Functional Nanomaterials Useful for Magnetic Refrigeration Systems Open Access
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Magnetic refrigeration is an emerging energy efficient and environmentally friendly refrigeration technology. The principle of magnetic refrigeration is based on the effect of varying a magnetic field on the temperature change of a magnetocaloric material (refrigerant). By applying a magnetic field, the magnetic moments of a magnetic material tend to align parallel to it, and the thermal energy released in this process heats the material. Reversibly, the magnetic moments become randomly oriented when the magnetic field is removed, and the material cools down. The heating and the cooling of a refrigerant in response to a changing magnetic field is similar to the heating and the cooling of a gaseous medium in response to an adiabatic compression and expansion in a conventional refrigeration system.One requirement to make a practical magnetic refrigerator is to have a large temperature change per unit of applied magnetic field, with sufficiently wide operating temperature. So far, no commercially viable magnetic refrigerator has been built primarily due to the low temperature change of bulk refrigerants, the added burden of hysteresis, and the system’s low cooling capacity.The purpose of this dissertation is to explore magnetic refrigeration system. First, the Active Magnetic Regenerator (AMR) system built by Shir et al at the GWU’s Institute for Magnetics Research (IMR) is optimized by tuning the heat transfer medium parameters and system’s operating conditions. Next, by reviewing literature and works done so far on refrigerants, a number of materials that may be suitable to be used in magnetic refrigeration technology were identified. Theoretical work by Bennett et al showed an enhancement in magnetocaloric effect of magnetic nanoparticles. Research was performed on functional magnetic nanoparticles and their use in magnetic refrigeration technology. Different aspects such as the size, shape, chemical composition, structure and interaction of the nanoparticle with the surrounding matrix and neighboring particles all have a profound effect on the magnetic behavior of a material. To carry out this research some nanoparticles, namely yttrium-iron and a Ni-Mn-In Heusler alloy, in the range of 10 to 200 nm were synthesized and characterized in order to determine the correlation between the size, shape, and the morphology of nanoparticles on their magnetic properties such as magnetization, magnetocaloric effect, Curie temperature, etc. Results showed a significant improvement in the AMR cooling performance when the heat transfer fluid parameters and system’s operating conditions are optimized. In addition, the magnetization results of the yttrium-iron nanoparticles revealed more than a six-fold increase in the amount of magnetization at room temperature when their size reduced from 42 to 21 nm. For Heusler alloy sample the magnetization improvement at room temperature was more than 5-folds when the size of the nanoparticles reduced from 200 to 30 nm. Hence, a larger magnetocaloric effect can be expected by decreasing the nanoparticles’ size. Furthermore, results presented a drop in the coercivity of the nanoparticles as their size reduced, therefore a reduction in the hysteresis. Nanoparticles, as compared to their bulk counterpart, have a larger magnetocaloric effect with less hysteresis.
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