Search

In this work, we explore the physics and evaluate the merits of strain engineering in two-dimensional van der Waals semiconductor-based FETs (field-effect-transistors) using DFT (density functional theory) to determine the modulation of the channel material properties under strain, and subsequently, their effect on carrier transport properties, i.e., scattering rates, mobility, and then finally simulate and analyze dissipative current transport with a non-equilibrium Green's function–Poisson's equation self-consistent solver. The scattering model includes the effects of charged impurities, intrinsic phonons, and remote phonons as well as the screening effect due to charged carriers. Impact of strain engineering on contact resistance is also incorporated into the transport simulations to determine the potential performance enhancements using strain in practical devices. Based on the comprehensive simulation results, we identify the materials and strain configuration that provide the best improvement in performance. We demonstrate an ON-current gain of 43.3% in a biaxially compressively strained monolayer MoSe2 device achieved through unique valley-crossing. Furthermore, implications of strain engineering for emerging energy-efficient devices based on band-to-band tunneling and spintronics are evaluated to explore uncharted frontiers in beyond-CMOS electron devices. [ABSTRACT FROM AUTHOR]

A precise description of the thermodynamic states of gaseous and solid detonation products is essential when using thermodynamic calculations to determine the detonation performance and destructive power of explosives. For high oxygen-lean explosives (the oxygen contained in explosives is insufficient to completely oxidize combustible elements and excess solid carbon will be generated in the detonation products), the phase state of solid carbon product affects the Chapman–Jouguet (CJ) detonation performance parameters, reaction zone, and energy release process. However, the recovery of detonation products demonstrates that the actual detonation carbon product is primarily a mixed state of diamond/graphite stack, as opposed to the existing thermodynamic codes, which essentially treat detonation carbon as single-phase carbon. To understand the thermodynamic effect of the mixed carbon phase state on the non-ideal detonation behavior, in this work, the matching relationship among the VINET equation of state parameters, thermodynamic potential parameters of the solid products of the equivalent system and the phase mixed system was constructed by using the nonlinear fitting method. The relationship between the carbon phase composition at the CJ point and the explosive composition structure was researched. Investigations were conducted into how the mixed carbon phase affected the volume and content of gas products as well as the composition at CJ points. Diamond formation in products is good for enhancing explosive's working capacity. Based on mixed-state potential parameters, the correlation mechanism between the mixed carbon phase and the chemical reaction zone was investigated, and it was found that intramolecular carbon/intermolecular carbon and more detonation graphite/diamond products all would lead to the extension of the reaction zone. [ABSTRACT FROM AUTHOR]

The catalytic hydrogenation of biomass-derived chemicals is essential in chemical industry due to the growing demand for sustainable and renewable energy sources. In this study, we present a comprehensive theoretical investigation regarding the hydrogenation of glycolaldehyde to ethylene glycol over a Ru/C catalyst by employing density functional theory and ab initio molecular dynamics simulations. With inclusion of explicit solvation, we have demonstrated that the glycolaldehyde hydrogenation is significantly improved due to the fast proton transfer through the hydrogen bond network. The enhanced activity could be attributed to the participation of the solvent water as the hydrogen source and the highly positively charged state of a Ru cluster in an aqueous phase, which are critical for the activation of aldehyde groups and proton-assisted hydrogenation. Overall, our findings provide valuable insights into glycolaldehyde hydrogenation over Ru/C catalysts in the aqueous phase, highlighting the importance of solvation effects in the biomass conversion. [ABSTRACT FROM AUTHOR]

The surface charges of catalysts have intricate influences on the thermodynamics and kinetics of electrochemical reactions. Herein, we develop a grand-canonical iteration method based on density functional theory calculations to explore the effect of surface charges on reaction kinetics beyond the traditional Butler–Volmer picture. Using the hydrogen evolution reaction on S vacancies of MoS2 as an example, we show how to track the change of surface charge in a reaction and to analyze its influence on the kinetics. Protons adsorb on S vacancies in a tough and charge-insensitive water splitting manner, which explains the observed large Tafel slope. Grand-canonical calculations report an unanticipated surface charge-induced change of the desorption pathway from the Heyrovsky route to a Volmer–Tafel route. During an electrochemical reaction, a net electron inflow into the catalyst may bring two effects, i.e., stabilization of the canonical energy and destabilization of the charge-dependent grand-canonical part. On the contrary, a net outflow of electrons from the catalyst can reverse the two effects. This surface charge effect has substantial impacts on the overpotential and the Tafel slope. We suggest that the surface charge effect is universal for all electrochemical reactions and significant for those involving interfacial proton transfers. [ABSTRACT FROM AUTHOR]

Effects of secondary gas injection on flame shape and combustion stability of a premixed swirl methane/air flame were investigated experimentally. Without secondary gas injection, an attached M-shaped flame was observed with pressure oscillations up to a sound pressure level of 68 dB. The flame exhibited significant oscillations due to flame–vortex interactions induced by velocity fluctuations. The secondary methane or air was separately injected into the outer recirculation zone to suppress the flame oscillations at a volume flow rate of 0.5% of the primary flow rates. For lower primary air flow rates ( Q air ≤ 60 L/min), both secondary methane and air injections were able to suppress the thermoacoustic instability and performed the sound pressure level reduction of 45 dB. Meanwhile, the flame transformed from an attached unstable M-shaped flame to a detached stable V-shaped flame. The secondary methane or air injection served to stabilize the flame and anchor its spatial heat release distribution by limiting the variation of flame shape in the outer recirculation zone, suppressing thermoacoustic oscillation. For higher primary air flow rates ( Q air = 80 L/min), the secondary methane injection could reduce sound pressure levels to noise levels, while secondary air injection could not. The velocity disturbances and heat release fluctuations introduced by secondary methane injection could lead to a transformation in flame shape and achieve the suppression of thermoacoustic oscillation. However, the velocity disturbances induced by secondary air injection were insufficient to stabilize the flame. [ABSTRACT FROM AUTHOR]

To investigate the unsteady shedding characteristics of a reentrant jet supercavity with a low Froude number, a high-fidelity numerical model based on the inhomogeneous multiphase model is developed to predict the complex supercavitation flow that occurs during supercavity development. The developed solver is validated quantitatively against experimental results in terms of supercavity geometry and closure mode. This study focuses on the initial generation and development process of a reentrant jet supercavity, revealing three distinct stages: foam cavity, transparent supercavity with rapid growth in dimensions, and fully developed supercavity exhibiting significant deformation. Owing to reverse flow of the gas–water mixture, interfacial instabilities arise from the unsteady cavity shedding, leading to fluctuations in supercavity shape. The types of large-scale cavity shedding observed in this work—wing-like and cloud-like—are caused by the concave deformation resulting from the reentrant jet. As the gas entrainment coefficient increases, the unsteady characteristics of pressure oscillation weaken, and the instance of wing-like cavities decreases. When the gas entrainment coefficient reaches a critical value, the twin-vortex closure mode occurs, resulting in a more stable flow behavior. In sum, we propose a theoretical model that elucidates the strength of the reentrant jet and reveals its unsteady shedding behavior during supercavity development. [ABSTRACT FROM AUTHOR]

Transition of hypersonic boundary layer flow is an important problem in aerodynamic research. To obtain the natural transition position using the flow stability analysis method, it is necessary to analysis the neutral curve and the perturbation growth rule. Accurate calculation of the basic flow is the basis and prerequisite for stability analysis. This paper proposes that surface radiation should be considered in the analysis of hypersonic boundary layer flow stability due to the high temperatures at the vehicle surface caused by aerodynamic heating. In order to obtain the influence of surface radiation on the flow stability, we assume that the surface satisfies the energy balance and the gas satisfies calorimetrically perfect gas properties. To simplify the analysis, gas radiation has been neglected. The boundary layer analysis of a hypersonic flat plate with and without radiation has been performed by a combination of flow stability theory and direct numerical simulation. It is found that high temperature radiation on the surface causes energy to transfer outwards. The results showed that surface temperature decreases along the streamwise direction and is no longer an adiabatic surface temperature. Instability interval decreases and the maximum growth rate increases. The range of the instability region expressed in dimensionless frequency values moves to lower frequency as the Mach number increases. Maximum growth rate shows a tendency to increase and then decrease as the Mach number increases. [ABSTRACT FROM AUTHOR]

Polymer film melt stretching exhibits a unique stress–strain behavior, featuring a stress plateau followed by a sudden increase with increasing the strain. This phenomenon poses a challenge to conventional melt-extension theories and methods. To address this, we derive a simplified flow theory according to the symmetric characteristics of thin melt-stretching, and propose a modified multi-mode Leonov model (m-Leonov) that accounts for molecular orientation effects, a crucial factor in accurately simulating the process. In order to improve the computational efficiency, an iterative algorithm was developed to decouple the interdependence between governing and constitutive equations. The computational results reveal that the viscoelastic nature of the film contributes to a delay in the transformation of stretching forces, leading to the observed stress plateau. Significantly, our m-Leonov model successfully predicts both the plateau and the subsequent abrupt increase in stress, across various temperatures and stretching rates. These simulations closely agree with experimental data, underlining the robustness of our approach. Our findings would enhance the understanding of rheological complexities in film stretching, offering potential applications in various materials and polymer processing. [ABSTRACT FROM AUTHOR]

During high-speed water-exit of the supercavitating projectile, the cavity interacts with the free surface and collapses, with instantaneous high collapse pressure impacting on the projectile. In order to study the cavity collapse characteristics during high-speed water-exit of the supercavitating projectile, the numerical study based on the Reynolds-averaged equation and the volume of fluid multiphase flow model is conducted in this paper. The results show that the cavity near the free surface will gradually become larger with the movement of the projectile during water-exit of the supercavitating projectile. The existence of attitude angles will cause the asymmetry of cavity to collapse. The cavity on the upstream side will first collapse and generate collapse pressure, while the cavity on the downstream side will collapse later but generate higher collapse pressure. The asymmetry of the cavity collapse becomes stronger with the increasing attitude angles. The time interval of the collapse pressure on the downstream and upstream sides of the projectile becomes shorter close to the projectile tail. [ABSTRACT FROM AUTHOR]

The maneuverability of supercavitating vehicles is significantly affected by disturbance from their control surfaces during the water-entry process. In this paper, experiments and simulations on cavity evolution models and vehicle dynamic characteristics are carried out to investigate the influence of the water-entry disturbance on the water-entry maneuvering motion process of supercavitating vehicles. In the water-entry experiment of medium speed projectiles based on the high speed photography, the shapes of cavity contours are extracted and the applicability of the cavity evolution models to the problem of water-entry maneuvering motion is verified. A kind of more correct prediction model of cavity shape is chosen by the comparison. A mathematical model on the water-entry maneuvering motions of vehicles is developed based upon certain simplifications, which makes an improvement on the three-dimensional cavity topology algorithm. Afterward, the simulations on the water-entry maneuvering motion of the vehicle are carried out, from which the real-time interaction between the vehicle and the cavity is extracted and the influence law of the fin deflection angle on the vehicle dynamic characteristics is analyzed. The influence mechanism of the desynchronization of fins' contacting water on the water-entry maneuvering motions of vehicles is revealed. It is found that the fin deflection angle of 10° is the most conducive to enhancing the vehicles' ability to stabilize their attitudes during water-entry maneuver by analyzing the fin efficiency and planning parameters. [ABSTRACT FROM AUTHOR]

A semi-sealed cylindrical shell is a hollow cylinder in which one end is open and the other end is sealed. In order to systematically study the cavity evolution, the hydrodynamic characteristics, and corresponding structural response of the semi-sealed cylindrical shell during high-speed vertical water entry, a numerical simulation is carried out based on a Star-CCM+ and ABAQUS collaborative simulation method. The results show that a nested cavity is formed that presents three different morphologies as the semi-sealed cylindrical shell penetrates the water. Moreover, a jet is formed under the shell, which profoundly influences the hydrodynamic and structural characteristics. Compared with the completely sealed case, the velocity and displacement of the semi-sealed cylindrical shell are significantly changed upon water entry, and the deformation at the top wall is more prominent. [ABSTRACT FROM AUTHOR]

In this study, the effects of entropy layer on the boundary layer over hypersonic blunt cones for a thermochemical equilibrium gas are investigated. The flow and stability characteristics of the entropy and boundary layers are presented considering the chemical reactions. It is found that the entropy layer has little influence on the inner layer inside the boundary layer. The inner layer thickness increases when chemical equilibrium is considered, which stems from the enhanced viscosity protection near the wall surface. At the leading edge of the blunt cone, due to the effects of the equilibrium gas, the temperature in the boundary and entropy layers decreases and the boundary-layer-edge parameters significantly change. The entropy layer gradually vanishes along the downstream direction, and the effects of chemical reactions are concentrated in the boundary layer. The entropy swallowing point of the equilibrium gas does not significantly differ from that of the perfect gas. For all the analyzed cases, chemical reactions stabilize the entropy layer instability modes. The instability region and the frequency range of the equilibrium gas decrease, and the growth rate of the most unstable perturbation of the equilibrium gas is considerably smaller than that of the perfect gas. Moreover, the results show that the equilibrium gas has a minor influence on the process of the blunt cone boundary layer solution tending to the sharp cone solution. [ABSTRACT FROM AUTHOR]

Instabilities of high-enthalpy flat-plate boundary layers in thermochemical equilibrium conditions are studied using a linear stability theory (LST) and parabolized stability equations. A supersonic radiation mode is found to be unstable at the downstream location of the second mode. This mode synchronizes with the second mode as its phase velocity approaches c r = 1 − 1 / M a . The amplification rate is expected to be comparable to the second mode when M a > 14 and T w < 14 T e . The results of disturbance evolution indicate that the radiation mode is generated by the second mode through a synchronization process. The coefficient for the amplitude response of the radiation mode to the forcing second mode is around 1. After the radiation mode is excited, the second mode itself still exists in the boundary layer, leading to a co-existence of dual unstable modes. Owing to the competition between the two modes, the disturbance amplitudes exhibit significant oscillations. A two-mode amplitude prediction model based on the LST is proposed by properly superposing the two modes' amplitudes. The accurate prediction of the N-factor demonstrates that the proposed model can be used to predict a high-enthalpy boundary layer transition involving these two modes. [ABSTRACT FROM AUTHOR]

Entropy swallowing is the phenomenon where the entropy layer is swallowed by the boundary layer developing downstream. In this work, the entropy-swallowing point over a 5 ° blunt hypersonic cone is investigated theoretically and numerically. An equation for obtaining the location of the entropy-swallowing point is derived herein that requires only the free-stream parameters. This equation also reveals the relationship between the entropy-swallowing point and the nose radius under the constant free-stream Mach number. The analytical results are entirely consistent with direct numerical simulation results. Furthermore, the influence of the Mach number and Reynolds number on the entropy-swallowing point is investigated. The location of the entropy-swallowing point depends more strongly on the Reynolds number than on the Mach number, with the entropy-swallowing point shifting further downstream as the Reynolds number increases. However, as the Mach number increases, its impact on the entropy-swallowing point decreases. Furthermore, the influence of the boundary-layer displacement also decreases with an increase of the Mach number. [ABSTRACT FROM AUTHOR]

Detection and analysis of volatile organic compounds' (VOCs) biomarkers lead to improvement in healthcare diagnosis and other applications such as chemical threat detection and food quality control. Here, we report on tri-molybdenum phosphide (Mo3P) and multiwalled carbon nanotube (MWCNT) junction-based vapor quantum resistive sensors (vQRSs), which exhibit more than one order of magnitude higher sensitivity and superior selectivity for biomarkers in comparison to pristine MWCNT junctions based vQRSs. Transmission electron microscope/scanning tunneling electron microscope with energy dispersive x-ray spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy studies reveal the crystallinity and the presence of Mo and P elements in the network. The presence of Mo3P clearly enhanced the performance of vQRS as evidenced in sensitivity and selectivity studies. The vQRSs are stable over extended periods of time and are reproducible, making them a potential candidate for sensing related applications. [ABSTRACT FROM AUTHOR]

The surface charges of catalysts have intricate influences on the thermodynamics and kinetics of electrochemical reactions. Herein, we develop a grand-canonical iteration method based on density functional theory calculations to explore the effect of surface charges on reaction kinetics beyond the traditional Butler-Volmer picture. Using the hydrogen evolution reaction on S vacancies of MoS2 as an example, we show how to track the change of surface charge in a reaction and to analyze its influence on the kinetics. Protons adsorb on S vacancies in a tough and charge-insensitive water splitting manner, which explains the observed large Tafel slope. Grand-canonical calculations report an unanticipated surface charge-induced change of the desorption pathway from the Heyrovsky route to a Volmer-Tafel route. During an electrochemical reaction, a net electron inflow into the catalyst may bring two effects, i.e., stabilization of the canonical energy and destabilization of the charge-dependent grand-canonical part. On the contrary, a net outflow of electrons from the catalyst can reverse the two effects. This surface charge effect has substantial impacts on the overpotential and the Tafel slope. We suggest that the surface charge effect is universal for all electrochemical reactions and significant for those involving interfacial proton transfers., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)