Metamaterials, with artificially engineered periodic structure, has attracted a great deal of research attention, for the ability of manipulating the path of propagation of light or electromagnetic wave, sound or acoustic / elastic wave, heat or thermal wave and the possibility of cloaking objects from a certain incoming physical radiation, which has brought the invisibility or stealth, a tantalizing concept for mankind over several centuries in the science fiction to a technological reality. As a relative new member of the metamaterials family, thermal metamaterial has also gained much attention from the very beginning, and has been intensively investigated in recent years, for the promising ability of con-trolling the conduction of heat or the distribution of temperature. Thermal metamaterials and their applications in thermal management are significant in the design of electronic devices and systems, such as supercomputer, solid-state laser and solid-state lighting, high power microwave devices, thermoelectric energy harvesters and thermal imagers, where the thermal design plays a key role in performance and device reliability. In this review, we first make a general introduction and brief summary of the main progresses in metamaterials, espe-cially in the area of optics and electromagnetics. Then the basic theory of coordinate transformation as well as the novel phenomenon of cloaking in metamaterials has been introduced, which is the key technology and original demonstration of all kinds of metamaterials. As a matter of fact, transformation optics has also made the hitherto inconceivable ad-vancements in the field of thermodynamics possible with the remarkable assistance of metamaterials. We mainly focus on the review of the brief advances, recent progresses and trend of development in the field of thermal metamaterials, both of theoretical simulation as well as experimental results. We have amply narrated the design, models, approaches, results and behaviors of thermal metamaterials, and their applications in heat flux manipulation, thermal cloaking or stealth, thermal mirage, thermal protection, thermal management, thermal Information and many other aspects in this paper. Among these reported progresses, thermal cloaking, which manipulates the heat flux in such a way that it can neither enter into the cloaked region nor be distorted outside, is a particularly important subject in studying the thermal metamaterials due to its potential multidimensional applications, and sort out to be transformation-based cloaking (with inhomogeneous and anisotropic parameters), three dimensional cloaking (with a spherical shell structure), irregu-lar-shaped cloaking (theoretical simulation), ultra-thin cloaking, active cloaking (controlled by external electric field), time-dependent cloaking, multi-physical cloaking (cloaking in both heat and current) etc. Finally, based on the research status and development trend of thermal metamaterials, we have systematically summarized, classified and compared the contents and characteristics of relevant research in recent years, both in China and abroad, and provide an outlook on the future directions as well as applications of thermal metamaterials in the fascinating fields of cloaking, thermal management, information and so on. This review is helpful for understanding and further developing thermal metamaterials in applicable concepts and practical techniques for a variety of different thermal devices and systems, and to pave a way for the new avenues that leads to new future technologies.
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