The influence of the surface modification of multi-walled carbon nanotubes (MWCNT) with nitrogen-containing compounds on the performance of 40 wt% Pt/MWCNT catalysts in the oxygen electroreduction ...reaction (ORR) was investigated using a rotating disk electrode (RDE) at 10–35 °C in 0.1 M HClO
4
as electrolyte in electrochemical cell, and in a hydrogen–oxygen polymer electrolyte membrane fuel cell (PEMFC) at 82 °C. The catalysts were characterized by low-temperature nitrogen adsorption, XPS, TEM, gas-phase CO titration, electrooxidation of the adsorbed CO monolayer, and cyclic voltammetry. It was shown that the modification of MWCNT with melamine–formaldehyde resin leads to the surface nitrogen concentration up to 8.3 at.% (CNT-MF sample). The 40 wt% Pt/CNT-MF catalyst with 0.1 mg cm
−2
Pt loading on the cathode showed a good performance in PEMFC (~ 0.61 W cm
−2
) and a high utilization ratio (0.84) of Pt in membrane electrode assembly as compared to Pt/CNT catalyst (~ 0.37 W cm
−2
and utilization of 0.29). The higher power density of nitrogen-modified catalysts was ascribed to a higher utilization of Pt in the electrode layer.
•Cryogenic quenching is enhanced by a structured capillary-porous coating of surface.•A new type of coating was produced by the directional plasma spraying technique.•Coating affects the process ...dynamics: reduces the total quench time by more than threefold.•Surface microstructure triggers the liquid-solid contacts at extremely high temperature.•The numerically obtained dynamic pattern of quenching correlates well with experimental data.
In the present study, the influence of structured capillary-porous coatings of surface on cryogenic quenching by the falling liquid nitrogen film is investigated. The coating was produced by the directional plasma spraying technique. The cryogenic quenching experiments were performed on high-temperature vertical copper slab with bare surface and on the surfaces with different orientation of coating protrusions. Thermocouples and a high-speed digital video camera were employed to obtain a synchronized data on the temperature-time history of the transient process and the pattern of quench front propagation. The peculiarities of quench front dynamics and heat transfer in the transient process are studied. The created numerical model determines the quench front velocity and the temperature fields in the heater, varying in space and time. The dynamic pattern of quench front propagation obtained numerically satisfactorily correlates with the observed in the experiments one. The heat transfer curves during quenching were determined for different surfaces based on the experimental cooling thermograms and visualization data. The results show that the cooling rate is influenced by the thermal properties of the coating as well as the geometry of the protrusions on the solid surface. The presence of capillary-porous coating significantly affects the dynamics of quenching, which results in the decrease more then threefold of the total quench time. In this way, the structured capillary-porous coating is a method for reducing the time and the total mass of cryogenic fluid required for a quenching process. The effect is due to the fact that the initialization of a quench front on a specimen with the coating occurs at a temperature significantly higher than the thermodynamic limit of a liquid superheat, when a stable solid-liquid contact is thermodynamically impossible. This phenomenon appears as a result of local contacts of the crests of wavy liquid flow at the liquid-vapor interface with the protrusions on the modified surface. The results indicated also the minimum film boiling temperature increase on heat transfer surface coated with capillary-porous layer. This increased temperature caused earlier transition to nucleate boiling, which results in the decrease in the quenching time. The results reliability is confirmed by direct comparison with experimental data on the quench front dynamics, velocity and geometry.
Amorphous carbon (AC) is present in the bulk and on the surface of nanostructured carbon materials (NCMs) and exerts a significant effect on the physical, chemical and mechanical properties of NCMs. ...Thus, the determination of AC in NCMs is extremely important for controlling the properties of a wide range of materials. In this work, a comparative study of the effect of heat treatment on the structure and content of amorphous carbon in deposited AC film, nanodiamonds, carbon black and multiwalled carbon nanotube samples was carried out by TEM, XPS, XRD and Raman spectroscopy. It has been established that the use of the 7-peak model for fitting the Raman spectra makes it possible not only to isolate the contribution of the modes of amorphous carbon but also to improve the accuracy of fitting the fundamental G and D
(D) modes and obtain a satisfactory convergence between XPS and Raman spectroscopy. The use of this model for fitting the Raman spectra of deposited AC film, ND, CB and MWCNT films demonstrated its validity and effectiveness for investigating the amorphous carbon in various carbon systems and its applicability in comparative studies of other NCMs.
Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order
. Unlike their nonmagnetic counterparts, magnetic topological ...insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics
, such as the quantum anomalous Hall effect
and chiral Majorana fermions
. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic
and electronic
properties of these materials, restricting the observation of important effects to very low temperatures
. An intrinsic magnetic topological insulator-a stoichiometric well ordered magnetic compound-could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi
Te
. The antiferromagnetic ordering that MnBi
Te
shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ
topological classification; ℤ
= 1 for MnBi
Te
, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi
Te
exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling
and axion electrodynamics
. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect
and chiral Majorana fermions
.
An approach for estimating at the DFT level of the standard redox potentials of the inclusion compounds based on Fe(III) and Fe(II) aqua complexes inside the cavities of cucurbitnurils (n = 6–8) ...has been proposed. These inclusion compounds were established to have compositions which can be described by the formulas Fe(H2O)63+/2+@CB6 and Fe(H2O)6·4H2O3+/2+@CB7,8. Redox potentials E 0 relative to the standard hydrogen electrode for the half-reaction Fe(III)/Fe(II) in the CBn cavities calculated at the PBE/TZVP level within the molecular-continuum solvation model are 1.607, 0.949, and 0.847 V for n = 6, 7, and 8, respectively. The obtained values indicate a relative increase of the oxidative ability of Fe(III) aqua-ions in the cavities of the examined CBn, especially in CB6, compared to the calculated value (E 0 = 0.786 V) for the same half-reaction in the bulk of aqueous solution. Possible causes of the detected trend are discussed. The calculations also showed that the Fe(III) aqua complex inside the CB6 changes its magnetic properties, transforming into a low-spin state with a total spin S = 1/2, whereas for all other systems high-spin states in accord with the classical ligand field theory are realized.
•The exact formula for the global attractor of the Lorenz system is obtained.•The validity of the formula for all classical values of parameters is proved.
The exact Lyapunov dimension formula for ...the Lorenz system for a positive measure set of parameters, including classical values, was analytically obtained first by G.A. Leonov in 2002. Leonov used the construction technique of special Lyapunov-type functions, which was developed by him in 1991 year.
Later it was shown that the consideration of larger class of Lyapunov-type functions permits proving the validity of this formula for all parameters, of the system, such that all the equilibria of the system are hyperbolically unstable. In the present work it is proved the validity of the formula for Lyapunov dimension for a wider variety of parameters values including all parameters, which satisfy the classical physical limitations.
The article presents the results of the experimental studies of the processes of removing moisture from fairly typical and widespread forest combustible materials, and woodworking waste under ...conditions of their thermal preparation for combustion. Based on the results of the experiments, the main regularities of the drying processes of woody biomass under conditions of radiation-convective heating have been established. The integral characteristics (characteristic times) of the drying process of forest biomass under typical (for this technology) heating conditions have been determined. The experiments were carried out on equipment that provides a low level of error (less than 3.3%) for determining the main characteristics of the biomass dehydration process. The values of the mass rates of moisture removal (Weva) of four promising (for modern and future power engineering) and fairly common types of biomass have been established in the ambient temperature range from 333 to 393 K, which corresponds to the operating modes of the drying equipment of power plants. According to the results of the experiments, it was shown that at relatively moderate ambient temperatures (333≤ Tg ≤ 353 K), the type of wood has a rather significant effect on the characteristics and conditions of the dehumidification process. At the same time, the process of drying wood chips is much faster compared to other types of biomass. As the ambient temperature rises (at 373≤Tg ≤ 393 K), the influence of the type of biomass decreases. As a result of the experiments, a physical model of the process of removing moisture from the bulk layer of biomass was formulated. According to the results of the experiments, it was found that before burning forest biomass in boilers of thermal power plants, it is most advantageous to carry out drying in conditions of relatively moderate (at 333≤Tg ≤ 353 K) ambient temperatures. The studies carried out provide grounds for justifying the use of forest combustible material as fuel (or one of the components of the fuel mixture) at thermal power plants currently operating on coal.
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•Forest fuel is a promising fuel for thermal power plants (TPP).•Established two temperature regimes for drying forest biomass.•At moderate ambient temperatures, the influence of the type of biomass is significant.•Low-temperature drying biomass drying is more efficient than high-temperature drying.•The rate of drying of all types of biomass at high temperatures are almost identical.
The insufficient data on a structure of the boron-doped diamond (BDD) has frustrated efforts to fully understand the fascinating electronic properties of this material and how they evolve with ...doping. We have employed X-ray diffraction and Raman scattering for detailed study of the large-sized BDD single crystals. We demonstrate a formation of boron-carbon (B-C) nanosheets and bilayers in BDD with increasing boron concentration. An incorporation of two boron atoms in the diamond unit cell plays a key role for the B-C nanosheets and bilayer formation. Evidence for these B-C bilayers which are parallel to {111} planes is provided by the observation of high-order, super-lattice reflections in X-ray diffraction and Laue patterns. B-C nanosheets and bilayers minimize the strain energy and affect the electronic structure of BDD. A new shallow acceptor level associated with B-C nanosheets at ~37 meV and the spin-orbit splitting of the valence band of ~6 meV are observed in electronic Raman scattering. We identified that the superconducting transitions occur in the (111) BDD surfaces only. We believe that the origin of Mott and superconducting transitions is associated with the two-dimensional (2D) misfit layer structure of BDD. A model for the BDD crystal structure, based on X-ray and Raman data, is proposed and confirmed by density functional theoretical calculation.
•The results of experiments on the ignition of coal/wood fuel particles are presented.•It is shown that biomass significantly accelerates the ignition of coal.•We have established optimal (in terms ...of ignition dynamics) biomass concentration values in the fuel mixture.•The optimum distance (for ignition) between the fuel particles has been established.
This article presents the results of experimental studies of the ignition of composite wood/coal fuel particles under high-temperature heating, corresponding to the internal furnace space of boiler units of thermal power plants. The ignition of a wood-coal mixture was studied based on coal and wood waste widely used in energy.
For the first time in world practice, the basic regularities of the joint ignition of a group of wood and coal particles located at a certain distance from one another have been experimentally established. The ignition delay times (tign) of each fuel particle in the group are determined. The influence of the concentration of wood and the relative position of the fuel particles relative to each other on the characteristics and conditions of ignition is analyzed. It was shown that in the temperature range Tg = 873–1273 K, the gaseous products of wood pyrolysis begin to burn first, the coal particles burn second. In this case, the time interval between the ignition of wood and coal does not exceed 1 s, with a distance between particles of up to 10 mm.
According to the results of experiments, it was also found that the optimal component ratio in the wood/coal system lies in the range from 30/70 to 35/65. It is shown that at a high concentration of coal in the wood-coal mixture, coal particles act as a kind of screen that blocks thermal radiation from the external environment to the wood particles.
In the shelf zone, an experimental study was conducted on the possibility of increasing the probability of detecting echo signals against an noise background as a result of taking into account the ...multipath waveguide model and design features of the reflector. The possibility of increasing the tracking range and reducing the probability of false alarms is experimentally substantiated with an algorithm that uses the duration of the time interval during which a sequence of signals reflected from one reflector is observed due to the multipath propagation of signals and multispecularity of the reflected signal, if the target structure is complex and includes several offset reflectors. Recommendations are given on the choice of intervals, the sampling lengths within which summation of the energy of the reflected signals is possible. It is shown that as a result of signal power accumulation using an algorithm that partially takes into account the echo signal model, the target tracking time increases and false alarms decrease.
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