|Title:||Magneto-caloric effect of fe-based metallic glasses at room temperature|
|Advisors:||Chan, K. C. (ISE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xv, 183 pages : color illustrations|
|Abstract:||The magnetocaloric effect (MCE) is the temperature change of magnetic materials corresponding to an external magnetic field change. Specifically, magnetic materials will heat up when they are magnetized and cool down when they are demagnetized. Magnetic refrigeration, based on the MCE, is a new technique which is more energy-saving and environmentally friendly than the conventional cooling method and has already been used in low temperature applications. Over the last decade, many researchers have further explored replacing the conventional gas expansion/compression cooling technique for room temperature applications by magnetic refrigeration. For working refrigerants, the magnetocaloric effect of magnetic materials is critical for the performance of a refrigerator. It has been reported that the refrigeration capacity (RC) of amorphous materials is generally larger than that of crystals. Among various amorphous MCE materials, Gd-based materials have attracted much research interest due to their large magnetocaloric effect. However, their Curie temperature is still too low to be applied for room temperature magnetic refrigeration. On the other hand, the MCE of Fe-based amorphous materials is generally smaller than that of Gd-based materials. The advantages of the Fe-based amorphous materials are easily tunable Curie temperatures and low cost. By adjusting the composition or alloying with some minor additions, the MCE of the Fe-based metallic glasses is able to be enhanced to a great extent. In this project, the prime aim is to develop the Fe-based metallic glasses with an enhanced magnetocaloric effect. To fulfill this objective, new compositions were tried on the basis of results in the literature. In order to ascertain the amorphous structure of the new materials, adjustment of the composition was carried out. With more Fe content, the magnetic properties are supposed to be better, while the glass forming ability (GFA) is worse. As a result, metallic ribbons were chosen instead of bulk rods in order to lower the critical cooling rate. FeZrB series amorphous materials were chosen as the master alloys in my study due to their excellent MCE among the Fe-based metallic glasses, after the MCE of FeZrB ternary amorphous materials were thoroughly reviewed in the literature. To enhance the MCE of the chosen master alloys, the effect of minor additions of different elements was thoroughly studied, for example Cr, Co, Cu, Sm, Mn and Er. With the addition of these elements, both the Curie temperatures and the peak magnetic entropy changes were affected. The results provided a feasible method to enhance the MCE and maintain the Curie temperatures around room temperature. To verify the experimental results as well as predict the MCE of new magnetic materials, Monte Carlo simulations were used in this project. The simulation was based on the importance sampling method. Since little work was done on the simulation of the MCE of amorphous materials, the simulation work in this project started from the binary Gd-based amorphous materials in order to simplify the codes and calculations. Then the simulation was used to calculate the MCE of Fe-based amorphous materials. For materials consisting of all magnetic elements, the results of the Curie temperatures obtained from the simulation fitted well with data from experiments, and the magnetic entropy changes were predicted. For materials consisting of magnetic and non-magnetic materials, the simulation still needs to be improved. The findings of the present study not only provide a better understanding of the MCE of the amorphous materials, but also form a strong foundation for further improvement of the simulation work.|
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