High Average Reflectance Research Articles

Passive daytime radiative cooling composite films with super-amphiphobic, friction-resistant and flame-retardant properties

Ensuring prolonged performance is a challenge in promoting passive daytime radiative cooling materials (PDRCMs) for addressing energy depletion. A single-layer inlay-structured passive daytime radiative cooling composite film (PDRCCF) was introduced with hexagonal boron nitride (h-BN) in polydimethylsiloxane (PDMS) and randomly semi-embedded fluorination-modified barium sulfate (M-BaSO4) particles on the substrate surface. The optimal sample demonstrated high average reflectivity of 94.73% in the solar spectrum (SS), average emissivity of 95.96% in the mid-infrared wavelength range (MIWR) (2.5–25 μm) and average daytime temperature drop of 4.21°C in cloudy weather, and also exhibited super-amphiphobic, friction-resistant and flame-retardant properties due to the h-BN and M-BaSO4 particles. This work offers a significant strategy for advancing PDRCMs.

Daytime radiative cooling aerogel with favorable amphiphobic surface properties

Passive daytime radiative cooling technology without any electricity input offers great promise to mitigate carbon emissions and greenhouse effects. As a type of daytime radiative cooling emitters (DRCEs), daytime radiative cooling aerogel attracts a lot of attention and has great advantages in minimizing parasitic solar absorption and reducing environmental heat gain. However, designing daytime radiative cooling aerogel with amphiphobic surface properties is still challenging. Herein, a novel daytime radiative cooling aerogel based on eco-friendly bacterial cellulose (BC) as the skeleton, modified barium sulfate (BaSO4) as functional particles and fluoride coating as amphiphobic surface modifier was developed by simple freeze-drying and spraying. The optimal daytime radiative cooling aerogel, designated as M-BC/BaSO4-14 (M-BC/BaSO4-14 refers to the modified BC/BaSO4 aerogel), exhibited a high average reflectivity (95.6%) and excellent average emissivity (98.1%). The M-BC/BaSO4-14 achieved a mean temperature drop of 6.64°C and maximum temperature drop of 13.30°C on a cloudy day, and a mean temperature drop of 8.49°C and maximum temperature drop of 12.34°C on a clear day. It should be noted that the temperature drop referred to the temperature inside the measurement box (called the inner ambient temperature throughout this article) not the true ambient temperature. Meanwhile, the M-BC/BaSO4-14 aerogel exhibited favorable amphiphobic surface properties (water contact angle of about 151°, ethylene glycol contact angle of 141° and N-Hexadecane contact angle of 139°) and daytime radiative cooling performance before and after self-cleaning changed insignificantly. This work provides a viable strategy for the simultaneous improvement of the spectral properties and amphiphobic surface properties of the DRCEs.

Scalable, flame-resistant, superhydrophobic ceramic metafibers for sustainable all-day radiative cooling

Passive daytime radiative cooling (PDRC), as a strategy to dissipate heat through an atmospheric transparency window (ATW) to outer space without any extra energy consumption, has been recently considered as a novel approach for global net-zero emissions. However, limited to expensive manufacturing, poor thermal/chemical stability, or insufficient weather-resistance, the development of a PDRC building material for long-term outdoor usages still remains a challenge. Here, a scalable superhydrophobic silica metafibers (sh-SMF) was fabricated via an electrospinning process combined with the fluorosilane-modification on fiber surface. The optically engineered sh-SMF could attain an extremely high average reflectivity (∼97 %) with near-zero absorption in the solar spectral region, due to the multiple backscattering at the fiber/air interfaces. In addition, the sh-SMF possessed a high average emissivity (∼90 %) in ATW, originated from the strong phonon resonances of the abundant Si-O bonds. Thus, the optimal sh-SMF realized a sub-ambient cooling performance of 6 °C (4 °C in nighttime) and the maximum cooling power of 112 W/m2 (87 W/m2 in nighttime) under a solar irradiance of ∼790 W/m2. Besides, the temperature decline for the sh-SMF-covered building and vehicle models could also achieve 12.7 °C and 17 °C under sunlight, respectively. Noteworthily, the ceramic sh-SMF could withstand high temperatures over 1200 °C, which might effectively prolong the time for resident to evacuate from buildings in fireground situation. Moreover, the superhydrophobic surface (contact angle=155°) of sh-SMF demonstrated attractive self-cleaning and anti-mildew properties. Furthermore, the excellent weather resistance against acid rain and ultraviolet exposure endowed the sh-SMF with long-term cooling performance. Finally, the sh-SMF with above mentioned properties opens a path for future energy-efficient and sustainable architectural applications.

Cryogenic mid-wave infrared hyperspectral Fabry-Perot filter based on a tensile-strained single-layer subwavelength grating mirror.

We report here the first demonstration of a cryogenic mid-wave infrared (MWIR) hyperspectral fixed-cavity Fabry-Perot filter based on a suspended tensile-strained single-layer 2-D subwavelength grating (SWG) mirror. Optical design optimization of the 2-D SWG mirror and parameter tolerance study are performed. For the first time, process control of grating air-hole sidewall angle and the grating air-hole fill-factor fabrication error caused by e-beam lithography electron-scattering effect is reported. At 80 K, namely the operating temperature of MWIR photodetectors, the as-fabricated suspended 2-D SWG mirror has achieved excellent surface flatness with a slight center-to-edge bowing of 15 nm over a 1-mm2 large mirror area and a high average reflectivity of 0.97 across a wavelength range of 3.72-5 µm, which represents an unprecedentedly wide fractional bandwidth Δλ/λc of 30%. The cryogenically cooled Fabry-Perot filter exhibits an unrivaled high spectral resolution of 10 nm that far exceeds the optical requirement for MWIR hyperspectral imaging applications.

Novel NaFeTiO4 cool pigment with high-infrared reflectance and low thermal conductivity

High near-infrared (NIR) reflectivity and low thermal conductivity are desired in cooling pigments used in exterior coating of buildings. In this work, brown-red whisker-like sodium iron titanate pigment were synthesized using Na2CO3, TiO(OH)2 and Fe2O3 as the raw materials and NaCl as the flux. We explored the optimal preparation conditions of sodium iron titanate, and investigated the near-infrared reflection characteristics, thermal conductivity and thermal stability of sodium iron titanate. The results show that single-phase sodium iron titanate whiskers can be obtained after calcination at 900 °C for 4 h. The growth of sodium iron titanate whiskers conforms to the series-parallel growth mechanism, and the well-developed sodium iron titanate whisker has a high average reflectivity (81.43%) in the near-infrared band of 780–2500 nm. The sodium iron titanate whisker has a low thermal conductivity (0.136 W/mK, at room temperature), which is mainly attributed to the inherent tunnel crystalline structure. In addition, sodium iron titanate has good thermal stability in the temperature range from room temperature to 900 °C. The present brown-red NaFeTiO4 whisker is expected to be a candidate pigment for building energy conservation.

Broadband, High-Temperature Stable Reflector for Aerospace Thermal Radiation Protection.

A simple and thermally stable photonic heterostructure exhibiting high average reflectivity (⟨R⟩ ≈ 88.8%) across a broad wavelength range (920-1450 nm) is presented. The design combines a thin, highly reflective and broadband metallic substrate (Ta) with an optimized dielectric coating (10 layers) to create an enhanced reflector with improved optical and thermal properties compared to its constituents. The heterostructure exhibits temperature-reversible reflective properties up to 1000 °C. In order to take advantage of the high reflectivity and temperature stable properties of this coating, in a wide range of non-photonic composite materials, we have fabricated heterostructure platelets as additives. By impregnating these additives into other types of materials, their response can be photonically enhanced. Platelets of such a heterostructure have been introduced inside an organic matrix toincreaseitsbroadband reflection performance. The platelet-impregnated matrix displays an average reflectivity improvement from 5% to an average of 55% over a 1000 nm range, making it a suitable additive for next generation thermal protection systems (TPS).

Experimental investigation on spectral splitting of photovoltaic/thermal hybrid system with two-axis sun tracking based on SiO2/TiO2 interference thin film

In a “thermally decoupled” PV/T system, photovoltaic and thermal absorbers are allowed to operate at different temperatures to utilize full-spectrum sunlight. In this study, a two-axis tracking PV/T system with a SiO2/TiO2 interference thin film (irradiated by natural sunlight) was designed and fabricated initially. The SiO2/TiO2 interference thin film designed by the authors achieved a high average reflectance (R ≥ 96.8%) for PV bands, with a high average transmittance (T ≥ 85%) at 1100–2500 nm. The photoelectric, photothermal, and overall efficiencies were tested and analyzed theoretically using the first and second laws of thermodynamics. Outdoor testing indicated that PV cells with SiO2/TiO2 interference thin film can decrease by 3.0 K, and that the power generation and efficiency of the PV cells can be improved by 10% and 0.65%, respectively. The overall energy and exergy efficiency of the PV/T system based on SiO2/TiO2 interference thin film can reach 22.72% and 18.81%, i.e., 4.94% and 1.03% higher than those without a SiO2/TiO2 interference thin film, respectively. The calculated payback time was 17 years for this PV/T system.

One dimensional photonic crystal based multilayer film with low IR and visible signatures

A multilayer film with high reflection wavebands and specific visible appearance has been designed based on principles of photonic crystals and optical interference. The angle and thickness sensitivities of this film, which is fabricated with germanium and zinc selenide, were studied and discussed. This multilayer film possesses a high average reflectance between 3 and 5 μm (92.1%) and 8–14 μm (84.2%) wavebands, thereby contributing to a low emissivity in those wavebands according to Kirchhoff's law of thermodynamics. As a result, IR energy from a hot object covered with this film would be suppressed, and low IR signature is expected to be realized. Chromaticity diagram shows that the chromaticity coordinates of this film is close to those of real green plants, which will make it more difficult to distinguish this film from green plant background. Combining the above features, this photonic crystal based film is promising to realize low IR and visible signatures under appropriate conditions.

Design, fabrication and characterization of a thin infrared-visible bi-stealth film based on one-dimensional photonic crystal

Based on the principle of photonic band gap and thin film equal inclination interference, a one-dimensional photonic crystal was designed. This material aims to achieve a specific spectrum within visible light range (present a color close to that of green plant) and infrared wavebands (mainly involves infrared atmospheric window). It was then fabricated with germanium and zinc selenide. The infrared spectral tests indicate the one-dimensional photonic crystal possesses a high average reflectance between 3-5μm (94.3%) and 8-14μm (85.5%) wavebands, thereby contributing to a low emissivity in those wavebands according to Kirchhoff’s law of thermodynamics. As a result, infrared energy would be controlled in this way and infrared stealth is expected to be realized. A chromaticity diagram shows that the chromaticity coordinates of this photonic crystal is close to that of real green plant, which is conducive to the blending of them. Combining the above two features, this PC is promising to realize infrared-visible bi-stealth under appropriate circumstances.

Effective strategy for visible-infrared compatible camouflage: surface graphical one-dimensional photonic crystal.

Herein, a novel design concept of a surface graphical photonic crystal (SGPC) has been proposed as an effective strategy to achieve angle-insensitive visible-infrared compatible camouflage. The SGPC, designed as a quasi-periodic Ge/ZnS photonic crystal following an arithmetic sequence in the physical thickness for each period, possesses a functionalized ZnS surface consisting of lithography-fabricated mosaic patterns with various etching depths. Our experiment data demonstrate the excellent infrared camouflage capability of the SGPC with a high average reflectance of 92.7% (surface emissivity ϵ=0.07) in 8-14μm, and related simulations further reveal the satisfying angle-insensitive reflection characteristic with a maximum effective relative photonic bandgap δBW=91.3% in 0°≤θ≤60°. Besides, the irregular mosaic patterns with various etching depths constitute a colorful digital camouflage on the surface of the SGPC, realizing an outstanding optical camouflage capacity without distinct angle dependency (dominant wavelength shift δλd=|λθ-λ0|/λ0≤2.33%).

Maximum length sequence dielectric multilayer reflector

This paper explores the wavelength-dependent reflectivity of alternating high and low refractive index multilayers with a thickness profile defined by a pseudo-random, maximum length sequence (MLS). An MLS contains all possible combinations of a binary sequence save one; thus, a multilayer with an MLS profile contains a superposition of a broad range of periods. The range of periodicities in an MLS multilayer should make these systems more effective broad wavelength reflectors as compared to purely periodic counterparts. We compute the reflection characteristics of MLS and periodic dielectric sequences at visible wavelengths over a range of incident angles using the transfer matrix method (TMM), a recursive multilayer calculation method. The materials SiO2 and TiO2 are chosen as the low and high refractive index materials, respectively, because these materials are commonly used in optical multilayers and because their wavelength-dependent refractive index is well known. Our results show that it is possible to create an MLS structure with high average reflectivity across the entire visible spectrum (400 nm – 700 nm) at all incident angles and polarizations. Finally, we compare the reflection characteristics of dielectric multilayers with metallic reflectors whose refractive index is based on a Brendel-Bormann (BB) model. The comparison shows that a seventh order MLS aperiodic multilayer exhibits slightly higher average reflectivity over the visible spectum than silver or aluminum metallic reflectors.

Metal-Based Graphical SiO₂/Ag/ZnS/Ag Hetero-Structure for Visible-Infrared Compatible Camouflage.

A brand-new approach to realizing visible-infrared compatible camouflage is proposed based on a metal-based graphical hetero-structure (MGHS) SiO2/Ag/ZnS/Ag. For different thicknesses (20, 40, and 60 nm) of color-controlling sub-layer, high-contract and large-span structure colors (yellow, navy, and cyan) were observed due to reintroducing constructive interference with a matching intensity of reflected waves. Ultra-low infrared emissivity values of 0.04, 0.05, and 0.04 (with high average reflectance values of 95.46%, 95.31%, and 95.09%) were obtained at 3–14 μm. In addition, the well-performing trisecting-circle structure further indicates that it is feasible to design on-demand compatible camouflage patterns using the easily-prepared MGHS.

The microstructure of white feathers predicts their visible and near-infrared reflectance properties.

Research on the optical properties of animal integuments, including fur, feather, skin and cuticle, has focussed almost exclusively on animal-visible wavelengths within the narrow range of 300–700 nm. By contrast, the near-infrared (NIR) portion of direct sunlight, spanning 700–2600 nm, has been largely ignored despite its potentially important thermal consequences. We quantified variation in visible and NIR reflectance and transmission for white body contour feathers of 50 bird species, and examined how well they are predicted by feather macro- and micro-structural morphology. Both visible and NIR reflectance of the feathers varied substantially across species. Larger, thicker, and sparser feathers that are characteristic of larger species, and feathers with rounder barbs and more closely spaced barbules, had high average reflectance, particularly within avian-visible wavelengths (300–700 nm). Feathers with rounder barbs and more closely situated barbules also had high average reflectance, particularly for NIR wavelengths. Barb roundness and barbule density were the only predictors of NIR reflectance after accounting for variation in visible reflectance and body size. Our results highlight the potential for adaptive variation in NIR reflectance mediated by feather structure, which may inform the design of functional materials to control light and heat.

One-dimensional photonic crystal with spectrally selective low infrared emissivity fabricated with Te and ZnSe

To restrain the infrared radiation from high temperature objects to decrease the probability of being discovered by infrared detectors operating in the mid- and far-infrared atmospheric windows, we design a one-dimensional heterostructure photonic crystal (PC) using low-cost coating materials Te and ZnSe, and test its reflection spectra and radiant temperature. The tested results show that this PC has high average reflectance in 3- to 5-μm and 8- to 14-μm wavebands, which is 86.72% and 72.91%, respectively, and the corresponding emissivity is 0.072 and 0.194, respectively. The radiant temperatures of the PC are always lower than those of the background, with the maximal difference of the radiant temperature being 31.97°C corresponding to a background radiant temperature of 75.64°C. The study confirms that the deposited PC can effectively decrease the infrared radiation in mid- and far-infrared bands.

Realization of compatible stealth material for infrared, laser and radar based on one-dimensional doping-structure photonic crystals

To inhibit the radiant infrared energy between 8 and 14μm, which is the infrared atmospheric window, and decrease the echo power of detecting laser and radar, to achieve compatible stealth, a doping structural one-dimensional photonic crystal (1-D PC) with Ge, ZnSe and Si was fabricated; and then combine it with radar absorbing material (RAM) to make a compound. After that, the reflection spectra of this compound was tested, and the result shows a high average reflectance (89.5%) in 8–14μm waveband, and a reflective valley (39.8%) in the wavelength of 10.6μm, which is the wavelength of CO2 laser; and the reflectance in radar band shows that at high frequency, especially between 7.8 and 18GHz, the radar power is strongly absorbed by this material and the reflected energy attenuate over 10dB within the range from 11.1GHz to 18.3GHz, even 24.5dB to the most in the frequency of 14.6GHz.

TiN x thin films for energy-saving application prepared by atmospheric pressure chemical vapor deposition

Non-stoichiometric titanium nitride (TiN x ) thin films were fabricated on glass substrates by atmospheric pressure chemical vapor deposition (APCVD) using titanium tetrachloride and NH 3 as precursors. The relationships between x of TiN x thin films and the structure, electrical and optical properties of films were investigated. The results showed that the crystalline property of over-stoichiometric films was better than that of sub-stoichiometric films. Electrical resistivity of films declined with increasing x from 0.83 to 1.25. Spectrophotometer curves of films demonstrated that the plasma wavelength ( λ p) shifted from the near-infrared region to the visible region and the peak value of transmittance increased with increasing x. The TiN x thin film with x = 1.25 had low sheet resistance of 5 Ω/sq, high average reflectivity of 95% in the mid-far infrared region, 53% average reflectivity in the near-infrared region, and low average reflectivity of 10% in the visible region, indicating it can be a promising candidate as energy-saving films combining the function of solar control and low emission.

Incident Light Metering in Computer Graphics

Every rendering process consists of two steps. The first is the computing of luminance values by methods like ray tracing or radiosity, and the second step is the mapping of the computed values to values appropriate for displaying. In the last years, as alternative to simple linear scaling which maps the average value to the medium luminance, some new ways of mapping were introduced. These new methods are based on photography analogies and on human vision models. All existing methods follow, implicitly or explicitly, the reflected light metering principle. The method introduced in this paper is the first that follows the incident light metering used in professional photography and in the movie industry. Actually the irradiances are measured using a set of diffusors, which are placed automatically in the scene, and a linear scale factor based on these measurements is used to map the computed radiances to the display device. The diffusors act as half space integrators, they collect the light energy from all half space directions. The light comes from the primary light sources, or it is the result of various interreflections. The newly introduced method reproduces original colors faithfully even for scenes with very low or very high average reflectivity.