Light (density <2.7g×cm−3) yet strong (tensile strength >350MPa) metal matrix composites (MMCs) are highly anticipated for aerospace and automotive industries. The MMCs application fields can be ...significantly expanded if they possess enhanced strength at elevated temperatures also. In the present study, Al-based composites loaded with either micro- or BN nanoparticles (BNMPs and BNNPs) with up to 10wt% of BN phase were produced via spark plasma sintering (SPS) of ball-milled Al-BN powder mixtures. A dramatic increase in the composite tensile strength compared to pure Al samples (up to 415%) was demonstrated during tensile tests both at 20°C and 500°C. BNMPs were found to be a more preferred additive compared with BNNPs due to the formation of more homogeneous and uniform morphologies within the ball-milled powder mixtures and resultant SPS products. The most impressive tensile strength of 170MPa at 500°C was achieved for an Al-7wt% BNMPs SPS composite, as compared to a value of only 33MPa for a pure Al SPS-produced sample. The reinforcement mechanism was uncovered based on detailed X-ray diffraction analysis, differential scanning calorimetry, Raman spectroscopy, scanning and high-resolution transmission electron microscopy and energy-dispersion X-ray analysis. Microscale BN, AlB2 and AlN inclusions acting within Al-matrices in the frame of Orowan strengthening mechanism, and pre-formed during ball-milling-induced pre-activation of Al-BN powder mixtures, finally crystallized during SPS processing and ensured the dramatically improved tensile strength and hardness of the resultant composites.
Density functional B3LYP/6-31G(d) and
ab initio
G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic ...aromatic hydrocarbons (PAH) chrysene and benzo
a
pyrene by various radicals abundant in combustion flames, such as H, CH
3
, C
3
H
3
, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol
−1
, followed by those with H and CH
3
with barriers of 16-17 kcal mol
−1
, and then with propargyl radicals with barriers of 24-26 kcal mol
−1
. Thus, the OH radical is predicted to be the fastest H abstractor from PAH. Even at 2500 K, the rate constant for H abstraction by H is still 34% lower than the rate constant for H abstraction by OH. The reaction with H is calculated to have rate constants 35-19 times higher than those for the reaction with CH
3
due to a more favorable entropic factor. The reactions of H abstraction by C
3
H
3
are predicted to be orders of magnitude slower than the other reactions considered and their equilibrium is strongly shifted toward the reactants, making propargyl an inefficient H abstractor from the aromatics. The calculations showed strong similarity of the reaction energetics in different H abstraction positions of benzo
a
pyrene and chrysene within armchair and zigzag edges in these molecules, but clear distinction between the armchair and zigzag sites. The zigzag sites appear to be more reactive, with H abstraction rate constants by H, CH
3
, and OH being respectively 37-42%, a factor of 2.1, and factors of 8-9 higher than the corresponding rate constants for the H abstraction reactions from armchair sites. Although the barrier heights for the two types of edges are similar, the entropic factor makes zigzag sites more favorable for H abstraction. Rate expressions have been generated for all studied reactions with the goal to rectify current combustion kinetics mechanisms.
A theoretical study of H abstraction reactions from benzo
a
pyrene and chrysene shows differences in kinetic effectiveness of various radicals and a clear distinction between zigzag and armchair edges.
•Nonlinear wave model based on the coupled-mode approach was developed.•We conducted an experimental study using the Brillouin light scattering technique.•Propagation of magnetostatic waves in two ...coupled magnonic crystals was studied.•Cases of normal and tangential magnetizing of magnonic crystals was cosidered.•With increasing input power in such structure two types of switches are discovered.•Single nonlinear switch are implement, when signal is divided between two outputs.•Double nonlinear switch are implement, when signal is divided between three outputs.
The features of spin wave processes in a layered structure based on two magnonic crystals (MC-1 and MC-2) separated by a dielectric layer have been studied theoretically and experimentally at different levels of input power. The theoretical model has been developed for the cases of magnetic field oriented in-plane and out-of-plane to MC surfaces in order to identify the mechanisms of nonlinear interactions. Two types of nonlinear switching are implemented in the structure MC-1/MC-2 with increasing input power. Depending on parameters of the structure the input signal can exit from three (“double nonlinear switching”) or two (“single nonlinear switching”) outputs. This feature allows to consider MC-1/MC-2 structure as a basic element in a multifunctional device for the spatial separation of signals with different power levels.
One of the goals of pump-probe spectroscopies is to determine how electrons relax after they have been driven out of equilibrium. It is challenging to determine how close electrons are to a thermal ...state solely by fitting their distribution to a Fermi-Dirac distribution. Instead, we propose that one compare the effective temperatures of both fermions and collective bosonic modes (derived from the fermions) to determine the distance from a thermal state. Measurements of effective fermionic and bosonic temperatures can be achieved directly via photoemission and nonresonant Raman scattering. Their difference quantifies the distance from thermal equilibrium.
•Band structure formation in the magnonic crystal-semiconductor structure are studied.•Possibility of electric and magnetic control of band gap properties is demonstrated.•Theory is built on methods ...of cross-linking surface conductivities and coupled waves.•Experimental data are obtained using microwave method.
The main features of the spin wave band structure formation and manipulation in the magnonic crystal – semiconductor layered heterostructure have been studied. We demonstrate possibility of double electric and magnetic control of band gap characteristics within the spectra of propagating spin waves. It is shown that in the magnonic crystal- semiconductor heterostructure, when a positive polarity voltage is applied to the semiconductor layer, the band gap in the spin-wave spectrum shifts to the low-frequency region, while for the negative polarity the band position is practically not changed. We show also that as the magnitude of the applied magnetic field is increased the shift of the band gap is decreased. Our results show the possibility of integration of magnonics and semiconductor electronics on the base of proposed structure.
The features of the propagation of spin waves in a layered periodic structure in the form of magnon crystals with a periodic system of grooves on the surface separated by an insulating layer are ...theoretically studied. The mechanism of band gap formation in the spectrum of spin waves in the structure under study is revealed. It is shown that one, two, three, or four band gaps can be formed near the first Bragg resonance. The positions of band gaps, their widths, and frequency interval between them are determined by the ratio of the geometric and magnetic parameters of the magnon crystals.
Comprehensive analysis of combustion and emission characteristics of HCCI engine operating on alternative fuels, including methane, syngas, methane/hydrogen and methane/syngas blends is conducted on ...the basis of CFD simulation with the use of detailed reaction mechanism. It is shown that the replacement of methane to methane/hydrogen blend with 50% H2 volume content makes it possible to increase the output energy during HCCI combustion and simultaneously to reduce the NO and CO emissions. The use of methane/syngas blend as a fuel also leads to an improvement of HCCI performance and to reducing the pollutant emissions, including the emission of CO2.
•2D CFD simulation of combustion and emission characteristics of HCCI engine is conducted.•CH4, syngas, CH4/H2 and CH4/syngas blends were considered as a fuel.•Replacement of CH4 by CH4/H2 blend increases output energy and decreases substantially CO emission.•The usage of CH4/syngas blend improves HCCI performance and reduces the pollutant emissions.•Addition of a certain amount of H2 or syngas to CH4 can decrease NO emission.
In this work, we studied the extraction systems for the separation f-elements based on the tetradentate N,O-donor ligand di(N-ethyl-4-ethylanilide) 2,2′-dipyridyl-6,6′-dicarboxylic acid (L). The ...organic phase of these systems was perspective fluorine-containing organic solvents–metanitrobenzotrifluoride (F-3), ionic liquid C4mimNTf2 (IL), and their mixture. The increase of Am(III) selectivity in the presence of Ln(III) in cases of the diluent mixture was shown. The mechanism of the f-element complexation leading to the improved properties of the extraction systems was studied by UV–visible, Raman-spectroscopy, XRD-study, and density functional theory calculations.
A theoretical study is performed of the interaction between spin current in a strongly spin-orbit-coupled platinum (Pt) conductor and a spin wave in ferromagnetic layers of yttrium iron garnet (YIG) ...with periodic modulation of their thickness under conditions of Bragg resonances and interlayer bond. It is shown that the conditions for two Bragg resonances in the first Brillouin zone in the spin wave spectrum are satisfied in the YIG/Pt/YIG sandwich structure. The spin current in Pt allows the frequency tuning of resonances and control over the depth of the spin wave band gap corresponding to the resonance conditions.
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