Guiding temperature waves with graded metamaterials

Temperature waves exist in many practical scenarios such as pulse heat sources and periodic temperature fluctuations, which have broad applications for thermal nondestructive detection. However, it is still challenging to guide temperature waves in irregularly shaped pipes because temperature waves might be strongly scattered. To solve the problem, we propose a scheme to guide temperature waves with graded metamaterials that have continuous parameter variations in space. Since graded metamaterials can ensure the same phases of thermal wavefronts despite different spatial distances, temperature waves can propagate in irregularly shaped pipes without scattering. We then demonstrate an elementary unit to realize the right-angle bending of temperature waves. Experimental suggestions are also provided to design the elementary unit with common materials and layered structures. We further combine the elementary units together to realize three practical functions including obstacle avoidance, thermal periscope, and reverse bending. These results have potential applications for thermal imaging and sensing in elbow pipes.