Physical properties of sintered neodymium iron boron

Sintered neodymium iron boron permanent magnets, as core functional components, are widely used in instruments and equipment such as motors, electroacoustics, magnets, and sensors. During the service process, magnets will be subjected to environmental factors such as mechanical forces, cold and hot changes, and alternating electromagnetic fields. If environmental failure occurs, it will seriously affect the functionality of the equipment and cause huge losses. Therefore, in addition to magnetic performance indicators, we also need to pay attention to the mechanical, thermal, and electrical properties of magnets, which will help us better design and use magnetic steel, and is of great significance for improving its stability and reliability in service.

Mechanical

The mechanical performance indicators of magnetic steel include hardness, compressive strength, bending strength, tensile strength, impact toughness, Young's modulus, etc. Neodymium iron boron is a typical brittle material. Magnetic steel has high hardness and compressive strength, but poor bending strength, tensile strength, and impact toughness. This causes the magnetic steel to easily drop corners or even crack during processing, magnetization, and assembly. Magnetic steel usually needs to be fixed in components and equipment using slots or adhesive, while also providing shock absorption and cushioning protection.

The fracture surface of sintered neodymium iron boron is a typical intergranular fracture, and its mechanical properties are mainly determined by its complex multiphase structure, as well as related to formula composition, process parameters, and structural defects (pores, large grains, dislocations, etc.). Generally speaking, the lower the total amount of rare earths, the worse the mechanical properties of the material. By appropriately adding low melting point metals such as Cu and Ga, improving the grain boundary phase distribution can enhance the toughness of magnetic steel. Adding high melting point metals such as Zr, Nb, Ti can form precipitates at grain boundaries, refine grains, and suppress crack extension, which helps improve strength and toughness; However, excessive addition of high melting point metals can cause excessive hardness of the magnetic material, seriously affecting processing efficiency.

In the actual production process, it is difficult to balance the magnetic and mechanical properties of magnetic materials, and due to cost and performance requirements, it is often necessary to sacrifice their ease of processing and assembly.

Thermal Properties

The main thermal performance indicators of neodymium iron boron magnetic steel include thermal conductivity, specific heat capacity, and thermal expansion coefficient.

Simulation of Magnetic Steel State under Motor Operation

The performance of magnetic steel gradually decreases with the increase of temperature, so the temperature rise of permanent magnet motors becomes a key influencing factor for the long-term load operation of the motor. Good thermal conductivity and heat dissipation ability can avoid overheating and maintain the normal operation of the equipment. Therefore, we hope that magnetic steel has a high thermal conductivity and specific heat capacity. On the one hand, heat can be quickly transmitted and dissipated, while also triggering lower temperature rise under the same heat.

Neodymium iron boron magnet is easy to magnetize in a specific direction (II-C axis), and in this direction, the magnetic steel will expand when heated; However, there is a negative expansion phenomenon in the two directions (Å C-axis) that are difficult to magnetize, namely thermal contraction. The existence of thermal expansion anisotropy makes the radiation ring magnetic steel prone to cracking during sintering; And in permanent magnet motors, soft magnetic material frames are often used as the support for magnetic steel, and the different thermal expansion characteristics of the two materials will affect the size adaptability after temperature rise.

Electrical Property

Magnet eddy current under alternating field

In the alternating electromagnetic field environment of permanent magnet motor rotation, the magnetic steel will generate eddy current loss, which leads to temperature rise. As the eddy current loss is inversely proportional to the resistivity, increasing the resistivity of neodymium iron boron permanent magnet will effectively reduce the eddy current loss and temperature rise of the magnet. The ideal high resistivity magnetic steel structure is formed by increasing the electrode potential of the rare earth rich phase, forming an isolation layer that can prevent electron transmission, achieving the encapsulation and separation of high resistance grain boundaries relative to the main phase grains, thereby improving the resistivity of sintered neodymium iron boron magnets. However, neither the doping of inorganic materials nor the layering technology can solve the problem of deteriorating magnetic properties, and currently there is still no effective preparation of magnets that combine high resistivity and high performance.