# Investigation of the influence of the annealing atmosphere on the structure of powders after recycling Nd-Fe-B magnets by solvothermal process

Abdelilah Chetouani, Fabienne Richomme, J.M. Lebreton, J.S. Thonikuzhiyil, P. Bernstein, J. Noudem

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## ► To cite this version:

Abdelilah Chetouani, Fabienne Richomme, J.M. Lebreton, J.S. Thonikuzhiyil, P. Bernstein, et al. Investigation of the influence of the annealing atmosphere on the structure of powders after recycling Nd-Fe-B magnets by solvothermal process. Journal of Magnetism and Magnetic Materials, 2024, 603, pp.172213. 10.1016/j.jmmm.2024.172213. hal-04709795

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Submitted on 21 May 2025

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Research article

# Investigation of the influence of the annealing atmosphere on the structure of powders after recycling Nd-Fe-B magnets by solvothermal process

A. Chetouani<sup>a, *</sup>, F. Richomme<sup>a</sup>, J.M. Le Breton<sup>a, *</sup>, J.S. Thonikuzhiyil<sup>b</sup>, P. Bernstein<sup>b</sup>,
J. Noudem<sup>b</sup>

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## ABSTRACT

The effect of annealing under vacuum and argon atmospheres on the structure of a Nd-Fe-B powder recycled by a solvothermal treatment was investigated by means of X-ray diffraction and Mössbauer spectrometry. The results show that vacuum annealing at 300 °C is accompanied by the complete desorption of hydrogen from the magnetic Nd₂Fe₁₄BHₓ phase that was obtained after the solvothermal treatment. Vacuum annealing above 650 °C leads to the decomposition of Nd₂Fe₁₄B into iron and Fe₂B. Annealing in argon atmosphere at 300 °C has almost no effect on the powder and the desorption of hydrogen from the Nd₂Fe₁₄BHₓ phase is partial. Argon annealing above 650 °C leads to the oxidation of the Nd₂Fe₁₄B phase.

## 1. Introduction

Recycling parts from used consumer goods is an eco-friendly way to both reduce waste and produce renewed materials. Rare earth materials, such as Nd-Fe-B magnets, are highly valued in various industries due to their high permanent magnet properties. They are commonly found in electrical devices like motors, hard disc drives, and even wind turbines [1]. These materials strike a good balance between magnetic performance and cost compared with other permanent magnet materials. The fact their use improves the efficiency and effectiveness of manufactured devices, leads to a higher demand for Nd-Fe-B magnets. Currently, significant amounts of rare earth elements are present in landfill waste, emphasizing the need for an environmentally friendly recycling process [2]. In the literature, several approaches have been investigated for recycling Nd-Fe-B magnets. Commonly discussed methods include hydrogen decrepitation [3], pyrometallurgical processes, and hydrometallurgical processes [4]. However, hydrogen decrepitation requires careful planning and appropriate management of the risks associated with the use of hydrogen. Pyrometallurgical processes involve high energy consumption and may generate emissions of polluting gases. Hydrometallurgical methods require large quantities of chemicals, such as acids, to dissolve Nd-Fe-B magnets. This high chemical consumption can result in additional costs and environmental impacts.

In this study, we specifically focus on the solvothermal method for recycling Nd-Fe-B magnets [5]. This approach offers significant advantages, including process simplification through a reduction in the

number of required steps. Additionally, it is characterized by low energy consumption, operating at temperatures generally below 250 °C. The solvothermal process consists of transforming magnets from WEEE (Waste from Electrical and Electronic Equipment) into a reusable powder, thus facilitating the manufacture of magnets by sintering having a variety of shapes and dimensions. In this perspective, the recycled powder was subjected to annealing under different atmospheres and at different temperatures in order to analyze its behavior before being used in the sintering process. This step is of crucial importance to evaluate the behaviour of the powder to heat treatment conditions and thus to optimize the subsequent sintering process.

The aim of this study is to investigate the structural behavior of the powder recycled by a solvothermal method, subjected to annealing under vacuum and argon atmospheres at different temperatures, in order to characterize the influence of the annealing conditions on the structure of the recycled powder. The powders were investigated by ⁵⁷Fe Mössbauer spectrometry, X-ray diffraction and scanning electron microscopy. The results are discussed as a function of both annealing atmosphere and temperature.

## 2. Experimental section

### 2.1. Preparation processes

The samples used for the recycling process are in the form of sintered arc segments, coated with a protective layer, consisting of Ni-Cu. These

<footnote><sup>a</sup> Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ, GPM UMR 6634, F-76000 Rouen, France</footnote>

<footnote><sup>b</sup> Normandie University, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000 Caen, France</footnote>

<footnote>* Corresponding authors at: GPM URM 6634 CNRS, Université de Rouen Normandie, 76000 Rouen, France.
E-mail addresses: abdelilah.chetouani@univ-rouen.fr (A. Chetouani), jean-marie.lebreton@univ-rouen.fr (J.M. Le Breton).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** XRD patterns of the initial Nd-Fe-B powder in the initially fragmented magnets (bottom), of powder treated in pure ethanol for 7 h (solvothermal) and of powders annealed at 300 °C, 650 °C, 1000 °C under vacuum and argon atmosphere. The unindexed peaks correspond to the Nd₂Fe₁₄B(Hₓ) phase. (Right) Enlargement over the 47-51° range (highlighted blue region in the XRD patterns), showing the shift of the Nd₂Fe₁₄B peaks down to low angles, in relation with the increase of the lattice parameters due to the insertion of H atoms, and the shift towards high angles due to partial or complete hydrogen desorption.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Rietveld refinement of the XRD pattern of the powder after solvothermal treatment.</figcaption></figure>
<table><caption>**Table 1**
Lattice parameters (deduced from the Rietveld method) of the phases present in the sample after solvothermal treatment.</caption><thead><tr><th>Phase</th><th>Structure</th><th>a (nm)</th><th>b (nm)</th><th>c (nm)</th><th>Unit cell volume (nm<sup>3</sup>)</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>B (*)</td><td>P42/<br>mnm</td><td>0.8804</td><td>0.8804</td><td>1.220</td><td>0.9456</td></tr><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub></td><td>P42/<br>mnm</td><td>0.8870</td><td>0.8870</td><td>1.230</td><td>0.9679</td></tr><tr><td>Nd(OH)<sub>3</sub></td><td>P 63/m</td><td>0.6429</td><td>0.6429</td><td>0.3741</td><td>0.1546</td></tr><tr><td>α-Fe</td><td>Im-3 m</td><td>0.2910</td><td>0.2910</td><td>0.2910</td><td>0.0246</td></tr></tbody></table>

(*) The lattice parameters of the Nd₂Fe₁₄B phase, obtained for the initial powder, are given for comparison.

magnets are obtained from computer hard disk drives (HDD) collected as WEEE. The separation of a Nd-Fe-B magnet from the hard disk drive in which it is integrated is carried out manually. Then, the magnet is heated at a temperature above the Curie temperature of the Nd₂Fe₁₄B phase (315 °C), to be thermally demagnetized.

The Ni-Cu coating of the magnet is removed using a scalpel. Then, the magnet is fragmented in a crusher into pieces having a size less than 5 mm. Fragmenting the magnets beforehand makes it possible to accelerate the pulverization kinetics [5]. The powder, weighing 2 g, is placed into a crystallization bomb, which contains 30 ml of ethanol, and heated at 225 °C for 7 h. At the end of the solvothermal reaction, the powder is separated by centrifugation and finally dried at 50 °C.

The powder obtained from solvothermal treatment was annealed at different temperatures (300 °C, 650 °C and 1000 °C) for 1 h, under vacuum (7.5 × 10⁻⁶ Torr, with continuous pumping) or argon atmosphere.

## 2.2. Characterization techniques

X-ray diffraction (XRD) analyses were performed by using Empyrean diffractometer equipped with Co Kα1 radiation (wavelength 0.178897 nm). ⁵⁷Fe Mössbauer spectrometry analyses were performed with a ⁵⁷Co source in a Rh matrix at room temperature. The parameters, including isomer shift (relative to metallic α-Fe at room temperature), quadrupolar splitting, quadrupolar shift, half width at half maximum and hyperfine field, are denoted as δ, Δ, ε, Γ₁/₂ and Bhf, respectively. The estimated errors associated with the hyperfine parameters are derived from statistical errors provided by the fitting program [6]. A JEOL 7900F scanning electron microscope (SEM) was employed for microstructural imaging of the magnets.

# 3. Results and discussion

## 3.1. Solvothermal treatment

The XRD pattern of the initial Nd-Fe-B powder corresponding to the initially fragmented magnet, as well as the pattern of the powder treated by solvothermal treatment, are shown in Fig. 1. The XRD analysis reveals that the pattern of the powder resulting from the solvothermal treatment is mainly composed of the peaks of the Nd₂Fe₁₄B phase. Peaks of the Nd(OH)₃ phase are also observed, with a low intensity. The presence of the Nd(OH)₃ phase is due to the reaction between the solvent and the Nd-rich phase. The evolution of the positions of the Nd₂Fe₁₄B peaks shows a shift towards smaller angles, indicating that the lattice parameters of the Nd₂Fe₁₄B phase increased after the solvothermal treatment. This shift is attributed to the hydrogenation of the Nd₂Fe₁₄B magnetic phase, resulting in the insertion of H atoms into the lattice, thus leading to Nd₂Fe₁₄BHₓ.

To quantify the expansion in lattice parameters, was used the Rietveld method. The resulting refined solvothermal powder diffractogram is presented in Fig. 2. The lattice parameters of the Nd₂Fe₁₄B, Nd(OH)₃ and α-Fe phases were determined, and the results are presented in

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** SEM image of the powder after solvothermal treatment.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Hysteresis loops of the initial powder and of the powder after solvothermal treatment.</figcaption></figure>

Table 1. The α-Fe phase was included in the model as its absence results in a high χ², indicating an unsatisfactory fit between the observed and calculated plots. Furthermore, a particular observation concerns the 51° angle, where the peak associated with metallic iron presents broadening, suggesting the formation of dispersed Fe precipitates. By determining the volume of the Nd₂Fe₁₄BHₓ magnetic phase, the hydrogen content can be estimated using insights from previous researches [5,7,8]. The hydrogen content is estimated to be x = 2.

To determine the proportion of the magnetic Nd₂Fe₁₄B phase, all the powders were analyzed by Mössbauer spectrometry. The room temperature Mössbauer spectra of the initial sample before and after solvothermal treatment, are presented in Fig. 5. The fitting of the Nd₂Fe₁₄B magnetic phase contribution considers the population of the different crystallographic sites (16:16:8:8:4:4). The results are listed as follows:

* Initial powder: six magnetic contributions corresponding to the Nd₂Fe₁₄B magnetic phase associated with the six iron sites in the tetragonal crystal structure, and one paramagnetic contribution corresponding to the Nd₁.₁Fe₄B₄ minor phase.
<figure><img src="image_8.png" /><figcaption>Fig. 5. Room temperature Mössbauer spectra of the initial Nd-Fe-B powder, of the powder after solvothermal treatment and of powders annealed at 300 °C, 650 °C, 1000 °C under vacuum and argon atmosphere.</figcaption></figure>

* After solvothermal treatment: six magnetic contributions for Nd₂Fe₁₄BHₓ, one magnetic contribution for α-Fe and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

The Mössbauer relative intensities obtained by this analysis, indicate that, in the initial powder, nearly (98 ± 1) % of the iron atoms are in the Nd₂Fe₁₄B magnetic phase, while the remaining (2 ± 1) % iron atoms are in the paramagnetic Nd₁.₁Fe₄B₄ phase. After the solvothermal treatment, (74 ± 1) % of the iron atoms are in the Nd₂Fe₁₄BHₓ phase, (24 % ± 1) % are in the α-Fe phase and (2 ± 1) % are in the Nd₁.₁Fe₄B₄ phase. The α-Fe phase results from the partial degradation of the Nd₂Fe₁₄B magnetic phase during the solvothermal treatment. The fact that the Nd₁.₁Fe₄B₄ phase is not affected by the solvothermal treatment is in agreement with the literature [9].

The analysis of the image of the powder resulting from the solvothermal treatment, shown in Fig. 3, reveals the presence of particles with an average size of approximately 10 µm.

The room temperature hysteresis loops of the powder before and after solvothermal treatment are presented in Fig. 4. Both remanence and coercivity are low, indicating that the solvothermal treatment leads to the complete pulverization of the initial magnet. The saturation magnetization increased after the solvothermal treatment, in relation with the formation of the α-Fe phase, the saturation magnetization of the α-Fe Fe phase being higher than that of the Nd₂Fe₁₄BHₓ phase. These results are consistent with both XRD and Mössbauer spectrometry analyses.

## 3.2. Annealing under vacuum

The XRD patterns of the powder obtained from solvothermal treatment followed by annealing under vacuum are shown in Fig. 1. After

annealing at 300 °C, the shift of the Nd₂Fe₁₄B peaks down to high angles, in relation with the decrease of the lattice parameters, is due to the release of H atoms from the Nd₂Fe₁₄BHₓ phase. Williams et al. [10] and Harris et al. [11] showed that hydrogen desorption from the magnetic phase is complete at 300 °C, which is in agreement with our results. On the other hand, the Nd(OH)₃ peaks are not observed in the XRD patterns, Nd₂O₃ peaks being observed instead. Based on these observation, the following reaction mechanisms are proposed:

$$ (300^{\circ}\text{C}) \text{Nd}_2\text{Fe}_{14}\text{BH}_x = \text{Nd}_2\text{Fe}_{14}\text{B} + \frac{x}{2}\text{H}_2 $$

$$ (300^{\circ}\text{C}) \text{Nd}(\text{OH})_3 = \text{Nd}_2\text{O}_3 + 3\text{H}_2\text{O} $$

It can be observed in the XRD pattern of the powder annealed at 300 °C that the large peak of α-Fe has a higher intensity than in the XRD pattern of the powder before annealing. This indicates that annealing at 300 °C results in a further degradation of the magnetic phase. Indeed, after annealing at 650 °C the powder is mostly composed of α-Fe, Nd₂Fe₁₄B, Fe₂B and NdHₓ/₂ peaks being detected with a low intensity. The formation of α-Fe and Fe₂B shows that the heat treatment under vacuum leads to the dissociation of the Nd₂Fe₁₄BHₓ phase, in agreement with the literature [10,12,13,14,15,16,17]. The following reaction mechanisms are thus proposed:

$$ (650^{\circ}\text{C}) \text{Nd}_2\text{Fe}_{14}\text{BH}_x = 2\text{NdH}_{x/2} + 12\text{Fe} + 2\text{Fe}_2\text{B} $$

The room temperature Mössbauer spectra of the annealed powders under vacuum at different temperatures are shown in Fig. 5. The fitting of the Nd₂Fe₁₄B magnetic phase contribution takes into account the population of the different crystallographic sites (16:16:8:8:4:4). The results of the fitting are listed below:
<caption>**Table 2**</caption>

Room temperature Mössbauer hyperfine parameters of the different contributions used to fit the spectra of the powders annealed at 300 °C, 650 °C, 1000 °C under vacuum.

<table><thead><tr><th>Powder annealed under vacuum at</th><th>Contribution</th><th>site</th><th>δ (mm/s) (*)</th><th>Δ or 2ε (mm/s)</th><th>Γ<sub>1/2</sub> (mm/s)</th><th>B<sub>hf</sub> (T)</th></tr></thead><tbody><tr><td rowspan="7">300 °C, 650 °C and 1000 °C</td><td rowspan="7">Nd<sub>2</sub>Fe<sub>14</sub>B</td><td>16k<sub>1</sub></td><td>-0.03</td><td>0.23</td><td>0.15</td><td>28.1</td></tr><tr><td rowspan="5">(**)</td><td>16k<sub>2</sub></td><td>-0.12</td><td>0.10</td><td>0.15</td><td>29.8</td></tr><tr><td>8j<sub>1</sub></td><td>-0.07</td><td>0.11</td><td>0.15</td><td>26.4</td></tr><tr><td>8j<sub>2</sub></td><td>0.13</td><td>0.56</td><td>0.15</td><td>33.8</td></tr><tr><td>4e</td><td>0.12</td><td>-0.26</td><td>0.15</td><td>25.4</td></tr><tr><td>4c</td><td>-0.11</td><td>0.04</td><td>0.15</td><td>24.5</td></tr><tr><td>Fe</td><td></td><td>0</td><td>0</td><td>0.20</td><td>33</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td></td><td></td><td>-0.10</td><td>0.60</td><td>0.21</td><td>-</td></tr><tr><td rowspan="2">Fe<sub>2</sub>B</td><td>A</td><td>0.09</td><td>0.02</td><td>0.17</td><td>23.2</td></tr><tr><td>B</td><td>0.11</td><td>0.03</td><td>0.17</td><td>24.2</td></tr></tbody></table>

(*) The values of the isomer shift δ are relative to α-Fe at room temperature.

(**) The mean value of the hyperfine field of the Nd₂Fe₁₄B phase is 28.7 T.

<caption>**Table 3**</caption>

Mössbauer relative intensities (%) of the different contributions used to fit the spectra of powders annealed at 300 °C, 650 °C and 1000 °C under vacuum.

<table><thead><tr><th>Contributions</th><th>300 °C<br>(±1%)</th><th>650 °C<br>(±1%)</th><th>1000 °C<br>(±1%)</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>B</td><td>44 %</td><td>16 %</td><td>8 %</td></tr><tr><td>Fe</td><td>55 %</td><td>73 %</td><td>80 %</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>1 %</td><td>1 %</td><td>2 %</td></tr><tr><td>Fe<sub>2</sub>B</td><td>-</td><td>10 %</td><td>10 %</td></tr></tbody></table>

* Powder annealed at 300 °C under vacuum: six magnetic contributions for Nd₂Fe₁₄B, one magnetic contribution for α-Fe and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

* Powder annealed at 650 °C and 1000 °C under vacuum: six magnetic contributions for Nd₂Fe₁₄B, one magnetic contribution for α-Fe, two magnetic contributions for Fe₂B and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

The hyperfine parameters resulting from the fitting of the spectra as well as the relative Mössbauer intensities of the iron-containing phases in the powders are given in Tables 2 and 3, respectively.

The hyperfine field of the magnetic phase in the powder annealed at 300 °C is determined to be approximately (29.5 ± 0.2) T. This value is lower than the value for the powder obtained from solvothermal treatment, which is estimated at (30.5 ± 0.2) T. This observation is consistent with the XRD analysis, providing confirmation of hydrogen desorption within the magnetic phase during the annealing process.

The Mössbauer relative intensities reveal that, in the powder annealed under vacuum at 300 °C, (44 ± 1) % of iron atoms are in the Nd₂Fe₁₄B phase. As the annealing temperature increases to 650 °C, the magnetic phase undergoes partial decomposition into Fe₂B and α-Fe. At 1000 °C, the magnetic Nd₂Fe₁₄B phase tends to disappear, the proportion of iron in the magnetic phase decreasing down to (8 ± 1) %. This indicates that vacuum annealing leads to an important degradation of the magnetic phase.

<caption>**Table 4**</caption>

Lattice parameters of the phases present in the powder annealed at 300 °C under argon atmosphere.

<table><thead><tr><th>Phase</th><th>Structure</th><th>a (nm)</th><th>b (nm)</th><th>c (nm)</th><th>Unit cell volume (nm<sup>3</sup>)</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>4</sub></td><td>P42/<br>mnm</td><td>0.8831</td><td>0.8831</td><td>1.2230</td><td>0.9540</td></tr><tr><td>Nd(OH)<sub>3</sub></td><td>P 63/m</td><td>0.6424</td><td>0.6424</td><td>0.3741</td><td>0.1338</td></tr><tr><td>α-Fe</td><td>Im-3 m</td><td>0.2902</td><td>0.2902</td><td>0.2902</td><td>0.0244</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Rietveld refinement of the XRD pattern of the powder annealed at 300 °C under argon atmosphere.</figcaption></figure>
<caption>**Table 5**</caption>

Room temperature Mössbauer hyperfine parameters of the different contributions used to fit the spectra of the powders annealed at 300 °C, 650 °C, 1000 °C under argon atmosphere.

<table><thead><tr><th>Powder annealed under argon at</th><th>Contribution</th><th>δ (mm/s) (*) ± 0.05</th><th>Δ or 2e(mm/s) ± 0.05</th><th>Γ<sub>1/2</sub> (mm/s)</th><th>B<sub>hf</sub>(T) ± 0.1(**300 °C)</th><th>B<sub>hf</sub>(T) ± 0.1(**650 °C)</th></tr></thead><tbody><tr><td rowspan="10">300 °C, 650 °C and 1000 °C</td><td>Nd<sub>2</sub>Fe<sub>14</sub>B (H<sub>x</sub>)</td><td>16k1<br/>-0.03</td><td>0.23</td><td>0.15</td><td>29.5 2</td><td>6.8</td></tr><tr><td rowspan="5">Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>16k2<br/>-0.12</td><td>0.10</td><td>0.15</td><td>31.6</td><td>31</td></tr><tr><td>8 J1<br/>-0.07</td><td>0.11</td><td>0.15</td><td>27.6</td><td>21.7</td></tr><tr><td>8j2<br/>0.13</td><td>0.56</td><td>0.15</td><td>35.1</td><td>35.5</td></tr><tr><td>4e<br/>0.12</td><td>-0.26</td><td>0.15</td><td>27.0</td><td>26</td></tr><tr><td>4c<br/>-0.11</td><td>0.04</td><td>0.15</td><td>25.2</td><td>24.2</td></tr><tr><td>Fe</td><td>-0.10</td><td>0.60</td><td>0.21</td><td>0</td><td></td></tr><tr><td>FeO</td><td>0</td><td>0</td><td>0.21</td><td>33</td><td></td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>1.01</td><td>0.18</td><td>0.5</td><td>0</td><td></td></tr><tr><td rowspan="3">650 °C and 1000 °C<br/>1000 °C</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>0.34<br/>A<br/>0.27</td><td>-0.15<br/>0.01</td><td>0.2<br/>0.16</td><td>50.9<br/>48.9</td><td></td></tr><tr><td>NdFeO<sub>3</sub></td><td>0.65<br/>B<br/>0.35</td><td>0.03<br/>-0.02</td><td>0.21<br/>0.2</td><td>45.9<br/>51.8</td><td></td></tr></tbody></table>

<caption>**Table 6**</caption>

Mossbauer relative intensities (%) of the different contributions used to fit the spectra of powders annealed at 300 °C, 650 °C and 1000 °C under argon atmosphere.

<table><thead><tr><th>Contributions</th><th>300 °C<br>(±1%)</th><th>650 °C<br>(±1%)</th><th>1000 °C<br>(±1%)</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>B(H<sub>x</sub>)</td><td>75 %</td><td>6 %</td><td>2 %</td></tr><tr><td>Fe</td><td>23 %</td><td>86 %</td><td>46 %</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>2 %</td><td>2 %</td><td>1 %</td></tr><tr><td>FeO</td><td>—</td><td>9 %</td><td>9 %</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>—</td><td>—</td><td>6 %</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub></td><td>—</td><td>—</td><td>26 %</td></tr><tr><td>NdFeO<sub>3</sub></td><td>—</td><td>—</td><td>10 %</td></tr></tbody></table>

### 3.3. Annealing under argon

The XRD patterns of powders annealed at different temperatures under argon atmosphere are shown in Fig. 1. Rietveld refinement of the XRD pattern and refined lattice parameters for the Nd₂Fe₁₄BHₓ, Nd (OH)₃ and α-Fe phases for the powder annealed at 300 °C are respectively shown in Fig. 6 and Table 4. The Nd₂Fe₁₄BHₓ volume is estimated to 0.954 nm³ which corresponds, according to [7,8], to a hydrogen content of x = 0.6.

The XRD pattern of the powder annealed at 650 °C reveals a high intense peak with a large width corresponding to the α-Fe phase. The XRD pattern of the powder annealed at 1000 °C shows the presence of six phases: FeO, NdFeO₃, α-Fe, Fe₂O₃, Fe₃O₄, and NdBO₃, indicating the presence of oxygen in the initial powder.

The Mossbauer spectra of the powders annealed under argon atmosphere at 300 °C, 650 °C and 1000 °C are presented in Fig. 5. The fitting results are listed as follows:

* Powder annealed at 300 °C: six magnetic contributions for Nd₂Fe₁₄B, one Fe magnetic contribution for α-Fe and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

* Powder annealed at 650 °C: six magnetic contributions for Nd₂Fe₁₄B, one magnetic contribution for α-Fe, one magnetic contribution for FeO and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

* Powder annealed at 1000 °C: six magnetic contributions for Nd₂Fe₁₄B, one magnetic contribution for α-Fe, one paramagnetic contribution for FeO, one magnetic contribution for Fe₂O₃, one magnetic contribution for NdFeO₃, two magnetic contributions for Fe₃O₄ and one paramagnetic contribution for Nd₁.₁Fe₄B₄.

The hyperfine parameters resulting from the fitting of the spectra as

well as the relative Mössbauer intensities of the iron-containing phases are respectively given in tables 5 and 6.

In the powder annealed at 300 °C, the mean hyperfine field of the magnetic phase is (30.1 ± 0.1) T. This value is lower than that of the powder resulting from solvothermal treatment, but higher than that of the powder annealed at 300 °C under vacuum. This confirms the results of the XRD analysis, indicating that there is no complete hydrogen desorption of the magnetic phase after annealing under argon at 300 °C.

The Mössbauer relative intensives show that, in the powder annealed under argon at 300 °C, (75 ± 1) % of iron atoms are present in Nd₂Fe₁₄BHₓ phase. This value is the same value as after solvothermal treatment. This indicates that annealing at 300 °C under argon atmosphere does not induce degradation of the Nd₂Fe₁₄BHₓ phase.

Upon annealing at 650 °C, the Nd₂Fe₁₄B magnetic phase is decomposed into Fe and FeO. At 1000 °C, the magnetic phase tends to disappear, and α-Fe, FeO, Fe₂O₃, Fe₃O₄ and NdFeO₃ are formed. This indicates that the treatment temperatures higher than 650 °C leads to oxidation of the magnetic phase. After solvothermal treatment, oxygen is retained in the form of Nd(OH)₃. Upon argon annealing at 650 °C and 1000 °C, the transformation of Nd(OH)₃ into Nd₂O₃ might releases oxygen, leading to the formation of oxide phases. This could be related to the fact that oxygen is not evacuated during the treatment, while upon vacuum annealing, the vacuum pump would discharge oxygen so that no iron oxides are formed. The absence of oxidation during vacuum annealing suggests that low pressure conditions are essential to avoid the formation of oxygen-containing phases.

### 4. Discussion

Powdered Nd-Fe-B magnets recycled via solvothermal treatment in pure ethanol were annealed for 1 h under vacuum or argon atmosphere to investigate the influence of annealing conditions on the structure and stability of the powder. The powders resulting from the solvothermal treatment have a particle size of about 10 µm and contain the hydrogenated NdFe₁₄BHₓ phase with x ≈ 2.

Annealing under vacuum at 300 °C leads to the complete desorption of hydrogen from the magnetic Nd₂Fe₁₄BHₓ phase and to a partial degradation of the Nd₂Fe₁₄BHₓ phase into α-Fe. Annealing under argon atmosphere at 300 °C induces partial hydrogen desorption of the Nd₂Fe₁₄BHₓ phase, the hydrogen content being x ≈ 0.6, and no degradation of the Nd₂Fe₁₄BHₓ phase is observed. This indicates the relative stability of the Nd₂Fe₁₄BHₓ phase at temperatures lower than 300 °C in inert conditions.

Upon annealing beyond 650 °C under vacuum, the Nd₂Fe₁₄BHₓ phase is decomposed into α-Fe and Fe₂B. Annealing above 650 °C under

<footnote>(*) The values of the isomer shift δ are relative to α-Fe at room temperature.</footnote>

<footnote>(**) The mean value of the hyperfine field of the Nd₂Fe₁₄BHₓ phase after annealing at 300 °C is 30.2 T.</footnote>

<footnote>(***) The mean value of the hyperfine field of the Nd₂Fe₁₄B phase after annealing at 650 °C is 28.3 T.</footnote>
<figure><img src="image_10.png" /><figcaption>Fig. 7. SEM images of the powders annealed at 1000 °C for 1 h under vacuum (top), and under argon atmosphere (bottom).</figcaption></figure>

argon atmosphere leads to the oxidation of the Nd₂Fe₁₄BHₓ phase. The formation of oxides is attributed to the release of oxygen that results from the transformation of Nd(OH)₃ into Nd₂O₃. In vacuum, oxygen is probably discharged by the vacuum pump, preventing the formation of iron oxides.

In the powder annealed under argon at temperatures higher than 650 °C, octahedral shaped crystals are observed, that correspond to Fe₃O₄, as observed during the treatment of Nd-Fe-B magnets in water and sodium chloride [12]. Such octahedral crystals were not observed in powders annealed under vacuum (Fig. 7).

These results indicate that the behavior of a solvothermal recycled Nd-Fe-B powder strongly depends on the annealing conditions. Annealing the powder under inert conditions at temperatures lower than 300 °C preserves the magnetic Nd₂Fe₁₄B phase. Annealing at higher temperatures under inert conditions leads to the oxidation of the magnetic phase, while no oxidation is observed in vacuum due to the continuous pumping.

The control of the annealing conditions (atmosphere, temperature and time) is thus crucial to preserve the structure of Nd-Fe-B recycled powders with the aim to obtain recycled Nd-Fe-B magnets with good magnetic properties.

## 5. Conclusion

The investigation of the influence of the annealing parameters (temperature and atmosphere) on the structure of Nd-Fe-B powders obtained from magnets recycled via solvothermal treatment in pure ethanol shows that the behavior of the powders strongly depends on the annealing conditions.

Vacuum annealing at 300 °C is accompanied by the complete desorption of hydrogen from the magnetic Nd₂Fe₁₄BHₓ phase together with a partial degradation of the Nd₂Fe₁₄BHₓ phase into α-Fe, while annealing above 650 °C leads to the decomposition of Nd₂Fe₁₄BHₓ into Nd hydride, α-Fe and Fe₂B.

Argon annealing at 300 °C leads to a partial desorption of hydrogen from the Nd₂Fe₁₄BHₓ phase (x ≈ 0.6) and no degradation of the Nd₂Fe₁₄BHₓ phase is observed, while annealing above 650 °C leads to oxidation of the Nd₂Fe₁₄B phase.

These results suggest that annealing under argon at temperatures up

to 300 °C may lead to the total desorption of hydrogen from the hydrogenated Nd₂Fe₁₄B phase, and prevents the magnetic Nd₂Fe₁₄B phase from significant degradation. The slight degradation of the Nd₂Fe₁₄B magnetic phase which is observed upon solvothermal treatment for 7 h could be limited or even suppressed by reducing the solvothermal treatment time.

Consequently, controlling the annealing parameters for a Nd-Fe-B powder containing pure Nd₂Fe₁₄B magnetic phase should make possible the production of bonded or sintered Nd-Fe-B magnets from recycled powders obtained by the solvothermal route.

## CRediT authorship contribution statement

A. Chetouani: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. F. Richomme: Validation, Investigation. J.M. Le Breton: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Funding acquisition. J.S. Thonikuzhiyil: Validation, Investigation. P. Bernstein: Validation, Project administration, Investigation, Funding acquisition. J. Noudem: Validation, Project administration, Investigation, Funding acquisition.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgements

The authors acknowledge the financial support from the French Agence Nationale de la Recherche and LabEx EMC3 through the project REFAIRE (Grant No. ANR-10-LABX-09-01)
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