双主相混合稀土基RE-Fe-B磁体的磁性能和抗腐蚀性改进

为了实现稀土资源的平衡应用且降低RE-Fe-B稀土永磁材料的价格,针对混合稀土基永磁材料进行研究,分别采用单、双主相工艺制备了名义成分[(Pr,Nd)_1-xMM_x]_30.3(Fe,Co)_balM_0.73B_0.98 （x=0.3,0.5和0.7,质量分数）的磁体,对比研究其磁性能和抗腐蚀性。研究发现：双主相工艺制备的磁体相比单主相工艺制备的同成分磁体展现了优越的磁性能和抗腐蚀性。当x=0.5,双主相磁体的磁性能为B_r=1.308 T,H_cj=799.98 kA/m和(BH)_max=325.6436 kJ/m³,远高于同成分的单主相磁体的性能（B_r=1.297 T,H_cj=746.8868 kA/m 和(BH)_max=317.8428 kJ/m³）。这种改进源于富稀土相分布的改进以及主相晶粒间和晶粒内部耦合作用的增强。当双主相磁体暴露在湿热环境下时,磁体中不仅存在富稀土相腐蚀,也存在主相晶粒的腐蚀成粉现象,这主要是由于富稀土相与水蒸气和氧气反应时产生氢气,导致主相晶粒被氢化,由于主相晶粒间和晶粒内部的镧铈分布差异,产生大的应力,导致其表现出区别于单主相磁体的腐蚀行为。     

Recent developments of high-performance NEOMAX magnets

For further improvement in achieving extremely high magnetic properties of Nd- Fe- B sintered magnets, extensive investigation has been done to densify the magnet up to the theoretical value, to increase the volume fraction of the Nd ₂Fe₁₄B matrix phase, and to achieve a high degree of alignment. By controlling chemical composition and the amount of constituent phases,improving particle size distribution, and adopting the isostatic pressing method to get better alignment of fine particles, we have succeeded in obtaining a high-performance magnet having residual flux density (B_r) of 1.495 T (14.95 kG), maximum energy product [(BH)_max] of 431 kj/m³ (54.2 MGOe), and intrinsic coercivity (iH_c) of 845 kA/m (10.62 kOe).     

The magnetic, structure and mechanical properties of rapidly solidified (Nd7Y2.5)-(Fe64.5Nb3)-B23 nanocomposite permanent magnet

The Nd₇Y_2.5Fe_64.5Nb₃B₂₃ nanocomposite permanent magnets in the form of rods with 2 mm in diameter have been developed by annealing the amorphous precursors produced by copper mold casting technique. The phase evolution, structure, magnetic and mechanical properties were investigated with X-ray diffractometry, differential scanning calorimetry, electron microscopy, magnetometry and universal uniaxial compression strength techniques. The heat treatment conditions under which the magnets attained maximum magnetic and mechanical properties have been established. The results indicate that magnet properties are sensitive to grain size and volume content of the magnetic phases present in the microstructure. The composite microstructure was mainly composed of soft α-Fe (20-30 nm) and hard Nd₂Fe₁₄B (45-65 nm) magnetic phase grains. The maximum coercivity of 959.18 kA/m was achieved with the magnets annealed at 760 °C whereas the highest remanence of 0.57 T was obtained with the magnets treated at 710 °C. The optimally annealed magnets possessed promising magnetic properties such as _jH_c of 891.52 kA/m, B_r of 0.57 T, M_r/M_s = 0.68, (BH)_max of 56.8 kJ/m³ as well as the micro-Vickers hardness (H_v) of 1138 ± 20 and compressive stress (σ_f) of 239 ± 10 MPa.     

Effect of additions of zinc stearate on the properties of sintered Nd-Fe-B magnets

Zinc stearate additions have been used to increase the remanence of sintered Nd-Fe-B magnets produced by the powder metallurgy without powder pressing. Zinc stearate acts as an internal lubricant, i.e., it decreases the friction forces between the particles and favors an increase in the degree of texture of the powders, which is induced by the magnetic field. It is shown that the density and the magnetic hysteresis characteristics of sintered magnets produced using additions of 0.15 wt % zinc stearate exceeds the corresponding values obtained for magnets produced without this addition at a filling density of powders in containers of more than 2.9 and 3.0 g/cm³ in dry and wet states, respectively. Using additions of zinc stearate in the amount of 0.15% with respect to the weight of the powder, magnets with a density of 7.55 g/cm³, B _r = 14.02 kG, H _c = 7.91 kOe, and (BH)_max = 46.1 MG Oe have been produced.     

Temperature stability of magnetic field for periodic permanent-magnet focusing system

In this study, finite element analysis based on an Ansoft Maxwell software was used to reveal the temperature stability of a magnet ring and the equivalent structural periodic permanent-magnet(PPM) focusing system. It is found that with the temperature increasing, the decrease rate of magnetic induction peak(Bz)maxof single magnet ring is greater than that of remanence Brof magnet in the range from room temperature to 200 °C, however,the PPM focusing system do have the same temperature characteristics of permanent-magnet materials. It indicates that the magnetic temperature properties of the PPM system can be effectively controlled by adjusting the temperature properties of the magnets. Moreover, the higher permeability of the magnets indicates the less Hcb, giving rise to lower magnetic induction peak (Bz)′max: Finally, it should be noted that the magnetic orientation deviation angle θ(\15°) of permanent magnets has little effect on the focusing magnetic field of the PPM system at different temperatures and the temperature stability. The obtained results are beneficial to the design and selection of permanent magnets for PPM focusing system.     

Enhancing magnetic properties of anisotropic NdDyFeCoNbCuB powder by applying magnetic field at high temperature during hydrogen desorption

Anisotropic powder was prepared with precursor (NdDy)-(FeCoNbCu)-B sintered magnets by hydrogen decrepitation, desorption, and subsequent annealing treatment. The hydrogen desorption was performed in magnetic fields of 0, 1, 3, and 5 T. The orientation of tetragonal phase grains of the powder was evaluated from the hysteresis loops measured by extraction magnetometer. Residual hydrogen content of the powder was evaluated by thermal-magnetic analysis. The powder with H _cj, B _r, and (BH)_max of 1138 kA·m⁻¹, 1.029 T, and 172.5 kJ·m⁻³, respectively, was achieved under the condition of the magnetic field of 3 T. Magnetic properties of the powder, especially, the remanence of the powder, are enhanced upon magnetic fields, which is due to better orientation of powder particles and less residual hydrogen in the powder resulted from the magnetic field during the hydrogen desorption process.     

The influence of alloying additions and process parameters on the magnetic properties of PrFeB-based bonded magnets

The effects of Co, Zr, Nb, Ga and Dy additions upon the magnetic properties of PrFeB-based alloys have been studied. Bonded magnets have been prepared from as-cast and homogenised alloys using an optimised hydrogenation disproportionation desorption and recombination (HDDR) process. In general, the HDDR bonded magnets from homogenised alloys exhibited higher remanence (B_r), squareness factor (SF) and intrinsic coercivity (_iH_c). In particular, the Pr_13.7Fe_63.5Co_16.7B₆Nb_0.1 HDDR magnet exhibited the best overall magnetic properties (B_r=1032±20 mT, _iH_c=793±20 kA/m and SF=0.51), indicating that Co and Nb additions, in these proportions have a beneficial effect on PrFeB-based magnets. Conversely, additions of Ga and Dy were observed to dramatically reduce the intrinsic coercivity of Pr-based HDDR magnets.     

Epoxy resin effect on anisotropic Nd-Fe-B rubber-bonded magnets performance

A novel anisotropic Nd-Fe-B flexible bonded magnet with epoxy resin lubricant was prepared by the two-step method to enhance its performance. Temperature characteristics of epoxy resin and its effect on magnetic properties and preparation of anisotropic Nd-Fe-B flexible bonded magnets were investigated and optimized. DOA of aligned flexible bonded magnets with epoxy resin lubricant increases significantly due to epoxy resin lower viscosity and subsequent better powder particles lubrication at a certain aligning temperature. Meanwhile, H_irr decreases sharply due to improved oxidation resistance of epoxy resin fully encapsulating magnetic powder during magnetic alignment process. Utilizing 1 wt% optimized encapsulating epoxy resin and heating unaligned flexible bonded magnets to 80 °C for 30 min during magnetic alignment resulted in the largest Δ(BH)_max and ΔDOA. Δ(BH)_max increased to over 126% along with ΔDOA increase to over 75%, much higher compared with unaligned flexible bonded magnets prepared exclusively by calendering.     

Preparation of sintered Nd-Fe-B magnets by pressless process

Pressless process used for the preparation of sintered Nd-Fe-B magnets has been studied. The effect of the average particle size D _av, filling density ρ_f, and powder-texturing conditions for obtaining density ρ and desired magnetic hysteretic properties of sintered magnets has been investigated for both traditional technology (TT) and low-oxygen technology (LOT). The ρ_f magnitude ensuring the optimum relation between the density ρ of sintered magnets and their degree of texture was shown to be 2.5–3.0 and 2.2 g/cm³ for the TT and LOT, respectively. At lower ρ_f magnitudes, no required density of sintered magnets is reached, whereas at higher filling densities, a low level of texture and low remanence are realized. Optimum parameters of pulsed field inducing the high degree of texture in powders have been determined. The following properties were achieved for magnets prepared by pressless LOT: B _r ≥ 14.2 kG, H _c ≥ 8 kOe, and (BH)_max ≥ 47.9 MG Oe.     

Formation of a high-coercive state in sintered Nd-Fe-B-Ga magnets by thermocycling

A comparative investigation of the effect of two types of low-temperature annealing, namely, (1) isothermal treatment and (2) thermocycling treatment (TCT), on the coercive force H _c of sintered magnets of an Nd-Fe-B-Ga alloy has been performed. Powders of an Nd_15.5Fe_77.7B_6.5Ga_0.3 alloy were prepared from materials taken in three different conditions: cast alloy (CA), hydrided alloy (HA), and partly dehydrided alloy (PDA). The coercive force of the sintered magnets reaches different maximum values, depending on the time of grinding, which after isothermal annealing at 550°C are 680, 688, and 800 kA/m in the magnets prepared from the CA, HA, and PDA, respectively. It has been revealed that the subsequent application of a TCT consisting in the variation of the temperature in a range of 20–550°C additionally increases H _c. In fine-grained magnets the increment in H _c is 10–15%; in coarse-grained, 100–300%. Thus, after a TCT the extremal dependence of H _c on the time of grinding of the powder virtually vanishes. To explain the observed effect of a TCT, an assumption is made on the effect of interphase stresses (arising at the boundaries between the grains and the intergrain phase enriched in Nd) on the mobility of atoms and the formation of defect-free grain boundaries, which favor an increase in H _c upon annealing.     

Effect of Ga addition on the magnetic properties and microstructure of nanocrystalline Nd_（12.3）Fe_（81.7）B_（6.0） ribbons

Nd 12.3 Fe 81.7 x Ga x B 6.0 (x = 0-1.8) ribbons were prepared by melt spinning at 22 m/s and subsequent annealing treatment. The influences of Ga addition and annealing conditions on the magnetic properties and microstructure of the nanocrystalline alloys were systematically investigated. After being annealed at 620℃ for 20 min, the J r and H ci increased from 0.85 T and 582.6 kA/m for Ga-free sample to 0.97 T and 734.6 kA/m for the x = 0.9 sample, respectively. The (BH) max for the x = 0.9 sample increased by about 40% from 96.3 to 135.5 kJ/m 3 compared with that of the Ga-free one. The significant improvement of magnetic properties originated from the refinement of grains in the samples by introducing Ga, which led to a stronger exchange coupling between the neighboring grains in comparison with that in Ga-free samples. The microstructure and magnetic properties of the samples depended strongly on annealing parameters, while the sensitivity of micro-structure to annealing conditions could be significantly suppressed by the addition of Ga element.     

晶界添加Ho63.3Fe36.7 对NdFeB再生磁体组织与性能的影响

采用一种高效、绿色的物理方法对NdFeB废旧磁体表面进行清理并回收利用。通过晶界添加低熔点Ho_63.3Fe_36.7合金制备NdFeB再生磁体。在未添加Ho_63.3Fe_36.7的磁体中,没有足够的富Nd相隔离Nd₂Fe₁₄B相,从而导致磁体性能较差;随着Ho_63.3Fe_36.7合金的加入,晶界相变得清晰且连续。在质量分数2%Ho_63.3Fe_36.7添加量下,钕铁硼再生磁体获得最佳磁性能[(BH)_max+H_cj=1756.07]。此时矫顽力增加123 kA/m（约提高9.1%）,磁体的最大能积由290.94 kJ/m³下降到281.07 kJ/m³,而剩磁少量下降。通过对再生磁体显微组织和成分的分析可知,磁体晶界处形成了(Nd, Pr, Ho)₂Fe₁₄B壳层,这能够提高磁体的矫顽力。而X射线衍射分析表明,磁体I₍₀₀₆₎/I₍₁₀₅₎的衍射峰强度比从0.92提高到1.32。这说明磁体取向度提高,可以减弱对剩磁的影响,从而使得再生磁体在保持剩磁的同时提高磁体的矫顽力。     

Improvement of the magnetic properties of nanocomposite permanent magnetic alloys with Dy and Ga substitutions

The Dy and Ga substituted NdFeB nanocomposite permanent magnetic alloys with high magnetic properties have been prepared by appropriate wheel speed of melt-spinning and post-annealing treatment. Under optimal conditions, compared with the best magnetic properties of ternary NdFeB alloy of J_r=1.18 T, H_ci=379.5 kA/m and (BH)_max=119.5 kJ/m³, the best magnetic properties of the alloy with Dy and Ga substitutions are J_r=1.16 T, H_ci=580.5 kA/m, and (BH)_max=162.7 kJ/m³. The XRD and TEM results showed that each of two alloys consists of hard magnetic 2:14:1 phase and soft magnetic α-Fe phase. The grain size of the 2:14:1 phase is about equal in the two alloys. The grain size and content of α-Fe phase in Dy and Ga substituted alloy are finer and lower, respectively.     

High-power (Nd,Dy)–(Fe,Co)–B magnets with a low temperature coefficient of induction

High-power (Nd, Dy)–(Fe, Co)–B permanent magnets with a low temperature coefficient of induction (α) were prepared using advantages of strip casting and low-oxygen technologies. The microstructure and temperature dependences of magnetic properties have been studied on sintered (Nd_{1 – x} Dy _x )_13.9(Fe_{1 – y} Co _y )_79.8Cu_0.1Ga_0.1B_6.1 magnets with 0.20 ≤ x ≤ 0.25 and 0 ≤ y ≤ 0.20. The increase in y from 0 to 0.20 is accompanied by an increase in the Curie temperature from 327 to 492°C. This favors a decrease in the value of α from 0.099 to 0.060%/°C, respectively. Magnets with an oxygen content of no more than 2500 ppm which were prepared from the (Nd_0.75Dy_0.25)_13.9(Fe_0.85Co_0.15)_79.8Cu_0.1 Ga_0.1B_6.1 alloy, have the following hysteresis characteristics at 140°C: B _r ≥ 11.3 kG, H _c ≥ 8 kOe, and (BH)_max ≥ 30 MGOe; in this case, α ≤ |–0.07%/°С|.     

Nanocrystallinity and magnetic property enhancement in melt-spun iron-rare earth-base hard magnetic alloys

Refinement of the grain size below ~35 nm mean diameter in melt-spun FeNdB-base alloys leads to en-hancement of remanent polarization,J _r, above the level predicted by the Stoner-Wohlfarth theory for an aggregate of independent, randomly oriented, and uniaxial magnetic particles. This article summarizes the results of the recent systematic research on this phenomenon, including the influence of alloy compo-sition and processing conditions on the crystallite size, degree of enhancement ofJ _r, and maximum en-ergy product(BH) _max. It has been shown that the effect can also occur in ternary FeNdB alloys, without the addition of silicon or aluminum, which was originally thought necessary, providing the nanocrystal-lites are not magnetically decoupled by a paramagnetic second phase. Values of(BH) _max above 160 kJ m_-3 have been achieved. The relationship between grain size,J _r, intrinsic coercivity,_JHc, and(BH) _max are discussed in terms of magnetic exchange coupling, anisotropy, and other parameters. Recent exten-sion of this work to the enhancement of properties in Fe-Mischmetal-Boron-base alloys and to bonded magnets with a nanocrystalline structure is also described.     

Effect of Co addition on the magnetic properties of nanocrystalline Fe-rich Fe-Nb-(Nd,Pr)-B alloys produced by crystallization of an amorphous phase

The effect of Co addition on the magnetic properties and microstructure for the nanocrystalline Fe_93-xNb₂(Nd,Pr)₂B₂ (x = 5–7) alloys produced by crystallization of an amorphous phase has been investigated. The melt-spun Fe_93-x-yCo_xNb₂(Nd,Pr)_yB₅ (x = 0–20 and y = 5–7 at.%) ribbons form a nanocomposite structure of bcc-(Fe,Co) and Pr₂(Fe,Co)₁₄B with grain sizes of 10–50 nm after annealing at 973-1023 K, showing a smooth J-H curve typical for the exchange-spring magnet. The alloys containing Co show a high energy product ((BH)_max) upon annealing at a relatively low temperature presumably because the precipitation temperature of each phase decreases by addition of CO. The (BH)_max values after annealing at an optimum temperature are improved by addition of 5–20 at. % Co for the Fe_86-xCo_xNb₂(Nd,Pr)₇B₅ and Fe_88-xCo_xNb₂Nd₅B₅ alloys. The improvement of (BH)_max by Co addition is attributed to the enhancement of J_r, presumably resulting from the increase in magnetization of each phase and the exchange-coupled region between the soft and hard magnetic phases.     

Fe72-xNd7B21Nbx(x=0~4.0)块体合金晶化过程和磁性能的研究

采用铜模吸铸法制备了直径为2mm的Fe_(72-x)Nd_7B_(21)Nb_x(x=0~4.0)块体合金,利用X射线衍射仪(XRD)、差示扫描量热仪(DSC)、振动样品磁强计(VSM)和多功能物理性质测量系统(PPMS)研究了Nb含量对该体系合金非晶形成能力、晶化行为及磁性能的影响。结果表明:适当的添加Nb有助于提高该体系合金的非晶形成能力,当Nb含量为2.0at%,2.2at%和2.5at%时,可以获得基本为非晶结构的块体合金。Nb含量对该体系合金的晶化行为有着重要影响,Nb含量为2.5at%的合金在晶化过程中能有效抑制非磁性相的析出,增强了晶粒间的交换耦合作用,使得其剩磁得到明显的提高,具有最佳的综合磁性能:B_r=0.63T,H_(ci)=448.97kA/m,(BH)_(max)=36.32kJ/m~3。     

Magnetic property enhancement of cobalt-free M-type strontium hexagonal ferrites by CaCO3 and SiO2 addition

An experiment was performed to investigate whether CaCO₃ and SiO₂ addition enhances the magnetic properties of cobalt-free M-type Sr ferrites, in which experimental parameters such as additive composition and the CaCO₃/SiO₂ ratio were changed. The specimens were prepared using conventional ceramic techniques. CaCO₃ addition promoted densification and uniform grain growth, resulting in high remanence; however, with the simultaneous addition of CaCO₃ and SiO₂, coercivity decreased to 3046 Oe in Sr ferrite hard magnets without Co₃O₄. SiO₂ addition suppressed grain growth. The addition of appropriate concentrations of CaCO₃ and SiO₂ as additives after calcination was shown to be very beneficial for tailoring a dense microstructure with relatively small grains. A CaCO₃/SiO₂ ratio of approximately 1.8 was found to be optimum, and the magnetic properties of B_r = 4.3 kG, iH_c = 3.7 kOe, and (BH)_max = 4.5 MGOe were obtained for Sr ferrite hard magnets without Co₃O₄, which are acceptable for motor applications under output power <1 kW.     

Preparation of high-power permanent magnets from platelike Nd-Fe-B alloys

The microstructure of platelike Nd-Fe-B alloys prepared by rapid quenching from the melt, namely, by the strip casting technique, has been studied. The processes of grain refinement, texture formation, and sintering were optimized. It is shown that the maximum energy product (BH)_max ≥ 50 MGOe can be realized in sintered magnets prepared from the alloy with 29 wt % Nd after a two-step heat treatment. The oxygen content in such magnets does not exceed 0.3 wt %; the degree of texture is α_t = 0.95.     

雾化喷涂沉积对晶界扩散Nd-Fe-B磁性能和微观结构的影响

提出了一种新的雾化喷涂沉积（SCD）方法,在Nd-Fe-B磁体表面均匀沉积TbF₃粉末,同时通过晶界扩散过程（GBDP）将Tb元素引入到磁体中。用这种方法（SCD+GBDP）处理厚度达5 mm的钕铁硼磁体。研究了TbF₃涂层增重比、扩散时间和扩散温度对烧结磁体组织和磁性能的影响。样品扩散温度和时间为940 ℃和10 h,退火温度和时间为480 ℃和5 h。TbF₃增重比（w）从0%增加到0.8%时,磁体的矫顽力从1201 kA/m 提高到1930 kA/m,剩磁下降约0.01 T。研究发现,随着TbF₃增重比的增加,磁体的矫顽力先增大后减小。SEM结果表明,在Nd₂Fe₁₄B晶粒边界区域,Tb取代Nd形成(Nd, Tb)₂Fe₁₄B核壳相。晶界相和核壳相中较高的磁晶各向异性对矫顽力的增强有积极的促进作用。核壳相的分布和浓度对矫顽力有密切的影响。当TbF₃增重比大于2.4%时,靠近磁体表面区域的晶界扩散明显增强。元素的SEM图像显示,进入磁体的Tb越多,晶核内的Tb浓度就越高。此外,大量Nd-F/Nd-O-F相的形成导致晶界相不像w=0.8% 时的样品那样连续,这可能是导致矫顽力下降的主要原因。