Understanding of the nanoparticle (NP) sintering mechanism at the atomic scale is of significance for improving various NP applications, such as printable nanoinks, catalysts, and electrode materials ...in energy devices. In this research, sintering dynamics of Cu–Ag core–shell NPs with various geometries are investigated through molecular dynamics simulations under different temperatures. The evolutions of local crystalline structure, characterized by common neighbor analysis, and potential energy during the sintering are studied to identify the sintering mechanisms. Sintering of two equally sized NPs is divided into three stages according to the shrinkage evolution, and depending on the sintering stage and condition, NP undergoes reorientation for achieving epitaxial layering, plastic deformation, surface diffusion, wetting, and crystallization–amorphization–recrystallization. Although the Cu core is coalescent neither in solid phase nor in surface-premelting-induced sintering, it can enhance the mobility of Ag shell atoms. The size-dependent optimal core radius/shell thickness ratio is proposed to achieve maximum densification and thus maximum bonding strength at room temperature.
While molecular dynamics (MD) has proven to be a promising approach to investigate the diffusion properties, the grand challenge resides in evaluating potential model parameters to accurately ...replicate experimentally measured properties. The Buckingham potential model with Columbic interaction is widely employed in MD simulations of chromia (Cr2O3) systems, as it allows for reasonable computational cost and accuracy. However, considering the well-known limitation of classical potential models in simultaneous reproduction of various physical phenomena, further comprehensive evaluation of the potential is required for calculation of diffusion properties. In this study, we benchmark the performance of three different Buckingham models with the experimental data by calculating structural, thermodynamic, and mechanical properties of defect-free Cr2O3, and diffusion properties of Cr2O3 with vacancy defects. Available Buckingham models display limited accuracies, consolidating the necessity of retraining the potential parameters for all properties impacting the diffusion dynamics. Oversimplification in parameterization procedures is suggested to impede the universal performance in property reproduction. This research also demonstrates effective guidelines for choosing a proper parameter set of current Buckingham potential for MD simulation with Cr2O3 depending on properties and for potential reparameterization.
High-energy optical phonons are preferred in phonon-absorbing transitions, and regarding their production we analyze the phonon upconversion processes under nonequilibrium created by heterojunction ...transmission. For heterojunctions, steady phonon flux from a low-cutoff-frequency layer (e.g., Ge) is transmitted to a high cutoff layer (e.g., Si), creating a nonequilibrium population of low-energy phonons for upconversion. Using quantum spectral phonon transmission and first-principles calculations of the phonon interaction kinetics, we identify the high-conversion efficiency channels, i.e., modes and wave vectors. Junction-transmitted phonons, despite suffering from the interface reflection and from spreading interactions with equilibrium native phonons, have a high upconversion rate to Brillouin zone-boundary optical phonons, while nonequilibrium native phonons are efficiently upconverted over most of the zone. So, depending on the harvested optical phonon, one of these nonequilibrium phonons can be selected for an efficient upconversion rate.
Thermal rectification in defect-engineered graphene with asymmetric hole arrangements is assessed via molecular dynamics simulations. Asymmetry in two different configurations (triangular and ...rectangular hole arrangements) is controlled by manipulating geometrical parameters, such as hole size; effects of geometry on the resultant rectification are investigated. Filtering of phonon propagation directions by geometrical confinement, and asymmetric relaxation distance induce a difference in heat transfer depending on transport direction, or thermal rectification. Increase in porosity, which results in additional confinement and larger difference in relaxation, produces more significant thermal rectification. While a rectangular arrangement of holes results in 70% of the maximum thermal rectification, up to 78% of rectification was achieved using a triangular arrangement within 47.5 nm of graphene, which can be attributed to more effective phonon-hole boundary scattering with a triangular arrangement. This study suggests a feasible approach to create thermal rectification and enables its fine control, contributing to the development of phononic devices and enhancement of thermal system design for electronics.
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Redirection of energy carrier propagation by geometric confinement is studied through the analysis of in-plane and cross-plane thermal transport within various graphene nanomesh (GNM) configurations ...using molecular dynamics (MD) simulations. As the transport channel width decreases with an increase in porosity, the effect of redirection increases; thus, the in-plane thermal conductivity of large-porosity GNM is more dependent on hole arrangement. Since higher porosities weaken the GNM structure due to a larger population of broken bonds, carbon atoms within the graphene structures are more easily influenced by interactions with the substrate silicon (Si) block. Subsequently, increase in porosity leads to the decrease of interfacial thermal resistance. At higher porosities, lower interfacial resistance and in-plane thermal conductivity cause diversions (and redirections) in heat flow from the GNM to the underlying Si substrate. Our study suggests that this method of heat flow redirection can be applied as an effective means to control and manage heat transfer within numerous applications; extension to the improved conductivity calculation accuracy can also be achieved through the inclusion of this diversion analysis.
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The electron-phonon interaction is the dominant mechanism of inelastic scattering in molecular junctions. Here we report on its effect on the thermoelectric properties of single-molecule devices. ...Using density functional theory and the nonequilibrium Green’s function formalism we calculate the thermoelectric figure of merit for a biphenyl-dithiol molecule between two Al electrodes under an applied gate voltage. We find that the effect of electron-phonon coupling on the thermoelectric characteristics strongly varies with the molecular geometry. Two molecular configurations characterized by the torsion angles between the two phenyl rings of 30° and 90° exhibit significantly different responses to the inelastic scattering. We also use molecular dynamics calculations to investigate the torsional stability of the biphenyl-dithiol molecule and the phonon thermal transport in the junction.
In recent years, the demand for small-scale remote sensing, which is used in disaster monitoring, agriculture, and ground subsidence has increased. A multichannel synthetic aperture radar (SAR) ...system can provide image and topographic information of the illuminated scene, regardless of adverse weather conditions. As a cost-effective solution to radar imaging, a multichannel W-band SAR system mounted on a multirotor unmanned aerial vehicle (UAV) is presented. The radar module was designed to operate at W-band to achieve small size and weight allowing the module to be mounted on multirotor UAVs with small payload. A detailed description of the design and measurement of the system is provided in this paper. The radar imaging capability of the developed system was verified by performing outdoor experiments using isolated buildings as targets. The multichannel functionality of the system was verified by measuring height of a point target placed above the ground. The measurements and experiments verified the feasibility of a multichannel radar mounted on a multirotor UAV for imaging and topographic applications.
Room temperature (
T
room
, 300 K) nanojoining of Ag has been widely employed in fabrication of microelectronic applications where the shapes and structures of microelectronic components must be ...maintained. In this research, the joining processes of pure Ag nanoparticles (NPs), Cu-Ag core-shell NPs, and nanowires (NWs) are studied using molecular dynamics simulations at
T
room
. The evolution of densification, potential energy, and structural deformation during joining process are analyzed to identify joining mechanisms. Depending on geometry, different joining mechanisms including crystallization-amorphization, reorientation, Shockley partial dislocation are determined. A three-stage joining scenario is observed in both joining process of NPs and NWs. Besides, the Cu core does not participate in all joining processes, however, it enhances the mobility of Ag shell atoms, contributing to a higher densification and bonding strength at
T
room
, compared with pure Ag nanomaterials. The tensile test shows that the nanojoint bears higher rupture strength than the core-shell NW itself. This study deepens understanding in the underlying joining mechanisms and thus nanojoint with desirable thermal, electrical, and mechanical properties could be potentially achieved.
Cu-Ag core-shell (CS) nanoparticles (NP) have been synthesized to replace pure Ag NP paste in order to lower the cost while maintaining excellent thermal and electrical conductivities for electronic ...applications. In this study, a multiple-CS-NP sintering model with molecular dynamics is employed to investigate the NP size and temperature dependency of the sintering process, as well as mechanical and thermodynamic properties of the sintered structures. Porosity and multiple particle effects are included, which allow for more accurate analysis than the conventional two- or three-NP sintering model. We unravelled the sintering mechanism at room temperature, and the interplay of liquid and solid surface diffusion during sintering at higher temperatures. Interfacial atoms have a higher mobility than surface atoms and contribute to a higher densification in the multiple-CS-NP model. A more densified structure yields higher Young's modulus, yield strength and Poisson's ratio, while lowering isothermal compressibility. The coefficient of thermal expansion and specific heat capacity exhibit grain-size and porosity independence. This multiple-CS-NP model provides a theoretical basis for determining NP configuration and sintering conditions for desirable properties.
Multiple-CS-NP sintered structure of 600 K yields similar porosity as the counterpart sintered at surface premelting temperature (900 K).
Ab initio molecular dynamics (AIMD) is a versatile and reliable computational approach to atomic-scale materials science. However, due to the expensive computational cost on the first-principles ...calculation at each time step, the temporal and spatial scales are significantly limited, hindering its broader applications. Therefore, to accelerate the simulation clock of AIMD, atomic data production in AIMD using a recurrent neural network (RNN) is studied in this research. We demonstrate the feasibility of incorporating RNN-predicted time steps in AIMD, while maintaining its accuracy. The RNN models, which are trained using AIMD simulation results, directly predict atomic velocities and positions of Si atoms, reducing errors by decoupling the position and velocity update procedures from the Newtonian mechanics. Not only the predicted atomic data but also material properties calculated using the predicted data, such as the radial distribution function, temperature, velocity autocorrelation function, phonon density of states, and heat capacity, exhibit excellent agreements with the ground-truth AIMD calculations. Since the RNN prediction is much faster than the first-principles calculation of AIMD, this approach is expected to effectively accelerate AIMD, contributing to computational materials research.
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