Korrosionsverhalten von Nd‐Fe‐B‐Dauermagneten

Corrosion behaviour of Nd‐Fe‐B permanent magnets The corrosion behaviour of Nd‐Fe‐B permanent magnetic alloys as well as of single phases of these alloys have been investigated in sulphuric acid at room temperature and humid air at 150 °C using mass loss and electrochemical techniques. Scanning electron microscopy and scanning probe microscopy were used to study the surface topography. The electrostatic surface potential was examined by scanning probe microscopy using tapping mode. A correlation between the electrostatic surface potential and the corrosion rate of these alloys was found. The higher the value of the electrostatic surface potential of the intergranular phases the higher is the corrosion sensitivity. The strength of the corrosion attack on the phases of sintered permanent magnetic alloys is as follows: Ferrmagnetic phase < B‐rich phase < Nd‐rich phase. The differences in the chemical composition as well as in the preparation of these magnetic alloys have distinct influence on the corrosion resistance of the magnetic Nd‐Fe‐B alloys.     

富钕相对烧结NdFeB磁体耐腐蚀性的影响

研究了Nd2Fe14B单晶、传统烧结NdFeB磁体和放电等离子烧结（简称SPS）NdFeB磁体在电解液溶液中的电化学特性。采用扫描电子显微镜和电子能谱分析了磁体的微观组织成分。结果表明在3．5％NaCI溶液的极化曲线中，Nd2Fe14B单晶具有最高的电化学腐蚀电位，放电等离子烧结NdFeB磁体的腐蚀电位高于传统烧结NdFeB磁体。与传统烧结NdFeB磁体相比，放电等离子烧结NdFeB磁体富Nd相具有独特的分布形态，主相Nd2Fe14B晶粒细小、均匀，富钕相在主相晶粒边界上分布较少，主要集中在三角晶界处。这种组织结构有效地抑制了磁体沿富钕相发生晶间腐蚀的过程，磁体因此具有良好的耐腐蚀性能。此外，从不同稀土含量的烧结NdFeB磁体的高压加速实验中可以看出磁体的腐蚀速度随稀土含量的增加而增大。以上结果表明富Nd相的化学特性及其分布状态和含量是决定合金耐蚀性能的关键，它在合金中以网络状分布在主相晶粒边界上，并决定了烧结NdFeB易于发生选择性晶间腐蚀，从而导致耐蚀性差。     

Electrochemical corrosion kinetics of spark plasma sintered Nd–Fe–B permanent magnets in NaCl electrolyte

Abstract

Static state immersion experiments, polarisation curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in NaCl electrolyte. Effect of microstructure modification on their chemical stability of the magnets was discussed. The electrochemical reactions of both magnets are controlled by the step of active substances adsorption process at the open circuit potential and the anodic potential, which turn to diffusion process at the cathodal potential. Although both magnets are susceptible to corrosion in saline electrolytes, SPS magnets are more corrosion resistant than conventional sintered magnets due to their special microstructure that is different from those of conventional sintered magnets. In SPS magnets, the grain size of the Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd rich phase form along the grain boundaries of Nd₂Fe₁₄B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure effectively restrains the aggressive intergranular corrosion along Nd rich phases. As a result, the SPS magnet possesses excellent corrosion resistance in NaCl electrolyte.     

烧结Nd—Fe—B永磁体化学镀Ni—Cu—P性能研究

通过适当的预处理工艺,在烧结Nd-Fe-B永磁体表面直接实用化学镀Ni-Cu-P,不需要预镀处理,用X－射线衍射仪,扫描电镜分析与镀层的结构和表面形貌、测试了镀层的耐蚀性能和结合强度。结果表明,Nd-Fe-B永磁体经化学镀Ni-Cu-P后,可以获得与基体结合良好,孔隙率低、耐蚀性高的镀层。     

Characterisation of electrochemical interactions between single phases of Nd‐Fe‐B permanent magnets

The electrode functions (anode or cathode) of coupled single phases (ferromagnetic phase: Nd₂Fe₁₄B; boron rich phase: Nd_1+εFe₄B₄; neodymium rich phase: Nd₄Fe) of Nd‐Fe‐B permanent magnets have been investigated. The electrochemical potential differences and current densities between the three phases were determined. Furthermore, the corrosion current density of the magnetic materials in 0.1 M H₂SO₄ was calculated from measurements of current density‐potential curves. The neodymium rich phase works as the anode in contact with both the ferromagnetic and the boron rich phase. The calculation of corrosion current densities for systems of three coupled phases from the current density‐potential curves of single phases measurements demonstrates very high values for the small area of the corroding neodymium rich phase at the grain boundaries. An increase of corrosion resistance of Nd‐Fe‐B‐magnets is possible only by an increase of the corrosion resistance of neodymium rich phase. The presented results do not demonstrate an important influence of the alloying elements gallium or dysprosium on the corrosion rate of the Nd‐Fe‐B magnets. Therefore, new concepts have to be developed to increase the corrosion resistance of these magnets.     

Effect of Dy2O3 intergranular addition on thermal stability and corrosion resistance of Nd–Fe–B magnets

Sintered Nd–Fe–B magnets with and without Dy₂O₃ were prepared by powder blending method. The temperature-dependent magnetic properties, thermal stability, microstructure and corrosion resistance of sintered magnets were investigated. The temperature-dependent magnetic properties revealed that with intergranular addition of Dy₂O₃, the reversible temperature coefficients β and α in the range of 293–373 K were both lowered, indicating that the thermal stability was effectively improved. This was also verified by the decreased irreversible flux loss (h_irr) and the increased maximum operating temperature (MOT). Moreover, the electrochemical and accelerated corrosion results clearly evidenced that the corrosion resistance of Nd–Fe–B magnet was also modified by addition of Dy₂O₃. Furthermore, the related mechanisms on improved thermal stability and corrosion resistance were systematically discussed.     

烧结NdFeB磁体相的继承性研究

设计成分为Nd32.5B1.04Febal(质量分数,%),经过熔炼,制粉,成型,烧结后制备了烧结NdFeB磁体,对样品的铸锭,烧结态样品以及高温回火态样品,低温回火态样品的微观组织采用SEM进行了仔细地分析。结果显示,烧结NdFeB磁体的相具有"继承性",在熔炼中产生的α-Fe相会被烧结回火后的磁体继承下去,而烧结中形成的Nd2Fe14B相和B-rich相在回火后数量和形态基本上变化不大,Nd-rich相虽然数量变化也不大,但是在高温回火中熔化流动,均匀分布在主相Nd2Fe14B周围,把主相Nd2Fe14B一个个分隔开来,在低温回火中,这种流动会延续,相的形态会得到巩固,使得磁体最终获得良好的综合磁性能。     

氢对Nd—Dy—Fe—B磁体制作过程的影响

研究了氢对Nd-Dy-Fe-B磁体制作过程的影响，主要包括氢对HD磁粉的氧化和对其后烧结磁体磁性能的影响。研究表明，氢不利于HD磁粉的氧化稳定性能，氢的存在促进磁粉吸湿和电化学腐蚀，加快氧的扩散。HD磁粉经过在一定温度下真空脱气处理后，氧化稳定性提高，特别是在较高温度下进行脱气处理的磁粉。氢的存在使磁粉容易粉碎，加快磁体的烧结过程，降低磁体的烧结温度。同时发现，未脱氢磁体的取向度较低。     

烧结(Nd,Tb)-Fe-B磁体的元素扩散、反磁化和矫顽力

采用双合金法将两种粉末混合制备烧结永磁体可提高磁体磁性能;但在烧结过程中两种粉末之间存在元素扩散,元素扩散对磁性能的影响程度需要进一步研究。本文将Nd13Fe81B6和TbHx粉末混合制备烧结磁体,Nd13Fe81B6磁体矫顽力为4.5 kOe。当TbHx混合量为3 wt.%,烧结磁体的矫顽力增加至20.0 kOe。通过热激活研究认为,磁畴壁的形核是反磁化需要经过的过程。由于热力学的原因Tb元素更容易扩散进入Nd2Fe14B主相而不是富集在晶间富稀土相。Tb元素进入主相替代Nd可形成具有更高各向异性场的(Nd,Tb)-Fe-B表层,在反磁化过程中晶粒表层磁畴壁的形核场会增加,因此矫顽力增加程度显著。但是,TbHx混合量超过5 wt.%,矫顽力增加幅度降低。对于TbHx混合量7 wt.%的磁体,元素分布显示在主相晶粒内部贫Tb区域明显增少,证实在烧结过程中更多Tb原子从晶粒表层扩散入晶粒内部,这样晶粒表层反磁化形核场的提高程度会减弱,因而磁体矫顽力增加幅度降低。本研究说明要提高双合金Nd-Fe-B磁体磁性能需进一步控制元素扩散并优化磁体的元素分布。     

添加Dy2O3对烧结NdFeB磁体微观结构的影响

研究了NdFeB粉末中添加1wt％Dy2O3粉末对烧结NdFeB磁体微观结构的影响，研究发现，在烧结过程中，Dy2O3中的Dy与Nd2Fe14B中的Nd发生了置换反应，Dy进入Nd2Fe14B相，形成了（Nd，Dy）2Fe14B相，提高了磁体的矫顽力。     

钕铁硼磁体表面电沉积纳米氧化硅复合镀锌层的研究

目的增强钕铁硼材料的耐腐蚀性能。方法在钕铁硼磁体材料表面电沉积含有氧化硅、氧化钛纳米粒子的复合锌层。结果在氯化物镀锌溶液中加入10 g/L的氧化硅纳米粒子,可以获得氧化硅质量分数在3%以上的氧化硅纳米粒子复合镀锌层,而加入10 g/L的氧化钛纳米粒子的镀液所获得的复合镀层中,氧化钛的质量分数只有0.3%。结论中性盐雾腐蚀和盐水浸泡腐蚀试验结果表明,含有氧化硅纳米颗粒的复合镀锌层的耐腐蚀性能得到了提高,而含有氧化钛纳米颗粒的复合镀锌层的耐腐蚀性能则没有提高。含有纳米粒子的复合镀液经过1年的放置和间断使用,仍然保持着纳米颗粒均匀分散的稳定性和在镀层中稳定析出氧化硅纳米粒子的特征。     

Nd-Fe-B合金的腐蚀及防蚀表面处理

研究了在环境腐蚀试验条件下Ｎｄ－Ｆｅ－Ｂ合金的腐蚀行为及腐蚀对其磁性能的影响,并提出了防止腐蚀的表面处理方法。试验结果表明,Ｎｄ－Ｆｅ－Ｂ合金有较高的腐蚀速率,腐蚀后Ｎｄ－Ｆｅ－Ｂ合金的磁性能降低。化学镀Ｎｉ－Ｐ及电泳涂装的复合表面处理方法,具有良好的防蚀效果。     

烧结NdFeB磁体热压变形后富Nd相的显微组织

采用热压变形法对NdFeB磁体晶间富Nd相的显微组织进行了研究,实验结果表明,NdFeB磁体经真空热压变形后,富Nd相不再平均地分布在磁体晶间,而是聚集成团块状或从磁体边缘渗出,显微组织分析表明,富Nd相主要是由α-Dd和Nd2Fe17两相组成,与Nd-Fe合金的共晶组织成分接近,对于晶间添加Al元素的磁体,Al溶入晶间形成Nd2Fe15Al2相弥散地分布在晶界上,这有益于磁体矫顽力的提高;对于晶间添加Cu元素的磁体,晶间没有发现有新相产生。     

HDDR法制备NdFeB永磁体的高矫顽力研究

采用X射线衍射分析、扫描电镜和BH测试仪分别研究HDDR法制备的NdFeB永磁体微结构、晶粒表面形貌及其磁性能。结果表明,HDDR法制备的磁粉再经1080 ℃高温真空烧结所获得的NdFeB永磁体,主要由四方相Nd2Fe14B（P42/mnm）和少量的富稀土相构成,扫描电镜相片显示主相平均晶粒尺寸约为12.3 μm;采用Horta法计算得到样品（006）晶面的极密度因子约为3.5,表明该样品具有较高的c轴取向;不同温度下退磁曲线研究表明,室温下合金有较好的磁性能：磁能积 (BH)max=264 kJ/m3,剩磁Br=1.17 T,矫顽力达到Hcj=2038 kA/m;随温度的升高,磁性能各参数都单调下降,特别是矫顽力降低最为显著,从295 K升温到448 K过程中其值下降了1496 kA/m;Hc(T)/Ms(T)与H(T)/Ms(T)(Kronmüller-Plot)关系曲线研究表明,该合金的矫顽力机制为畴成核反转机制,其中微磁参数αk和Neff分别为1.39和1.75,是决定该合金高矫顽力的关键因素。     

NdFeB永磁体表面磁控溅射铝防护镀层性能研究

目的研究一种NdFeB永磁体表面腐蚀防护技术。方法采用磁控溅射技术,在烧结NdFeB永磁体表面沉积一层纯铝防护薄膜,然后对纯铝薄膜进行阿洛丁化学转化复合处理,表征膜层的表面和截面形貌,并研究结构及耐腐蚀性能。结果沉积的Al中间层和Al薄膜均结构致密,膜/基界面平整,膜层的自腐蚀电流密度为3.5×10-6A/cm2,说明纯Al薄膜能够对NdFeB永磁体提供有效的防护。阿洛丁化学转化可使铝薄膜表面更加致密,自腐蚀电流密度低至7.9×10-7A/cm2,进一步提高了纯铝薄膜的防护性能。结论 NdFeB永磁体表面磁控溅射镀铝是一种有效且环保的防护技术,可用于替代不环保的电镀防护。     

添加Dy烧结NdFeB磁体的研究进展

介绍了添加Dy烧结NdFeB磁体的制备方法,包括单相合金粉末烧结法、双相合金粉末烧结法和晶界扩散法,并总结了Dy元素对烧结NdFeB磁体显微结构和磁性能的影响。添加Dy能细化磁体晶粒,并且在Nd2Fe14B晶粒周围形成富稀土层,从而显著提高磁体的矫顽力性能。     

三相烧结工艺对NdFeB磁体结构与性能的影响

本文采用Nd2Fe14B、Nd55FeBCo和Nd86FeCu53相粉末共烧结的方法制得NdFeB磁体。研究发现通过3相烧结工艺制得NdFeB磁体的晶界相比常规工艺制得NdFeB磁体的晶界相更均匀。由于晶界存在大量的Nd-Cu4、NdCo3和NdCo2相,晶界相的平均电位大大升高,超过了主相Nd2Fe14B的电位。因而NdFeB磁体发生电化学腐蚀时,电流密度大大降低,耐腐蚀性能得到了显著的改善。磁体腐蚀失重从一般工艺的60.47 mg.cm-2下降至1.2 mg.cm-2,而磁体的磁性能基本保持不变。     

Nd3+对烧结Nd-Fe-B化学镀Ni-P合金结合力和耐蚀性的影响

在化学镀液中添加Nd3+,研究其浓度对Ni-P镀层与烧结Nd-Fe-B磁体的结合力和施镀后磁体耐蚀性的影响.测定添加不同浓度Nd3+镀液中所得Ni-P镀层与磁体的结合力,以及镀层在3.5%NaCl(质量分数,下同)溶液中的极化曲线,并结合中性盐雾实验表征施镀后磁体耐蚀性.结果表明,添加2.5 g·L-1Nd3+时,Ni-P镀层与Nd-Fe-B磁体的结合力从6.4 MPa提高至25.2 MPa:施镀后磁体的自腐蚀电位从-0.382 V升高到-0.148 V,自腐蚀电流密度从4.52μA·cm-2降低到0.07μcm-2,耐盐雾腐蚀时间达到256 h,磁体耐蚀性显著提高.     

Effects of Ti and(or) Cu on microstructures and magnetic properties of sintered Nd-Fe-B magnets

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镝的分布对烧结和时效含镝钕铁硼永磁合金性能的影响

本文研究了烧结和时效含镝钕铁硼磁体。通过对不同状态磁体的磁滞曲线,场发射电镜以及能谱分析对磁体的磁性能,微结构以及成份进行了分析。结果表明,除了主晶相外,镝元素主要分布在富钕相,钕镝氧化物和位于晶界的富镝颗粒中。优化时效过程促进了镝在磁体中合理的扩散以及分布,镝在烧结磁体,高温时效磁体以及优化时效磁体中富钕相,钕镝氧化物以及含镝颗粒中的含量减少,证明了优化时效后,镝元素在磁体中的合理分布,导致了含镝钕铁硼永磁合金矫顽力的提高。