Encrypted Thermal Printing with Regionalization Transformation

Analytical realization of complex thermal meta-devices

Fourier's law dictates that heat flows from warm to cold. Nevertheless, devices can be tailored to cloak obstacles or even reverse the heat flow. Mathematical transformation yields closed-form equations for graded, highly anisotropic thermal metamaterial distributions needed for obtaining such functionalities. For simple geometries, devices can be realized by regular conductor distributions; however, for complex geometries, physical realizations have so far been challenging, and sub-optimal solutions have been obtained by expensive numerical approaches. Here we suggest a straightforward and highly efficient analytical de-homogenization approach that uses optimal multi-rank laminates to provide closed-form solutions for any imaginable thermal manipulation device. We create thermal cloaks, rotators, and concentrators in complex domains with close-to-optimal performance and esthetic elegance. The devices are fabricated using metal 3D printing, and their omnidirectional thermal functionalities are investigated numerically and validated experimentally. The analytical approach enables next-generation free-form thermal meta-devices with efficient synthesis, near-optimal performance, and concise patterns.

Ultrawide Spectra Camouflage Coatings from Metallic Flake Powder

Ultrawide-spectra-compatible camouflage materials are imperative for military science and national security due to the continuous advancement of various sophisticated multispectral detectors. However, ultrawide spectra camouflage still has challenges, as the spectral requirements for different bands are disparate and even conflicting. This work demonstrates an ultrawide spectra camouflage material compatible with visible (VIS, 400-800 nm), infrared (IR, 3-5 and 8-14 μm), and microwave (S-Ku bands, 2-12 GHz). The carbon nanotubes adsorbed on porous anodic alumina/aluminum flake powder (CNTs@PAA/AFP) material for ultrawide spectra camouflage is composed of bioinspired porous alumina surface layers for low visible reflection and aluminum flake powder substrate for low infrared emissivity, while the surface of the porous alumina layers is loaded with carbon nanotubes for microwave absorption. Compared with previous low-emissivity materials, CNTs@PAA/AFP has omnidirectional low reflectance (Ravg = 0.29) and high gray scale (72%) in the visible band. Further, it exhibits low emissivity (ε3-5μm = 0.15 and ε8-14μm = 0.18) in the dual infrared atmospheric window, which reduces the infrared lock-on range by 59.6%/49.8% in the mid/far-infrared band at high temperatures (573 K). The infrared camouflage performance calculated from the radiation temperature of CNTs@PAA/AFP coatings is enhanced to over 65%, which is at least 4 times greater than that of its substrate. In addition, the CNTs@PAA/AFP coating achieves high microwave absorption (RLmin = -42.46 dB) and an effective absorption bandwidth (EAB = 7.43 GHz) in the microwave band (S-Ku bands) due to the enhancement of interfacial polarization and conductive losses. This study may introduce new insight and feasible methods for multispectral manipulation, electromagnetic signal processing, and thermal management via bioinspired structural design and fabrication.

Higher-Order Topological In-Bulk Corner State in Pure Diffusion Systems

Compared with conventional topological insulator that carries topological state at its boundaries, the higher-order topological insulator exhibits lower-dimensional gapless boundary states at its corners and hinges. Leveraging the form similarity between Schrödinger equation and diffusion equation, research on higher-order topological insulators has been extended from condensed matter physics to thermal diffusion. Unfortunately, all the corner states of thermal higher-order topological insulator reside within the band gap. Another kind of corner state, which is embedded in the bulk states, has not been realized in pure diffusion systems so far. Here, we construct higher-dimensional Su-Schrieffer-Heeger models based on sphere-rod structure to elucidate these corner states, which we term "in-bulk corner states." Because of the anti-Hermitian properties of diffusive Hamiltonian, we investigate the thermal behavior of these corner states through theoretical calculation, simulation, and experiment. Furthermore, we study the different thermal behaviors of in-bulk corner state and in-gap corner state. Our results would open a different gate for diffusive topological states and provide a distinct application for efficient heat dissipation.

Brochosome-inspired binary metastructures for pixel-by-pixel thermal signature control

Microscale thermal signature control using incoherent heat sources remains challenging, despite recent advancements in plasmonic materials and phase-change materials. Inspired by leafhopper-generated brochosomes, we design binary metastructures functioning as pixel twins to achieve pixelated thermal signature control at the microscale. In the infrared range, the pixel twins exhibit distinct emissivities, creating thermal counterparts of "0-1" binary states for storing and displaying information. In the visible range, the engineered surface morphology of the pixel twins ensures similar scattering behaviors. This renders them visually indistinguishable, thereby concealing the stored information. The brochosome-like pixel twins are self-emitting when thermally excited. Their structure-enabled functions do not rely on the permittivities of specific materials, which distinguishes them from the conventional laser-illuminated plasmonic holographic metasurfaces. The unique combination of visible camouflage and infrared display offers a systemic solution to microscale spatial control of thermal signatures and has substantial implications for optical security, anticounterfeiting, and data encryption.

Can Adhesion Energy Optimize Interface Thermal Resistance at a Soft/Hard Material Interface?

Thermal resistance at a soft/hard material interface plays an undisputed role in the development of electronic packaging, sensors, and medicine. Adhesion energy and phonon spectra match are two crucial parameters in determining the interfacial thermal resistance (ITR), but it is difficult to simultaneously achieve these two parameters in one system to reduce the ITR at the soft/hard material interface. Here, we report a design of an elastomer composite consisting of a polyurethane-thioctic acid copolymer and microscale spherical aluminum, which exhibits both high phonon spectra match and high adhesion energy (>1000 J/m²) with hard materials, thus leading to a low ITR of 0.03 mm²·K/W. We further develop a quantitative physically based model connecting the adhesion energy and ITR, revealing the key role the adhesion energy plays. This work serves to engineer the ITR at the soft/hard material interface from the aspect of adhesion energy, which will prompt a paradigm shift in the development of interface science.

Photoresponsive Diarylethene-Containing Polymers: Recent Advances and Future Challenges

Photoresponsive polymers have attracted increasing interest owing to their potential applications in anticounterfeiting, information encryption, adhesives, etc. Among them, diarylethene (DAE)-containing polymers are one of the most promising photoresponsive polymers and have unique thermal stability and fatigue resistance compared to azobenzene- and spiropyran-containing polymers. Herein, the design of DAE-containing polymers based on different types of structures, including main chain polymers, side-chain polymers, and crosslinked polymers, is introduced. The mechanism and applications of DAE-containing polymers in anti-counterfeiting, information encryption, light-controllable adhesives, and photoinduced healable materials are reviewed. In addition, the remaining challenges of DAE-containing polymers are also discussed.

Homogeneous Zero-Index Thermal Metadevice for Thermal Camouflaging and Super-Expanding

The infinite effective thermal conductivity (IETC) can be considered to be an equivalence of the effective zero index in photonics. A recent highly rotating metadevice has been discovered to approach near IETC, subsequently demonstrating a cloaking effect. However, this near IETC, related to a rotating radius, is quite inhomogeneous, and the high-speed rotating motor also needs a high energy input, limiting its further applications. Herein, we propose and realize an evolution of this homogeneous zero-index thermal metadevice for robust camouflaging and super-expanding through out-of-plane modulations rather than high-speed rotation. Both the theoretical simulations and experiments verify a homogeneous IETC and the corresponding thermal functionalities beyond cloaking. The recipe for our homogeneous zero-index thermal metadevice involves an external thermostat, which can be easily adjusted for various thermal applications. Our study may provide meaningful insights into the design of powerful thermal metadevices with IETCs in a more flexible way.

Polarization-driven thermal emission regulator based on self-aligned GST nanocolumns

The increasing advances in thermal radiation regulators have attracted growing interest, particularly in infrared sources, thermal management, and camouflage. Despite many advances in dynamic thermal emitters with great controllability, sustained external energy is required to maintain the desired emission. In this study, we present a polarization-driven thermal emission regulator based on a two-way control: i) phase change and ii) polarization tuning. Based on a conventional, non-volatile phase change material, i.e., Ge₂Sb₂Te₅ (GST), we newly introduce an anisotropic medium for facile emissivity regulation without heat energy consumption. A rigorous coupled-wave analysis method provides design guidelines for finding optimal structural parameters. We utilized a simple glancing angle deposition process which induces tilted self-aligned nanocolumns with anisotropic properties. The fabricated sample shows polarization-sensitive thermal regulation through thermal imaging spectroscopic measurement. Additionally, we manufactured a multispectral visibly/thermally camouflaged patch that identifies encrypted information at a specific polarization state for a proof-of-concept demonstration.

Nonlinear thermal responses in geometrically anisotropic metamaterials

Nonlinear metamaterials have great potential in heat management, which has aroused intensive research interest in both theory and application, especially for their response to surroundings. However, most existing works focus on geometrically isotropic (circular) structures, limiting the potential versatile functionalities. On the other hand, anisotropy in architecture promisingly offers an additional degree of freedom in modulating directional heat transfer. Here, we investigate nonlinear composition effects in geometrically anisotropic (confocal elliptical) thermal medium under the framework of effective medium approximation, and deduce a series of general formulas for quantitatively predicting nonlinearity enhancement. Enhancement coefficients are analytically derived by the Taylor expansion method in different nonlinearity cases. In particular, we find that some coupling conditions can greatly promote the nonlinear modulation coefficients, introducing stronger enhancement beyond isotropic construction. Our theoretical predictions are verified by finite-element simulation, and feasible experimental suggestions are also given. For extending these results to practical scenes, two intelligent thermal metadevices are designed in proof of concept and demonstrated by numerical simulation. Our works provide a unified theory for anisotropic nonlinear thermal metamaterial design and may benefit flexible applications in self-adaptive thermal management, such as switchable cloaks, concentrators, or macroscopic thermal diodes.

Mass Diffusion Metamaterials with "Plug and Switch" Modules for Ion Cloaking, Concentrating, and Selection: Design and Experiments

The outstanding abilities of metamaterials to manipulate physical fields are extensively studied in both wave-based and diffusion-based fields. However, mass diffusion metamaterials, with the ability to manipulate diffusion with practical applications associated with chemical and biochemical engineering, have not yet been experimentally demonstrated. In this work, ion cloaking, concentrating, and selection in liquid solvents are verified by both simulations and experiments, and the concept of a "plug and switch" metamaterial is proposed based on scattering cancellation (SC) to achieve switchable functions by plugging modularized functional units into a functional motherboard. Plugging in any module barely affects the environmental diffusion field, but the module choice impacts different diffusion behaviors in the central region. Cloaking strictly hinds ion diffusion, and concentrating increase diffusion flux, while cytomembrane-like ion selection permits the entrance of some ions but blocks others. In addition, these functions are demonstrated in special applications like the catalytic enhancement by the concentrator and the protein protection by the ion selector. This work not only experimentally demonstrates the effective manipulation of mass diffusion by metamaterials, but also shows that the "plug and switch" design is extensible and reconfigurable. It facilitates novel applications including sustained drug release, catalytic enhancement, bioinspired cytomembranes, etc.

Tunable Thermoresponsive Flexible Films for Adaptive Temperature Management and Visual Temperature Monitoring

Effective temperature management is essential for human thermal comfort and health. Although various temperature regulation materials have been proposed previously, there are few materials that have the dual functions of temperature monitoring and thermal management. Herein, a thermoresponsive form-stable flexible film based on phase-change materials (PCMs) and polydimethylsiloxane (PDMS) is rationally designed. The resultant versatile PCM@PDMS film is able to absorb and release heat responding to temperature stimuli and good mechanical strength. Moreover, optical visibility of the PCM@PDMS film can be reversibly converted between opaque and transparent states to monitor temperature. The switching principle is that solid PCMs embedded in the PDMS would be melted into liquid PCMs to enable light through the PCM@PDMS. The thermal experiment results suggest that the PCM@PDMS films can effectively regulate the human body temperature to adapt to the demanding environment (self-heating more than 3 °C in the cold environment or self-cooling more than 4 °C in the hot environment). Such dual-function films open a pathway to develop smart personalized thermoregulation materials for human body thermal management and temperature monitoring.

A Real-Time Self-Adaptive Thermal Metasurface

Emerging metamaterials have served as an efficient strategy for the realization of unconventional heat control and management using structural thermal properties, and many functional thermal metadevices have been investigated. However, thermal functions are usually fixed or limited in the switching range. Thus far, real-time thermal regulation is elusive for thermal metamaterials because of deterministic artificial metastructures and uncontrollable phase transitions, coupled with the absence of dynamic adaptability. Here, a self-adaptive metasurface platform to implement programmable thermal functions via the automatic evolution of thermoelectric heat sources and real-time control of the driven voltage is reported. The proof-of-concept smart platform experimentally demonstrates arbitrary switching between elaborate thermal patterns consolidated into an active thermoelectric element matrix. Further, thermal pixels and feedback control systems are integrated into printed circuit boards, resulting in self-adaptability to any thermal requirements. This study sets up a new paradigm for arbitrary transitions between exquisite thermal patterns and is expected to pave the way for real-time thermal management in a programming formation.

Passive Ultra-Conductive Thermal Metamaterials

Ultra-conductive heat transport showcases significant potentials in popular thermal managements with convection, phase change, and heat source. However, it is captivated impossible for passive thermal manipulation, usually bounded by intrinsic thermal conductivities of natural materials, to outperform these active recipes in need of extra energy payload. Here, a robust recipe to create passive ultra-conductive thermal metamaterials consisting of nothing but bulk natural materials is reported. Thanks to the local thermal resistance regulation by vertical thermal transport channel, the proof-of-concept thermal metamaterials experimentally demonstrate extreme effective conductivity (1915 W m^-1  K^-1 ) solely with naturally occurring materials. The purely conductive modulation without any external energy is comparable to active counterparts, and further reveals its robustness and unexpected convenience. The findings construct a high-efficient paradigm of passive thermal management with ultra-conductive heat transport, and further hint potentials in other Laplace fields, e.g., DC and magnetostatics.

Study on Phonon Localization in Silicon Film by Molecular Dynamics

In recent years, nanoscale thermal cloaks have received extensive attention from researchers. Amorphization, perforation, and concave are commonly used methods for building nanoscale thermal cloaks. However, the comparison of the three methods and the effect of different structural proportions on phonon localization have not been found. Therefore, in this paper, an asymmetrical structure is constructed to study the influence of different structure proportions on phonon localization by amorphization, perforation, and concave silicon film. We first calculated the phonon density of states (PDOS) and the mode participation rate (MPR). To quantitatively explore its influence on phonon localization, we proposed the concept of the degree of phonon localization (DPL) and explored the influence of center and edge effects on phonon localization. We found that for different processing methods, the degree of phonon localization increased with the increase in the processing regions. Compared to the edge, the center had a stronger influence on phonon localization, and the higher the degree of disorder, the stronger the phonon localization. Our research can guide the construction of a nanoscale thermal cloak.

Thermal Cloaking in Nanoscale Porous Silicon Structure by Molecular Dynamics

Nanoscale thermal cloaks have great potential in the thermal protection of microelectronic devices, for example, thermal shielding of thermal components close to the heat source. Researchers have used graphene, crystalline silicon film, and silicon carbide to design a variety of thermal cloaks in different ways. In our previous research, we found that the porous structure has lower thermal conductivity compared to bulk silicon; thus, so we tried to use the porous structure to construct the functional region to control the heat flux. We first calculated the thermal conductivity of crystalline silicon and porous silicon films by means of nonequilibrium molecular dynamics, proving that the porous structure satisfied the conditions for building a thermal cloak. A rectangular cloak with a porous structure was constructed, and a crystalline silicon film was used as a reference to evaluate its performance by the index of the ratio of thermal cloaking. We found that the thermal cloak built with a porous structure could produce an excellent cloaking effect. Lastly, we explain the mechanism of the cloaking phenomenon produced by a porous structure with the help of phonon localization theory. Porous structures have increased porosity compared to bulk silicon and are not conducive to phonon transport, thus producing strong phonon localization and reducing thermal conductivity. Our research expands the construction methods of nanocloaks, expands the application of porous structure materials, and provides a reference for the design of other nanodevices.

Novel connections and physical implications of thermal metamaterials with imperfect interfaces

Thermal metamaterials are of great importance in advanced energy control and management. Previous studies mainly focused on interfaces with perfect bonding conditions. In principle, imperfectness always exists across interface and the effect is intriguingly important with small-length scales. This work reports the imperfect interface effect in thermal metamaterials thoroughly. Low conductivity- and high conductivity-type interfaces are considered. We show that an object can always be made thermally invisible, with the effect of imperfect interface, as that of a homogeneous background material. This unprecedented condition is derived in an exact and analytic form, systematically structured, with much versatile and physical implications. Conditions for thermal shielding and enhancements are analytically found and numerically exemplified, highlighting the specific role of material and geometric parameters. We find that both types of interfaces are complementing with each other which, all together, will constitute a full spectrum to achieve the thermal invisibility. The analytic finding offers a general perception that adds to the understanding of heat transport mechanism across interfaces in thermal metamaterials, in ways that drastically distinct from that of ideal interfaces. This finding opens up new possibilities for the control and management of thermal metamaterials with imperfect bonding interfaces.

Nanoscale Thermal Cloaking in Silicon Film: A Molecular Dynamic Study

Nanoscale thermal shielding is becoming increasingly important with the miniaturization of microelectronic devices. They have important uses in the field of thermal design to isolate electronic components. Several nanoscale thermal cloaks based on graphene and crystalline silicon films have been designed and experimentally verified. No study has been found that simultaneously treats the functional region of thermal cloak by amorphization and perforation methods. Therefore, in this paper, we construct a thermal cloak by the above methods, and the ratio of thermal cloaking and response temperature is used to explore its cloaking performance under constant and dynamic temperature boundary. We find that compared with the dynamic boundary, the cloaking effect produced under the constant boundary is more obvious. Under two temperature boundaries, the thermal cloak composed of amorphous and perforated has a better performance and has the least disturbance to the background temperature field. The phonon localization effect produced by the amorphous structure is more obvious than that of the perforated structure. The phonon localization of the functional region is the main reason for the cloaking phenomenon, and the stronger the phonon localization, the lower the thermal conductivity and the more obvious the cloaking effect. Our study extends the nanoscale thermal cloak construction method and facilitates the development of other nanoscale thermal functional devices.

Robustly printable freeform thermal metamaterials

Thermal metamaterials have exhibited great potential on manipulating, controlling and processing the flow of heat, and enabled many promising thermal metadevices, including thermal concentrator, rotator, cloak, etc. However, three long-standing challenges remain formidable, i.e., transformation optics-induced anisotropic material parameters, the limited shape adaptability of experimental thermal metadevices, and a priori knowledge of background temperatures and thermal functionalities. Here, we present robustly printable freeform thermal metamaterials to address these long-standing difficulties. This recipe, taking the local thermal conductivity tensors as the input, resorts to topology optimization for the freeform designs of topological functional cells (TFCs), and then directly assembles and prints them. Three freeform thermal metadevices (concentrator, rotator, and cloak) are specifically designed and 3D-printed, and their omnidirectional concentrating, rotating, and cloaking functionalities are demonstrated both numerically and experimentally. Our study paves a powerful and flexible design paradigm toward advanced thermal metamaterials with complex shapes, omnidirectional functionality, background temperature independence, and fast-prototyping capability.

Generalized multi-channel scheme for secure image encryption

The ability of metamaterials to manipulate optical waves in both the spatial and spectral domains has provided new opportunities for image encoding. Combined with the recent advances in hyperspectral imaging, this suggests exciting new possibilities for the development of secure communication systems. While traditional image encryption approaches perform a 1-to-1 transformation on a plain image to form a cipher image, we propose a 1-to-n transformation scheme. Plain image data is dispersed across n seemingly random cipher images, each transmitted on a separate spectral channel. We show that the size of our key space increases as a double exponential with the number of channels used, ensuring security against both brute-force attacks and more sophisticated attacks based on statistical sampling. Moreover, our multichannel scheme can be cascaded with a traditional 1-to-1 transformation scheme, effectively squaring the size of the key space. Our results suggest exciting new possibilities for secure transmission in multi-wavelength imaging channels.

Adaptive visible and short-wave infrared camouflage using a dynamically tunable metasurface

In this Letter, a dynamically tunable metasurface, which is based on antimony trisulfide, is introduced. In this structure, first a metal-insulator-metal (MIM) nanocavity is optimized in a way that, upon phase change, the visible response switches from a transmissive colored window into a reflective mirror. Later, an indium tin oxide nanoantenna is integrated on the MIM cavity to provide antireflection in the short-wave infrared (SWIR) range for SWIR mode thermal camouflage. The MIM host acts as a thermally tunable substrate to tune the SWIR response of the design.

High-Temperature Skin Softening Materials Overcoming the Trade-Off between Thermal Conductivity and Thermal Contact Resistance

The trade-off between thermal conductivity (κ) and thermal contact resistance (R_c ) is regarded as a hurdle to develop superior interface materials for thermal management. Here a high-temperature skin softening material to overcome the trade-off relationship, realizing a record-high total thermal conductance (254.92 mW mm^-2 K^-1 ) for isotropic pad-type interface materials is introduced. A highly conductive hard core is constructed by incorporating Ag flakes and silver nanoparticle-decorated multiwalled carbon nanotubes in thermosetting epoxy (EP). The thin soft skin is composed of filler-embedded thermoplastic poly(ethylene-co-vinyl acetate) (PEVA). The κ (82.8 W m^-1 K^-1 ) of the PEVA-EP-PEVA interface material is only slightly compromised, compared with that (106.5 W m^-1 K^-1 ) of the EP core (386 µm). However, the elastic modulus (E = 2.10 GPa) at the skin is significantly smaller than the EP (26.28 GPa), enhancing conformality and decreasing R_c from 108.41 to 78.73 mm² K W^-1 . The thermoplastic skin is further softened at an elevated temperature (100 °C), dramatically decreasing E (0.19 GPa) and R_c (0.17 mm² K W^-1 ) with little change in κ, overcoming the trade-off relationship and enhancing the total thermal conductance by 2030%. The successful heat dissipation and applicability to the continuous manufacturing process demonstrate excellent feasibility as future thermal management materials.

Smart Materials for Dynamic Thermal Radiation Regulation

Thermal radiation in the mid-infrared region profoundly affects human lives in various fields, including thermal management, imaging, sensing, camouflage, and thermography. Due to their fixed emissivities, radiance features of conventional materials are usually proportional to the quadruplicate of surface temperature, which set the limit, that one type of material can only present a single thermal function. Therefore, it is necessary and urgent to design materials for dynamic thermal radiation regulations to fulfill the demands of the age of intelligent machines. Recently, the ability of some smart materials to dynamically regulate thermal radiation has been evaluated. These materials are found to be competent enough for various commands, thereby, providing better alternatives and tremendously promoting the commercial potentials. In this review, the dynamic regulatory mechanisms and recent progress in the evaluation of these smart materials are summarized, including thermochromic materials, electrochromic materials, mechanically and humidity responsive materials, with the potential applications, insufficient problems, and possible strategies highlighted.

A Triple-Mode Midinfrared Modulator for Radiative Heat Management of Objects with Various Emissivity

Thermal management is ubiquitous in the modern world and indispensable for a sustainable future. Radiative heat management provides unique advantages because the heat transfer can be controlled by the surface. However, different object emissivities require different tuning strategies, which poses challenges to develop dynamic and universal radiative heat management devices. Here, we demonstrate a triple-mode midinfrared modulator that can switch between passive heating and cooling suitable for all types of object surface emissivities. The device comprises a surface-textured infrared-semiabsorbing elastomer coated with a metallic back reflector, which is biaxially strained to sequentially achieve three fundamental modes: emission, reflection, and transmission. By analyzing and optimizing the coupling between optical and mechanical properties, we achieve a performance as follows: emittance contrast Δε = 0.58, transmittance contrast Δτ = 0.49, and reflectance contrast Δρ = 0.39. The device can provide a new design paradigm of radiation heat regulation for wearable, robotics, and camouflage technologies.

Metamaterials-Enabled Sensing for Human-Machine Interfacing

Our modern lives have been radically revolutionized by mechanical or electric machines that redefine and recreate the way we work, communicate, entertain, and travel. Whether being perceived or not, human-machine interfacing (HMI) technologies have been extensively employed in our daily lives, and only when the machines can sense the ambient through various signals, they can respond to human commands for finishing desired tasks. Metamaterials have offered a great platform to develop the sensing materials and devices from different disciplines with very high accuracy, thus enabling the great potential for HMI applications. For this regard, significant progresses have been achieved in the recent decade, but haven’t been reviewed systematically yet. In the Review, we introduce the working principle, state-of-the-art sensing metamaterials, and the corresponding enabled HMI applications. For practical HMI applications, four kinds of signals are usually used, i.e., light, heat, sound, and force, and therefore the progresses in these four aspects are discussed in particular. Finally, the future directions for the metamaterials-based HMI applications are outlined and discussed.

Thermal Metamaterial: Fundamental, Application, and Outlook

Thermal 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.

Selective Emitter with Engineered Anisotropic Radiation to Minimize Dual-Band Thermal Signature for Infrared Stealth Technology

Selective emitters comprising plasmonic resonators have been exploited for cooling devices or infrared stealth technology. While selective emitters have been designed using odd-order resonances, even-order resonances also emit anisotropic thermal radiation signals. Thermal radiation by even-order resonances in selective emitters can be experimentally detected by thermal imaging cameras, and such thermal emissions often degrade the observability of infrared detectors, rendering them inapplicable to infrared stealth technology. Here, a selective emitter with extremely low thermal radiation signature in a dual-band range, a detection range by an infrared detector, is proposed with engineering anisotropic thermal radiation by even-order resonances. To minimize infrared signature in a dual-band range, the characterization of even-order resonances of gap plasmon metasurfaces is achieved based on vectorial diffraction within a relative error of 10%. Thermal radiation by even-order resonance has been shown to be highly directional and can be experimentally measured using mid-wave infrared images. Based on model prediction, the proposed selective emitter reduces mid-wave infrared signatures and long-wave infrared signatures by factors of 37.95 and 38.06, respectively, compared with those of blackbody surfaces. In addition, numerically confirmed thermal signature reduction and captured mid-wave infrared images indicate excellent thermal camouflage performance of the selective emitter with background medium. Thus, the characterization of even-order resonances provides a basis for the design of metasurfaces that can be employed for multispectral applications, especially infrared stealth technology.

Dipole-assisted thermotics: Experimental demonstration of dipole-driven thermal invisibility

Thermal management has made considerable progress in the past decade for the emerging field of thermal metamaterials. However, two severe problems still handicap the development of thermal metamaterials. That is, thermal conductivities should be singular and uncommon as required by corresponding theories. To solve these problems, here we establish the theory of dipole-assisted thermotics. By tailoring the thermal dipole moment, thermal invisibility can be achieved without the requirements of singular and uncommon thermal conductivities. Furthermore, finite-element simulations and laboratory experiments both validate the theoretical analyses. The performance of the dipole-driven scheme is excellent in both two and three dimensions, and in both steady and transient states. Dipole-assisted thermotics not only offers a distinct mechanism to achieve thermal invisibility, but also has potential applications in thermal management such as infrared signature reduction, thermal protection, and infrared camouflage.

Radiative metasurface for thermal camouflage, illusion and messaging

Thanks to the conductive thermal metamaterials, novel functionalities like thermal cloak, camouflage and illusion have been achieved, but conductive metamaterials can only control the in-plane heat conduction. The radiative thermal metamaterials can control the out-of-plane thermal emission, which are more promising and applicable but have not been studied as comprehensively as the conductive counterparts. In this paper, we theoretically investigate the surface emissivity of metal/insulator/metal (MIM, i.e., Au/Ge/Au here) microstructures, by the rigorous coupled-wave algorithm, and utilize the excitation of the magnetic polaritons to realize thermal camouflage through designing the grating width distribution by minimizing the temperature standard deviation of the overall plate. Through this strategy, the hot spot in the original temperature field is removed and a uniform temperature field is observed in the infrared camera instead, demonstrating the thermal camouflage functionality. Furthermore, thermal illusion and thermal messaging functionalities are also demonstrated by resorting to using such an emissivity-structured radiative metasurface. The present MIM-based radiative metasurface may open avenues for developing novel thermal functionalities via thermal metasurface and metamaterials.

Thermal surface transformation and its applications to heat flux manipulations

A new method to control heat flux, called thermal surface transformation (TST), is introduced from transformation thermodynamics. Compared with transformation thermodynamics, TST has many advantage. First, there is no mathematical calculation during the whole process in TST (novel thermal devices can be designed graphically in a surface-to-surface way). Second, all thermal devices of various functions, shapes and sizes designed by TST only require one homogenous anisotropic thermal medium, i.e., thermal-null medium (TNM). With the help of the effective medium theory, TNM can be realized by layered copper and expanded polystyrene, whose performance on controlling heat flux by TST is verified by numerical simulations. Many examples are given, including thermal imaging devices, thermal unidirectional cloak, concentrator, rotator and thermal focusing devices.