Self-adaptive thermoregulation, mimicking the temperature-balancing mechanisms of living organisms, offers a promising pathway towards decarbonizing heating and cooling. Controlling radiative heat transfer via emissivity modulation is a key strategy. While various passive systems utilizing thermochromic materials (metal-oxides like VO2, chalcogenides, perovskites, and liquid crystals) have been developed, they often lack scalability, cost-effectiveness, and design flexibility. Current approaches, such as multilayered Fabry-Perot cavities, metasurfaces, and thin films, frequently necessitate specialized equipment and cleanroom environments, limiting their widespread adoption. Furthermore, these often require rigid substrates, restricting their applicability to planar surfaces. Existing solutions using spherical particles achieve only modest emissivity modulation. This paper aims to address these limitations by introducing a novel approach based on infrared thermochromic dipole antennas.

Significant research focuses on utilizing thermochromic materials for passive thermoregulation, leveraging their thermally driven phase change around a critical temperature (Tc). Metal oxides (e.g., VO2), chalcogenides (e.g., GST and IST), and perovskites (e.g., SmNiO3 and LSMO) are prominent examples. Thermochromism leads to self-adaptive heat emission due to varying absorption and emissivity profiles across the phase transition, governed by Kirchhoff's law. The key design parameter is the hot-to-cold emissivity contrast (Δε), which must be positive for passive thermoregulation. Existing strategies to maximize Δε often employ resonant structures like multilayered cavities, metasurfaces, or thin films. However, scaling these approaches presents challenges due to fabrication complexities and cost, typically requiring specialized equipment and cleanroom environments, as well as rigid substrates incompatible with non-planar surfaces. Previous work with spherical thermochromic particles demonstrated only limited emissivity modulation.

This study proposes using infrared thermochromic dipole antennas to enhance self-adaptive heat radiation. Vanadium dioxide (VO2), known for its insulator-to-metal transition (IMT), served as the proof-of-concept material. Rod-shaped VO2 antennas were synthesized via hydrothermal synthesis, allowing for precise control over length and aspect ratio. The synthesis involved a two-step process: reducing V2O5 to form a transparent blue solution, followed by hydrothermal reaction to produce the black precipitate. The phase transformation of VO2 (A) to VO2 (M) was achieved through annealing. Flexible composite films were fabricated by hot-pressing a mixture of high-density polyethylene (HDPE) powder and VO2 antennas against an aluminum back reflector. The spectral emissivity of the composite films was measured using Fourier transform infrared spectroscopy (FTIR). A second proof-of-concept involved spray-coating a VO2 antenna solution onto surfaces. Numerical simulations were performed using both a simplified Beer-Lambert's law model and first-principle radiative transfer calculations (using mc-photon) to correlate the antenna absorption cross-section ratio with the composite emissivity. The simulations account for scattering effects and polydispersity of the antenna size distribution. The morphology of the antennas was further investigated by synthesizing VO2 nanostars through modifying the hydrothermal synthesis parameters. The impact of antenna morphology (rods, stars, flakes, core-shell spheres, and spheres) on emissivity switching was evaluated via simulations.

The research demonstrates that non-spherical dipole antennas significantly enhance the absorption cross-section ratio, leading to improved emissivity switching. Rod geometries were found to be optimal, achieving a ~200-fold increase in absorption cross-section ratio as temperature rises. The high surface-area-to-volume ratio of these antennas is crucial for suppressing non-radiative modes and enhancing radiative dissipation. Composite films made with only a small concentration of VO2 rod antennas embedded in HDPE exhibit a dramatic emissivity switching (Δε ≈ 0.56 at λ = 10 μm) and a narrow hysteresis width (ΔThyst = 7.7 °C). First-principle radiative transfer calculations showed good agreement with experimental results. The spray-coating technique demonstrated the versatility of the approach for creating free-form composites. Simulations indicated that the absorption cross-section ratio can be tuned by adjusting antenna width and length. The study also investigated the impact of antenna morphology, comparing rods, stars, flakes, and core-shell spheres. Rods exhibited the largest absorption cross-section ratio (132.4), followed by stars (56.7) and flakes (52.1). Scattering effects were shown to either enhance or reduce the emissivity contrast depending on the ratio of absorption and scattering cross-sections. The study reveals that only a small volume fraction of the antennas is necessary to achieve high Δε and that the method is compatible with various scalable manufacturing techniques.

The findings demonstrate a significant advancement in passive thermoregulation technology. The use of infrared thermochromic dipole antennas provides a scalable, cost-effective, and design-flexible solution for creating self-adaptive heat radiators. The high emissivity switching achieved, combined with the ease of fabrication, opens up various applications. The ability to tune the spectral response through antenna geometry adjustment allows for the development of wavelength-selective radiators. The compatibility with diverse manufacturing methods makes it suitable for various product forms such as coatings, fibers, membranes, and films. The success with VO2 suggests the broader applicability of the approach to other thermochromic materials with IMT transitions.

This research successfully demonstrates a novel approach to self-adaptive thermoregulation using infrared thermochromic dipole antennas. The method offers significant advantages in terms of scalability, cost-effectiveness, and design freedom compared to existing techniques. Future research could focus on exploring other thermochromic materials and antenna morphologies to further optimize emissivity switching and expand the range of applications. Investigating the use of these materials for thermal camouflage and exploring novel manufacturing techniques to improve the scalability and cost-effectiveness could also prove beneficial.

While the study demonstrates significant emissivity switching, the long-term stability of the composite materials under various environmental conditions needs further investigation. The impact of polydispersity in antenna size on emissivity contrast is considered in the simulations but further experimental verification is warranted. The current research predominantly focuses on two specific antenna morphologies (rods and stars). Exploring a broader range of shapes and their resulting properties could potentially lead to further optimizations.