Thermoelectric materials in particular hold vast potential for use in energy applications because they generate electricity from waste heat, such as that generated by industrial processes, by car and truck engines, or simply by the sun. Reducing the thermal conductivity of these materials by a factor of three, for example, would completely revolutionize existing waste-heat recovery, and also all refrigeration and air-cooling technology.
In the paper Unified theory of thermal transport in crystals and glasses, three researchers discuss a theory that finally unifies the fundamental, atomistic origin of heat conduction. Up to now, different formulations needed to be used depending on the system, for example for ordered materials such as a silicon chip, or disordered such as in a glass. The new theory derives directly from the quantum mechanics of dissipative systems a transport equation that covers on equal footing diffusion, hopping, and tunneling of heat.
This fundamental understanding will allow scientists and engineers to accurately predict the thermal conductivity of any insulating material but particularly with thermoelectric materials that can convert heat into electricity as these have both crystal- and glass-like properties.
“Up to now, two different equations have been used for calculating thermal properties: one describes perfectly crystalline materials – that is, materials with highly ordered atomic structures – and the other one completely amorphous materials like glass, whose atoms do not follow an ordered pattern,” said Michele Simoncelli, a PhD student at EPFL’s Theory and Simulation of Materials (THEOS) Laboratory who worked with Nicola Marzari, a professor at EPFL’s School of Engineering and head of THEOS and of the MARVEL NCCR, and Francesco Mauri, a professor at the University of Rome–Sapienza.
These equations happened to work well in those special cases. “But between these two extremes lie a plethora of interesting cases, and neither equation worked – this is really where our contribution makes a profound difference,” he said.