Electro-Thermal Parameters of Graphene Nano-Platelets Films for De-Icing Applications
, Gianmarco Trezza 1, Gaspare Giovinco 3, Francesco Bertocchi 4, Sergio Chiodini 4, Francesco Cristiano 4 and Antonio Maffucci 1,*Round 1
Reviewer 1 Report
1.Authors have reported similar types of work in reference [13] and [14].
2. What is new in this work compared to reference [13] and [14].
3. How the film is developed please explain briefly.
4. The authors have used a GNP of fixed size, but why? What will be the
effect of GNP size and thickness on the results that is essential to the
scientific community?
5. What is the requirement of Figure-2.
6. Please suggest alternative binders that can be used.
7. Whether the authors have conducted its application for deicing.
8. Authors may refer the below mentioned papers in the introduction section.
a. Journal of composites Science, 2021, 5(7), 181.
b. Nanoscale,2010, 3, 86–95.
c. Applied Sciences, 2018, 8(9), 1438
d. Applied Sciences 2020, 10(5), 1753
Author Response
> REVIEWER:
1. Authors have reported similar types of work in reference [13] and [14].
2. What is new in this work compared to reference [13] and [14].
>>AUTHORS’ REPLY
Note that in the revised paper, references [13] and [14] are now numbered as [22]-[23].
We have better clarified in the introduction what is the novel content with respect to previous works, see the following sentences starting from row #78
“This paper follows previous works done by the Authors, see [22]-[23]. Compared to such references, here a more accurate analysis of the electrical resistivity behavior is provided, including a new material, assessing a sensitivity analysis on the effect of strip lengths, and proposing a physically-meaningful method to explain the obtained negative derivative of the resistivity with respect to temperature.
Furthermore, a new experimental setup is presented that is specifically intended to discuss and assess the Joule heating capability with respect to the power density targets for de-icing and anti-icing applications. This new setup provides much more controllable and stable conditions, so that a transient thermal analysis can be performed. Finally, another novel contribution is a detailed description of a methodology proposed for deriving the thermal emissivity, by exploiting the feature of the set-up realized for the Joule heating assessment”.
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>REVIEWER:
3.How the film is developed please explain briefly.
>> AUTHORS’ REPLY
We have provided more details about the film fabrication, see Subsection 2.1, rows from #115:
“These films are realized by Nanesa [25], through the following industrial process:
(i) Graphene NanoPlatalets (GNPs) are synthesized from a low-cost graphitic precursor (intercalated expandable graphite), through a process of thermal expansion followed by liquid exfoliation;
(ii) a mixture is obtained by dispersion of GNPs in a solvent (acetone) or aqueous solution, with magnetic stirring and a final sonication phase. In case of inclusion of any polymeric binder, this is added during sonication phase. From a mechanical point of view, the binders that are found to be suitable to the pur-poses of this work are polyurethane (used here) or epoxy;
(iii) the strips are then obtained by spraying the mixture at a controlled pressure, by using a semiautomatic 3-axes pantograph (Computer Numeric Control plotter EXTREMA, model Basic);
(iv) a final step of calendering (optionally joined to annealing) is applied, to com-pact the strip and provide optimized thickness/alignment ratio”
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> REVIEWER:
4. The authors have used a GNP of fixed size, but why? What will be the effect of GNP size and thickness on the results that is essential to the scientific community?
5. What is the requirement of Figure-2.
>> AUTHORS’ REPLY
By addressing these comments, we have now better clarified that the size and thickness of GNPs plays an important role in determining the physical property of the final composite. We have also added new references ([26] and [28]) to support this point. Specifically, the following sentences have been added to the manuscript, see Subsection 2.1, rows starting from # 132:
“It is known that size and thickness of the GNPs have significant influence on the physical properties of the final composite material. The effect of GNP thickness has been for instance discussed in [26], showing that thinner GNPs are recommended for improving global mechanical and thermal performance. The impact of size/thickness aspect ratio on the electrical conduction has been instead analysed in [27], showing that the percolation threshold in the composite increases with increasing values of aspect ratio. Finally, in [28] the electrical conductivity of the composite is demonstrated to im-prove as the GNP size and surface area increase. Therefore, in order to ensure stability to the behaviour of the GNP films, it is important to assess a fabrication process that is able to control the GNP dimensions. The GNPs produced by the above-mentioned process exhibit an average particle size of 30 µm, with a standard deviation of 5µm (Fig.2), and an average thickness around 12 nm, with a standard deviation of 3 nm.”
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> REVIEWER:
6. Please suggest alternative binders that can be used.
>> AUTHORS’ REPLY
The binder is used in these films with the purpose of providing mechanical properties to the strips. An alternative binder that has been successfully used to realize these films is epoxy, that shows similar performance as polyurethane. We have added a specific sentence into the paper (see row #123):
“From a mechanical point of view, the binders that are found to be suitable to the purposes of this work are polyurethane (used here) or epoxy;”
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> REVIEWER:
7. Whether the authors have conducted its application for deicing.
>> AUTHORS’ REPLY
We are carrying out the first tests in view of using these strips in de-icing systems. The graphene strips analyzed in this paper have been encapsulated in GFRM layers and have been intensively tested to acquire a preliminary data set of heating performances to be compared with the project requirements. Tests have been executed overlapping the heater unit with aluminium layers of different thickness and size to emulate the real process of de-icing and anti-icing at the external side of a wing or rotor blade profiles. Anyway, these results must be assessed and will be shown in future works. Preliminary considerations about the de-icing anti-icing applications can be however done with the present results. Therefore, following the reviewer’s suggestion, we have added a more detailed discussion about the heating results also adding a new reference [39], see Subsection 3.2, from row #352:
“Let us now discuss the Joule heating capability in view of the use of such strips as heating elements. Standard requirements for the heat flux needed for de-icing and anti-icing are falling in the range of tens of kW/m2, depending on the specific systems and conditions [39]. For our purposes, the project targets are 50 W/m2 and 25 W/m2 for de-icing and anti-icing, respectively. As mentioned, the power dissipated by the strip is about 1.05W, that provide an average value of the power density of 95.8 kW/m2 (assuming a uniform distribution of the temperature along the whole strip). This value would be enough both for de-icing and anti-icing applications. However, it can be observed from Fig.6 that the graphene heater is characterized by an almost flat distribution of the temperature on almost all the strip, except for two regions of few mm in touch with the electrodes. Considering this behavior, if we refer to this inner zone the power density can be estimated as equal to 213 W/m2, so providing an excellent performance.
Note that in real de-icing and anti-icing applications, these values of power intensity have to be imposed in continuous mode with no failure for hot-spots over the whole heating surface. Therefore, the uniform distribution of temperature in the inner part of the strip and the absence of hot spots (Fig.6) is a highly desirable feature for a heater. Note that this behavior is observed not only at steady state but also at each time instant during the transient (see the flat areas in Fig.7). Similar behavior is observed for the other two graphene films.”
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>REVIEWER:
8. Authors may refer the below mentioned papers in the introduction section.
- Journal of composites Science, 2021, 5(7), 181.
- Nanoscale,2010, 3, 86–95.
- Applied Sciences, 2018, 8(9), 1438
- Applied Sciences 2020, 10(5), 1753
>> AUTHORS’ REPLY
We thank the reviewer for suggesting us such references, that have been added to the reference list (see [8]-[9], [11]-[12]) and commented in the introduction.
Reviewer 2 Report
Authors have presented their interesting results from testing Nanesa's films. Here are some thoughts to improve the quality of the work:
1) line 87 opf -> of
2) Authors mention the tested films as "graphene" films, but from the dimensions of the flakes the term does not look right. The flakes could be called as graphite micro platelets. See please here: https://iopscience.iop.org/article/10.1088/2053-1583/aafc6e
Of course I understand that Nanesa sells these films by this name and you may leave it as it is, but an explanatory comment should be added to manuscript.
3) line 232 an->and
4) In Table 4, please explain column 4.
5) A comment on results of Table 2 should be added: as Cu looks that has two orders of magnitude lower ρ, why these films are novel?
6) line 275: please add examples of conventional conductors and values for their performance for comparison.
7) what electrical power was need for this experiment? It should be mentioned.
8) a better discussion for thermal emissivity evaluation and Joule heating capability results is necessary.
Finally, as these films are candidates for aerospace applications, mechanical characterization could be performed, perhaps in future work. The brittle character of Nanesa's films could be a negative factor for future applications.
Author Response
> REVIEWER:
Authors have presented their interesting results from testing Nanesa's films. Here are some thoughts to improve the quality of the work:
1) line 87 opf -> of
>> AUTHORS’ REPLY: Fixed, thank you.
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>REVIEWER:
2) Authors mention the tested films as "graphene" films, but from the dimensions of the flakes the term does not look right. The flakes could be called as graphite micro platelets. See please here: https://iopscience.iop.org/article/10.1088/2053-1583/aafc6e
Of course I understand that Nanesa sells these films by this name and you may leave it as it is, but an explanatory comment should be added to manuscript.
>> AUTHORS’ REPLY
We agree with the reviewer, as this material cannot be classified as graphene, and the suggested name (graphite microplatelets) would be more appropriate. However, following the reviewer’s suggestion, we have left the commercial name used by Nanesa, but we have now clarified that this is not graphene but a commercial graphene-related material. To clarify this point, we have added the following sentences into the introduction, citing the reference suggested by the reviewer (see [10]):
From row #40:
“However, the materials that can be strictly denoted as graphene or graphene-related are of a limited interest for industrial applications, given the well-known issues related to their fabrication and integration costs. Therefore, recently attention has been paid to the so-called commercial graphene materials, that strictly speaking are more correctly to be classified as graphitic materials, whose performance are not so outstanding as the real graphene, but can provide a satisfactory compromise between the needs of improving the performance and of using an industrially appealing material [10]. Materials based on Graphene Nanoplatelets (GNPs) are among the most promising candidates to meet the above trade-off. Indeed, GNPs are irregular flakes of few-layers graphene [11]-[12] that can be produced in several ways, including high-yield industrially scalable techniques such as the microwave irradiation discussed in the present paper. Industrial materials can be produced from GNPs, such as for instance nano-composites.”
and from row #67:
"The materials analyzed in this paper are macroscopic strips, industrially fabricated by assembling GNPs produced from expandable graphite. Strictly speaking, this material should be classified as graphite micro-platelets, but for the sake of simplicity hereafter we will refer to as industrial graphene, with the meaning so far discussed.
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> REVIEWER:
3) line 232 an->and
>> AUTHORS’ REPLY. Fixed, thank you.
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>REVIEWER:
4) In Table 4, please explain column 4.
>> AUTHORS’ REPLY.
Column 4 of Table 4 refers to the results obtained by considering the composite with a higher fraction of binder (polyurethane), specifically 20% (compared to 5% in column 3 and 0% in column 2). The emissivity of polyurethane is about 0.90, hence the result in column 4 is coherent with the presence of a higher percentage of binder. We have added the following sentence (see Subsection 3.2, from row #345):
“Note that the emissivity of the binder (polyurethane) can be assumed equal to 0.90, hence by increasing the percentage of binder, the emissivity is expected to increase, as coherently reported in Table 4.”
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> REVIEWER:
5) A comment on results of Table 2 should be added: as Cu looks that has two orders of magnitude lower ρ, why these films are novel?
6) line 275: please add examples of conventional conductors and values for their performance for comparison.
>> AUTHORS’ REPLY
We reply to both questions, since the answer comes from the same consideration. Graphene or graphitic resistors are candidate to realize heating elements in such applications not because of their electrical properties but mainly because of the huge reduction of weight compared to conventional conductor.
Considering the reviewer’s comment, we added the following sentence as a comment to Table 2 to clarify this point, see Subsection 3.1, from row #300.
“Note that the electrical performance of the industrial graphene is far from those exhibited by conventional conductors like copper, see Table 2. However, although the resistivity is much higher, one of the main reasons for proposing graphene instead of copper is the possibility of reducing the weight, that is a crucial constraint in aeronautical applications. Indeed, copper density is about 9 g/cm3, whereas that for the strips analysed here is in the range 0.0020-0.045 g/cm3. Therefore, given the same dimensions, a copper strip weight will be about 2100 or 2500 times larger than the graphene strips.”
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> REVIEWER:
7) what electrical power was need for this experiment? It should be mentioned.
>> AUTHORS’ REPLY
We have specified the electrical power dissipated during the experiment, see subsection 2.3, from row #250:
“This fixed current level of 1A corresponds to a variable value of the electrical power, as the equivalent resistance of the strip varies as an effect of temperature rise. In the experiment carried out here and described in the next Section, the baseline temperature was T=26.8 °C (see Fig.7): the electrical power adsorbed at such a temperature was 1.05 W.”
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> REVIEWER:
8) a better discussion for thermal emissivity evaluation and Joule heating capability results is necessary.
>> AUTHORS’ REPLY
We have provided a more detailed discussion about the procedure that is adopted to estimate the thermal emissivity (see sentences highlighted in red in subsection 2.3, starting from row #216). As it is shown, the emissivity comes from the solution of the system of equations (4) and (5), whose parameters are derived from the experiment that now is described with more details.
As for the Joule heating capability, we have provided a more extensive discussion of the obtained results, in view of using the strips as heaters, see 3.2, from row #352:
“Let us now discuss the Joule heating capability in view of the use of such strips as heating elements. Standard requirements for the heat flux needed for de-icing and anti-icing are falling in the range of tens of kW/m2, depending on the specific systems and conditions [39]. For our purposes, the project targets are 50 W/m2 and 25 W/m2 for de-icing and anti-icing, respectively. As mentioned, the power dissipated by the strip is about 1.05W, that provide an average value of the power density of 95.8 kW/m2 (assuming a uniform distribution of the temperature along the whole strip). This value would be enough both for de-icing and anti-icing applications. However, it can be observed from Fig.6 that the graphene heater is characterized by an almost flat distribution of the temperature on almost all the strip, except for two regions of few mm in touch with the electrodes. Considering this behavior, if we refer to this inner zone the power density can be estimated as equal to 213 W/m2, so providing an excellent performance.
Note that in real de-icing and anti-icing applications, these values of power intensity have to be imposed in continuous mode with no failure for hot-spots over the whole heating surface. Therefore, the uniform distribution of temperature in the inner part of the strip and the absence of hot spots (Fig.6) is a highly desirable feature for a heater. Note that this behavior is observed not only at steady state but also at each time instant during the transient (see the flat areas in Fig.7). Similar behavior is observed for the other two graphene films.”
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> REVIEWER:
Finally, as these films are candidates for aerospace applications, mechanical characterization could be performed, perhaps in future work. The brittle character of Nanesa's films could be a negative factor for future applications.
>> AUTHORS’ REPLY.
Actually, mechanical performance is under investigation but we would like to point out that the proposed strips will not be used "as they are" but enveloped in a glass-fibre reinforced material GFRM layers to guarantee either mechanical strength than electrical connections protective system. In addition, in order to enhance the mechanical performances of such multi-layered structures, different methodologies are under examination.
In particular, we are investigating graphene coatings of different non-woven supports; they are all lightweight, advanced non-woven fiber veils, bonded together in a random fiber matrix. For a stable integration with graphene, an epoxy GNP added dispersion has been selected and formulated to be sprayed on the fiber surface.
The results of these investigations will be published in future works, as they are not assessed at this time.
However, following the suggestion of the reviewer, we have added the following sentence to the conclusion, to highlight the importance of studying the mechanical performance:
“Work is in progress to assess the mechanical performance of the strips studied in this paper, when considering them in a realistic assembly, where they will be enveloped in a glass-fibre reinforced material. In order to enhance the mechanical performance of such multi-layered structures, different methodologies are under examination, and will be discussed in future works.”
Round 2
Reviewer 1 Report
The authors have well revised the manuscript.
