Tailoring of Fe-based magnetic refrigerants based on magnetic characteristics in active regenerative actions*

Among recent developments and investigations of caloric phenomena and their applications, magnetocaloric effect (MCE)-based practical products come closer to the demonstration stage, because of the advantage of non-contact external field control, that requires no lead wire, clamp, or presser plate as a source of heat leak.  The non-contacting feature is also very convenient to know “what’s going on” in the refrigeration cycle.  Although the materials evaluation is still performed by using isothermal field induced entropy change ΔS_M as a criterion, there is no isothermal field-induced pass in an active magnetic regenerative (AMR) cycle.  Instead, hot/cold blows generate a change in temperature of magnetic materials under constant/zero magnetic fields.  A change in magnetic state in the AMR bed can be detected by a susceptibility change captured outside of the bed.  Such observations give fruitful information which could not get through naïve temperature measurements.  For instance, a final state of the adiabatic temperature change (= an initial state for the hot/cold blow process) is a unsteady/non-equilibrium state, where an equilibrium thermodynamical route never approaches.   From these results, an empirical S-T diagram inside of the AMR bed can be drawn and the cycle trace was assigned on the diagram.  This procedure is also applicable even when heat load is applied at the cold end, and the change of magnetic state at various parts of the bed was observed up to zero temperature-span heat load.  From these results, new evaluation direction for the magnetocaloric materials in terms of role in the active regenerative action.