Physical properties of sintered neodymium iron boron
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The mechanical performance indicators of magnetic steel include hardness, compressive strength, flexural 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 lose corners or even crack during processing, magnetization, and assembly. Magnetic steel is usually fixed in components and equipment using card slots or adhesive, while also providing shock absorption and buffering protection. The fracture surface of sintered neodymium iron boron is a typical transgranular fracture, and its mechanical properties are mainly determined by its complex multiphase structure, as well as the formulation composition, process parameters, and structural defects (pores, large grains, dislocations, etc.). Generally speaking, the lower the total amount of rare earths, the poorer the mechanical properties of the material. By adding low melting point metals such as Cu and Ga in moderation, improving the grain boundary phase distribution can enhance the toughness of magnetic steel. Adding high melting point metals such as Zr, Nb, Ti, etc. can form precipitation phases at grain boundaries, refine grain size, and suppress crack propagation, which helps improve strength and toughness; However, excessive addition of high melting point metals can cause excessive hardness of magnetic materials, seriously affecting 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 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.
The performance of magnetic steel gradually decreases with increasing temperature, so the temperature rise of permanent magnet motors becomes a key factor affecting whether the motor can operate under load for a long time. Good thermal conductivity and heat dissipation 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, which can quickly conduct and dissipate heat, while also causing lower temperature rise under the same amount of heat. Neodymium iron boron magnetic steel is easy to magnetize in a specific direction (∥ C axis), and the magnetic steel 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 contraction. The existence of thermal expansion anisotropy makes it prone to cracking during the sintering process of radiation ring magnetic steel; 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 dimensional adaptability after temperature rise.
Electrical performance
In the alternating electromagnetic field environment of permanent magnet motor rotation, eddy current losses will be generated in the magnetic steel, leading to temperature rise. As eddy current losses are inversely proportional to electrical resistivity, increasing the electrical resistivity of neodymium iron boron permanent magnets can effectively reduce the eddy current losses and temperature rise of the magnets. The ideal high resistivity magnetic steel structure is achieved by increasing the electrode potential of the rare earth rich phase, forming an isolation layer that can prevent electron transfer, and realizing the wrapping 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 doping nor layering techniques of inorganic materials can solve the problem of deteriorating magnetic properties. Currently, there is still no effective preparation of magnets that combine high resistivity and high performance