Magnetic Properties of Nd-Fe-B Sintered Magnets by Chemical Reagents Adding Method
Song Xiaoming Zhang Ting'an Dou Zhihe Pei Wenli Jv Xiangming Zhou Lian
School of Materials Science and Engineering,Northeastern University
Ningbo Qiyuan New Materials Technology Co.,Ltd
School of Metallurgy,Northeastern University
Abstract:
Nd-Fe-B materials were applied widely in civil,aerospace and other fields.High properties Nd-Fe-B sintered magnets were required sustainably in the market.Most commercial magnets were cut from large blocks.Magnetic consistency was one of the fundamental market requirements.There were many reasons that affected the internal consistency of blank magnets.One of the reasons was that magnetic powders and chemical agents were mixed uneven.Nd-Fe-B powder was chemically active and easy to oxidize.The whole process of making powders was carried out in nitrogen atmosphere.There was no proper equipment,so it was very difficult to add chemical reagent evenly.The researchers were unable to research further due to inadequate equipment.In order to improve the magnetic properties and internal consistency of sintered blank magnets,new chemical reagent addition technology and new additive equipment should be developed.According to the characteristics of Nd-Fe-B powders preparation,at present,it was feasible to add multiple times and add less each time,and atomize the reagent to further improve the mixing uniformity.After a lot of experiments,new atomizing additive technology and the patented product "the auto fog-doser" were developed.The dosage,frequency,interval time and times were adjusted through the special programmable logic controller(PLC).Specially designed spiral-percussion nozzle and the corresponding pipeline combination improved the atomization effect of the reagent.The auto fog-doser worked synchronously with the jet-mill to realize that the whole surface of magnetic powder adsorbed chemical reagent.At the same time,the mixing time was reduced from 5 h to 1 h,which reduced the oxygen absorption of magnetic powder in this process.In actual operation,the spraying frequency was set for one or several times according to the fixed frequency of the jet-mill.The magnets were obtained by the dual-alloy technology and the commonly techniques of strip-casting,hydrogen crushing,jet-mill grinding,orientation molding and vacuum sintering.The cylindrical magnet of Φ10 mm×10 mm was gotten out in the center and eight apexes angle of the rectangular magnets;the magnetic properties were tested for calculating the change of different locations in the same magnets.The auto fog-doser was used in the magnets preparation process.The mixing uniformity of magnetic powder and chemical reagent was greatly improved with the increase of dosing frequency and the increase of the reagent layers.The remanence Br increased from 1.403 T to 1.426 T,with an increase of 1.64%.The maximum magnetic energy product(BH)max increased from 394.66 to 406.99 kJ·m-3,with an increase of 3.12%.The coercivity Hcj decreased from 1377.9 to 1373.1 kA·m-1,with an decrease of 0.35%.The changes of magnetic properties of the cylindrical Φ10 mm×10 mm in the center and eight apex angles of the rectangular magnets were:ΔBr=0.77%,ΔHcj=0.76%,Δ(BH)max=0.84%,the values were all less than 1%.The internal magnetic properties of the magnet had good consistency.The new atomizing additive technology and the patented product "the auto fog-doser" could improve the mixing uniformity of magnetic powder and chemical reagent.The mixing time was reduced from 5 to 1 h to better protect the magnetic powder.Nd-Fe-B sintered magnets with better internal consistency and higher magnetic properties were obtained:Br increased 1.64%,(BH)max increased 3.12%,Hcj decreased 0.35%.The change values of magnetic properties in the same magnet were all less than 1%.The internal magnetic properties of the magnet had good consistency.The internal magnetic properties consistency improvement of the blank was significant for small commercial magnets.The auto fog-doser had been applied in Nd-Fe-B sintered magnets fields.
根据以上问题,项目组自行开发出“自动雾化加剂设备”(已申请发明专利)。通过改变化学试剂加入方式,研究了磁粉和化学试剂混合均匀性对磁体内部一致性、剩磁等磁性能的影响,得到内部一致性更好、磁性能更高的烧结钕铁硼磁体:剩磁Br=1.426 T,内禀矫顽力Hcj=1373.1 k A·m-1,最大磁能积(BH)max=406.99 k J·m-3。开发的“自动雾化加剂设备”有助于生产高性能、高稳定性烧结钕铁硼磁体,已经得到广泛应用。
根据Br的关系式可以看出Br与f成正比。图4显示的是加剂方式1,2,3对应样品的Br,Hcj和(BH)max变化。随着加剂频率的提高:Br从1.403 T提高到1.426 T,提高了1.64%;(BH)max从394.66k J·m-3提高到406.99 k J·m-3,提高了3.12%;Hcj从1377.9 k A·m-1下降到1373.1 k A·m-1,只下降了0.35%。在取向成型工序中磁粉在磁场作用下向易取向方向转动,由于加剂频率的提高,使得化学试剂与磁粉混合均匀性更好,难取向的磁粉量减小,从而磁体取向度增加
[13];另外试剂对磁粉的保护也会更好,导致2∶14∶1主相的比例提高,两个方面都会提升剩磁和磁能积。
图4 不同加剂方式对应的磁体磁性能
Fig.4 Magnetic properties of magnets prepared by different adding methods
(1)Method 1;(2)Method 2;(3)Method 3
实验得到样品的磁性能:Br=1.426 T,Hcj=1373.1 k A·m-1,(BH)max=406.99 k J·m-3。该性能的烧结钕铁硼磁体已经实现批量生产。