The stability of permanent N52 Neodymium magnet material
The stability of permanent N52 Neodymium magnet material properties is an important indicator of permanent magnet materials. Stability mainly refers to the process of changing the magnetic properties of a magnet under the influence of internal and external factors after magnetization. It is commonly expressed by the rate of change of performance indicators.
Common causes of changes in magnetic properties include temperature, time, electromagnetic field, radiation, mechanical vibration and impact, etc.
In the previous issue, Magnetics Emperor has introduced to you the temperature stability of permanent magnet materials (remanent magnetization temperature coefficient, coercive force temperature coefficient) , reversible temperature coefficient, click for details), in this issue we will learn about the time stability of permanent magnet materials.
If the magnet works or is left for a long time, the surrounding environment (such as temperature, humidity, corrosive liquids, etc.) may cause the physical and chemical properties of the magnet to change.
After the permanent magnet is magnetized, most areas are magnetized to a specific direction, but there are still some small magnetic domains whose magnetization directions are chaotic (called reverse magnetization nuclei).
Under the action of various environmental factors, the original reverse magnetization nuclei As it grows, new countermagnetized nuclei will be produced, which causes the magnetic properties of the permanent magnet to decay N52 Neodymium magnet.
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 coercive force, coercive force or maximum magnetic energy product, and even leads to complete failure of the magnet.
This loss of magnetic performance is irrecoverable, and even if the magnet is re-magnetized, it cannot return to the level before it was left for a long time.
In recent years, with the wide application of NdFeB permanent magnet materials in fields requiring long service life such as aerospace N52 Neodymium magnet, electric vehicles, and high-power wind power generation, application designers are increasingly concerned about the time stability of NdFeB permanent magnets. Come pay more and more attention to it.
Long term stability at room temperature
A research result released by Finnish scholars in 2013 showed that samples of sintered NdFeB magnets (HcJ=15.6kOe) placed at room temperature for 1 year (10000h) with different Pc values (Pc=-0.33,-1.1, -3.3) without any detectable loss of magnetization N52 Neodymium magnet.
The Tri-Huan Research Institute also conducted a similar measurement study, which lasted for more than 12 years (4441 days). The sintered NdFeB magnet used in the experiment had an intrinsic coercive force HcJ=18kOe, and the sample was an uncoated one with a side length of 10.2mm. Cube, magnetic permeability coefficient Pc=-2 (click on magnetic moment, magnetic flux and residual magnetism to learn what the Pc value is), the number of samples is 8, directly exposed to the atmospheric environment of the laboratory, the temperature is 22℃~28 ℃, observations and measurements are made once a year for 12 years N52 Neodymium magnet.
From the above data, it can be found that the relative magnetic flux loss measured in the first 6 years is basically not large, and an inflection point appears near 2208 days (about 6 years). From the outside, rust spots can be seen on the surface of the black magnet after being placed for 6 years, which means that the surface and interior of the magnet have begun to oxidize and corrode N52 Neodymium magnet.
As time goes by, the scope of oxidation or corrosion will continue to expand, and the rate of performance degradation will also be 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. (Hollow dots in the picture above) N52 Neodymium magnet.
This result shows that if the service life of the magnet is defined as the time corresponding to the magnetic flux loss rate equal to 5%, even if the magnet is on the surface without corrosion-resistant coating, the currently measured sintered NdFeB magnets still have a very long life. The lifespan is conservatively estimated to be 30-50 years N52 Neodymium magnet.
Usually, large magnetic flux loss comes from oxidation or corrosion of the magnet surface, which is an irrecoverable loss. Among various types of rare earth permanent magnet materials, this loss of sintered NdFeB is the most serious. However, after composition optimization and Surface protection treatment, oxidation resistance and corrosion resistance of sintered NdFeB magnets have been greatly improved. Therefore, if the magnet surface is well protected, the service life of sintered NdFeB with sufficiently high HcJ can exceed 30 to 50 years. (This is under the condition that the operating temperature is not exceeded~)
Estabilidad a largo plazo a altas temperaturas
The figure below shows the relative flux loss as a function of time at 80°C, 120°C and 150°C for magnets with different Pc values and HcJ=20.1 kOe.
It is not difficult to find from the above figure that under the same Pc value, the higher the magnet storage temperature, the faster the relative flux loss decreases. The initial magnetization loss and long-term magnetization loss of a magnet with a lower absolute value of Pc are significantly greater than that of a magnet with a higher Pc, and both losses increase significantly as the temperature increases N52 Neodymium magnet.
If HcJ cannot be further improved due to technical and cost reasons, the Increasing the absolute value of Pc can effectively suppress magnetization loss.
It can be seen from the time relationship of the relative magnetization loss of different HcJ and different Pc magnets at different temperatures that HcJ has an important impact on high-temperature magnetization loss. The higher the HcJ, the lower the magnetization loss. High-temperature stability requires that the magnet must have a higher HcJ. At the same time, the magnetic permeability coefficient Pc can also determine the magnetization loss of the N52 Neodymium magnet high temperatures and for a long time.
Stability of permanent magnets
External conditions that cause changes in magnetic properties include temperature, time, electromagnetic fields, mechanical vibration or impact, radiation, chemical effects, etc. Correspondingly, the stability of magnets includes temperature stability, time stability, vibration and impact stability, and electromagnetic fields. Performance requirements in terms of stability and chemical stability are generally described by the change in the magnet’s performance parameters (such as the percentage change of the remanent magnet for every 1°C increase in temperature, and the annual attenuation of the magnet’s remanent magnet at room temperature) N52 Neodymium magnet.
Changes in magnetic properties caused by changes in environmental conditions are mainly in two aspects:
Changes in magnetic properties caused by changes in magnetic domain structure (also known as magnetic aging). This change is reversible. When the magnet is magnetized or magnetized again, most of the magnet’s performance can be restored.
Changes in magnetic properties caused by changes in the microstructure of the magnet (also called tissue aging) are irreversible. When the magnet is magnetized or magnetized again, the magnet performance cannot be restored N52 Neodymium magnet.
Any change in the magnetic properties of a magnet caused by changes in environmental conditions includes two types: magnetic aging and tissue aging.
Magnets are used in different environmental conditions and require different performance stability. For example, magnets used on spacecraft generally focus on performance stability under vibration and impact conditions. They also require stability under radiation, temperature and time conditions; they must be stable under conditions of radiation, temperature and time. Magnets working in acidic and alkali environments are generally required to have chemical stability; in places where the working environment temperature changes, attention is paid to whether the magnet has temperature stability N52 Neodymium magnet.
Temperature stability of permanent magnets
It is generally impossible for instruments and equipment made of permanent magnet materials to work at a constant temperature, and changes in ambient temperature have a direct impact on the magnetic properties of the magnet. In order to make the instrument and equipment work normally when the temperature changes, when designing the magnetic circuit, it is necessary to Know how the magnetic properties of a magnet change with temperature.
In order to quantitatively reflect the influence of temperature on magnet performance, some temperature stability parameters related to ambient temperature have been defined, such as the remanence temperature coefficient αBr, the intrinsic coercive force temperature coefficient αHcJ, and the reversible loss of open-circuit flux density Lrev and irreversible loss Lirr, reversible temperature coefficient of open-circuit flux density, heat-resistant temperature or maximum continuous operating temperature Tm, etc. Among them, the residual magnetization temperature coefficient αBr and the intrinsic coercive force temperature coefficient αHcJ are among the performance indicators that commercial permanent magnets must provide. one.
Residual magnetization temperature coefficient and intrinsic coercive force temperature coefficient N52 Neodymium magnet.
As the name suggests, the temperature coefficient is the relative rate of change of physical quantities with temperature, in the temperature range from the reference temperature T0 to a certain high temperature T. The temperature coefficient of the remanent magnetism and the temperature coefficient of the intrinsic coercive force are defined as follows, in units of %/°C N52 Neodymium magnet.
Among them, Br(T) and Br(T0) are the residual magnetism at the temperature T and the reference temperature point T0 respectively (the same is true for HcJ). Room temperature or 20°C is usually selected as T0, while the value of high-temperature T needs to be determined by supply and demand according to the use environment. Both parties confirmed. If αBr is positive, it means that the remanence increases as the temperature increases; if it is negative, it means that the remanence decreases as the temperature increases.
Magnets usually work in an open-circuit state with an air gap, and the characteristics of open-circuit residual magnetism (or open-circuit flux) changing with temperature are of more practical significance. When the ambient temperature rises from room temperature T0 to a given temperature T1, the open-circuit magnetic flux drops from B(T0) to B(T1). If the temperature returns to room temperature T0, the open-circuit magnetic flux will generally return to a value higher than B(T0). T0) lower B`(T0) value, as shown below. Experiments have shown that when the temperature changes repeatedly between T0 and T1, and ΔT is not very large, the change in B is linearly reversible.
In the big concept of temperature coefficient, special attention should be paid to distinguishing the differences between the small concepts of temperature coefficient, reversible temperature coefficient and irreversible temperature coefficient N52 Neodymium magnet.
Aging treatment can significantly reduce hT, hirr, hrev and α. Aging treatment (heating at a certain temperature for a period of time) of permanent magnets before use or testing can eliminate the unstable structure of the N52 Neodymium magnet. The temperature and time of the aging treatment need to be determined according to the type and purpose of the magnet.
The reversible temperature coefficient αB (T) or the remanence temperature coefficient αBr (T) depends on the intrinsic magnetism of the material. By adding some elements, the relationship between the saturation magnetization intensity of the main magnetic phase and temperature can be changed, which can change the temperature coefficient of the magnet. For example, replacing part of Fe with Co in N52 Neodymium magnet can significantly increase the Curie temperature of the main phase; replacing part of Nd with Dy, αB (T) will also be improved.
During the entire heating process, the total magnetic flux loss from room temperature to high temperature is hT=(B(T1)-B(T0))/B(T0)×100%, which can be decomposed into two parts: reversible magnetic flux Flux loss hrev=(B(T1)-B`(T0))/B`(T0)×100% and irreversible flux loss hirr=(B`(T0)-B(T0))/B(T0)× 100% N52 Neodymium magnet.
It can be seen from the PB`(T0) line that when the temperature changes within the range of T0~T1, the change of B is linear. The average reversible loss of open-circuit flux is expressed by the reversible temperature coefficient α.
