关键词： Graphene foams   Molecular dynamics   Porosity   Stress-strain curve   Thermal conductivity  

摘要： Graphene foams have recently attracted a great deal of interest for their possible use in technological applications, such as electrochemical storage devices, wearable electronics, and chemical sensing. In this work, we present computational investigations, performed by using molecular dynamics with reactive potentials, of the mechanical and thermal properties of graphene random nanofoams. In particular, we assess the mechanical and thermal performances of four families of random foams characterized by increasing mass density and decreasing average pore size. We find that the foams' mechanical performances under tension cannot be rationalized in terms of mass density, while they are principally related to their topology. Under compression, higher-density foams show the typical slope change in the stress-strain curve at 5 - 10% strain, moving from linear elasticity to a buckling region. At variance, lower density foams display a quasi-linear behavior up to 35% strain. Furthermore, we assess the thermal conductivity of these random foams using the Green-Kubo approach. While foam thermal conductivity is affected by both connectivity and defects, nevertheless we obtain similar values for all the investigated families, which means that topology is the critical factor affecting thermal transport in these structures. (c) 2018 Elsevier Ltd. All rights reserved.

关键词： NC   DBP   miscibility   T-g   mechanical properties   MD simulation  

摘要： Molecular dynamics (MD) simulations were performed to research the miscibility, glass transition temperature (T-g) and mechanical properties of nitrocellulose/dibutyl phthalate (NC/DBP) binary systems. The solubility parameters () of NC and DBP were calculated to predict the miscibility. NC and DBP are miscible as a result of a small (<2.0MPa(0.5)) between the two components. The free volumes (V-F) and density () of NC/DBP system were simulated to study the T-g. It is found that the V-F and of NC/DBP changes regularly with the increase of DBP mass fraction and the transition occurs at the turning point. The simulation results demonstrate that DBP makes significant contribution to the reduction of the T-g of NC. Additionally, the mechanical properties of NC/DBP systems including Young's moduli (E), Bulk moduli (K), Shear moduli (G) and Poisson's ratio (v) were calculated. The results reveal that with the increase of DBP mass fraction the ductility of NC/DBP improves while the brittleness decreases. Overall, a new and systematical method is proposed to study the miscibility, T-g and mechanical properties of polymers.

关键词： GLASS fibers   REINFORCED thermoplastics   FIREPROOFING agents   OXYGEN index of materials   THERMAL analysis  

摘要： Long glass fiber reinforced polypropylene composites containing 2,2',3,3',4,4',5,5' ,6,6'-decabromobibenzy flame retardants (LGF/DBDPE/PP) are prepared by melt blending method. The flame-retardant properties of the composites with various thermal-oxidative exposure time (0-50 days) at 140 degrees C were investigated by the limiting oxygen index (LOI) and UL-94 vertical burning test. The LOI values vary slightly with increasing the aging time, while the UL-94 level maintains a constant V-0 rating. The results demonstrate that the flammability of LGF/DBDPE/PP composites can remain at a relatively stable level in long time thermal-oxidative aging process, which can be attributed to the excellent efficiency of DBDPE fire retardant and the impacts of thermal-oxidative aging on the thermal stability of the composites turned out to be small. The thermal degradation kinetics of the unaged and aged LGF/DBDPE/PP composites are investigated by the Flynn-Wall-Ozawa and Kissinger methods. The obtained activation energy values can further provide objective evidence for revealing the regularities about the flammability of the LGF/DBDPE/PP composites during the thermal-oxidative aging process. However, the crystallinity and mechanical properties of LGF/DBDPE/PP composites show a slow downtrend after thermal-oxidative aging. (C) 2018 Society of Plastics Engineers

关键词： Resins   Cure behaviour   Mechanical properties   Pultrusion  

摘要： This paper analyses the effect of curing kinetics on the mechanical properties of composites manufactured by out of die ultraviolet (UV) cured pultrusion. Curing kinetics have been modified using two combinations of depth curing (BAPO) and surface curing (a aminoketone) photoinitiators. The rate constant of the autocatalytic model k, shows that the curing kinetics of the formulation with the higher content of BAPO and lower content of a aminoketone is faster in the studied intensity range. Spectrometry results justify the differences in curing kinetics, since the light transmission through thickness is higher during the whole UV curing process due to photobleaching effect of BAPO. Faster UV curing kinetics generates lower expansion at the exit of the pultrusion die, reducing the void content approximately 90%. Consequently, flexural and interlaminar shear strength, as well as specific energy absorption index of the fast curing resin, are higher.

关键词： ion pair amphiphile   phosphatidylcholine   bilayer   molecular dynamics  

摘要： An ion pair amphiphile (IPA), a molecular complex composed of two oppositely charged amphiphiles, is a phospholipid mimic which differs from a phospholipid only in the hydrophilic compositions. Here, we utilized molecular dynamics (MD) simulations to compare the bilayer systems composed of phosphatidylcholines (PC) and alkyltrimethylammonium-alkylsulfate IPAs with various alkyl chain lengths. The membrane properties for both liquid-disordered (Ld) and gel (S) phase bilayers were examined via running simulations above and below the main transition temperatures. The electrostatic attraction between the IPA hydrophilic groups leads to a more ordered molecular packing within both S and Ld phase IPA membranes, as revealed by the molecular area, deuterium order parameter, and gauche conformation analyses. Furthermore, IPA bilayers possess a higher area compressibility modulus, molecular tilt modulus, and effective bending rigidity than PC systems. The variation of hydrophilic groups of IPA also leads to fewer hydrogen bonds on the membrane surface and smaller electrostatic potentials for the biomimetic bilayer. The non-covalently linked head groups of IPA further decouple alkyl tilting and surface water retention. The combined results reveal the importance of hydrophilic groups of amphiphiles on modulating the membrane properties, which also provides insights for designs of biomimetic membranes.

关键词： Ultra-high temperature ceramics (UHTC)   Spark plasma sintering (SPS)   Tantalum carbide (TaC)   Densification kinetics  

摘要： Sintering kinetics and densification of tantalum carbide (TaC) based ultra-high temperature ceramics (UHTC) processed using spark plasma sintering (SPS) were investigated. Samples sintered in the temperature ranging from 1600 degrees C to 2300 degrees C for 15 min under uniaxial pressure of 50 MPa showed increasing densification (similar to 77% to 99% from 1600 degrees C to 2300 degrees C) accompanied by substantial grain growth (similar to 0.57 mu m to similar to 4.05 mu m). Below 1900 degrees C, the densification mechanism was grain-boundary sliding with an apparent activation energy of 97.3 +/- 10.2 kJ/mol, However above 1900 degrees C, the activation energy was found to be 232.7 +/- 19.6 kJ/mol with grain boundary diffusion to be an active densification mechanism. The flexural strength (from 161.3 +/- 10.4 MPa to 326.5 +/- 10.5 MPa) and elastic modulus (from 375.7 +/- 3.1 GPa 535.4 +/- 4.7 GPa) was observed to increase with the increasing sintering temperature. The study explicitly establishes that the dense bulk TaC ceramic can be produced by SPS in a shorter time and lower sintering temperature, while maintaining the compositional stoichiometry.

关键词： 动力设备   船舶机械   混合隔振   自适应控制  

摘要： 船舶动力设备机械单层隔振装置能够减轻一部分振动噪声,但距动力设备较近的舱室其振动噪声明显,大量的振动噪声会直接影响船舶动力装置的使用寿命。为此提出船舶动力设备机械双层混合隔振自适应控制研究。利用物理减振层和电磁阻振层,搭建双层混合自适应控制隔板;依托Richard Henderson理论,构建双层混合自适应噪音控制算法,利用系统磁路完成双层混合隔振自适应控制研究。实验数据表明,提出的双层混合隔振自适应控制方法比单层隔振自适应控制方法振动噪声降低9倍,且船舶动力装置的使用寿命有效的延长。

关键词： Exoskeleton Control   Gait   Kinematics   Power   Electromyography  

摘要： Background: Controllers for assistive robotic devices can be divided into two main categories: controllers using neural signals and controllers using mechanically intrinsic signals. Both approaches are prevalent in research devices, but a direct comparison between the two could provide insight into their relative advantages and disadvantages. We studied subjects walking with robotic ankle exoskeletons using two different control modes: dynamic gain proportional myoelectric control based on soleus muscle activity (neural signal), and timing-based mechanically intrinsic control based on gait events (mechanically intrinsic signal). We hypothesized that subjects would have different measures of metabolic work rate between the two controllers as we predicted subjects would use each controller in a unique manner due to one being dependent on muscle recruitment and the other not. Methods: The two controllers had the same average actuation signal as we used the control signals from walking with the myoelectric controller to shape the mechanically intrinsic control signal. The difference being the myoelectric controller allowed step-to-step variation in the actuation signals controlled by the user's soleus muscle recruitment while the timing-based controller had the same actuation signal with each step regardless of muscle recruitment. Results: We observed no statistically significant difference in metabolic work rate between the two controllers. Subjects walked with 11% less soleus activity during mid and late stance and significantly less peak soleus recruitment when using the timing-based controller than when using the myoelectric controller. While walking with the myoelectric controller, subjects walked with significantly higher average positive and negative total ankle power compared to walking with the timing-based controller. Conclusions: We interpret the reduced ankle power and muscle activity with the timing-based controller relative to the myoelectric controller to

关键词： Protein   Mineral   Interface   Mechanical property   Molecular dynamics  

摘要： Biological composite materials, such as bone, tooth and nacre, are comprised of a mixture of nano-sized hard components (e.g. mineral platelets) and soft components (e.g. protein molecules). Their mechanical behaviour greatly depends on the protein-mineral interfaces. This paper investigates the effects of mineral surface nanostructures on the interfacial interaction and mechanical behaviour of protein-mineral nanocomposites. Interfacial shear between osteopontin (OPN) and hydroxyapatite (HA) mineral layers with surface nanostructures is investigated using the atomistic molecular dynamics (MD) simulations. The results indicate that the OPN residues can be attached to HA surfaces but the surface nanostructures greatly affect the interfacial interaction and mechanical behaviour. The HA layers with a higher number of nano-sized grooves (defects) increase the surface roughness but reduce the pulling force and energy dissipation. (C) 2018 Elsevier Ltd. All rights reserved.