Temperature-independent thermal radiation

Abstract

Thermal emission is the process by which all objects at nonzero temperatures emit light and is well described by the Planck, Kirchhoff, and Stefan-Boltzmann laws. For most solids, the thermally emitted power increases monotonically with temperature in a one-to-one relationship that enables applications such as infrared imaging and noncontact thermometry. Here, we demonstrated ultrathin thermal emitters that violate this one-to-one relationship via the use of samarium nickel oxide (SmNiO₃), a strongly correlated quantum material that undergoes a fully reversible, temperature-driven solid-state phase transition. The smooth and hysteresis-free nature of this unique insulator-to-metal phase transition enabled us to engineer the temperature dependence of emissivity to precisely cancel out the intrinsic blackbody profile described by the Stefan-Boltzmann law, for both heating and cooling. Our design results in temperature-independent thermally emitted power within the long-wave atmospheric transparency window (wavelengths of 8 to 14 µm), across a broad temperature range of ∼30 °C, centered around ∼120 °C. The ability to decouple temperature and thermal emission opens a gateway for controlling the visibility of objects to infrared cameras and, more broadly, opportunities for quantum materials in controlling heat transfer.

Keywords: heat transfer; phase transition; quantum materials; thermal emission; thermal radiation.

Fig. 1.

Comparison between a typical thermal emitter and a zero-differential thermal emitter (ZDTE). (A and B) For a typical emitter, for example comprising a semiconductor or insulator (cartoon band diagram in B, Top), any change in emission from a temperature-dependent change in materials properties is dwarfed by the $T^4$ dependence in the Stefan–Boltzmann law. Conversely, a ZDTE decouples temperature and thermal radiation over some temperature range and thus can only be made using a material with a very strong temperature dependence. In our implementation, we use the hysteresis-free insulator-to-metal phase transition in samarium nickelate (SmNiO₃) to achieve this behavior (B, Bottom). DOS, density of states. (C and D) LWIR images of samples mounted to hang off the edge of a heater stage, such that a temperature gradient is established from hot to cold. (C) A reference sample with a constant emissivity—in this case, a sapphire wafer—and (D) a ZDTE based on SmNiO₃. The color bar encodes the apparent temperature, obtained by assuming a particular set emissivity, ${\epsilon }_{set}$, which was chosen such that the sample region just below the heat stage appeared to be at 130 °C, which is the actual temperature at that point (see more discussion in Methods). For sapphire, there is a one-to-one relationship between temperature and thermally emitted power. Conversely, the ZDTE exhibits a constant emitted power over a range of temperatures, here ∼100 to 135 °C.

Fig. 2.

Hysteresis-free insulator-to-metal phase transition in SmNiO₃. (A) Normalized temperature-dependent electrical resistance of our SmNiO₃ thin film grown on a sapphire substrate and (B) mid-infrared reflectance at several representative wavelengths, during both heating and cooling, showing the hysteresis-free nature of the IMT in SmNiO₃. The insets in A are nanoscale XAS maps at 105 and 120 °C, where the ratio of X-ray absorption at 848 eV to that at 849 eV is plotted as an indication of the metallic/insulating properties; no features other than detector noise are observed, indicating a gradual transition with no observable domain texture. (C and D) Temperature-dependent (C) real and (D) imaginary parts of the complex refractive index of the SmNiO₃ film, as a function of wavelength across the mid infrared, extracted using spectroscopic ellipsometry.

Fig. 3.

Zero-differential thermal emission. (A) Calculated temperature derivative of the emitted radiance, integrated over the 8- to 14-µm atmospheric transparency window, of an SmNiO₃ film with thicknesses $d$ from 50 nm to infinity, on a semi-infinite sapphire substrate. (B) Measured wavelength- and temperature-dependent emissivity of our ZDTE, comprising a ∼220-nm film of SmNiO₃ on a sapphire substrate, via direct emission (dotted) and Kirchhoff’s law using reflectance measurements (solid). (C) The temperature-dependent spectral radiance of the ZDTE, which is the product of the spectral emissivity in B and the Planck distribution. (D) Thermally emitted radiance of our ZDTE, integrated over 8 to 14 µm, compared to that of a blackbody.

Fig. 4.

Long-wavelength infrared (LWIR) images of samples held at temperatures from 100 to 140 °C. The emissivities of the laboratory blackbody (carbon nanotube forest), sapphire wafer, and fused SiO₂ wafer do not change appreciably over this temperature range. The emissivities of our SmNiO₃-based ZDTEs change as a function of temperature, and thus effectively mask the temperature differences from the camera. The apparent temperature is plotted (like in Fig. 1 C and D) with ${\epsilon }_{set}$ for each sample selected such that for a stage temperature of 100 °C the infrared camera returned this value as the temperature reading. The dark squares on the bottom row are metal electrodes that were used for the resistance measurements in Fig. 2A.