Métodos para la caracterización de materiales magnetocalóricos multifásicos

Abstract

This thesis is aimed to study the characteristics of thermomagnetic phase transitions and the resulting magnetocaloric effect in multiphase magnetocaloric materials, which is a very common scenario in the research of the magnetocaloric effect and can lead to complex situations that need to be properly approached. On the one hand, composites that combine various phases with Curie transitions close to each other are interesting and highly studied as they can lead to an enhanced refrigerant capacity with respect to the pure phases while keeping a large enough response. In this thesis, newly developed Gd + Gd7Pd3 composites are presented as a case in which the mentioned improvement of the refrigerant capacity is achieved. This intentional situation of overlapping Curie transitions has been used for the development of a method for the deconvolution of the magnetocaloric responses by means of the scaling laws of the magnetocaloric effect. On the other hand, materials exhibiting magnetostructural transformation (first-order phase transition) can also show the Curie transitions of the phases (second-order phase transition), where the phase transitions of different type can be concurrent. This situation is approached using Heusler alloys undergoing magnetostructural transformation located close to the Curie transitions of the martensitic and austenitic phases. The competing effect of both types of phase transitions in this case is investigated by experimental and analytical methods. The deconvolution method used for the composites is successfully applied here, enabling the subtraction of the Curie transition to the total response. It has to be noted that these situations of overlapping phase transitions can also occur unintentionally (i.e., due to the presence of impurities during synthesis). Therefore, the developed methods can also be used in these situations to know the effect of additional phase transitions and to gauge the actual response of the desired phase. Additionally, it has also been shown how the overlapping Curie transitions of the phases affect the hysteretic signature of the alloys undergoing magnetostructural transformation. This study is addressed through the emerging Temperature first-order reversal curves (TFORC) method for the study of magnetocaloric materials, using again Heusler alloys as a model case. This part combines experimental results with results from the modelling of the thermomagnetic behavior of the material. This has enabled a direct correlation between the characteristics of the thermomagnetic behavior of the alloys and the features of the TFORC distributions. Finally, the emerging topic of using polymer-based composites for the 3D printing of functional parts is addressed. Here, the main existing problem lies in the fabrication of uniform composite materials that allow the 3D printing of parts with predictable and repeatable functionality without relying on industrial techniques. A novel manufacturing method of polymer-based filaments containing functional fillers is proposed. Soft-magnetic steel particles have been used to manufacture polymer-based magnetic composites as proof of concept of the validity of the method. Once validated, the method is used to manufacture polymer-based magnetocaloric composites for additive manufacturing. The relatively simple method provides highly homogeneous filaments that preserve the magnetic functionality of the fillers. However, the property of the polymeric matrix is significantly affected by the addition of the metallic fillers, which has an important influence on the processability parameters both for the extrusion and printing.