Bulk Nanocomposite Magnets Research Articles

High-performance anisotropic Sm2Co17/Fe(Co) bulk nanocomposite magnets fabricated by two-step high-pressure thermal compression deformation

In the realm of Sm2Co17 nanocomposite magnetic materials, it remains a challenge to fabricate anisotropic magnets by forming nanocrystalline Sm2Co17 phases with strong texture, alongside soft phases of small size and high soft phase content. In this paper, we present a novel approach for fabricating anisotropic Sm2Co17/Fe(Co) nanocomposite bulk magnets with a prominent (00 l) texture of the Sm2Co17 phase, the 25 wt% content of the Fe(Co) phase, and a refined grain size of 25 nm. This fabrication is achieved using a two-step high-pressure thermal compression (HPTC) deformation process. The fabricated magnets exhibit a maximum energy product [(BH)max] of 20.0 MGOe with a pronounced magnetic anisotropy (Br///Br⊥ = 1.23). This result is 53 % higher than the previously reported largest value [(BH)max = 13.1 MGOe] for Sm2Co17-based nanocomposites. The magnets also exhibit a low remanence temperature coefficient (α = −0.014 %/°C) and a low coercivity temperature coefficient (β = −0.23 %/°C), demonstrating exceptional thermal stability. Our findings may improve the fabrication of anisotropic bulk Sm2Co17 nanostructure magnets for practical applications.

High magnetic energy product in isotropic nanocomposite powders with high percent of soft phase towards ultrastrong magnets

Nanostructure of magnetically hard and soft materials is fascinating for exploring next-generation ultrastrong permanent magnets with less expensive rare-earth elements. However, the resulting hard/soft nanocomposites often exhibit a low remanence/energy product due to the challenge in obtaining ideal phase components and appropriate soft phase fraction. In this work, a novel microstructure of multiple hard phases consisted of SmCo5 and SmCo3 as main phase, SmCo7 and Sm2Co17 as minor phase and a high ratio of FeCo (27 wt.%-48 wt.%) as soft phase was obtained. The grain size of both hard and soft phase below 15 nm was observed. The optimal energy product for Sm-Co/Fe(Co) nanocomposite is 2.1 times (an increase of 107%) of the corresponding single-hard-phase powders without soft phase. It reports that the isotropic nanocomposite powders exhibit a new record of magnetic energy product larger than 25 MGOe (the highest value is 28.6 MGOe). The high performance and the microstructure achieved in this work for the isotropic powders will shed light on and provide a good premise for synthesizing high performance anisotropic bulk nanocomposite magnets.

Production of Nd-Fe-B bulk nanocomposite magnets by hot deformation

In this study, the effects of Nd70Cu30 alloy powder addition on the microstructures and magnetic properties of Nd4Fe77.5B18.5 hot-deformed magnets were investigated. The Nd4Fe77.5B18.5 hot-deformed magnets consisted of the α-Fe, Fe3B, and Nd2Fe14B phases and were found to be nanocomposite. The coercivity of the Nd4Fe77.5B18.5 bulk nanocomposite magnets increased with Nd70Cu30 alloy powder addition. It was found that the Nd70Cu30 alloy reacted with the α-Fe and Fe3B phases in the Nd4Fe77.5B18.5 bulk nanocomposite magnets and formed the Nd2Fe14B phase. The Nd-Fe-B bulk nanocomposite magnet with 30% Nd70Cu30 alloy exhibited a high remanence of 9.72 kG and a high coercivity of 2.65 kOe.

Achieving ultrastrong anisotropy in bulk Pr2Fe14B/α-Fe nanocomposite magnets by high-pressure thermal compression

Aligning the hard phase has always been a great challenge in fabricating bulk nanocomposite magnets due to the high soft-phase fractions. In this work, anisotropic bulk Pr2Fe14B/α-Fe nanocomposite magnets have been fabricated from amorphous precursors by high-pressure thermal compression deformation. The synthesised magnet exhibits an ultrastrong magnetic anisotropy with a high soft-phase fraction of ∼29 wt.% and a dual-morphology grain structure that consists of equiaxed and lath-shaped grains. The anisotropy is originated from the strong (00l) crystallographic texture for Pr2Fe14B nanocrystals on account of its preferential nucleation and growth during the deformation with high stress and large strain. This study is favourable for the development of bulk nanocomposite magnets with superior energy products on the premise of high soft-phase fractions.

High energy product of isotropic bulk Sm-Co/α-Fe(Co) nanocomposite magnet with multiple hard phases and nanoscale grains

Nanocomposite magnets consisting of hard and soft magnetic phases have potential applications to be the next generation of permanent magnets with very high energy product and less expensive rare-earth elements. But it is still a big challenge to obtain bulk magnets with ideal microstructure and high performance. In this work, two - step warm processing at relative low temperatures had been adopted to obtain nearly theoretical density bulk nanocomposite magnets from amorphous/nanocrystalline powder precursors. Novel nanostructures consisting of multiple Sm-Co hard phases (SmCo5 as main phase, SmCo3, SmCo7, Sm2Co17 as minor phases) and 25 wt% α-Fe(Co) soft phase, nanoscale grain size below 20 nm for both the hard phase and soft phase, and the diffusion of Fe and Co compositions had been obtained in bulk isotropic magnets. Besides the ideal nanostructures, a high coercivity of 5.9 kOe, Mr/Ms value of 0.78 and a high square degree of demagnetization curve S = 0.47 were obtained. All of these factors together brought a new record-high energy product (BH)max of 23.6 MGOe. These results make an important step toward fabricating novel nanostructure with high performance.

Structural and Magnetic Studies of Bulk Nanocomposite Magnets Derived from Rapidly Solidified Pr-(Fe,Co)-(Zr,Nb)-B Alloy.

In the present study, the phase constitution, microstructure and magnetic properties of the nanocrystalline magnets, derived from fully amorphous or partially crystalline samples by annealing, were analyzed and compared. The melt-spun ribbons (with a thickness of ~30 µm) and suction-cast 0.5 mm and 1 mm thick plates of the Pr9Fe50Co13Zr1Nb4B23 alloy were soft magnetic in the as-cast state. In order to modify their magnetic properties, the annealing process was carried out at various temperatures from 923K to 1033K for 5 min. The Rietveld refinement of X-ray diffraction patterns combined with the partial or no known crystal structures (PONKCS) method allowed one to quantify the component phases and calculate their crystalline grain sizes. It was shown that the volume fraction of constituent phases and their crystallite sizes for the samples annealed at a particular temperature, dependent on the rapid solidification conditions, and thus a presence or absence of the crystallization nuclei in the as-cast state. Additionally, a thermomagnetic analysis was used as a complementary method to confirm the phase constitution. The hysteresis loops have shown that most of the samples exhibit a remanence enhancement typical for the soft/hard magnetic nanocomposite. Moreover, for the plates annealed at the lowest temperatures, the highest coercivities up to ~1150 kA/m were measured.

Evolution of microstructures and magnetic properties of SmCo/Fe(Co) nanocomposite magnets prepared by HPTC with different soft-phase contents

abstract Evolution of microstructures and magnetic properties of SmCo/Fe(Co) bulk nanocomposite magnets fabricated by high-pressure thermal compression (HPTC) with different soft magnetic phase contents and height reductions was investigated. With increasing α-Fe content from 22 wt% to 30 wt%, the (00l) texture of the SmCo7 phase decreases and the grain size of SmCo7, SmCo3 and Fe(Co) phases remains nearly unchanged. The remanence of the magnets increases and their coercivity decreases with increasing α-Fe content, yielding a maximum energy product of 26 MGOe at the α-Fe content of 26 wt%. With the height reduction from 0% to 78% during the HPTC process, the (00l) texture of the SmCo7 phase increases, but no obvious change in the grain size of SmCo7, SmCo3 and Fe(Co) phases; moreover, both the remanence and coercivity increase, yielding a maximum energy product of 25.6 MGOe at the height reduction of 78%.

Tuning the microstructure and magnetic properties of bulk nanocomposite magnets with large strain deformation

In this study, we investigated the effect of strain on the microstructure and magnetic properties of bulk α-Fe/Nd2Fe14B nanocomposite magnets produced by a combination of severe plastic deformation at room temperature and subsequent thermal annealing. Experiment results indicate that severe plastic deformation can induce the formation of α-Fe and Nd2Fe14B nanocrystals in the amorphous matrix and then suppress the formation of metastable intermediate phases during thermal annealing. The volume fraction of α-Fe phase in the magnets increases as the strain increases，and the grain size of α-Fe and Nd2Fe14B phases significantly decreases. As a result, the bulk magnets made at ε=6.2 show enhanced magnetic properties, (BH)max=17.8 MGOe and Hc=7.2kOe, compared with that of directly annealed partially amorphous (Nd-Pr)-Fe-Co-Nb-B, (BH)max=12.2 MGOe and Hc=6.2kOe.

Microstructure evolution and magnetic properties of the Nd9Fe81–xTi4C2Nb4Bx (x=11, 13, 15) bulk magnets prepared by copper mold suction casting

The bulk nanocomposite magnets of Nd9Fe81–xTi4C2Nb4Bx (x=11, 13, 15) in sheet form with the thickness of 0.7 mm were prepared by copper mold suction casting and subsequently annealing. The microstructure evolution and magnetic properties of bulk magnets were studied. It was shown that the as-cast microstructure of bulk alloys were composed of Nd2Fe14B, α-Fe, Fe3B crystalline phases and an amorphous matrix, and that the glass formability of alloy was improved with increasing the B content. The DSC analysis showed that the as-cast bulk alloys had the crystallization behavior of a two-step process. After annealing at the temperatures which was 40–63 K higher than their onset temperatures of the second exothermic peak, Nd9Fe81–xTi4C2Nb4Bx (x=11, 13, 15) bulk alloys obtained a finely mixed structure which were composed of Nd2Fe14B, α-Fe, Fe3B, (Nb,Ti)C crystalline phases and a residual amorphous phase, whose magnetic properties were significantly enhanced. For the bulk magnets of Nd9Fe81–xTi4C2Nb4Bx (x=11, 13, 15), the optimal magnetic properties of Br=0.63 T, iHc=155.1 kA/m, (BH)max=18.73 kJ/m3 could be achieved when x=13 after annealing at 983 K for 10 min.

Effects of gadolinium and silicon substitution on magnetic properties and microstructure of Nd–Fe–B–Nb bulk nanocomposite magnets

Abstract The magnetic properties, phase evolution and microstructure of Fe70−xMxB19Nd7Nb4 (M=Si, Gd, Si+Gd; x=0–2.5 at%) bulk nanocomposite permanent magnets in the form of rods produced by annealing the amorphous precursor have been investigated systematically. Microstructural examination, three-dimensional atom probe microanalysis, δM-plots, X-ray diffraction analysis and magnetometer studies deduced that good magnetic properties in the magnets originate from the homogenous microstructure consisting of exchange coupled, soft magnetic (α-Fe, Fe3B) and hard magnetic (Nd,Gd)2Fe14B nanophases. Optimally annealed Fe70B19Nd7Nb4 rod magnets exhibit magnetic properties of Br=0.61 T, iHc=876 kA/m and (BH)max=50.2 kJ/m3. Gadolinium and silicon addition to quaternary Fe70B19Nd7Nb4 alloy increased the mass fraction of hard magnetic phase, strengthened the exchange coupling interactions and enhanced the magnetic properties. Gadolinium and silicon segregated into hard magnetic phase which led to enhance coercivity up to 1115 kA/m. Enhancement in the coercivity is mainly resulted by hard phase increment as well as domain wall pinning, while strengthening of exchange coupling is caused by grain size refinement and increase in Curie temperature of the magnetic phases. The Fe67B19Nd7Gd2Nb4Si1 magnetic rods of 1.2 mm in diameter demonstrated the best magnetic properties such as intrinsic coercivity, iHc of 1115 kA/m, remanence, Br of 0.57 T and maximum energy product, (BH)max of 65.7 kJ/m3.

Thermal stability, thermal expansion and grain-growth in exchange-coupled Fe–Pt–Ag–B bulk nanocomposite magnets

Rare-earth free (RE-free) exchange coupling nanocomposite magnets are intensively studied nowadays due to their potential use in applications demanding stable high-temperature operation and corrosion resistance. In this respect, the FePt alloy system is one of the most actively addressed potential permanent magnet solutions. In FePt alloys, promising magnetic features arise from the co-existence of hard magnetic L10 FePt and soft magnetic L12 Fe3Pt phases emerged from the same metastable precursor. The present work deals with an in-situ temperature-resolved synchrotron radiation study of the thermal stability, thermal expansion and microstructure evolution in exchange-coupled FePtAgB alloys. The as-cast microstructural state as well as the optimized magnetic behavior are given as reference and correlated to the observed microstructural evolution with temperature. The melt-spun Fe48Pt28Ag6B18 alloy ribbons were examined in situ by synchrotron X-ray powder diffraction from ambient temperature up to 600°C. The FePt–Fe3Pt exchange-coupled microstructure achieved by rapid solidification is not significantly altered during the high temperature exposure. The thermal expansion of the FePt L10 unit cell has been found to be strongly anisotropic, being essentially an in-plane expansion which may be seen as an anisotropic invar effect. For the FePt L10 phase, a significant deviation from linear thermal expansion is observed at the Curie temperature TC=477°C. This non-linear behavior above TC is tentatively linked to a diffusion/segregation mechanism of Ag. The promising hard magnetic properties as well as the direct formation of the L10 phase from the as-cast state are directly related to the presence of Ag in the intergranular regions.

Iron oxide nanocomposite magnets produced by partial reduction of strontium hexaferrite

Isotropic bulk nanocomposite permanent magnets were produced with strontium hexaferrite, SrO·6Fe2 O3 , and magnetite, Fe3 O4 , as the magnetically hard and soft components. A novels synthesis scheme based on the partial reduction of SrO·6Fe2 O3 was employed. In two parallel experiments, nano- and microcrystalline SrO·6Fe2 O3 particles were compacted into pellets along with a controlled, understoichiometric amount of potato starch as a reducing agent. The pellets were then sintered in a passive atmosphere. Based on XRD and room temperature magnetic hysteresis measurements, it was concluded that a fraction of the SrO·6Fe2 O3 input material had been reduced into Fe3 O4 . In comparison with pure SrO·6Fe2 O3 control pellets, these composites exhibited maximum energy product increases in excess of 5 % due to remanence boosting. The improvement of magnetic properties was attributed to an efficient exchange spring coupling between the magnetic phases. Interestingly, as the synthesis scheme also worked for microcrystalline SrO·6Fe2 O3 , the method could presumably be adapted to yield crystallographically oriented bulk nanocomposite magnets.

Simultaneously increasing the magnetization and coercivity of bulk nanocomposite magnets via severe plastic deformation

In general, there is a trade-off between magnetization and coercivity in nanocomposite magnets. Here, we demonstrate a simultaneous enhancement of both the magnetization and coercivity in bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared via a severe plastic deformation (SPD) compared with thermally annealed magnets. The enhanced magnetization results from a high fraction (>30%) of α-Fe phase induced by SPD, while the increase in coercivity from 4.6 to 7.2 kOe is attributed to an enhancement in domain wall pinning strength. This study shows an increase in energy product is possible in the nanocomposite magnets for a large inclusion of soft-magnetic phase.

Effect of Soft Phase on Magnetic Properties of Bulk Sm–Co/$\alpha$–Fe Nanocomposite Magnets

Bulk Sm-Co/α-Fe nanocomposite magnets were fabricated by high energy ball milling and subsequent quick hot-pressing. Three different parameters were investigated which are soft phase content, type, and distribution. The effect of Fe soft phase content on the magnetic properties of the final bulk samples was examined. Increasing the Fe content significantly increased the saturation magnetization but at the cost of reduced coercivity. The optimum Fe addition for the highest maximum energy product was determined to be 15 wt% under the present processing conditions. Besides pure Fe a Fe49Co49V2 powder was also studied as a soft phase addition. The results indicated that FeCoV powder was more readily alloyed with the Sm-Co phase during the milling procedure. For the final bulk magnets, nanoscale Fe particles existed in Sm-Co matrix with pure Fe addition, but with Fe49Co49V2 addition the soft phase particles were undetectable. Two methods for adding soft phase Fe into hard phase SmCo5 were utilized. The investigation showed that milling Fe and SmCo5 together resulted in a more uniformly distributed mixture of nanoscale soft phase particles, improved squareness of the demagnetization curves of bulk nanocomposite magnets, and higher (BH)max as compared to blending Fe with SmCo5.

High temperature magnetic properties of SmCo5/α-Fe(Co) bulk nanocomposite magnets

To find alternative high temperature magnets containing no heavy rare earths for power applications, SmCo5/Fe bulk nanocomposite magnets with enhanced energy density and high thermal stability have been produced by using a ball-milling plus warm-compaction route. Up to 30% of the Fe soft magnetic phase has been added to the composites with grain size <20 nm distributed homogenously in the matrix of the SmCo5 hard magnetic phase. It was observed that the microstructure does not change with temperature up to 500 °C. It is also observed that the thermal stability of bulk nanocomposite samples is closely related to bulk density. Energy products above 11 MGOe have been obtained at 300 °C in fully dense bulk SmCo5/Fe nanocomposite magnets, which is 65% higher than that of a single-phase counterpart at the same temperature.

Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The glass forming ability and magnetic properties of Nd5Fe68−xB23Mo4Yx (x=0, 2, 4, 6) alloys prepared by copper mold casting technique have been studied. Amorphous rods with a diameter of 2mm were obtained in the Nd5Fe64B23Mo4Y4 alloy. After annealing for 10min at 1013K, the Nd5Fe64B23Mo4Y4 alloy showed optimal hard magnetic properties with the coercivity of 764.2kA/m, remanence of 0.6T and maximum energy product of 57.3kJ/m3, respectively. The enhanced magnetic properties can be ascribed to the strong exchange coupling among the magnetically soft α-Fe (25–30nm), Fe3B (30–35nm) and hard Nd2Fe14B (40–50nm) grains present in the magnet microstructure. Large size bulk nanocomposite magnets with sound magnetic properties make the Nd–Fe–B–Mo–Y alloy system a promising candidate for industrial applications.

Fe65B22Nd9Mo4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The Fe65B22Nd9Mo4 nanocomposite permanent magnets in the form of a rectangular cross sectioned rod have been prepared by annealing the amorphous precursors. The thermal behavior, structure and magnetic properties of the magnets have been investigated by differential scanning calorimetry, X-ray diffractometry, electron microscopy and magnetometry techniques. The as-cast Fe65B22Nd9Mo4 alloy showed soft magnetic properties, which changed into magnetically hard after annealing. Results provoke that the magnetic properties of the alloy are sensitive to thermal processing conditions. The optimum hard magnetic properties with a remanence (Br) of 0.56T, coercivity (iHc) of 920.7kA/m and maximum energy product (BH)max of 50.15kJ/m3 were achieved after annealing the alloy at 983K for 10min. The good magnetic properties of Fe65B22Nd9Mo4 magnets are ascribed to the exchange coupling between the nano-scaled soft α-Fe, Fe3B and hard Nd2Fe14B magnetic grains.

Bulk anisotropic nanocomposite magnets prepared by the thermo-mechanical processing of Nd3.6Pr5.4Fe83Co3B5 with different microstructures

Bulk nanocomposite magnets with enhanced magnetic properties and (0 0 l) crystallographic texture for the (Nd,Pr)2Fe14B hard magnetic phase were prepared by thermo-mechanical process of Nd3.6Pr5.4Fe83Co3B5 ribbons. Ribbons with different microstructures, i.e. an amorphous state and amorphous matrix with a few α-(Fe,Co) nanocrystals, were employed in this study. Optimum magnetic properties were obtained in the magnets made by the hot deformation of the ribbons with nanocrystals in the amorphous matrix; the maximum energy product (BH)max = 118.6 kJ m−3. The magnets made from ribbons with a fully amorphous structure showed the largest magnetic anisotropy, and the values of (BH)max measured parallel (∥) and perpendicular (⊥) to the stress direction were ∼102.7 kJ m−3 and 79.6 kJ m−3, respectively. This study is of importance for the development of bulk anisotropic nanocomposite magnets with enhanced magnetic properties.

Magnetic properties of Pr-Fe-Ti-B nanocomposite magnets produced by spark plasma sintering method

Amorphous melt-spun ribbons of Pr8Fe86–xTixB6 (x = 0–10) alloys were successfully consolidated into bulk materials by the spark plasma sintering method. X-ray diffraction and transmission electron microscopic studies revealed that the resultant bulk materials consisted of nanosized grains of the α-Fe and Pr2Fe14B phases. The coercivity of the Pr-Fe-Ti-B bulk nanocomposite magnets increased with increasing Ti content. The Pr8Fe80Ti6B6 bulk nanocomposite magnet exhibited a high remanence of 10.2 kG with a coercivity of 4.9 kOe, while the Pr8Fe76Ti10B6 bulk magnet exhibited a remanence of 6.5 kG with a high coercivity of 8.2 kOe.

Synthesis, structure, and magnetic properties of Nd-Y-Fe-Mo-B bulk nanocomposite magnets

NdxY6−xFe68Mo4B22 (x = 1–5) nanocomposites were prepared directly by the devitrification of amorphous rods. The effects of Y doping on the glass-forming ability, microstructure, and magnetic properties of the alloys were also investigated. GFA of the alloys was found to get enhanced through substituting Nd with Y and to increase with the growth of Y contents. Results also showed that the best glass former was Nd1Y5Fe68Mo4B22 with a critical diameter of 4 mm. The coercivity first increased after subsequent crystallization and then decreased with the reduction of Y content, which was closely related to the phase transition during the crystallization. The devitrified Nd3Y3Fe68Mo4B22 exhibited a maximal coercivity of 364.1 kA/m, resulting from strong exchange coupling between the hard and the soft phases.