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Highlights • Sintered Nd-Fe-B strip casting flakes (SC) and waste sintered magnets (SM) show the approximate HD process, which accelerates with increasing pressure. • The surface activation process accelerates with increasing pressure, and almost disappears when the initial hydrogen pressure reaches to 6 MPa. • The hydrogen content of SM is lower than SC, and increases with increasing pressure. • For the SM, blasting power of HD process decreases with the increase of the initial hydrogen pressure. Abstract Hydrogen treatment technology is developed to recycle the waste Nd-Fe-B sintered magnets in the way of short route, low cost, and high efficiency. In present study, effect of hydrogen pressure on hydrogen decrepitation (HD) process of sintered Nd-Fe-B strip casting flakes (SC) and waste sintered magnets (SM) was systematically studied. Both SC and SM show the approximate HD process, which accelerates with increasing pressure. The surface activation process accelerates with increasing pressure, and almost disappears when the initial hydrogen pressure reaches to 6 MPa. The hydrogen content of SM is lower than SC, and increases with increasing pressure. For the SM, blasting power of HD process decreases with the increase of the initial hydrogen pressure. It provides beneficial reference for recycling waste Nd-Fe-B sintered magnets. [ABSTRACT FROM AUTHOR]

Shaping process is crucial for the application of MM′X systems which are commonly fragile due to the large volume effect during phase transition. We adopted spark plasma sintering as a new method to produce MnNiSi-based bulks and investigated the variations of both structural and magnetic properties. The sintering process creates many micro-cracks inside the large particles and changes the residual strain in the small particles. It leads to the reduction of the entropy change, while the relative cooling power is increased by 7%. This work provides reference for further investigations about the effects of different shaping processes on the performance of MM′X systems. [ABSTRACT FROM AUTHOR]

Structure and magnetic properties were studied for Tm2(Co1-xFex)17 bulk nanocrystalline magnet synthesized by spark plasma sintering technique. X-ray diffraction results indicate both ingot and sintered magnet of Tm2(Co0.7Fe0.3)17 alloy exhibit hexagonal Th2Ni17 type structure. The microstructure of the magnet is composed of Th2Ni17 type nanograins with an average size of 35 nm detected by transmission electron microscopy and selected area electron diffraction patterns. Magnetic measurement shows that the remanence of the magnet is higher at 573 K than at 300 K, indicating there is a positive remanence temperature coefficient in the magnet. The anisotropy field of Tm2(Co0.7Fe0.3)17 alloy is 3.8 T, which is much higher than the HA of 2.3 T for the pure Tm2Co17alloy. The coercivity of the sintered magnet increases from 0.25 to 0.354 T after optimal annealing. [ABSTRACT FROM AUTHOR]

The structure and magnetic properties were studied for bulk nanocrystalline SmCo5 sintered magnet produced by spark plasma sintering technique. X-ray diffraction patterns show that the sintered magnet exhibits CaCu5 crystal structures. Transmission electron microscopy (TEM) observation indicates the average grain size of the magnet of about 40 nm. Magnetic measurement shows that the coercivity reaches as high as 2.85 T; the saturation magnetization and the remanence are 0.71 and 0.5 T, respectively. The magnet exhibits good thermal stability with a coercivity of 0.9 T at 773 K and a coercivity coefficientβ of -0.15%/K. [ABSTRACT FROM AUTHOR]

The crystal structure and magnetic transition were studied for the bulk Mn1.1Fe0.9P0.8Ge0.2 compound prepared by a simple blending and subsequent spark plasma sintering route. X-ray diffraction analysis and refinement show that the compound crystallizes in the hexagonal Fe2P-type structure, in which the Mn atoms occupy all the 3g sites and some 3f sites, the Fe atoms occupy the rest of the 3f sites, and P and the Ge atoms randomly occupy the 2c and 1b sites. Magnetic measurement indicates that the Curie temperature TC is at 253 K and the thermal hysteresis of M-T curves at TC upon heating and cooling, a signature of a first-order magnetic phase transition, is about 15 K. The maximum magnetic entropy change of the compound reaches as high as 49.2 J/kg K in a field change from 0 to 5 T at 253 K, while the adiabatic temperature change of the compound reaches only 1.2 K in a field change from 0 to 1.5 T at the same temperature. [ABSTRACT FROM AUTHOR]

Polarization curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in different electrolytes. Effect of microstructure modification on the chemical stability of the magnets was discussed. Both magnets exhibit evident passive behavior in alkaline electrolyte but are susceptible to corrosion in weak acid and saline electrolytes. SPS magnets are more corrosion resistant than conventional sintered magnets due to a remarkable microstructure modification formed in the SPS process. Further investigation shows that the unique microstructure in SPS magnets effectively restrains the aggressive intergranular corrosion which takes place along Nd-rich phases in conventional magnets. [ABSTRACT FROM AUTHOR]

The correlation between preparation conditions and microstructure as well as magnetic properties were studied for Sm(Co 0.665 Fe 0.25 Cu 0.06 Zr 0.025 ) 7 magnets. Increase of sintering temperature from 1468 to 1493 K promotes the amalgamation of small grains into big ones, resulting in an enlargement of average grain size from 10 to 25 μm and diffusion of Cu from inner grains to Cu-lean grain boundaries. As a result, the demagnetization curve squareness of the magnets improves substantially, leading to a remarkable enhancement of the maximum energy product from 21.3 to 27.1 MGOe. The coercivity of the magnets drops from 25.3 to 16.0 kOe at the same time, which was verified by the increase of the average domain width from 1.1 μm in small grains to 2.5 μm in big grains. It is therefore concluded that tuning of the preparation conditions causes a more homogeneous distribution of Cu, which is crucial for the improvement of magnetic properties. [ABSTRACT FROM AUTHOR]

Spark plasma sintering technique has been applied to synthesize composite Gdx(Gd5Si2Ge2)1-x (x=0.3, 0.5, and 0.7) with layer structure using high-purity gadolinium (Gd) metal and Gd5Si2Ge2 alloys. The magnetocaloric effect of the composites was studied by directly measuring adiabatic temperature change (ΔTad) with a magnetic-field change of 1.5 T using a homemade magnetocaloric effect measuring apparatus. With increasing x from 0.3 to 0.7, ΔTad peak temperature shifted from 286 to 293 K and the peak values of ΔTad slowly increased from 1.6 to 2.0 K at a magnetic-field change of 1.5 T. Meanwhile, the ΔTad peak of the composite broadened as Gd content was increased. For x=0.7, the composite showed a ΔTad ranging from 1.1 to 2.0 K at a magnetic-field change of 1.5 T in a temperature range of 260–300 K, which adapted to work as a room-temperature refrigerant. [ABSTRACT FROM AUTHOR]

Commercial NdFeB powders mixed with DyH x nanoparticles are hot pressed and hot deformed into anisotropic magnets by Spark Plasma Sintering (SPS). The hot deformed magnet exhibits strong c -axis crystallographic texture. The coercivity of the magnet doped with 1.0 wt% DyH x is increased by 66.7%, compared with the magnet without DyH x , while the remanence decreases only by 3%. TEM observation shows that there exists a continuous (Nd,Dy) 2 Fe 14 B layer between Nd-rich phase and NdFeB main phase. [ABSTRACT FROM AUTHOR]

Crystal structure and magnetic properties have been studied for bulk MnFePSi compounds prepared by the spark plasma sintering method. The effect of Si content on the crystal structure and the magnetic properties of the compounds were investigated. The MnFePSi compounds crystallize in a hexagonal Fe2P-type structure without obvious secondary phase. Magnetic measurements indicate that the compounds undergo a ferromagnetic to paramagnetic first-order transition near their Curie temperature ( TC) . With the increase in Si content, the TC rises from 343 to 381 K, while \Delta T\mathrm{ hys} decreases simultaneously from 27 to 15 K. For MnFeP0.5Si0.5 compound, the magnetic entropy change ( - \Delta SM) for a magnetic field change from 0 to 3 T is 12 J/kg \cdot \textK , demonstrating its good potential for practical application. [ABSTRACT FROM AUTHOR]

Nanocomposite Nd2Fe14B/SmCo5 permanent magnets with different weight ratios have been prepared by spark plasma sintering mixtures of homemade amorphous SmCo5 powders and commercially available Nd2Fe14B powders. The magnetic properties of the nanocomposite magnets are sensitive to the ratio of the two hard phases. The peak (BH)max value of 14.4 MGOe is obtained at the SmCo5/Nd2Fe14B ratio of 4:1, which is $\sim 50$ % higher than that of the single-phase SmCo5 magnet prepared under the same preparation conditions. Hysteresis loop of the nanocomposite magnet shows the characteristics of single-phase magnet due to strong exchange coupling between the two phase grains. At high temperatures, the nanocomposite magnets exhibit greatly improved thermal stability compared with the single-phase Nd2Fe14B magnet. [ABSTRACT FROM AUTHOR]

Abstract: Crystal structure, and thermal and magnetic properties have been studied for bulk MnFePSiGe compounds with different silicon contents prepared by the spark plasma sintering method. The MnFePSiGe compounds crystallize in a hexagonal Fe2P-type structure without obvious secondary phase. The compounds undergo a ferromagnetic to paramagnetic first-order transition near their Curie temperature (T C). With increase of Si content, both T C and magnetic entropy change (−ΔS M) of the compounds rise remarkably, while the thermal hysteresis (T hys) remains almost invariant. For MnFeP0.63Si0.26Ge0.11 compound, the T C and −ΔS M for a magnetic field change from 0 to 2T are 318K and 27.9J/KgK, respectively, and its phase transition entropy change (ΔS) obtained from DSC is 32.3J/KgK. [Copyright &y& Elsevier]

An anisotropic Nd-Fe-B magnet was prepared by the spark plasma sintering (SPS) technique. A magnet with the same composition was produced using the conventional sintering method for comparison. The oxidation behavior in oxidizing environments for both magnets was studied. The rnicrostructure and composition of the magnets were investigated using scanning electron microscopy with an energy-dispersive x-ray detector. Compared with the conventional sintered Nd-Fe-B magnet, the SPS magnet possesses a lower oxygen diffusion coefficient, which leads to a lower oxidation rate, a thinner oxidation layer, and lower oxygen content. Microstructure observation shows that the SPS magnet possesses a unique microstructure due to the special sintering mechanism. In detail, the grain size of the Nd2Fe14B main phase isfine and untform. Only a little of the Nd-rich phase forms along the grain boundaries of the Nd2Fe14B phase, while most of it agglomerates into triple junctions as small particles. Therefore, the pathways for oxidation propagation are restricted and the intergranular oxidation process through the Nd-rich phase in the magnet is suppressed effectively. As a result, the SPS Nd-Fe-B magnet possesses better oxidation resistance than that of the conventional magnet. [ABSTRACT FROM AUTHOR]

Melt spun ribbons of Nd4·5Fe77B18·5 alloy were consolidated into bulk permanent magnets by spark plasma sintering (SPS) method. Effects of heat treatment on the density, microstructure and magnetic properties of the magnets are studied. Further observation shows that the sample consolidated by SPS possesses ultrafine nanocrystalline structure and small fraction of amorphous phase. After proper heat treatment, magnets with fine crystal structure and good magnetic properties were obtained simultaneously. On the other hand, excessive grains growth, which will lead to undesirable magnetic properties, can occur due to improper heat treatment. [ABSTRACT FROM AUTHOR]

Sm2Fe17Nx sintered magnets were fabricated by spark plasma sintering (SPS) technique. Effects of sintering pressure, sintering temperature and heating rate on the magnetic properties and crystal structures of the magnets were investigated. The results showed that the density of the magnet was obviously improved with increasing sintering pressure, but the coercivity dropped simultaneously because Sm2Fe17Nx decomposed into SmN, α-Fe and N2. The coercivity decreased rapidly when sintering temperature was above 200°C under 1 GPa sintering pressure, which indicated that high pressure promoted the decomposition of Sm2Fe17Nx even at low temperature. In addition, the decomposition could not be effectively restrained even if the heating rate reached 450°C min-1. [ABSTRACT FROM AUTHOR]

The crystal structure and magnetic properties were studied for the bulk nanostructural permanent magnet SmCo6.6Nb0.4 prepared by spark plasma sintering method. The magnet crystallizes in a single phase with a hexagonal TbCu7-type crystal structure after the sintering process. Rietveld fitting results show that the alloying element of Nb prefers to occupy the 3g site. Microstructure observation indicates that the average crystal grain size is about 30 nm. The coercivity decreases almost linearly from 2.8 to 0.5 T with increasing temperature from 300 to 773 K. A remanence enhancement resulting from intergrain exchange coupling among the fine grains was also observed. [ABSTRACT FROM AUTHOR]

Bulk nanocrystalline gadolinium metal was prepared using spark plasma sintering (SPS) and subsequent annealing process. The SPS Gd metal had a single phase with a mean grain size of about 10 nm. A pressure-induced C-axis crystal texture was found in the SPS Gd sample. With the decrease of Gd grains from micrometer to nanometer range, the Curie temperature TC decreased by 10.7 K and the magnetic moment per Gd atom at 5 K dropped noticeably from 7.81μB to 6.31μB. These results indicate the remarkable influence of the nanostructure on the magnetism of Gd due to the finite size effect. [ABSTRACT FROM AUTHOR]

In this paper, bulk nanocomposite permanent magnets are prepared by spark plasma sintering (SPS) technique using melt-spun powders with a nominated composition of Pr4.2Tb0.3 Fe78B17.5. Effect of processing conditions on the density, microstructure, and magnetic properties of the magnets are studied. Higher sintering temperature improves the densification of the magnets, while deteriorates their magnetic properties simultaneously due to the excess crystal grain growth. An increased compressive pressure leads to better magnetic properties and higher density for the magnets. Magnet possesses a Br of 10.4 kGs and a Hci of 2.76 kOe under the optimal sintering condition. Observation shows the microstructure of the magnet contains fine grains with an average size of about 20 nm. The fine grain size in the microstructure and good magnetic properties are ascribed to the rapidly sintering speed and high compressive pressure of SPS, which effectively restrains the grain growth and allows the formation of fine crystalline materials with full dense in a short time. [ABSTRACT FROM AUTHOR]

Anisotropic Nd15.5Dy1.0FeBalCo3.0B6.8Al1.0 magnets were produced by the spark plasma sintering (SPS) technique. The effects of processing conditions on the microstructure, magnetic properties, dimensional precision and density of the magnets were studied. The magnetic properties, microstructure and constituents were investigated by means of a magnetic flux density - magnetic field strength (B-H) loopline instrument, scanning electron microscopy and energy dispersive X-ray analysis. The density of the magnets was determined by the Archimedes method, and the dimensional precision of the magnets was measured by micrometer. It was found that the microstructure of SPS processed Nd-Fe-B magnets is unique; the grain size is fine and uniform while distribution of the neodymium rich phase is heterogeneous. The optimal magnetic properties of SPS processed Nd-Fe-B magnets obtained so far are maximum energy product of 240 kJ m-3 and coercive force of 1260 kA m-1. The dimensional precision of the magnets is ~ 20 μm, and the density of the magnets reaches 7.58 g cm-3. [ABSTRACT FROM AUTHOR]

Nd-Fe-B permanent magnets doped with a small amount of praseodymium (Pr) nanoparticles were prepared by conventional sintering method. Effects of the Pr nanoparticles doping amount and the sintering temperature on the magnetic properties of the magnets were studied. As the doping amount increases, the coercivity of the magnets rises gradually, while the remanence and the maximum energy product of the magnets rise first, peak at 1.0 wt. % of Pr nanoparticles doping, then drop. The optimal sintering temperature was determined as 1040 °C. Under optimal doping amount and sintering temperature, the Nd-Fe-B magnet bears good magnetic properties of Br of 1.386 T, Hci of 1170 kA/m and (BH)max of 371 kJ/m3, which are better than the Pr-free magnet's magnetic properties of Br of 1.358 T, Hci of 1097 kA/m and (BH)max of 354 kJ/m3, suggesting the doping of Pr nanoparticles is a promising way to enhance the magnetic properties of sintered Nd-Fe-B magnets. [ABSTRACT FROM AUTHOR]

Anisotropic hybrid bulk magnets of MnBi/NdFeB with different composition ratio have been prepared with starting MnBi and Nd 2 Fe 14 B powders as well as epoxy resin as a binder in case it is needed to form bulk samples. It has been found that the ratio between the two phases in content has a remarkable influence on the magnetic properties, the thermal stability and the density of the bulk magnets. With increasing MnBi content the binder addition can be reduced. When the MnBi content is larger than 30 wt%, no binder is needed. On the other hand, the coercivity and saturation magnetization were increased significantly with increasing NdFeB content. When the NdFeB content was increased from 0% to 50%, the maximum energy product was enhanced from 4.7 to 10.0 MGOe, respectively. The energy product then decreased gradually with the NdFeB content due to the reduced density of the hybrid magnet. The thermal stability measurements showed that the temperature coefficient of coercivity grew with the MnBi content and became positive with MnBi=80 wt%. [ABSTRACT FROM AUTHOR]

We studied the influence of Al doping on the crystal structure and magnetocaloric properties of Mn 1.2 Fe 0.8 P 1 − x Ge x Al y compounds prepared by spark plasma sintering, with x = 0.24, 0.25, 0.26 and y = 0, 0.004, 0.008, 0.010 or 0.12. These compounds have the same Fe 2 P-type structure (space group P 6 ¯ 2m ) as the undoped system, and we found that the addition of Al had significant effects on the hexagonal unit cell of the compound and its physical properties. As y increased, the lattice parameter a and cell volume increased, the lattice parameter c decreased, the thermal hysteresis (Δ T hys ) decreased, and the Curie temperature ( T c ) became significantly higher than at y = 0. The variations of T c and Δ T hys correlated almost linearly with variation in the c / a ratio, indicating that T c can be increased with Al doping. In addition, Al doping did not have any adverse effects on other magnetocaloric properties, such as the entropy change, adiabatic temperature change, and the temperature range of the coexistence of the paramagnetic and ferromagnetic phases. We also found that the magnetocaloric properties of Mn 1.2 Fe 0.8 P 1 − x Ge x Al y compounds could be improved by homogenous annealing. The particular composition Mn 1.2 Fe 0.8 P 0.74 Ge 0.26 Al 0.010 exhibited a T c close to room temperature (292.2 K) with a hysteresis of Δ T hys 2.1 K, making it a very promising material for magnetic refrigeration around room temperature. [ABSTRACT FROM AUTHOR]

Abstract: Nd–Fe–B permanent magnets with a small amount of Dysprosium (Dy) nanoparticles doping were prepared by conventional sintered method, and the microstructure and magnetic properties of the magnets were studied. Investigation shows that the coercivity rises gradually, while the remanence decreases simultaneously with increased Dy doping amount. As a result, the magnet with 1.5wt.% Dy exhibits optimal magnetic properties. Further investigation presumed that Dy is enriched as (Nd, Dy)2Fe14B phase in the surface region of the Nd2Fe14B matrix grains indicated by the enhancement of the magneto-crystalline anisotropy field of the Nd2Fe14B phase. As a result, the magnet doped with a small amount of Dy nanoparticles possesses remarkably enhanced coercivity without sacrificing its magnetization noticeably. [Copyright &y& Elsevier]