Abstracts Engineering

On the magnetocaloric properties of Heusler compounds: Reversible,time- and size-dependent effects of the martensitic phase transition

by Tino Gottschall

Large magnetocaloric effects can be obtained in the Heusler alloy systems Ni-Mn-In and Ni-Mn-In-Co during the magnetostructural phase transformation between the low temperature paramagnetic martensite and the high temperature ferromagnetic austenite phase. The martensitic transition takes place by a nucleation and growth process and can be tuned in a wide temperature window by varying the chemical composition. It is furthermore sensitive to a magnetic field but also to hydrostatic pressure. The phase transformation can therefore be induced by those external stimuli, which is investigated in this thesis by means of a phenomenological model. The martensitic transition is related to a significant thermal hysteresis, which limits the reversible adiabatic temperature and isothermal entropy change of the material. However, the magnetocaloric effect under cycling can be enhanced when the material remains all the time in a mixed-phase state, in so-called minor loops of hysteresis. On the contrary, in very high magnetic-field rates as well as in micrometer-sized single particles, the thermal hysteresis increases significantly, which needs to be considered in terms of application. In order to understand the contrasting behavior of small fragments in comparison to their bulk representatives, a finite element model is introduced, from which the importance of mechanical stress during the first-order transition becomes apparent. Furthermore, an attempt is made to improve the sustainability of magnetocaloric Heusler alloys by the substitution of critical elements to move this interesting material class further towards application. Advisors/Committee Members: Gutfleisch, Oliver (advisor), Wende, Heiko (advisor).