Impact of TiB2 particle size on microstructure and magnetic properties of Nd-Ce-Fe-B sintered magnets

Abstract

The incorporation of titanium as an additive in Nd-Ce-Fe-B sintered magnets significantly enhances the coercivity through grain refinement. However, the in-situ formed TiB₂ secondary phase consumes boron atoms from the matrix during the sintering process, thereby limiting the remanence enhancement of the magnets. Here, we investigate the impact of TiB₂ addition with various particle sizes (50 nm, 500 nm, and 5 μm) on the microstructure and magnetic properties of Nd-Ce-Fe-B sintered magnets. As the size of TiB₂ particles declines, the remanence (B_r) decreases while the coercivity (H_cj) increases. The interfacial stress induced by TiB₂ particles promotes the segregation of the ZrB₂ phase, reducing the concentration of B atoms in regions adjacent to the TiB₂ phase. This reduction inhibits the decomposition of the Nd₂Fe₁₅Ga₂(B) phase into the Nd₂Fe₁₄B phase, leading to a decrease in the proportion of Nd₂Fe₁₄B phase. Additionally, the irregular growth direction of the acicular ZrB₂ phase is detrimental to the orientation of the 2:14:1 phase, causing the deterioration of the B_r. As the TiB₂ particle size decreases, the contact area and compressive stress increase, along with a rise in the content of Nd₂Fe₁₅Ga₂(B) phase, and ZrB₂ phase. The acicular ZrB₂ phase exerts a pinning effect on the grain boundaries (GBs), hindering the motion of magnetic domain walls, and thereby enhancing the H_cj and thermal stability of the magnet. Controlling the TiB₂ particle size is crucial for balancing the H_cj and B_r. However, the reduction of particle size inevitably leads to agglomeration, posing significant challenges for future efforts to enhance magnetic properties and refine the microstructure through mitigating TiB₂ agglomeration.