Al2O3 Nanoparticles to Heating: ReaxFF

Article Cite This: J. Phys. Chem. C 2018, 122, 9191−9197

pubs.acs.org/JPCC

Responses of Core−Shell Al/Al2O3 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 ‡

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S 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/Al2O3 nanoparticles. According to atomic mean square displacements, the reaction initialization of core−shell Al/Al2O3 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, and 5) clusters were ejected from the system surface at later stages, suggesting 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/Al2O3 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 comparison 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 to that of flat aluminum samples. Coulet and coworkers13 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 © 2018 American Chemical Society

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 of an amorphous oxide layer with 4 nm thick. Puri and Yang16 employed the MD method to explore the effect of particle size on the melting point of nanosized aluminum particles. ReaxFFMD simulations were performed by Hong and van Duin17 to research the oxidation of aluminum nanoparticles. The results showed that the hot-spots and high-temperature regions were responsible for the oxidation mechanisms of pure aluminum nanoparticles. However, a passivation oxide layer (i.e., Al2O3) 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 Al2O3-coated aluminum nanoparticles. At present, many scientists have summarized two distinct explanations for the oxidation and combustion mechanisms of Received: January 31, 2018 Revised: April 1, 2018 Published: April 4, 2018 9191

DOI: 10.1021/acs.jpcc.8b01088 J. Phys. Chem. C 2018, 122, 9191−9197

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The Journal of Physical Chemistry C

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 accurately 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 Esystem in the ReaxFF potential can be formulated by eq 1

aluminum nanoparticles or nanoclusters on account of those experimental phenomena and observations. First, diffusion oxidizer mechanism was chiefly presented by Park24 and Rai25,26 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 groundthat the melting point of aluminum core and some transitions of alumina shell. Second, Levitas et al.27−29 put forward the melt dispersion mechanism 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 buildup 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 ignitions of aluminum 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 violent chemical reaction, 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 Yang32 surveyed the thermomechanical 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 nanosized 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/Al2O3 nanoparticles by means of the reaction simulation of core−shell Al/Al2O3 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, coordination number, and space charge distribution among atoms and so on.

Esystem = E bond + Eover + Eunder + E lp + Eval + Etors + EvdWaals + ECoulomb

(1)

where the total energy term, Esystem, contains Ebond denoted the bond energy, Eover denoted the energy penalty for overcoordinated atoms, Eunder denoted the energy contribution for undercoordinated atoms, Elp denoted the lone-pair energy, Eval denoted the three-body valence angle term, Etors denoted the four-body torsion term, EvdWaals denoted the van der Waals interactions, and ECoulomb denoted the Coulomb interactions. The changing atomic charges are counted using the electronegativity equilibration method approach as described by Mortier et al.40 and Janssens et al.41 with a view to the Coulomb 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/Al2O3 nanoparticles are composed of a nanosized face-centered cubic (fcc)-crystalline aluminum core with a diameter of 4 nm containing 2334 atoms and a nanocrystalline Al2O3 shell possessing three different thicknesses (δ = 0.6, 0.8, and 1.0 nm) to research the effect of shell thickness on the reaction characteristics of core−shell Al/Al2O3 nanoparticles. Using the alumina shell with 1.0 nm, as an example, the total diameter of core−shell Al/Al2O3 nanoparticles is on the order of 6.6 nm with 14 813 atoms. The space intervals between Al core and Al2O3 shell were chosen to be about 0.3 nm, as plotted in Figure 1. In this investigation, all

Figure 1. Snapshots of the initial structures of core−shell Al/Al2O3 nanoparticles. (a) Cross-sectional diagram; (b) three-dimensional stereogram. The yellow and blue spheres represent Al and O atoms, respectively.

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/Al2O3 nanoparticles were then heated up to the initial temperature of 2000 K at the heating rate of 100 K/ps. The Nosé−Hoover thermostat scheme43,44 was carried out for this simulation procedure. In addition, a temperature damping

2. COMPUTATIONAL DETAILS The whole mimic computations are performed by the largescale atomic/molecular massively parallel simulator35 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 9192


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The Journal of Physical Chemistry C constant for the Nosé−Hoover thermostat was set as 20 fs. For the sake of analyzing the structural evolutions and reaction mechanisms of core−shell Al/Al2O3 nanoparticles after heated, the targeted temperature was held at 2000 K for another 100 ps. All of the MD simulations were carried out on the highperformance computing resources provided by National Supercomputer Center in Tianjin, using 12-144 processor cores.

clusters sputtering latterly. To further study the effect of shell thickness on the atomic diffusivities of the system, Figure 3

3. RESULTS AND DISCUSSION At first, to explore deeply the atomic diffusion mechanisms of core−shell Al/Al2O3 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

Figure 3. 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. 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.

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 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 smallsized 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 because the thicker shell has more obstructive influence on the diffusivity of shell O atoms. In the MD simulations, the nanostructured evolutions of core−shell Al/Al2O3 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/Al2O3 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

at the core−shell interfaces of Al/Al2O3 nanoparticles. Then, we mark out the common aluminum atoms between the annular region and the aluminum core region, i.e., gbcal. Meanwhile, we also mark out the common 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. On the basis of these data, the atomic diffusion coefficient D is computed by using eq 2 ∂⟨r 2(t )⟩ = 6D ∂(t )

(2)

where t is the elapsed time and ⟨r (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, whereas 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 atoms driven by the electric field discussed next. Additionally, the diffusivity of shell aluminum atoms also enhances visibly as a result of the 2

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Figure 4. Snapshots of the structural evolutions of core−shell Al/Al2O3 nanoparticles for δ = 1.0 nm. (a) Relaxed configuration; (b) 5.2 ps; (c) 9.5 ps; (d) 50 ps; (e) 80 ps.

occurence 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 literatures32,34 reported that the melting point of aluminum oxide for small particle size also is below that of bulk alumina shell. At that moment, Chakraborty and Zachariah presumed that the shell is no longer alumina but a suboxide 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 the rapid outward diffusion of core Al atoms. The void space is made up of multiple adjacent smaller cavities in favor of explaining its net volume. Figure 4d displays that the system surface is beginning to eject at 50 ps. In a later stage of the simulation, Figure 4e shows that a considerable number of nanoclusters in the form of distorted (AlO)n (n = 3, 4, and 5) sputter from the surface of the system, stating clearly that the nanoclusters are detonating. As Lynch et al.47 reported experimentally, AlO clusters were detected in absorption during nanosized aluminum combustion at temperatures as low as 2000 K. Moreover, 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 are 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

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

Figure 6. Radial distribution functions of Al−O and O−O bonds in the shell region at different times for δ = 1.0 nm. 9194


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at the core−shell interfaces and associated electric field are calculated for all of the considered systems 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

ps, a no long-range order pattern is observed, which illustrates that the shell also 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 suboxides are composed of mixed AlO4 tetrahedra and AlO6 octahedra configurations; this is similar to the available literatures.50,51 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

Table 1. 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 δ (Å)

Ts (ps)

Tm (K)

Qint (C × 10−19)

E (N/C × 109)

10 10 10 10 10

0 5.2 9.5 18 50

300 730 1153 2000 2000

9.73 2.07 0.21 0 0

1.98 1.51 1.21 0 0

electric field drops with simulated time development. Above all, the associated electric field equals 0 when heating to 2000 K. Therefore, the results mentioned above allow us to conjecture that the associated electric field accounts 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

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

variation of melting points as a function of the shell thickness. It can be seen that the shell thickness has no effect on the melting temperature of aluminum core with the settled value of 730 K, at 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/Al2O3 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 eq 3 E=

1 Q int re 4πε0 r 2

Figure 8. 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.

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 are 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/Al2O3 nanoparticles are fused together when the shell has melted and the electric potential difference of the system approaches 0 at the moment, as listed in Figure 8c,d. These observations could have important implications in the electric field-induced atomic diffusions of shell O atoms and core Al atoms.

(3)

where E represents the electric field at the core−shell interfaces, ε0 represents this permittivity constant of a vacuum, Qint represents the total charge at the core−shell interface regions, r represents the radial position of the interfacial aluminum atom, and re represents the radial unit vector. The total charge 9195



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4. CONCLUSIONS In present article, the thermal induced reaction process was simulated to systematically study the atomic diffusion mechanisms of core−shell Al/Al2O3 nanoparticles by means of the 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 core−shell Al/Al2O3 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/ Al2O3 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/Al2O3 nanoparticles are exploding. The presence of an importance void space was observed when the alumina shell has molten, 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 mechanisms of core−shell Al/Al2O3 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/Al2O3 nanoparticles may prove useful to understand the thermodynamic properties of aluminum-coated alumina nanoparticles.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 11774248 and 11474207) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11502244).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01088. Total charge at the core−shell interfaces and associated electric field are given for all of 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 profiles 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 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.C.). *E-mail: [email protected] (Z.L.). ORCID

HuaDong Zeng: 0000-0001-7517-7589 ChaoYang Zhang: 0000-0003-3634-7324 Notes

The authors declare no competing financial interest. 9196


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Al2O3 Nanoparticles to Heating: ReaxFF

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