A large-scale continuous detonation combustor (CDC) has been designed, fabricated and tested to study the effect of different design elements on the operation process and CDC propulsion performance. ...It has been shown experimentally that widening of the air-inlet slit in the annular combustion chamber from 2 to 15 mm leads to a decrease in the number of detonation waves (DWs) simultaneously circulating in the combustor from four to one and, finally, to transition to the operation mode with intermittent (pulse) longitudinal reaction waves resembling pulse detonations. The number of DWs and the thrust produced by the CDC can be increased by installing a shaped obstacle at the CDC exit nozzle providing the blockage of the combustor cross section. The maximum net thrust produced by the CDC attained 6 kN at the total mass flow rate of fuel components of 7.5 kg/s, whereas the maximum fuel-based specific impulse attained ∼3000 s.
•We conduct experiments in a large-scale continuous-detonation combustor operating on hydrogen–air mixture.•We vary a size of air-inlet slit to observe changes in operation process and propulsion performance.•Widening of slit from 2 to 15 mm leads to decrease in a number of rotating detonation waves and to detonation failure.•Maximum fuel-based specific impulse obtained in experiments is 3000 s.
We observe linear and nonlinear light localization at the edges and in the corners of truncated moiré arrays created by the superposition of periodic mutually twisted at Pythagorean angles square ...sublattices. Experimentally exciting corner linear modes in the femtosecond-laser written moiré arrays we find drastic differences in their localization properties in comparison with the bulk excitations. We also address the impact of nonlinearity on the corner and bulk modes and experimentally observe the crossover from linear quasilocalized states to the surface solitons emerging at the higher input powers. Our results constitute the first experimental demonstration of localization phenomena induced by truncation of periodic moiré structures in photonic systems.
Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are ...limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron–hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm.
A coating of high-entropy Cantor alloy FeCoCrNiMn of nonequiatomic composition was formed on a 5083 aluminum alloy substrate by wire-arc additive manufacturing (WAAM). The methods of physical ...materials science were applied to analyze the structure, elemental composition, microhardness, and wear resistance of the coating–substrate system. The deposition of the FeCoCrNiMn high-entropy coating on the 5083 alloy surface is accompanied by the formation of microhardness and elemental composition gradients. Microcracks and micropores were revealed in the cross section of the coating. Microhardness in the volume of the coating is 2.5–3.5 GPa and increases to 9.9 GPa at the boundary with the substrate. In the middle part of the coating, the wear factor is 2.3 × 10
–4
mm
3
/N m; the friction coefficient is 0.7. A transition layer up to 450 µm thick is formed at the interface between the coating and the substrate. We analyzed the elemental composition gradient of the transition layer and noted a high level of chemical homogeneity of the coating. The found doping of the coating with substrate elements (aluminum) leads to the formation of a FeC-oCrNiMnAl high-entropy coating, causing a lamellar structure at the interface between the transition layer and the substrate.
The mechanism of thermal defunctionalization of multiwalled carbon nanotubes (CNTs) oxidized by nitric acid was studied. X-ray photoelectron spectroscopy and thermal analysis under different heating ...rates combined with mass spectrometry of evolved gases (TGA–MS) were used to reveal the transformations on the CNT surface. Hydrogen–deuterium exchange and mathematical handling of TGA–MS curves were carried out to evaluate the impact of a small amount of residual oxygen on CNT defunctionalization. Water, CO, CO2, and NO/CH2O mass curves recorded during TGA–MS study were curve fitted. The resultant peaks were attributed to the different stages of CNT defunctionalization. Deuterium exchanged CNTs allowed one to reveal the mechanism of water release during heating. Kissinger’s model was applied to estimate the activation energy of the decomposition of different functional groups on the surface of CNTs.
The creation of connecting tunnel diodes with a peak tunneling-current density higher than the density of the short-circuit current of photoactive
p
–
n
junctions is an important task in the ...development of multijunction photoconverters (III–V) of high-power optical radiation. Basing on numerical simulation of the
J–U
characteristics of tunnel diodes, a method is proposed for increasing the peak tunneling-current density by including a thin undoped
i
-type layer with a thickness of several nanometers between degenerate layers of the tunnel diode. The
p
–
i
–
n
-GaAs/Al
0.2
Ga
0.8
As structures of the connecting tunnel diodes with a peak tunneling-current density of up to 200 A/cm
2
are grown by molecular-beam epitaxy.
The ability to synthesize VO2 in the form of single-crystalline nanobeams and nano- and microcrystals uncovered a number of previously unknown aspects of the metal−insulator transition (MIT) in this ...oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg−Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO2 in the vicinity of the R−M2−M1 triple point using the Ginzburg−Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO2 nanostructures in desired phase states and to use of external fields in the control of the VO2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO2 nanoplatelets, and theoretically predict phases that have not been previously observed.
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•Kinetics of CNT oxidation was studied.•Highest SBET and O content in CNTs promotes the best catalyst performance in FTS.•Surface geometry of CNT support affects Co stability stronger ...than functional groups.•Turnover frequency and activity show opposite dependencies on Co particle size.•CNT evolution during catalyst preparation, reduction and FTS was investigated.
CNT oxidation, Co/CNT preparation and transformation of both catalyst and support in Fischer-Tropsch synthesis (FTS) were thoroughly studied. CNTs functionalized by nitric acid for 1–15h were used as supports for Co catalysts for FTS at 190°C and atmospheric pressure. Co supported on CNTs with the highest specific surface area and oxygen content showed the highest activity and C5+ yield. According to TEM results after 70h of FTS this catalyst was the most stable to Co sintering due to the large number of surface defects and oxygen groups on a support surface combined with the preserved integrity of CNTs. Activity of catalysts increased and selectivity to C5+ decreased when the particle size diminished from 26.5 to 4.3nm. Study of the evolution of CNT support revealed that most of the carboxylic groups decomposed at calcination. Etching of support surface during the catalyst reduction and partial pore blocking after FTS were detected.
We propose a design of a terahertz-frequency signal generator based on a layered structure consisting of a current-driven platinum (Pt) layer and a layer of an antiferromagnet (AFM) with easy-plane ...anisotropy, where the magnetization vectors of the AFM sublattices are canted inside the easy plane by the Dzyaloshinskii-Moriya interaction (DMI). The dc electric current flowing in the Pt layer creates due to the spin Hall effect, a perpendicular spin current that, being injected in the AFM layer, tilts the DMI-canted AFM sublattices out of the easy plane, thus exposing them to the action of a strong internal exchange magnetic field of the AFM. The sublattice magnetizations, along with the small net magnetization vector m(DMI) of the canted AFM, start to rotate about the hard anisotropy axis of the AFM with the terahertz frequency proportional to the injected spin current and the AFM exchange field. The rotation of the small net magnetization mDMI results in the terahertz-frequency dipolar radiation that can be directly received by an adjacent (e.g., dielectric) resonator. We demonstrate theoretically that the radiation frequencies in the range f = 0.05-2 THz are possible at the experimentally reachable magnitudes of the driving current density, and we evaluate the power of the signal radiated into different types of resonators. This power increases with the increase of frequency f, and it can exceed 1 mu W at f similar to 0.5 THz for a typical dielectric resonator of the electric permittivity epsilon similar to 10 and a quality factor Q similar to 750.
We report on the thorough investigation of light emitting diodes (LEDs) made of core-shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) in the outer shell, which are grown on patterned ...substrates by metal-organic vapor phase epitaxy. The multi-bands emission of the LEDs covers nearly the whole visible region, including UV, blue, green, and orange ranges. The intensity of each emission is strongly dependent on the current density, however the LEDs demonstrate a rather low color saturation. Based on transmission electron microscopy data and comparing them with electroluminescence and photoluminescence spectra measured at different excitation powers and temperatures, we could identify the spatial origination of each of the emission bands. We show that their wavelengths and intensities are governed by different thicknesses of the QWs grown on different crystal facets of the NRs as well as corresponding polarization-induced electric fields. Also the InGaN incorporation strongly varies along the NRs, increasing at their tips and corners, which provides the red shift of emission. With increasing the current, the different QW regions are activated successively from the NR tips to the side-walls, resulting in different LED colors. Our findings can be used as a guideline to design effectively emitting multi-color NR-LEDs.
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