First-principles Study Of Ce-doped NdFeB Magnets And Its Heat Treatment Experiment

The NdFeB permanent magnets are the permanent magnet with the largest output currently.It has a wide range of applications and a promising development prospect.It is one of several new materials that have received much attention.However,the problems brought about by high production volumes are also becoming increasingly prominent.Rapidly increasing production makes the rare earth resources even more scarce,resulting in higher production costs.The use of more abundant Ce elements instead of Nd not only makes the use of rare earth elements more balanced,but also reduces the production cost of NdFeB,which is very meaningful for Middle and low markets for permanent magnet materials.In this paper,the First-principles calculation method was used to study the changes of the stability of the unit cell system after using Ce to replace Nd at different positions in the Nd₂Fe₁₄B unit cell.It was found that the stability of the unit cell system was decreased by the order of Ce substitution 4f,4g and the substitution of both positions gradually.At the same time,the preferential occupation of Ce atoms in the Nd₂Fe₁₄B unit cell is studied.When Ce atoms substitute for Nd,they will preferentially occupy the 4f position.After that,Ce atoms occupy other positions in sequence as Ce increases.On the other hand,substitution of Ce for Nd atoms increases the bond between the rare earth and Fe atoms and decreases the bond energy.The Ce atom also affects the magnetic moment of the Fe atoms around it,which decreases the magnetic moment of the Fe atoms.It would make the magnetic moment drops.The heat treatment process of Ce-doped NdFeB magnets was also studied in this paper.Magnets with nominal composition(Nd_0.8Ce_1.2)Fe₁₄B and(Nd_0.5Ce_1.5)Fe₁₄B were used as the basic samples to study the crystallization behavior,microstructure and magnetic properties changes of magnets in terms of temperature and time.In the heat treatment process,as the temperature increases,the amorphous structure in the quenched ribbon decomposes fistly to precipitate theα-Fe phase and the metastable phase Re₂Fe₂₃B₃,and then at about 680℃,and then the Re₂Fe₂₃B₃ phase is completely decomposed to produce the hard magnetic phase.And in the heat treatment,the atoms in the amorphous structure are in a thermally activated state,and after sufficient driving force of crystallization is obtained,gradual crystallization begins.While maintaining the low crystallization efficiency,the crystallinity is gradually increases with time,and the appropriate control time and temperature can obtain about 150 nm crystal grains.At T=700℃,t=5 min,there is better performance:Br=9.33 kGs,Hci=10.60 kOe,（BH）max=17.64 MGOe.In order to reduce the magnetic properties ofα-Fe in the process of crystallization,the effects of the addition of Zr elements and the temperature rise rate on theα-Fe phase were studied.The results show that Zr element can not only reduce the formation ofα-Fe phase in the process of rapid quenching,but also inhibit the growth ofα-Fe phase grains in heat treatment,and effectively reduce the soft magnetic phase content.Increasing the heating rate can reduce the time of precipitation and growth ofα-Fe phase,and effectively reduce its volume ratio;but too fast heating up will make the amorphous phase crystallization in thin band incomplete,which is not conducive to the improvement of magnetic properties of magnets.