| Sm2Co17-based sintered permanent magnet with the mechanism of domain wall pinnig, known as the second generation of rare earth permanent magnet, has attracted much attention due to its distinguished high temperature magnetic properties, extremely low temperature coefficient, excellent corrosion resistance and oxidation resistance. Microwave tubes, gyroscopes and accelerometers, reaction and momentum wheels to control and stabilize satellites, magnetic bearings, sensors and actuators are examples of applications in which high-energy density permanent magnets with magnetic fields stable over a variety of environmental conditions and wide temperature ranges are required [120]. Based on the consensus that pursuing high performance (including high remanence Br, intrinsic coercivity HCj and the maximum magnetic energy products) is the eternal melody in the development history of permanent magnets, the magnetic properties and microstructure of high-performance Sm2Co17-based sintered permanent magnet have been investigated systematically. The following conclusions are drawn:(1) Sm2Co17 sintered permanent magnets of high Hcj have been prepared using jet-milling technique due to effective oxygen control.(2) TEM BF images of magnet with the high solution temperature (1195℃) shows that there is small cellular microstructure with the size ～50 nm, which is deem to decrease the volume fraction of 1:5H pinning phase, and then the HCj. The magnets with the comprehensive magnetic properties (which equals to (BH)max+HCj) of 63.01 ((BH)max=31.64 MGOe and Hcj=31.37 kOe) have been prepared.(3) The study shows that with the increasing isothermal annealing time tia, both the saturation magnetic polarization Js and the magneto-crystalline anisotropy constant K] of 1:5H phase decrease, but the value of Js/K1 may increases. It is held that the increasing Hcj is related to the changes of the factors which are structure-sensitive. In detail, it is related to the more integrated cellular microstructure, the increasing thickness of 1:5H cell boundary phase from ～3 nm to ～7 nm, and the increasing number density of Z-phase from ～0.023 nm-1 to～0.038 nm-1.(4) For magnet made of c-5 ball-milled powder, the optimum isothermal annealing temperature Tia and time tia are 800℃ and 20 h, respectively. And the corresponding magnetic properties are:Br= 11.57 kGs, Hcj=9.46 kOe, and (BH)max=27.8 MGOe. The Sm2Co17 sintered permanent magnets, with high remanence-11.6 kGs, have been prepared. To be specific, the results of magnetic properties show that with the increasing tia, the change rule of Br, Hcj, the knee-point coercivity Hk, the magnetic induction coercivity Hcb and (BH)max of magnet with T,a of 830 and 800℃ appear to be similar to each other. In detail, Br remains almost the same, Hcj increases gradually, Hk first increases and then decreases, Hcb increases monotonously, and (BH)max first increases and then decreases.(5) Grade 32H (30 MGOe<(BH)max≤32 MGOe, Hcj≥25 kOe) Sm2Co17-based sintered permanent magnets have been successfully fabricated using c-5 jet-milled powder. According to section 4.3.4, (BH)max of most of magnets are about 31 MGOe. The optimal (BH)max>31.64 MGOe, and the corresponding treatment regime is: solution temperature Ts=1187℃, and Tia=785℃. It is reasonable to deduce that the optimal (BH)max approaches 32 MGOe. On the premise of (BH)max of 31.64 MGOe, Sm2Co17-based sintered permanent magnet with ultrahigh Hcj of 31.64 kOe has been prepared. Sm2Co17-based sintered permanent magnet with ultrahigh Hcj 32.83 kOe has been prepared, of which (BH)max=30.4 MGOe tested by NIM-2000HF hysteresigraph.(6) To obtain magnet with bigger (BH)max, ingots with composition c-6 and c-8 have been designed. The magnetic properties of the final state magnet made of c-6 and c-8 jet-milled powder show that the iron content should not be too much. |
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