UC San Diego

The control and manipulation of heat flux could lead to reduced energy losses as well as pave the way for creation of thermal analogues to electronic and optical elements such as diodes and lenses. Additionally, the developed principles can be employed for enhancing thermal to electrical energy conversion efficiencies as well as for efficient cooling. While recent work on thermal energy control seeks to understand atomic scale paradigms that control transport processes, e.g., phononics, with the broad objective of manipulating heat flow with electronic analogs and beyond, our approach is nominally distinct and involves ideas borrowed from transformation optics. The principle that heat flux takes the path of least thermal resistance, analogous to the Fermat principle for light, was used to fabricate thermal devices, which could be used to concentrate heat similar to a converging lens. Finite Element analysis (FEA) based simulations as well as analytical relations were used to trace the path of heat flow in the thermal lens. For demonstrating such effects experimentally, fabricated thermal lenses consisted of a multi-layered structure made of carbon nanotube (CNT) polymer composites, constituted from layers with gradually increasing CNT composition and concomitant increased thermal conductivity. The heat flux concentration, due to a temperature gradient across the thermal lens was monitored through a infrared (IR) imaging technique, and found to be in excellent accord with the simulations. From our experiments, we show 40 % increase in flux concentration using a rectangular geometry and a 50 % increase using a reducing geometry setup, which was transduced to electrical energy, through a thermoelectric generator. area / cross-section, which was demonstrated experimentally. Our results pave the way for further understanding of ways to control and manipulate heat propagation