Mechanical activation process for self-propagation high-temperature synthesis of ceramic-based composites

Journal of Thermal Analysis and Calorimetry - In this study, the mechanical activation process of the Al–TiO2–H3BO3 thermite mixture was modeled and optimized before self-propagation...     

The influence of hydrogen- and lithium-bonding on the cooperativity of chalcogen bonds: A comparative ab initio study

ABSTRACT

The aim of this study is to investigate the influence of a hydrogen- or lithium-bonding interaction on the cooperativity of chalcogen bonds in linear NCH···(OCX)_2–5 and NCLi···(OCX)_2-5 clusters (X?=?S, Se). The nature of interactions in the optimised structures is analysed by means of molecular electrostatic potential, quantum theory of atoms in molecules, natural bond orbital and electron density difference methods. According to our results, the formation of a lithium-bonding interaction in NCLi···(OCX)_2-5 clusters induces a large increase in the strength of X···O chalcogen bonds, and hence their cooperativity. This can be mainly rationalised in terms of the electrostatic nature of chalcogen bonds as well as the fundamental orbital interaction between the interacting OCX subunits. The results of this study provide a theoretical evidence for the tuning of chalcogen bonds cooperativity by a hydrogen- or lithium-bonding.     

The strengthening effect of a halogen,chalcogen or pnicogen bonding on halogen–π interaction: a comparative ab initio study

The aim of this work is to study the possible cooperative effects between Z···N and X···π interactions (Z = Cl, S, P and X = Cl, Br) in some model complexes, where both these interactions coexist. The nature of the interactions in these complexes is characterised by means of molecular electrostatic potential, electron localisation function, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. According to the results, the formation of an Z···N interaction in these systems makes a significant shortening of X···π distance. The cooperative enhancement of the X···π bonding in the ternary complexes depend on the strength of the Z···N interaction, and it becomes larger in the order Z = Cl > S > P. The mechanism of the cooperativity between the Z···N and X···π interactions is studied using the parameters derived from the QTAIM and NBO analyses.     

The triel bond: a potential force for tuning anion–π interactions

Using ab-initio calculations, the mutual influence between anion–π and B···N or B···C triel bond interactions is investigated in some model complexes. The properties of these complexes are studied by molecular electrostatic potential, noncovalent interaction index, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. According to the results, the formation of B···N or B···C triel bond interactions in the multi-component systems makes a significant shortening of anion–π distance. Such remarkable variation in the anion–π distances has not been reported previously. The strengthening of the anion–π bonding in the multi-component systems depend significantly on the nature of the anion, and it becomes larger in the order Br^? > Cl^? > F^?. The parameters derived from the QTAIM and NBO methodologies are used to study the mechanism of the cooperativity between the anion–π and triel bond interactions in the multi-component complexes.     

Numerical simulations of gas–liquid–solid flows in a hydrocyclone separator

The flow behavior in hydrocyclones is quite complex. In this study, the computational fluid dynamics (CFD) method was used to simulate the flow fields inside a hydrocyclone in order to investigate its separation efficiency. In the computational fluid dynamics study of hydrocyclones, the air-core dimension is a key to predicting the mass split between the underflow and overflow. In turn, the mass split influences the prediction of the size classification curve. Three models, the model, the Reynolds stress model (RSM) without considering the air-core, and the Reynolds stress turbulence model with the volume of fluid (VOF) multiphase model for simulating the air-core, were compared in terms of their predictions of velocity, axial and tangential velocity distributions, and separation proportion. The RSM with air-core simulation model, since it reproduces some detailed features of the turbulence and multiphase, clearly predicted the experimental data more closely than did the other two models.     

Effect of Fe2O3 as an accelerator on the reaction mechanism of Al–TiO2 nanothermite system

Thermite reactions between aluminum and metal oxides could lead to the formation of intermetallic matrix composites used in high-temperature industrial applications. Thermite reaction in Al–TiO₂ system needs a considerable amount of energy to take place by mechanochemical or by the combustion synthesis (CS) method due to the low amount of reaction enthalpy in Al–TiO₂ system. In this study, Fe₂O₃ was chosen as a accelerator for this system, to generate a high amount of heat which could be released between Fe₂O₃ and Al, leading to a more convenient reaction between Al and TiO₂ in the CS process. The results of XRD, SEM, and DSC analyses indicated that both the mechanical activation of Al–TiO₂ system in a high-energy ball mill and the Fe₂O₃ addition led to considerable effects of reduction in the reaction temperature and increase in the reaction intensity in Al–TiO₂ nanothermite system. Finally, it was shown that Fe₃Al intermetallic compounds as well as γ-AlTi and alumina phases in the final products were formed after the CS of the milled powders.     

Molecular dynamics simulation of the diffusion of nanoconfined fluids

A new molecular dynamics simulation technique for simulating fluids in confinement [H. Eslami, F. Mozaffari, J. Moghadasi, F. Müller-Plathe, J. Chem. Phys. 129 (2008) 194702] is employed to simulate the diffusion coefficient of nanoconfined Lennard-Jones fluid. The diffusing fluid is liquid Ar and the confining surfaces are solid Ar fcc (100) surfaces, which are kept frozen during the simulation. In this simulation just the fluid in confinement is simulated at a constant temperature and a constant parallel component of pressure, which is assumed to be equal to the bulk pressure. It is shown that the calculated parallel (to the surfaces) component of the diffusion coefficients depends on the distance between the surfaces (pore size) and shows oscillatory behavior with respect to the intersurface separations. Our results show that on formation of well-organized layers between the surfaces, the parallel diffusion coefficients decrease considerably with respect to the bulk fluid. The effect of pressure on the parallel diffusion coefficients has also been studied. Better organized layers, and hence, lower diffusion coefficients are observed with increasing the pressure.     

The influence of halogen-bonding cooperativity on the hydrogen and lithium bonds: an ab initio study

ABSTRACT

Ab initio calculations are carried out to study linear NCH···(NCX)_1–5 and NCLi?…?(NCX)_1–5 clusters (X?=?F, Cl, Br). The aim is to study the influence of halogen-bonding cooperativity on the strength and bonding properties of hydrogen or lithium bond. Particular attention is given to parameters such as binding distances, interaction energies and cooperative energies in these systems. According to our results, the halogen-bonding cooperativity between the NCX molecules has an enhancing effect on the strength of hydrogen and lithium bonds, with an increase of 0.33–0.93 and 0.19–0.43?kcal/mol in NCH···(NCX)_n and NCLi···(NCX)_n, respectively. The enhancing effect of halogen bond on the hydrogen and lithium bond is dependent on the nature of halogen atom, and increases as X?=?F?相似文献   

Comparing the Effect of Micro- and Nano-Scale Silica on the Rheological,Dynamic Mechanical Properties and Flame Resistance of Nylon 6,6

Nylon 6,6 micro- and nano-silica composites were prepared by melt processing using a twin-screw extruder. Three nanocomposites containing 4, 8, and 12 wt.% of nanosilica were prepared. In order to compare the effect of size, a microcomposite containing 4 wt.% of micron-size silica was also prepared. The effects of particle type (micro- and nano-size) on the dynamic thermomechanical and rheological properties, morphology, and flame resistance of the composites were examined. The dynamic thermomechanical properties (DMTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic rheometry, thermogravimetry analysis (TGA), and limiting oxygen index (LOI) data are reported. The particles were observed to be dispersed uniformly, but with a different level of coalescence, by means of SEM and TEM. The DMTA results showed that the damping factor peak positions of the nanocomposites at low content of nanofiller shifted more to higher temperature compared to those of nanocomposites containing high concentrations of nanofiller. Dynamic rheometry, using a parallel plate rheometer, showed that the rheological moduli of the nanocomposites increased with increase in nanofiller concentration; however, this increase was greater in the high-frequency region. These results showed that increasing the concentration of nanofiller, and the consequent coalescence effect within the nanocomposites, led to rheological moduli values similar to those of the microcomposite. The TGA and LOI results of the microcomposite and nanocomposite containing 4wt.% of nanosilica showed that nanosilica had a more significant effect to enhance the heat and flame resistance of nylon 6,6 compared to that of micron-sized silica.