Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications

Abstract

Magnetocaloric materials (MCM) are the 'heart' of every magnetic refrigeration or heat-pump application. Apart from having a crucial role in the heat-regeneration process, they also exhibit a special and vital phenomenon for magnetic refrigeration called the magnetocaloric effect. As mentioned in the previous chapter on the Thermodynamics of magnetocaloric energy conversion and as described later in the book in the Chap. 7 (Overview of existing magnetocaloric prototype devices) the first real magnetic devices working near room temperature were not built until the middle of the 1970s. However, the discovery of the magnetocaloric effect (MCE) in ferromagnetic materials dates back more than 90 years to 1917.The MCE was discovered by the French and Swiss physicists Weiss and Piccard [1,2]. This is an important historical fact that needs to be emphasized, since in the past 15 years a misconception has arisen in the magnetocaloric research community, wrongly attributing the discovery of the MCE to the work of Warburg [3]. At this point we have to address the recent paper of Smith [4], who did a thorough and interesting review of the research on thermodynamics that led to the discovery of the MCE. The next few lines are a brief summary of the historical events according to Smith's findings. He reviewed the original works of scientists that date back to the nineteenth century, starting with Joule in 1843 [5]. Joule observed that heat was evolved from iron samples when they were subjected to a magnetic field. Later, in 1860, Thomson (Lord Kelvin) [6] was already aware of the fact that ferromagnetic materials lose their magnetic properties when heated above a certain temperature (now known as the Curie temperature). Thomson correctly predicted that ferromagnetic materials would experience a heating effect when magnetized and a cooling effect when demagnetized, and that these effects would be the largest around the temperature where they lose their magnetization. However, he did not associate these predictions with the magnetocaloric effect. Then, in 1881, Warburg published a paper [3], which is nowadays wrongly cited when referencing the discovery of the MCE. Nevertheless, the work of Warburg was of great significance, since he was the first to explain magnetic hysteresis. He correctly predicted that the magnetization of a material is larger when the magnetic field is decreasing than when it is increasing. One year later, in 1882, Ewing [7] discovered the same phenomenon and was the first to name it "hysteresis". It was not until the works of