Large cryogenic magnetocaloric effect in the blocking state ofGdAl2∕Al2O3nanocapsules

Abstract

The nanocapsules with crystalline cores of GdAl 2 compound and shells of amorphous Al 2 O 3 were prepared by evaporating Gd x Al 100−x ͑x = 50, 60, 70, 80, and 90͒ alloys using a modified arc-discharge technique. The morphologies, average sizes, lattice constants, and surface characteristics of GdAl 2 /Al 2 O 3 nanocapsules were studied in detail by means of x-ray diffraction, energy dispersive spectroscopy, x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The formation mechanism of the nanocapsules was analyzed in detail. The differences in Curie temperatures and anisotropy constants of these nanocapsules were discussed with respect to their different structural characteristics. From 180 to 5 K, the magnetic entropy change of the GdAl 2 /Al 2 O 3 nanocapsules continuously increases with decreasing temperature T and rapidly enhances when the temperature tends to 5 K. The largest entropy change −⌬S at 7.5 K can respectively reach 18.02, 18.71, and 31.01 J kg −1 K −1 by varying the magnetic field from 7 to 1 T for the nanocapsules synthesized by arc-discharging Gd 70 Al 30 , Gd 80 Al 20 , and Gd 90 Al 10 alloys. The appearance of a large entropy change at low temperatures was ascribed to a lower anisotropy energy barrier and a high magnetic-moment density of the nanocapsules. The linear relation between the magnetic entropy change and the reciprocal of the temperature ͑1/T͒ was discussed in terms of superparamagnetism and magnetocaloric theory.

Magnetic properties of Dy nanoparticles and Al2O3-coated Dy nanocapsules

“…Dy nanoparticles/nanocapsules were synthesized by a modified arc-discharge process similar to the one used in our previous study (Zhang 2007;Liu et al 2008aLiu et al , b, 2009aLiu et al , b, 2010Geng et al 2003;Zhang et al 2006;Ma et al 2007Ma et al , 2008Si et al 2004). Master DyAl alloy was prepared by arc melting Dy and Al bulks of 99.9 wt% purity under a high purity argon atmosphere.…”

“…Therefore, it is significant to search an easy and substrate-free fabrication technique, which can prevent RE nanoparticles from severe oxidization. As a well-known method for producing nanoparticles/ nanocapsules, the arc-discharge technique has been widely used in the last decade (Zhang 2007;Liu et al 2008aLiu et al , b, 2009aGeng et al 2003;Zhang et al 2006;Ma et al 2007;Si et al 2004). Nanocapsules, which are normally composed of a core and a shell in the size of nanometer that are made of different materials, are excellent for preventing the core from further oxidization (Zhang 2007).…”

“…Nanocapsules, which are normally composed of a core and a shell in the size of nanometer that are made of different materials, are excellent for preventing the core from further oxidization (Zhang 2007). In the previous studies, we successfully introduced ceramics Al 2 O 3 as the shell and fabricated Al 2 O 3 -coated REAl 2 (RE = Dy, Gd, Ho, and Tb) compound nanocapsules by a modified arc discharge technique on the basis of evaporation pressures (Liu et al 2008a(Liu et al , b, 2009aZhang et al 2006;Ma et al 2007). Recently, Gd and Dy nanocapsules, DyCo 2 and HoCo 2 nanoparticles were synthesized by the arc discharge technique and their magnetic properties were investigated (Liu et al 2009a(Liu et al , b, 2010Si et al 2004;Ma et al 2007).…”

“…In the previous studies, we successfully introduced ceramics Al 2 O 3 as the shell and fabricated Al 2 O 3 -coated REAl 2 (RE = Dy, Gd, Ho, and Tb) compound nanocapsules by a modified arc discharge technique on the basis of evaporation pressures (Liu et al 2008a(Liu et al , b, 2009aZhang et al 2006;Ma et al 2007). Recently, Gd and Dy nanocapsules, DyCo 2 and HoCo 2 nanoparticles were synthesized by the arc discharge technique and their magnetic properties were investigated (Liu et al 2009a(Liu et al , b, 2010Si et al 2004;Ma et al 2007). These nanoparticles/nanocapsules can be stable in air without any special protection and they also exhibit large cryogenic magnetocaloric effect (MCE) induced by superparamagnetism (SPM), which can be applied for the magnetic refrigeration at low temperatures (Liu et al 2009a(Liu et al , b, 2010Ma et al 2007Ma et al , 2008.…”

“…Recently, Gd and Dy nanocapsules, DyCo 2 and HoCo 2 nanoparticles were synthesized by the arc discharge technique and their magnetic properties were investigated (Liu et al 2009a(Liu et al , b, 2010Si et al 2004;Ma et al 2007). These nanoparticles/nanocapsules can be stable in air without any special protection and they also exhibit large cryogenic magnetocaloric effect (MCE) induced by superparamagnetism (SPM), which can be applied for the magnetic refrigeration at low temperatures (Liu et al 2009a(Liu et al , b, 2010Ma et al 2007Ma et al , 2008. Utilization of magnetic refrigeration depends on the development of materials with sufficiently large changes of magnetic entropy at sufficiently low applied fields.…”