Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y40: Invited Session: New Views of Thermal Transport |
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Sponsoring Units: DCMP Chair: Barry Zink, University of Denver Room: Mile High Ballroom 2B-3B |
Friday, March 7, 2014 8:00AM - 8:36AM |
Y40.00001: Heat under the microscope: towards a microscopic understanding of thermal transport in solids Invited Speaker: Austin Minnich The thermal conductivity of a solid is typically the observable quantity used to study heat conduction. While knowledge of thermal conductivity is important, a wealth of microscopic information remains obscured because the thermal conductivity represents an average over a thermal distribution. Recent efforts have demonstrated that this microscopic information, in the form of the phonon mean free path distribution, can in fact be measured directly by systematically observing the transition from the diffusive to the ballistic transport regimes. In this talk, I will describe our recent efforts to develop this thermal conductivity spectroscopy technique as a general tool, as well as how the technique is giving a detailed picture of phonon heat conduction in a variety of solids. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y40.00002: Nanophononics at low temperature: manipulating heat at the nanoscale Invited Speaker: Olivier Bourgeois Nanophononics is an emerging field of condensed matter that deals with transport of thermal phonons at small length scales. When the section of a waveguide becomes smaller than the mean free path or the phonon wavelength, heat transfer are strongly affected. Here, I will present the results we obtained by ultra- sensitive measurements of thermal conductance of suspended nano-objects (nanowires and membranes) using the 3$\omega$ method. This experimental set-up allows the measurement of power as small as a fraction of femtoWatt (10$^{-15}$ Watt). These experiments show that the concepts of mean free path and dominant wavelength are crucial to understand the phonon thermal transport below 10K. The phonon transport, at this temperature, is well described by the Casimir-Ziman model used here to treat the data. The contribution of the thermal contact between a nanowire and the heat bath has been estimated to be close to one, thanks to the fact that the nanowire are made out of monolithic single crystal. Strong reduction of thermal conductance has been obtained in serpentine nanowire where the transport of ballistic phonons is blocked. Moreover, in corrugated silicon nanowire, we showed that the corrugations induce significant backscattering of phonon that severely reduces the mean free path, beating in some cases, the Casimir limit. These experiments demonstrate the ability to manipulate ballistic phonons by adjusting the geometry of thermal conductors, and hence manipulate heat transfer. Finally, the use of these new concepts of engineering ballistic phonons at the nanoscale allows considering the development of new nanostructured materials for thermoelectrics at room temperature, opening exciting prospects for future applications in the energy recovery. J.-S. Heron, T. Fournier, N. Mingo and O. Bourgeois, Nano Letters 9, 1861 (2009). J-S. Heron, C. Bera, T. Fournier, N. Mingo, and O. Bourgeois, Phys. Rev. B 82, 155458 (2010). C. Blanc, A. Rajabpour, S. Volz, T. Fournier, and O. Bourgeois, Appl. Phys. Lett. 103, 043109 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y40.00003: Engineering thermal conductance using a two-dimensional phononic crystal Invited Speaker: Ilari Maasilta Controlling thermal transport has become very relevant in recent years, in light of the strong push to develop novel energy harvesting techniques based on thermoelectricity, the need to improve the heat dissipation out of semiconductor devices, and the push to increase the sensitivity of bolometric radiation detectors. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centers in the material (nanoparticles, impurities etc.). Recently we have taken another approach and demonstrated that one can also use coherent bandstructure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. Working at around 1 Kelvin where the wavelength of the dominant thermal phonons is more than two orders of magnitude longer than at room temperature, we have created phononic crystals with a period of 1 $\mu$m that strongly reduce the thermal conduction. In addition, we performed theoretical calculations that accurately determine the ballistic thermal conductance in a phononic crystal device, showing full quantitative agreement with the experiments. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y40.00004: Thermal transport in amorphous nanostructures: the (enduring) role of low-energy phonons Invited Speaker: Jason Underwood Micromachined amorphous solid structures have proven to be ideal platforms for physicists to challenge their understanding of phonon transport. Such nanostructures have been exploited for early experimental demonstrations of the quantum of thermal conductance. These structures also serve important technological functions. Amorphous silicon nitride (SiN$_x$) nanostructures, in particular, are increasingly critical to the operation of state-of-the-art low temperature detector arrays. Achieving control over which phonon modes propagate in a given structure --- phononics --- is a major goal for engineering better thermoelectric materials, for regulating heat flow in ever-shrinking microprocessors, and for the developing field of caloritronics. At very low temperatures, it is generally accepted that phonons with energy much lower than the Debye energy (i.e., $\omega \ll 10^{13}$~Hz) dominate thermal transport. At room temperature, the preponderance of higher energy modes is usually reason enough to assume that the low energy modes do not contribute substantially to the overall thermal conductance. While generally true for crystals, the efficient scattering of high-energy phonons in amorphous solids means that the remaining low-energy modes may acquire comparably long mean free paths. Recent measurements of SiN$_x$ nanostructures strongly suggest that this bias in mean free paths leads to the result that low-energy phonons may contribute up to 50\% of the overall thermal conductance of the structure --- even at room temperature. After a brief review of thermal transport in the low-energy regime, I will discuss these results, as well as other recent experiments where low-energy phonons play an important role. [Preview Abstract] |
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