A Novel Experimental Platform for the Study of Near-Field Radiative Transport and Measurements from Thin Dielectric Coatings.

Abstract

Near-field radiative heat transfer (NFRHT) is an active area of research with implications for heat transfer and thermal management technologies in the future. Previous experiments observed that when the gap-size between a hot surface, the emitter, and a cold one, the receiver, reduces to micrometer dimensions significant enhancements in radiative heat flow between the two surfaces, above the value predicted by Stefan-Boltzman law, are observed. Subsequent theoretical studies supported these results and predicted orders-of-magnitude enhancement in radiative heat flow if the gap-size is further decreased to nanoscale. A range of other interesting phenomena are also predicted for this near-field regime. One of the most intriguing of these theoretical predictions is that pertaining to NFRHT enhancements calculated for nanoscale-thin dielectric coatings. In particular, when the gap-size between the emitter and receiver becomes comparable to film thickness, the enhancements in radiative heat flow are predicted to be as large as those for bulk materials, which can result in heat transfer coefficients that are ~20 times that of far-field values for a gap size of ~20 nm. No experiment has proved the validity of theoretical predictions pertaining to NFRHT enhancement from nanoscale-thin dielectric films. Here, a new experimental platform to perform NFRHT experiments is presented. The platform consists of two major components; a microfabricated resistive picowatt-resolution calorimeter and a six degree-of-freedom nanopositioner that can parallelize two planes with ~6 µrad of resolution. While this platform is designed to eventually perform NFRHT measurements between parallel plates, here it is used to measure enhancements of radiative heat flow between a spherical emitter and thin dielectric receiver with varying thickness. Consequently, for the first time, a dramatic increase in near-field radiative heat transfer from thin dielectric films is observed, which is comparable to that obtained between bulk materials, even for very thin dielectric films (50–100 nm) when the spatial separation between the hot and cold surfaces is comparable to the film thickness. These results are attributed to the spectral characteristics and mode shapes of surface phonon polaritons, which dominate near-field radiative heat transport in polar dielectric thin films.