Key Takeaways
- Researchers at Korea Institute of Materials Science (KIMS) have devised the world’s first sandwich‑structured grain‑boundary diffusion and bonding process for Nd‑Fe‑B magnets.
- The technology simultaneously boosts coercivity, raises electrical resistivity, and bonds multilayer magnets in a single step.
- By using a light rare‑earth alloy (praseodymium) at both surfaces and internal interfaces, diffusion initiates from within the magnet, delivering uniform performance even in thick sections.
- The high‑resistivity microstructure suppresses eddy‑current formation, cutting heat generation and improving motor efficiency.
- The approach reduces reliance on costly heavy rare‑earth elements, lowers manufacturing complexity, and opens pathways for high‑power applications such as EV traction motors, wind‑turbine generators, and electric ships.
Background
Neodymium‑iron‑boron (Nd‑Fe‑B) permanent magnets are the cornerstone of modern high‑power electromechanical systems, providing exceptional magnetic strength for electric vehicle (EV) traction motors, wind‑turbine generators, and industrial drives. As demand for higher torque and power density grows, manufacturers must increase magnet size and thickness. However, thicker magnets suffer from two interrelated problems: a drop in coercivity toward the interior, which makes the magnet vulnerable to demagnetizing fields, and intensified eddy‑current losses during high‑speed rotation, which generate heat and erode efficiency. These challenges have traditionally been mitigated by adding heavy rare‑earth elements (HREEs) such as dysprosium or terbium, but this solution is costly, supply‑constrained, and limited in its ability to homogenously improve bulk properties.
Conventional Limitations
The prevailing method to enhance high‑temperature performance is grain‑boundary diffusion (GBD). In GBD, a thin coating of an HREE‑rich alloy is applied to the magnet surface; upon heating, the HREE atoms migrate inward along grain boundaries, locally raising coercivity. While effective for thin or moderately thick magnets, the diffusion depth is intrinsically limited by the surface‑centric nature of the process. Consequently, the core of a thick magnet receives insufficient HREE enrichment, leading to a coercivity gradient that undermines overall magnetic stability. Moreover, achieving the desired resistivity increase to curb eddy currents typically requires a separate insulating or segmentation step, adding complexity and cost to production.
Sandwich‑Structured Grain‑Boundary Diffusion and Bonding Process
KIMS researchers overcame these constraints by introducing a sandwich‑structured approach. Multiple Nd‑Fe‑B layers are stacked, and a low‑melting‑point light‑rare‑earth (LRE) alloy—praseodymium (Pr)—is deposited not only on the outer surfaces but also at each internal interface between layers. During the subsequent heat treatment, Pr atoms diffuse inward from both the exterior surfaces and the internal interfaces, effectively creating diffusion fronts that meet in the magnet’s core. This bidirectional diffusion yields a more uniform Pr distribution throughout the bulk, enabling consistent coercivity enhancement even for magnets several millimeters thick. Simultaneously, the Pr‑enriched grain boundaries act as barriers to electron flow, raising the material’s electrical resistivity.
Uniform Coercivity Enhancement in Thick Magnets
Experimental results demonstrated that magnets processed with the sandwich technique achieved coercivity values comparable to those obtained by conventional HREE‑based GBD in thin samples, but with far less variation across the thickness. Magnetic characterization showed a flat coercivity profile from surface to center, indicating that the internal regions received adequate Pr to pin domain walls and resist demagnetizing fields. This uniformity is critical for high‑power motors where the magnetic flux must remain stable under large, rapidly varying loads; any weak spot could precipitate localized demagnetization and torque ripple.
Heat‑Generation Mitigation via High‑Resistivity Structure
In addition to magnetic benefits, the Pr‑rich grain boundaries significantly increase the magnet’s electrical resistivity—by up to a factor of three in tested specimens. Higher resistivity suppresses the formation of eddy currents that are induced when the magnet rotates at high speeds within an alternating magnetic field. Reduced eddy‑current losses translate directly into lower Joule heating, which preserves magnetic performance and improves overall motor efficiency. Notably, this resistivity increase is achieved without an extra insulating layer or magnet segmentation; the same grain‑boundary diffusion step that delivers coercivity enhancement simultaneously imparts the desired electrical property.
Process Integration and Economic Advantages
The sandwich strategy consolidates three traditionally separate operations—surface coating for GBD, internal diffusion for bulk property improvement, and insulating bonding or segmentation for eddy‑current control—into a single, streamlined heat‑treatment cycle. This integration reduces manufacturing steps, lowers energy consumption, and minimizes the risk of defects introduced during multiple handling stages. By substituting a light rare‑earth (Pr) for expensive heavy rare‑earths, the process also mitigates material cost and supply‑chain vulnerability. Pr is more abundant and less price‑volatile than dysprosium or terbium, making the technology attractive for large‑scale production.
Applications and Broader Impact
The enhanced magnets are poised to benefit any system that demands strong, stable magnetization under high thermal and mechanical loads. Primary targets include EV traction motors, where higher efficiency extends driving range and reduces cooling requirements; wind‑turbine generators, in which improved magnet performance enables larger, more efficient direct‑drive designs; and high‑speed industrial spindles or servo motors that suffer from thermal demagnetization. Moreover, the ability to produce large, high‑performance magnets without prohibitive HREE usage opens emerging markets such as electric ship propulsion, where massive drive motors demand both strength and thermal stability.
Research Support, Publication, and Follow‑Up
The work was conducted by a team led by senior researchers Su‑Min Kim and Jung‑Goo Lee at KIMS, under the auspices of the Materials and Components Technology Development Program funded by the Ministry of Trade, Industry and Energy. Findings were reported in the peer‑reviewed journal Scripta Materialia (Impact Factor 5.6) on March 18, 2026. The article details the microstructural characterization, magnetic measurements, and resistivity assessments that validate the sandwich approach. The researchers note that ongoing studies focus on scaling the process to industrial‑scale magnet furnaces and integrating the resulting magnets into prototype motor assemblies to evaluate real‑world performance gains.
Expert Perspective and Outlook
Su‑Min Kim emphasized the significance of the breakthrough: “This study demonstrates a breakthrough in simultaneously achieving high coercivity and reduced heat generation in thick magnets. What differentiates this technology is that it integrates coercivity enhancement, resistivity improvement, and structural bonding into a single process.” He added that the method’s versatility extends beyond EVs to any application requiring large, robust magnets, positioning it as a key enabling material technology for the next generation of high‑efficiency electromechanical systems. With further development, the sandwich‑structured grain‑boundary diffusion strategy could help reduce the automotive and renewable energy sectors’ dependence on scarce heavy rare‑earths while delivering tangible gains in power density, efficiency, and reliability.