Table-like Magnetocaloric Effect in Ho36Co48Al16 Multiphase Alloy

Abstract

Magnetocaloric effect (MCE) has recently gained wide attention in both fundamental research [1, 2], and application in magnetic refrigeration (MR) systems. It is promising as an efficiently competitive technology for the replacement of the potentially environmentally risky gas compression/expansion refrigeration [3–5]. Ericsson cycle implementation of MR is one of the most suitable thermodynamic cycles for enhanced performance. However, its requirement that magnetic entropy change (–ΔSM) of the magnetic refrigerant is constant (table-like MCE) over the operating temperature range severely limits the availability of appropriate magnetocaloric materials (MCMs). The search for MCMs with such property is a daunting challenge. Some proposed research approach include multilayered and powder composite mixtures but the low density of powder composites significantly deteriorates the magnitude of –ΔSM. Moreover, the lifespan of sintered or glued layer refrigerants is shortened by repeated differential expansion and contraction at the interfaces. Single bulk multiphase alloys with table-like feature could be a solution to overcoming the issues associated with layered and ex-situ composite mixtures. However, the challenge of identifying such alloy systems with temperature independent –ΔSM or improved refrigerant capacity (RC) can be overwhelming. Their discovery is strictly dictated by the nature of the related phase diagrams. Several Ho(Co, Al)-based binary and quasi-binary crystalline alloys are known to have good magnetocaloric properties. Therefore, we examine the Ho-Co-Al system to identify a suitable multiphase region for study in expectation of a single bulk multi-crystalline alloy that exhibits table-like MCE. In particular, HoCo 2 , HoCoAl, and Ho 2 Co 2 Al compounds have good magnetocaloric properties [6–8]. Consequently a carefully selected region of the Ho-Co-Al system, bounded by the three compounds and expected to contain all three phases, was studied. Results show that the title multiphase alloy, which comprises the HoCo 2 , HoCoAl, and Ho 2 Co 2 Al main phases, exhibits table-like MCE. The effect of interatomic substitution on the magnetic properties of relevant phases and its contributions to the attainment of the table-like MCE and RC are discussed in detail.