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Karl Sandeman

Dr Karl Sandeman

Dr Karl Sandeman

Research Fellow

Interests and expertise (Subject groups)

Grants awarded

Phase transition materials for room temperature magnetic refrigeration

Scheme: University Research Fellowship

Organisation: Imperial College London

Dates: Oct 2010-Sep 2013

Value: £308,906.23

Summary: The physics of conventional refrigeration is that heat is absorbed by a liquid when it evaporates to become a gas. This is the cold sensation felt when using an aerosol can. Liquid refrigerants therefore need to be fairly volatile, and so if their application is at room temperature, e.g. for domestic cooling or air conditioning, they can readily escape as gases. The problem is that many of today's refrigerants are greenhouse gases. By using a magnetic solid that changes temperature when exposed to a magnetic field, we can have a non-volatile refrigerant. There is also the opportunity to tailor the device power in such a way to achieve efficiency benefits. Both of these factors could enable a significant reduction in the production of CO2 and CO2-equivalents if magnetic cooling is adopted. I research the physics of such solid magnetic refrigerants. At a magnetic phase transition there is a release or take-up of heat which can be used in a cooling cycle analagous to that when a volatile liquid is alternately expanded and compressed in a conventional refrigerator. The phase transition can be triggered by an applied magnetic field. However, there are complexities. A magnetic solid has many different interacting "degrees of freedom" including atomic order, atomic vibrations, electrons and quantum mechanical spins. By seeking to understand the interplay of these degrees of freedom I aim not only to help in the development of magnetic refrigerants but also to discover and tailor new phenomena occuring when different degrees of freedom couple. Magnetic refrigerants have another potential application: as energy conversion materials. By harnessing the temperature-induced (rather than magnetic field-induced) change of state at a magnetic transition, waste heat can be converted into power. My research is starting to address the materials that could be considered for this newer function.

Exploring metamagnetic systems for room temperature magnetic refrigeration

Scheme: University Research Fellowship

Organisation: Imperial College London

Dates: Oct 2005-Sep 2010

Value: £235,794.30

Summary: I study solids that change their magnetic properties when they are exposed to an external stimulus (e.g. a magnetic field). At such so-called magnetic phase transitions, there is a release or take-up of heat which can be used in a cooling cycle analagous to that when a volatile liquid is alternately expanded and compressed in a conventional refrigerator. In short, the materials I study are the working substance for a future magnetic fridge. The attraction of this research is the promise of a more efficient cooling cycle that is also free of greenhouse gas refrigerants. From a fundamental viewpoint, the reason for studying solid magnetic materials is their complexity. A magnetic solid has many different interacting "degrees of freedom" including atomic order, atomic vibrations, electrons and quantum mechanical spins. By seeking to understand the interplay of these degrees of freedom I aim not only to help in the development of magnetic refrigerants but also to discover and tailor new phenomena occuring when different degrees of freedom couple. Most magnetic phase transitions have been observed in single-phase alloys where degrees of freedom interact within one phase on the atomic scale. Now, however, developments in nano-fabrication mean that we can examine these short-lengthscale interactions not just between atoms but also between nano-scale phases of different materials. If we move from considering atoms as building blocks to the use of material phases in this regard, the range of materials we can make is potentially very large. I am currently collaborating with researchers who are able to assemble new materials from the “bottom up” in a number of different ways. Some involve making mixtures of phases with well-controlled interfaces. Others use one “host” material to act as a molecular scaffold for another “guest” material, thus controlling inter-molecular interactions. Such developments could result in a whole new class of tailored magnetic materials.

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