The Magnetic Properties And Coercivity Mechanism Of Nd-Ce-Fe-B Rapidly Quenched Alloys

Large amount of rare earth resources have been consumed since rare earth permanent magnetic materials?PM?have been applied invarious fields such as wind power generation and new energy vehicles.Due to the high pressure on the demand,availability and cost of rare earth metals,particularly Nd,Pr,Dy,and Tb,being vital to the high performance PM materials,the low-cost PM materials developed by substituting Ce for the aforementioned expensive elements have come into being.However,the low coercivity and poor thermal stability are still main obstacles to extend the application area of Nd-Ce-Fe-B based PM materials.In addition,the reason why the magnetic property of the Ce-substituted Nd-Fe-B based material deteriorates is not only related to the reduction of the intrinsic properties of alloys,but there are other factors to be needed to clarify.In this thesis,we first explore the mechanism of the magnetic properties of Nd-Ce-Fe-B with Ce addition.The evolution of coercivity mechanisms has also been investigated with different Ce content.Comprehensive analysis of TEM microstructure characterization and magnetic measurement simplies that the decrease in magnetic properties of Nd-Ce-Fe-B alloys with low Ce content is mainly attributed to the decrease in the intrinsic magnetic properties of Nd-Ce-Fe-B alloys.However,in the high Ce-content Nd-Ce-Fe-B alloys,the decrease in magnetic properties is related to the decrease in the magnetic domain pinning effect of the grain boundaries as well as the reduction in the intrinsic magnetic properties.The reduction in pinning effectiveness may be related to the composition change of the grain boundary phase of the alloy.To enhance the magnetic properties of alloys,the composition of the grain boundary phase and microstructure of Nd-Ce-Fe-B alloy have been regulated by the characteristics of Nb which tends to combine?-Fe to form Fe₂Nb phases.The room and elevated temperature magnetic properties of(Nd_0.8Ce_0.2)₁₃Fe_82-xNb_xB₅ alloys have been significantly improved with the addition of Nb element.The optimum magnetic properties obtained with 0.5 at.%Nb doping are H_cj=13.1 kOe,J_r=0.79 T and?BH?_max=13.3 MGOe,respectively.The enhanced J_r of the alloys at x=0.5 and 1.0 may result from the synergistic function between the increase of?-Fe with high saturation magnetization M_S and the enhancement of the exchange-coupling interaction.In this thesis,also investigated are the effects of Zr addition on the microstructures,magnetic properties,exchange coupling and coercivity mechanisms of Nd-Ce-Fe-B alloys fabricated by melt-spinning technique.Furthermore,it is found out that the H_cj is enhanced significantly by Zr substitution in the(Nd_0.8Ce_0.2)₁₃Fe_82-xZr_xB₅ alloys while the remanence J_r is reduced slightly.H_cjj increases from 12.2 to 13.7 kOe by adding Zr element up to 1.5 at.%content whereas H_cjj decreases by the further increase of Zr content.The larger lattice constants and unit cell volume of the matrix phase indicates that Zr atoms enter into the hard magnetic phase by substituting Fe sites.The weakened intergranular exchange coupling of the Zr added alloysis attributed to the formation of non-magnetic intergranular phase.The synergistic function between pinning effect and exchange coupling leads to the improved magnetic properties.To conclude,the influences of Ce,Nb,and Zr additions on the microstructures and magnetic properties of Nd-Ce-Fe-B alloys have been studied systematically.The current research may shed light on the further development of Nd-Ce-Fe-B based magnets.