Temperature Field Modulations Based On Coordinate Transformation Theory And Non-Hermitian Thermal Diffusion Theory

Most of the current research in thermal metamaterials at macroscopic size is based on coordinate transformation theory and effective medium theory,focusing on the constructing of effective thermal conductivity.These modulation methods have enriched the paths of heat flow manipulation and realized novel functions/effects,such as thermal cloaking,thermal camouflage,thermal concentration,thermal collection,etc.However,due to the limitation and discreteness of natural materials,it is hard to modulate the heat flows continuously.Since the spatial distribution of the temperature field in ring structures can be analogous to the periodic wave in channels,the study of wave-like effects in thermal diffusion can offer new paths and freedoms for temperature field modulation.The diffusive coupled-mode theory provides theoretical guidance for temperature field manipulation in amplitude and phase precisely.Due to the diffusion nature of heat transfer,the elements of the effective Hamiltonian for thermal diffusion are pure imaginary,and the system is inherently nonHermitian.In wave systems,two ways are usually employed to introduce non-Hermitian potentials,direct gain/loss modulation or indirect asymmetric coupling.In thermal diffusion systems,these non-Hermitian potentials can be realized by convection and mass density ratio modulation,respectively.On this basis,the physical symmetry and non-Hermitian topology in thermal diffusion systems can be further explored.In this thesis,the three-dimensional(3D)wide-angle thermal radiation camouflage helmet is proposed based on coordinate transformation theory(effective anisotropic thermal conductivity).The thermal reciprocity in the temporal system is discussed based on periodical effective thermal conductivity/mass density modulation.The diffusive nonHermitian skin effect is discovered based on asymmetric coupling modulation.The exceptional point in different ring structures are investigated based on convection and asymmetric coupling modulation.Details are as follows:Firstly,a 3D anisotropic thermal conductivity modulated wide-angle radiation camouflage helmet is proposed based on the coordinate transformation theory.By a stretching transformation and a compression transformation,the temperature background can be directly mapped to the up Gaussian surface of the structure.In this way,the object placed inside the structure can be camouflaged on the horizon of the infrared camera through thermal radiation.Compared with the thermal camouflage in the conduction regime,the proposed thermal radiation camouflage does not need to destroy the background structure,works well in wide-angle view,and is adaptive to the background temperature change.Thermal diffusion reciprocity in temporal systems is discussed based on dynamical periodic space-time thermal conductivity and mass density modulation.Since this modulation leads to material migration,thus the thermal nonreciprocity caused through temporal modulation will be counteracted by the introduced equivalent convection,and the system still holds reciprocity.This work proves theoretically and experimentally that thermal reciprocity cannot be broken only with temporal modulation,without the introduction of net directional material transport,which corrects the previous incomplete cognition of thermal nonreciprocity.Secondly,the asymmetric coupling mechanism of the temperature field is proposed based on the non-equivalence of heat and temperature,and the diffusive non-Hermitian skin effect is then discovered.Deducing on the diffusive coupled-mode theory,the asymmetric coupling of temperature field can be achieved by the rings with different mass densities directly contacted.On this basis,the diffusive skin effect in the tight-bounding model is explored,and being explained theoretically with the non-Bloch mode theory.A new type of temperature collecting device,the topological heat funnel is presented,which can localize the initial input temperature field at the specified position robustly.Finally,the exceptional points(EPs)in different thermal diffusion systems are investigated based on convection and asymmetric coupling modulation.As the splitting of the real part of eigenvalues at the EP is proportional to the perturbation strength,with the power of 1/n(n is the degeneracy degree),EPs can benefit the high-precision thermal sensing.Starting from the double-channel ring structure containing a second-order EP with convection and asymmetric coupling,we investigate the third-order EP and the anti-paritytime symmetry in a four-channel system.Moreover,we present the arbitrary-order EP in the asymmetric coupling multi-channel systems,and clarify the diffusive skin effect as the unbalanced temperature field distribution in the broken phase after the high-order EP.Above all,the mechanism of three-dimensional radiation stealth,thermal reciprocity,and temperature field amplitude/phase regulation in the macroscopic heat conduction systems are investigated in this thesis,based on coordinate transformation theory,periodic temporal modulation,and non-Hermitiion thermal diffusion theory.This work provides a new path for the research and exploration of thermal metamaterials,and it introduces new physics for heat manipulation.