Dongchao Xu1 Fabian Medina1 Yue Xiao1 Qing Hao1

1, University of Arizona, Tucson, Arizona, United States

Nanomaterials introduce new opportunities in tuning the thermal transport for various applications. Among various nanostructured materials, phonon transport within periodicnanoporous materials has been intensively studied for its potential applications in thermoelectrics, heat waveguides, thermal diodes, and heat imaging.1,2It is now acknowledged that wave effects of lattice vibrations, i.e., phononic effects, are only important for ultrafine nanoporous structures and at cryogenic temperatures. Experimental evidence is usually found by comparing the thermal conductivities of periodic and aperiodic nanoporous Si films,3,4or by measuring the specific heat to justify the phonon dispersion variation.5
In this work, a new approach is proposed to validate the wave effects, simply by comparing the thermal conductivity of the same Si thin film with increased number of nanopores. When phononic effects exist, it is anticipated that the thermal resistance can be largely increased from single to multiple rows of nanopores. Without phononic effects, the thermal resistance of a patterned Si film should linearly increase with the number of nanopore rows. The measured thermal conductivities are compared to frequency-dependent phonon Monte Carlo simulations that assumes incoherent phonon transport and diffusive pore-edge and film-surface phonon scattering.

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2. Anufriev, R., Ramiere, A., Maire, J. & Nomura, M., Heat guiding and focusing using ballistic phonon transport in phononic nanostructures. Nature communications8, 15505 (2017).
3. Maire, J., Anufriev, R., Yanagisawa, R., Ramiere, A., Volz, S. & Nomura, M. Heat conduction tuning by wave nature of phonons. Science advances3, e1700027 (2017).
4. Lee, J., Lee, W., Wehmeyer, G., Dhuey, S., Olynick, D. L., Cabrini, S., Dames, C., Urban, J. J. & Yang, P. Investigation of phonon coherence and backscattering using silicon nanomeshes. Nature communications8, 14054 (2017).
5. Hao, Q., Xu, D., Zhao, H., Xiao, Y. & Medina, F. J., Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm —Comparison between Different Pore-Drilling Techniques. Scientific Reports8, 9056 (2018).