Low Weight Hollow Microspheres of Iron with Thin Dielectric Coating: Synthesis and Microwave Permeability
, Dmitry Petrov 1, Polina Zezyulina 1, Konstantin Pokholok 4, Dmitry Filimonov 4 and Mangui Han 5Round 1
Reviewer 1 Report
A report of reviewing a manuscript, Coatings-948708
This manuscript reports the preparation of hollow iron microspheres via ultrasonic spray pyrolysis followed by coating with a protective SiO2 layer with thickness of approximately 20 nm, and their various physical properties including morphology, magnetic and microwave characteristics. Since there are no publications in literature on the synthesis of iron powders using the two-stage method, which consists of (i) the production of metal spheres and (ii) their encapsulation by inorganic protective shell, this manuscript gives technologically useful and valuable knowledges to the current microsphere science and engineering communities. The Introduction, material, and method sections of the manuscript are satisfactorily prepared, but the result and discussion sections have several materials to be improved/added before publication in Coatings, which are indicated as follows.
(1) In L. 158 of subsection 3.3 (SEM analysis), a term “th” is use in “1/4th”. Is it necessary?
(2) In L. 166 of subsection 3.3 (SEM analysis), how did the authors confirm the presence of SiO2 spheres from Fig. 3(c). Please describe the method of classification between “Fe@SiO2” and “SiO2” spheres.
(3) In LL. 171-172 of subsection 3.3 (SEM analysis), why does the presence of individual SiO2 particles influence the particle size distribution of “Fe@SiO2” spheres.
(4) In LL. 178-179 of subsection 3.4 (TEM analysis), how did the authors confirm the presence of SiO2 spheres from Fig. 5. Please describe the method of classification between “Fe@SiO2” and “SiO2” spheres.
(5) In L. 188 of subsection 3.5 (Magnetic properties), a term “(CL)“ is needed immediately after “the carbonyl iron” because a term “CL” is used in L. 202 without its definition.
(6) In L. 199 of subsection 3.5 (Magnetic properties), a symbol “÷” is used. Is it necessary?
(7) In LL. 215-221 of subsection 3.6 (Microwave measurements), the proportionality with respect to the concentration of “Fe” and “Fe@SiO2” particles is qualitatively argued for the observed permeability. A quantitative judgement is should be made instead of the qualitative argument. An additional figure, which shows the permeability as a function of “Fe” and “Fe@SiO2” particles, supports the authors judgement concerning the proportionality.
(8) In L. 221 of subsection 3.6 (Microwave measurements), the authors mention a noticeable magnetic interaction between particles. Which type interaction(s) is(are) expected between the magnetic particles?
(9) In L. 247 of subsection 3.6 (Microwave measurements), “Figure 6” should be corrected to “Figure 7” because “Figure 6” is used for the magnetization curve results.
(10) In L. 250 of subsection 3.6 (Microwave measurements), “Figure 7” should be corrected to “Figure 8”.
(11) Generally, the skin depth effect influences the microwave measurement of metallic materials. Nevertheless, no comment(s) on the skin effect is made in subsection 3.6 (Microwave measurements).
(12) In section 1 (Introduction), the authors mention the studies on Co and Ni cases. However, no comparative arguments are made between Co/Ni and the present Fe cases.
Author Response
Reply to the Reviewer 1
Dear Editor and Reviewer, thank you for your time working on the manuscript. We appreciate all suggestions, comments, and remarks, especially those that are pointed toward misprints and other technical errors, which regrettably leaked to the first version of the manuscript. Both the referees required additional discussion of the microwave measurements (permeability vs filler concentration dependencies for both fillers), and this is now given in the manuscript. All changes in the manuscript are marked as “corrections” in MS Word. Below is a point-by-point reply to each remark.
Sincerely yours,
On behalf of the authors,
Anastasia Kosevich.
(1) In L. 158 of subsection 3.3 (SEM analysis), a term “th” is use in “1/4th”. Is it necessary?
(5) In L. 188 of subsection 3.5 (Magnetic properties), a term “(CL)” is needed immediately after “the carbonyl iron” because a term “CL” is used in L. 202 without its definition.
(6) In L. 199 of subsection 3.5 (Magnetic properties), a symbol “÷” is used. Is it necessary?
(9) In L. 247 of subsection 3.6 (Microwave measurements), “Figure 6” should be corrected to “Figure 7” because “Figure 6” is used for the magnetization curve results.
(10) In L. 250 of subsection 3.6 (Microwave measurements), “Figure 7” should be corrected to “Figure 8”.
Thanks a lot for pointing out these errors! We have made changes to the text.
(2) In L. 166 of subsection 3.3 (SEM analysis), how did the authors confirm the presence of SiO2spheres from Fig. 3(c). Please describe the method of classification between “Fe@SiO2” and “SiO2” spheres.
The Fe particles had a porous structure (Fig. 3b). When coated, the porous morphology of iron particles still can be distinguished through a thin SiO2 layer (Fig. 3c). Pure SiO2 particles appeared as semi-transparent spheres that contrasted vividly to the complicated nano-structure of Fe@SiO2 particles. This is now indicated within the text.
(3) In LL. 171-172 of subsection 3.3 (SEM analysis), why does the presence of individual SiO2 particles influence the particle size distribution of “Fe@SiO2” spheres.
Individual, or “pure”, SiO2 particles were lower than 0.6 um in diameter, and thus contributed to small-size regions of the distribution. Additional SEM images, taken with lower magnification, are now added to the manuscript to improve a “feeling” of the size of the particles.
(4) In LL. 178-179 of subsection 3.4 (TEM analysis), how did the authors confirm the presence of SiO2 spheres from Fig. 5. Please describe the method of classification between “Fe@SiO2” and “SiO2” spheres.
Explanation is now given in the manuscript, lines 170-173.
(7) In LL. 215-221 of subsection 3.6 (Microwave measurements), the proportionality with respect to the concentration of “Fe” and “Fe@SiO2” particles is qualitatively argued for the observed permeability. A quantitative judgement is should be made instead of the qualitative argument. An additional figure, which shows the permeability as a function of “Fe” and “Fe@SiO2” particles, supports the authors judgement concerning the proportionality.
(8) In L. 221 of subsection 3.6 (Microwave measurements), the authors mention a noticeable magnetic interaction between particles. Which type interaction(s) is(are) expected between the magnetic particles?
Additional graph and a table, depicting the μs vs filler concentration dependencies is now given in the manuscript.
(11) Generally, the skin depth effect influences the microwave measurement of metallic materials. Nevertheless, no comment(s) on the skin effect is made in subsection 3.6 (Microwave measurements).
For a conductive material, skin-effect effects permeability. The samples studied were non-conductive composites with a dielectric matrix. Thus, skin-effect was negligible. Further studies concerning intrinsic permeability of the particles are outside of the scope of the study.
(12) In section 1 (Introduction), the authors mention the studies on Co and Ni cases. However, no comparative arguments are made between Co/Ni and the present Fe cases.
In the section 5 (Magnetic properties), the typical coercivity of hollow Co particles was compared to the coercivity of iron particles obtained here. Also, typical size of Co, Ni and the newly reported Fe particles is now discussed within the “particle size distribution section” (lines 184-191).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript is clear well structured and well written, and fits with the scope of the journal. However, the results are not conclusive and they need deep revision.
In general in the whole paper, atomic, weight and volumic concentrations must be distinguished. Please scpecify in each % (after XRD, Mössbauer spectroscopy, EDX and magnetization) to which one it refers, to correlate all them with microwave characterization.
The XRD spectrum is not well measured, as the 2-theta peak positions are too large and do not agree with the JPCDS cards. This fact requires a deep revision.
In Mössbauer spectroscopy, the obtained % of each compound is not clearly stated. Are there three phases: Fe, Fe3O4 and FeO? In this case, do the sum of Fe3O4 and FeO give the 17% cited in the text? How this correlates with XRD results? Please clarify these issues in the text.
Referring to SEM, the spheres' dimension need a more coherent discussion, as the body states "approximate diameter of 1 um" for all the three products (suggesting that all spheres are similar, as can be concluded from Fig. 4 but not fom Fig. 3), while the Conclusion stablishes "an average diameter of 0.7 um". In this direction, showing a larger area in SEM (and TEM, Figs. 3 and 5) pictures would allow a better statistical view. Finally, please add the specific picture (a, b or c) to each reference to Fig. 3 in the SEM discussion (lines 155-166).
From the analysis of the magnetic properties, authors obtain a mass concentration for SiO2 different from the ones obtained from other techniques (in particular from EDX). This discrepancy needs some explation. And at the end of this paragraph (line 202), what is CI? Please consider if the density value for CI needs a reference.
Similarly, please define epsilon', epsilon'', mu' and m'' in the Microwave measurements section (Fig. 6 and 7, not 7-8 as written in line 207!). The difference in mu" for "Fe" and "Fe@SiO2" is larger than the 15% stated in the text (please revise line 210); and its reason -related to the presence of the diamagnetic phase- needs some assessment or reference (lines 210-211). Moreover, please show Fig. 6 and 7 at the same scale, for comparison.
In the same section, I do not agree with the proportionality argument presented in lines 215-218. The proposed proportionality/non-proportionality is not clearly seen in Figs. 6 and 7. Consequently, this statement requires line-trends in the graphs, numerical data, or to be removed from the body.
Finally, please revise Conclusion to clarify the previous points, mainly the spheres dimensions and the concentration weight/volumic/atomic. The sentence "The ratio of the real permittivity to the real permeability for the “Fe@SiO2” composite (66 wt. % of the filler) was 11." (lines 269-270) must be explained in the Results section if included in the Conclusion one.
Author Response
Reply to the Reviewer 1
Dear Editor and Reviewer, thank you for your time working on the manuscript. We appreciate all suggestions, comments, and remarks, especially those that are pointed toward misprints and other technical errors, which regrettably leaked to the first version of the manuscript. Both the referees required additional discussion of the microwave measurements (permeability vs filler concentration dependencies for both fillers), and this is now given in the manuscript. All changes in the manuscript are marked as “corrections” in MS Word. Below is a point-by-point reply to each remark.
Sincerely yours,
On behalf of the authors,
Anastasia Kosevich.
In general in the whole paper, atomic, weight and volumic concentrations must be distinguished. Please scpecify in each % (after XRD, Mössbauer spectroscopy, EDX and magnetization) to which one it refers, to correlate all them with microwave characterization.
Thank you for your comment. Now We have really found several bugs and fixed them.
The XRD spectrum is not well measured, as the 2-theta peak positions are too large and do not agree with the JPCDS cards. This fact requires a deep revision.
Some misunderstanding probably appeared here. To start with, the XRD was measured using Cr radiation. Moreover, the XRD graph is given in semi-logarithmic coordinates to reveal reflections of low intensity. These two facts cause “unusual” appearance of the Fig. 1. The more “typical” presentation of the XRD is given here (https://drive.google.com/file/d/188n-f6w7dyPXZEY7LQYgt1XldJPMQITn/view?usp=sharing). The lattice constant 2.8633(3) Å coincided fairly well with the tabular one 2.867 Å.
In Mössbauer spectroscopy, the obtained % of each compound is not clearly stated. Are there three phases: Fe, Fe3O4 and FeO? In this case, do the sum of Fe3O4 and FeO give the 17% cited in the text? How this correlates with XRD results? Please clarify these issues in the text.
High deficiency and small (even nano-) size of the particles caused broadening of the Mossbauer spectrum. To decrease influence of thermal effects on the results of this analysis technique, additional measurements were carried out at 78K. Unfortunately, even this advanced technique could not distinguish thoroughly the oxide phases. Total content of oxide phases was estimated to be 15-17 at.%, this “oxide phases” were probably mixed of Fe, Fe3O4 and FeO, which is now given in the text.
Referring to SEM, the spheres' dimension need a more coherent discussion, as the body states "approximate diameter of 1 um" for all the three products (suggesting that all spheres are similar, as can be concluded from Fig. 4 but not fom Fig. 3), while the Conclusion stablishes "an average diameter of 0.7 um". In this direction, showing a larger area in SEM (and TEM, Figs. 3 and 5) pictures would allow a better statistical view. Finally, please add the specific picture (a, b or c) to each reference to Fig. 3 in the SEM discussion (lines 155-166).
Fair remark. Additional images of a larger area have been added to the text, which provides a better statistical idea of the particle size distribution.
From the analysis of the magnetic properties, authors obtain a mass concentration for SiO2 different from the ones obtained from other techniques (in particular from EDX). This discrepancy needs some explation.
We recalculated the mass percent EDX, the results were 16 mass.% for SiO2, which is in agreement with the results obtained from the saturation magnetization (15 mass.%).
And at the end of this paragraph (line 202), what is CI? Please consider if the density value for CI needs a reference.
CI is Carbon Iron, the abbreviation is now given in a proper place. The density of CI was measured in exactly the same conditions as was measured the density of the “Fe” powder.
Similarly, please define epsilon', epsilon'', mu' and m'' in the Microwave measurements section (Fig. 6 and 7, not 7-8 as written in line 207!). The difference in mu" for "Fe" and "Fe@SiO2" is larger than the 15% stated in the text (please revise line 210); and its reason -related to the presence of the diamagnetic phase- needs some assessment or reference (lines 210-211). Moreover, please show Fig. 6 and 7 at the same scale, for comparison.
The μ′, μ″, ε′ and ε″ are now determined. Figure captions have been corrected. According to the newly inserted graph No.9 and table No.1, the difference in permeability was 14%. We also decided to not to change scale on the ε” graphs because in this case nothing can be seen from such a picture. A comprehensive study considering interparticle interactions could be the subject of a separate paper. Here, “magnetic interactions” are now substituted with the “dipole-dipole interactions”.
In the same section, I do not agree with the proportionality argument presented in lines 215-218. The proposed proportionality/non-proportionality is not clearly seen in Figs. 6 and 7. Consequently, this statement requires line-trends in the graphs, numerical data, or to be removed from the body.
Additional graph and a table are now given in the text.
Finally, please revise Conclusion to clarify the previous points, mainly the spheres dimensions and the concentration weight/volumic/atomic.
Figure 3 is added to the text of the manuscript, giving information about the distribution of particles (Fig. 4). the Average size is 0.6 microns with a sample of more than 600 particles for each sample.
Mass/volume/atomic concentrations have been corrected in the conclusions and in the main text.
The sentence "The ratio of the real permittivity to the real permeability for the “Fe@SiO2” composite (66 wt. % of the filler) was 11." (lines 269-270) must be explained in the Results section if included in the Conclusion one.
Indeed, this statement is debatable and it is now deleted.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript has been improved as compared to the previous version. However, it deserves still some minor corrections.
There is still some mixing in %, and in particular in lines 151-153 it should be stated what type of % are authors speaking about. Authors must be sure about the coherence of all the % the different techniques provide.
Line 230 still needs some assessment, as a reference of a short explanation of why authors are so sure that the reason is only the presence of the diamagnetic phase SiO2.
Finally, the bibliography needs also some revision. Refs. 3 and 36 are not cited in the text, and ref. 10 is cited after refs. 11, 12 and 13.
Author Response
Reply to the Reviewer 2: Round 2
Dear reviewer, thank you for the additional comments. Below is a point-by-point reply to each remark from the “Round 2” section.
Sincerely yours,
On behalf of the authors,
Anastasia Kosevich.
There is still some mixing in %, and in particular in lines 151-153 it should be stated what type of % are authors speaking about. Authors must be sure about the coherence of all the % the different techniques provide.
All percentage values are now double-checked and stated clearly in the text. Considering the coherence, the following is to be said:
- Studying the “Fe” powder, the quantitative estimation of the “ordered” oxide phase, Fe3O4, was established at 5 mass % through XRD measurements. This value may be converted to 11 atomic% of Iron. For the same sample, the quantity of “disordered”, or metastable oxide phase was found to be 15-17 at.%. These two values match well.
- Investigating the “Fe@SiO2” powder, quantity of the “SiO2” phase was found to be 15 mass % according to the difference in saturation magnetization between Fe and Fe@SiO2. At the same time, the very value was estimated at 15 atomic % (which is roughly equal to 17 mass %). These two values also coincided well.
This short discussion is now in the manuscript. Thanks a lot for the remark.
Line 230 still needs some assessment, as a reference of a short explanation of why authors are so sure that the reason is only the presence of the diamagnetic phase SiO2.
Evaluation of the presence of the diamagnetic phase is based on assumption that the chemical state of the iron core was not subjected to any changes during the SiO2 shell deposition. Two facts confirm this assumption. (1) according to the reference [23], the chemical stability of iron powder during the modified Stober process was directly proven through the gamma-resonance studies. (2) in this study chemical stability of iron core was indirectly shown through the unchanged value of coercivity in the comparison between the “Fe” and “Fe@SiO2” samples.
These assumptions allowed us to state that the difference in the saturation magnetization (MS) of the “Fe” and “Fe@SiO2” was due to the presence of the dielectric phase, SiO2.
Finally, the bibliography needs also some revision. Refs. 3 and 36 are not cited in the text, and ref. 10 is cited after refs. 11, 12 and 13.
Thanks a lot! These pitiful mistakes are now revised.
Author Response File: Author Response.pdf
