Physical Properties of Sintered NdFeB Magnet
As the core functional component, sintered NdFeB permanent magnets are widely used in instruments and equipment such as motors, electroacoustics, magnets and sensors. During the service process, the magnet will be subjected to environmental factors such as mechanical force, cold and heat changes, alternating electromagnetic field, etc. If environmental failure occurs, it will seriously affect the function of the equipment and cause huge losses. Therefore, in addition to the magnetic performance index, we also need to pay attention to the mechanical, thermal and electrical properties of the magnet, which will help us better design and use the magnetic steel, and is of great significance for improving the stability and reliability of its service. .
The mechanical properties of magnetic steel include hardness, compressive strength, flexural strength, tensile strength, impact toughness, Young’s modulus, etc. NdFeB is a typical brittle material. The hardness and compressive strength of magnetic steel are high, but the bending strength, tensile strength and impact toughness are poor. This makes the magnetic steel prone to drop corners or even cracks during processing, magnetization and assembly. Magnetic steel in components and equipment usually needs to be fixed with slots or adhesives, and at the same time provide shock absorption and buffer protection.
The fracture surface of sintered NdFeB is a typical intergranular fracture, and its mechanical properties are mainly determined by its complex multiphase structure, and are also related to formula components, process parameters and structural defects (holes, large grains, dislocations, etc.) . Generally speaking, the lower the total amount of rare earth, the worse the mechanical properties of the material. By adding Cu, Ga and other low-melting point metals in an appropriate amount, improving the grain boundary phase distribution can enhance the toughness of the magnetic steel. Adding high-melting-point metals such as Zr, Nb, and Ti can form precipitated phases at grain boundaries, which can suppress crack extension while refining grains, and help improve strength and toughness; but excessive addition of high-melting-point metals will cause hardness of magnetic materials If it is too high, it will seriously affect the processing efficiency.
In the actual production process, it is difficult to balance the magnetic and mechanical properties of magnetic materials. Due to cost and performance requirements, it is often necessary to sacrifice its ease of processing and assembly.
The main thermal performance indicators of NdFeB magnets include thermal conductivity, specific heat capacity and thermal expansion coefficient.
The performance of the magnetic steel gradually decreases with the increase of temperature, so the temperature rise of the permanent magnet motor becomes the key factor affecting the long-term load operation of the motor. Only good heat conduction and heat dissipation capabilities can avoid overheating and maintain the normal operation of the equipment. Therefore, we hope that the magnetic steel has a high thermal conductivity and specific heat capacity. On the one hand, the heat can be quickly conducted and dissipated, and at the same time, the same heat will cause a lower temperature rise.
NdFeB magnets are easy to magnetize in a specific direction (∥C axis), and the magnets will expand when heated in this direction; but there is a negative expansion phenomenon in the two directions (⊥C axis) that are difficult to magnetize, that is, thermal shrinkage. The existence of thermal expansion anisotropy makes the radiant ring magnetic steel easy to crack during sintering; and in permanent magnet motors, the frame of soft magnetic material is often used as the support of the magnetic steel, and the different thermal expansion characteristics of the two materials will affect the temperature rise. Size fit.
In the alternating electromagnetic field environment where the permanent magnet motor rotates, the magnet will generate eddy current loss and cause temperature rise. Since the eddy current loss is inversely proportional to the resistivity, increasing the resistivity of the NdFeB permanent magnet will effectively reduce the eddy current loss of the magnet and the magnet. temperature rise. The ideal high-resistivity magnetic steel structure is to increase the electrode potential of the rare earth-rich phase to form an isolation layer that can prevent electron transmission, and realize the wrapping and separation of high-resistance grain boundaries relative to the main phase grains, thereby improving the sintered NdFeB magnet. resistivity. However, neither the doping of inorganic materials nor the layering technology can solve the problem of deterioration of magnetic properties, and there is still no effective preparation of magnets with both high resistivity and high performance. Physical Properties of Sintered NdFeB
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