Designing Multicaloric Materials with Martensitic Phase Transitions for Future Cooling Applications

Abstract

The demand for cooling devices and the corresponding energy costs are constantly expanding, driven by the growth of global population and the economies of fast-developing countries in warm climates. Novel caloric cooling solutions are an alternative that do not rely on environmentally harmful refrigerants and can provide a better energy-efficiency compared to the conventional vapor-compression technology. Especially magnetocaloric cooling is in focus of research activities including material research and well-performing device development. In order to optimize the material, many requirements need to be taken into account and each material system behaves differently under the application of an external magnetic field. This work focuses on the development of the MM'X material family with a conventional magnetocaloric effect and the various systems of Heusler alloys showing the inverse magnetocaloric effect. Both systems have in common that they experience a martensitic transition between a high-magnetization and a low-magnetization state. The MM'X base system of MnNiGe can be tuned by the isostructural alloying method, which is investigated in detail in this work. Therefore, Mn is substituted partially by Fe as well as Ge by Si. This enhances the magnetization change of the magnetostructural phase transition, reduces the amount of expensive Ge and allows for tailoring the transition temperature. The resulting alloys show very large isothermal entropy changes for small pieces of material. A difficulty that arises for this system is the mechanical integrity together with the low magnetic-field dependence of the transition temperature. The very good sensitivity of the transition towards hydrostatic pressure reveals barocaloric purposes as a very attractive field of application for these materials. A direct comparison with the versatile family of Ni-Mn-based Heusler alloys underlines their high potential for magneto- and multicaloric applications. With stoichiometric changes, the phase transition can be adjusted and also the magnetic-field dependence of the transition temperature is found to scale directly with the difference of the transition temperature to the austenite Curie temperature. The most promising system for low magnetic field changes is Ni(-Co)-Mn-In. Even though Ni(-Co)-Mn-Sn shows similar isothermal entropy changes, adiabatic temperature changes cannot compete. The drawback of a significant thermal hysteresis of around 10 - 15 K, which hinders a good cyclic performance of Heusler alloys, can be turned into an advantage by considering the novel approach of a multi-stimuli cycle, which exploits the thermal hysteresis to lock the material completely in its transformed state after a magnetic-field application. The necessary reverse transformation can be carried out by the application of pressure/stress as a second stimulus requiring a good pressure/stress-sensitivity of the transition temperature. Among the Heusler alloys, the novel all-d Heusler alloys of Ni-Co-Mn-Ti represent a promising material system for this approach. Their magnetocaloric performance is compared to the other Heusler alloys in small magnetic field changes of 2 T as well as for higher and faster field changes since the multi-stimuli approach allows for concentrated magnetic fields. Detailed investigations on the microstructure give insights that are crucial in order to understand the transition behavior. Analyzing the temperature-, magnetic field-, and pressure-induced phase transitions allows for assessing the potential of using the different Heusler systems for magnetocaloric and/or multicaloric cooling applications. This thesis puts the general properties of different material systems in a broad context and aims at providing principal design rules for the studied systems in order to develop and tailor well-performing caloric materials.