# Influence of post-sintering annealing on microstructure and magnetic properties of recycled Nd-Fe-B magnets by solvothermal treatment

Abdelilah Chetouani, Jefry-Samson Thonikuzhiyil, Jean-Marie Le Breton, Pierre Bernstein, Jacques Noudem

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Abdelilah Chetouani, Jefry-Samson Thonikuzhiyil, Jean-Marie Le Breton, Pierre Bernstein, Jacques Noudem. Influence of post-sintering annealing on microstructure and magnetic properties of recycled Nd-Fe-B magnets by solvothermal treatment. Journal of Materials Research and Technology, 2025, 39, pp.7386-7395. 10.1016/j.jmrt.2025.11.038. hal-05365807

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# Influence of post-sintering annealing on microstructure and magnetic properties of recycled Nd-Fe-B magnets by solvothermal treatment

Abdelilah Chetouani<sup>a, *, ID</sup>, Jefry-Samson Thonikuzhiyil<sup>b</sup>, Jean-Marie Le Breton<sup>a, ID</sup>, Pierre Bernstein<sup>b</sup>, Jacques Noudem<sup>b</sup>

## ABSTRACT

The impact of sintering and post-sintering annealing on the microstructure and coercivity of powders obtained by solvothermal treatment of recycled Nd-Fe-B magnets has been investigated. It was observed that a high pressure helps to reduce the degradation of the Nd₂Fe₁₄B magnetic phase upon sintering. Mixing the recycled Nd-Fe-B powder obtained by solvothermal treatment with a commercial Nd-Fe-B powder allows minimizing the degradation of the Nd₂Fe₁₄B phase upon sintering. For these mixed materials, a post-sintering vacuum annealing of 30 min at 750 °C resulted in sintered recycled materials with a coercive field of 0.53 T and a maximum energy product of about 42 kJ/m³. The increase in coercivity after post-sintering annealing is associated with the presence of an oxidized Nd-rich phase, which is distributed around the Nd₂Fe₁₄B grains, inducing magnetic disconnection between these grains.

## 1. Introduction

Nd-Fe-B permanent magnets are crucial materials for the energy of the future. Their exceptional magnetic properties, characterized by high coercivity, strong remanent magnetization, and a high maximum energy product, make them essential in the production of motors for electric vehicles and wind turbines [1]. The development of sustainable technologies leads to a growing demand for these magnets, highlighting their strategic importance for the energy transition. In view of this demand and the landfilling of large quantities of rare earth elements, recycling becomes essential to reduce material losses and provide secondary raw materials. However, this requires environmentally friendly recycling processes [2]. In this context, several approaches have been suggested in the literature to recycle Nd-Fe-B magnets, including reuse of the magnets, recycling of the alloy or extraction of the rare earth elements. Reusing the magnets preserves their magnetic properties and efficiency. However, it can pose a problem of compatibility between the shape and dimensions of the magnets recovered from waste electrical and electronic equipment (WEEE), and the shape and dimensions required for the new equipment. The extraction of rare earth elements can be done by hydrometallurgical or pyrometallurgical processes. Hydrometallurgical process separates the rare earth elements using chemical solutions, but it requires large quantities of chemicals, such as acids, which can have environmental impacts. Pyrometallurgical process overcomes these problems, but it requires high-temperature

treatment to cause physical and chemical transformations to recover rare earths. To overcome these limitations, recycling the magnetic phase is the most environmentally friendly process. This can be done by Hydrogen Decrepitation (HD) techniques [3–5]. Solvothermal treatment is an innovative process that has been developed in the GPM laboratory. This process has several advantages: it uses an environmentally friendly solvent, consumes little energy, and is applicable to Nd-Fe-B permanent magnets [6–8]. It consists in reducing the magnet into powder to produce, by sintering, other magnets with variable shapes and dimensions. The impact of thermal treatments on the hard-magnetic properties of Nd-Fe-B sintered magnets has been extensively investigated [9–11], showing that such treatments have a beneficial effect on the magnetic properties. Heat treatment plays a crucial role in optimizing coercivity. Since coercivity is heavily influenced by microstructure, it is anticipated that thermal treatments induce significant microstructural changes, thereby influencing coercivity. The temperature, time, and atmosphere of the heat treatment must be adjusted according to the composition of the magnet.

In order to investigate the possibility of obtaining sintered permanent magnets from recycled Nd-Fe-B powders obtained by solvothermal treatment, we examine here the influence of sintering parameters and post-sintering annealing conditions on the microstructure and magnetic properties of sintered powders obtained from recycled Nd-Fe-B magnets.

<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, Caen, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: abdelilah.chetouani@univ-rouen.fr (A. Chetouani), jean-marie.lebreton@univ-rouen.fr (J.-M. Le Breton).</footnote>
## 2. Experimental section

### 2.1. Preparation processes

The samples utilized for the recycling process in this study are sintered rotors magnets obtained from WEEE. These magnets were fragmented into 1 mm size pieces using a crusher. Fragmenting the magnets beforehand makes it possible to accelerate the pulverization kinetics [12]. Subsequently, the crushed powder was introduced into a crystallization bomb containing 30 mL of pure ethanol. The crystallization bomb was then heated at 225 °C for 30 min. During the process, the autogenous pressure inside the crystallization bomb reaches up to 125 bar. The resulting powder obtained after the solvothermal process has a particle size of less than 200 µm. The details of this process have been previously described [6].

The recycled powder was sintered using Spark Plasma Sintering (SPS) under pulsed DC current mode. Sintering was carried out under vacuum at 900 °C with a heating rate of 100 °C/min. Pressures of 25, 50, and 75 MPa were applied. The holding time at 900 °C was fixed to 15 min. For each sample, a mass of 3.5 g of recycled powder was placed into a 15 mm cylindrical graphite die, and graphite foils were placed as a protective layer to prevent contact with the die or spacers.

The sintered powders were subsequently subjected to annealing under various conditions of time (15 min, 30 min and 45 min), temperature (550 °C, 650 °C, 750 °C and 1000 °C), and atmosphere (argon and vacuum). The influence of post-sintering temperature, atmosphere, and time on the structural and magnetic properties of the sintered

powders is discussed.

### 2.2. Characterization techniques

X-ray diffraction (XRD) analysis of the samples was performed with an Empyrean diffractometer equipped with Co $Kα_1$ radiation (wavelength 0.178897 nm). $^{57}$Fe Mössbauer spectrometry analyses were performed with a $^{57}$Co source in a Rh matrix at room temperature to determine the relative proportions of the Fe-containing phases. The estimated errors associated with the hyperfine parameters are derived from statistical errors provided by the fitting program [13]. A JEOL 7900F scanning electron microscope (SEM) with EDS analysis was employed for microstructural imaging of the sintered powders. Magnetic measurements were performed at room temperature using a Quantum Design MPMS-XL5 SQUID magnetometer.

## 3. Results and discussion

### 3.1. Investigation of sintered recycled powders

The room temperature Mössbauer spectra of the initial crushed powder, after solvothermal treatment, and of the powders sintered at different pressures (25 MPa, 50 MPa, 75 MPa) are presented in Fig. 1. 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:

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Room temperature Mössbauer spectra of, the Nd-Fe-B crushed powder (Initial), the powder obtained after solvothermal treatment (Recycled powder), and the powder sintered under different pressures (25 MPa, 50 MPa and 75 MPa). The spectrum of the recycled powder was fitted with the Nd₂Fe₁₄BHₓ contribution, in relation with the hydrogenation of the magnetic phase during solvothermal treatment. The spectra of the sintered powders were fitted with the Nd₂Fe₁₄B contribution, in relation with the dehydrogenation of the magnetic phase after sintering.</figcaption></figure>
* Initial crushed powder and powder obtained after solvothermal treatment:

* six magnetic contributions corresponding to the Nd₂Fe₁₄B (initial crushed powder) or Nd₂Fe₁₄BHₓ (powder after solvothermal treatment [7]) magnetic phase associated with the six iron sites in the tetragonal crystal structure

* one paramagnetic contribution corresponding to the Nd₁.₁Fe₄B₄ minor phase.

* Powder sintered at 25 MPa:

* six magnetic contributions for the Nd₂Fe₁₄B phase

* one magnetic contribution for the α-Fe phase

* two magnetic contributions for the Fe₂B phase

* one paramagnetic contribution for Nd₁.₁Fe₄B₄.

* Powders sintered at 50 MPa and 75 MPa:

* six magnetic contributions for Nd₂Fe₁₄B

* one magnetic contribution for α-Fe

* one paramagnetic contribution for Nd₁.₁Fe₄B₄.

The Mössbauer relative intensities indicate that, in the initial powder, nearly (98 ± 1) % of the iron atoms are in the Nd₂Fe₁₄B or Nd₂Fe₁₄BHₓ magnetic phase, while the remaining (2 ± 1) % iron atoms are in the Nd₁.₁Fe₄B₄ paramagnetic phase. It is observed that as the pressure increases, the relative intensity of the α-Fe phase resulting the degradation of the magnetic phase is reduced, decreasing from 65 % at 25 MPa to 15 % at 75 MPa. This can be explained by the fact that as the pressure increases from 25 MPa to 75 MPa, the density of the sintered sample increases [14], which reduces the undesirable reactions that could occur at the surface of the Nd₂Fe₁₄B grains upon sintering.

The hyperfine field of the magnetic phase in the sintered powder is (28.1 ± 0.1) T. This value is lower than that of the powder resulting from solvothermal treatment (30 ± 0.1) T and close to the value of the initial powder (28 ± 0.1) T. These results indicate complete dehydrogenation of the magnetic phase following the vacuum annealing above 300 °C, in agreement with previous observations [7,15,16].

These results show that sintering at 75 MPa leads to minimized degradation of the Nd₂Fe₁₄B magnetic phase.

The XRD patterns of the initial crushed Nd-Fe-B powder extracted from the rotor magnets, the powder obtained after solvothermal treatment, and the powder sintered at 75 MPa are shown in Fig. 2. The analysis of the XRD patterns reveals that the solvothermal treated powder is primarily composed of Nd₂Fe₁₄BHₓ peaks, with additional low-intensity peaks corresponding to the Nd(OH)₃ phase. The formation of the Nd(OH)₃ phase is attributed to a reaction between the solvent and the Nd-rich phase upon the solvothermal treatment. Previous studies [6, 7] have reported an increase in lattice parameters of the Nd₂Fe₁₄B phase after the solvothermal treatment. Such an increase is associated with the hydrogenation of the Nd₂Fe₁₄B magnetic phase, in relation with the insertion of H atoms into the crystal lattice, forming a hydrogenated Nd₂Fe₁₄BHₓ phase. The XRD pattern of the sintered powder shows Nd₂Fe₁₄B peaks, accompanied by low intensity peaks of Nd₂O₃ phase. The presence of the Nd₂O₃ phase is attributed to the dehydrogenation of Nd(OH)₃. These results are consistent with the Mössbauer analysis and corroborate the previous analyses by Chetouani et al. [7] and Williams et al. [16], which reports the complete desorption of hydrogen from the magnetic phase during vacuum annealing above 300 °C.

### 3.2. Investigation of sintered recycled powders mixed with a commercial Nd-Fe-B powder

Using a high pressure to obtain sintered powders after solvothermal treatment limits the degradation of the Nd₂Fe₁₄B magnetic phase. With the aim at further reducing the proportion of degraded magnetic phase after sintering, the powder obtained after solvothermal treatment (so-called recycled powder) was mixed with a commercial Nd-Fe-B powder, with different mass proportions, and sintered at 75 MPa. The green compacts for SPS were prepared using a mixture of recycled and

<figure><img src="image_6.png" /><figcaption>Fig. 2. XRD patterns of, the Nd-Fe-B crushed powder (Initial), the powder obtained after solvothermal treatment (Recycled powder), and the powder sintered at 75 MPa (as-sintered). The unindexed peaks correspond to the Nd₂Fe₁₄B(Hₓ) phase.</figcaption></figure>

commercial powders in mass proportions of 50:50 and 25:75 respectively.

The room-temperature Mössbauer spectra for the commercial powder and the sintered samples are compared in Fig. 3. The spectra of the sintered recycled/commercial powder mixture were fitted with the same contributions as those applied to the sintered recycled powder. The results show a reduction in the relative intensity of the α-Fe phase when the mass fraction of recycled powder in the mixture decreases. For the sintered material with 25 % of recycled powder, the relative intensity of the α-Fe phase is zero, indicating that the proportion of iron resulting from the degradation of the magnetic phase is below the detection threshold of Mössbauer spectrometry (2 %). Fig. 4 illustrates the evolution of the Mössbauer relative intensity of Nd₂Fe₁₄B and α-Fe phases in the sintered material as a function of the mass fraction of recycled powder. It is observed that, as the proportion of the recycled powder in the mixture decreases, the proportion of α-Fe resulting from the degradation of the magnetic phase also decreases, being nearly suppressed in the sintered material with 25 % of recycled powder.

The microstructure of the sintered material with 25 % of recycled powder was investigated by SEM, and the corresponding micrographs are shown in Fig. 5. Three distinct zones can be observed, that appear in grey (region A), light grey (region B), and dark grey (region C) contrast. The grey regions, that occupy the largest area, correspond to the Nd₂Fe₁₄B phase. EDS analysis indicates that light grey regions contain O, while O is absent from the Nd₂Fe₁₄B grains. This is in agreement with the presence of Nd₂O₃, as observed in the sintered recycled powder from XRD analysis (Fig. 2). The light grey regions thus correspond to Nd-oxide regions.

Regions in dark grey contrast are observed in zones that also contain Nd-oxides. Their presence indicates that the magnetic phase has been degraded upon sintering. It is known that the degradation process of the
<figure><img src="image_7.png" /><figcaption>Fig. 3. Room temperature Mössbauer spectra of, the commercial Nd-Fe-B powder, the sintered powder/commercial powder mixtures with 25 %/75 % (R-25 %) and 50 %/50 % (R-50 %) mass proportions, and the sintered recycled powder (R-100 %). The powders were sintered at 900 °C under 75 MPa.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 4. Mössbauer relative intensities (%) of the commercial Nd-Fe-B powder and of the sintered powder/commercial powder mixtures with varying proportions R of recycled powder.</figcaption></figure>

Nd₂Fe₁₄B phase at high temperature leads to the formation of α-Fe and Nd oxides, which can be described by equation (1) [17]. Because the presence of α-Fe has been observed in recycled powders sintered at 75 MPa (Fig. 1), the dark grey regions are attributed to regions containing the α-Fe phase that results from the degradation of the Nd₂Fe₁₄B magnetic phase. A deeper structural investigation could confirm this

hypothesis but, for the present investigation, the SEM observations show that in mixed powders sintered at 75 MPa, the degradation of a reduced fraction of the Nd₂Fe₁₄B magnetic phase (too low to be detectable by Mössbauer spectrometry) occurs.

$$Nd_2Fe_{14}B + \frac{3}{2}O_2 \rightarrow Nd_2O_3 + 14Fe + B \quad (Eq.1)$$

The hysteresis loops at 295 K of the recycled powder after solvothermal treatment, and the sintered Nd-Fe-B material with 25 % mass proportion of recycled powder, are shown in Fig. 6. One can notice that the saturation magnetization of the recycled powder is close to that of the commercial powder. The magnetization at 5 T of the powder sintered with 25 % recycled powder (136.4 Am²/kg) is very close to that of both commercial powder (138.5 Am²/kg) and recycled powder after solvothermal treatment (134.4 Am²/kg). This confirms the absence of a significant proportion of pure α-Fe phase, indicating that the magnetic phase in the sintered material is not significantly degraded, in agreement with Mössbauer analysis. The fact that the magnetization of the sintered material does not saturate at 5 T is attributed to the existence of a random crystallographic alignment of the Nd₂Fe₁₄B particles in the analyzed sample, as the particles in this sample have not been magnetically aligned prior to the measurement. To reach saturation, it would be necessary to apply a magnetic field higher than the anisotropy field of the magnetic phase, which is about 7 T [18].

### 3.3. Post sintering annealing

Post-sintering annealing is considered as an effective method for improving the magnetic properties of Nd-Fe-B sintered magnets, as these properties are significantly influenced by the microstructure. We examine here the effects of atmosphere, time and temperature of the
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** SEM back-scattered images of the Nd-Fe-B sintered material with 25 % proportion (in mass) of recycled powder. (A) grey regions correspond to the Nd₂Fe₁₄B matrix phase. (B) light grey regions correspond to Nd oxides located at the grain boundaries. (C) indicates the regions degraded after sintering. An EDS elemental map of O is also shown.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Hysteresis loops at 295 K of the powder obtained after solvothermal treatment (Recycled powder) and the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder (As-sintered).</figcaption></figure>

heat treatment, on the magnetic properties and microstructure of sintered Nd-Fe-B materials obtained from a mixture of recycled and commercial powders in mass proportions of 25 % and 75 %.

Following the sintering process, the sintered Nd-Fe-B material underwent heat treatment at various temperatures (550 °C, 650 °C, 750 °C and 1000 °C) for different times (15 min, 30 min and 45 min) under argon atmosphere or vacuum. The structural and magnetic properties of the annealed materials were analyzed to characterize the influence of the annealing conditions on microstructure, coercivity, remanent magnetization, and saturation magnetization. Subsequently, these

measurements were compared to those obtained from the as-sintered materials.

### 3.3.1. Influence of annealing time under argon atmosphere

The annealed materials were investigated by Mössbauer spectrometry. The spectra are shown in Fig. 7. They were fitted with the following contributions:

* six magnetic contributions for Nd₂Fe₁₄B, accounting for the population of the different crystallographic sites (16:16:8:8:4:4)

* one magnetic contribution for α-Fe

* one paramagnetic contribution for Nd₁.₁Fe₄B₄.

The relative Mössbauer intensities of the different contributions are reported in Table 1.

These results show that post-sintering annealing leads to the formation of the α-Fe phase, with a relative proportion that increases with the annealing time. This suggests the degradation upon annealing of a fraction of the Nd₂Fe₁₄B phase, which could be related to the evolution of both the intergranular Nd-oxide regions (B regions in Fig. 5) and the degraded intergranular regions (C regions in Fig. 5). A deeper structural investigation could confirm this hypothesis.

Fig. 8 shows the hysteresis loops at 295 K of the sintered Nd-Fe-B material before and after annealing under argon at 750 °C for 15, 30 and 45 min. The values of magnetization at 5 T (denoted as M<sub>S</sub>), remanent magnetization (M<sub>r</sub>) and coercive field (μ<sub>0</sub>H<sub>C</sub>) are reported in Table 2.

Compared with the properties of the as-sintered material, post-sintering annealing results in an increase of all the magnetic properties. The highest M<sub>S</sub> and M<sub>r</sub> values are achieved after 45 min of annealing, while the maximum μ<sub>0</sub>H<sub>C</sub> is obtained after annealing for 30 min corresponding to a gain of 540 % as compared with the as-sintered magnet.

In agreement with the Mössbauer analysis, the observed increase of M<sub>r</sub> and M<sub>S</sub> with the annealing time can be related to the higher
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Room temperature Mössbauer spectra of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder, after annealing under argon at 750 °C for 15, 30 and 45 min.</figcaption></figure>

<caption>**Table 1**</caption>

Relative intensities of the contributions fitted in the Mössbauer spectra of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder, before and after annealing under argon at 750 °C for 15, 30 and 45 min.

<table><thead><tr><th>Contribution</th><th>As-sintered</th><th>15 min annealing</th><th>30 min annealing</th><th>45 min annealing</th></tr></thead><tbody><tr><td>Nd<sub>2</sub>Fe<sub>14</sub>B</td><td>(97 ± 1) %</td><td>(91 ± 1) %</td><td>(90 ± 1) %</td><td>(74 ± 1) %</td></tr><tr><td>α-Fe</td><td>-</td><td>(7 ± 1) %</td><td>(8 ± 1) %</td><td>(24 ± 1) %</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>(3 ± 1) %</td><td>(2 ± 1) %</td><td>(2 ± 1) %</td><td>(2 ± 1) %</td></tr></tbody></table>

proportion of the α-Fe phase in the annealed materials, since the saturation magnetization of α-Fe is higher than that of Nd₂Fe₁₄B.

It is worth mentioning that although the post-sintered annealed samples contain both Nd₂Fe₁₄B hard phase and α-Fe soft phase, their

hysteresis loops do not exhibit a biphasic signature. This could suggest the presence of an exchange coupling between the hard and soft phases, highlighted by Kneller [19] and further studied in detail by Skomski et al. [20].

<caption>**Table 2**</caption>

Magnetic properties measured at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder before (As-sintered) and after annealing under argon at 750 °C for 15, 30 and 45 min.

<table><thead><tr><th>Parameter</th><th>As-sintered</th><th>15 min annealing</th><th>30 min annealing</th><th>45 min annealing</th></tr></thead><tbody><tr><td>M<sub>S</sub>(ST) (Am<sup>2</sup>/kg)</td><td>136.4</td><td>148.8</td><td>149.8</td><td>159.1</td></tr><tr><td>M<sub>R</sub> (Am<sup>2</sup>/kg)</td><td>22.7</td><td>50.5</td><td>48.7</td><td>55.1</td></tr><tr><td>μ<sub>0</sub>H<sub>c</sub> (T)</td><td>0.05</td><td>0.22</td><td>0.32</td><td>0.28</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Hysteresis loops at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder before (As-sintered) and after annealing under argon at 750 °C for 15, 30 and 45 min.</figcaption></figure>
SEM images of the sintered material annealed at 750 °C for 15 min under argon are shown in Fig. 9. Compared to the SEM images of the as-sintered samples in Fig. 5, the SEM images of the annealed material reveal noticeable modifications in the morphology of the Nd-oxide regions. As observed in Fig. 9, the microstructure of the sintered material reveals the presence of Nd-oxide regions at the grain boundaries. Some Nd-oxide regions are quite continuous, surrounding several magnetic grains. The increase of coercivity upon annealing could be related to the presence of these Nd-oxide regions between some grains of the Nd₂Fe₁₄B, as these non-magnetic Nd-rich regions facilitate the magnetic disconnection between the Nd₂Fe₁₄B grains.

Additionally, some dark grey regions, similar to the degraded intergranular regions observed in the as-sintered material (C regions in Fig. 5) are present in the SEM images of the sintered magnet (Fig. 9). This is consistent with the presence of the α-Fe phase in the annealed material, as detected by Mössbauer spectrometry.

### 3.3.2. Influence of annealing temperature under argon atmosphere

The annealing temperature of 750 °C is higher than the solidification temperature of the Nd-rich phase, which is about 665 °C [21]. To confirm the positive influence of the presence of Nd-oxides intergranular regions on the magnetic properties of the annealed materials, heat treatments were conducted for 30 min at 550 °C and 650 °C that is, below the solidification temperature of the Nd-rich phase.

The room temperature Mössbauer spectrum of the Nd-Fe-B sintered material annealed at 550 °C is shown in Fig. 10, where it is compared with the spectrum of the same material annealed at 750 °C. The two spectra are similar, showing that the proportion of pure α-Fe phase is the same in both annealed materials.

The hysteresis loops of the sintered Nd-Fe-B material after annealing under argon at 550 °C, 650 °C and 750 °C for 30 min are shown in Fig. 11. The values of magnetization at 5 T (M<sub>S</sub>), remanent magnetization (M<sub>R</sub>) and coercive field (μ<sub>0</sub>H<sub>C</sub>) are given in Table 3.

The results show that the value of M<sub>S</sub> does not significantly change with the annealing temperature. This indicates that the phase composition of the sintered Nd-Fe-B material is the same for the three annealing temperatures, in agreement with the results of the Mössbauer analysis. However, the values of M<sub>R</sub> and μ<sub>0</sub>H<sub>C</sub> after annealing at 550 °C and 650 °C are significantly lower than those obtained after annealing at 750 °C, and are comparable to the properties of the as-sintered material. This indicates that annealing at 550 °C or 650 °C does not induce an increase in the magnetic properties of the sintered material.

SEM images of the sintered material annealed at 550 °C for 30 min are shown in Fig. 12. The microstructure is markedly different from the microstructure of the sintered material annealed at 750 °C as no Nd-oxide regions are observed that surround the Nd₂Fe₁₄B grains as in Fig. 9. Instead, the microstructure appears similar to that of the as-sintered material (Fig. 5), showing the presence of grey regions corresponding to the Nd₂Fe₁₄B grains, light grey regions corresponding to Nd-oxides and dark grey regions corresponding to degraded regions. These

<figure><img src="image_13.png" /><figcaption>Fig. 10. Room temperature Mössbauer spectra of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing under argon at 550 °C and 750 °C for 30 min.</figcaption></figure>

observations indicate that annealing at 550 °C or 650 °C does not induce an evolution of the microstructure of the sintered material. This is in agreement with the results of the magnetic measurements.

Therefore, it appears that the enhanced coercivity of the sintered material after annealing at 750 °C can be related to the microstructural evolution upon annealing at a temperature higher than the solidification temperature of the Nd-rich phase.

Annealing the sintered material at a temperature lower than the solidification temperature of the Nd-rich phase does not allow obtaining an increase of the magnetic properties of the sintered material.

### 3.3.3. Influence of the annealing atmosphere

Based on the results shown above, it can be concluded that annealing at 750 °C for 30 min under argon is the most optimal treatment for improving the magnetic properties of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder.

A vacuum annealing treatment was conducted to investigate the influence of the annealing atmosphere on the structural and magnetic properties of the sintered Nd-Fe-B material.

The room temperature Mössbauer spectra of the sintered material after annealing under vacuum for 30 min at 750 °C and 1000 °C are presented in Fig. 13, along with the spectrum of the sintered material annealed under argon for 30 min at 750 °C, for comparison.

The spectrum of the sample annealed under vacuum at 750 °C is similar to the spectrum of the sample annealed under argon at 750 °C, indicating that the proportions of the Fe containing phases are similar in both samples. In particular, the relative intensity of the α-Fe phase is 8 ± 1 %. The spectrum of the sintered material annealed under vacuum at 1000 °C reveals a relative intensity of the α-Fe contribution equal to 15

<figure><img src="image_14.png" /><figcaption>Fig. 9. SEM back-scattered images of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing under argon at 750 °C for 15 min.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Hysteresis loops at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing under argon at 550 °C, 650 °C and 750 °C for 30 min.</figcaption></figure>

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

Magnetic properties measured at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder before (As-sintered) and after annealing under argon at 550 °C, 650 °C and 750 °C for 30 min.

<table><thead><tr><th>Parameter</th><th>As-sintered</th><th>550 °C annealing</th><th>650 °C annealing</th><th>750 °C annealing</th></tr></thead><tbody><tr><td>M<sub>S</sub>(5T) (Am<sup>2</sup>/kg)</td><td>136.4</td><td>148.1</td><td>148.6</td><td>149.8</td></tr><tr><td>M<sub>R</sub> (Am<sup>2</sup>/kg)</td><td>22.7</td><td>7.1</td><td>7.7</td><td>48.7</td></tr><tr><td>μ<sub>0</sub>H<sub>C</sub> (T)</td><td>0.05</td><td>0.01</td><td>0.02</td><td>0.32</td></tr></tbody></table>

± 1 %, which is significantly higher than that observed after annealing at 750 °C. This suggests that increasing the vacuum annealing temperature does not lead to a reduction of the proportion of α-Fe in the sintered material.

The hysteresis loops at 295 K of the sintered material after vacuum annealing for 30 min at 750 °C and 1000 °C are shown in Fig. 14, along with the hysteresis loop of the sintered material after annealing under argon for 30 min at 750 °C, for comparison. The values of magnetization at 5 T (denoted as M<sub>S</sub>), remanent magnetization (M<sub>R</sub>) and coercive field (μ<sub>0</sub>H<sub>C</sub>) are reported in Table 4.

It is observed that annealing under vacuum at 750 °C, for the same treatment time, leads to a higher coercivity than annealing under argon. Specifically, μ<sub>0</sub>H<sub>C</sub> increases from 0.32 T to 0.53 T, representing an increase of 66 %, and M<sub>R</sub> increases from 48.7 Am<sup>2</sup>/kg to 71.8 Am<sup>2</sup>/kg, representing an increase of 47 %. These variations could be related to different modifications of the intergranular regions, depending on the annealing atmosphere. However, M<sub>S</sub> remains unchanged, in agreement with the fact that the proportion of α-Fe phase is the same in both

samples, as indicated by Mössbauer spectrometry. The energy product (BH)<sub>max</sub> for the material annealed under vacuum at 750 °C is estimated to 42 kJ/m<sup>3</sup>.

Vacuum annealing at 1000 °C for 30 min leads to values of M<sub>S</sub> and M<sub>R</sub> higher than after vacuum annealing at 750 °C for 30 min, in relation with the fact that the proportion of α-Fe phase in the material is higher after annealing at 1000 °C than after annealing at 750 °C, as indicated by Mössbauer spectrometry. Nevertheless, the coercive field μ<sub>0</sub>H<sub>C</sub> is lower after vacuum annealing at 1000 °C than after vacuum annealing at 750 °C, suggesting that the microstructural changes induced by vacuum annealing at 1000 °C are not favorable to increasing the coercivity of the sintered material.

### 3.4. Discussion and conclusion

This investigation shows that the recycled Nd-Fe-B powders obtained by solvothermal treatment can be successfully consolidated by spark plasma sintering. However, the structural and magnetic properties of the obtained material are strongly dependent on the sintering parameters. For the sintering parameters that were considered, a partial degradation of the Nd<sub>2</sub>Fe<sub>14</sub>B magnetic phase into α-Fe and Nd-oxides is observed, which can be reduced on increasing the applied pressure.

Optimizing the sintering conditions is therefore essential to produce a magnet with useful magnetic properties. Furthermore, the application of a magnetic field upon sintering could allow increasing the coercivity of the sintered material through the magnetic alignment of the Nd<sub>2</sub>Fe<sub>14</sub>B grains.

Mixing the recycled Nd-Fe-B powder obtained by solvothermal treatment with a commercial Nd-Fe-B powder also allows obtaining a consolidated material upon sintering. A limited degradation of the

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** SEM back-scattered images of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing under argon at 550 °C for 30 min.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 13. Room temperature Mössbauer spectra of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing for 30 min under vacuum at 750 °C and 1000 °C, and under argon at 750 °C for 30 min.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 14. Hysteresis loops at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after annealing for 30 min under argon at 750 °C, and under vacuum at 750 °C and 1000 °C.</figcaption></figure>

Nd₂Fe₁₄B phase was observed with a mixture of 25 % of recycled powder and 75 % of commercial powder (in mass proportions).

The low coercivity obtained in the sintered materials could result from the formation of a nanometric amorphous layer due to surface oxidation of Nd-Fe-B particles [22]. Furthermore, the presence of a high oxygen content in the sintered magnets has been related to soft magnetic behavior, as demonstrated by Xia et al. [23] and Thonikuzhiyil et al. [24]. A deeper microstructural investigation of the sintered materials

could allow clarifying this point.

Post-sintering annealing under argon or vacuum significantly improves the magnetic properties of the sintered materials. An increase of the magnetic properties is observed for the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after vacuum annealing for 30 min at 750 °C. The increase in saturation magnetization is related to the formation of α-Fe upon the heat treatment. The increase in coercivity is related to structural changes at the grain boundaries that
<caption>**Table 4**</caption>

Magnetic properties measured at 295 K of the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder before annealing (As-sintered) and after annealing for 30 min under argon at 750 °C and under vacuum at 750 °C and 1000 °C.

<table><thead><tr><th>Parameter</th><th>As-sintered</th><th>750 °C (Argon)</th><th>750 °C (vacuum)</th><th>1000 °C (vacuum)</th></tr></thead><tbody><tr><td>M<sub>s</sub>(5T) (Am<sup>2</sup>/kg)</td><td>136.4</td><td>149.8</td><td>149.9</td><td>159.8</td></tr><tr><td>M<sub>r</sub> (Am<sup>2</sup>/kg)</td><td>22.7</td><td>48.7</td><td>71.6</td><td>68.1</td></tr><tr><td>μ<sub>0</sub>H<sub>c</sub> (T)</td><td>0.05</td><td>0.32</td><td>0.53</td><td>0.35</td></tr></tbody></table>

lead to the formation of oxidized Nd-rich regions surrounding several Nd₂Fe₁₄B grains, which may facilitate the magnetic disconnection between the Nd₂Fe₁₄B grains. Such an evolution was observed only after annealing at a temperature higher than the solidification temperature of the Nd-rich phase. It has to be noted that although the sintered materials annealed at 750 °C and 1000 °C contain both hard magnetic Nd₂Fe₁₄B and soft magnetic α-Fe phases, their hysteresis loops do not show a biphasic signature, suggesting the occurrence of exchange coupling between the hard and soft phases [19,20].

With a (BH)max product estimated to 42 kJ/m³, the magnetic properties obtained for the sintered Nd-Fe-B material with 25 % proportion in mass of recycled powder after vacuum annealing for 30 min at 750 °C are encouraging, being consistent with some requirements, especially for isotropic magnets.

However, improvements are necessary in order to obtain sintered permanent magnets from the recycled Nd-Fe-B powders obtained by solvothermal treatment.

A further increase of the magnetic properties should be obtained by optimizing both sintering and annealing parameters, through the minimization of the oxygen content and the obtaining of magnetically aligned magnetic grains through the application of a magnetic field upon sintering.

## 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.

## 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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