Substrate effects on the near-field radiative heat transfer between two hBN films

Near-field radiative heat transfer (NFRHT) could surpass the blackbody limit defined by Stefan-Bolzmann’s law by several orders of magnitude, which has potential applications in thermal switching, thermal management, and photovoltaics. To further develop the NFRHT from theory to application, the substrate, which could enhance the stability of the structure, is a critical factor not to be ignored. However, the substrate effect on the NFRHT is still rarely discussed. In this work, we investigate the NFRHT between hexagonal boron nitride (hBN) films with different permittivities of the substrate. Results demonstrate that when the thickness of the film is 1 nm, increasing the permittivity of the substrate will suppress the NFRHT. In contrast, when the thickness of the film is larger (>2 nm), the high-permittivity substrate could enhance the NFRHT. The spectral heat flux (SHF) corresponding to substrates with different permittivities was investigated. The SHF in Type I hyperbolic band of hBN increases with the increase in the permittivity of the substrate, while that in Type II hyperbolic band is completely opposite. This competitive relationship leads to the above-mentioned phenomenon of NFRHT. The underlying physics mechanism can also be explained by the hyperbolic phonon polaritons (HPPs), which are analyzed by the energy transmission coefficients and dispersion relations. The findings in this work will deepen the understanding of the substrate on HPPs and pave a novel way for near-field radiation devices based on hyperbolic materials.