Magnetic properties of a mischmetal-cobalt alloy

The magnetic properties of a mischmetal-cobalt alloy are studied. A mischmetal-64% cobalt alloy was melted in induction furnace under a protective atmosphere of argon. The melt was solidified in a magnetic field of 7 kOe, and cooled to a temperature of 950° C and was thereafter quenched in water. Specimens of suitable dimensions were machined, heat-treated in the temperature range 1000–1100° C for various lengths of time and the magnetic properties evaluated. The (BH)_max value was 10·3 MGOe which was associated with aB _r value of 7 kG and H_c value of 5 kOe. The results obtained offer feasibility to design new MM-Co₅ magnets through developments in processing technique.     

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.     

Anisotropic NdFeB Magnet Fabricated by Single Stroke Hot Deformation

1. IntroductionHard magnetic properties of the materials, such ascoercivity (He), remanence (B,), and energy product[(BH)...] depend strongly on the composition andthe processing conditionslll. The optimally quenchedmelt-spun ribbons of NdFeB alloys produced by rapidqllenching process have equiaxed and randomly oriellted NdZFe14B grains. In order to obtain anisotropicNdFeB grains, the general method is followed by hotpressing and die-upsetting of the ribbon powder oneafter anotherll'2]. …     

Magnetic properties,microstructures and domain structures of arc-plasma sprayed Nd-Fe-B permanent magnet

Thin Nd-Fe-B films prepared by arc-plasma spraying at different substrate temperatures were investigated for their magnetic and structural properties. The isotropic magnets with the best magnetic properties (_M H _c=1.2 MA m^–1, _o M _r=0.6 T, (BH)_max=64 kJ m^–3), were obtained after plasma spraying the Nd-Fe-B powders on water-cooled copper substrates and subsequently annealing the films for 0.5 h at 750 °C. The optimum magnetic properties of the anisotropic Nd-Fe-B films, i.e. _M H _c=1.2 MA^–1, _o M _r=0.9 T and (BH)_max=180 kJ m^–3, were obtained in films sprayed on to heated to 600 °C substrates. The magnetic properties of the sprayed films were strongly influenced by the microstructure. The domain structure of these films is also presented.     

Role of lanthanum in microstructure evolution and enhanced magnetic properties of cerium-containing sintered magnets

In this study, lanthanum was applied to strip cast Ce−Fe−B alloy to improve its phase composition and microstructure. The results reveal that lanthanum doping can significantly enhance the proportion of 2 : 14 : 1 phase and improve the microstructure of Ce−Fe−B alloy. Besides, the influence of the starting alloys structure on the microstructure and magnetic properties of final multi-phases cerium-containing magnets was also systematically investigated. Compared to the multi-phases magnet without lanthanum addition, a pronounced coercivity increment could be distinguished in lanthanum-doping multi-phases magnet, which could be attributed to the finer grain size together with ideal grain boundary. In this work, the superior performance of H_cj = 701.28 kA/m B_r = 1.30 T, and (BH)_max = 313.70 kJ/m³ were obtained by blending Nd−Fe−B alloy with (La_0.35Ce_0.65)-Fe−B alloy to meet 25.0 wt.% lanthanum-cerium utilization content, suggesting that the possibility to develop high abundant rare earth permanent magnetic materials.     

HDD法制备各向异性NdFeB磁粉的研究

本文探讨了用吸氢—歧化—脱氢(HDD)工艺制取各向异性NdFeB永磁粉末的可能性。研究了成份、氢处理工艺以及后处理等对各向异性现象的影响。研究结果表明,在Nd_xFe_(87)-x-yCo_6B_7Zr_y的合金中经适当的氢气处理,初步获得了各向异性磁粉,由M115型振动样品磁强计所测磁粉的磁性能为:B_r=0.92emu/gm,iH_c=720kA/m(9kOe),(BH)_(max)=112kJ/m~3(14MGOe)。由该粉与3％(重量百分比)的环氧树脂混合制成的各向异性粘结磁体的磁性能为B_r=0.65T,iH_c=744kA/m(9.3kOe),(BH)_(max)=64k3/m~3(8MGOe)。本文还观察了各向异性粉的微观结构。     

Effect of forms of B2O3-SiO2 as an additive on magnetic properties of SrFe12O19 ferrites

Strontium ferrites (SrO · 5.5Fe₂O₃) were prepared using hot-rolled mill scale as a source of iron oxides. Calcination was performed at 1200°C for 2 h. The fused B₂O₃-SiO₂, with various mole ratios of B₂O₃ to SiO₂, was added to the calcined ferrites during the milling stage. The ferrites were formed anisotropically. The fused additives are quite effective to enhance the magnetic properties; (BH)_max can reach 3.0 MGOe for the ferrite sintered at 1220°C for 2 h. In general, (BH)_max is independent of the mole ratio of B₂O₃ to SiO₂ and dominantly influenced by the sintering temperature. The addition of 0.3 or 0.5 wt% fused additives showed no significant difference in the magnet quality. The quality of the magnet was decreased with increasing mole ratio of B₂O₃ to SiO₂ when unfused B₂O₃-SiO₂ was used under the same processing conditions.     

A practical classification of ferromagnets in relation to composition,temperature and fabrication process variations

The classification of sets of ferromagnets may be usefully discussed in a graphical form by plotting theH _cB/B _r andH _cl/B _r ratios of the individual magnets. For each set, a curve of the formH _cB/B _r=K(H _cl/B _r) ^L , can be fitted, whereK andL are constants of fit. This general form is discussed in detail with respect to the theoretical limiting cases. An appraisal of data from several sets of rare earth-based ferromagnets indicates that these ferromagnetic materials can be classified into three general types, especially when the powder metallurgical processes and conditions for fabricating the actual magnets are taken into consideration. Effects of composition and temperature variations on the magnetic properties can also be consistently discussed within this classification.     

Ga对HDDR磁粉热变形磁体磁性能和微观结构的影响

利用HDDR工艺制备出Nd32FebalBGax(x=0.0、0.2%、0.4%、0.6%、0.8%(质量分数))磁粉,并且对HDDR磁粉进行热压/热变形处理制备出全密度各向异性磁体。研究了热变形温度和Ga含量对Nd-Fe-B热变形磁体磁性能的影响,观测了不同Ga含量热变形磁体的微观结构,探讨了微量元素Ga的添加对用HDDR磁粉制备的热变形磁体微观结构和磁性能的影响机制。研究发现,Ga的添加能够明显减小热变形磁体的主相晶粒尺寸,改善磁体的微观织构,并可以同时提高热变形磁体的剩磁和矫顽力。当Ga含量为0.6%(质量分数)时,热变形磁体的磁能积达到最大值228.3kJ/m3。     

The Influence of Co and Nb Additions on the Magnetic Properties and Thermal Stability of Ultra-high Intrinsic Coercivity Nd-Fe-B Magnets

The effect of Co and Nb additions on the magnetic properties and thermal stability of ultra-high intrinsic coercivity Nd-Fe-B magnets has been investigated. The results show that the remanence (B _r ) and maximum energy product (BH)_max of sintered Nd-Fe-B magnets first increases, and reaches a maximum, then starts to decrease with increasing Co content. The intrinsic coercivity and temperature coefficients of remanence (α) decrease, and the temperature coefficients of coercivity (β) increase with increasing Co content. The minimum reversible magnetic flux loss, temperature coefficients of reversible magnetic flux, total magnetic flux loss and irreversible magnetic flux loss are obtained when the Co content is 7.5 at.%. The temperature coefficients of coercivity (β) and remanence (α) for sintered Nd₉Dy₆Fe_79?x Nb _x B₆ magnets from 300 K to 513 K are 0.36 %/K and 0.127 %/K respectively when the content of Nb is 1.0 at.%. The reasons for the variation on magnetic properties and temperature coefficients were analyzed.     

Nanocomposite Magnets and their Preparation under High Pressure

This article reports current studies on nanocomposite magnets and a newly developed route for their preparation, known as crystallization of amorphous alloy under high pressure (CAAHP). The microstructure and magnetic properties of nanocomposite magnets prepared by CAAHP are presented, and the influences of pressure on the microstructures of the magnets are discussed. α‐Fe/Sm₂(Fe,Si)₁₇C_x nanocomposite magnets with a grain size <10 nm have been successfully prepared by CAAHP, and a maximum energy product of about 25 MGOe is obtained.     

Improving the hard magnetic properties by intragrain pinning for Ta doped nanocrystalline Ce-Fe-B alloys

To develop Ce based permanent magnets with high performance/cost ratio, Ta doping is was employed to enhance the magnetic performance of Ce-Fe-B alloys. For melt spun Ce₁₇Fe_78-xTa_xB₆ (x = 0–1) alloys, the coercivity H_c increases from 439 to 553 kA/m with increasing x value from 0 to 0.75. Microstructure characterizations indicate that Ta doping is helpful for grain refinement. A second phase of TaB₂ is observed in Ce₁₇Fe_77.25Ta_0.75B₆ alloy, which acts as the pinning center of the magnetic domains, resulting in the change of coercivity mechanism from nucleation type to nucleation + pinning type. The micromagnetic simulation confirms that non-magnetic particles within hard magnetic phase can increase the demagnetization field around them and it is crucial for preventing the further magnetization reverse by pinning effect. Take the advantage of Ta doping for enhancing the coercivity, Ce content of Ce-Fe-B alloy can be further cut down to increase the remanence J_r due to the reduced volume fraction of CeFe₂ phase and increased Fe/Ce ratio. As a result, a good combination of magnetic properties with H_c = 514 kA/m, J_r = 0.49 T, and the maximum energy product (BH)_max = 36 kJ/m³ have been obtained in Ce₁₅Fe_79.25Ta_0.75B₆ alloy. It is expected that the present work can serve as a useful reference for designing new permanent magnetic materials with low-cost.     

High magnetic performance NdFe10.5Mo1.5Nx prepared by mechanical alloying

Spherical or nearly regular hexagonal Nd powders with the size of 20–150 nm, Fe powders with the size of 20–150 nm and Mo powders with the size of 80–100 nm were prepared by Argon and Hydrogen arc plasma. The nanostructural NdFe_10.5Mo_1.5 compound with ThMn₁₂-type structure have been formed after having been mechanically alloyed for 6–12 hours and crystallized at 700–850 °C under high purified Argon atmosphere. After having been nitrided at 400–500 °C for 2 hours, NdFe_10.5Mo_1.5N _x nanopowders with ThMn₁₂-type structure can be obtained, which have excellent permanent magnetic properties such as iH _c within range of 360.4–716.2 kA/m (4528–8997 Oe), B _r within range of 0.6363–0.9831 T (6363–9831 Gs), (BH)_max within range of 42.87–166.2 kJ/m³ (5.386–20.88 MGOe).     

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.     

Appreciable Magnetic Moment and Energy Density in Single-Step Normal Route Synthesized MnBi

We study the structural and magnetic properties of the MnBi intermetallic compound. The LTP (Low Temperature Phase) MnBi compound is successfully synthesized in single step by vacuum encapsulation technique and rapid quenching from phase formation temperature. The phase purity and the magnetic moments of MnBi are highly dependent on heat treating schedule. The best phase purity and the magnetic moment are found for a heat-treated sample at 310 °C for 48 h. Rietveld fitted X-ray diffraction (XRD) patterns revealed that the studied MnBi compound is crystallized in hexagonal P63/mmc space group with minute presence of unreacted Bi and Mn phases. The scanning electron microscopy (SEM) study is carried out to visualize the grains morphology and phase identification. The bulk MnBi powder showed appreciable magnetic moment of ～62 emu/g at 6 Tesla and maximum energy product BH _max of 4.01 MGOe at 6 Tesla. The magnetic properties of synthesized MnBi show that it could be a potential candidate for rare earth free permanent magnets.     

Cobalt ferrite nanoparticles in a mesoporous silicon dioxide matrix

We have studied magnetic nanoparticles of cobalt ferrite obtained by the extraction-pyrolysis method in a mesoporous silicon dioxide (MSM-41) molecular sieve matrix. The X-ray diffraction data show evidence for the formation of CoFe₂O₄ particles with a coherent scattering domain size of ∼40 nm. Measurements of the magnetization curves showed that powders consisting of these nanoparticles are magnetically hard materials with a coercive field of H _c(4.2 K) = 9.0 kOe and H _c(300 K) = 1.8 kOe and a reduced remanent magnetization of M _r/M _s(4.2 K) = 0.83 and M _r/M _s(300 K) = 0.49. The shape of the low-temperature (4.2 K) magnetization curves is adequately described in terms of the Stoner-Wohlfarth model for randomly oriented single-domain particles with a cubic magnetic anisotropy.     

Magnetic properties, phase evolution, and microstructure of melt spun Hf-substituted Sm(Co0.97Hf0.03)(x)Cy (x = 5-9; y = 0-0.1) nanocomposites

Magnetic properties, phase evolution, and microstructure of melt spun Hf-substituted Sm(Co0.97Hf0.03)(x)Cy (x = 5-9; y = 0-0.1) ribbons quenched at the wheel speed of 40 m/s are investigated. X-ray diffraction analysis shows that the main phases existed in Sm(Co0.97Hf0.03)(x) ribbons are 1:5 phase for x = 5-5.5; 1:5 and 1:7 phases for x = 6; 1:7 phase for x = 6.5-7.5; 1:7 and 2:17 phases for x = 8; and only 2:17 phase for x = 8.5-9, respectively. For Sm(Co0.97Hf0.03)(x) (x = 5-9) ribbons, the optimum magnetic properties of B(r) = 5.6 kG, (i)H(c)= 15.6 kOe and (BH)(max) = 7.1 MGOe are obtained for Sm(Co0.97Hf0.03)6.5 ribbons. Furthermore, a slight amount of C addition in Sm(Co0.97Hf0.03)(x) ribbons slightly modify phase constitution and effectively refine the grain size from 200-700 nm for C free ribbons to 10-70 nm, strengthening the exchange coupling effect between magnetic grains of the ribbons. As a result, magnetic properties are further improved. The magnetic properties of B(r) = 6.9 kG, (i)H(c) = 9.2 kOe and (BH)(max) = 10.0 MGOe can be achieved for Sm(Co0.97Hf0.03)7.5C0.1 nanocomposites.     

Improvement of corrosion resistance of Cu and Nb co-added Nd–Fe–B sintered magnets

Cu and Nb powders are co-added as intergranular modifiers to improve the corrosion resistance of Nd–Fe–B sintered magnets. For the magnet co-added with 0.2 wt.% Cu and 0.8 wt.% Nb, mass loss of accelerated corrosion test in 120 °C, 2 bar and 100% relative humid atmosphere for 96 h drops from 2.47 mg/cm² to 0.49 mg/cm² in comparison with the Cu/Nb free one. The corrosion potential E_corr in 3.5% NaCl aqueous solution increases from −1.115 V to −0.799 V, which indicates the better resistance against electrochemical corrosion. The improved corrosion resistance is ascribed to the enhanced stability of the intergranular phase by forming high electrode potential Cu-containing phase and reduced Nd-rich phase at triple junctions. Besides, the distribution of (Pr, Nd)-rich phases along the grain boundaries becomes more clear and continuous through Cu/Nb co-addition, maintaining fairly good magnetic properties of B_r = 13.6 kGs, H_cj = 11.4 kOe, (BH)_max = 46.2 MGOe. Further investigation demonstrates that Nb is effective to refine the grains of hard magnetic Nd₂Fe₁₄B phase and Cu is beneficial for optimizing the distribution of the intergranular phases.     

Magnetoresistance and Irreversibility Fields of Bismuth-Based 1G Tape

The magnetoresistance and irreversibility fields of commercial bismuth-based 1G tape were studied on the basis of the temperature dependencies of the magnetoresistance at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape are the following: T _{c50 %} = 110.3 K and T _c0 = 109.9 K, and the width of superconducting transition is ΔT = 0.5 K. The widths of the transition to the superconducting state versus applied magnetic fields were derived for both orientations. The experimental data were fitted using the formula ΔT = C H ^m + ΔT ₀. The irreversibility field values were obtained and successfully fitted as a function of temperature. At 77 K, they were found to amount to H _{i r r} = 72.8 kOe and H _{i r r} = 5.5 kOe for the parallel and perpendicular directions, respectively.     

Effect of the Ce Content on the Magnetic Properties and Microstructure of CeCo<Subscript>5</Subscript>-based Sintered Bulk Magnets

CeCo₅-based magnets have recently attracted much attention due to their moderate magnetic performance and low cost. Nevertheless, there have been few studies on the effects of Ce content on the magnetic properties and microstructures of CeCo₅-based magnets. In response to this, the magnetic properties of sintered bulk magnets with nominal compositions of Ce(Co_0.73Cu_0.135Fe_0.135) _z (z =?4.95, 5.15, 5.35, and 5.55), prepared by the conventional powder metallurgy method, were investigated here. Based on experimental findings, it was shown that Ce(Co_0.73Cu_0.135Fe_0.135)_5.15 sintered bulk magnets had comprehensive magnetic properties—maximum energy product of 80 kJ m^??3 (10 MGOe) and intrinsic coercivity (H _cj) of 452 kA m^??1 (5.69 kOe)—superior to those previously reported by us. For z =?5.35 and 5.55, due to the presence of the minor Ce₂Co₁₇ phase (which has a Curie temperature (T _c ) <?20 °C), magnets had low H _cj values. Based on x-ray diffraction and scanning electron microscopy observations, it was suggested that the volume fraction of the 1:5 matrix phase was the main factor determining the H _cj of CeCo₅-based sintered bulk magnets obtained with different Ce contents. Furthermore, the importance of the dispersion characteristics of the Ce₂O₃ phase within the matrix was emphasized. Uniform dispersion of the Ce₂O₃ phase can significantly improve the overall magnetic performance of CeCo₅-based magnets.