62 research outputs found
Polymer-stable magnesium nanocomposites prepared by laser ablation for efficient hydrogen storage
Hydrogen is a promising alternative energy carrier that can potentially
facilitate the transition from fossil fuels to sources of clean energy because
of its prominent advantages such as high energy density (142 MJ per kg), great
variety of potential sources (for example water, biomass, organic matter), and
low environmental impact (water is the sole combustion product). However, due
to its light weight, the efficient storage of hydrogen is still an issue
investigated intensely. Various solid media have been considered in that
respect among which magnesium hydride stands out as a candidate offering
distinct advantages. Recent theoretical work indicates that MgH2 becomes less
thermodynamically stable as particle diameter decreases below 2 nm. Our DFT
(density functional theory) modeling studies have shown that the smallest
enthalpy change, corresponding to 2 unit-cell thickness (1.6 {\AA} Mg/3.0{\AA}
MgH2) of the film, is 57.7 kJ/molMg. This enthalpy change is over 10 kJ per
molMg smaller than that of the bulk. It is important to note that the range of
enthalpy change for systems that are suitable for mobile storage applications
is 15 to 24 kJ permolH at 298 K. The important key for the development of
air/stable Mg/nanocrystals is the use of PMMA (polymethylmethacrylate) as an
encapsulation agent. In our work we use laser ablation, a non-electrochemical
method, for producing well dispersed nanoparticles without the presence of any
long range aggregation. The observed improved hydrogenation characteristics of
the polymer/stable Mg-nanoparticles are associated to the preparation procedure
and in any case the polymer laser ablation is a new approach for the production
of air/protected and inexpensive Mg/nanoparticles.Comment: Hydrogen Storage, Mg - Nanoparticles, Polymer Matrix Composites,
Laser Ablation, to appear in International Journal of Hydrogen Energy, 201
Элемент согласования волноводной линии передачи с диодом
The article deals with the issues of matching a waveguide transmission line with millimeter wave diodes. The implementation of the transition of a rectangular waveguide to a microstrip switching line in the form of a line of the fin-line type is described. The computer simulation results of the transmission coefficient module of the developed element and the standing wave coefficient by voltage separately for the transition from a rectangular waveguide to a microstrip transmission line and as part of an amplitude detector are shown. A joint application of the developed element with a microstrip detector based on a Schottky diode in the frequency range from 118.1 to 178.4 GHz is proposed. The results of measuring its parameters and the results of determining the sensitivity are given, as well as the evaluation of the results obtained is performed. The obtained results proved the possibility of using the matching element of a waveguide transmission line with a diode in the millimeter wavelength range. Рассмотрены вопросы согласования волноводной линии передачи с диодами миллиметрового диапазона длин волн. Описана реализация перехода с прямоугольного волновода на микрополосковую линию передачи в виде линии типа фин-лайн. Показаны результаты компьютерного моделирования модуля коэффициента передачи разработанного элемента и коэффициента стоячей волны по напряжению отдельно перехода с прямоугольного волновода на микрополосковую линию передачи и в составе амплитудного детектора. Представлена реализация совместного применения разработанного элемента с микрополосковым детектором на базе диода Шоттки в диапазоне частот от 118,1 до 178,4 ГГц. Приведены результаты измерения его параметров и определения чувствительности. При оценке полученных данных подтверждена возможность применения разработанного элемента согласования волноводной линии передачи с диодом в миллиметровом диапазоне длин волн
Experimental and Numerical Modeling of Segregation in Metallic Alloys
International audienceElectromagnetic levitation (EML) has been used as an experimental technique for investigating the effect of the nucleation and cooling rate on segregation and structure formation in metallic alloys. The technique has been applied to aluminum-copper alloys. For all samples, the primary phase nucleation has been triggered by the contact of the levitated droplet with an alumina plate at a given undercooling. Based on the recorded temperature curves, the heat extraction rate and the nucleation undercooling for the primary dendritic and the secondary eutectic structures have been determined. Metallurgical characterizations have consisted of composition measurements using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry and the analysis of SEM images. The distribution maps drawn for the composition, the volume fraction of the eutectic structure, and the dendrite arm spacing (DAS) reveal strong correlations. Analysis of the measurements with the help of a cellular-automaton (CA)-finite-element (FE) model is also proposed. The model involves a new coupling scheme between the CA and FE methods and a segregation model accounting for diffusion in the solid and liquid phases. Extensive validation of the model has been carried out on a typical equiaxed grain configuration, i.e., considering the free growth of a mushy zone in an undercooled melt. It demonstrates its capability of dealing with mass exchange inside and outside the envelope of a growing primary dendritic structure. The model has been applied to predict the temperature curve, the segregation, and the eutectic volume fraction obtained upon single-grain nucleation and growth from the south pole of a spherical domain with and without triggering of the nucleation of the primary solid phase, thus simulating the solidification of a levitated droplet. Predictions permit a direct interpretation of the measurements
Estimation of cooling rates during close-coupled gas atomization using secondary dendrite arm spacing measurement
Al-4 wt pct Cu alloy has been gas atomized using a commercial close-coupled gas-atomization system. The resulting metal powders have been sieved into six size fractions, and the SDAS has been determined using electron microscopy. Cooling rates for the powders have been estimated using a range of published conversion factors for Al-Cu alloy, with reasonable agreement being found between sources. We find that cooling rates are very low relative to those often quoted for gas-atomized powders, of the order of 10 K s for sub-38 μm powders. We believe that a number of numerical studies of gas atomization have overestimated the cooling rate during solidification, probably as a consequence of overestimating the differential velocity between the gas and the particles. From the cooling rates measured in the current study, we estimate that such velocities are unlikely to exceed 20 m s
Mushy zone coarsening with fluid flow
Cylindrical samples of a near-eutectic AlCu30 alloy are annealed applying constant axial temperature gradients to directionally molten samples in an aerogel furnace. During annealing with various times and gradients also a rotating magnetic field (RMF) of 6 mT was applied leading to azimuthal and meridional flows of well known magnitude. The specific surface area of the primary phase was measured on metallographic in section perpendicular to the sample axis with a fixed amount of fraction solid. The specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and compared to isothermal coarsening measurements of Voorhees and co-workers
Monotectic Growth Morphologies and Their Transitions
Alloys of exact monotectic composition can decompose at the monotectic temperature into three different two phase microstructures. The minority phase can be arranged as fibers similar to eutectics, as string of pearls or as irregularly distributed droplets. The solidification of three Al-base monotectic alloys was investigated, Al-Pb, Al-In and Al-Bi. The solidified microstructures were characterized as a function of temperature gradient ahead of the solidification front and the solidification velocity. The experimental results are presented in form of stability diagrams for the three microstructures. The inter fiber spacing is analyzed and presented in from of Jackson and Hunt type relations. The transitions between the microstructures are discussed in view of new hypotheses for the transition from fibers to string of pearls and the origin of irregular droplets
Psychological features of teaching the negotiating activity in the system of additional vocational education
In the article, the authors consider the need for timely updating the content of training programs and improving the skills of negotiating subjects, taking into account the rapidly changing conditions of professional activity. The use of situational, system-situational approaches in training contributes to the formation of professional competences among negotiators and the development of professionally important qualities necessary for effective implementation of negotiating activities in various professionally significant situations.
Collection and analysis of empirical material was carried out in several stages from 1995 to 2014, and included a survey of employees of law enforcement agencies and the conduct of a systematic analysis of the bank in 1705 problem situations. Based on the analysis of the bank of crisis situations in the negotiation process, depending on the characteristics of the negotiating situation, priority strategies and possible crisis zones are singled out, which makes it possible to effectively implement anti-crisis talks. Vocationally important negotiating situations and ways of formation of negotiating competence in the process of preparation and retraining have been determined
An inelastic threshold in electron - alkali cluster collisions
An analysis of integral cross sections for slow electron collisions with
neutral sodium clusters and nanoparticles reveals that, in addition to an effective
negative ion formation channel, there exists a strong inelastic threshold-type
process which appears above a collision energy of 1-1.3 eV. We show that it
can be plausibly associated with the onset of direct electron-induced cluster
fragmentation. This result highlights the importance of understanding the
dynamics of electron-vibrational energy transfer in nanoclusters, including the
relative probability of direct vs. statistical energy transfer
Investigation of relaxation process in elastomers filled with ground vulcanizate
In the paper, the relaxation processes in the el astomeric compositions based on nitrile rubber and filed with ground vulcanizate are studied. The ground vulcanizate is obtained from waste products of molded rubber products. The particle size is 0.63–1.00 mm. The main elastic and stress-strain properties of elasomeric compositions under research are determined. The dependence of the properties on the ground vulcanizate dosage is considered. It is shown that the growth of ground vulcanizate dosage increases the viscosity of the rubber compounds. At the same time the stress relaxation rate was practically not changed. The processes of compression and tension
stress relaxation in vulcanizates are studied. It is revealed that the nature of the process of physical relaxation does not change with increasing the dosage of the ground vulcanizate. There is the growth of the modulus with the increasing of the filler dosage. It is determined that the degree of stress relaxation in tensile strain is practically independent on the do sage of the filler and it has an extreme character for
the compressed samples. It is shown that the use of ground vulcanizate as a filler for elastomeric compositions based on NBR can slow processes of the chemical stress relaxation in the elastomeric matrix, as well as it can improve the physical and mechanical properties of vulcanizates. The results of studies suggest that
ground vulcanizate can be used as a filler for sealing rubber
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