What is the difference between conventional NdFeB magnets and cerium-containing magnets?
In recent years, the prices of rare earth raw materials such as praseodymium and neodymium have fluctuated greatly, which has caused great troubles and constraints in terms of costs for NdFeB production companies and terminal application companies. Cerium Ce and praseodymium and neodymium PrNd, both rare earth elements, have similar structural characteristics and are extremely abundant in the earth's crust. Using them to replace Pr and Nd in neodymium iron boron magnets can not only effectively realize the balanced utilization of rare earth resources, but also It can significantly reduce the production cost of sintered NdFeB. At present, almost all domestic NdFeB manufacturers have been involved in the development and production of cerium magnets. The annual output of new cerium magnets has reached about 50,000 tons, and the scale has a trend of continuous expansion.
What is the difference between cerium magnets and sintered NdFeB permanent magnets produced by conventional processes?
Is there a difference in magnetic properties?
Will it be more brittle and prone to breaking?
This is the question that many magnet users are most concerned about. In today’s article, we will explain it to you in detail.
The element Ce has variable valence characteristics and a small ionic radius. When the Ce content is high, it is easy to form the CeFe2 phase, making it difficult for the magnet to obtain a high coercive force. Due to the low saturation magnetization and anisotropic field of CeFeB, a diffusion process is usually required after adding Ce to the magnet to further improve the performance of the cerium magnet.
Differences in diffusion magnetic properties between conventional magnets and cerium-containing magnets:
| Cerium content | Diffuse source | Performance improvements | Squareness | |
| Normal NdFeB magnet | 0% | Dy | 6.0-8.0KOe | >95% |
| Cerium-containing magnet | 0-5% | Dy | 6.0-8.0KOe | >95% |
| Cerium-containing magnet | 5-8% | Dy | 5.0-7.0KOe | 95% |
| Cerium-containing magnet | 8-12% | Dy | 4.0-6.0KOe | <95% |
| Cerium-containing magnet | >12% | Dy | <5.0KOe | <95% |
When the Ce element is added in a small amount, the impact on the diffusion performance is basically negligible. When the Ce magnet is added in a large amount, especially after more than 12%, the microstructure of the magnet deteriorates more seriously, which not only greatly reduces the diffusion performance improvement Amplitude, there will also be deterioration of magnet squareness due to microstructure inhomogeneity.
From the perspective of use, when the base material contains a low Ce content, under the same Br and Hcj conditions, there is no obvious difference in the magnetic moment and high-temperature demagnetization effect between the Ce-containing magnet and the non-cerium-containing magnet, and their usage characteristics are basically the same. . When the cerium content of the base material is greater than 8%, especially 12%, special attention needs to be paid to the phenomenon that the magnet cannot be fully saturated and demagnetized due to the dual factors of low Hcj and low squareness to avoid residual magnetism. However, the magnetic moment is not enough and the coercivity is satisfied but the thermal demagnetization is not enough.
There are also certain differences in temperature resistance between cerium magnets prepared by conventional processes and diffusion processes. Let’s take 38MT, 30*15*3.2mm as an example. Temperature resistance characteristics of non-diffusive and diffused cerium magnets:
| Cerium content | Diffuse source | Hcj | 100°C*2h open circuit magnetic loss | |
| Normal NdFeB magnet | <6% | Non-Diffuse | 16KOe | 6-12.5% |
| Cerium-containing magnet | 6-10% | Non-Diffuse | 16KOe | 15-20% |
| Cerium-containing magnet | >10% | Dy | 16KOe | 8-13% |
As can be seen from the above figure, the thermal demagnetization of products after cerium magnets are diffused is significantly better than the non-diffusion process of cerium magnets. The thermal demagnetization of products after cerium magnets are diffused is equivalent to the thermal demagnetization of conventional magnets using the non-diffusion process.
Compared with conventional magnets, the mechanical properties of Ce-added magnets will deteriorate as the Ce content changes during processing and use.
| Cerium content | Diffuse source | Mechanical properties | |
| Normal NdFeB magnet | 0% | Dy/Pr/Nd | Excellent |
| Cerium-containing magnet | 0-5% | Dy/Pr/Nd | Excellent |
| Cerium-containing magnet | 5-8% | Dy/Pr/Nd | Common |
| Cerium-containing magnet | 8-12% | Dy/Pr/Nd | Common |
| Cerium-containing magnet | >12% | Dy/Pr/Nd | Poor |
The deterioration of the mechanical properties of Ce-added magnets is mainly due to the fact that after too much Ce is added, the formation of the CeFe2 phase greatly destroys the infiltration and coupling effect of the grain boundaries relative to the main phase grains, resulting in a significant decrease in the mechanical properties. Relevant experimental data shows that when the Ce addition amount is greater than 10%, the mechanical properties of Ce magnets even decrease by 20-50%. Mechanical performance indicators include hardness, compressive strength, flexural strength, tensile strength, impact toughness, Young's modulus, etc. The decline in mechanical properties makes the already brittle NdFeB magnets difficult to process during processing, magnetization and assembly. Corners are more likely to fall off or even crack.
To sum up, when we use ultra-high Ce magnets, we must pay great attention to problems such as poor diffusion effect caused by high Ce, uneven microstructure of the product, local weak magnetic areas, easy demagnetization at high temperatures, and reduced mechanical properties. At present, with the continuous advancement of process technology, CeFe2 technical problems are being paid attention to and overcome by more and more companies, and these problems associated with high Ce are also slowly being weakened.
