Sm2Co17-based magnets are still playing an irreplaceable role in the permanent magnet industry due to its unique high temperature magnetic properties and superior magnetic stability, then it always serving to the high-speed motors, electronic communication, and aerospace. High energy product magnets are the important basis to accelerate miniaturization and high efficiency of the device. Thus, obtain high performance SmCo magnets have always been a goal since the advent of Sm2Co17.
Sm2Co17-based magnets are composed of Samarium, Cobalt, Iron, Copper, and Zirconium. The content of each element has different influence on the magnetic performance. The microstructure of Sm2Co17-based magnets is a type of cellular structure. Cellular structure is made up of 2:17R cell phase, 1:5H cell boundary phase, and Zr-rich 1:3R platelet phase. 2:17R cell is a rhomboid which long axis along the easy magnetization axis. Its internal is Fe-rich Th2Zn17-type rhombohedral Sm2(Co, Fe)17 main phases. 2:17R main phase gives high saturation magnetization Ms to the magnet which eventually determines the Br. 1:5H phase is a Cu-rich CaCu5-type Sm(Co, Cu)5 cell boundary phases, and it gives higher coercivity to magnets via domain wall pinning. Zr-rich 1:3R platelet phase is perpendicular to the c-axis and running across the cellular structure. Zr-rich 1:3R platelet phase provides diffusion paths for Copper to enter the cell boundary phase, then would be beneficial to expand cellular phase and density variations of cell boundary phase’s domain wall energy, thereby enhance coercivity.
Besides dimension of cellular phase, cell boundary phase’s quantity, thickness, and composition will all affect comprehensive magnetic properties of the magnets. The theoretical value of permanent magnet’s maximum energy product is proportional to the square of Ms. Enhancement of Ms is the prerequisite to obtain high energy product. As a structure sensitive parameter, energy product can be also improved through optimization of cellular structure. That is to say, Ms and energy product of SmCo magnets can be effectively enhanced by composition optimization and heat treatment process modification. Fe in the Sm2(Co, Fe)17 main phases are mainly served to improve Ms and Br of the magnets. Saturation magnetization Js of Sm2Co17 phase is merely around 12kGs. As the content of Fe increased, Js of Sm2(Co0.8Fe0.2)17 and Sm2(Co0.7Fe0.3)17 can achieve 13.5kGs and 16.3kGs, respectively. However, cellular structure grew abnormally once the content of Fe in Sm(Co, Fe, Cu, Zr)z higher than 25wt%. Oversized cellular structure is detrimental to the homogeneity of the cellular structure and result in a sharp deterioration of coercivity and demagnetization curve’s squareness. With the application of jet milling and modification of heat treatment, SDM has already mastered mass production of high performance SmCo magnets. Their magnetic performance is comparable with Electron Energy Corporation (EEC) and Arnold Magnetic Technologies.