Controlling macroscopic heat transfer with thermal metamaterials: Theory, experiment and application

Classical thermodynamics often helps to passively describe macroscopic heat phenomena of natural systems, which means people almost cannot change the heat phenomena, but understand them according to the four thermodynamic laws. In contrast, thermal metamaterials, together with the governing theories, make it possible to actively manipulate macroscopic heat phenomena of artificial systems, which enables people to change the heat phenomena at will. Such metamaterials or metamaterial-based devices refer to those artificial structures that yield novel functions in controlling heat transfer. Since the concept of thermal cloak was proposed in 2008, this field has been developed rapidly with fruitful research results, which range from both theoretical models and experimental techniques in scientific research to practical applications in industry, such as radiative cooling and infrared camouflage. In this review, we comb through the research history of thermal metamaterials, and present novel functions and their associated theories in four areas. Such theories include both transformation theories and their extended theories, which are called theoretical thermotics for convenience. The four areas are classified according to the different ways of heat transfer, namely, heat conduction, heat conduction–convection,heat conduction–radiation, and heat conduction–convection–radiation. The corresponding experiments and applications are also introduced. At last, we provide our views on future opportunities and challenges in thermotics of metamaterials.