Nanocrystalline Nd–Fe–B Anisotropic Magnets by Flash Spark Plasma Sintering

Flash spark plasma sintering (flash SPS) is an attractive method to obtain Nd–Fe–B magnets with anisotropic magnetic properties when starting from melt-spun powders. Compared to the benchmark processing route via hot pressing with subsequent die upsetting, flash SPS promises electroplasticity as an additional deformation mechanism and reduced tool wear, while maximizing magnetic properties by tailoring the microstructure—fully dense and high texture. A detailed parameter study is conducted to understand the influence of Flash SPS parameters on the densification and magnetic properties of commercial MQU-F powder. It is revealed that the presintering conditions and preheating temperature before applying the power pulse play a major role for tailoring grain size and texture in the case of hot deformation via Flash SPS. Detailed microstructure and magnetic domain evaluation disclose the texture enhancement with increasing flash SPS temperature at the expense of coercivity. The best compromise between remanence and coercivity (1.37 T and 1195 kA m⁻¹, respectively) is achieved through a combination of presintering at 500 °C for 120 s and preheating temperature of 600 °C, resulting in a magnet with energy product (BH)_max of 350 kJm⁻³. These findings show the potential of flash SPS to obtain fully dense anisotropic nanocrystalline magnets with high magnetic performance.     

Microstructure and properties of Nd-Fe-B magnets prepared by spark plasma sintering

Abstract

Anisotropic Nd₁₅.₅Dy₁.₀Fe_BalCo₃.₀B₆.₈Al₁.₀ magnets were produced by the spark plasma sintering (SPS) technique. The effects of processing conditions on the microstructure, magnetic properties, dimensional precision and density of the magnets were studied. The magnetic properties, microstructure and constituents were investigated by means of a magnetic flux density - magnetic field strength (B-H) loopline instrument, scanning electron microscopy and energy dispersive X-ray analysis. The density of the magnets was determined by the Archimedes method, and the dimensional precision of the magnets was measured by micrometer. It was found that the microstructure of SPS processed Nd-Fe-B magnets is unique; the grain size is fine and uniform while distribution of the neodymium rich phase is heterogeneous. The optimal magnetic properties of SPS processed Nd-Fe-B magnets obtained so far are maximum energy product of 240 kJ m^-3 and coercive force of 1260 kA m^-1. The dimensional precision of the magnets is ~ 20 μm, and the density of the magnets reaches 7.58 g cm^-3.     

Coercivity and domain structure of nanograined Fe–C alloys after high-pressure torsion

The microstructure and magnetic properties of binary hypo- and hyper-eutectoid Fe–C alloys were studied. The investigations have been carried out on the samples in the as-cast state, after a long annealing at 725 °C and on the specimens after the high-pressure torsion (HPT). The deformation was carried out at the ambient temperature and the pressure of 5 GPa. The grain size after HPT is in the nanometer range. Long annealing leads to a drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H _c increases with increasing carbon content. The distance L between pinning points for domain walls decreases with increasing carbon content. Increase of the coercivity and decrease of L are more pronounced below the eutectoid concentration. The coercivity of the nanostructured samples is higher than that of the as-cast alloys. Due to the pinning of domain walls by the cementite particles, the hysteresis loop in the coarse-grained alloys both in as-cast and annealed states has a narrowing near the zero magnetization.     

Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (H _c) is roughly inversely proportional to the grain size (D _g) for grain sizes exceeding ∼0·1−1 μm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesD _g<∼100 nm,H _c decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.     

Magnetic properties and microstructure of radially oriented Sm(Co,Fe,Cu,Zr)z ring magnets

Radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets are prepared by powder metallurgy with appropriate magnetic field molding, sintering process and aging treatment. The results indicate that radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets have obvious anisotropy of thermal expansion and sintering shrinkage, which easily lead to the splits and deformation of the ring magnets. So, slow heating, vacuum pre-sintering in sintering process and various quenching processes at different steps during quenching are adopted. The magnets have excellent magnetic properties: B_r = 10.8 kGs, H_cj = 27.6 kOe, BH_max = 28.1 MGOe. Besides, there is a uniform magnetization field on the surface of the ring magnets. The average surface magnetization field () is 1.502 kGs. The deviation from average (α) is only 4.2%. The microstructure of the magnets consists of a mixture of homogeneous cellular and lamella structures.     

Analysis on deformation and texture formation mechanism of hot-deformed Nd-Fe-B magnets based on heterogeneous structure evolution

In this paper,microstructure,micromagnetic structure,texture,together with magnetic properties of the hot-deformed (HD) Nd-Fe-B magnets were systematically studied to understand the deformation process and the formation mechanism of c-axis texture.The results show that the platelet grains are formed in the fine-grain regions at the initial stage of the deformation.As the amount of deformation increases,the proportion of platelet grains increases and arranges gradually,causing the formation of c-axis texture,till the grain merging occurres when the deformation is excessive.It should be noted that the rare earth-rich phase in the fine-grained region slowly diffuses to the coarse-grained region where only grain growth can be observed during deformation.The deformation mechanism and formation of c-axis texture in HD Nd-Fe-B magnets can be deduced to be accomplished by the processes of dissolution-precipitation diffusion,grain rotation and grain arrangement,based on the characterization of microstructure and texture evolution.Also,approaches to optimize the preparation process and magnetic properties of the hot-deformed Nd-Fe-B magnets were discussed.     

Coercivity dependence of a Sm2(Co,Cu, Fe,Zr)17 type alloy on magnetic processing procedure

The results of an investigation into the effects of the magnetic processing procedure on the intrinsic coercivity of a Sm(Co_0.673Cu_0.080Fe_0.222Zr_0.025)_8.92 (217-type) alloy are reported. Two basic types of magnet were investigated: polymer-bonded fine powder magnets and cast (solid) magnets. The fine particles which were processed into polymer-bonded magnets were prepared by two different methods; i.e. by milling or by a hydrogen treatment. The cast magnets were manufactured from selected parts of solidified ingots exhibiting preferrred orientation and along directions parallel to the preferred orientation of magnetization. Magnetic properties and Vickers microhardness measurements on the solid solution treated (1170° C) and isothermally aged (800° C) samples, revealed that there was a clear similarity between the variations of the intrinsic coercivity and microhardness values versus ageing period. This suggests a coercivity mechanism for the present 217-type alloy which is predominantly controlled by general domain wall pinning by a critical dispersion of coherent precipitates. Certain aspects of the intrinsic coercivity against ageing time variations of the variously processed magnets as well as the corresponding microhardness variations have been attributed to a partial conversion of coherent precipitates to semi- or in-coherent particles during processing.     

Soft magnetic property and magnetization reversal mechanism of Sm doped FeCo thin film for high-frequency application

A series of (Fe_0.67Co_0.33)_1 − xSm_x (0 ≤ x < 0.25) thin films with thickness around 110 nm have been fabricated on silicon(111) substrates by magnetron co-sputtering at ambient condition with a 2.4 kA/m magnetic field applied in the film plane during deposition. With the Sm concentration increasing, FeCo grain size gradually decreases and FeCoSm film eventually becomes amorphous, while the isotropic magnetic property changes to in-plane uniaxial anisotropy as long as Sm is doped. The investigation of the angular dependence of coercivity and switching field indicates that the magnetization reversal mechanism of FeCoSm film is domain-wall depinning and coherent rotation when the applied field is close to the easy axis and hard axis, respectively. The anisotropy field and the resonance frequency of FeCoSm films can be tuned in the range of 15.0-109.5 kA/m and 5.2-11.8 GHz, respectively, by controlling the content of Sm, indicating that FeCoSm films have much potential in high-frequency applications.     

烧结钕铁硼磁体溅射渗镝工艺与磁性能研究

采用直流磁控溅射的方法,在烧结Nd-Fe-B磁体表面制备了Dy薄膜,对比研究了N35烧结态与回火态磁体晶界扩散后组织形貌与性能的变化。N35烧结态与回火态磁体经溅射渗Dy处理后,在剩磁仅降低0.009T和0.03T的情况下,矫顽力大幅度提高,分别提高了708.44kA/m和665.46kA/m,渗Dy处理后磁体中的Dy元素平均质量分数增加不超过0.4%。SEM和EDS能谱的分析结果表明,晶界组织形貌的改善和(Nd,Dy)2Fe14B外延层的形成是矫顽力提升的主要原因。EPMA元素面分布结果显示,Dy主要富集在富Nd相处,三叉型富Nd相处Dy含量最高,而Dy没有扩散到主相晶粒内部,不会导致剩磁大幅度降低,从而有效提高了磁体的综合磁性能。     

SmCo_5/α-Fe双相复合纳米晶磁体的结构与磁性研究

超声波分解Fe（CO）5的产物Fe纳米颗粒,通过非均相沉淀获得包覆型SmCo5/α-Fe双相复合磁粉,采用放电等离子快速热压技术（Spark Plasma Sintering,SPS）制备出全致密的各向同性Sm-Co5/α-Fe双相复合纳米晶磁体,研究发现,软磁相α-Fe添加后,磁体的剩磁Mr有所提高,矫顽力Hci则有所减小,随后通过对各向同性磁体进行热变形制备出各向异性磁体,形成了较好的C轴晶体织构。软磁相α-Fe名义含量为10%时,磁体磁性能为：μ0Ms=1.01T、μ0Mr=0.86T、Hci=0.1708T。     

Influence of Ru/Ru-SiO2 underlayers on the microstructure and magnetic properties of CoPt-SiO2 perpendicular recording media

SiO₂ was doped into Ru underlayer to reduce the grain size of CoPt-SiO₂ perpendicular media. The effects of SiO₂ volume fraction and sputtering deposition pressure of Ru-SiO₂ underlayer on the microstructure and magnetic properties of CoPt-SiO₂ media were studied. Increasing SiO₂ volume fraction in Ru-SiO₂ layer decreased the grain sizes of Ru and CoPt. Adding 5% SiO₂ to Ru-SiO₂ layer increased the coercivity and enhanced the exchange decoupling and thermal stability of CoPt-SiO₂ layer. A further increase in SiO₂ volume fraction caused the deterioration of magnetic properties of CoPt-SiO₂ layer. Deposition of Ru-SiO₂ layer at 1.3 Pa resulted in a smaller activation volume and higher thermal stability of the CoPt-SiO₂ media than that deposited at 0.4 Pa.     

Magnetic, dielectric, and complex impedance properties of nanocrystalline Mn-Zn ferrites prepared by novel combustion method

The electromagnetic and micro-structural properties of nanocrystalline spinel ferrite Mn_xZn_1 − xFe₂O₄ (x = 0.0-1.0) prepared by the novel route of combustion method were investigated. The microstructure and morphology were characterized by X-ray diffraction and scanning electron microscopy, respectively. The magnetic properties were measured using vibrating sample magnetometer. The analysis indicates that the permittivity, saturated magnetization and coercivity increase as the content of manganese rises. Further, the analysis of complex impedance spectra by an equivalent circuit model was used to investigate the AC electrical conduction mechanism. The results show that as-synthesized Mn-Zn ferrites with low conductivity and good magnetic properties have excellent potential for applications in electromagnetic devices.     

各向异性Nd2Fe14B／α-Fe纳米晶复合磁体的制备和性能

采用声化学法、放电等离子烧结技术(SPS)和热变形工艺制备致密各向同性和各向异性Nd_2Fe_(14)B/αFe复合磁体,研究了软磁相包覆对磁体的结构和性能的影响.结果表明,软磁相α-Fe对各向同性Nd_2Fe_(14)B/α-Fe复合磁体的影响主要表现为增强两相间的交换耦合作用,从而提高剩磁.当α-Fe体积分数的数值适当(不超过2%)时,各向异性Nd_2Fe_(14)B/α-Fe磁体形成较好的c轴晶体织构,具有较高的磁性能.α-Fe体积分数为1%的磁体性能最高:B_r=1.367 T,H_(ci)=712 kA/m,(BH)_m=327 kJ/m~3.     

Structure and magnetic property of CoFe2−xSmxO4 (x = 0–0.2) nanofibers prepared by sol–gel route

CoFe_2−xSm_xO₄ (x = 0–0.2) nanofibers with diameters about 100–300 nm have been prepared using the organic gel-thermal decomposition method. The composition, structure and magnetic properties of the CoFe_2−xSm_xO₄ nanofibers were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, inductive coupling plasma mass analyzer and vibrating sample magnetometer. The CoFe_2−xSm_xO₄ (x = 0–0.2) nanofibers obtained at 500–700 °C are of a single spinel structure. But, at 800 °C with a relatively high Sm content of 0.15–0.2 the spinel CoFe_2−xSm_xO₄ ferrite is unstable and the second phase of perovskite SmFeO₃ occurs. The crystalline grain sizes of the CoFe_2−xSm_xO₄ nanofibers decrease with Sm contents, while increase with the calcination temperature. This grain reduction effect of the Sm³⁺ ions doping is largely owing to the lattice strain and stress induced by the substitution of Fe³⁺ ions with larger Sm³⁺ ions in the ferrite. The saturation magnetization and coercivity increase with the crystallite size in the range of 8.8–57.3 nm, while decrease with the Sm content from 0 to 0.2 owing to a smaller magnetic moment of Sm³⁺ ions. The perovskite SmFeO₃ in the composite nanofibers may contribute to a high coercivity due to the interface pinning, lattice distortion and stress in the ferrite grain boundary fixing and hindering the domain wall motion.     

Spark plasma sintering of Li/Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics: Post-annealing temperature effect on phase structure, electrical properties and grain growth behavior

Highly dense Li/Ta co-doped (K,Na)NbO₃ lead-free ceramics were prepared by spark plasma sintering (SPS) in a relatively wide temperature window. The influences of post-annealing temperature on phase structure, electrical properties and grain growth were investigated. Through tailoring post-annealing temperature, relative dielectric constant ?_r, and piezoelectric coefficient d₃₃ of SPSed KNN-based samples can be enhanced to 960 and 225 pC/N for the same composition compared with normally sintered values (771 and 206 pC/N). The enhanced electrical properties were discussed from the point view of microstructure (grain size and its distribution, porosity) and phase transition, which occurred when post-annealing temperature exceeded 1050 °C. Also, with increasing post-annealing temperature, three grain growth behavior stages were investigated on the balance between the volatilization of alkali metal oxides and secondary grain growth.     

Comparison on the magnetic properties and microstructure of directly casted Nd-Fe-Ti-B and Nd-Fe-Ti-Zr-Cr-B-C rod magnets with various diameters

Magnetic properties and microstructure of directly quenched Nd_9.5Fe_72.5Ti₃B₁₅ and Nd_9.5Fe_71.5Ti_2.5Zr_0.5Cr₁B_14.5C_0.5 rod magnets with various diameters of 0.7-1.5 mm have been investigated. TMA shows that all the studied alloy rods mainly consist of a large amount of a 2:14:1 phase. Although the magnetic properties of these two alloys are decreased with the increase of diameter, the latter alloy, consisting of more elements, can preserve attractive magnetic properties up to a larger diameter of 1.3 mm, due to the grain refinement effect induced by multi-component substitutions.     

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

TaC and TaC-1 wt.% B₄C powders were consolidated using spark plasma sintering (SPS) at 1850 °C and varying pressure of 100, 255 and 363 MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B₄C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255 MPa and higher at 1850 °C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B₄C leads to an increase in the density of 100 MPa sample from 89% to 97%. B₄C nano-powder resists grain growth even at high pressure of 363 MPa. The formation of TaB₂/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B₄C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B₄C addition.     

Microstructural, compositional and magnetic characterization of electrodeposited and annealed Co-Pt-based thin films

Near-equiatomic Co-Pt thin films with thicknesses of 520 nm were deposited from a single electrolyte onto glass-based Au-coated substrates using the electrodeposition method. The as-deposited Co-Pt-based films were annealed at temperatures from 500 ºC to 700 ºC for 1 h. The phase formation, microstructure and the magnetic properties were analyzed. It was found that with an increase of the annealing temperature the coercivity increases up to 1.18 T due to the transformation to the L1₀ phase. In contrast to the coercivity the saturation magnetization and the remanence were found to decrease upon annealing, due to the decrease of the Co concentration in the Co-Pt film because of the oxidation and the interdiffusion reactions between the Co-Pt thin film and the substrate.     

Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12 − xPx nanocrystalline soft-magnetic alloys

The P content dependences of the crystallization behavior, thermal stability and soft-magnetic properties of high Fe content Fe_83.3Si₄Cu_0.7B_12 − xP_x (x = 0 to 8) nanocrystalline soft-magnetic alloys were investigated. P addition is very effective in widening the optimum annealing temperature range and refining of bcc-Fe grain size in addition to the increasing of nanocrystalline grain density. Uniform nanocrystalline bcc-Fe grains with average size of about 20 nm and number density of 10²³-10²⁴ /m³ were prepared at around x = 6-8 for the annealed Fe_83.3Si₄Cu_0.7B_12 − xP_x alloys. The coercivity H_c markedly decreases with increasing x and exhibits a minimum at around x = 6-8, while the saturation magnetic flux density B_s shows a slight decrease. Fe_83.3Si₄Cu_0.7B₆P₆ nanocrystalline alloy exhibits excellent soft-magnetic properties with a high saturation magnetic flux density B_s of 1.77 T, low coercivity H_c of 4.2 A/m and high effective permeability μ_e of 11,600 at 1 kHz.     

Study of microstructure and correlative magnetic property in bulk Fe61Nd10B25Nb4 permanent magnet

The correlation between microstructure and magnetic property of a bulk Fe₆₁Nd₁₀B₂₅Nb₄ alloy are investigated. The microstructure of the as-cast Fe₆₁Nd₁₀B₂₅Nb₄ alloy shows a small amount of NbFeB phase with a grain size of 500 nm embedded in an amorphous matrix. The as-cast sample shows soft magnetic behavior at room temperature, after a heat treatment the hard magnetic properties are observed. A fully dense bulk Fe₆₁Nd₁₀B₂₅Nb₄ permanent magnet is obtained with an intrinsic coercivity (_iH_c) of 1191 kA/m and a maximum energy product ((BH)_max) of 31.7 kJ/m³ after annealing at 943 K for 20 min. The corresponding microstructure consists of Nd₂Fe₁₄B, NdFe₄B₄ and NbFeB phases. The existence of the hard magnetic Nd₂Fe₁₄B phase is the reason resulting in a high value of _iH_c. On the other hand, the influences of NdFe₄B₄ and NbFeB phases in the annealed specimen on the magnetic properties are also discussed.