Nd-Fe-B基纳米复合永磁材料的制备及矫顽力的探索
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摘要
本文采用熔体快淬、高能球磨、热压的方法制备出纳米复合磁体(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo,研究了磁体软磁相质量百分比、晶粒尺寸与磁性能的关系,对晶化相变过程的热力学性质、反磁化过程的形核、自钉扎作用以及弱交换耦合作用进行了分析讨论。
高能球磨使粉末颗粒细化和均匀化,同时增加系统的能量,在晶化过程中提高了晶化的形核率,热压也具有提高晶化形核率的作用并抑制原子的长程扩散,这两者使晶粒更为细小,晶粒尺寸分布更为集中。虽然高能球磨热压纳米晶(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6单相磁体晶粒之间的交换耦合作用较强,一定程度地使磁体矫顽力下降,但磁体晶粒尺寸分布集中,晶粒界面相对较多,磁体仍能保持高的矫顽力,达到1631kA/m,与未经过高能球磨热压的(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6磁体的矫顽力1616kA/m相当。随着(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo复合磁体中FeCo质量百分比的增加,磁体饱和磁化强度提高,矫顽力下降。通过讨论分析,在(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo纳米复合磁体中软磁相质量百分比小于20%时,反磁化过程中自钉扎作用较为突出,复合磁体矫顽力高,在FeCo的质量百分比为10%和20%时,(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo复合磁体的矫顽力分别达到1357kA/m和801kA/m;当软磁相质量百分比较大于30%时,磁偶极作用比较突出,反磁化过程以在软磁相晶粒内形核机制为主,复合磁体矫顽力低,当FeCo质量百分比为40%时,复合磁体的矫顽力仅为(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6单相磁体的9.1%。复合磁体软磁相α-Fe的晶粒尺寸偏大,在40nm~70nm之间,比硬磁相Nd_2Fe_(14)B的晶粒尺寸大,这是由于Nd_2Fe_(14)B和α-Fe在从非晶向纳米晶相变过程中热力学特性的差异决定的,α-Fe先于Nd_2Fe_(14)B形核晶化,导致α-Fe晶粒生长时间长,晶粒尺寸比较大,这也是复合磁体矫顽力低的一个主要原因。
Nd-Fe-B基纳米复合磁体的软、硬磁相晶粒界面附近存在的缺陷会减弱晶粒之间的交换耦合作用,从而造成复合磁体矫顽力下降。为从根本上提高纳米复合磁体的磁性能,应探索新的方法以减弱Nd_2Fe_(14)B和α-Fe在相变过程中热力学特性差异对复合磁体晶粒尺寸的影响,将软磁相α-Fe的晶粒尺寸控制在小于硬磁相晶粒尺寸的范围内,同时减少晶粒之间界面中原子空位等缺陷。
(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo magnets were prepared using the procedure of high energy ballmilling, hot pressing following melt spinning. The dependence of magnetic properties on weightpercent of soft magnetic phase and grain size has been studied. The nucleation and self-pinningduring the process of demagnetization, the phase transition thermodynamics ofα-Fe and Nd_2Fe_(14)Band weakened exchange coupling between soft and hard magnetic phase grains have beendiscussed.
High energy ball milling (HEBM) makes the powder particles much refine and homogeneous,increases the energy of system, thus improves nucleation rate of nanograins during the period ofcrystallization. Hot pressing also improves nucleation rate of nanograins and suppresses the longrange diffusion of atoms. Owing to the effects of both HEBM and hot pressing on grain size, thenanograins of (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets are much finer and its grain size distribution is muchmore centralized. In the nanocrystalline (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets with HEBM and hotpressing, where the exchange coupling between grains is much stronger and would make theircoercivities decrease, but it keep the coercivity value up to 1631kA/m, almost equal to that ofnanocrystalline (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets without HEBM and hot pressing, 1616kA/m,because of its grain size distribution more centralized and the amount of grain interface relativelymuch greater than the latter. With increasing the weight percent of FeCo, the saturationmagnetization of nanocomposite magnets (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo increases but the coercivitydecreases. In this paper on the (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets with a limitedamount ofα-Fe phase no more than 20 weight percent, the self-pinning is dominant during theprocess of demagnetization with relatively high coercivity value. In the(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets with 10% and 20% of weight percent ofFeCo, the coercivity value is 1357kA/m and 801kA/m respectively. While in the(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets withα-Fe phase more than 30 weight percent,the role of dipolar reaction will play an important role and the nucleation mechanism is dominantduring the process of demagnetization, thus the coercivity of nanocomposite magnets decreasesabruptly. And the coercivity value of nanocomposite magnets with 40 weight percent of FeCo isonly 9.1% of that of nanocrystalline single phase magnets. In the prepared nanocomposite magnets,the grain size of soft magnetic phaseα-Fe, in the range of 40nm to 70nm, is more than that of hard magnetic phase Nd_2Fe_(14)B. This is owed to the big difference of thermodynamics betweenNd_2Fe_(14)B andα-Fe during the period of phase crystallisation, which forcesα-Fe to crystallizefirstly, thusα-Fe grains have much longer time to grow. This is an important reason contributing tothe poor coercivity of nanocomposite magnets especially with higher percent ofα-Fe phase.The defects near grain boundary could weaken the effect of exchange coupling betweengrains, resulting in a decreased coercivity of nanocomposite magnets. To fundamentally improvethe magnetic properties, new methods should been explored to weaken the effects of difference ofthermodynamics between Nd_2Fe_(14)B andα-Fe on the grain size of nanocomposite magnets, thus tokeep the grain size of soft magnetic phase less than the range of hard magnetic one. And newmethods should been taken to reduce the amount of defects, such as structure free volumes nearinterface between grains.
Key Words: nanocomposite magnets, melt spinning, high energy ball milling, hotpressing,thermodynamics, coercivity, exchange coupling
高能球磨使粉末颗粒细化和均匀化,同时增加系统的能量,在晶化过程中提高了晶化的形核率,热压也具有提高晶化形核率的作用并抑制原子的长程扩散,这两者使晶粒更为细小,晶粒尺寸分布更为集中。虽然高能球磨热压纳米晶(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6单相磁体晶粒之间的交换耦合作用较强,一定程度地使磁体矫顽力下降,但磁体晶粒尺寸分布集中,晶粒界面相对较多,磁体仍能保持高的矫顽力,达到1631kA/m,与未经过高能球磨热压的(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6磁体的矫顽力1616kA/m相当。随着(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo复合磁体中FeCo质量百分比的增加,磁体饱和磁化强度提高,矫顽力下降。通过讨论分析,在(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo纳米复合磁体中软磁相质量百分比小于20%时,反磁化过程中自钉扎作用较为突出,复合磁体矫顽力高,在FeCo的质量百分比为10%和20%时,(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo复合磁体的矫顽力分别达到1357kA/m和801kA/m;当软磁相质量百分比较大于30%时,磁偶极作用比较突出,反磁化过程以在软磁相晶粒内形核机制为主,复合磁体矫顽力低,当FeCo质量百分比为40%时,复合磁体的矫顽力仅为(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6单相磁体的9.1%。复合磁体软磁相α-Fe的晶粒尺寸偏大,在40nm~70nm之间,比硬磁相Nd_2Fe_(14)B的晶粒尺寸大,这是由于Nd_2Fe_(14)B和α-Fe在从非晶向纳米晶相变过程中热力学特性的差异决定的,α-Fe先于Nd_2Fe_(14)B形核晶化,导致α-Fe晶粒生长时间长,晶粒尺寸比较大,这也是复合磁体矫顽力低的一个主要原因。
Nd-Fe-B基纳米复合磁体的软、硬磁相晶粒界面附近存在的缺陷会减弱晶粒之间的交换耦合作用,从而造成复合磁体矫顽力下降。为从根本上提高纳米复合磁体的磁性能,应探索新的方法以减弱Nd_2Fe_(14)B和α-Fe在相变过程中热力学特性差异对复合磁体晶粒尺寸的影响,将软磁相α-Fe的晶粒尺寸控制在小于硬磁相晶粒尺寸的范围内,同时减少晶粒之间界面中原子空位等缺陷。
(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo magnets were prepared using the procedure of high energy ballmilling, hot pressing following melt spinning. The dependence of magnetic properties on weightpercent of soft magnetic phase and grain size has been studied. The nucleation and self-pinningduring the process of demagnetization, the phase transition thermodynamics ofα-Fe and Nd_2Fe_(14)Band weakened exchange coupling between soft and hard magnetic phase grains have beendiscussed.
High energy ball milling (HEBM) makes the powder particles much refine and homogeneous,increases the energy of system, thus improves nucleation rate of nanograins during the period ofcrystallization. Hot pressing also improves nucleation rate of nanograins and suppresses the longrange diffusion of atoms. Owing to the effects of both HEBM and hot pressing on grain size, thenanograins of (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets are much finer and its grain size distribution is muchmore centralized. In the nanocrystalline (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets with HEBM and hotpressing, where the exchange coupling between grains is much stronger and would make theircoercivities decrease, but it keep the coercivity value up to 1631kA/m, almost equal to that ofnanocrystalline (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6 magnets without HEBM and hot pressing, 1616kA/m,because of its grain size distribution more centralized and the amount of grain interface relativelymuch greater than the latter. With increasing the weight percent of FeCo, the saturationmagnetization of nanocomposite magnets (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo increases but the coercivitydecreases. In this paper on the (Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets with a limitedamount ofα-Fe phase no more than 20 weight percent, the self-pinning is dominant during theprocess of demagnetization with relatively high coercivity value. In the(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets with 10% and 20% of weight percent ofFeCo, the coercivity value is 1357kA/m and 801kA/m respectively. While in the(Nd_(10.5)Pr_(2.5))Fe_(80)Nb_1B_6/FeCo nanocomposite magnets withα-Fe phase more than 30 weight percent,the role of dipolar reaction will play an important role and the nucleation mechanism is dominantduring the process of demagnetization, thus the coercivity of nanocomposite magnets decreasesabruptly. And the coercivity value of nanocomposite magnets with 40 weight percent of FeCo isonly 9.1% of that of nanocrystalline single phase magnets. In the prepared nanocomposite magnets,the grain size of soft magnetic phaseα-Fe, in the range of 40nm to 70nm, is more than that of hard magnetic phase Nd_2Fe_(14)B. This is owed to the big difference of thermodynamics betweenNd_2Fe_(14)B andα-Fe during the period of phase crystallisation, which forcesα-Fe to crystallizefirstly, thusα-Fe grains have much longer time to grow. This is an important reason contributing tothe poor coercivity of nanocomposite magnets especially with higher percent ofα-Fe phase.The defects near grain boundary could weaken the effect of exchange coupling betweengrains, resulting in a decreased coercivity of nanocomposite magnets. To fundamentally improvethe magnetic properties, new methods should been explored to weaken the effects of difference ofthermodynamics between Nd_2Fe_(14)B andα-Fe on the grain size of nanocomposite magnets, thus tokeep the grain size of soft magnetic phase less than the range of hard magnetic one. And newmethods should been taken to reduce the amount of defects, such as structure free volumes nearinterface between grains.
Key Words: nanocomposite magnets, melt spinning, high energy ball milling, hotpressing,thermodynamics, coercivity, exchange coupling
引文
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