Nonreciprocal thermal metamaterials:Methods and applications

Nonreciprocity of thermal metamaterials has significant application prospects in isolation protection,unidirectional transmission,and energy harvesting.However,due to the inherent isotropic diffusion law of heat flow,it is extremely difficult to achieve nonreciprocity of heat transfer.This review presents the recent developments in thermal nonreciprocity and explores the fundamental theories,which underpin the design of nonreciprocal thermal metamaterials,i.e.,the Onsager reciprocity theorem.Next,three methods for achieving nonreciprocal metamaterials in the thermal field are elucidated,namely,nonlinearity,spatiotemporal modulation,and angular momentum bias,and the applications of nonreciprocal thermal metamaterials are outlined.We also discuss nonreciprocal thermal radiation.Moreover,the potential applications of nonreciprocity to other Laplacian physical fields are discussed.Finally,the prospects for advancing nonreciprocal thermal metamaterials are highlighted,including developments in device design and manufacturing techniques and machine learning-assisted material design.

General three-dimensional thermal illusion metamaterials

Thermal illusion aims to create fake thermal signals or hide the thermal target from the background thermal field to mislead infrared observers,and illusion thermotics was proposed to regulate heat flux with artificially structured metamaterials for thermal illusion.Most theoretical and experimental works on illusion thermotics focus on two-dimensional materials,while heat transfer in real three-dimensional(3D)objects remains elusive,so the general 3D illusion thermotics is urgently demanded.In this study,we propose a general method to design 3D thermal illusion metamaterials with varying illusions at different sizes and positions.To validate the generality of the 3D method for thermal illusion metamaterials,we realize thermal functionalities of thermal shifting,splitting,trapping,amplifying and compressing.In addition,we propose a special way to simplify the design method under the condition that the size of illusion target is equal to the size of original heat source.The 3D thermal illusion metamaterial paves a general way for illusion thermotics and triggers the exploration of illusion metamaterials for more functionalities and applications.

Designing nonlinear thermal devices and metamaterials under the Fourier law: A route to nonlinear thermotics

Nonlinear heat transfer can be exploited to reveal novel transport phenomena and thus enhance people’s ability to manipulate heat flux at will.However,there has not been a mature discipline called nonlinear thermotics like its counterpart in optics or acoustics to make a systematic summary of relevant researches.In the current review,we focus on recent progress in an important part of nonlinear heat transfer,i.e.,tailoring nonlinear thermal devices and metamaterials under the Fourier law,especially with temperature-dependent thermal conductivities.We will present the basic designing techniques including solving the equation directly and the transformation theory.Tuning nonlinearity coming from multi-physical effects,and how to calculate effective properties of nonlinear conductive composites using the effective medium theory are also included.Based on these theories,researchers have successfully designed various functional materials and devices such as the thermal diodes,thermal transistors,thermal memory elements,energy-free thermostats,and intelligent thermal materials,and some of them have also been realized in experiments.Further,these phenomenological works can provide a feasible route for the development of nonlinear thermotics.

Multiple control of thermoelectric dual-function metamaterials

Thermal metamaterials based on transformation theory offer a practical design for controlling heat flow by engineering spatial distributions of material parameters,implementing interesting functions such as cloaking,concentrating,and rotating.However,most existing designs are limited to serving a single target function within a given physical domain.Here,we analytically prove the form invariance of thermoelectric(TE)governing equations,ensuring precise controls of the thermal flux and electric current.Then,we propose a dual‐function metamaterial that can concentrate(or cloak)and rotate the TE field simultaneously.In addition,we introduce two practical control methods to realize corresponding functions:one is a temperature‐switching TE rotating concentrator cloak that can switch between cloaking and concentrating;the other is an electrically controlled TE rotating concentrator that can handle the temperature field precisely by adjusting external voltages.The theoretical predictions and finite‐element simulations agree well with each other.This work provides a unified framework for manipulating the direction and density of theTE field simultaneously and may contribute to the study of thermal management,such as thermal rectification and thermal diodes.

热扩散系统中非厄米趋肤效应的观测

The paradigm shift of Hermitian systems into the non-Hermitian regime profoundly modifies inherent property of the topological systems, leading to various unprecedented effects such as the nonHermitian skin effect(NHSE). In the past decade, the NHSE has been demonstrated in quantum, optical and acoustic systems. Beside those wave systems, the NHSE in diffusive systems has not yet been observed, despite recent abundant advances in the study of topological thermal diffusion. In this work,we design a thermal diffusion lattice based on a modified Su-Schrieffer-Heeger model and demonstrate the diffusive NHSE. In the proposed model, the asymmetric temperature field coupling inside each unit cell can be judiciously realized by appropriate configurations of structural parameters. We find that the temperature fields trend to concentrate toward the target boundary which is robust against initial excitation conditions. We thus experimentally demonstrated the NHSE in thermal diffusion and verified its robustness against various defects. Our work provides a platform for exploration of non-Hermitian physics in the diffusive systems, which has important applications in efficient heat collection, highly sensitive thermal sensing and others.