Nano Focus: Theoretical thermocrystals control heat like sound
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M
artin Maldovan of the Massachusetts Institute of Technology has produced a theoretical framework that could lead to improved control of heat flow in materials. Thermocrystals, comprising alloys containing nanoparticles, are materials that can manipulate thermal energy flow by exploiting the coherent reflections of phonons from internal surfaces. Potential applications including heat waveguides, heat lensing, thermal diodes, and thermal cloaking may become possible. “The theory outlines a completely new way of manipulating heat,” Maldovan says. “When they created photonic crystals, it was a completely new way of manipulating light. Then, they created phononic crystals as a completely new way to manipulate sound. This is equivalent to that, but for heat.” The key to the theory, as reported in the January 11 issue of Physical Review Letters (025902; DOI: 10.1103/ PhysRevLett.110.025902), is to transfer thermal energy flow from standard short wavelength particle transport to long wavelength wave transport—to make “heat behave like light,” according to Maldovan. Particle transport occurs when a phonon hits an interface and scatters diffusely in all directions. Wave transport happens when a phonon hits an interface and reflects and transmits co-
herently, like light in a mirror. Most of the time, heat phonons are scattered diffusely when they encounter an interface because their wavelengths are so small. For coherent scattering to occur, the interface has to be almost perfect (defect-free), rendering it almost impossible to make. Instead of trying to make the perfect interface, Maldovan decided to try to make the phonon wavelength larger by reducing its frequency. Such phonons should transmit and reflect like light from even an imperfect interface. From previous experience, Maldovan knew that in thermoelectrics, researchers use alloys and nanoparticles in order to block all frequencies of phonons. In this work, he used Si1–xGex alloys with Ge nanoparticles to block only select frequencies. The mass-difference scattering in the alloy blocks some high-frequency phonons, and the nanoparticles block another portion. “I use the nanoparticles in such a way to kill only the really highfrequency phonons,” he says, “so the nanoparticles in my case must be very, very small.” In this work, he considered Ge nanoparticles with 1 nm diameter. After killing the high-frequency phonons, Maldovan’s theory was still left with a large number of wavelengths of heat that it could not handle, so he decided to narrow the frequency range by requiring the material to be a thin film, which kills the very low phonon frequencies. Having chopped off the highest and lowest frequencies, the heat that was left was concentrated into a narrow, interme-
diate band of wavelengths. Specifically, for Si90Ge10 thermocrystal thin films containing Ge nanoparticles, the heat spectrum was concentrated into a relatively narrow, low frequency window between 0.1 THz and 2.0 THz. Up to 40% of this heat was restricted to a narrow hypersonic range of 100–300 GHz. N
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