Materials discovery holds the key to efficient refrigeration

The world needs more efficient refrigeration. Could solid state refrigeration be the answer? Researchers in the Department of Materials Science and Engineering are developing new materials that could help this to become a reality.

Conventional refrigeration methods are very inefficient.
Conventional refrigeration methods are very inefficient. With approximately 10% of all electricity generated used to power refrigeration equipment, they have a significant environmental impact.

Fridges and freezers can be found in practically every domestic kitchen, but refrigeration technology is not just restricted to household use. Fridges and freezers are used in hospitality and catering, food preparation and storage and retail settings, air conditioning, heat pumps and cryogenics to name but a few.

However, the technology used is still based on a heat transfer process in which a refrigerant circulates through pipework removing heat from the system as it turns from liquid to gas by evaporation. This process is not particularly efficient with only about 10% of the power going into the refrigeration unit being used for cooling, and since appliances tend to be left on permanently, this adds up to a lot of energy. 

With approximately 10% of all electricity generated used to power refrigeration equipment, they have a significant environmental impact. Furthermore, the refrigerants used tend to contain greenhouse gases, so when it comes to disposal there is additional risk of environmental damage.

As a result, there is a drive to develop a more efficient method of achieving the cooling processes that make refrigeration possible. One such method, which has been in development for some time, uses the magnetocaloric effect - a magneto-thermodynamic phenomenon in which a temperature change of a suitable material is caused by exposing the material to a changing magnetic field.

In other words, for a closed (adiabatic system), where no energy is allowed to enter or escape, when the magnetic field applied to a magnetocaloric material is reduced, the magnetic domains within the material become disorientated. Then, when the magnetic field is then increased again, the domans realign, absorbing thermal energy and therefore cause a cooling effect.

Rather than relying on a liquid refrigerant, this technology offers solid state refrigeration, therefore negating the need for a pump in the system to constantly draw on electricity and removing potentially harmful chemicals from the products.

The strongest effects are observed in materials containing the rare earth metal gadolinium or alloys of praseodymium and nickel or lanthanum-iron-silicon alloys. This method of cooling has been proven in small scale trials, but is yet to make any inroads towards commercialisation, since the materials are either extremely expensive, scarce, difficult to extract or harmful to the environment. Currently, the cost of a magnetic refrigerator is thought to be five to ten times more expensive than its conventional equivalent.

Therefore, it is the discovery and development of new affordable materials capable of exhibiting strong enough magnetocaloric effects, which holds the key to a step change in these technologies.

Here at the University of Sheffield, we are investigating a number of discovery routes to find the next viable materials which could make widespread magnetic refrigeration a reality. 

A recent Leverhulme grant funded project aimed to identify high entropy alloys (alloys made up of similarly large proportions of five or more elements) with the potential to possess properties approaching those of known magnetocaloric alloys. These alloys would be cheap, sustainable and would avoid the use of elements on the European Commission’s list of critical raw materials, such as gallium and all rare earth elements.

The alloys under consideration contain combinations of iron, silicon, aluminum, chromium, nickel and copper. Because of its strong magnetic properties, cobalt - another element on the EU critical list - is still included in some compositions, but it is hoped that further developments will mean that this can be removed too.

This project focuses on the elimination of alloy combinations that won’t meet these property requirements through the use of semi-empirical modelling, which takes known physical property data and models how compositions might behave. The elimination of possible alloy compositions means that those with greater potential can be modelled in greater detail and then actually synthesised for physical testing.

A second approach to materials discovery involves high-throughput combinatorial fabrication and characterisation . Thin films of high entropy alloys are produced by sputter deposition. These films vary in composition across their width. When magnetic fields are applied across these films, it is possible to analyse the responses of the alloy compositions using magneto-optic Kerr effect (MOKE) techniques. By using this method, it is possible to investigate many tens of compositions in a relatively short time, highlighting those that are worthy of further exploration.

Both of these methods will inevitably speed up the materials discovery and development process, thereby making the possibility of more effective and environmentally friendly refrigeration a reality a lot sooner than it may otherwise have been.


Details of the research can be found in the following published articles:

  • A. Quintana-Nedelcos, Z. Leong, N.A. Morley, “Study of dual-phase functionalisation of NiCoFeCr-Alx multicomponent alloys for the enhancement of magnetic properties and magneto-caloric effect”, Materials Today Energy, Volume 20, 2021, https://doi.org/10.1016/j.mtener.2020.100621
  • Morley, N.A., Lim, B., Xi, J. et al. “Magnetic properties of the complex concentrated alloy system CoFeNi0.5Cr0.5Alx.” Sci Rep 10, 14506 (2020). https://doi.org/10.1038/s41598-020-71463-3
  • James Harris et al, “Investigation into the magnetic properties of CoFeNiCryCux alloys”, 2021 J. Phys. D: Appl. Phys. 54 395003. https://doi.org/10.1088/1361-6463/ac1139 

Professor Nicola Morley will be presenting at this year’s Institute of Physics Physics in the Spotlight conference titled Physics’s Role in Realising the Green Economy. Nicola will be talking about The Sustainability Challenges in Magnetic and Polymeric Electronic Materials on Thursday 25 November. More information about the conference can be found here: http://spotlight2021.iopconfs.org/Home

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