The grain boundary diffusion(GBD) technology in sintered Nd-Fe-B magnets

Jinghui Di   2023/09/15 

To address this, grain boundary diffusion (GBD) technology comes into play. This process involves the diffusion of heavy rare earth (HRE) elements, such as Dysprosium (Dy) and Terbium (Tb), leading to a desired distribution in the magnets macroscopically and microscopically. It aids in conserving rare and expensive HRE, maintains the magnet's remanence, and improves thermal stability—attributes that are critical in ensuring the efficiency and longevity of the magnets in various operating conditions.

The grain boundary diffusion process can be implemented post-sintering, making it an economically viable solution to improve the high-temperature performance of Nd-Fe-B magnets without significantly increasing the HRE content and overall rare earth (RE) content.

The process begins with the preparation of the sintered Nd-Fe-B magnet. This magnet is produced by powder metallurgy techniques, which involve the compaction of Nd-Fe-B powder into a desired shape, followed by sintering at high temperatures. The end result is a magnet composed of microcrystalline Nd2Fe14B grains.

Application of HRE Coating: The sintered Nd-Fe-B magnet is then coated with a layer of HRE, such as Dy or Tb. The coating is applied using methods such as physical vapor deposition (PVD), electroplating, or ion implantation.

Heat Treatment for Diffusion and Formation of High-Coercivity Shell: The coated magnet is subjected to a heat treatment process, typically under a controlled atmosphere. The high temperature facilitates the diffusion of the HRE atoms from the coating layer into the Nd2Fe14B grains. The HRE atoms substitute the Nd atoms at the grain boundaries. This diffusion process, followed by a cooling stage, results in the formation of a high-coercivity shell around the Nd2Fe14B grains. This shell enhances the magnet's resistance to demagnetization, particularly at elevated temperatures.

During the GBD process, the HRE diffuse into the interior along the boundaries and the outer regions of the Nd2Fe14B grains. The HRE substitute Nd atoms at the outer regions of the grain, thus the grains are microscopically magnetically hardened. Besides, the regions at the surfaces which are more likely to be demagnetized are macroscopically hardened by the layers of grains with higher HRE concentration. Thus, the intrinsic coercivity of the bulk magnet are enhanced.

1. Screen Printing

Screen printing is a versatile method used for depositing rare-earth elements onto the surface of the magnet. Using a mesh to transfer ink onto a substrate, this method allows for the precise application of materials like Tb or Dy onto specific areas of the magnet. After the application, a heat treatment process follows to facilitate the diffusion of these elements into the grain boundaries. The main advantage of screen printing is its ability to apply uniform coatings over complex geometries.

2. PVD

PVD is a vacuum deposition method where the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase. In the context of GBD, Tb or Dy is vaporized and then deposited onto the magnet's surface. The resulting thin film is then diffused into the grain boundaries through a subsequent heat treatment process. PVD provides excellent control over the thickness of the coating, which can be beneficial in managing the diffusion process. Thus, it has the identical level of precision in terms of thickness uniformity or the ability to coat complex geometries.

3. Rotary Coating

Rotary coating involves rotating the magnet in a solution containing the rare-earth elements. This rotation ensures a uniform coating of Tb or Dy on the magnet's surface. The heat treatment process that follows encourages the diffusion of the elements into the grain boundaries of the magnet. Rotary coating is advantageous due to its simplicity and ability to provide a uniform coating, especially for products with complex geometries.

Each of these methods offers unique advantages and can be chosen based on the specific requirements of the sintered Nd-Fe-B magnet's production process.

GBD is a way in efficient utilization of HRE with its distinct coercivity mechanism and the geometry of the product plays a vital role in the successful implementation of the GBD process, making it an essential factor in the production of high-performance sintered Nd-Fe-B magnets. 

The GBD products, if not properly evaluated, could pose the risk of performance degradation at elevated temperatures which may further induce shortened product lifespan, safety risks and economic loss. Therefore, it is crucial to conduct thorough evaluation and testing of GBD products to ensure their performance, safety, and reliability. 

Our magnet engineer team has gained deep understanding of the coercivity mechanism and accumulated relative experiences in the magnet manufacturing. 

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