Grain Boundary Diffusion Sources and Their Coating Methods for Nd-Fe-B Permanent Magnets

Round 1

Reviewer 1 Report

• For the different GBD sources, it should be added under which conditions (temperature, time duration, single/multi-stage, ...) the respective diffusion heat treatment is to be performed. At least for the "HRE-based diffusion sources" variant, some temperatures are already specified at which diffusion/reaction takes place.
• Does the choice of tempering program also depend on the type of coating method illustrated in Figure 5?
• It is known from the literature that after heat treatment often a part of the surface of the magnet is removed. It would therefore be beneficial for the paper if the authors could add in the paper how this is usually done in the different variants, provided that there is information on this aspect in the literature.

Author Response

Response to the reviewer #1’s comments:

1. For the different GBD sources, it should be added under which conditions (temperature, time duration, single/multi-stage, ...) the respective diffusion heat treatment is to be performed. At least for the "HRE-based diffusion sources" variant, some temperatures are already specified at which diffusion/reaction takes place.

Response: Thanks for your suggestion, we have added the relevant descriptions in the paragraph 1 of the section 2 “Development of Diffusion Sources and Their Fabrication”.

2. Does the choice of tempering program also depend on the type of coating method illustrated in Figure 5?

Response: The choice of tempering does not have a direct relationship with the type of coating method, it mostly depends on the type of diffusion sources.

3. It is known from the literature that after heat treatment often a part of the surface of the magnet is removed. It would therefore be beneficial for the paper if the authors could add in the paper how this is usually done in the different variants, provided that there is information on this aspect in the literature.

Response: This is an interesting issue. Yes, for some diffusion sources, a part of the magnet surface may be detached after the diffusion heat treatment. However, there is almost no investigation about this phenomenon. The relationship between the type of diffusion source and the detachment of magnet surface is not clear. Therefore, we did not discuss this topic in this manuscript. However, we think that this issue should be noticed by researchers.

Reviewer 2 Report

Presented article is a review article about processing of Nd based magnets. Article can be interesting for researchers as well as industry working in this field. The topic is quite relevant. However, the structure of Nd based magnets is not explained in detail. The GBD process is not explained in detail. There more remarks on manuscript. As for review article the list of citing literature is quite short. There are wide discussion on grain boundary diffusion, but what about bulk diffusion, i.e. the diffusion inside of grains. Is it neglected, does it not plays any role?

1. That is a review article and from the first sentence in abstract it must be obvious, that this is a review article.
2. What is Nd2Fe14B 35 (2:14:1 phase) compound Curie point (Tc)?
3. Line 36 “ high temperature sensitivity of 36 anisotropy field (HA)” needs to clarify
4. line 34 “ without addition of heavy rare earth (HRE) have insufficient coercivity” word “elements” is missing.

5 Statement line 40 “  Grain boundary diffusion (GBD) process, which was firstly proposed in 2005”, please comment more detail, because GBD itself is known much more earlier. Explain what is Hq?

6. sentence line 55-56 needs corrections.
7. line 73 “which is lower than the diffusion temperature”, what means “diffusion temperature”?
8. line 271, the statement “the coating methods for GBD sources can be mainly classified into three types: adhesive coating, electrochemical deposition, and physical vapor deposition.”. Why chemical vapor deposition CVD is not suitable process?

Author Response

Response to the reviewer #2’s comments:

Presented article is a review article about processing of Nd based magnets. Article can be interesting for researchers as well as industry working in this field. The topic is quite relevant.

Response: Thanks for your positive comments.

However, the structure of Nd based magnets is not explained in detail. The GBD process is not explained in detail. There more remarks on manuscript. As for review article the list of citing literature is quite short.

Response: Please note that this manuscript mainly discussed the development of diffusion sources and their coating methods, but not the structure-properties relationships. Several review articles [12-16] have discussed the structure-properties relationships of the GBD process in detail. Based on your advice, we have added more remarks in the manuscript.

There is wide discussion on grain boundary diffusion, but what about bulk diffusion, i.e., the diffusion inside of grains. Is it neglected, does it not play any role?

Response: The so-called “grain boundary diffusion process” for Nd-Fe-B magnets mainly employs the grain boundary diffusion for enhancing the coercivity. In this process, the diffusion mainly occurs in the grain boundaries, which enhances the coercivity by modify the grain boundary, although bulk diffusion may be also involved. The body diffusion mainly modifies the 2:14:1 phase lattice with the introduced elements. Based on your advice, we have added some description about the bulk diffusion in some paragraphs of section 2 “Development of Diffusion Sources and Their Fabrication”.

1. That is a review article and from the first sentence in abstract it must be obvious, that this is a review article.

Response: Thanks for your advice. As suggested, we have modified the Abstract. One sentence “Here we give a special review on the grain boundary diffusion sources and their coating methods” has been added.

2. Line 35, what is Nd₂Fe₁₄B (2:14:1 phase) compound Curie point (T_c)?

Response: The Curie point refers to the temperature at which the spontaneous magnetization of magnetic materials decreases to zero. It is the critical point at which ferromagnetic or ferromagnetic materials become paramagnetic. The Curie point of Nd₂Fe₁₄B compound is ~312 ℃.

3. Line 36 “high temperature sensitivity of 36 anisotropy field (H_A)” needs to clarify.

Response: The sentence has been revised as: “The magnets in the motors and generators should operate at the temperatures greater than 150 ℃ [3], but the Nd-Fe-B magnets without addition of heavy rare earth (HRE) elements have insufficient coercivity (H_cj) to withstand the demagnetization field at high such temperatures because the hard magnetic Nd₂Fe₁₄B (2:14:1 phase) compound has a low Curie point (T_c) of ~312 ℃, and its anisotropy field (H_A) decreases drastically with the increasing temperature [5, 6].”

4. line 34 “without addition of heavy rare earth (HRE) have insufficient coercivity” word “elements” is missing.

Response: Thanks to point out this. We have added the word “elements” into this sentence.

5. Statement line 40 “Grain boundary diffusion (GBD) process, which was firstly proposed in 2005”, please comment more detail, because GBD itself is known much earlier. Explain what is Hq?

Response: Thank you. We have modified this sentence as: “Grain boundary diffusion (GBD) process for the Nd-Fe-B magnets, which was firstly proposed in 2005,”.

  In addition, the “H_q” is actually “H_cj”, which means the intrinsic coercivity and has been defined at Line 34.

6. sentence line 55-56 needs corrections.

Response: Thanks for your suggestion. This sentence has been modified as: “It gave an idea that the non-rare earth (non-RE) compound or alloy can be used to modify the grain boundary (GB) phase as the next generation of diffusion source.”

7. line 73 “which is lower than the diffusion temperature”, what means “diffusion temperature”?

Response: We have revised the phrase of “diffusion temperature” to “temperature of the diffusion heat treatment”.

8. line 271, the statement “the coating methods for GBD sources can be mainly classified into three types: adhesive coating, electrochemical deposition, and physical vapor deposition.”. Why chemical vapor deposition CVD is not suitable process?

Response: The coating methods of diffusion sources mainly depends on the types of diffusion sources. Compared with CVD, other coating methods can deposit the reported diffusion source much easier at room temperature. In contrast, CVD generally needs high reaction temperatures, which is not beneficial to control the magnetic properties of the Nd-Fe-B magnets. Therefore, at present, few chemical vapor deposited diffusion source was reported.

The sentence of: “As shown in Fig. 5, the coating methods for GBD sources can be mainly classified into three types: adhesive coating, electrochemical deposition, and physical vapor deposition.” has been revised as: “As shown in Fig. 5, at present, the coating methods for GBD sources can be mainly classified into three types: adhesive coating, electrodeposition, and vapor deposition.”