Thermal Stability of Anisotropic Bonded Magnets Prepared by Additive Manufacturing

Abstract

In this research, anisotropic NdFeB + SmFeN hybrid and NdFeB bonded magnets are additively printed in a polyphenylene sulfide (PPS) polymer binder. Printed NdFeB + SmFeN PPS bonded magnets displayed excellent magnetic properties (Br [remanence] = 6.9 kG [0.69 T], Hcj [coercivity] = 8.3 kOe [660 kA/m], and BHmax [energy product] = 9.9 MGOe [79 kJ/m3]) with superior corrosion resistance and thermal stability. The anisotropic NdFeB bonded magnet shows a high coercivity of 14.6 kOe (1162 kA/m) with a BHmax of 8.7 MGOe (69 kJ/m3). The coercivity and remanence temperature coefficients for NdFeB + SmFeN hybrid bonded magnets are −0.10%/K and −0.46%/K, and for NdFeB bonded magnets are −0.14%/K and −0.53%/K in the range of 300–400 K, indicating that the hybrid bonded magnets are thermally stable. The average flux aging loss for hybrid magnets was also determined to be very stable over 2000 h at 448 K (175°C) in air with 2.04% compared to that of NdFeB magnets with 3.62%. Thermal Stability of Anisotropic Bonded Magnets

1 INTRODUCTION

Permanent magnets such as Nd2Fe14B (referred to as NdFeB) and Sm2Fe17N3 (referred to as SmFeN) are used in a variety of technological applications ranging from voice coil motors in consumer electronics, wind power generators, and radial or axial gap flux machines in electric vehicles.1-3 In general, commercial magnets are manufactured for different applications via sintering and polymer bonding. Bonded magnets are prepared with magnet powders derived from hydrogenation–disproportionation–desorption–recombination, and/or melt-spinning processes. They are preferred in specific applications due to their high electrical resistivity (suppresses eddy current losses) and suitability for near-net shape fabrication.4

Recent progress in additive manufacturing has offered new opportunities for the development of bonded magnets.5-11 It offers numerous advantages in tailoring design and achieving near-net shape magnets, meeting tight tolerance in the desired dimensions, minimizing waste, rapid prototyping, and so on. The excellent magnetic properties of Nd2Fe14B, namely, saturation magnetic induction, Js = 1.61 T, anisotropy field, μ0HA = 7.7 T, maximum energy product, (BH)max = 56 MGOe (445 kJ/m3), and Curie temperature, Tc = 588 K, make it a preferred choice for manufacturing bonded magnets.12 Sm2Fe17N3 has a high (BH)max of 47 MGOe (375 kJ/m3), comparable with the NdFeB magnet, but a higher operating temperature than NdFeB due to its higher Tc.13 The typical temperature coefficient of intrinsic coercivity, β is −0.36%/K.14 Therefore, isotropic and anisotropic bonded magnets of Nd2Fe14B, Sm2Fe17N3, or their hybrid, with different thermal stability values, can be produced using polyphenylene sulfide (PPS) or nylon polymer binders.15-19

Li et al. have demonstrated a (BH)max of 43.49 kJ/m3 (5.47 MGOe) with good thermal stability for isotropic bonded permanent magnets with temperature coefficients of remanence, α = 0.11%/K and coercivity, β = 0.34%/K for 65-vol% isotropic NdFeB in nylon.20 We have recently demonstrated anisotropic hybrid bonded magnets of 11 MGOe (87.5 kJ/m3) using a 65-vol% NdFeB + SmFeN hybrid bonded in nylon.21 The performance exceeds those of the bonded magnets prepared using other additive manufacturing methods including direct-write, binder-jet, and fused deposition modeling.6, 22-24 The energy product (BH)max scales with the square of magnetization, being proportional to (1/4)μ0Ms2, (μ0 is the permeability of free space and Ms is saturation magnetization) and remanence scales with binder phases as Br ∝ f × Φp × Mr (f = magnet volume fraction, Br = remanence, and Φp = degree of alignment, Mr = remanence of the magnetic powder, respectively). Therefore, the performance efficiencies of randomly oriented and magnetically aligned bonded magnets are 18% and 70%, respectively, compared to assuming nearly 100% in sintered magnets.13, 25 We previously reported a highly dense anisotropic bonded magnet using 70-vol% NdFeB with an energy product of ∼20 MGOe (159 kJ/m3).26 Therefore, the bonded NdFeB magnets with magnetic properties lie in the middle of lower energy product ferrites and low-grade sintered NdFeB magnets. Thermal Stability of Anisotropic Bonded Magnets

Magnetic properties can be improved further with fine-tuning of both remanence and energy product by varying the ratio of the components of hybrid bonded magnets. Density can be improved by filling the smaller voids between coarse NdFeB particles in bonded magnets with finer SmFeN particles. The commercial anisotropic NdFeB powder has a typical size of several tens to hundreds of microns, whereas that of SmFeN is about several microns. One reason is that the coercivity of NdFeB powder reduces rapidly when its size is less than 20 μm. On the other hand, SmFeN powder can only gain high coercivity when particle size is less than 10 μm. Also, due to the high reactivity of finer NdFeB particles in air and safety concerns, particles >10 μm are preferred for large-scale manufacturing. Hence, coarse NdFeB particles are used in this study. This research aims to develop additive manufacturing methods to produce high-performance anisotropic bonded magnets with good thermal stability. Here, we report the magnetic and thermal properties of anisotropic bonded magnets: (1) hybrid NdFeB + SmFeN and (2) NdFeB, both bonded in PPS binders using Big Area Additive Manufacturing (BAAM). The BAAM system deposits its customized thermoplastic composites and high-performance engineered thermoplastics via melt extrusion processing, which allows rapid manufacturing of objects almost completely unbounded in size. In the BAAM process, the nozzle deposits layers of hybrid magnetic materials, which are fused with PPS polymer and solidify to form the desired shape. Instead of using pre-extruded filament feedstock commonly used on other typical standard FDM 3D printing systems, the BAAM system combines melting, compounding, and extruding functions to deposit polymer products at a controlled rate.16, 20

2 EXPERIMENTAL PROCEDURES

Composites of (1) extruded anisotropic NdFeB + SmFeN magnet powders (hereinafter referred to as hybrid magnet) in PPS (Source: Tengam Engineering, Inc., USA); and (2) anisotropic NdFeB powder in PPS (hereinafter referred to as NdFeB magnet) (Source: Kolektor Magnet Technology, Germany) have been used for additive manufacturing.16, 20, 26, 28 The corrosion resistance of anisotropic hybrid and NdFeB PPS bonded magnets with Scotch-Weld DP100 coatings was tested under the following conditions: (1) The magnets were dropped in a pH solution of 1.35 for 24 h and (2) exposed to flowing argon gas with high humidity (95%) at 353 K (80°C) for >100 h. Magnetic measurements were conducted after testing. The magnetic properties were measured using a vibrating sample magnetometer. The hysteresis loop and AC loss fraction of up to 10 kHz of the printed magnets were measured at 300 K. For the flux loss measurements, the AM magnets were cut into rectangular specimens and were aligned in a 1.0-T magnet field using an electromagnet at 583 K. The magnetized samples were coated with the ∼10-μm thick protective resin coating (3-M Scotch-Weld DP100) and studied for flux loss at various operating temperatures. Thermal Stability of Anisotropic Bonded Magnets

3 RESULTS AND DISCUSSION

Figure 1 shows the scanning electron microscopy micrographs and elemental mapping of both as-printed hybrid and NdFeB bonded magnets. As shown in Figure 1A, a hybrid bonded magnet comprises large platelike NdFeB particles and fine particles of SmFeN. The particle size distribution of anisotropic NdFeB magnet powders is of about 20–100 μm. Figure 1B shows the enlarged view (350× magnification) of fine SmFeN particles, and the inset shows a higher magnification (3500× magnification). The average diameter of the finer particles is in the range of about 1–3 μm. The elemental mapping of the hybrid bonded magnets presented in Figure 1C shows that the large particles of NdFeB with the red contrast (Nd), and the fine particles of SmFeN with the blue contrast (Sm). The platelike morphology of the particle tends to self-align during printing, whereas the finer particles fill in the voids to improve the packing density.22, 28-30 The morphology of a NdFeB PPS magnet is shown in Figure 1D. The diameter of irregular-shaped NdFeB particles is in the range of about 100–200 μm.

FIGURE 1

fig-0001 Scanning electron microscopy (SEM) micrographs of the as-printed

Scanning electron microscopy (SEM) micrographs of the as-printed: (A) Hybrid magnets: NdFeB + SmFeN powder in polyphenylene sulfide (PPS) matrix; (B) the enlarged view of fine SmFeN particles; (C) EDS elemental mapping of the hybrid bonded magnet; and (D) NdFeB bonded magnet: NdFeB powder in PPS matrix
Most of the commercial magnet manufacturing processes adopt Nylon as the binder because higher magnetic powder loading can be obtained due to the low viscosity of Nylon compared to PPS.31 On the other hand, PPS bonded magnets exhibit excellent anti-corrosion ability and resistance in a corrosive environment (due to their low water absorbability compared to nylon and can perform better than nylon magnets at higher temperatures and humid environments).16, 32, 33 The magnetic properties of these printed magnets are presented in Figure 2. The data in Table S1 illustrate that with increasing alignment field (μ0H) from 0 to 2.0 T, the Br of the hybrid bonded magnets increases from 4.2 to 6.9 kG (0.42–0.69 T); (BH)max increases from 3.7 to 9.9 MGOe (29.4–79 kJ/m3), whereas Hc decreases from 9.5 kOe (756 kA/m) to 8.3 kOe (660 kA/m). Similarly, with an increasing alignment field from 0 to 1.5 T, the Br of the NdFeB bonded magnets increases from 3.1 to 6.1 kG (0.31–6.1 T); (BH)max from 2 to 8.7 MGOe (15.9–79 kJ/m3), whereas Hc decreases from 15.3 to 14.6 kOe (1217–1162 kA/m). The irreversible losses of the printed magnets were characterized by temperature coefficients of remanence Br, as α (%/K), and coercivity Hc, as β (%/K), using the second quadrant demagnetization characteristics obtained between 300 and 400 K, for the hybrid magnets (Figure 2B) and NdFeB magnets (Figure 2D). For NdFeB bonded magnets, α = −0.14%/K and β = −0.53%/K, whereas for hybrid bonded magnet, α = −0.10%/K and β = −0.46%/K. The values show that the hybrid bonded magnet has better thermal stability than the NdFeB bonded magnet.

FIGURE 2

fig-0002 magnetization curve of aligned (post-printing) anisotropic NdFeB bonded magnets at elevated temperatures

(A) Room temperature magnetic hysteresis of both as-printed and aligned (post-printing) anisotropic hybrid bonded magnets at 2.0-T field and (B) magnetization curve of post-aligned anisotropic hybrid bonded magnets at 300–400 K, (C) Magnetic hysteresis of magnets aligned at 1.5-T field strength and (d) magnetization curve of aligned (post-printing) anisotropic NdFeB bonded magnets at elevated temperatures
Corrosion resistance is critical for using bonded magnets in harsh environmental conditions. Usually, the corrosion resistance of bonded NdFeB is much better than that of sintered NdFeB due to the protection of polymer binders. Figure S1 shows the magnetic properties of additively printed magnets. The room temperature magnetic properties, that is, coercivity and magnetization at 30 kOe magnetic field of these magnets are listed in Table S2. The magnetic properties are maintained when the sample is treated in a corrosive environment. Coated and uncoated magnets were not degraded in both aggressive conditions. These results show that PPS binders (even without coating) provide sufficient protection for the magnets in the corrosive environment, an added advantage of bonded magnets over sintered magnets. Thermal Stability of Anisotropic Bonded Magnets

The thermal stability of post-printing magnetic field annealed BAAM hybrid magnets has been analyzed by measuring the flux density at ambient temperature before and after aging at 448 K (175°C) and 2000 h. The magnetization reversal process with increasing temperatures can contribute to flux loss. They could also be related to losses due to compositional degradations, for example, oxidation of the magnetic powder. The average percentage of flux loss at a given temperature is direct evidence of the thermal stability of the BAAM printed hybrid PPS magnets. The average flux aging loss over time for both hybrid and NdFeB magnets is shown in Figure 3. Additively fabricated magnets were determined to be very stable over 2000 h with less than 5% within the industry standards. This demonstrates that the printed hybrid and NdFeB anisotropic magnets can be operated up to 448 K (175°C) if the properties of the magnet are sufficient for an intended application at the temperature.

FIGURE 3
fig-0003 Flux aging loss (%) of aligned (post-printing magnetic field annealed) hybrid Big Area Additive Manufacturing (BAAM) printed magnets
Flux aging loss (%) of aligned (post-printing magnetic field annealed) hybrid Big Area Additive Manufacturing (BAAM) printed magnets with ∼10-μm thick 3-M Scotch-Weld DP100 protective resin coatings over 2000-h annealing at 448 K. Industry standard16 is ≤5%

4 CONCLUSION

Anisotropic NdFeB + SmFeN hybrid and NdFeB bonded magnets in PPS polymer were additively printed using a BAAM process. Further increased in remanence of 6.9 kG (0.67 T) and 6.4 kG (0.64 T), and coercivity of ∼8.3 kOe (∼660 kA/m) and 14.6 kOe (1162 kA/m), respectively, were obtained after optimizing post-printing magnetic field alignment. Anisotropic hybrid bonded magnets in PPS polymer exhibited a maximum energy product of ∼10 MGOe (79.5 kJ/m3), and anisotropic NdFeB bonded magnets in PPS polymer showed ∼8.7 MGOe (69 kJ/m3). The hybrid and NdFeB bonded magnets investigated showed good corrosion resistance under high-temperature and harsh chemical conditions.

ACKNOWLEDGMENTS

This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. Part of the printing efforts at Oak Ridge National Laboratory was also supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Wind Energy Technologies Office Program. Thanks are due to Brian Post and John Lindahl with BAAM printing of magnets. This manuscript has been authored, in part, by UT-Battelle, LLC, under contract DE-AC05-00OR22725 and Ames Laboratory, operated by Iowa State University under contract DE-AC02-07CH11358 with the US Department of Energy (DOE).

This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US DOE. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

The Coming Revolution in MnBi High-Strength Magnets

Rare Earth Free Magnets Neodymium Disc Single Pole Magnet

Custom Made Blank Metal Magnetic Name Badge with Three Magnets Backing

D30X6mm SmCo2:17 350 degree C High Temperature SMT Disc Rare Earth Magnets

IATF Certificated High Performance Free Sample SmCo Rod Permanent Magnets

Three Plated Steel Sandwiched Assembly Permanent Magnets

Block Neodymium Hybrid Vehicle Motor Magnets for Honda FREED

High Temperature 160 Degree Rare Earth Bonded Neodymium Ring Magnets BNP-8SR

Grain Boundary Diffusion- Conserving HREE in NdFeB Magnets

Red/Green, Go/No Go, Yes/No Magnets

90 Degree Neodymium Welding Magnets

Reed Sensors Magnets AlNiCo Rod

Neodymium Micro Magnets N52 for Coin Vibrating Motor

N35H Micro Motor Magnets for Mobile Phone Pager Tablet Household