Thermal Metamaterials

The ability to steer the heat flux in a desired path requires materials with anisotropic thermal conductivity, but most conventional materials found in nature are isotropic. Given such limitations, my thesis is aimed at designing and fabricating anisotropic composites based on the concepts of transformation optics and effective medium theory. We show that when the orientation of the layers in the composite is physically rotated with respect to a constant temperature gradient, there would then be a corresponding introduction of off-diagonal components in the thermal conductivity tensor and thermal anisotropy is induced. Experimental evidence of the bending of heat to desired purpose, in analogy to that of light, through designed placement and orientation of nominally isotropic material is presented. An upward or downward heat flux bending of up to26°, in close agreement with theoretical estimates, was obtained in a metamaterial constituted from thin, stacked layers of copper and stainless steel. Transient observations of heat flow indicate anisotropic energy transport hinging on the relative differences between the elements of the thermal diffusivity tensor. Next, we discuss the possibility of bending of heat flux in a multilayered composite, corresponding to positive and negative refraction, according to whether the horizontal and the vertical components of the incident and refracted heat flux vectors point in the same or opposite direction, respectively. We propose practical designs where both positive and negative refraction phenomena may be observed. Next, a method for the most efficient removal of heat, through an anisotropic composite, is proposed. It is shown that an optimal placement of constituent materials, in the radial and the azimuthal variation, at a given point in the composite yields a uniform temperature distribution in spherical diffusers, accompanied by a very significant reduction of the source temperature, in principle, to infinitesimally above the ambient temperature, and underlies the design of a perfect thermal diffuser. Orders of magnitude enhanced performance, compared to that obtained through the use of a diffuser constituted from a single material with isotropic thermal conductivity has been observed and the analytical principles underlying the design were validated with simulations