Innovative Methodological Approaches for evaluating the Magnetocaloric Effect in Functional Materials

The experimental characterization of magnetocaloric effect has a fundamental role in the development of an efficient, environmentally friendly and cost-effective room-temperature magnetic refrigeration technology. The proper measure of the magnetocaloric effect as a function of temperature and magnetic field, in terms of adiabatic temperature change and isothermal entropy change, is required to compare the potentiality of different materials and to lead to their development. Moreover, the test of materials performance under operative conditions and the characterization of other physical and chemical features of magnetocaloric materials are essential to optimize the design of refrigeration machines and to prefigure new technological applications. This Thesis deals with advanced characterization methods of the magnetocaloric effect, which go beyond the standard magnetocaloric measurements and which have the purpose to test the materials response under real and extreme operative conditions and to investigate the relationship between the magnetocaloric effect and other magnetic, thermodynamic and structural properties of materials. The first part of the Thesis reports an in-depth thermodynamic analysis of the experimental setups for the direct measurement of the adiabatic temperature change. The influence of the temperature sensor, of non-ideal adiabatic conditions and of the sample thermal conductivity are discussed. Two innovative experimental setups for the measurement of the adiabatic temperature change in thin sheets and under pulsed magnetic fields are presented. Both the instruments are based on non-contact measurement techniques: the first employs an IR temperature detector, while the second exploits a thermo-optical effect, which is named “Mirage Effect”. The description and evaluation of the experimental setups are reported, together with the characterization of some of the most promising magnetocaloric materials (Gd, Fe2P-based compounds, Heusler alloys, La-Ca manganites, La-Fe-Co-Si, Mn-based antiperovskites). The second part of the Thesis analyses some of the materials properties that could be detrimental for application in magnetic refrigeration devices. In particular, the influence of the transition width on the magnetocaloric effect at first- and second-order transitions is discussed. Theoretical predictions, derived from phenomenological models of the transformations, are compared with experimental analysis of several Ni-Mn-based Heusler alloys. The obtained results reveal a strong correlation between the structural and microstructural characteristics of these alloys with their magnetic and magnetocaloric properties, both at the magnetostructural martensitic transformation and at the Curie transition.