| 研究実績の概要 |
Firstly, to overcome the experimental challenges and explore the hydrodynamic flow in micro-and nano-scale and at elevated temperatures, I utilized graphite as perfect host owing to its dominant normal scattering in nature. I built up a suspended microstructure system and fabricated 30 um-long and 65 nm-thick graphite ribbons with various widths from 500 nm to 5 um. Based on a steady-state u-TDTR setup, I measured the thermal conductivity of the graphite ribbons from 10 to 200 K.
Secondly, to confirm the occurrence of phonon Poiseuille flow, I advocated a more unambiguous criterion for ascertaining phonon hydrodynamic flow in graphite - a faster rise of thermal conductivity (k) than the ballistic thermal conductance (G-ballistic). In light of this explicit criterion, I demonstrarted the value of k/G-ballistic is enhanced by 16% from 30 to 50 K in the ribbon with a width of 5 um, which indicates the emergence of hydrodynamic phonon transport.
Furthermore, I also investigated the impact of isotopes on the formation of phonon Poiseuille flow in submicron graphite ribbons with both natural (1.1% 13C) and purified (0.02% 13C) carbon isotope concentrations. I concluded that with the temperature increases, k is enhanced over the ballistic case from 30 to 60 K, attributed to the hydrodynamic transport of phonons in the isotopically purified sample. Whereas k/G-ballistic monotonously decreases in the natural graphite sample, which indicates the absence of phonon Poiseuille flow resulting from the sufficient momentum-destroying isotope scattering.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
In the current work, we utilize the isotopically-enriched graphite crystals to minimize the isotope effect on phonon Poiseuille flow, carefully design the submicronscale graphite ribbon structure to satisfy the window condition of phonon Poiseuille flow, and demonstrate the first unambiguous experimental evidence of phonon Poiseuille flow based on a novel understanding of the occurrence criterion (κ/G-ballistic) for anisotropic crystals in the 5 μm-wide isotopically-purified graphite ribbon at temperature up to 60 K, which is much improved compared to those of other 3D materials. We believe that our study also provide the experimental evidence of isotope effect on the observation of phonon Poiseuille flow for the first time to the best of our knowledge.
The suspended microstructure system built up in this work provides a good platform for further experiments to investigate the extraordinary super-linear width dependence of thermal conduction in the hydrodynamic regime or the phonon Knudsen minimum phenomenon.
In summary, we have developed an integrated experimental platform to investigate the steady-state phonon hydrodynamics in suspended graphite submicron structures. Our joint theoretical and experimental study on phonon hydrodynamics in graphitic materials thus deepens the understanding of the collective physics of phonons in anisotropic solids. The experimental platform will also open innovative possibilities for tuning and manipulation of phonon hydrodynamics, as well as its application in thermal management of the modern micro- and nanoelectronics.
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| 今後の研究の推進方策 |
To provide a comprehensive investigation of phonon hydrodynamic flow in solid-state materials, further experiments will be carried out based on the suspended graphite microstructure system built up in this work.
On the one hand, another important aspect to evidence the phonon hydrodynamic flow is the super-ballistic width dependence of thermal conductivity. To this end, proper graphite ribbons with a fixed length and a width varying from several hundreds of nanometers to several tens of micrometers are desirable to validate the study. A relevant experiment is going on to investigate and observe the super-linear width dependence of thermal conduction in the hydrodynamic regime.
On the other hand, similar to the fluids (gas or liquid) flowing in a microscale channel, the heat flow rate has been predicted to experience a minimum as well when the Knudsen number equals to one (i.e., molecule mean free path is comparable to the channel width). The Knudsen minimum also points the transition from ballistic to hydrodynamic phonon transport regime. However, the observation of phonon Knudsen minimum in solid-state materials remains experimental difficulty and lacking direct confirmation due to the additional size effect from the channel length. An appropriate update of the experimental platform is under construction to challenge the observation of phonon Knudsen minimum in graphite ribbon (channel) structures with isotopic enrichment.
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