LUE - Nouveaux alliages à haute entropie pour des applications magnétocaloriques/New High Entropy Alloys for Magnetocaloric Applications

Keywords

High Entropy Alloys , Magnetocaloric materials, metallurgy, magnetism, materials for energy

Subject details

The magnetocaloric effect (MCE) describes the reversible temperature change of a magnetic substance in response to the application or removal of a magnetic field. It is an intrinsic material's property. The magnetocaloric effect is maximum around the magnetic ordering temperature, and can be further enhanced if a first order structural transition is concomitant with the magnetic transition. It is linked to a strong variation of the magnetic entropy, which, due to the magnetothermal coupling, induces a variation in the temperature of the material. In some cases, this effect is giant and can be exploited in solid-state magnetic refrigeration systems around room temperature. In this case, the thermodynamic cycle of compression/expansion of the refrigerant gas used in conventional systems is replaced by a thermomagnetic cycle of magnetization/demagnetization of a material with a magnetocaloric effect that plays the role of refrigerant. In recent years, magnetic refrigeration has attracted growing interest because it represents a more efficient and less polluting alternative to traditional cold production technologies. In particular, the solid-state magnetic refrigeration alternative does not use harmful gases and is a low noise solution that can also be made very compact. Magnetocaloric research is extremely timely in the context of global warming, considering that 20% of total worldwide energy consumption involves the use of refrigeration and air conditioning, and that the global energy demand for air conditioning devices is expected to be multiplied by a factor of 3 in the next decade. Different classes of magnetocaloric materials are currently under scrutiny, each presenting advantages and disadvantages. Many different considerations have to be taken into account for an optimal magnetocaloric material, both of intrinsic and extrinsic origins. The ideal material should exhibit high isothermal entropy change and large adiabatic temperature change obtainable by permanent magnets (ideally less than 2T), tunable operating T and broad operating T range, low hysteresis, highly reversible effect, excellent mechanical stability, excellent heat exchanger capacity with fast dynamics, be stable on the long run (good resistance to corrosion), rely on readily available, non-toxic, non-strategic chemical elements, and be low cost material for large scale production. In this context, a new class of materials is currently being considered. These are high entropy alloys (HEA), a new concept of alloy design in which, unlike common alloys, there is no principal element. These materials can exhibit not only excellent mechanical properties but also remarkable physical and chemical functionalities as demonstrated by recent reports. In particular, HEAs composed of rare-earth or ferromagnetic transition metal elements could have great advantages as candidates for magnetocaloric applications. The objective of the thesis will focus on the development of new HEA materials for magnetocaloric refrigeration. The foreseen advantages of HEAs for magnetocaloric applications are that they may possess enhanced MCE because of extreme disorder that could result in sluggish magnetic transitions, combined with improved mechanical properties. The strategy adopted during the thesis to search for new HEA compositions will be based on isostructural chemical substitutions of elements starting from ternary intermetallic compounds presenting the desired magnetostructural transitions in order to bring them into the composition domain of high entropy alloys. Other factors known to favor the formation of solid solutions, such as the configurational entropy, the pair enthalpies of mixing between the different elements, the size differences in atomic radii, the valence electron concentration, etc…, will also be taken into account.