D J Kang

Fluorescence and microwave-absorption properties of multi-functional ZnO-coated α-Fe solid-solution nanocapsules

Fluorescence (FL) and microwave-absorption properties of multi-functional α-Fe solid-solution nanocapsules have been investigated. High-resolution transmission electron microscopy and x-ray photoelectron spectroscopy analysis show that the nanocapsules have a shell/core structure with α-Fe solid-solution nanoparticles as the core and amorphous ZnO as the shell. The nanocapsules are ferromagnetic at room temperature. There is a tendency to redshift for the peak at 388 nm in the FL spectra as the Zn concentration decreases. The in-depth study of relative permittivity and permeability reveals that the ZnO-coated α-Fe solid-solution nanocapsules exhibit excellent microwave-absorption properties, because of a proper electromagnetic match in the microstructure, strong natural resonances and dipolar polarization mechanisms.

Electromagnetic-wave absorption properties of FeCo nanocapsules and coral-like aggregates self-assembled by the nanocapsules

FeCo nanocapsules with FeCo alloy as core and amorphous Al(2)O(3) as shell have been synthesized by a modified arc-discharge technique. Three-dimensional coral-like aggregates are self-assembled by the nanocapsules in the arc-discharge process. The FeCo nanocapsules are ferromagnetic at room temperature. The electromagnetic-wave absorption properties of FeCo nanocapsules were investigated in the frequency range from 2 to 18 GHz. A reflection loss exceeding -20 dB was obtained in the frequency range of 5.2-15.9 GHz for absorber thicknesses of 2-4 mm. An optimal reflection loss of -44.8 dB was reached at 12.8 GHz for an absorber thickness of 3 mm. As a result, these FeCo nanocapsules may be applied in high-frequency electromagnetic-wave absorption

Electromagnetic-wave-absorption properties of wire-like structures self-assembled by FeCo nanocapsules

Al2O3-coated FeCo nanocapsules were synthesized by the arc-discharge method and three-dimensional wire-like macro-aggregates were self-assembled simultaneously by nanocapsules. The electromagnetic (EM) parameters (i.e. the relative permeability and permittivity) were measured at 2–18 GHz. As a consequence of the surface- and shape-anisotropy energy of the nanocapsules, a natural resonance at 6 GHz is dominant in the microwave absorption properties. A reflection loss (RL) exceeding −20 dB was obtained in the frequency range 4.2–18 GHz for absorber thicknesses of 1.3–6 mm. An optimal RL of −51 dB was found at 10.2 GHz for an absorber thickness of 2 mm. As a result, the Al2O3-coated FeCo nanocapsules show good prospects of being applied in EM wave absorptive devices. (Some figures in this article are in colour only in the electronic version)

Large reversible magnetocaloric effect in Dy2In

A large reversible magnetocaloric effect over a wide temperature region has been observed in the compound Dy2In. For a magnetic-field change of 5?T, the maximum magnetic-entropy change ??SM is 9.2?J?kg?1?K?1 at 126?K. The full width at half maximum (?TFWHM) of the ??SM versus T curve has a high value of 80?K and the relative cooling power (RCP) is as high as 736?J?kg?1 with no hysteresis losses. In particular, a large reversible ??SM (4.6?J?kg?1?K?1), large ?TFWHM (50?K) and large RCP (230?J?kg?1) are also achieved for a low field change of 2?T. Therefore, Dy2In may be an interesting material for low-temperature magnetic refrigeration.

Formation and large cryogenic magnetocaloric effect of HoAl(2)/Al(2)O(3) nanocapsules

HoAl(2)/Al(2)O(3) nanocapsules with a core of HoAl(2) and a shell of amorphous Al(2)O(3) have been synthesized. The formation of the rare-earth compound nanocapsules can be ascribed to the different melting points and the optimal proportions of Ho and Al atoms during the arc-discharging process. Furthermore, the formation of the Al(2)O(3) shell protects the rare-earth atoms from serious oxidation. The HoAl(2)/Al(2)O(3) nanocapsules display superparamagnetic properties between the blocking temperature of 6 K and the Curie temperature of 26 K. The absolute value of the magnetic-entropy change in the HoAl(2)/Al(2)O(3) nanocapsules rapidly increases with the decrease in temperature and reaches 14.7 J kg(-1) K(-1) at 3K for a magnetic-field change from 0 to 70 kOe. As a result, this new type of nanocapsule may be applied, in the temperature range studied, in cryogenic magnetic-refrigerator devices.