Wenpeng Song

Controllably Manipulating Three-Dimensional Hybrid Nanostructures for Bulk Nanocomposites with Large Energy Products

Hybrid nanostructures that comprise two or more nanoscale functional components are fascinating for applications in electronics, energy conversion devices, and biotechnologies. Their performances are strongly dependent on the characteristics of the individual components including the size, morphology, orientation, and distribution. However, it remains challenging to simultaneously control these structural properties in a three-dimensional (3D) hybrid nanostructure. Here, we introduce a robust strategy for concurrently manipulating these characteristics in a bulk SmCo/Fe(Co) nanocomposite. This method can tune nanocrystals in size (down to sub-10 nm), morphology (sphere, rod, or disc), and crystallographic orientation (isotropic or anisotropic). We have therefore achieved the desired nanostructures: oriented hard magnetic SmCo grains and homogeneously distributed soft magnetic Fe(Co) grains with high fractions (∼26 wt %) and small sizes (∼12.5 nm). The resulting anisotropic nanocomposite exhibits an energy product that is approximately 50% greater than that of its corresponding pure SmCo magnet and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Our findings pave a new way to manipulating 3D hybrid nanostructures in a controllable manner.

Novel Bimorphological Anisotropic Bulk Nanocomposite Materials with High Energy Products

Nanostructuring 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 random crystallographic orientations and monomorphological equiaxed grains, leading to inferior magnetic performances compared to corresponding pure rare-earth magnets. This study describes the first fabrication of a novel bimorphological anisotropic bulk nanocomposite using a multistep deformation approach, which consists of oriented hard-phase SmCo rod-shaped grains and soft-phase Fe(Co) equiaxed grains with a high fraction (≈28 wt%) and small size (≈10 nm). The nanocomposite exhibits a record-high energy product (28 MGOe) for this class of bulk materials with less rare-earth elements and outperforms, for the first time, the corresponding pure rare-earth magnet with 58% enhancement in energy product. These findings open up the door to moving from a pure permanent-magnet system to a stronger nanocomposite system at lower costs.

Effects of Annealing Pressures on the Ordering and Microstructures of FePt:Ag Nanocomposite Films

FePt:Ag (100 nm) nanocomposite thin films were prepared on naturally-oxidized Si substrates by dc magnetron sputtering at room temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to investigate the effects of annealing pressures on the ordering processes and microstructures of these films. The average sizes for the L1 0 ordered domains and the FePt grains are reduced to d = 9 nm and D = 13 nm from d = 19 nm and D = 34 nm, respectively, when the annealing pressure is enhanced to 0.6 GPa from room pressure at 873 K. Furthermore, the size distribution is improved into a narrow range. With increasing pressure, the coercivity of L1 0 -FePt:Ag thin films decreases from 15.1 to 7.6 kOe. In the present study, the effects of high pressure on the L1 0 ordering processes and microstructures of FePt:Ag nanocomposite films were discussed.

Anisotropic bulk SmCo7 nanocrystalline magnets with high energy product

Realizing grain alignment along easy magnetization axis in bulk SmCo7 nanocrystalline materials is crucial for their development as high-performance high-temperature magnets, yet it remains challenging. Here, we report the fabrication of anisotropic bulk SmCo7 nanocrystalline magnets with a small grain size of ∼20 nm and a (00l) texture using high-pressure thermal compression starting from partially amorphous precursors. The synthesized magnet exhibits a high energy product of 18.4 MGOe, 40% larger than the reported highest value (13 MGOe) for bulk nanostructured SmCo7 magnets, and outperforms its anisotropic coarse-grained counterpart. Moreover, our magnet shows a low coercivity temperature coefficient of β = −0.19%/°C. These findings make an important step toward the fabrication of oriented bulk nanostructures for practical applications.

Phase transition of FePt/Ag nanocomposite thin films: Kinetics, activation volume, and atomic processes

Understanding the L10 phase transformation of FePt nanocrystals in FePt:X (X = nonmagnetic materials) nanocomposite thin films is of particular importance for controlling and designing these films as ultrahigh density magnetic recording media. In this study, by employing temperature- and pressure-dependence of the rate constant of phase transition, the nucleation (En = 0.3 eV) and growth (Eg = 0.9 eV) activation energies of L10 ordered domains, as well as an activation volume ΔV* ≈ 0.96 Ω (Ω = average atomic volume) for L10 ordering transition in FePt:Ag nanocomposite thin films have been successfully determined. These data suggest that the growth of L10 ordered domains is predominantly dependent on the diffusion of Fe atoms in FePt crystals, and that the diffusion of Ag atoms out of FePt lattice prompts the nucleation of the L10 ordered domains. These findings have a direct implication for designing FePt:Ag nanocomposite thin films with high pack fraction of L10-FePt nanocrystals.

Stress-dependent deformation behaviour in bulk nanocrystalline titanium–nickel alloys

In this study, the deformation mechanisms operating with stress in bulk nanocrystalline (NC) titanium–nickel with an average grain size below a critical size of 10–20 nm have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary (GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. These deformation mechanisms are different from the previous understanding that below a critical grain size of 10–20 nm, GB sliding and grain rotation govern plastic deformation of NC materials.