Magnetocaloric Effect Caused by Paramagnetic Austenite–Ferromagnetic Martensite Phase Transformation

Abstract

In the present work, the magnetization of Ni 50 Mn 17.5 Ga 25 Cu 7.5 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 Ni 50 Mn 17.5 Ga 25 Cu 7.5 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.Keywords: inverse magnetocaloric effect; magnetic free energy; magnetic entropy change; strong magnetic filed; metamagnetic shape memory alloy should be expected in the temperature range corresponding to the abrupt change of magnetization value. This feature of magnetization behavior has attracted the attention of researchers to martensitic transformations (MTs) from ferromagnetic austenite to paramagnetic martensite, observed in the metamagnetic SMAs [1,2]. However, the abrupt change in magnetization also appears in the temperature range of first-order phase transformation of SMA, from paramagnetic austenite to ferromagnetic martensite [4][5][6]. Therefore, a theoretical estimation of the magnetic-field-induced entropy change for SMA undergoing such phase transformation may be a guide for experiments aimed at the improvement of magnetic refrigeration technology. However, sound arguments against the applicability of thermodynamic Maxwell relations to ferromagnetic solids undergoing first-order phase transitions were presented (see Refs. [1,7,8] and references therein). It was argued, firstly, that these relations were obtained for the equilibrium thermodynamic phase, and therefore, 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]. This feature of MCE obviously cannot follow from the equation obtained for the equilibrium thermodynamic phase. In this connection, other ways of evaluating MCE deserve careful attention.In the present study, we analyze the possibility of evaluating MCE from the fundamental thermodynamic relationshipwhich relates entropy function S to the temperature derivative of Helmholtz free energy F. The expression for free energy is a starting point of the Landau-type theories of magnetic, structural, and magnetostructural phase transitions. Due to this, Equation (2) was used recently for the evaluation of the magneti...

Influence of Structural Defects on the Properties of Metamagnetic Shape Memory Alloys

“…Although other approaches have been proposed in the literature based on the temperature derivative of the Helmoltz-free energy [54,55], the MCE, defined as the field induced entropy change ∆S iso in isothermal conditions, has been calculated according to classical thermodynamics through the temperature dependence of magnetization (curves not shown here but similar to those shown in Figure 4 but at different applied magnetic fields). The measured ∆S iso for a powder milled during 15á s a function of temperature and magnetic field is shown in Figure 6.…”

Forming of metastable austenitic stainless steel tubes with axially graded martensite content by flow-forming

“…Austenitic stainless steel offers favorable preconditions for this, since the metastable austenitic phase is not in an equilibrium state and can thus be transformed into α'-martensite with different properties in terms of mechanical and magnetic parameters through deformation-or temperature-induced energy [3,4]. The ferromagnetic nature of α'-martensite is fundamental for the work within this paper, since this can be detected and measured with micromagnetic measurement methods [5]. The objective of the first experimental investigations is to identify a correlation between the process parameters of a flow-forming process and the resulting deformationinduced α'-martensite content of the formed seamless tubes in metastable austenitic steel AISI 304L (X2CrNi18-9, 1.4307).…”

Finite Element Analysis of Magnetic Field Exciter for Direct Testing of Magnetocaloric Materials’ Properties

“…However, these materials exhibit the MCE over a narrow temperature range which makes it difficult Energies 2021, 14, 2792 2 of 17 to develop functional magnetic refrigeration systems. For this reason, intensive research is being conducted to develop better and better MCMs (with higher entropy and a wider range of temperature changes) [14][15][16][17]. Currently, the hybrid MCMs are seen as a potential solution enabling the development of effective magnetic refrigeration systems [18][19][20] [21].…”