Al2O3 Nanoparticles to Heating: ReaxFF

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C: Physical Processes in Nanomaterials and Nanostructures 2

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Responses of Core-Shell Al/AlO Nanoparticles to Heating: ReaxFF-Molecular Dynamics Simulations Huadong Zeng, Xin-Lu Cheng, Chaoyang Zhang, and Zhipeng Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01088 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Responses of Core-Shell Al/Al2 O3 Nanoparticles to Heating: ReaxFF-Molecular Dynamics Simulations HuaDong Zeng,† XinLu Cheng,∗,† ChaoYang Zhang,‡ and ZhiPeng Lu∗,‡,¶ †

Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 91–311, Mianyang 621900, China ¶ Department of Mathematics and Physics, Officers College of CAPF, Chengdu 610213, China ‡

Supporting Information ABSTRACT: Molecular dynamics simulations combined with the ReaxFF reactive force field were implemented to detailedly study the atomic diffusion behaviors of core-shell Al/Al2 O3 nanoparticles. According to atomic mean square displacements, the reaction initialization of core-shell Al/Al2 O3 nanoparticles substantially resulted from the inward diffusion of shell oxygen atoms. In particular, the effect of shell thickness on the atomic diffusivities of the system was investigated. The results demonstrated that the diffusivities of core Al atoms and shell O atoms at the core-shell interfaces were irrelevant to the shell thickness during heating process; however, corresponding atomic diffusivities decreased as increasing the shell thickness after heating. A majority of distorted (AlO)n (n = 3, 4, 5) clusters were ejected from the system surface at later stages, suggesting that the detonation of the nanoparticles. The presence of a significant void space was observed when the alumina shell melted, which was in agreement with the experimental evidence. The alumina shell with 1 nm melts at 1153 K, and the melting point enhances with the augment of shell thickness. Furthermore, the electric field induced atomic diffusion mechanisms of core-shell Al/Al2 O3 nanoparticles are obtained as is reported, further providing extensive insights into the ignition mechanisms of passivated aluminum nanoparticles.

1. INTRODUCTION

Over the years, aluminum nanoparticles have attracted widespread attention in the scientific community due to remarkable advantages in high energy density, high reactivity, high combustion temperature, relative safety and low cost, and thereby extensively applied in both industrial and military fields such as solid rocket propellants, explosives and nanothermites, and so forth. 1–5 As a consequence, the ignition and combustion mechanisms of aluminum nanoparticles have been constantly characterized by studying the melting or oxidation of aluminum nanoparticles. Enormous experimental and theoretical researches have been reported in recent years. 6–10 For instance, Eckert et al. 11 exploited differential scanning calorimetry and transmission electron microscopy to measure the melting behaviors of nanocrystalline aluminum powders and shown that the melting temperature for a grain size of 13 nm was reduced to 836 K, in compared to the value of 933 K for bulk Al. The oxidation behaviors of ultrafine grain aluminum powders with average particle diameters from 24 to 65 nm were conducted by Aumann et al. 12 They disclosed that the activation energy for oxidation of aluminum nanopowders decreased by about 70% as compared with that of flat aluminum samples. Coulet and coworkers 13 experimentally studied the oxidation mechanisms of aluminum nanopowders under air at different temperatures and proposed a schematically two-step oxidation mechanism. On the other hand, Campbell et al. 14,15 first utilized molecular-dynamics (MD) approach to simulate the oxidation process of aluminum nanoclusters and uncovered that large negative pressure in the oxide derived from charge transfer took shape an amorphous oxide layer with 4 nm thick. Puri and Yang 16 employed MD method to explore the effect of particle size on the melting point of nano-sized aluminum particles. ReaxFFMD simulations were performed by Hong and van Duin 17 in order to research the oxidation of aluminum nanoparticles. The results shown that the hot-spots and high-temperature regions were re-

sponsible for the oxidation mechanisms of pure aluminum nanoparticles. However, a passivation oxide layer (i.e., Al2 O3 ) is typically formed on the surface of aluminum nanoparticles at oxygen environment. Commonly, the alumina film thickness was constant to about 0.5–1.0 nm for small aluminum nanoparticles over the range of 4–12 nm. 18–20 On the other side, the alumina film thickness of larger aluminum nanoparticles was about 2–4 nm at room temperature. 21–23 Consequently, plenty of researchers have focused on the ignition and combustion mechanisms of Al2 O3 -coated aluminum nanoparticles. At present, many scientists have summarized two distinct explanations for the oxidation and combustion mechanisms of aluminum nanoparticles or nanoclusters on account of those experimental phenomena and observations. Firstly, diffusion oxidizer mechanism (DOM) was chiefly presented by Park 24 and Rai 25,26 for the ignition mechanisms of aluminum coated alumina nanoparticles. When the reaction temperature is less than the melting point of aluminum, oxygen atoms pass through the alumina shell to spread inwardly, resulting in a relatively slow oxidation reaction. Subsequently, aluminum atoms diffuse through the shell to boost the fast oxidation of aluminum nanoparticles on the ground that the melting point of aluminum core and some transition of alumina shell. Secondly, Levitas et al. 27–29 put forward the melt dispersion mechanism (MDM) for the fast reaction of aluminum nanoparticles. Under high heating rate (106 –108 K/S), the variation of volume caused by the melting of aluminum core gives rise to a pressure build-up of 0.1–4.0 GPa and dynamic spallation of the oxide shell. Then, plentiful aluminum nanoparticles are unloaded as some small liquid clusters due to the high tensile pressures. In contrast, Trunov et al. 21,30 ascertained that the ignition of aluminium nanoparticles were primarily attributed to polymorphic phase transformations in the alumina layer. Even so, in virtue of the limitation of the spatiotemporal resolution in experimental apparatus and violently chemical reaction,

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the microscopic melting or oxidation or burn process of aluminum nanoparticles cannot be real-time observed to date. As a result, it is a great controversial issue with respect to the oxidation and combustion properties of aluminum nanoparticles. To the best of our knowledge, there also have been several theoretical studies with regard to the dynamics melting or oxidation process of passivated aluminum nanoparticles. For example, MD simulations were carried out by Li et al. 31 to examine the size effect on the oxidation of nanometer sized aluminum. Puri and Yang 32 surveyed the thermo-mechanical behaviors of aluminum nanoparticles with oxide layers. In addition, Hwang et al. 33 studied the numerically superheating temperatures and melting behaviors of aluminum core-oxide shell nanoparticles among a broad range of heating rates. Reactive MD simulations were executed by Chakraborty et al. 34 to elaborate the quantitative effect of nano-sized aluminum sintering on a time scale as compared to the characteristic reaction time. However, the underlying reaction mechanisms of passivated aluminum nanoparticles are remain rather scattered because of the inconsistency and uncertainties of multifarious existing theories. Therefore, the goal of this work is to theoretically provide much useful information about the atomic diffusion behaviors of core-shell Al/Al2 O3 nanoparticles by means of the reaction simulation of core-shell Al/Al2 O3 nanoparticles with the size range of 6–8 nm based on the classic MD simulations with the use of the ReaxFF potential. Particularly, the effect of the alumina shell thickness on the reaction characteristics was roundly investigated by a combination of the time evolutions of various physical quantities involving the atomic configuration, mean square displacement (MSD), radial distribution functional (RDF), coordination number and space charge distribution among atoms and so on.

taining 2334 atoms and a nano-crystalline Al2 O3 shell possessing three different thicknesses (δ = 0.6, 0.8 and 1.0 nm) in order to research the effect of shell thickness on the reaction characteristics of core-shell Al/Al2 O3 nanoparticles. Using the alumina shell with 1.0 nm as an example, the total diameter of coreshell Al/Al2 O3 nanoparticles is on the order of 6.6 nm with 14813 atoms. The space intervals between Al core and Al2 O3 shell were chosen to be about 0.3 nm as plotted in Figure 1. In this investigation, All MD simulations adopted the canonical ensemble with the nonperiodic boundary condition and a time-step of 0.2 fs. The considered system was relaxed at 300 K for equilibration. The core-shell Al/Al2 O3 nanoparticles was then heated up to the initial temperature of 2000 K at the heating rate of 100 K/ps. The Nose/Hoover thermostat scheme 43,44 was carried out for this simulation procedure. In addition, a temperature damping constant for the Nose/Hoover thermostat was set as 20 fs. For the sake of analyzing the structural evolutions and reaction mechanisms of core-shell Al/Al2 O3 nanoparticles after heated, the targeted temperature was held at 2000 K for another 100 ps. All the MD simulations were carried out on the high performance computing resources provided by National Supercomputer Center in Tianjin, using 12-144 processor cores.

2. COMPUTATIONAL DETAILS

Figure 1. The snapshots of the initial structures of core-shell Al/Al2 O3 nanoparticles. (a) Cross-sectional diagram; (b) three-dimensional stereogram. The yellow and blue spheres represent Al and O atoms, respectively.

The whole mimic computations are performed by the largescale atomic/molecular massively parallel simulator (LAMMPS) 35 based upon classic molecular dynamics simulations in association with the ReaxFF reactive force field proposed by van Duin et al. 36 The ReaxFF is used not only to correctly elaborate the relationship between bond distance and bond order, but also to expound the correlativity between bond order and bond energy. The ReaxFF potential has the outstanding advantage of being able to accurate describe the formation and dissociation of dynamic bonds and charge transfer among atoms in the chemical systems. 37 Hence, The ReaxFF parametrizations for O/Al are extracted from Shin et al. 38 and Rahaman et al. 39 The general expression for total energy E system in the ReaxFF potential can be formulated by the equation 1. Esystem =Ebond + Eover + Eunder + Elp + Eval + Etors + EvdWaals + ECoulomb

(1)

where the total energy term, E system , contains E bond denoted the bond energy, E over denoted the energy penalty for overcoordinated atoms, E under denoted the energy contribution for undercoordinated atoms, E lp denoted the lone-pair energy, E val denoted the threebody valence angle term, E tors denoted the four-body torsion term, E vdwaals denoted the van der Waals interactions, and E Coulomb denoted the Coulomb interactions. The changing atomic charges are counted using the Electronegativity Equilibration Method (EEM) approach as described by Mortier et al. 40 and Janssens et al. 41 with a view to the Coulcomb interactions. In addition, the polarization and charge transfer effects are taken into account in the formulation. The concrete details of the ReaxFF potential are formulated in van Duin and partners. 36,42 The core-shell Al/Al2 O3 nanoparticles are composed of a nanosized fcc-crystalline aluminum core with a diameter of 4 nm con-

3. RESULTES AND DISCUSSIONS

At first, to explore deeply the atomic diffusion mechanisms of coreshell Al/Al2 O3 nanoparticles, we calculate the temporal evolutions of atomic MSDs at the core-shell interfaces for δ = 1.0 nm as shown in Figure 2. An annular region is plotted out at the coreshell interfaces of Al/Al2 O3 nanoparticles. Then, we mark out the commonable aluminum atoms between the annular region and the aluminum core region, i.e., gbcal. Meanwhile, we also mark out the commonable aluminum and oxygen atoms between the annular region and the alumina shell region, i.e., gbsal and gbso, respectively. The MSD of shell O atoms is far greater than that of core Al atoms at the beginning of the simulated reaction. Based on these data, the atomic diffusion coefficients D are computed by using the equation 2. ∂< r2 (t) > = 6D (2) ∂(t) where t is the elapsed time, and < r2 (t) > is the atomic MSD. The diffusion coefficient of shell O atoms is 6.04 × 10−5 cm2 /s at 50 ps, which is larger than that of core Al atoms with 4.53 × 10−5 cm2 /s. Thus, those results manifest that the initialization of chemical reaction of the system fundamentally derives from the rapid inward diffusion of shell O atoms. This is in great accordance with the available experimental literature. 26 However, It demonstrates a dramatic change that the diffusion coefficient of core Al atoms markedly increases to 2.42 × 10−4 cm2 /s at 80 ps, while that of shell O atoms is almost unchanged. We deduce that the qualitative variation is mainly attributed to the vast out-migration of core Al


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The Journal of Physical Chemistry atoms driven by the electric field discussed next. Additionally, the diffusivity of shell aluminum atoms also enhances visibly as a result of the clusters sputtering latterly. In order to further study the effect of shell thickness on the atomic diffusivities of the system, Figure 3 exhibits the temporal evolutions of atomic MSDs at the core-shell interfaces for different shell thicknesses (δ = 0.6, 0.8 and 1.0 nm). Analyses of the figure on the top conclude that the atomic MSDs of core Al atoms both are consistent for three shell thicknesses during the heating process, suggesting that the diffusivity of core Al atoms is irrelevant to the shell thickness presumably due to the simulated time constraint and small-sized nanoparticles; but, as the shell thickness increases, the atomic MSDs of core Al atoms sharply decreases after 20 ps. In other words, the diffusion coefficient of core Al atoms drops from 1.75 × 10−4 cm2 /s for δ = 0.6 nm to 1.12 × 10−4 cm2 /s for δ = 0.8 and 1.0 nm. Similar to core Al atoms, the diffusivity of shell O atoms reduces with increasing the shell thickness from the bottom of Figure 3, since the thicker shell has more obstructive influence on the diffusivity of shell O atoms.

Figure 3. The time evolutions of the mean square displacements of (a) core Al atoms and (b) shell O atoms at the core-shell interfaces for different shell thicknesses.

Figure 2. The time evolutions of atomic mean square displacements at the core-shell interfaces for δ = 1.0 nm. Black, red and blue lines represent gbcal, gbsal and gbso, respectively.

In the MD simulations, the nanostructured evolutions of coreshell Al/Al2 O3 nanoparticles can contribute to clearly understand the reaction characteristics and even related phase transformations. Figure 4 displays the snapshots of the reaction process of core-shell Al/Al2 O3 nanoparticles for δ = 1.0 nm at different reaction times. Following equilibration, the relaxed atomic configuration is listed in Figure 4a. After imitating to 5.2 ps, in consequence of the sudden inflexion of the potential energy curve and structural disorders seen from Figure 4b, it is observed that nanoaluminum core melts with the melting temperature of 730 K, which is in line with the experimental and theoretical works. 7,16,45 When heating to 9.5 ps, the occurance of the melting of alumina shell with the value of ca. 1153 K, is also received as expressed in Figure 4c. This is lower than the experimental melting point of bulk alumina shell (2350 K). Nevertheless, several theoretical literatures 32,34 reported that the melting point of aluminum oxide for small particle size also belows that of bulk alumina shell. At that moment, Chakraborty and Zachariah presume that the shell is no longer alumina but a sub-oxide of aluminum. Interestingly, as the simulated temperature increases an apparent void space is formed near the center, which is in accord with the previous observation. 46 One possible explanation is that the rapid outward diffusion of core Al atoms. The void space is made up of multiple adjacent smaller cavities in favour of explaining its net volume. Figure 4d displays that the system surface is beginning to eject at the 50 ps. In a later stage of the simulation, Figure 4e shows that a considerable number of nanoclusters in form of distorted (AlO)n (n = 3, 4 and 5) sputter from the surface of the

system, stating clearly that the nanoclusters is detonating. As Lynch et al. 47 reported experimentally, AlO clusters was detected in absorption during nano-sized aluminum combustion at temperatures as low as 2000 K. What’s more, Lam et al. 48 found that (AlO)n cluster is the most preponderant structure for high temperature. As previously mentioned, the local structures of the molten shell is different from alumina and remain ambiguous. Motivated by this, we compute and analyze the radial distribution functions g(r) of the system at different times. On account of providing the local atomic arrangement, g(r) is frequently used as an approach to discern between solids and liquids. Figure 5 shows the radial distribution functions of Al-Al bond in the aluminum core region for δ = 1.0 nm at different times. A long-range order pattern is characterized by a repeating sequence of sharp peaks after relaxed, signifying that the aluminum core is a crystalline solid. The position of the first peak gives the first nearest neighbor distance among Al atoms to be around 2.85 Å. The corresponding coordination number for Al in the core region is 11. The results are basically matching to that of fcc Al lattice. In the case of 5.2 ps, we catch sight of a no long-range order pattern and apparently broad peaks, indicating that the aluminum core is melted, which corroborates the preceding results, and exists in the liquid state (shown in S2 of Supporting Information). The first nearest neighbor distance among Al atoms becomes large with the value of 2.95 Å. The corresponding coordination number for Al in the core region is 10.5. Furthermore, we also calculate the atomic radial distribution functions of Al-O and O-O bonds in the alumina shell region at different times for δ = 1.0 nm as provided in Figure 6. For the g(r) of Al-O bond in the shell region, the top of Figure 6 shows that the first peak locates 1.75 Å after relaxed and the corresponding coordination number for Al is 4.2 with a similar value to the previous works. 8,49 For 9.5 ps, a no long-range order pattern is observed, which illustrates the shell al-



Figure 4. The snapshots of the structural evolutions of core-shell Al/Al2 O3 nanoparticles for δ = 1.0 nm. (a) Relaxed configuration; (b) 5.2 ps; (c) 9.5 ps; (d) 50 ps. (e) 80 ps

so has melted. The first nearest neighbor distance among Al atoms remains 1.75 Å, but the corresponding coordination number for Al becomes 3.6. For the g(r) of O-O bond in the shell region as listed in the bottom of Figure 6, the first peak locates 2.85 Å after relaxed, and the corresponding coordination number for O is 11.5. In the case of 9.5 ps, the first peak shifts slightly toward larger r and is not periodic. The first nearest neighbor distance among Al atoms is 2.95 Å and the corresponding coordination number for O becomes 10.2. Therefore, the results allow us to assume that the possible sub-oxide are composed of mixed AlO4 tetrahedra and AlO6 octahedra configurations; this is similar to the available literatures. 50,51

Figure 6. The radial distribution functions of Al-O and O-O bonds in the shell region at different times for δ = 1.0 nm.

Figure 5. The radial distribution functions of Al-Al bond in the core region at different times for δ = 1.0 nm.

In addition, a variation tendency of shell thickness dependence of the melting of aluminum core and alumina shell is obtained by means of the sudden jumps of the potential energy curves and atomic radial distribution functions of the systems (shown in S3 of Supporting Information). Figure 7 shows the variation of melting points as a function of the shell thickness. It can be seen that that the shell thickness has no effect on the melting temperature of aluminum core with the settled value of 730 K, which relatively error is less than 5%. However, as the shell thickness increases, the melting temperature enhances. The alumina shell with 1 nm melts at approximately 1153 K. Subsequently, the atomic diffusion mechanisms of core-shell Al/Al2 O3 nanoparticles are studied. As is well-known, the kinetics of growth of thin oxide films on metal crystals are described commendably by the conventional Cabrera-Mott model, especially in the nascent oxidation process of metal particles. 52 To this end, Coulomb’s Law is used to quantitatively compute the electric field at the core-shell interfaces through the equation 3. E=

1 Qint re 4πε0 r2

(3)

Figure 7. The variation of melting points as a function of the shell thickness.

where E represents the electric field at the core-shell interfaces, ε0 represents this permativity constant of a vacuum, Qint represents the total charge at the core-shell interfaces region, r represents the radial position of the interfacial aluminum atom, and re represents the radial unit vector. The total charge at the core-shell interfaces and associated electric field are calculated for all the considered sys-


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The Journal of Physical Chemistry tems during the different reaction times (shown in S2 of Supporting Information). In the case of δ = 1.0 nm from Table 1, we notice that the associated electric field drops with simulated time development. Above all, the associated electric field euqal zero when heating to 2000 K. Therefore, the results mentioned above allow us to conjecture that the associated electric field account for the inward diffusion of shell O atoms and the outward diffusion of core Al atoms. Furthermore, we analyze the space charge distributions of the system for δ = 1.0 nm at different stages as listed in Figure 8. After equilibrium, we observe from Figure 8a that there exists an electric field in the alumina shell and presents neutral in the aluminum core. The reaction is initiated on the ground that shell oxygen atoms diffuse into the aluminum core and combine with core aluminum atoms to form Al-O bonded interfaces, resulting in that the neutral aluminum atoms is positively charged and then promote the outward diffusions of core aluminum atoms at the core-shell interfaces. As simulated reaction processes, the electric potential difference of the system decreases (see Figure 8b). Despite mostly retaining spherical shape, the core-shell Al/Al2 O3 nanoparticles are fused together when the shell has melted and the electric potential difference of the system approaches zero at the moment as listed in Figure 8c and d. These observations could have important implications that the electric field induced atomic diffusions of shell O atoms and core Al atoms. Table 1. The total charge at the core-shell interfaces and associated electric field are given here for the system of δ = 1.0 nm during the different reaction times. Ts (ps)

Tm (K)

Qint (C*10−19 )

E (N/C*109 )

10

0

300

9.73

1.98

10

5.2

730

2.07

1.51

10

9.5

1153

0.21

1.21

10

18

2000

0

0

10

50

2000

0

0

δ (Å)

core-shell Al/Al2 O3 nanoparticles by means of molecular dynamics simulations method in integration with the ReaxFF reactive force field. On the basis of the atomic mean square displacement, the results obviously confirmed that the reaction initialization of coreshell Al/Al2 O3 nanoparticles mainly derived from the inward diffusion of oxygen atoms. The concentration was taken into account in exploring the effect of shell thickness on the reaction characteristics of core-shell Al/Al2 O3 nanoparticles. A combination of the temporal evolutions of the atomic configuration, mean square displacement, radial distribution function and atomic charge distribution was concretely investigated. Analyses of those calculations testified that the diffusivities of core Al atoms and shell O atoms at the core-shell interfaces were not relevant to shell thickness during heating process. However, the corresponding atomic diffusions reduced with increasing the shell thickness after heating. Especially, a majority of distorted (AlO)n (n = 3, 4 and 5) clusters were ejected from the surface of the system at the end of the simulation, implying that the core-shell Al/Al2 O3 nanoparticles is exploding. The presence of an of importance void space was observed when the alumina shell has molted, which was in great agreement with the experimental evidence. The alumina shell melts at approximately 1153 K and corresponding melting temperature enhanced with the augment of shell thickness. Moreover, the electric field induced atomic diffusion mechanism of core-shell Al/Al2 O3 nanoparticles are obtained as is reported. In short, the results obtained in this work allow us to draw the conclusion that the novel atomic diffusion mechanism of core-shell Al/Al2 O3 nanoparticles may prove useful to understand the thermodynamic properties of aluminum coated alumina nanoparticles.

∎ ASSOCIATED CONTENT Supporting Information The total charge at the core-shell interfaces and associated electric field are given for all the considered systems during the different reaction times. The time evolution of radial distribution functions of Al-Al bond in the core region at different times for δ = 1.0 nm. Potential energy profile of all atoms in the shell region for different shell thicknesses during the heating process. The time evolutions of radial distribution functions of Al-O and O-O bonds in the shell region for different shell thicknesses.

∎ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (C.X.L.). *E-mail: [email protected] (L.Z.P.). Notes The authors declare no competing financial interests.

∎ ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 11176020 and 11374217) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11502244) Figure 8. The time evolutions of steric charge distributions of the system for δ = 1.0 nm. (a) Relaxed; (b) 5.2 ps; (c) 9.5 ps; (d) 50 ps.

4. CONCLUSIONS

In present article, the thermal induced reaction process was simulated to systematically study the atomic diffusion mechanisms of

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The Journal of Physical Chemistry

Synopsis: The snapshots of the atomic configurations of core-shell Al/Al2 O3 nanoparticles have been acquired by OVITO (Open Visualization Tool) software during the thermally simulated reaction process. A majority of distorted (AlO)n (n = 3, 4, 5) clusters were ejected from the system surface, which is in excellent agremment with the experimental observation.


Al2O3 Nanoparticles to Heating: ReaxFF

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