The Influence of Processing Conditions on Gas Transport and Thermal Properties of Graphite Foil Compressed from Exfoliated Graphite
, Andrey L. Yurkov 1,2, Igor L. Kalachev 1, Natalia V. Maksimova 1, Artem P. Malakho 1,3, Svetlana I. Volkova 1 and Victor V. Avdeev 1
Round 1
Reviewer 1 Report
This manuscript systematically investigated the influence of Graphite foil (GF) processing conditions (the GIC stage number and the exfoliated graphite (EG) preparation temperature) on the main properties (gas permeability and thermal oxidation stability) of GF compressed from EG. Detailed chemical compositions and structural characterizations as well as physicochemical properties measurements were performed and also well supported the authors’ claim. Overall, the manuscript is well-organized, and thus I recommend the paper to be published in Processes.
Author Response
Thank you for your comments and recommendation
Reviewer 2 Report
The paper introducs how processing conditions can influence gas transport and thermal properties of graphite foil compressed from exfoliated graphite. The scientific soundness is good and the results are adequateky explained. However, there are some suggestions for the authors to improve the paper:
1) in the abstract the aims of the different optimizations should be better indicated
2) in the introduction the gaps that led to this study should be more highlighted which are not clear
3) in the conclusions should include the different applications in which optimizations are most useful and the motivations
4) An overall english and formatting revision is needed.
Author Response
Thank you for your comments
1) We have extended the abstract and added the text below:
The preparation of graphite foil is a complex process, which includes the intercalation of graphite, water washing, thermal exfoliation and pressing of intermediate products. The preparation conditions significantly influence the structure of the material and its physicochemical properties. Thus, the aim of work was to reveal the correlation between GF processing conditions, its crystalline structure, porosity and gas permeability as well as thermal stability. Sealability of the material is connected with low value of gas permeability, while thermal stability allow to use the material in high-temperature processes. Optimization of these parameters allow to obtain a reliable material and expand the areas of its application.
Added text is indicated in the article in green
2) Expanding the scope of application of sealing materials based on GF requires control of their gas permeability and thermal stability to create a material with the desired functional properties. It is known that the GF preparation conditions have the greatest influence on the structure of EG and GF. Thus, varying the conditions can allow to obtain the material with the minimum gas permeability (i.e. maximum sealability) and high thermal stability. The ability to control these characteristics is therefore important in both developing new materials and improving existing ones. On the other hand, understanding which structural parameters affect these properties makes the mechanisms of gas transport and oxidation more clear.
The corresponding text is added in the introduction.
3) The optimum preparation temperature of EG and GF is 800 oC, at which the minimum of H2 and N2 permeability is observed for each GIC stage number. On the other hand, GF based on GIC of II stage has lower thermal stability. Thus, for relatively low-temperature applications such as gas condensation and separation, using overheated steam GF based on GIC of II stage can be used. For high-temperature processes such as oil refining a GF based on GIC of a higher stage with a high temperature resistance should be selected.
The corresponding text is added in the conclusion.
4) We have improved the language and some sentences that were difficult to understand have been rephrased.
Author Response File: 
Author Response.docx
Reviewer 3 Report
2.2. Investigation techniques
2.2.1. Characterization
The crystallite size along the c axis (Lc, nm) of the obtained samples was calculated using the Scherrer equation [37]: Lc = 0.91 λ/(β cosθ) (1) where λ is the wavelength of the copper Kα1 X-ray radiation (0.154 nm), θ is the diffraction angle of (006) peak.
Actually, the Θ is not diffraction angle. Θ is the grazing angle, which sometimes equals to a HALF of diffraction angle 2Θ, but only in the case of symmetric scan.
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The authors neglect the doublet structure of Kalpha radiation, whereas the wavelength ration is 1.54056/1.54439=0.997. The value is comparable with ratio of measured values of an interlayer distance of expandable graphite and initial graphite - 0.34 nm and 0.336 nm. The comparison of such a small sizes needs not to neglect of the doublet structure of the diffraction maxima.
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The XRD analysis should be improved.
Author Response
Thank you for your comments
We have improved the quality of the article according your recommendation
1) In our case, we use the symmetric scan mode to obtain diffraction angle 2Θ for all graphite materials
2) We used for analysis the (006) peak, which is resolved on Kalpha1 and Kalpha2 doublet according work *. Indeed, if we measure an interlayer distance of expandable graphite based on Kalpha1 compotent of the (006) peak, it is 0.336 nm for expandable graphite as well as for initial graphite and graphite foil. We have corrected the text according to these calculations. In addition, we added the XRD patterns of (006) peak of expandable graphite and XRD patterns of compressed exfoliated graphite in Figure 1. Added corrections is indicated in the article in green
We also made changes to the abstract, introduction and conclusions, which are designated in green in the text
* Cançado, L.G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y.A.; Mizusaki, H.; Speziali, N.L.; Jorio, A.; Pimenta, M.A. Measuring the Degree of Stacking Order in Graphite by Raman Spectroscopy. Carbon N. Y. 2008, 46, 272–275, doi:10.1016/j.carbon.2007.11.015.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
1) "Θ is the angle of (006) peak, which equals to a half of diffraction angle 2Θ"
What does it mean, "Θ is the angle of (006) peak? A peak has no any angle. The text looks like a set of jargon.
2) "βm is the measured half-height width of (006) peak"
It should be clarified which line shape model was used during the approximation or fitting.
3) I do not understand why the authors deliberately ignore the doublet structure of Kalpha line of the copper anode X-ray tube. It should be clarified. The best way is to take into account the doublet structure of the source.
4) The authors write about (422) peak of silicon standard, nevertheless there are at least two types of crystal structure of silicon. The authors could check by using any database, a free one indeed.
5) The reason of absence of the difraction maxima (001) and (005) should be clarified.
...
Author Response
1) Such formulations are typical for many publications, for example:
“From the X-ray diffraction peaks, the interlayer spacing [d(006)] can be obtained as d(006) = λ/2sinθ, where λ is the wavelength of the copper Ka1 X-ray line (λ = 0.154 nm), and θ is the diffraction angle of the (006) peak.”
Cançado, L.G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y.A.; Mizusaki, H.; Speziali, N.L.; Jorio, A.; Pimenta, M.A. Measuring the Degree of Stacking Order in Graphite by Raman Spectroscopy. Carbon 2008, 46, 272–275, doi:10.1016/j.carbon.2007.11.015.
“The accuracy of the values of interlayer spacing depends predominantly on that of the diffraction angle of each peak.”
Inagaki M., Kang F. Materials Science and Engineering of Carbon Characterization. 2016 Elsevier Inc. doi: 10.1016/C2014-0-03769-0
“the stress along the horizontal direction in the AZO film is related to the location of the diffraction angle of peak (002)”
Shen H.-L., Zhangl H., Lu L.-F., Jiang F. Yang C. Preparation and properties of AZO thin films on different substrates. Progress in Natural Science: Materials International 2010, 20, 44-48. doi: 10.1016/S1002-0071(12)60005-7
“θhkl is the diffraction angle of peak (hkl)”
Kahn, G. M. Michal, F. Ernst, A. H. Heuer. Poisson Effects on X-Ray Diffraction Patterns in Low-Temperature-Carburized Austenitic Stainless Steel. Metallurgical and Materials Transactions A, 2009, 40, 1799-1804. doi: 10.1007/s11661-009-9814-4
According [Inagaki M. and Kang F. Materials Science and Engineering of Carbon Characterization.], the formulation of “Θ is the diffraction angle of peak (006) position, which equals to a half of goniometer angle 2Θ” is more correct. We introduce this sentence in the article.
2) A pseudo-Voigt function was used for fitting the diffraction line
3) X-ray employed is the CuKa radiation. It is a double of diffraction lines caused by Ka1 and Ka2 radiation with a small difference in wavelength (0.15405 and 0.15444 nm, respectively). The intensity of Ka1 radiation is twice that of Ka2. The diffraction lines caused by Ka1 and Ka2 radiation have been separated. For graphite foil splitting due to Ka1 and Ka2 has been observed on the 006 line. We used for measurement of Lc the Ka1 component of (006) peak, which was resolved on Ka1 and Ka2 doublet.
4) Silicon with cubic crystal system and Fd3m space group was used as standard
5) The lines with 00l indices are due to the reflection from crystallographic basal planes (hexagonal carbon layers of graphite), where only the even l = 2n indices are allowed because of the extinction rule due to the parallel stacking of the layers ABABAB. The XRD pattern of hexagonal graphite is characterized by the presence of line (00l), l = 2n, while the remaining reflections (00l) are forbidden due to the action of the helical axis 63 in a cell with an alternating structure ABABAB. The entire set of peaks (00l) is visible on the XRD pattern of the GICs and their extinction does not occur due to a change in the symmetry of the space group of the crystal lattice and the appearance of axis 6 in a cell with an alternating structure AαAαAα… (A is a graphite layer, α is an intercalate layer). (001) peak is also observed only on GICs XRD patterns at 2θ = 5-7 o and has very low intensity
Savoskin M.V., Yaroshenko A.P., Whyman G.E., Mestechkin M.M., Mysyk R.D., Mochalin V.N. Theoretical study of stability of graphite intercalation compounds with Brønsted acids. Carbon, 2003, 41, 2757-2760.
Doyen-Lang S., Charlier A., Lang L., Charlier M.F. Theoretical study of charge transfer in graphite intercalation compounds. Synth. Met, 1993, 58, 95-107.
Added text is indicated in the article in yellow
Author Response File: Author Response.docx
Round 3
Reviewer 3 Report
1) "the Scherrer equation [37]: Lc = 0.91 λ/(β cosΘ) (1) where λ is the wavelength of the copper Kα1 X-ray radiation (0.15405 nm), Θ is the diffraction angle of peak (006) position"
I advise for authors to read at leas Wikipedia page about Scherrer equation (https://en.wikipedia.org/wiki/Scherrer_equation).
The theta is NOT the diffraction angle, theta is the Bragg angle, the diffraction angle is 2theta. In the picture, the diffraction is presented, 2theta is the diffraction angle.
cubic crystal system
2) "cubic crystal system" - Incorrect jargon.
3) It should be proofed by structure analysis that the reflection 005 is forbidden. Actually, now it is entered with no evidence.
Author Response
1) Unfortunately, Wikipedia is not a reliable source to refer to in a publication. As we have previously given examples from the literature, many authors in both books and journals use this “jargon” regarding the diffraction angle. They use the same definition “diffraction angle” for both the angle theta Θ and the angle two theta 2Θ. To avoid inaccuracies in the definition, we propose the following clarifying definition:
Θ is the angle (the Bragg angle, the angle of the diffracted wave, the angle between the primary X-ray beam (with λ wavelength) and the family of lattice planes, with interplanar spacing d), which equals to a half of the angle 2Θ (the goniometer angle, the diffraction angle, the angle between the direction of incident beams and resulting diffracted beam) of the peak (006) position
2) Such “incorrect jargon” is used by many authors:
Five branching growth patterns in the cubic crystal system: A direct observation of cuprous oxide microcrystals (https://doi.org/10.1016/j.actamat.2006.11.032 )
Comparative study of grids based on the cubic crystal system for the FDTD solution of the wave equation (https://doi.org/10.1016/j.matcom.2019.06.014 )
On the spectral decomposition of the elasticity tensor for the cubic crystal system (https://doi.org/10.1016/j.ijengsci.2011.03.005 )
In addition, it is used by Wikipedia: https://en.wikipedia.org/wiki/Cubic_crystal_system
3) The detailed structural analysis of graphite crystalline lattice, which had previously been carried out by many groups, was not included in the aims of this work. The peak (005) does not occur on the XRD pattern of hexagonal graphite, see previous publication, for example:
Howe J.Y., Rawn C.J., Jones L.E., Ow H. Improved crystallographic data for graphite. // Powder Diffr. 2003. V. 18. P. 150-154. (https://doi.org/10.1154/1.1536926 )
Author Response File: Author Response.docx
Round 4
Reviewer 3 Report
It could be accepted.
