【作者】 崔熙贵；

【导师】 严密；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2009， 博士

【摘要】 烧结Nd-Fe-B磁体是具有高磁性能和高性价比的新一代稀土永磁材料,广泛应用于各种高新技术领域。然而,矫顽力低、温度稳定性差和易腐蚀的缺点严重限制了其进一步发展和在各种重要领域的应用。烧结Nd-Fe-B材料的性能除与磁体成分直接相关外,显微结构也是一个重要的影响因素。因此,如何通过调整磁体显微结构来提高其综合性能是一个兼具重要理论和实际意义的科学问题。从调整和改善烧结Nd-Fe-B材料的显微结构出发,本文系统研究了晶界改性对材料磁性能、耐腐蚀性能和温度稳定性的影响,建立了纳米添加物在磁粉表面的理想分布模型,结合热力学计算,揭示了不同纳米添加物对主相Nd₂Fe₁₄B晶粒尺寸、分布形态及主相-富Nd相的界面结构的影响机制及其对磁体性能的作用机理;同时,通过气流磨过程中回收超细磁粉的晶界添加,明确了磁体性能和显微结构随添加量的变化规律,为降低磁体成本提供理论依据;此外,在研究磁体温度系数随其内禀性能和结构因素变化规律的基础上,掌握了高温度稳定性烧结Nd-Fe-B磁体的设计原则,并成功制备出低温度系数磁体。本文的主要研究结果如下:纳米添加物的晶界改性能够同时提高烧结Nd-Fe-B磁体的磁性能和耐腐蚀性能。研究发现,各种纳米粉的添加都能够提高磁体的矫顽力H_cj,其中纳米Cu、SiO₂、ZnO和AlN的晶界添加能够有效细化主相晶粒,并使其分布更加均匀,有效增大了反磁化畴的形核场;纳米Cu、Zn、SiO₂和ZnO的晶界添加还能改善晶界富Nd相的性质,使其在主相周围分布更加均匀,减弱了晶粒间的磁交换耦合作用;而Dy₂O₃纳米粉的晶界添加,能够使主相晶粒表面磁硬化。纳米Cu、Zn、SiO₂和ZnO的晶界添加促进磁体的烧结致密化,能够有效提高密度,从而提高磁体的剩磁B_r和磁能积（BH）_max;而纳米Dy₂O₃和AlN的作用则恰恰相反。此外,纳米Cu、Zn、SiO₂、ZnO和AlN的晶界添加能够提高晶界相的电极电位和优化显微结构,从而抑制磁体的晶间腐蚀,提高了磁体的耐腐蚀性能。根据热力学分析,阐述了各种纳米粉的添加对晶界富Nd相组成和性质的影响机制。研究发现,添加的金属Cu纳米粉和Zn纳米粉,在烧结过程中主要与晶界富Nd相发生化学反应,形成新的晶界相或溶入富Nd相中,而并没有进入主相。热力学计算表明,在晶界中添加的SiO₂和ZnO氧化物纳米粉与Nd在烧结温度下反应生成稳定的Nd₂O₃颗粒,以及元素Si或Zn;Nd₂O₃颗粒抑制了晶粒长大,Si和Zn改善了富Nd相的性质,进而优化了磁体显微结构。根据规则熔体Miedema理论模型计算,晶界添加的Dy₂O₃纳米粉与富Nd相反应还原生成的Dy易于扩散进入主相晶粒表面生成磁晶各向异性场更大的Dy₂Fe₁₄B,起到表面磁硬化的作用。采用气流磨过程中回收的超细磁粉的晶界添加制备了高性能低成本烧结Nd-Fe-B磁体,并揭示了超细磁粉添加量对磁体性能和显微结构的作用机制。研究表明,气流磨过程中回收超细磁粉的成分以稀土元素Nd、Pr和Dy为主,在晶界中添加能够提高磁体磁性能,尤其是矫顽力H_cj。超细磁粉的晶界添加能够减缓磁体液相烧结过程中溶质的扩散传质速度,抑制主相晶粒长大,细化了主相晶粒,提高了反磁化畴的形核场;同时有助于在主相晶粒表面形成高磁晶各向异性场的Dy₂Fe₁₄B和Pr₂Fe₁₄B,能够起到表面磁硬化的作用,从而显著提高磁体的矫顽力H_cj;在添加量低于5 wt%时,也能提高磁体的磁能积（BH）_max。此外,超细粉的晶界添加还能有效降低磁体的矫顽力温度系数β和磁通不可逆损失h_irr,提高了烧结Nd-Fe-B磁体的温度稳定性。建立了烧结Nd-Fe-B磁体温度系数与内禀性能和显微结构参量的定量关系,为设计低温度系数磁体提供了理论指导,并制备出高温度稳定性烧结Nd-Fe-B磁体。通过理论分析建立了矫顽力可逆温度系数β与磁晶各向异性场H_A和显微结构参量c/N_eff的定量关系,发现H_A越高、c/N_eff越大,β就越小。在此基础上,通过成分设计和磁体显微结构优化,成功制备了低温度系数的烧结Nd-Fe-B磁体,在20～150℃温度区间,β仅为-0.385%/℃;且在220℃时,磁体的矫顽力H_cj仍有557 kA/m,表明该磁体具有很高的温度稳定性。此外,根据磁体的最高工作温度与β的理论关系阐述了烧结Nd-Fe-B磁体矫顽力温度系数对其最高工作温度的影响规律。在相同的H_cj下,最高工作温度随β的降低而显著升高,表明降低β是制备高温度稳定性磁体行之有效的方法。更多还原

【Abstract】 Sintered Nd-Fe-B magnet is a new generation of rare earth permanent magnetic material with outstanding magnetic properties and high cost performance,and has been applied widely in various hi-tech fields.However,the disadvantages of low coercivity,poor thermal stability,and easy corrosion seriously restrict its further development and application.In addition to the magnet composition,microstructure is also an important factor influencing the properties of sintered Nd-Fe-B magnets.Therefore,how to improve its overall properties through adjusting microstructure of the magnet is of both theoretical and practical significance.In this dissertation,aiming to adjust and modify the microstructure of sintered Nd-Fe-B materials,the effects of intergranular phase modification on the magnetic properties,corrosion resistance and thermal stability were systematically investigated.An ideal distribution model of nano-sized additives on the surface of magnetic powders was proposed.Several possible influencing mechanisms of different nano-sized additives on grain size and distribution of the main phase Nd₂Fe₁₄B,on the interface structure of main phase/Nd-rich phase,and on the properties of magnet were revealed on the basis of thermodynamic calculation.Meanwhile,the variation law of the properties and microstructure of magnet with additive content of ultrafine powders were made clear by intergranular addition.Furthermore,through investigating the variation of temperature coefficient with intrinsic property and microstructral parameter,the design principle of sintered Nd-Fe-B magnets with high thermal stability was suggested,and the magnet with low temperature coefficient was successfully prepared.The main results are as follows:Both the magnetic properties and corrosion resistance of sintered Nd-Fe-B magnets can be improved simultaneously by the intergranular phase modification of nano-sized additives.The results show that the additions of various nanopowders can all improve the coercivity H_cj of magnets.The intergranular additions of Cu,SiO₂,ZnO and AIN nanopowders can refine the grains of main phase effectively and make them distribute more homogeneously,increasing the nucleation field of reversed domain.The intergranular additions of Cu,Zn,SiO₂ and ZnO nanopowders can also improve the characteristics of Nd-rich phase,and make it distribute more homogeneously around the main phase,weakening the magnetic exchange coupling interaction. The intergranular additions of Dy₂O₃ nanopowders can harden the grain surface of the main phase.The intergranular additions of Cu,Zn,SiO₂ and ZnO nanopowders can promote the sintering densification of magnets,increasing the magnet density effectively,thereby improving the remanence B_r and magnetic energy product（BH）_max;whereas the additives of Dy₂O₃ and AlN nanopowders played an opposite role.Furthermore,all the intergranular additions of Cu, Zn,SiO₂,ZnO and AlN nanopowders can increase the electrode potential of the intergranular phase and optimize the microstructure,consequently inhibiting intergranular corrosion and thus improving the corrosion resistance of magnet.On the basis of thermodynamic analysis,the influencing mechanism of various nanopowders addition on the composition and the physico-chemical characteristics of the intergranular Nd-rich phase have been described.It is found that the added Cu and Zn nanopowders reacted mainly with the intergranular Nd-rich phase to form new grain boundary phase or dissolved into the Nd-rich phase during sintering.Both of them do not enter the main phase.Thermodynamic calculations show that SiO₂ and ZnO nanopowders added into grain boundaries react with Nd-rich phase and form Nd₂O₃ particles,as well as the elements Si or Zn at the sintering temperature.These Nd₂O₃ particles can inhibit the grain growth of the main phase,while Si and Zn can improve the physico-chemical characteristics of the Nd-rich phase, consequently optimizing the microstructure.According to the Miedema theoretical models for the regular melt,Dy formed through reduction reaction between Dy₂O₃ and the Nd-rich phase can easily diffuse into the main phase grain surface to form Dy₂Fe₁₄B with higher magnetocrystalline anisotropy field,playing a role of surface magnetic hardening.The high performance,low cost sintered Nd-Fe-B magnet has been prepared through intergranular addition of ultrafine powders recycled during jet-milling process.The influencing mechanism of additive content of ultrafine powders on the properties and microstructure is also revealed.The results show that the composition of ultrafine powders consists mainly of rare earth elements of Nd,Pr and Dy.The intergranular addition can improve the magnetic properties, especially the coercivity H_cj.The intergranular addition of ultrafine powders can decrease the diffusion mass transfer rate of solute during sintering,hindering the grain growth of the main phase,and thus improving the nucleation field of reversed domain.The intergranular addition of ultrafine powders is helpful to the formation of Dy₂Fe₁₄B and Pr₂Fe₁₄B with higher magnetocrystalline anisotropy field on the grain surface of the main phase,playing a role of surface magnetic hardening,consequently improving the coercivity H_cj of magnet significantly. When the additive content is below 5 wt%,the magnetic energy product（BH）_max can also be improved.Moreover,the intergranular addition of ultrafine powders also results in the decrease of the temperature coefficient of coercivityβand the irreversible flux loss h_irr,which improves the thermal stability of sintered Nd-Fe-B magnets.The quantitative relationship between temperature coefficient and intrinsic property and microstructral parameter has been established,which provide a theoretical guidance for designing low temperature coefficient magnet.Sintered Nd-Fe-B magnet with high thermal stability has also been prepared.The relationship between temperature coefficient of coercivityβand magnetocrystalline anisotropy field H_A and microstructral parameter c/N_eff have been established through theoretical analysis,it is found that the higher H_A is and the bigger c/N_eff is, the smallerβis.Based on this,the sintered Nd-Fe-B magnet with low temperature coefficient have been successfully prepared through composition design and microstructure optimization,βis only -0.385%/℃at the temperature interval of 20～150℃.Its coercivity H_cj can reach 557 kA/m at 220℃,indicating that the magnet possesses very high thermal stability.Furthermore, according to the theoretical relationship between highest operating temperature andβ,the influence of temperature coefficient of coercivity for sintered Nd-Fe-B magnets on the highest operating temperature is described.At the same H_cj,the highest operating temperature increases significantly with the decrease ofβ,suggesting that decreasingβis an effective method to prepare magnet with high thermal stability.更多还原