Enhanced Magnetic Cooling through Optimising Local Interactions

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

Refrigeration is central to modern society by making hot climates habitable, preserving food, and facilitating medical scanners and quantum computing. Over a tenth of Britain's electricity is estimated to go to cooling, at a cost of over £5 billion a year, with nearly 12 million (m) people globally employed in refrigeration related industries. Cryogenic refrigeration, which provides temperatures close to absolute zero, is becoming a major industry; the research council STFC have predicted that contributions to the UK economy from cryogenic refrigeration will increase from £324m to £3300m in the decade to 2025. Solid state cooling based on caloric materials promises higher energy efficiencies than current technologies based on fluid refrigerants and do not suffer from inevitable escape of their active components as gases, such as scarce liquid helium used for cryogenic applications. Caloric refrigerants rely on a change in entropy, a measure of the universe's tendency to disorder, in response to external stimuli such as applied magnetic and electric fields or pressure. Practical caloric cooling requires new materials that exhibit the maximum change in their entropy for readily achievable external stimuli.

In magnetocalorics, the cooling process is driven by applied magnetic fields. This has advantages, including high cyclability compared to other calorics as materials do not tend to deteriorate when exposed to magnetic fields. Magnetocalorics have been known for over a century, but their use has mostly been restricted to refrigeration at ultra-low temperatures, measured in milli-kelvins, and with very large magnetic fields, limiting their application. Developing magnetocalorics that work at higher temperatures and under lower magnetic fields will enable magnetisation based cryogenic cooling to be more used much more widely.

We can systematically optimise new magnetocalorics for desired cryogenic temperatures by tuning interactions in these materials. This aches large entropy changes for small magnetic field changes at key operating temperatures. While this approach is well known for magnetocalorics for near room temperature cooling its importance when optimising magnetocalorics for cryogenic cooling has only been shown recently. Enhanced cryogenic magnetocalorics will have greater efficiency than alternative cooling technologies enabling us to greatly decrease dependence on liquid helium, which is increasingly expensive and prone to supply disruptions. Realising the best magnetocalorics requires an interdisciplinary approach that combines chemistry and physics; this is well matched by our team's expertise in materials synthesis, structural and physical properties characterisation and prototype testing.

Magnetocalorics with strong interactions within chains of magnetic ions and weaker competing interactions between chains appear particularly suited to replace liquid helium. We will take advantage of this recent discovery by developing framework magnetocalorics, a new class of materials that enable enormous freedom to optimise properties due to their very flexible compositions and structures. We will systematically investigate how these magnetocalorics are best optimised for use above 4 K by tuning the extent of competing interactions between units with strong magnetic coupling and modifying the dimensionality of the strongly coupled units. These magnetocalorics will be optimised further by changing the magnetic ions incorporated to directly tune their magnetic interactions. The best magnetocalorics to emerge from this screening process will then be assessed to determine their maximum cooling capacity and power. This key information will enable us to establish their utility in practical cooling devices, which will be demonstrated by incorporation into a prototype magnetocaloric refrigerator. Understanding gained from this project will enable development of precise design rules for tailored magnetocalorics.

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