Artificial cooling using conventional gas compression has been around in commercial household applications for about one hundred years. However, the technology has barely changed during all this time. Experts estimate that around one billion refrigerators based on this technology are in worldwide use today, in ever-growing numbers. “Cooling technology is now regarded as the largest power consumer in our own four walls. The potential for environmental pollution entailed by typical coolants is just as problematic”, Dr. Tino Gottschall, scientist at HZDR, describes the motivation of the research.
The “magnetocaloric effect” could become the heart of future cooling technologies: Certain elements and alloys suddenly change their temperature when exposed to a magnetic field. There is a whole series of such magnetocaloric substances already known from research. “But whether they are suitable for household and industrial applications on a large scale – this is a whole different question”, says Prof. Oliver Gutfleisch, Professor for Functional Materials Science at Technische Universität Darmstadt.
Substance database for cooling materials
The scientists were collecting data on substance properties to clarify these issues. However, they quickly ran into difficulties. “We were particularly surprised that only a few results from direct measurements can be found in the specialist literature”, reports Gottschall. "In most cases, these parameters were indirectly derived from the observed magnetization data. We found that neither the measurement conditions, such as the strength and the profile of the applied magnetic field, nor the measuring regimes, are comparable. Consequently, the results do not match“.
To dispel the inconsistencies in the previously published material parameters, the scientists came up with an elaborate measurement program, which covers the entire spectrum of the currently most promising magnetocaloric materials and their relevant material properties.
By coupling high-precision measurements with thermodynamic considerations, the scientists from Darmstadt and Dresden were able to generate consistent material data sets. They are now presenting a solid database that can facilitate the selection of suitable materials for various magnetic cooling applications.
Which materials can take on gadolinium?
The suitability of a material for magnetic cooling purposes is ultimately determined by various parameters. Only its proper combination opens competition with well-established cooling technologies. “The temperature change achieved at room temperature should be large, and as much heat as possible should be dissipated at the same time” – this is how Gottschall describes the most preferable properties of tomorrow's cooling materials.
To enter future mass applications, these substances must not possess harmful characteristics, both in terms of environment and health. “In addition, they should not consist of raw materials that are classified as critical due to supply risks and no easy replacement in technological applications,” explains Gutfleisch. “In the overall assessment of technological processes, this aspect is often neglected. A mere focus on physical properties is no longer sufficient today. In this respect, magnetic cooling is also a prime example of the fundamental challenges coming along with the current energy transition, which will not be possible without sustainable access to suitable materials”.
At ambient temperature, the prime magnetocaloric standard is still made of gadolinium. If the rare–earth element is brought into a magnetic field of 1 Tesla, the scientists measure a temperature change of almost 3 degrees centigrade. Keeping the economic viability of future magnetic cooling devices in mind, the generation of such field strengths will be the job of commercial permanent magnets.
Suitable materials: A look into the future
Despite its outstanding properties, the prospects of using gadolinium in household cooling devices are rather unrealistic. The element is one of those rare-earth metals that are classified as critical when it comes to a secure, long-term supply. Given an equal design, heat exchangers made of iron-rhodium alloys could dissipate even larger quantities of heat per cooling cycle. Nevertheless, due to their content of the platinum group metal rhodium, they are also among the raw materials singled out by the European Commission to have a high criticality.
However, the researchers found candidates readily available in the near future and, at the same time, with a promising performance: Intermetallic compounds consisting of the elements lanthanum, iron, manganese and silicon, for example, in which hydrogen is stored in the crystal lattice can even outperform gadolinium in terms of heat that could be pumped out of the refrigerator compartment.
Others could follow suit: Researchers at the HZDR and TU Darmstadt are working hard on expanding the range of magnetic cooling materials. In close cooperation, scientists of both institutions are preparing new series of experiments on the properties of magnetocaloric substances. On the Dresden High Magnetic Field Laboratory for example, they are set to study how these substances behave in pulsed high magnetic fields. The wider focus of future research lies on a given material´s cross-response to different stimuli like field, strain and temperature, as well as the construction of efficient demonstrators.
The work is funded by the European Research Council (ERC Advanced Grant) from the European Union's Research and Innovation Program “Horizon 2020” (Project No. 743116 – Cool Innov), by the Deutsche Forschungsgemeinschaft within the Priority Program 1599 (DFG, SPP 1599) and the Darmstadt Excellence Graduate School of Energy Science and Energy Engineering (Scholarship No. GSC 1070), through the 7th Framework Program of the European Community (Scholarship No. 310748 DRREAM) and through the Dresden High Magnetic Field Laboratory at the HZDR, a member of the European Magnetic Field Laboratory (EMFL).
T. Gottschall, K.P. Skokov, M. Fries, A. Taubel, I. Radulov, F. Scheibel, D. Benke, S. Riegg, O. Gutfleisch, „Making a cool choice: the materials library of magnetic refrigeration“ in Advanced Energy Materials, 2019.