CHARACTERIZATION AND TABLE-LIKE MAGNETOCALORIC EFFECT IN SELECTED HoCoAl ALLOYS

ESMOND AGURGO BALFOUR (PhD. Candidate)

ABSTRACT: Magnetocaloric effect (MCE) finds technological application in magnetic refrigeration (MR). Ericsson cycle is one of the effective thermodynamic implementation cycles for MR. It requires that magnetic entropy change (ΔS_M) is constant (tablelike MCE) over the working temperature range. Considering that multiple transitions which are useful in attaining tablelike MCE can be induced in multiphasic alloys, carefully chosen regions of the HoCoAl ternary system are examined for suitable multiphase areas with tablelike feature. A synthesis technique for Ho₂Co₂Al compound that suppresses precipitation of the well-known HoCoAltype impurity phase is proposed. Composition tuning in selected region of the HoCoAl system and its effect on magnetic and magnetocaloric properties are investigated. The molecular meanfield theory predicts that the peak position of ΔS_M in MCMs shifts in response to changes in magnetic properties. However, variation in ΔS_M peak magnitude has been achieved over a wide range at constant peak occurrence temperature in the Ho₃₆Co_64xAl_x (x = 19, 20, 22) multiphase alloys. Table-like MCE is obtained for the Ho₃₆Co₄₈Al₁₆ alloy of the lower Al content Ho₃₆Co_64xAl_x (x = 15.5, 16, 17) alloy series. Magnetic properties and MCE of meltspun Ho₃₆Co₄₄Al₂₀ flakes has also been assessed and determined. Fully amorphous Ho_53.8Co_17.3Al_28.9 was prepared by injection casting and then hydrogenized to study its magnetic and magnetocaloric properties. The ΔS_M profile is table-like. Over all, the fully hydrogenized amorphous Ho_53.8Co_17.3Al_28.9 and crystalline multiphase Ho₃₆Co₄₈Al₁₆ of the Ho-Co-Al system are practically suitable as magnetic refrigerants for Ericsson cycle MR applications.  

BRIEF INTRODUCTION: Esmond Agurgo Balfour is a PhD candidate in the field of bulk magnetocaloric materials

SUPERVISOR: Prof. Fu Hao

TIME: 09:30

DATE: 2018-04-18

VENUE: Faculty of Physics 204

HOST: Prof. Hao Fu