基于变换热力学的三维任意形状热斗篷设计

在变换热力学的基础上,通过坐标变换的方法,推导出三维任意形状热斗篷导热系数的通解表达式,并进行了全波仿真验证.结果表明:热流均能绕过保护区域流出,保护区域的温度保持不变,而且热斗篷外的温度场并没有破坏,具有很好的热保护和热隐身的效果.这一方法把变换热力学从二维拓展到三维,具有普遍的适用性.同时,这种技术为热流流动路径和目标温度场的控制奠定了理论基础,在微芯片、电动机的保护以及目标热隐身上有潜在应用.     

Thermal Magnifier and Minifier

For thermal conduction cases,one can detect the size of an object explicitly by measuring the temperature distribution around it.If the temperature is the only signature we can obtain,we will give an incorrect judgment on the shape or size of the object by disturbing the distribution of it.According to this principle,in this article,we develop a transformation method and design a dual-functional thermal device,which can create a thermal illusion thai the object inside it"seems"to appear bigger or smaller than its original size.This device can functionally switch among magnifier and minifier at will.The proposed device consists of two layers:the cloak and the complementary material.A thermal cloak can make the internal region thermally"invisible"while the complementary layer offsets this effect.The combination leads to the illusion of magnification and minification.As a result of finite element simulations,the performances of the illusions are confirmed.     

Quasi-invisible thermal cloak based on homogenous materials

Invisible thermal cloak, which cancels distortions of temperature distribution caused by objects, has many potential applications in thermal engineering. In this letter, we theoretically proposed and simulatively verified a new design method for quasi-invisible thermal cloak. Different from conventional transformation thermodynamics that focus on complete invisibility, our method only decreases the effective scale of objects to small enough and realizes a quasi-invisible cloaking effect in thermal conduction regime. However, this quasi-invisible cloak has the same effect as that of invisible thermal cloak in practical engineering. More important, our cloak is easy to construct by natural materials due to its homogenous thermal properties and can cloak objects with different shapes and properties. These characters make the clock more comfortable for engineering applications.     

While rotating while cloaking

Invisible optical and thermal cloaking have been explored as the typical demonstrations of the transformation optics and thermotics theory. However, the existing cloaks are realized by only one-coordinate transformation, and the cloaking layout, i.e. the form of electromagnetic wave/heat passing around the invisible region, is single for a long time. Here, we propose a new rotated thermal cloak which can unify the conventional cloaking and rotating together, and realize the while-rotating–while-cloaking effect. The required anisotropic thermal conductivity tensor is deduced from the new geometric mapping. Though rotated, the heat flux can be tuned around the central invisible region perfectly by the proposed rotated thermal cloak. The underlying physics is explored by comprehensive analysis of the distribution of the thermal conductivity tensor, which is further compared with those of the conventional cloak and rotator. The experimental feasibility is also discussed by validating the practical while-rotating–while-cloaking effect through a proof-of-concept design. The proposed rotated thermal cloak is expected to extend the possibility of cloaking scheme, and open avenues for the multiple coordinate transformation in counterpart physical fields, like optics, electrics, acoustics, magnetics, mechanics, etc.     

任意横截面柱形热斗篷研究与设计

基于变换热力学,导出了具有非共形任意横截面的柱形热斗篷热导率表达式,并在此基础上设计了具有非共形横截面的柱形热斗篷.全波仿真结果表明,热斗篷迫使外部热流绕过斗篷,导致隐身区域热通量为零,从而具有热保护功能;同时,热流绕过斗篷后将恢复原来的温度场分布,使其具有完美热隐身功能.此外,基于所导出的变换媒质热导率表达式,设计并求解了具有规则共形和非规则共形横截面的柱形热斗篷,发现它们同样具有热保护和完美热隐身功能.这表明通过选择适当的边界函数,所得变换媒质热导率表达式可用于设计任意横截面柱形热斗篷,具有普遍的适用性,这种技术在计算机芯片、卫星和航天器等的热保护中有潜在应用.     

Multi-window invisible cloaks

This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation. A hollow region can be hidden after this transformation. It was recently shown (Ma H, Qu S B, Xu Z and Wang J F 2009 \wx{Appl. Phys. Lett.}{94} 103501) that when the original point moves to the boundary of a cloak, the cloak can be designed to be open. Based on this theory, we propose multi-window invisible cloaks which can conceal a group of objects. Full wave simulations for invisible cloaks with regular and irregular shapes verified this method.     

Nonsingularity in two-dimensional cylindrical invisible cloaks

The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation. The exterior boundary of a cylindrical invisible cloak is unchanged and the interior boundary is extended from that of a point to that of a cylindrical region in coordination transformation. This transformation process makes perfect cloaks, but it causes singularity in the constitutive material parameters of cloaks. This singularity makes the cloaks impossible to realize in practice. In order to remove this singularity, this paper sets a small cylindrical region replacing a point in the space transformation. The cylindrical region is so small that it does not affect the invisibility effects, but it can remove the singularity for material parameters. Full wave simulations based on the finite element method were used to verify the designed cloaks.     

非均匀背景中任意柱状热斗篷的研究与设计

在变换热力学的基础上,通过坐标变换的方法严格推导出在层状背景和渐变背景下二维任意形状热斗篷导热系数的通解表达式,并在此基础上设计出非均匀背景下二维非共形热斗篷.全波仿真结果表明:在不同背景下,热流均能绕过保护区域流出,保护区域的温度保持不变,而且热斗篷外的温度场并没有破坏,具有很好的热保护和热隐身的效果.这一方法考虑到背景的复杂性,更加贴近工程实际应用,为未来灵活控制热流传递提供了一种可行的方法,对目标热隐身和热保护具有重要借鉴意义.     

Bandwidth evaluation of dispersive transformation electromagnetics based devices

In this paper, the transient responses of some devices which are based on transformation electromagnetics are studied, such as invisible cloaks and concentrators, by using the Finite-Difference Time-Domain (FDTD) numerical technique. In particular, effects of the inherent losses as well as the coating size of the ideal cylindrical cloak on its bandwidth and cloaking performance are examined. In addition, it is demonstrated that the performance of transformation electromagnetics based devices is affected by the material parameters in the design, although they may behave nicely under monochromatic plane wave illuminations. The obtained results are of interest for the future practical implementation of these structures.     

Design of two-dimensional elliptically cylindrical invisible cloaks with multiple regions

Two-dimensional(2D) elliptically cylindrical invisible cloaks with multiple regions are designed based on the transformation optics and the complementary media theory. Multiple invisible cloak regions can be obtained by properly using the compressed or folded transformation in each space layer. The constitutive parameter tensor expressions for each region have been obtained. The results of full wave simulations by using finite element software confirm the validity of the constitutive parameter tensor expressions. In addition, the parameters are relatively easier to realize.     

基于超材料的定向传热结构研究与设计

在热斗篷研究的基础上, 提出定向传热结构的研究. 基于变换热力学, 采用坐标斜变换推导了定向传热结构热导率分布的表达式. 数值计算结果表明, 外部热流流经定向传热区域时, 热流向设计的高温面流动, 而设计的低温面温度较低. 此外, 在导出的热导率分布表达式的基础上, 进一步进行坐标旋转变换, 得到的热导率表达式只有相互垂直的两个分量. 计算结果表明, 沿高温面法向的热导率增大时, 传热效率提高, 而且经过坐标旋转变换后, 高温面与低温面的温差增大. 定向传热在红外隐身、热防护领域具有潜在的应用价值.     

Design of diamond-shaped transient thermal cloaks with homogeneous isotropic materials

Transformation thermodynamics as a major extension of transformation optics has recently received considerable attention. In this paper, we present two-dimensional (2D) and three-dimensional (3D) diamond-shaped transient thermal cloaks with non-singular homogeneous material parameters. The absence of singularity in the parameters results from the fact that the linear coordinate transformation is performed by expanding a line segment rather than a point into a region, while the mechanism behind the homogeneity is the homogeneous stretching and compression along orthogonal directions during the transformation. Although the derived parameters remain anisotropic, we further show that this can be circumvented by considering a layered structure composed of only four types of isotropic materials based on the effective medium theory. Numerical simulation results confirm the good performance of the proposed cloaks.     

新型椭圆形互补隐身斗篷设计

基于变换光学理论和互补媒质理论,提出了新型椭圆形互补隐身斗篷的设计方法,并得到了本构参数张量表达式,利用基于有限元算法的电磁仿真软件对该模型进行了全波仿真验证,结果证实了所得到的表达式的正确性,这种隐身斗篷不仅能够实现对位于其内部的物体隐身,而且电磁波能透进隐身斗篷内部进而可以与外界进行通信。     

Invisible anti-cloak with elliptic cross section using phase complement

Based on the theory of phase complement,an anti-cloak with circular cross section can be made invisible to an object outside its domain.As the cloak with elliptic cross section is more effective to make objects invisible than that with circular cross section,a scaled coordinate system is proposed to design equivalent materials of invisible anticloak with elliptic cross section using phase complement.The cloaks with conventional dielectric and double negative parameters are both simulated with the geometrical transformations.The results show that the cloak with elliptic cross section through phase complement can effectively hide the outside objects.     

热超构材料：几何结构、工作机制与新奇性质

因为在热保护、热探测和热管理领域存在重要的应用价值,自由操控宏观热流一直是人类的 一个梦想。热超构材料正是为此目的应运而生,它是电磁超构材料在热学领域的延伸。在此,我将 综述该领域自2008 年诞生以来取得的若干研究进展,其将主要包括以下新奇热现象或功能器件： 热隐身;热聚集;热旋转;宏观热二极管;热伪装;热透明;热晶体;环境温差中零能耗保温;宏 观热网络中反常热传导;热对流隐身、聚集、伪装;热辐射制冷。我将介绍与之相关的微观或宏观 传热机制,这些机制可以通过以下理论或方法来理解或阐述：变换热学理论、Laplace 方程、热声 子能带理论、相变理论、变换热对流理论、热辐射制冷理论。我也将介绍这些材料从基础研究到工 业应用的发展前景。     

Broadband acoustic cloak for ultrasound waves

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and shadow. Because of the nonresonant nature of the building elements, this low-loss (～6 dB/m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64?kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.     

Two-dimensional thermal illusion device with arbitrary shape based on complementary media

On the basis of transformation thermodynamics and compensation medium theory, we develop a method to design a two-dimensional thermal illusion device with arbitrary shape, and the general expression of thermal conductivity in the each region is obtained. Simulation results show that when an object is covered with the thermal illusion device, it will accurately perform the same temperature distribution signature as another object we have predetermined. Owing to the property of deceiving and interfering with the observer, the thermal illusion device can achieve generalized thermal stealth by using thermal metamaterials, which may have a potential application in military field.     

An arbitrary-shaped acoustic cloak with merits beyond the internal and external cloaks

Based on transformation acoustics, an arbitrary-shaped acoustic cloak capable of functioning as an information exchange-enabling internal cloak and a movement-allowing external cloak is presented. The general expressions of material parameters for the acoustic cloaks with arbitrarily conformal or non-conformal boundaries are derived, and then the performances of developed cloaks are validated by full-wave simulations. Finally, the different characteristics of the linear and nonlinear transformations-based cloaks are compared and analyzed. The proposed cloak could lead to wider applications beyond that of normal cloaks, since it effectively compensates the insufficiencies of traditional internal and external cloaks. Besides, this work also provides a new method to design bifunctional device and suggests an alternative way to make a large object invisible.     

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.     

Construction and mechanism analysis on nanoscale thermal cloak by in-situ annealing silicon carbide film

In recent years,there is a strong interest in thermal cloaking at the nanoscale,which has been achieved by using graphene and crystalline silicon films to build the nanoscale thermal cloak according to the classical macroscopic thermal cloak model.Silicon carbide,as a representative of the third-generation semiconductor material,has splendid properties,such as the high thermal conductivity and the high wear resistance.Therefore,in the present study,we build a nanoscale thermal cloak based on silicon carbide.The cloaking performance and the perturbation of the functional area to the external temperature filed are analyzed by the ratio of thermal cloaking and the response temperature,respectively.It is demonstrated that silicon carbide can also be used to build the nanoscale thermal cloak.Besides,we explore the influence of inner and outer radius on cloaking performance.Finally,the potential mechanism of the designed nanoscale thermal cloak is investigated by calculating and analyzing the phonon density of states(PDOS)and mode participation rate(MPR)within the structure.We find that the main reason for the decrease in the thermal conductivity of the functional area is phonon localization.This study extends the preparation method of nanoscale thermal cloaks and can provide a reference for the development of other nanoscale devices.