Thermoelectric devices might be made more efficient

Heat — vibration of atoms and molecules in a material — usually travels in a difficult to control "random walk" The new observations show a very different pattern, called coherent flow, more like ripples moving across a pond in an orderly way. The research opens the feasibility of new materials in which heat flow could be precisely tailored. Such research might lead to new ways of shedding the heat generated by electronic devices and semiconductor lasers, which currently hampers performance, even destroying the devices. The new work, by graduate Maria Luckyanova, postdoc Jivtesh Garg and professor Gang Chen, all of MIT's Department of Mechanical Engineering — along with other students and professors at MIT, Boston University, the California Institute of Technology and Boston College — is reported in Science. The study involves a nanostructured superlattice: a stack of alternating thin layers of GaAs (gallium arsenide) and AlAs (aluminium arsenide) each deposited in turn through MOCVE Imetal-organic chemical vapor deposition). Chemicals containing these elements are vaporized in a vacuum, deposited on a surface, their thicknesses precisely controlled through the the deposition process. Resulting layers are 12 nanometers thick, and the entire structures ranged 24 to 216 nanometers thick. Researchers had believed that roughness at the interfaces between the layers would scatter heat-transporting quasi-particles, called phonons, as they moved through the superlattice. In many layer materials, such scattering effects would accumulate, it was thought, and "destroy the wave effect" of the phonons, but this assumption had never been proved, so the researchers re-examined the process. Experiments and computer simulations showed that while phase-randomizing scattering takes place among high-frequency phonons, wave effects were preserved among low-frequency phonons and "coherent conduction of heat is really happening." Understanding the factors that control this coherence could, in turn, lead to better ways of breaking the coherence, reducing the conduction of heat, desirable in thermoelectric devices to harness unused heat energy in everything from powerplants to electronics. Such applications require materials that conduct electricity very well, but conduct heat very poorly. The work could also improve the shedding of heat, as in the cooling of computer chips. The ability to focus and direct heat flow could lead to better thermal management for such devices. The new work not only provides the possibility of controlling the flow of heat (mostly carried by short wavelengths phonons) but also controlling the movement of sound waves (primarily carried by longer-wavelength phonons). The insights that made the work possible arose in large part through interactions between researchers in different disciplines, facilitated through the Solid State Solar-Thermal Energy Conversion Center, an Energy Frontier Center funded by the U.S. Department of Energy, which holds regular cross-disciplinary meetings at MIT. "Those meetings provided long, fruitful discussions that really strengthened the paper," Luckyanova says. The variety of people in the group "really encouraged us to attack this problem from all sides." Massachusetts Institute of Technology