ELECTRIC-FIELD ENHANCED DEPOSITION IN FLAME-SYNTHESIZED MATERIALS MANUFACTURING

Theoretical and experimental studies of electric field enhanced deposi tion of flame-synthesized silica onto a disk target were carried out i n relation to applications in optical waveguide preform manufacturing. The deposition method utilized an imposed electric field to induce a charge on the particle and cause electrophoretic drift in addition to thermophoretic drift towards the deposition target, An analytical mode l utilizing an axisymmetric, viscous stagnation-point flow analysis wa s developed. From the model calculations, the overall thermophoretic d eposition rate (mass/time) onto the target was found to be proportiona l to the target size to the power of 3/2. In the presence of an applie d electric field the model calculation results showed that for a const ant particle density and a constant particle charge the overall deposi tion rate increased as the strength of the applied electric field incr eased. The results also showed that for a constant particle density an d a constant applied potential the deposition rate increased as the de gree of particle charging increased. To confirm the analytical results , experiments were carried out. After preliminary experiments to estab lish optimal conditions for deposition measurements, silica deposition rates onto targets were measured both in the absence and in the prese nce of applied electric fields. The experimental results for thermopho retic deposition were found to be within 12% with respect to the depos ition rates predicted by the model. When a potential of -1.6 kV was ap plied to the 9 cm target, for particle average charge of 2.78e (e: ele ctronic charge) the deposition rate (0.28 g min(-1)) was increased by approximately 35% compared to the thermophoretic deposition rate (0.2 g min(-1)). Related to the deposition measurements, interesting aspect s of silica charging mechanisms were discussed.