Design and development of Ni-based heusler alloys for magnetic refrigeration

Toraman, Sedanur
Magnetic refrigeration has attracted increasing interest in the materials research communities because of its higher cooling efficiency and environmentally friendliness. In this thesis study, it is aimed to develop Ni-based Heusler alloys for use in magnetic refrigeration systems, which consists of two parts; the theoretical and experimental part. In the theoretical part of this thesis, in order to characterize the order-order (L21-B2) and order-disorder (B2-A2) phase transitions in A2BC type full Heusleralloys, statisco-thermodynamical theory of ordering by means of Bragg-Williams-Gorsky (BWG) method combined with electronic theory in the pseudopotential approximation were employed. The effect on ternary alloy element addition on ordering characteristics in Ni-Mn-C (C=Ga, In, Sb, Sn) Heusler alloys were studied and the L21-B2 and B2-A2 critical transformation temperatures were determined by calculating the partial ordering energies using the electronic theory of alloys in pseudopotential approximation. The results of these calculations were utilized to predict the most suitable potential alloying element (C) and its composition for the development of Ni-Mn-C magnetocaloric materials. In the experimental part of this thesis, by using the results obtained from the theoretical predictions, Ni-Mn-In alloy system was chosen and structural and magnetic analyses of Ni51Mn34In15 alloy were performed. Within this context, the effect of heat treatment processes on structural and magnetic properties of Ni-rich Ni51Mn34In15 Heusler alloy have been analysed by means of XRD, SEM, EDS and VSM techniques. While L21-type ordered crystal structure could not be detected in the as-cast alloy, however, after applying a proper heat treatment processes, formation of stable L21-type ordered structure in Ni51Mn34In15 alloy was achieved, which is most desirable structure for magnetocaloric applications. To determine the magnetocaloric effect (MCE), the magnetic entropy changes ( SM) of the samples were calculated from the magnetic field dependent magnetization measurements. It was shown that the maximum of SM reaches the magnitudes of 4.8 J=kg/K, 5.6 J=kg/K and 12.8 J=kg/K at 271 K, 294 K and 305 K temperatures at magnetic field change of H=18kOe for the as-cast, 24 hours-aged and 48 hours-aged Ni51Mn34In15 alloy, respectively. Consequently, large magnetic entropy changes with positive sign were observed in wide temperature ranges and these positive SM values indicate that this alloy exhibits inverse MCE around the martensitic transformation temperature (TM). In addition to that, the relative cooling power (RCP) of the magnetocaloric material was calculated according to the magnetic entropy change. Results of the calculations reveal that application of heat treatment processes tends to increase magnitude of RCP parameter of Ni51Mn34In15 Heusler alloy.