Thermodynamics of magnetic systems and some applications

摘要

Until recently, the theory of continuous media dealt mainly with diamagnetic and paramagnetic fluid. The researchers did not consider strongly magnetizable fluids, probably because such media had not yet been discovered in natural conditions and did not exist until recently as manufactured products. However, in the early 1960s magnetic fluids (MF) could be synthetically prepared, and then technological interest in MF has grown rapidly. Besides, magnetic refrigeration has been used for over 60 years as a technology to achieve temperatures below 1 K, employing the magnetocaloric effect of some solid materials. However, in the last twenty years, the technology has been developing for refrigeration applications above 1 K up to including heat pumps above room temperature. The work has been multinational in scope and has focused on the analysis of magnetic thermodynamic systems, investigation of magnetic materials suitable for refrigerants, and development of prototype refrigerators. After a synthetic review of the basic properties of MF, and of magnetic refrigerators and heat pumps, the aim of this paper is a thermodynamic examination of these systems in comparison with the relations usually applied to fluids. Three independent variables are generally considered, whereas usually only two variables have been considered till now for what is known to the author. Furthermore, some applications of MF are taken into account; in particular, possibilities of converting thermal into mechanical energy are examined.