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(SmCo_(5)as main phase,SmCO_(3),SmCo_(7),Sm_(2)Co_(17)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,M_(r)/M_(s)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.

Study on properties of sinter mineral phases

In this study,the properties of sinter mineral phases were investigated by X-ray diffraction,optical microscopy quantitative observation,electron probe microanalysis,and the nanoindentation technique. The mechanisms that form return fines are discussed with respect to the factors of microregion composition,sintering temperature,and the reactive behavior of pisolite. The study results indicate the following：（ 1） Sinter mineral assemblage mainly comprises hematite,magnetite,calcium ferrite,and glass. In addition,the mineral assemblage of sinter products includes a great deal of calcium ferrite and melt-erosive magnetite,an abundance of secondary hematite,and a small amount of primary iron ore; whereas the mineral assemblage of return fines contains plentiful amounts of euhedral magnetite and secondary hematite,a large amount of relic pisolite with particle sizes less than1 mm,and relatively less calcium ferrite. In particular,some calcium ferrite was found to coexist with relic iron ore in a fiber-like microstructure.（ 2） Dentritic calcium ferrite has less SiO_2 and Al_2O_3,higher basicity（ w_（CaO）/w_（SiO_2））and a mole ratio of Fe_2O_3/CaO,whereas platy and blocky calcium ferrites have more SiO_2 and Al_2O_3,lower basicity and a mole ratio of Fe_2O_3/CaO.（ 3） The hardness of hematite is the highest（ around 18-22 GPa）,those of calcium ferrite and magnetite are relatively lower,and that of glass is the lowest. In terms of the formation mechanism of return fines,because of their weak ability to resist external shocks,these sorts of mineral phases and microstructures-（1） euhedral magnetite and glass formed in microregions with low basicity;（2） SFCA-Ⅰand relic iron ore formed in regions with a relatively low sintering temperature; and（3） relic pisolite and its nearby reaction regions-are inclined to form return fines.

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd30–xDyxFe69B1（x=2, and 4）（wt.%） were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction（XRD）. The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy（SEM） and transmission electron microscopy（TEM） analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd2Fe14 B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases（Dy-rich and Dy-lean phases） coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.