Since the introduction of NdFeB magnets around the 1980s, NdFeB Magnets have been widely used in various fields (including automobiles) due to their excellent magnetic properties. In recent years, the demand for Nd magnets, including motors in hybrid automobiles and electric vehicles, has increased abruptly, which presents the following requirements:
– Higher requirements on Coercivity – Hcj (Coercivity – the resistance of a magnetic material to changes in magnetization)
– Stability at higher temperatures above 200°C
– High reverse field environments
– High-speed, high induced losses environments
– Less weight
To meet these requirements, R&D efforts are being put towards advanced technologies. This article, by Saravanan R, takes a closer look at the application of grain boundary diffusion technology in NdFeB magnets.
Need for new technologies
1. A permanent magnet is characterized by Br, Hcj & BHmax values.
– Higher Br means stronger magnetic field strength
– Higher Hcj means the better anti-interference ability
– Higher BHmax means stronger magnetic field strength
2. The temperature of a permanent magnet in a motor could rise above 150 deg C under maximum load conditions, which would cause a magnet with lower intrinsic coercivity to become de-magnetized.
3. To increase the heat resistance and improve coercivity, part of the Nd is being substituted with dysprosium (Dy), terbium (Tb), or other heavy rare earth elements (HREEs).
4. With added Dy/Tb/HREE, the magnet will suffer less loss in the same temperature condition but will be more expensive and have a lower residual magnetic induction or magnetic remanence, otherwise known as Br and BHmax.
5. Even though Tb is more effective than Dy for improving coercivity in high-temperature environments around 200 °C, Dy is used in most, as it has better availability and is less expensive.
6. The slight addition of Tb, Dy, or other HREEs to Nd sintered magnets improves coercivity, but due to the high sintering temperature (1100°C), HREEs diffuse and reduce remanence.
7. Rather than adding expensive Dysprosium to the total mix and wasting it in the middle of the grain, new technology allows the Dysprosium to diffuse from the surface around all of the grain boundaries.
Coercivity enhancement of sintered neodymium magnets
New energy vehicle requires sintered Neodymium magnets with high (BH)max, superior Hcj. It is a major challenge to enhance Hcj while still maintaining high Br and (BH)max. The frequently-used adding methods include:
– Traditional Alloying Process
– Grain Boundary Modification Process
– Grain Boundary Diffusion Process
Traditional alloying process
Addition of HREE Dy or Tb to the raw material. During melting, all elements show homogenization of composition.
– Uniform distribution of HREE will result in a waste of resources and an increase in cost.
– Antiferromagnetic coupling between Fe atoms and Dy atoms will generate a serious magnetic dilution effect and substantially deteriorate Br and (BH)max.
Grain boundary modification process
NdFeB main alloy and HREE-rich auxiliary alloy are manufactured separately, mixed, pressed, and sintered. Dy and Tb will diffuse to the main phase grain from the grain boundary during the sintering process, thus forming magnetic hardening layers at the boundary areas of the main phase.
– Decrease nucleation of the reversed magnetic domain.
– Even though the grain boundary modification process has promoted the utilization ratio or HREE, HREE still inevitably exists in the interior of the main phase grain and gives rise to the magnetic dilution effect.
Grain boundary diffusion process
– Rather than add expensive Dysprosium/ Terbium/HREE to the total mix and waste it in the middle of the grain, new technology allows the Dysprosium/ Terbium/HREE to diffuse from the surface around all of the grain boundaries. The end goal is to increase the coercivity (resistance to demagnetization) of the magnet without compromising its induction (magnetic strength).
– Grain boundary diffusion treatment will be implemented after the machining process.
– The HREE layer can be obtained by spraying, physical vapor deposition (PVD), electrophoresis, and thermal evaporation methods.
– Vacuum heat treatment above the melting point of the Nd-rich phase.
– HREE elements diffuse into the magnet along the grain boundaries
– The grain boundary phase becomes more continuous and straight, which will weaken the magnetic exchange coupling between the main phases.
– Allowing the magnet to increase Hcj while simultaneously maintaining high Br.
– Unlike alloying process, HREE elements do not need to enter the main phase, thus creating a major reduction in the amount of HREE and cost price in conventional high-coercivity sintered Neodymium magnets.
– The grain boundary is capable of manufacturing some new grades which were previously unimaginable via the alloying process.
– Improved HREE utilization ratio.
– Magnetic dilution effect avoided.
– It can repair and increase the magnetic properties of the machined magnet surface.
– By reducing the HREE in the grain and ensuring it is only at the boundary, the Br and BHmax remain unchanged, and less Dysprosium is used in the production process, reducing the manufacturing cost.
– Increasing the coercivity of a magnet material after sintering and machining by heating magnets close to the diffusion element.
– Can create magnets smaller, thinner, and have higher resistance to opposing magnets or coils without risking demagnetization.
Cons or limitations
– Constrained by the thickness of the magnet.
– The enhancement degree of intrinsic coercivity is inverse proportion to the thickness.
– Raising diffusion temperature or prolonging diffusion time can boost the depth and concentration of diffused HREE. However, it is not recommended due to the grain growth of the main phase and changes in phase structure & distribution of the Nd-rich phase.
– Higher Process time.
– Uneven coercivity throughout the volume of the magnet due to the penetration not throughout the entire magnet’s volume.
Traditional method vs Grain boundary diffusion method
|Traditional Method||Grain Boundary Diffusion Method|
|Method||HREE has been added to the entire alloy at an early stage of the production cycle. This ensures that the HREE will be distributed evenly throughout the melt.||HREE is diffused on machined magnets; GBD concentrates HREE in the metallurgical phases of the magnet where it is most effective.|
|Use of HREE||Higher Volume||Low Volume|
|Impact on Br||Reduced Br||GBD allows a magnet to increase Coercivity while simultaneously maintaining high Remanence.|
|Option||To compromise at some level between Remanence and Coercivity. It’s a compromise between magnetic field strength (Br) on the one hand and resistance to demagnetization (Hcj) on the other.||GBD allows materials engineers to increase the maximum energy product to levels not possible with traditional technology.|
|Technical Limitation||[BHMax + Hcj <70]||[BHMax + Hcj >= 80]|
|Impact on Higher Temperature||Magnet’s performance decreases with increasing temperature rating @ same remanence value.||Magnet’s performance is stable even with increasing temperature rating @ same remanence value.|
Buyer’s Views – Grades & Methods
1. Grades M and H previously required approximately 1- 3% Dysprosium before these changes were made. Now both grades are achievable with zero HREE due to the following methods
– Grain Size Reduction
– Grain Refinement and
– Grain Modification.
2. Higher temperature grades that previously required over 7% Dysprosium now require less than 3% Dy.
Source Credit: https://www.yunshengusa.com
About the author
Saravanan R has a track record of nearly 20 years in multiple aspects of Global Sourcing & Strategic Procurement in the Electric Vehicle (EV) & Automotive Industries. He currently works with Okaya Electric Vehicle and can be reached at email@example.com.
Also read: Neodymium magnets in electric vehicles
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