The Nd-Fe-B magnets are indispensable components for modern technologies, which are widely used in numerous devices such as electric motors, hybrid vehicles, wind generators, etc., because they have high coercivity (Hcj), high remanence (Br) and high maximum magnetic energy product ((BH)max). Although Nd-Fe-B magnets are the most widely used permanent magnetic material at present, they still have many shortcomings, such as poor thermal stability, easy oxidation and corrosion, and low resistivity, which make the application of Nd-Fe-B magnets limited. Bulk Nd-Fe-B magnets are usually developed with magnetic powder as precursors, which are then sintered, hot pressed/hot deformed or bonded. The surface modification of Nd-Fe-B magnetic powder before the preparation of bulk Nd-Fe-B magnets is an effective means to solve the existing shortcomings of Nd-Fe-B magnets. In this report, Nd-Fe-B hot-deformed magnets with enhanced resistivity and thermal stability are fabricated by hot pressing and hot deformation of H3PO4 phosphating and then MgF2 precipitation coating Nd-Fe-B magnetic powders. The effects of different H3PO4 concentrations and MgF2 precipitation coating under preferred H3PO4 concentration on the resistivity, thermal stability, magnetic properties, and microstructure of the magnets are studied. The results show that the preferred H3PO4 concentration for the preparation of the hot-deformed Nd-Fe-B magnets is 0.04 mol/L. At this time, the maximum magnetic energy product (BH)max, remanenceBr, coercivity Hcj, and resistivity of the magnets are 44.81 MGOe, 13.78 kGs, 9.05 kOe and 229.4 μΩ·cm respectively. Their resistivity is 58.2% higher than that of the Nd-Fe-B magnets produced by untreated magnetic powder. The optimal concentration of MgF2 precipitation-coated Nd-Fe-B magnetic powder is 3 mol/L (that is, the concentration of MgCl2 solution is 3 mol/L, and the concentration of KF solution is 6 mol/L) under the optimal H3PO4 concentration (0.04 mol/L). At this point, the (BH)max, Br, Hcj, and resistivity are 40.46 MGOe, 12.91 kGs, 13.54 kOe, and 267.5 μΩ·cm respectively. Their resistivity is 84.5% higher than that of the Nd-Fe-B magnets produced by untreated magnetic powder. The remanence temperature coefficients a(20-120 ℃) of magnets without treatment and those prepared by optimum phosphating treatment, and optimum MgF2 precipitation coating after optimum phosphating treatment are 0.097, 0.094, and 0.086 respectively. The coercivity temperature coefficients β(20-120 ℃) of prepared magnets without treatment and those prepared by optimum phosphating treatment, and optimum MgF2 precipitation coating after optimum phosphating treatment are 0.608, 0.653, and 0.581 respectively. Based on the values of the remanence and coercivity temperature coefficients, the hot-deformed magnets prepared by the powder through optimum MgF2 precipitation coating after optimum phosphating treatment possess enhanced thermal stability. The enhanced thermal stability can mitigate the magnetic attenuation of the magnets in high-temperature environments. The enhanced resistivity can resist the eddy current losses generated by the magnets during the operation of the motor, thereby reducing the temperature rise of the magnets and lowering the thermal demagnetization of the magnets. Therefore, in this work, the resistivity and thermal stability of Nd-Fe-B magnets can be optimized through the phosphating-coating co-modified magnetic powder process. The prepared magnets have the potential to be applied in high-temperature working conditions.
Key words
surface modification /
Nd-Fe-B magnets /
resistivity /
magnetic properties /
thermal stability
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Funding
National Key Research and Development Program of China (2022YFB3505600)
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