关键词： Shape-memory polyurethane   Semi-crystalline polymer   Polymer crystallization   Multiscale simulation   Coarse-grained molecular dynamics simulation   Melting temperature   Thermo-mechanical deformation  

摘要： Shape-memory polyurethanes (SMPUs) are smart semi-crystalline polymers with heat-induced shape memory. The segmented SMPU copolymer simulated in this study consisted of an isocyanate (hard segment) that acts as a netpoint to stabilize the network, and a polyol (soft segment) that, owing to polymer crystallization, acts as a molecular switch. The ratio of hard to soft segments is a determining factor in the thermo-mechanical behavior and shape-memory performance of SMPU copolymers. However, the limitations of the scale of the conventional all-atom molecular dynamics simulation makes it difficult to observe mesoscale phenomena such as the phase-segregated morphology and polymer crystallization. To address this problem, we have developed a coarse-grained (CG) molecular dynamics (MD) model with fewer degrees of freedom by treating multiple atoms as a single bead. We derived the CG intra- and inter-bead potentials of the CG MD model such that the structural and thermodynamic properties would be identical to those of the all-atom reference model. Consequently, polymer crystallization was verified to occur at room temperature with the developed CG potentials. We subsequently investigated the effects of hard-segment content (HSC) on the thermal transition at SMPU melting temperature. We also investigated the influence of HSC on the thermo-elastic behavior and shape-memory properties of SMPU copolymers by simulating a 4-step thermo-mechanical cycle. The results revealed that the microstructure significantly affects macroscopic shape-memory behavior. We expect that this study will be used to improve the prediction and design of the thermo-mechanical behavior of segmented polyurethane or other semi-crystalline polymers.

摘要： The high sensitivity and effective frequency discrimination of sound detection performed by the auditory system rely on the dynamics of a system of hair cells. In the inner ear, these acoustic receptors are primarily attached to an overlying structure that provides mechanical coupling between the hair bundles. Although the dynamics of individual hair bundles has been extensively investigated, the influence of mechanical coupling on the motility of the system of bundles remains underdetermined. We developed a technique of mechanically coupling two active hair bundles, enabling us to probe the dynamics of the coupled system experimentally. We demonstrated that the coupling could enhance the coherence of hair bundles' spontaneous oscillation, as well as their phase-locked response to sinusoidal stimuli, at the calcium concentration in the surrounding fluid near the physiological level. The empirical data were consistent with numerical results from a model of two coupled nonisochronous oscillators, each displaying a supercritical Hopf bifurcation. The model revealed that a weak coupling can poise the system of unstable oscillators closer to the bifurcation by a shift in the critical point. In addition, the dynamics of strongly coupled oscillators far from criticality suggested that individual hair bundles may be regarded as nonisochronous oscillators. An optimal degree of nonisochronicity was required for the observed tuning behavior in the coherence of autonomous motion of the coupled system.

关键词： Tungsten   Graphene   Helium   Diffusion   Point defects   First principles calculation  

摘要： In Tokamak device, helium atoms caused by the fusion reaction will generate helium bubbles and point defects after entering the first wall material composed of tungsten metal, which will seriously affect the stability of the first wall material. Therefore, we designed the tungsten/graphene/tungsten interface material as a new first wall material. It was found from first-principles calculations that the interface can trap helium atoms to reduce the diffusion of helium atoms from the interface to the tungsten phase again. The interface also traps vacancies and accelerates the recombination of vacancies and self-interstitial atoms. Using the quasi-harmonic Debye model, we found that the presence of the graphene layer can also improve the ductility, at a cost of reducing the mechanical modulus of the tungsten metal. (C) 2020 Elsevier B.V. All rights reserved.

关键词： C-S-Hs-PCE   Sodium sulfate   Hydration kinetics   Mechanical properties  

摘要： This study reports the effects of combined application of calcium silicate hydrate seed-Polycarboxylate (C-S-Hs-PCE) and sodium sulfate (SS) on the hydration kinetics and mechanical properties of cement paste. The setting time, compressive and flexural strengths, and chemical shrinkage were evaluated. The mechanism of synergistic utilization of C-S-Hs-PCE and SS was explored on basis of the hydration heat releases, pore structures and crystalline phases. The results show that the setting time is shortened and the strengths of cement pastes are improved dramatically with the addition of C-S-Hs-PCE and SS. The incorporation of C-S-Hs-PCE increases the chemical shrinkage, while the co-doped C-S-Hs-PCE and SS can mitigate it. Overall, it accelerates the hydration process especially at early age, the pore structures are refined and the volume of gel micro pores increases due to more formation of gel. In the hardened pastes, the content of calcium hydroxide (CH) increases with the C-S-Hs-PCE and SS dosages, attributed to the further hydration of dicalcium silicate (C2S) and tricalcium silicate (C3S). This study could be viewed as a reference for the application of C-S-Hs-PCE and SS in cement paste, which is of great significance for its enhancement of early strengths. (C) 2020 Elsevier Ltd. All rights reserved.

关键词： Polymer composite   Mechanical properties   Interface   Molecular dynamics  

摘要： We investigate the mechanical behavior of polypropylene/silica composites under mode I and II loading by using molecular dynamics simulations, focusing on the effects of two typical surface coupling agents of HMDZ and APTES on the interface deformation and failure. It is found that under mode I loading, the silica surface functionalized with APTES can enhance the tensile strength of polypropylene/silica composites effectively. In contrast, the silica surface functionalized with HMDZ has little effect on the tensile strength. The simulation results are supported by experiments. Under mode II loading, it is found that both HMDZ and APTES can enhance the shear strength of polypropylene/silica composites with APTES being more effective. The underlying mechanisms for the enhancement of interface strength are analyzed. Our work provides a deep understanding of the interface structure, deformation and failure and will be useful for selecting suitable surface coupling agents in enhancing the interface strength of polymer composites.

关键词： Nickel doping   Adsorption regeneration   In situ decontamination   Cationic and anionic dyes   Synergistic effect  

摘要： Iron-nickel bimetallic organic frameworks (FeNix-BDC, H2BDC: terephthalic acid) were developed as bifunctional materials for adsorption and photo-Fenton degradation of organic dyes with different charge properties. Significantly enhanced adsorption capacity of FeNi1/15-BDC towards methylene blue (MB) and methyl orange (MO) was achieved, 5.3 and 2.6 times higher than that of pristine Fe-BDC, which was attributed to enlarged specific surface area and pore volume and the decreased surface charges induced by Ni doping. The adsorption kinetics demonstrated that chemisorption was dominant and intra-particle diffusion was the rate-controlling step. Two-stage degradation including slow induction stage and rapid oxidation stage fitted with pseudo-zeroorder kinetics well. The increased rate constants (2.472 vs. 1.188 min(-1) for MB;0.616 vs. 0.421 min(-1) for MO) in the induction stage as well as the superior removal capability by asynchronism relative to synchronism jointly corroborating the improved adsorption performance was favor for subsequent degradation. Notably, this heterogeneous system not only exhibited obvious advantages like wider pH working range (3-9), better stability and reusability of catalysts, but also achieved the dual objectives of in-situ decontamination and adsorbent regeneration. The coupling of adsorption and degradation along with synergism between photocatalysis and Fenton-like process are responsible for the reinforced removal of organic contaminants.

关键词： 2-Amino-3-cyanopyridine   Aldehyde   Amines   Chemically-Controlled   Cyclohexanone   Fructose   Kinetics   Malononitrile  

摘要： 2-Amino-3-cyanopyridine derivatives have been synthesized via a four-component reaction of malononitrile (1), aromatic aldehyde (2), cyclohexanone (3), and amines (4) in the presence of fructose and maltose catalysts with good to excellent yields. In addition, for the first time we describe the experimental kinetics study and a full mechanistic investigation of title reaction. To determine the right position of each component in the reaction path, kinetic study of the reaction was carried out using UV-vis spectrophotometry technique. Based on temperature effects on the reaction rate, activation energy (E-a= 28.97 +/- 0.21 kJ mol(-1)) and its parameters (Delta H-double dagger = 26.49 +/- 0.23 kJ mol(-1), Delta S-double dagger = -122.88 +/- 0.67 J mol(-1 )K(-1) and Delta G(double dagger) = 63.12 +/- 0.46 kJ mol(-1)) were determined, so the reaction was enthalpy and chemically controlled and also its mechanism was associative. By the change in the solvent polarity, it was found that polar solvents increase the reaction rate and the mixture of water/ethanol (50/50) was the best choice. Changing the structure of aniline (4) and benzaldehyde (2) by the substituent groups exhibited that the reaction proceeds faster in a poor electronic ring, regarding benzaldehyde (2), which is the result of the electron-withdrawing groups and a rich electronic ring for aniline, which is because of the electron donating groups. Further investigations on the partial orders of reactants approved the independency of the rate law to the concentrations of components (3) and (4), so the reaction was second-order kinetics corresponding to compounds (1) and (2) with the right rate law = k7k2k112Catk-6. Step(7) was recognized as the RDS.

关键词： 产业化应用   商用车   混合动力系统   城际交通   复合驱动   能量回馈   节能减排   尾气排放  

摘要： 商用车是我国城市、城乡、城际交通与运输的重要运载工具。2017年全国商用车保有量达2800万辆,产销量420万辆(占全球20%)。据统计,我国商用车燃油消耗达1.22亿吨/年(占石油总消耗21.9%),北京等城市的大气颗粒物中,商用车尾气排放占40%以上。因此,商用车节能减排需求迫切。混合动力(简称"混动")系统利用发动机与电机复合驱动,以及能量回馈复合制动等新技术,可显著降低内燃动力车辆能耗和排放,是解决该问题最重要的途径之一。然而,我国混合动力技术严重缺失,商用车并联混动市场被美国伊顿公司垄断。

关键词： 动力设备   机械振动控制   位移量   振动信号  

摘要： 为增强船舶动力设备机械振动控制效果,设计一个船舶动力设备机械振动控制模型。设定船舶动力设备机械振动控制模型,依据采集的振动信号模型得到机械振动位移量,同时依据位移量提出控制计算方法,以此完成船舶动力设备机械振动控制。实验结果表明,所设计的船舶动力设备机械振动控制模型加强了船舶在低频激励下的效果,并降低了机械振动控制的控制误差,具备实际应用的意义。

关键词： 湿喷台车机械臂   三维模型   运动学   动力学  

摘要： 针对混凝土湿喷台车机械臂的结构特点,采用D-H法建立连杆坐标系,推导出运动学和动力学方程。在此基础上,基于MATLAB Robotic Toolbox对混凝土湿喷台车机械臂进行运动学仿真分析,建立仿真模型并求解工作空间,验证所建运动学模型的正确性。采用ADAMS建立湿喷台车机械臂动力学模型,对两种最危险工况分别进行研究,计算动作油缸的最大载荷,并探究其随油缸位移的变化情况,仿真结果为机械臂与油缸的设计提供了一定的参考依据。