Magnetic refrigeration

Mn 3-x Fe x Sn system (0.00 x 3.00 with Dx = 0.25) alloys present the Curie temperature (T C) or transition temperature (T T) tuneable by the Fe content. A piece-wise linear profile for T C,T as a function of x is observed in a two wide temperature ranges, between 155 K up to 759 K and 259 K up to 155 K. Their equations are T C,T = (59 ± 15) + (240 ± 7)Áx and T C,T = (257 ± 1) À (206 ± 4)Áx, respectively. The alloys are low cost and easy manufacturing, rare earth free, with second order magnetic transition (SOMT), and have good magnetic properties. These features suggest an immediate application of the material in cascade thermomagnetic motors that operate with a large temperature range between hot and cold sources. Furthermore, SOMT Mn-Fe-Sn system materials are also reported with advantages that could make alloys of the Mn 3-x Fe x Sn system, (0.88 x 1.20), promising candidate for magnetic refrigeration. The typical ferromagnetic behaviour is achieved only by samples with x ! 1. The samples with x between 0.00 and 0.75 do not show the saturation magnetization even using fields up to 13 T. Thermomagnetic motors [1–3] convert thermal energy into mechanical energy using the thermomagnetic effect [1] in which ferromagnetic materials change their magnetic property in a temperature interval near the Curie temperature (T C). If the magnetic material is in a region with a magnetic field gradient, when in the ferromagnetic state, it suffers a strong force which can move it and do work. When the material is in the paramagnetic state, the force is small, and no movement occurs. Some works were done to simulate the thermomagnetic motors performance [2– 8]. Ferreira et al. [9] showed that a thermomagnetic motor powered by solar energy is a promising clean renewable energy alternative. According to theoretical and numerical studies about the efficiencies of magnetic power conversion plants operating with porous wheels [10], the thermodynamic efficiency is correlated with the magnetic field variation and the temperature range between the hot and cold sources. This range presents a limited value and is centred on the Curie temperature. Therefore, to work with a thermomagnetic motor with large temperature difference between hot and cold sources, a cascade arrangement of thermo-magnetic motors was proposed [1,10] using materials with distinct T C 's. In addition, for each thermomagnetic motor the temperature span among hot and cold sources is limited around the Curie temperature of each applied specific material. In this context, finding a material with tuneable T C is important. Magne-tization study done by Kouvel [11] for the Mn 2.5 Fe 0.5 Sn and Mn 2.0 Fe 1.0 Sn compounds exhibited interesting magnetic properties led us to consider the Mn 3-x Fe x Sn alloys as candidates for application in thermomagnetic motors and in magnetic refrigerators. Kouvel [11] described the Mn 3-x Fe x Sn alloys as pseudo-binary compounds between antiferromagnetic Mn 3 Sn [12–29] and ferromagnetic Fe 3 Sn [30–32] compounds. We studied the effects of the substitution of Mn by Fe, from x = 0.00 up to x = 3.00, Dx = 0.25 (Fig. 1) and we chose to prepare the Mn-Fe-Sn alloys based upon some points: the similarities between iron and manganese and their mutual solubility [33]; Fe is cheaper and more abundant than Mn [34]; ferromagnetic materials and their alloys at temperatures near T C were studied as components of thermomagnetic energy harvesting [35], and the resulting alloys are of easy manufacturing, low cost and rare earth free. This paper points out a material with tuneable transition temperature in a large temperature range and the main magnetic results for the Mn 3-x Fe x Sn compounds. Mn 3-x Fe x Sn (0.00 x 3.00, Dx = 0.25) polycrystalline samples were prepared from high purity elements (>99.99%). Each composition was melted in an arc-furnace under high purity Ar atmosphere. The molten buttons were turned and remelted at least three times to ensure homogeneity. Then, samples were wrapped in Ta foil and encapsulated in sealed quartz tubes in high purity Ar atmosphere and were heated-treated at 800 °C for 10 days