Ultrathin oxynitride coatings as transmissive mid-infrared emitter for passive radiative cooling

Abstract

Embodiments of the present technology may include radiative cooling devices (300). The radiative cooling devices (300) may include a substrate (302). The radiative cooling devices (300) may also include a thermal emission layer (304) of silicon-containing material disposed on the substrate (302). The thermal emission layer (304) of silicon-containing material may emit infrared radiation of wavelengths within an atmospheric infrared transmission window.

Description

PATENT Attorney Docket No.097226-1490048 (007910US) Client Ref. No.2022-050-02 ULTRATHIN OXYNITRIDE COATINGS AS TRANSMISSIVE MID- INFRARED EMITTER FOR PASSIVE RADIATIVE COOLING CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application No.63 / 552,848, filed February 13, 2024, the entire contents of which are hereby incorporated by reference for all purposes in its entirety. TECHNICAL FIELD

[0002] The present technology relates to passive radiative cooling. More specifically, the present technology relates to coatings to serve as transmissive to visible light and mid- infrared emitter materials. BACKGROUND OF THE INVENTION

[0003] Temperature regulation can be a major source of energy consumption. Currently, nearly 20% of global energy consumption can be associated with cooling. By the year 2050, global energy consumption associated with cooling may rise to 60%. A high-power demand for active cooling can be linked to an increase in global warming, and refrigerants in cooling systems can create significant carbon footprints. Space, with a temperature of about 3 Kelvin, can be a potential ideal heat sink for heat rejection. Earth has an atmosphere that can be remarkably transparent to electromagnetic radiation with wavelengths in a mid-infrared wavelength range between about 8 microns and about 14 microns. The mid-infrared wavelength range can overlap with peak wavelengths of thermal emissions from terrestrial objects at typical ambient temperatures. A sky-facing object can have access to the potential ideal heat sink of space for radiative cooling.

[0004] There is a need for improved systems and methods that can be used for passive radiative cooling. These and other needs are addressed by the present technology.BRIEF SUMMARY OF THE INVENTION

[0005] In one example, a radiative cooling device can cool objects. The radiative cooling device can include a substrate. Additionally, the radiative cooling device can include a layer of silicon-containing material disposed on the substrate. The layer of silicon-containing material can emit infrared radiation of wavelengths within an atmospheric infrared transmission window.

[0006] In another example, a method can involve cooling an object. The method can include fabricating a radiative cooling device by forming a layer of silicon-containing material on a substrate. The method can further include placing the radiative cooling device on the object. Additionally, the method can include cooling the object by emitting infrared radiation of wavelengths within an atmospheric infrared transmission window. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG.1 is a graph depicting electromagnetic radiations incident upon Earth’s atmosphere according to certain aspects of the present disclosure.

[0008] FIG.2 is a set of graphs depicting optical constants over a range of infrared wavelengths for several different ceramic thin films according to certain aspects of the present disclosure.

[0009] FIG.3 is a schematic perspective view of a device with a radiative coating according to certain aspects of the present disclosure.

[0010] FIG.4 is a schematic perspective view showing plasma-enhanced chemical vapor deposition (PECVD) or co-sputtering fabrication approaches for forming a silicon oxynitride (SiOxNy) ceramic coating for radiative cooling according to certain aspects of the present disclosure.

[0011] FIGS.5A-5B are illustrations of exemplary absorptivity / emissivity spectra of exemplary photonic structures with (SiOxNy) ceramic coatings according to certain aspects of the present disclosure.

[0012] FIGS.6A-6B are illustrations of exemplary absorptivity spectra of exemplary photonic structures with SiOxNycoatings formed by PECVD under different conditions according to certain aspects of the present disclosure.

[0013] FIGS.7A-7B are illustrations of exemplary absorptivity spectra of exemplary photonic structures with either SiN or SiO2 single layer coatings formed by sputtering according to certain aspects of the present disclosure.

[0014] FIGS.8A-8B are illustrations of exemplary Fourier transform infrared spectroscopy (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) survey spectra according to certain aspects of the present disclosure.

[0015] FIG.9 is an illustration of exemplary XPS survey spectra focusing on a silicon 2p peak according to certain aspects of the present disclosure.

[0016] FIGS.10A-10B are illustrations of exemplary XPS survey spectra focusing on a nitrogen 1s peak and oxygen 1s peak according to certain aspects of the present disclosure.

[0017] FIGS.11A-11F are illustrations of exemplary cooling characteristics of SiON layers according to certain aspects of the present disclosure.

[0018] FIG.12 is a flow chart of an exemplary process that can be implemented to manufacture radiative cooling coatings according to some examples of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0019] Radiative cooling, otherwise known as passive cooling, can involve nighttime, or more recently, daytime cooling applications. Efficient passive cooling may be achieved by engineering optical properties of materials to either i) minimize absorption of sunlight and other surrounding thermal radiations, ii) enhance emittance in a mid-infrared (IR) wavelength range, or iii) accomplish both. A variety of organic, inorganic, or hybrid material photonic structures for radiative cooling can be developed by using surface engineering and nano / micro-structured technologies. Examples of such photonic structures can include multilayer structures, microsphere-based polymeric films, microsphere-periodic arrays, metal-dielectric nanophotonic structures, double-layer nanoparticle-based coatings, aerogels, porous synthetic polymer-based coatings, and hybrid dielectric-polymer materials. Most of these photonic structures can involve complicated fabrication processes that involve multiple materials with high engineering and fabrication costs. The high engineering and fabrication costs can prohibit large-scale production and wide deployments in real applications.

[0020] Certain aspects and examples of the present disclosure relate to optically transparent ceramic single-layer silicon oxynitride (SiOxNy) composite coatings with high mid-IR emissivity for passive radiative cooling. Oxygen content can be described by , which can include values between 1.00 and 2.00, and Nitrogen content can be described by , which can include values between 0.1 and 0.8. The SiOxNycoatings can exhibit a broadband spectral emissivity of between 0.83 and 0.9 in the mid-IR wavelength range of between about 8 and about 14 microns. A single-layer of the SiOxNy coatings can have a thickness of less than or about 2 microns. Mid-IR broadband absorption characteristics of the SiOxNy coatings can be tuned by adjusting chemical intermixing of silicon oxide (SiO2) and silicon nitride (SiN) during physical or chemical vapor deposition processes. When deposited on a solar reflective substrate, the SiOxNycoatings may exhibit a cooling power of greater than or about 140while in an ambient temperature of about 27 . The single-layer SiOxNy coatings can provide radiative cooling solutions for many applications including, but not limited to, solar cell systems, automobiles, or building window systems.

[0021] In the following description, various embodiments will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified in order not to obscure the embodiment being described.

[0022] FIG.1 is a graph 100 depicting electromagnetic radiations incident upon Earth’s atmosphere according to certain aspects of the present disclosure. FIG.1 also depicts transparency of Earth’s atmosphere for various incoming (e.g. radiations within an optical window) and outgoing (e.g., radiations within an infrared transparency window) electromagnetic radiations. The electromagnetic radiation may include incoming solar radiations through an optical window within a wavelength range of between about 0.25 microns and about 2.5 microns. The electromagnetic radiation may also include outgoing thermal radiations through an atmospheric transparency window in a broad wavelength range between about 8.0 microns and about 14.0 microns. A large overlap among blackbody radiation spectra for typical ranges of outdoor temperatures (e.g., between about 290 Kelvin and about 325 Kelvin) and an atmospheric transparency window in the mid-IR wavelength range (e.g., between about 8 microns and about 14 microns) is evident in FIG.1. A photonicstructure with a high selective emissivity in the mid-IR range (see curve 102) may effectively dissipate heat out into space by radiative cooling. The photonic structure can minimize absorption of downward radiation from the atmosphere with a low emissivity outside the mid-IR transparency window. Additionally, an ideal cooling system may have zero absorptivity in a solar wavelength range to minimize photothermal absorption from the Sun, which can diminish any cooling effect if absorbed by a surface of an emitter.

[0023] FIG.2 is a set of graphs depicting optical constants over a range of infrared wavelengths for various ceramic thin films according to certain aspects of the present disclosure. The range of infrared wavelengths covers between about 2 microns and about 14 microns which includes wavelengths of an atmospheric transparency window (e.g., between about 8 microns and about 14 microns). The various dielectric thin films in FIG.2 include aluminum nitride (AlN), hafnium oxide (HfO2), titanium oxide (TiO2), aluminum oxide(Al2O3), SiN, and SiO2. The optical constants include a refractive index ( ) and an extinctioncoefficient ( ). Unlike broadband absorption of polymers, the various dielectric thin filmsshown in FIG.2 may depict narrow absorption bands due to phonon-polariton excitation associated with ionic lattice vibrations.

[0024] Kirchoff’s law of thermal radiation states that for a body of any arbitrary material emitting and absorbing thermal electromagnetic radiation at every wavelength in thermal equilibrium, a ratio of emissive power of the body to a dimensionless coefficient of absorption associated with the body can be equal to a universal function only of radiative wavelength and temperature. Thus, the absorption modes observed in FIG.2 can contribute to an increase in absorption of wavelengths in the atmospheric transparency window of the 8 microns to 14 microns range, subsequently increasing emissivity in the same 8 microns to 14 microns range. Note that an extinction coefficient curve 202 for SiN can have a broad absorption that nearly covers the entire atmospheric transparency window but can be reduced in the 8 microns to 10 microns range. Meanwhile, an extinction coefficient curve 204 for SiO2can have a narrow absorption peak that spans the 8 microns to 10 microns range. Thus, data depicted in FIG.2 suggests that a ceramic film of SiN and SiO2(such as silicon oxynitride) can form a photonic structure with a high and selective emittance in the atmospheric transparency window. Based on the extinction coefficients of pure thin films of SiN and SiO2, as fabricated single-layered SiOxNy coatings can leverage complementary contributions of vibrational modes of Si-N and Si-O in the mid-IR wavelength range to achieve broadband selective emissivity.

[0025] FIG.3 is a schematic perspective view of a device 300 with a radiative cooling according to certain aspects of the present disclosure. The radiative cooling device 300 can include a reflective substrate 302 and a thermal emission layer 304. In other examples, the radiative cooling device 300 can include a substrate that can be highly reflective, highly absorptive, or both highly reflective and highly absorptive of electromagnetic radiation with a wide range of wavelengths. The reflective substrate 302 can have a reflective surface that efficiently reflects a range of wavelengths, such as visible light. The reflective substrate 302 can also absorb certain wavelengths that may not be efficiently reflected by the reflective surface, such as visible radiations.

[0026] The thermal emission layer 304 can be a single material layer. For example, the thermal emission layer 304 can be a single-layer silicon oxynitride (SiOxNy) coating. The SiOxNycoatings may be characterized by a thickness less than or about 20 microns, less than or about 10 microns, less than or about 5.0 microns, less than or about 2.0 microns, less than or about 1.8 microns, less than or about 1.6 microns, or less, such as between about 1.6 microns and about 20 microns. The SiOxNycoating can emit infrared radiation of wavelengths within an atmospheric infrared transmission window.

[0027] The radiative cooling device 300 can be coated on a surface of an object and cool the object through radiative cooling even in daylight. The radiative cooling device 300 with additional bottom reflective layer can protect the object from heating via solar radiation. Solar radiation, particularly visible light, can be transmitted through the thermal emission layer 304 and then reflected by a reflective surface of the reflective substrate 302. The thermal emission layer 304 can emit radiation in a mid-IR range that can pass through an atmospheric transparency window into space. Other incident atmospheric radiation can be either reflected off the reflective substrate 302.

[0028] FIG.4 is a schematic perspective view showing PECVD or co-sputtering fabrication approaches for forming a SiOxNy layer of a device structure for radiative cooling according to certain aspects of the present disclosure. The PECVD approach can fabricate SiOxNy coatings with various oxygen to nitrogen ratios onto various substrates. The PECVD approach can include ammonia (NH3), silane (SiH4), and nitrous oxide (N2O) diluted with nitrogen gas as precursor gases. In embodiments, a gas flow for the silane can be maintained between about 20 standard cubic centimeters per minute (sccm)and about 30 sccm. NH3 and N2O gas flowscan be varied to keep a total gas flow constant. Different SiOxNy stoichiometries can be obtained by varying an oxygen ratio, defined in terms of gas flow rates by: (1)Oxygen ratio can be an indicator of oxygen content as well as nitrogen content within as- deposited SiOxNylayers. As previously discussed, the SiOxNyfilms can be deposited on substrates with reflective coatings. For example, SiOxNyfilms can be deposited on silicon wafers with a (100) crystal orientation and a thickness of 525 20 . In embodiments, the silicon wafers can be coated with a 100 nm thick silver film. Prior to deposition of the SiOxNy layer, a silicon oxide thin film may be deposited by E-beam evaporation to passivate the silver metal film. For example, the silicon oxide thin film can be about 85 nm thick. E-beam evaporation can be performed using commercially available evaporators, such as Temescal BJD-2000 E-beam evaporator. Exemplary gas flow parameters for various device configurations are shown in Table 1. Sample Name SiH4 flow rate N2O flow rate NH3 flow rate Oxygen ratio (sccm) (sccm) (sccm) SiOxNy (0.5) 20 40 40 0.5 SiOxNy(0.67) 30 120 60 0.67 SiOxNy (0.75) 20 60 20 0.75 SiOxNy (0.84) 20 80 15 0.842 SiOxNy(1) 30 80 0 1 Table 1. Gas flow rates during deposition of SiON films using PECVD.

[0029] The co-sputtering fabrication approach can produce SiO2-SiN ceramic films and can control a concentration of each material by varying input powers. Commercial sputterers such as an AJA Orion 300 sputterer can be used. Prior to a co-sputtering deposition process, asputtering chamber can be evacuated to a pressure lower than or about 3 10 Torr. RFpower values of 170 and 200 W may be supplied to SiN and SiO2 sputtering targets, respectively. Each of the sputtering targets may be characterized by a 3-inch diameter andcan have a purity of greater than or about 99%, such as greater than or about 99.999%. Co- sputtering deposition processes can be performed in an argon (Ar) plasma environment at a pressure of about 4 mTorr.

[0030] FIGS.5A-5B are illustrations of exemplary absorptivity / emissivity spectra of exemplary photonic structures with SiOxNy layers according to certain aspects of the present disclosure. A graph 500 in FIG.5A shows absorptivity for a photonic device that includes a SiOxNy layer. The SiOxNy layer may be about 1.57 microns thick and may be formed on a silver-coated silicon substrate by PECVD deposition with an oxygen ratio of about 0.75. An inset 510 in the graph 500 shows a cross-sectional scanning electron microscopy (SEM) image of the photonic structure with the SiOxNylayer. The as-fabricated single-layered ceramic SiOxNylayer can exhibit an average value for selective mid-IR broadband emissivity of greater than or about 0.8, greater than or about 0.82, greater than or about 0.84, greater than or about 0.86, greater than or about 0.88, greater than or about 0.9, or more in a wavelength range of between about 8 microns and about 14 microns. The as-fabricated single-layered ceramic SiOxNylayer can exhibit a very high reflectivity (indicated by low absorptivity / emissivity in FIG.5A) in a wavelength range of between about 0.25 microns and about 2.0 microns. The mid-IR emissivity can be optimized by changing the oxygen ratio during the PECVD process as well as varying a thickness of the SiOxNy layer.

[0031] A graph 502 in FIG.5B shows absorptivity for a photonic device that includes a co- SiO2-SiN ceramic layer. The co-SiO2-SiN ceramic layer may be about 1.6 microns thick and may be formed on silver-coated silicon by co-sputtering SiO2 and SiN targets. An inset 520 in the graph 502 shows a cross-sectional SEM image of the photonic structure with the co-SiO2- SiN ceramic layer. Oxygen content in as-deposited co-sputtered co-SiO2-SiN films can be varied by changing a deposition rate of one material by changing input power to each target compared to the other. Based on individual deposition rates, the roughly 1.6 micron thick co- SiO2-SiN ceramic layer containing about 72% SiO2 compound within the ceramic layer can show an average mid-IR emissivity of greater than or about 0.75, greater than or about 0.77, greater than or about 0.79, greater than or about 0.81, greater than or about 0.83, or more in the wavelength range of between about 8 microns and about 14 microns., co-SiO2-SiN ceramic layer may maintain high reflectivity in the wavelength range of between about 0.25 microns and about 2.0 microns.

[0032] FIGS.6A-6B are illustrations of exemplary absorptivity spectra of exemplary photonic structures with SiOxNy layers formed by PECVD under different conditions according to certain aspects of the present disclosure. A graph 600 in FIG.6A shows absorptivity for photonic devices that include a SiOxNylayer. The SiOxNylayer in each of the photonic devices is about 1.5 microns thick and is formed on a silver coated silicon substrate by PECVD deposition with differing oxygen ratios from about 0.5 to about 1.0. The mid-IR emissivity can be optimized by changing the oxygen ratio in the precursor gases during the PECVD process. A graph 602 in FIG.6B shows absorptivity for photonic devices that include a SiOxNylayer. The SiOxNylayer in each of the photonic devices is formed on a silver-coated silicon substrate by PECVD deposition an oxygen ratio of about 0.5 with differing thicknesses due to varied deposition times. Results in FIG.6B indicate that the deposition time corresponding to the thickness of as-deposited SiOxNy layer can also be important for achieving selectivity in mid-IR selectivity.

[0033] FIGS.7A-7B are illustrations of exemplary absorptivity spectra of exemplary photonic structures with either SiN or SiO2layers formed by sputtering according to certain aspects of the present disclosure. A graph 700 in FIG.7A shows absorptivity for photonic devices that include a SiO2 layer with varying thicknesses of about 0.53 microns, about 0.82 microns, or about 1.40 microns. A graph 702 in FIG.7B shows absorptivity for photonic devices that include a SiN layer with varying thicknesses of about 0.62 microns, about 0.87 microns, or about 1.37 microns. Pure SiO2 layers can show a narrow absorption peak centered at about 10 microns. Pure SiN coatings can show a narrow absorption peak shifted to a longer wavelength of about 13 microns compared to the SiO2 peak. By introducing a low concentration nitrogen within co-SiO2-SiN ceramic layers, a narrow band absorption can be broadened to achieve a higher mid-IR emissivity while maintaining a desired spectral selectivity.

[0034] FIGS.8A-8B are illustrations of exemplary FTIR spectra and X-ray photoelectron spectroscopy (XPS) survey spectra according to certain aspects of the present disclosure. A graph 800 in FIG.8A shows the FTIR spectra for various coatings. A comparison of the FTIR spectra demonstrates an optical broadening in SiOxNyand co-SiO2-SiN thin films, as highlighted by shaded section 804 of the graph 800. In a case of pure SiN, an absorption band at about 800 cm-1highlighted by shaded section 806 of the graph 800 can correspond to a Si- N stretching mode, while a band at about 1000 cm-1highlighted by shaded section 808 can be attributed to Si-O stretching dominant in pure SiO2films. In cases of SiOxNyand co-SiO2-SiN ceramic films, a broader absorption band appears in a range between about 800 cm-1and about 1000 cm-1owing to a combinatorial effect of Si-N and Si-O stretching modes. The broadening may confirm chemical intermixing during fabrication, which can be important for achieving a desired emissivity while keeping film thickness to a minimum. Increasing the film thickness may have a negative impact on emissive selectivity of the films and may even induce higher possibility of delamination.

[0035] Elemental chemical states in SiOxNy and co-SiO2-SiN ceramic films can beexplored using XPS. The XPS survey spectra of various SiOxNy, pure SiN, and co-SiO2-SiNfilms shown in graph 802 of FIG.8B can demonstrate a presence of intense peaks from Si, N, C and O. All the samples appear to show a C 1s peak with similar intensity possibly due to a presence of a hydrocarbon-like pollutant on any film surface. As observed in the FTIR spectroscopic data of graph 800, there does not appear to be any carbon in the bulk film.

[0036] FIG.9 is an illustration of exemplary XPS survey spectra focusing on a silicon 2p peak according to certain aspects of the present disclosure. Graph 900 of FIG.9 shows silicon 2p level photoelectron spectra, indicating a presence of three peaks that can be attributed to binding energies associated with silicon 2p in silicon (99.4 eV), silicon nitride (101.7 eV), and silicon oxide (103.2 eV). After purging higher flow rates of SiH4and N2O gases to increase oxygen ratio from about 0.5 to about 1.0, an as-fabricated film can show stronger signals for Si+3(103.2 eV) and Si+0(99.4 eV), which may indicate a significant enrichment in silicon oxide and silicon, respectively. In addition, a stronger dominance of silicon 2p in silicon oxide (103.2 eV) for co-SiO2-SiN may indicate an abundance of Si+3(103.2 eV) in comparison with Si+0(99.4 eV) and Si+2(101.7 eV).

[0037] FIGS.10A-10B are illustrations of exemplary XPS survey spectra focusing on a nitrogen 1s peak and oxygen 1s peak according to certain aspects of the present disclosure. A relative intensity of the 101.7 eV signal (FIG.9) in the XPS silicon 2p, as well as an intensity of nitrogen 1s at 397.5 eV (FIG.10A), decreases in a sequence of samples SiOxNy (oxygen ratio of about 0.5), SiOxNy (oxygen ratio of about 0.75), and SiOxNy (oxygen ratio of about 1), when compared to pure SiN film. These decreasing trends can indicate a decrease in Si-N bond concentration and nitrogen content in as-fabricated films. In addition, an absence of any peak around 404 eV in FIG.9A may confirm a lack of metastable O-N-O bonding, and a peak shift to higher energies can confirm a presence of stable Si2-N-O and N-(SiOx)3 configurations. In order to identify an oxygen chemical state in composite samples, adeconvolution for oxygen 1s is shown in FIG.9B. Considering a relatively high content of carbon in the near-surface region, a main oxygen 1s peak at about 532.5 eV can be attributed to Si-O bonds, while peaks at about 531.3 eV and about 533.5 eV can correspond to C=O and C-O functional groups, respectively. These results may indicate slightly higher concentration of oxygen in co-sputtered films compared to those fabricated through PECVD.

[0038] FIGS.11A-11F are an illustration of exemplary cooling characteristics of SiON layers according to certain aspects of the present disclosure. FIG.11A shows an increasingtrend of normalized emissivity for pure SiN, SiO2, SiOxNy, co-SiO2-SiN thin films in awavelength range of about 8 microns to about 14 microns. For the SiOxNyfilms fabricated by the PECVD approach, an increase in emissivity is observed along with an increase in oxygen content. With an intrinsic advantage of design simplicity, single-layer SiOxNyand co-SiO2- SiN thin film coatings can exhibit strong selectivity and emissivity compared to existing multilayer films and photonic structures. Based upon optical properties of optimized SiOxNyand co-SiO2-SiN ceramic films, radiative cooling performance can be assessed by employing analytical calculations with experimentally determined emissivity / absorptivity data as shown in FIG.11B. Radiative cooling performance can be evaluated through a temperature difference between the emitter surface and ambient, . A more negative can represent a higher net cooling flux, while a positive can stand for an above-ambient cooling situation. FIG. 11C shows net cooling power ( ) and temperaturedifference ( ) as a function of various ambient temperatures ( ) undermeasured emissivity profiles , of spectrally selective emitters SiOxNy and co-SiO2-SiN.Under ideal insulation for conduction and convection, a SiOxNyfilm having an oxygen ratio of about 0.75 with a highest selective emissivity of about 0.9 can also show a highest net cooling power of greater than or about 140 W / m2when ambient temperature is about 27oC( =300K). For co-SiO2-SiN thin film, the net cooling power may be beyond 110 W / m2 at27 oC. Owing to spectral selective emissivity and high , all thin films of SiOxNyand co-SiO2-SiN can contribute to sub-ambient cooling, as plotted in FIG.11D. Under realistic environmental conditions, by considering non-radiative heat transfer coefficient in a range of between about 5 W / m2-K and about 15 W / m2-K, a net can be plotted for SiOxNy(0.75) (FIG.11E) and co-SiO2-SiN thin films (FIG.11F). Facile fabrication approaches and design simplicity of single-layer spectrally selective ceramic emitter can pave a way for many practical applications, such as solar cells (PV and TPV) cooling, energy- efficient vehicle and glass protective coatings, and temperature-sensitive electronic devices.

[0039] FIG.12 is a flow chart of an exemplary process that can be implemented to manufacture radiative cooling devices according to some examples of the present disclosure. FIG.12 may describe operations previously discussed with respect to the components in FIG. 3. Accordingly, any of the following operations of method 1200 may include features or characteristics of radiative cooling device 300 previously with regard to FIG.3.

[0040] Operation 1202 of method 1200 may include fabricating a radiative cooling coating by forming a layer of silicon-containing material on a substrate with a reflective layer. The layer of silicon-containing material can be characterized by a thickness of less than or about 20 microns. The layer of silicon-containing material can include silicon oxynitride. The silicon oxynitride layer can be formed by a PECVD approach or by a co-sputtering approach. The PECVD approach can fabricate SiOxNycoatings with various oxygen to nitrogen ratios onto various substrates. The PECVD approach can include a gas mixture of ammonia (NH3), silane (SiH4), and nitrous oxide (N2O) diluted with nitrogen gas as precursor gases. However, any other nitrogen-containing gases, silicon-containing gases, or oxygen-containing gases may also be used. A gas flow for SiH4can be maintained between about 20 sccm and about sccm. NH3 and N2O gas flows can be varied to keep a total gas flow constant. Different SiOxNy stoichiometries can be obtained by varying an oxygen ratio. The oxygen ratio is defined in equation (1) previously discussed above. Radiative cooling devices formed from the PECVD approach can exhibit high emissivity values for wavelengths associated with an atmospheric infrared transmission window. A standard atmospheric infrared transmission window can be defined by a range of infrared wavelengths between 8 microns and 14 microns. For example, radiative cooling devices formed from the PECVD approach can exhibit an average emissivity of about 0.9 for the 8 microns to 14 microns range of wavelengths.

[0041] The co-sputtering fabrication approach using SiO2 and SiN targets can produce SiO2-SiN ceramic films and can control a concentration of each material by varying input powers. Prior to a co-sputtering deposition process, a sputtering chamber can be evacuated toa pressure lower than or about 3 10 Torr. RF power values of about 170 and about 200 Wcan be supplied to SiN and SiO2 sputtering targets, respectively. Each of the sputtering targets may be characterized a 3-inch diameter and can have a purity of greater than or about 99%, such as greater than or about 99.999%. Co-sputtering deposition processes can be performed in an argon (Ar) plasma environment at a pressure of about 4 mTorr. Radiative coolingdevices formed from the PECVD approach can exhibit an average emissivity of about 0.83 for the 8 microns to 14 microns range of wavelengths.

[0042] Operation 1204 of method 1200 can include placing the radiative cooling coating on an object. The object can be a window of a building, window of an automobile, or some surface. The object can be outdoors and exposed to sunlight. The substrate of the radiative cooling device can be silver / aluminum-coated silicon. The silver / aluminum-coated silicon can protect the object from absorbing radiation by reflecting electromagnetic radiation transmitted by the layer of silicon-containing material in the radiative cooling device. For example, the layer of silicon-containing material can transmit solar radiation and the silver coating of the silicon substrate can reflect the solar radiation. Solar radiation can be described by a range of electromagnetic wavelengths including a range from about 0.25 microns to about 2.5 microns. In some examples, a surface of the object can serve as the substrate of the radiative cooling device.

[0043] Operation 1206 of method 1200 can include cooling the object by emitting infrared radiation of wavelengths within the atmospheric transmission window. The layer of silicon- containing material can emit radiation in a mid-IR range that can pass through an atmospheric transparency window into space. Other incident atmospheric radiation including solar radiation can be either reflected off the thermal emission layer or the substrate. Thus, the radiative cooling device can be effective at cooling the object even during the daytime.

[0044] The order of the operations of method 1200 presented in the examples above can be varied. For example, operations can be re-ordered, combined, and / or broken into sub- operations. Certain operations of method 1200 may also be performed in parallel.

[0045] Examples

[0046] In order to characterize sample, absorptivity, reflectivity, and transmissivity of samples were recorded in a range of about 250 nm to about 2500 nm using an ultraviolet- visible-near infrared (UV-vis-NIR) spectrophotometer (e.g., LAMBDA 1050). A specular reflectance in a wavelength range of about 2.5 microns to about 20 microns was recorded using a specular reflectance module (e.g., Bruker VERTEX, 80v model) at an incident angleof 10 o. A normalized solar absorptance ( ) and thermal emittance ( ) was calculated fromEquations (2) and (3) below by using the measured spectral reflectance data...1 (2)..., 1 (3).,In Equations (2) and (3), is a radiation intensity in AM 1.5 standard spectrum, ,is a radiation intensity in a blackbody spectrum at a certain surface temperature , and is a measured spectral reflectivity. The AM 1.5 standard spectrum refers to two standard terrestrial solar spectral irradiance spectra. The two spectra define a standard direct normal spectral irradiance and a standard total spectral irradiance. High- resolution scanning electron microscopy (measured with e.g., a Nova NanoSEM 650 FEI) was used to check the top morphology and cross-sectional thicknesses. X-ray photoelectron spectroscopy (XPS) (measured with e.g., a Thermo ESCALAB 250Xi) was used to characterize the elemental compositions in the ceramic films.

[0047] Cooling Performance Evaluation

[0048] Selective emitter based photonic structures, when given access to a clear sky, can reach a sub-ambient temperature even under direct sunlight. A cooling power of the selectiveemitter can be estimated using a power balance Equation (4).(4)is the power that a surface of the emitter surface absorbs from the sun. can bedetermined from Equation (7) below. is power emitted from the emitter due to thermal emission as a function of surface temperature. can be determined from Equation (5) below. is a power that layers absorb due to thermal emission from the atmosphere. can be determined from Equation (6) below. is power absorbed from surroundings due to conduction and convection as a result of a temperature difference between the emitter and an environment. d, , (5)Here, is a solid angle, denotes an angle between a direction of the solid angle and anormal direction of the surface, ( , ) is an emissivity of the object at a wavelength andangle , and IBB(T, ) is spectral irradiance of a blackbody per unit area of emitting surface. d, , , (6)Equation (6) describes a portion of downwards radiation from the atmosphere that isabsorbed by the emitter. Here, the emissivity of the atmosphere is , 1 / , with being a transmission coefficient of the atmosphere in a zenith direction.The incident solar power absorbed by the cooler is expressed as: ,(7)I , used to calculate from Equation (7), is a ASTM G173-03 AM 1.5 global tiltspectrum. Atmospheric transmittance data used for the model is for a water vapor column of 5 mm and an air mass of 1.5. Table 2 below summarizes power balance results for various SiON ceramic coatings. Sample Tssat Tssat Tssat Tssat T / m ) (W / m ) (W / m )ambTambT T (W / m2) (W2 2 2amb amb(300K) (300K) (300K) (300K) hc=0 hc=5 hc=10 hc=20 W / m2.K W / m2.K. W / m2.K. W / m2.K SiOxNy 7.89 178.50 15.50 155.11 -108.00 -21.75 -13.00 -7.00 (0.75) SiOxNy 14.61 128.30 9.90 103.79 -86.00 -15.25 -8.75 -4.75 (0.5) SiOxNy14.69 137.00 10.30 112.01 -87.50 -16.25 -9.25 -5.00 (1) Co- 39.21 168.15 12.78 116.16 -64.00 -15.75 -9.25 -5.25 SiO2- SiNTable 2. Power balance results of various SiON ceramic coatings.

[0049] While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, it should be understood that the present disclosure has been presented for purposes of example rather than limitation, and does not preclude inclusion of such modifications, variations, and / or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art. Indeed, the methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes in the form of the methods and systems described herein may be made without departing from the spirit of the present disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present disclosure.

[0050] Conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain examples include, while other examples do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements and / or steps are in any way required for one or more examples or that one or more examples necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and / or steps are included or are to be performed in any particular example.

[0051] Disjunctive language such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is otherwise understood within the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, and / or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain examples require at least one of X, at least one of Y, or at least one of Z to each be present.

[0052] Use herein of the word “or” is intended to cover inclusive and exclusive OR conditions. In other words, A or B or C includes any or all of the following alternative combinations as appropriate for a particular usage: A alone; B alone; C alone; A and B only; A and C only; B and C only; and all three of A and B and C.

[0053] The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosed examples (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. The use of “adapted to” or “configured to” herein is meant as open and inclusive language that does not foreclose devices adapted to or configured to perform additional tasks or steps. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Additionally, the use of “based on” is meant to be open and inclusive, in that a process, step, calculation, or other action “based on” one or more recited conditions or values may, in practice, be based on additional conditions or values beyond those recited. Similarly, the use of “based at least in part on” is meant to be open and inclusive, in that a process, step, calculation, or other action “based at least in part on” one or more recited conditions or values may, in practice, be based on additional conditions or values beyond those recited. Headings, lists, and numbering included herein are for ease of explanation only and are not meant to be limiting.

[0054] The various features and processes described above may be used independently of one another or may be combined in various ways. All possible combinations and sub- combinations are intended to fall within the scope of the present disclosure. In addition, certain method or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described blocks or states may be performed in an order other than that specifically disclosed, or multiple blocks or states may be combined in a single block or state. The example blocks or states may be performed in serial, in parallel, or in some other manner. Blocks or states may be added to or removed from the disclosed examples. Similarly, the example systemsand components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed examples.

[0055] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

Claims

WHAT IS CLAIMED IS:

1. A radiative cooling device comprising: a substrate; and a layer of silicon-containing material disposed on the substrate, wherein the layer of silicon-containing material emits infrared radiation of wavelengths within an atmospheric infrared transmission window.

2. The radiative cooling device of claim 1, the layer of silicon-containing material transmits solar radiation.

3. The radiative cooling device of claim 1, wherein the layer of silicon- containing material is characterized by a thickness of less than 20 microns.

4. The radiative cooling device of claim 1, wherein the layer of silicon- containing material comprises silicon oxynitride (SiOxNy).

5. The radiative cooling device of claim 4, wherein the layer of silicon- containing material is formed through co-sputtering silicon oxide (SiO2) and silicon nitride (SiN).

6. The radiative cooling device of claim 4, wherein values for comprise a first range of values between 1.00 and 2.00, and wherein values for comprise a second range of values between 0.1 and 0.

8.

7. The radiative cooling device of claim 4, wherein the layer of silicon- containing material is formed through plasma-enhanced chemical vapor deposition (PECVD).

8. The radiative cooling device of claim 7, wherein a PECVD gas mixture comprises silane (SiH4), ammonia (NH3), and nitrous oxide (N2O).

9. The radiative cooling device of claim 8, wherein the layer of silicon- containing material is characterized by an oxygen ratio of between about 0.5 and about 1.0 by controlling gas flow rates during deposition.

10. The radiative cooling device of claim 1, wherein the layer of silicon- containing material is characterized by an average emissivity of greater than or equal to 0.8 for wavelengths in the atmospheric infrared transmission window.

11. The radiative cooling device of claim 1, wherein the substrate comprises silver-coated silicon.

12. A method comprising: fabricating a radiative cooling device by forming a layer of silicon-containing material on a substrate; placing the radiative cooling device on an object; and cooling the object by emitting infrared radiation of wavelengths within an atmospheric infrared transmission window.

13. The method of claim 12, wherein the layer of silicon-containing material is characterized by a thickness of less than 20 microns.

14. The method of claim 12, wherein the layer of silicon-containing material comprises silicon oxynitride (SiOxNy).

15. The method of claim 14, wherein the layer of silicon-containing material is formed through co-sputtering using silicon oxide (SiO2) and silicon nitride (SiN).

16. The method of claim 14, wherein values for comprise a first range of values between 1.00 and 2.00, and wherein values for comprise a second range of values between 0.1 and 0.

8.

17. The method of claim 12, wherein layer of silicon-containing material is formed through plasma-enhanced chemical vapor deposition (PECVD).

18. The method of claim 17, wherein a PECVD gas mixture comprises silane (SiH4), ammonia (NH3), and nitrous oxide (N2O).

19. The method of claim 18, wherein the layer of silicon-containing material is formed using an oxygen ratio between about 0.5 and about 1.0 by controlling gas flow rates during deposition.

20. The method of claim 12, wherein the layer of silicon-containing material is characterized by an average emissivity of greater than or equal to 0.8 for wavelengths in the atmospheric infrared transmission window.

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