Hongliang Ge

Structure and magnetic properties of bulk anisotropic SmCo5/α-Fe nanocomposite permanent magnets with different α-Fe content

Chemical coating, hot compaction, and hot deformation techniques have been applied to prepare bulk anisotropic SmCo5/α-Fe nanocomposite magnets. The effects of α-Fe content on the structure and magnetic properties of the magnets were studied. With the increase of the α-Fe content, both the saturation magnetization (Ms) and remanence (Mr) of the magnets rise first, peak at 10 vol. % α-Fe content and then fall while the coercivity (Hci) of the magnets drops simultaneously. Crystal structure analysis shows that the magnets exhibit a strong c-axis crystal texture of the SmCo5 phase, which, however, weakens gradually as the α-Fe content increases. Microstructure observation also shows that there are many SmCo5 equiaxial grains even after hot deformation in the magnets with 15 vol. % α-Fe.

Enhancement in the coercivity in Nd2Fe14B/α-Fe nanocomposite alloys by Ti doping

Nanocomposite α-Fe/Nd2Fe14B alloys with high coercivity were prepared by melt spinning. The influence of Ti addition on the microstructure and magnetic properties of these nanocomposite alloys were investigated. Ti addition has proved to result in relevant improvements in the microstructure and magnetic properties, especially in the coercivity Hc. It is shown that the magnetic properties of α-Fe/Nd2Fe14B nanocomposite are improved by an additional 5 at. % Ti, in particular, Hc from 595 up to 1006 kA/m, and (BH)max from 126.7 up to 135.3 kJ/m3. The coercivity mechanism of the α-Fe/Nd2Fe14B nanocomposite magnets was analyzed by studying the behavior of Hc(T)/Ms(T) versus HNmin(T)/Ms(T) (Kronmuller plot) and the dependence of Mirrev(H)/2Mr on the reverse field H. The results of this analysis show that nucleation is the dominating mechanism for the magnetization reversal in these nanocomposites. The Kronmuller plot gives evidence for a reduction in the exchange coupling resulting in an increase in the coercivity.

Goethite (α-FeOOH) nanopowders synthesized via a surfactant-assisted hydrothermal method: morphology, magnetic properties and conversion to rice-like α-Fe2O3 after annealing

Acicular goethite (α-FeOOH) nanopowders were synthesized at a high temperature via a CTAB-assisted hydrothermal method. Their morphologies, mechanism of saturation magnetization and coercivity, and conversion to rice-like α-Fe2O3 after annealing are studied. It is found that the α-FeOOH should be synthesized at a temperature lower than 240 °C, and the acicular α-FeOOH structure actually consists of slender α-FeOOH nanorods. With the increase of CTAB content from 0.2 g to 0.4 g, the synthesized α-FeOOH nanorods become more and more slender, accompanying with the enhancement of the saturation magnetization from about 0.05 emu g−1 to about 0.25 emu g−1, which can be ascribed to the increase of the surface area of the α-FeOOH nanorods. The as-synthesized acicular α-FeOOH powders exhibit a large coercivity with a maximum value of about 120 kA m−1 (∼1500 Oe) due to the shape anisotropy. It is also found that the acicular α-FeOOH powders gradually convert to rice-like α-Fe2O3 particles after annealing at high temperatures.

The Origin of Coercivity Enhancement of Sintered NdFeB Magnets Prepared by Dy Addition

The effect of Dy addition on the microstructure and magnetic properties of the sintered NdFeB magnets was investigated. The results of the microstructure analysis showed that Dy-free and Dy-doped samples are composed of Nd₂Fe ₁₄B (P42/mnm) and a trace of Nd-rich phase. Dy addition reduces significantly the pole density factor of (004), (006) and (008) crystal faces as estimated by the Horta formula. Accordingly, the coercivity of the Dy-doped sample increases from 2038 kA·m?1 up to 2288 kA·m?1. The Hcj(T)/Ms(T) versus /Ms(T) (Kronmuller-plot) behavior shows that the nucleation is the dominating mechanism for the magnetization reversal in these two kinds of magnets, and two microstructural parameters of αk and Neff are obtained. The Kronmuller-Plot gives evidence for an increase of the αk responsible for an increase of the coercivity as the result of the increase of the magnetic field as the magnetic domain reversed.

Magnetization reversal behavior in high coercivity Zr doped α-Fe/Nd2Fe14B nanocomposite alloys

The magnetization reversal behavior for rapidly solidified Zr-doped α-Fe/Nd2Fe14B alloys with high coercivity has been investigated by analyzing hysteresis curves and recoil loops of demagnetization curves. A drastic increase in the coercivity Hc from 620 to 855 kA/m at room temperature by an addition of 1 at. % Zr in α-Fe/Nd2Fe14B alloys has been observed. The maximum value of the integrated recoil loop area for Zr-doped samples of 3.05 kJ/m3 is much lower than that of the Zr-free sample. This result can be explained by a larger recoverable portion of the magnetization remaining in the Zr-free sample as long as the applied reversal field is below the coercivity Hc, i.e., it is an effect of an increased exchange-coupling in the Zr-free sample. The coercivity mechanism of the α-Fe/Nd2Fe14B nanocomposite magnets was analyzed in terms of the Kondorsky model and the plot of Hc(T)/Ms(T) versus HNmin(T)/Ms(T) (Kronmuller plot), respectively.

Large scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts

The FeS coated Fe nanoparticles were prepared by using high temperature reactions between the commercial Fe nanoparticles and the S powders in a sealed quartz tube. The simple method developed in this work is effective for large scale synthesis of FeS/Fe nanoparticles with tunable shell/core structures, which can be obtained by controlling the atomic ratio of Fe to S. The structural, magnetic and photocatalytic properties of the nanoparticles were investigated systematically. The good photocatalytic performance originating from the FeS shell in degradation of methylene blue under visible light and the high saturation magnetization originating from the ferromagnetic Fe core make the FeS/Fe nanoparticles a good photocatalyst that can be collected and recycled easily with a magnet. An exchange bias up to 11 mT induced in Fe by FeS was observed in the Fe/FeS nanoparticles with ferro/antiferromagnetic interfaces. The enhanced coercivity up to 32 mT was ascribed to the size effect of Fe core.

Formation of FexOy hollow nanospheres inside cage type mesoporous materials: a nanocasting pathway

FexOy hollow nanospheres were synthesized inside cage type mesoporous silica LP-FDU-12 through nanocasting. The sample was reduced to Fe3O4 and then further oxidized to γ-Fe2O3 with little changes in morphology and size. The Verwey transition is observed in the Fe3O4 hollow nanosphere system for the first time with magnetic measurement.

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.

Synthesis and Magnetic Properties of Cobalt Oxide Nanowires Array With Columnar SBA-15

Using hexagonal ordered straight-pore SBA-15 silica as hard template, mesoporous cobalt oxide (Co₃O₄) nanowires SBA-15 materials are synthesized by a nanoreplication route, and X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), field-emission scanning electron microscopy (FESEM), and N₂ physisorption isotherms are used to characterize the microstructure of columnar SBA-15 particles, Co₃O₄-doped SBA-15 (Co₃O₄/SBA-15) and Co₃O₄ nanowires array. The results indicate that Co₃O₄ exist in the mesochannels of SBA-15 to form Co₃O₄/SBA-15 nanocomposites and Co₃O₄ nanowires present mesostructure. Furthermore, the magnetic properties of Co₃O₄ nanowires array are measured and discussed by vibrating sample magnetometer (VSM) and surperconducting quantum interference device (SQUID), and the prepared Co₃O₄ nanowires array present superparamagnetism. Owing to surface effect of the uncompensated spins at the surface of Co₃O₄ nanowires, it presents a weak ferromagnetic behavior at low temperature.

In situ Observation of Phase Transformation in MnAl(C) Magnetic Materials

The phase transformation in two modes, including both displacive and massive growth of τ-phase from ε-MnAl(C), was observed by in situ transmission electron microscopy. The exact temperature range for different phase transformation modes was determined by magnetic measurements. The displacive growth of ε→τ in Mn54Al46 (or Mn54Al46C2.44) occurs at temperatures below 650 K (or 766 K), above which both modes coexist. One-third or less of the ε-phase can be transformed into τ-phase via displacive mode while the remaining two-thirds or more via massive mode. In bulk τ-phase, most τ-nanocrystals formed via displacive mode are distributed in the matrix of large τ-grains that formed via massive mode. The typical massive growth rate of the τ-phase is 8–60 nm/s, while the displacive growth rate is low. A more complete understanding of the ε→τ phase transformations in the MnAl-based magnets was provided in this work, based on which the annealing process for ε→τ was optimized and thus high purity τ-phase with high saturation magnetization was obtained.