Scientist uncover key mechanism in magnetic cooling crystals

Scientist at Imperial College have uncovered a key mechanism in magnetic cooling crystals, predicting performance can be improved by nanostructuring. Refrigerators could benefit.

Certain materials get hot under the influence of a magnetic field, and then self-cool when the field is removed - the magnetocaloric effect.

Notably in Gd₅Si₂Ge₂, the effect is large and better at extracting heat than conventional fridges.

"The system is said to be 30% more efficient than the vapour compression cycle used today," Professor Lesley Cohen told Electronics Weekly.

Using a microscopic scanning Hall probe on crystals of the material, the team found that the exothermic transition from paramagnetic to ferromagnetic state always start in or near 200nm thick plates of Gd₅Si_1.5Ge_1.5 that always naturally occur in the crystals, occupying about 1% of the volume.

The researchers also showed that the endothermic reverse transition, when the field is removed, has nothing to do with the platelets, but is seeded by physical features like cracks and edges.

"This is an exciting discovery because it means we may one day be able to tailor-make a material from the bottom up, starting with the microstructure, so it ticks all the boxes required to run a magnetic fridge," said Cohen.

Magnetocaloric cooling becomes a practical proposition when permanent magnets rather than energy-consuming electromagnets can be used.

Currently the operational threshold of Gd₅Si₂Ge₂ is close to the to the 1T achievable by permanent magnets, said Cohen.

Bottom up design might engineer materials that operate better with permanent magnets.

A practical magnetocaloric heat pump, said Cohen, could consist of a disc or ring of Gd₅Si₂Ge₂ rotating through a strong permanent magnet.

Heat would be absorbed where the ring leaves the magnet and emitted as it enters.

"It is certainly capable of cooling large objects, abattoirs could be one application," said Cohen. "the main problem is penetration of a very large stabilised market."

The research was carried out in collaboration with the US Ames Laboratory.