Enhanced Mid-Infrared Emissivity Modulation in CaF2−VO2 Core−Shell Particle Coatings for Temperature-Adaptive Radiative Control

Abstract

Temperature-adaptive radiative coatings necessitate large and reversible emissivity modulation in the 8−13 μm atmospheric window to suppress radiative heat loss at low temperatures while enhancing thermal emission at elevated temperatures. Phase-change VO2 is a promising material for this purpose, but a large emissivity contrast across the atmospheric window in a simple, coating-compatible architecture remains challenging. Here, we propose CaF2−VO2 core−shell particles embedded in a polyethylene matrix on an aluminum back reflector to amplify the temperature-dependent mid-infrared absorption of an ultrathin VO2 shell. By combining Lorenz-Mie theory with coating-scale radiative transfer modeling, we perform a stepwise parameter search over the particle geometry, particle volume fraction, and coating thickness to identify a high-contrast coating design. The optimized CaF2−VO2 core−shell particle coating, based on particles with a CaF2 core radius of 490 nm and a VO2 shell thickness of 10 nm, achieves a blackbody-weighted emissivity contrast of 0.84 over the 8−13 μm atmospheric window at a particle volume fraction of 0.015 and a coating thickness of 100 μm, evaluated using a 298 K blackbody weighting. Near-field absorption analysis reveals that the CaF2 core reshapes the local mid-infrared field distribution, while the VO2 shell serves as the dominant temperature-dependent absorption region. Compared with an optimized pure VO2 particle coating, which shows a corresponding emissivity contrast of 0.45, the CaF2−VO2 core−shell architecture substantially enhances the atmospheric-window emissivity contrast with VO2 confined to an ultrathin shell. This work provides a core−shell phase-change particle strategy for efficient adaptive control of mid-infrared thermal radiation in coating-compatible nanomaterial architectures.