Development of non-rare earth grain boundary modification techniques for Nd-Fe-B permanent magnets

The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure.Intergranular addition and grain boundary diffusion(GBD)process are effect...     

Towards the diffusion source cost reduction for NdFeB grain boundary diffusion process

Aiming at improving the performance/cost ratio in grain boundary diffusion process(GBDP),the critical RE containing Pr-Al-Cu alloy,less expensive RE containing La-Al-Cu alloy and non-RE Al-Cu alloy were employed as the diffusion sources.The preliminary results show that the coercivity was successfully enhanced from 1000 kA/m to 1695,1156 and 1125 kA/m by Pr70Al20Cu10,La70Al20Cu10 and Al75Cu25(at.%) alloys diffusion,respectively,due to the formation of(Nd,Pr)-Fe-B,La2 O3 and c-Nd2 O3 phases respectively,after diffusion.It is also found that the corrosion resistance can be improved by Al-Cu diffusion due to the positive effects of Al and Cu elements in grain boundary.The present results demonstrated the various coercivity enhancement mechanisms for the GBDP based on different diffusion sources,and provided feasible solutions for cost reduction of GBDP and NdFeB production by saving RE resource.     

烧结NdFeB铸造新工艺-薄片铸锭法

合金铸造技术是制备高性能烧结NdFeB磁体的关键工艺之一 .根据高性能磁体制造的设计要求 ,NdFeB合金铸锭微观组织应该具有柱状晶完整 ,没有α -Fe偏析相 ,富钕相弥散分布的特点 .本文比较了现有几种生产中常用的铸造方法 -平板铸造、柱状铸造、薄板铸造等得到的合金铸锭 ,然后介绍了目前国际上高性能NdFeB磁体制备的铸造新工艺 -薄片铸锭法 ,并通过扫描电镜观察了不同Nd含量薄片铸锭的组织结构 .最后展望了薄片铸锭在我国的应用前景 .     

SC铸片微结构对烧结NdFeB结构与磁性能的影响

研究了铸片工艺SC(StripCasting)制备的合金铸片的微结构对烧结钕铁硼磁体微结构与磁性能的影响。结果表明:冷却速度过高时铸片厚度变薄,同时在急冷面产生细小的等轴晶,使烧结磁体容易出现固固烧结现象和主相晶粒的反常长大,降低了磁体的永磁性能;采用合适的冷却速度制备的铸片几乎全部由厚度3～5μm片状晶组成,且被富钕相薄层均匀隔开,采用该类铸片可以获得高永磁性能的烧结磁体,其永磁性能达到:Br=1.44T,jHc=877KA/m,(BH)max=398kJ/m3(50MGOe)。     

添加润滑剂对烧结钕铁硼磁体性能的影响

研究了烧结钕铁硼永磁材料的粉末流动性及添加润滑剂对粉末流动性与磁体取向度和磁性能的影响.结果表明:影响松装状态磁粉流动性的主要因素是粉末颗粒团聚,影响密实磁粉流动性的主要因素是颗粒间的摩擦力.添加适量的润滑剂可以防止粉末颗粒团聚、明显地减小摩擦力,改善粉末流动性,提高磁体的取向度、剩磁与磁能积.采用添加润滑剂和橡皮模等静压制成型工艺,批量生产的烧结钕铁硼磁体性能达到:Br=1.457 T,jHc=1148 kA/m(14.43 kOe),(BH)max=408 kJ/m3(51.3 MGOe).     

车载永磁传感器件中NdFeB烧结永磁的稳定性分析

研究了电磁干扰、振动和温度等环境因素对NdFeB永磁性能的影响。研究结果表明:(1)场强或频率越高,失效越大,而交变磁场比稳恒磁场更为严重;(2)交变场下因涡流的"趋肤"效应,导致永磁出现不可逆的热退磁现象,当外场为30kA/m、500Hz干扰时,其失效瞬间即超过15%;(3)振动和高温的共同作用干扰,使得退磁失效情况加剧。     

A study of Nd2Fe14B and a neodymium-rich phase in sintered NdFeB magnets

The microstructure and properties of NdFeB sintered permanent magnets were analysed by different methods. Samples analysed were sintered and thermally treated. The hard magnetic Nd₂Fe₁₄B phase and amorphous neodymium-rich phase were observed by TEM. The neodymium-rich phase contained iron and boron, in elemental and in B₂O₃ form, which is known as a glass former. At the sintering temperature, Nd₂Fe₁₄B and the neodymium-rich phase are supersaturated with iron, which should be dissolved at the annealing temperature to react with neodymium and boron and form additional Nd₂Fe₁₄B phase. Iron precipitates of size up to 2 nm were detected in the Nd₂Fe₁₄B phase. These superparamagnetic precipitates of -Fe could affect the hard magnetic properties of NdFeB magnets.     

微观组织调整和稀土含量对烧结NdFeB永磁腐蚀的影响

采用放电等离子烧结技术(SPS)制备了不同稀土含量的新型烧结NdFeB磁体,为了对比,采用传统烧结工艺制备了相同成分的磁体。测量了磁体在高压湿热环境的腐蚀行为。结果表明SPS磁体的耐腐蚀性明显优于传统烧结磁体,而且随着稀土含量的降低,烧结NdFeB磁体的耐腐蚀性能显著提高。     

Origin of grain boundary motion during diffusion bonding by hot pressing

The microstructural changes during diffusion bonding of Al₂O₃ fibres with boron-doped Ni₃Al plates by hot pressing were investigated. Particular attention was paid to the conversion of the bonding surfaces to an interior grain boundary and its migration during the hot-pressing treatment. The microstructure changes were found to depend on the grain size of the matrix material prior to hot pressing. Fine-grained matrix material led to a fast migration of the bonded interface in order to establish force equilibrium at grain boundary junctions at the prior surfaces. In coarse-grained material, the bonded interface moved after much larger hot pressing time by strain-induced boundary migration due to the accumulated plastic-strain during prolonged hot-pressing treatment. The fibres were found to strongly impede migration of the bonded interface, which can interfere with the perfection of the bonding process.     

Lattice and grain boundary diffusion of copper in thin aluminum films

The lattice D₁ and grain boundary δD_b diffusivities of Cu in Al thin films at 130–185°C are calculated from measurements employing Auger electron spectroscopy and Ar ion beam etching. The calculated values are D₁ = 0.065 cm² s^-1 × exp(-122 kJ/RT) and δD_b = 4.5 × 10^-9 cm³s^-1 exp(-97.4 kJ/RT). The D₁ value is 3–5 times larger at 130–185°C than that predicted by an extrapolation of radioactive tracer measurements of large grain bulk specimens at 433–652°C. The higher value measured here is attributed to the higher density of subgrain defect structures in the thin film.     

Applications of grain boundary diffusion studies to soldering and thermocompression bonding

This paper describes the application of grain boundary diffusion and oxidation to the technologically important topics of solderability and thermocompression bondability. Copper leads plated with 0.73 μm of gold were studied. Arrhenius plots were constructed and extrapolated to expected storage conditions. A storage life of greater than 2 a at 50°C is predicted for both properties. The time to loss of solderability is given by t_s = 1.6 x 10^-7 exp(0.8 eV/kT) h and the time to loss of bondability by t_B = 4 x 10^-10 exp(1.0 eV/kT) h. The effective activation energies 0.8 and 1.0 eV compare with 0.91 eV for the grain boundary diffusion of copper through gold. Loss of solderability and loss of bondability were modeled as a two-step series event. First the copper diffuses to the gold surface via its grain boundaries. The copper then oxidizes. Auger analysis indicates that the surface oxide is Cu₂O.     

The properties of aluminium coating on sintered NdFeB by DC magnetron sputtering

Pure aluminium coating was deposited on sintered NdFeB magnets by direct current (DC) magnetron sputtering to improve the corrosion resistance. The corrosion behaviour of sintered NdFeB coated with aluminium was characterized by potentiodynamic polarization measurement. The adhesive strength between the aluminium coating and the sintered NdFeB was characterised by vertical tensile test and high-low temperature shock test. The influence of the coating on the magnetic properties of the sintered NdFeB was also characterised. It was found that the aluminium coating can improve the corrosion resistance of the sintered NdFeB evidently. The aluminium coating was well adhesive with the substrate and did not deteriorate the magnetic properties of the sintered NdFeB magnets. These two characters may overcome the disadvantages of NiCuNi coating which is usually electroplated on NdFeB in industry.     

Oxygen diffusion blocking of single grain boundary in yttria-doped zirconia bicrystals

Yttrium-doped zirconia bicrystals with [001] symmetric tilt Σ 5 grain boundaries were fabricated by a diffusion bonding technique, and oxygen diffusion behavior across the grain boundary was measured by secondary ion mass spectrometry (SIMS), tracing ¹⁸O isotope. It was found that the ¹⁸O fraction across the boundary exhibited explicit decrease around the boundary plane, indicating that the oxygen-diffusion is blocked by the presence of the Σ 5 grain boundary. This is the first experimental detection of oxygen diffusion blocking at a single grain boundary in zirconia ceramics. From high-resolution transmission electron microscopy observations and energy dispersive X-ray spectroscopy analysis, neither amorphous layers nor Si impurity segregation were found at the grain boundary. The grain boundary blocking effect of the Σ 5 boundary must be an intrinsic feature arising from its core structure and yttrium solute segregation of the grain boundary.     

Atomic mechanisms of grain boundary diffusion: Low versus high temperatures

We analyze recent results of atomistic computer simulations of grain boundary (GB) diffusion in metals. At temperatures well below the bulk melting point T_m GB diffusion occurs by random walk of individual vacancies and self-interstitials. Both defects are equal participants in the diffusion process and can move by a large variety of diffusion mechanisms, many of which are collective transitions. GB diffusion coefficients can be computed by kinetic Monte Carlo simulations. At high temperatures, the presence of large concentrations of point defects is likely to alter the diffusion mechanisms. Molecular dynamics simulations of GB structure and diffusion in copper reveal a continuous GB premelting in close vicinity of T_m. However, diffusion in high-energy GBs becomes almost independent of the GB structure (“universal”) at temperatures well below T_m. This behavior can be tentatively explained in terms of heterophase fluctuations from the solid to the liquid phase. The exact diffusion mechanisms in the presence of heterophase fluctuations are yet to be established.