Molecular Dynamics Study of Melting Behavior of Planar Stacked Ti–Al Core–Shell Nanoparticles

Round 1

Reviewer 1 Report

The paper seems to be interesting. The authors have performed a serious MD research of planar Ti@Al stacked 2D models, i.e. they investigated the sintering behavior in these models. A number of relevant parameters were calculated and presented in numerous figures and tables. However, I have some comments which should be taken into account by the authors:

1. The choise of Ti/Al nanoalloys and of Ti@Al core-shell NPs should be explained in Introduction. Why just Ti@Al instead of Al@Ti?
2. All the notations in formula (1) should be explained. E.g.: “A function F_i of electron density ρ_i(R)” etc.
3. The highest temperature in MD experiments was 1100K. At temperatures lower than 1155K Ti has hcp structure. But the authors assumed that Ti has fcc structure. The fcc and hcp structure are close to each other. However, a loose enough assumption that Ti has fcc structure should be commented in the manuscript.
4. In accordance with figures, the authors really calculated the radial distribution function (RDF) introduced by J. Kirkwood. Now some other terms are employed as well and RDF is sometimes missed with radial distribution of density. But RDF is a function of distance of an atom from another atom, i.e. not from a reference point as it started at p. 8 line 240.

An additional comment that can be taken into account:

Figs. 6 and 7 illustrate “the atomic configurations”. However, Ti and Al atoms are not presented by sphere of different colors. So, it is not clear what happens with atoms of different components. However, I do not insist on taking into account this comment: it may be difficult for the authors.

After minor revision I recommend the paper for publication.

Author Response

Please see attached Word file.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript used molecular dynamics method to study the sintering behaviour of 2D stacked Ti/Al Core/Shell nanoparticles (NPs). Authors considered two different stacking modes, that is, the square-aligned (A-type) and the closed packed (C-type), which have different number of neighbour connections for each NP. For these two differently stacked core/shell nanoparticle model systems, the effects of core volume fraction and heating rates on the sintering behaviour are investigated by analysing mean square displacement (MSD), radial distribution function (RDF), radius of gyration (Rg), and common neighbour analysis (CNA). The authors state that the main finding of their study is that the closed-packed (C-type) NPs can form a more stable, completely sintered product at a relatively low sintering temperature given that the heating rate is relatively slow.

Even though the study employs many different analysis methods to draw reasonable conclusions, it seriously lacks an appropriate literature survey, which makes it difficult for the reviewers to judge whether the work adds any new findings and knowledge to the existing literature regarding the nanoparticle sintering. It appears that the authors tried to make some form of literature survey, but it is entirely self-citations (line 48 – 55) without a proper discussion of other groups’ work in the field.

Therefore, I advise that the authors conduct a proper literature survey to outline the significance of their findings based on the existing literature and cite them throughout the manuscript. For example, the current manuscript reminds me of the previous work by Wang and Shin (RSC Adv., 2017,7, 21607-21617,  doi.org/10.1039/C7RA02611K) of sintering behaviour and mechanical properties of Cu-Ag core-shell nanoparticles arranged in simple cubic (SC) in 3D. They observed that simultaneous interactions with multiple particles accelerate the sintering process which seems essentially the same conclusion as in the current manuscript (observed by comparing A-type and C-type in 2D). By explicitly stating what is already known in the field, and what hasn’t been done or is unknown in the introduction of the manuscript, the motivation of the work will be clear. Authors also should compare their results and findings with existing literature to make it clear where this work stands. It may be the case that the effects of different core volume fraction on the sintering behaviour are the main findings of the current manuscript rather than the stacking (porosity or density) of the nanoparticles. If that is the case, the abstract and the conclusion should be adjusted as well.

Comments focused on the scientific content of the manuscript:

• Line 122: Was the 1000 ps of equilibration long enough? Can authors prove that by showing the potential energy curve and put it in the supporting information?

• In Figure 3, it is interesting that the MSD of the NPs with higher core volume fraction takes longer time than other CVFs before it significantly increases which is easily seen in Figure 3 (a). Can authors comments on this regarding the solid-state diffusion?
  
  
• Line 204: It is not clear how authors determined the T_a, ‘quick aggregation’, i.e., the temperature at which the NPs aggregate together tightly. In other words, how did authors quantify ‘tightness’ of the aggregates? Without a description of the method, it seems too arbitrary to tabulate the results and discuss the implications.

Other minor comments:

• Line 169: In “This finding also supports the previous conclusion that more …”, it is not clear which conclusion the authors are referring to, in other words, whether it is from another paper, or it is from the previous paragraphs.  

• Figure 8. There is a typo in the figure legend; ‘T_nh’

• Line 265, ‘This temperature makes the stacked-NPs fully melt’ is confusing, because the RDF in Figure 8 was only for the Al Shell and 950 K is referred to as the temperature that exceeds the melting point of Al. Should it say that ‘This temperature makes the Al shell fully melt?

• Line 314, ‘Figures 9 (a)&(d) …’ should be ‘Figures 10 (a)&(d)…’

Author Response

Please see the attached Word file.

Author Response File: Author Response.docx

Reviewer 3 Report

The article by Huadian Zhang, Yu-Chen Su, Yiwei Han, Shan Jiang is devoted to molecular dynamics study of melting behavior of planar stacks of bimetallic nanoparticles. The particles were prepared with Ti/Al core/shell structure. For all calculations the authors use the LAMMPS package. The article is of interest to readers and can be recommended for publication after a minor revision. My comments are placed below.

1) The authors use nanoparticles with a diameter of 50 Å. Particles of such small size (compared to particle sizes of 10 to 200 µm used in selective laser sintering processes) have the strong size dependence of melting and crystallization temperatures. The created systems are heated up to the final temperature of 1100 K, but this temperature can significantly exceed the melting point of the nanoparticles used. Thus, the results obtained may be of limited value and cannot be generalized to the entire laser sintering process. This fact should be reflected in the manuscript and be sure to comment.

2) The heating rate of nanoparticles used by the authors is too high in comparison with the real process, so it cannot be said that this is a real laser sintering process. This should be explained to the reader that, perhaps, using such heating rates dictated the limitations of the molecular dynamics method.

3) Lines 200-201. The authors write “Hence, in this work, it is regarded as the complete sintering of the stacked-NPs when all the pre-existing voids vanish”. Perhaps the complete sintering is just due to the fact that the authors consider nanoparticles, and not microparticles that  are used in real processes and using unrealistic heating rates. This needs to be commented on.

4) The data presented in Table 1 does not demonstrate the effect of the core/shell structure on aggregation temperature and “no-void” temperature. The difference in the results for different volume fractions of the nucleus can be explained by the calculation error. It is possible that the presence of a core for the selected sizes of NPs does not appear clearly. What estimates for the error of simulations can the authors make?

5) The previous remark also applies to the results collected in Table 5.

Author Response

Please see the attached Word file.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report