Microstructure of sintered ND-Fe-Ga-B magnets with Mo and MoS2 addition

The effect of Mo and MoS₂ additions on the magnetic and microstructure properties has been investigated in Nd-Fe-Ga-B sintered magnets. Coercivity can be increased by both the additions, but the MoS₂ addition provides the larger increase per Mo atom for up to 0.6 at.% Mo. Microstructure investigation reveals a new amorphous intergranular Ga rich phase. This phase forms a thin layer in the grain boundaries and leads to a wetting behavior of the grain boundary phase, therefore increasing the coercivity. Molybdenum addition in the form of MoS₂ is found to modify the Nd₂Fe₁₄B phase, rather than form new minority phases, and the coercivity enhancement of the magnet is due to the increased anisotropy field of the hard magnetic phase.     

Texture analysis of ultra-high coercivity Sm_2Co_7 hot deformation magnets

Bulk anisotropic Sm_2 Co_7 nanocrystalline magnets were successfully prepared by hot deformation process using spark plasma sintering technology.The coercivity of the isotropic Sm_2 Co_7 nanocrystalline magnet is 34.76 kOe,further,the ultra-high coercivity of 50.68 kOe is obtained in the anisotropic hot deformed Sm_2 Co_7 magnet when the height reduction is70%,which is much higher than those of the ordinarily produced hot deformed Sm_2 Co_7 magnet.X-ray diffraction(XRD)analysis shows that all the samples are Sm_2 Co_7 single phase.The investigation by electron backscatter diffraction indicates that increasing the amount of deformation is beneficial to the improvement of the(001) texture of Sm_2 Co_7 magnets.The Sm_2 Co_7 nanocrystalline magnet generates a strong c-axis crystallographic texture during large deformation process.     

The origin of coercivity decrease in fine grained Nd-Fe-B sintered magnets

Microstructures of fine grained Nd-Fe-B sintered magnets that were produced by the pressless process were investigated to understand the origin of the sudden coercivity decrease below a certain grain size. The intrinsic coercivity is inversely proportional to ln D² with the highest coercivity of 17 kOe at D∼4.5 μm, below which the coercivity drops as the grain size decreases. We found that the degradation of the coercivity of the magnet with a grain size of 3 μm was mainly caused by the inhomogeneous distribution of fcc-Nd oxide whose volume fraction increased with respect to the dhcp Nd-rich phase.     

Microstructure of precipitation hardened cobalt rare earth permanent magnets

A transmission electron microscope study of several precipitation hardened cobalt rare earth magnets has been undertaken. The magnets were in peak aged condition and varied in the chemical composition. The studies reveal a fine cellular microstructure. The shape and size of cells depend on the heat treatment and alloying elements. The cell interiors consist of at least two plate-shaped 17:2 phases and are surrounded by a 5:1 boundary phase. The replacement of zirconium with hafnium does not alter the cellular morphology and also leads to a high coercivity. The coercive force is strongly influenced by the shape and size of the cellular structure as well as by the lattice misfit between the 5:1 and 17:2 phases. The influence of the microstructure on the coercivity is discussed.     

The magnetic properties,thermal stability and microstructure of Nd–Fe–B/Ga sintered magnets prepared by blending method

The effect of gallium added by blending method on the magnetic properties, thermal stability and microstructure of Nd_16.5Dy_16.0Fe_53.45Co_13.0B_1.05 (wt%) sintered magnets was investigated. The experimental results show that an appropriate Ga addition can markedly increase the coercivity, reduce the irreversible loss and slightly enhance the remanence. For instance, by adding 0.5 wt% Ga, the coercivity is increased from 1232 to 1819 kA/m; the irreversible loss after being exposed at 200°C for 0.5 h is reduced from above 33% to below 5%. Microstructure analyses show that the grain boundaries of the magnets with and without Ga addition are substantially different. The grain boundaries of the Ga-free magnet are meandrous. On the other hand, most of the boundaries of Ga-containing magnets are straight and smooth. These characteristics can be explained by the appearance of new phases during sintering process.     

Effect of very small additions on the coercivity of Dy-free Nd–Fe–(Co)–B-sintered magnets

The effect of small substitutions on the coercivity of Nd–Fe–B and Nd–Fe–Co–B-sintered magnets has been investigated. Addition of 0.18 at% Ga was found to be the most effective for improving the coercivity in both Nd–Fe–B and Nd–Fe–Co–B without decreasing much the remanent magnetization. Scanning electron microscopy on Ga-free and Ga-added magnets did not reveal a noticeable difference in phase morphology. However, Fe and especially Co concentrations in the intergranular Nd-rich phase were found to be markedly increased after the small Ga addition. Combined addition of Co with a small amount of Cu did not increase the coercivity of the Dy-free magnets. However, none of the examined very small additives can yet be considered as an alternative to Dy for extending operating temperature range of the high-energy Nd–Fe–B magnets.     

Effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets

The effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets has been investigated. The magnetic flux loss of two kinds of magnets before and after irradiation was measured. Results show that the effect of γ-ray irradiation on the magnetic properties of sintered NdFeB is not so obvious as that on Fe–Cr–Co magnet. Irradiation-induced damage from γ-ray for the Fe–Cr–Co magnets was characterized for the first time. The decline of permanent magnetic properties of Fe–Cr–Co magnet induced by γ-ray irradiation is reversible except for the maximum energy product (BH)_max. The difference of coercivity mechanism between these two kinds of permanent magnets is responsible for the different dependence of magnetic properties loss induced by γ-ray irradiation.     

Sm(Co,Cu,Fe,Zr)z反磁化过程的微磁学分析

通过微磁学有限元方法研究了微结构对各向异性的Sm(Co,Cu,Fe,Zr)z磁性能的影响， 并 对不同温度下的退磁曲线进行了计算.计算结果表明，矫顽力随着2∶17相晶粒尺寸的增大 而增大，随1∶5晶界相厚度的增大而减小；通过减小晶界相厚度或增大晶粒尺寸可以有效提 高 磁能积.反磁化的物理机制主要为形核机制，主要表现为首先在晶界相形成反磁化核，随 着 磁场的增大反磁化核不断长大，最后导致整个磁体的磁化反转；而当温度升高时，晶界相逐 渐变成非磁性相，使得反磁化核难以形成，因此出现了反常的矫顽力温度依赖关系. 关键词： 微磁学 有限元 微结构 磁性能     

稀土含量对速凝工艺制备(Nd,Dy)-(Fe,Al)-B合金结构和磁性能的影响

采用双合金法制备了不同稀土含量的(Nd,Dy)-(Fe,Al)-B合金系列,研究了稀土含量的多寡对薄带和最终烧结磁体的微观结构和磁性能的影响.研究结果表明,适量的稀土含量(～3145%)不仅能改善速凝带主相(Nd,Dy)₂Fe₁₄B的单相性,还有利于富稀土(Nd,Dy)相在晶界的均匀分布.当主相薄带的稀土含量在接近2∶14∶1相稀土含量时,磁体的内禀矫顽力和磁能积显著提高.当稀土含量较少时,对速凝薄带的合适时效热处理将能有效地提高最终烧结NdFeB磁体的磁特性. 关键词： 双相烧结 速凝技术 微观结构 磁性能     

Effects of post-sinter annealing on microstructure and magnetic properties of Nd–Fe–B sintered magnets with Nd–Ga intergranular addition

We investigate the effects of post-sinter annealing on the microstructure and magnetic properties in B-lean Nd–Fe–B sintered magnets with different quantities of Nd–Ga intergranular additions. The magnet with fewer Nd–Ga additions can enhance 0.2 T in coercivity, with its remanences nearly unchanged after annealing. With the further increase of the Nd–Ga addition, the annealing process leads coercivity to increase 0.4 T, accompanied by a slight decrease of remanence. With the Nd–Ga addition further increasing and after annealing, however, the increase of coercivity is basically constant and the change of remanence is reduced. Microstructure observation indicates that the matrix grains are covered by continuous thin grain boundary phase in the magnets with an appropriate Nd–Ga concentration after the annealing process. However, the exceeding Nd–Ga addition brings out notable segregation of grain boundary phase, and prior formation of part RE6 Fe13 Ga phase in the sintered magnet. This prior formation results in a weaker change of remanence after the annealing process.Therefore, the diverse changes of magnetic properties with different Nd–Ga concentrations are based on the respective evolution of grain boundary after the annealing process.     

A study of Sm-substituted SrM magnets sintered using hydrothermally synthesised powders

Anisotropic SrM magnets with Sm substitution, which is observed to have the largest beneficial effect both on the coercivity and on the inhibition of grain growth at high temperature among the other elements such as La, Nd and Pr, were investigated. The average grain size of the samples decreases with increasing Sm/Sr ratio. All the magnets with Sm additions exhibit a bigger coercivity and remanence than those of the SrM magnet without Sm and the coercivity of the magnets increases with increasing Sm/Sr ratio. EDX quantitative analysis suggests that the solubility of Sm³⁺ in the SrM-type structure is very small and that the Sm³⁺ preferably goes into SrFeO_3−x, which is probably located around the SrM grain boundaries. The coercivity mechanism of the magnets is nucleation controlled. The formation and the distribution of the SrFeO_3−x phase around the SrM grain boundraies probably provides the inhibition of SrM grain growth, the reduction of the reverse domain nucleation at the grain surface and the isolation of the SrM grains. All these factors would contribute to the improvements of the coercivity of the magnets with Sm additions.     

相分布和晶粒尺寸对Nd2Fe14B/ɑ-Fe纳米复合永磁材料矫顽力的影响

本文采用统计平均方法研究了软、硬磁性晶粒尺寸及相分布对Nd2Fe14B/α-Fe纳米复合永磁材料矫顽力的影响。计算结果表明：对于单相纳米硬磁材料，磁体矫顽力随着硬磁性晶粒尺寸的减小而降低；对于软、硬两磁性相组成的Nd2Fe14B/a-Fe纳米复合永磁材料，两相的随机分布将导致磁体矫顽力随硬磁性晶粒尺寸的减小呈现极大值。本文的计算结果还表明当硬磁性晶粒尺寸大于软磁性晶粒的最佳尺寸时(15nm)，具有多层膜结构的Nd2Fe14B/a-Fe纳米复合永磁材料将比两相随机分布时具有更大的矫顽力。     

Coercivity enhancement of HDDR-processed Nd-Fe-B permanent magnet with the rapid hot-press consolidation process

High coercivity, fully dense anisotropic permanent magnets of submicron grain sizes were produced by rapid hot-press consolidation of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-Co-B powders. In the hot-press process, the coercivity of the consolidated material showed a sharp minimum prior to full densification. Thereafter, it reached a value 25% higher than that of the initial powder. Scanning electron microscopy and transmission electron microscopy observations revealed that the variation in H_cJ was caused by a redistribution of Nd along the grain boundaries during hot pressing and that the high coercivity was attributable to the formation of thin, continuous Nd-rich phase along the grain boundaries.     

利用Pr_(70)Cu_(30)晶界扩散改善烧结钕铁硼废料矫顽力的研究

钕铁硼磁体制备过程中出现的部分块体废料由于矫顽力较低,性能难以满足使用要求.本文主要通过晶界扩散技术来提高废料磁体的矫顽力.采用Pr_(70)Cu_(30)合金作为扩散介质,对烧结钕铁硼废料磁体进行了晶界扩散处理,研究了扩散温度、扩散时间和回火时间对扩散后的磁体性能的影响.结果显示,800℃下扩散3 h,磁体的矫顽力从原来的7.88 kOe(1 Oe=79.5775 A/m)提升至11.55 kOe,提升幅度为46.6%,同时剩磁没有明显降低.扩散后回火对矫顽力的提升有一定的作用.800℃下扩散4h后的磁体在500℃回火3h后,最高矫顽力可达11.97 kOe,比原磁体废料提高了51.9%,接近成品磁体的水平.显微组织分析证实了晶界扩散的作用.扩散处理后的磁体中,主相晶粒间形成了连续晶间相,起到有效的磁隔离作用,有利于矫顽力的提高.研究还发现,Pr_(70)Cu_(30)晶界扩散虽然可以使磁体腐蚀电位上升,但也会增加腐蚀电流密度,不利于磁体抗腐蚀性的改善.本文工作对于提高材料的成品率具有重要意义.     

Coercivity enhancement of sintered Nd-Fe-B magnets by grain boundary diffusion with Pr80-xAlxCu20 alloys

A grain boundary diffusion (GBD) process with Pr_80-xAl_xCu₂₀ (x = 0, 10, 15, 20) low melting point alloys was applied to commercial 42M sintered Nd-Fe-B magnets. The best coercivity enhancement of a diffused magnet was for the Pr₆₅Al₁₅Cu₂₀ GBD magnet, from 16.38 kOe to 22.38 kOe. Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary (GB) phase, optimizing the microstructure to enhance the coercivity. The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains. Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process. Meanwhile, the introduction of Al can effectively promote the infiltration of Pr into the magnet, which increases the diffusion rate of rare-earth elements within a certain range. This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.     

Effects of terbium sulfide addition on magnetic properties,microstructure and thermal stability of sintered Nd-Fe--B magnets

To increase coercivity and thermal stability of sintered Nd–Fe–B magnets for high-temperature applications, a novel terbium sulfide powder is added into(Pr_(0.25)Nd_(0.75))_(30.6)Cu_(0.15)Fe_(bal)B_1(wt.%) basic magnets. The effects of the addition of terbium sulfide on magnetic properties, microstructure, and thermal stability of sintered Nd–Fe–B magnets are investigated.The experimental results show that by adding 3 wt.% Tb_2S_3, the coercivity of the magnet is remarkably increased by about 54% without a considerable reduction in remanence and maximum energy product. By means of the electron probe microanalyzer(EPMA) technology, it is observed that Tb is mainly present in the outer region of 2:14:1 matrix grains and forms a well-developed Tb-shell phase, resulting in enhancement of HA, which accounts for the coercivity enhancement.Moreover, compared with Tb_2S_3-free magnets, the reversible temperature coefficients of remanence(α) and coercivity(β) and the irreversible flux loss of magnetic flow(hirr) values of Tb_2S_3-added magnets are improved, indicating that the thermal stability of the magnets is also effectively improved.     

The effects of blending additions of copper and cobalt to Nd16Fe76B8 milled powder to produce sintered magnets

A blending process involving the mixing of powders of NdFeB and pure copper and pure cobalt has been developed. This process has been shown to be an effective and simple way of adding copper or cobalt and copper to the composition. This allows the composition and hence properties of the finished magnets to be adjusted subsequent to the casting and milling of the basic alloy. Additions of more than 0.4 at% Cu resulted in poor densification. The coercivities of the magnets containing Cu were very much dependent on the heat treatment. A treatment of 500°C for 1 h followed by rapid cooling, yielded an excellent coercivity for 0.25 at% addition of Cu; however, the same heat treatment decreased the coercivity significantly for an addition of 0.15 at% Cu. Examination of the microstructure showed that Cu was affecting the grain size and nature of the grain boundary phases. A combined addition of 0.25 at% Cu and Co allowed Co to be added without excessive loss in coercivity. Increased Co content led to improved Curie temperature and remanence. The combined addition led to the formation of Nd₃Co as the main grain boundary phase. The optimal heat treatment for magnets containing both Co and Cu was 900°C for 10 h followed by 500°C for 1 h and a rapid cool. However, the lower temperature treatment on its own also yielded excellent properties.     

纯ZrO_2纳米粒子结构的拉曼光谱研究

本文对粒径在５８～１０ｎｍ范围内不同粒度的纯ＺｒＯ＿２粒子进行了拉曼散射光谱研究。结果表明，粒子的点阵结构受表面效应的影响，粒径的减小，粒子表面层结构严重畸变，但仍是有序的结构。同时，表面效应将进一步引起ＺｒＯ＿２粒子点阵结构变化，在室温下，使单斜相的粒子自发地转变为四方相结构。ＺｒＯ＿２粒子由单斜相向四方相转变的临界直径为１５ｎｍ。     

Phase distribution and computed magnetic properties of high-remanent composite magnets

Numerical micromagnetic calculations using finite-element techniques allow a quantitative treatment of the correlation between the microstructure and the basic magnetic properties of two-phase permanent magnets such as the remanence, the coercive field and the maximum energy product. For the investigation of (A) the role of the amount of the soft magnetic phase, and (B) the effect of grain shape, realistic three-dimensional grain arrangements have been used. The numerical results show that both short-range exchange and long-range magnetostatic interactions determine the magnetic properties. The optimal microstructure of an isotropic nanocrystalline permanent magnet was found to consist of soft magnetic particles with a large spontaneous magnetization embedded between hard magnetic grains. Exchange interactions than enhance the remanence of isotropic, composite magnets of Nd₂Fe₁₄B and -Fe by about 60%. Because of exchange hardening the soft magnetic phase can be increased up to 50% without a significant loss of coercivity. A uniform grain structure suppresses strong demagnetizing fields and this increases coercivity by 30% as compared with irregular shaped particles.     

Formation of L10-FeNi hard magnetic material from FeNi-based amorphous alloys

L1₀-FeNi hard magnetic alloy with coercivity reaching 861 Oe was synthesized through annealing Fe₄₂Ni_41.3Si₈B₄P₄Cu_0.7 amorphous alloy, and the L1₀-FeNi formation mechanism has been studied. It is found the L1₀-FeNi in annealed samples at 400 ℃ mainly originated from the residual amorphous phase during the second stage of crystallization which could take place over 60 ℃ lower than the measured onset temperature of the second stage with a 5 ℃/min heating rate. Annealing at 400 ℃ after fully crystallization still caused a slight increase of coercivity, which was probably contributed by the limited transformation from other high temperature crystalline phases towards L1₀ phase, or the removal of B from L1₀ lattice and improvement of the ordering quality of L1₀ phase due to the reduced temperature from 520 ℃ to 400 ℃. The first stage of crystallization has hardly direct contribution to L1₀-FeNi formation. Ab initio simulations show that the addition of Si or Co in L1₀-FeNi has the effect of enhancing the thermal stability of L1₀ phase without seriously deteriorating its magnetic hardness. The non-monotonic feature of direction dependent coercivity in ribbon segments resulted from the combination of domain wall pinning and demagnetization effects. The approaches of synthesizing L1₀-FeNi magnets by adding Si or Co and decreasing the onset crystallization temperature have been discussed in detail.