Spectroscopic Ellipsometry Measurement of Spectrum-Dependent Refraction Index of Thermochromic Asphalt Binder

Innovative thermochromic asphalt binder has been developed to modulate the surface temperature of asphalt pavement and therefore to improve the durability and environmental impacts. The multifunctional design of thermochromic asphalt binder requires the material to achieve the appropriate solar reflectance, which is related to its optical constants. This paper studies the optical constants of thermochromic asphalt binder. Various thermochromic powder-doped asphalt binder films were deposited onto glass substrates by spin coating technique. The thickness histograms of the films were firstly characterized by a stylus profilometer. As the powder concentration in films increased from 0 to 50% by weight of asphalt binder, the range of thickness distribution varied from 0–2 μm to 0–18 μm, and the peak value of the thickness distribution increased from 0.8 to 1.6 μm. The optical constants (n, k) of various asphalt binder films were obtained by fitting the ellipsometric parameters (Ψ and Δ) data with New Amorphous model in the 1–6 eV (206–1240 nm) spectral range. It was found that the doping 10 wt.% thermochromic powders decreased refractive indices of the asphalt films between the visible and near infrared region while increasing the extinction coefficients in the infrared region. Moreover, with increasing thermochromic powder concentration from 20 to 50 wt.%, the optical constants of the films increased, and then decreased at the percolation threshold of concentration. The research from this study will help to optimize the design of optical properties of thermochromic asphalt binder to achieve desired thermal performance for pavement.