Sugrue, Maria (2000) Sol-Gel routes to transparent conductive coatings. (PhD thesis), Kingston University, .
Abstract
A novel sol-gel process has been developed for producing transparent conductive tin dioxide films and for their application to polymer substrates. Initially, tin oxide was produced by hydrolysis of tin tetrachloride using amended versions of methods found in the literature. The tin oxide produced from the procedure did not adhere well to glass and was not transparent. The procedure was also very time consuming. Preliminary experiments using tin bis (acetylacetonate) dichloride as a precursor were very promising and all efforts were soon concentrated on that route. After a lot of experimentation optimising the method, it was found that colloids capable of producing conductive transparent tin oxide coatings were obtained approximately 10 days after beginning the procedure. Indium and antimony dopants were added to the tin oxide at the sol-gel stage in order to investigate their effect on the DC conductivities of the tin oxide coatings. Bulk conductivities in the ranges 1 to 100 Sm[sup]-1 and 13 to 800 Sm[sup]-1 were obtained for p- and n-type doped annealed tin oxide coatings on glass substrates respectively. The percentage transparencies of the coatings remained high for unannealed coatings (~80%) but decreased to ~ 40% after annealing. Polymer pre-treatments were carried out on the poly (methyl methacrylate), polycarbonate and poly (ethylene terephthalate) substrates. The contact angles and surface energies of the substrates were studied to investigate the effects of the surface pre-treatment techniques. Treatment of the poly (methyl methacrylate) and poly (ethylene terephthalate) substrates consisted of hydrolysis by aqueous sodium hydroxide. A combination of UV/ozone oxidation was used for the polycarbonate substrates. The tin oxide coatings were applied to the polymer substrates by spin-coating, solvent casting and spray coating. From optical microscopy studies, it was shown that the spin-coated films were the most uniform and least cracked. To further increase the adhesion of the coatings on the polymeric substrates, a systematic study was made of the effects of coupling agents, both on the substrate surfaces and in the sol-gel solutions themselves. Short pulses of heating were given to the coatings in an infrared furnace. The annealing did not increase the bulk conductivities greatly but the coatings retained a very high transparency (greater than 90%). To investigate the effect of infrared annealing on tin oxide powder, it was shown from X-ray diffraction that the crystallinity of the tin oxide increased significantly as a result of annealing in the infrared furnace.
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