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543 records · Page 31

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DOE OSTI2015

Effect of shape in near-field thermal transfer for periodic structures

In this paper, the effect of the geometrical shape on the radiative thermal transfer between a periodic array of beams and a planar substrate is investigated. Specifically, we analyze the changes in the thermal transfer that occur when the cross-sectional shape of SiC beams is modified from rectangular to ellipsoidal and finally triangular. Numerical calculations are done based on the rigorous coupled wave analysis. These exact results from this analysis are compared to modified proximity and far-field approximations, which become valid for small and large spacings, respectively. Moreover, these results are also compared to effective medium theory, which becomes increasingly accurate in the limit of small periodicities. We show that a reduction in the periodicity will lead to a reduced thermal transfer for triangular and ellipsoidal shaped beams. Even though, in the limit of very small periodicity, thermal transfer for the case of rectangular shaped beams also decreases by decreasing the periodicity, this decrease is slower as compared to other cross-sectional shapes. Lastly, we show that even though changing periodicity will change the magnitude of thermal transfer, the scaling law for its variation with the beam to substrate spacing is primarily determined by the cross-sectional shape rather than the periodicity. We analytically prove this fact by investigating the large and small periodicity regimes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Near-field heat transfer between graphene/hBN multilayers

We study the radiative heat transfer between multilayer structures made by a periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN) slab. Surface plasmons in a monolayer graphene can couple with a hyperbolic phonon polaritons in a single hBN lm to form hybrid polaritons that can assist photon tunneling. For periodic multilayer graphene/hBN structures, the stacked metallic/dielectric array can give rise to a further e ective hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior of hBN. The e ective hyperbolicity can enable more hyperbolic polaritons that enhance the photon tunneling and hence the near- eld heat transfer. However, the hybrid polaritons on the surface, i.e. surface plasmon-phonon polaritons, dominate the near- eld heat transfer between multilayer structures when the topmost layer is graphene. The e ective hyperbolic regions can be well predicted by the e ective medium theory (EMT), thought EMT fails to capture the hybrid surface polaritons and results in a heat transfer rate much lower compared to the exact calculation. The chemical potential of the graphene sheets can be tuned through electrical gating and results in an additional modulation of the heat transfer. We found that the near- eld heat transfer between multilayer structure does not increase monotonously with the number of layer in the stack, which provides a way to control the heat transfer rate by the number of graphene layers in the multilayer structure. The results may bene t the applications of near- eld energy harvesting and radiative cooling based on hybrid polaritons in two-dimensional materials.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Tuning Solid Electrolyte Interphase Formation before Plating Onset in Anode-Free Sodium Batteries

Sodium (Na) batteries are of growing interest due to the higher earth abundance of sodium than lithium, as well as their promising theoretical energy density when metallic Na anodes are used. However, Na plating and stripping are heavily influenced by the physicochemical properties of the solid electrolyte interphase (SEI), which is directly influenced by the solvent and salt used for the electrolyte. While most studies focus on the SEI that forms on the surface of Na metal after plating, we expand this analysis by identifying a nanoscale “pre-plating” SEI that forms on the current collector (CC) prior to the onset of Na plating. Here, we systematically investigate an array of Na salt and glyme solvents in the electrolyte and determine the associated impacts on pre-plating SEI formation on aluminum CCs. By combining analytical electrochemistry approaches with a multimodal suite of spectroscopy techniques (X-ray, infrared, and Raman), supported by density functional theory calculations, we reveal a direct correlation between the Na + coordination environment and pre-plating SEI composition. We find that longer-chain glymes produce larger proportions of organic alkoxide products in the interphase, consistent with increased Na + −glyme interactions, while the fraction of salt-derived inorganic products (e.g., NaF) correlates with Na + −anion coordination. These insights highlight the critical influence of electrolyte composition particularly solvent identity and Na + coordination on the initial SEI formation in anode-free Na batteries.

anode-free batteries↗