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While hollow microspheres and various porous structures have received much attention for solar reflection in the recent literature, their fundamental determinants of optical properties and material selection criteria are relatively little known. Here, we study hollow microspheres with varying refractive index and extinction coefficient and identify their role in determining the solar reflectivity. Our simulations based on finite-difference time-domain method show the effects of refractive index between 1.5 and 100 and extinction coefficient between 10−6–100 in the wavelength region of 0.2–2.4 μm and explain how the reflectivity of hollow microspheres is attributed to a combination of strong backscattering and limited absorption. Our analysis indicates that ceramic materials with a high refractive index and a low extinction coefficient such as Y2O3 are promising. When Y2O3 hollow microspheres are randomly distributed with the diameter ranging from 0.5 to 1 μm, our simulation shows the solar reflectivity reaches 0.97 even at 300 μm thickness, and a diffusion theory-based model predicts the solar reflectivity to exceed 0.98 at 500 μm or 0.99 at 1 mm thickness. Our findings can guide optimal designs of hollow microspheres and related porous structures toward complete solar reflection and enable breakthroughs in thermal management and deep-space applications.

While solid and hollow microsphere composites have received significant attention as solar reflectors or selective emitters, the driving mechanisms for their optical properties remain relatively unclear. Here, we study the solar reflectivity in the 0.4–2.4 μm wavelength range of solid and hollow microspheres with the diameter varying from 0.125 μm to 8 μm. SiO2 and TiO2 are considered as low- and high-refractive-index microsphere materials, respectively, and polydimethylsiloxane is considered as a polymer matrix. Based on the Mie theory and finite-difference time-domain simulations, our analysis shows that hollow microspheres with a thinner shell are more effective in scattering the light, compared to solid microspheres, and lead to a higher solar reflectivity. The high scattering efficiency, owing to the refractive-index contrast and large interface density, in hollow microspheres allows low-refractive-index materials to have a high solar reflectivity. When the diameter is uniform, 0.75 μm SiO2 hollow microspheres provide the largest solar reflectivity of 0.81. When the diameter is varying, the randomly distributed 0.5–1 μm SiO2 hollow microspheres provide the largest solar reflectivity of 0.84. The effect of varying diameter is characterized by strong backscattering in the electric field. These findings will guide optimal designs of microsphere composites and hierarchical materials for optical and thermal management systems.

In this Letter, we show that the heat transferred from a thermal composite hot body emitter may be selectively directed in the far field toward a cold body receiver of choice through the enhanced design of both the anisotropic thermal conductivity of the emitter body and its surface emissivity. Specifically, focused radiosity of a representative cylindrical emitter in a preferential direction is attained by optimizing the layout of high and low thermally conductive materials within the solid in combination with an angularly varying emissivity surface profile. The relationship between the multi-body view factor scene and the thermal metamaterial design is clarified by way of numerical experiments. Subsequent gradient-based co-optimization of the thermal composite confirms the working principles of the device and reveals non-intuitive material and surface coating layouts that further enhance the directional radiative intensity of the emitter. The principles are extendable to the engineering of arbitrarily shaped advanced composites for thermal protection systems, energy conservation, or spacecraft low-energy deep-space maneuvering by way of radiation/conduction heat flow control.

While various silicon nanocomposites with their low thermal conductivity have received much attention for thermoelectric applications, the effects of inclusion interface and shape on thermal transport remain unclear. Here, we investigate thermal transport properties of silicon nanocomposites, in which metal silicide inclusions are periodically arranged within silicon. Using the known phonon dispersion relations and the diffuse mismatch model, we explore the effects of different silicide-silicon interfaces, and using Monte Carlo ray tracing simulations, we explore the effects of silicide inclusion shapes. Our investigations show that the thermal conductivity of silicon nanocomposites can be reduced to the range of nanoporous silicon of the same geometry, depending on the interface density, crystal orientation, and acoustic mismatch. For instance, CoSi2 inclusions of [111] orientation can reduce the nanocomposite thermal conductivity more effectively than inclusion materials with lower intrinsic thermal conductivity, such as NiSi2, when the inclusion density is up to 12.5% with an interface density of 7.5 μm−1. Among the silicide inclusion materials investigated in this work, Mn4Si7 leads to the lowest nanocomposite thermal conductivity due to a combination of low intrinsic thermal conductivity and high acoustic mismatch. Compared to widely spaced and symmetric inclusions such as a circular shape, narrowly spaced and asymmetric inclusions such as a triangular shape are more effective in limiting the phonon mean free path and reducing the nanocomposite thermal conductivity. These findings regarding thermal transport in silicon nanocomposites with respect to inclusion interface and shape will guide optimal material designs for thermoelectric cooling and power generation.

Nanoporous structures with a critical dimension comparable to or smaller than the phonon mean free path have demonstrated significant thermal conductivity reductions that are attractive for thermoelectric applications, but the presence of various geometric parameters complicates the understanding of governing mechanisms. Here, we use a ray tracing technique to investigate phonon boundary scattering phenomena in Si nanoporous structures of varying pore shapes, pore alignments, and pore size distributions, and identify mechanisms that are primarily responsible for thermal conductivity reductions. Our simulation results show that the neck size, or the smallest distance between nearest pores, is the key parameter in understanding nanoporous structures of varying pore shapes and the same porosities. When the neck size and the porosity are both identical, asymmetric pore shapes provide a lower thermal conductivity compared with symmetric pore shapes, due to localized heat fluxes. Asymmetric nanoporous structure...