Refrigeration Contributes to Climate Change. This Material Might Help.
Chlorofluorocarbons, or CFCs, were once widely used as refrigerants. Though good at cooling, CFCs damaged the ozone layer in the Earth’s stratosphere. In order to reduce harm to the ozone layer, which absorbs the sun’s ultraviolet radiation, the industry gradually turned from CFCs to hydrofluorocarbons (HFCs).
But there was a trade-off. HFCs, while less harmful to the ozone layer, are very effective at trapping heat in the atmosphere. In other words, they have a high global warming potential.
A paper published in Advanced Materials described a type of material called a spin-crossover compound that could someday be a more ecofriendly alternative to HFCs.
“We want to find solid-state refrigerant materials that have a very low ecological footprint and ideally enable cooling devices to work in an efficient manner,” said Professor Karl Sandeman (Brooklyn College and The Graduate Center, CUNY). Sandeman and Ph.D. students Steven Vallone and Anthony Tantillo collaborated on the work with colleagues at Oak Ridge National Laboratory in Tennessee, and The University of Leeds and Heriot-Watt University in the United Kingdom.
The solids they wrote about are made of iron atoms surrounded by molecules called ligands. Their electrons—negatively-charged subatomic particles—can be configured in one of two ways: in a “high-spin” or a “low-spin” state.
When scientists apply pressure to the material, the paper says, two things happen. The material shrinks, and its electrons switch from the high-spin to low-spin state. Both of these things make the material more “ordered”—the smaller volume, low-spin state is less chaotic than the larger, high-spin state. In physics terms, the material’s entropy decreases.
However, the laws of thermodynamics insist that a system’s entropy must increase over time. To compensate for the loss, then, some of the energy that went into compressing the material is transformed into heat, adding disorder anew. The reverse—releasing the pressure and letting the material expand—produces a cooling effect.
While scientists have been studying spin-crossover materials for a long time, Sandeman said, using them for cooling is a new, cutting edge part of the field.
“There’s so much to do,” Sandeman said. “There is a huge amount of molecular tuning we can do that can have an impact on the transition temperature and volume change. We would also like to acquire structural data as a function of pressure because then we could construct computer models to find ways to further improve the refrigerant properties of the materials.”