Influence of Carbon Nanotube Attributes on Carbon Nanotube/Cu Composite Electrical Performances

Round 1

Reviewer 1 Report

I thoroughly enjoyed reading the manuscript. I recommend publication as it is.

Author Response

We would like to thank the reviewer for favorably considering our paper and recommending the publication of our manuscript as is.

Reviewer 2 Report

Overall, the paper presents an interesting study between MW-CNT/Cu Wires, SW-CNT/Cu Wires, and SW-CNT/Cu Pillars that have not been comparatively explored in the literature. It is a useful publication in both the specific field of copper-CNT composite conductors as well as CNT electrical conductors more broadly. Generally, the experimentation is sound and conclusions are supported, but there are parts of the paper that require some additional rigor or clarification. Suggestions are mentioned below:
1) A first area for improvement deals with lines 247-248 where the authors claim that the void fraction of the SW-CNT/Cu Wires make an accurate assessment of the volume fraction of CNTs difficult to ascertain. This is a key aspect of the comparison between SW-CNT/Cu Wires and MW-CNT/Cu Wires that a seemingly large amount of the analysis depends on. One possible solution would be to densify the SW-CNT/Cu Wires and CNT templates through radial drawing dies or a rolling mill to compress the empty space and then re-measure the cross-sectional area. As an interesting result, the impact of the void space or lack thereof on the conductivity and TCR could be determined. As a second option, the deposited volume percent could be determined through the deposited copper mass, density of copper, mass of the starting SW-CNT template, and the skeletal density of the SW-CNT template. 
2) The deposited mass of copper as a percent of each composite’s mass is also an important factor for comparison. If the mass percent of copper is vastly different, it needs to be emphasized and included in the paper’s discussion. It appears using the electrodeposition times and currents (lines 152-156) that a similar total number of coulombs per cm^2 are delivered. However, the percentage of deposition charge delivered from organic vs. aqueous plating is very different. The MW-CNT wire seems to receive 216 C/cm^2 from organic plating and 36 C/cm^2 from aqueous plating, while the SW-CNT composites receive 21.6 C/cm^2 from organic plating and 216 C/cm^2 from aqueous plating. What are the approximate deposition efficiencies from each technique as determined by deposited mass? Do the composites have similar deposited copper mass from each technique? For example, if the quality of the deposited copper from the aqueous deposition is more pure than that from the organic deposition and deposition efficiencies are similar, it would be expected that the composites produced from a majority of aqueous deposition would have better conductivity. This should be discussed or explored further in the paper. Conductivity measurements of the seeded composites could help understanding of the process. Also, a supplemental table of deposited mass percent from each technique on each composite would be beneficial.
3) In applications where conductor mass is important, the specific conductivity (the reciprocal of the resistivity-density product) is useful. As this metric eliminates the cross-sectional area from its calculation, it is immune to issues of void space within the composites encountered in the SW-CNT/Cu Wires. From Table 1, it appears that the specific conductivity of the SW-CNT/Cu Wire is over 50% higher than that of the copper control. This certainly bears mention and discussion in the text.
4) The method of cross-sectional measurement is mentioned on lines 161-172, but the measured values are not given. This makes verification of the results difficult. They should be added into the text or onto the respective images of the cross-sectional areas.
5) The authors mention the carbonaceous impurities deposited onto the SW-CNT pillars as a possible reason for their lower conductivity compared to the SW-CNT wires. Since all three carbon templates are produced through different methods, it is possible that other impurities (e.g. residual catalyst) may have an impact on the copper-CNT interface. Were EDX scans taken during cross-sectional analysis, and did they reveal any other impurity signals? Are any other impurities known or expected from these synthesis methods? Discussion of other expected impurities would be beneficial.

Author Response

We thank the referee and editors for considering our paper.

Please find attached our point-by-point response to the referee-comments. 

Our responses are in purple. 

Changes made in the manuscript based on reviewer suggestions are in red font. Copies of the altered text are furnished in the document under each point-by-point response along with corresponding section- and line-numbers in the revised manuscript (without tracking). 

We hope the revisions are satisfactory to qualify our article’s acceptance for publication.

Looking forward to the final decision on our paper.

Regards,

Rajyashree Sundaram, Atsuko Sekiguchi and Takeo Yamada

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors studied the effect of CNT structural parameters on the electrical properties of Cu-CNT composites. It is known that the weak adhesion between CNTs and Cu matrix is one of the main issues which hinders the application of the CNT/Cu composites as high-performance conductors. Therefore, the subject of this study can be important and shed more light on the possible methodologies for improving the electrical properties of these hybrid materials. The manuscript is well written and presents results in a convincing manner. It also provides a comparison between structural and electrical properties of fabricated composites using various CNT-based templates. I think the manuscript has some interesting findings which may be suitable for publication after the following revisions:

1. The previous theoretical results (J. Phys. Chem. C 2014, 118, 13936) on the transport properties of CNTs showed that the conductivity of MW-CNTs is much larger than that for the corresponding SW-CNT due to more transport channels available in MW-CNTS. In addition, the report suggests that MW-CNTs are less sensitive to applied deformation in comparison with SW-CNTs. Based on this report, one should expect that MW-CNTs are better candidates for Cu-composite than SW-CNT if the interface properties are similar. The authors should explain the role of interfacial geometries in the transport properties of the different samples together with the discussion on the mentioned paper.

2. As mentioned in the manuscript, it is surprising that SW-CNT/Cu wires show copper-level electrical conductivities despite voids seen in the cross-section. In order to be better understand the TCR results of SW-CNT/Cu wires, the authors should discuss the effect of the phonon-mediated scattering in the transport properties of these composites. Did the authors compare the thermal conductivity of the SW-CNT/Cu wires with pure copper?

3. The authors wrote that “we hypothesize that better CNT lattice quality can also enhance CNT-Cu interactions and electron transport across the interface”. In principle, the presence of defects or functionalization of the CNTs may improve bonding with the composite matrix and this may be beneficial for electrical properties. The logic of the authors should be explained in more detail.

4. The authors discussed different methodologies for better incorporation of the CNTs into the Cu matrix. Indeed, a uniform integration of CNTs into the metal matrix is still a significant challenge. The recent report showed that that alloying the Cu with Ni or Cr can significantly improve wettability, adhesion and conductivity in Cu-CNT composites (Nanoscale 2017, 9, 8458). This reference should be added due to the direct relevance with the subject of this work.

Author Response

We thank the referee and editors for considering our paper.

Please find attached our point-by-point response to the referee-comments. 

Our responses are in purple. 

Changes made in the manuscript based on reviewer suggestions are in green font. Copies of the altered text are furnished in the document under each point-by-point response along with corresponding section- and line-numbers in the revised manuscript (without tracking). 

We hope the revisions are satisfactory to qualify our article’s acceptance for publication.

Looking forward to the final decision on our paper.

Regards,

Rajyashree Sundaram, Atsuko Sekiguchi and Takeo Yamada

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I am satisfied with the authors' response to my initial review and recommend the paper for publication after a final proofing check by the publisher. I would look forward to further studies investigating the impact of carbon nanotube type and morphology on composite conductor electrical properties in the future.

Reviewer 3 Report

The authors have made point-by-point responses to my comments. Therefore, I recommend the publication of this work in C.