Thermal Cloak Research Articles

Mechanism and performance evaluation of a thermoelectric-chip-based cloak for active regulation of heat flow

In the field of heat manipulation, active thermal cloaks based on thermoelectric chips (TECs) have received much less attention than passive thermal cloaks based on metamaterials. This work focuses on active thermal cloaks and explores in-depth their mechanism of heat flow regulation. For a two-dimensional circular object located in a rectangular background plate with uniform temperature gradient, the theoretical design of an active cloak was realized by analytically solving for the target temperature field. A parameter study was conducted numerically for the designed thermal cloak. The results show that the object-to-background thermal conductivity ratio (m) is the key parameter that determines the working mode of the cloak. When m < 1, the TECs work in a heating mode with less power consumption; when m > 1, the TECs work in a chilling/heat pump mode with higher power consumption. If the change of a certain parameter increases the heat flow regulation, it will also cause performance degradation and an increase of the thermal cloak’s power consumption, especially when m is greater than 1. Improvement can be achieved by increasing the number of TECs (N), but the impact of N diminishes rapidly beyond a certain value. The systematic study in this work reveals the high flexibility and wide adaptability of the TE-chip-based thermal cloak in heat flow regulation.

Topology optimization of thermal cloak using the adjoint lattice Boltzmann method and the level-set method

This paper presents a topology optimization (TO) method for thermal cloak design based on the adjoint lattice Boltzmann method and the level-set method. The gradient-type TO method presented is of high computational efficiency and of strong parallelism scalability since it completely circumvents large-scale matrix operations. After validated, the TO method is applied to design the thermal cloak composed of common isotropic bulk materials instead of anisotropic meta-materials. A clearly defined material distribution is obtained without non-physical grayscales. Excellent camouflage performance is achieved in the final design with the objective, namely the dimensionless squared norm of the temperature deviation, reduced from 1 to 7.57 × 10−5. Effects of the dam structures and bypass structures in the designed cloak on the heat transfer are discussed in detail. All results in this work are verified by the finite volume method with the relative error lower than 2 %. A comprehensive parametric study is conducted to reveal the effects of the initial structure, mesh number, thermal conductivity, the subdomain size, as well as the location, shape and topology of the cloaking object. The results have demonstrated the effectiveness, robustness and flexibility of the presented TO method on the practical design of thermal cloak.

Exact thermal invisibility for spherical cloaks with imperfect interfaces

We present new exact results for the design of spherical thermal cloaks with the effect of imperfect interfaces. Thermal metamaterials are of great importance in advanced energy control and management. However, nearly all relevant studies considered that interfaces are ideally perfect. In principle, bonding imperfectness always exists at interfaces, and this effect is particularly important in small-length scales. Here, we will examine in detail the effect of bonding imperfectness on the performance of thermal functionality. The thermal metamaterial is made of a homogeneous spherically anisotropic material with a constant conductivity tensor. Low conductivity- and high conductivity-type interfaces are considered. We show how the anisotropic layer, together with the effect of imperfect bonding interfaces, can be made thermally invisible. An exact condition for thermal invisibility is derived in a simple algebraic form. Conditions for thermal shielding or enhancement are theoretically analyzed and numerically exemplified, in which relevant material and geometric parameters can be tuned to achieve the functionality. In addition, numerical simulations based on finite element calculations are carried out to validate our analytic solutions. The present findings offer a general guideline in the design of spherical thermal metamaterials with imperfect interfaces.

Experimental demonstration of thermal cloaking metastructures designed by topology optimization

Recent developments in information science have contributed to several innovative advances in thermal flow manipulation. Topology optimizations provide numerically optimal structures enabling superior performance from this manipulation. For cloaking and concentrating heat flux, we describe the experimental implementation of thermal metastructures designed by topology optimization. These metastructures are composed of steel, copper, and polydimethylsiloxane (PDMS), with their spatial compositions optimized through topology optimization without requiring metamaterials. Their performances in experiments were compared with those obtained in numerical simulations employing two different qualitative evaluations, one adopting objective functions based on temperature distribution measurements, the other comparing temperature distribution data around the thermal metastructures. The thermal metastructures designed by topology optimization performed as well in the experiments conducted as in the simulations. The differences in performance between numerical simulations and experiments and their causes are discussed.

Experimental investigation of thermal architected metamaterials for regulating transient heat transfer

Metamaterials have innovative functionalities as a result of artificial design. By arbitrarily designing thermal conductivity, thermal metamaterials have achieved thermal invisible cloaks, camouflages, and deceptions in steady-state. However, density and specific heat capacity have sometimes been overlooked, despite the fact that they are critical for managing transient heat flow and that density are an indicator of creating lightweight materials. The traditional materials are incapable of synchronously controlling thermal conductivity, density, and specific heat capacity. In this research, we experimentally investigate a novel thermal architected metamaterial that can modulate (accelerate or decelerate) transient heat transfer velocity without disrupting the steady-state. The novel type of thermal architected metamaterial achieves efficiency using a topology based on neutral inclusion theory. The transient heat transfer velocity is regulated by altering the effective density and specific heat capacity on the one hand, while the steady-state heat transfer is maintained by keeping the effective thermal conductivity on the other. The density of the architected metamaterial could be adjusted by selecting an appropriate specific heat capacity. The method we provide can be utilized to create lightweight energy-efficient materials (by accelerating transient heat transfer) as well as lightweight engineered thermal protection materials (by decelerating transient heat transfer). Moreover, the proposed thermal architected metamaterial could control heat flow velocity continuously, and this characteristic covers the gaps left by natural materials.

Janus functional thermal metadevice with predictable thermal rotation

The Janus thermal metadevice, which displays path-dependent functionalities, has received growing attention. However, the previous design of a metadevice with Janus features required a complex anisotropic parameter arrangement and challenging fabrication realizations, which inevitably resulted in overmuch thermal contact resistance. Using natural inclusion iteration, we present a bilayer Janus thermal metadevice with isotropic geometry and homogenous parameters that exhibit thermal concentration on the x-axis and thermal cloak on the y-axis. The proof-of-concept experimental results unambiguously demonstrate the path-dependent functionality, which was achieved by the compact bilayer structure of thermal diffusion media. Additionally, a metadevice with configurable anisotropic parameters can exhibit predictable thermal rotation functions in additional directions.

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.

Analytical solution of cloaking in creeping flow inside composite porous media with simultaneous heat transfer

ABSTRACT This paper presents a theoretical analysis of steady-state creeping flow in a composite porous domain with coupling heat transfer. The problem arises in thermal cloaking studies in which it is desirable to hide an obstacle from the surrounding hydrodynamic and thermal fields, by suitably covering the obstacle with a cloak having specific material properties. The particular system under investigation consists of a porous composite region, with a thermal-hydro field driven by imposed temperature and pressure difference in the surrounding. Both two dimensional circular and three-dimensional axisymmetric spherical annuli are considered. Under the validity of Darcy law for a porous medium, an exact solution satisfying the continuity, momentum and energy equations at any Péclet number is derived. The analytical results have been numerically verified using the commercial code COMSOL.

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.

The Design of Dynamic Thermal Metadevice with Vertical Rotation Coupling Mechanisms for Switchable Effects

Recently, thermal metamaterials which can control heat flux at will have attracted many researchers and they can be used in many thermal application scenarios such as thermal concentration and thermal cloak. In this paper, this dynamic thermal metadevice with a vertical rotation coupling component can achieve the target of switchable thermal effect based on effective medium theory and scattering cancellation method. Firstly, this metadvice can adjust the effective thermal conductivities in the region with two novel vertical rotation rings by tuning the rotational velocity. Additionally, an almost undistorted and nonspinning thermal field in the central region can be obtained via this novel design. Furthermore, compared with the previous work, this metadevice can implement the continuous modulation of the effective thermal conductivity at will by adopting a vertical rotation scheme, realizing various thermal functionalities such as thermal concentration and thermal cloak. Then, we demonstrate the switchable effect of this dynamic thermal metadevice in simulations. Meanwhile, the variations of temperature along the measured horizonal line are also shown in simulations. In contrast to previous approaches, this dynamic thermal metadevice may provide more dynamic thermal manipulation inspirations for designing novel multifunctional thermal metadevices.

Design of thermal cloaks with isotropic materials based on machine learning

Thermal manipulation has been widely researched due to its potentials in novel functions, such as cloaking, illusion and sensing. However, thermal manipulation is often realized by metamaterials which entails extreme material properties. Here, we propose a machine learning based thermal cloak consisting of a finite number of layers with isotropic materials. An artificial neural network is established to intelligently learn the relation between each layer’s constitutive properties and the cloaking performances. Optimal material properties are retrieved so that heat flows can be directed to detour the cloaked object without any invasion, as if the object is not there. The designed cloak demonstrates both easiness to implement in applications and excellent performances in thermal invisibility, which are verified by simulations and experiments. The proposed method can be flexibly extended to other physical fields, like acoustics and electromagnetics, providing inspiration for metamaterials design in a wide range of communities.

Temperature-dependent switchable thermal bifunctions in different diamond-shaped devices

The different temperature-dependent switchable thermal devices are designed in this paper, which can automatically realize the switchable thermal concentrating or cloaking functions at different temperature intervals. Based on the temperature-dependent transformation thermotics, we first theoretically derive the material parameters of the diamond-shaped switchable thermal concentrators and cloaks in the transformed space, thus achieving the construction of the mathematical models of temperature-dependent switchable thermal concentrators and cloaks. Then, we further integrate the switchable thermal concentrating and cloaking functions described above into a single device to construct the path-dependent switchable devices, which can intelligently control the thermal concentrating and cloaking functions along different axes simultaneously as the temperature changes. Finally, the temperature-dependent properties of the switchable thermal concentrators, cloaks and path-dependent devices are validated numerically. The results show that the switchable thermal concentrators, cloaks, and path-dependent devices can perfectly realize the thermal concentrating, cloaking, and x-directional concentrating and y-directional cloaking functions at specific different temperature intervals, respectively. Once the specific temperature intervals have changed, the thermal bifunctions described above are turned off automatically. The temperature-dependent switchable properties can also be extended to other physical fields to design the intelligent multi-physical metadevices.

Fast active thermal cloaking through PDE-constrained optimization and reduced-order modelling

In this paper, we show how to efficiently achieve thermal cloaking from a computational standpoint in several virtual scenarios by controlling a distribution of active heat sources. We frame this problem in the setting of PDE-constrained optimization, where the reference field is the solution of the time-dependent heat equation in the absence of the object to cloak. The optimal control problem then aims at actuating the space–time control field so that the thermal field outside the obstacle is indistinguishable from the reference field. In particular, we consider multiple scenarios where material’s thermal diffusivity, source intensity and obstacle’s temperature are allowed to vary within a user-defined range. To tackle the thermal cloaking problem in a rapid and reliable way, we rely on a parametrized reduced order model built through the reduced basis method, thus entailing huge computational speedups compared with a high-fidelity, full-order model exploiting the finite-element method while dealing both with complex target shapes and disconnected control domains.

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.

Design of thermal cloak and concentrator with interconnected structure

Heat flux manipulation based on thermal metamaterials under diffusive heat conduction has been extensively studied due to their extraordinary potential in energy utilization. However, the requirement of extreme material parameters or even infinite singularity may significantly limit their practical applications. In this work, we propose a new method for uniformly designing thermal cloak and concentrator with interconnected structure. The condition for realizing the undisturbed temperature distribution of the background medium in thermal cloak is strictly deduced. Through adjusting two geometric parameters of the interconnected structure, we analytically demonstrate that the thermal cloak and concentrator are realizable, where the heat flux can be shielded or concentrated in the target region. The desired thermal functionalities are confirmed both by the finite element simulations and heat transfer experiments. Furthermore, the results of interconnected structure are in excellent agreement with that of conventional thermal devices, and demonstrate that the interconnected structure provides an efficient method for manipulating heat flux over a very wide range (from cloaking to concentrating) despite having a simple composition and structural characteristics. The feasibility of interconnected structure can enhance the selection of bulk natural materials in addition to easy manufacturability for the heat flux manipulation, which might be extended to other disciplines, such as electromagnetics and acoustics.

Radial Basis Function表現とレベルセット関数によるCMA-ESに基づくトポロジー最適化の加速

In this paper, we developed topology optimization that represents the level-set functions using the radial basis function（RBF）and attempted to speed up the solution search using covariance matrix adaptation evolution strategy（CMA-ES）. By using a continuous and smooth level-set functions represented by RBF to represent the boundary between solid and void, the number of design variables and the accuracy of the finite element analysis can be separated, and the number of design variables can be reduced while maintaining the accuracy. Using thermal cloaks as numerical examples, it was confirmed that the optimal configurations and fitness are almost the same as those of conventional design methods, even when RBF is introduced.

Enhanced Thermal Invisibility Effect in a Isotropic Thermal Cloak with Bulk Materials

Enhanced Thermal Invisibility Effect in a Isotropic Thermal Cloak with Bulk Materials

Topology Optimization of Thermal Cloaks Using Level Set Method

Topology Optimization of Thermal Cloaks Using Level Set Method

Performance investigation of nanoscale thermal cloak by the perforated silicon film

In recent years, nanoscale thermal cloak, as a representative of nanoscale heat flux regulation devices, has attracted a lot of attention from researchers. However, the existing design methods are relatively complicated and all adopt constant temperature boundaries, the temperature changes constantly in the real environment, which greatly hinders its engineering applications. In this paper, inspired by phonon localization theory, we construct a nanoscale thermal cloak by a perforated silicon membrane and evaluate its cloaking performance and dynamic response. Results show that when the perforated area is fixed, the more the number of holes, the better the cloaking performance. In addition, the nanoscale thermal cloak still exhibits good cloaking performance in the dynamic environment. Finally, the cloaking mechanism is analyzed by calculating the phonon density of states (PDOS) and mode participation rate (MPR), and the reduction of thermal conductivity in the functional region is attributed to phonon localization.

Thermal Cloak: Theory, Experiment and Application.

In the past two decades, owing to the development of metamaterials and the theoretical tools of transformation optics and the scattering cancellation method, a plethora of unprecedented functional devices, especially invisibility cloaks, have been experimentally demonstrated in various fields, e.g., electromagnetics, acoustics, and thermodynamics. Since the first thermal cloak was theoretically reported in 2008 and experimentally demonstrated in 2012, great progress has been made in both theory and experiment. In this review, we report the recent advances in thermal cloaks, including the theoretical designs, experimental realizations, and potential applications. The three areas are classified according to the different mechanisms of heat transfer, namely, thermal conduction, thermal convection, and thermal radiation. We also provide an outlook toward the challenges and future directions in this fascinating area.