Continuous Grain Boundary Phase Research Articles

Coercivity enhancement of sintered Nd-Fe-B magnets by grain boundary diffusion with Pr80–x Al x Cu20 alloys

A grain boundary diffusion (GBD) process with Pr80 – x Al x Cu20 (x = 0, 10, 15, 20) low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets. The best coercivity enhancement of a diffused magnet was for the Pr65Al15Cu20 GBD magnet, from 16.38 kOe to 22.38 kOe. Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary (GB) phase, optimizing the microstructure to enhance the coercivity. The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains. Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process. Meanwhile, the introduction of Al can effectively promote the infiltration of Pr into the magnet, which increases the diffusion rate of rare-earth elements within a certain range. This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.

Influence of Pr-Al-Co alloy diffusion source on magnetic properties and microstructure of sintered Nd-Fe-B magnets processed by grain boundary diffusion

In this study, the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr70Al30–xCox (x = 0 at%, 10 at%, 15 at%, 20 at%, 30 at%) alloys. When the Co content in the diffusion source increases from 0 at% to 10 at%, the coercivity enhancement in the Pr70Al20Co10 diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr70Al30 diffused magnet. With further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr70Al15Co15, Pr70Al20Co10 and Pr70Co30 diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr70Al20Co10 diffused magnet under the synergistic effect of Al and Co, which leads to the enhancement of magnetic isolation between more adjacent grains. However, the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.

Microstructural Effects on the Thermal Conductivity of Polycrystalline Aluminum Nitride

Polycrystalline AIN bodies were made with a range of porosities from various AIN powders by sintering or hot‐pressing. Thermal conductivity data for material produced without sintering aids showed a gradual, yet definite, porosity dependence with scatter similar to other property‐porosity studies. The thermal conductivity‐porosity data for ALN with sintering aids showed the existence of two distinct regions: (1) a higher‐porosity region (greater than approximately 2%) which was similar to the data for material without sintering aids, and (2) a low‐porosity region where grain boundaries were seen to dominate thermal conductivity. Auger spectroscopy was used to investigate fracture surfaces for dense CaO‐doped materials. Thermal conductivity was observed to correlate to effective grain‐boundary thickness, which was calculated from quantitative analysis of the Auger data. A model consisting of cubic grains in a continuous grain‐boundary phase accurately describes these data.