Thermal Metamaterials Research Articles

Inverse design of rotating metadevice for adaptive thermal cloaking

Thermal metamaterials have been extensively studied due to their extraordinary properties beyond natural materials and offered great flexibilities to tune heat flow for desired thermal functionalities, like thermal cloaking, concentrating, rotating, etc. Whereas, the thermal properties of thermal metamaterials are usually fixed once the configuration and the constituent materials are designed and fabricated. For instance, the thermal cloaking effect may be deteriorated when background changes, which limits its practical application significantly. By deducing the effective thermal conductivities of rotating objects, we propose an adaptive thermal cloaking metadevice that is composed by three rotating layers with different roles. The joint effect of three rotating layers makes the effective thermal conductivity a real number on the reciprocity line for feasible implementation. When background changes, we only need change the angular velocities rather than change the configuration or the constituent materials to restore the cloaking effect, which is much more convenient and real-time for practical applications. The underlying physics of the rotating thermal cloak is discussed to identify the key parameters and upper and lower limits of the effective thermal conductivity for further improving the cloaking effect. The present study can trigger more rotating metadevices for novel applications beyond thermal cloaking.

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 peo-ple's ability to manipulate heat flux at will. However, there hasn't been a mature discipline called nonlinear thermotics like its counterpart in optics or acoustics to make a systematic summary of rele-vant 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's 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.

Topology Optimization Design of Adjustable Thermal Expansion Metamaterial Based on Independent Continuous Variables

This paper presents a topology optimization method to construct adjustable thermal expansion metamaterials. The negative thermal expansion (NTE) and extreme positive thermal expansion (EPTE) microstructures are designed, respectively. First, the effective elastic modulus and the effective thermal expansion coefficient of microstructure are derived by the multiscale asymptotic homogenization theory based on the periodic characteristics of the metamaterial. Second, a topology optimization model aiming at extreme thermal expansion coefficient is established and solved based on independent continuous topological variables, which is subjected to the effective elastic modulus and structure weight fraction. Then a cross-precision progressive optimization method is provided to save the computation time and make a smoother boundary. In addition, the specific topological feature for the initial configuration is discussed. Five holes are the necessary condition to obtain adjustable thermal expansion metamaterials for four-axisymmetric structures. Finally, a transformation strategy from NTE to EPTE and the change law of thermal expansion coefficient bounds with effective elastic modulus are studied. Numerical examples show that the NTE and EPTE can be designed in the range of effective elastic modulus from 0.01 to 0.05. The proposed method provides a reference for the novel configuration design of metamaterials.

Selective laser melting 3D printed thermal shield metamaterial structure

Due to ingenious design and elaborate construction of artificial structures, thermal metamaterials commonly possess extraordinary properties beyond natural materials under the conventional heat transfer conditions. Based on the transformation thermodynamics theory and the effective medium theory, in this paper, a plane-structure thermal shield aiming to manipulate heat flow was artificially designed and manufactured with the selective laser melting (SLM) 3D printing technology. The well-designed thermal shield allows the heat flow to bypass the central area of the structure for transmission and can be applied for heat protection in industries. Furthermore, the thermal shield metamaterial was fabricated and experimentally tested through the thermal monitor platform. And the experimental findings were in good agreement with the thermal simulation results. Therefore, the research results and the use of SLM 3D printing technology verified the applicability and feasibility of the thermal shield in the fields of thermal engineering and thermal control.

High-efficient heat flux manipulation of micro-scale thermal metamaterials with facile functional unit design

Based on the novel macro-thermophysical properties exhibited by its peculiarly artificially design structure, thermal metamaterials have been widely used in the harvest and management of thermal energy. Herein, a facile micro-scale thermal functional unit structure is proposed assisted by an advanced picosecond laser technology to realize the fabrication of thermal metamaterials with thermal shielding and thermal concentrating. To obtain the optimal design criterion of thermal functional units, the effects of thermal conductivity ratio, layer orientation angle, and laminated thickness on thermal conduction are investigated. Both simulation and experiment validate that thermal metamaterials assembled by diverse thermal functional units with different orientation angles exhibit completely neoteric performances of thermal concentrating and shielding. Besides, the results verify that the micro-scale thermal metamaterials exhibit better heat flux regulation capability and higher thermal concentration efficiency. Additionally, the effect of inevitable air convection on the thermal performance of these metamaterials has also been evaluated. This work provides an effective strategy to achieve small-scale thermal energy harvesting and protection by using thermal metamaterials with micro-structure.

Transforming heat transfer with thermal metamaterials and devices

The demand for sophisticated tools and approaches in heat management and control has triggered the fast development of fields that include conductive thermal metamaterials, nanophononics, and far-field and near-field radiative thermal management. In this Review, we offer a unified perspective on the control of heat transfer, summarizing complementary paradigms towards the manipulation of physical parameters and the realization of unprecedented phenomena in heat transfer using artificial structures. The Review is divided into three parts that focus on the three main categories of heat flow control. Thermal conduction and radiation, at both the macroscale and microscale, are emphasized in the first and second parts. The third part discusses efforts to actively introduce heat sources or tune the material parameters with multiphysical effects in conduction, radiation and convection. We conclude by analysing the challenges in this research area and surveying new possible directions, in particular topological thermal effects, heat waves and quantum thermal effects.

Self-adaptive field manipulation with thermal logic material

Emerging thermal metamaterials have been deployed to realize advanced thermal field manipulations and opened up the possibilities of flexible control of thermal processes. However, the state-of-art thermal metamaterials only possess fixed and passive functionalities, and the artificial interventions of structural adjustments are also inescapable. In this paper, we propose a class of thermal logic material to overcome such long-standing problems. The proof-of-concept scheme can be created with varied configurations of shape memory alloy (SMA) and conventional thermal media. Thanks to the shape changes of SMA and multifarious medium configurations, self-adaptive switching between thermal transparency and obvious signature can be observed under appropriate working-temperature ranges of SMA with the logic operations of AND, NOR, and XOR-NOT gates. The findings provide a constructive paradigm of creating logic operations in thermal field, and might further pave the way of intelligent thermal manipulation and management with varied configurations of thermal logic gates.

Thermal transparency with periodic particle distribution: A machine learning approach

The periodic interparticle interaction mechanism has been previously proposed for heat management, especially the practical application of thermal transparency. In our mechanism for engineering and manipulating thermal metamaterials, particles are arranged in periodic lattices with symmetric interactions. In this work, we relax the constraints in the previous work and allow rectangle lattice and arbitrary relative positioning between the two types of particles. We use a machine learning-based approach to solve the inverse design problem by training autoencoders to compress the dimensionalities of both the design space and the response space and training a neural network tailored for the inverse design problem. We employ the finite-element method for generating the training set for the neural network and for validating the calculated design parameters for a given thermal transparency problem. We also discuss the possibility of extending the machine learning-based workflow to other problems, such as thermal camouflage.

Adaptive Radiative Thermal Camouflage via Synchronous Heat ConductionSupported by the National Natural Science Foundation of China (Grant No. 52076087).

The advent of transformation thermotics has seen a boom in development of thermal metamaterials with a variety of thermal functionalities, including phenomena such as thermal cloaking and camouflage. However, most thermal metamaterials-based camouflage devices only tune in-plane heat conduction, which may fail to conceal a target from out-of-plane detection. We propose an adaptive radiative thermal camouflage via tuning out-of-plane transient heat conduction, and it is validated by both simulation and experiment. The physics underlying the performance of our adaptive thermal camouflage is based on real-time synchronous heat conduction through the camouflage device and the background plate, respectively. The proposed concept and device represent a promising new approach to fabrication of conductive thermal metamaterials, providing a feasible and effective way to achieve adaptive thermal camouflage.

Active Thermal Wave CloakSupported by the National Natural Science Foundation of China (Grant Nos. 11725521 and 12035004), and the Science and Technology Commission of Shanghai Municipality (Grant No. 20JC1414700).

Active metamaterials have shown huge advantages to control electromagnetic and acoustic waves. However, how to use active metamaterials to control thermal waves has not been explored, though thermal waves are significant in various fields. To address the problem, here we report an active scheme for thermal wave cloaks. The thermal waves are based on conduction and convection, which are dominated by the Fourier and Darcy laws, respectively. By calculating the propagation of thermal waves in a free space, we can derive the global temperature and pressure distributions. We then apply these calculation results to actively control the boundary temperature and pressure, and active thermal wave cloaks can be obtained. Compared with existing passive schemes to control thermal waves, the present active scheme is more flexible for switching on/off and changing geometries. This work provides active and controllable components to thermal wave cloaks, which can be further used to design more active thermal wave metamaterials.

Tunable analog thermal material

Naturally-occurring thermal materials usually possess specific thermal conductivity (κ), forming a digital set of κ values. Emerging thermal metamaterials have been deployed to realize effective thermal conductivities unattainable in natural materials. However, the effective thermal conductivities of such mixing-based thermal metamaterials are still in digital fashion, i.e., the effective conductivity remains discrete and static. Here, we report an analog thermal material whose effective conductivity can be in-situ tuned from near-zero to near-infinity κ. The proof-of-concept scheme consists of a spinning core made of uncured polydimethylsiloxane (PDMS) and fixed bilayer rings made of silicone grease and steel. Thanks to the spinning PDMS and its induced convective effects, we can mold the heat flow robustly with continuously changing and anisotropic κ. Our work enables a single functional thermal material to meet the challenging demands of flexible thermal manipulation. It also provides platforms to investigate heat transfer in systems with moving components.

Experimental Demonstration of Thermal Chameleonlike Rotators with Transformation-Invariant Metamaterials

Intelligent metamaterials have aroused intensive research interest due to their adaptive responses to environmental changes. However, existing transformation-thermotics-based metamaterials have almost no intelligence because their parameters are crucially dependent on environmental parameters. Therefore, once the environment changes, these thermal metamaterials can no longer work, which limits practical applications. To solve the problem, we propose a mechanism for intelligent thermal metamaterials based on transformation-invariant metamaterials. As an application, we design intelligent thermal rotators, which can guide the direction of heat flux with different environmental parameters. Since the adaptive behavior is similar to chameleons, the present rotators are also called chameleonlike rotators. We further perform finite-element simulations and laboratory experiments to validate the scheme, and the chameleonlike behavior is clearly demonstrated. These results have potential applications for the implementation of adaptive and adjustable thermal metamaterials. Similar behaviors can also be expected in other fields like hydrodynamics.

Thermal Metamaterial: Fundamental, Application, and Outlook.

SummaryThermal metamaterials have amazing properties in heat transfer beyond naturally occurring materials owing to their well-designed artificial structures. The idea of thermal metamaterial has completely subverted the design of thermal functional devices and makes it possible to manipulate heat flow at will. In this perspective, we review the up-to-date progress of thermal metamaterials starting from 2008. We focus on both the key theoretical fundamentals and techniques for applications and give a perspective of scale-based classification on thermal metamaterials' theories and applications. We also discuss the junction between macroscale and microscale design methods and propose some prospects for the future trend of this field.

A Continuously Tunable Solid-Like Convective Thermal Metadevice on the Reciprocal Line.

The emerging thermal metamaterials and metadevices demonstrate significant potential to transform thermal conduction. However, the thermal conductivities of existing devices are all restricted at fixed values if the configuration or constituent materials are static. Thermal convection provides an additional tool to boost and flexibly modify the heat transfer in moving matter, but it is essentially distinct from thermal conduction since the Onsager reciprocity is generally broken in the former but preserved in the latter. Therefore, it is difficult to use convective components for sophisticated control of conductive heat. Here, it is shown that a convective system can be made undistinguishable from a conductive one in principle, by discovering and operating on the reciprocal line of mechanically rotating systems. The realized thermal metadevice can thus mimic a solid-like material whose thermal conductivity dynamically covers a wide range. It offers great possibilities of real-time smooth control over heat transfer for broad applications.

Thermal wave crystals based on the dual-phase-lag model

Thermal wave crystals based on the dual-phase-lag model are investigated in this paper by both theoretical analysis and numerical simulation to control the non-Fourier heat conduction process. The transfer matrix method is used to calculate the complex dispersion curves. The temperature response is calculated by the finite difference time domain method. The results show that thermal band-gaps exist due to the Bragg-scattering. The thermal wave impedance and center frequencies are introduced to predict band-gaps theoretically. The key parameters characterizing the band-gaps are analyzed. Our results show that the larger the difference in the thermal wave impedances is, the wider of the thermal band-gaps will be. This study demonstrates a type of the thermal metamaterials which have potential innovative applications such as thermal imagining, thermal diodes and thermal waveguides for energy transmission.

Omnithermal metamaterials switchable between transparency and cloaking

Transparency and cloaking are two typical functions of thermal metamaterials that have attracted intensive research interest. However, two restrictions remain to be solved. The first one is that existing studies considered only one or two modes of heat transfer, which may not be consistent with practical conditions because conduction, radiation, and convection often coexist. The second one is that transparency and cloaking cannot be switched between at will. To solve these problems, we propose an effective medium theory to handle conductive, radiative, and convective (herein called omnithermal) processes simultaneously, which is based on the Fourier law, the Rosseland diffusion approximation, and the Darcy law. With the present theory, we further design an omnithermal metamaterial switchable between transparency and cloaking, which results from the nonlinear properties of radiation and convection. Finite-element simulations indicate that our scheme is robust under different boundary conditions. These results have potential applications such as in thermal camouflage, nonlinear thermotics, and intelligent thermotics.

High-performance infrared thermal radiation suppression metamaterials enabling inhibited infrared emittance and decreased temperature simultaneously

Infrared thermal radiation suppression techniques are vital to the survival of various vehicles/targets, and low-emissivity materials are conventionally employed to reduce infrared radiant power of targets. However, infrared radiant power depends not only on infrared emittance but also heavily on the temperature according to Steven-Boltzmann law (P=εσT4). In this work, it is for the first time, a novel type of infrared stealth material based on tailoring radiative properties in an ultra-broadband ranging from 0.4 µm to 14 µm is proposed. A low emittance in atmosphere window (3–5 µm, 8–14 µm) is achieved to suppress infrared radiation, and a high emittance from 5 to 8 µm is obtained to reduce temperature via radiative cooling from metamaterial surface to the atmosphere. Meanwhile, low absorptance in the solar spectra (0.4–2.5 µm) can help to resist the solar heat. As a result, the infrared radiant power in the atmospheric window is prominently reduced benefiting from low emittance and decreased temperature. This work helps guide the design of more effective infrared stealth materials and paves the way for the applications of metamaterials in infrared stealth applications.

Design of Thermal Metamaterials with Excellent Thermal Control Functions by Using Functional Nanoporous Graphene

Thermal metamaterials can effectively manipulate heat flux to achieve different thermal management functions, such as thermal cloak, concentrator, and rotator. To date, most of these metamaterials are based on macroscopic compound structures, such as metal/polymer. Herein, the concept of thermal metamaterials is extended to two‐dimensional (2D) graphene‐based systems because of their fast response speeds, in contrast to traditional three‐dimensional metamaterials. Three thermal metamaterials with heterogeneous thermal parameters are constructed using nano‐holed graphene, and some extraordinary thermal phenomena, such as effective shielding, accumulation and rotation of heat flux, are observed due to the significant anisotropic thermal conductivity of these 2D systems. Moreover, these thermal phenomena are insensitive to external disturbances. For designing thermal metamaterials, this study provides a novel approach, which can be applied to other 2D thermal functional materials.

Effective medium theory for thermal scattering off rotating structures.

Controlling heat transfer with artificial functional materials has been a promising route towards the efficient and smart utilization of thermal energy in modern society. At the macroscopic scale, thermal metamaterials have demonstrated versatile functionalities in manipulating thermal conduction. One major method is the effective medium theory, which provides a reliable approximation for the material parameters of the composite. Although most of thermal metamaterials use static components, recent devices with integrated moving parts are attracting great interest thanks to their high efficiency and flexibility. However, the effective medium theory for thermal scattering off such devices has not been well established, due to the fundamental difference between thermal convection and conduction. Here, we provide a thorough study on heat transfer through mechanically rotating structures. It is shown that the effective thermal conductivity of a rotating structure can be rigorously described in a complex plane. The analytical expressions of the effective thermal conductivity for structures with rotating multiple layers are formulated, which explicitly capture their influences on the surrounding temperature field. We validate the theory and numerically demonstrate the rotated and unrotated temperature distributions generated around a single structure. Our theory is expected to become a design recipe for novel thermal metamaterials and meta-devices.

Sensitively tuning the thermal conductivity of diamane via engineering the mass of functional groups

Sensitively tuned thermal conductivity is highly desired for many practical applications, e.g. thermoelectricity and thermal metamaterials. Our theoretical calculations reveal that the thermal conductivities of functionalized two-dimensional (2D) materials show a rapid response to the increasing mass of functional groups, in which we take the newly synthesized diamane as an example and artificially tune the mass of hydrogen atoms to mimic the surface ligands of different masses. Our results indicate the sensitive influence of the mass of functional groups on the vibration and phonon transport properties of 2D materials. As the mass increases, the phonons are appreciably softened which results in reduced group velocities and the squeezed distribution of phonons in the frequency domain. The latter factor leads to an enlarged phonon-scattering phase space. Consequently, the intensified phonon–phonon scattering combined with the reduced group velocities could make the thermal conductivities decrease by ∼80%. Our findings suggest an intriguing strategy to sensitively tune the thermal conductivities of 2D materials via tailoring the mass of functional groups.