Nonequilibrium Molecular Dynamics Research Articles

Mechanical and Lattice Thermal Properties of Si-Ge Lateral Heterostructures.

Two-dimensional (2D) materials have drawn extensive attention due to their exceptional characteristics and potential uses in electronics and energy storage. This investigation employs simulations using molecular dynamics to examine the mechanical and thermal transport attributes of the 2D silicene-germanene (Si-Ge) lateral heterostructure. The pre-existing cracks of the Si-Ge lateral heterostructure are addressed with external strain. Then, the effect of vacancy defects and temperature on the mechanical attributes is also investigated. By manipulating temperature and incorporating vacancy defects and pre-fabricated cracks, the mechanical behaviors of the Si-Ge heterostructure can be significantly modulated. In order to investigate the heat transport performance of the Si-Ge lateral heterostructure, a non-equilibrium molecular dynamics approach is employed. The efficient phonon average free path is obtained as 136.09 nm and 194.34 nm, respectively, in the Si-Ge heterostructure with a zigzag and armchair interface. Our results present the design and application of thermal management devices based on the Si-Ge lateral heterostructure.

Thermal conductivity of boronated-holey graphene under mechanical strain: Insights from molecular dynamics

Non-equilibrium molecular dynamics is used to elucidate the thermal properties of 2D boronated-holey graphene (C2B) nanostructures under different mechanical strain levels. First, the thermal conductivity of strain-free monolayers was explored over a wide range of sheet width and length and system temperature. At room temperature, the thermal conductivity of C2B was calculated to be ∼40 W/(m.K) which was lower than its C2N counterpart with k = ∼60 W/(m.K).Next, the thermal conductivity of C2B (and C2N) monolayer was examined under both uniaxial and biaxial tensile strains. Interestingly, the thermal conductivity shows a maximum in both types of strains. Further examinations revealed that the rippling in C2B (C2N) decreases under applying strain up to 2 % (12 %) and 2–6% (4 %) for uniaxial and biaxial strains, respectively, which is responsible for the primarily enhanced thermal conductivities. Finally, a machine-learning model was applied to estimate the thermal conductivity of C2B at desired length and strain level.

An approach for tailoring the interfacial thermal conductance of copper-water nanofluids through ion additions and the underlying mechanism

Smaller Kapitza resistance is known to be the most crucial factor responsible for the larger thermal conductivity of nanofluids. But what the underlying mechanism is remains ambiguous. Non-equilibrium molecular dynamics simulations were performed in the present work to examine the effect of ionic addition on the Kapitza resistance of Cu-water nanofluids. The temperature profile indicates that the concentration and type of the additive slightly affects the thickness of the interfacial layer, but the temperature drops significantly. For concentrations of 1 M, the interfacial thermal conductance (ITC, the reciprocal of the Kapitza resistance) of Cu-water with NaCl, NaOH and HCl increases by 48.1 %, 29.2 % and 22.6 % respectively, compared to that without additives. The greatest ITC improvement of 123.7 % can be achieved by adding 3 M NaCl. The density profiles of Cu-water nanofluids with various additives suggest that the changes in the interfacial structure are primarily due to the positions of the added cations and anions. It is important to note, however, that the ITC is closely related to the orderliness of the interfacial layer and the matching of the phonon density of states, since the addition of NaCl can increase the PDOS at lower frequencies 1–5 THz, where that for the Cu nanoparticle appears. The findings provide a practical approach to tailor the thermal conductivity of nanofluids by the addition of appropriate ions.

Influence of perturbative intertube interactions on ballistic and quasi-ballistic phonon transports in double-walled carbon nanotubes

Assembling single-walled carbon nanotubes (SWCNT) into multiwalled carbon nanotubes (MWCNT) reduces the intrinsically high thermal conductivity of individual SWCNTs. Typically, this reduction can be explained by structural imperfections, e.g., defects, and van der Waals (vdW) intertube interactions between the constituent SWCNTs are considered to have negligible impacts on the reduction. However, intertube interactions should alter the transport characteristics of low-frequency phonons responsible for heat conduction, which implies that intertube interactions may also reduce the thermal conductivity of MWCNTs when low-frequency phonons modulated by intertube interactions participate in the overall heat conduction. In this study, by applying a combination of the atomistic Green's function method and nonequilibrium molecular dynamics simulation to double-walled carbon nanotubes (DWCNT) with different chiral configurations of SWCNTs and various lengths, we investigated the length scale and chiral configuration where perturbative intertube interactions can suppress phonon transports. We found that thermal resistance attributed to intertube interactions manifests in the low-temperature ballistic regime and quasi-ballistic regime for DWCNT lengths greater than 1 μm. Revisiting the influence of the intertube interactions helps comprehend the intrinsic origin of the reduced thermal conductivity of MWCNTs. In addition, the reported findings are beneficial for the thermal engineering of SWCNT-assembled materials, including emerging one-dimensional vdW heterostructures.

Phonon transport in vacancy induced defective stanene/hBN van der Waals heterostructure

In this study, Non-Equilibrium Molecular Dynamics (NEMD) simulation is employed to investigate the phonon thermal conductivity (PTC) of Sn/hBN van der Waals heterostructures with different vacancy-induced defects. We deliberately introduce three types of vacancies in Sn/hBN bilayer point vacancies, bivacancies, and edge vacancies at various concentrations ranging from 0.25% to 2%, to examine their effects on PTC across temperatures from 100 K to 600 K. The key findings of our work are (i) PTC declines monotonically with increasing vacancy concentration for all types of vacancies, with a maximum reduction of ∼62% observed at room temperature compared to its pristine form. (ii) The position of defects has an impact on PTC, with a larger decrease observed when defects are present in the hBN layer and a smaller decrease when defects are in the Sn layer. (iii) The type of vacancy also influences PTC, with point vacancies causing the most substantial reduction, followed by bivacancies, and edge vacancies having the least effect. A 2% defect concentration results in a ∼62% decrease in PTC for point vacancies, ∼51% for bivacancies, and ∼32% for edge vacancies. (iv) Finally, our results indicate that for a given defect concentration, PTC decreases as temperature increases. The impact of temperature on thermal conductivity is less pronounced compared to the effect of vacancies for the defective Sn/hBN bilayer. The presence of vacancies and elevated temperatures enhance phonon-defect and phonon–phonon scattering, leading to changes in the phonon density of states (PDOS) profile and the distribution of phonons across different frequencies of Sn/hBN bilayer, thus affecting its thermal conductivity. This work offers new insights into the thermal behavior of vacancy-filled Sn/hBN heterostructures, suggesting potential pathways for modulating thermal conductivity in bilayer van der Waals heterostructures for applications in thermoelectric, optoelectronics, and nanoelectronics in future.

How caged motion in the contact layer enhances thermal tunneling across a liquid/solid interface

Ever more powerful and densely packed chips for applications like cryptocurrency mining and artificial intelligence generate such enormous heat fluxes that designers are pivoting from gas to liquid cooling to forestall damage from thermal runaway. Even with optimal flow patterns, however, the intrinsic thermal boundary resistance at the liquid/solid (L/S) interface poses an additional source of thermal impedance. There is a lingering misconception in the field that the higher the liquid contact density, the more frequent the L/S collision rate and the smaller the thermal slip length. Here we present an insightful counterexample based on nonequilibrium molecular dynamics simulations of a simple liquid confined between two face centered cubic crystals at different temperatures aligned with the [001], [011] or [111] facet plane. Measurements of various static and dynamic quantities of the contact layer reveal the ways in which long-range order, anisotropy of the L/S potential, and correlated motion act to reduce thermal boundary resistance. Systems with the smallest thermal slip length exhibit two distinct features: 2D caged motion with stringlike alignment of liquid particles, unlike that observed in glassy systems, and larger nonergodicity parameter with shorter, not longer, caging times. This trapping and release mechanism suggests a paradigm for the design of L/S interfaces to maximize thermal exchange across a classical L/S interface. Published by the American Physical Society 2024

Thermal Rectification in Graphene-Boron Nitride Nanotube Hybrid Structures: An Independent Control Mechanism for Forward and Backward Heat Flux.

The weak van der Waals interactions in the out-of-plane direction result in markedly low thermal conductivity in one-dimensional (1D) and two-dimensional (2D) materials, which substantially restricts their applications. Developing three-dimensional (3D) columnar hybrid structures, featuring high thermal conductivity both within and beyond the plane, effectively addresses this challenge. This study investigated a 3D hybrid structure composed of graphene and boron nitride nanotubes (GR-BNNTs) using non-equilibrium molecular dynamics simulations. This approach allowed the examination of the formation mechanisms and key factors influencing thermal rectification (TR) in these materials. Our findings reveal a novel mechanism for independently regulating forward and backward heat fluxes in GR-BNNTs. By manipulating the thermal properties of the BNNTs and the graphene layer, the TR ratio can be controlled flexibly. Additionally, we identify specific strategies for independently adjusting the heat flux, such as altering the intercolumn distance of BNNTs, which impacts the backward flux merely, while applying strain to affect the forward flux merely. This research introduces a novel concept of independent regulation of forward and backward heat fluxes, providing significant insights into phonon thermal transport in 3D hybrid structures.

Molecular dynamics simulation on thermal and transport properties of LiF-KF

Molten fluoride is considered as an attractive heat transfer and storage medium in advanced energy utilization facilities, but it is short of the necessary thermal and transport properties at high temperature. In this paper, the thermal and transport properties of the binary LiF-KF mixtures at 1073–1473 K are obtained by classical molecular dynamics. The deviation of simulated thermal conductivity of binary LiF-KF at 1073 K from experimental value is less than 19 %. Moreover, the deviation of simulated viscosity of pure LiF at different temperatures from experimental value is around 10 %. It is indicated that reverse non-equilibrium molecular dynamics (RNEMD) is acceptable for describing fluoride salts. An exponential correlation is established between molar fraction of LiF and thermal conductivity at different temperatures. Furthermore, the macroscopic thermal and transport properties correlated with local structure are analyzed, especially the dependence of local structure on temperature and composition of LiF is discussed deeply. With temperature increasing, ion clusters are considered to be formed and overall structure become loose, which leads to reductions in density, thermal conductivity and viscosity. An increase in composition of LiF results in higher specific heat capacity, thermal conductivity and viscosity, which is related to the reduction of Li-Li bond length and the enhancement of interactions between F−. The comparison of properties enhancement of different compositions is completed through the analysis of the synergy coefficient of macroscopic properties. This work provides the basic data for the accurate design of high-temperature heat transfer system.

Going beyond an old shockwave conjecture for improving upon Navier-Stokes.

Nonequilibrium molecular dynamics (NEMD) computer simulations of steady shockwaves in dense fluids and rarefied gases produce detailed shockwave profiles of mechanical and thermal properties. The Boltzmann equation, under the assumption of local thermodynamic equilibrium (LTE), leads to the first-order (linear) continuum theory of hydrodynamic flow: Navier-Stokes-Fourier (NSF). (Expansion of the LTE Boltzmann equationin higher powers of gradients yields so-called Burnett second-order terms, etc.) NEMD simulations of strong shockwaves with high gradients are not well modeled by NSF theory. Many years ago, a conjecture for going "beyond Navier-Stokes" was proposed, applying the empirical observation of anisotropic thermal enhancement in the shock front to the temperature dependence of the NSF transport coefficients, whose dissipation determines the slope at the center of the shock profile: for weak shocks, the actual coefficients in NEMD simulations appear to be smaller than in NSF predictions, leading to steeper gradients being observed, while for strong shocks, the NEMD coefficients appear to be larger, leading to less steep shock rises than predicted by NSF calculations. In this paper, we show that adding significant Burnett nonlinearity into an LTE continuum theory reproduces the early shock rise and slope of NEMD profiles, for both weak and strong shocks in dense fluids, as well as strong shockwaves in the ideal gas. Moreover, we show that "Holian's conjecture" incorporates significant Burnett nonlinearity, but like all the other LTE continuum theories, it fails to describe the slow NEMD return to equilibrium beyond the shock front. We show that Maxwell relaxation has to be applied to the hydrodynamic variables themselves (rather than attempting indirect relaxation of their gradients) in order to more accurately model the entire shockwave profile. Non-LTE Maxwell relaxation is the only way to bring the entire profile into agreement with NEMD, most noticeably for strong shockwaves.

Differential study on the thermal–physical properties of metal and its oxide nanoparticle-formed nanofluids: Molecular dynamics simulation investigation of argon-based nanofluids

Abstract The influence of nanoparticle shape, volume fraction, and temperature on the thermal properties of nanofluids plays a pivotal role in engineering applications. However, there remains a considerable lack of systematic research comprehensively considering these factors to study the similarities and differences in the thermal properties of nanofluids composed of metals and their oxides and to conduct in-depth analyses of their internal mechanisms and characteristics. In this study, molecular dynamics simulations were conducted, employing reversing perturbation non-equilibrium molecular dynamics and non-equilibrium molecular dynamics methods. The thermal conductivity and viscosity of Al–Ar and Al2O3–Ar nanofluids were thoroughly investigated under the various influencing factors. Results reveal that under identical conditions, the thermal conductivity of Al–Ar nanofluid surpasses that of Al2O3–Ar nanofluid, exemplified by values such as 0.1832 W/m K (Al–Ar, 1.5%, cylinder, 86 K) versus 0.17745 W/m K (Al2O3–Ar, 1.5%, cylinder, 86 K). Furthermore, the viscosity of Al–Ar nanofluid is lower than that of Al2O3–Ar nanofluid, demonstrated by values such as 0.0004882 Pa S (Al–Ar nanofluid, 86 K, 2.5%, platelets) compared to 0.008975 Pa S (Al2O3–Ar nanofluid, 86 K, 2.5%, platelets). Subsequently, this study analyzed the difference in thermal conductivity between the two nanofluids from the perspective of microscale interface heat conduction by comparing the phonon density of states curves of Al, Ar, and Al2O3 in the two nanofluids for overlap. Subsequently, through radial distribution function analysis, the viscosity difference between Al–Ar and Al2O3–Ar nanofluids is explained based on nanofluid–solid interface and microstructural considerations. This research addresses the comprehensive lack of comparative studies on the thermal properties of nanofluids formed by metals and their oxides. The internal mechanisms underlying the thermal property differences of nanofluids formed by metals and their oxides were revealed from a microscopic perspective, which holds significant implications for the engineering applications of nanofluids.

Life and death of a thin liquid film

Thin films, bubbles and membranes are central to numerous natural and engineering processes, i.e., in solar cells, coatings, biosensors, foams, and emulsions. Yet, the characterization and understanding of their rupture is limited by the scarcity of atomic detail. We present here the complete life-cycle of freely suspended films using non-equilibrium molecular dynamics simulations of a simple atomic fluid free of surfactants and surface impurities, thus isolating the fundamental rupture mechanisms. We identified a short-term ‘memory’ by rewinding in time from a rupture event, extracting deterministic behaviors from apparent stochasticity. A comprehensive investigation of the key rupture-stages including both unrestrained and frustrated propagation is made—characterization of the latter leads to a first-order correction to the classical film-retraction theory. The highly resolved time window reveals that the different modes of the morphological development, typically characterized as nucleation and spinodal rupture, continuously evolve seamlessly with time from one into the other.

Mass transfer at vapor-liquid interfaces of H2O + CO2 mixtures studied by molecular dynamics simulation

Abstract In many industrial applications as well as in nature, the mass transfer of CO2 at vapor-liquid interfaces in aqueous systems plays an important role. In this work, this process was studied on the atomistic level using non-equilibrium molecular dynamics simulations. In a first step, a molecular model of the system water + CO2 was developed that represents both bulk and interfacial equilibrium properties well. This system is characterized by a very large adsorption and enrichment of CO2 at the vapor-liquid interface. Then, non-equilibrium mass transfer simulations were carried out using a method that was developed recently: CO2 is inserted into the vapor phase of a simulation box which contains a liquid slab. Surprising effects are observed at the interface such as a net repulsion of CO2 particles from the interface and a complex time dependence of the amount of CO2 adsorbed at the interface.

Thermomechanical Properties of Transition Metal Dichalcogenides Predicted by a Machine Learning Parameterized Force Field.

The mechanical and thermal properties of transition metal dichalcogenides (TMDs) are directly relevant to their applications in electronics, thermoelectric devices, and heat management systems. In this study, we use a machine learning (ML) approach to parametrize molecular dynamics (MD) force fields to predict the mechanical and thermal transport properties of a library of monolayered TMDs (MoS2, MoTe2, WSe2, WS2, and ReS2). The ML-trained force fields were then employed in equilibrium MD simulations to calculate the lattice thermal conductivities of the foregoing TMDs and to investigate how they are affected by small and large mechanical strains. Furthermore, using nonequilibrium MD, we studied thermal transport across grain boundaries. The presented approach provides a fast albeit accurate methodology to compute both mechanical and thermal properties of TMDs, especially for relatively large systems and spatially complex structures, where density functional theory computational cost is prohibitive.

Molecular dynamics study on the thermal properties of DGEBA/DETA/Ag/SWCNT-Ag composite materials.

Traditional conductive adhesives based on epoxy resin system often encounter problems such as high brittleness and low heat resistance. Therefore, it is particularly important to improve the thermal and mechanical properties of the conductive adhesive. In this study, the effects of SWCNT-Ag and SWCNT fillers on the thermal properties of DGEBA/DETA/Ag conductive adhesive system were studied by using molecular dynamics to construct different cross-linking models. The final results show that the addition of SWCNT and SWCNT-Ag can significantly improve the thermal properties of the conductive adhesive. However, the nanosilver particles on the surface of SWCNT-Ag act as a bridge for the connection between SWCNT and Ag in the conductive adhesive. Therefore, SWCNT-Ag has a more positive impact on the thermal properties of DGEBA/DETA/Ag conductive adhesive system. In this paper, the influence of SWCNT-Ag on the thermal properties of traditional DGEBA/DETA/Ag conductive adhesive system was studied by using Materials Studio software. The volume shrinkage, glass transition temperature, thermal expansion coefficient, and thermal conductivity of the material were calculated based on COMPASS force field. The thermal conductivity is calculated by using reverse non-equilibrium molecular dynamics method. Finally, it is found that SWCNT-Ag has a positive effect on the thermal properties of the conductive adhesive system by comparing several groups of calculation data.

Molecular simulations of organic solvent transport in dense polymer membranes: Solution-diffusion or pore-flow mechanism?

We employed non-equilibrium molecular dynamics simulations and theoretical predictions to elucidate organic solvent and water transport mechanisms in dense polymer membranes. Our study investigated the transport of water and 14 organic solvents across 16 dense polymer membranes with varying fractional free volumes. Simulations demonstrated that the dependence of organic solvent or water flux on applied pressure exhibited minimal deviation from linearity, contrary to the prediction of a “ceiling flux” at high pressures, based on the solution-diffusion model. Notably, our findings underscore the paramount influence of solvent size and membrane pore size on solvent permeance, significantly diverging from the solvent solubility-based predictions of the solution-diffusion model. Overall, our results agree with the solution-friction model for characterizing water and organic solvent transport in dense polymer membranes such as reverse osmosis and organic solvent nanofiltration. These fundamental insights into the transport mechanisms of water and organic solvents offer molecular-level guidance for designing next-generation high-performance polymer membranes for various separation applications.

One-Dimensional van der Waals Heterojunction Comprising Carbon Nanotube Half-Wrapped in Boron Nitride Nanotube: Deep Investigation of Thermal Rectification.

One-dimensional van der Waals (vdWs) heterostructures are celebrated for their exceptional thermal management capabilities, garnering significant research interest. Consequently, our research focused on the one-dimensional vdWs heterojunction comprising carbon nanotube half-wrapped in boron nitride nanotube (BNCNT), specifically their thermal rectification (TR) properties. We employed non-equilibrium molecular dynamics to explore the TR mechanism and assess the impacts of temperature, strain, and coupling strength on heat flux and TR ratio. Our findings reveal that the backward heat flux demonstrates greater atomic vibration instability, as indicated by mean square displacement (MSD), compared to forward heat flux. This instability leads to a higher concentration of localized phonons, thereby diminishing the backward heat flux and enhancing TR. Additionally, we utilized MSD to shed light on the negative differential thermal resistance phenomenon and the influence of stress on forward and backward heat fluxes. Remarkably, TR ratios reached 344% at 3% strain and 400% at -1% strain. Calculations of phonon density of states revealed a competitive mechanism between in-plane and out-of-plane phonons coupling in the inner carbon nanotube and an overlap degree of out-of-plane phonon spectra between the inner carbon nanotube and outer boron nitride nanotube. This accounts for the differing trends in forward and backward heat fluxes as coupling strength χ increases, with TR ratios exceeding 1000% at χ = 7.5. This study provides vital insights for advancing one-dimensional vdWs thermal rectifiers.

Effects of interlayer spacing and applied pressure on the lanthanide transport in MoS2-based two-dimensional channels

ABSTRACT Rare-earth elements (REEs) are critical to modern industry but difficult to separate due to their subtle and monotonic changes in physicochemical properties. MoS2-based two-dimensional (2D) materials offer novel opportunities for enhancing REE separation, exhibiting a distinct volcano-shaped transport performance distribution that peaks at Sm3+. However, the specific contributions of thermodynamic and kinetic factors to ion transport within 2D confinement remain unclear. In this study, we conducted a series of non-equilibrium all-atom molecular dynamics (MD) simulations to explore the effects of interlayer spacing and external pressure on the transport of lanthanide ions in Å-scale acetate functionalized 2D MoS2 (MoS2-COOH) channels. We examined ion entry and permeation rates, water flux, dehydration, and binding modes. The simulation results reveal that the transport trends of lanthanide ions are jointly driven by the dehydration degree and the relative-binding strengths of ions to water and to the acetate within the 2D channels. Notably, the dehydration pattern of lanthanide ions during permeation is closely linked to kinetic factors. Overall, this study provides a detailed atomistic understanding of the mechanisms underlying lanthanide ion transport under confinement. These findings point to the significant potential for tuning confinement and chemical functionalization within Å-scale channels for more efficient REE separation.

Chemical heterogeneity size effects at nanoscale on interface thermal resistance of solid–liquid polymer interface via molecular dynamics simulations

The shrinking size of integrated chips poses thermal management challenges. Understanding the size effect of chemical heterogeneity on solid–liquid interfacial thermal transfer is essential for heterogeneous chip design, yet the underlying mechanisms remain lacking. The present work used the liquid n-alkanes as the thermal interface material between solid platinum substrates. To characterize chemical heterogeneity, periodic solid surface patterns composed of patches with alternating solid–liquid affinities were constructed. By using non-equilibrium molecular dynamics simulations, we investigated the size effect of chemically heterogeneous patterns on interfacial thermal resistance (ITR) at the nanoscale. At larger heterogeneity sizes, i.e., larger patch sizes, most alkane molecules directly in contact weak interaction patches cannot interact with strong interaction patches due to long atomic distances. In the case of alkanes in contact a cold substrate, alkanes in contact weak interaction patches transferred thermal energy to the substrate at a lower rate than those in contact strong interaction patches. The different rates resulted in the higher temperature of alkanes in contact weak interaction patches than those in contact strong interaction patches and, therefore, a larger disparity between temperature jump at the strong interaction areas and that at the weak interaction areas. The non-uniformity of temperature jump distribution increased ITR when compared to the heterogeneous surface system characterized by a smaller patch size with a more uniform temperature distribution in the plane perpendicular to the heat flux direction. In addition, the classical parallel thermal resistance model predicted ITR accurately for the heterogeneous surface systems with small size patches but overestimated overall thermal resistance.

Rheological behaviors of Na-montmorillonite considering particle interactions: A molecular dynamics study

Understanding the rheology of bentonite suspensions is crucial for ensuring the safety of engineering practices. However, the rheological mechanisms of bentonite remain unclear due to the limitations of conventional experimental techniques, particularly in assessing the microscopic interactions between clay particles and their impact on rheological properties. In this paper, the rheological behaviors of Na-montmorillonite were studied with a focus on interparticle interactions. Both equilibrium molecular dynamics (MD) and non-equilibrium MD simulations were conducted to understand the physical properties of Na-montmorillonite under zero shear and various shear rates, respectively. The interaction between two parallel clay particles was determined in simulations, indicating that the classical Darjaguin-Landau-Verwey-Overbeek (DLVO) theory underestimates the interactions for a small separation distance. Na-montmorillonite exhibits a typical shear thinning behavior under shearing. However, as water content increases, it begins to behave more like liquid water. The yield stress of montmorillonite, as determined by the Bingham model, was found to be linearly related to the interaction pressures between clay particles. Besides MD simulations, the microstructure of clay suspension was further quantified using the separation distance and incline angle between non-parallel clay particles. Based on MD results and the quantified clay structure, a model was developed to estimate the yield stress of montmorillonite considering various influence factors, including electrolyte concentration, temperature, and solid fraction. Finally, from a comparison with calculated and experimental data, the results confirm the good performance of the proposed model. These findings provide significant insights for understanding the rheological soil behaviors and evaluating the yield stress of bentonite suspensions.

Orientation-dependent multi-spall performance of monocrystalline NiTi alloys under shock compression

The primary objective of this work was committed to large-scale non-equilibrium molecular dynamics modeling for the investigation on multiple spall performance, microstructural evolutions, and melting state of NiTi alloys along three typical crystal orientations. We discovered that there was a single-wave structure present in the [100] orientation, whereas elastic-plastic two-wave structures were found in [110] and [111], exhibiting a significant anisotropy in elastic-plastic wave velocity. Compared to [110] and [111] orientations, multi-spallation was more likely to be generated in the [100] direction. During accumulative dynamic damage, there was also a strong sensitivity of void evolution to orientation. For [100], voids tended to be densely distributed in the material and exhibited a strip-shaped manner. In contrast, a more scattered void distribution was observed for the other two orientations, showing a flaky pattern. Interestingly, dislocations in the spall region were almost annihilated before approaching void nucleation. According to void statistics, the peak number of voids, void nucleation rate, and spall region size were identified to be in a good uniformity along [110], [100], and [111]. For all the cases, the samples experienced partial melting in the released state and were followed by a recovery of structural order degree. However, there was an insignificant anisotropy in the melting state for each crystal orientation during multi-spallation.