Research On Grain Boundary Modification And Property Regulation Of Sintered NdFeB Magnets

NdFeB permanent magnets have been widely applied in the industries such as aerospace, energy, information communication, computer, and transportation because of their good magnetic properties. The requirements for better magnets with higher properties are being pushed forward by the increasing and expanding market demands.It is very important to develop the NdFeB magnets with low heavy rare earth(HRE)elements and excellent high temperature magnetic properties when the rear earth,especially HRE, raw materials have been so expensive continuously. We have studied the effect on structure, morphology and magnetic properties of the NdFeB magnets by TbF3 coating or Tb sputtering and also the following heat treatment technologies. We have discussed the mechanisms on the coercivity improvement and micro-morphology evolution. Furthermore, we have investigated the grain boundary diffusion process(GBDP) and its influences on micro-morphology and magnetic properties.We studied the NdFeB permanent magnets with TbF3 coating and different heat treatment temperature and time. The result showed that Tb exists in the grain boundary but not in the matrix. It indicates that the heave rare earth element can be diffused into the magnetic interior successfully by the surface coating of compounds with heavy rare earth element at various temperatures. In different heat treatment temperatures(700℃、750℃、800℃、850℃),Hcj increased firstly and then decreased with the increase of annealing time. The improvement of Hcj comes from the formation of the new Tb2Fe14 B phase in the grain boundary with much higher magneto-crystalline anisotropy(22T)than Nd2Fe14B(6.7T). The maximum Hcj 1847 kA/m was obtained by annealing at 850℃ for 7h,which is 35% higher than that of the original magnet. Br drops by 1.6%、2.4% 、 2.4% at 700 ℃, 750 ℃, 800 ℃ respectively as the annealing time prolonged while 12% Br slump was the biggest for the magnet by annealing at 850℃ for 9 hours.The higher the annealing temperature and the longer the time, the more Tb being diffused into the interior of Nd2Fe14 B grains. The strong anti-ferromagnetic coupling between Tb and Fe are attributed to the decline of Br.We also studied the NdFeB permanent magnets with Tb sputtering and different heat treatment processes. The EDS result showed that Tb exist in the grain boundarybut not in the matrix for the magnet after heat treatment of 800 ℃ for 3 hours. It indicates that the heavy rare earth elements can be diffused into the interior grain boundary successfully by the sputtering and heat treatment. At 800℃ and 850℃, the Hcj increased firstly and then decreased with the increase annealing time while Br and(BH)max decreased inversely. Under the condition of 800℃/7h,Hcj is up to 1516kA/m,which is 10.8% higher than that of the original magnet while Br is 8% lower than the counterpart. The increase of the magnetization reversal nucleation field is considered as the improvement of Hcj. The sharp decline of Br is due to the Tb diffusion from grain boundary to the Nd2Fe14 B matrix.We tried PrTbCu sputtering for the same magnets to further observe the phenomena and clarify the mechanism. The results, however, showed that Hcj are lower than those of the original magnets under all annealing conditions, which suggests that Pr60Tb20Cu20 did not enter the interior of the magnets beyond our initial expectation. It is possible that the Pr60Tb20Cu20 phase was not formed even after heat treatment but the actual formed phase was something hard to be diffused. More work need to be done in this respect.