Magnetic refrigeration materials are often rare-earth metals or their compounds, and they are one of the key technologies for magnetic refrigeration. Researching efficient magnetic refrigeration materials is a top priority for developing magnetic refrigeration technology.
Principle of Magnetic Refrigeration Technology
Without an external magnetic field, the magnetic moments of atoms or ions within magnetic materials are disordered, resulting in a larger magnetic entropy change. In an isothermal environment, applying a magnetic field to the magnetic material produces magnetization, causing the internal magnetic moments of atoms or ions to align in an orderly fashion along the direction of the applied magnetic field, thus increasing the order and decreasing the entropy of the magnetic material while releasing heat.
If the external magnetic field is removed, the thermal motion of the internal atoms or ions causes the magnetic moments to revert to a disordered state, increasing magnetic entropy. At this moment, under isothermal conditions, the magnetic material absorbs heat from the external environment, thereby achieving the cooling effect.
Role of Magnetic Refrigeration Materials
Magnetic refrigeration materials are substances with magnetocaloric effects used in refrigeration systems. Magnetic refrigeration relies on the magnetocaloric effect of magnetic materials to achieve cooling results.
The magnetocaloric effect refers to the phenomenon where the temperature of a magnetic material changes with variations in the magnetic field strength. When the strength of the magnetic field changes, the material undergoes heat absorption and release processes. This temperature change can be transferred to the surrounding air, thereby achieving the cooling effect.
Common Magnetic Refrigeration Materials
The research on room-temperature magnetic refrigeration materials primarily focuses on Gd metal compounds, Mn-based metal compounds, La metal compounds, and Heusler alloys.
| Gd Metal Compounds: Gd metals and their alloy compounds have always been a research hotspot of magnetic refrigeration materials. The Curie temperature of metallic Gd is 293 K, and at this Curie temperature, the maximum magnetic entropy change ΔSmax during a magnetic field change of 0-5T is 9.5 J/(kg·K), with a maximum adiabatic temperature change ΔTad of 12 K. Gd is often used as a reference material for studying other magnetic refrigeration materials.
| La-Based Compounds: Compared to other rare earth elements, La metal is inexpensive and exhibits a noticeable magnetocaloric effect. Among the alloys and compounds with first-order phase transitions, La(Fe, Si)13-based compounds are considered the most suitable materials for room-temperature magnetic refrigeration technology. The primary feature of La(Fe, Si)13 series compounds is that their first-order phase transition belongs to a ferrimagnetic transition process, and the phase transition is accompanied by isotropic volume changes. Due to the presence of volume changes, these alloys are prone to fracture during phase transitions.
Another key challenge for this series of compounds is the stability of La(Fe, Si)13-based compounds. During the preparation of the alloy, prolonged heat treatment can easily lead to the formation of the α-Fe phase, which can destabilize the compound phase.
La(Fe, Si)13-based compounds have low raw material costs and exhibit significant magnetocaloric effects. Under a 2 T magnetic field change, the magnetic entropy change of the LaFe11.6Si1.4 alloy can reach 13 J/(kg·K), with negligible thermal hysteresis. However, the low Curie temperature TC (around 200 K) does not meet the requirements for room temperature magnetic refrigeration. Doping with Co can enhance the performance of La(Fe, Si)13-based compounds in aspects such as structure and magnetocaloric properties, as demonstrated in the compounds LaFe11.4-xCoxSi1.6 and LaFe11.8-xCoxSi1.2, but heat treatment should be minimized since prolonged heat treatment can lead to excessive α-Fe formation.
By substituting a small amount of Pr for La in the LaFe11.44Si1.56 compound, it has been found that the thermal hysteresis ΔThys increases, the Curie temperature TC decreases, but the magnetic entropy change ΔSM increases. This series of compounds should aim to reduce thermal hysteresis while keeping the Curie temperature near room temperature.
| Mn-Based Compounds: The research on Mn-based metal compounds primarily focuses on substituting As in the compounds and adding filler atoms. As x increases, the Curie temperature TC of MnFeP1-xSix materials gradually rises, and the thermal hysteresis ΔThys gradually decreases. Under a magnetic field of 0-2T, the maximum magnetic entropy change of the compounds ranges from 7 to 13 J/(kg·K), which is small compared to Gd metals, but their cooling capacity (RCP) is 1.5 to 2.0 times larger.
| Heusler Alloys: Heusler alloys mainly consist of half-Heusler alloys with a 1:1:1 ratio and full Heusler alloys with a 2:1:1 ratio of intermetallic compounds. Heusler alloys possess characteristics such as giant magnetocaloric effects and shape memory effects, making them a research hotspot in the field of magnetic refrigeration, particularly for NiMnZ (Z=In, Sn, Sb) and NiMnGa alloys. When the alloy Ni54.8Mn20.3Ga24.9 experiences a magnetic field strength of 1.2 T at a temperature of 332 K, ΔSmax can reach 7.0 J/(kg·K). When Co is incorporated into the alloy, ΔS improves, and the maximum magnetic entropy value reaches 17.8 J/(kg·K) in the Ni41Co9Mn32Ga18 alloy.
Challenges of Magnetic Refrigeration Materials
Under the current magnetic field processes and manufacturing conditions, the commercialization of magnetic refrigeration materials still faces many practical issues.
| Although Gd compounds exhibit a large magnetocaloric effect and achieve ideal magnetic entropy changes, their low Curie temperature is a limitation. Gd alloys can control the Curie temperature to be near room temperature, making them suitable for room temperature magnetic refrigeration. However, the high cost of Gd is a disadvantage for practical applications.
| In Mn-based compounds, MnAs has a significant magnetocaloric effect, but the pronounced thermal hysteresis phenomenon severely impacts its practical value. Furthermore, As contradicts the concept of safe and environmentally friendly energy.
| La-based compounds have been the most researched room-temperature magnetic refrigeration materials in recent years, and compared to Gd, La is relatively inexpensive. La(Fe, Si)13 alloy has a significant magnetocaloric effect, but it operates within a narrow temperature range, exhibits notable thermal hysteresis, and has poor chemical stability.
| Heusler alloys are low-cost raw materials that show considerable magnetocaloric effects, but they also feature lower adiabatic temperature changes, and the Mn element in the alloy is prone to volatilization.
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