The effects of electrical fields on growth of titania particles formed in a CH4-O-2 diffusion flame

The effects of a DC electric field on the growth processes of titania parti cles formed in a premixed CH4-O-2 flame were investigated for three basic t ypes of electrode geometry, namely pairs of needle tips, spheres and plates . The conclusions are based on measurements of the primary particle diamete r, agglomerate mobility distribution and fractal dimension, agglomerate cha rge distribution, as well as flame temperature profile and electrical curre nt, wherever possible with spatial resolution. Changes in primary particle diameter and rutile content were found to correlate well in magnitude and s ign with shifts in flame temperature profile induced by the electric field. When measured just above the flame, needle electrodes were found to increa se agglomerate sizes (in terms of average number of primary particles per a gglomerate) by up to 3-fold, explainable by a redistribution of aerosol mas s into more but smaller primary particles (as a result of lower flame tempe ratures). Electrostatic enhancement did not play a significant role as part icle charge distributions remained bipolar and little affected by the field . Plate electrodes lead to a slowing of agglomerate growth which compensate d the increased number of smaller primary particles, resulting in a little net change in number of primary particles as compared to no field. This is caused by separation of the aerosol into a positively and a negatively char ged plume as suggested by Vemury et al., however without addition of charge carriers. Aside from the slowing of coagulation in the plate configuration , which can be considered a primary electrostatic process, the effects of e lectrical fields resulted mostly from secondary mechanisms, especially shif ts in local flame temperature, which reduced or enhance the sintering rate in primary particles. Their magnitude depended on applied potential differe nce alone, not on electrode polarity. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.