Characterizing REE-Rich Phases in Recycled Magnets

Nd-Fe-B permanent magnets (Fig. 1) are indispensable for the development of motors for electric vehicles and generators for wind turbines. As the energy transition progresses and the automotive market shifts towards green technologies, demand for these magnets has risen sharply. Rare earth elements (REE) are an essential component of these magnets and some of them are considered critical elements, i.e., of great economic and strategic importance. The recycling of used magnets has therefore become crucial. To better optimize the recycling process while maintaining magnet performance, it is important to accurately characterize the various REE-rich phases that form during this process.

Here we present the results of the EDS and WDS microanalysis of the heterogeneous recycling material (Fig. 2). The analytical challenge is twofold: both high spatial and high spectral resolution is required for accurate quantification of the different phases within the samples.

For efficient excitation of the K-lines of Fe and Ga as well as the L-lines of REE, which are all constituents of the samples, an accelerating voltage of at least 15 kV would be required. However, these conditions limit the spatial resolution to approximately 600 nm in the present material (Fig. 3a), which is much larger than many inclusions (Fig. 2). A further complication for the quantification of REE under these conditions results from the strong overlap of the Dy-L line with the Fe-K line.

The use of 5 kV acceleration voltage leads to a significantly higher spatial resolution (Fig. 3b): Objects as small as 125 µm in diameter can be analyzed discretely without including information from the surrounding matrix. With EDS, however, there are strong peak overlaps at the low X-ray energies with the given element selection (Fig. 4, 5, 6). Deconvolution can only partially assist with the quantification of the elements present; their exact concentration or relative ratio is difficult to determine (Tables 1, 2).

WDS results in significantly better spectral resolution and consequently resolves the M-lines of the REE and the L-lines of the metals (Fig. 4, 5, 6). This enables a more precise quantification of the complex phases, as the respective stoichiometries determined demonstrate (Tables 1, 2).

Due to its high sensitivity for the light elements, QUANTAX WDS can also reliably determine low concentrations of boron.

Fig. 1: Nd-Fe-B permanent magnets are essential for motors of electric vehicles and for generators of wind turbines.

Fig. 2: SEM micrograph of recycled battery material showing inclusions of Nd-rich phases (green) and Dy-rich phases (red) in ferrite. Note that some of the inclusions are as small as a few hundred nm.

Fig. 3a: The overvoltage requirements for the excitation of the REE L lines and Fe- and Ga K lines in the current samples demand for at least 15 kV acceleration voltage. This high voltage results in a minimum spatial resolution of 600 nm.

Fig. 3b: For excitation of the REE M lines, Fe- and Ga L lines as well as B K line, an acceleration voltage of 5 kV is sufficient. These low kV conditions result in higher spatial resolution for the analysis. Small inclusions down to 125 nm size can be analyzed discretely (avoiding contributions from the surrounding matrix).

Fig. 4: Spectra of Dy-rich phase 1 (large red inclusions in Fig. 2) recorded by EDS (top) and WDS (bottom). Note the high spectral resolution of WDS and its high sensitivity for B detection.

Table 1: Quantitative results for phase 1 derived by EDS and WDS analysis. Note the determination of B, better totals and correct stoichiometry for the WDS data.

Fig. 5: Spectra of Dy-rich phase 2 (small red inclusion in Fig. 2) recorded by EDS (top) and WDS (bottom). Note the high spectral resolution of WDS and its high sensitivity for B detection.

Table 2: Quantitative results for phase 2 derived by EDS and WDS analysis. WDS data show better totals and the expected stoichiometry for the 1:4:4 phase.

Fig. 6: EDS (top) and WDS (bottom) spectra of the Nd-rich phase (large green inclusion in Fig. 2). Using 10 kV acceleration voltage was in accordance with the larger size of the analyzed phase and resulted in a higher X-ray yield.

This EDS spectrum shows severe overlaps of the Fe, Co, Nd, Pr and Cu X-ray lines. In this case it is difficult to determine the element ratios correctly using deconvolution.

WDS instead can well resolve the different X-ray lines and thus allows more accurate quantification of the complex phase. Even low concentrations of B can be reliably determined.

Samples & References:

Data and sample curtesy of Dr. Eric Robin (CEA-IRIG, Université Grenoble Alpes, France)

Further Resources:

Webinar: Advanced Materials Microanalysis using QUANTAX WDS with a Grazing Incidence X-ray Optic

QUANTAX WDS

QUANTAX WDS is a parallel-beam wavelength-dispersive X-ray spectrometer that is optimized for the determination of low X-ray energies. The system is fully integrated in the ESPRIT software, allowing simultaneous acquisition and combined quantification with EDS.

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