晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

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.     

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.     

晶粒大小分布对纳米晶磁体Pr2Fe14B剩磁和矫顽力的影响

用微磁学有限元法计算了纳米晶磁体Pr2Fe14B的退磁曲线，文中模拟了三种具有不同晶粒大小分布的样品。计算结果表明，剩磁增强与磁体的晶粒尺寸分布情况无关而依赖于平均晶粒直径。晶粒尺寸分布对矫顽力的影响与磁体中的晶间交换耦合作用密切相关。     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

Effect of the intergrain interactions on the coercivity and its angular dependence for Nd(FeCo)B sintered magnets

Effects of interactions between grains with different alignment degrees on the coercivity and its angular dependence for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets have been studied. The experiments show that the intrinsic coercivity _jH_c decreases with enhancing grain alignment (decreasing alignment coefficient σ), the coercivity _jH_c(θ) increases with increasing angle θ between the applied field and the texture axis of the magnets and the variation ratio is larger for the magnets with better grain alignment. The coercivity of the magnets should be determined by the critical field making the moment of individual grains reverse and the interactions between the grains. For the sintered magnets composed of the grains with μm size, the magnetostatic interaction between the grains is stronger than the exchange coupling interaction and it makes the coercivity of magnet increase with increasing alignment coefficient σ. Taking into account the intergrain interactions, the starting field theory of coercivity is in good agreement with the experimental results for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets.     

Improved magnetic properties and fracture strength of NdFeB by dehydrogenation

Effects of the dehydrogenation of the hydrogen decrepitated (HD) powders on the magnetic properties and the fracture strength of sintered NdFeB magnets were studied. It was found that the lattice parameters and the crystal phase of NdFeB changed significantly with the various hydrogen contents of the resultant HD powders due to the different degrees of dehydrogenation. The magnetic properties and fracture strength increased with decreasing hydrogen content, reaching the maximum increases of 200% for both intrinsic coercivity and bending strength, which can be ascribed to the improved microstructure of the sintered NdFeB magnets. The hydrogen remaining in the HD powders diffused out and affected drastically the grain and grain boundaries by the hydrogen out-take channel during the subsequent sintering process.     

Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets

The effect of Tb on the coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets has been investigated. The results showed that the addition of Tb enhanced the intrinsic coercivity, reduced the remanence and improved the impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.0 at% Tb was incorporated. The possible reasons for increasing the intrinsic coercivity and improving impact toughness of sintered magnets were analyzed, and the relations between the microstructure and impact toughness of sintered magnets were studied.     

Deformation-induced texture in nanocrystalline 2:17, 1:5 and 2:7 Sm-Co magnets

The effect of crystal structures on texture induced by hot plastic deformation was studied for Sm-Co, Sm-Zr-Co, Sm-Zr-Co-Fe and Sm-Co-Fe-Mn nanocrystalline alloys with 9-22 at% Sm. Nanocrystalline precursors were obtained via high-energy ball milling and subsequent hot consolidation; deformation was carried out at 800-1150 °C. The analysis of X-ray diffraction and magnetic measurements showed that the degree of the axial [0 0 1] texture after deformation was negligible for the ordered 2:17 structure, but became increasingly noticeable for the disordered 2:17 (“1:7”), 1:5 and 2:7 structures. Because of interplay of several factors including the [0 0 1] texture, saturation magnetization and magnetocrystalline anisotropy, there was no universal trend in the hard magnetic properties with the Sm content. Optimum compositions for the maximum energy product varied from Sm₁₁(Co, Fe, Mn)₈₉ in the Sm-Co-Fe-Mn series to Sm₁₁Zr₂(Co, Fe)₈₇ in the Sm-Zr-Co-Fe series to Sm₁₇(Co, Fe)₈₃ in the Sm-Co-(Fe) series. Iron substitution for cobalt strongly suppresses the 1:5 structure, whereas the Fe-free magnets based on the SmCo₅ compound showed by far the highest room-temperature coercivity.     

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。     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Magnetic properties of Nd–Fe–B film magnets prepared by RF sputtering

Highly oriented films of ∼6 μm in thickness consisting of the Nd₂Fe₁₄B compound phase were obtained by a three-dimensional sputtering method at room temperature and the subsequent crystallization by annealing. The c-axis orientation and coercivity of film samples were sensitive to the sputtering parameters and annealing conditions. The optimum temperature and time for annealing were 650 °C and 30 min to show the highest coercivity without any deterioration for the orientation of Nd₂Fe₁₄B grains, and furthermore the degree of c-axis orientation was increased by decreasing the Ar gas pressure or input power for sputtering. The resultant film magnets with good magnetic properties of B_r=∼1.06 T, H_C=∼371 kA/m, and (BH)_max=∼160 kJ/m³ were obtained under the optimized parameters for sputtering.     

Effects of the degree of grain alignment on the hard magnetic properties of sintered NdFeB magnets

r increases monotonously with an enhancement in the degree of grain alignment (i.e. decrease in the orientation coefficient σ), whereas the intrinsic coercivity _jH_c decreases monotonously. This dependence leads to a non-monotonic behaviour of the magnetic-inductive coercivity _bH_c and a saturation of the maximum energy product (BH)_max. Received: 28 August 1997/Accepted: 12 January 1998     

Effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets

The effect of Co on the thermal stability and impact toughness of sintered Nd–Fe–B magnets has been investigated. The results showed that the addition of Co decreased the intrinsic coercivity and the temperature coefficient of remanence (α), and increased the temperature coefficient of coercivity (β) for sintered Nd–Fe–B magnets. The impact toughness of sintered Nd–Fe–B magnets with the addition of Co first decreases, reaches a minimum, and then starts to increase. The possible reasons for increasing the temperature coefficients of coercivity (β) for sintered Nd–Fe–B magnets were analyzed, and the relations between the microstructure and impact toughness of sintered Nd–Fe–B magnets were studied.     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

Investigation on magnetic properties of orientated nanocomposite Pr2Fe14B/α-Fe permanent magnets by micromagnetic finite-element method

Demagnetization curves for nanocomposite Pr₂Fe₁₄B/α-Fe permanent magnets with different hard grain alignment are calculated by a micromagnetic finite-element method. The results show that both remanence and coercivity increase with improving hard grains alignment. The demagnetization curves show a single-phase demagnetization behavior for the samples with grain size d of 10 nm and two-phase behavior for the samples with d of 20 and 30 nm. H_ex (reflecting the magnetic hardening of α-Fe) and H_irr (expressing the irreversible reversal of hard phase) are both enhanced with improving the hard grain alignment. The magnetic reversal in orientated nanocomposite permanent magnets is mainly controlled by inhomogeneous pinning of the nucleated type.     

Effects of grain size distribution on coercivity and permeability of ferromagnets

Grain size dependence of coercivity and permeability (GSDCP) theory is extended to include grain size distribution in ferromagnets. It is found that the experimental data do not agree with the GSDCP theory on the transition location of different grain size ranges (The GSDCP theory has three different grain size ranges for different magnetization processes.). Correspondingly, including the grain size distribution the GSDCP theory fits the experimental data very well. These results prove that the grain size distribution indeed affects the magnetic properties of nanocrystalline ferromagnets.     

Effect of SiO2 nanopowders on magnetic properties and corrosion resistance of sintered Nd-Fe-B magnets

To improve the magnetic properties and corrosion resistance of magnet, SiO₂ nanopowders were added into Nd_28.25Dy_2.75Fe_balAl_0.15Ga_0.1Nb_0.1B_0.98-sintered magnets as grain boundary modifiers. It was found that the intrinsic coercivity (H_cj), remanence (B_r) and corrosion resistance were all improved with addition of certain amount of SiO₂ nanopowders. The optimum addition amount of SiO₂ nanopowders was found to be 0.01 wt%. Thermodynamic calculation in combination of energy dispersive X-ray spectroscopy (EDX) analyses indicated that SiO₂ nanopowders reacted with Nd of Nd-rich phase. As a result, the intergranular phase was modified, and the grain growth of main phase was restrained. It led to the optimization of microstructure.     

Effective anisotropy and coercivity in nanocrystalline single-phase NdFeB permanent magnetic material

The effect of exchange-coupling interaction on the effective anisotropy and its varying tendency in nanocrystalline single-phase NdFeB permanent magnetic material have been investigated. The results show that the exchange-coupling interaction between grains makes the effective anisotropy of material, Keff, decrease with the reduction of grain size. The variation of Keff is basically the same as that of coercivity. The decrease in effective anisotropy is the main reason of the reduction of coercivity for nanocrystalline single-phase NdFeB permanent magnetic material. In order to get high anisotropy and coercivity in nanocrystalline single-phase NdFeB permanent material, the grain size should be larger than 35 nm.     

Interactive contribution of grain size and grain orientation to coercivity of melt spun ribbons

During melt spinning process, the improvement of certain grain orientation and the refinement of grain size with surface velocity have interactive and contradictory effects on the magnetic properties. The contributions of these effects have seldom been taken into account and they were discussed in this paper via Fe-2, 4, 6.5 wt% Si alloys. Heat treatment at 1173 K for 1 h was performed to show the annealing impact. The X-ray diffraction patterns show that the high surface velocity and heat treatment increase the intensity ratio of line (2 0 0) to (1 1 0) of A2 phase. The (2 0 0) line corresponds to (2 0 0) plane in 〈0 0 1〉 direction, easy magnetization direction of α-Fe phase in Fe-Si alloy. The improvement of this grain orientation with the surface velocity decreases the coercivity, which should increase due to the grain refinement. It is revealed that the 〈0 0 1〉 texture promoted by the anisotropic heat release during melt spinning process is one factor to improve the magnetic properties and should be considered when preparing soft magnetic materials.