The temperature stability of permanent magnets: remanence temperature coefficient, coercivity temperature coefficient, reversible temperature coefficient
Permanent magnets are generally used as a magnetic field source to provide a constant magnetic field in a certain space. For some precision instruments and magnetic devices, the stability of the magnetic field of the magnet is very important, and it will directly affect the accuracy and reliability of the instrument.
Stability of permanent magnet
The external conditions that cause changes in magnetic properties include temperature, time, electromagnetic field, mechanical vibration or shock, radiation, chemical action, etc. Correspondingly, the stability of magnets includes temperature stability, time stability, vibration and shock stability, and electromagnetic field Performance requirements such as stability and chemical stability are generally described by the change in the performance parameters of the magnet (such as how many percent of the remanence changes for every 1°C increase in temperature, and how much the remanence of the magnet decays per year at room temperature).
Changes in magnetic properties caused by changes in environmental conditions are mainly in two aspects:
The change in magnetic properties caused by the change in the magnetic domain structure (also called magnetic aging) is reversible. When the magnet is magnetized or magnetized again, most of the performance of the magnet can be restored.
The change in magnetic properties caused by the change of the microstructure of the magnet (also called tissue aging) is irreversible. When the magnet is magnetized or magnetized again, the performance of the magnet cannot be restored.
Any changes in the magnetic properties of magnets caused by changes in environmental conditions include magnetic aging and tissue aging.
The environmental conditions of the use of magnets are different, and the required performance stability is also different. For example, the magnets used on spacecraft generally pay attention to the performance stability under vibration and shock conditions, and also require stability under radiation, temperature and time conditions; Magnets working in acid and alkali environments generally require chemical stability; and where the temperature of the working environment changes, attention is paid to whether the magnet has temperature stability.
Temperature stability of permanent magnet
Instruments and equipment made of permanent magnet materials generally cannot 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 equipment work normally when the temperature changes, it is necessary to design the magnetic circuit Know the change in magnetic properties of magnets with temperature.
In order to quantitatively reflect the influence of temperature on the performance of magnets, some temperature stability parameters related to ambient temperature are defined, such as the temperature coefficient of remanence αBr, the temperature coefficient of intrinsic coercivity αHcJ, and the reversible loss of open-circuit magnetic flux density Lrev And irreversible loss Lirr, reversible temperature coefficient of open circuit magnetic flux density, heat-resistant temperature or maximum continuous working temperature Tm, etc. Among them, the temperature coefficient of remanence αBr and the temperature coefficient of intrinsic coercivity αHcJ are among the performance indicators that commercial permanent magnets must provide One.
Temperature coefficient of remanence and temperature coefficient of intrinsic coercivity
The temperature coefficient, as the name implies, is the relative change rate of a physical quantity with temperature. From the reference temperature T0 to a certain high temperature T, the temperature coefficient of remanence and the temperature coefficient of intrinsic coercivity are defined as follows, and the unit is %/℃.
Among them, Br(T) and Br(T0) are the remanence at temperature T and reference temperature T0 respectively (HcJ is the same), usually room temperature or 20℃ is selected as T0, and the value of high temperature T needs to be supplied and demanded according to the use environment Both parties determine. If αBr is positive, it means that the remanence increases with increasing temperature; if it is negative, it means that the remanence decreases with increasing temperature.
During the whole 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 magnetism pass loss hrev=(B(T1)-B`(T0))/B`(T0)×100% and irreversible magnetic flux loss hirr=(B`(T0)- B(T0))/B(T0)×100%.
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 magnetic flux is represented by the reversible temperature coefficient α.
In the big concept of temperature coefficient, special attention should be paid to distinguish the differences between the small concepts of temperature coefficient, reversible temperature coefficient and irreversible temperature coefficient.
Aging treatment can significantly reduce hT, hrr, hrev and α. The aging treatment (heating at a certain temperature for a period of time) of the permanent magnet before use or testing can eliminate the unstable structure of the magnet. The temperature and time of the aging treatment should 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 magnetic properties of the material. By adding some elements, the relationship between the saturation magnetization of the magnetic main phase and the temperature can be changed, that is, the temperature coefficient of the magnet can be changed. For example, substituting part of Co for Fe in neodymium iron boron magnets can significantly increase the Curie temperature of the main phase; substituting Dy for part of Nd, αB (T) will also be improved.
Source: China Permanent Magnet Manufacturer – www.rizinia.com