Formation and large cryogenic magnetocaloric effect of HoAl₂/Al₂O₃nanocapsules

Abstract

HoAl2/Al2O3 nanocapsules with a core of HoAl2 and a shell of amorphous Al2O3 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 Al2O3 shell protects the rare-earth atoms from serious oxidation. The HoAl2/Al2O3 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 HoAl2/Al2O3 nanocapsules rapidly increases with the decrease in temperature and reaches 14.7 J kg−1 K−1 at 3 K 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.

Giant reversible magnetocaloric effect in flower-like β-Co(OH)2 hierarchical superstructures self-assembled by nanosheets

“…Currently, there is a great deal of interest in utilizing the magnetocaloric effect (MCE) as an alternate technology for refrigeration, replacing the common gas-compression expansion technology, due to higher efficiency and environmental concerns [1][2][3][4][5][6][7][8][9][10][11] . An ideal material for magnetic refrigeration should be composed of relatively inexpensive raw materials, have a high MCE demonstrated by a high change in magnetic entropy (ΔS M ) and a high adiabatic temperature change, and have little or no thermal/magnetic hysteresis 1,8 .…”

Magnetic susceptibility and parameters of electronic structure of Al2REM (Gd, Dy, and Ho) intermetallic compounds at high temperatures

“…It was established that the given compound undergoes magnetic ordering at T < 4.2 K. The effective magnetic moment per holmium atom μ eff = 8.1μ B was also determined in [1]. In the last few years, the Al 2 REM compounds in amorphous and nanocrystal states have actively been studied [3][4][5]. In particular, the Al 2 Gd and Al 2 Ho compounds in the form of nanocrystal capsules were used to obtain ultralow temperatures [4,5].…”

“…In the last few years, the Al 2 REM compounds in amorphous and nanocrystal states have actively been studied [3][4][5]. In particular, the Al 2 Gd and Al 2 Ho compounds in the form of nanocrystal capsules were used to obtain ultralow temperatures [4,5].…”

Microstructure and magnetic properties of graphite-coated Gd nanocapsules

“…In contrast to the compression/expansion cycles characteristic of gas refrigerators, magnetic refrigeration relies on the magnetocaloric effect ͑MCE͒, the change in temperature associated with the magnetization/ demagnetization of a magnetic refrigerant. [1][2][3][4][5][6] Recently, giant MCEs have been observed near room temperature at first-or second-order phase transitions. At lower temperatures ͑T Ͻ 20 K͒, however, magnetic refrigeration can seldom be realized by employing these types of MCE because these phase transitions usually occur at higher temperatures.…”

“…GdAl 2 / Al 2 O 3 , TbAl 2 / Al 2 O 3 , and HoAl 2 / Al 2 O 3 nanocapsules with large −⌬S m at low temperatures have been prepared. 2,4,6 However, the low thermal conductivity of the Al 2 O 3 shell may decrease their application potential in a refrigeration process. Due to its metal-like thermal conductivity, graphite is a better candidate for the protective shell.…”