Passive radiative cooling using optical thin film coatings

Radiative cooling is a passive way of cooling by which a body loses heat by emitting energy. When a body is exposed to sky, heat transfer between the body and sky occurs depending on transparency of the atmosphere through radiation. During nighttime, due to extremely low incident solar irradiation cooling can be achieved. However, during daylight nearly 940 W/m2 energy is present in Istanbul, due to sun and since emission by the object is not as high as this energy, cooling cannot be achieved. So, in order to achieve radiative cooling during daylight, incident solar energy has to be reflected strongly which prevents heating of the object. Also, by maximizing emission in the atmospheric transparency window in 8‐13 μm range, in which very low amount of solar energy is carried, radiative cooling can be achieved. In this study, design studies about thin film filters are carried out whose focus is to achieve high reflection in the visible and near‐infrared spectrums in which high amount of solar energy is present and maximize absorption/emissivity in 8‐13 μm spectrum where atmospheric transparency window is present. For these purposes, different design methods are examined, e.g. quarter wavelength stacks for high reflection and an impedance matching technique, Chebyshev transform, is used to increase emission in 8‐13 μm spectrum. For the performance evaluations, radiative heat transfer dynamics are examined and cooling powers are compared with a design results given in the literature. It is observed that significant performance improvement can be observed by proposed design methods

Design of spectrally selective surfaces

Tailoring the spectral reflection, absorption, and transmission of the surfaces, as well as their emission, in broadband has been attracted great attention with the recent advancements in micro/nanotechnology. Such advancements provide the opportunity of realizing structures that are comparable to wavelength in terms of geometrical dimensions, which allow manipulation of incident or emitted waves from visible to infrared spectrum in small scale. Therefore, understanding the physical mechanisms that are responsible from altered spectral behaviors achieved by various type of optical filters/coatings and designing those become equally important. Aim of this thesis is to propose design methods to selected energy and thermal applications, including daytime passive radiative cooling, broadband reflectance with refractory metals, absorption mechanisms of black silicon and high temperature broadband thermal emitter, all of which require engineering of electromagnetic spectrum in broadband. Various phenomena are considered during the design stages and utilized to evaluate the resulting characteristics. Methods used in this thesis generate novel designs to be considered in various application

Enhancing the spectral reflectance of refractory metals by multilayer optical thin-film coatings

Good thermomechanical properties of refractory metals, including tungsten, tantalum, molybdenum, and niobium, make them attractive candidates for operation in extreme environments. In addition, their ability to reflect thermal radiation in the infrared region beyond 1.5 mu m makes them attractive for use as reflective surfaces in extreme environments. These refractory metals, however, have relatively poor reflectivity in the visible and near-infrared spectral regions, making them absorb incident thermal radiation. Average absorption percentages of these metals at the 300-1500 nm spectrum is in the range of 40%-50%. In this paper, we propose and demonstrate that by using periodic thin-film dielectric coatings deposited over refractory metals, the absorption of thermal radiation can be drastically reduced. Various thin-film optical filter designs are investigated to engineer and improve the spectral reflectivity of refractory metals in the visible and near-infrared spectral regions without deteriorating the performance beyond 1.5 mu m. TiO2, Al2O3, and SiO2 are used as materials, which are dielectrics that do not absorb incident radiation in the visible and near-infrared with high melting points and Young's modulus. Our results indicate that a combination of several periodic segments, designed at different wavelengths around which high reflectivity is desired, can be utilized to generate high reflectivity in the broadband spectrum. Bandwidth and magnitude of the reflectivity over the spectrum are highly dependent on materials, number of segments, and number of layers in the segments of the filters

Broadband high-temperature thermal emitter/absorber designed by the adjoint method

Broadband thermal emitters/absorbers are of great interest for thermal management in high-temperature applications including thermophotovoltaics and hypersonics due to the dominance of radiative heat transfer. Several studies have been reported, which benefit from the effectiveness of photonic structures on wave coupling tuned by parametric sweeps and intuition. However, the higher emission/absorption potential of structures with nonintuitive geometries due to increased number of available design parameters has not been explored. To address this need, we have studied gradient-based topology optimization for the design of high-temperature thermal emitters/ absorbers. By utilizing the adjoint method, which allows gradient calculation only with two simulations, more complex geometries that exhibit higher emission/absorption in the broadband spectrumare designed. ZrB2 is chosen as the coating material, which belongs to the family of ultrahigh-temperature ceramics (UHTC), due to its very good thermomechanical properties. Emission/absorption rates reaching up to 85% levels in the broadband spectrum are achieved, which is around 40% levels in film form. Electromagnetic phenomena that give rise to elevated emission/absorption are also analyzed. Our findings demonstrate the potential of effectiveness of adjoint-based topology optimization in the broadband spectrumand high emission of ZrB2 when patterned, which, to the best of our knowledge, was not previously explored

Spectrally selective filters and their applications

Engineering the spectral characteristics of surfaces is of great importance for different applications including but not limited to energy and electronics. Advancements in nano−/micro-fabrication techniques accelerated the progress made in the field by allowing the realization of structures that are comparable to wavelengths of operation, which can exhibit desired characteristics in a broad range of wavelengths. Therefore, understanding both fundamental mechanisms and design principles of such spectrally selective surfaces becomes very important. The aim of this chapter is to demonstrate some examples of such spectrally selective surfaces, as well as their design principles and physical mechanisms, for applications such as passive radiative cooling and highly reflecting/absorbing coatings for high-temperature applications. Having broadband features is attractive for thermal control, plasmonic particles, and surfaces which can bring additional functionality to these broadband spectrally selective filters. Potential uses of such surfaces are also discussed

Heat-sink designs for plasmonic transducers in heat assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a promising technique to extend the areal density of hard drives. In HAMR, localized optical spots are obtained via plasmonic transducers and these plasmonic transducers are utilized to heat the magnetic medium during the recording process. One potential challenge in a HAMR system is the heating of plasmonic transducers and performance reduction due to such heating. The heating of the plasmonic transducers can result in both performance and reliability issues in a HAMR system, including structural distortions of the slider and transducer. In this study, to overcome the aforementioned performance and reliability issues in HAMR, we designed heat-sinks for plasmonic transducers and reduced the temperature of the plasmonic transducer and surroundings by cooling techniques. We discuss various plasmonic transducers and provide heat-sink designs to reduce their heating

Origins of the enhanced broadband absorption in black silicon

Although black silicon is utilized in a wide range of applications due to its broadband spectral emission and absorption, the underlying electromagnetic mechanisms are not well explored. In this study, the underlying phenomena that are responsible for these enhanced spectral features are investigated. The absorption spectra of the black silicon with random textures are analyzed, and the electromagnetic mechanisms that drive elevated absorption are explored. Our findings reveal that two separate electromagnetic phenomena occur in the textures, effective wavelength matching and waveguide modes. Detailed analysis reveals that the occurrence condition of those phenomena is highly dependent on the dimensions of the textures in the transverse direction. The effect of the texture dimensions and doping concentration both on absorption characteristics and physical phenomena is analyzed in detail. The findings of this study explain the absorption mechanisms of black silicon observed in experimental studies, which can lead to designer materials with rough surfaces for the desired spectral emissivity

Passive radiative cooling design with broadband optical thin-film filters

The operation of most electronic semiconductor devices suffers from the self-generated heat. In the case of photovoltaic or thermos-photovoltaic cells, their exposure to sun or high temperature sources make them get warm beyond the desired operating conditions. In both incidences, the solution strategy requires effective radiative cooling process, i.e., by selective absorption and emission in predetermined spectral windows. In this study, we outline two approaches for alternative 2D thin film coatings, which can enhance the passive thermal management for application to electronic equipment. Most traditional techniques use a metallic (silver) layer because of their high reflectivity, although they display strong absorption in the visible and near-infrared spectrums. We show that strong absorption in the visible and near-infrared spectrums due to a metallic layer can be avoided by repetitive high index-low index periodic layers and broadband reflection in visible and near-infrared spectrums can still be achieved. These modifications increase the average reflectance in the visible and near-infrared spectrums by 3-4%, which increases the cooling power by at least 35 W/m(2). We also show that the performance of radiative cooling can be enhanced by inserting an Al2O3 film (which has strong absorption in the 8-13 mu m spectrum, and does not absorb in the visible and near-infrared) within conventional coating structures. These two approaches enhance the cooling power of passive radiative cooling systems from the typical reported values of 40 W/m(2)-100 W/m(2) and 65 W/m(2) levels respectively

Spectral emissivity profiles for radiative cooling

Passive radiative cooling, an innovative approach for cooling buildings and devices, has attracted considerable attention in recent years. In particular, the spectral emissivity distribution of surfaces plays a crucial role for an object to radiate at wavelengths for which the atmosphere is transparent and solar irradiance is low. Here, we study the role of spectral emissivity distributions using different performance metrics: cooling power (CP) and equilibrium temperature (TEq). We investigated the roles of environmental factors, such as ambient temperature and level of thermal insulation from surroundings, on spectral emissivity distributions. Based on these emissivity distributions, we report the conditions at which the suitable profile for cooling power maximization and equilibrium temperature minimization changes. We discuss the realization of spectral emissivity distributions using various optical materials for cooling power maximization and equilibrium temperature minimization separately under different environmental conditions. The impacts of material selection on the realization of desired emissivity profiles and corresponding outcomes are analyzed. As progress in this emerging field gains traction, development of radiative cooling structures with suitable spectral emissivity profiles under different circumstances will become essential

Adjoint-based optimization of dielectric coatings for refractory metals to achieve broadband spectral reflection

Refractory metals, which include niobium, tantalum, molybdenum, and tungsten, are critical components in applications in extreme environments due to their attractive thermomechanical properties. However, their low reflectivity below 1500 nm has prompted researchers to focus on increasing their reflection at shorter wavelengths. In this study, we applied an adjoint-based optimization technique to improve the spectral reflectivity of refractory metals in the broadband spectrum (300–3000 nm). An optimized periodic multilayer consisting of SiO2/TiO2 is selected as a starting point for the process. Then, the adjoint-based method is implemented to enhance the reflection of the surfaces. This approach involves an iterative procedure that guarantees improvement in every iteration. In every iteration, both the direct and adjoint solutions of Maxwell’s equations are computed to predict the scattering characteristics of a particular microstructure on a surface and measure its effectiveness. The results of our study indicate that the final designs not only increase reflectivity to over 90% but also have thermomechanical benefits that make them suitable for use in harsh environments. We also explored the effect of initial geometry on the results. Overall, our study shows that the adjoint-based optimization technique is an effective method for creating high-performing broadband reflectors with refractory metal substrates coated with dielectric multilayers