BEsT - Investigation of multicaloric materials by adiabatic temperature change and heat-capacity measurements

• Applicant: Dr. Tino Gottschall
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456263705

We live in a time in which mankind is dependent on artificially produced cold. On average, German households have around 1.8 refrigerators. There are currently more than 3 billion refrigerators and air conditioners in use worldwide, and this figure rises continuously. Already today, around 17 % of the world's electrical energy is consumed by refrigeration systems, and the trend is rising. New, more efficient technologies, therefore, have the potential to save enormous amounts of energy.Magnetic materials may be of help here. When exposed to a magnetic field, they change their temperature. This so-called magnetocaloric effect can be used to construct alternative, environmentally friendly cooling systems. Heusler alloys based on nickel, manganese, indium, and cobalt have enormous cooling effects at room temperature and in moderate magnetic fields. The reason for the strong cooling is the transformation of the non-magnetic martensite phase into magnetic austenite. However, this effect can only be produced once, since these materials exhibit a large thermal hysteresis. Recently, it has been shown that this hysteresis can be exploited under the influence of two different stimuli in a so-called multicaloric cooling process. Since the crystal structure changes during the conversion as well as the magnetization, the reverse transformation can be induced by applying mechanical pressure. The combination of a magnetic field and uniaxial load thus enables enormous savings in expensive Nd-Fe-B permanent magnets, since the material only needs to be magnetized shortly. The experimental proof of this multicaloric cooling process was obtained on the laboratory scale in collaboration with scientists from the University of Barcelona.The BEsT project aims to gain a profound understanding of the coupling mechanisms of the different ferroic orders in multicaloric materials in order to enable their further development. However, the necessary characterization is extensive and challenging, since the different external fields have to be applied both individually and simultaneously. Due to the high complexity of these measurements, only a few experimental setups exist so far, and they are still in the test phase. Based on the long experience and the preliminary work of the applicant in the field of calorimetry in static and pulsed magnetic fields, existing measurement setups will be further developed or newly constructed within the project to enable multicaloric investigations in magnetic and electric fields as well as under uniaxial load on both bulk samples and thin films. With these new experimental tools, we will be able to better understand multicaloric materials, determine their coupling constants and thus further advance the development of the solid-state cooling process by exploiting the thermal hysteresis.

• DFG Programme: Research Grants