Author:Site Editor Publish Time: 2020-09-16 Origin:Site
One of the important parameters of permanent magnet materials is the stability of magnetism, which mainly refers to the influence of internal and external factors on the magnetic properties of permanent magnet materials after magnetization: temperature, time, chemical corrosion, mechanical vibration and shock, radiation, etc. The most significant impact on the stability of neodymium permanent magnet materials. The three parameters to measure magnetic performance are: remanence, coercivity, and maximum magnetic product energy. First, let's understand the concept of related parameters.
Remanence: The symbol is Br. The neodymium permanent magnet is magnetized to technical saturation, and the surface field Br retained after the external magnetic field is removed is called residual magnetic induction.
Coercive force: The symbol is HC, which means that after the magnetic material is saturated and magnetized, when the external magnetic field returns to zero, the magnetic induction intensity B does not return to zero. Only by adding a certain magnitude of magnetic field to the opposite direction of the original magnetization field can the magnetic induction intensity Back to zero, this magnetic field is called coercive magnetic field, also called coercive force.
Maximum magnetic energy product: The symbol is (BH)max. The product of B and H at any point on the demagnetization curve, namely Bm, Hm, and (BH), represents the magnetic energy density established by the magnet in the air gap space, that is, the air gap unit volume Magnetostatic energy, because this energy is equal to the product of the magnet Bm and Hm, it is called the magnetic energy product. The relationship curve of the magnetic energy product changing with B is called the magnetic energy curve, and the product of Bd and Hd corresponding to one point has the maximum value. Is the maximum magnetic energy product.
One of many magnetic parameters, its direct industrial significance is that the larger the magnetic energy product, the less magnetic material is needed to produce the same effect. The visual representation on the hysteresis loop is: the line between the intersection of the perpendicular line of Hc and Br and O. The product of Br and Hc corresponding to the intersection on the demagnetization line is the largest, which is called the maximum energy product.
Curie temperature: Curie temperature or magnetic transition point refers to the temperature at which a material can change between ferromagnetic and paramagnetic, that is, the phase transition temperature at which ferroelectrics transform from ferroelectric phase to paraelectric phase. The temperature corresponding to the disappearance of ferromagnetism is the Curie point temperature. If the magnet is heated to the Curie temperature, the dipoles become disordered and they cannot return to their original state.
Hard magnetization: Usually Hc>1000A/m is difficult to magnetize, and after removing the external field, it can still retain the material with high residual magnetization.
Paramagnetic material: There are unpaired electrons outside the atomic nucleus, which causes the magnetic effect generated by the electron spin to not be offset. That is, under the action of a magnetic field, the thermal disorder magnetic moment of adjacent atoms or ions in the substance is to a certain extent with the magnetic field strength The phenomenon of oriented arrangement in the same direction.
Since neodymium iron boron magnets are sensitive to working temperature, both the instantaneous maximum temperature and the continuous maximum temperature of the environment may demagnetize the magnet to different degrees, including reversible and irreversible, recoverable and irreversible.
The temperature coefficient of remanence of NdFeB material is -0.01%/degree, and the temperature coefficient of intrinsic coercivity is -0.45~-0.6%/degree.
An important part of the relationship between magnet and temperature is heating the magnet to make its molecules more disordered.
Magnetic dipoles, which means they have opposite charges, or the direction of the magnetic field, at each end. This is due to most of the magnetic molecules facing the same direction. When we heat our magnet, these polar molecules start to move around. Because these magnetic molecules are no longer the polarities of the magnets facing the same direction, the average direction becomes a bit messy.
If magnets are heated to the Curie temperature, they lose their ability to be magnetized.
The Curie temperature of iron is 1417°F. The Curie temperature of NdFeB is 320°C-460°C.
When you cook the magnetic cooling from 100°C back to the boiling point of the room temperature, it will return to its normal magnetic field strength. Cooling the magnet even 0°C ice water or 78°C dry ice will cause the magnet to become stronger. Cooling causes less kinetic energy in the molecular magnet. This means that the molecules in the magnet vibrate less, allowing them to create a magnetic field that is more uniformly concentrated in a given direction. When the temperature rises to the Curie temperature of the material, it changes from hard magnetic to paramagnetic material.
The absolute value of the average temperature coefficient of remanence α (Br) increases with the increase in temperature, and the rate of decrease of remanence is also faster. When it exceeds 100°C, the decrease of remanence increases sharply.
The absolute value of the average temperature coefficient of coercivity β (Hcj) gradually decreases with the increase of temperature, and the rate of decrease of coercive force changes from fast to slow.
With the increase of temperature, the open circuit magnetic induction of permanent magnets gradually loses, and the irreversible loss gradually increases. When the experiment is below 50℃, the baking time and length have almost no effect on the irreversible loss of neodymium iron boron magnets. When the experiment temperature is 80℃ and 100℃, the 24h irreversible loss increases to 1%, and when the temperature reaches 100℃, the 24h irreversible loss Up to 35%. When the temperature reaches 200℃, the irreversible loss in 24h reaches 44%.
When the experimental temperature reaches 40℃, the reduction rate of remanence and (BH)max is 3.7%, the (BH)max decreases 6.0% when the temperature reaches 55℃, and the (BH)max decreases 14.9% when the temperature reaches 100. When the temperature rises to 200, the (BH)max is reduced by about 49.4%.
So, you should know that NdFeB magnets are better stored at low temperatures!
The temperature limitation of NdFeB magnets has led to the development of a series of grades of magnets to meet different operating temperature requirements. Please refer to our performance catalog to compare the operating temperature ranges of various grades of magnets. It is necessary to confirm the maximum operating temperature before selecting a neodymium permanent magnet.