Detecção acústica do efeito magnetocalórico : parâmetros que influenciam a ciclagem térmica

Abstract

In recent years there has been a growing concern with the environment, boosting the global scientific community to search more efficient technologies and seek alternatives to the main processes of modern society that are sources of environmental degradation. One such process is the current method of refrigeration. Among the possible alternatives, the magnetic refrigeration has emerged as the most promising. It is based on the so called magnetocaloric effect (MCE), that is, the adiabatic temperature variation or the isothermal entropy change experimented by a magnetic material when it is submitted to magnetic field variation. One of the methods of investigation and characterization of the magnetocaloric materials is the acoustic detection technique. In this approach, still little explored in the literature, the temperature variation experienced by the magnetocaloric sample under a modulated magnetic field is captured acoustically. In this thesis, a magnetoacoustic setup was built and applied to the study of thermal cycling in three magnetocaloric materials: the Gd, the alloy Gd5,09Ge2,03Si1,88 as cast and an unpublished compound annealed Gd4,7Nd0,3Si4,0. Bulk and powder samples were analyzed and it was verified a significant difference between the heating and cooling curves which acted even in second-order transitions. Two causes were discovered: the moisture present in the atmospheric air when it is used as transducer and a reorientation of the particles when the powder samples transits from ferromagnetic to paramagnetic or vice versa. If no precaution is taken, errors can exceed 50 %. Thus, here is presented a methodology to drying the magnetoacoustic system in order to completely eliminate the influence of moisture. Solving the problem of particles movement we propose to use a leaked rod, having on its bottom end a screen clipping of a fine mesh, pressed against the powder sample in order to restrict it. Furthermore, by means of the Gd4,7Nd0,3Si4,0 compound it was determined how to distinguish the conventional MCE of the inverse one through a 180 degrees signal phase inversion. A parallel study aiming to process the Gd5,09Ge2!03Si1 83 alloy in a manufacturing route similar to powder metallurgy also developed in this thesis. Six ranges of particle size were evaluated and through two completely different techniques were possible to establish a minimum particle size of 40 um where the MCE is not drastically affected. Moreover, contrasting the cooling capacity with the transition enthalpy of the powders we concluded that a maximization of MCE occurs in particle size between 45 and 63 µm.