Sol Gel Dip Coating Process

Sol-gel dip-coating is a process where the substrate to be coated is immersed in a liquid and then withdrawn with a well-defined withdrawal speed under controlled temperature and atmospheric conditions, Figure 3 [4]. Vibration-free mountings and very smooth movement of the substrate are essential for the dip-coating process. The coating thickness is mainly defined by the withdrawal speed, the solid content, and the sol viscosity.

The dip-coating route can be divided into three stages: (i) pulling phase, (ii) drying, and (iii) densification.

(i) Pulling Speed: The entrainment of the draining fluid (sol) film during the dip-coating process establishes a layer, which increases in thickness with the substrate withdrawal speed. Draining and simultaneous evaporation accompanying pulling, strongly influences the thickness as well as the resulting solid volume fraction of the densified film. The thickness-speed relation is directly linked to the complex time-dependent exchanges near the meniscus. The higher humidity and viscosity of the sol increase the film thickness under given pulling rate.

(ii) Drying: Drying is necessary in order to remove the solvent completely before densification of the film to avoid any cracking. Minimum drying temperature of 150 °C is required for complete solvent removal from the thin films. During the drying stage, the structure of the deposited film depends on the competition between two phenomena such as evaporation, which compacts the film, and the condensation reactions (elimination of —OH groups), which strengthen the film, increasing its resistance to compaction.

(iii) Densification: Annealing temperature range of 200600 °C with annealing time of 30-60 min are generally selected for final densification and crystallization of thin films. Increasing annealing temperature reduces the film thickness. The average nanocrystallite size and its distribution increase with increasing annealing temperature. The atomic density of the film and its refractive index also increase with increasing annealing temperature.

Figure 3. Steps involved in the sol-gel dip-coating: (a) dipping, (b) withdrawal and concurrent deposition, and (c) evaporation.

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