Scientists investigate cooling behavior of promising solid-state cooling material

A research team led by the Department of Energy’s Oak Ridge National Laboratory has bridged a knowledge gap in the movement of heat at the atomic scale. This new understanding holds promise for improving materials to advance an emerging technology called solid-state cooling. The findings are published in the journal Advances in science.

An environmentally friendly innovation, solid-state refrigeration can efficiently cool many things in everyday life, from food to vehicles to electronics – without traditional cooling liquids and gases or moving parts. The system would work through a quiet, compact and lightweight system that allows precise temperature control.

Although the discovery of improved materials and the invention of higher-quality equipment are already helping to fuel the growth of the new cooling method, a deeper understanding of material improvements is essential. The research team used a variety of neutron scattering instruments to examine at the atomic scale a material that scientists consider to be an optimal candidate for use in solid-state cooling.

The material, a magnetic nickel-cobalt-manganese-indium shape memory alloy, can be deformed and then returned to its original shape by causing it to undergo a phase transition either by increasing the temperature or by applying a magnetic field. When subjected to a magnetic field, the material undergoes a magnetic and structural phase transition during which it absorbs and releases heat, a behavior known as the magnetocaloric effect.

In solid state cooling applications, the effect is exploited to provide cooling. A key characteristic of the material is its proximity to disordered states known as the ferroic glassy state, because they represent a way to increase the material’s ability to store and release heat.

Magnons, also known as spin waves, and phonons, or vibrations, join in a synchronized dance in tiny areas scattered throughout the disordered arrangement of atoms that make up the material. The researchers found that the patterns of behavior in these small regions, referred to as hybrid localized magnon-phonon modes in the team’s paper detailing the research, have important implications for the material’s thermal properties.

The scientists discovered that the modes cause the phonons to be altered or shifted significantly by the presence of a magnetic field. Modalities also modify the phase stability of the material. These changes can result in substantial changes in material properties and behavior that can be tuned and tailored.

“Neutron scattering shows that the cooling capacity of the magnetic shape-memory alloy is tripled by the heat contained in these local hybrid magnon-phonon modes that form due to disorder in the system,” said ORNL’s Michael Manley, who led the study. . “This discovery reveals a path to making better materials for solid-state cooling applications for society’s needs.”

The shape memory magnetic alloy the team studied is in a phase that has almost created disordered conditions known as spin glass and strain glass—not the familiar glass used in windows and elsewhere, but unconventional phases of matter that have no order. . The magnetic moments, or small magnets, associated with the atoms in the spin glass phase are randomly oriented instead of pointing in the same direction.

Comparatively, in the strain glass phase, the lattice of atoms is strained on the nanometer scale in a disordered and irregular pattern. Torsion glass and strain glass are referred to as frustrated states in a material because they arise from competing interactions or constraints that prevent the material from reaching a stable ordered state.

“As the material approaches this frustrated state, the amount of heat that is stored increases,” Manley said. “The long- and short-range interactions manifest as localized vibrations and spin waves, which means they are being trapped in small areas. This is important because these additional localized vibrational states store heat. The changing magnetic field causes another phase transition in which this heat is released”.

Controlling the functions of a shape memory magnetic alloy so that it can be used as a heat sponge could be a way to allow efficient solid-state cooling without the need for traditional heatsinks or mechanical components.