Magnetic minibars

Hotel minibars are probably the first market to adopt magnetic cooling on a large scale. These machines currently use silent but very inefficient thermoelectric Peltier cooling. Replacing those machines by equally silent magnetocaloric fridges would save lots of energy, while adding to the hotel’s green credentials.

Dr Nguyên Thàn Trung, a former PhD student at the faculty of Applied Sciences, has developed a material featuring a ‘giant magnetocaloric effect’. Its temperature rise under the application of a magnetic field is ten times greater than in other materials. Trung’s PhD supervisor, Professor Ekkes Brück, says Trung’s new material is the first of a second generation. An unnamed industrial partner is even considering introducing magnetocaloric fridges, based on Trung’s alloy, on the market next year.

By applying an intermittent magnetic field, the magnetocaloric effect can be used for cooling. Switch the field on, and the magnetocaloric material will heat up. Switch the magnets off, and the temperature will drop, so that the material can absorb heat from its surrounding. A clever cycle results in magnetic refrigeration. However, until now, the effect was too small to compete with the standard compressor-driven fridges (in which expanding gas will absorb heat, and compressed gas will release it).

The magic of the new material, an alloy of manganese, cobalt, and germanium with boron doping, is the synchronisation of two phase transitions: the magnetic phase change plus a structural one. “The material can switch from hexagonal into orthorhombic crystal structure [‘square’, ed.] ones”. Prof. Brück explains. When the material changes from hexagonal to orthorhombic, entropy falls and heat is given off. Normally, this happens at relatively high temperatures, but by inserting boron atoms into the crystal, Trung has succeeded in stabilising the hexagonal form at lower temperatures, and coinciding the structural change with the magnetic phase change, resulting in a giant magnetocaloric effect.

Nguyên Thàn Trung, First-order phase transitions and giant magnetocaloric effect, PhD supervisor Professor Ekkes Brück.