Research On The Asymmetric Heat Transfer Of Thermal Metamaterials

Asymmetric heat transfer indicates that different physical phenomena arise when heat flow is conducted along different paths;therefore,the study of asymmetric heat transfer phenomena is important to further investigate the directional manipulation of heat flow,while macroscopic asymmetric heat transfer devices,represented by thermal rectifiers and thermal diodes,have potential and wide application prospects.In this paper,we look at how heat flows from different places in space into thermal metamaterials and shows asymmetric heat transfer behavior.Because of the different ways in which the physical problems show up,they can be put into two groups: angular asymmetry and opposite directional asymmetry.Asymmetries based on the angular presentation of heat transfer were studied in Chapters 2 and 3 by drawing on concepts from the field of optics,such as the Janus structure,and asymmetric heat transfer presented in the opposite directions is studied in Chapters 4 and 5.Janus materials or structures have been developed in the field of optics for a long time,and the concept of "Janus" mainly refers to the asymmetry of the light propagation process.Their core concept is to introduce asymmetry along the wave propagation direction,by stacking different materials or layers of meta-atoms,or breaking out-of-plane mirror asymmetry with external biases.Nevertheless,it has been hitherto elusive to realize a diffusive Janus metadevice,since scalar diffusion systems such as heat conduction normally operate in the absence of polarization control,spin manipulation,or electric-field stimuli,which all are widely used in achieving optical Janus devices.There are more multifunctional thermal metamaterials available today in the field,however,a given thermal metadevice only possesses either omnidirectional or unidirectional functionality.It is even more challenging,if not impossible,for a single diffusive metadevice to exhibit more than two thermal functions.Here,a path-dependent thermal metadevice beyond Janus functionalities proposed using successive 3D transformations that can exhibit three distinct thermal behaviors(cloaking,concentrating,and transparency)under different directions of heat flow.The rotation transformation mechanism of thermal conductivity provides a robust platform to assign a specific thermal behavior in any direction.The proof-of-concept experiment of anisotropic in-plane conduction successfully validates such a path-dependent tri-function thermal metamaterial device.It is anticipated that this path-dependent strategy can provide a new dimension for multifunctional metamaterial devices in the thermal field,as well as for a more general diffusion process.In order to solve the problem of the large range of contact thermal resistance that is difficult to avoid in the construction of anisotropic parameters of the designed Janus metadevice,a bilayer Janus metadevice is designed using natural materials by applying the neutral inclusion iteration method to minimize the effect of contact thermal resistance.Additionally,a metadevice with configurable anisotropic parameters can exhibit predictable thermal rotation functions in additional directions.At the moment,there are usually two ways to get asymmetric heat transfer in opposite directions.One of them is to use the nonlinearity of the material reference to break the symmetry of the heat transfer process.However,due to the thermal physical limitations of materials,it is difficult to find ideal nonlinear materials for the realization process,and their demanding parameter requirements limit the further development and application of this method.The other is to achieve asymmetric heat transfer in both positive and negative directions by active modulation,such as the spatiotemporal modulation method,but this method requires an active energy payload,thus increasing the difficulty and cost of experimental operation.So,we propose a nonlinear perturbation method to find a direct way to implement passively asymmetric heat transfer using graded metamaterials with linear conductivity.The nonlinear perturbation model can be designed in a flexible way to couple effective disturbances in the heat transfer process as a way to mimic and get around the nonlinear parameter limitations found in natural materials.Also,the method makes it easier to figure out how traditional designs with nonlinear materials work when it comes to thermal rectification.Validation experiments of surface thermal radiation and thermal convection show that the heat exchange between gradient linear thermal metamaterials and the environment can be controlled to achieve macroscopic asymmetric heat transfer.Meanwhile,the asymmetric heat transfer behavior of thermal metamaterials exists not only in the steady state but also in the transient state at the macroscopic scale,which will directly affect the efficiency and operating state of the asymmetric heat transfer devices.While thermal metamaterials have symmetric temperature distributions in the steady state,asymmetric temperature distributions can occur in the transient state.Therefore,the final chapter of this paper focuses on the transient asymmetric heat transfer behavior of thermal metamaterials.It is found that gradient metamaterials satisfying the parameter mismatch criterion and with a constant thermal diffusion coefficient still have transient asymmetric heat transfer behavior and exhibit the strongest transient asymmetric heat transfer effect.The transient heat transfer experiments validate the designed transient asymmetric heat transfer effect.Our work will contribute to the development of the fields of advanced thermal management and thermal computing.Meanwhile,the asymmetric heat transfer behavior of thermal metamaterials exists not only in the steady state,especially at the macroscopic scale,and the transient heat transfer behavior will directly determine the efficiency and working state of asymmetric heat transfer devices.While thermal metamaterials exhibit symmetric temperature distributions in the steady state,it is still possible to exhibit asymmetric temperature distributions in the transient state.Therefore,the final chapter of this paper focuses on the time-dependent nonreciprocal heat transfer behavior of thermal metamaterials.It is found that graded metamaterials that satisfy the parameter mismatch criterion and have a constant thermal diffusion coefficient exhibit the strongest time-dependent nonreciprocal heat transfer.Transient heat transfer experiments validate the designed time-dependent nonreciprocal heat transfer effect.Our work will contribute to the development of the fields of advanced thermal management and thermal computing.