Manufacturing technology of high-performance magnets

Development of high-performance magnets

The first rare earth magnetic material that has been widely used is Cobalt (S-Co). The use of alloy technology, including the addition of praseodymium (Pr) to the alloy, can further increase the magnetic energy of rare earth magnets, but the price is still relatively high because rare earth materials are rare materials.

In order to solve the price problem, research work has been developed in two directions to improve the performance of the cobalt magnets, which actually reduces the magnetic materials used. In addition, the composition of the alloy is adjusted to reduce the consumption of cobalt. Research on rare earth magnetic materials that do not use expensive cobalt. This is neodymium iron boron (NdFeB) that was born in the early 1980s. In this material, iron accounts for about two-thirds of the composition, and boron accounts for only a small proportion. In this way, the price of neodymium iron boron alloy was lower than that of cobalt at the beginning, and as the price of neodymium fell, the price of neodymium iron boron magnets also fell. In addition, NdFeB has higher magnetic energy than Shan cobalt and consumes less material.

Demand for high-performance magnets

High-performance magnets are currently used in many occasions, such as: increasing the clamping force, improving the sealing performance of the gasket, and the magnetic separation of materials. With the increase in the output of battery-powered electric tools, materials with small volumes and high magnetic energy have become more important. Many types of motors have or will benefit from high-performance permanent magnet technology. DC motors can adopt the form of winding excitation or permanent magnet. At present, among general industrial DC motors, permanent magnet motors are no longer limited to the fractional horsepower range, but have been expanded to 6 horsepower, with various speeds and protection forms.

By the mid-1970s, high-precision positioning drives using DC servo motors replaced hydraulic equipment on many occasions. Hydraulic drive equipment requires a very high maximum torque. These motors must be made into a hydromagnetic method. Because the motor requires a large maximum torque, a high-energy magnetic field is required. If the winding excitation method is used, it will cause overheating problems. In addition, the use of permanent magnets can save space and increase the diameter of the electric drive.

With the development of new types of magnets, the high coercivity of rare earth materials can make DC servo motors produce a larger maximum torque, without worrying about the demagnetization effect produced by the electric drive magnetic field. After adopting a new type of magnet, it is possible to arrange the magnets radially, so that the magnetic circuit is very compact, the magnetic leakage is very low, and only a few magnetic lines of force escape.

Due to the almost linear demagnetization curve of the rare earth magnet, it can be magnetized before assembling, and there is no need to use a soft ferromagnetic retainer to prevent magnet demagnetization when disassembling the motor.

Rare earth magnets are most advantageous for brushless DC motors. After using it, the volume of the magnet decreases, and the specific gravity decreases, so the moment of inertia also decreases. The torque inertia ratio of brushless motors using rare earth magnets and conventional DC motors is 2:1.

The rotating magnet in the permanent magnet motor has another function. The magnetic flux generated by the magnet can generate a position feedback signal together with the Hall effect element to control the brushless DC servo motor.

Because the magnet of the brushless DC servo motor works at low temperature, the temperature limit value of the new high-performance magnet such as neodymium iron boron will not affect its use. NdFeB magnets can work at 120°C. This temperature index is ideal for DC brushless motors. If a stepping motor is made of rare earth magnets, the stepping motor can reach 200 steps/revolution (that is, 1.8 degrees/revolution). Combined with microprocessor control technology, the motor can reach 2500 steps per revolution. If the magnetic flux leakage is reduced to a minimum, the high-speed loss that used to limit the step rate of the motor will be further reduced.

Another potential market for permanent magnet motors is automotive motors. After using rare earth magnets, the volume and weight of the starter will be reduced. In addition, rare earth motors can be used for window openers, rain cover motors, ventilators and fans.

With the development of material manufacturing technology, rare earth high-performance magnet technology is gradually being improved. The use temperature is also increasing.

Research on high-performance thermal deformation ndfeb magnet

In the field of rare earth permanent magnetic materials, it has been relatively mature to use the coupling mechanism of the magnetic phase in the nano or sub-micron micro-scale to research and develop the macro-magnetic uniform magnetic material technology. However, the research on the magnetic coupling phenomenon on a larger scale, especially Using this long-range coupling mechanism, there are few reports on the design and development of new high-performance permanent magnet materials.

The Permanent Magnet Research Group of the Rare Earth Magnetic Functional Materials Laboratory, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, through the structural design to control the long-range magnetic coupling between the magnetic phases, so as to achieve the micro-to-macro-scale "soft" and "hard" phase recombination. The composite structure of the new high-performance permanent magnetic material, and a good interpretation of many magnetic problems that are difficult to explain by short-range exchange coupling in the rare earth permanent magnetic material system.

Aiming at the characteristics of large and small grain size of the original powder particles of thermally deformed Nd-Fe-B magnets, the research team first used permanent magnet powders rich in high-abundance rare earth such as La and Ce to achieve the difference between a few microns and tens of microns. The effective coupling of Fe-B powder has successfully prepared high La and Ce thermally deformable magnets with excellent macroscopic magnetic properties. When 30wt.% mixed rare earth is substituted, the maximum magnetic energy product of the magnet is 43.5MGOe and the coercivity is 1.07T; when 20wt.% Ce is substituted, the maximum magnetic energy product is 39.1MGOe and the coercivity is 1.20T.

Following this work, the researchers used (NdPr)-Cu and Dy-Cu eutectic alloy diffusion technology to prepare a macroscopic "core-shell" structure of non-heavy rare earth high coercivity thermally deformed Nd-Fe-B magnets and High magnetic energy product thermally deformed Nd-Fe-B magnet. The structure shows a unique gradient structure in terms of element distribution and grain size, and the gradient range is 2-6mm. However, the overall magnetic performance of the magnet does not show obvious decoupling due to the macroscopic "core-shell" structure. On the contrary, the magnetic behavior shows good consistency, which proves the strong long-range magnetic coupling of the magnet from the millimeter scale.

In order to further verify and utilize this long-range coupling effect, the researchers selected two magnetic phases with significant differences in intrinsic magnetic properties, analyzed and realized the multi-scale coupling between the magnetic phases with the help of the macro-layered structure design, and found sub-millimeters on the basis of experiments. The best coupling distance of the two phases on the order of magnitude, the preparation of excellent thermal deformation Nd-Fe-B magnets.

The magnetic properties of the macroscopic "core-shell" thermally deformed Nd-Fe-B magnet prepared by Dy-Cu diffusion (A) and its near-surface and central area microstructure (B); the macroscopic layered structure is designed as a composite multilayer of thermally deformed magnets Schematic diagram and demagnetization curve (C); and low coercivity layer (a, b, c, d), low coercivity magnet (e, f, g, h) and high coercivity in a composite structure with coupling effect The domain structure evolution (D) of magnets (i, j, k, l) in the process of magnetization reversal shows that the magnetic coupling of the high coercivity layer in the composite multilayer structure makes the low coercive force layer produce stronger "nails" The "stripping effect" has gained strong demagnetization resistance.

The development of high-performance magnets in the future

With people’s reliance on high-tech products, just as the reason why today’s earphones can go from the moving coil type to the electrostatic type in 1937 to the current wired and TWS, it relies on the fixed magnetic field formed by the magnet and the audio current. The interaction between changes. In the future, TWS headsets are likely to be not only mobile phone accessories but also smart terminals and even the gateway to the Internet of Everything. This also means that there will be more sensors inside, which means that the requirements for magnets will be higher and higher: the size should be smaller, the magnetic field strength and the anti-interference ability should be stronger. The basic materials of high-performance magnets need high-performance, small-sized neodymium iron boron, which can be better, smaller, and more refined in the future.