Microstructure Design And Magnetic Properties Of Sm₂Fe₁₇N_x Magnets

Permanent magnets are widely used in many fields of technology.High-performance rare earth permanent magnetic materials are one of the key materials for high-tech,such as electric vehicles,direct-drive wind generators,industrial motors,magnetic machinery,magnetic resonance imaging equipment,maglev trains,and consumer electronics.Sintered Nd-Fe-B magnet is the most widely used high performance permanent magnet materials.In order to produce Nd-Fe-B magnet,Pr and Nd metals are mined in large quantities,while symbiotic La,Ce,Y,and Sm elements are excessively abundant,the market price of these elements is very low.The development and application of rare earth resources are seriously out of balance.Both the Th2Zn17 type Sm2Fe17N3 and TbCu7 Sm-Fe-N compounds possess excellent intrinsic magnetic properties.Using these nitrides is expected to produce permanent magnes that are more resistant to high temperatures than Nd-Fe-B.The isotropic TbCu7-type Sm-Fe-N powder is usually prepared by nitriding the rapidly-quenched alloy.The key to the preparation of high-performance TbCu7-type magnetic powder is to form an alloy with ultra-fine nanocrystalline structure.In this paper,the phase structure in rapidlly quenched Sm-Zr-Fe-Co alloy,the preparation technology of nitride powder and the coercivity mechanism were studied.The amorphous forming ability of Sm-Zr-Fe-Co alloy was improved by micro-alloying and the microstructure was refined through amorphous crystallization.Sm2Fe17N3 is generally prepared by nitridation of a single-phase Sm2Fe17 powder.In the Sm-Fe binary alloys,a-Fe and SmFe3 coexist in equilibrium with Sm2Fe17.Therefore,in the preparation of Sm2Fe17N3 by Sm-Fe alloy,it is difficult to completely avoid the formation of harmful soft magnetic phase a-Fe.If an alloy consisting of Sm2Fe17N3 and non-magnetic phase can be prepared from nitridation recation directly,it would be easy to prepare anisotropic Sm2Fe17N3 magnetic powders without a-Fe;more importantly,in view of the nucleus-type coercivity mechanism of micron-sized Sm2Fe17N3 magnets,the microstructure consisting of Sm2Fe17N3 and non-magnetic grainboundary phase would facilitate the improvement of coercivity through the modification of the boundary microstructure,removal of susceptibility nucleation sites and magnetic decoupling.For these reasons,this paper proposes that Cu is added to the Sm-Fe binary alloy to change the phase equilibrium relationship,forming nonmagnetic low-melting phases that are in equilibrium with Sm2Fe17 phase.In this way,a new type of mcirostrure that is characteristic of consisting of Sm2Fe17N3 and nonmagnetic phase can form directly by nitriding reaction.The main research contents and results are as follows:(1)A partial isothermal phase diagram of Sm-Fe-Cu ternary system at 450 ?was determined by investigating the phase equilibrium relationship.The phase diagram contains a three-phase region of Sm2Fe17+SmCu2+SmCu.Based on this phase equilibrium,nonmagnetic low-melting Sm-Cu phases were designed.(2)It was found that Sm2Fe17/Sm-Cu alloys can still be nitrided according to the nitridation behaviour of SmCu and SmCu2 phases in Sm2Fe17/Sm-Cu alloys.When the Sm-Cu phases are dominated by SmCu,no a-Fe phase precipitate after the Sm2Fe17/Sm-Cu alloy is nitrided,and the relevant mechanism was investigated.Based on this discovery,nitrided alloys consisting of Sm2Fe17N3 and nonmagnetic phase can be prepared by nitriding reaction directly.(3)The effects of strip-cast and heat treatment on the microstructure of Sm2Fe17/Sm-Cu alloys were investigated,it was confirmed that this kind of alloys have adjustable component tolerance and uniform strip-casted microstructure,the small amount of SmFe2 and a-Fe impurity phases can be eliminated at a low heat treatment temperature of 900?.These advantages help to significantly reduce or even completely remove the a-Fe phase in Sm2Fe17Nx powders.Based on the strip-casted alloy with a composition of(Sm10.5Fe89.5)97(Sm45Cu55)3,a Sm2Fe17NX powder without a-Fe phase was prepared by heat treatment,crushing and nitriding.Anisotropic fine Sm2Fe17Nx powders with a squareness of 53%to 56%were prepared by ball milling.The coercive force,squareness and maximum energy product of the 0.9 ?m sized magnetic powder are 11.5 kOe,52.9%and 32.9 MGOe,respectively.When Sm2Fe17N3 fine magnetic powder is used to prepare sintered magnets,Sm2Fe17N3 particles easily oxidize and precipitate the a-Fe phase,causing a sharp decrease in coercive force.Therefore,the influence of SmCu powder as an oxygen-removable binder on the magnetic properties of sintered Sm2Fe17N3 during the spark plasma sintering(SPS)process was studied.It was found that the SmCu powder can suppress the oxidative decomposition of Sm2Fe17N3,the coercive force and squareness of the SmCu powder added anisotropic sintered magnet are comparable to those of the raw Sm2Fe17N3 fine powder.