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.     

Microstructure and magnetic properties of Fe72.5Cu1M2V2Si13.5B9 (M=Nb, Mo, (NbMo), (MoW)) nanocrystalline alloys

The microstructure and magnetic properties of Finemet-type Fe_72.5Cu₁M₂V₂Si_13.5B₉ (M=Nb, Mo, (NbMo), (MoW)) alloys have been systematically studied. Results show that the nanocrystalline alloy with M=NbMo has the smallest grain size of about 8 nm. The order of the effect of Nb, Mo and W additions in decreasing the α-Fe grain size in nanocrystalline alloys is Nb>Mo>W. The best DC soft magnetic properties are obtained in the alloy with M=Nb. In the case of AC soft magnetic properties, the Nb alloy also exhibits a very low core loss comparing with typical Finemet alloy. Therefore, the Nb alloy is suitable for use as a transformer core material. In addition, it is shown that a narrow grain size distribution and a uniform dispersion of α-Fe grains in the amorphous matrix are very crucial for the development of new Finemet-type nanocrystalline alloys with good soft magnetic properties.     

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.     

Control of crystallographic orientation in L10 FePt films by using Cr and CrW underlayers

The microstructure and magnetic properties of FePt films grown on Cr and CrW underlayers were investigated. The FePt films that deposited on Cr underlayer show (2 0 0) orientation and low coercivity because of the diffusion between FePt and Cr underlayer. The misfit between FePt magnetic layer and underlayer increases by small addition of W element in Cr underlayer or using a thin Mo intermediate layer, which is favorable for the formation of (0 0 1) orientation and the transformation of FePt from fcc to fct phase. A good FePt (0 0 1) texture was obtained in the films with Cr₈₅W₁₅ underlayer with substrate temperature of 400 °C. The FePt films deposited on Mo/Cr underlayer exhibit larger coercivity than that of the films grown on Pt/Cr₈₅W₁₅ because 5 nm Mo intermediate layer depressed the diffusion of Cr into magnetic layer.     

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.     

Enhanced alignment and magnetic properties of die-upset nano-crystal Nd2Fe14B magnets with Nb addition

It is difficult to obtain the crystallographic alignment for stoichiometric Nd₂Fe₁₄B alloys by applying the melt-spun and subsequent hot-pressing and hot-deformation techniques. However, the enhanced alignment and magnetic properties of die-upset nano-crystal Nd₂Fe₁₄B magnets have been obtained by Nb addition in the present paper. The magnetic properties studies show that Nb addition leads to the remarkable increase of remanence B_r and intrinsic coercivity H_ci, which is due to the improvement of c-axis texture and refinement of microstructure. Microstructure studies using transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that Nb atoms are enriched at grain boundary and the NbFeB phase is observed with increasing Nb content. Since some Fe atoms in the Nd₂Fe₁₄B phase participate in the formation of NbFeB phase, the excessive Nd atoms may be enriched at grain boundary, which may improve the physical property of grain boundary and provide a mass transport pass for preferential growth of oriented Nd₂Fe₁₄B grains, thus leading to the enhanced alignment and magnetic properties.     

Characterization of K2CO3/Co–MoS2 catalyst by XRD,XPS, SEM,and EDS

MoS₂, Co–MoS₂ and K₂CO₃/Co–MoS₂ catalysts have been characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD analysis indicates that Co–MoS₂ is a primary phase in K₂CO₃/Co–MoS₂ catalyst and the diffraction lines of Co–MoS₂ are not changed by the addition of K₂CO₃. Co₉S₈ phase is not present at Co/Mo mole ratio of 0.5 using a co-precipitation method for preparation of cobalt–molybdenum catalyst. The binding energies (BEs) of chemical species present on the surface of the catalysts are compared through the course of catalyst preparation. K₂CO₃/Co–MoS₂ catalyst has been investigated as a function of dispersion of K on the surface and exposure to a mixture of carbon monoxide and hydrogen (syngas) by scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The distribution of potassium on the surface of the K-promoted catalyst is not uniform.     

时效工艺对PtCo合金磁性能的影响

研究了时效温度对PtCo合金磁性能的影响.磁性能测量结果显示,随着时效温度的增加合金的剩磁逐渐减小,时效温度为690 ℃时,合金的矫顽力达到最大值.第一步时效处理后,合金的有序度S随时效温度的提高而增加,长程有序度的增加说明合金有序相的晶粒尺寸也在增加,这是合金矫顽力变化的根本原因.第二步时效处理后,PtCo合金的微观组织结构不发生改变,有序相的各向异性场增大,使合金的矫顽力增大. 关键词： 时效温度 PtCo合金 磁性能 有序度     

Magnetization reversal mechanism of anisotropic HDDR Nd2Fe14B-based magnet powder

The coercivity mechanism of anisotropic Nd₂Fe₁₄B-based magnetic powders prepared by hydrogenation–decomposition–esorption–recombination process was studied. Polarization and corresponding differential susceptibility curves of the powders in its thermally demagnetized state were measured. Microstructure and constituents of the powders were investigated by means of transmission electron microscope and scanning electron microscope with energy dispersive X-ray detector. In addition, theoretical calculation of the intrinsic coercive force of the magnetic powders was performed. It is concluded that the magnetic hardening mechanism of the powders is the pinning of domain walls at grain boundaries of the Nd₂Fe₁₄B main phase and Nd-rich phase that distributes homogeneously around some conglomerations composed of fine Nd₂Fe₁₄B grains. The coercive force of the powders is mainly determined by the pinning of domain walls by Nd-rich boundary phase.     

Magnetic-Electric Metamirror and Polarizing Beam Splitter Composed of Anisotropic Nanoparticles

The emergence of new materials and fabrication techniques provides progress in the development of advanced photonic and communication devices. Transition metal dichalcogenides (e.g., molybdenum disulfide, MoS₂) are novel materials possessing unique physical and chemical properties promising for optical applications. In this paper, a metasurface composed of particles made of bulk MoS₂ is proposed and numerically studied considering its operation in the near-infrared range. In the bulk configuration, MoS₂ has a layered structure being a uniaxial anisotropic crystal demonstrating an optical birefringence property. It is supposed that the large-scale and uniform MoS₂ layers are synthesized in a vertical-standing morphology, and then they are patterned into a regular 2D array of disks to form a metasurface. The natural anisotropy of MoS₂ is utilized to realize the splitting of electric and magnetic dipole modes of the disks while optimizing their geometric parameters to bring the desired modes into overlap. At the corresponding resonant frequencies, the metasurface behaves as either an electric or a magnetic mirror, depending on the polarization of incident light. Based on the extraordinary reflection characteristics of the proposed metasurface, it can be considered an alternative to traditional mirrors and optical splitters when designing compact and highly efficient metadevices, which provide polarization and phase manipulation of electromagnetic waves on a subwavelength scale.     

高矫顽力型FeCrCo合金相变化的穆斯堡尔谱研究

通过穆斯堡尔谱、透射电镜、x射线和磁测量等手段研究了将普通FeCrCo合金中Cr,Co元素含 量提高、并加入Mo,Zr元素后合金矫顽力的变化，发现其值76kA/m比普通值40—52kA/m高50% 以上.对该合金不同热处理阶段的穆斯堡尔谱的测试表明，合金的微观结构与普通FeCrCo合 金有显著的不同.固溶处理后合金单相性好，不产生对磁性起破坏作用的γ相；磁场热处理 过程中，发生原子的重新排布，α₂相开始出现，两相结构基本确定；分级回火 后，调幅 分解进行得更彻底，合金内部完全形成调幅结构，与低矫顽力合金相比，铁磁性的α₁ 相更容易形成. 关键词： 穆斯堡尔谱 调幅分解 磁超精细场 同质异能移     

Effect of precursor on growth and morphology of MoS2 monolayer and multilayer

The rise of two-dimensional (2D) material is one of the results of successful efforts of researchers which laid the path to the new era of electronics. One of the most exciting materials is MoS₂. Synthesis has been always a major issue as electronic devices need reproducibility along with similar properties for mass productions. Chemical vapor deposition (CVD) is one of the successful methods for 2D materials including graphene. Furthermore, the choice of starting materials for Mo and S source is crucial. The different source has different effects on the layers and morphology of MoS₂ films. In this work, we have extensively studied the CVD technique to grow few layers of MoS₂ with two precursors MoO₃ and MoCl₅, show remarkable changes. The MoO₃ source gives a triangular shaped MoS₂ monolayer while that of MoCl₅ can achieve uniform MoS₂ without triangle. The absence of geometric shapes with MoCl₅ is poorly understood. We tried to explain with MoCl₅ precursor, the formation of continuous monolayer of MoS₂ without any triangle on the basis of chemical reaction formalism mostly due to one step reaction process and formation of MoS₂ from gas phase to the solid phase. The film synthesized by MoCl₅ is more continuous and it would be a good choice for device applications.     

Complete suppression of metastable phase and significant enhancement of magnetic properties of B-rich PrFeB nanocomposites prepared by devitrifying amorphous ribbons

The effect of refractory element addition on phase transformation, crystallization behavior and magnetic properties of Pr_8.5Fe_81.5B₁₀ (addition-free) and Pr_8.5Fe_81.5M₂B₁₀ (M=V, Cr, Nb, Zr, Ti) ribbons has been investigated. The annealed addition-free ribbon as well as the samples with V or Cr additions are mainly composed of the metastable Pr₂Fe₂₃B₃ phase, whereas annealed ribbons with Nb, Zr or Ti additions primarily consist of Pr₂Fe₁₄B and a minor amount of Fe₃B/boride. The complete suppression of the metastable Pr₂Fe₂₃B₃ phase due to Nb, Zr or Ti additions leads to a significant enhancement of the magnetic properties. For example, the remanence, the coercivity and the energy product are remarkably increased from 2.5 kG, 0.4 kOe and 0.2 MG Oe for the addition-free material to 9.2 kG, 4.7 kOe and 7.6 MG Oe for the specimens with Nb addition. The successful elimination of the metastable Pr₂Fe₂₃B₃ phase is believed to profit from two factors: (a) Nb, Zr or Ti atoms substitute the Pr site, comparatively increase the Pr content, and thus inhibit the nucleation of Pr-lean Pr₂Fe₂₃B₃ phases, and (b) the formation of Nb, Zr, or Ti borides consumes some part of B, which hinders the generation of the B-rich Pr₂Fe₂₃B₃ phase.     

High coercive Nd-Fe-B magnets fabricated via two-step sintering

The fabrication of Nd₁₁Dy₄Fe_76.5TM_2.5B₆ (TM=Co, Cu, Al, and Nb: at%) sintered magnets via a two-step sintering process was demonstrated. A densification of greater than 99% was achieved using the two-step sintering of the Nd-Fe-B powder. This process produced a sample with an average grain size of 6.2 μm in the Nd₂Fe₁₄B phase, resulting in an enhancement of the coercivity compared to that of the isothermally sintered sample. The suppression of grain growth was considered to be affected by the relatively low temperature during the two-step sintering and by the presence of a B-rich phase at the triple junction. Consequently, the two-step sintering process led to suppression of grain growth and a uniform grain size distribution, which resulted in improved magnetic properties, in particular in the intrinsic coercivity.     

Phase composition, microstructures and magnetic properties of melt-spun Nd1.2Fe10.5Mo1.5 ribbons and their nitrides

The effect of wheel speed on the phase compositions and microstructures of melt-spun Nd_1.2Fe_10.5Mo_1.5 ribbon was investigated. It is found that the Nd(Fe,Mo)₁₂ phase can be obtained at the wheel speed of 10 m/s, and TbCu₇-type Nd(Fe,Mo)₇ phase is formed with the wheel speed higher than 10 m/s. The amorphous phase is achieved at 65 m/s. The average grain size of phases decreases linearly with increasing wheel speed. The Nd(Fe,Mo)₁₂N_1.0 nitride obtained from annealed ribbons quenched at 65 m/s shows a coercivity much higher than that from the ribbons quenched at 10 m/s, which is due to the smaller grain size in the former ribbons.     

Phase conversion of chemically exfoliated molybdenum disulfide

Multilayer MoS₂ is exfoliated by Li treatment. A thin film of Li-treated MoS₂ contains a large portion of 1T phases. This is attributed to the atomic structural change caused by Li insertion, which is investigated using X-ray photoelectron spectroscopy (XPS). In a phase recovery via thermal annealing at 523 K in air, we observe another phase, referred to as the relaxed 1T phase, with a slightly larger binding energy than the 1T phase. After annealing at 523 K in air, the peak intensity of the relaxed 1T phase is reduced, accompanying a strong MoO_x peak and weak S 2s peak, according to XPS. This indicates that the annealing of Li-treated MoS₂ in air yields sulfur vacancies that induce the oxidation of Mo. However, after annealing at 523 K in vacuum, no MoO_x is observed, and the considerable peak intensity of the relaxed 1T phase remains, which starkly contrasts Raman-spectroscopy results supporting a full recovery from the 1T phase to the 2H phase. The absence of a gating effect of the Li-treated MoS₂ device supports the possibility of an incomplete phase change of Li-treated MoS₂ annealed in a vacuum.     

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。     

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.     

The effect of bias voltage and working pressure on S/Mo ratio at MoS2-Ti composite films

S/Mo ratio has a crucial effect on the tribological properties of MoS₂-Ti composite films. The deposition parameters as such bias voltage and working pressure play a dominant role on the change of this ratio value. To determine the effect of working pressure and bias voltage on S/Mo ratio, MoS₂-Ti composite films were deposited on glass wafers by pulsed-dc magnetron sputtering (PMS). The deposition process was performed for nine different test conditions at various levels of target current, working pressure, and substrate voltage using the Taguchi L₉(3⁴) experimental method. It was observed that the chemical composition of MoS₂-Ti composite films was significantly affected by sputtering parameters. It was also observed that S/Mo ratio decreased as the bias voltage increased at a constant working pressure and S/Mo ratio increased with increasing working pressure at a constant bias voltage.     

Enhancement of coercivity with reduced grain size in CoCrPt film grown by pulsed laser deposition

We report a pulsed laser deposition (PLD) growth of VMn/CoCrPt bilayer with a magnetic coercivity (H_c) of 2.2 kOe and a grain size of 12 nm. The effects of VMn underlayer on magnetic properties of CoCrPt layer were studied. The coercivity, H_c, and squareness, S, of VMn/CoCrPt bilayer, is dependent on the thickness of VMn. The grain size of the CoCrPt film can also be modified by laser parameters. High laser fluence used for CoCrPt deposition produces a smaller grain size. Enhanced H_c and reduced grain size in VMn/CoCrPt is explained by more pronounced surface phase segregation during deposition at high laser fluence.