Influence of Dopants on the Suspension Properties and Reactive Electrophoretic Deposition (REPD)

Doped glasses are usually manufactured by melting of silica. As the melting point of silica is 2100 °C, most of the suitable dopants evaporate. Hence, alternative processes for the fabrication of glasses are needed. Sintering of nano-sized silica powders to full densety and transparently occurs at 1400 °C because of its large sintering activity. Green bodies can be doped by means of gas infiltration, by soaking of green bodies with salt solutions or by using mixtures of different powders. A further development of the electrophoretic deposition (EPD) is the reactive electrophoretic deposition (REPD), where soluble salts are added into a suspension. The dissolved ions are adsorbed on the surface of the particles and can be deposited with the particles resulting in a homogeneously doped green body. Shaping of a green body and doping can be achieved within only one process step via REPD. The feasibility of this process was shown first for a suspension of SiO2 containing different amounts of boric acid or/and cobalt chloride. First of all, the influence of different quantities of boron acid or cobalt chloride on suspension properties like viscosity, stability and zeta-potential was investigated. Secondly, green bodies were shaped by REPD and characterized regarding deposition rate, green density and amount of dopant. Finally, sintering was carried out at different temperatures. Thus, it was shown that green bodies doped with boric acid could be sintered to full density at lower temperatures compared to undoped ones. However, the sintering temperature depends on the amount of boric acid added into the suspension.