We indicate the fundamental rationale underlying the control of temperature and the manipulation of thermal flux, with reference to a multilayered composite material. 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. The consequent bending of the heat flux lines is found to depend on both the (i) composite rotation angle, as well as the (ii) ratio of the thermal conductivities of the constituent materials.
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. This was done by inducing anisotropy in an effective thermal medium through off-diagonal components in the thermal conductivity tensor. An upward or downward heat flux bending of up to ± 26°, 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.
Multilayered oriented composites constituted from two materials of different thermal conductivities are shown to have the ability to direct thermal energy. The composites behave as effective media with anisotropic thermal conductivity. The guiding of the heat flux is shown experimentally with bending angle ranging from ∼25° to the maximum possible value of ∼45° (for the considered prototypical composites) with respect to a horizontal temperature gradient—in excellent accord with theoretical estimates and computational simulations. Such thermal metamaterials lay the basis for efficient manipulation of heat and for thermal elements, such as thermal concentrators and cloaks.
The utility of a metamaterial, assembled from two layers of nominally isotropic materials, for thermal energy re-orientation and harvesting is examined. A study of the underlying phenomena related to heat flux manipulation, exploiting the anisotropy of the thermal conductivity tensor, is a focus. The notion of the assembled metamaterial as an effective thermal medium forms the basis for many of these investigations and will be probed. An overarching aim is to implement in such thermal metamaterials, functionalities well known from light optics, such as reflection and refraction, which in turn may yield insights on efficient thermal lensing. Consequently, the harness and dissipation of heat, which are for example, of much importance in energy conservation and improving electrical device performance, may be accomplished. The possibilities of energy harvesting, through exploiting anisotropic thermopower in the metamaterials is also examined. The review concludes with a brief survey of the outstanding issues and insights needed for further progress.
We discuss the possibility of bending of heat flux in a multilayered composite typical to abnormal negative refraction, according to which the horizontal and the vertical components of the incident and refracted heat flux vectors point in the opposite direction. The engineered anisotropy of the thermal conductivity tensor is integral to such effects. We propose practical designs where such anomalous refraction phenomena may be observed and be used for heat flux redirection. V
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