||Antireflection (AR) coating is needed on glass substrate to improve light transmission, which benefits applications such as optical filters, photovoltaics, windows, eye-wear, and display screens. AR coatings have been produced by chemical etching, physical vapor deposition, sputtering and by sol-gel method. For a glass substrate (ns=1.52), an ideal homogeneous AR layer would require a refractive index of 1.23. The easiest way to achieve such a low index is a porous coating. Taking Silica as the matrix (ns=1.5), this would mean a porosity higher than 60%, which usually leads to a degraded mechanical strength.|
The objective of this research is to fabricate AR coatings with microporous crystalline material, zeolite, which is known to have a refractive index ~1.3. Since it is a crystalline material, the mechanical strength would be strong. To reach the desired refractive index of 1.23, inter-particle void below scattering limit can be introduced by using <60 nm nanocrystals.
In this research, AR layer was prepared on both sides of glass substrates by dip- coating with sols made form a combination of fully dispersed zeolite nanocrystals, concentrated zeolite precursors, surfactant template for inter-particle void and appropriate solvents. The major variables investigated were the recipe (type of surfactants, the surfactant to zeolite ratio, Aging time of zeolite precursor and the zeolite to precursor ratio), the dip coating parameters (sol concentration, pulling rate), and the heat treatment conditions after coating. The reflectivity of the obtained coatings was measured to identify the best recipe and procedures, along with critical issues such as adhesion strength, scratch resistance, durability in harsh environment and anti-soil ability.
The AR film formed using precursor sol (CP) along is comparatively dense and mechanically strong due to the excessive hydroxyl sites on precursor particles, but the associated reflective index is high. To reduce its refractive index, one has to add surfactant as pore forming template, but this quickly diminishes the original mechanical strength.
Zeolite nanocrystals, on the other hand, from a film with very low refractive index, thanks to its > 60% intercrystal and intracrystal porosity. However, fully crystalized particles have practically no surface hydroxyl group to bond with the substrate, and the adhesion of the film is very weak.
The obvious next step is to combine the reactive precursor as glue and the nanocrystals as porous brieck to form a strong and porous AR layer. Under the best combination tested so far, we were able to prepare coating that has less than 1% average reflection in the visible range. The adhesion strength, scratch resistance of the coating were also comparable to commercial requirements. The coating prepared currently is super-hydrophilic in nature, but can be easily made hydrophobic by reacting with flourosilane.
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