Application of Magnetic Field in Fabrication Process of Anisotropic Bonded Nd-Fe-B Magnet

This work studied the application of the different magnetic field used in the compaction process for die fabrication of anisotropic Nd-Fe-B bonded magnet. The static field made from Nd-Fe-B permanent magnets was used in the blending process to separate the particles each other. The SEM observation gave intuitionistic results about it. The anisotropic Nd-Fe-B bonded magnets were fabricated with warm-compaction under the electromagnetic field about 2.5 T. It is known that magnetic field is necessary for anisotropic materials fabrication for alignment. And warm compaction was used to decrease the viscousness of binder, to enhance alignment magnetic particle while press, and to get high density materials. For coercivity of Nd-Fe-B magnets decrease largely with the temperature increasing, press in proper temperature and oriented field is benefit to the magnetic characteristics and the mechanical properties of the anisotropic bonded Nd-Fe-B magnets. Finally solidifying process was performed under the pulse field of 4 T. The increment for solidifying in the field was about 15% for maximum energy product of the bonded magnet. The magnetic properties of anisotropic bonded Nd-Fe-B magnets from d-HDDR powders compact at 90 °C in alignment field of 2.5 T were: B_r=8.55 kGs, _iH_c=12.0 kOe, (BH)_max=14.57 MGOe.     

New Systems of Rare Earth Permanent Magnets for Generation of Extrahigh Magnetic Fields

Various systems, which are composed of rare-earth permanent magnets and generate strong magnetic fields, are calculated. Strong fields are taken to mean the fields with the strength, which exceeds the magnet induction. Calculated values of the strength of strong fields can mount to 20–150 kOe. Depending on form of domains where this fields are localized they can be classified as linear and point fields, the corresponding systems can be named linear and point as well. It was shown that the maximal linear field can be obtained in Halbach cylinder (prism), in which this field tends to H_x = 4πM_s ln(a/x) with increasing of number of component sectors. The maximal point field is reached in systems composed of many conical magnets, divided by many sectors. The strength of such field tends to the value H_x = 8πM_s ln(a/x). Experimental values of strong fields measured in some systems by various methods are in good agreement with calculated data.     

Effect of Thermal Treatment on Formation of High Magnetic Parameters of Sintered Magnets Dd-Fe-B

The present paper is dedicated to study effect of thermal treatment parameters on the properties (residual magnetic induction fir, coercive force on magnetization _iH_c, temperature coefficient of magnetic induction α_b within the temperature range 300–373 K) of sintered permanent magnets [Dd_1–x(Dy, Tb, Ho)_x]₁₅(Fe_1–yCo_y)_bal(Al,Cu,Ga,Ti)_0.1–1.4B_5.0–8.5 (at.%), where x=0.2–0.4; y = 0.2–0.3; Dd - didymium. For the magnets (Dd_1–xDy_x)₁₅(Fe_1–yCo_y)_baiB_5.0–5.6 discontinuities were observed at demagnetization curves caused by appearance of magnetic phases with reduced magnetic anisotropy. _iH_c value nearly does not depend on temperature of thermal treatment within the range 750–1275 K at increasing of boron content (7.0–7.5at.%). Magnets doping by such elements as aluminium, copper, gallium shifts optimal temperature of the last step of thermal treatment from 900 K to 725–825 K, at the same time magnetic properties (B_r, _iH_c α_b) improve, especially with increasing of Co and Dy content. It was possible to attain the following properties for the magnets (Dd_0.6Dy_0.4)₁₅(Fe_0.74Co_0.26)_bai(Al,Cu,Ga)_1.1B_8.5 after optimal thermal treatment {1175 K (7, 2 ks) with further cooling at the rate 1–2 K/min, then 900 K (3.6 ks) - hardening + 775 K (3.6 ks) - hardening}: B_r= 1,0 T, _iH_c = 1600 kA/m, BH_max = 192 kJ/m³, α_b= −0,024 %/K in the temperature interval 293–373 K.     

Magnetic properties and microstructure of Fe_(69.5-x)Nd_7B_(21)Nb_(2.5)Ga_x(x=0-1)alloys

The Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)permanent magnets in the form of rods were prepared by annealing the bulk amorphous alloys.The magnetic properties,phase evolution and microstructure of the alloys were investigated systematically.It is found that the glass forming ability(GFA), microstructure and magnetic properties are sensitive to Ga content for Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)bulk alloys.The annealed alloys are mainly composed of soft α-Fe,hard Nd_2 Fe_(14)B and nonmagnetic Nd_(1.1)Fe_4 B_4 phases.When x = 0.3,the optimally annealed magnets exhibit magnetic properties of the remanence Br = 0.63 T,intrinsic coercivity H_(cj) = 368.68 kA/m and maximum energy product(BH)_(max) = 33.73 kJ/m~3.Furthermore,magnetic field heat treatment at the temperature close to Curie temperature of Nd_2 Fe_(14)B phase was applied to the annealed Fe_(69.2)Nd_7 B_(21)Nb_(2.5)Ga_(0.3) magnet.The results of X-ray diffraction(XRD)and transmission electron microscopy(TEM)indicate that the magnetic field heat treatment can be beneficial for the precipitation of α-Fe.Thus,the B_r,H_(cj) and(BH)_(max) are enhanced by 8.7%,6.3% and 16.3%,respectively.     

Structure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation

Structure and magnetic properties were studied for bulk nanocrystalline Nd-Fe-B permanent magnets that were prepared at 650 °C for 3 min under 300 MPa using the SPS-3.20-MK-V sintering machine and the hot pressed magnets were then submitted to hot deformation with height reduction of 50%,60%,70%,80%,and 85%.Effects of height reduction(HR) and deformation temperature on the structure and magnetic properties of the magnets were investigated.The crystal structure was evaluated by means of X-ray diffraction(XRD) and the microstructure was observed by transmission electron microscopy(TEM).The magnetic properties of the magnets were investigated by vibrating sample magnetometer(VSM).As the height reduction increased,the remanence(B r) of the magnets increased first,peaks at 1.3 T with HR=60%,then decreased again,and the coercivity(H ci) of the magnets decreased monotonically.On the other hand,as the deformation temperature increased,the B r of the magnets increased first,peaks at 1.36 T with HR=60%,then decreased again,and the H ci of the magnets decreased monotonically.Under optimal conditions,the hot deformed magnet possessed excellent magnetic properties as B r =1.36 T,H ci =1143 kA/m,and(BH) max =370 kJ/m 3,suggesting the good potential of the magnets in practical applications.     

Model for Calculating J(H) Curves of Ni Coated Nd-Fe-B Magnets

An analytical model to describe the influence of surface degradation and the Ni layer itself on the magnetic properties of Ni coated Nd-Fe-B magnets is presented. Starting from the bulk magnetic properties, the dimensions, the thickness of Ni coating and the affected surface layer, J(H) demagnetization curve is calculated. Subsequently the expected values of (BH)_max, and the reversible permeability are deduced from the calculated J(H) curves. For flat magnets the surface effects lead to a decrease of B_r and an increase of the permeability which lowers (BH)_max. For strait magnets a step in the J(H) curve appears at H = 0. The deteriorating effect of Ni coating and the surface layer scale with the dimensions of magnet and the thickness of these layers, which depend on the processing and the grain size of magnet. These effects can not be neglected if one or more dimensions of a Ni coated magnet are less than about 5 mm. SmCo₅ magnets show similar effects but the coercivity of the damaged surface layer is higher. Pinning type Sm₂Co₁₇ magnets show almost no deterioration on surface due to machining. As a result, Sm-Co magnets are better suited for applications with dimensions smaller than about 2 mm.     

Effect of niobium substitution on microstructures and thermal stability of TbCu7-type Sm–Fe–N magnets

This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm_(8.5)Fe_((85.8-x)Co_(4.5)Zr_(1.2)Nb_x(x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu_7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase a-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous micro structure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H_(cj) is achieved in the 1.2 at% substituted samples.The irreversible flux loss for 2 h exposure at 120 ℃ declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution.     

Tremendous enhancement of magnetic performance for Sm(CoFeCuZr)z magnet based on multiscale copper redistribution

Microstructure and magnetic properties were studied for the commercial Sm(CoFeCuZr)_z magnets before and after post annealing treatment. The results show that the phases composition and orientation of the magnet do not change after post annealing treatment, but the substantial redistribution of Cu element within multiscale (the microscale crystal grain and the nanoscale cellular structure) is observed simultaneously. In detail, along with the Cu redistribution, the thickness of the Cu-rich Sm(Co,Cu)₅ cell boundary becomes thinner, and the Cu concentration in the boundary increases sharply. The pinning field of domain walls and corresponding coercivity increase remarkably with slight remanence and maximum energy product loss, and the overall magnetic performance of (BH)_max (MGOe)+H_cj (kOe) increases by 54.3% as a result. Moreover, the thermal stability of the magnet improves as well. On the other hand, Cu-lean phenomenon was observed along the grain boundary region, triggering to magnetic domain reversal process and slightly undermining the squareness of the demagnetization curve of the magnet.     

Effect of yttrium substitution on magnetic properties and service performances of Nd-Fe-B sintered magnets

We successfully fabricated partial Y substituted NdY-Fe-B magnets with nominal compositions of(Nd_1-xY_x)_13.80Fe_ba1Al_0.24Cu_0.1B_6.04(at%,x=0,0.1,0.2,0.3,0.4) by powder metallurgy process and the magnetic properties as well as service performances of the magnets were also systematically investigated.The phase constituents of the magnets have no obvious variation within the whole range of Y content,while the main phase grain...     

Study on Fabrication Process of Anisotropic Injection Bonded Nd-Fe-B Magnets

This study is on the injection molding process for the fabricating anisotropic Nd-Fe-B bonded magnets. The effects of powder loading, particle size of the magnetic powder, polymer binder and the fabricating process on the magnetic and the mechanical properties of anisotropic Nd-Fe-B magnets were investigated. The proper powder loading, particle size and binder are 60%(vol%), 75–106 μm and PA 1010, respectively. The optimum condition for good magnetic properties of anisotropic injection bonded Nd-Fe-B magnets is mixing the binder and the chemicals in the temperature between 205–215 °C, injection temperature of 265 °C, the injection pressure of 5–6 MPa, the press time of 5 second, and molding temperature of 80 °C. The magnetic properties of anisotropic bonded Nd-Fe-B magnets made in above conditions from d-HDDR powder were: B_r=0.72 T, _iH_c=983 kA/m, (BH)_max=75 kJ/m^c.     

Magnetic Microstructures of 2：17 Type Sm（Co,Fe,Cu,Zr）z Magnets Detected by Magnetic Force Microscopy

The magnetic microstructures of 2：17 type Sm （Co, Fe, Cu, Zr）z magnets were detected by magnetic force microscopy. Comparing the microstructures of the specimens eoated with and without Ta thin film before and after heat-treatment, it is found that： （a） as a protection layer, Ta coating layer about 20 nm thick can effectively restrain Sm volatilization under high temperature; （b） the stress built in the 2.17 type Sm-Co magnets during specimen preparation only affects some local parts of the domain structures; （c） the magnetic microstructures vary largely for specimens heat-treated at high temperature without Ta film coating due to Sm volatilization. In addition, by comparing with high coercivity Fe-Pt point tips, it is found that the Co-Cr thin-film tips are not suitable for detecting the magnetic microstructures of strong permanent magnets.     

Slow magnetic relaxation in a 3D dysprosium(III)-fluoro-oxalate framework containing zig-zag [Dy–F]n chains

Dysprosium complexes based on [F–Dy] or near-linear [F–Dy–F] unit are of great concern in the field of single-molecule magnets due to their large magnetic anisotropy. Here, the crystal structure and the magnetic relaxation dynamics were reported for a three-dimensional (3D) metal–organic framework (MOF): [DyF(C₂O₄)(H₂O)₂]_n·2nH₂O (1), which is the unique MOF containing zig-zag [Dy–F]_n chains. Magnetic susceptibility characterization reveals that 1 is one of the few 3D MOFs which show slow magnetic relaxation under zero dc field. And the effective energy barrier of 72 K for 1 is also higher than most Dy-based 3D MOFs. The diamagnetic ion dilution study shows that the ferromagnetic exchange couplings mainly transmitted by F^? bridges in 1 contribute little to the energy barrier, but effectively suppress the quantum tunneling process and result in a smooth hysteresis loop with no waist-restricted step.     

Selective Leaching Process for Neodymium Recovery from Scrap Nd-Fe-B Magnet

Neodymium-iron-boron (Nd-Fe-B) magnets were most widely applied to permanent magnetic products in the world due to their high magnetic force. The increasing growth of scrap Nd-Fe-B magnets resulted in disposal problems and the reduction of neodymium (Nd) valuable resources. In this study, we developed a simple hydrometallurgical precipitation process with pH adjustment to separate and recover Nd 100 pct recovery from scrap Nd-Fe-B magnets. Several physical and chemical methods such as demagnetization, grinding, screening, and leaching processes were also adopted to investigate the recovery of Nd and other metals from scrap Nd-Fe-B magnets. The leaching process was carried out with four leaching reagents such as NaOH, HCl, HNO₃, and H₂SO₄. Batch studies were also conducted to optimize the leaching operating conditions with respect to leaching time, concentration of leaching reagent, temperature, and solid/liquid ratio for both HCl and H₂SO₄ leaching reagents. Nd was successfully separated and recovered with 75.41 wt pct from optimized H₂SO₄ leaching solution through precipitation. Further, the purity and weight percentage of the obtained Nd product was analyzed using scanning electron microscopy–energy-dispersive spectroscopy (SEM-EDS) analysis. An X-ray diffraction (XRD) study confirmed the obtained product of Nd was in the form of NdOOH and Nd(OH)₃.     

Structural and Magnetic Characterization of High Performance Die-Upset Forged Nd-Fe-B Magnets

The processing method of anisotropic Nd-Fe-B magnets, based on rapid solidification of a molten alloy, followed by hot pressing and die-upset forging is currently well established and commonly used for the processing of high performance magnets. In this method uniaxial stress, created in an isotropic, polycrystalline alloy, leads to texture formation with the crystallographic c axis, of the tetragonal structure in each grain, being parallel, to the deformation direction (DD). However, the mechanisms leading to the anisotropic structure and accompanying processes are still not fully understood. In this study the Fe_73.7Nd_13.6Co_6.6Ga_0.6B_5.5 alloy, after rapid solidification by melt spinning and hot pressing was subjected to die-upset forging with strains 30% and 65%, respectively. Systematic studies of the structure, microstructure and magnetic domain structure versus processing parameters were performed for this alloy, using magnetic measurements, transmission electron microscopy and atomic force microscopy.     

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (H_cj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ contents were studied. The experimental results demonstrate that the coercivity (H_cj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from ?0.5634%/℃ to ?0.4506%/℃ and ?0.1276%/℃ to ?0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Effect of Terbium addition on the coercivity of the sintered NdFeB magnets

The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction (XRD) and BH magnetometers. The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd₂Fe₁₄B (space group P42/mnm) and a trace amount of Nd-rich phase. Accordingly, addition of Tb led to a decrease of the pole density factor of (004), (006) and (008) crystal plane of the Nd₂Fe₁₄B phase calculated by Horta formula, but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition. The study of the H_c(T)/M_s(T) versus H^min_N/(M_s(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets, and the microstructural parameters of α_k and N_eff were obtained also. The Kronmüller-plot showed an increase of the α_k responsible for an increase of the coercivity.     

Optimization of both coercivity and knee-point magnetic field of Sm2Co17-type magnets via solid solution process

It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm–Co sintered magnets. In this work, the influence of solid solution process on microstructure and magnetic properties of the Sm(Co_balFe_0.233Cu_0.073Zr_0.024)_7.6 sintered magnets was systematically studied. With the solid-solution treating duration (t_s) increasing from 0 to 4 h, intrinsic coercivity (H_cj) increases from 12.83 to 36.54 kOe, magnetic field at knee-point (H_knee) increases from 2.76 to 19.14 kOe, and the maximum energy product increases from 19.79 to 29.48 MGOe. The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides “white” rare earth-rich phase, and the content of Sm, Fe and Cu elements for the two kinds of regions changes a lot for the specimens. Furthermore, with t_s increasing up to 4 h, the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at% to 0.27 at%, 7.66 at% to 0.21 at% and 7.27 at% to 0.16 at% for Sm, Fe and Cu elements, respectively. Moreover, transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when t_s is 4 h. It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient (1.94 at%/nm). It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features, which are the key points for obtaining both high H_cj and H_knee.     

Effect of titanium addition on the magnetic property of Nd2Fe14B/α-Fe nanocomposite alloys

Rapidly solidified nanocrystalline α-Fe/Nd2Fe14B alloys with enhanced coercivity were obtained by melt spinning.The effects of Ti addition on the microstructore and magnetic properties of the nanocomposite α-Fe/Nd2Fe14B alloys were investigated by X-ray diffraction(XRD)and superconducting quantum interference device(SQUID)magnetometer.The analysis of XRD showed that Vα-Fe estimated to be about 35.3% in the Ti-free α-Fe/Nd2Fe14B nanocomposites decreased down to 26.5% as the addition of was 5 at.% Ti.Accordingly,adding Ti resulted in relevant improvements of magnetic properties,especially of the coercivity Hc from 595 kA/m up to 1006 kA/m.The dependence of Mirrev(H)/2Mr on the reverse field H indicated that nucleation was the dominating mechanism for the magnetization reversal in these nanocomposites.The analysis of the temperature dependence of the demagnetization curve in the α-Fe/Nd2Fe14B nanocomposite magnets indicated that a reduction of αex could play a leading role in an increase in the coercivity of Ti-doped sample.     

Coercivity of 2:17 Based Permanent Magnets

High-quality permanent magnets either are based on large nucleation fields or strong pinning forces of domain walls (dws). In the case of 2:17 based magnets depending on the temperature range considered both hardening mechanisms have been discussed. The dominant hardening mechanisms at temperatures below around 700 K are repulsive or attractive pinning of domain walls at the cell walls (1:5 structure) between the nanostructured pyramidal cells (2:17 structure). Micromagnetic calculations of the pinning forces sensitively depend on the shape of the interaction potential between the dw and the microstructure. In the case of the 2:17 based magnets these calculations become rather complex if the domain wall width is of the order of the potential width. In this case the domain wall is modified due to the space dependent local anisotropy which changes from K₁^2:17 to K₁^1:5 of the cell boundary phase over a distance D of the intergranular phase boundary. Using the K₁(r)-profiles as determined from the chemical composition obtained by high-resolution EDX measurements the coercive field due to domain wall pinning is determined self-consistently as a function of the pinning-potential parameters taking into account the modification of the domain wall by the space dependent material parameters. It is shown that the coercive field of the high-temperature magnets as a function of the width of the cell boundary phase varies between 3 T for a sharp boundary (D = 0 nm) and 1 T for a wide boundary (D = 4 nm). On the basis of micromagnetism the condition for the dominance of pinning or nucleation is investigated. It is shown that in the case of 2:17 based high-temperature magnets with increasing temperature the hardening mechanisms change from repulsive to attractive pinning and nucleation starts at around 700 K. The temperature ranges where the transitions take place sensitively depend on the Cu content and on the annealing parameters.     

Model of temperature field for the preparation process of melt-spun NdFeB powders

Melt-spun ribbons which are the important raw material for hot-deformed magnets can be prepared by single-roller melt-spinning. In order to prepare well-structured ribbons, the model of temperature field for single-roller melt-spinning process was constructed in this work. The heat conduction in this process was simplified as one dimensional heat conduction problem. It was shown by modeling that, the temperature field in the melt-spinning before solidification in this model could be described as this equation T(x,t)=Tmoexp[–k(x–x0)–k2αt]+T0. The temperature T(x,t) of the alloy melts decreased with increased position x and cooling time t exponentially from the wheel-free surface to the wheel-side surface. The constant k determined the decrease speed of alloy temperature T(x,t), which was proportional to the interfacial heat transfer coefficient h and the interfacial area of heat conduction A0, but inversely proportional to the thermal conductivity K. x0 was the thickness of the alloy melt. With increased x0, the temperature difference between wheel-free surface and the wheel-side surface became larger, which would lead to larger difference in grain size. In experiments, the influence of melt-spinning process parameters on the temperature field model was discussed, such as cooling roller materials, wheel speed, and so on. Melt-spun ribbons prepared by single-roller melt spinning at different wheel speed were investigated and magnetic properties of die-upset magnets from melt-spun ribbons on different cooling roller were analyzed. The variation of grain size in the depth direction consisted with temperature field model. This model provided directions for the preparation of melt-spun ribbons with uniformly distributed fine grains, which were very necessary for producing hot-deformed magnets with high magnetic performance.