Faster Relaxation Mechanism Research Articles

Low temperature mechanical behaviour of PVDF: cryogenic pre-treatment, quasi-static, cyclic and dynamic experimental testing and modelling

PVDF has historically drawn the interest of the scientific community because of its mechanical response to external stimuli. Recently, novel 3D printing techniques have been proposed to manufacture responsive PVDF components, leading to the reconsideration of this polymer in many applications (i.e., sensor-actuator or energy harvesting systems). During manufacturing and/or service conditions, PVDF structures can be subjected to large deformations that, eventually, can involve low temperature loading (e.g., piezoelectric sensors for aircrafts). In this work, a deep mechanical characterisation of PVDF specimens is carried out under low temperature conditions. To this end, we first evaluate potential effects of cryogenic pre-treatment going beyond the glass transition temperature of the polymer. Then, mechanical tests are conducted at different loading conditions and a wide range of testing temperatures from room to temperatures below glass transition: quasi-static compression tests, cyclic loading tests, and high strain rate tests. The complete set of experiments is analysed together to identify slow and fast relaxation mechanisms within the polymeric structure and motivate a new constitutive model. Finally, taking the experimental observations as formulation's basis, a thermodynamically consistent constitutive model is developed for finite deformations. This model describes the mechanical behaviour of PVDF as the combination of slow and fast response and accounts for strain rate dependency, temperature sensitivity, hysteresis and thermal evolution during the deformation process. The results from this work provide a full study of the mechanical behaviour of PVDF at low temperature, considering effects of both pre-treatment and testing temperature, and a new modelling tool to predict its response under a wide range of room-to-low temperature loading conditions.

Influence of Composition on Nanoindentation Response of Ni-Zr Alloy Thin Films

Variation in composition of an alloy thin film can alter its microstructure, which provides control over its nanomechanical behavior. To explore this idea, we fabricate thin films of Ni-Zr binary alloys with three different compositions and degrees of crystallinity. At low Zr-content, the microstructure is nanocrystalline, which becomes a mixture of amorphous and nanocrystalline phases at intermediate Zr-content. Further, the increase in Zr-content yields a predominantly amorphous film. Nanoindentations of the films reveal negative strain rate sensitivities over the investigated range of composition, although the effect becomes more pronounced with an increase in the Zr-content. Furthermore, the experiments render a closer view of the nanoindentation creep deformation of these Ni-Zr thin films. In particular, we have examined the influence of loading strain rate and composition on the creep compliance and retardation spectra, which provide valuable insight into the timescales associated with the time-dependent relaxation mechanisms. While the decrease in crystallinity mitigates the creep resistance, an increase in the loading strain rate is found to give rise to fast relaxation mechanisms corresponding to relatively smaller timescales. This study also introduces and highlights the prospects of analyzing the instantaneous strain rate sensitivity measured during the nanoindentation creep, which shows temporal features qualitatively analogous to that of the retardation spectra.

Accelerated Carrier Relaxation through Reduced Coulomb Screening in Two-Dimensional Halide Perovskite Nanoplatelets

For high-speed optoelectronic applications relying on fast relaxation or energy-transfer mechanisms, understanding of carrier relaxation and recombination dynamics is critical. Here, we compare the differences in photoexcited carrier dynamics in two-dimensional (2D) and quasi-three-dimensional (quasi-3D) colloidal methylammonium lead iodide perovskite nanoplatelets via differential transmission spectroscopy. We find that the cooling of excited electron–hole pairs by phonon emission progresses much faster and is intensity-independent in the 2D case. This is due to the low dielectric surrounding of the thin perovskite layers, for which the Fröhlich interaction is screened less efficiently leading to higher and less density-dependent carrier-phonon scattering rates. In addition, rapid dissipation of heat into the surrounding occurs due to the high surface-to-volume ratio. Furthermore, we observe a subpicosecond dissociation of resonantly excited 1s excitons in the quasi-3D case, an effect which is suppressed in the 2D nanoplatelets due to their large exciton binding energies. The results highlight the importance of the surrounding environment of the inorganic nanoplatelets on their relaxation dynamics. Moreover, this 2D material with relaxation times in the subpicosecond regime shows great potential for realizing devices such as photodetectors or all-optical switches operating at THz frequencies.

Formation of satellites from cold collapse

We study the collapse of an isolated, initially cold, irregular (but almost spherical) and (slightly) inhomogeneous cloud of self-gravitating particles. The cloud is driven towards a virialized quasi-equilibrium state by a fast relaxation mechanism, occurring in a typical time $\tau_c$, whose signature is a large change in the particle energy distribution. Post-collapse particles are divided into two main species: bound and free, the latter being ejected from the system. Because of the initial system's anisotropy, the time varying gravitational field breaks spherical symmetry so that the ejected mass can carry away angular momentum and the bound system can gain a non-zero angular momentum. In addition, while strongly bound particles form a compact core, weakly bound ones may form, in a time scale of the order of $\tau_c$, several satellite sub-structures. These satellites have a finite lifetime that can be longer than $\tau_c$ and generally form a flattened distribution. Their origin and their abundance are related to the amplitude and nature of initial density fluctuations and to the initial cloud deviations from spherical symmetry, which are both amplified during the collapse phase. Satellites show a time dependent virial ratio that can be different from the equilibrium value $b\approx -1$: although they are bound to the main virialized object, they are not necessarily virially relaxed.

Physical Characterization of a Commercial Suspoemulsion as a Reference for the Development of Suspoemulsions

AbstractViscoelasticity, low shear flow, microstructure, and physical stability of a commercial suspoemulsion pesticide are studied to serve as a point of reference for the development of new suspoemulsions. Creep compliance tests allow the zero‐shear rate viscosity, η0, to be calculated. As expected, the values of η0 decrease with temperature. The storage modulus is higher than the loss modulus until a crossover frequency, ω*, which is located at the lower frequencies studied. The dynamic parameters decrease with higher temperature and ω* increases in agreement with a faster relaxation mechanism. Multiple light scattering predicts the occurrence of a creaming destabilization process, whose kinetics depends on temperature. Rheological measurements are demonstrated to be a powerful tool to assist in the prediction of destabilization processes together with the multiple light scattering technique.

Hierarchical supramolecular structuring and dynamical properties of water soluble polyethylene glycol–perylene self-assemblies

The structural and dynamical properties of dilute aqueous solutions of poly(ethylene glycol)-perylene diimides (PEG(n)-PDI) have been investigated by means of static and dynamic light scattering, TEM microscopy, and small-angle X-ray scattering experiments. The amphiphilic PEG(n)-PDI molecules first self-assemble into stable and compact primary stacks of a few units of planar PDI through hydrophobic and π-π interactions. These primary stacks subsequently arrange in large and globular aggregates of typically 100-250 nm via weak PEG chain interpenetration. Surprisingly, the scattered electric field autocorrelation function g((1))(q,t) measured by dynamic light scattering evolves over very long periods of times (several months) and up to a bimodal distribution. The fast relaxation mechanism is associated to the diffusion of free primary stacks, whereas the slower relaxation still indicates the presence of large self-assemblies. Kinetic experiments show that the large supramolecular aggregates slowly release the free primary stacks whose proportion increases with time. This dissociation depends on several parameters such as PEG side chain length, total concentration, and shaking.

Effect of temperature on the rheology of wormlike micelles in a mixed surfactant system

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a mixed surfactant system of alkyl ethoxylate sulfate (AES), C 12H 25(OCH 2CH 2) 3OSO − 3Na +, and polyoxyethylene dodecyl ether, C 12EO 3, and the unusual effect of temperature on the rheological behavior have been studied. Upon successive addition of C 12EO 3 to the dilute micellar solution of AES, viscosity increases swiftly and reaches its peak where a viscoelastic solution with nearly Maxwellian behavior is formed. With the further addition of C 12EO 3, viscosity decreases sharply, which is attributed to the formation of micellar joints. With increasing temperature, the extent of micellar growth increases and the viscosity maximum is achieved at a lower mixing fraction of C 12EO 3, but the maximum viscosity attained by the system decreases. The evolution of relaxation time and network density of the viscoelastic network also suggests that with increasing temperature, enhanced micellar growth takes place, but an additional, faster relaxation mechanism becomes increasingly favorable at high concentrations of C 12EO 3. These results can be explained in terms of the increase in free energy of hemispherical end-caps (end-cap energy) of the micelles with increasing temperature.

Copper selenide nanowires and nanocrystallites in alumina: Carrier relaxation, recombination, and trapping

Nonequilibrium carrier dynamics in copper selenide (Cu2−δSe δ=0.15, Cu3Se2) nanowires (diameter ≈18 nm, height ≈2 μm) and nanocrystallites (diameter≈18 nm) in femto- and picosecond time domains by the means of a transient dynamic grating technique were investigated. Bulk and quantum confinement approaches were used to fit the experimental results using nonequilibrium carrier fast relaxation, recombination, and trapping mechanisms. A nonradiative Auger recombination was concluded to be the main mechanism of nonequilibrium carrier recombination. The Auger coefficient for copper selenide was estimated of the order of 10−30−10−29 cm6 s−1. Hole trapping at shallow impurity centers in nanowires was interpreted. From calculating the experimental results the trapping parameters and high concentration of centers >1020 cm−3 were evaluated. Finally, direct measurement of carrier lifetime in copper selenide nanostructures showed values of the order of ≈10−10 s. Samples were characterized by the means of transmission electron microscopy, scanning electron microscopy, x-ray diffraction, and optical spectroscopy.

Double-Channel Contact Recombination of Radical Pairs Subjected to Spin Conversion via the Δg Mechanism

The contact recombination from both singlet and triplet states of a radical pair is studied assuming that the spin conversion is carried out by the fast transversal relaxation and Delta g mechanism. The alternative HFI mechanism is neglected as being much weaker in rather large magnetic fields. The magnetic-field-dependent quantum yields of the singlet and triplet recombination products, as well as of the free radical production, are calculated for any initial spin state and arbitrary separation of radicals in a pair. The magnetic field effect is traced and its diffusional (viscosity) dependence is specified.

Stimulated emission and ultrafast carrier relaxation in InGaN multiple quantum wells

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different QW In compositions x. SE threshold energy densities (Ith) increased with increasing x-dependent QW depth. Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of Ith on x. Carriers are captured from the barriers to the QWs in <1 ps, while carrier recombination rates increased with increasing x. For excitation above Ith, an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing Ith.

DISORDER-INDUCED RELAXATION OF FRENKEL EXCITONS IN MOLECULAR AGGREGATES

A model, based on the intraband scattering of excitons in one-dimensional J-aggregates is proposed to describe the lengthening of the experimentally observed radiative lifetime with temperature. According to this mechanism, the exciton-phonon scattering transfers the oscillator strength fromt he lowest k≅0 optically allowed state to the other states within the excitonic band. A Pauli master equation, in which the hopping rates are calculated, is used to describe the thermalization of the excitonic band. Assuming a fast relaxation mechanism, the temperature dependence of the exciton radiative lifetime is simulated for various chain lengths.

Photoconduction in Langmuir–Blodgett films of octasubstituted metal-free phthalocyanine molecules

In-plane steady-state photoconduction in Langmuir-Blodgett (LB) films of octasubstituted metal-free phthalocyanine molecules have been studied over a temperature range between 78 and 293K. Deep-level traps localised at grain boundaries of multicrystalline films are believed to be responsible for both dark conduction and photoconduction. Slow and fast relaxation mechanisms are identified from measurements of transient currents. Small changes of substituting chains in the molecule are found to have significant influences on rate and quantum yield of photogeneration of charge carriers in LB films.

Dielectric Relaxation of Surfactant Micellar Solutions

Dielectric relaxation behavior of spherical micelles of cationic surfactants, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and cetyltrimethylammonium bromide, in aqueous solution was examined in the frequency range from 106 to 109 Hz. Each surfactant solution shows pronounced dielectric dispersions with two distinct relaxation times above the critical micelle concentrations. A slow relaxation mode with a relaxation time τs ∼ 5 ns reduced its magnitude with increasing the concentration of surfactants (CD). On the other hand, the magnitude of a fast relaxation mechanism with the relaxation time τf ∼ 0.5 ns was proportional to CD. The slow relaxation mode was attributed to the fluctuation of a counterion distribution around the spherical micelle or that of a distribution of dissociated cationic surfactant headgroups on the micellar surface. The fast relaxation mode was assigned to the rotational motion of ionic pairs consisting of surfactant cations and counteranions in the spherical micelle.

Temperature Dependence of the High‐Frequency Viscoelastic Behavior of a Soda‐Lime‐Silica Glass

The high‐frequency viscoelastic behavior of a soda‐lime‐silica glass has been investigated using the Brillouin scattering technique. The temperature dependence of instantaneous elastic constants and very short‐time relaxation mechanisms have been measured in a temperature range between 100° and 1000°C. Variations in the elastic constants in the transition region are significant, and have to be taken into account for numerical computations of tempering and annealing of glasses. A complex thermorheological behavior is observed for very fast relaxation mechanisms in the transition range, whereas intermediate and slow relaxation mechanisms exhibit a simple thermorheological behavior. The viscoelastic behavior of this glass can be adequately represented by assuming a different thermorheological behavior in the alpha‐ and β‐relaxation regimes.

Intervalley scattering rates in GaAs measured by time-resolved four-wave mixing spectroscopy

A three-pulse transient grating technique was used to measure the carrier dynamics in photoexcited GaAs. The four-wave mixing signal exhibits a two component relaxation of different magnitudes for various probe energies. The fast relaxation mechanism is due to electrons in the L valleys scattering back to the Γ valley. The effective transfer time for L→Γ was found to be ≊8 ps. The slower relaxation arises from carrier recombination.

Decomposition of mechanical spectra into elementary cole—cole domains. Application to the viscoelastic behaviour of a styrene—butadiene—styrene block copolymer

The dynamic viscosity of a styrene—butadiene—styrene triblock copolymer melt in the cylindrical phase was measured in the frequency range from 10 −3 to 10 3 rad s −1 for various temperatures between 130°C and 190°C. The Argand plots (η″(ω) versus η′(ω)) show two domains of the Cole—Cole type, and we developed a method of decomposition of these plots into elementary Cole—Cole domains by the minimization technique of Davidon—Fletcher—Powell. The faster relaxation mechanism is attributed to the relaxation of the entangled polybutadiene chains in their primitive tubes, but modified by the terminal polystyrene blocks. The longer relaxation time is attributed to constraint release at the polybutadiene chains due to the diffusion of the polystyrene blocks along the cylindrical domains.

Memory effects in glass transition

Behavior of glass in the glass transition region is reviewed with the emphasis on memory effects. Three examples of memory effects analyzed are the behavior of the fictive temperature in the cross-over, sub- T g relaxation and simple reheating experiments. In all cases, the non-equilibrium system reveals a temporary neglect of the equilibrium state by moving away from it. A multiexponential model of structural relaxation is employed to model this phenomenon and to investigate the behavior of separate relaxation mechanisms contributing to a total effect. The model characterizes the glassy state by a broad distribution of partial fictive temperatures - low, produced by fast relaxation mechanisms, and high, produced by slow mechanisms. During reheating, each mechanism approaches the equilibrium line T f= T in two stages: first from above (because T f i > T) and then from below (when due to the overshoot T f i < T. In the lower part of the glass transition region where slow mechanisms are still in the first stage of a very slow decrease of their T f i 's, the fast mechanisms are finishing their approach to equilibrium by a vigorous increase of their T f i 's, The T f measured in the experiment is the weighted average of all T f i 's and therefore reflects the behavior of fast mechanisms by increasing its value although T f is still greater than the actual temperature T. A complex thermal treatment designed as a combination of quenching and annealing steps will emphasize different groups of mechanisms and produce many intriguing effects. All these effects can be explained as a superposition of quenched and annealed mechanisms approaching the equilibrium state from opposite directions.

Ferromagnetic Resonance of Electrically Conductive Manganese Ferrites

Resonance experiments have been performed between 77–350 K at 9.3 GHz on single crystal spheres of manganese ferrite of low resistivities (0.1∼0.3Ω cm at room temperature). The linewidth shows a maximum at a temperature from 100 K to 200 K, with a marked dependence upon the sample size as well as upon the resistivity of specimen. A characteristic deviation of the resonance field in from that expected by Kittel's formula is also noted. Detailed description is given to show the observed results to be attributable to an eddy current effect rather than due to the so-called slow or fast relaxation mechanisms. The linewidth maximum is interpreted to occur in a temperature region over which the excitation of resonant mode varies from “surface type” to “volume one” corresponding to the change in skin depth.

THE INFLUENCE OF RARE EARTH IONS ON FERRIMAGNETIC RESONANCE

本文利用关联函数的方法(久保理论),讨论了强交换耦合系统的亚铁磁共振,给出了系统总磁化率张量的一般表达式,由此可以定出铁磁支与交换支的共振场H0(或共振频率)和峯宽2△ω。所得结果表明,所谓快弛豫及慢弛豫机理不过是铁磁共振的两个分支(横分支与纵分支)。横分支相应于J及S的横向磁矩之间的耦合运动J,S分别为希土离子及铁离子的磁矩),而纵分支相应于J的纵向分量与S的横向分量之间的耦合运动。由于晶场及各向异性交换场的作用,J的量子化方向与S的量子化方向偏离一个角度φ。此外由于交换作用的各向异性,在交换作用哈密顿J·λ·S中,张量λ的非对角元可以相当大。结果表明,纵分支对峯宽的贡献近似地正比于φ2及λi3(i=1,2)。根据2△ω的一般表达式,在极低温下(4.2°K以下),峯宽主要是由横分支决定的。沿某些晶轴方向?a,当希土离子最低两个能级接近“交叉”时,共振场及峯宽应该出现反常峯值,这在实验上已经得到了证实。当温度升高时,纵分支将逐渐“压过”横分支。当纵向弛豫频率达到高频场的频率ω时,峯宽将出现极大值,一般实验中观察到的就是这个极大值。当温度继续升高时,横分支又将起主要作用。当横向弛豫频率接近相应于希土离子最低两个能级之间的间距ω21时(?=1),峯宽将出现第二个极大值。实验上只有沿希土离子最低两个能级接近交叉的方向进行测量时,才有可能观测到第二个峯值。当频率足够高,满足|ω21(?a)-ω|<<ω的条件时,在极低温下,将出现由横分支决定的尖锐的峯宽极大值。根据所得理论结果,除上述现象外,还可以统一地解释在希土石榴石铁氧体中观测到的下列实验事实:有效旋磁比随温度的显著变化;在抵消点附近峯宽的急剧上升;在镱铁氧体中观测到的在峯宽极大值出现的温度共振场显著上升等。指出了经典磁矩运动方程的局限性,在铁氧体中,晶场的作用与交换场其大小可以相比时,利用经典方程求解所得出的结果只能定性地解释某些与希土离子具体能级结构无关的现象。