Method for preparation of a sintered type NdFeB permanent magnet with an adjusted grain boundary

A preparation method for a sintered type NdFeB permanent magnet, the method including the steps of:

• (1) Covering a NdFeB magnet with a diffusion source to form a NdFeB magnet intermediate, the chemical composition of the NdFeB magnet intermediate is expressed in weight percentage as [R1xR2yR31-x-y]aMbBcFe100-a-b-c,
• with 0.8≤x≤1, 0≤y≤0.08, 32≤a≤38, 0.5≤b≤7, and 0.95≤c≤1.2,
• R1 being one or more of Nd, Pr, and Ce,
• R2 being one or more of La and Sm,
• R3 being one or more of Tb, Dy, and Ho,
• M being one or more of Al, Cu, Ga, Ti, Co, Mg, Zn, Nb, Mo, and Sn, and
• wherein the diffusion source refers to an alloy including two or more of the alloying elements Nd, Pr, Ce, La, Ho, Tb, Dy, Ga, Al, Cu, and Mg;
• (2) Putting the NdFeB magnet intermediate into a furnace and performing a diffusion treatment and subsequently an aging treatment, wherein the aging treatment is divided into a heating step and a cooling step, and
• wherein the cooling step is carried out by means of argon gas positive pressure circulation cooling such that NdFeB magnets with a thickness of grain boundaries in the range of 10 nm to 1 μm are formed, and a structure of the grain boundaries includes a main phase, grain boundary (a), grain boundary (b), and grain boundary (c),
• wherein grain boundary (a) meets the following conditions:
• R≥55 wt % or 35 wt %≤R≤40 wt %, 10 wt %≤M≤28 wt %, where 3:1≤Nd/(Pr or Ce or La)≤2:1, and (Cu+Al+Ga)/M≥0.8;
• grain boundary (b) meets the following conditions:
• 40 wt %≤R≤55 wt %, 10 wt %≤M≤20 wt %, 9:10≤Nd/(Pr or Ce or La)≤2, and (Cu+Co+Al)/M≥0.9;
• and
• grain boundary (c) meets the following conditions:
• 25 wt % R≤50 wt % or R≥60%, 0≤M≤10 wt %,
• where R in the grain boundaries (a) to (c) refers to the total amount of rare earths, and M refers to the total amount of Al, Cu, Ga, Ti, Co, Mg, Zn, Nb, Mo, and Sn.