Theoretical Modeling Of Radiative Heat Transfer Involving Micro/Nanoscale Complex Structures

Thermal radiative heat transfer is one the most common forms of energy exchange in nature.The need to enhance,suppress or selectively tailor the spectral radiative transfer exists widely not only in scientific and industrial applications but also in everyday life.Radiative heat transfer involving micro/nanoscale complex structures spans from astronomical scale to the size of basic particles and is the frontier in the current research of heat transfer.The complex interactions between materials and electromagnetic waves as well as the near-field radiative transfer with its subwavelength energy transfer path have a myriad of problems to be answered.The focus of this dissertation is on two types of representative problems as defined by the length of path in radiative trasnfer: macroscopic radiative transfer problems with micro/nanoscale particulate system as participating media,and the near-field radiative transfer with a subwavelength path involving complicated structures.For the first problem,the opacified silica aerogel,a complex particulate media used for thermal insulation,is selected as the studied system.Composed by silica nanoparticle skeleton and micrometer sized opacified particles,this composite material spans multiple size scales with structural randomness.To characterize the impact of electromagnetic interactions between particles,a silica nanoparticle aggregation is generated to represent the optical properties of the aerogel skeleton.The non-spherical shapes and non-uniform sizes of actual opacifier particles are considered,showing that a practical model should fully take the particle size distribution into account.Finally,with an improved physical model of particle aggregation,a pseudo-realistic microstructure is generated,containing thousands of aerogel particles and one opacifier grain,whose radiative properties are rigorously calculated by the multi-sphere T-matrix method,showing the difference from single sphere Mie scattering.The results lead to an appropriate solution of the radiative transfer equation,yielding the prediction of measurable bulk parameters,which are validated by carefully designed optical experiment.For the second problem,near-field thermophotovoltaics(TPV)is selected as the studied system,where the spectral radiative transfer should be tailored to enable selectively high flux within a narrow band above the cell's bandgap.The complexity of near-field radiative transfer modelling as well as the poor absorption of III-V semiconductor brings challenge to the optimization of the TPV system.This work designed a simple four-layer planar thermal emitter and optimized the structural paramters for the system to reach a higher conversion efficiency than the current state-ofthe-art designs,with discussion focused on the difference from designing a far-field thermal emitter.Secondly,for hyperbolic metamaterial emitters enabled by subwavelength periodic structures,rigorous methods are applied to calculate the resultant near-field radiative flux,showing the limitations of effective medium theories in the area of near-field radiative transfer that is mostly neglected in past publications.Finally,this work creatively designed an equivalent of anti-reflection structures for near-field applications,adding sub-wavelength surface structures including nanowire,naohole and pillar arrays to the GaSb cell.Rigorous calculation revealed the additional surface structures can selectively tailor the near-field radiative transfer in a TPV system with a maximal improvement of 78.3% in conversion efficiency.This is due to the light-trapping mechanism coupled with surface wave excitations which only exists in the scenario of near-field radiative transfer.