Search Results (5,868)

The single-layer MoS₂ is a highly sought-after semiconductor material in the field of photoelectric performance due to its exceptional electron mobility and narrow bandgap. However, its photocatalytic efficiency is hindered by the rapid recombination rate of internal photogenerated electron–hole pairs. Currently, the construction of heterojunctions has been demonstrated to effectively mitigate the recombination rate of photogenerated electron–hole pairs. Therefore, this paper employs the first principles method to calculate and analyze the four heterojunctions formed by MoS₂/WSe₂, MoS₂/MoSe₂, MoS₂/AlN, and MoS₂/ZnO. The study demonstrates that the four heterojunctions exhibit structural stability. The construction of heterojunctions, as compared to a monolayer MoS₂, leads to a reduction in the band gap, thereby lowering the electron transition barrier and enhancing the light absorption capacity of the materials. The four systems exhibit II-type heterojunction. Therefore, the construction of heterojunctions can effectively enhance the optical properties of these systems. By forming heterojunctions MoS₂/WSe₂ and MoS₂/MoSe₂, the absorption coefficient in the visible light region is significantly increased, resulting in a greater ability to respond to light compared to that of MoS₂/ZnO and MoS₂/AlN. Consequently, MoS₂-based heterojunctions incorporating chalcogenide components WSe₂ and MoSe₂, respectively, exhibit superior catalytic activity compared to MoS₂ heterojunctions incorporating non-chalcogenide components ZnO and AlN, respectively. The absorption spectrum analysis reveals that MoS₂/MoSe₂ exhibits the highest light responsivity among all investigated systems, indicating its superior photoelectric performance. Full article

In this study, we evaluated the nonlinear dynamics of convection flow using the thermodynamic variational principle, focusing on scenarios where multiple external forces, such as a thermal gradient and rotational field, are applied to a shallow annular pool. We observed that with the increase in the thermal gradient, the flow changed from an axial flow to a rotational oscillatory flow with the wave amplitudes aligned. Further increasing the temperature difference led to a rotational oscillatory flow characterized by alternating wave generation and annihilation. Our analysis of the flow, considering heat fluxes orthogonal to the thermal gradient, allowed us to describe the flow state as a phase at equilibrium. The state transition of the flow was accompanied by a discontinuous jump in the heat flux, which occurred at the intersection of the entropy production curves. The first transition occurred at a temperature difference $\Delta T=12.4 \mathrm{K} \left(\mathrm{M}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{o}\mathrm{n}\mathrm{i} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r},\mathrm{M}\mathrm{a}=1716\right)$ and the second at ΔT = 16.3 K $\left(\mathrm{M}\mathrm{a}=2255\right)$. Analysis based on entropy production could accurately predict the observed transition points. Full article

Oxide-dispersion-strengthened (ODS) alloys generally exhibit extraordinary service performance under severe conditions through the formation of ultrafine nano oxides. Y₂Ti₂O₇ has been characterized as the major strengthening oxide in Fe-based ODS alloys. First-principles energetic analyses were performed to investigate the structural, elastic and interface properties of Y₂Ti₂O₇ in either Fe-based or Ni-based ODS alloys. Y₂Ti₂O₇ has comparable elastic constants to bcc-Fe and fcc-Ni and similar elastic deformation compatibility in Y₂Ti₂O₇-strengthened Fe-based and Ni-based ODS alloys is therefore expected. The Ni/oxide interface has generally better thermostability than Fe/oxide across the whole range of the concerned oxygen chemical potential. Further interface bonding and adhesion calculations revealed that Y₂Ti₂O₇ can enhance the bonding strength of Ni/Y₂Ti₂O₇ through d-d orbital interaction between the interfacial YTi layer and Ni layer, while the interface bonding between the Fe layer and YTi layer is weakened compared to the metal matrix. First-principles calculations suggest that Y₂Ti₂O₇ can be a candidate for strengthening nano-oxides in either Fe-based or Ni-based ODS alloys with well-behaved mechanical properties for fourth-generation fission reactors and further experimental validations are encouraged. Full article

The by-products of green coffee processing are rich in compounds that can be recycled for their possible use in the production of beverages, fertilizers and weed control in production areas. The objective of this work was to identify the organic and inorganic bioactive compounds of green coffee and the coffee by-products related to the production of origin, such as dried cascara (skin-pulp), parchment and silverskin (unroasted), in order to investigate the role their biomolecules may have in reuse through practices and local knowledge, not yet valued. The metabolomic profile by HPLC-ESI-HRMS of the aqueous extract of the dried cascara highlighted 93 non-volatile molecules, the highest number reported for dried cascara. They belong to groups of organic acids (12), alkaloids (5), sugars (5), fatty acids (2), diglycerides (1), amino acids (18), phospholipids (7), vitamins (5), phenolic acids (11), flavonoids (8), chlorogenic acids (17), flavones (1) and terpenes (1). For the first time, we report the use of direct analysis in real-time mass spectrometry (DART-MS) for the identification of metabolites in aqueous extracts of dried cascara, parchment, silverskin and green coffee. The DART analysis mainly showed the presence of caffeine and chlorogenic acids in all the extracts; additionally, sugar adducts and antioxidant compounds such as polyphenols were detected. The mineral content (K, Ca, P, S, Mg and Cl) by EDS spectrometry in the by-products and green coffee showed a relatively high content of K in the dried cascara and green coffee, while Ca was detected in double quantity in the silverskin. These metabolomic and mineral profile data allow enhancement of the link between the quality of green coffee and its by-products and the traditional local practices in the crop-growing area. This consolidates the community’s experience in reusing by-products, thereby minimizing the impact on the environment and generating additional income for coffee growers’ work, in accordance with the principles of circular economy and bioeconomy. Full article

Dissolved gas analysis (DGA) is a vital method for the online detection of transformer operation state. The adsorption performance of a SnP₃ monolayer modified by transition metal Cr regarding six characteristic gases (CO, C₂H₄, C₂H₂, CH₄, H₂, C₂H₆) dissolved in oil was studied. The study reveals the relevant adsorption and gas-sensing response mechanisms through calculations of the adsorption energy, density of states, differential charge density, energy gap, and recovery time. The results display a considerable increase in the adsorption effect of the Cr-SnP₃ monolayer on six gases. The CO, C₂H₂, and C₂H₄ gases lead to chemical adsorption, and the CH₄, H₂, and C₂H₆ gases lead to physical adsorption. Combined with the recovery time, the Cr-SnP₃ monolayer has a strong adsorption effect on CO and C₂H₂ gases at normal temperatures and even high temperatures, and the adsorption is stable. C₂H₄ gas can be rapidly desorbed from the Cr-SnP₃ monolayer at 398 K. Therefore, the Cr-SnP₃ monolayer can be expected to serve as a CO and C₂H₂ gas adsorbent and a resistive gas sensor for C₂H₄ gas. This research offers a theoretical foundation for the development of the Cr-SnP₃ monolayer in gas-sensitive materials. Full article

Stator faults are one of the common issues in pumped storage generators, significantly impacting their performance and safety. To ensure the safe and stable operation of pumped storage generators, a stator fault diagnosis method based on an improved short-time Fourier transform (STFT)-support vector data description (SVDD) hybrid algorithm is proposed. This method establishes a fault model for inter-turn short circuits in the stator windings of pumped storage generators and analyzes the electrical and magnetic states associated with such faults. Based on the three-phase current signals observed during an inter-turn short circuit fault in the stator windings, the three-phase currents are first converted into two-phase currents using the principle of equal magnetic potential. Then, the STFT is applied to transform the time-domain signals of the stator’s two-phase currents into frequency-domain signals, and the resulting fault current spectrum is input into the improved SVDD network for processing. This ultimately outputs the diagnosis result for inter-turn short circuit faults in the stator windings of the pumped storage generator. Experimental results demonstrate that this method can effectively distinguish between normal and faulty states in pumped storage generators, enabling the diagnosis of inter-turn short circuit faults in stator windings with low cross-entropy loss. Through analysis, under small data sample conditions, the accuracy of the proposed method in this paper can be improved by up to 7.2%. In the presence of strong noise interference, the fault diagnosis accuracy of the proposed method remains above 90%, and compared to conventional methods, the fault diagnosis accuracy can be improved by up to 6.9%. This demonstrates that the proposed method possesses excellent noise robustness and small sample learning ability, making it effective in complex, dynamic, and noisy environments. Full article

Based on domain decomposition, a semi-analytical method (SAM) is applied to analyze the free vibration of double-curved shells of revolution with a general curvature radius made from graphene nanoplatelet (GPL)-reinforced porous composites. The mechanical properties of the GPL-reinforced composition are assessed with the Halpin–Tsai model. The double-curvature shell of revolution is broken down into segments along its axis in accordance with first-order shear deformation theory (FSDT) and the multi-segment partitioning technique, to derive the shell’s functional energy. At the same time, interfacial potential is used to ensure the continuity of the contact surface between neighboring segments. By applying the least-squares weighted residual method (LWRM) and modified variational principle (MVP) to relax and achieve interface compatibility conditions, a theoretical framework for analyzing vibrations is developed. The displacements and rotations are described through Fourier series and Chebyshev polynomials, accordingly, converting a two-dimensional issue into a suite of decoupled one-dimensional problems. The obtained solutions are contrasted with those achieved using the finite element method (FEM) and other existing results, and the current formulation’s validity and precision are confirmed. Example cases are presented to demonstrate the free vibration of GPL-reinforced porous composite double-curved paraboloidal, elliptical, and hyperbolical shells of revolution. The findings demonstrate that the natural frequency of the shell is related to pore coefficients, porosity, the mass fraction of the GPLs, and the distribution patterns of the GPLs. Full article

Heterostructures are highly promising photocatalyst candidates for water splitting due to their advanced properties than those of pristine components. The ZnO/Sc₂CF₂ heterostructure was designed in this work, and its electronic structure was investigated to explore its potential for water splitting. The assessments of binding energy, phonon spectrum, ab initio molecular dynamics, and elastic constants provide strong evidence for its stability. The ZnO/Sc₂CF₂ heterostructure has an indirect band gap of 1.93 eV with a type-Ⅰ band alignment. The electronic structure can be modified with strain, leading to a transition in band alignment from type-Ⅰ to type-Ⅱ. The heterostructure is suitable for water splitting since its VBM and CBM stride over the redox potential. The energy barrier and built-in electric field, resulting from the charge transfer, facilitate the spatial separation of photogenerated carriers, enhancing their utilization efficiency for redox processes. The photogenerated carriers in the heterostructures with lattice compression greater than 6% follow the direct-Z transfer mechanism. The ZnO/Sc₂CF₂ heterostructure is confirmed with high photocatalytic activity by a Gibbs free energy change of HER, which is 0.89 eV and decreases to −0.52 eV under an 8% compressive strain. The heterostructure exhibits a remarkable enhancement in both absorption range and intensity, which can be further improved with strains. All these findings suggest that the ZnO/Sc₂CF₂ heterostructure is an appreciated catalyst for efficient photocatalytic water splitting. Full article

The internal combustion engines of long-endurance UAVs are optimized for cruises, so they are prone to overheating during climbs, when power requests increase. To counteract the phenomenon, step-climb maneuvering is typically operated, but the intermittent high-power requests generate repeated heating–cooling cycles, which, over multiple missions, may promote thermal fatigue, performance degradation, and failure. This paper deals with the development of a model-based monitoring of the cylinder head temperature of the two-stroke engine employed in a lightweight fixed-wing long-endurance UAV, which combines a 0D thermal model derived from physical first principles with an extended Kalman filter capable to estimate the head temperature under degraded conditions. The parameters of the dynamic model, referred to as nominal condition, are defined through a particle-swarm optimization, minimizing the mean square temperature error between simulated and experimental flight data (obtaining mean and peak errors lower than 3% and 10%, respectively). The validated model is used in a so-called condition-based extended Kalman filter, which differs from a conventional one for a correction term in section prediction, leveraged as degradation symptom, based on the deviation of the model-state derivative with respect to the actual measurement. The monitoring algorithm, being executable in real-time and capable of identifying incipient degradations of the thermal flow, demonstrates applicability for online diagnostics and predictive maintenance purposes. Full article

Inverter-fed machines are widely used in electric vehicle drive systems and have shown a trend toward high voltage and frequency in recent years. They are subjected to multiple types of stress during operation, causing potential short-circuit fault damage to the stator winding insulation. Online condition monitoring of the insulation before or in the early stage of the short circuit fault can effectively reduce maintenance costs and ensure its health. This paper reviews and summarizes the deterioration mechanism and the recent online electrical monitoring techniques. First, four types of failure stress and each type’s failure factors and mechanisms are analyzed. The coupling effect and overall process of multi-physical fields on stator insulation failure are considered. Secondly, the latest online electrical monitoring technologies are summarized. Each technique’s principles, methods, advantages, and disadvantages are analyzed. Finally, existing problems and possible directions for improvement in current research are discussed, focusing on their feasibility and accuracy in practical applications. Full article

The performance of the cathodic hydrogen evolution reaction (HER) in alkaline water electrolysis, an attractive hydrogen production technology, is highly dependent on efficient catalysts. Ruthenium (Ru), which is more affordable than platinum (Pt) and has a metal–hydrogen bond strength comparable to that of Pt, shows exceptional catalytic activity for the alkaline HER. Consequently, in recent years, research in the field of hydrogen production through alkaline water electrolysis has increasingly focused on Ru as a key element. This review first discusses the fundamentals of the alkaline HER, including principles, factors affecting its performance, and regulation strategies for its performance improvement. The research progress of ruthenium-based catalysts for the alkaline HER is then summarized with selected examples. The electronic structures of various ruthenium nanoparticles, ruthenium-M (M = noble metals and transition metals) heterogeneous catalysts, and ruthenium-based compounds are regulated by modulating the components and ligands of Ru atoms, aiming to achieve low water dissociation energies and optimal binding energies for hydrogen (H) and hydroxyl (−OH) groups, thereby enhancing the alkaline HER catalytic performance. Finally, the problems, challenges, and future development directions of the alkaline HER are proposed. Full article

In wireless power transfers (WPTs), it is challenging to obtain a constant output of power (COP) and constant transmission efficiency (CTE) in large coupling variation ranges. In this study, the eight-coil WPT system achieves a uniform magnetic field (UMF) in the transmitter and receiver sides using two transmitting (Tx) coils and two receiving (Rx) coils, respectively. COP and CTE are then achieved with large coupling variation ranges. The circuit model and equations of the transmission characteristics are first obtained based on the structure and working principle of the Helmholtz coil. The model of the mutual inductance and equation of the impedance coupled factor are then developed. The laws of the transmission characteristic are also determined by adopting a simulation tool and equations of the transmission characteristics. Finally, the eight-coil WPT experimental system is designed. In a fixed-frequency mode, the COP and CTE are achieved when the coupling and misalignment distances are changed within a quarter or one-fifth of the relay coil diameter, respectively. This topology provides an efficient solution for problems faced in practical applications, such as wireless chargers of kitchen appliances and automatic mobile robots of small size. Full article

Using first-principles density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we performed this study on the phase stability, the intrinsic redox stability, and the Li⁺ conductivity of Li₁₀Ge_xMo_1−xP₂S₁₂ (x = 0~1) superionic conductors. Molybdenum (Mo) is expected to replace expensive germanium (Ge) to lower tmaterial costs, reduce sensitivity to ambient water and oxygen, and achieve acceptable Li⁺ conductivity. The ab initio first principle molecular dynamics simulations show that room-temperature Li⁺ conductivity is 1.12 mS·cm⁻¹ for the Li₁₀Ge_0.75Mo_0.25P₂S₁₂ compound, which is comparable to the theoretical value of 6.81 mS·cm⁻¹ and the experimental measured one of 12 mS·cm⁻¹ of the Li₁₀GeP₂S₁₂ (LGPS) structure. For Li₁₀Ge_xMo_1−xP₂S₁₂ (x = 0, 0.25, 0.5 and 1) compounds, the density of states and the projection fractional wave state density were calculated. It was found that when Ge atoms were partially replaced by Mo atoms, the band gap remained unchanged at 2.5 eV, but deep level defects appeared in Mo-substituted compounds. Fortunately, this deep level defect is difficult to ionize at room temperature, so it has no effect on the electronic conductivity of Mo substitute compounds, making Mo substitution a suitable solution for electrolyte materials. The projection fractional wave state density calculation shows that the covalent bond between Mo and S is stronger than that between Ge and S, which reduces the sensitivity of Mo-substituted compounds to water and oxygen contents in the air. In addition, the partial state density coincidence curve between Li and S elements disappears in the 25% Mo-substituted compound with energies of 4–5 eV, indicating that the Li₂S by-product is decreased. Full article

This entry first provides an overview of the historical, cultural and epistemological background that is key for Hamilton’s positions on mechanics. We consider the investigations on geometrical optics in the 17th and 18th centuries, Euler’s and Lagrange’s foundations of variational calculus in the 18th century to find extrema of physical quantities expressed as infinite sums of infinitesimals (today, we would say ‘definite integrals’), and Lagrange’s introduction of a revolutionary analytical mechanics, all of which are all fertile grounds for Hamilton’s steps—first, in what we could call analytical optics, then in an advanced form of analytical mechanics. Having provided such an overview, we run through some of Hamilton’s original papers to highlight how he posed his principle(s) in the wake of his forerunners and how his principles are linked with the search for a unitary view of physics. Full article

Oil refining is a branch of industry that delivers energy to move our vehicles. The transportation of people and goods by airplanes, ships, trains, trucks, buses, and cars is unthinkable for modern mankind without the use of refined petroleum automotive fuels. Thus, the optimal functioning of this industrial branch is vital to contemporary human society. The modeling of processes that take place during refined oil products’ manufacturing, which are parallel in their essence, by generalized nets enables their activity optimization and better management. The generalized nets, which are in principle extensions of Petri nets, are applied in this research as a toolkit to model all processes from crude oil selection and delivery to a high complex refinery (Nelson index of 10.6) to the production of a great diversity of fuels, propylene, and polypropylene. The proposed article is a continuation and extension of the articles, published in Mathematics Journal in 2021 and 2023. It is the first (theoretical part) of our comprehensive study of modeling petroleum products’ production processes in a refinery, and the second part will discuss the results of the software implementation of the model. Full article