Hydrogen absorption and desorption characteristics of high coercivity NdDyFeCoNbCuB sintered magnet. I. Low temperature hydrogen decrepitation treatments

Hydrogen absorption/adsorption properties of high coercivity NdDyFeCoNbCuB sintered magnets were determined. Hydrogenation kinetics were analyzed using both differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Hydrogenation of the Nd-rich intergranular phase results in a rather broad and large peak at ∼100 ± 50 °C, then the tetragonal main phase (Φ phase) reacts readily close to 195 °C. The disproportionation process of the whole magnet initiates at T ∼ 500 °C, then accelerates in the vicinity of 600 °C and finally ends at T ∼ 780 °C. Furthermore, the first hydrogenation reaction, which is associated with the hydrogen diffusion in the bulk via the intergranular Nd-rich phase, was seen to proceed quite differently depending on the heating rate, or the applied plateau temperature. While hydrogen absorption at 50 °C is rather slow, it results in higher hydrogen uptake than at 150 °C, though there it happens much faster. Three modes of hydrogenation of sintered magnets are discussed in terms of practical operability. Using the optimized hydrogen decrepitation/desorption annealing route leads to a demonstration that the anisotropic NdDyFeCoNbCuB powders obtained by the HD/D technique have recovered most of the magnetic performance initially displayed in the bulk magnets.     

Structure and magnetic properties of sputtered Nd-Fe-B alloys

Studies were conducted on the relationship between the composition of Nd-Fe-B alloys, the temperature of the substrate, additional heat treatment and magnetic properties, crystalline phases, and microstructure and texture of films 30 to 300 μm thick produced by ion-plasma sputtering of cast targets. The sputtering rate was maintained at ~30 μm/h. The axial crystalline texture (001) along the normal to the sputtering plane is formed at a sputtering temperature within the limits ranging from 300 to 450 °C and higher than 600 °C. In the intermediate domain of sputtering temperatures, there is a deterioration of the texture (001), or such a change of the indices of the texture when the normal to one of the planes of the zone [010] becomes its axis. Within the framework of the classical theory of crystallization, a model of growth texture formation during the course of sputtering is suggested. The model explains the nonmonotonic change of texture with the change of sputtering temperature. The data obtained about the structure and properties of films have made it possible to optimize the sputtering process with a view to obtaining sputtered magnets with very good permanent magnet properties. Examples of magnetic systems and devices with sputtered magnets are given.     

合金成分对Pr2Fe14B/α-Fe纳米复合永磁材料组织与磁性的影响

用XRD、TEM、Mossbauer谱和VSM等实验方法,研究了不同Pr含量、B含量和Cu含量的Pr2Fe14B/α Fe型纳米复合快淬带的显微结构与磁性。结果表明:PrxFe94-xB6合金在x=8(α Fe体积分数约30%)时磁性能最佳,Br=1.29T,Hci=461.7kA/m,(BH)max=165.6kJ/m3;Pr8.5(Fe0.8Co0.2)86.5-xCuxB5合金在x=0.5时获得最佳的磁性能;随B含量增加,富B相在晶界分布,Pr8Fe92-xBx交换耦合减弱,磁性能单调下降。     

Effects of oxygen and carbon on the magnetic properties and microstructure of Sm2Co17 permanent magnets

The research on the sintered Sm2Co17 permanent magnets prepared by metal injection molding is still at the exploratory stage. Carbon and oxygen are two key factors that influence the magnetic properties. In this article, the effects of oxygen and carbon on the properties and microstructure of the magnets have been studied. The results indicate that oxygen consumes the effective Sm content of the magnets and forms Sm2O3-the non-magnetism phase, which result in the deterioration of the magnetic properties. Besides, the magnetic properties decrease in evidence with increasing carbon content. The main factor that affects the magnetic properties is the deterioration of the microstructure of the magnets. The Sm（Co, Cu）5 phase decreases, whereas the cell size increases with the increase of the carbon content. When the carbon content is above 0.43 wt.%, the Sm（Co, Cu）5 phase is not enough to form a uniform cellular microstructure. Thus the magnetic properties disappear. ZrC is detected in the magnets by XRD when the carbon content is above 0.21 wt.%. ZrC also reduces the properties of the magnets.     

Magnetic properties and thermal stability of anisotropic bonded Nd-Fe-B magnets by warm compaction

Anisotropic bonded magnets were prepared by warm compaction using anisotropic Nd-Fe-B powder. The forming process, magnetic properties, and temperature stability were studied. The results indicate that the optimal temperature of the process, which was decided by the viscosity of the binders, was 110℃. With increasing pressure, the density of the magnets increased. When the pressure was above 700 MPa, the powder particles were destroyed and the magnetic properties decreased. The magnetic properties of the anisotropic bonded magnets were as follows: remanence Br = 0.98 T, intrinsic coercivity iHc=1361 kA/m, and maximum energy product BHmax = 166 kJ/m3. The magnets had excellent thermal stability because of the high coercivity and good squareness of demagnetization curves. The flux density of the magnets was 35% higher than that of isotropic bonded Nd-Fe-B magnets at 120℃ for 1000 h. The flux density of the bonded magnets showed little change with regard to temperature.     

Influence of heat treatment on fracture and magnetic properties of radially oriented Sm2Co17 permanent magnets

The quenching, fracture and aging treatment of radially oriented Sm2Co17 ring magnets were investigated. The results indicate that the ring magnets have obvious anisotropy of thermal expansion, which easily leads to the splits of the magnets during quenching. The fracture is brittle cleavage fracture. The difference （Aa） of the expansion coefficient reaches the maximum value at 800-850 ℃. So, various quenching processes at different steps are adopted in order to reduce the splits. When the magnets are aged, 1：5 phase precipitates from the 2：17 matrix phase and forms a cellular microstructure with 2：17 phase. BHmax and JHc reach the maximum value 226 kJ/m^3 and 2 170 kA/m after being aged at 850 ℃ for 4 h and 8 h, respectively. The aging treatment at 850 ℃ has little influence on remanence（Br）, which can always keep a high value （≥1.0 T）. Through appropriate heat treatment, the ring magnets have uniform cellular microstructure and excellent magnetic properties： Br ≥ 1.0T, JHc ≥2 100 kA/m, BHmax ≥ 220 kJ/m^3.     

Magnetic anisotropy in Fe-25Cr-12Co-1Si alloy induced by external magnetic field

Structural and magnetic properties of Fe-25Cr-12Co-1Si alloy thermo-magnetically treated under different external magnetic field conditions were investigated. Orientation and morphology of the ferromagnetic α1 phase embedded in α2 phase matrix before and after step ageing are characterized by transmission electron microscope（TEM）. The results show that the ellipsoidal particles of ferromagnetic al phase are aligned along the direction of external magnetic field during isothermal magnetic ageing. Approximately 28% of the total coercivity can be attributed to the shape anisotropy of al phase particles induced by external magnetic field for Fe-25Cr-12Co-1Si alloy thermo-magnetically treated with a parallel magnetic field.     

NdFeB粘结磁体的使用温度及磁性能

对磁粉进行表面处理,利用冷、热模压法制备了金属基及塑料基两种粘结NdFeB磁体,研究了表面处理前后及不同基体磁体的使用温度和磁性能.研究结果表明磁粉的表面处理可以提高磁体的磁性能及使用温度,塑料基磁体的磁性能低于金属基磁体的,但其使用温度却较高,可达180 ℃左右.     

Rate determining step in the absorption and desorption of hydrogen by magnesium

The kinetics of hydrogen absorption and desorption by magnesium has been investigated by a volumetric technique. Experimental data have been analysed in order to find the rate determining step for both the absorption and desorption processes. It is shown that a nucleation and growth (NG) mechanism, with exponent values n=2 for desorption and n=0.5 to n=l for absorption provides suitable equations in order to fit the experimental data. The influence of hydrogen pressure and temperature on the process rate has been studied to obtain an expression for the driving force of the reaction and its activation energy. The driving force for desorption seems to follow a parabolic law though the experimental data are also compatible with a linear law. According to our data, the rate determining step in the desorption of hydrogen by magnesium, and probably also in the absorption process, seems to be the hydrogen diffusion through the β phase. An activation energy for such a diffusion process of 100±10 kJ mol⁻¹H has been obtained from the desorption data.     

Structure and magnetic properties of strontium ferrite anisotropic powder with nanocrystalline structure

The phase composition, nanocrystallite size, lattice microstrain and particles morphology of a SrFe₁₂O₁₉ powder subjected to milling and subsequent annealing were studied by various methods. The investigations showed that the high-temperature annealing of the preliminarily milled powder resulted in the increase in the coercive force (μ₀Н_сi) of the SrFe₁₂O₁₉ powder up to 0.4 T owing to the formation of nanocrystalline structure (D ∼ 10³ nm) with low lattice microstrains. However, the annealed powder cannot be textured in an applied magnetic field because of random orientations of the crystallites in powder particles. A processing technique, which includes the low-temperature annealing of powder in an applied magnetic field, was suggested. It allowed us to produce the anisotropic powder of the strontium ferrite with the nanocrystalline structure that ensures the high coercive force of the powder (∼0.4 T) and possibility of the powder texturing in the magnetic field. The prepared samples textured in a magnetic field exhibit the higher both remanence (by a factor of 1.4) and energy product (by a factor of 2.1) as compared to those of isotropic SrFe₁₂O₁₉ samples.     

成膜时外加稳恒磁场对Fe-Ga-Al-Y磁致伸缩薄膜磁性能的影响

借助自行设计的磁场发生器,以铍青铜为衬底,采用离子束溅射沉积法在不同稳恒磁场环境下制备Fe-Ga-Al-Y磁致伸缩薄膜(MF)样品,其成分为Fe_(74.34~79.33)Ga_(11.45~13.73)Al_(5.33~6.95)Y_(3.27~4.36)。采用原子力显微镜(AFM)与扫描电镜(SEM)观测Fe-Ga-Al-Y MF表面形貌,通过激光微位移传感器和交变梯度磁强计分别对薄膜样品的悬臂梁自由端偏转量与磁滞回线进行测定,研究Fe-Ga-Al-Y MF表面形貌以及成膜时外加稳恒磁场方向及大小对Fe-Ga-Al-Y MF磁性能的影响。研究结果表明:通过IBSD法制备的薄膜样品表面光滑平整,无明显的缺陷,薄膜组织结构均匀致密,具有优异的成膜质量。成膜时施加的稳恒磁场能调控薄膜的易磁化轴方向,Fe-Ga-Al-Y MF的易磁化轴方向与成膜时所施加的稳恒磁场方向一致;成膜时施加与衬底短轴方向平行的稳恒磁场,可以显著提高Fe-Ga-Al-Y MF的饱和磁致伸缩性能,且随着磁场的增大,薄膜的饱和磁致伸缩性能逐渐增加。薄膜面内(沿着x轴方向、y轴方向)的饱和磁化强度、矫顽力也随着磁场的增大而逐渐增大。     

Development of permanent magnetic refrigerator at room temperature

A reciprocating magnetic refrigerator was developed based on the active magneticregeneration technology. Rare earth metal Gd and intermetallic compound LaFe11.2Co0.7Si1.1 were used as the magnetic operating materials in the machine. The particles of the magnetic operating materials, with diameter of 0.5- 2 mm and total mass of 950 g, were mounted in the cooling bed. A magnetic field was assembled using NdFeB rare earth permanent magnets. It had the magneticfield space of Φ 34×200 and the magnetic induction of 1.5 T. The water at pH=10 is used as a heat transfer fluid. When the ambient temperature is 296 K, a temperature span of 18 K was achieved after operation of 45 min at a frequency of 0.178 Hz. The temperature span and the output power increase significantly with the increasing velocity of heat transfer.     

Plasma deposition of powder materials in a magnetic field

The effect of the longitudinal magnetic field on the properties of deposited metal is investigated. The experimental results show that a longitudinal magnetic field with a specific value of magnetic induction has a positive effect on various parameters such as the burn out of the alloying elements, the penetration depth of the parent metal and the hardness and microhardness of the coating.     

Effects of Mo substituted Nb on magnetic properties of Finemet alloy

Different Mo contents have been added into traditional Finemet alloy to form Fe73.5Cu1Nb3-x MoxSil3B9.5( x = 0 - 3) alloys. The change in DC and AC magnetic properties with Mo for Nb substimtlon was investigated. The results show that, with adding Mo, although the DC relative permeability decreases and the coercive force increases slightly,the saturation flux density Bs can be increased, and the core loss of the alloy can be decreased. The AC permeability of samples contained Mo is higher than that of alloy without Mo content. Fe73.sCu1Nb1Mo2Si13B9.5 alloy has the highest saturation flux density Bs. Fe73.sCu1Nb2Mo1Si13B9.5 alloy has the best frequency dependence on the AC permeability and core loss.     

Structure and magnetic properties ofhigh-performanced Sm2（Co, Fe, Cu, Zr）17 alloys

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磁场热处理对纳米复相Nd2Fe14B/α-Fe永磁体磁性能的影响

研究了磁场热处理工艺对Nd10.5Fe76.4Co5Zr2B6.1永磁体组织结构和磁性能的影响,采用XRD、AFM等方法对合金的组织结构、晶化行为进行了研究.结果表明:与传统热处理工艺相比,晶化过程中外加磁场可促进快淬NdFeB粉末的晶化,降低晶化温度,缩短晶化时间;磁场热处理可细化晶粒,增强晶粒间磁交换耦合作用,提高磁性能;在外加磁场为0.28 T,经670℃/10 min晶化处理后,可获得最佳磁性能,Br=0.670 T,Hcj=687kA/m,Hcb=427 kA/m,(BH)m=75 kJ/m3.     

Annealing temperature dependance of magnetic properties and magneto-impedance effect in CoZrB alloys

The influence of conventional annealing on the magnetic properties and the magneto-impedance (MI) effect of Co₇₂Zr₈B₂₀ alloys has been intensively studied in this paper. The as-quenched Co₇₂Zr₈B₂₀ ribbon is identified to be amorphous alloy. After Co₇₂Zr₈B₂₀ being annealed at 495 °C for 10 min, there is almost no change in the XRD pattern. When the annealing temperature (T_a) reaches 540 °C, a diffraction peak representing some crystalline phase appears. After the sample being annealed at 630 °C, Co, Zr and B₁₂Zr crystalline phase are formed in the alloy. Magnetic measurements reveal that the as-quenched Co₇₂Zr₈B₂₀ possesses smaller coercivity, higher permeability and larger saturation magnetization. With the increasing annealing temperature, the soft magnetic properties of the sample greatly deteriorate. The study on the MI ratio (ΔZ/Z(%)) versus external magnetic field curves indicates that the MI behavior changes from single-peak to double-peak shape when T_a increases from 495 to 540 °C. The most drastic MI ratio about 90% is obtained in the as-quenched Co₇₂Zr₈B₂₀ at 1110 kHz. The mechanism of thermal treatment affecting the magnetic properties and the MI effect in Co₇₂Zr₈B₂₀ alloys will be discussed in this paper.     

Microstructures and magnetic properties of hydrogenation disproportionation desorption recombination-processed Nd–Fe–B materials with different Nd content of 11.0 and 12.6 at.%

The hydrogenation disproportionation desorption recombination (HDDR) process was performed on the generally used alloy composition of Nd_12.6Fe_63.1Co_17.4Zr_0.1Ga_0.3B_6.5 and a low rare earth content alloy composition of Nd_11.0Fe_65.0Co_17.8Zr_0.1Ga_0.3B_5.8. A detailed evaluation was made of the relationship between the microstructure and magnetic properties of these HDDR-processed magnetic powders with respect to their different rare earth element concentrations. The HDDR-processed powders of both alloy compositions were transformed to the Nd₂Fe₁₄B phase consisting of fine recombined crystal grains of around 400–500 nm in size and maintained the anisotropic magnetic characteristic that was present before HDDR processing. However, reduction of the rare earth content drastically reduced coercivity, and the alloy composition of Nd_11.0Fe_65.0Co_17.8Zr_0.1Ga_0.3B_5.8 did not manifest magnetic properties. From the results of an examination of their microstructures, it was inferred that the coercivity decreased due to a decline in the concentration of the rare earth element at the grain boundaries of the fine Nd₂Fe₁₄B grains with the reduction of the rare earth content of the alloys. Accordingly, in magnetic powders obtained by the HDDR process, the nucleation type of coercivity mechanism predominates, in which rare earth-rich regions present at the grain boundaries of fine Nd₂Fe₁₄B grains play a large role in the manifestation of coercivity.     

Investigation of hydrogen absorption/desorption properties of ZrMn0.85−xFe1+x alloys

The crystal structures and hydrogen absorption/desorption properties of the ZrMn_0.85−xFe_1+x alloys (x = 0, 0.2, 0.4) were investigated systematically. The pressure–composition (PC) isotherms and absorption kinetics were measured at 273–333 K by the volumetric method. Besides the crystal structure, the plateau pressure and the hydrogen intake capacity, this article also discussed the absorption kinetics, the pulverization resistance and the thermodynamic properties. XRD patterns revealed that ZrMn_0.85Fe and ZrMn_0.65Fe_1.2 were formed as hexagonal C14 laves phase structure while ZrMn_0.45Fe_1.4 possessed cubic C15 laves phase structure. With the increase of Fe and decrease of Mn, the plateau pressure increased while the hydrogen intake capacity lowered and the hydrogen absorption kinetics degraded. On the other hand, the hysteresis alleviated, the pulverization resistance improved and the stability of the hydrides decreased. The decomposition pressure was increased to more than 160 times for ZrMn_0.85Fe and more than 2500 times for ZrMn_0.65Fe_1.2 compared with that of the ZrMn₂ alloy.