Stability of NdFeB performance
The performance stability of permanent magnet material is an important index of permanent magnet material. The stability mainly refers to the process of magnetic properties change under the influence of internal and external factors after the magnet is magnetized, which is usually expressed by the change rate of performance index. The common causes of magnetic properties change include temperature, time, electromagnetic field, radiation, mechanical vibration and impact. In the last issue, yueci has introduced the temperature stability of permanent magnet materials (remanence temperature coefficient, coercivity temperature coefficient, reversible temperature coefficient). In this issue, we will learn about the time stability of permanent magnet materials.
If the magnet is working for a long time or placed for a long time, the surrounding environment (such as temperature, humidity, corrosive liquid, etc.) may cause the physical and chemical properties of the magnet to change. After the permanent magnet is magnetized, most of the area is magnetized to a specific direction, but the magnetization direction of some small magnetic domains is chaotic (called reversal magnetization nucleus). Under the action of various environmental factors, the original reversal magnetization nucleus As it grows up, a new magnetization nucleus will be produced, which makes the magnetic properties of the permanent magnet attenuate. This change is generally a slow and irreversible change from the surface to the inside, which directly affects the main performance parameters of the magnet, remanence, intrinsic coercivity, coercivity or maximum magnetic energy product, and even leads to complete failure of the magnet. This loss of magnetic properties is irreversible, even if the magnet is re-magnetized, it cannot be restored to the level before being placed for a long time. In recent years, as NdFeB permanent magnet materials have been widely used in aerospace, electric vehicles, high-power wind power and other fields with long service life requirements, application designers have become more aware of the time stability of NdFeB permanent magnets. Pay more attention to it.
Temperature stability
When the temperature of the working environment of the neodymium iron boron permanent magnet changes within a certain range, the magnetic flux Φ (TotalFlux) of the magnet will change accordingly, as shown below:
We use the reversible temperature coefficient of remanence αBr, the temperature coefficient of HcJ βHcJ and the irreversible loss of magnetic flux hirr to measure the change in the magnetic properties of NdFeB with temperature.
Remanence temperature coefficient
Reversible temperature coefficient of remanence αBr: When the working environment temperature rises from room temperature T0 to temperature T1, the remanence Br of neodymium iron boron also decreases from B0 to B1; when the ambient temperature returns to room temperature, Br cannot return to B0, and Only to B0′. After that, when the ambient temperature changes between T0 and T1 (assuming the amount of change is not very large), the change of Br is linear and reversible.
In the same way, we can get the temperature coefficient βHcJ of the intrinsic coercive force HcJ as follows:
The temperature coefficients α and β only measure the reversible change of the magnetic properties, that is, the magnetic properties can be restored after the temperature is restored.
Flux formula
In reality, we often see irreversible changes, especially when the magnetic flux (TotalFlux) changes with temperature to T1 when the magnet is open, the irreversible relative change, we call it the magnetic flux at temperature T1 The irreversible loss hirr, the formula is:
From the perspective of use, it is hoped that αBr, βHcJ and hirr are all as small as possible. But in fact, in the open circuit state, for the NdFeB magnet at a specific operating point (ie the size and shape of the magnet element), the αBr is higher, generally -0.11-0.12%/℃; βHcJ is also higher, generally -0.6- 0.7%/℃ (but it is directly related to the temperature range). So which is more important for αBr and βHcJ? This depends on the choice of operating point. If the operating point of the magnet is higher, that is, when B/H>>1, αBr plays a major role, and when B/H<<1, βHcJ plays a major role in the stability of the magnetic field. . As for the irreversible loss of magnetic flux hirr, it is usually required to be >1. Under the maximum temperature allowed for the magnet material, the hirr of the magnet should be ≤5%. For example, the 33SH performance standard block (2″×2″×1″) After a constant temperature of 150℃×1 hour, return to normal temperature, and its hirr<5%.
When the external temperature rises from room temperature, the initial loss of magnetic properties is reversible, and the temperature can be restored to restore the magnetic properties; later includes irreversible but recoverable loss, that is to say, the loss of magnetic properties at this time cannot be recovered by the temperature. It can be recovered, but it can be recovered by remagnetization; if the temperature rises above the Curie temperature of the magnet, the structure of the magnet is irreversibly damaged, which is an irreversible and irreversible loss of magnetic properties.
Research progress
In general use, the solution to temperature stability is to do aging treatment to eliminate the instability of the magnet (of course, this is at the expense of part of the magnetism, generally 10%). The temperature and time of the aging treatment are based on the application or user requirements. For example: it can be boiled in boiling water for 3 hours, or heated and aged in an oven with an iron plate, or it can be accurately kept constant at 125℃×1.5 hours in a high vacuum sintering furnace. There are also some ways to directly improve the temperature stability of the magnet itself by adding certain elements. For example, in the application field of microwave communication devices, the lower the temperature coefficient of magnetic induction αBr, the better. In recent years, research in this area has made great progress:
- ① Adding Co can effectively increase the Curie temperature (generally adding 1at.% Co can increase Tc by about 10°C); at the same time, adding Co can strengthen the exchange effect between 3d sublattices, thereby increasing αBr. The addition of Dy, although it will lower the Curie temperature, can also improve αBr due to the antiparallel coupling between its magnetic moment and the Fe sublattice magnetic moment. Such as adding at the same time: replace Fe with Co and replace Nd with Dy, and when the ratio is appropriate, the αBr of the NdFeB magnet can be reduced to zero. If the composition is (Nd0.5Dy0.5) 15.5Fe51Co26B7.5 magnet, its magnetic performance can reach: Br=0.88T; HcJ=1.23MA/M-1 (15kOe), Hcb=525.4KAM-1; BHm= 119.4KJ/M3, αBr=0.00%/℃; irreversible loss of magnetic flux ≤5%.
- ②On this basis, adding Ga and W can obtain low αBr sintered NdFeB magnets.
- ③The addition of Tb to the magnet can not only obtain low αBr, but also maintain high HcJ and BHm.
Motor magnet
Another example is the magnetic steel used in motors, which does not require much αBr, but requires βHcJ as low as possible. It is difficult to improve βHcJ, but some research results show that:
① Adding Dy, Tb, Ga can improve the βHcJ of the sintered magnet;
②Adding Sn can improve βHcJ: NdFeB magnets of sintered magnets or NdFeB magnets containing Al and Dy. Adding Sn can reduce the local effective demagnetization factor Neff, thereby reducing the temperature coefficient of coercive force βHcJ. But the reduction effect of βHcJ value is limited. Therefore, in practical applications, βHcb is mainly increased by increasing HcJ and the irreversible loss of magnetic flux is reduced. Experience shows that when the operating point Pc=2 and HcJ≥17kOe, βHcb can be reduced from -0.6%/℃ to -0.2%/℃.
③About the irreversible loss of magnetic flux hirr: Using the knowledge of the phenomenological theory of magnetism, the calculation formula for the irreversible loss of magnetic flux can be derived:
hirr=(where Hd(T) is the demagnetizing field)
If it is assumed that αBr and βHcJ vary linearly with temperature, there are further:
Irreversible loss of magnetic flux hirr=(CGS)
Path to reduce magnetic flux
According to the above formula, there are several ways to reduce the irreversible loss of magnetic flux:
Add Dy, Nb, V, Ga and other trace elements to reduce βHcJ, thereby reducing the irreversible loss of magnetic flux.
Adding trace elements to reduce Neff: both reduce the value of D and βHcJ, thereby ultimately reducing the irreversible loss of magnetic flux: studies have shown that adding a trace of Sn to a neodymium iron boron magnet can reduce the local effective demagnetization field inside the alloy, and it can also reduce The temperature coefficient of coercive force βHcJ, so that the irreversible loss of magnetic flux of the magnet can be reduced.
By improving the magnet size distribution and crystal grain uniformity, the difference of Br-Mk can be reduced, thereby reducing the irreversible loss of magnetic flux.
Choose the appropriate aspect ratio to get the appropriate D value.
Choose an appropriate temperature to control the irreversible loss of magnetic flux within the required range.
Long-term stability at room temperature
A study published by Finnish scholars in 2013 showed that sintered NdFeB magnets (HcJ=15.6kOe) placed at room temperature for 1 year (10000h), samples with different Pc values (Pc=-0.33,-1.1, -3.3) There is no noticeable loss of magnetization. The Sanhuan Research Institute also conducted a similar measurement and research, which lasted more than 12 years (4441 days). The experimental sintered NdFeB magnet has an intrinsic coercivity HcJ=18kOe, and the sample is an uncoated sample with a side length of 10.2mm. Cube, permeability coefficient Pc=-2 (magnetic moment, magnetic flux and remanence to understand what Pc value), the number of samples is 8 pieces, directly exposed to the atmospheric environment where the laboratory is located, the temperature is between 22°C and 28°C, and observation and measurement shall be carried out once a year within 12 years.
The relative magnetic flux loss measured in the previous 6 years was basically not large, and an inflection point appeared near 2208 days (about 6 years). From the outside, rust spots can be seen on the surface of the black magnet after 6 years of storage, which means that the surface and the inside of the magnet have begun to oxidize and corrode. As time goes by, the range of oxidation or corrosion will continue to expand, and the rate of performance degradation is also obvious. accelerate. In addition, the experiment also extrapolated the magnetic flux loss from the currently measured 4441 days (12 years and 2 months) to 30-50 years. The estimated magnetic flux loss in 30 years is less than 1%, and the magnetic flux loss in 50 years is about 1.3 %, 2% corresponds to about 150 years.
This result shows that if the service life of the magnet is defined as the time corresponding to the flux loss rate equal to 5%, even if the magnet is on the surface without corrosion-resistant coating, the currently measured sintered NdFeB magnet still has a very long Life span is conservatively estimated to be 30-50 years.
Generally, the larger magnetic flux loss comes from the oxidation or corrosion of the magnet surface, which is an irreversible loss. Among all kinds of rare earth permanent magnet materials, the loss of sintered NdFeB is the most serious, but after composition optimization and Surface protection treatment, oxidation resistance and corrosion resistance of sintered NdFeB magnets have been greatly improved. Therefore, under the condition that the surface of the magnet is well protected, for a sintered NdFeB with sufficiently high HcJ, the service life can completely exceed 30-50 years. (This is when the operating temperature is not exceeded)
Long-term stability at high temperatures
With the same Pc value, the higher the magnet storage temperature, the faster the relative magnetic flux loss will decrease. The initial magnetization loss and long-term magnetization loss of magnets with lower absolute value of Pc are significantly greater than those of magnets with higher Pc, and the two types of losses increase greatly due to temperature rise. In the case that the HcJ cannot be further improved due to technical and cost reasons, the The increase in the absolute value of Pc can effectively suppress the magnetization loss.
HcJ has an important influence on the high-temperature magnetization loss. The higher the HcJ, the lower the magnetization loss. The high-temperature stability requires the magnet to have a higher HcJ. At the same time, the permeability coefficient Pc can also determine the high temperature and long-term magnetization loss of the magnet.
External magnetic field stability
The length and volume of the air gap change during the operation of the magnet for the motor, which is a dynamic magnetic circuit. The magnet is not only affected by temperature changes, but also by the reverse demagnetization of the armature magnetomotive force. Since the operating point changes back and forth on the return line, the magnet is in a cyclic demagnetization state. This requires us to consider not only the effects of temperature changes when designing the motor magnetic circuit, but also the additional effects of dynamic demagnetization. For general-power DC motors, due to the armature effect, their lowest operating point is about -0.6. At this time, the BH curve of the magnet is required to be straight under the continuous operating temperature of the motor. If the BH curve is bent, the magnet will be Permanent demagnetization is caused by the armature effect; in addition, there is an irreversible loss of magnetic flux hrr≤5% for the magnet at the limit temperature of the motor.
Chemical stability
The so-called chemical stability refers to the oxidation and corrosion resistance of permanent magnet materials. Compared with traditional ferrite or Sm-based permanent magnets, the chemical stability of sintered NdFeB is the worst. If directly exposed to the atmosphere, it will continue to oxidize and rust. Since the sintered NdFeB magnet is a composite structure made of powder metallurgy technology and composed of three phases, there is a deterioration layer produced by grinding and some pores and oxidation of the material itself. The moisture in the air corrodes from the B-rich phase and pores on or near the surface of the magnet.
The current solution to the chemical stability of neodymium iron boron permanent magnets is mainly to add certain alloying elements such as Co, Ni, Al and Cr, and at the same time increase the density and reduce the pores as much as possible in the sintering process. For example, through the composite addition of Co+Ni+Al, the alloy magnet is tested for 48 hours in an environment of 70°C and 95% relative humidity, and the surface still has a metallic luster without corrosion. Another way to solve the chemical stability is to perform surface treatment on the magnet, such as electroplating, electroless plating and other surface treatments, so that the magnet can obtain practical corrosion resistance.
Source: China Permanent Magnet Manufacturer – www.rizinia.com
