CONCLUSIONS AND OUTLOOKS

The aim that has driven this Thesis was the development of new experimental methodologies for the characterization of functional materials to be used as active elements in magnetic cooling systems at room temperature. The attention was mainly paid to the test of magnetocaloric features of materials in shapes and under conditions closer to those of real applications. Moreover, the necessity to precisely control the magnetocaloric performance of materials, moved me to investigate the effect of several properties of magnetic transitions, which can be modified during materials’ synthesis and manufacturing and which have a key role in determining the exploitable magnetocaloric effect.

The main goal of this Thesis was the realization of two innovative experimental setups to directly measure the MCE, as adiabatic temperature change, in thin samples and with fast magnetic field changes.

In the first realized instrument, based on a non-contact IR temperature sensor, very good quasi-adiabatic conditions were obtained, thus allowing the measurement of field-induced temperature variations in gadolinium sheets with a thickness down to 27 µm. Thermal simulations of the measurement system confirmed the experimental results and allowed to estimate the time-scale of magnetic field change required for measuring thinner samples. The designed technique represents an ideal solution both to directly measure the MCE of samples with reduced thickness and to reproduce high frequency (up to 6 Hz) thermomagnetic cycles for testing the material response in operating conditions.

The use of thermopiles moreover can be extended to directly measure other caloric effects in thin samples. The proposed non-contact technique, when studying Electrocaloric, Barocaloric and Elastocaloric effects,may become an undeniable solution to isolate the temperature sensor from electric or mechanical stresses, thus improving the measurement quality and the lifetime of the technique.

The second designed experimental setup represents an original idea, which takes advantage of the thermo-optical “Mirage effect” to directly measure the adiabatic temperature change induced in a magnetocaloric material by a pulsed magnetic field. This innovative, simple and versatile non-contact technique allows to test the response of magnetocaloric materials to fast magnetic field changes, in this way simulating high frequency operating conditions (1 T at a frequency of about 150 Hz).

Measurements performed on some Fe2P-based compounds, demonstrated the suitability of the realized experimental setup and the outstanding dynamic response of these materials, which makes them ideal for applications at high frequencies. Moreover, the reduction of the measurement time constant below one millisecond ensures very good adiabatic conditions, making this technique suitable also for characterizing samples with low dimensionality (foils, ribbons and freestanding thin films). This possibility was proved through the direct characterization of the magnetocaloric effect near the Curie transition of a series of NiMn(In,Sn) ribbons.

The combination of the principles of the two presented instruments, may bring, in the future, to the development of experimental setups in which IR-temperature sensors and magnetic field pulses are exploited to investigate the MCE induced by very fast magnetic stimuli. This may offer important information to design innovative technological applications and to extend the understanding of the kinetics of magnetic transformations.

The second main result of this Thesis was the demonstration of the negative influence on the MCE of the broadening of first- and second-order magnetic transitions. This conclusion has been reached by exploiting phenomenological and numerical models of the transformations and complete magnetocaloric characterizations performed on different MC materials. In particular, the magnetostructural martensitic transition and the Curie transition of (Ni,Mn)-based Heusler alloys were investigated. Concerning the magnetostructural transformation of these alloys, the combination of bulk measurements techniques and microscopy analysis demonstrated that the broadness of the transition is characteristic of a microscopic scale and cannot be ascribed to macroscopic chemical inhomogeneities. Moreover, the study of the degrading effect of cold-working on the MCE of a NiMnIn Heusler alloy at the Curie transition suggested the existence of a correlation between the density of structural defects, and in particular of antiphase boundaries, with the magnetic and magnetocaloric features of these alloys.

For both the transitions, it resulted evident a strong relationship between the magnetic and magnetocaloric properties of these alloys with their structural and microstructural features. A thorough study is needed to understand these correlation. Results from bulk measurement techniques have to be compared with local direct observations (e.g.: high resolution electron microscopy and crystallography, Lorentz microscopy, magnetic atomic force microscopy) and experiments with statistical probes of local states (e.g.: high-resolution X-rays diffraction, elastic and inelastic neutron diffraction and solid-state nuclear magnetic resonance). Furthermore, these studies have to be carried out

considering different synthesis protocols, annealing processes and mechanical stresses induced by cold manufacturing. This work would result fundamental in the perspective to optimize the MC performance of materials and to develop mass production routes with the scaling-up of synthesis protocols. Moreover, the idea of realizing complex structures consisting of layered magnetocaloric materials, or of their smart composites, requires a careful evaluation of the manufacturing influence on the functional properties of materials.

Finally, an unresolved topic that has still to be studied is the correlation between hysteresis and broadness of first-order transformations. Partial transformations between metastable mixed-states, induced by temperature or by magnetic field under different conditions have to be explored on both the macroscopic and the microscopic scale, in order to optimize the thermomagnetic cycles exploitable in energy conversion applications. Moreover, this study would result in a clearer understanding of the kinetics of first-order magnetic transformations.