关键词： WOOD -- Research   HEAT treatment   POPULUS alba   THERMOCOUPLES   EQUIVALENCY tests  

摘要： The kinetics of heat treatment as well as its effect on some physical and mechanical properties of poplar wood (Populus alba L.) were analysed in this research. Kinetic tests were performed at different treatment temperatures and two different air ventilation settings [low and high air exchange rate (AER) with the exterior]. The treatment kinetics was studied, starting from the oven-dry condition, according to the mass loss during time. The time-temperature equivalency was checked, the mass loss versus time formalised through a master curve. The analysis clearly showed how the heat treatment at low and high AER presents different degradation kinetics even if similar activation energy values were found. Some physical and mechanical properties of wood after treatments up to a mass loss of 7 and 10 % starting from oven-dry or standard environmental conditions state were also studied. All of the treated samples showed statistically significant differences compared to the untreated one. The treatments performed at 7 or 10 % of dry mass loss showed homogeneous behaviour. The same tendency was observed for the treatments starting at oven-dry or standard environmental conditions with the exception of Young's modulus, which resulted in smaller reductions in wet starting condition when compared to dry samples.

关键词： Monoclinic hydroxyapatite   Uniaxial tension   Uniaxial compression   QEq charge equilibration method   Strain rate  

摘要： Inorganic biomedical materials are of great interest in the biomedical field. One such material, hydroxyapatite (HAP), is the main inorganic substance in the hard tissue of bones and teeth in the human body. Until recently, studies of the mechanical properties of HAP by uniaxial tension testing and compression molecular dynamics (MD) simulation had remained difficult. In addition, electric charges used in such simulations alter the molecular structure of HAP. Here, we present the mechanical properties and new charges of the monoclinic form of HAP using the self-consistent charge equilibration (QEq) scheme proposed by Rappe and Goddard in 1991 (J Phys Chem 95:3358-3363), and a successful MD simulation of the uniaxially tensile and compressive properties of monoclinic HAP. Also presented is the change in potential energy in the cell and how temperature and strain rate affect the uniaxial tension and compressive properties of HAP. Additionally, we compare the mechanical properties of this substance along different directions, and some conclusions useful for further studies of the mechanical properties of HAP composite materials are discussed.

关键词： POLYTHIOPHENES   NANOSTRUCTURED materials   MOLECULAR physics   COMPOSITE materials   THIOPHENES  

摘要： Carbon nanotubes (CNTs) have very important applications in ultrastrong lightweight materials. CNTs can improve mechanical properties of polymer matrix such as breaking stress and Young's modulus. In this article, we studied the interaction between polythiophene (PT)/polyethylene (PE)/poly(p-phenylenevinylene) (PPV) and CNTs by molecular dynamics (MD) simulation based on a reactive force field (ReaxFF). We studied the influence of CNT diameter, polymer type, and temperature on interaction energy. We found that a large radius CNT at low temperature shows the strongest interaction energy with PT. In addition, we computed the mechanical properties of CNTs-polymer composites such as the breaking stress, breaking strain, and Young's modulus. Our results show that there is a direct relation between mechanical properties and interaction energy. We found that the mechanical properties of CNTs-PT composite are better than CNTs-PPV and CNTs-PE and it is a good candidate for ultrastrong lightweight materials. We studied the influence of temperature on the mechanical properties. Our results show that CNTs-polymer composites show stronger mechanical properties at low temperature. We found that ReaxFF can reproduce the other force fields results and it is a very powerful force field to study the various properties of CNTs-polymer composites. (C) 2014 Society of Plastics Engineers

关键词： Cataclastic fault zone   mylonitic shear zone   chemical reactions   state variables  

摘要： Shear zones in outcrops and core drillings on active faults commonly reveal two scales of localization, with centimeter to tens of meters thick deformation zones embedding much narrower zones of mm-scale to cm-scale. The narrow zones are often attributed to some form of fast instability such as earthquakes or slow slip events. Surprisingly, the double localisation phenomenon seem to be independent of the mode of failure, as it is observed in brittle cataclastic fault zones as well as ductile mylonitic shear zones. In both, a very thin layer of chemically altered, ultra fine grained ultracataclasite or ultramylonite is noted. We present an extension to the classical solid mechanical theory where both length scales emerge as part of the same evolutionary process of shearing the host rock. We highlight the important role of any type of solid-fluid phase transitions that govern the second degree localisation process in the core of the shear zone. In both brittle and ductile shear zones, chemistry stops the localisation process caused by a multiphysics feedback loop leading to an unstable slip. The microstructural evolutionary processes govern the time-scale of the transition between slow background shear and fast, intermittent instabilities in the fault zone core. The fast cataclastic fragmentation processes are limiting the rates of forming the ultracataclasites in the brittle domain, while the slow dynamic recrystallisation prolongs the transition to ultramylonites into a slow slip instability in the ductile realm.

关键词： Flame-retardant epoxy   DOPO-based epoxy   Non-isothermal curing   Isothermal curing   Autocatalytic kinetics  

摘要： A halogen-free flame retardant epoxy resin was prepared consisting of DOPO-based glycidyl ether of cresol formaldehyde novolac and diglycidyl ether of bisphenol A cured by micronized dicyandiamide with accelerator U-52. The optimized epoxy resin formula contained 1.5 wt% of phosphorus and achieved UL-94 V-0 grade and LOI of 32%, with high tensile strength and impact strength of 48 MPa and 14.5 kJ/m(2), respectively. Curing reaction kinetics for the DOPO-containing epoxy with 1.5 wt% of P was investigated by DSC,through both non-isothermal and isothermal scanning. A two-parameter autocatalytic equation of the Sestak-Berggren model was constructed with experimentally determined kinetics parameters, which described the non-isothermal curing reaction rate quantitatively. For the isothermal curing kinetics, the Kamal-Sourour model was adopted to express the autocatalytic effect. The parameters of the reaction rate constant and reaction order were evaluated by curve fitting, and the calculated curve described the observed reaction rate fairly well. The determined kinetics parameters reflected the characteristics of the autocatalytic reaction for the curing of the phosphorus-containing epoxy system. (C) 2014 Elsevier Ltd. All rights reserved.

关键词： BMGs   uniaxial   core-shell   mechanical strength  

摘要： In the present study, a three-dimensional molecular dynamics simulation is performed to elaborate the mechanical strength of bulk metallic glasses (BMGs). The radial distribution function (RDF) is used to predict the structural disorder that appeared during the quenching provided for BMGs processing. The mechanical behavior is investigated using uniaxial tensile loading through stress-strain curves. It is observed that during tensile loading, the yield strength of Cu50Zr50 increases with the increase in the strain rate, and it quickly attains the maximum value. Soon after the sample fractures without entering into the plastic region. To elucidate the effects of component concentration, we design BMGs with the following three configurations: Cu25Zr75, Cu50Zr50 and Cu75Zr25. It is revealed from the results that samples with a lower Cu concentration have a higher degree of short-range ordering and lower yield strength, and vice versa. To analyze the significance of crystalline-amorphous interfaces, we designed four cylindrical core-shell nanorods with Cu cores and BMGs shells. It is observed that the mechanical strength of the core-shell nanorod is significantly higher compared to the pure BMGs nanorod.

关键词： lattice dynamics   soft mode   mechanical boundary conditions  

摘要： The role of different mechanical boundary conditions in the soft mode dynamics of ferroelectric PbTiO3 is systematically investigated using first-principles-based simulations and analytical model. The change in the soft mode dynamics due to hydrostatic pressure, uniaxial and biaxial stresses and biaxial strains is studied in a wide temperature range. Our computations predict: (i) the existence of Curie-Weiss laws that relate the soft mode frequency to the stress or strain;(ii) a non-trivial temperature evolution of the associated Curie-Weiss constants;(iii) a qualitative difference between the soft mode response to stresses/strains and hydrostatic pressure. The latter finding implies that the Curie-Weiss pressure law commonly used for residual stress estimation may not apply for the cases of uniaxial and biaxial stresses and strains. On the other hand, our systematic study offers a way to eliminate this difficulty through the establishment of Curie-Weiss stress and strain laws. Implications of our predictions for some available experimental data are discussed.

关键词： allyl ligands   homogeneous catalysis   kinetico-mechanistic studies   metallamacrocycles   self-assembly  

摘要： The square-like homo- and heterometallamacrocycles [{Pd((3)-2-Me-C3H4)(L-n)(2)}(2){M(dppp)}(2)](CF3SO3)(6) (dppp=1,3-bis(diphenylphosphino)propane) and [{Pd((3)-2-Me-C3H4)(L-1)(2)}(2){M(PPh3)(2)}(2)](CF3SO3)(6) [py=pyridine, M=Pd, Pt, L(n=)4-PPh(2)py (L-1), 4-C(6)F(4)PPh(2)py (L-2)] containing allyl corners were synthesised by antisymbiotic self-assembly of the different palladium and platinum metallic corners and the ambidentate N,P ligands. All the synthesised assemblies displayed a complex dynamic behaviour in solution, the rate of which is found to be dependent on the electronic and/or steric nature of the different building blocks. A kinetico-mechanistic study by NMR line shape analysis of the dynamics of some of these assemblies was undertaken in order to determine the corresponding thermal activation parameters. Both an enhanced thermodynamic stability and slower dynamics were observed for platinum-pyridine-containing species when compared with their palladium analogues. Time-dependent NMR spectroscopy in combination with ESI mass spectrometry was used to study the exchange between the assemblies and their building blocks, as well as that occurring between different metallamacrocycles. Preliminary studies were carried out on the activity of some of the metallamacrocyclic compounds as catalytic precursors in the allylic substitution reaction, and the results compared with that of the monometallic allylic corner [Pd(eta(3)-2-Me-C3H4)(L-1)(2)](+).

关键词： pillared graphene   molecular dynamics   contact stiffness  

摘要： The deformation behaviour and mechanical properties of three-dimensional (3D) pillared graphene are investigated using molecular dynamics simulations. The Tersoff-Brenner many-body potential model is employed to evaluate the interactions between 3D pillared-graphene carbon atoms and nanotube carbons. The Lennard-Jones potential model is used to compute the interactions between a conical indenter and 3D pillared-graphene carbon atoms. The effects of the size and geometric structure of 3D pillared-graphene are evaluated in terms of the indentation force and contact stiffness. The simulation results for an armchair nanotube of 3D pillared graphene show that the contact stiffness increases with increasing chiral vector of the 3D-pillared graphene. However, the adhesive force sharply decreases with increasing chiral vector of the 3D-pillared graphene. A zigzag nanotube of 3D-pillared graphene exhibits better mechanical properties compared with those of the armchair nanotube.

关键词： Grain size effect   Molecular dynamics   Mechanical properties   Nanocrystalline copper  

摘要： Molecular dynamics (MD) simulation has been used to study effects of grain size and shape on mechanical properties of nanocrystalline copper with mean grain size varying from 2.6 to 53.1 nm. Three grain size regions are identified according to the plot of flow stress versus mean grain size d. In region I (d approximate to 20-53 nm) flow stress increases with the decrease of d. Deformation twinning process and extended dislocation are observed in this region. In region II (d approximate to 8-20 nm) detwinning process appears as a competitive deformation mechanism with the twinning process. The flow stress begins to decrease slightly in this region. In region III (d < 8 nm) plastic deformation mainly occurs in grain boundaries and grain rotation is observed to accommodate the stress buildup induced by the grain boundary sliding. Deformation twin formed by sequent emission of partial dislocations at nearby sliding planes is not observed in this region. Young's modulus has a linear relation with d(-1) in the simulated range of mean grain size, and it directly depends on the volume fraction of grain boundaries. The Young's modulus of the grain boundary component is found to be similar to 25% of that of the grain interior. MD simulations on samples with spherical and cylindrical grain shapes are also carried out. The influence of grain shape on flow stress is hardly observed, indicating that for different grain shapes the plastic deformation mechanism is the same. The grain shape has an obvious effect on Young's modulus, which is attributed to the difference of the volume fraction of grain boundaries for samples with the different grain shapes. (C) 2014 Elsevier B.V. All rights reserved.