Study On The Diffusion Of Nd₂Fe₁₄B-Ce₂Fe₁₄B Pseudo-Binary System

With the development of the world economy and the population,the demand for new energy and new materials is continuously growing.Among new materials,permanent magnet is of strategy importance for a country,especially for a country rich in rare earth like China.The R-Fe based permanent magnet is represented by Nd-Fe-B,which processed outstanding comprehensive magnetic properties and reasonable price.However,the high temperature coercivity can’t meet the demand used in electrical cars due to its low Curie Temperature and high temperature coefficient of coercivity.In recently years,research of(Nd_1-xCe_x)₂Fe₁₄B single crystals,sintered magnets and melt-spun powders both domestic and overseas found that under a specific substitution amount x,the magnetic properties has anomaly change.Many researchers suggested that there is phase separation in(Nd_1-xCe_x)₂Fe₁₄B compounds due to the valence difference of Nd and Ce.To clarify the anomaly of(Nd_1-xCe_x)₂Fe₁₄B compounds,a Ce₂Fe₁₄B-Nd₂Fe₁₄B diffusion couple was fabricated in this study.The aim is to study the Ce substitution of Nd₂Fe₁₄B and the structure,composition and magnetic properties of(Nd_1-xCe_x)₂Fe₁₄B compounds by high throughput method.A Ce₂Fe₁₄B-Nd₂Fe₁₄B diffusion couple is successfully fabricated by hot-pressed solid welding method and the interface has atomic-level bond.After a long time heat treatment,the structure is homogenized and the diffusion time is adequate.Optical microscopy,X-ray diffraction and Scanning electron microscopy are used to characterize the microstructure and composition of the samples.Vibrating Sample Magnetometer is used to measure the magnetic properties.All the results were analyzed to make sure the reliability.After heat treatment,the amount of-Fe phase and R-rich phase are tiny,which optimizes the matrix for diffusion and analysis.The composition of Ce and Nd is not gradient as expectation,but has a turning step for both elements.On the original Nd-Fe-B side,the matrix is still ternary compound Nd₂Fe₁₄B,no trace of Ce.On this side Ce element exists in R-rich phase and oxidized phase.While on the original Ce-Fe-B side,Nd atoms substitute Ce in Ce₂Fe₁₄B and results in two 2-14-1 phase with different Ce/Nd ratio,(Ce_0.835Nd_0.165)₂Fe₁₄B and(Ce_0.385Nd_0.615)₂Fe₁₄B compounds.And the structure is the latter surrounding the former,like the core-shell structure which increases coercivity of the magnet.This is the evidence that(Nd_1-xCe_x)₂Fe₁₄B compound is not a continuous miscibility system.The magnetizationμ₀Ms and the bulk anisotropy of magnet are improved after long time heat treatment.Ce-Fe-B shows stronger bulk anisotropy than Nd-Fe-B for both as-cast samples and homogenized samples.The sample cut at the(Ce_0.835Nd_0.165)₂Fe₁₄B and(Ce_0.385Nd_0.615)₂Fe₁₄B two-phase region has the highest bulk magnetization anisotropy,reaching 7.21 T.And the magnetizationμ₀Ms is 1.48 T.The magnetizationμ₀Ms of single phase(Ce_0.835Nd_0.165)₂Fe₁₄B and(Ce_0.385Nd_0.615)₂Fe₁₄B is 1.2 T and 1.5 T respectively and the two samples show lower bulk anisotropy than the two-phase sample.Therefore the strong bulk magnetization anisotropy of the two-phase sample is not attributed to the two composing phase,but resulting from the interaction of the two phases.And this is different from the core-shell structure,whose shell is of high coercivity.