. In addition, physical aging below the glass transition temperature Tg induces a reduction of intensity of the β relaxation. Evolution of the concentration of “defects” of metallic glasses was analyzed in the framework of Interstitialcy theory. Our research provides a new insight on understanding of the correlation between the structural heterogeneity and β relaxation of metallic glasses.

On the Dynamic Viscosity Coefficients and Relaxation Processes in Magnetic Fluids

On the Dynamic Viscosity Coefficients and Relaxation Processes in Magnetic Fluids

Visco-elasticity and mechanical relaxation behavior in La-based metallic glasses

Dynamic mechanical relaxation process is one of the important issues to understand mechanical/physical properties of metallic glasses. In the current research, dynamic mechanical relaxation behavior of La62Al14(Cu5/6Ag1/6)20(Ni1/2Co1/2)4 and (La0.7Ce0.3)65Al10Co25 bulk metallic glasses were investigated by mechanical spectroscopy. Unlike the typical Zr- or Cu-based metallic glasses, (La0.7Ce0.3)65Al10Co25 metallic glass shows an evident β relaxation process. On the other hand, La62Al14(Cu5/6Ag1/6)20(Ni1/2Co1/2)4 metallic glass presents a “shoulder” relaxation process. Physical aging below the glass transition temperature Tg induces a decrease of the concentration of “defects”, which corresponds to a reduction of the intensity of the β relaxation. During aging process, the amplitude of β relaxation (△E") decreases, while the relaxation time (τβ) increases. In organic glasses, it was reported that the changes of In(τβ) and In(△Emax) are linearly correlated with In(τβ)=b-a In(△E"max), this correlation has also been found in metallic glass. This behavior is discussed in the framework of the asymmetric double well potential (ADWP) model, and U and △ are used to characterize ADWP. It is suggested that during the aging process, the U/△ ratio remains basically unchanged, which is close to the coefficient describing the linear correlation between In(τβ) and In(△Emax) (U/△~a).

Quantum-Sized Objects in the Form of Exciton and Trion Traps: Their Role in Excitonic Partial Blocking of the Memristor Properties of Thin Zinc Oxide Films

Possible mechanisms of the emergence of exciton and trion traps at room temperature and the dynamics of relaxation processes occurring in them are discussed, based on an analysis of the results from experiments on photon echoes in thin films of zinc oxide and doped silicon. The current–voltage characteristics recorded for such films with traps displaying the memristor effect are studied, and the influence traps exert on the parameters of such characteristics is established.

Нейросетевое моделирование реологии сплава АМг6 в условиях проявления барьерного эффекта дисперсоидами и замедления динамических релаксационных процессов

The paper deals with a neural network to model the flow stress of the AlMg6 alloy at temperatures ranging between 300 and 500 C and strain rates from 1 to 25 s−1. In this temperature–strain-rate range, the movement of free dislocations is blocked and dynamic relaxation processes are inhibited. The results of training the neural network and its verification at a temperature not used in the training show that neural networks with a single hidden layer can correctly approximate and predict the rheological behavior of the AlMg6 alloy for the studied temperature–strain-rate range of deformation.

Effect of Pulse Current on Residual Stresses in AMg6 Aluminum Alloy in Electrodynamic Treatment

The development of energy-efficient methods of controlling the stress state of metallic materials and welded joints is topical for modern production. One of such methods is electrodynamic treatment (EDT). It is based on the passage of electric current pulses (ECP) through the metal, time-synchronized with an impact load initiated by electrodynamic forces. In this case, an electrodynamic deformation process and a process of formation of elastic strain waves occur simultaneously in the material being treated; they are defined as “electrodynamic impact”, the result of which is a local stress relaxation in the metal. The developed mathematical model of the formation of stress and strain fields during EDT, which is based on the motion of an elastoplastic medium, does not take into account the effect of ECPs on dynamic relaxation processes. A computational and experimental assessment of the effect of ECPs, which is based on the experimental assessment of the stress state and microindentation of AMg6 aluminum alloy plates after EDT without and with the passage of ECPs through the metal being treated, has been performed by the mathematical modeling of the EDT process taking into account microindentation results. The passage of ECPs contributes to an increase in tensile plastic strains and hence to an increase in residual compressive stresses (provided that the specimens to be treated are rigidly fixed). The reliability of the procedure has been confirmed by the results of evaluating the distribution of residual welding stresses in AMg6 alloy butt joints after EDT.

DYNAMIC RELAXATION CHARACTERISTICS AND HIGH TEMPERATURE FLOW BEHAVIOR OF ZR-BASED BULK METALLIC GLASS 1)

Dynamic mechanical relaxation processes of amorphous alloys are very important to understand plastic deformation, glass transition phenomenon, diffusion behavior and crystallization. How to establish the correlation between mechanical properties and mechanical relaxation modes is one of key issues. In the current research, with the help of dynamic mechanical analysis (DMA), dynamic mechanical behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ bulk metallic glass from room temperature to supercooled liquid region was probed. In parallel, based on the uniaxial tensile tests, high-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass around glass transition temperature were investigated. Dynamic mechanical behavior and high temperature deformation behavior were discussed in the framework of quasi-point defects theory. The results demonstrated that main $\alpha $ relaxation process of metallic glass can be well described by the quasi-point defects theory. Based on internal friction of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass, activation energy of elementary movement of atoms $U_\beta$ is 0.63 eV. In addition, correlation factor $\chi $ corresponding to concentration of the quasi-point defects in solid glass remains almost constant below the glass transition temperature. When the temperature above the glass transition temperature, the correlation factor $\chi $ increases by increasing the temperature (below the crystallization temperature). Finally, high temperature flow behavior in tensile mode near the glass transition temperature of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was studied. The normalized viscosity decreases with increasing strain rate at low temperatures or high strain rates, indicating a non-Newtonian flow behavior. Whereas Newtonian flow behavior is observed at higher temperatures and lower strain rates. The apparent viscosity is affected by temperature and strain rate. High-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was described by stretched exponential function and free volume theory. Specifically, experimental master curve of the high temperature flow behavior of metallic glass is in good agreement with the prediction of the quasi-point defects theory, which provides a new insight on understanding of viscous effects during high temperature deformation of solid glasses.

Dielectric Relaxation Processes in Mn2O3/Reduced Graphene Oxide Nanocomposites

One essential for developing electronic materials for device applications is an understanding of the materials’ dynamic relaxation processes. This paper seeks to improve our understanding of the relaxation processes in pure Mn2O3 and Mn2O3/reduced graphene oxide (RGO) nanocomposites on the basis of the capacitive contributions of grain and grain boundaries (GBs). To achieve this, the dielectric properties for pure Mn2O3 and Mn2O3/xRGO (x = 5, 10 and 20 wt.%) nanocomposites, prepared theby sol–gel method, were investigated over a wide frequency range at room temperature using a broadband dielectric spectrometer (BDS). Crystalline structure, molecular structure and microstructure of samples were characterized by x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscope, respectively. XRD analysis showed the formation of orthorhombic crystalline structure in all samples with no considerable changes in the crystalline structure. FTIR spectra did not show new absorption bands upon introducing the graphene content. Characteristic peaks corresponding to Mn-O vibration were found to have shifted to lower wavenumbers, indicating an increase of particle size. The dielectric study showed some changes in the dielectric properties of the Mn2O3/RGO nanocomposites. For pure Mn2O3, two relaxation processes were clearly observed; the slower one positioned at high relaxation time (τ = 1.7 × 10−2 s) is associated to the GBs capacitive contribution, while the faster one of lower τ (= 2.5 × 10−5 s) is associated to the grain capacitive contribution. For Mn2O3/RGO nanocomposites, τ for both process slightly increased with increasing RGO up to 10 wt.% then remarkably decreased to much lower values at 20 wt.%. Interestingly, BDS, rather than XRD and FTIR techniques, is considered as a powerful and sensitive technique, even for small changes in Mn2O3/RGO nanocomposites.

Dynamic relaxation process of a 3D super crystal structure in a Cu:KTN crystal

In this Letter, we report the existence and relaxation properties of a critical phenomenon on called a 3D super crystal that emerges at T = TC − 3.5°C, that is, in the proximity of the Curie temperature of a Cu:KTN sample. The dynamics processes of a 3D super crystal manifest in its formation containing polarized nanometric regions and/or polarized clusters. However, with strong coupling and interaction of microcomponents, the characteristic relaxation time measured by dynamic light scattering demonstrates a fully new relaxation mechanism with a much longer relaxation time. As the relaxation mechanism of a relaxator is so-far undetermined, this research provides a novel perspective. These results can help structure a fundamental theory of ferroelectric relaxation.

Counter anions influence the relaxation dynamics of phenoxy-bridged Dy2 single molecule magnets.

The syntheses, structures and magnetic properties of a series of dimeric dysprosium(iii) complexes [Dy2L2(CH3OH)(H2O)]·2X·solvent {X = Cl (1), NO3 (2), ClO4 (3)} and [Dy2L2(CH3OH)2]·2X·solvent {X = CF3SO3 (4)}, formed from the 1 : 1 reactions of the H2L ligand with the corresponding dysprosium salts, are reported. Structural and magnetic studies reveal that counter anions on the periphery play a significant role in determining the dynamic magnetic relaxation process of these complexes. The coordination geometries of the Dy(1)(iii) centers are eight-coordinate triangular dodecahedra in 1-4. All compounds exhibit single-molecule magnet (SMM) behavior under a zero dc field and optimal applied dc field except 3, which displays only slow relaxation of magnetization. A comparison of the magnetic properties and structural parameters of the four compounds shows that the short Dy-Ophen distances and the large Ophen-Dy-Ophen angles create an axial ligand field in which dysprosium(iii) complexes exhibit magnetic anisotropy and SMM properties.

PRETRANSITIONAL PHENOMENA IN REGION OF STRUCTURAL PHASE TRANSITION IN SODIUM PERCHLORATE

Structural and dynamic properties and molecular relaxation processes in crystalline sodium NaClO4 perchlorate in the temperature range from 300 K to 650 K were studied by Raman spectroscopy. The temperature dependences of the position of the maximum v (frequency), the width w and the intensity I of the spectral band, corresponding to the fully symmetric vibration v1(A) of the ClO4–1 perchlorate ion, in the spectral range from 933 cm–1 to 944 cm–1 were analyzed. The frequency v and intensity I decrease, and the width w increases with the increasing temperature. It is shown that these temperature dependences have certain features at a temperature of 460 K. With a further increase in temperature, the frequency v decreases more rapidly, the width w increases, and the intensity I decreases more intensively. In the temperature range from 460 K to the temperature Tstr = 581 K of the first order structural phase transition, we observe a deviation of the temperature dependence of the frequency and width from the linear dependences characteristic of lower temperatures. These deviations appear at a temperature of 460 K and increase with the increasing temperature and approaching the phase transition temperature. It has been established that in the crystalline sodium perchlorate NaClO4 a structural first-order phase transition is stretched. At the phase transition temperature (Tstr = 581 K), the width increases sharply, and the frequency decreases sharply, decreasing with a further increase in temperature. The existence of a pretransitional region in the studied crystalline sodium perchlorate NaClO4 was found. This pre-transition region occurs in the temperature range from 460 K to Tstr = 581 K.

Investigation of microstructure and temperature and frequency dependent dielectric relaxation of Molybdenum-zinc-selenite glass nanocomposite systems

Transition metal oxide doped glass nanocomposite bulk samples of the composition xMoO3–(1-x) (0.5ZnO–0.5SeO2), for x = 0.05, 0.1, 0.2 and 0.3 weight percent, have been developed via the process of slow cooling of the melt. The dependency of dielectric properties on temperature and frequency has been studied. Using transmission electron microscopy (TEM) micrographs, the growth of nanocrystallites and their lattice fringes have been noticed and selected area electron diffraction images have as well been observed. The results of the differential scanning calorimetric (DSC) process describe the thermal property of these compositions. The dielectric constant (ε/) and dielectric loss (ε//) value increases with the increment in temperature and reduces as frequency increases considerably. The dynamic conductivity relaxation process is non-Debye type as Kohlrausch stretching exponent (β) value is not equal to unity and obtained relaxation time shows Arrhenius behavior. The scaled electric okmodulus (complex) spectra discloses that the dynamic conductivity relaxation process depends on the composition but does not depend on temperature.

EFFECT OF SILVER ION REDUCTION ON ELECTRICAL MODULUS OF CHITOSAN-SILVER TRIFLATE SOLID POLYMER ELECTROLYTE MEMBRANE

Utilizing impedance spectroscopy, chitosan-based solid polymer electrolyte containing CF 3 SO 3 Ag was investigated for their electric modulus properties. Using the highest conductivity sample, the lowest conductivity relaxation time (τ σ ) was determined. The high capacitivity of the material is shown by the electrical modulus of the real part. A non-Debye type relaxation is predicted by the asymmetric peak of electric modulus's imaginary part ( ). Utilizing Argand plot, a deformed arc for the relaxation times distribution is demonstrated. Silver nanoparticles formed from silver ions causes a rise in the values of M ' and M above 358 K. The ultraviolet-visible (UV-vis) absorption and complex impedance of silver nanoparticles in chitosan-silver triflate solid electrolyte are temperature dependent. Transmission electron microscopy (TEM) confirmed the silver nanoparticles formation. The dynamical relaxation process for a particular composition is proven by the scaling behavior of spectra to be independent of temperature. The conductivity relaxation is shown by a β exponent to be non-exponential.

Dynamic properties and relaxation processes in surface layer of pulmonary surfactant solutions

In this work, dynamic surface properties of spread monolayers of DPPC and adsorbed layers of PS solutions were investigated in a broad range of surface pressure. Application of a modified Langmuir trough and a recently developed approach for the analysis of a non-linear response of the adsorption layer to large surface deformations gives a possibility to determine the dilational surface elasticity in the region of high surface pressures corresponding to the physiological state of the lung alveoli surface. Although DPPC is the main component of the complex pulmonary surfactant (PS) mixture, the dynamic surface properties of the adsorption layers of these two compounds are different. Other components in the PS adsorption layer lead to a decrease of the dynamic surface elasticity due to a looser packing of the layer, as confirmed by ellipsometry and infrared reflection absorption spectroscopy. Moreover, in the region of high surface pressures the main relaxation time of the surface stresses is much lower than that for spread DPPC monolayers. The fast relaxation in the adsorption layer can be connected with the redistribution of molecules between the surface and the subsurface layer in the course of compression. The acceleration of the relaxation processes can lead to a decrease of the number and size of collapsed parts of the layer and thereby to a decrease of the number of molecules with lost functional properties. This effect can be one of the reasons for the lower efficiency of synthetic pharmaceutical drugs, which do not have proteins in their composition, as compared with the natural PS extracted from animal lungs.

The high-speed ultraviolet photodetector of ZnO nanowire Schottky barrier based on the triboelectric-nanogenerator-powered surface-ionic-gate

Nanowires have a high specific surface area and high surface activity, and the surface states are key factors controlling the nanowire Schottky barrier. Therefore, adjusting the surface states is an important strategy for developing high-performance electronic and optoelectronic devices based on nanowire Schottky barriers. Here, a surface-ionic-gate modulation technique was developed to modulate the surface states and Schottky barrier of nanowires, based on the gas discharge induced by triboelectric nanogenerators (TENGs). It is demonstrated that the Schottky barrier height and electrical transport characteristics of the Ag/ZnO nanowires can be modulated by effectively controlling the surface states. The experimental results from different atmospheres show that O2− ions and electrons generated by gas discharge can form local ions on nanowires surface, which is the source for modulating surface states. Reversible modulation of the surface states and barrier height could be achieved by combining the surface-ionic-gate modulation with ultraviolet (UV) light. In addition, the recovery time of the ZnO nanowire Schottky barrier UV photodetector was reduced from 87 s to 0.3 s using the surface-ionic-gate modulation technique, the recovery speed increased 290 times, making it a universal method for achieving fast UV detection in different environments, such as in air, in oxygen, and in nitrogen. The TENG-based surface-ionic-gate modulation technique proposed here can modulate the surface states of nanowires in real time and in situ. This provides a novel method to study the dynamic generation and relaxation process of surface states and to develop high-performance electronic and optoelectronic nanodevices with broad application prospects.

Main α relaxation and slow β relaxation processes in a La30Ce30Al15Co25 metallic glass

Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La30Ce30Al15Co25 bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La30Ce30Al15Co25 metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature Tg leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed.

Interface effects and relaxation processes in nanocomposites based on CdSe/ZnS semiconductor quantum dots and porphyrin molecules

Controllable self-assembly and properties of nanocomposites based on CdSe/ZnS semiconductor quantum dots (QDs) and tetrapyridylporphyrin molecules (H2P) as well as the dynamics of relaxation processes in these systems were studied for solutions and single nanoobjects in the temperature range of 77–295 K. It was proved that the formation of surface states of different nature is crucial to nonradiative relaxation of exciton excitation in QDs. The efficiency of QD→Н2Р energy transfer was shown to be at most 10–15%. Regularities of photoluminescence (PL) quenching for QDs in nanocomposites in solutions of different polarity correlate with the dependences of PL blinking for single QDs. A scheme was proposed of excited states and main relaxation channels of exciton excitation energy in semiconductor QDs and QD–Н2Р nanocomposites.

The effect of compaction of a porous material confiner on detonation propagation

The fluid mechanics of the interaction between a porous material confiner and a steady propagating high explosive (HE) detonation in a two-dimensional slab geometry is investigated through analytical oblique wave polar analysis and multi-material numerical simulation. Two HE models are considered, broadly representing the properties of either a high- or low-detonation-speed HE, which permits studies of detonation propagating at speeds faster or slower than the confiner sound speed. The HE detonation is responsible for driving the compaction front in the confiner, while, in turn, the high material density generated in the confiner as a result of the compaction process can provide a strong confinement effect on the HE detonation structure. Polar solutions that describe the local flow interaction of the oblique HE detonation shock and equilibrium state behind an oblique compaction wave with rapid compaction relaxation rates are studied for varying initial solid volume fractions of the porous confiner. Multi-material numerical simulations are conducted to study the effect of detonation wave driven compaction in the porous confiner on both the detonation propagation speed and detonation driving zone structure. We perform a parametric study to establish how detonation confinement is influenced both by the initial solid volume fraction of the porous confiner and by the time scale of the dynamic compaction relaxation process relative to the detonation reaction time scale, for both the high- and low-detonation-speed HE models. The compaction relaxation time scale is found to have a significant influence on the confinement dynamics, with slower compaction relaxation time scales resulting in more strongly confined detonations and increased detonation speeds. The dynamics of detonation confinement by porous materials when the detonation is propagating either faster or slower than the confiner sound speed is found to be significantly different from that with solid material confiners.

Dynamics of water bound to crystalline cellulose

Interactions of water with cellulose are of both fundamental and technological importance. Here, we characterize the properties of water associated with cellulose using deuterium labeling, neutron scattering and molecular dynamics simulation. Quasi-elastic neutron scattering provided quantitative details about the dynamical relaxation processes that occur and was supported by structural characterization using small-angle neutron scattering and X-ray diffraction. We can unambiguously detect two populations of water associated with cellulose. The first is “non-freezing bound” water that gradually becomes mobile with increasing temperature and can be related to surface water. The second population is consistent with confined water that abruptly becomes mobile at ~260 K, and can be attributed to water that accumulates in the narrow spaces between the microfibrils. Quantitative analysis of the QENS data showed that, at 250 K, the water diffusion coefficient was 0.85 ± 0.04 × 10−10 m2sec−1 and increased to 1.77 ± 0.09 × 10−10 m2sec−1 at 265 K. MD simulations are in excellent agreement with the experiments and support the interpretation that water associated with cellulose exists in two dynamical populations. Our results provide clarity to previous work investigating the states of bound water and provide a new approach for probing water interactions with lignocellulose materials.

Tunable backflow in chiral nematic liquid crystals via twist-bend nematogens and surface-localised in-situ polymer protrusions

ABSTRACTDynamic electro-optic response of the liquid crystal (LC) director shows a backflow effect that is manifested as an optical bounce in chiral nematic LCs (N*LC) during field-induced homeotropic-twisted transition. The bend elastic constant (K33) strongly influences the dynamics of backflow at the N*LC in homeotropic-twisted transition. The cyanobiphenyl LC dimers – CB7CB, CB9CB and CB11CB – possess a unique characteristic of inherent bend molecular configuration that lowers K33. With the modulation of the effective K33 in dimer-doped N*LCs, we report the tunability of the optical bounce that decreases with the increase in the length of flexible spacers in LC dimers. The doped LC dimers with short spacer lengths not only generate a strong backflow with an enhanced twist degeneracy of the LC director across the cell, but also prolong the time of disappearance of the optical bounce. Furthermore, we demonstrate the suppression of the optical bounce with surface localised polymer protrusions having 50–100 nm diameters, which allow faster dynamic relaxation process and reduced backflow. We envision a novel design of a tunable microfluidic device for precise flow control of organic or inorganic matter in LC medium that exploits the tunable backflow in LC dimer-doped N*LCs.