Metallic solution to keeping cool

As a result of climate change, population growth, and rising expectations regarding quality of life, energy requirements for cooling processes are growing. Now a team from Germany has developed a cooling cycle based on the 'magnetic memory' of alloys.

The magnetic properties of metals can change when they are heated or cooled, and it is this effect that the team say can be utilised in a new generations of coolant technologies.

"Iron, for example, is only ferromagnetic below 768 degrees Celsius; nickel's transformation temperature is 360 degrees Celsius," says Oliver Gutfleisch, Professor of Functional Materials at the Technische Universität (TU) Darmstadt. "Conversely, some alloys become ferromagnetic when they warm up. This phase transition is connected with the so-called magnetocaloric effect when these shape-memory alloys are placed in an external magnetic field just below their transformation temperature and they spontaneously jump to their magnetic order and simultaneously cool down. The stronger the magnetic field, the more they cool."

The scientists used an alloy of nickel, manganese, and indium for their experiments because its conversion can be triggered at room temperature. They generated the magnetic field using the strongest permanent magnets known which generate magnetic fields up to 2 tesla – that is 40,000 times stronger than the Earth's magnetic field. "Under such conditions, our alloy cools down by several degrees," explains Professor Gottschall. "Measurements we have made have shown that a millisecond in the magnetic field is already enough for permanent transformation."

The team is convinced that the future of air conditioning will involve solid coolants. "We have been able to show that shape-memory alloys are highly suitable for cooling cycles," says Professor Gottschall. "We need far fewer neodymium magnets but can nevertheless generate stronger fields and a correspondingly greater cooling effect. In the meantime, we have found alloys that combine all the desired properties, including a large magnetocaloric effect, without using any rare earths or other critical raw materials at all."

The work is published in Nature Materials.