Thermal resistance of nanoporous antireflective coatings on silica glass for solar tower receivers

Quarter-wave anti reflectance (AR) coatings were applied on silica glass by a sol–gel dip coating process to minimize reflective losses of the receiver cover materials for solar tower power plants. To investigate the thermal resistance of the coatings prepared from silica sols of 20 nm, mean particle sizes, solar transmittance, thickness, and density were determined as a function of isothermal heat treatments using UV–vis–NIR spectroscopy, profilometry, and X-ray reflectometry, respectively. The initial solar transmittance τ of the AR coated silica glass, cured at 773 K, was 0.973 for a film thickness d=111 nm. Heat treatments in the range from 1273 to 1423 K for 15–240 min reduce both film thickness and solar transmittance of the crystal-free films. The isothermal shrinkage rate of the film thickness was described for a relative density >0.634 using Mackenzie–Shuttleworth (MS) model. Viscosity data calculated from the sintering rate constant of the MS model were compared with literature data of silica glasses of known water content prepared by sintering and fusion. This analysis indicates that the sintering kinetics of the nanoporous film were controlled by viscous flow of amorphous silica particles of a water content of ≈1100 ppmw associated with an activation energy of 321 kJ mol−1.