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Abstract
The characteristic and electrophoretic deposition (EPD) of anatase TiO2 dispersed in water at pH 7 was investigated. The purpose of this study was to determine an effective method to produce high-quality TiO2 coatings.
Sedimentation tests (solids loading, pH, and concentration of deflocculant) were done to assess optimal EPD parameters. EPD was done using Ti foil substrates as cathode and anode, using variable parameters: Voltage, deposition time, electrode distance, substrate flatness, and substrate roughness. The films obtained were characterised by optical microscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and profilometry.
The characteristic studies showed that the TiO2 suspensions were very stable when the pH value was ≥5. The effect of deflocculant ammonium polyacrylate (NH4PA) was found to be insignificant in optimising the suspension. The use of a solids loading of 0.03 g/mL and distilled water at pH 7 (no deflocculant) was determined to be suitable for the EPD process.
The different parameters were found to affect the film roughness in two different ways: (1) At low applied voltages and short deposition times, the films deposited followed the contours of the substrates; (2) at high applied voltages and long deposition times, rougher films were formed owing to greater deposition of agglomerates and/or electrolysis. The overall behaviour of the roughness as a function of time was sigmoidal, with increasing voltage increasing the roughness.
Different pH values also affected the film roughness, where pH 3 had rougher surfaces than those deposited at higher pH values owing to the formation of soft agglomerates. Other effects, such as substrate flatness, substrate roughness (ground at different SiC paper grades), and handling during the EPD all exhibited an influence on the roughness of the films.
The thickness of the films increased linearly with increasing voltage, increasing time, and decreasing electrode distance. However, the deposition rate gradually decreased as the thickness of the deposition increased owing to the formation of an electrically insulating layer, giving an overall logarithmic function. The significance of experimental parameters to the coating thickness was in the order: Voltage > time >> electrode separation.