In recent years, the prices of rare earth raw materials such as praseodymium and neodymium have fluctuated significantly, causing considerable cost challenges and constraints for NdFeB manufacturers and end-user companies. Cerium (Ce), also a rare earth element, shares similar structural characteristics with praseodymium and neodymium (PrNd) and is extremely abundant in the Earth’s crust. Its use in NdFeB magnets to replace Pr and Nd can not only effectively achieve balanced utilization of rare earth resources but also significantly reduce the production cost of sintered NdFeB. Currently, almost all NdFeB manufacturers in China have ventured into the development and production of cerium magnets, with an annual output of approximately 50,000 tons of new cerium magnets, and this scale continues to expand.
What are the differences between cerium magnets and sintered NdFeB permanent magnets produced using conventional processes?
Are there differences in magnetic properties?
Will they be more brittle and prone to breakage?
These are the most pressing questions for many magnet users. This article will provide a detailed explanation.
Cerium (Ce) exhibits variable valence and a small ionic radius. When Ce content is high, it readily forms the CeFe2 phase, making it difficult for magnets to achieve high coercivity. Since CeFeB has low saturation magnetization and anisotropic field, magnets typically require a diffusion process after Ce is added to further improve their performance.
Differences in diffusion properties between conventional magnets and cerium magnets
| Cerium content of substrate | diffusion source | Performance improvement | Rechtwinkligkeit | |
| conventional magnets | 0% | Dy | 6,0–8,0 KOe | >95% |
| Cerium magnet | 0%-5% | Dy | 6,0–8,0 KOe | >95% |
| Cerium magnet | 5%-8% | Dy | 5,0–7,0 KOe | 95% |
| Cerium magnet | 8%-12% | Dy | 4,0–6,0 KOe | ≤95% |
| Cerium magnet | >12% | Dy | <5KOe | <95% |
When the amount of Ce added is small, its impact on diffusion performance is negligible. However, when the Ce content is large, especially above 12%, the microstructure of the magnet deteriorates significantly. This not only drastically reduces the improvement in diffusion performance but also leads to a deterioration in magnet squareness due to microstructural inhomogeneity.
From a usage perspective, when the substrate contains low Ce content, under the same Br and Hcj conditions, there is no significant difference in magnetic moment and high-temperature demagnetization effect between Ce-containing and non-cerium-containing magnets, and their usage characteristics are basically the same. When the substrate Ce content is greater than 8%, especially 12%, special attention needs to be paid to the inability to fully saturate the magnet and the phenomenon of high-temperature demagnetization caused by the combined effects of low Hcj and low squareness. This is to avoid the phenomenon of sufficient remanence but insufficient magnetic moment and sufficient coercivity but insufficient thermal demagnetization.
There are also some differences in temperature resistance between conventional and diffusion-processed cerium magnets. Taking 38MT, 30*15*3.2mm as an example:
Temperature resistance characteristics of non-diffusion and diffusion cerium magnets
| Cerium content of substrate | diffusion source | Hcj | Open circuit magnetic loss | |
| conventional magnets | <6% | Dy | 16KOe | 6%-12.5% |
| Cerium magnet | 6%-10% | Non-diffusion | 16KOe | 15%-20% |
| Cerium magnet | >10% | Non-diffusion | 16KOe | 8%-13% |
As shown in the figure above, the thermal demagnetization of cerium magnets after diffusion is significantly better than that of cerium magnets without diffusion, while the thermal demagnetization of cerium magnets after diffusion is comparable to that of conventional magnets without diffusion.
Compared to conventional magnets, the mechanical properties of Ce-added magnets deteriorate with changes in Ce content during processing and use.
Mechanical properties of non-diffusion and diffusion cerium magnets
| Cerium content of substrate | diffusion source | Mechanische Eigenschaften | |
| conventional magnets | 0% | Dy/Pr/Nd | good |
| Cerium magnet | 0%-5% | Dy/Pr/Nd | good |
| Cerium magnet | 5%-8% | Dy/Pr/Nd | medium |
| Cerium magnet | 8%-12% | Dy/Pr/Nd | medium |
| Cerium magnet | >12% | Dy/Pr/Nd | poor |
The deterioration in the mechanical properties of Ce-added magnets is mainly due to the formation of the CeFe2 phase when the Ce content is too high. This significantly disrupts the wetting and coupling effect of grain boundaries on the main phase grains, leading to a substantial decrease in mechanical properties. Related experimental data shows that when the Ce content exceeds 10%, the decrease in mechanical properties of Ce magnets can reach 20-50%. Mechanical properties include hardness, compressive strength, flexural strength, tensile strength, impact toughness, and Young’s modulus. This decline in mechanical properties makes the already brittle NdFeB magnets more prone to chipping and cracking during processing, magnetization, and assembly.
In summary, when using ultra-high Ce magnets, we must pay close attention to the problems caused by high Ce, such as poor diffusion, uneven microstructure, localized weak magnetic areas, easy demagnetization at high temperatures, and reduced mechanical properties. Currently, with continuous advancements in process technology, the technical challenges associated with CeFe2 are being increasingly addressed and overcome by more and more companies, and these problems associated with high Ce are gradually being mitigated.