Preparation and Properties of Nd-Fe-B/Fe Nanocomposite Magnets

Nd-Fe-B/α-Fe nanocomposite magnets with high magnetic properties were successfully fabricated through a sonochemical process with carbonyl iron as Fe precursor and subsequently SPS. Experimental results show that α-Fe can increase the remanence of Nd-Fe-B/α-Fe nanocomposite magnets while decrease the coercivity. The demagnetizing curve indicates that the hard and the soft phases did not coupled very well, even though the remanence was improved. The magnetic properties of Br 8.61 kGs, Hcj 8.59 kOe and (BH)_max 12.05 MGOe were obtained for Nd-Fe-B/α-Fe nanocomposite magnets with the nominal Fe content of 5 wt.%. It is noted, the exchange coupling was obviously enhanced by a MA process before SPS, and the magnets properties were increased to Br 9.42 kGs and (BH)_max 14.27 MGOe for Nd-Fe-B/α-Fe nanocomposite magnets with the same Fe content.     

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.     

Progress of Research on Bonded Nd-Fe-B Magnets

Some progress of research on bonded Nd-Fe-B magnets in National University of Defense Technology(NUDT) is presented in this paper. The contents include B-rich R₂Fe₁₄B-based nano composite with good performance; a model to determine of the least amount of binder; resin for high temperature application; resin encapsulating magnetic powders for long-term storage; thermoplastic polymer used for mold-pressing magnets; hybrid bonded Nd-Fe-B/Sm₂Co₁₇ magnet with a potentially useful improvements in remanence and magnetic energy product.     

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.     

Magnetic properties and microstructure of nanocrystalline bulk Nd-Fe-B with Ce-Co addition

The influence of Ce-Co alloy addition and sintering holding time on permanent magnetic properties and micro structure of nanocrystalline Nd-Fe-B bulk alloy were investigated.The coercivity of Nd-Fe-B bulk alloy can be enhanced greatly by more than 100% after adding Ce-Co powders.However,when the concentration of Ce-Co is up to 30 wt%,the density of the magnet can reach the maximum value of 7.58 g/cm~3,but the coercivity does not increase significantly.On the other hand,with the increase of holding time to 10 min,the density and coercivity of magnets increase gradually,reaching up to 7.55 g/cm~3 and 1134.3 kA/m,respectively.After the addition of Ce-Co alloy,Ce-Co may easily diffuse into the Nd-Fe-B matrix during hot-pressing and under the high pressure and temperature,thus increasing the content of grain boundary phase and the pinning effect of grain boundary,which leads to the increase of coercivity.The extension of the hot-pressing holding time may be more conducive to the diffusion of CeCo into the Nd-Fe-B matrix.In addition,the effect of Ce-Co addition on the magnetic properties of Nd-FeB with different content of rare earth was also studied.The addition of Ce-Co can effectively increase the coercivity of nanocomposite Nd_2 Fe_(14)B/α-Fe magnets.The addition of Nb to the parent alloy can further improve the coercivity.For Nd_(11)Fe_(81.5)Nb_1 Ga_(0.5)B_6 alloy with 10 wt% Ce-Co addition,the coercivity can increase from 740.28 to 1098.48 kA/m.     

Preparation and control mechanism of anisotropic Sm-Pr-Co/Co nanocomposites with unusual continuous soft-phase coatings

To produce nanocomposite materials with high magnetic properties, studies concerning nanostructural processing technologies and control mechanisms are urgently required in aspects of achieving perfect alignment of the hard phase while keeping desired sizes and distributions of the soft phase. In the present study, a designed low-rate electroless deposition method is found to be an effective way in producing strong textured anisotropic Sm-Pr-Co/Co nanocomposites with unusual continuous soft-phase coatings when assembling Co particles on the ball-milled anisotropic Sm-Pr-Co hard phase. The average particle size of the soft-phase coatings is 18–50 nm and the obtained Sm-Pr-Co/Co composites exhibit a high intrinsic coercivity of H_ci = 748 kA/m with an enhanced remanence of M_r = 79 A·m²/g, as compared to H_ci = 836 kA/m and M_r = 68 A·m²/kg for uncoated Sm-Pr-Co hard phase. Moreover, the coating process study reveals a nucleation control mechanism for the formation of the continuous coating structures. Down-sized Sm-Pr-Co/Co nanocomposites with tailored size below 300 nm or even below 100 nm were also produced by this designed method. This study is of theoretical and practical importance for developing advanced nanostructures including the next generation permanent magnets.     

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 properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering.The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity,namely diffusion treatment and annealing treatment.The coercivity increases significantly from 18.3 to 28.0 kOe,the remanence decreases slightly from 14.1 to 14.0 kGs,and the comprehensive magnetic properties are higher than 75(Hcj+(BH)_max=76.7).SEM results indicate that,on the one hand,950℃is the optimal diffusion temperature.Lower diffusion temperature results in insufficient diffusion of Tb element.Higher diffusion temperature can lead to the main phase grain growth,the decrease of Nd-rich phase,and forming holes in the magnet.On the other hand,500℃is the optimal annealing temperature.Lower annealing temperature can result in the reduction of Nd-rich phase.Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.     

Recycled Nd-Fe-B sintered magnets prepared from sludges by calcium reduction-diffusion process

Environmental friendly recycling process for Nd-Fe-B sintered magnet sludges generated in the manufacturing process, which contain large amount of rare earth, including Nd, Pr and Dy, is badly needed so far. In present study, we have developed an effective route to obtain recycled sintered magnets from Nd-Fe-B sintered magnet sludges by calcium reduction-diffusion (RD) process. Compared to conventional recycling process, our research is focused on recovering most of the useful elements, including Nd, Pr, Dy, Co, and Fe together instead of just rare earth elements. To improve the recycling efficiency and reduce pollution, the co-precipitating parameters were simulated and calculated using MATLAB software. Most of useful elements were recovered by a co-precipitation method, and the obtained composite powders were then directly fabricated as recycled Nd-Fe-B powders by a calcium reduction-diffusion (RD) method. The recovery rates are 98%, 99%, 99%, 93%, and 99%, for Nd, Pr, Dy, Co, and Fe, respectively. The amount of useful elements contained in the recovered composite powders is greater than 99.71 wt%. The process of RD for synthesizing Nd₂Fe₁₄B and subsequently removing CaO was thoroughly investigated. Furthermore, the recycled Nd-Fe-B magnet exhibits a remanence of 1.1 T, a coercivity of 1053 kA/m, and an energy product of 235.6 kJ/m³, respectively, indicating that recycled Nd-Fe-B sintered magnet was successfully recovered from the severely contaminated sludges via an effective recycling route.     

Multilayer Magnets

After experimental evidence of intergrain exchange coupling was reported, nanocomposite magnets with high remanence and large energy products were predicted. However, the experimental values of the maximum magnetic energy product of nanocomposite bulk magnets have been much less than the theoretically predicted ones. We gave a brief review on advances in multilayer magnets. The exchange coupling and remanence enhancement were realized in nanocomposite (Nd,Dy)(Fe,Co,Nb,B)_5,5/α-Fe thin films prepared by sputtering and heat treatments. Well-designed multilayer films consist of magnetically hard Nd₂Fe₁₄B-type phase with the grain size of 40 nm and magnetically soft α-Fe phase existing in the form of the continuous layers. Furthermore, we reported the structural and magnetic properties of anisotropic Nd-Fe-B thin films. The effects of thickness, deposition rates, substrate temperature, annealing temperature were studied. A high maximum energy product of (BH)_max = 270 kJ/m³ was obtained for anisotropic Nd-Fe-B thin films.     

Preparation and characterization of sodium silicate/epoxy resin composite bonded Nd-Fe-B magnets with high performance

As an organic binder for bonded Nd-Fe-B magnets, epoxy resin(EP) has poor heat resistance but good moisture resistance, while sodium silicate(SS) has poor moisture absorption but better heat resistance and corrosion resistance. In order to improve high temperature stability and decrease moisture absorption of bonded Nd-Fe-B magnets, EP/SS composites were applied as the binder to prepare bonded Nd-Fe-B magnets. The magnetic properties, moisture absorption, corrosion resistance, compressive strength and microstructure of composite bonded magnets were investigated. The results show that EP/SS bonded magnets can obtain excellent magnetic properties at room temperature, and even useable magnetic properties a thigh temperature environments at 200°C. EP/SS composite binder effectively improves heat resistance and corrosion resistance of bonded Nd-Fe-B magnets, and reduces the hygroscopic properties. The molecule of sodium silicateis rigid and keeps it original shape at high temperature environments. In addition, SS in composite binder improves the mobility of the magnetic powders during the pre-pressing process, which makes the magnetic powders attain a more regular structure. These two factors will increase the mechanical properties. Moreover, sodium silicate in the composite binder can also cover the surfaces protecting the magnetic powders from oxidation and corrosion. EP in composite binder can cover SS surface to reduce the water absorption of SS as epoxy is a hydrophobic material. The EDX analysis shows that the composite binder has accumulated in the gaps of the magnet powders, which not only improves heat resistance and corrosion resistance, but also increases the mechanical properties. Therefore, EP/SS composite binder endows bonded Nd-Fe-B magnets excellent comprehensive properties.     

Large magnetocaloric effect in Er_2O_3 compounds at cryogenic temperature

We investigated the magnetocaloric effect in commercial Er_2 O_3 powders which presents almost no hysteresis losses at low temperature.At a magnetic field change of 5 T,it displays large magnetic entropy change(-ΔS_M)_(max) of 15.02 J/(kg·K) and a refrigerant capacity(RC) of 311 J/K at Neel temperature T_N=3.32 K.The magnetic transition was found to be of a second-order.The maximum values of adiabatic temperature change(ΔT_(ad))_(max) reach 0.70 K for a magnetic field change of 1 T.The large value,of(-AS_M)_(max) as well as no hysteresis loss,makes Er_2 O_3 a promising material as a magnetic refrigerant at low temperature.     

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 Al82.8Cu17Fe0.2 alloy doping on structure and magnetic properties of SmCo5-based ribbons

This work tries to improve the magnetic properties by multi-element doping in the form of a ternary alloy.SmCo₅₊χwt%Al-Cu-Fe(x=0-7)ribbons melt-spun at 40 m/s were produced by adding Al_82.8Cu₁₇Fe_0.2alloy into SmCo₅ matrix,and their phases,microstructure,and magnetic properties were investigated.The results show that both x=0 and 3 ribbons form a cellular microstructure.Al-Cu-Fe addition reduces the content of the Sm₂(Co,M)₇ cell wall,narrows its width,and forms the local disordered micro-regions and solute-segregation nanoclusters in the Sm(Co,M)₅ grains.With x increasing to5,Al-Cu-Fe addition promotes the phase separation between and within grains of the SmCo₅-based alloy.The Al-Cu-Fe addition can simultaneously improve the coercivity and magnetization of the SmCo₅-based ribbons,in particular,the magnetization of the x=3 ribbons increases by 35%,while the coercivity of the x=5 ribbons increases by 3.9 times.Finally,the microstructure evolution models are built up,and the relationship between the microstructure and the magnetic properties is discussed.     

Synthesis of Invar 36 type alloys from elemental and prealloyed powders by mechanical alloying

Invar 36 (Fe₆₄Ni₃₆) nanocrystalline powders were successfully obtained by the mechanical alloying process. The mechanically alloyed Invar 36 powders were obtained from both, Fe–Ni elemental and Fe–Ni₃Fe prealloyed powders. XRD, DSC and magnetic measurements were used to characterise the Invar 36 powders. The lattice parameter evolution versus temperature of Invar 36 powders was investigated by in-situ high-temperature X-ray diffraction (HT-XRD). For both, Invar 36 (Fe, Ni) and Invar 36 (Fe, Ni₃Fe) powders, the lattice parameter values are constant up to about 350°C. The magnetic measurement also indicated that the Invar 36-type alloys are formed after 16?h of milling.     

Wetting of grain boundaries in hard-magnetic Nd-Fe-B alloys

Samples of Nd-Fe-B alloys, which have been the main hard-magnetic materials with the highest magnetic energy since the end of the 1980s, are investigated. Magnets based on them are obtained by liquid-phase sintering or spin coating. In this article, wetting of the Nd₂Fe₁₄B grains by the neodymium-enriched liquid phase is investigated. The microstructure of the Fe-12.3 at %Nd-7.6 at %B alloy quenched after annealing at T = 700?1100°C in the presence of a neodymium-enriched melt is studied. The acquired data indicate that the transition from incomplete to complete wetting of grain boundaries occurs as the temperature increases. The results are compared with the reference data for alloys of the Nd-Fe-B system obtained by liquid-phase sintering. The relation between the wetting phase transition of grain boundaries and magnetic properties is also discussed.     

Magnetic Field-Induced Precipitation Behaviors of Alloy Carbides M2C,M3C,and M6C in a Molybdenum-Containing Steel

The effect of a 12-T high magnetic field on alloy carbide precipitation in an Fe-C-Mo alloy during tempering at an intermediate temperature was investigated. Thin foils and carbon extraction replicas of the treated specimens were examined by transmission electron microscopy (TEM). The results show that the applied high field effectively promoted the precipitation of (Fe,Mo)₆C alloy carbide. The concentration of Fe atom in Fe_6?x Mo _x C carbide is increased whereas that of Mo atom decreased when the high magnetic field was applied. However, the high magnetic field almost had no detectable influence on the atom concentration in (Fe,Mo)₂C and (Fe,Mo)₃C carbides. First principle calculations have been performed to calculate the magnetic moment per iron atom of the carbides to explore the origin of the effect of the magnetic field. The influence of the high magnetic field on the precipitation behaviors of alloy carbides was closely related to the magnetic moment of (Fe,Mo)₂C, (Fe,Mo)₃C, and (Fe,Mo)₆C. The magnetic field promotes the formation of the carbides with high total magnetic moment. The effect of the high magnetic field on the substitutional solute atom (Fe and Mo) concentration change in the three alloy carbides was attributed to their magnetization differences per Fe atom.     

Sintering of highly dispersed titanium diboride and composite titanium diboride-calcium hexaboride powders

The sintering behavior in vacuum of ultrafine nonmilled titanium boride powder and a composite of the latter with CaB₆ was investigated. It was shown that such titanium boride powders and TiB₂?CaB₆ composites poses better sinterability than conventional powders; the temperature of rapid densification was approximately 200°C lower. The introduction of ～1% each of Fe and Ni into the TiB₂?5% CaB₆ composite noticeably accelerated densification due to the formation of a transient liquid phase. The effect was further enhanced by the activation of surface and volume diffusion by the solution of Fe and Ni in TiB₂, as well as recrystallization processes.     

Coercivity enhancement of Nd2Fe14B/α-Fe nanocomposite magnets through neodymium diffusion under annealing

The coercivity enhancement of ball-milled Nd2Fe14B/α-Fe nanocomposite magnets was investigated. It was found that the coercivity could be enhanced through mixing a small amount of Nd powder with as-milled Fe-rich Nd-Fe-B powders. The annealed samples were investigated by means of X-ray diffraction, scanning electron microscopy and magnetic measurement systems. Under annealing, some of Nd powders promoted the formation of hard magnetic phase Nd2Fe14B. On the other hand, a few of Nd would diffuse into the interface of Nd2Fe14B/α-Fe nanocomposite to compensate for the loss of the interfacial magnetic anisotropy. These two features are all beneficial to the coercivity.     

Effect of MgCl2 on electrophoretic deposition of TbF3 powders on Nd-Fe-B sintered magnet

Using electrophoretic deposition(EPD) method,the TbF₃ powders were deposited on the surface of sintered Nd-Fe-B magnets,and the effects of MgCl₂ on electrophoretic deposition and grain boundary diffusion were investigated.The results show that addition of 5 wt% MgCl₂ can significantly improve the EPD efficiency and improve the adhesion of the coating by releasing local stress through the formation of special gully morphology.Combining with Biesheuvel equation,the...