Takehisa Minowa

Coercivity Enhancement by the Grain Boundary Diffusion Process to Nd–Fe–B Sintered Magnets

In this paper, changes in magnetic properties and microstructures of sintered Nd-Fe-B magnets that had undergone grain boundary diffusion will be discussed.Changes in magnetic properties after applying grain boundary diffusion process to Nd-Fe-B sintered magnets with a thickness of several millimeters were investigated, and their microstructures were also observed. Coercivities of the magnets coated with heavy rare earths (HRE=Dy and Tb) oxide or fluoride powder and then heat-treated, were enhanced significantly without a considerable reduction in a remanence. During the heat treatment, HRE element diffused into the magnet through the grain boundary, and formed HRE-rich shells around the grain boundary of the Nd2Fe14B phase by substituting Nd. The coercivity increased by more than 400 kA/m with a thickness of up to 3 mm for the magnet processed with TbF3

Coercivity distributions in Nd–Fe–B sintered magnets produced by the grain boundary diffusion process

The grain boundary diffusion (GBD) process is an effective process for enhancing the coercivity of Nd–Fe–B sintered magnets. When the GBD process is applied to a thick magnet, the magnet exhibits a coercivity distribution. Partial coercivity at the centre is the same as that of the original magnet, but the partial coercivity increases on going towards the magnet surface. In this paper, the effects of conditions of the GBD process on the characteristics of coercivity distributions were investigated. Surface coercivities increase with increasing amount of coatings, while diffusion depths are almost the same. The diffusion treatment temperature varied from 800 to 900 °C, and does not affect the coercivity distributions. A long-time diffusion treatment is effective in enhancing the inner coercivities, but a very long treatment causes a significant reduction in surface coercivity.

Microstructure of Nd-rich phase in Nd-Fe-B magnet containing oxygen and carbon impurities

Abstract The microstructure of Nd-Fe-B was examined by Scanning Electron Microscopy (SEM) with a Wavelength Dispersive X-ray Spectrometer (WDS). Investigations were carried out on (Nd 0.9 Dy 0.1 ) 16 Fe bal B 8 magnets with various oxygen and carbon contents. Electron Probe Micro Analysis (EPMA) revealed a complex microstructure in the Nd-rich phase at grain boundary junctions. Carbon and oxygen appear to be concentrated in the Nd-rich phase. After annealing, the Nd-rich phase separated into two phases: one is a condensed neodymium-carbon phase and the other is a neodymium-oxygen phase. Carbon and oxygen contents were quantitatively analyzed by EPMA. Heat treatment increases the carbon content in the Nd-rich phase. Magnetic properties of Nd-Fe-B magnets were influenced to a considerable extent by the impurity elements-oxygen and carbon. In particular, the coercive force of the magnets decreased significantly with increasing carbon content.