Size effect in the melting and freezing behaviors of Al/Ti core-shell nanoparticles using molecular dynamics simulations

The thermal stability of Ti@A1 core/shell nanoparticles with different sizes and components during continuous heating and cooling processes is examined by a molecular dynamics simulation with embedded atom method. The thermodynamic properties and structure evolution during continuous heating and cooling processes are investigated through the character- ization of the potential energy, specific heat distribution, and radial distribution function （RDF）. Our study shows that, for fixed Ti core size, the melting temperature decreases with A1 shell thickness, while the crystallizing temperature and glass formation temperature increase with A1 shell thickness. Diverse melting mechanisms have been discovered for different Ti core sized with fixed A1 shell thickness nanoparticles. The melting temperature increases with the Ti core radius. The trend agrees well with the theoretical phase diagram of bimetallic nanoparticles. In addition, the glass phase formation of A1-Ti nanoparticles for the fast cooling rate of 12 K/ps, and the crystal phase formation for the low cooling rate of 0.15 K/ps. The icosahedron structure is formed in the frozen 4366 A1-Ti atoms for the low cooling rate.

Research progress of molecular dynamics simulation for nanoparticles

With the continuous development of high-power electronic devices,the traditional tin-lead brazing materials no longer meet the conditions of use,and sintered nanometal solder paste is promising for a new generation of packaging materials.The mechanism of microstructural changes of nanoparticle sintering during the sintering process has not been well studied at present.Molecular dynamics(MD)simulations can effectively track the diffusion process of metal atoms during the sintering process and help to reveal the dynamic evolution of nanoparticles.This review presents many MD simulations of nanoparticle sintering,including the growth mechanism of nanoparticles,the effect of different sintering parameters on the performance of sintered joints,the connection mechanism between the reinforced phase and nanoparticles and the performance of composite sintered joints.The low temperature and low pressure sintering of nanopaste are still in face of some problems,and MD simulations are very helpful for improving the sintering process and verifying the mechanism of the reinforcing phase.

Melting Behaviour of Shell-symmetric Aluminum Nanoparticles： Molecular Dynamics Simulation

Molecular dynamics simulations with embedded atom method potential were carried out for A1 nanoparticles of 561 atoms in three structures： icosahedron, decahedron, and truncated octahedron. The total potential energy and specific heat capacity were calculated to estimate the melting temperatures. The melting point is 540＋10 K for the icosahedral structure, 500±10 K for the decahedral structure, and 520±10 K for the truncated octahedral structure. With the results of mean square displacement, the bond order parameters and radius of gyration are consistent with the variation of total potential energy and specific heat capacity. The relaxation time and stretching parameters in the Kohlraush-William-Watts relaxation law were obtained by fitting the mean square displacement. The results show that the relationship between the relaxation time and the temperatures is in agreement with standard Arrhenius relation in the high temperature range.

Effect of TiB_(2)Nanoparticles on Microstructure and Mechanical Properties of Ni_(60)Cr_(21)Fe_(19)Alloy in Rapid Directional Solidification Process:Molecular Dynamics Study

Molecular dynamics(MD)simulations are employed to delve into the multifaceted effects of TiB2 nanoparticles on the intricate grain refinement mechanism,microstructural evolution,and tensile performance of Inconel 718 superalloys during the rapid directional solidification.Specifically,the study focuses on elucidating the role of TiB2 nanoparticles in augmenting the nucleation rate during the rapid directional solidification process of Ni60Cr21Fe19 alloy system.Furthermore,subsequent tensile simulations are conducted to comprehensively evaluate the anisotropic behavior of tensile properties within the solidified microstructures.The MD results reveal that the incorporation of TiB₂nanoparticles during the rapid directional solidification of the Ni_(60)Cr_(21)Fe_(19)significantly enhances the average nucleation rate,escalating it from 1.27×10^(34)m^(-3)·s^(-1)to 2.55×10^(34)m^(-3)·s^(-1).Notably,within the face centered cube(FCC)structure,Ni atoms exhibit pronounced compositional segregation,and the solidified alloy maintains an exceptionally high dislocation density reaching up to 10^(16)m^(-2).Crucially,the rapid directional solidification process imparts a distinct microstructural anisotropy,leading to a notable disparity in tensile strength.Specifically,the tensile strength along the solidification direction is markedly superior to that perpendicular to it.This disparity arises from different deformation mechanisms under varying loading orientations.Tensile stress perpendicular to the solidification direction encourages the formation of smooth and organized mechanical twins.These twins act as slip planes,enhancing dislocation mobility and thereby improving stress relaxation and dispersion.Moreover,the results underscore the profound strengthening effect of TiB2 nanoparticles,particularly in enhancing the tensile strength along the rapid directional solidification direction.

Sintering reaction and microstructure of MAl(M=Ni,Fe,and Mg)nanoparticles through molecular dynamics simulation

The sintering-alloying processes of nickel(Ni),iron(Fe),and magnesium(Mg) with aluminum(Al) nanoparticles were studied by molecular dynamics simulation with the analytic embedded-atom model(AEAM) potential.Potential energy,mean heterogeneous coordination number NAB,and surface atomic number Nsurf-A were used to monitor the sintering-reaction processes.The effects of surface segregation,heat of formation,and melting point on the sinteringalloying processes were discussed.Results revealed that sintering proceeded in two stages.First,atoms with low surface energy diffused onto the surface of atoms with high surface energy;second,metal atoms diffused with one another with increased system temperature to a threshold value.Under the same initial conditions,the sintering reaction rate of the three systems increased in the order MgAl <FeAl <NiAl.Depending on the initial reaction temperature,the final core-shell(FeAl and MgAl) and alloyed(NiAl and FeAl) nanoconfigurations can be observed.

Anti-sintering behavior and combustion process of aluminum nano particles coated with PTFE:A molecular dynamics study

The characteristic of easy sintering of aluminum nanoparticle(ANP)limits its application in solid propellants.Coating ANP with fluoropolymer could effectively improve its combustion performance.To find out how the coating layer inhibits sintering and promotes complete combustion of particles from an atomic view,a comparative study has been done for bare ANP and PTFE coated ANP by using reactive molecular dynamics simulations.The sintering process is quantified by shrinkage ratio and gyration radius.Our results show that,at the same heating rate and combustion temperatures,bare ANPs are sintered together after the temperature exceeds the melting point of aluminum but the decomposition of PTFE coating layer pushes particles away and increases reaction surface area by producing small Al-F clusters.The sintering of ANPs which are heated in PTFE is alleviated compared with particles heated in oxygen,but particles still sinter together due to the lack of intimate contact between PTFE and alumina surface.The effect of temperature on the combustion of PTFE coated ANPs is also studied from 1000 to3500 K.The number density analysis shows the particles will not be sintered at any temperature.Aluminum fluoride prefers diffusing to the external space and the remained particles are mainly composed of Al,C and O.Fast ignition simulations are performed by adopting micro canonical ensemble.With the expansion of aluminum core and the melting of alumina shell,bare ANPs are sintered into a liquid particle directly.For PTFE coated ANPs,the volatilization of gaseous aluminum fluoride products continually endows particles opposite momentum.

Melting mechanisms of Pt-based multimetallic spherical nanoparticles by molecular dynamics simulation

The melting mechanisms of Pt-based multimetallic nanoparticles(NPs)are important to help determine their optimal melting processes.To understand the melting and coalescence behaviors of heterogeneous NPs(Pd-Pt NPs)with various sizes and compositions,molecular dynamics(MD)simulation was employed.The MD results for larger Pd-Pt NPs with an effective diameter of4.6-7.8 nm show that PtPd alloy can form at Pd/Pt interface before Pd NP melted completely,while for Pt-core/Pdshell NP and Pd-core/Pt-shell NP,PtPd alloy formed only after Pd portion melted completely.For smaller Pd-Pt NPs with an effective diameter of 2.5-4.0 nm,PdPt alloy is not formed until both Pd and Pt NPs melted completely.Besides,the coalescence process of Pd-Pt NPs depends on the melting temperature of Pt NP when Pt composition is higher than 20 at%.Furthermore,the melting mechanisms of Pd/Pt/Ir trimetallic NPs are investigated.A two-step melting process occurs in Pd-Pt-Ir NPs and Ir-core/Ptshell/Pd-shell NP,and the melting sequence of Pd-core/Ptshell/Ir-shell NP and Pt-core/Pd-shell/Ir-shell NP is different from Pd/Pt bimetallic NPs.

Impact Analysis of Microscopic Defect Types on the Macroscopic Crack Propagation in Sintered Silver Nanoparticles

Sintered silver nanoparticles(AgNPs)arewidely used in high-power electronics due to their exceptional properties.However,the material reliability is significantly affected by various microscopic defects.In this work,the three primary micro-defect types at potential stress concentrations in sintered AgNPs are identified,categorized,and quantified.Molecular dynamics(MD)simulations are employed to observe the failure evolution of different microscopic defects.The dominant mechanisms responsible for this evolution are dislocation nucleation and dislocation motion.At the same time,this paper clarifies the quantitative relationship between the tensile strain amount and the failure mechanism transitions of the three defect types by defining key strain points.The impact of defect types on the failure process is also discussed.Furthermore,traction-separation curves extracted from microscopic defect evolutions serve as a bridge to connect the macro-scale model.The validity of the crack propagation model is confirmed through tensile tests.Finally,we thoroughly analyze how micro-defect types influence macro-crack propagation and attempt to find supporting evidence from the MD model.Our findings provide a multi-perspective reference for the reliability analysis of sintered AgNPs.

Molecular dynamics studies on spreading of nanofluids promoted by nanoparticle adsorption on solid surface

Abstract Spreading of nanofluids on solid substrate was studied via molecular dynamics simulations. Simulation models for two immiscible fluids （oil and water based nanofiuids） confined in a slit between two planar solid walls were set up. The influence of the volume concentration of the nanoparticles on the three-phase contact line motion was investigated. We found that the larger volume concentration results in more visible nanoparticle adsorption on solid surface. This effect further induces an advancing displacement of the contact line compared with the meniscus profiles in low concentration case and that with the absence of nanoparticles. These findings are consistent with the previous experimental and theoretical results and provide the atomic-scale understanding on nanofluid spreading.

Molecular Dynamics of Free and Graphite-Supported Pt-Pd Nanoparticles

The thermal characteristics of bimetallic Pt-Pd nanoparticles, both free and graphite-supported, were investigated through molecular dynamics simulations using quantum Sutton-Chen many-body potentials for the metal-metal interactions. The graphite substrate was represented as layers of fixed carbons sites and modeled with the Lennard-Jones potential model. The melting temperatures for bimetallic nanoparticles were estimated based on variations in thermodynamic properties such as potential energy and heat capacity. Melting temperatures of the nanoparticles were found to be considerably lower than those of bulk Pt and Pd. The Pt-Pd clusters exhibited a two-stage melting, where surface melting of the external atoms is followed by homogeneous melting of the internal atoms. The melting transition temperature was found to increase when the particle is on the graphite support, with an increase at least 180 K higher than that of the same-sized free nanoparticle. The results of the density distributions perpendicular to the surface indicate that the Pd atoms have a tendency to remain at the surface, and the Pd atoms wet the graphite surface more than the Pt atoms, while root mean squares suggest that surface melting starts from the cluster surface, and surface melting was seen in both free and graphite-supported nanoparticles. Structural changes accompanying the thermal evolution were studied by the bond-orientational order parameter method.

Effect of Ellipsoidal Particle Shape on Tribological Properties of Lubricants Containing Nanoparticles

Adding nanoparticles can significantly improve the tribological properties of lubricants.However,there is a lack of understanding regarding the influence of nanoparticle shape on lubrication performance.In this work,the influence of diamond nanoparticles(DNPs)on the tribological properties of lubricants is investigated through friction experiments.Additionally,the friction characteristics of lubricants regarding ellipsoidal particle shape are investigated using molecular dynamics(MD)simulations.The results show that DNPs can drastically lower the lubricant's friction coefficientμfrom 0.21 to 0.117.The shearing process reveals that as the aspect ratio(α)of the nanoparticles approaches 1.0,the friction performance improves,and wear on the wall diminishes.At the same time,the shape of the nanoparticles tends to be spherical.When 0.85≤α≤1.0,rolling is ellipsoidal particles'main form of motion,and the friction force changes according to a periodic sinusoidal law.In the range of 0.80≤α<0.85,ellipsoidal particles primarily exhibit sliding as the dominant movement mode.Asαdecreases within this range,the friction force progressively increases.The friction coefficientμcalculated through MD simulation is 0.128,which is consistent with the experimental data.

Phonon resonance modulation in weak van der Waals heterostructures:Controlling thermal transport in graphene-silicon nanoparticle systems

The drive for efficient thermal management has intensified with the miniaturization of electronic devices.This study explores the modulation of phonon transport within graphene by introducing silicon nanoparticles influenced by van der Waals forces.Our approach involves the application of non-equilibrium molecular dynamics to assess thermal conductivity while varying the interaction strength,leading to a noteworthy reduction in thermal conductivity.Furthermore,we observe a distinct attenuation in length-dependent behavior within the graphene-nanoparticles system.Our exploration combines wave packet simulations with phonon transmission calculations,aligning with a comprehensive analysis of the phonon transport regime to unveil the underlying physical mechanisms at play.Lastly,we conduct transient molecular dynamics simulations to investigate interfacial thermal conductance between the nanoparticles and the graphene,revealing an enhanced thermal boundary conductance.This research not only contributes to our understanding of phonon transport but also opens a new degree of freedom for utilizing van der Waals nanoparticle-induced resonance,offering promising avenues for the modulation of thermal properties in advanced materials and enhancing their performance in various technological applications.

Molecular Dynamics Simulations for Melting Temperatures of SrF2 and BaF2

The shell-model molecular dynamics method was applied to simulate the melting temper- atures of SrF2 and BaF2 at elevated temperatures and high pressures. The same method was used to calculate the equations of state for SrF2 and BaF2 over the pressure range of 0.1 MPa-3 GPa and 0.1 MPa-7 GPa. Compared with previous results for equations of state, the maximum errors are 0.3% and 2.2%, respectively. Considering the pre-melting in the fluorite-type crystals, we made the necessary corrections for the simulated melting temper- atures of SrF2 and BaF2. Consequently, the melting temperatures of SrF2 and BaF2 were obtained for high pressures. The melting temperatures of SrF2 and BaF2 that were obtained by the simulation are in good agreement with available experimental data.

Molecular dynamics of MgSiO3 perovskite melting

The melting curve of MgSiO3 perovskite is simulated using molecular dynamics simulations method at high pressure. It is shown that the simulated equation of state of MgSiO3 perovskite is very successful in reproducing accurately the experimental data. The pressure dependence of the simulated melting temperature of MgSiO3 perovskite reproduces the stability of the orthorhombic perovskite phase up to high pressure of 130GPa at ambient temperature, consistent with the theoretical data of the other calculations. It is shown that its transformation to the cubic phase and melting at high pressure and high temperature are in agreement with recent experiments.

Molecular dynamics simulation of Ni_3Al melting

With the Voter-Chen version of embedded-atom model （EAM） potential and molecular dynamics, the melting of Ni3Al alloy was simulated by one-phase （conventional） and two-phase approaches. It is shown that the simulated melting point is dependent on the potential and the simulation method. The structures of the melts obtained by different simulation methods were analyzed by the pair correlation function, the coordination number, and the distribution of atom pair type （indexed by the Honeycutt-Andersen pair analysis technique）. The results show that the structures are very similar.

Molecular Dynamics Investigation of Differences in Melting Behaviors of Cu57 and Cu58 Clusters

Within the framework of the embedded-atom method, we performed molecular-dynamics calculations to investigate the structural transformation during melting of two copper clus- ters containing 57 and 58 atoms. The simulation results reveal how their different structural changes can strongly influence internal energy and radial distribution functions. The local structural patterns of different regions during the temperature increase, determined by atom density profiles, are identified for the melting of each cluster. The simulations show sensi- tivities of the structural changes for these two small size clusters with different structures.

Molecular Dynamics Simulation of MgO Melting at High Pressure

Molecular dynamics simulation was used to study the melting of MgO at high pressures. The melting temperature of MgO was accurately obtained at elevated temperature and high pressure after corrections based on the modern theory of melting. The calculated melting curve was compared with the available experimental data and other theoretical results at the pressure range of 0-135 GPa. The corrected melting temperature of MgO is in good agreement with the results from Lindemann melting equation and the two- phase simulated results below 15 GPa.

Local structure of calcium silicate melts from classical molecular dynamics simulation and a newly constructed thermodynamic model

The distributions of local structural units of calcium silicate melts were quantified by means of classical molecular dynamics simulation and a newly constructed structural thermodynamic model. The distribution of five kinds of Si-O tetrahedra Qi from these two methods was compared with each other and also with the experimental Raman spectra, an excellent agreement was achieved. These not only displayed the panorama distribution of microstructural units in the whole composition range, but also proved that the thermodynamic model is suitable for the utilization as the subsequent application model of spectral experiments for the thermodynamic calculation. Meanwhile, the five refined regions mastered by different disproportionating reactions were obtained. Finally, the distributions of two kinds of connections between Qi were obtained, denoted as Qi-Ca-Qj and Qi-[Ob]-Qj, from the thermodynamic model, and a theoretical verification was given that the dominant connections for any composition are equivalent connections.

Ab Initio Two-Phase Molecular Dynamics on the Melting Curve of SiO_2

Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20-160 GPa and temperatures of 2 500-6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature （Tin） of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary （CMB） of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD （molecular dynamics） simulations. For liquid silica, the O-O coordination number is found to be 12 along the Tm and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10^-9-3.3×10^-9 m2/s, and the viscosity is 0.02-0.03 Pa＇s along the Tin. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.

Superheating and melting behaviors of Ag clusters with Ni coating studied by molecular dynamics and experiments

Using molecular dynamics with embedded-atom-type interatomicpotentials, we simulated the melting behavior of a spherical Ag3055 cluster coated with Ni. The semi-coherent Ag/Ni interface formed at low temperatures acts as an effective barrier against the surface melting and leads to a substantial superheating of the Ag cluster. The melting point was found to be about 100 K above the equilibrium melting point of the bulk Ag crystal (1230 K±15 K) and about 290 K above that (1040 K) of the free Ag3055 cluster. A superheating of 70 K was observed in the high-temperature differential scanning calorimetry measurement for Ag particles with a mean size of 30 nm embedded in Ni matrix prepared by means of melt-spinning. Melting is initiated locally at the defective interfacial area and then propagates inwards, suggesting a heterogeneously nucleated melting event at the Ag/Ni interface.