A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal-Insulator Transition.

Abstract

Thermal radiation from a black body increases with the fourth power of absolute temperature (T⁴ ), an effect known as the Stefan-Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (ε_int ), is insensitive to temperature (|dε_int /dT| ≈ 10^-4 °C^-1 ). The resultant radiance bound by the T⁴ law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which ε_int can be engineered to decrease in an arbitrary manner near room temperature (|dε_int /dT| ≈ 8 × 10^-3 °C^-1 ), enabling unprecedented manipulation of infrared radiation. As an example, ε_int is programmed to vary with temperature as the inverse of T⁴ , precisely counteracting the T⁴ dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.