Wet chemical deposition of transparent conducting coatings in glass tubes

A modified dip coating process was developed to deposit transparent conducting sol-gel SnO2:Sb (ATO, antimony doped tin oxide) coatings inside tubes and cavities in optical quality. The necessity for this improvement arises from the fact that the conventional dip coating process applied to tubes results in coatings with poor and non-reproducible properties, showing large variations in the thickness, an increased roughness and an island-like morphology. The main reason for this is found in a delayed and restricted solvent evaporation, due to an impediment of a laminar flow and a progressive saturation of the atmosphere inside the tube. By means of an additional exhausting pipe, a forced laminar flow was, therefore, generated inside the tube, which guarantees a fast evaporation and hydrolysis of the sol-gel films by a continuous renewal of the atmosphere. In this way, ATO coatings with an excellent thickness uniformity and a low roughness of Ra ≈ nm could be deposited on borosilicate glass tubes of 300 mm length and inner diameters down to 11 mm. After a heat treatment at temperatures up to 550 °C, single layer coatings exhibit a high optical transmission (T>85%) and a resistance down to 10 kΩ over a length of 22 cm