ORALS

SESSION: SISAMFriPM2-R3 F: Metastability & sustainability	Kobe International Symposium on Science of Innovative and Sustainable Alloys and Magnets (5th Intl. Symp. on Science of Intelligent and Sustainable Advanced Materials (SISAM))
Fri Oct, 25 2019 / Room: Dr. Christian Bernard
Session Chairs: Michael J. Zehetbauer; Session Monitor: TBA

16:45: [SISAMFriPM211] Invited
Towards High Coercivities in Heavy Rare Earth Free Nd-Fe-B Ribbons
Marko Soderžnik¹ ; Matic Korent² ; Urska Ročnik³ ; Boris Saje⁴ ; Spomenka Kobe⁵ ; ¹Jožef Stefan Institute, Ljubljana, Slovenia; ²Jozzef Stefan Institute, LJUBLJANA, Slovenia; ³Department for Nanostructured Materials, Jožef Stefan Institute, Ljubljana, Slovenia; ⁴Kolektor Magnet Technology GmbH, Essen, Germany; ⁵Josef Stefan Institute, Ljubljana, Slovenia;
Paper Id: 367 [Abstract]

Reasonable magnetic performance to weight ratio makes polymer-bonded magnets indispensable in automotive applications [i]. The magnetic powders, used for bonded magnets are mainly produced by the gas atomization and melt-spinning [ii]. Several magnetic powders can be used for such purposes, namely ferrites, SmCo, Sm-Fe-N, Nd-Fe-B and/or combinations of all of them. Since the magnetic powder is blended with non-magnetic binder, the remanent magnetization is diluting as the volume percent of the binder is increasing. Therefore, they can be classified as medium-performance isotropic bonded magnets. The coercivity of the magnet, however, is not related to the magnetic powder/non-magnetic binder ratio but to the chemistry and microstructural features. Melt-spun ribbons of Nd-Fe-B material are composed of randomly oriented Nd₂Fe₁₄B grains within the size of single magnetic domain [iii]. Therefore, they have a huge potential for higher coercivity compared to sintered Nd-Fe-B magnets in which a typical grain size is measured in microns [iv]. There exist several ways to improve the coercivity of Nd-Fe-B magnets. One way is to decouple the Nd2Fe14B grains by infiltration of low eutectic Nd-based alloys which we propose within this study. Detailed microstructural analyses showed that non-ferromagnetic Nd₇₀Cu₃₀ was successfully infiltrated between the grains, which prevented the physical contact between the grains leading to weaker intergrain exchange coupling. The results of such a process show more than 20 % improvement in coercivity while the remanence is increased as expected due to the lower amount of the ferro-magnetic phase. Significant increase in coercivity compensates lower remanence, and the energy product is also increased. In comparison to the basic powder, the coercivity at 150 °C is significantly improved, which enables these magnets to be used at a higher temperature.

References:
[i] J. J. Croat, 8-Major applications for rapidly solidified NdFeB permanent magnets, Woodhead Publishing Series in Electronic and Optical Materials (2018) 325–361.\n[ii] G. Sarriegui, J. M. Martín, M. Ipatov, A. P. Zhukov, J. Gonzalez, Magnetic Properties of NdFeB Alloys Obtained by Gas Atomization Technique, IEEE Trans. Magn. 54 (2018) 2103105.\n[iii] J. D. Livingston, Magnetic domains in sintered Fe-Nd-B magnets, J. Appl. Phys. 57 (1985) 4137–4139.\n[iv] M. Soderžnik, M. Korent, K. Žagar Soderžnik, M. Katter, K. Üstüner, S. Kobe, Acta Mat. 115 (2016) 178–284.

SESSION: SISAMSatPM1-R3 H: Characterisation	Kobe International Symposium on Science of Innovative and Sustainable Alloys and Magnets (5th Intl. Symp. on Science of Intelligent and Sustainable Advanced Materials (SISAM))
Sat Oct, 26 2019 / Room: Dr. Christian Bernard
Session Chairs: A. Lindsay Greer; Session Monitor: TBA

14:00: [SISAMSatPM105]
Correlative Characterization from Atoms to Magnetic Fields in Tb-Doped Nd-Fe-B Magnets
Kristina Zagar Soderznik¹ ; Saso Sturm² ; Andras Kovacs³ ; Aleksei Savenko⁴ ; Marko Soderžnik⁵ ; Rafal Dunin Borkowski⁶ ; Joachim Mayer⁷ ; Spomenka Kobe⁸ ; ¹Jozef Stefan Institute, Ljubljana, Slovenia; ²Head of Department for Nanostructured Materials, Ljubljana, Slovenia; ³Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Juelich, Germany; ⁴Thermo Fisher Scientific, Erlangen, Germany; ⁵Jožef Stefan Institute, Ljubljana, Slovenia; ⁶Forschungszentrum Jülich, Jülich, Germany; ⁷Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Jülich, Germany; ⁸Josef Stefan Institute, Ljubljana, Slovenia;
Paper Id: 347 [Abstract]

High coercivity Nd-Fe-B permanent magnets play an important role in the rapidly-growing renewable energy sector. To retain the coercivity at high operating temperatures, heavy-rare-earth elements (HRE), such as Dy and Tb, are added using the grain-boundary diffusion (GBD) process. The addition of HRE results in a significant improvement of the coercivity due to the increase of the intrinsic resistance to demagnetization. [1] In the present study, we report on the correlation between magnetic properties and the distribution of Tb₄O₇ in the Nd₂Fe₁₄B magnet. The Nd₂Fe₁₄B magnet was coated with Tb₄O₇ powder and annealed. During the annealing process, Tb diffused along grain boundaries (GB) into the outer parts of Nd-Fe-B grains, thus forming core-shell grains with the Tb-rich shell and Nd-Fe-B core. Magnetometry measurements were performed to observe the Tb concentration gradient from the surface of the magnet into its central part. It was found that the coercivity gradually decreases towards the central part where it is still around 30% higher when compared with the untreated magnet. [2,3] Although magnetic measurements clearly indicate the presence of Tb, it is not clear what the actual amount of Tb is in central regions of magnets or how they are distributed in the microstructure and if it is possible to distinguish the magnetisation flux between soft magnetic shells and hard magnetic cores. For that purpose, we applied the Cs-corrected STEM: FEI Titan 80-200 equipped with SuperX electron dispersive X-ray (EDX) spectrometer and electron energy-loss (EEL) spectrometer and FEI Titan 80-300 equipped with electron biprism to perform electron holography. As a complementary method, atom probe tomography (APT) was used using 3D atom probe LEAP 4000x HR. In order to analyse the core-shell region, a lamella was prepared from the representative core-shell grains and the interface between the shell and the core was examined using EELS and APT. Detailed line-scans and spectrum image maps were performed at this interface. The estimated width of the transition area between the shell and the core was 20 nm. Further studies focused on the electron holography of core-shell grains. The magnetic fluxes were within the core and the shell was determined. The thickness and the composition of the shell were determined as a function of the specimen position within the magnet.

References:
[1] P. J. McGuiness, et al., JOM 67 (2015) 1306-1317.
[2] M. Soderžnik, et al., Intermetallics 23 (2012) 158-162.
[3] M.Soderznik, et al., Acta Materialia 115 (2016) 278-284

SESSION: SISAMSatPM2-R3 H: Characterization cont.	Kobe International Symposium on Science of Innovative and Sustainable Alloys and Magnets (5th Intl. Symp. on Science of Intelligent and Sustainable Advanced Materials (SISAM))
Sat Oct, 26 2019 / Room: Dr. Christian Bernard
Session Chairs: Petra Jenus; Session Monitor: TBA

16:45: [SISAMSatPM211]
Metal-Bonded Magnets Based on YCo5-Type Nanocrystals
Marko Soderžnik¹ ; Matic Korent² ; Kristina Zagar Soderznik³ ; Jean-marie Dubois⁴ ; Pelin Tozman⁵ ; M. Venkatesan⁶ ; Michael Coey⁷ ; Spomenka Kobe⁸ ; ¹Jožef Stefan Institute, Ljubljana, Slovenia; ²Jozzef Stefan Institute, LJUBLJANA, Slovenia; ³Jozef Stefan Institute, Ljubljana, Slovenia; ⁴JSI - K7 Dpt for Nanostructured Materials, Ljubljana, Slovenia; ⁵NIMS, Tsukuba, Japan; ⁶School of Physics and CRANN, Trinity College, Dublin, Ireland; ⁷School of Physics, Dublin, Ireland; ⁸Josef Stefan Institute, Ljubljana, Slovenia;
Paper Id: 342 [Abstract]

Metal-bonded magnets based on YCo₅-type nanocrystals [i] were produced by hot-compaction using a spark plasma-sintering device. Zn and Zn/Al metallic binders with a melting temperature of ̴ 420°C were employed to fabricate dense cylindrical magnets. Two different pressures were used for compaction. The pressure of 400 MPa provided a metal-bonded magnet with Vickers hardness (HV10) of 460 ± 20 Vickers. The temperature coefficients for remanence (α) and coercivity (β) were derived from magnetization vs. magnetic field measurements in the temperature range of 20°C – 150°C. Temperature coefficients α and β for the Zn/Al-bonded magnet pressed with 400 MPa were -0.055 %/°C and -0.201 %/°C, respectively. The field emission gun scanning electron microscope revealed a ‘core-shell’-type microstructure. The pure YCo_4.8Fe_0.2 phase was detected in the core region whereas the shell was enriched with non-ferromagnetic Zn or Zn/Al phases. The high-resolution transmission electron microscope revealed the presence of clusters with ̴ 20 nm YCo_4.8Fe_0.2 grains. In the Zn/Al-bonded magnet, fabricated at 400 MPa, the coercivity <i>µ₀H_ci</i>, remanent magnetization σ and energy product (BH)_max were 0.87 T, 39.3 Am²/kg and 23.4 kJ/m³, respectively.[ii]

References:
[i] P. Tozman, M. Venkatesan, J. M. D. Coey, Optimization of the magnetic properties of nanostructured Y-Co-Fe alloys for permanent magnets, AIP Adv. 6 (2016) 056016.\n[ii] M. Soderžnik, M. Korent, K. Žagar Soderžnik, J.-M. Dubois, P. Tozman, M. Venkatesan, J. M. D. Coey, S. Kobe, Hot-compaction of YCo4.8Fe0.2 nanocrystals for metal-bonded magnets, J. Mag. and Magn. Mat. 460 (2018) 401-408.