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81.
Strain steps are applied to elastomers in a pneumatic relaxometer and monitored by small‐angle X‐ray scattering (SAXS). The relaxometer provides a rise time of 13 ms for strain pulses of step height Δε = ±1 in strain. The basic character of the 2D SAXS frames is examined and corresponding invariants Q(t) are analyzed. Three thermoplastic polyurethanes (TPU) of hardness 85 Shore A with different soft segments are studied both unannealed and annealed. The first response of all materials is a fast morphology conversion which finishes within tmc =250 ms. Because it has been untraceable, it is characterized by a settling stroke Q(tmc) ? Q(0). The second response is a slow morphology adjustment process which complies with logarithmic relaxation. It is characterized by a relaxation rate DQ = Q(10 t)/Q(t) ? 1. Comparison indicates that the nanoscopic morphology relaxation processes appear to have little direct relation to the macroscopic stress relaxation curves. The materials differ with respect to hard‐domain morphology stability and morphology recovery. Most unstable is the morphology of the annealed polyether‐based material. It forms nanofibrillary entities when strained. 相似文献
82.
Eshwar Reddy Cholleti Jonathan Stringer Piaras Kelly Chris Bowen Kean Aw 《Polymer Engineering and Science》2020,60(12):3086-3094
The stress relaxation behavior of barium titanate (BTO)-elastomer (Ecoflex) composites, as used in large strain sensors, is studied using the generalized Maxwell-Wiechert model. In this article, we examine the stress relaxation behavior of ceramic polymer composites by conducting stress relaxation tests on samples prepared with varying the particle loading by 0, 10, 20, 30, and 40 wt% of 100 and 200 nm BTO ceramic particles embedded in a Ecoflex silicone-based hyperelastic elastomer. The influence of BTO on the Maxwell-Wiechert model parameters was studied through the stress relaxation results. While a pristine Ecoflex silicone elastomer is predominantly a hyperelastic material, the addition of BTO made the composite behave as a visco-hyperelastic material. However, this behavior was shown to have a negligible effect on the electrical sensing performance of the large strain sensor. 相似文献
83.
Kaijun Hao Wenwei Ge Zhenan Ren Xiaojuan Liu Laihui Luo Xiaobing Li Haosu Luo Dwight Viehland 《Journal of the American Ceramic Society》2020,103(5):3349-3360
The structural and dielectric properties of Na0.5Bi0.5TiO3 (NBT) ceramics and crystals have been investigated and are compared to that of Pb(Zr0.55Ti0.45)O3 (PZT55/45) and Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMNT 72/28) ceramics. X-ray diffraction (XRD) profiles for (100), (110), (111), (200), (220), and (222) (referred to cubic structure) reveal that the monoclinic structure with Cc space group exists both in the NBT single crystal and ceramics. The diffraction profile obtained with high resolution laboratory XRD for the NBT single crystal can be well described, using Cc model instead of R3c model. The dielectric constant of NBT below Thump shows some similarity to that of PZT45/55 ceramics below 50°C in which oxygen octahedron rotations cause the frequency dispersion of the dielectric constant. The temperature-dependent dielectric constant for NBT can be deconvolved into two independent processes. The lower temperature process shows a typical relaxor characteristic and follows the Vogel-Fulcher relationship. The other process at higher temperature shows less frequency-dependent behavior. Comparing the dielectric constant of NBT with that of PZT55/45 and PMNT72/28 reveals that both oxygen octahedral rotations and random electric fields play an important role in the frequency dispersion of the dielectric constant for NBT relaxor feroelectric. 相似文献
84.
85.
《Ceramics International》2020,46(12):19682-19690
Nano-particles of GdFeO3 (GFO) and Gd0.4Sm0.6FeO3 (GSFO) were synthesized by sol-gel auto-combustion method to study the effect of Sm3+doping on physical properties of GFO. Rietveld refinement of x-ray diffraction pattern confirms the proper phase formation of samples and the average octahedral <Fe–O1–Fe> angle of GSFO is found to be 145.54⁰ which is larger compared to GFO 142.53⁰. Temperature dependent DC magnetization measurements showed that Sm doping in GdFeO3introduces spin-reorientation transition which is absent in pure GdFeO3 phase. The dielectric constant of GSFO is found to be larger than that of GFO because of change in hybridization between the O-2p states and Fe-3d states. In both the ceramics at higher temperatures (above 150 °C) conduction mechanism is taking place via oxygen defect charge carrier hoping. The complex impedance analysis revealed that the distribution of relaxation time of charge carriers is temperature independent. Sm doping in GFO not only changed the dielectric properties but also changed the magnetic coercive field and shape of the magnetic isotherm of GFO. 相似文献
86.
In this paper we are concerned with the assessment of sub-models within a two-phase continuum mechanical FE framework for process modeling of composites manufacturing. In particular, the framework considers the inclusion of two deformation dependent models describing resin flow related to: (1) meso-scale wetting and compaction of individual plies and (2) overall preform deformation and macroscopic Darcian flow. Using micro-mechanical modeling, we model the physics of these sub-processes in relation to the recently developed Out-Of-Autoclave (OOA) prepergs. The models are placed in context with a compression–relaxation experiment, employed to study the preform deformations considered separated from other sub-processes. Finally, calibrations and model validations are carried out against the relaxation experiment to relate the FE framework to the mechanical response of the preform. Therefore, using the above experiment, parameter values out of the literature and those estimated from micrographs gave a fair agreement between the simulation and experiments. 相似文献
87.
Small and large scale segmental motion in polymers: estimating cooperativity length by ordinary relaxation experiments 下载免费PDF全文
Marco Pieruccini Andrea Alessandrini Simone Sturniolo Maurizio Corti Attilio Rigamonti 《Polymer International》2015,64(11):1506-1512
We derive a suitable expression for estimating the size of the cooperatively rearranging regions (CRRs) in supercooled polymer melts by fitting data worked out by ordinary relaxation experiments carried out in isothermal conditions. As an example, the average CRR size in poly(n‐butyl methacrylate) in proximity to the glass transition temperature is derived from a stress relaxation experiment performed by means of an atomic force microscopy setup. Good agreement is found with results in the literature derived from measurements of temperature fluctuations (the so‐called Donth method). The temperature dependence of the CRR size is explored for poly(butadiene); in this case the segmental relaxation function is derived through a novel method for the analysis of the efficiency with which free induction decay echoes are refocused in 1H NMR experiments. It is found that the CRR size increases upon cooling. The results derived from the analysis of the NMR data are found to be in satisfactory agreement with those worked out from broadband dielectric spectroscopy data in the literature. © 2015 Society of Chemical Industry 相似文献
88.
We consider the switched-affine optimal control problem, i.e., the problem of selecting a sequence of affine dynamics from a finite set in order to minimize a sum of convex functions of the system state. We develop a new reduction of this problem to a mixed-integer convex program (MICP), based on perspective functions. Relaxing the integer constraints of this MICP results in a convex optimization problem, whose optimal value is a lower bound on the original problem value. We show that this bound is at least as tight as similar bounds obtained from two other well-known MICP reductions (via conversion to a mixed logical dynamical system, and by generalized disjunctive programming), and our numerical study indicates it is often substantially tighter. Using simple integer-rounding techniques, we can also use our formulation to obtain an upper bound (and corresponding sequence of control inputs). In our numerical study, this bound was typically within a few percent of the optimal value, making it attractive as a stand-alone heuristic, or as a subroutine in a global algorithm such as branch and bound. We conclude with some extensions of our formulation to problems with switching costs and piecewise affine dynamics. 相似文献
89.
This paper investigates the stochastic stability of fuzzy Markovian jumping neural networks with mixed delays in mean square. The mixed delays include time-varying delay and continuously distributed delay. By using the Lyapunov functional method, Jensen integral inequality, the generalized Jensen integral inequality, linear convex combination technique and the free-weight matrix method, several novel sufficient conditions are derived to ensure the global asymptotic stability of the equilibrium point of the considered networks in mean square. The proposed results, which do not require the differentiability of the activation functions, can be easily checked via Matlab software. Finally, two numerical examples are given to demonstrate the effectiveness and less conservativeness of our theoretical results over existing literature. 相似文献
90.
《International Journal of Hydrogen Energy》2021,46(58):30101-30111
W-doped La0·5Sr0·5Fe0·9W0·1O3-δ (LSFW) was prepared and evaluated as a symmetric electrode for solid oxide fuel cells (SSOFCs). Phase and structural stability of LSFW under both reducing and oxidizing atmospheres was studied. The oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) mechanisms were investigated by using electrochemical impedance spectra (EIS) and distribution of relaxation times (DRT). Electrode polarization resistance (Rp) of LSFW are 0.08 and 0.16 Ω cm2 in air and wet hydrogen at 800 °C, respectively. DRT results indicate that the rate-limiting step of LSFW at 800 °C in cathodic conditions and anodic conditions are related to oxygen diffusion and hydrogen adsorption/diffusion, respectively. A La0·8Sr0.2Ga0.8Mg0·2O3-δ (LSGM) electrolyte-supported single cell using LSFW electrodes shows a maximum power density of 617.3 mW cm−2 at 800 °C with considerable stability and reversibility, which enables LSFW a promising SOFCs symmetric electrode material. 相似文献