Transformation thermotics: thermal metamaterials and their applications

Abstract

Heat transportation is one of the most ubiquitous phenomenon in the mother nature. Manipulating heat flow at will is of tremendous value in industry, civil life and even military. It would be a common sense that in different materials thermal properties are different. According to this knowledge people may design thermal materials to control heat conduction. One of the most common and successful example is blanket, which has been invented for thousands of years to keep us warm in cold days and keep icecream cool in summer. However, those great inventions are not powerful enough to manipulate heat flow at will. So there are still a lot of demands for designing the so-called metamaterials which have special properties that should not exist in nature. In 2006, Leonhardt and Pendry's research group (Pendry, Schurig and Smith) independently proposed a type of optical metamaterial which is also called invisible cloak. This device is well known for bending light around an object to make it invisible. Such a significant progress soon enlightened a lot of scientists in different aspects since it offers a powerful approach to design metamaterials. The principle of invisible cloak, which is concluded as transformation optics has been applied to light waves, acoustic, seismic, elastic waves, hydrodynamics and even matter waves as they all satisfy with wave equation. Although the conduction equation which governs the process of heat conduction is totally different from wave equation, from 2008 to 2012, Fan's group and Guenneau's group established the theoretical system of transformation thermotics. Since then, many thermal metamaterials with novel thermal properties have been figured out. Therefore, a boom in transformation thermotics and thermal metamaterials has begun. In this article, we will introduce some most recent achievements in this field, including novel thermal devices, simplified experimental method, macro thermal diode based on temperature dependent transformation thermotics, and the important role that soft matters play in the experimental confirmations of thermal metamaterials. These works pave the developments in transformation mapping theory and can surely inspire more designs of thermal metamaterials. What is more, some approaches proposed in this article provide more flexibility in controlling heat flow, and it may also be useful in other fields that are sensitive to temperature gradient, such as the Seebeck effect and many other domains where transformation theory is valid.