Isotropy and Anisotropy of Magnets
Magnetic materials are divided into two categories: isotropic magnets and anisotropic magnets:
🔸Isotropic magnets have the same magnetic properties in any direction and can be magnetized arbitrarily;
🔸The magnetic properties of anisotropic magnets will be different in different directions. The direction in which it can obtain the best magnetic properties is called the orientation direction of the magnet.
Common anisotropic magnets are mainly hard magnetic materials such as sintered neodymium iron boron and sintered samarium cobalt.
Orientation is an important process link in the production of sintered NdFeB magnets
The magnetism of magnets originates from magnetic ordering (the orderly arrangement of magnetic domains in one direction). Sintered NdFeB is formed by pressing magnetic powder in a mold. Put the magnetic powder into a mold to give a given shape, apply a strong magnetic field through an electromagnet, and at the same time, the press gives a certain pressure to the magnetic powder so that the easy magnetization axis of the magnetic powder is oriented in the same direction. After the pressing is completed, the blank is demagnetized and then demoulded to obtain a blank with a good orientation in the easy magnetization direction, which can then be cut into finished magnetic steel products of specified sizes according to the user's needs.
Powder orientation is a key process for preparing high-performance NdFeB permanent magnets. Whether the magnet orientation is good during the blank production stage is affected by many factors, including: orientation magnetic field strength, powder particle shape and size, molding method, and the relationship between orientation field and molding pressure. direction, bulk density of oriented powder, etc.
The magnetic declination angle produced by the post-processing process has a certain impact on the magnetic field distribution of the magnet steel.
Magnetization is the last step in obtaining magnetism from sintered NdFeB
The magnet blank is cut to the size required by the user, and then undergoes anti-corrosion treatment such as electroplating to become a finished magnet. However, at this time, the magnet itself does not show magnetism to the outside world, and the magnet needs to be "magnetized" through the process of magnetization.
The equipment we use to magnetize magnetic steel is a magnetizer, also called a magnetizing machine. The magnetizer first charges the capacitor with DC high-voltage voltage (ie, energy storage), and then discharges it through a coil (magnetization fixture) with very small resistance. The peak value of the discharge pulse current is very high, reaching tens of thousands of amperes. This current pulse creates a strong magnetic field within the magnetizing fixture, which permanently magnetizes the magnet placed in the magnetizing fixture.
Unexpected situations also occur during the magnetization process, such as unsaturated magnetization, explosion of the pole head of the magnetizer, and broken magnets.
🔸 The main reason for non-saturated magnetization is that the magnetizing voltage is not enough, and the magnetic field generated by the coil cannot reach 1.5~2 times the saturation magnetization intensity of the magnet.
🔸 If it is multi-pole magnetization, it is difficult to magnetize the magnet with a thicker orientation to saturation, because the distance between the upper and lower pole heads of the magnetizer is too large, and the magnetic field intensity generated by the pole heads is not enough to form a normal magnet. Because of the closed magnetic circuit, the magnetic field passing through the magnet cannot penetrate the magnet, so it will cause confusion of the magnetic poles and insufficient magnetic field strength.
🔸 The main reason why the magnetizing pole head bursts is because the set voltage is too high and exceeds the safe voltage of the magnetizing machine.
Unsaturated magnets or magnets that have been demagnetized will be more difficult to saturate, because the magnetic domains in the original state are chaotic and do not show magnetism to the outside world. To be saturated, you only need to overcome the resistance of the displacement and rotation of the own magnetic domains. . However, when the magnet is not saturated, or has been demagnetized but not completely demagnetized, there will be a reverse magnetic field area inside it. Whether it is forward magnetization or reverse magnetization, there will be some magnetized areas that need to be reversely magnetized, which requires additional To overcome the intrinsic coercive force in the reverse magnetic field region, a magnetic field stronger than the theoretical magnetizing magnetic field is required.
