Thermal Metamaterials Research Articles

Estimating interfacial thermal conductivity in metamaterials through heat flux mapping

The variability of the thickness as well as the thermal conductivity of interfaces in composites may significantly influence thermal transport characteristics and the notion of a metamaterial as an effective medium. The consequent modulations of the heat flux passage are analytically and experimentally examined through a non-contact methodology using radiative imaging, on a model anisotropic thermal metamaterial. It was indicated that a lower Al layer/silver interfacial epoxy ratio of ∼25 compared to that of a Al layer/alumina interfacial epoxy (of ∼39) contributes to a smaller deviation of the heat flux bending angle.

Layered thermal metamaterials for the directing and harvesting of conductive heat

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.

Converting the patterns of local heat flux via thermal illusion device

Since the thermal conduction equation has form invariance under coordinate transformation, one can design thermal metamaterials with novel functions by tailoring materials’ thermal conductivities. In this work, we establish a different transformation theory, and propose a layered device with anisotropic thermal conductivities. The device is able to convert heat flux from parallel patterns into non-parallel patterns and vice versa. In the mean time, the heat flux pattern outside the device keeps undisturbed as if this device is absent. We perform finite-element simulations to confirm the converting behavior. This work paves a different way to manipulate the flow of heat at will.

Guiding conductive heat flux through thermal metamaterials

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.

Experimental evidence for the bending of heat flux in a thermal metamaterial

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.

Full Control and Manipulation of Heat Signatures: Cloaking, Camouflage and Thermal Metamaterials

Thermal camouflage and cloaking can transform an actual heat signature into a pre-controlled one. A viable recipe for controlling and manipulating heat signatures using thermal metamaterials to empower cloaking and camouflage in heat conduction is demonstrated. The thermal signature of the object is thus metamorphosed and perceived as multiple targets with different geometries and compositions, with the original object cloaked.

Unconventional thermal cloak hiding an object outside the cloak

All the thermal cloaks reported in the literature can be used to thermally hide an object inside the cloak. However, a common limitation of this kind of thermal cloaks is that the cloaked object cannot feel the external heat flow since it is located inside the cloak; thus we call these cloaks “conventional thermal cloaks”. Here we manage to overcome this limitation by exploiting a class of unconventional thermal cloaks that enable the cloaked object to feel the external heat flow. Our finite-element simulations in two dimensions show the desired cloaking effect. The underlying mechanism originates from the complementary effect of thermal metamaterials with negative thermal conductivities. This work suggests a different method to design thermal devices where heat conduction can be controlled at will.

Geometrical considerations in the control and manipulation of conductive heat flux in multilayered thermal metamaterials

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 and the (ii) ratio of the thermal conductivities of the constituent materials.