Active thermal surfaces that can be electronically switched between states of high and low infrared emissivity have been made by a team of researchers from Turkey, the US and the UK (Salihoglu, O. et al. Nano Lett. 18, 4541–4548; 2018). The team says that the electronically controllable graphene surfaces can camouflage the thermal signature of an object they cover so that it blends in with background temperatures and becomes invisible to an infrared thermal camera. The surfaces can also make hot objects appear cold and vice versa.
The surfaces are made from multilayer graphene on top of an ionic-liquid-soaked polyethylene membrane and a thin bottom layer of gold. On application of an electrical bias of ~3 V between the graphene and the gold, intercalation of positive ions into the graphene occurs. This serves to dope the graphene and increase the charge density and raise the Fermi level to higher energies, thus suppressing infrared absorption and emissivity. The result is that the surface switches to an effective cold state. The effect is reversible and switching off the electrical bias causes the emissivity of the surface to rise back up to its original value.
Tests with the surface placed on a hot plate at a temperature of 55 °C show the emissivity at a wavelength of 10 μm changes between values of 0.76 to 0.33 as the voltage is ramped up from 0 V to 3.5 V. The switching takes <1 s and is accompanied by a step-like variation in the electrical sheet resistance from 33 Ω to 0.6 Ω.
Furthermore, by pixelating the rear gold electrode, the surface can be turned into an array of independently controllable thermal pixels, allowing spatial temperature patterns to be created on demand. In experiments with a 5 × 5 array of square pixels (each with an area of 2 × 2 cm2), the researchers were able to write the word ‘HELLO’ with a temperature contrast of ~10 °C (35–45 °C).
As the surfaces are lightweight (30 g m–2), thin (50 μm thick), flexible and can potentially be fabricated in large areas, the team believes that a wide range of potential applications are possible, including their use in thermal camouflage and adaptive heat shields for satellites.
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Synthesis and fine spectroscopy tuning of the hyperspectral simulation material based on organic anions intercalated Mg-Al layered double hydroxide
Infrared Physics & Technology (2020)