The microscopic mechanisms of femtosecond laser ablation of an Al target are investigated in large-scale massively parallel atomistic simulations performed with a computational model combining ...classical molecular dynamics technique with a continuum description of the laser excitation and subsequent relaxation of conduction band electrons. The relatively large lateral size of the computational systems used in the simulations enables a detailed analysis of the evolution of multiple voids generated in a sub-surface region of the irradiated target in the spallation regime, when the material ejection is driven by the relaxation of laser-induced stresses. The nucleation, growth, and coalescence of voids take place within a broad (
∼
100 nm) region of the target, leading to the formation of a transient foamy structure of interconnected liquid regions and eventual separation (or spallation) of a thin liquid layer from the bulk of the target. The thickness of the spalled layer is decreasing from the maximum of
∼
50 nm while the temperature and ejection velocity are increasing with increasing fluence. At a fluence of
∼
2.5 times the spallation threshold, the top part of the target reaches the conditions for an explosive decomposition into vapor and small clusters/droplets, marking the transition to the phase explosion regime of laser ablation. This transition is signified by a change in the composition of the ablation plume from large liquid droplets to a mixture of vapor-phase atoms and clusters/droplets of different sizes. The clusters of different sizes are spatially segregated in the expanding ablation plume, where small/medium size clusters present in the middle of the plume are followed by slower (velocities of less than 3 km/s) large droplets consisting of more than 10,000 atoms. The similarity of some of the characteristics of laser ablation of Al targets (e.g., evolution of voids in the spallation regime and cluster size distributions in the phase explosion regime) to the ones observed in earlier simulations performed for different target materials points to the common mechanical and thermodynamic origins of the underlying processes.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Short-pulse laser irradiation of a colloidal solution of nanoparticles is an effective method for fragmenting the nanoparticles and producing a population of smaller nanoparticles and atomic clusters ...with properties desired in various fields of applications, including biology, medicine, and catalysis. To investigate the mechanisms involved in the fragmentation, we develop a computational model capable of realistic treatment of a variety of interrelated processes occurring on different time and length scales, from the electronic excitation by the laser pulse, to the electron–phonon energy transfer and an explosive phase decomposition of the superheated nanoparticle, and to the generation and collapse of a nanobubble in a liquid environment. The application of the model to simulation of laser fragmentation of a Au nanoparticle in water has revealed two distinct channels of the formation of the fragmentation products. The first channel involves the direct injection of compact nanodroplets propelled by the phase explosion of the irradiated nanoparticle deep into the water environment. The second channel of the nanoparticle formation involves a more gradual growth through agglomeration of numerous atomic clusters embedded into a narrow region of water surrounding the laser-induced nanobubble. This channel produces irregularly shaped nanoparticles and leads to a rapid decline of the population of atomic clusters on the timescale of nanoseconds. All the clusters and nanoparticles experience an ultrafast quenching by the water environment and feature a high density of twin boundaries and other crystal defects, which may enhance the density of active sites for the catalytic applications of the nanoparticles. The computational predictions of the prompt generation of a high concentration of the fragmentation products in a relatively narrow shell-like region on the outer side of the nanobubble, as well as the rapid solidification of atomic clusters and nanoparticles at the early stage of the nanobubble formation, have important practical implications for the design of new methods aimed at achieving an improved control over the size, shape, and defect structures of nanoparticles produced by laser fragmentation in liquids.
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A clear understanding of carbon fiber (CF) microstructure is necessary for the development of high strength CFs. Here, we present an atomistic approach for generating and characterizing realistic ...microstructures of CFs. Large-scale reactive molecular dynamics simulations are used to generate a set of distinct CF microstructures. Comprehensive characterization of the simulated microstructures is enabled by the development of a suite of computational structural analysis tools capable of evaluation of hybridization states of carbon atoms, populations and orientations of individual carbon rings, degree of graphitization, and pore size distribution. The calculation of X-ray diffraction profiles provides a direct link between the structural features of simulated samples and experimental data available for CFs. The CF generation algorithm is shown to produce microstructures with experimental densities and with structural characteristics matching those of PAN-based CFs. The key structural features affecting the properties of CFs, such as the relative fractions of graphitic, turbostratic and amorphous microconstituents, degree of alignment, pore size distributions, and chemical cross-linking can be effectively controlled in simulations, thus enabling efficient exploration of structure–properties relationships in CFs. The capabilities of the developed approach are illustrated by performing computational analysis of the mechanical deformation and fracture of CFs under axial tensile loading.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The generation of colloidal solutions of chemically clean nanoparticles through pulsed laser ablation in liquids (PLAL) has evolved into a thriving research field that impacts industrial ...applications. The complexity and multiscale nature of PLAL make it difficult to untangle the various processes involved in the generation of nanoparticles and establish the dependence of nanoparticle yield and size distribution on the irradiation parameters. Large-scale atomistic simulations have yielded important insights into the fundamental mechanisms of ultrashort (femtoseconds to tens of picoseconds) PLAL and provided a plausible explanation of the origin of the experimentally observed bimodal nanoparticle size distributions. In this paper, we extend the atomistic simulations to short (hundreds of picoseconds to nanoseconds) laser pulses and focus our attention on the effect of the pulse duration on the mechanisms responsible for the generation of nanoparticles at the initial dynamic stage of laser ablation. Three distinct nanoparticle generation mechanisms operating at different stages of the ablation process and in different parts of the emerging cavitation bubble are identified in the simulations. These mechanisms are (1) the formation of a thin transient metal layer at the interface between the ablation plume and water environment followed by its decomposition into large molten nanoparticles, (2) the nucleation, growth, and rapid cooling/solidification of small nanoparticles at the very front of the emerging cavitation bubble, above the transient interfacial metal layer, and (3) the spinodal decomposition of a part of the ablation plume located below the transient interfacial layer, leading to the formation of a large population of nanoparticles growing in a high-temperature environment through inter-particle collisions and coalescence. The coexistence of the three distinct mechanisms of the nanoparticle formation at the initial stage of the ablation process can be related to the broad nanoparticle size distributions commonly observed in nanosecond PLAL experiments. The strong dependence of the nanoparticle cooling and solidification rates on the location within the low-density metal-water mixing region has important implications for the long-term evolution of the nanoparticle size distribution, as well as for the ability to quench the nanoparticle growth or dope them by adding surface-active agents or doping elements to the liquid environment.
The effect of the laser pulse duration on the nanoparticle generation in laser ablation in liquids is investigated; three mechanisms operating at different stages of the ablation process and in different parts of the cavitation bubble are identified.
Short pulse laser irradiation of metal targets can trigger a cascade of highly nonequilibrium processes leading to the formation of unique surface structures of interest to various practical ...applications. In this paper, we report the results of a large-scale atomistic simulation predicting the generation of a ∼200 nm long frozen nanospike on the surface of a Ag target irradiated by a femtosecond laser pulse. The simulation provides detailed information on the mechanisms responsible for the formation of the nanospike and the processes that define its complex nanostructure. The competing contributions of the epitaxial regrowth of the solid part of the target and the homogeneous nucleation of new crystallites triggered by the strong undercooling of the liquid regions are found to produce a remarkable variability of the structural motifs coexisting in different regions of the frozen nanospike. The homogeneous solidification, in particular, proceeds along two distinct paths selected at the nucleation stage and produces markedly different nanostructures in different parts of the nanospike, namely, nanograins with mixed fcc/hcp structure and a continuous network of pentagonal twinned structural elements arranged into a polyicosahedral structure.
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Laser ablation in liquids is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms of the laser ablation in ...liquids and the key processes that control the nanoparticle structure, composition, and size distribution, however, are not yet fully understood. In this paper, we report the results of first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of the liquid environment (parameterized for water), a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed and applied for simulation of laser ablation of a thin silver film deposited on a silica substrate. The simulations, performed at two laser fluences in the regime of phase explosion, predict a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface. The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of a cavitation bubble. Two distinct mechanisms of the nanoparticle formation are predicted in the simulations: (1) the nucleation and growth of small (mostly ⩽10nm) nanoparticles in the metal-water mixing region and (2) the formation of larger (tens of nm) nanoparticles through the breakup of the superheated molten metal layer triggered by the emergence of complex morphological features attributed to the Rayleigh-Taylor instability of the interface between at the superheated metal layer and the supercritical water. The first mechanism is facilitated by the rapid cooling of the growing nanoparticles in the supercritical water environment, resulting in solidification of the nanoparticles located in the upper part of the mixing region on the timescale of nanoseconds. The computational prediction of the two mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes is consistent with experimental observations of two distinct nanoparticle populations appearing at different stages of the ablation process.
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The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of ...laser excitation, electron−phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase explosion is signified by an abrupt change in the composition of the ejected plume (from liquid layers and/or large droplets to a mixture of vapor-phase atoms, small clusters and droplets), and results in a substantial increase in the duration of the melting process. In simulations performed with longer, 50 ps, laser pulses, when the condition for stress confinement is not satisfied, the spallation regime is absent and phase explosion results in smaller values of the ablation yield and larger fractions of the vapor phase in the ejected plume as compared to the results obtained with a 1 ps pulse. The more vigorous material ejection and higher ablation yields, observed in the simulations performed with the shorter laser pulse, are explained by the synergistic contribution of the laser-induced stresses and the explosive release of vapor in phase explosion occurring under the condition of stress confinement.
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The mechanisms of picosecond pulse laser ablation in liquid are investigated in a series of large-scale atomistic simulations performed for FeNi targets irradiated in a liquid environment by ...picosecond laser pulses at a broad range of fluences. The simulations reveal the existence of three fluence regimes featuring different dominant mechanisms of material ejection and nanoparticle formation. These are (1) the low fluence regime, where atomic clusters and small nanoparticles form through the evaporation of metal atoms followed by condensation in a low-density region at the front of the ablation plume, (2) the medium fluence regime, where roughening and decomposition of a top part of a transient spongy structure of interconnected liquid regions leads to the formation of large nanoparticles, and (3) the high fluence regime, where the nanoparticles form primarily at the phase separation front propagating through the ablation plume cooled from the supercritical state by expansion against the liquid environment and mixing with the liquid. The generation of the largest nanoparticles is observed in the medium fluence regime, and both the maximum size of the nanoparticles and the energy efficiency of the material conversion into nanoparticles decrease upon transition to the high fluence regime. Some of the nanoparticles experience extreme quench rates and rapidly solidify under conditions of deep undercooling, yielding a population of defect-rich nanoparticles of interest for practical applications. The results of the simulations are mapped to the conditions realized within a laser spot irradiated by a beam with a Gaussian spatial profile, where different ablation regimes are activated simultaneously in different parts of the laser spot. The spatially and time-resolved maps of the transient nonequilibrium states predicted in the simulations provide a comprehensive picture of the ablation dynamics and a solid foundation for interpretation of the results of time-resolved experimental probing of the initial stage of the ablation process.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Predicting thermal conductivity, k, of carbon nanotubes (CNTs) has been the focus of many molecular dynamics (MD) simulation studies reported in the literature. The values of k obtained in these ...studies exhibit a large, up to an order of magnitude, variability that is commonly attributed to the variations in the computational setups adopted in different studies. The sensitivity of the computational results to the choice of individual parameters of the simulation setups, however, has not been systematically investigated and is often overlooked when the predicted values of k are compared across the literature. Here we present the results of several series of simulations specifically designed to evaluate the effects of common computational parameters of non-equilibrium MD (NEMD), such as the type of boundary conditions, size and location of heat bath regions, definition of the CNT length, and the choice of interatomic potential, on the computational predictions. The length dependence of thermal conductivity is found to exhibit a gradual transition from a strong increase of k with CNT length for nanotubes that are shorter than ∼200nm to a much weaker dependence for longer CNTs, reflecting the transition from ballistic to diffusive-ballistic heat transport regimes. The effect of increasing length of thermal bath regions is found to be nearly indistinguishable from the effect of increasing length of the unperturbed region between the bath regions, suggesting that the value of k is defined by the total length of the CNT (including the length of the heat bath regions) in NEMD simulations employing uni-directional heat flux. The choice of interatomic potential is shown to be responsible for an up to fourfold variability in predictions of k for otherwise identical simulation conditions. Overall, the results of this study help elucidate the cause of quantitative discrepancies across published data and provide recommendations on the choice of simulation setups that may improve the consistency of the computational predictions.
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•Hydrodynamic simulations of CW laser melting are performed.•Only a marginal effect of the Marangoni stresses is revealed.•The recoil pressure effect is capable of strongly decreasing the ...melt-through time.•A simple one-dimensional model of CW laser melting is developed.•The model captures all qualitative trends revealed in the hydrodynamic simulations.
The relative contributions of evaporation and melt expulsion due to the recoil vapor pressure and Marangoni effects to the laser damage and melt dynamics in continuous wave (CW) laser interactions with free-standing aluminum films are evaluated in two-phase hydrodynamic simulations. In order the establish the dominant damage mechanisms in different irradiation regimes, the results of hydrodynamic simulations are compared with the predictions of several simplified models that only account for a subset of the involved processes. The hydrodynamic simulations performed in the range of film thickness from 0.2mm to 4mm and laser spot radius from 0.1mm to 1cm reveal only a marginal effect of the Marangoni stresses on the overall picture of melt flow and the melt-through time. The recoil pressure effect, on the contrary, is capable of strongly decreasing the melt-through time in a certain range of laser intensity. At laser intensities below this range the melting process is largely defined by heat transfer in the radial direction, while at laser intensities above this range the thickness of the molten pool and the efficiency of melt expulsion decrease and evaporation becomes the primary mechanism of material removal from the center of the laser spot. The range of laser intensities where the melt-through time is controlled by the recoil pressure effect is not unique and depends on the film thickness. A simple two-phase one-dimensional thermal model of laser melting, where melt expulsion due to the recoil pressure effect is accounted for based on the Bernoulli integral, is developed and found to be capable of accurate prediction of the melt-through time above a certain level of laser intensity. The one-dimensional thermal model captures all qualitative trends revealed in the direct hydrodynamic simulations and can be used as a robust engineering tool for the first-order estimation of the conditions for CW laser damage of metal films.
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