P.Z. Si

Synthesis and characteristics of carbon-coated iron and nickel nanocapsules produced by arc discharge in ethanol vapor

Fe(C) and Ni(C) nanocapsules with low carbon content have been produced via an arc discharge process in ethanol vapor. It is clarified by X-ray diffraction that the core of the Fe(C) nanocapsules consists of gamma-Fe, alpha-Fe and Fe3C phase, while that of the Ni(C) nanocapsules contains only nickel. High-resolution transmission electron microscopy imaging confirms that these particles have a broad size distribution and the core/shell structure. Besides mutually independent nanocapsules with segregate graphitic shells, those with sharing shells are also observed in the Fe(C) nanocapsules. The remanence and the coercivity at room temperature of both the nanocapsules are higher than those of the corresponding microcrystallines, while the saturation magnetization is lower

Unconventional exchange bias in oxide-coated manganese nanoparticles

We report unconventional exchange bias in oxide-coated manganese nanoparticles, in which the Curie temperature of Mn3O4 shell is lower than the Neel temperature of the antiferromagnetic core. The coercivity (873 kA/m) of the nanoparticles, which is more than four times greater than that of bulk Mn3O4, has been enhanced significantly. A considerable enhancement in Curie temperature compared to the bulk was also observed for Mn3O4 in nanoscale. An exchange bias field as large as 400 kA/m was observed due to the strong interfacial exchange coupling. A simple phenomenological model is given to understand these phenomena in this ferri/antiferromagnetic system

Al2O3 coated α-Fe solid solution nanocapsules prepared by arc discharge

Al2O3 coated alpha-Fe solid solution nanocapsules are prepared by arc-discharging a bulk AlNiCo permanent magnet. The size of the nanocapsule is in range of 3-300 nm and the thickness of the shell is 1-6 nm. Al atoms in the AlNiCo magnet form the shell of amorphous Al2O3 to prevent the nanocapsules from further oxidation. The magnetic properties of saturation magnetization J(s) = 85 A m(2)/kg and coercive force H-j(c) = 27.5 kA/m are achieved for the nanocapsules

Amorphous boron nanoparticles and BN encapsulating boron nano-peanuts prepared by arc-decomposing diborane and nitriding

Amorphous boron nanoparticles were prepared by arc-decomposing diborane, which had ideal morphologies in comparison with that of those fabricated by furnace or laser heating diborane. Peanut-shaped boron nitride encapsulating boron nanocapsules were fabricated by nitridation of amorphous boron nanoparticles. Unique core/void/shell structure of the nanocapsules was observed by using a high-resolution transmission electron microscopy. The mechanism of growing the BN nanocapsules by a catalyst free process was distinctly different from the process of arc discharge or laser heating. The broadening of nonpolar intralayer Raman line of hexagonal BN at about 1370 cm−1 was observed, which was attributed to the small crystal size of BN.

Structure and magnetic properties of Gd nanoparticles and carbon coated Gd/GdC2 nanocapsules

Gd nanoparticles and carbon coated Gd/GdC2 nanocapsules have been prepared by means of arc discharge in argon and methane, respectively. In the Gd nanoparticles, gadolinium oxide was detected by x-ray diffraction, while the presence of metallic gadolinium was proved by oxygen analysis and x-ray photoelectron spectra. In the nanocapsules, the existence of GdC2 and Gd was detected by x-ray diffraction. Shell/core structures were observed for either the nanoparticles (with diameters larger than 20 nm) or the nanocapsules. The magnetization data above 25 K, when scaled as a function of mu(0)H/T, fall on a universal curve while the deviation of the magnetization curves at 4.2 K from the universal curves was ascribed to the superparamagnetism-ferromagnetism transition of small Gd particles. The temperature dependence of magnetization supports the theoretical predictions for very tiny Gd nanoparticles. The presence of metallic Gd in these nanocapsules results in comparatively high magnetization of about 60.5 A m2/kg at 4.2 K, compared with the carbon-coated gadolinium carbide nanocapsules without the metallic Gd core. The possibility of the existence of the spin-glass-like state is discussed for Gd(O) nanoparticles and Gd(C) nanocapsules

Effect of microstrain on the magnetism and magnetocaloric properties of MnAs0.97P0.03

In the compound MnAs0.97P0.03, prepared by mechanical milling, a large microstrain of 0.68%, calculated by quantitative x-ray diffraction analysis, induces a recoverable helimagnetic state at low temperatures and suppresses the temperature/field-induced orthorhombic-hexagonal phase transition. This leads to a remarkable reduction of both the thermal and the magnetic hysteresis at the Curie temperature, TC. Around the helimagnetic-ferromagnetic transition temperature and at TC, a large inverse magnetocaloric effect (MCE) with magnetic entropy change ΔSm of 5.6 J/kg K at 208 K and a normal MCE with ΔSm of −4.4 J/kg K at 253 K for a 5 T field change are observed. After annealing, MnAs0.97P0.03 exhibits a large MCE near room temperature with ΔSm of ∼14 J/kg K for a field change from 0 to 5 T.

Giant low-field magnetostriction of epoxy/TbxDy1-x(Fe0.8Co0.2)2 composites (0.20 ≤ x ≤ 0.40)

Spin configuration, magnetocrystalline anisotropy compensation, and magnetostriction of TbxDy1−x(Fe0.8Co0.2)2 (0.20 ≤ x ≤ 0.40) compounds have been investigated. Experimental evidence for the anisotropy compensation has been observed directly by performing x-ray diffraction on magnetic-field aligned powders and by evaluating the Mossbauer spectra. The easy magnetization direction (EMD) at room temperature rotates from the ⟨100⟩ (x ≤ 0.27) to the ⟨111⟩ axis (x ≥ 0.32), subjected to the anisotropy compensation between Tb3+ and Dy3+ ions. The strong grain-⟨111⟩-oriented pseudo-1–3 epoxy/composite has been fabricated by curing under a moderate magnetic field. A giant low-field magnetostriction, longitudinal λ|| and linear anisotropic λa (=λ|| − λ⊥) up to 550 and 760 ppm at 3 kOe, respectively, is obtained for Tb0.32Dy0.68(Fe0.8Co0.2)2 composite, which can be attributed to anisotropy compensation, a large magnetostriction coefficients λ111, EMD lying along ⟨111⟩ direction, the strong ⟨111⟩-textured orientation...

High magnetic-refrigeration performance of plate-shaped La0.5Pr0.5Fe11.4Si1.6 hydrides sintered in high-pressure H2 atmosphere

La(Fe, Si)13 hydride is regarded as one of the most promising room-temperature refrigerants. However, to use the alloys in an active magnetic regenerator machine, it is vital to prepare thin refrigerants. In this work, a high H2 gas pressure of 50 MPa was employed to suppress the desorption of hydrogen atoms during the sintering process of plate-shaped La0.5Pr0.5Fe11.4Si1.6 hydrides. At 330 K, a high-density sintered thin plate shows a large magnetic-entropy change ΔSm of 15.5 J/kg K (106 mJ/cm3 K) for a field change of 2 T. The volumetric ΔSm is almost twice as large as that of bonded La(Fe,Si)13 hydrides. Favorably, hysteresis is almost absent due to the existence of micropores with a porosity of 0.69% which has been analyzed with high-resolution X-ray microtomography.

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.

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.