For the first time, the efficiency of an object’s absorption and emission of thermal radiation have been made not to match, thus breaking Kirchhoff’s law of thermal radiation. Scientists have suspected for a while that Kirchhoff’s law is not universal, but this is the first proof. The discovery could help us create more efficient methods for harvesting energy from sunlight, as well as improving camouflage.
People worked out a long time ago that light colors reflect more heat than dark ones, making them better clothes in a hot environment. Newton started the process of putting the idea on a scientific footing with his law of cooling. Gustav Kirchhoff developed this further, defining a capacity known as the emissivity, the ratio between the capacity of a body to emit heat relative to a black body radiator of the same size and shape at the same temperature. He also showed the emissivity matches how much thermal energy the object absorbs under the same conditions, creating one of his many laws (others relate to electric circuits and spectroscopy).
Physicists phrase the law as: “For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.” The law only applies when an object is in thermodynamic equilibrium – that is, no net heating up or cooling down. The law has been useful in identifying objects that can be used to maintain temperatures, such as thermal blankets that can reflect heat away, but also keep it in.
There are few things physicists enjoy more than finding exceptions to scientific laws, and Professor Harry Atwater of California Institute of Technology claims his graduate student Komron Shayegan has done just that by placing an engineered material in a magnetic field.
"Kirchhoff's law has been upheld for more than 150 years, and while theoretical proposals for its violation have been advanced before, this is the first experimental proof that this law can be broken," Atwater said in a statement.
Shayegan notes that most of the time, Kirchhoff’s law is useful. “[B]y designing around and measuring the absorptive properties of a material, we get the emissive properties for free,” he said.
The energy crisis has changed that. “If an energy-harvesting object, like a photovoltaic (solar panel), is re-emitting some of its absorbed energy back toward the energy source (the Sun) as heat, that energy is lost to human purposes,” Shayegan explained. A device that re-emitted radiation away from the source would allow us a second chance at capturing it – for example, by placing a second solar panel under the first.
A feature of Kirchhoff’s law is that absorption and emission are equal not just in total, but at every wavelength.
Shayegan’s product has a patterned structure that increases its absorption and emission in the infrared, but also has a strong magnetic-field response. When placed in a magnetic field of 1 Tesla (similar to that used in a loudspeaker and a 16th of what is required to levitate a frog) and heated above room temperature its emissive efficiency exceeded its absorptivity. Results were repeated at 50°, 100° and 150°C (122°, 212° and 302°F). The effect depends on the angle of the magnetic field, opening up lots of opportunity for fine-tuning.
Besides little things like helping wean humanity off fossil fuels, the work could lead to new ways to make invisibility cloaks, absorbing specific wavelengths while preventing their emission.
The study is published in Nature Photonics.