Additive manufacturing of magnetocaloric materials for magnetic refrigeration

Sun, Kun (2023). Additive manufacturing of magnetocaloric materials for magnetic refrigeration. University of Birmingham. Ph.D.

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Abstract

Magnetic refrigeration is a promising environmentally friendly technology that may help address global warming and energy challenges worldwide. Compared with conventional cooling systems, magnetic refrigeration devices emit less mechanical noise and vibrations and theoretically achieve higher energy efficiency (60% of the Carnot cycle). This technique is based on the magnetocaloric effect (MCE), which is a change in the magnetic entropy (\(ΔS\)) upon applying/removing an external magnetic field, resulting in a temperature change (\(ΔT\)). The magnetocaloric materials (NiMnSn Heusler alloys and LaCe(Fe,Mn,Si)\(_{13}\) alloys) are very interesting due to their high MCE. However, the materials also have limited uses due to poor mechanical properties. Laser powder bed fusion (PBF-LB) technology can overcome the disadvantage of hard-to-machine magnetocaloric materials to produce parts with complex shapes and extremely porous magnetocaloric materials with predefined external shapes. This work explores and investigates the possibility of the PBF-LB technique in creating NiMnSn and LaCe(Fe,Mn,Si)\(_{13}\) samples. We also examined different post-treatment processes to improve PBF-LBed parts' MCE.

The PBF-LB technology can be used to build NiMnSn Heusler alloys with different shapes, particularly blocks, microchannel (MC) cylinders, lattices, and LaCe(Fe,Mn,Si)\(_{13}\) samples. The PBF-LB processing parameters were optimised for these samples' minimum defects, cracks, and maximum densification. It has demonstrated that PBF-LB technology can create a fully dense NiMnSn block and LaCe(Fe,Mn,Si)\(_{13}\) samples with a competitive MCE, although it is challenging to make cracks disappear. The low laser powers (60, 70, and 80 W) are more suitable for manufacturing NiMnSn Heusler alloys with MC cylinders and lattices. The appropriate energy density with low laser power, low scanning speed, and low hatch is good for building LaCe(Fe,Mn,Si)\(_{13}\) block samples with the lowest porosity fraction and crack formation. The behaviour and mechanism of different cracks and defects within PBF-LBed NiMnSn alloys and LaCe(Fe,Mn,Si)\(_{13}\) alloys were revealed.
The chemical composition of PBF-LBed in-situ NiMnSn alloys is more uniform when the heat treatment time increases. Although the long heat treatment process did not significantly affect the MCE of NiMnSn alloys around the curie temperature. The different PBF-LB process parameters did not significantly affect the \(ΔS_m\) around \(T^C_A\). The \(ΔS_m\) for the three PBF-LB samples around \(T^C_A\) is approximately 1.0 Jkg\(^{-1}\)K\(^{-1}\) when applied to the 1 T field. Two different heat treatment processes were used to improve the MCE and change the transition temperature of PBF-LBed LaCe(Fe,Mn,Si)\(_{13}\). The heat treatment processes in an argon atmosphere enhance the form of NaZn13-type LaCe(Fe,Mn,Si)\(_{13}\) phases and improve the MCE. The heat treatment in a hydrogen atmosphere incorporates hydrogen atoms into NaZn\(_{13}\)-type LaCe(Fe,Mn,Si)\(_{13}\) interstitially. This treatment improves the MCE and increases the curie temperature. Moreover, the heat treatment in a hydrogen atmosphere can decrease oxides volume, improving the magnetic properties. At T\(_c\) (298 K), the \(ΔS_m\) of HTH LaCe(Fe,Mn,Si)\(_{13}\) sample was -3.68 Jkg\(^{-1}\)K\(^{-1}\) when applied to 1 T field. Compared with the pure Gd and LaFeSi-based alloys reported in other literature, the \(∆S\) of the second-order phase transition (SOPT) in this work was promising T\(_c\) (room temperature, 298 K) found in this study makes the material an ideal choice for realising room-temperature magnetic refrigeration.

In this work, the PBF-LB technology is a promising manufacturing route for producing magnetic refrigerant. Furthermore, the PBF-LBed magnetocaloric materials with a large surface area, such as MC cylinders and lattices, can increase the efficiency of heat exchange and magnetic refrigeration.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):