【作者】 张士岩；

【导师】 董远达； 徐晖；

【作者基本信息】 上海大学 ， 材料学， 2008， 博士

【摘要】 纳米晶复合永磁材料是由硬磁相和软磁相在纳米尺度内,通过两相间的交换耦合作用复合而成,它由硬磁相提供高的矫顽力,软磁相提供高的饱和磁化强度,具有高剩磁、高最大磁能积以及低稀土含量等优点,有望发展成为新一代高性能的稀土永磁材料。为改善纳米晶复合永磁合金的微观结构以提高磁性能,本论文从合金成分的优化和晶化工艺的改进两个方面,对纳米晶复合Nd₂Fe₁₄B/α-Fe永磁合金进行了较为深入的研究。通过采用X射线衍射（XRD）、差示扫描量热仪（DSC）、透射电镜（TEM）、三维原子探针技术（3DAP）、振动样品磁强计（VSM）和穆斯堡尔谱仪等分析仪器和手段,重点研究了微合金化元素Zr、Nb、Ga对纳米晶复合Nd₂Fe₁₄B/α-Fe永磁合金的晶化行为、微观结构、磁性能及其温度稳定性的影响规律和作用机理.通过对快淬Nd₂Fe₁₄B/α-Fe永磁合金进行脉冲磁场退火,探讨了磁退火对合金的晶化过程、交换耦合作用、显微结构以及磁性能的影响机制,寻求到一种新的有效提高纳米晶复合永磁材料磁性能的晶化工艺方法。本文的主要研究结果如下:在纳米晶复合Nd_9.5Fe_79-xCo₅Zr_xB_6.5（X=0～4）合金中,Zr元素的添加优化了合金的晶化过程,抑制了软磁相晶粒的长大,改善了材料的微结构,从而增强软、硬磁相之间的交换耦合作用,有效提高合金的磁性能。同时,Zr在一定程度上提高了磁体的温度稳定性,改善了磁体的不可逆磁通损失。制备的纳米晶复合Nd_9.5Fe₇₆Co₅Zr₃B_6.5粘结磁体的温度系数α=—0.13%/℃,β=—0.35%/℃,在150℃环境温度下时效100h后,磁体的不可逆磁通损失δ_im=—4.50%。Nb元素的添加,明显提高了纳米晶复合Nd_9.5Fe_79-xCo₅Nb_xB_6.5（X=0～3）合金的矫顽力,细化了合金晶粒尺寸,从而有利于减小材料内部的散磁场,改善了合金的温度稳定性。3DAP研究结果发现,在纳米晶复合Nd_9.5Fe₇₇Co₅Nb₂B_6.5永磁合金中,添加的Nb原子在晶间产生明显的富集,形成了晶间NbFeB相,从而增强了硬磁相的磁晶各向异性场,提高了合金的磁性能。在纳米晶复合Nd_8.5Fe_77.6-xCo₅Zr_2.7Ga_xB_6.2（X=0～1）永磁合金中,添加微量的Ga元素后,提高了合金的居里温度,从而改善合金的温度系数和不可逆磁通损失。快淬速度为18m/s的合金经710℃退火处理,制备的纳米晶复合Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2粘结磁体具有优异的磁性能,J_r=0.73T,_iH_c=643kA/m,（BH）_max=82kJ/m³,α=—0.095%/℃,β=—0.35%/℃,δ_im=—4.06%。对快淬非晶Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2合金进行脉冲磁场退火发现,同常规退火相比,磁退火降低了合金的最佳退火温度,提高了非晶晶化时的形核率,改善了合金的微结构,从而增强了软、硬磁性晶粒间的交换耦合作用,明显提高了合金的磁性能,经670℃磁退火后合金具有最佳的磁性能,即_iH_c=586kA/m,J_r=1.01T,（BH）_max=138kJ/m³,最大磁能积比常规退火提高15%。对Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2合金在居里温度以下进行脉冲磁场退火发现,磁退火后样品的磁性能明显提高,尤其是剩磁和最大磁能积,其中最大磁能积较常规退火最大可提高24.8%。利用脉冲磁场在合金的居里温度以下进行退火为制备高性能纳米晶复合永磁材料提供了一种新的工艺方法。更多还原

【Abstract】 Nanocomposite magnets consisting of a hard magnetic phase with high anisotropy and a soft magnetic phases with high saturation magnetization have been widely studied experimentally and theoretically owing to their unusually high remanence,energy product and low cost.The high remanence in nanocomposites arises from the exchange coupling between the magnetically hard and soft phases.In this dissertation,the influences of alloy compositions and crystallization techniques on the magnetic properties and microstructure for nanocomposite Nd₂Fe₁₄B/α-Fe permanent magnets have been investigated clearly.X-ray diffraction （XRD）,differential scanning calorimeter（DSC）,transimission electron microscope （TEM）,three-dimensional atom probe（3DAP）,vibrating sample magnetometer（VSM） and Mrssbauer spectroscopy have been employed.Special attention has been paid to the effects of Zr,Nb and Ga on the crystallization behavior,microstructure,magnetic properties and temperature stability for nanocomposite Nd₂Fe₁₄B/α-Fe magnets. Pulsed magnetic field was employed when annealing,and the effects of manetic annealing on the exchange coupling,microstructure and magnetic properties for nanocomposite Nd₂Fe₁₄B/α-Fe magnets have been studied in detail.The results show that:The addition of Zr changed the crystallization behavior of amorphous phase, inhibited the grain growth,and thus enhanced the exchange coupling bwteen magnetically soft and hard phases for nanocomposite Nd_9.5Fe_79-xCo5Zr_xB_6.5（x=0～4） magnets.The temeperature stability and irreversible flux aging loss were improved by proper Zr addition.The optimal magnetic properties for nanocomposite Nd_9.5Fe₇₆Co₅Zr₃B_6.5 magnet are:α=-0.13%/℃,β=-0.35%/℃andδ_irr=-4.50%, respectively. The addition of Nb element improved obviously the coercivity of nanocomposite Nd_9.5Fe_79-xCo₅Nb_xB_6.5（x=0～3） magnets.Nb addition made grains smaller and led to the homogenization and regularization of grains,which is benifical to reduce inner dispersal magnetic field and improve the thermal stability of the alloys.An intergranular NbFeB phase with Nb-enriched atoms between magnetic phases was observed by 3DAP technique,which is the main reason for microstructure refinement and magnetic properties improvement for nanocomposite Nd_9.5Fe₇₇Co₂Nb₂B_6.5 alloy.Proper Ga addition improved the Curie temperature of nanocomposite Nd_8.5Fe_77.6-xCo₅Zr_2.7Ga_xB_6.2（x=0～1） alloys,thus the temperature coefficient and irreversible flux aging loss were improved.The optimal magnetic properties of J_r= 0.73T,_iH_c=643kA/m,（BH）_max=82kJ/m³,α=-0.095%/℃,β=-0.35%/℃andδ_irr =-4.06%were obtained for nanocomposite Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2 alloy.The melt-spun Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2 alloy ribbons were annealed in a pulsed magnetic field.The results revealed that the optimal annealing temperature was degraded and the nucleation rate was elevated by magnetic annealing.The magnetic annealing also led to a refined microstructure and an enhanced exchange coupling between magnetically hard and soft phases.The optimal magnetic properties of _iH_c= 586kA/m,J_r= 1.01T and（BH）_max=138kJ/m³ were obtained for nanocomposite Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2 alloy ribbons annealed with a pulsed magnetic field at 670℃for 4rain.The（BH）_max was enhanced by 15%compared with that of samples without magnetic annealing.The melt-spun Nd_8.5Fe₇₇Co₅Zr_2.7Ga_0.6B_6.2 alloy ribbons were annealed in a pulsed magnetic field at temperatures near the Curie temperature of the alloy.The results demonstrated that the remanence and the maximum energy product were obviously improved when annealed below the Curie temperature of the alloy.The （BH）_max was enhanced by 24.8%compared with that of samples without magnetic annealing at 300℃.It provides a new way to improve the magnetic properties of nanocomposite permanent magnets produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys.更多还原