An Overview Of Grain Boundary Diffusion

Table of Contents

What Is Grain Boundary Diffusion?

Grain boundary diffusion is a technique that has been used to improve the performance of permanent magnets. Distributing heavy rare-earth elements into the intergranular material around the grains of a sintered magnet makes it possible to increase the magnet’s coercivity (resistance to demagnetization) without compromising its induction (magnetic strength).

This is typically accomplished by applying a high-temperature treatment to the magnet, which causes the heavy rare-earth elements to diffuse through the grain boundaries and into the intergranular material. This process can be controlled to achieve the desired distribution of the heavy rare-earth elements within the magnet, which can improve its overall performance.

It is worth noting that grain boundary diffusion is just one of many techniques that can be used to improve the performance of permanent magnets. Other techniques include the use of different magnetic alloys, the optimization of the microstructure of the magnet, and the application of various surface treatments.

Image Credit: Sciencedirect

Grain Boundary Diffusion vs. Traditional Technology

  • Traditional Method

One traditional method of increasing the coercivity of a permanent magnet is to mix heavy rare-earth elements such as dysprosium and terbium into the initial grain structure of the magnet when it is being produced. These elements tend to end up in the intergranular material around the grains of the magnet, where they can improve the magnet’s resistance to demagnetization.

However, this process also decreases the induction (Br) of the magnet, as the heavy rare-earth elements can displace some of the neodymium in the grain structure. This trade-off between coercivity and induction is a common challenge in the design of permanent magnets, and it is why techniques such as grain boundary diffusion can help improve the performance of these materials.

  • Grain Boundary Diffusion

Grain boundary diffusion allows the heavy rare-earth elements to be distributed into the intergranular material without affecting the grain structure of the magnet, which can help preserve the magnet’s induction while still increasing its coercivity. This can be a more efficient way to improve the magnet’s performance, as it allows the heavy rare-earth elements to be targeted specifically to the regions where they are most effective.

Hence, grain boundary diffusion technology has the potential to create new magnet grades with higher Br and Hcj (coercivity) and BH Max (maximum energy product) that were previously unattainable with traditional technology, making it a valuable tool in the production of high-performance neodymium magnets.

Advantages Of Grain Boundary Diffusion

Grain boundary diffusion is a technique that has several advantages in producing permanent magnets.

One of the main advantages is that it improves the magnet’s coercivity after sintering and even after machining by heating the magnet close to the diffusion element. This can be done without affecting the grain structure of the magnet, which can help to preserve its induction while still increasing its coercivity.

This ability to increase the coercivity of a magnet after it has already been sintered and machined can be particularly useful in designing smaller, thinner magnets that require a high resistance to demagnetization. Using grain boundary diffusion, it is possible to create magnets more resistant to opposing magnets or coils without compromising their other performance characteristics.

Overall, grain boundary diffusion is a valuable technique for improving the performance of permanent magnets and can be used in various applications where a high level of coercivity is required.

Disadvantages Of Grain Boundary Diffusion

The grain boundary diffusion (GBD) process has some drawbacks or trade-offs.

One disadvantage is the time and energy required to perform the process. The magnets and diffusion element must be heated to very high temperatures (800-1000 degrees C) for several hours to achieve the desired diffusion of the heavy rare-earth elements. This can be a bottleneck in mass production, as it limits the throughput of GBD magnets.

Another disadvantage is the cost of the heavy rare-earth elements, such as dysprosium and terbium, used in the GBD process. These elements are relatively expensive, which can increase the overall cost of producing GBD magnets.

Finally, the GBD process does not uniformly penetrate the entire volume of the magnet. It primarily diffuses within several hundred microns of the surface layer, which can result in an uneven distribution of coercivity throughout the magnet. This can be important to consider when designing with GBD magnets, especially if the GBD process is performed on the sintered block, which is the most common method.

Improvements To NdFeB Magnets By Grain Boundary Diffusion

Grain boundary diffusion (GBD) is a technique that offers many improvements to neodymium-iron-boron (NdFeB) magnets. Here is a summary of some of the critical benefits of GBD:

  • GBD significantly reduces the amount of heavy rare-earth elements (HREE), such as dysprosium and terbium, used in NdFeB magnets. The exact reduction depends on various factors but ranges from 70% to 100%.
  • GBD concentrates the HREE in the metallurgical phases of the magnet, where they are most effective in improving the magnet’s performance.
  • GBD allows materials engineers to increase the magnet’s maximum energy product (BH Max) to impossible levels with traditional technology.
  • GBD allows the magnet to have increased coercivity (resistance to demagnetization) while maintaining high remanence (magnetic strength). This is a feature that is not possible with traditional technology.
  • GBD enables the production of new grades of magnets that were previously unimaginable through conventional metallurgy.
Image Credit: Sciencedirect

Making Neodymium Magnets With GBD Technology

To make neodymium magnets using the grain boundary diffusion (GBD) process, it is generally necessary to follow a specific set of steps:

  1. Start with a sintered neodymium magnet: GBD is typically performed on a sintered neodymium magnet, produced using a powder metallurgy process that involves pressing and sintering neodymium, iron, and boron powders together to form a solid magnet.
  1. Add heavy rare-earth elements: The next step is to add heavy rare-earth elements such as dysprosium and terbium to the sintered magnet. These elements can be mixed into the magnet powder before sintering, or they can be added as a coating or thin film on the surface of the magnet.
  1. Perform the GBD treatment: The magnet is then subjected to a high-temperature treatment, during which the heavy rare-earth elements diffuse through the grain boundaries and into the intergranular material around the grains of the magnet. The temperature, time, and other processing parameters of the GBD treatment are carefully controlled to achieve the desired distribution of the heavy rare-earth elements in the magnet.
  1. Finish and test the magnet: Once the GBD treatment is complete, the magnet may undergo additional processing steps such as machining, grinding, or polishing to achieve the desired final shape and surface finish. The magnet is tested to confirm that it meets the desired performance specifications.
Image Credit: Mdpi

Grain Size, Grain Refinement, And Grain Modification

Reducing the grain size, refining the grain structure, and modifying the grain shape are essential steps in producing high-performance neodymium magnets. These steps can help to optimize the microstructure of the magnet, which can, in turn, improve its overall performance.

Reducing the grain size involves decreasing the size of the individual grains in the microstructure of the magnet. This can be achieved through advanced equipment and careful control of multiple process factors. Reducing the grain size reduces the ratio of volume-to-grain-boundary-surface area can help improve the magnet’s performance.

Grain refinement involves regulating the size of the individual grains so that the grain size distribution is narrowed, resulting in more uniform grain sizes. This can help to create uniformity of properties in the entire magnet, as it promotes the formation of uniform magnetic domains.

Grain modification involves making the shape of the grains as similar as possible, which can also promote the formation of uniform magnetic domains and improve the overall uniformity of the magnet.

These three steps – grain size reduction, grain refinement, and grain modification – are often used in combination in the production of neodymium magnets, and they can help to increase the ratio of grain boundary phase-to-main phase, which can reduce or eliminate the need for heavy rare-earth elements (HREE) such as dysprosium and terbium. Combined with techniques like grain boundary diffusion (GBD), these steps can be powerful tools for optimizing the performance of neodymium magnets.

Future Thoughts

As technology advances, new developments in grain boundary diffusion will likely allow materials engineers to optimize the performance of neodymium magnets further and create new magnet grades with even higher performance. In addition, as market demand for high-performance magnets continues to grow, grain boundary diffusion will likely continue to be an essential tool in meeting that demand.

We’re glad to have been able to provide you with information about grain boundary diffusion (GBD) technology. Talk to an expert from JdaMagnet, and we’re happy to answer any additional questions.

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