Morphological Characteristics of W/Cu Composite Nanoparticles with Complex Phase Structure Synthesized via Reactive Radio Frequency (RF) Thermal Plasma

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The experimental method is not clear. A process diagram describing the stages should be included. It is not clear whether the W and Cu oxides are commercial or have been obtained by chemical or physical synthesis. The industrial application of the composite, electrodes, nuclear industry, etc. is not clear.

A small modification to the explanation of the experimental procedure would be necessary. Clarifying the production of the oxides and their chemical reduction to obtain the nano composite would also allow comparing this manufacturing method with other existing ones. Likewise, the mechanical and wear properties would be missing, in the case of being used as electric welding electrodes.

Author Response

I appreciate your valuable comments. The manuscript was modified based on your suggestion as follows:

1. Page 2

A process diagram is inserted in the manuscript as Fig. 1.

Fig. 1. Schematic illustration of a reactive RF thermal plasma system.

 

2. Page 2, 3^rd paragraph, 2^nd line

We used commercial oxide powder as raw materials. We added the information of WO3, and CuO in the manuscript:

WO₃ (>99.99%, LTS Inc., USA) and CuO (>99.99%, LTS Inc., USA)

3. Cu-coated W composite nanoparticles could be utilized for various applications such as an electrode for electrical discharging machine, earthing material for electronic devices. This work mainly aims to study the morphology and sintering mechanism so that the mechanical and wear properties will be dealt with future study.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. The detailed information must be given for the raw oxide powder (such as purity and morphology).

2. In Materials and Methods section, the author mentioned “A detailed schematic image of the system is shown in Fig. 1.” However, the Fig. 1. was the morphology and EDS results of the synthetic powders. Where is the reactive RF thermal plasma system overview image?

3. As author mentioned in the manuscript, the W-Cu composite was the nanoparticles. At the same time, the nano-particle size was about from 1nm to 100nm. Therefore, the Laser particle size analyzer should be used to confirm its particle size distribution range.

4. Please indicate which areas in the Fig.1 are targeted by the point probe analysis.

5. During the evaporation process, tungsten oxide evaporates first and then copper oxide evaporates. Therefore, W may deposit on the surface of unevaporated copper oxide. Has the author considered this ?

6. The diffraction peak of β-W was ambiguity in Fig. 2a.

7. Similarly, please indicate which areas in the Fig.3 and Fig.4 are targeted by the point probe analysis.

8. The EDS result in Fig. 5 was ambiguity.

9. Obviously, the Cu segregation was also related to the melted Cu (its fine particle size)  at 1000℃.

10. According to the author’s study, the W/Cu composite powder will exhibit elemental segregation, resulting in non-uniform materials when sintered at high temperatures, which may affect their performance. May I ask what the purpose of this powder is and where the innovation of this article lies?

Comments on the Quality of English Language

Language needs further refinement.

Author Response

1. The detailed information must be given for the raw oxide powder (such as purity and morphology).

 

I appreciate your valuable comment. The detailed information of raw material are presented in the manuscript as follow:

 

Page 2, 3^rd paragraph, 2^nd line

WO₃ (>99.99%, LTS Inc., USA) and CuO (>99.99%, LTS Inc., USA) with irregular shape

 

2. In Materials and Methods section, the author mentioned “A detailed schematic image of the system is shown in Fig. 1.” However, the Fig. 1. was the morphology and EDS results of the synthetic powders. Where is the reactive RF thermal plasma system overview image?

 

Thank you for your comment. The schematic image is inserted in the manuscript.

Page 2

Fig. 1. Schematic illustration of a reactive RF thermal plasma system.

 

 

 

3. As author mentioned in the manuscript, the W-Cu composite was the nanoparticles. At the same time, the nano-particle size was about from 1nm to 100nm. Therefore, the Laser particle size analyzer should be used to confirm its particle size distribution range.

 

I appreciate your sharp comment. Since the particles have a particle size of several nm to several hundred nm, the uncertainty is high with the current particle size analyzer due to the aggregation phenomenon between nano-sized particles. Therefore, the equivalent diameter was measured using the image obtained through TEM microstructure analysis and the particle size was calculated. As you suggested, the average particle size of W-Cu composite is added in the manuscript as follow:

Page 3, 3^rd paragraph, 5^th line

The cuboid and spherical W-Cu nanoparticles are well observed in the TEM images (Fig. 2) with average particle sizes of 26.2 nm.

 

4. Please indicate which areas in the Fig.1 are targeted by the point probe analysis.

I appreciate your sharp comment. In Fig.1, only elemental mapping was performed using SEM. Therefore, the content related to the point analysis was deleted and substituted to the elemental mapping in the manuscript.

I appreciate your valuable comment. As you indicated, Fig. 1 only shows elemental mapping results. Therefore, the manuscript was revised as you suggested. The phrases related to the point analysis were deleted.

 

5. During the evaporation process, tungsten oxide evaporates first and then copper oxide evaporates. Therefore, W may deposit on the surface of unevaporated copper oxide. Has the author considered this?

I appreciate your sharp point. We also considered your opinion. XRD results showed that peaks for oxides were not found. Therefore, we assumed that copper oxide was evaporated.

 

6. The diffraction peak of β-W was ambiguity in Fig. 2a.

As you indicated, peaks of β-W was ambiguous for the samples heat-treated at 800 °C. We added the insets for the region showing peaks of β-W.

Thank you for your valuable comment. Figure was modified to minimize ambiguity. 

Page 5. Figure 3

Figure 3. (a) X-ray diffraction pattern showing phase evolution and (b) phase fractions of α-W, β-W with respect to the heat treatment temperature in W/Cu composite nanoparticles synthesized via RF thermal plasma process.

 

7. Similarly, please indicate which areas in the Fig.3 and Fig.4 are targeted by the point probe analysis.

As same as previous query, the manuscript was revised as you suggested. The phrases related to the point analysis were deleted.

 

8. The EDS result in Fig. 5 was ambiguity.

Fig. 5 was revised. EDS result in Fig. 5 was reinserted in the figure.

Page 7

Figure 6. (a) Cross-sectional morphology of Cu segregated region in FIBed sample heat-treated at 1,000 ℃ showing severe segregation of Cu particlse and W particle growth. EDS results at the position from 1 to 6 in (a) are represented in (b).

 

9. Obviously, the Cu segregation was also related to the melted Cu (its fine particle size) at 1000℃.

I appreciate your opinion and we also agree with you. Possibly, nanosized-Cu particle could be melted and then segregation occurs.

Page 8, 1^st paragraph, 4^th line

and local melting Cu particles may be responsible for the segregation.

 

10. According to the author’s study, the W/Cu composite powder will exhibit elemental segregation, resulting in non-uniform materials when sintered at high temperatures, which may affect their performance. May I ask what the purpose of this powder is and where the innovation of this article lies?

As indicated in the introduction section, RF thermal plasma offers the broad material selectivity compared to the conventional method. RF thermal plasma makes it easy to apply Cu to the W surface in the form of a core shell, and the composite synthesized in this way has the advantage of excellent sintering properties and re-arrangement and dispersibility of W nanoparticles during the sintering process. When this process is compared to other methods, the process is simple, clean, and productive.

In case of infiltration, the process is complicated, the loss of Cu is high, and there are disadvantages of having to make a W skeleton. Also, surface friction occurs during the process, resulting in the existence of pores.
In case of the mechanical alloying method, it is difficult to manufacture a composite by using nanoparticles, and there is a disadvantage in that the composition is uneven and it is difficult to make a core shell.
The mechano-chemical method uses stainless steel containers and balls during the ball milling process, so the milling powder is likely to be contaminated by Fe, Ni, Cr, etc., which act as an active sintering additive.

In addition, W and Cu do not make a solution and this makes the formation of W-Cu composite difficult. In this study, W-Cu composite nanoparticles were successfully fabricated and the segregation mechanism was suggested.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article of C. Han et al. describes the preparation of W-Cu composite using the reactive radio frequency (RF) thermal plasma method as well as the phase and morphological changes of the synthesized metal particles on annealing. The novelty of the research consists in tailoring the RF plasma method for fabrication of mentioned composite, aiming to find an alternative to commonly used infiltration and liquid phase sintering technologies. The targeted composite material is known for its excellent electrical and thermal performance, which justifies the interest to undertaken research. However, the article has a number of shortcomings listed below and needs to be thoroughly revised prior to publication.

 1) It should be precised in part 2, mass or molar ratio of WO3:CuO 4:1 in the feedstock is indicated.

 2) It is not clear why only one precursor composition (4:1) was tested. Is this optimal ratio? An explanation should be given in the manuscript text.

 3) An evaluation of particle size distribution in the material according to STEM data should be provided and discussed.

4) It is strongly recommended to use the diffraction data measured by the authors for determination of the crystallite sizes of metals after preparation and at different steps of the heat treatment.

5) The particles dimensions evidenced by STEM and XRD should help the authors to clarify following important questions in the paper. (i) What are the initial particle/crystallite sizes of the copper and tungsten formed by the RF plasma synthesis? (ii) How the initial size characteristic changes upon annealing?

6) The homogeneity is known to be a key parameter for the W-Cu composites. By this reason, it appears to be necessary to discuss the homogeneity characteristics of the obtained material in comparison with those resulted from other synthesis methods.

Author Response

The article of C. Han et al. describes the preparation of W-Cu composite using the reactive radio frequency (RF) thermal plasma method as well as the phase and morphological changes of the synthesized metal particles on annealing. The novelty of the research consists in tailoring the RF plasma method for fabrication of mentioned composite, aiming to find an alternative to commonly used infiltration and liquid phase sintering technologies. The targeted composite material is known for its excellent electrical and thermal performance, which justifies the interest to undertaken research. However, the article has a number of shortcomings listed below and needs to be thoroughly revised prior to publication.

• It should be precised in part 2, mass or molar ratio of WO3:CuO 4:1 in the feedstock is indicated.

Thank you for your valuable comment. The molar ratio was calculated and added in the manuscript.

Page 2, 3^rd paragraph, 9^th line

of which molar ratio was 0.56:0.44 ((WO₃: CuO).

 

• It is not clear why only one precursor composition (4:1) was tested. Is this optimal ratio? An explanation should be given in the manuscript text.

I appreciate your sharp indication. The selected composition is optimal when the densification and electrical conductivity are considered. The manuscript was revised as follow:

Page 2, 3^rd paragraph, 10^th line

The selected composition is an optimal condition considering densification and high electrical conductivity. Complete densification is difficult under a composition of 20 wt% Cu or less because the contact angle of Cu with respect to W is large [15].

 

• An evaluation of particle size distribution in the material according to STEM data should be provided and discussed.

I appreciate your sharp comment, and I totally agree with your opinion. Since the particles have a particle size of several nm to several hundred nm, the uncertainty is high with the current particle size analyzer due to the aggregation phenomenon between nano-sized particles. Therefore, the equivalent diameter was measured using the image obtained through TEM microstructure analysis and the particle size was calculated. As you suggested, the average particle size of W-Cu composite is added in the manuscript as follow:

Page 3, 3^rd paragraph, 5^th line

The cuboid and spherical W-Cu nanoparticles are well observed in the TEM images (Fig. 2) with average particle sizes of 26.2 nm.

 

• It is strongly recommended to use the diffraction data measured by the authors for determination of the crystallite sizes of metals after preparation and at different steps of the heat treatment.

As same as query no.3, Average particle size was measured using TEM images. Thank you again for your valuable comment.

 

• The particles dimensions evidenced by STEM and XRD should help the authors to clarify following important questions in the paper. (i) What are the initial particle/crystallite sizes of the copper and tungsten formed by the RF plasma synthesis? (ii) How the initial size characteristic changes upon annealing?
• Initial particle size of composite particles was 26.2 nm and this is added to the manuscript.
• During annealing, particle size does not much vary due to the low probability of contact and the existence of W. The manuscript was revised as follow:

Page 6, 2nd paragraph, 7th line

Although the particles of W grow, the pressure and low temperature of liquid Cu and the low probability of contact with small and large particles of W hinder particle growth, so there is little change in particle size due to heat treatment.

 

• The homogeneity is known to be a key parameter for the W-Cu composites. By this reason, it appears to be necessary to discuss the homogeneity characteristics of the obtained material in comparison with those resulted from other synthesis methods.

Thank you for your valuable comment. As you suggested, the homogeneity was compared and discussed with other processes.

Page 8, 1^st paragraph, 7^th line

In addition, the element distribution of the sample made by the RF plasma process in this study is relatively more uniform compared to the nanoparticles made by the ball milling process [21]. Since stainless steel containers and balls are used during the ball milling process, so the milling powder is likely to be contaminated by Fe, Ni, Cr, etc., which act as an active sintering additive.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

This  manuscript can be accepted and published.

Reviewer 3 Report

Comments and Suggestions for Authors

In the revised version, the main drawbacks were corrected. In my opinion, the articles is now acceptable for publication.