Thermal radiation from an object with temperature above absolute zero originates from the random vibrational and rotational transitions of molecules in the object. Its electromagnetic wave nature allows to manipulate heat exchange among objects with sophisticated contactless long-range strategies, which can transport and control heat flows efficiently even in the vacuum. For example, the earth keeps its energy and temperature balance through accepting the sun’s radiation and emitting infrared radiation to the cold space under the atmospheric regulation. In general, spectral reflection, transmittance and absorption/emission determine the radiative thermal processes that can passively heat up or cool down terrestrial objects, and these spectral responses of materials can be artificially designed to achieve advanced thermal management. This doctoral research focuses on understanding the fundamentals of radiative thermal design of polymeric composites and how the modified spectral responses enhance the capacity of radiative thermal management, aiming at the applications on energy saving and conversion.

The main works presented in this thesis are categorized under three chapters including (1) gradient structure enhanced polymeric nanocomposite for radiative cooling, (2) soft radiative cooling coatings for advanced thermal management in wearable electronics and (3) thermoelectric generation harnessing both the solar heating and the cold universe. Prior to these sections, a general introduction on the fundamentals related to radiative thermal design is given; a summary of the current works and outlooks for future research are also presented.

In the first work, the spectral selectivity of a radiative cooler is discussed by considering the varied cooling power under thermal loads and dynamic atmospheric radiation as well as the advantages of broadband radiators for building envelope applications. To understand the influence of structure gradience in the radiative cooling capability of polymeric nanocomposites, their spectral responses, including solar reflectance and infrared emittance, are investigated by a modified effective multilayer Monte-Carlo method incorporating Mie theory. The gradient dispersion of nanoparticle fillers in polymeric nanocomposites can be naturally formed through the sedimentation effect during coating curing. Four types of gradient structures, density-gradient and size-gradient with both downward and upward profiles are studied and compared with common polymer coatings of randomly distributed dielectric nanoparticles. Their corresponding cooling performance is also evaluated according to their spectral responses. The calculation results reveal that the downward size-gradient and upward density-gradient profiles exhibits a significant improvement in both solar reflectance and infrared radiation, while the upward size-gradient and downward density-gradient profiles perform the opposite. An enhanced cooling power of ~36 W/m² can be obtained at a broad size distribution for the downward size-gradient structure. The impact of gradient profiles on the spectral properties of radiative coolers is attributed to the important role of the near-surface region that interacts initially with the incident light. The above findings help the community understand the radiative thermal requirement under various demands and the divergence of actual spectral performance as well as theoretical predictions of polymer coatings, which shed light on the optimization of radiative cooling coatings for real-world applications.

In the work on soft radiative cooling coatings for advanced thermal management in wearable electronics, a thin, soft polymeric coating is designed and optimized to enhance the radiative thermal management capacity. The scattering properties of a series of dielectric nanoparticles are calculated and compared for materials selection of the polymer coating. Other key parameters, including the size distribution and volume fractions of functional fillers are also investigated. The optimized coating consists of a polymer matrix (poly-styrene-acrylic) and three functional fillers (hollow SiO₂ microspheres, TiO₂ nanoparticles, and fluorescent pigments) and is applied as an ultrathin, ultralight and highly flexible cooling interface by considering the thermal management requirements of wearable electronics with dense integration, multifunctionality and miniaturization. A collection of demonstrations is explored to reveal the radiative thermal management potential of the designed cooling coating including the Joule heat dissipation in flexible circuits, working efficiency improvement for epidermal electronics and signal stabilization for skin-interfaced wireless sensors. A remarkable temperature reduction greater than 56 ℃ is observed in a flexible circuit capped with a 600 μm thick coating. The enhanced performance of wearable devices proves the effectiveness of our cooling interface and radiative thermal management strategy.

For the work on thermoelectric generation harnessing both the solar heating and the cold universe, the radiative exchange processes at both sides of a thermoelectric generator are purposely designed to enlarge their temperature difference and hence the energy harvesting efficiency. A solar absorptive and infrared-transparent layer consisting of polyethylene and Ge powders is designed as the hot side to harness the solar energy and an infrared emissive layer of polydimethylsiloxane is applied as the cold side to dissipate heat to the cold space through the atmosphere. By analyzing the heat fluxes under thermal equilibrium, the geometric parameters and thermal conductivity of TE elements can be determined. Compared to the radiative cooling enhanced TE structure, our structure with both solar heating and radiative cooling can provide over 40 ℃ improvement on temperature difference and in the meanwhile maintain its thermal management capacity. The results reveal the huge potential of radiative thermal designs (using the sun as a heat source and the cold space as a heat sink) in realizing multifunctional energy conversion systems.