A bioinspired solution for spectrally selective thermochromic VO_2 coated intelligent glazing

Abstract

We present a novel approach towards achieving high visible transmittance for vanadium dioxide (VO(2)) coated surfaces whilst maintaining the solar energy transmittance modulation required for smart-window applications. Our method deviates from conventional approaches and utilizes subwavelength surface structures, based upon those present on the eyeballs of moths, that are engineered to exhibit broadband, polarization insensitive and wide-angle antireflection properties. The moth-eye functionalised surface is expected to benefit from simultaneous super-hydrophobic properties that enable the window to self-clean. We develop a set of design rules for the moth-eye surface nanostructures and, following this, numerically optimize their dimensions using parameter search algorithms implemented through a series of Finite Difference Time Domain (FDTD) simulations. We select six high-performing cases for presentation, all of which have a periodicity of 130 nm and aspect ratios between 1.9 and 8.8. Based upon our calculations the selected cases modulate the solar energy transmittance by as much as 23.1% whilst maintaining high visible transmittance of up to 70.3%. The performance metrics of the windows presented in this paper are the highest calculated for VO(2) based smart-windows.

Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

“…70 A survey of earlier work has been given in the literature. 19 We investigated the effect of AR coatings with thicknesses in the range 0 < d AR < 300 nm, characterized by a refractive index n AR = 1.5 (typical for SiO 2 and many polymers), positioned on top of VO 2 -based films with different values of z and d as specified in the first two columns in Table II. The outcomes of the calculations were visualized as trajectories in plots of T lum (τ < τ c ) vs ΔT sol and T lum (τ > τ c ) vs ΔT sol , as shown in Fig.…”

“…Below this temperature, at τ < τ c , VO 2 is semiconducting and infrared transmitting, whereas it is metallic and infrared reflecting at τ > τ c . As τ c is rather close to room temperature, VO 2 -based materials are of interest for applications on energy efficient windows, [12][13][14][15][16][17][18][19] specifically for passively modulating the solar energy throughput depending on temperature. The potential energy saving can be high, especially in warm climates.…”

“…2,3 Recent modeling studies on "nanothermochromics" 4,5 have shown that VO 2 nanoparticles can offer significantly higher values of T lum and ΔT sol than films, and these new possibilities have led to a burst of research on such nanoparticles. 6,9,19,22,25,[30][31][32][33][34] 25 This paper is outlined as follows: Section II reports on the samples and treats sputter deposition, compositional and structural data, and optical characterization and properties; this section also contains a discussion of sample quality which-as found in recent work of ours 9 -may be critically dependent on several aspects of the thin film preparation. Section III contains results and discussions for undoped and Mg-doped VO 2 thin films without and with AR coatings as well as for nanoparticle composites containing the same VO 2 -based materials.…”

Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings

“…Particularly good properties were obtained with multilayers of TiO 2 and VO 2 [99][100][101]. "Bio-inspired" cone-shaped surfaces represent another possibility to increase T lum [102]. Mg-containing VO 2 films have improved properties, which can be further enhanced by AR layers.…”

[{VOCl2(CH2(COOEt)2)}4] as a molecular precursor for thermochromic monoclinic VO2 thin films and nanoparticles

“…68 1C, which is typical of undoped VO 2 (M). 60 Tungsten doped films exhibited a reduction in MST, to ca. 50 1C.…”