Near-Field Radiative Heat Transfer Characteristic Of Gradient Index Slab

Near-field radiative heat tranfer,whose heat flux can exceed that of blackbody’s limit by several orders of magnitude,holds promise for nanaoscale device for energy conversion and management.Previous studies on near-field heat transfer of slab mainly focus on medium whose dielctric function is uniform.There are few studies considering media with gradient index distribution,which can be caused by uneven coating,rougness,or field effect.To treat this problem,this dissertation uses a multilayer medium to approximate gradient index slab,thus the near-field radiation theory of the former is adopted to study the near-field heat transfer characteristic of the latter.The surface roughness is a common form of gradient distribution of dielectric function in engineering practice.In this dissertation,based on effective medium theory(EMT),the rough surface of Si C is modeled as a multilayer medium with gradient index distribution,and its near-field heat transfer characteristics are studied.The research shows that with the increase of the root mean square height(σ)of the rough surface,the peaks of local density of states and spectral heat flux are red-shifted.The physical mechanism is due to the coupling of low-frequency surface polarizations supported on the internal interface of the effective gradient index medium that models the rough layer.For a given distance,the net radiative heat flux between rough surfaces exceeds that between smooth plates and increases with increasing σ.However,the proposed effective method is unable to study the effect of the correlation length of rough surface on near-field heat transfer.The research on the near-field heat transfer between rough surfaces is an application example of the method for gradient index slab.This dissertation further studies the near-field heat transfer mechanism of general gradient index medium whose dielectric function is described by the Drude model.On the one hand,through parametric studies,it is found that as the total depth of changing index(h)increases,the near-field heat flux increases first and then decreases.Meanwhile,the spectral heat flux red-shifts as h increases.As the plates spacing(d)increases,the penetration depth of the heat flux increases,and the spectral heat flux blue-shifts.On the other hand,through the analysis of the electromagnetic wave amplitude,the mechanism of surface polaritions in gradient index medium is studied.It is proved that internal surface polaritons with the frequency zeroing the dielectric function at the corresponding location can be supported,which causes the total heat flux absorption distribution to peak inside the medium.The modes of the internal surface polaritons is more abundant than that on vacuum/bulk interface and can be altered by the gradient of dielectric function.By analyzing the local density of states,this dissertation also studies the modulation of gradient distribution on the surface polarritons on the vacuum/gradient index medium interface.It is found that the gradient distribution of dielectric function changes the dispersion relation: the wave vector magnitude is determined by the distribution of dielectric function,and its value is taken between the corresponding values of the surface and the inner medium,while the dielectric property of the inner medium shows greater influence on the dispersion relation than that of the surface.The results of this dissertation are helpful to deepen the understanding of near-field heat transfer of gradient index medium,and provide reference for the design and evaluation of micro-scale energy equipment in practical engineering.