This blog post explores the principles of the magnetic heat effect and how phase transitions in magnetic materials led to the development of magnetic refrigerator technology operable at room temperature, illuminating the future of eco-friendly cooling technology.
Discovered in the late 19th century, the magnetic heat effect was confirmed to be applicable to magnetic cooling technology by the early 20th century. This breakthrough spurred the development of magnetic cooling technology into the advanced techniques used today to achieve cryogenic temperatures. Conventional refrigerators utilize a thermodynamic cycle where a gas refrigerant releases heat when compressed and absorbs heat when expanded, transferring heat from inside the refrigerator to the outside. However, gas refrigerants can solidify below a certain temperature, losing their function as a refrigerant, and pose environmental pollution risks if leaked. Recently, magnetic cooling technology has been identified as a potential candidate for developing new refrigerators that could replace conventional models. The magnetic material used in magnetic cooling determines the refrigerator’s operating temperature range based on its magnetic properties, making the development of suitable magnetic materials a critical challenge. Particularly, active research is underway to develop new magnetic materials aimed at realizing room-temperature magnetic refrigerators that can operate at ambient temperatures.
Magnetic materials refer to substances that become magnetized by an external magnetic field. The magnetization of a material is proportional to the strength of the applied external magnetic field and the number of magnetic dipoles per unit volume within the magnetic material. Here, a magnetic dipole refers to a tiny magnet present within the magnetic material. Magnetic materials are broadly classified into ferromagnets and ferrimagnets. Ferromagnets retain their magnetic properties even after the external magnetic field is removed, whereas ferrimagnets lose their magnetic properties once the external field is removed. Ferromagnets undergo a phase transition to ferrimagnetism at a specific temperature as the temperature rises, during which the entropy of the magnetic material increases.
The magnetic heat effect originates from the phenomenon where a magnetic material dissipates heat when subjected to an external magnetic field. Magnetic refrigerators combine this effect with a thermodynamic cycle to expel heat from the refrigerator’s interior to the outside. This cycle consists of two processes with no heat exchange and two processes where the external magnetic field is maintained constant. In these thermodynamic processes with no heat exchange, no entropy change occurs. The thermodynamic cycle of a magnetic refrigerator proceeds through four processes: I, II, III, and IV. In Process I, when a magnetic field is applied to a working substance at temperature T, which previously had randomly arranged magnetic dipoles, while blocking heat exchange with the outside, the dipoles within the working substance align in the direction of the magnetic field, generating heat and causing its temperature to rise. The stronger the magnetic field, the greater the heat generated in the working substance. In process II, while maintaining the external magnetic field, allowing heat exchange between the working substance and the external environment causes the working substance to release heat and its temperature to decrease. In process III, after blocking heat exchange between the working substance and the external environment again, removing the external magnetic field causes the dipole arrangement to become disordered, leading to a further decrease in the working substance’s temperature. In Process IV, when heat exchange between the working substance and the external environment is permitted again, the working substance absorbs heat, its temperature rises, and it returns to its initial temperature T, thus completing one cycle. During this cycle, when the working substance absorbs heat, it is in contact with the interior of the refrigerator; when it releases heat, it is in contact with the exterior. Repeating this process enables the working substance to act as a heat pump, transferring heat from the refrigerator interior to the exterior.
To create a highly efficient magnetic refrigerator, it is essential to use a magnetic material as the working substance that exhibits a large entropy change in response to variations in the external magnetic field at a specific temperature. The amount of heat escaping to the outside during one cycle in a magnetic refrigerator is closely linked to the entropy change before and after the external magnetic field is applied. Generally, entropy changes most dramatically at the critical temperature where a substance changes its magnetic state. Therefore, when the critical temperature at which the working material undergoes a phase transition is near the refrigerator’s operating temperature, the magnetic cooling effect of that material is maximized. Recently, various new materials with critical temperatures close to room temperature have been discovered, and based on these, active research is underway to develop room-temperature magnetic refrigerators. These research advances are further broadening the commercialization potential of magnetic cooling technology and paving the way for a significant breakthrough in next-generation eco-friendly cooling technology.
