Efficient Warming Textile Enhanced by a High‐Entropy Spectrally Selective Nanofilm with High Solar Absorption

Abstract Solar and radiative warming are smart approaches to maintaining the human body at a metabolically comfortable temperature in both indoor and outdoor scenarios. Nevertheless, existing warming textiles are ineffective in frigid climates because the solar absorption of selective absorbing coating is significantly reduced when coated on rough textile surface. Herein, for the first time, high‐entropy nitrides based spectrally selective film (SSF) is introduced on common cotton through a one‐step magnetron sputtering method. The well‐designed refractive index gradient enables destructive interference effects, offering a roughness‐insensitive high solar absorptance (92.8%) and low thermal emittance (39.2%). Impressively, the solar absorptance is 9.1% higher than the reported best‐performing selective nanofilm‐based textile. As a result, such a textile achieves a record‐high photothermal conversion efficiency (82.2% under 0.6 suns, at 0 °C). This textile yields a 3.5 °C drop in the set‐point of indoor air‐conditioner temperature. Besides, in a winter morning with an air temperature of 7.5 °C, it warms up the human skin by as large as 12 °C under weak sunlight (350 W m−2). More importantly, such a superior radiative warming performance is achieved by engineering the widely used cotton without compromising its breathability and durability, showing great potential for practical applications.

process, the target current and substrate bias keep constant, and substrate temperature does not change at room temperature. Before being placed into the vacuum chamber, all SS substrates were cleaned with alcohol, acetone, and de-ionized water in an ultrasonic agitator.
The base pressure was pumped down to 5.5 × 10 -6 mtorr by a cryopump. The thickness of those films was controlled by the method that calculates the film growth rate through deposition time. The detailed deposition parameters can be seen in Table S1. The reflective Al film was deposited on common cotton to decrease IR thermal loss. Subsequently, the optimized tri-layer ZrNbMo-Al-N based absorber was deposited on the reflective Al decorated cotton.

Characterization:
The reflectance and transmittance spectra in the solar spectrum range (0.3-2.5 μm) were measured by a Perkin Elmer Lambda 950 UV/Vis/NIR Spectrometer with an integration sphere (module 150 mm). Reflectance spectra in the infrared region (2.5-17 μm) were measured on a Bruker TENSOR 27 FT-IR Spectrometer, equipped with an integrating sphere (A562-G/Q) using a gold plate as a standard for diffuse reflectance. According to experimental spectra, the normal α s and ε T values were obtained by Eqs. (S1) and (S2). where λ is the specific wavelength, R(λ) presents reflectance, and I sol (λ) is the direct normal solar irradiance which is defined according to ISO standard 9845-1, normal radiance, AM 1.5. Normal thermal emittance ε T is equally a weighted fraction, but between emitted radiation and the Planck black body distribution, I b (λ, T), at temperature T.
It is worth noting that the absorbers' reflectance spectra before and after annealing were measured at 82 °C. Accordingly, solar absorptance and thermal emittance of the absorbers at high temperatures were calculated based on the reflectance spectra.
To evaluate the solar-thermal conversion performance of cotton/Al/SSF textile in practical applications, photothermal conversion efficiency (η) was employed to quantitatively weigh the influence caused by solar absorptance (α) and high-temperature thermal emittance (ԑ), which is shown below: In formula (S3), C, I, and σ represent the solar concentration ratio, solar flux intensity (AM1.5G), and Stefan-Boltzmann constant. T represents the operating temperature, i.e., skin temperature, while T 0 represents ambient temperature.
Based on the measured spectra (reflectance and transmittance) of ZrNbMo-Al-N film deposited on glass, CODE software is utilized to calculate the optical constants (n, k) by fitting the experimental transmittance and reflectance spectra. X-ray diffraction (XRD) Water vapor transmission rate test: The test was performed using ASTM E96 with modification. A beaker filled with 50 ml distilled water was sealed by those textile samples respectively using rubber bands. The sealed beaker was then put into an environmental chamber whose temperature was kept at 25 °C and relative humidity inside at around 46%.
The beaker was weighed periodically with an electronic balance. The water vapor transmission rates were calculated from the mass loss, which was equal to the mass of evaporated water.

Supplement 2. Heat transfer model analysis
The heat transfer analysis for outdoor radiative warming is based on the one-dimensional steady-state heat transfer model, 7,8 which is carried out to determine the total heat dissipation rate of the human body wearing textile of different optical properties ( Figure S28). In this model, sunlight illumination, metabolic heat generation, thermal radiation, conduction, and convection are included to simulate the heat dissipation from the body to ambient air. Among them, the heat gain of the human body is from solar radiation and metabolic heat generation.
Heat radiation, conduction, and convection are included to simulate the heat dissipation from a clothed human body to the ambient air. The human body is assumed to be in a sedentary state with a uniform skin temperature and heat generation. All optical properties are assumed to be gray and diffuse. The skin and environment are assumed to be an ideal blackbody emitter and absorber. All radiative view factors are equal to 1. Internal scattering and selfabsorption effects are neglected within the cloth. It is assumed that the absorption and emission profile is linear within the cloth. The primary unknown variables in this model are the inner surface textile temperature , the outer surface textile temperature , and the environment temperature .
The energy balance at the skin surface: The energy balance at the textile outer surface: Thermal radiation of textile can be divided into thermal radiation to the ambient environment ( in q ) and outer space ( out q ) through an atmospheric window.
Where () is the transmittance of the atmospheric window.
To determine the temperature profile within the textile, heat conduction and radiative heat transfer must be included in the heat transfer analysis. If a differential volume element is taken within the cloth, the heat equation will take the following form, where t  is the textile thermal conductivity and is the net radiative transfer within the textile. The net radiative heat transfer will consist only of incident radiative absorption and outgoing radiative emission as follows, We assume the absorption and emission profile to be linear as follows, where A and B are unknown coefficients that will depend on the boundary conditions assumed for each radiative heat flux, and the boundary conditions are as follows, Therefore, the final temperature relation can be derived,