Extreme Near-Field Heat Transfer between Silica (SiO₂) Surfaces

This study investigates extreme near-field thermal transport between two parallel silica (SiO₂) plates, exploring gap distances ranging from ~ 0.5 nm to 2 nm. In the near-field regime, where distances are comparable to or smaller than the thermal wavelength, tunneling evanescent modes result in radiative heat transfer several orders of magnitude higher than predicted by black body radiations. This phenomenon holds significant relevance for applications such as scanning thermal microscopy, thermally assisted magnetic recording, and near-field thermophotovoltaics. Recent experiments achieved over an 18000-fold enhancement in radiative heat flux between parallel planar silica surfaces with gaps as small as ~ 10 nm. Despite these advancements, the investigation of gap distances below 10 nm remains unexplored. In this study, we employ non-equilibrium molecular dynamics (NEMD) simulations to characterize heat transfer between SiO₂plates in both amorphous and crystalline forms. Results are compared with fluctuational electrodynamics (FE) theory, showing a relative good agreement, especially for large gap sizes. The relative deviations to fluctuational electrodynamics theory are interpreted in terms of acoustic tunneling and non-local effects. The heat transfer coefficient is observed to decrease ~ 20-fold as the gap size varies from 0.5 nm to 2 nm. Furthermore, the study explores the impact of temperature on heat transfer, revealing temperature-dependent variations, particularly at low temperatures. Spectral analysis of heat transfer coefficients across various temperatures provides insights into the dominance of low-frequency modes as temperatures decrease.