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晶界渗Dy高矫顽力烧结NdFeB磁体耐蚀特性研究
中文摘要

摘要:NdFeB磁体具有高剩余磁化强度、高磁能积、高能量密度以及良好的机械性能,在信息电子、航空航天、能源交通、医疗保健等诸多领域得到了广泛应用。但是,目前研制的烧结NdFeB的矫顽力还未及理论值的1/3;而且,含稀土钕元素的NdFeB磁体因表面易氧化而导致耐腐蚀性下降。因此,提高矫顽力和改善耐蚀性已成为提高NdFeB磁体性能并拓宽其应用领域的关键。针对上述两方面问题,本文围绕高矫顽力、高耐蚀性NdFeB磁体的研制,开展了系统的研究工作,在研究NdFeB磁体矫顽力机制及获得高矫顽力NdFeB磁体的晶界渗Dy技术方法的基础上,深入探讨了影响高矫顽力NdFeB磁体的腐蚀行为及作用规律,进一步提出了改善烧结NdFeB磁体耐蚀性能的方法。本文的主要研究内容及结论如下: 1、在烧结NdFeB永磁材料的表面进行了晶界渗Dy处理以提高材料的矫顽力,研究了渗Dy处理后合金的磁性能、显微结构及Dy元素的晶界扩散机理,建立了晶界扩散方程,并在此基础上进一步优化了Dy元素晶界扩散工艺。利用电子束蒸发技术在磁体表面沉积了一层厚约39m的稀土镝(Dv)薄膜,在氩气气氛条件下于700~900℃进行扩散处理。结果表明,稀土Dy沿Nd₂Fe₁₄B主相晶界扩散,并在Nd₂Fe₁₄B晶粒表面形成一层(Nd,Dy)₂Fe₁₄B壳层。基于扩散方程,发现随着热处理温度升高,扩散系数增大,900℃热处理的样品扩散系数为2.7×10⁻⁷c㎡/s;建立了不同温度条件下扩散深度与扩散时间的关系,证实了在晶界扩散技术中Dy的最大扩散深度小于3㎜。在最佳温度900℃条件下扩散8h的样品,矫顽力H〓由1020kA·m⁻¹增加到1450kA·m⁻¹,剩余磁化强度B〓无明显变化,磁性能得到明显提高。该方法可改善烧结NdFeB磁体的耐蚀性能,也能有效降低约11wt%重稀土元素用量。 2、在制备出高矫顽力烧结NdFeB磁体的基础上,采用化学镀在该磁体表面成功沉积了单层和双层镍磷(Ni-P)镀层。结果表明:超声波辅助施镀的中性单层化学镀Ni-P镀层与基体结合力最强,抗热振次数可达12次,但耐蚀性较差;无超声波辅助施镀的单层酸性化学镀Ni-P层呈现非晶组织结构,稳定后的开路电位为﹣0.314V,具有较优越的耐蚀性。在此基础上,本论文进一步制备出了超声波辅助施镀中性化学镀Ni-P/酸性化学镀Ni-P双层膜,发现沉积有超声波辅助施镀中性化学镀Ni-P(30min)/酸性化学镀Ni-P(60min)双层镀膜的烧结NdFeB磁体抗热震次数可达6次、腐蚀电流仅为5.4×10⁻⁶A·cm⁻²,有效耐盐雾时间长达320h,综合耐腐蚀性能更优。 3、为了满足NdFeB磁体长效防腐的要求,本论文分别采用离子溅射真空镀膜技术、电子束蒸发技术在晶界渗Dy高矫顽力烧结NdFeB磁体的表面沉积了一层耐腐薄膜,并对其耐腐蚀性进行了系统研究。首先,采用室温下离子溅射真空镀膜方法,以O.8nm/s沉积速率镀膜3h,成功在NdFeB磁体表面沉积一层厚度约为9μm的非晶态Ni膜。该镀膜对磁体磁性能无明显影响,具有较高的膜一基结合强度,且腐蚀电流密度仅为2.2×10⁻⁷A·cm⁻²,表明抗腐蚀性能良好;但镀有Ni膜的NdFeB磁体在磁化状态下,腐蚀电流密度明显增加。为了降低磁体的腐蚀电流密度,采用电子束蒸发技术,以0.6nm/s的沉积速率,在NdFeB磁体表面沉积了约4μm厚的Al₂O₃薄膜,使NdFeB磁体腐蚀电流密度由2.49×10⁻⁵A·cm⁻²下降至3.47×10⁻⁶A·cm⁻²;但沉积的Al₂O₃薄膜易出现开裂问题。采用电子束蒸发技术,在0.4nm/s沉积速率下,在烧结NdFeB磁体表面沉积了一层6.6μm的Al膜。实验表明,其与基体的结合强度高达13.8MPa,可耐中性盐雾96h,自腐蚀电流密度值仅为3.03×10⁻⁷A·cm⁻²,具有最佳的防腐性能。 4、为达到烧结NdFeB服役要求,研究了磁化状态下晶界渗Dy高矫顽力NdFeB磁体的耐蚀特性。首先,研究了磁化状态与退磁状态下NdFeB磁体在不同酸碱度溶液中的腐蚀行为,发现磁化后样品在NaCl介质中的腐蚀电流密度由1.24×10⁻⁵A·cm⁻²增加到4.31×10⁻⁵ A·cm⁻²,而在H₃PO₄和NaOH介质中的腐蚀速率会降低。其次,基于ANSYS软件仿真了磁化状态下NdFeB磁体周围磁场,并探讨了NdFeB磁体表面磁场分布规律对其耐腐蚀性能影响,结果表明:磁化后的NdFeB磁体表面呈现非均匀磁场以及存在磁场强度梯度分布,是NdFeB磁体耐腐蚀性能差的原因,但镀有单层Al薄膜或AlN/Al复合涂层的磁化NdFeB磁体的自腐蚀电流密度值分别仅为7.16×10⁻⁷ A·cm⁻²和9.30×10⁻⁷A·cm⁻²,可以达到改善NdFeB磁体耐腐蚀性能的目的。 图80幅,表33个,参考文献184篇。 关键词:烧结NdFeB;晶界扩散渗Dy;高矫顽力;耐腐蚀性 分类号:TM273

英文摘要

Abstract: Possessing high remanent magnetization and coercivity, large energy density and excellent mechanical properties, NdFeB magnets are used in information electronics, aerospace, energy transportation and health care, etc. In spite of wide applications, conventional NdFeB magnets face two limitations. In addition to easy surface oxidation which leading to poor corrosion resistance associating to chemical active element Nd, their coercivities are less than 1/3 of theoretical values. Hence, the present work mainly focused on preparation of NdFeB magnets with high coercivity and corrosion resistance. Coercivity mechanisms and possible factors on corrosion behavior of the magnets were studied in detail. The main contents and conclusions of this thesis are as follows: 1.To increase coercivity of sintered NdFeB magnets, Electron Beam Evaporation (EBE) method was used to deposit single layer of Dysprosium (Dy) film with thickness of about 3 μm on surface of the magnets, followed by diffusion heat treatment of the film under Argon (Ar) atmosphere. Microstructure of the as-prepared samples indicated that Dy diffused along Nd₂Fe₁₄B grain boundaries to form a layer of (Nd, Dy)₂Fe₁₄B shell on the grain surface. By solving Diffusion Equation, it was found that diffusion coefficient of Dy increased with heat treatment temperature. Maximum value of 2.7×10⁻⁷ ㎝²/s was obtained when samples were heating at 900 ℃. Relationship between time and depth of Dy diffusion under different temperatures was also established, demonstrating that maximum depth of Dy diffusion along grain boundaries was less than 3 μm. After being heated at 900 ℃ for 8 h, which is the optimal condition of heat treatment, coercivity of the samples increased from 1020 to 1450 kA·m⁻¹ with remanent magnetization almost unchanged, indicating remarkable improvement of magnetic properties. It can then be concluded that EBE method can reduce content of heavy rare earth elements by about 11%, and improve corrosion resistance of NdFeB magnets at the same time. 2.After obtaining sintered NdFeB magnets with high coercivity, single layer of Ni-P coatings were successfully deposited on surface of sintered NdFeB magnets by electroless plating rout. Results showed that neutral Ni-P coating with ultrasonic-assisted plating was able to withstand 12 times of thermal shock (quenched into cold water from 300 ℃), suggesting strong binding force between the coating and substrate. Corrosion resistance of the coating, however, was confirmed to be relatively poor. Acid Ni-P coating was also deposited on surface of sintered NdFeB magnets without ultrasonic-assisted plating. The coating showed amorphous structure with open circuit potential to be -0.314 V, meaning superior corrosion resistance. Neutral Ni-P/acid Ni-P bilayer coatings were plated on surface of NdFeB samples as well, finding that the bilayer coatings were able to withstand 6 times of thermal shock (quenching into cold water from 300℃) and 320 h of salt spray testing with corrosion current density to be 5.4×10⁻⁶ A/㎝². These suggested that the bilayer coatings had acceptable bonding strength to substrate and good corrosion resistance. 3.To meet requirement of long-lasting corrosion resistance of NdFeB magnets, a layer of Ni film with thickness of about 9 μm was deposited on surface of NdFeB samples by vacuum ion sputtering coating at room temperature with deposition rate of 0.8 nm/s and deposition time of 3h. It was found that the film was in amorphous state, which had little effects on magnetic properties of the samples and exhibited high bonding strength to substrate. Corrosion current density of the film was determined to be only 2.2×10⁻⁷ A·㎝⁻² when the samples were unmagnetized, indicating excellent corrosion resistance. The value, however, increased remarkably after magnetization of the samples. Non-metal Al₂O₃ film with thickness of about 4 μm was deposited on surface of NdFeB magnets by Electron Beam Evaporation (EBE) method at deposition rate of 0.6 nm/s. Despite of the fact that the coatings were easy to crack, corrosion current density of the coated sample was 3.47×10⁻⁶ A·㎝⁻², much smaller than the uncoated one of 2.49×10⁻⁵. Al metal film with thickness of about 6.6 pm was deposited on surface of NdFeB samples by EBE method at deposition rate of 0.4 nm/s. The film, with bonding strength to substrate determined to be about 13.8 MPa, was able to withstand up to 96 h of neutral salt spray testing. Self corrosion current density of the film was 3.03×10⁻⁷ A·㎝⁻², indicating good corrosion resistance. 4.Corrosion resistance of NdFeB magnets under magnetization state was discussed by comparing corrosion behaviors between magnetized and demagnetized samples in different solutions. It was found that, after magnetization, corrosion current density of samples increased from 1.24×10⁻⁵ to 4.31×10⁻⁵ A·㎝⁻² in NaCl solution; whereas their corrosion rate decreased in H3PO4 and NaOH solutions. Magnetic field around NdFeB samples after magnetization were simulated using HFSS software to study impacts of surface distribution of magnetic field on corrosion resistance. It was confirmed that magnetic field of NdFeB samples showed gradient distribution, leading to deterioration of corrosion resistance. After coating single layer of Al film or AlN/Al bilayer films, self corrosion current density of the samples were estimated to be 7.16×10⁻⁷ and 9.30×10⁻⁵ A·㎝⁻², respectively, which are much lower than the values of uncoated samples. These demonstrated that Al single layer and AlN/Al bilayer coatings can effectively improve corrosion resistance of NdFeB magnets. Key words: Sintered NdFeB; Grain boundary diffusion of Dy; High coercivity; Corrosion resistance Classification:TM273

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