【作者】 张龙凤；

【导师】 刘颖；

【作者基本信息】 四川大学 ， 材料学， 2007， 硕士

【摘要】 双相纳米晶永磁材料以其优异的性能引起了广泛关注，如何制备具有高性能且廉价的双相纳米晶永磁材料是目前研究的热点之一。本文针对纳米复相永磁材料发展中存在的问题，采用快淬、传统热处理及放电等离子烧结工艺制备了纳米复相Fe₃B／Nd₂Fe₁₄B永磁体，并在此基础上采用XRD、DSC、AFM、SEM等方法研究了制备工艺参数及合金元素对纳米复相Fe₃B／Nd₂Fe₁₄B永磁材料的组织结构和性能的影响。Nd_4.5Fe₇₇B_18.5三元合金在较低淬速下，晶态相含量较高，这些晶核在随后的热处理过程中很容易长大、粗化，合金磁性能较低。当快淬速度较高时结晶度很低，退火过程中形核中心减少，形核率降低，导致Fe₃B粒子奇异长大，不利于磁性能的提高。只有在适当的快淬速度8m／s下才能获得细小均匀的晶粒组织和最佳磁性能。Nd_14.5Fe₇₇B_18.5合金在较低晶化温度下，晶化不完全，存在部分非晶相，交换耦合作用较弱，磁性能较低。但温度过高会导致软磁相Fe₃B急剧长大，也不利于磁性能的提高。合金的磁性能随晶化时间的延长先增而减，Nd_14.5Fe₇₇B_18.5合金的最佳热处理工艺参数为：710℃×10min。本文研究了Nd、B、Zr、Cu和Co元素对合金性能的影响。Nd_xFe_81.5-xB_18.5合金随着Nd含量的增加磁性能先增而减，Nd=4.5at％时达到最高，B_r=0.80T，H_cb=206kA／m，H_cj=259A／m，（BH）_m=48kJ／cm³。Nd含量过高会抑制硬磁相Nd₂Fe₁₄B和软磁相Fe₃B的出现，提高非磁性相Nd₂Fe₂₃B₃的稳定性。B含量和Zr含量的增加有利于非晶态物质的形成和稳定。B含量超过一定量后，晶化过程中会出现Nd_1.1Fe₄B₄非磁性相，且随着B含量的增加逐渐增多，它的存在稀释了软、硬磁相间的交换耦合作用，导致磁性能降低。当硼B=18.5at％的纳米晶Nd_4.5Fe_95.5-yB_y永磁合金具有最佳磁性能。Zr的添加恶化了四元合金的磁性能，较高含量的Zr会抑制Nd₂Fe₁₄B和Fe₃B的出现，提高非磁性相Nd₂Fe₂₃B₃的稳定性。Cu元素有利于降低合金的非晶形成能力。Cu原子形成了Cu串晶，分布在Fe₃B晶体和非晶之间，与Fe₃B粒子直接接触，为Fe₃B的析出提供了形核核心，Fe₃B的析出量增多，交换耦合作用加强，磁性能提高。Co元素与Fe原子发生置换作用溶入Nd₂Fe₁₄B相和Fe₃B相，使合金的磁性能得到提高。综合比较，传统热处理下添加Cu元素的四元合金可获最佳磁性能。与传统热处理相比，放电等离子烧结法促进了快淬NdFeB粉末的晶化，降低了晶化温度，缩短了晶化时间，因而可加强交换耦合作用，提高磁性能。Nd_4.5Fe_76.9Cu_0.1B_18.5合金快淬粉末经50MPa，670℃×5min放电等离子烧结后，致密化程度明显提高，获得最佳磁性能：磁体密度为6.89g／cm³，B_r=0.95T，H_cb=233kA／m，H_cj=275kA／m，（BH）_max=69kJ／cm³。更多还原

【Abstract】 Double-phase nanocomposite permanent magnet materials cause wide attention due to excellent magnetic properties. How to produce permanent magnets with high properties and cheapness is one of the hot points at present.Considering the problem existing in the development of nanocrystalline composite magnets, nanocomposite Fe₃B/Nd₂Fe₁₄B permanent magnets are prepared by rapidly quenching, post heat treatment and Spark plasma sintering （SPS） in this paper. Effect of preparation parameters and component on microstructure and magnetic properties of nanocomposite permanent magnets have been investigated by X-ray diffraction （XRD）, differential scanning calorimetry （DSC）, and atomic force microscopy （AFM）, scanning electron microscopy （SEM）.For Nd_4.5Fe₇₇B_18.5, at lower quenching speed, the amount of nuclear increases. It is easy to grow coarsely for crystallized compound, and result in worsen the magnetic properties. While at the higher quenching speed, and the smaller nucleation center the as-spun alloy has during the crystallization process. As a result, the magnetic properties are low because of coarse grain of Fe₃B. Only when the quenching speed is 8m/s, which has a fine and homogenous microstructure and excellent combination of magnetic properties are obtained.When the crystallization temperature is low, the magnetic phases precipitate incompletely, and amorphous phases existing in Nd_4.5Fe₇₇B_18.5 ternary alloys. And thereby the exchange coupling interaction is weak and the magnetic properties are low. When the crystallization temperature is high, the exchange coupling interaction is weak due to the coarse grain of Fe₃B, and the magnetic properties such as B_r and （BH）_m degraded. Magnetic properties, such as B_r, H_cj and （BH）_m, increase with the crystallization time increasing at first, and then decrease, that is to say it exists a maximum. As a result, the optimal magnetic properties of Nd_4.5Fe₇₇B_18.5 are obtained after dynamic crystallization heat treatment at 710℃for 10min.The influence of Nd,B,Zr, Cu and Co to the magnetic properties are also discussed in this paper. With the increasing of atom percent of Nd for Nd_xFe_81.5-xB_18.5 alloy, the magnetic properities first increase and then decrease. When atom percent of Nd is 4.5, the magnetic properties reach the maximum: B_r =0.80T, H_cb=206kA/m, H_cj=259 kA/m and （BH）_m=48kJ/cm³. At the same time,the high content of Nd retards the formation of Fe₃B and Nd₂Fe₁₄B, promote the stability of the nonmagnetic phase Nd₂Fe₂₃B₃.Increasing the B content or Zr content can promote the formation of amorphous phases. But if the content of B exceeds a certain level, nonmagnetic phase Nd_1.1Fe₄B₄ occurs during crystallization of the alloys, and increase with the increase of atom percent of B. As a result, the exchange coupled interaction between the grains becomes weak and the magnetic properties decrease with the B content increasing. The optimal magnetic properties of Nd_4.5Fe_95.5-yB_y are obtained when y=18.5. The magnetic properties decrease by adding Zr element.The high content of Zr can also retard the formation of Fe₃B and Nd₂Fe₁₄B and promote the stability of the nonmagnetic phase Nd₂Fe₂₃B₃.Adding Cu can reduce the formation of amorphous phases, which is helpful for the alloy crystallising at low temperature. It can be clearly seen that the Cu atoms form clusters. They are located at the interface between the Fe₃B crystals and the amorphous matrix, and each cluster is in direct contact with the Fe₃B particles. Those clusters serve ad heterogeneous nucleation sites for the Fe₃B particles,thereby increasing the number density of the particles. The exchange coupled interaction enhances and the magnetic properties increases too.The element Co can dissolve in Nd₂Fe₁₄B and Fe₃B phase by replacing Fe atom, which improve the magnetic properties. The Nd_4.5Fe_76.9Cu_0.1B_18.5 quartemary alloy obtained the optimal properties by adding Cu after treated by conventional heat treatment in the mass:B_r=0.82T, H_cb=219kA/m,H_cj=277kA/m,（BH）_max=53kJ/cm³.It has been determined that, compared with the conventional heat treatment,spark plasma sintering can promote crystallization of melt-spun NdFeB powder from amorphous phase, decrease the crystallization temperature and shorten the crystallization time. Moreover SPS can enhance the exchange coupled interaction between the grains and increase the magnetic properties. As a result, the optimal magnetic properties of Nd_4.5Fe_76.9Cu_0.1B_18.5 (B_r=0.95T, H_cb=233kA/m,H_cj=275kA/m,（BH）_max=69kJ/cm³,the density is 6.89g/cm³) are obtained after sintered at 50MPa, 670℃for 10min.更多还原