Research On The Magnetocaloric Effect Of The Rare-earth Based Multiphase Alloys

Magnetic refrigeration based on the magnetocaloric effect(MCE) of magnetic materials is a potential technique that has prominent advantages over the currently used gas compression-expansion technique in the sense of its high efficiency and environment friendliness. Magnetocaloric materials(MCMs) are essential key in the realization of magnetic refrigeration. There are many multi-phase regions bounded by ferromagnetic phases showing good magnetocaloric potential in the Gd-Co(Ni)-Al ternary phase diagram. In this work, a careful selection of the ternary Gd-Co(Ni)-Al alloy samples have be prepared to obtain multi-phase structure and their relationships between structural, magnetic and magnetocaloric properties established. By employing the successive magnetic phase transitions, we successfully obtained large refrigerant capacity(RC) and even “table-like” MCE feature, which is necessary for the Ericsson thermodynamic cycle for the magnetic refrigerator operating above 20 K, in the selected single buck alloys. Our study has the potential to contribute to fundamental understanding of MCE in multi-phase alloys and also advance the state-of-the-art in magnetic refrigeration. The topics focusing on and some of the main conclusions are as follows:1. Two sorts of Gd-Co-Al ternary alloys of multi-phase structure with table-like MCE suitable for the ideal Ericsson cycle have been developed. One is prepared by arc-melting Gd53Co19Al28 with the resulting alloy composed of Gd2 Al, Gd2Co2 Al and GdCo0.74Al1.26 crystalline phases and the other is synthesized by suck-casting Gd52.5Co16.5Al31 which has a glassy structure and crystalline phases of Gd2 Al and Gd2Co2 Al. Metamagnetic transition of antiferromagnetic Gd2 Al phase near 50 K and ferromagnetic transitions of Gd2Co2 Al, GdCo0.74Al1.26 and the glassy phase near 80 K result in the occurring of table-like magnetocaloric effect with magnitude of ~7 J/kg K over the temperature range from 47.5 K to 77.5 K for a magnetic field variation of 0 to 50 kOe. The fitting of the temperature dependence of magnetic entropy change(ΔSM) and the experimental results suggest that the magnetocaloric response of the Gd2 Al phase in the ternary alloys have been improved, which is crucial for the composite alloys to obtain table-like magnetocaloric effect.2. The multiphase GdNixAl2-x(0.35≤x≤0.70) compounds were synthesized, and the relationship between the composition and magnetic and magnetocaloric properties of the as-cast alloys was subsequently established. For the alloys with composition close to the GdNiAl-end, large ΔSMs occur. Decreasing ratio of Ni to Al induces considerable RC due to the large broadening of temperature range of the phase transitions. In particular, a plateau-shaped magnetocaloric feature was found in the fully crystalline GdNi0.40Al1.60 alloy composed of three ferromagnetic phases. The maximum magnetic entropy change(-ΔSM-max) and RC of the as-cast GdNi0.40Al1.60 alloy are 8.8 J/kg K and 567 J/kg, respectively, for a magnetic field change of 50 kOe.3. Based on the structure-properties relationships that have been established in the Gd-Ni-Al ternary system, this research line presents a comparative analysis on the magnetic and magnetocaloric properties of carefully chosen Gd28Ni24Al48 and Gd33Ni13Al54 multiphase alloys. Both as-cast alloys are composed of GdNiAl2, GdNiAl and Ni-substituted GdAl2 ferromagnetic phases. The component ferromagnetic phases have closely adjacent ordering temperatures near 40 K resulting in reasonably large ΔSM for the Gd28Ni24Al48 alloy, while the widened ferromagnetic phase transitions contribute to the “table-like” ΔSM profile and considerable RC in the Gd33Ni13Al54 alloy. The above difference is principally due to the influence of Ni-Al substitution on the transition temperatures of GdNiAl and GdAl2 phases. The-ΔSMs and RCs of 14.5 J/kgK, 500 J/kg and 9.2 K/kgK, 591 J/kg were obtained, for a 50 kOe field change, for the Gd28Ni24Al48 and Gd33Ni13Al54 alloys, respectively. The plateau-shaped-ΔSM and considerable RC qualify the Gd33Ni13Al54 alloy as a competitive refrigerant candidate for regenerative Ericsson cycle based magnetic refrigeration at low temperatures.