Sol–gel multilayer coatings for spectral control of thermal radiation

Optical properties, shrinkage, and crack formation up to 800 °C were studied on TiO2 and SiO2 sol–gel films for application to thermal radiation control. Using precursor sols comprising hydrolyzed alkoxides and polymers, high-transparency films were obtained for TiO2 and SiO2. However, after heating at high temperatures, TiO2 films exhibited a slight degradation in transmittance due to light scattering by rutile grains. For both materials film thickness decreased by nearly half by increasing the heating temperature from 130 to 800 °C, giving rise to a much smaller critical cracking thickness than that obtained at low temperatures. Within the maximum thickness of the crack-free films, we fabricated TiO2/SiO2 alternating stacks as very near-infrared optical filters that allowed the control of the radiation spectra from a plate heater. The emissivity spectra measured on the film/heater system at 750 °C indicated fair agreement with the designed optical transmittance of the stacks, proving the feasibility of applying sol–gel films to thermal radiation control.