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Article
Peer-Review Record

Low-Temperature Large-Area Zinc Oxide Coating Prepared by Atmospheric Microplasma-Assisted Ultrasonic Spray Pyrolysis

Coatings 2021, 11(8), 1001; https://doi.org/10.3390/coatings11081001
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Coatings 2021, 11(8), 1001; https://doi.org/10.3390/coatings11081001
Received: 18 July 2021 / Revised: 15 August 2021 / Accepted: 20 August 2021 / Published: 22 August 2021
(This article belongs to the Special Issue Intelligent Tribological and Functional Coatings)

Round 1

Reviewer 1 Report

  1. There is no addition to the literature or novelty.
  2. A number of works are available more on micro-plasma-assisted ultrasonic spray pyrolysis techniques, moreover, the material under study is explored more.
  3. “After vacuum treatment, the ZnO reached a resistivity of 1.0 × 10-3 Ω⋅cm and a transmittance of more than 80%”, give it as quantitatively as increase or decrease of resistivity in abstract.
  4. No explanation is provided why the morphology of the particle prepared by the atmospheric plasma-assisted process is different from the conventional spray technique.
  5. The change of Tribology Behavior between the films is not clear.
  6. Overall the presentation needs improvement.

Author Response

Point-to-Point Response to Referees’ Comments

Comments from Referee 1

  1. There is no addition to the literature or novelty.

Response:

We are very thankful to the referee for this precious comment. This article integrates the atmospheric micro-plasma system with the traditional ultrasonic spray pyrolysis deposition into a large-area ZnO synthesis system. The main goal of the new synthesis system is a large-scale and energy-efficient ZnO preparation process. In this study, various process parameters are studied and demonstrated. The parameters that meet the requirements of transparent conduction oxide are proposed. At the same time, the effect of process parameters on the characteristics of ZnO coating is also studied. The results presented in this article are of reference value for the development of new equipment for equipment.

 

  1. A number of works are available more on micro-plasma-assisted ultrasonic spray pyrolysis techniques, moreover, the material under study is explored more.

Response: We do appreciate the referee for this professional comment. This study focuses on the design and development of new integrated process equipment. It is therefore essential to understand whether the new integrated system can synthesize ZnO that meets TCO requirements. 

 

3.“After vacuum treatment, the ZnO reached a resistivity of 1.0 × 10-3 Ω⋅cm and a transmittance of more than 80%”, give it as quantitatively as increase or decrease of resistivity in abstract.

Response: We truly thank the referee for the crucial correction. In the revised manuscript, this sentence was modified as follows. “After vacuum treatment, the ZnO reached a 1.0 × 10-3 Ω⋅cm resistivity and a transmittance of 82%.”

 

  1. No explanation is provided why the morphology of the particle prepared by the atmospheric plasma-assisted process is different from the conventional spray technique.

Response:  Thank you for your professional comment. We added the discussion to 3.1 and the results and discussion of Figure 5 in the revised version. With the AP process, the film formation of ZnO can be effectively improved, and the synthesis temperature can be effectively reduced. However, a large amount of argon blown the reactant out of the reaction area, reducing the thickness of the coating.

 

  1. The change of Tribology Behavior between the films is not clear.

Response: We are very thankful for the referee for the valuable comment. The addition of AP makes the ZnO coating complete and robust. Therefore, the better load capacity in the same process temperature than the conventional ultrasonic spray pyrolysis process coating. The better load capacity is a significant factor in the better tribology properties of the hard coating.

 

  1. Overall the presentation needs improvement.

Response: We are highly thankful to the referee for the professional comment. Thank you for your valuable comments. The primary purpose of this article is to demonstrate the ability of the integrated system to prepare TCO ZnO. The effect of each process parameter on coating properties is discussed by synthesizing ZnO coatings with different process parameters.

Reviewer 2 Report

In this manuscript, the authors prepared ZnO coatings using the atmospheric pressure micro-plasma-assisted ultrasonic spray method at low temperature. The ZnO coatings were characterized by SEM, XRD. The electrical, optical, and tribology properties of them were measured and compared with these ZnO2 coatings prepared by regular spray pyrolysis and with vacuum annealing. Here are my comments:

  1. Since the authors already labeled the equipment with numbers in Figure 1, it would be better to explain each of them by using the number.
  2. What is the thickness of ZnO2 coatings in Figure 2 and 3? What is the ZnO coating growth rate with and without plasma? Does the temperature affect the growth rate?
  3. We suggest the authors also compare the grain size of ZnO coatings in Figure 2, 3, and 4.
  4. In Figure 7, why does ZnO400-APA show a higher transmittance at the beginning of the measurement (0 nm)? Does this offset contribute to the difference between ZnO400-APA and ZnO400-AP in the IR range?

Author Response

Comments from Referee 2

In this manuscript, the authors prepared ZnO coatings using the atmospheric pressure micro-plasma-assisted ultrasonic spray method at low temperature. The ZnO coatings were characterized by SEM, XRD. The electrical, optical, and tribology properties of them were measured and compared with these ZnO2 coatings prepared by regular spray pyrolysis and with vacuum annealing. Here are my comments:

 

  1. Since the authors already labeled the equipment with numbers in Figure 1, it would be better to explain each of them by using the number.

Response: Thank you for your valuable comment. In Figure 1 of the revised manuscript, the subsystems were illustrated by numbers.

 

  1. What is the thickness of ZnO2 coatings in Figure 2 and 3? What is the ZnO coating growth rate with and without plasma? Does the temperature affect the growth rate?

Response: We genuinely appreciate the referee for the comment. The evidence of film thickness and grain size is demonstrated in newly added figure 5.  The AP process does not increase the thickness of the coating, instead of decreasing the film thickness. It is because that a large amount of AP gas blows the reactant away from the reaction area.

 

  1. We suggest the authors also compare the grain size of ZnO coatings in Figure 2, 3, and 4.

Response: We are highly thankful for the referee for the comment. In the revised manuscript, the grain size of ZnO under various synthesis processes was demonstrated in the newly added figure 5. Furthermore, the AP increases the grain size comparing to the same substrate temperature between AP/spray and conventional spray pyrolysis deposition, indicating the improvement of the ZnO property.

 

  1. In Figure 7, why does ZnO400-APA show a higher transmittance at the beginning of the measurement (0 nm)? Does this offset contribute to the difference between ZnO400-APA and ZnO400-AP in the IR range?

Response: Thank you for your valuable comment. Figure 8 (because of the addition of Figure 5) represents the transmittance of the material for incident light at different wavelengths. The transmittance rate is higher, the lower absorption rate. The horizontal axis represents the wavelength of the incident light, ranging from 450-2600 nm, from visible light to infrared is used in this article. According to reference 38 (Serier, H. Gaudon, M.; Ménétrier, M. Al-doped ZnO powdered materials: Al solubility limit and ir absorption properties.  Solid State Sci. 2009, 11, 1192-1197.), the free carrier absorbs visible to infrared light,  and the higher the concentration of the free carrier, the stronger the absorption.  ZnO400-APA has a higher concentration of free carriers and therefore has a lower transmittance in the visible to the infrared light band.

Round 2

Reviewer 1 Report

The authors have addressed well all the necessary comments from my side.

Reviewer 2 Report

The authors have responded to my comments properly and revised the manuscript accordingly.

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