Magnetocaloric Effect Caused by Paramagnetic Austenite–Ferromagnetic Martensite Phase Transformation

Anna Kosogor,Patricia Lázpita,Victor L’Vov,Concepció Seguí,Eduard Cesari

doi:10.3390/met9010011

Abstract

In the present work, the magnetization of Ni50Mn17.5Ga25Cu7.5 alloy undergoing the first-order phase transition from paramagnetic austenite to ferromagnetic martensite was measured to evaluate the magnetic-field-induced entropy change (MFIEC) and refrigerant capacity (RC) of the alloy. A standard method (SM) of evaluation of MFIEC is based on thermodynamic Maxwell relation. In view of the criticism of SM expressed by some scientists, the alternative method (AM), which is based on thermodynamic relationships for free energy, was proposed recently for the determination of MFIEC. We developed this method and computed MFIEC in two ways—by AM and SM. The values of MFIEC obtained for Ni50Mn17.5Ga25Cu7.5 alloy by these methods appeared to be large but very different from each other. Moreover, AM reveals the possibility of both normal and inverse magnetocaloric effects in the adjoining temperature ranges, while SM results only in the normal magnetocaloric effect.

Highlights

The normal/inverse magnetocaloric effect (MCE) is the decrease/increase of the temperature of a solid in the presence of a decreasing or increasing magnetic field
Sound arguments against the applicability of thermodynamic Maxwell relations to ferromagnetic solids undergoing first-order phase transitions were presented. That these relations were obtained for the equilibrium thermodynamic phase, and that they are not applicable to the mixed two-phase thermodynamic state arising in the temperature range of first-order phase transitions
For a detailed thermodynamic analysis of MCE observed during martensitic transformations (MTs), one must take into account the fact that MT is a first-order phase transition, which goes through the mixed austenitic-martensitic state, and calculate the field-induced entropy change ∆S1→2 from the free energies Faust and Fmart of austenitic and martensitic phases, respectively

Summary

Introduction

The normal/inverse magnetocaloric effect (MCE) is the decrease/increase of the temperature of a solid in the presence of a decreasing or increasing magnetic field. That these relations were obtained for the equilibrium thermodynamic phase, and that they are not applicable to the mixed two-phase thermodynamic state arising in the temperature range of first-order phase transitions It was found, secondly, that the MCE value strongly depends on the shift of phase transition temperature under magnetic field [9]. For the detailed theoretical analysis of MCE we used the experimental temperature dependences of magnetization of a Ni50 Mn17.5 Ga25 Cu7.5 alloy, which undergoes the phase transformation from paramagnetic austenite to ferromagnetic martensite These dependences enabled the estimation of the MCE value from Equation (2), but from the Equation (1) as well.

Experimental

Temperature of magnetization oflines

Model Formulation

Theoretical Basis

MCE in the Ni50 Mn18 Ga25 Cu7 Ribbon

Refrigerant Capacity