A thermochromic film embedded with size-optimized microcapsules autonomously switches between passive heating and cooling modes while maintaining high infrared emission for all-season thermal regulation.
(Nanowerk Spotlight) Every summer, air conditioners around the world consume staggering amounts of electricity to keep buildings and vehicles cool. Global residential cooling demand continues to rise sharply, creating a feedback loop where the energy used for climate control accelerates the warming that drives the need for more cooling.
Against this backdrop, passive radiative cooling has emerged as an elegant potential solution. Certain materials can channel heat directly into outer space by emitting infrared radiation through a narrow transparency “window” in Earth’s atmosphere, where wavelengths between 8 and 13 micrometers escape largely unimpeded. No electricity required.
But a fundamental problem has stymied progress. Materials engineered to reject solar heat in summer become liabilities when temperatures drop, continuing to radiate warmth into space precisely when buildings need to retain it.
Scientists have pursued numerous workarounds. Vanadium dioxide coatings can shift their infrared properties in response to temperature, but they demand expensive fabrication and offer limited spectral control. Hydrogel-based thermochromic windows, including promising PNIPAM systems that modulate solar transmission based on temperature, can reduce indoor temperatures and cut cooling energy use (see our previous Nanowerk Spotlight: “A step towards a greener future: Thermochromic smart windows for energy efficiency“). Yet these materials only control how much sunlight passes through; they cannot actively cool a building or provide winter heating benefit, and they face challenges with durability and scalability.
Nanophotonic structures with precision-engineered multilayers achieve impressive spectral selectivity, but they require complex manufacturing and struggle with angular performance.
A research team from the University of Science and Technology of China and collaborating institutions now reports a material architecture that breaks this constraint. Published in Advanced Energy Materials (“Mie‐Resonant Thermochromic Safe Architectures for All‐Season Radiative Thermal Management”), their work describes a thermochromic composite film that autonomously switches between heating and cooling modes based on ambient temperature, while maintaining consistently high thermal emission in the atmospheric window regardless of operating state.
The film accomplishes this through precise engineering at the microscale. The researchers embedded temperature-responsive microcapsules within a porous polymer matrix made of poly(vinylidene fluoride-co-hexafluoropropylene), a fluorinated material with inherent infrared absorption properties.
Particle-size–selective thermochromic microcapsule strategy for radiative thermal management. (a) Bio-inspired Mie scattering structures in white swan feathers and beetle shells. (b) Spectral overlap between atmospheric emissivity and surface thermal emission. (c) Calculated scattering efficiency map as a function of particle size and wavelength. (d) Mid-IR scattering resonance shifts induced by the dielectric environment. Scattering spectra of 3, 6, and 9 μm microcapsules in air (solid) and embedded in PVDF-HFP (dashed). (e) Near-field electric field distribution showing strong wave–structure coupling. (f) Resonance broadening via a Gaussian particle-size distribution compared to discrete sizes. (g) Far-field scattering directionality showing dominant forward lobes. (h) Measured mid-infrared emissivity of PVDF–HFP composite films compared with a transparent conductive film reference. (i) Thermochromic switching mechanism of microcapsules: low-temperature crystalline state yields broadband absorption, while high-temperature melted state yields high reflectance with maintained long-wave infrared emissivity. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
Each microcapsule contains a phase-change mixture that reversibly shifts color through a chemical mechanism involving a dye, a developer compound, and fatty alcohols. At low temperatures, the fatty alcohol crystallizes, separating the dye from the developer and producing a dark state that absorbs sunlight for heating. At higher temperatures, the alcohol melts, allowing the dye and developer to recombine and suppressing visible absorption, which yields a light, reflective state that rejects solar energy.
The critical innovation lies in decoupling the visible and infrared responses. Using Mie scattering theory, which describes how particles interact with light waves of comparable wavelength, the team calculated that microcapsules with diameters between 4 and 6 micrometers would generate strong scattering effects precisely within the 8 to 13 micrometer atmospheric window.
Particles in this size range excite multiple resonance modes that extend the path length of infrared light traveling through the material. This enhancement boosts the film’s effective emissivity, its ability to radiate heat, without interfering with the thermochromic switching that occurs at visible and near-infrared wavelengths. The two spectral channels operate independently.
The porous polymer matrix contributes additional functionality. Air voids within the structure create sharp contrasts in refractive index at polymer-air interfaces, amplifying mid-infrared scattering. The matrix also develops a gradient in pore size through its thickness during fabrication, a consequence of controlled evaporation where acetone escapes faster than water.
This gradient produces directionally dependent thermal conductivity. Heat spreads readily in the horizontal plane but moves poorly through the film’s thickness. In heating mode, this architecture traps photothermally generated warmth, preventing it from escaping to cold ambient air. In cooling mode, the same structure promotes efficient radiative release while suppressing convective losses.
Laboratory measurements confirmed bidirectional performance. Under 600 W/m² of simulated sunlight, the film in its cold, absorbing state achieved a theoretical net heating power of approximately 245 W/m² when non-radiative losses were minimized. In its warm, reflective state at 40 °C ambient temperature under identical illumination, it delivered roughly 86 W/m² of net radiative cooling.
The switching transition proved sharp and reversible. As temperature rose from −5 °C to 40 °C, visible reflectance climbed from below 10% to above 90%, with minimal hysteresis. Throughout this range, emissivity in the atmospheric window remained high in both states, approaching near-blackbody levels.
Field testing in Hefei, China validated these laboratory results under real atmospheric conditions. Over 180 days of outdoor exposure, the film maintained sub-ambient surface temperatures with maximum differentials of approximately 10 °C. Mean cooling power reached about 102.7 W/m², averaging 113.6 W/m² at night and 94.9 W/m² during daytime.
The material showed no measurable degradation in equilibrium temperature or heating rate across the test period. Statistical analysis revealed that cooling power correlated positively with atmospheric transparency in the infrared window and negatively with cloud cover, consistent with the underlying physics.
The team demonstrated meter-scale continuous fabrication, addressing a persistent barrier that has kept radiative cooling materials confined to laboratory benches.
Building energy simulations using EnergyPlus software showed the thermochromic film outperforming conventional static radiative coolers across diverse climates. Benefits proved greatest in mid-latitude regions with strong seasonality, where heating and cooling demands alternate. Regional modeling projected potential annual energy savings exceeding 30 MJ/m² in some areas, with corresponding carbon dioxide reductions reaching 5 kg/m² per year in dense urban environments.
The applications extend beyond buildings. Simulations of electric vehicles and unmanned aerial vehicles, where climate control systems heavily drain battery range, showed similar advantages. In winter, the film’s absorbing state could reduce electrical heating loads. In summer, its reflective state could lower cabin temperatures without drawing power.
Thermal safety testing demonstrated stability to approximately 350 °C, with the material retaining more than 80% of its reversible function after 60 seconds of high-heat exposure. The porous architecture with embedded microcapsules provided intrinsic flame retardancy and visual over-temperature warning through its color-changing behavior.
The platform permits aesthetic customization as well. By modifying the molecular structure of the encapsulated dyes, the researchers produced films in multiple colors while preserving thermal performance, a feature that could facilitate adoption in architectural contexts where appearance matters.
This work establishes that thoughtful microstructural design can resolve the longstanding tension between passive heating and cooling. By exploiting size-dependent scattering to maintain high infrared emissivity while enabling independent thermochromic control of the solar band, the composite film points toward intelligent, zero-energy climate control that adapts to seasons and environments without external input.
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Jixin Zhu (University of Science and Technology of China)
, 0000-0001-8749-8937 corresponding author
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