Near-field heat transfer at nanoscale level have been extensively investigated. Despite the agreement of most study findings, there are still numerous unresolved problems and discrepancies that need to be urgently addressed. As such, researchers have been looking for alternative channels for activating heat transfer in extreme-near fields and have identified phonon tunneling as a promising solution. There are numerous mechanisms for phonon tunneling including van der Waal interactions, the electrostatic potential difference between surfaces, and polar dielectrics for radiative heat transfer. Even though most metal surfaces can support Rayleigh acoustic waves propagating near the medium surface, the contribution of Rayleigh waves to near-field radiative heat transfer is not fully explored. This is mainly due to a lack of coupling between the acoustic waves and thermal radiation.
Professor Aleksandr Volokitin from the Samara State Technical University recently investigated the contribution of acoustic waves to near-field heat transfer. He determined the radiative and phonon heat transfer between two identical gold plates in an extreme near-field in the presence of electrostatic potential differences. The gold plates were separated by a vacuum gap of relative thickness. The contribution of the Rayleigh waves was compared to that of bulk contribution. The paper is currently published in the research Journal of Physics: Condensed Matter.
Experimental findings showed that the coupling between the acoustic waves and radiation field was caused by the potential difference as it increased from 0 -10 V. As such, the radiative heat transfer increased by many orders of magnitude with the increase in the potential difference. Due to the van der Waals interactions and electrostatic forces between the surface displacements, it was easier to compare the radiative heat transfer and phonon heat transfer. For instance, the radiative heat transfer was reduced to electrostatic phonon heat transfer for smaller thickness distances and large potential differences.
Conditions were established for the relationship between surface phonon polaritons and Rayleigh waves interactions. The contributions from Rayleigh and bulk acoustic waves were found to be of the same order and nearly equal distances. Although the experimental results could not be explained by conventional radiative heat transfer theories, the observed large contributions were generally attributed to the fluctuation of the dipole moments induced by the potential difference on the surface. Furthermore, it was worth noting that the exclusion of the electron tunneling from the heat transfer mechanism was possible due to the exponential current growth for distances below 0.5nm and the dependence of the heat flux on power in the distance range.
In summary, Professor Volokitin calculated the radiative and phonon heat transfer between metal plates in an extreme near-field to determine the contribution of Rayleigh acoustic waves with applications in different fields such as high-frequency signal processing and sensors. Most importantly, results showed coupling between the radiation field and acoustic waves due to potential difference. Additionally, the radiative heat transfer was comparable to the phonon heat transfer where both Rayleigh and bulk waves produced contributions of the same order. In a statement to Advances in Engineering (AIE) fraternity, Professor Volokitin noted that the study will be useful in niche applications including using the potential difference to control heat fluxes at nanoscale levels.
Volokitin, A. (2020). Contribution of the acoustic waves to near-field heat transfer. Journal of Physics: Condensed Matter, 32(21), 215001.